Review Article | Open Access
Stem cells in Osteoporosis: From Biology to New Therapeutic Approaches
Osteoporosis is a systemic disease that affects the skeleton, causing reduction of bone density and mass, resulting in destruction of bone microstructure and increased risk of bone fractures. Since osteoporosis is a disease affecting the elderly and the aging of the world’s population is constantly increasing, it is expected that the incidence of osteoporosis and its financial burden on the insurance systems will increase continuously and there is a need for more understanding this condition in order to prevent and/or treat it. At present, available drug therapy for osteoporosis primarily targets the inhibition of bone resorption and agents that promote bone mineralization, designed to slow disease progression. Safe and predictable pharmaceutical means to increase bone formation have been elusive. Stem cell therapy of osteoporosis, as a therapeutic strategy, offers the promise of an increase in osteoblast differentiation and thus reversing the shift towards bone resorption in osteoporosis. This review is focused on the current views regarding the implication of the stem cells in the cellular and physiologic mechanisms of osteoporosis and discusses data obtained from stem cell-based therapies of osteoporosis in experimental animal models and the possibility of their future application in clinical trials.
Osteoporosis is the most common systemic disease of the skeleton with the main feature of the uninterrupted disturbance of bone microarchitecture, resulting, particularly high in the elderly, in an increased risk of bone fractures [1, 2]. Osteoporosis is endemic in developed countries with about 50% of women and 20% of men, over the fifties, being affected in the UK (National Osteoporosis Society, UK) and up to 50 million individuals meeting the criteria of the World Health Organization (WHO) for osteoporosis in industrialized countries in North America, Europe, Japan, and Australia . According to the WHO criteria, the diagnosis of osteoporosis is confirmed when the values of bone mineral density (BMD) are equal to or less than 2.5 standard deviations below the normal, and it is expressed as a , with a normal being equal of zero . Despite the current pharmaceutical intervention available, bone fractures are still very common in patients with osteoporosis and the associated medical costs are relatively high. In 2005 in the United States of America, the annual cost for the management of patients with fractures from osteoporosis reached 19 billion dollars. Given the increase in life expectancy in the coming years, it is estimated that the cost in 2025 will be higher than $25 billion .
The pathophysiological mechanism in osteoporosis is a disturbance of bone metabolism, by several risk factors that cause a dysregulation between bone resorption and new bone formation and disruption of the well-orchestrated procedure of osteogenesis and bone remodeling . Optimal bone metabolism is a dynamic mechanism throughout life where a constant process between the balance of proteolytic bone digestive activity of osteoclasts and the bone-secreting function of osteoblasts takes place . The physiological bone remodeling process that regenerates the entire skeleton has been calculated to take place every 10 years . Furthermore, this bone remodeling also controls the reshaping of the bone following injuries such as major fractures or stress-induced microfractures . When this bone remodeling is imbalanced, where bone formation is decreased and/or bone resorption is increased, osteoporosis occurs. The recruitment and activation of specialized bone cells and their progenitor cells at the site of the remodeling are orchestrated by multiple signal and receptor activation pathways, which is quite complicated and not fully elucidated. These pathways are recently targets for the development of new therapies, based on the regenerative medicine, for the management of osteoporosis . The main two specialized cells that have been mostly studied in bone remodeling are the osteoclast (bone resorption) and osteoblast (bone formation). These two cell types were found to have an interdependent relationship via a variety of mechanisms [11, 12]. Osteoporosis is mediated by several factors that induce both the onset of the disease and its maintenance, such as hormonal, nutritional, and behavioral factors, as well as a genetic predisposition [7, 13]. Bone mass reaches its maximum development within the first decade of adulthood and maintains it for the next years, but with increasing age, both women and men lose bone, with aging having a paramount influence on the onset and the progression of osteoporosis . Osteoporosis is quite clearly more aggressive in postmenopausal women, with a dramatic increase of the rate of bone turnover and a continuous bone loss due to estrogen deficiency [15, 16]. Estrogen deficiency and glucocorticoids represent the most common cause of osteoporosis .
During the last decades, stem cells have gained considerable attention, both for their broad use in basic research and for their potential capabilities in the regenerative medicine to develop therapeutic strategies for a large number of pathophysiological disorders . Stem cell therapies have been investigated in almost every degenerative disorder [19–22], and promising results from preclinical studies and clinical trials in several diseases have led to the development of new therapeutic strategies [23, 24]. This review focuses on the field of regenerative medicine that is attempting to develop new stem cell therapeutic strategies for osteoporosis to prevent and repair bone loss and to reduce the susceptibility of fractures in osteoporotic patients .
2. Overview of Current Therapies
The current therapeutic management of osteoporosis is based on a range of pharmacological agents that inhibit bone resorption or anabolic drugs that induce bone formation . Studies show that the antiabsorption drugs do not restore the loss of bone mass but inhibit its further absorption and bone degradation. Bisphosphonates, which are nonhydrolysable pyrophosphate analogs, inhibit the absorption of bone mass by disrupting the function of osteoclasts and affecting their survival via a disorder of the subcellular localization and the normal function of certain signaling proteins which leads to accumulation of toxic nucleotide metabolites . Although bisphosphonates are the most common treatment of osteoporosis, generally well tolerated and reduce the risk of osteoporotic-related fractures, a number of serious adverse events have been observed to occur , such as osteonecrosis of the jaw, gastrointestinal upsets, renal complications, and atypical femoral fractures [29–33]. Another agent that reduces bone loss and minimizes the risk of bone fracture is calcitonin, which is a hormone produced and secreted by the C cells of the thyroid gland. However, the use of calcitonin in long-term treatments has been implicated in an increased risk of malignancies, such as prostate cancer, and has also been found to be less effective in the increase of bone mass or the reduction of bone turnover when administered to postmenopausal women [34–36]. Recently, the European Medicines Agency (EMA) recommended that the benefits from calcitonin did not outweigh their risks in the treatment of osteoporosis, and thus, calcitonin is not typically described or developed as a treatment for osteoporosis [36, 37]. Selective estrogen receptor modulators (SERMs), a class of drugs that bind the estrogen receptor acting as either antagonists or agonists, are used in the treatment of osteoporosis. These factors appear to reduce the risk of osteoporotic bone fractures due to the reduction of the bone loss. Raloxifene and its chemical cousin arzoxifene are therapeutically administered for the prevention of fractures to postmenopausal women suffering from osteoporosis [38, 39]. However, a large randomized study has associated the administration of raloxifene to women with cardiovascular disease, after menopause, with an increased risk of thromboembolic events and strokes . Data suggest that both raloxifene and arzoxifene reduce vertebral fracture risk by improving BMD and improve osteoporosis in postmenopausal women; however, careful consideration is required when prescribing SERMs, mainly to patients with previous history of venous thromboembolism [41, 42].
Parathyroid hormone (PTH) is one of the two essential hormones that regulate calcium and phosphate homeostasis in the bones, it is an 84-amino acid polypeptide, and it is secreted by the parathyroid glands. Administration of PTH in an intermittent manner and at low doses shows an anabolic effect inducing bone remodeling . Teriparatide is a recombinant form of PTH that consists of the bioactive part of the hormone, the first 34 amino acids of the N-terminus. This agent is used for osteoporosis treatment and, in contrast to drugs that act as inhibitors of bone loss, mentioned above, induces bone formation by targeting recruitment of new osteoblasts (osteoblastogenesis) and osteoblast survival . The use of teriparatide, however, is limited for up to 2 years , and upon termination of treatment, the patient must continue with antiresorptive drugs as discontinuation of the treatment with PTH rapidly results in a decrease of the BMD . Despite the limitation, the use of teriparatide for the treatment of osteoporosis seems to increase the rate of healing of bone fractures resulting in improved functionality in osteoporotic patients . Calcium sensing receptor (CaSR), a G-protein-coupled receptor, is expressed on the surface of parathyroid cells, and it is activated by millimolar concentrations of extracellular Ca2+, which primarily regulates the secretion of PTH. Calcimimetics are a class of drugs that activate this receptor and inhibit the secretion of PTH, while calcilytics are CaSR antagonists and they stimulate the secretion of PTH. Accumulating evidence indicates that this receptor is implicated in the regulation of bone metabolism , and it has been suggested that this receptor could be a potential target in drug development for the management of bone metabolism disorders [48, 49]. Activation of CaSR, by a calcimimetic compound, using amniotic fluid-derived human mesenchymal stem cells, demonstrated a significant enhancement of their osteogenic differentiation after exposure in the osteogenic environment . Calcilytics were proposed for the management of osteoporotic patients , because these compounds have shown an anabolic efficacy, in OVX rat models of osteoporosis [51, 52]. However, the results from phase II clinical trials in postmenopausal osteoporotic women were disappointing [53, 54] and their clinical development was stopped, due to the lack of clinical efficacy . Despite the experimental evidence that compounds that act on the PTH receptor are involved in bone regeneration, there is no clinical evidence to support their efficacy in the treatment of osteoporosis. Recently, a metanalysis has demonstrated that combination therapy with PTH analogs and antiresorptive agents was more successful than monotherapy in osteoporotic patients . However, it is based in a small number of patients and large-scale studies are necessary to evaluate these results.
Strontium is a chemical element with similar chemical properties to those of calcium, its vertical neighbor in the periodic table. This similarity gives the strontium the ability to substitute calcium in the bone tissue without disturbing the physiology of bone metabolism . Strontium ranelate, a strontium salt of ranelic acid, is used in the treatment of osteoporosis, as it has the capacity to reduce the risk of bone fractures, including vertebral compression fractures and to increase BMD . This therapeutic effect seems to be the result of dissociation of bone resorption from the bone formation, leading to the increase of functional osteoblasts and simultaneous decrease of osteoclasts . Strontium ranelate is an alternative treatment, with a positive benefit-harm relationship, for osteoporosis, but consideration should be given to the contraindications, the revised indications, and the possible cardiovascular risk [58, 60]. Skin rashes, myocardial infarction, and embolic and thrombotic venous events have been reported as side effects from the use of strontium ranelate in the treatment of osteoporosis .
The introduction of the biological therapies in the last decades to clinical practice has also given new impetus to new therapeutic approaches and the development of new agents in the management of osteoporosis. Denosumab, an IgG2 human monoclonal antibody, is a biological agent for the treatment of osteoporosis. This agent exhibits high specificity and affinity for the receptor activator of nuclear factor kappa-B ligand (RANKL) . It exerts antiresorptive properties by inhibiting the RANKL to interact with its receptor on the surface of osteoclasts and by blocking formation, function, and survival of osteoclasts and by reducing bone resorption . Denosumab is considered as a relatively safe treatment of osteoporosis . Monoclonal antibodies directed to inhibiting sclerostin and cathepsin K inhibitors have been generated for the treatment of osteoporosis. Three humanized neutralizing antibodies to sclerostin, a glycoprotein secreted by osteocytes with a negative effect on the formation of bone tissue, are currently in clinical trials, under development, with preclinical and clinical evidence supporting their use in osteoporosis . A number of compounds that inhibit cathepsin k have also been used to treat osteoporosis. However, most of them caused adverse reactions and drug interactions, resulting in withdrawal . Odanacatib, a cathepsin k inhibitor, which directly inhibits bone resorption and maintains bone formation, was developed up to phase III clinical trials in osteoporotic postmenopausal women  but has been discontinued due to the increased risk of stroke and major adverse cardiovascular events .
The limited efficacy and the undesirable side effects of both the existing antiresorptive and the anabolic drugs, combined with the failed efforts of newly developed drugs in clinical trials on new therapeutic agents, increase the importance of developing new therapeutic strategies that could promote bone regeneration in individuals with osteoporosis. Recently, extensive research has focused on bone regeneration through biomaterial scaffolds, small osteoinductive molecules, and stem cells of various origin, for the therapy of orthopedic-related diseases, mainly osteoporosis [68, 69]. Various biomaterials, natural or synthetic, have been examined for their use as grafts for the repair of bone fractures in osteoporotic patients . Thus, scaffolds from ceramics or from bioderived materials, such as the extracellular matrix, and synthetic scaffolds from polymers or fibers have been designed and used to mimic native bone and provide the necessary biochemical and mechanical support for stem cells to adhere, proliferate, and differentiate for ingrowth of new bone [71–75]. Further research is needed to examine which types of scaffolds can reconstitute native tissue biological properties and what stem cell combinations could induce and maintain a functional and stable osteoblast population to generate new bone tissue . Osteoinductive molecules, such as growth factors, peptides, and small molecules, with osteogenic properties have also been used in tissue regeneration and tissue engineering . These molecules can induce both angiogenesis and osteogenesis simultaneously, and a number of small osteoinductive molecules alone or combined with other osteogenic techniques are under investigation for the treatment of osteoporosis [77–80]. However, stem cells have gained considerable experimental and clinical attention for the development of cell-based therapies due to their promising contribution to regenerative medicine to date.
3. Stem Cells in the Pathogenetic Mechanism of Osteoporosis
3.1. Stem Cells in Bone Regeneration and Pathophysiology of Osteoporosis
Osteogenesis and bone remodeling are complex processes that involve multiple mechanisms and interactions between distinct cell populations, not only the osteoblastic and osteoclastic cell lineages . An interplay between progenitor stem cells that maintain the bone cell repertoire and hematopoietic and tissue immune cells also occurs, via the secretion of local cytokines and growth factors [82–84] and the activation of transcription factors [85, 86]. Osteoporosis is a metabolic disorder, which implicates not only the osteoblast and osteoclast imbalance but also other functions, such as bone-vessel coupling and bone-adipocyte coupling . Despite the complex nature of the pathogenesis of osteoporosis, osteoblast generation is the most important mechanism of its pathogenetic process since osteoid secretion by osteoblasts is the main procedure for producing bone tissue. Accumulating evidence has indicated an important role of a stem cell population, derived from bone marrow, in bone maintenance and remodeling. Recently, it has been identified and a population of self-renewing and multipotent human skeletal stem cells (SSCs) that generates progenitors of bone, cartilage, and stroma, but not fat has been reported . These multipotent SSCs reside in the postnatal bone marrow and are implicated in skeletal physiology . The term mesenchymal stem cells (MSCs) has been used for years in literature to characterize a multipotent stromal cell population that can differentiate into a variety of cell types, including osteoblasts (bone cells) and chondrocytes (cartilage cells), and most articles use this term when they refer to multipotent skeletal stem cells [90–92]. The bone tissue homeostasis, under physiological conditions, is maintained due to the osteogenetic and adipogenetic abilities of the MSCs . Various factors such as menopause and aging or ovariectomy could disrupt this homeostatic ability of the MSCs that would lead to the accumulation of a large number of bone marrow adipocytes resulting in bone mass loss and osteoporosis . The proliferative response and the osteogenic differentiation of MSCs have been found significantly affected in osteoporosis . The differentiation of MSCs into osteoblasts promotes osteogenesis; however, a balance between their differentiation into osteoblasts and adipocytes is necessary to maintain the balance between bone and adipose tissues. If the number of adipocytes increases and that of osteoblasts decreases, osteoporosis could result . Understanding the regulatory factors of the osteogenic and adipogenic abilities of MSC is very important in the study of the underlying mechanisms governing osteoporosis. The differentiation of MSCs involves the transformation from osteogenic cells to preosteoblasts and finally into osteoblasts. Following differentiation, the osteoblasts proliferate, maturate, and secrete extracellular matrix proteins to induce bone matrix mineralization . The sequential process of MSC differentiation to bone-forming osteoblasts and how it is affected by factors such as hormones, cytokines, drugs, and physiotherapy are very important for understanding normal growth and maintenance of bone tissue . Dysfunction at any stage of this sequential procedure leads to a disorder of the bone metabolism and osteoporosis, as a result of an imbalance between bone formation and resorption . Thus, the therapeutic strategies for osteoporosis must target the improvement of MSC differentiation into osteoblasts and the osteoblast proliferation.
In healthy individuals, these MSC-derived osteoblast progenitors occur at a very low rate in the bone marrow . In elderly people, the number of osteoblast progenitor MSCs is further decreased, leading to reduction in bone formation and osteoporosis . Apart from the progenitor number reduction, age-related osteoporosis is characterized by diminished ability of MSCs to proliferate and differentiate giving osteoblasts capable of forming bone . The capacity of MSCs from osteoporosis patients to differentiate into osteoblasts was found to be lower than that from healthy people, as they are found to have a lower growth rate than control cells and to exhibit a deficient ability to differentiate into the osteogenic lineage . Therefore, bone formation and remodeling are dependent not only on the appropriate number but also on the function of the osteoblast progenitor MSCs . MSCs from postmenopausal women with osteoporosis were found to have less sensitivity to insulin-like growth factor and a weaker ability to differentiate into osteoblasts, compared to normal controls, while postmenopausal women with osteoporosis were found to express a differential responsiveness of MSCs to leptin, resulting to a variability of the MSC osteogenic potential .
Many studies have shown that two factors, estrogen and glucocorticoids, have an important regulatory effect on the pathophysiology of osteoporosis affecting the differentiation of MSCs. Estrogen receptor, a type of nuclear hormone receptor, is expressed in the osteoblasts, and it has an extremely important role in maintaining the equilibrium between bone resorption and formation . Various studies have confirmed that estrogen, in certain concentrations, can induce MSC proliferation and osteogenic differentiation . Another study demonstrated that estrogen, in parallel with the induction of osteogenic differentiation of the MSCs, inhibited adipogenic differentiation . Estradiol was found to promote MSC-derived osteoblast proliferation and maturation and the expression and secretion of bone morphogenetic protein 2 (BMP2) . These studies suggest that hormone replacement therapy could have a therapeutic effect on osteoporosis, but long-term administration of estrogen in postmenopausal women has been found responsible for increased incidences of coronary heart disease, stroke, and breast cancer . Recently, a role of IL-17 has been suggested in the estrogen-deficient osteoporosis. IL-17 was found to promote osteoclastogenesis, in a mice model of ovariectomy- (OVX-) induced osteoporosis, by stimulating RANKL expression by osteoblasts via the IL-17RA SEFIR/TILL. Act1mRNA expression that is augmented in estrogen deficiency increases the RANKL expression and causes bone resorption via an IL-17RA interaction . Another study demonstrated that estrogen deficiency increases IL-17 production, modulating differentiation of Th17 cells, and results to production of osteoclastogenic mediators, including IL-6, TNF-α, and RANKL from osteoblasts and bone loss. Blocking of IL-17 function, with a neutralizing monoclonal antibody, was found to prevent bone resorption in an OVX mouse experimental model .
Transplantation of MSCs in systemic lupus erythematosus mice was found to ameliorate secondary osteoporosis, rendering IL-17 production in bone marrow and improving the function of MSCs. This resulted in a balance between osteoblasts and osteoclasts and maintenance of a positive bone metabolism . In addition to the implication of estrogens in the pathophysiology of postmenopausal osteoporosis, these results support the evidence that osteoporosis is associated with disorders of innate immunity.
Glucocorticoids also have a regulatory effect on the differentiation of MSCs and osteoporosis. Long-term use or/and high-doses of glucocorticoids have been found to induce osteoporosis via differentiation of MSCs to adipocytes against the differentiation to osteoblasts . It was also reported that high dosage of dexamethasone could lead to apoptosis of bone marrow MSCs, which would result to osteoporosis . Ginsenoside, a pharmacologically active compound found in ginseng, was demonstrated to inhibit the high dosage of dexamethasone-induced apoptosis in primary murine bone marrow MSCs . Thus, when patients are treated with corticosteroids, the dose, the duration of the treatment, and the postmenopausal status should be taken into account . Data from glucocorticoid-induced osteoporotic rats demonstrated for the first time that tetramethylpyrazine (TMP), an extract from a traditional Chinese herb with antiapoptotic properties, could protect MSCs from exposure to glucocorticoids by promoting autophagy and might be an effective agent for the prevention and/or treatment of induced osteoporosis due to the use of glucocorticoids . Recently, another study, using local delivery of TMP in bone marrow of aging mice, demonstrated that TMP could eliminate the senescent phenotype of bone marrow MSCs and could be a potential treatment to ameliorate human age-related skeletal diseases . However, corticosteroids are used for a large number of inflammatory conditions and the corticosteroid-induced osteoporosis represents an important problem from their use.
Current knowledge shows that osteoporosis is a multifactorial process. Recently, microRNAs and long non-coding RNA were reported to be implicated in the pathogenetic mechanisms of skeletal disorders, including osteoporosis [119–121]. Osteoclasts were found to regulate the function of osteoblasts via the secretion of microRNA-containing exosomes that selectively inhibit osteoblast activity . let-7, a miRNA family, was found markedly to promote in vitro the osteogenetic and to suppress the adipogenetic capacities of MSCs, and the let-7/HMGA2 axis has a pivotal role in the maintenance of the balance between the MSC-orchestrated osteogenesis and adipogenesis , while miRNA-214-5p/TGF-β/Smad2 signaling promotes adipogenic differentiation of bone marrow MSCs and might be a new therapeutic target in postmenopausal osteoporosis . Recently, long noncoding RNA named HOXA-AS2 was found to regulate osteogenesis of MSCs via the transcriptional activity of NF-κB and they suggested that HOXA-AS2 might be a new therapeutic target . Another study, using bone marrow MSCs from 3-week-old Sprague Dawley rats, demonstrated that overexpression of the long noncoding RNA X-inactive-specific transcript (XIST), a major effector of the X chromosome inactivation process, significantly inhibited the differentiation of bone marrow MSCs into osteoblasts. In addition, XIST was found significantly upregulated in plasma and monocytes from osteoporotic patients . Teng et al., using an estrogen deficiency-induced osteoporosis mice model, demonstrated a regulatory effect of miRNAs on the multidirectional differentiation ability of MSCs . Another study demonstrated that miR-106b negatively regulated in vitro the osteogenic differentiation of MSCs and that silencing of miR-106b signaling protected mice against glucocorticoid-induced osteoporosis, through promoting bone formation and inhibiting bone resorption . Both studies suggested that microRNAs might be a potential new strategy for treating osteoporosis and bone defects.
Extensive research experience from experimental studies and clinical data indicate a central role for stem cells in osteogenesis and bone tissue remodeling. Pathophysiological disorders and external factors that disturb the normal functioning of MSC differentiation into osteoblasts could lead to the induction of osteoporosis. Thus, stem cells might be proven key players in the search for new therapeutic strategies in the management of osteoporosis.
3.2. The Role of Very Small Embryonic-Like Stem Cells to Osteogenesis and Skeletal Regeneration
A decade ago, a population of very small stem cells, with pluripotent properties, was described for the first time as very small embryonic-like stem cells (VSELs). They were first identified in murine adult bone morrow  and later in most murine tissues . In humans, VSELs were first isolated from umbilical cord blood , and few years later, they were also found in bone marrow and peripheral blood . VSELS are very small in size (3-6 microns) and express the chemokine receptor CXCR4, other markers that are characteristic for embryonic, epiblast, and primordial germ cells, while they do not express characteristics of the haematopoietic lineage . It has been suggested that VSELs are stored during embryonic development in the bone marrow and they work as a deposit of PSCs. They play a crucial role in postnatal tissue regeneration, giving cells for both hematopoietic and nonhematopoietic tissues and renewing the cell populations of all three germ layers. It has been postulated that VSELs are stored during the organogenesis in organs and tissues as pluripotent SCs that during the postnatal life give rise to oligopotent or omnipotent SCs to support tissue turnover and regeneration. It has also been suggested that this pluripotent stem cell population decreases with age . Therefore, VSELs appear to be significantly involved in the renewal of cellular elements in adult tissues and in the repair of damaged organs, while the reduction in their number with age may be associated with aging.
Given these characteristics, VSELs have gain attention on their ability to generate new tissues, both in in vitro and in experimental models. They have demonstrated that human and murine VSELs could express markers of the osteoblastic phenotype and to differentiate into cells with the capacity to generate skeletal structures and to participate in skeletal repair [132, 133]. Havens et al. have shown that VSELs were capable of differentiating into cells that expressed osteocalcin, which is involved in the mineralization of the bone tissue, and serum circulating osteocalcin reflected the level of bone formation, that is indicative of osteoblastic differentiation. Injected VSELs were localized to the murine bone marrow space adjacent to bony trabeculae and produced robust mineralized tissue by 3 months [133, 134]. The development of mineralized bone tissue and the achieved bone volume were found to be dependent on the number of the injected cells . VSELs seem to be promising for use in regenerative medicine, as autologous treatment of skeletal disorders and to decelerate the aging processes in bone metabolism that leads to osteoporosis.
Several studies have focused on the use of several pharmaceutical agents to recruit endogenous SCs to the osteoporotic tissues, instead of transplanting exogenous stem cells. As we mentioned above, PTH and parathyroid hormone-related protein (PTHrP) are anabolic agents and their administration at low doses and in an intermittent manner induce bone remodeling. It is worth to notice that MSCs have been found to express the PTH receptor on their membranes . These data may help to explain an old observation that PTH exerts its anabolic action, among other mechanisms, stimulating, the first week after its administration, the proliferation and differentiation of MSCs into osteoblasts . In addition, PTH recruits endogenous bone marrow MSCs and induces their adhesion on the trabecular bone surface, starting within 2 weeks from the time of its administration . Furthermore, we have recently observed in our laboratory that a population of VSELs, isolated from peripheral blood and bone marrow of healthy donors, express the PTH receptor (unpublished data). Taken together the physiological role of SCs in bone regeneration and the effects of PTH receptor analogues on osteogenesis, these cells might lead to better and more efficacious cell-based treatment for osteoporosis. A plausible therapeutic option worth investigating would be the use stem cells from young healthy donors with intermittent PTH receptor analogues, such as PTH and PTHrP.
3.3. Stem Cell Therapy: Learning from Animal Models of Osteoporosis
Experimental animal models are key tools for the development of new drugs and therapies as they provide important information on the preclinical assessment of their efficacy and safety. Despite the important information provided by animal models in preclinical studies, there are significant differences between the pathophysiological and immune disorders of experimental animals and human diseases. These differences must be carefully taken into account by researchers who would like to translate findings from preclinical studies into therapeutic strategies, mainly when developing biologics or cell therapies . Stem cell transplantation and cell-based therapies have been suggested, and several studies, using experimental animal models, have been conducted for the treatment of the common bone and joint diseases, including osteoporosis [25, 101]. Given that preclinical studies are essential for organizing future clinical trials , it is very important to use experimental animal models for osteoporosis to check for safety and to prove the effectiveness of stem cell-based therapy, taking into account the stem cell origin, the route of administration for transplant, and the osteoporotic animal model that has been used (Table 1).
MSCs: mesenchymal stem cells; ADSCs: adipose-derived mesenchymal stem cells; VSELs: very small embryonic-like stem cells.
The majority of experimental animal studies were conducted in small animal models such as mice [140, 141] and rats , while only a small number of studies used rabbits [143, 144]. Osteoporosis was induced mostly by ovariectomy [143, 145] or by administration of glucocorticoid [13, 146], while Aggarwal et al. developed an immune-deficient osteoporotic murine model . Among these osteoporosis animal models, the rat ovariectomy model seems to be the most preferred animal, with mice to follow, as ovariectomy leads to bone loss without hormonal or drug intervention, mimicking postmenopausal osteoporosis in women. Although there are substantial differences between the rat and human skeleton, these could be overcome with the extensive knowledge of the rat skeleton and various experimental techniques as body mineral density (BMD) is mainly used to prove the effectiveness of the stem cell-based treatment . For studies into the treatment of osteoporotic animals, in addition of embryonic mesenchymal stem cells (ESCs), stem cells of various origin have mostly been used, such as mesenchymal stem cells derived from adipose tissue (ADSCs), from bone marrow (BM-MSCs), and from umbilical cord blood (UCB-MSCs).
MSCs were found to exert significant therapeutic potential through differentiating into various musculoskeletal tissues and thereby mediating tissue repair or by secreting a plethora of soluble mediators, including growth factors and immunomodulatory cytokines . These properties have inspired researchers to investigate the effect of transplanting MSCs locally (to the bone surface) and systemically in osteoporotic animal models [149, 150]. However, MSCs do not have the capability of spontaneous engraft and differentiation depending on the therapeutic target and the relevant target areas. Thus, genetically modified MSCs were used to increase engraftment and differentiation at osteoporotic bone areas [13, 151]. In an ovariectomy (OVX) mouse model, bone-marrow derived MSC’s were observed to be home to bone, after 2 months of intravenous injection leading to increased BMD and restored bone volume . Adipose-derived MSCs injected intravenously in OVX mice induced an increase of BMD at 4 and 8 weeks post injection . Transplantation of adipose-derived MSCs, infected with lentiviral vectors expressing alpha-1 antitrypsin, was found to reduce bone loss in OVX mice. Interestingly, the migration of the transplanted cells into the bone tissue resulted to alpha-1 antitrypsin secretion, a significant decrease of the serum IL-6 and IL-1β cytokines, and a reduction in the gene expression of RANK in the bone . Intravenous transplantation in OVX osteoporotic mice of MSCs that overexpress, after retroviral transduction, the SDF-1 receptor CXCR4 or the RANK-Fc, an antagonist for RANK-L, was found to increase the cell trafficking into the bone tissue. This procedure protected against bone loss and enhanced the positive effect of RANK-Fc in osteoporosis treatment . Another similar study, using a senescence-accelerated mouse model that expresses a phenotype of progressive aging and injecting through the tail vein MSCs or conditioned media from MSC, demonstrated a decrease in osteoporosis progression as a result of both maintaining osteocalcin production and inhibiting bone marrow adipose tissue accumulation . These data suggest that MSC transplantation may prevent or treat human age-related osteoporosis providing a model for developing cell-based therapies using MSCs. There is evidence of a substantial decrease of the proliferation rate and of the ability to differentiate in aged MSCs. This is additionally supported by data from experimental models where MSCs from bone marrow of young and old rats transplanted in osteoporotic mice were compared. Transplantation of young MSCs to osteoporotic mice resulted in significantly higher BMD compared to the administration of aged MSCs . This raises the question are MSCs from a younger source more plausible to be used for therapy than autologous older cells or cells that have been expanded (as expansion ages cells)?
A number of experimental studies of stem cell-based therapy in osteoporotic animals have used ADSCs. These cells are more easily isolated, compared to their bone marrow counterparts, and easier to produce a larger number of isolated stem cells for transplantation . ADSC transplantation in osteoporotic rats and mice significantly increased BMD , while the transplantation of young ADSCs showed a statistically significant improvement in bone regeneration, compared to that in mice receiving aged transplants, with a BMD improvement of 24.3% on average . In one of the few studies of stem cell therapies conducted in larger animals, ADSCs transplanted in OVX osteoporotic rabbits were found to enhance bone regeneration and osteogenesis in vivo and to stimulate in vitro proliferation of bone marrow MSCs, followed by osteogenic differentiation . These data combined to similar results from MSC studies, mentioned above, suggest that in stem cell therapy, stem cells isolated from young donors might be more effective in the treatment of osteoporosis.
UCB-MSCs are another rich source of stem cells that hold distinct inherent characteristics making them a first choice for the development of stem cell-based treatments for musculoskeletal disorders . Human UCB-derived CD34(+) cells that were expanded using a nanofiber-based culturing system systematically were transplanted to osteoporotic mice, and they achieved to increase bone deposition and BMD and to improve bone microarchitecture . Four and eight weeks after systemic application of UCB-derived MSCs or conditioned media from their cultures into OVX, osteoporotic mice showed an increase in BMD levels. In addition to UCB-MSCs, other fetal adnexa, such as placenta and amniotic fluid, are an innovative source of nonembryonic SCs, probably suitable for cell-based therapies, mainly in perinatal disorders. MSCs derived from fetal adnexa seem to have higher proliferative potential and multilineage differentiation capacity compared to adult tissue sources [162, 163]. These cells share similar properties with adult MSCs, but in vitro, they have a higher expansion rate and differentiation capacity, compared to adult MSCs that could give a number of cells of the adipose and skeletal tissues [162, 163]. Amnion and chorion, placenta-derived structures, are important sources of MSCs, but the expansion ability of these SCs is dependent on the gestational age . Amniotic fluid-derived stem cells, deriving from dermal fibroblast and fibrous connective tissues, express similar properties and embryonic and adult SC markers with MSCs derived from other tissue sources . MSCs derived from chorion differentiate more easily into osteocytes , while all the types of fetal adnexa-derived MSCs differentiated into osteogenic, myogenic, chondrogenic, and adipogenic lineages . Another study demonstrated amelioration in bone fractures, using intrauterine human embryonic MSC transplants isolated from blood of the first trimester donors, in osteogenesis imperfecta mice . Finally, mesenchymal stem cells, derived from fetal adnexa, are less implicated in ethical issues than embryonic stem cells, as these tissues are discarded after delivery .
There is very little evidence in the literature, and it is under investigation, whether MSCs differentiate into their target tissue, even when desired clinical effects are observed. In most studies, implantation of MSCs in the bone was not detected and it has been suggested that the improvement of the bone loss is a result of a secretory paracrine mechanism instead of direct differentiation into target tissue . In fact, nearly all evidence suggests that the mechanism of regeneration by MSCs is via a paracrine effect on the multitude of growth factors and immunomodulatory cytokines and microparticles, such as exosomes, that they secrete . A recent study, using green fluorescent protein-labeled bone marrow MSCs demonstrated that donor MSCs homed and inhabited, for at least 4 weeks, recipient bone marrow and prevented recipient bone marrow cell apoptosis, while donor MSCs committed to Osterix+ osteoblast progenitors and induced osteoblastogenesis .
Despite that several studies have successfully directed iPSCs cells in specific cell lineages, the results from attempts to specifically induce osteogenic differentiation of iPSC cells are poor and limited so far. A number of studies have demonstrated differentiation of iPSCs into osteoblasts, using similar protocols that induced differentiation of embryonic SCs and MSCs into osteoblasts [170–172]. Similarly, scaffolds were found to increase differentiation potential of iPSCs into osteoblasts with less problems compared to MSC cultures [173, 174]. The potential functional benefit of iPSC-derived bone tissue for cell replacement therapy has been confirmed in animal models. Human iPSCs, implanted on long bone segmental defects in rats, demonstrated an osteogenic differentiation and an in vivo osteogenic potential . Implantation in mice of iPSC-derived osteoblasts, after a 12-week period, resulted in differentiation of iPSCs into mesenchymal lineages of the bone, cartilage, and fat . Another study, using a coculture of human-induced pluripotent stem cell-derived mesenchymal stem cells (hiPSC-MSCs) with human umbilical vein endothelial cells implanted on in a cranial bone defect model in nude rats demonstrated an increased () new bone generation . These characteristics suggest that iPSCs have the potential to be used in future bone regenerative therapies, provided that they would meet the clinical safety standards .
Existing data from experimental studies have shown that the use of SCs is very promising for developing cell-based therapies for the treatment and/or probably the prevention of osteoporosis. Recently, a meta-analysis that examines 12 animal model studies of osteoporosis indicates that treatments based on SC therapies significantly improved the BMD . This meta-analysis includes studies that used SCs of various origins, and the SCs were mainly transplanted by intrabone marrow injection and intratail venous injection. Analysis of the specific characteristics of this study revealed that the SC therapies significantly improved the BMD, regardless of the route of transplantation, the method of osteoporosis induction, and the type and amount of used stem cells, with the only exception of the transplantation of embryonic SCs that did not significantly improve the BMD of the osteoporotic animals.
Studies from animal models so far have yielded important knowledge on the methodology of using SCs in the treatment of osteoporosis and strong evidence suggesting a positive future contribution of SC therapy to the severe global medical issue of osteoporosis . In addition, we have learned many mechanisms underlying the significant improvement of the BMD in osteoporosis after SC-based treatments. Current knowledge indicates two possible beneficial effects of stem cells. On one hand, transplanted SCs migrate to the site of injury regardless the route and they are differentiated to assist in tissue repair via their regenerative properties. On the other hand, migrating SCs, producing immunomodulatory cytokines and growth factors, affect the local environment and “renovate” the niche, helping local cells to recover and inducing the recruitment of new cells in the area . Transplantation of UCB-MSCs in collagen-induced arthritis mice with osteoporosis has been found to improve impaired osteogenic differentiation ability via the inhibition of TNF-α . More knowledge from basic research and in vivo experiments must be acquired to further understand the pathophysiology of bone regeneration and the integration of the underlying mechanisms of SC involvement in this process. However, the existing animal model data from the preclinical studies along with some early clinical information of SC transplantation provide a pathway for organizing specific clinical trials and using SCs in the treatment and the management osteoporosis.
3.4. Stem Cell Therapy: Clinical Trials
Results from experimental animal models and sporadic clinical reports suggest that the use of SCs and regenerative medicine could provide a potential new therapeutic strategy in the management of osteoporosis. However, although this idea has recently received a considerable attention, clinical trials of stem cell transplantation in osteoporotic humans have not been published so far. In humans, the available information mainly comes from studies that transplanted intravenously MSCs for the treatment of osteogenesis imperfecta in children. Transplantation of MSCs seems to improve the bone structure and stability, growth, and fracture healing, but there is limited experience in this area so far . A clinical study of allogeneic MSC transplantation in six children with osteogenesis imperfecta demonstrated that MSCs were safely administered and differentiated to osteoblasts, capable of extending the clinical benefits of bone marrow transplantation, with five out of six patients showing improvement during the first 6 mo post infusion . Another study reported the cases of prenatal human fetal transplantation in two patients with osteogenesis imperfecta, who also received a postnatal boosting with the same cell population. They demonstrated that prenatal transplantation of allogeneic MSCs appears to be safe and of clinical benefit and that a retransplantation is feasible . Taketani et al. administered allogeneic MSCs, ex vivo expanded, in two patients with severe hypophosphatasia, performing multiple MSC infusions. Both patients had been transplanted with bone marrow, received from asymptomatic relatives. They demonstrated that multiple MSC infusions were effective with clinical improvement of the patients with lethal hypophosphatasia and without adverse events . These clinical studies of MSC transplantation for the treatment of osteogenetic disorders present promising clinical results for the use of stem cell therapies in osteoporosis. However, the limited information to date means that it is not possible to be conclusive and that further experimental and clinical studies are required. Currently, only two clinical trials in phase I/II are being conducted that are related to the use of SCs in the management of osteoporosis, as assessed in https://www.clinicaltrials.gov.
Contemplating the use of SCs in the development of cell-based therapies for osteoporosis, two main questions arise. First, which source of SCs is the most appropriate to be used, and second, which administration route would better drive stem cells in the target tissue? A large recent meta-analysis of animal model studies of osteoporosis demonstrated that transplantation of SCs significantly improved osteoporosis regardless of the administration route that they used . Implantation of MSCs, derived from adipose tissue, bone marrow, or umbilical cord blood, was found to significantly increase the BMD compared to controls; in contrast, the use of transplants from embryonic SCs did not induce BMD improvement . Ideally, MSCs should be administrated via systemic transplantation in order to allow them to reach the osteoporotic tissue and treat osteoporosis. There are currently three main sources of MSCs, placental cord blood, bone marrow, and adipose tissue that have been used for research into stem cell therapies. Although very little research has yet to be performed in humans, these sources of MSCs have been used clinically in trials in humans for exploring effectiveness in other diseases [158, 185]. ADSCs have received particular attention in recent years because these cells are easier to be isolated and in large quantities compared to bone marrow MSCs and their inherent characteristics and safety make them attractive for cell-based therapies [157, 160, 186]. This has led many researchers to use ADSCs in bone research, such as in the treatment of osteoporosis. The use of ADSCs is limited when it comes to individuals with very low percentage of body fat and complexities of repeat lipoaspirations when further treatments are required. In these cases, MSCs derived from umbilical cord blood are an important source of SCs in the cell-based therapies. UCB-MSCs have particular characteristics that make them a distinct source of stem cells. First, it is easy to be isolated without requiring any risk procedure for the donor; second, they have strong differentiation capacity and low immunogenic potential; and third, they are from a youthful source [187–189]. Although, these characteristics make UCB-MSCs a distinct source of SCs for cell-based treatment of osteoporosis, we have to take into attention the results from experimental animals that demonstrated a limited effect of embryonic stem cell transplantation in osteoporotic animals . On the other hand, due to the safety and ethical issues raised by the use of induced pluripotent stem cells (iPSCs) and embryonic SCs, many researchers use MSCs in clinical studies of cell therapies . Furthermore, since MSCs could be easily received from several tissues, as mentioned above, and they are multipotent adult cells, over the past decade, more and more clinical studies have been using MSCs. In addition, the therapeutic effect of MSCs has been examined in immune disorders due to their ability for immunosuppression . Possible clinical applications for MSCs in the musculoskeletal system have mainly focused on acute or chronic disorders, such as bone fractures, osteoporosis, cartilage lesions, or ligament injuries . Finally, as mentioned above, the new population of VSELs may hold promise in regenerative medicine and cell-based therapies and it would be possibly efficient to use these cells in clinical studies for treating osteoporosis, because they could be used as an autologous transplantation from peripheral blood.
Data from basic research studies as well as from the experimental animal models show encouraging results for the use of SCs for the development of cell-based therapies for osteoporosis. However, before proceeding with clinical studies on the development of these therapeutic approaches for osteoporosis or other degenerative diseases, several factors should be taken into account and a number of issues should be examined and addressed regarding the isolation and the retention of their reparative capacity [93, 191]. The safe and effective use of MSC should calculate the optimal number of cells to be transported in order to achieve the best clinical outcome in patients with osteoporosis without any side effect or lack of efficacy. If large numbers are required (as expected), MSCs would need an in vitro expansion with a cocktail of growth factors and cytokines. These protocols have been well characterized and can produce high quality of cultures that could produce large numbers of cells for the clinical studies. However, these techniques, used for MSC culture and expansion, have also been shown to lead to an alteration in their reparative capacity [192, 193]. Furthermore, long-term cold storage of MSCs and how that affects the function of MSCs will need to also be examined, if “banking” of MSCs is required [194, 195]. Another thing to consider, from an autologous perspective, is the age of the patient. The older the patient, the more likely that the isolated MSCs will be less in number and less in reparative function due to the loss of lineage specificity, depletion due to self-renewal over the years, and depletion due to senescence that has been seen with age [191, 196, 197]. This suggests that from an autologous approach, the patients should be at a coherent age when doing trials. These are complex issues, and probably, it will take a long experimental and clinical research time to resolve, before we establish new therapeutic strategies, using stem cells, obtained from various sources, like adipose tissue, embryonic vessels, VSELs, and iPSCs, for the treatment of damaged tissues, including osteoporosis.
Osteoporosis is a common disease in both the developing and developed countries that primarily affects elderly people. Current treatments attempt to target the imbalance in the osteoblast-osteoclast axis. Although these treatments have demonstrated some benefits, limited efficacy and adverse effects complicate them. The development of new strategies is rapidly becoming a dire necessity to meet the rising human and economic toll of osteoporosis. Despite the multifactorial nature of this disorder, stem cells maintain a central role in the pathogenetic mechanisms as their differentiation into osteoblasts promotes osteogenesis and bone remodeling. As osteoporosis is related to a decline in the number and function of osteoblasts, substantial evidence indicates that stem cell stimulation and transplantation have the ability to reverse bone demineralization. Current knowledge supports the concept of using stem cell therapy in osteoporosis, providing evidence of its potential as a new and promising type of treatment with unique advantages.
Conflicts of Interest
Vasilis Paspaliaris and George Kolios declare that they have no conflict of interest
- J. E. M. Sale, D. Beaton, and E. Bogoch, “Secondary prevention after an osteoporosis-related fracture: an overview,” Clinics in Geriatric Medicine, vol. 30, no. 2, pp. 317–332, 2014.
- P. Sáez-López, I. Etxebarria-Foronda, M. P. Mesa Lampre, N. Alonso García, and N. Sánchez Hernández, “Efficacy, cost, and aspects to take into account in the treatment of osteoporosis in the elderly,” Revista Española de Geriatría y Gerontología, vol. 54, no. 3, pp. 156–167, 2019.
- S. W. Wade, C. Strader, L. A. Fitzpatrick, M. S. Anthony, and C. D. O’Malley, “Estimating prevalence of osteoporosis: examples from industrialized countries,” Archives of Osteoporosis, vol. 9, no. 1, p. 182, 2014.
- J. A. Kanis, E. V. McCloskey, H. Johansson, A. Oden, L. J. Melton III, and N. Khaltaev, “A reference standard for the description of osteoporosis,” Bone, vol. 42, no. 3, pp. 467–475, 2008.
- D. W. Dempster, “Osteoporosis and the burden of osteoporosis-related fractures,” American Journal of Managed Care, vol. 17, Supplement 6, pp. S164–S169, 2011.
- L. G. Raisz, “Pathogenesis of osteoporosis: concepts, conflicts, and prospects,” The Journal of Clinical Investigation, vol. 115, no. 12, pp. 3318–3325, 2005.
- S. K. Sandhu and G. Hampson, “The pathogenesis, diagnosis, investigation and management of osteoporosis,” Journal of Clinical Pathology, vol. 64, no. 12, pp. 1042–1050, 2011.
- S. C. Manolagas and A. M. Parfitt, “What old means to bone,” Trends in Endocrinology & Metabolism, vol. 21, no. 6, pp. 369–374, 2010.
- O. D. Kennedy, B. C. Herman, D. M. Laudier, R. J. Majeska, H. B. Sun, and M. B. Schaffler, “Activation of resorption in fatigue-loaded bone involves both apoptosis and active pro-osteoclastogenic signaling by distinct osteocyte populations,” Bone, vol. 50, no. 5, pp. 1115–1122, 2012.
- J. C. Crockett, M. J. Rogers, F. P. Coxon, L. J. Hocking, and M. H. Helfrich, “Bone remodelling at a glance,” Journal of Cell Science, vol. 124, Part 7, no. 7, pp. 991–998, 2011.
- N. A. Sims and T. J. Martin, “Coupling signals between the osteoclast and osteoblast: how are messages transmitted between these temporary visitors to the bone surface?” Frontiers in Endocrinology, vol. 6, p. 41, 2015.
- W. Sun, C. Zhao, Y. Li et al., “Osteoclast-derived microRNA-containing exosomes selectively inhibit osteoblast activity,” Cell Discovery, vol. 2, article 16015, 2016.
- C.-Y. Lien, K. Chih-Yuan Ho, O. K. Lee, G. W. Blunn, and Y. Su, “Restoration of bone mass and strength in glucocorticoid-treated mice by systemic transplantation of CXCR4 and cbfa-1 co-expressing mesenchymal stem cells,” Journal of Bone and Mineral Research, vol. 24, no. 5, pp. 837–848, 2009.
- K. Lippuner, “The future of osteoporosis treatment – a research update,” Swiss Medical Weekly, vol. 142, article w13624, 2012.
- N. E. Lane, “Epidemiology, etiology, and diagnosis of osteoporosis,” American Journal of Obstetrics & Gynecology, vol. 194, Supplement, no. 2, pp. S3–S11, 2006.
- J. P. Rodriguez, P. Astudillo, S. Ríos, and A. M. Pino, “Involvement of adipogenic potential of human bone marrow mesenchymal stem cells (MSCs) in osteoporosis,” Current Stem Cell Research & Therapy, vol. 3, no. 3, pp. 208–218, 2008.
- E. M. Curtis, R. J. Moon, E. M. Dennison, N. C. Harvey, and C. Cooper, “Recent advances in the pathogenesis and treatment of osteoporosis,” Clinical Medicine, vol. 15, Supplement 6, pp. s92–s96, 2015.
- G. Kolios and Y. Moodley, “Introduction to stem cells and regenerative medicine,” Respiration, vol. 85, no. 1, pp. 3–10, 2013.
- I. Drosos and G. Kolios, “Stem cells in liver regeneration and their potential clinical applications,” Stem Cell Reviews and Reports, vol. 9, no. 5, pp. 668–684, 2013.
- E. K. Zuba-Surma, W. Wojakowski, Z. Madeja, and M. Z. Ratajczak, “Stem cells as a novel tool for drug screening and treatment of degenerative diseases,” Current Pharmaceutical Design, vol. 18, no. 18, pp. 2644–2656, 2012.
- H. Shyam, S. K. Singh, R. Kant, and S. K. Saxena, “Mesenchymal stem cells in regenerative medicine: a new paradigm for degenerative bone diseases,” Regenerative Medicine, vol. 12, no. 2, pp. 111–114, 2017.
- V. Holan, B. Hermankova, and J. Kossl, “Perspectives of stem cell-based therapy for age-related retinal degenerative diseases,” Cell Transplantation, vol. 26, no. 9, pp. 1538–1541, 2017.
- A. Terzic, C. M. Harper Jr., G. J. Gores, and M. A. Pfenning, “Regenerative medicine blueprint,” Stem Cells and Development, vol. 22, Supplement 1, pp. 20–24, 2013.
- B. McCall, “UK announces strategy for regenerative medicine,” The Lancet, vol. 379, no. 9822, p. 1183, 2012.
- Y. Liu, J. Wu, Y. Zhu, and J. Han, “Therapeutic application of mesenchymal stem cells in bone and joint diseases,” Clinical and Experimental Medicine, vol. 14, no. 1, pp. 13–24, 2014.
- L. Liu and T. J. Webster, “In situ sensor advancements for osteoporosis prevention, diagnosis, and treatment,” Current Osteoporosis Reports, vol. 14, no. 6, pp. 386–395, 2016.
- M. J. Rogers, J. C. Crockett, F. P. Coxon, and J. Monkkonen, “Biochemical and molecular mechanisms of action of bisphosphonates,” Bone, vol. 49, no. 1, pp. 34–41, 2011.
- R. E. Marx, “A decade of bisphosphonate bone complications: what it has taught us about bone physiology,” The International Journal of Oral & Maxillofacial Implants, vol. 29, no. 2, pp. e247–e258, 2014.
- A. A. Khan, G. K. Sándor, E. Dore et al., “Bisphosphonate associated osteonecrosis of the jaw,” The Journal of Rheumatology, vol. 36, no. 3, pp. 478–490, 2009.
- J. C. Lo, F. S. O'Ryan, N. P. Gordon et al., “Prevalence of osteonecrosis of the jaw in patients with oral bisphosphonate exposure,” Journal of Oral and Maxillofacial Surgery, vol. 68, no. 2, pp. 243–253, 2010.
- A. Saleh, V. V. Hegde, A. G. Potty, and J. M. Lane, “Bisphosphonate therapy and atypical fractures,” Orthopedic Clinics of North America, vol. 44, no. 2, pp. 137–151, 2013.
- R. Hirschberg, “Renal complications from bisphosphonate treatment,” Current Opinion in Supportive and Palliative Care, vol. 6, no. 3, pp. 342–347, 2012.
- A. Ghirardi, L. Scotti, A. Zambon et al., “Risk of severe upper gastrointestinal complications among oral bisphosphonate users,” PLoS One, vol. 8, no. 12, article e73159, 2013.
- R. A. Overman, M. Borse, and M. L. Gourlay, “Salmon calcitonin use and associated cancer risk,” Annals of Pharmacotherapy, vol. 47, no. 12, pp. 1675–1684, 2013.
- J. J. Body, “Calcitonin for the long-term prevention and treatment of postmenopausal osteoporosis,” Bone, vol. 30, Supplement 1, no. 5, pp. 75–79, 2002.
- K. Henriksen, I. Byrjalsen, J. R. Andersen et al., “A randomized, double-blind, multicenter, placebo-controlled study to evaluate the efficacy and safety of oral salmon calcitonin in the treatment of osteoporosis in postmenopausal women taking calcium and vitamin D,” Bone, vol. 91, pp. 122–129, 2016.
- J. Y. Reginster, F. Pelousse, and O. Bruyere, “Safety concerns with the long-term management of osteoporosis,” Expert Opinion on Drug Safety, vol. 12, no. 4, pp. 507–522, 2013.
- V. C. Jordan, “Beyond raloxifene for the prevention of osteoporosis and breast cancer,” British Journal of Pharmacology, vol. 150, no. 1, pp. 3-4, 2007.
- S. R. Cummings, M. McClung, J. Y. Reginster et al., “Arzoxifene for prevention of fractures and invasive breast cancer in postmenopausal women,” Journal of Bone and Mineral Research, vol. 26, no. 2, pp. 397–404, 2011.
- E. Barrett-Connor, L. Mosca, P. Collins et al., “Effects of raloxifene on cardiovascular events and breast cancer in postmenopausal women,” The New England Journal of Medicine, vol. 355, no. 2, pp. 125–137, 2006.
- P. D'Amelio and G. C. Isaia, “The use of raloxifene in osteoporosis treatment,” Expert Opinion on Pharmacotherapy, vol. 14, no. 7, pp. 949–956, 2013.
- D. L. Kendler, S. Palacios, D. A. Cox et al., “Arzoxifene versus raloxifene: effect on bone and safety parameters in postmenopausal women with osteoporosis,” Osteoporosis International, vol. 23, no. 3, pp. 1091–1101, 2012.
- J. Reeve, P. J. Meunier, J. A. Parsons et al., “Anabolic effect of human parathyroid hormone fragment on trabecular bone in involutional osteoporosis: a multicentre trial,” BMJ, vol. 280, no. 6228, pp. 1340–1344, 1980.
- S. Lou, H. Lv, G. Wang et al., “The effect of teriparatide on fracture healing of osteoporotic patients: a meta-analysis of randomized controlled trials,” Biomed Research International, vol. 2016, Article ID 6040379, 10 pages, 2016.
- R. Lindsay, J. H. Krege, F. Marin, L. Jin, and J. J. Stepan, “Teriparatide for osteoporosis: importance of the full course,” Osteoporosis International, vol. 27, no. 8, pp. 2395–2410, 2016.
- E. Canalis, A. Giustina, and J. P. Bilezikian, “Mechanisms of anabolic therapies for osteoporosis,” The New England Journal of Medicine, vol. 357, no. 9, pp. 905–916, 2007.
- T. Yamaguchi, “The calcium-sensing receptor in bone,” Journal of Bone and Mineral Metabolism, vol. 26, no. 4, pp. 301–311, 2008.
- E. F. Nemeth, “Allosteric modulators of the extracellular calcium receptor,” Drug Discovery Today: Technologies, vol. 10, no. 2, pp. e277–e284, 2013.
- S. J. Steddon and J. Cunningham, “Calcimimetics and calcilytics—fooling the calcium receptor,” The Lancet, vol. 365, no. 9478, pp. 2237–2239, 2005.
- C. Pipino, P. D. Tomo, D. Mandatori et al., “Calcium sensing receptor activation by calcimimetic R-568 in human amniotic fluid mesenchymal stem cells: correlation with osteogenic differentiation,” Stem Cells and Development, vol. 23, no. 24, pp. 2959–2971, 2014.
- M. R. John, L. Widler, R. Gamse et al., “ATF936, a novel oral calcilytic, increases bone mineral density in rats and transiently releases parathyroid hormone in humans,” Bone, vol. 49, no. 2, pp. 233–241, 2011.
- J. E. Fisher, K. Scott, N. Wei et al., “Pharmacodynamic responses to combined treatment regimens with the calcium sensing receptor antagonist JTT-305/MK-5442 and alendronate in osteopenic ovariectomized rats,” Bone, vol. 50, no. 6, pp. 1332–1342, 2012.
- J. Halse, S. Greenspan, F. Cosman et al., “A phase 2, randomized, placebo-controlled, dose-ranging study of the calcium-sensing receptor antagonist MK-5442 in the treatment of postmenopausal women with osteoporosis,” Journal of Clinical Endocrinology & Metabolism, vol. 99, no. 11, pp. E2207–E2215, 2014.
- L. A. Fitzpatrick, C. E. Dabrowski, G. Cicconetti et al., “Ronacaleret, a calcium-sensing receptor antagonist, increases trabecular but not cortical bone in postmenopausal women,” Journal of Bone and Mineral Research, vol. 27, no. 2, pp. 255–262, 2012.
- E. F. Nemeth and W. G. Goodman, “Calcimimetic and calcilytic drugs: feats, flops, and futures,” Calcified Tissue International, vol. 98, no. 4, pp. 341–358, 2016.
- S. Lou, H. Lv, P. Yin, Z. Li, P. Tang, and Y. Wang, “Combination therapy with parathyroid hormone analogs and antiresorptive agents for osteoporosis: a systematic review and meta-analysis of randomized controlled trials,” Osteoporosis International, vol. 30, no. 1, pp. 59–70, 2019.
- G. Boivin, P. Deloffre, B. Perrat et al., “Strontium distribution and interactions with bone mineral in monkey iliac bone after strontium salt (S 12911) administration,” Journal of Bone and Mineral Research, vol. 11, no. 9, pp. 1302–1311, 1996.
- J. Y. Reginster, M. L. Brandi, J. Cannata-Andía et al., “The position of strontium ranelate in today's management of osteoporosis,” Osteoporosis International, vol. 26, no. 6, pp. 1667–1671, 2015.
- T. C. Brennan, M. S. Rybchyn, W. Green, S. Atwa, A. D. Conigrave, and R. S. Mason, “Osteoblasts play key roles in the mechanisms of action of strontium ranelate,” British Journal of Pharmacology, vol. 157, no. 7, pp. 1291–1300, 2009.
- J. Yu, J. Tang, Z. Li et al., “History of cardiovascular events and cardiovascular risk factors among patients initiating strontium ranelate for treatment of osteoporosis,” International Journal of Women's Health, vol. 7, pp. 913–918, 2015.
- R. Rizzoli, U. Yasothan, and P. Kirkpatrick, “Denosumab,” Nature Reviews Drug Discovery, vol. 9, no. 8, pp. 591-592, 2010.
- A. Capozzi, S. Lello, and A. Pontecorvi, “The inhibition of RANK-ligand in the management of postmenopausal osteoporosis and related fractures: the role of denosumab,” Gynecological Endocrinology, vol. 30, no. 6, pp. 403–408, 2014.
- J. Chen and P. Smerdely, “Hypocalcaemia after denosumab in older people following fracture,” Osteoporosis International, vol. 28, no. 2, pp. 517–522, 2017.
- N. M. Appelman-Dijkstra and S. E. Papapoulos, “Sclerostin inhibition in the management of osteoporosis,” Calcified Tissue International, vol. 98, no. 4, pp. 370–380, 2016.
- S. Das and J. C. Crockett, “Osteoporosis – a current view of pharmacological prevention and treatment,” Drug Design, Development and Therapy, vol. 7, pp. 435–448, 2013.
- J. L. Pérez-Castrillón, F. Pinacho, D. de Luis, M. Lopez-Menendez, and A. Dueñas Laita, “Odanacatib, a new drug for the treatment of osteoporosis: review of the results in postmenopausal women,” Journal of Osteoporosis, vol. 2010, Article ID 401581, 5 pages, 2010.
- A. Mullard, “Merck & Co. drops osteoporosis drug odanacatib,” Nature Reviews Drug Discovery, vol. 15, no. 10, p. 669, 2016.
- O. H. Jeon and J. Elisseeff, “Orthopedic tissue regeneration: cells, scaffolds, and small molecules,” Drug Delivery and Translational Research, vol. 6, no. 2, pp. 105–120, 2016.
- L. A. Jevons, F. D. Houghton, and R. S. Tare, “Augmentation of musculoskeletal regeneration: role for pluripotent stem cells,” Regenerative Medicine, vol. 13, no. 2, pp. 189–206, 2018.
- J. A. Sterling and S. A. Guelcher, “Biomaterial scaffolds for treating osteoporotic bone,” Current Osteoporosis Reports, vol. 12, no. 1, pp. 48–54, 2014.
- B. Ostrowska, A. di Luca, L. Moroni, and W. Swieszkowski, “Influence of internal pore architecture on biological and mechanical properties of three-dimensional fiber deposited scaffolds for bone regeneration,” Journal of Biomedical Materials Research Part A, vol. 104, no. 4, pp. 991–1001, 2016.
- K. K. Gómez-Lizárraga, C. Flores-Morales, M. L. del Prado-Audelo, M. A. Álvarez-Pérez, M. C. Piña-Barba, and C. Escobedo, “Polycaprolactone- and polycaprolactone/ceramic-based 3D-bioplotted porous scaffolds for bone regeneration: a comparative study,” Materials Science and Engineering: C, vol. 79, pp. 326–335, 2017.
- X. Yu, X. Tang, S. V. Gohil, and C. T. Laurencin, “Biomaterials for bone regenerative engineering,” Advanced Healthcare Materials, vol. 4, no. 9, pp. 1268–1285, 2015.
- M. Ojansivu, L. Johansson, S. Vanhatupa et al., “Knitted 3D scaffolds of polybutylene succinate support human mesenchymal stem cell growth and osteogenesis,” Stem Cells International, vol. 2018, Article ID 5928935, 11 pages, 2018.
- Y. Kim, S. H. Lee, B. J. Kang, W. H. Kim, H. S. Yun, and O. K. Kweon, “Comparison of osteogenesis between adipose-derived mesenchymal stem cells and their sheets on poly-ɛ-caprolactone/β-tricalcium phosphate composite scaffolds in canine bone defects,” Stem Cells International, vol. 2016, Article ID 8414715, 10 pages, 2016.
- E. R. Balmayor, “Targeted delivery as key for the success of small osteoinductive molecules,” Advanced Drug Delivery Reviews, vol. 94, pp. 13–27, 2015.
- J. Shen, A. W. James, X. Zhang et al., “Novel Wnt regulator NEL-like molecule-1 antagonizes adipogenesis and augments osteogenesis induced by bone morphogenetic protein 2,” The American Journal of Pathology, vol. 186, no. 2, pp. 419–434, 2016.
- M. E. Bateman, A. L. Strong, R. S. Hunter et al., “Osteoinductive effects of glyceollins on adult mesenchymal stromal/stem cells from adipose tissue and bone marrow,” Phytomedicine, vol. 27, pp. 39–51, 2017.
- M. Li, X. Yang, W. Wang et al., “Evaluation of the osteo-inductive potential of hollow three-dimensional magnesium-strontium substitutes for the bone grafting application,” Materials Science and Engineering: C, vol. 73, pp. 347–356, 2017.
- Q. Zhang, X. Chen, S. Geng et al., “Nanogel-based scaffolds fabricated for bone regeneration with mesoporous bioactive glass and strontium: in vitro and in vivo characterization,” Journal of Biomedical Materials Research Part A, vol. 105, no. 4, pp. 1175–1183, 2017.
- S. L. Teitelbaum, “Stem cells and osteoporosis therapy,” Cell Stem Cell, vol. 7, no. 5, pp. 553-554, 2010.
- X. Li, Z. Y. Zhou, Y. Y. Zhang, and H. L. Yang, “IL-6 contributes to the defective osteogenesis of bone marrow stromal cells from the vertebral body of the glucocorticoid-induced osteoporotic mouse,” PLoS One, vol. 11, no. 4, article e0154677, 2016.
- H. Huang, H. J. Kim, E. J. Chang et al., “IL-17 stimulates the proliferation and differentiation of human mesenchymal stem cells: implications for bone remodeling,” Cell Death & Differentiation, vol. 16, no. 10, pp. 1332–1343, 2009.
- T. Itkin, K. B. Kaufmann, S. Gur-Cohen, A. Ludin, and T. Lapidot, “Fibroblast growth factor signaling promotes physiological bone remodeling and stem cell self-renewal,” Current Opinion in Hematology, vol. 20, no. 3, 2013.
- L. Vicari, G. Calabrese, S. Forte et al., “Potential role of activating transcription factor 5 during osteogenesis,” Stem Cells International, vol. 2016, Article ID 5282185, 8 pages, 2016.
- M. Bionaz, E. Monaco, and M. B. Wheeler, “Transcription adaptation during in vitro adipogenesis and osteogenesis of porcine mesenchymal stem cells: dynamics of pathways, biological processes, up-stream regulators, and gene networks,” PLoS One, vol. 10, no. 9, article e0137644, 2015.
- M. Yang, C. J. Li, X. Sun et al., “MiR-497∼195 cluster regulates angiogenesis during coupling with osteogenesis by maintaining endothelial Notch and HIF-1α activity,” Nature Communications, vol. 8, no. 1, article 16003, 2017.
- C. K. F. Chan, G. S. Gulati, R. Sinha et al., “Identification of the human skeletal stem cell,” Cell, vol. 175, no. 1, pp. 43–56.e21, 2018.
- P. Bianco and P. G. Robey, “Skeletal stem cells,” Development, vol. 142, no. 6, pp. 1023–1027, 2015.
- P. Bianco, P. G. Robey, I. Saggio, and M. Riminucci, “"Mesenchymal" stem cells in human bone marrow (skeletal stem cells): a critical discussion of their nature, identity, and significance in incurable skeletal disease,” Human Gene Therapy, vol. 21, no. 9, pp. 1057–1066, 2010.
- R. S. Mahla, “Stem cells applications in regenerative medicine and disease therapeutics,” International Journal of Cell Biology, vol. 2016, Article ID 6940283, 24 pages, 2016.
- J. A. Ankrum, J. F. Ong, and J. M. Karp, “Mesenchymal stem cells: immune evasive, not immune privileged,” Nature Biotechnology, vol. 32, no. 3, pp. 252–260, 2014.
- L. Hu, C. Yin, F. Zhao, A. Ali, J. Ma, and A. Qian, “Mesenchymal stem cells: cell fate decision to osteoblast or adipocyte and application in osteoporosis treatment,” International Journal of Molecular Sciences, vol. 19, no. 2, 2018.
- Y. Tokuzawa, K. Yagi, Y. Yamashita et al., “Id4, a new candidate gene for senile osteoporosis, acts as a molecular switch promoting osteoblast differentiation,” PLoS Genetics, vol. 6, no. 7, article e1001019, 2010.
- Q. Wang, B. Zhao, C. Li, J. S. Rong, S. Q. Tao, and T. Z. Tao, “Decreased proliferation ability and differentiation potential of mesenchymal stem cells of osteoporosis rat,” Asian Pacific Journal of Tropical Medicine, vol. 7, no. 5, pp. 358–363, 2014.
- C. Wang, H. Meng, X. Wang, C. Zhao, J. Peng, and Y. Wang, “Differentiation of bone marrow mesenchymal stem cells in osteoblasts and adipocytes and its role in treatment of osteoporosis,” Medical Science Monitor, vol. 22, pp. 226–233, 2016.
- S. Ariffin, T. Manogaran, I. Abidin, R. Wahab, and S. Senafi, “A perspective on stem cells as biological systems that produce differentiated osteoblasts and odontoblasts,” Current Stem Cell Research & Therapy, vol. 12, no. 3, pp. 247–259, 2017.
- H. Kang, Y. R. V. Shih, M. Nakasaki, H. Kabra, and S. Varghese, “Small molecule–driven direct conversion of human pluripotent stem cells into functional osteoblasts,” Science Advances, vol. 2, no. 8, article e1600691, 2016.
- X. D. Chen, V. Dusevich, J. Q. Feng, S. C. Manolagas, and R. L. Jilka, “Extracellular matrix made by bone marrow cells facilitates expansion of marrow-derived mesenchymal progenitor cells and prevents their differentiation into osteoblasts,” Journal of Bone and Mineral Research, vol. 22, no. 12, pp. 1943–1956, 2007.
- O. Katsara, L. G. Mahaira, E. G. Iliopoulou et al., “Effects of donor age, gender, and in vitro cellular aging on the phenotypic, functional, and molecular characteristics of mouse bone marrow-derived mesenchymal stem cells,” Stem Cells and Development, vol. 20, no. 9, pp. 1549–1561, 2011.
- J. Kiernan, J. E. Davies, and W. L. Stanford, “Concise review: musculoskeletal stem cells to treat age-related osteoporosis,” Stem Cells Translational Medicine, vol. 6, no. 10, pp. 1930–1939, 2017.
- J. P. Rodríguez, S. Garat, H. Gajardo, A. M. Pino, and G. Seitz, “Abnormal osteogenesis in osteoporotic patients is reflected by altered mesenchymal stem cells dynamics,” Journal of Cellular Biochemistry, vol. 75, no. 3, pp. 414–423, 1999.
- Z. Wang, J. Goh, S. D. de et al., “Efficacy of bone marrow–derived stem cells in strengthening osteoporotic bone in a rabbit model,” Tissue Engeneering, vol. 12, no. 7, pp. 1753–1761, 2006.
- Z. M. Zhang, L. S. Jiang, S. D. Jiang, and L. Y. Dai, “Osteogenic potential and responsiveness to leptin of mesenchymal stem cells between postmenopausal women with osteoarthritis and osteoporosis,” Journal of Orthopaedic Research, vol. 27, no. 8, pp. 1067–1073, 2009.
- F. L. Yuan, R. S. Xu, D. L. Jiang et al., “Leonurine hydrochloride inhibits osteoclastogenesis and prevents osteoporosis associated with estrogen deficiency by inhibiting the NF-κB and PI3K/Akt signaling pathways,” Bone, vol. 75, pp. 128–137, 2015.
- A. Martin, J. Xiong, T. Koromila et al., “Estrogens antagonize RUNX2-mediated osteoblast-driven osteoclastogenesis through regulating RANKL membrane association,” Bone, vol. 75, pp. 96–104, 2015.
- R. Okazaki, D. Inoue, M. Shibata et al., “Estrogen promotes early osteoblast differentiation and inhibits adipocyte differentiation in mouse bone marrow stromal cell lines that express estrogen receptor (ER) α or β,” Endocrinology, vol. 143, no. 6, pp. 2349–2356, 2002.
- R. Y. Kim, H. J. Yang, Y. M. Song, I. S. Kim, and S. J. Hwang, “Estrogen modulates bone morphogenetic protein-induced sclerostin expression through the Wnt signaling pathway,” Tissue Engineering Part A, vol. 21, no. 13-14, pp. 2076–2088, 2015.
- M. A. Khan, M. W. Liu, D. Singh et al., “Long-term (three years) effect of estrogen replacement therapy on major adverse cardiac events in postmenopausal women after intracoronary stenting,” The American Journal of Cardiology, vol. 86, no. 3, pp. 330–333, 2000.
- C. J. DeSelm, Y. Takahata, J. Warren et al., “IL-17 mediates estrogen-deficient osteoporosis in an Act1-dependent manner,” Journal of Cellular Biochemistry, vol. 113, no. 9, pp. 2895–2902, 2012.
- A. M. Tyagi, K. Srivastava, M. N. Mansoori, R. Trivedi, N. Chattopadhyay, and D. Singh, “Estrogen deficiency induces the differentiation of IL-17 secreting Th17 cells: a new candidate in the pathogenesis of osteoporosis,” PLoS One, vol. 7, no. 9, article e44552, 2012.
- L. Ma, R. Aijima, Y. Hoshino et al., “Transplantation of mesenchymal stem cells ameliorates secondary osteoporosis through interleukin-17-impaired functions of recipient bone marrow mesenchymal stem cells in MRL/lpr mice,” Stem Cell Research & Therapy, vol. 6, p. 104, 2015.
- H. Zhuang, X. Zhang, C. Zhu et al., “Molecular mechanisms of PPAR-γ governing MSC osteogenic and adipogenic differentiation,” Current Stem Cell Research & Therapy, vol. 11, no. 3, pp. 255–264, 2016.
- I. H. Song, A. I. Caplan, and J. E. Dennis, “Dexamethasone inhibition of confluence-induced apoptosis in human mesenchymal stem cells,” Journal of Orthopaedic Research, vol. 27, no. 2, pp. 216–221, 2009.
- B. Gao, Q. Huang, Q. Jie et al., “Ginsenoside-Rb2 inhibits dexamethasone-induced apoptosis through promotion of GPR120 induction in bone marrow-derived mesenchymal stem cells,” Stem Cells and Development, vol. 24, no. 6, pp. 781–790, 2015.
- M. Duyvendak, M. Naunton, E. N. van Roon, G. A. W. Bruyn, and J. R. B. J. Brouwers, “Systematic review of trends in prophylaxis of corticosteroid-induced osteoporosis: the need for standard audit guidelines,” Osteoporosis International, vol. 19, no. 10, pp. 1379–1394, 2008.
- L. Wang, H. Y. Zhang, B. Gao et al., “Tetramethylpyrazine protects against glucocorticoid-induced apoptosis by promoting autophagy in mesenchymal stem cells and improves bone mass in glucocorticoid-induced osteoporosis rats,” Stem Cells and Development, vol. 26, no. 6, pp. 419–430, 2017.
- B. Gao, X. Lin, H. Jing et al., “Local delivery of tetramethylpyrazine eliminates the senescent phenotype of bone marrow mesenchymal stromal cells and creates an anti-inflammatory and angiogenic environment in aging mice,” Aging Cell, vol. 17, no. 3, article e12741, 2018.
- C. Seeliger, E. R. Balmayor, and M. van Griensven, “miRNAs related to skeletal diseases,” Stem Cells and Development, vol. 25, no. 17, pp. 1261–1281, 2016.
- X. Zhu, J. Yu, J. du, G. Zhong, L. Qiao, and J. Lin, “LncRNA HOXA-AS2 positively regulates osteogenesis of mesenchymal stem cells through inactivating NF-κB signalling,” Journal of Cellular and Molecular Medicine, vol. 23, no. 2, pp. 1325–1332, 2019.
- X. Chen, L. Yang, D. Ge et al., “Long non‑coding RNA XIST promotes osteoporosis through inhibiting bone marrow mesenchymal stem cell differentiation,” Experimental and Therapeutic Medicine, vol. 17, no. 1, pp. 803–811, 2019.
- J. Wei, H. Li, S. Wang et al., “let-7 enhances osteogenesis and bone formation while repressing adipogenesis of human stromal/mesenchymal stem cells by regulating HMGA2,” Stem Cells and Development, vol. 23, no. 13, pp. 1452–1463, 2014.
- J. Qiu, G. Huang, N. Na, and L. Chen, “MicroRNA-214-5p/TGF-β/Smad2 signaling alters adipogenic differentiation of bone marrow stem cells in postmenopausal osteoporosis,” Molecular Medicine Reports, vol. 17, no. 5, pp. 6301–6310, 2018.
- Z. W. Teng, Y. Zhu, Q. Na et al., “Regulatory effect of miRNA on multi-directional differentiation ability of mesenchymal stem cell in treatment of osteoporosis,” Journal of Biological Regulators and Homeostatic Agents, vol. 30, no. 2, pp. 345–352, 2016.
- K. Liu, Y. Jing, W. Zhang et al., “Silencing miR-106b accelerates osteogenesis of mesenchymal stem cells and rescues against glucocorticoid-induced osteoporosis by targeting BMP2,” Bone, vol. 97, pp. 130–138, 2017.
- M. Kucia, R. Reca, F. R. Campbell et al., “A population of very small embryonic-like (VSEL) CXCR4+SSEA-1+Oct-4+ stem cells identified in adult bone marrow,” Leukemia, vol. 20, no. 5, pp. 857–869, 2006.
- M. Z. Ratajczak, B. Machalinski, W. Wojakowski, J. Ratajczak, and M. Kucia, “A hypothesis for an embryonic origin of pluripotent Oct-4+ stem cells in adult bone marrow and other tissues,” Leukemia, vol. 21, no. 5, pp. 860–867, 2007.
- M. Kucia, M. Halasa, M. Wysoczynski et al., “Morphological and molecular characterization of novel population of CXCR4+ SSEA-4+ Oct-4+ very small embryonic-like cells purified from human cord blood – preliminary report,” Leukemia, vol. 21, no. 2, pp. 297–303, 2007.
- H. Sovalat, M. Scrofani, A. Eidenschenk, S. Pasquet, V. Rimelen, and P. Hénon, “Identification and isolation from either adult human bone marrow or G-CSF−mobilized peripheral blood of CD34+/CD133+/CXCR4+/ Lin−CD45− cells, featuring morphological, molecular, and phenotypic characteristics of very small embryonic-like (VSEL) stem cells,” Experimental Hematology, vol. 39, no. 4, pp. 495–505, 2011.
- E. K. Zuba-Surma, W. Wu, J. Ratajczak, M. Kucia, and M. Z. Ratajczak, “Very small embryonic-like stem cells in adult tissues—Potential implications for aging,” Mechanisms of Ageing and Development, vol. 130, no. 1-2, pp. 58–66, 2009.
- M. Z. Ratajczak, E. K. Zuba-Surma, D. M. Shin, J. Ratajczak, and M. Kucia, “Very small embryonic-like (VSEL) stem cells in adult organs and their potential role in rejuvenation of tissues and longevity,” Experimental Gerontology, vol. 43, no. 11, pp. 1009–1017, 2008.
- R. S. Taichman, Z. Wang, Y. Shiozawa et al., “Prospective identification and skeletal localization of cells capable of multilineage differentiation in vivo,” Stem Cells and Development, vol. 19, no. 10, pp. 1557–1570, 2010.
- A. M. Havens, H. Sun, Y. Shiozawa et al., “Human and murine very small embryonic-like cells represent multipotent tissue progenitors, in vitro and in vivo,” Stem Cells and Development, vol. 23, no. 7, pp. 689–701, 2014.
- A. M. Havens, Y. Shiozawa, Y. Jung et al., “Human very small embryonic-like cells generate skeletal structures, in vivo,” Stem Cells and Development, vol. 22, no. 4, pp. 622–630, 2013.
- M. Ahlström, M. Pekkinen, and C. Lamberg-Allardt, “Dexamethasone downregulates the expression of parathyroid hormone-related protein (PTHrP) in mesenchymal stem cells,” Steroids, vol. 74, no. 2, pp. 277–282, 2009.
- S. Nishida, A. Yamaguchi, T. Tanizawa et al., “Increased bone formation by intermittent parathyroid hormone administration is due to the stimulation of proliferation and differentiation of osteoprogenitor cells in bone marrow,” Bone, vol. 15, no. 6, pp. 717–723, 1994.
- J. Davies and T. J. Chambers, “Parathyroid hormone activates adhesion in bone marrow stromal precursor cells,” Journal of Endocrinology, vol. 180, no. 3, pp. 505–513, 2004.
- G. Kolios, “Animal models of inflammatory bowel disease: how useful are they really?” Current Opinion in Gastroenterology, vol. 32, no. 4, pp. 251–257, 2016.
- V. Valatas, G. Bamias, and G. Kolios, “Experimental colitis models: insights into the pathogenesis of inflammatory bowel disease and translational issues,” European Journal of Pharmacology, vol. 759, pp. 253–264, 2015.
- J. T. Taiani, H. R. Buie, G. M. Campbell et al., “Embryonic stem cell therapy improves bone quality in a model of impaired fracture healing in the mouse; tracked temporally using in vivo micro-CT,” Bone, vol. 64, pp. 263–272, 2014.
- R. Aggarwal, V. J. Pompili, and H. Das, “Generation of osteoporosis in immune-compromised mice for stem cell therapy,” in Animal Models for Stem Cell Therapy, B. Christ, J. Oerlecke, and P. Stock, Eds., vol. 1213 of Methods in Molecular Biology, pp. 209–214, Humana Press, 2014.
- Z. Yu, T. Zhu, C. Li et al., “Improvement of intertrochanteric bone quality in osteoporotic female rats after injection of polylactic acid-polyglycolic acid copolymer/collagen type I microspheres combined with bone mesenchymal stem cells,” International Orthopaedics, vol. 36, no. 10, pp. 2163–2171, 2012.
- X. Ye, P. Zhang, S. Xue, Y. Xu, J. Tan, and G. Liu, “Adipose-derived stem cells alleviate osteoporosis by enchancing osteogenesis and inhibiting adipogenesis in a rabbit model,” Cytotherapy, vol. 16, no. 12, pp. 1643–1655, 2014.
- S. Kikuta, N. Tanaka, T. Kazama et al., “Osteogenic effects of dedifferentiated fat cell transplantation in rabbit models of bone defect and ovariectomy-induced osteoporosis,” Tissue Engineering Part A, vol. 19, no. 15-16, pp. 1792–1802, 2013.
- J. H. An, H. Park, J. A. Song et al., “Transplantation of human umbilical cord blood-derived mesenchymal stem cells or their conditioned medium prevents bone loss in ovariectomized nude mice,” Tissue Engineering Part A, vol. 19, no. 5-6, pp. 685–696, 2013.
- R. Aggarwal, J. Lu, S. Kanji et al., “Human umbilical cord blood-derived CD34+ cells reverse osteoporosis in NOD/SCID mice by altering osteoblastic and osteoclastic activities,” PLoS One, vol. 7, no. 6, article e39365, 2012.
- P. P. Lelovas, T. T. Xanthos, S. E. Thoma, G. P. Lyritis, and I. A. Dontas, “The laboratory rat as an animal model for osteoporosis research,” Comparative Medicine, vol. 58, no. 5, pp. 424–430, 2008.
- K. Čamernik, A. Barlič, M. Drobnič, J. Marc, M. Jeras, and J. Zupan, “Mesenchymal stem cells in the musculoskeletal system: from animal models to human tissue regeneration?” Stem Cell Reviews and Reports, vol. 14, no. 3, pp. 346–369, 2018.
- M. Guan, W. Yao, R. Liu et al., “Directing mesenchymal stem cells to bone to augment bone formation and increase bone mass,” Nature Medicine, vol. 18, no. 3, pp. 456–462, 2012.
- J. Kiernan, S. Hu, M. D. Grynpas, J. E. Davies, and W. L. Stanford, “Systemic mesenchymal stromal cell transplantation prevents functional bone loss in a mouse model of age-related osteoporosis,” Stem Cells Translational Medicine, vol. 5, no. 5, pp. 683–693, 2016.
- L. You, L. Pan, L. Chen et al., “Suppression of zinc finger protein 467 alleviates osteoporosis through promoting differentiation of adipose derived stem cells to osteoblasts,” Journal of Translational Medicine, vol. 10, p. 11, 2012.
- F. S.-H. Hsiao, C. C. Cheng, S. Y. Peng et al., “Isolation of therapeutically functional mouse bone marrow mesenchymal stem cells within 3 h by an effective single-step plastic-adherent method,” Cell Proliferation, vol. 43, no. 3, pp. 235–248, 2010.
- S. W. Cho, H. J. Sun, J. Y. Yang et al., “Human adipose tissue-derived stromal cell therapy prevents bone loss in ovariectomized nude mouse,” Tissue Engineering Part A, vol. 18, no. 9-10, pp. 1067–1078, 2012.
- M. A. Akbar, Y. Lu, A. S. Elshikha et al., “Transplantation of adipose tissue–derived mesenchymal stem cell (ATMSC) expressing alpha-1 antitrypsin reduces bone loss in ovariectomized osteoporosis mice,” Human Gene Therapy, vol. 28, no. 2, pp. 179–189, 2017.
- S. W. Cho, H. J. Sun, J. Y. Yang et al., “Transplantation of mesenchymal stem cells overexpressing RANK-Fc or CXCR4 prevents bone loss in ovariectomized mice,” Molecular Therapy, vol. 17, no. 11, pp. 1979–1987, 2009.
- Y. H. Kim, M. Park, K. A. Cho et al., “Tonsil-derived mesenchymal stem cells promote bone mineralization and reduce marrow and visceral adiposity in a mouse model of senile osteoporosis,” Stem Cells and Development, vol. 25, no. 15, pp. 1161–1171, 2016.
- H. T. Chen, M. J. Lee, C. H. Chen et al., “Proliferation and differentiation potential of human adipose-derived mesenchymal stem cells isolated from elderly patients with osteoporotic fractures,” Journal of Cellular and Molecular Medicine, vol. 16, no. 3, pp. 582–592, 2012.
- A. Tzouvelekis, G. Koliakos, P. Ntolios et al., “Stem cell therapy for idiopathic pulmonary fibrosis: a protocol proposal,” Journal of Translational Medicine, vol. 9, no. 1, p. 182, 2011.
- F. Li, C. Zhou, L. Xu, S. Tao, J. Zhao, and Q. Gu, “Effect of stem cell therapy on bone mineral density: a meta-analysis of preclinical studies in animal models of osteoporosis,” PLoS One, vol. 11, no. 2, article e0149400, 2016.
- H.-Y. Liu, J.-F. Chiou, A. T. H. Wu et al., “The effect of diminished osteogenic signals on reduced osteoporosis recovery in aged mice and the potential therapeutic use of adipose-derived stem cells,” Biomaterials, vol. 33, no. 26, pp. 6105–6112, 2012.
- U. Hempel, K. Müller, C. Preissler et al., “Human bone marrow stromal cells: a reliable, challenging tool for in vitro osteogenesis and bone tissue engineering approaches,” Stem Cells International, vol. 2016, Article ID 7842191, 14 pages, 2016.
- L. Gucciardo, R. Lories, N. Ochsenbein-Kölble, E. Done’, A. Zwijsen, and J. Deprest, “Fetal mesenchymal stem cells: isolation, properties and potential use in perinatology and regenerative medicine,” BJOG: An International Journal of Obstetrics & Gynaecology, vol. 116, no. 2, pp. 166–172, 2009.
- J. S. Heo, Y. Choi, H. S. Kim, and H. O. Kim, “Comparison of molecular profiles of human mesenchymal stem cells derived from bone marrow, umbilical cord blood, placenta and adipose tissue,” International Journal of Molecular Medicine, vol. 37, no. 1, pp. 115–125, 2016.
- N. Beeravolu, C. McKee, A. Alamri et al., “Isolation and characterization of mesenchymal stromal cells from human umbilical cord and fetal placenta,” Journal of Visualized Experiments, no. 122, article e55224, 2017.
- P. De Coppi, G. Bartsch, M. M. Siddiqui et al., “Isolation of amniotic stem cell lines with potential for therapy,” Nature Biotechnology, vol. 25, no. 1, pp. 100–106, 2007.
- A. Somal, I. A. Bhat, B. Indu et al., “A comparative study of growth kinetics, in vitro differentiation potential and molecular characterization of fetal adnexa derived caprine mesenchymal stem cells,” PLoS One, vol. 11, no. 6, article e0156821, 2016.
- P. V. Guillot, O. Abass, J. H. D. Bassett et al., “Intrauterine transplantation of human fetal mesenchymal stem cells from first-trimester blood repairs bone and reduces fractures in osteogenesis imperfecta mice,” Blood, vol. 111, no. 3, pp. 1717–1725, 2008.
- S. Rani, A. E. Ryan, M. D. Griffin, and T. Ritter, “Mesenchymal stem cell-derived extracellular vesicles: toward cell-free therapeutic applications,” Molecular Therapy, vol. 23, no. 5, pp. 812–823, 2015.
- B. Sui, C. Hu, X. Zhang et al., “Allogeneic mesenchymal stem cell therapy promotes osteoblastogenesis and prevents glucocorticoid-induced osteoporosis,” Stem Cells Translational Medicine, vol. 5, no. 9, pp. 1238–1246, 2016.
- K. Tashiro, M. Inamura, K. Kawabata et al., “Efficient adipocyte and osteoblast differentiation from mouse induced pluripotent stem cells by adenoviral transduction,” Stem Cells, vol. 27, no. 8, pp. 1802–1811, 2009.
- F. Li and C. Niyibizi, “Cells derived from murine induced pluripotent stem cells (iPSC) by treatment with members of TGF-beta family give rise to osteoblasts differentiation and form bone in vivo,” BMC Cell Biology, vol. 13, no. 1, 2012.
- T. Foroutan, “Comparison of differentiation of induced pluripotent stem cells and bone-marrow mesenchymal stem cells to osteoblast: osteogenesis versus pluripotency,” International Journal of Organ Transplantational Medicine, vol. 7, no. 2, pp. 91–96, 2016.
- G. Z. Jin, T. H. Kim, J. H. Kim et al., “Bone tissue engineering of induced pluripotent stem cells cultured with macrochanneled polymer scaffold,” Journal of Biomedical Materials Research Part A, vol. 101A, no. 5, pp. 1283–1291, 2013.
- X. Qi, J. Zhang, H. Yuan et al., “Exosomes secreted by human-induced pluripotent stem cell-derived mesenchymal stem cells repair critical-sized bone defects through enhanced angiogenesis and osteogenesis in osteoporotic rats,” International Journal of Biological Sciences, vol. 12, no. 7, pp. 836–849, 2016.
- J. Y. Ko, S. Park, and G. I. Im, “Osteogenesis from human induced pluripotent stem cells: an in vitro and in vivo comparison with mesenchymal stem cells,” Stem Cells and Development, vol. 23, no. 15, pp. 1788–1797, 2014.
- G. Bilousova, D. H. Jun, K. B. King et al., “Osteoblasts derived from induced pluripotent stem cells form calcified structures in scaffolds both in vitro and in vivo,” Stem Cells, vol. 29, no. 2, pp. 206–216, 2011.
- X. Liu, W. Chen, C. Zhang et al., “Co-seeding human endothelial cells with human-induced pluripotent stem cell-derived mesenchymal stem cells on calcium phosphate scaffold enhances osteogenesis and vascularization in rats,” Tissue Engineering Part A, vol. 23, no. 11-12, pp. 546–555, 2017.
- A. F. Steinert, L. Rackwitz, F. Gilbert, U. Noth, and R. S. Tuan, “Concise review: the clinical application of mesenchymal stem cells for musculoskeletal regeneration: current status and perspectives,” Stem Cells Translational Medicine, vol. 1, no. 3, pp. 237–247, 2012.
- H. Zigdon-Giladi, U. Rudich, G. Michaeli Geller, and A. Evron, “Recent advances in bone regeneration using adult stem cells,” World Journal of Stem Cells, vol. 7, no. 3, pp. 630–640, 2015.
- C. Liu, H. Zhang, X. Tang et al., “Mesenchymal stem cells promote the osteogenesis in collagen-induced arthritic mice through the inhibition of TNF-α,” Stem Cells International, vol. 2018, Article ID 4069032, 10 pages, 2018.
- M. Westgren and C. Götherström, “Stem cell transplantation before birth - a realistic option for treatment of osteogenesis imperfecta?” Prenatal Diagnosis, vol. 35, no. 9, pp. 827–832, 2015.
- E. M. Horwitz, P. L. Gordon, W. K. K. Koo et al., “Isolated allogeneic bone marrow-derived mesenchymal cells engraft and stimulate growth in children with osteogenesis imperfecta: implications for cell therapy of bone,” Proceedings of the National Academy of Sciences, vol. 99, no. 13, pp. 8932–8937, 2002.
- C. Götherström, M. Westgren, S. W. S. Shaw et al., “Pre- and postnatal transplantation of fetal mesenchymal stem cells in osteogenesis imperfecta: a two-center experience,” Stem Cells Translational Medicine, vol. 3, no. 2, pp. 255–264, 2014.
- T. Taketani, C. Oyama, A. Mihara et al., “Ex vivo expanded allogeneic mesenchymal stem cells with bone marrow transplantation improved osteogenesis in infants with severe hypophosphatasia,” Cell Transplantation, vol. 24, no. 10, pp. 1931–1943, 2015.
- A. Tzouvelekis, V. Paspaliaris, G. Koliakos et al., “A prospective, non-randomized, no placebo-controlled, phase Ib clinical trial to study the safety of the adipose derived stromal cells-stromal vascular fraction in idiopathic pulmonary fibrosis,” Journal of Translational Medicine, vol. 11, no. 1, p. 171, 2013.
- B. Levi and M. T. Longaker, “Concise review: adipose-derived stromal cells for skeletal regenerative medicine,” Stem Cells, vol. 29, no. 4, pp. 576–582, 2011.
- D. Ilic, C. Miere, and E. Lazic, “Umbilical cord blood stem cells: clinical trials in non-hematological disorders,” British Medical Bulletin, vol. 102, no. 1, pp. 43–57, 2012.
- C. K. Rebelatto, A. M. Aguiar, M. P. Moretao et al., “Dissimilar differentiation of mesenchymal stem cells from bone marrow, umbilical cord blood, and adipose tissue,” Experimental Biology and Medicine, vol. 233, no. 7, pp. 901–913, 2008.
- P. V. Guillot, C. de Bari, F. Dell'Accio, H. Kurata, J. Polak, and N. M. Fisk, “Comparative osteogenic transcription profiling of various fetal and adult mesenchymal stem cell sources,” Differentiation, vol. 76, no. 9, pp. 946–957, 2008.
- A. Trounson, R. G. Thakar, G. Lomax, and D. Gibbons, “Clinical trials for stem cell therapies,” BMC Medicine, vol. 9, no. 1, 2011.
- A. Shakouri-Motlagh, A. J. O'Connor, S. P. Brennecke, B. Kalionis, and D. E. Heath, “Native and solubilized decellularized extracellular matrix: a critical assessment of their potential for improving the expansion of mesenchymal stem cells,” Acta Biomaterialia, vol. 55, pp. 1–12, 2017.
- H. Majd, T. M. Quinn, P. J. Wipff, and B. Hinz, “Dynamic expansion culture for mesenchymal stem cells,” in Mesenchymal Stem Cell Assays and Applications, M. Vemuri, L. Chase, and M. Rao, Eds., vol. 698 of Methods in Molecular Biology, pp. 175–188, Humana Press, 2011.
- C. A. Pacak, M. T. Eddy, L. Woodhull et al., “Microcarrier-based expansion of adult murine side population stem cells,” PLoS One, vol. 8, no. 1, article e55187, 2013.
- M. Serra, C. Correia, R. Malpique et al., “Microencapsulation technology: a powerful tool for integrating expansion and cryopreservation of human embryonic stem cells,” PLoS One, vol. 6, no. 8, article e23212, 2011.
- N. I. Nikolaev, Y. Liu, H. Hussein, and D. J. Williams, “The sensitivity of human mesenchymal stem cells to vibration and cold storage conditions representative of cold transportation,” Journal of The Royal Society Interface, vol. 9, no. 75, pp. 2503–2515, 2012.
- K. Lynch and M. Pei, “Age associated communication between cells and matrix: a potential impact on stem cell-based tissue regeneration strategies,” Organogenesis, vol. 10, no. 3, pp. 289–298, 2014.
- H. Liu, X. Xia, and B. Li, “Mesenchymal stem cell aging: mechanisms and influences on skeletal and non-skeletal tissues,” Experimental Biology and Medicine, vol. 240, no. 8, pp. 1099–1106, 2015.
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