Abstract

Neuromuscular diseases are a heterogeneous group of diseases that lead to significant disability in effected individuals. Pharmacological treatments failed to provide any significant improvement to date. Recently, the introduction of stem cells into the field of health sciences raised the hopes for a new treatment for neuromuscular diseases. In theory, stem cells, owing to their multilineage differentiation capacity, could differentiate into myofibers and neurons and replace the degenerated cells leading to recovery of the patients. Results obtained from the preclinical studies supported this theory. However, clinical trials with stem cells could not meet the expectations mainly because of early mortality, limited migration, and differentiation of the implanted cells. Modification of the stem cells before implantation, such as introduction of deficient genes or commitment to a precursor cell line provided little improvement. The biggest barrier to overcome for a successful of stem cell treatment, which also should be the focus of the future studies, is to increase the functional integration of the donor cells with the recipient tissues. Understanding the underlying pathogenic mechanisms of the neuromuscular diseases is essential to achieve this goal.

1. Introduction

The term neuromuscular diseases define a wide range of conditions characterized by the weakness or wasting of the body muscles. Problems may primarily originate from the spinal cord, the peripheral nerves (neuropathies), the muscle fibers (myopathies), or the neuromuscular junction. Some of these diseases are hereditary, while others are acquired. The diagnosis is mainly done by clinical observation, electromyography, muscle biopsy, and in some instances molecular genetic studies. Some of the major types of neuromuscular diseases are amyotrophic lateral sclerosis (ALS), myasthenia gravis (MG), multiple sclerosis (MS), and muscular dystrophies (Duchenne’s muscular dystrophy (DMD) and Becker’s muscular dystrophy (BMD)). Despite the long lasting research on the pathogenesis and the molecular mechanisms of neuromuscular diseases, no satisfactory treatment has been offered yet for these diseases [1].

Stem cell research is relatively new in the medical field but holds a great potential for the treatment of a variety of diseases that remained untreatable so far. Stem cells are believed to exist in all tissues in human body and may be totipotent, pluripotent, or multipotent, depending on tissue type. Neuromuscular degeneration itself seems to promote proliferation, migration, and transdifferentiation of autologous stem cells. Production of neurotropic and growth factors and stimulation of the regenerative processes by stem cells have been demonstrated in neurodegenerative diseases [2]. Therefore, human skeletal muscle tissue and nerve tissue have a limited regeneration capacity via the muscle stem cells, also called satellite cells and nerve derived stem cells, respectively [3]. However, this regeneration capacity is not enough to reverse the pathological process in case of neuromuscular diseases. Following the discovery of multilineage plasticity of the stem cells that can be obtained in large numbers from easily accessible tissues, several attempts have been performed to treat neuromuscular diseases via the local or systemic injection of the stem cells. Despite the promising results obtained from in vitro studies, these attempts have yielded limited success in vivo as well as in clinical trials. This in part is due to the complexity of the microenvironment needed to ensure stem cell integration and function [3].

The aim of this paper is to summarize the translational research on the most common types of neuromuscular diseases.

2. Anatomy of Neuromuscular Unit

The formation of skeletal muscle begins during the fourth week of embryonic development by the rapid mitotic division of specialized mesodermal cells, termed myoblasts. By week nine of development, multinucleated skeletal muscle cells, termed muscle fibers, can be identified. By month five, the muscle fibers begin to accumulate protein filaments important in muscle contraction. Muscle fibers aggregate into bundles as the fetus grows, and by birth myoblast activity, so the formation of new muscle fibers stops. Muscle fibers contain longitudinally arranged myofilaments, named actin and myosin. Muscle contraction on a subcellular level is a complex interaction of myofilaments coupled with the influx and efflux of calcium ions following the excitation by nerve fibers. Each muscle fiber is bound to adjacent fibers to form bundles and accumulation of muscle bundles forms the muscle belly. Supporting connective tissue surrounding the individual fibers is named endomysium, the perimysium encloses the fascicles, and the epimysium surrounds the muscle belly [3].

Nerve fibers innervating the muscles originate from brain stem nuclei or the spinal cord and meet the muscle fibers in neuromuscular junctions. Neuromuscular junctions are composed of presynaptic nerve membrane, synaptic cleft, and postsynaptic muscle membrane. Myelinated motor nerve fibers enter their target muscles and they divide into 20–100 unmyelinated terminal fibers, each of which innervates a single muscle fiber. The terminal fiber from a motor axon and the muscle fiber it innervates are called a motor unit. The terminal nerve fibers contain the neurotransmitter acetylcholine (ACh) stored in synaptic vesicles. When the nerve fibers are stimulated by an action potential, ACh is released into the synaptic cleft and binds to the ACh receptors on the postsynaptic muscle membrane initiating the muscle contraction [6].

3. Stem Cells

Stem cells can be classified according to the way that they are derived and the source tissues. Embryonic stem cells (ESCs), mesenchymal stem cells (MSCs), haemopoietic stem cells (HSCs), and induced pluripotent stem (iPS) cells are the major types of stem cells. Each one of these stem cell types possesses certain advantages and disadvantages which should be weighed carefully before the desired applications.

ESCs cells are derived from the inner cell mass of a developing blastocyst and are pluripotent, possessing the capacity to give rise to all 3 germ layers. Concerns about in vivo tumor formation and ethical concerns regarding their harvest have thus far restricted the use of ESCs [3]. MSCs and HSCs are multipotent, self-renewing cells derived from adult tissues that can form a number of cells or tissues that are usually restricted to a particular germ layer. The major advantages of the MSCs and HSCs over ES cells are the ease of harvest and lack of immunoreactivity since they are derived from autologous sources. However, they may be less desirable for the treatment of genetic diseases since they may possess the same genetic predisposition to the disease. For example, MSCs derived from ALS patients exhibit diminished growth and differentiation capacity [7].

Most recent advancement in the field of stem cell research is the development of iPS cells [8]. iPS cells are reprogrammed fibroblasts that are transfected by selected transcription factors delivered by vector-, virus-, protein-, or RNA-mediated approaches. Original protocol utilized transcription factors Oct 3/4, Klf, Sox2, and c-Myc; however multiple research groups have now accomplished successful reprogramming of fibroblasts using various combinations of factors [912]. These cells are not only an option for disease modeling but also provide a novel source for autologous cellular therapies [7].

Stem cells are used in two different ways in neurodegenerative diseases: direct differentiation and regeneration or paracrine anti-inflammatory activity. Moreover, stem cells might become eventually carriers of pharmacological and gene treatments [2].

4. Amyotrophic Lateral Sclerosis

ALS is a neurodegenerative disease caused by the selective loss of both spinal and upper motor neurons [13]. It is the most common motor neuron disease in adults and usually diagnosed in the sixth decade of life. Initial presentation is limb weakness which progresses gradually to generalized muscle atrophy and paralysis. Respiratory muscle involvement leads to death usually within 5 years. The definitive treatment of the disease is the replacement of the lost motor neurons. Many pharmacologic agents have been tried for the treatment of ALS but none of them proved to be effective up to date.

Stem cells have been extensively experimented for the treatment of ALS in animal models and some provided significant benefit in preclinical level (Table 1). The types of stem cells that have been tested include neural stem and progenitor cells (NSCs, NPC), umbilical cord blood stem cells (UCBCs), bone marrow stem cells (BMCs), human glial restricted progenitors (hGRPs), embryonal stem (ES) cells, glial restricted progenitors (GRPs), and olfactory bulb neural progenitor cells (OB-NPCs). There are around ten clinical trials reported so far placing the amyotrophic lateral sclerosis to the top of the list of clinical trials for neurodegenerative disorders (Table 2). Even though some of these trials reported a limited success in terms of prolonged life span and functional improvement, the desired level of success has not been reached yet. Mazzini et al. reported the first clinical application of BMCs to the patients with ALS but mainly focused on the safety of the procedure in this preliminary study [14]. They later on published larger patient series along with the long-term follow-up results [2, 15, 16], and in their final paper in 2012 they stated that BMC application does not improve the symptoms of ALS in the long term, however, stabilizes the symptoms of the disease thus prolonging the life span of the patients [17]. On the other hand, Nefussy et al. could not detect any benefit of stem cell treatment in case of ALS, but it should be mentioned that they mobilized the BMCs via injection of granulocyte colony stimulating factor (G-CSF) but did not carry out direct injection of the cells [18]. A common finding of these clinical trials was the lack of major side effects of intraspinal/intracerebral cell injection that makes the clinicians more comfortable in continuing their efforts for the refinement of the stem cell therapies for ALS [14, 15, 19, 20]. Therefore, more clinical trials are on the way and there are nine more in various stages listed in the NIH database for clinical trials.

5. Multiple Sclerosis

MS is a chronic inflammatory demyelinating disease of the central nervous system that leads to cumulative and irreversible damage. MS is characterized by self-reactive lymphocytes and demyelination. Therefore, any method for the treatment of MS, to be effective, should reverse the demyelination as well as suppressing self-reacting lymphocytes. Available drug therapies are only partially effective in these terms. However, stem cells, considering their immunosuppressive effects and multilineage differentiation capacity, may be applicable for meeting some of these goals. Preclinical data and ongoing clinical trials suggest that selected patients may respond positively to autologous stem cell transplantation. But it is still uncertain if adult stem cells can repair existing neurological deficits in patients with MS [21].

The stem cell types that have been used for the treatment of MS in animal models were BMCs, iPS, NSCs, NPCs, HSCs, ESCs, adipose-derived stem cells (ASCs), embryonal carcinoma cells, bone marrow transplantation (BMT), Schwann cells (SCs), Schwann cell precursors (SCPs), and oligodendrocyte progenitor (OCP) cells. It was Bonilla et al. who first used stem cells for the treatment of MS [22]. They have injected BM-derived HSCs into the mouse brain and obtained encouraging results. Numerous studies have followed and they are listed in Table 3. Oka et al. used primates for their experiments and injected monkey NPCs into the central nervous system of the monkeys to treat MS [4] (Figure 1). They have seen significant number of myelinating fibers in the transplantation zone. In another primate study it was reported that transplanted human NPCs decreased disability and increased survival of the animals [23]. The results of these two experiments were important in terms of the proximity of the animal model to human beings. The potential of iPS cells to differentiate into functional OPCs, as documented recently, is another milestone development that would ameliorate the stem cell donor site issues [24]. There is only one controversial study in the literature published by Reekmans et al. in which mouse NSCs were transplanted to a mouse MS model but no therapeutic effect or improvement in the disease state could be detected [25].

Clinical trials on the treatment of MS with stem cells are listed in Table 4. In the standard protocol, patients are first treated with a combination of immunosuppressive agents to eliminate all lymphocytes including self-reactive lymphocytes. Myeloablative treatment is followed by stem cell transplantation. However, direct intrathecal injection of stem cells as well as nonmyeloablative stem cell transplantation has also been tried with favorable results [26, 27]. The largest series published so far includes 183 patients that are recorded in the database of the European Blood and Marrow Transplantation Group [28]. All these patients were treated with HSCs, and overall transplant-related mortality was 5.3% with an improvement or stabilization of neurological conditions in 63% of patients during a median followup of 41.7 months. The only reported serious side effect of stem cell treatment of MS was the Epstein-Barr virus-associated posttransplantation lymphoproliferative disorder [29]. In summary, both myeloablative or nonmyeloablative stem cell applications seem to be safe and both provide a certain level of clinical benefit to the patients.

6. Muscular Dystrophies

DMD is characterized by a progressive degeneration of the whole body musculature due to a deficit in the dystrophin gene. Dystrophin is a large protein of skeletal muscle tissue that is expressed under the sarcolemma and contributes to the stability of the giant syncytial muscle fibers. Natural course of the disease is confinement to wheelchairs around the age of 12 and death ensues in the later stages due to cardiomyopathy. During the course of the disease, effected skeletal muscle tissue regenerates to a limited extend through the activation of satellite cells but multiple cycles of degeneration-regeneration eventually lead to exhaustion of the myogenic reservoir. Replenishment of the myogenic reservoir is the principle of the stem cell transplantation to the skeletal muscle in muscular dystrophies. BMD is very similar toDMD except that it gets worse at a much slower rate and it is less common [30].

DMD and BMD are the most extensively studied diseases in the group of neuromuscular disorders. Therefore it would be overwhelming to cite all the experimental papers on treatment of muscular dystrophies in this paper. Instead, a general summary of the preclinical studies is given in Table 5. The most commonly used cell type in preclinical studies was myoblasts that originate from myogenic progenitors (MPCs), and the main MPCs in skeletal muscles are satellite cells. These normally quiescent cells are located beneath the muscle fiber basal lamina, and upon activation they either differentiate into committed progenitors (myoblasts) or self-renew by asymmetric division [3, 30, 31]. However, the first experimental report by Ferrari et al. utilized BM-derived MPCs in a mouse model of muscle degeneration [32]. Lafreniere et al. carried out the only primate study and reported that the grafted cells fused only with the recipient fibers at the injection site but did not migrate for so long distances limiting the success of the treatment [33]. Gene delivery methods using stem cells as vehicles proved some improvement [5, 3437]. Tedesco et al. transplanted genetically corrected mesoangioblasts in SCID/mdx mice intramuscularly [5]. They have successfully documented the dystrophin production and amelioration of morphological defects in tibialis anterior muscle of SCID/mdx mice following cell transplantation (Figure 2). While most of the other studies documented the limited success of stem cell treatment in muscular dystrophies, some others reported no benefit [3840].

Despite the high number of preclinical studies and encouraging data, clinical trials have been hampered by poor survival and limited migratory ability of the cells (Table 6). Neumeyer et al. reported no positive effect of myoblast transplantation into the tibialis anterior muscles of the patients with BMD [41]. Skuk et al. could detect fusion of the donor and recipient muscle fibers and subsequent synthesis of donor dystrophin following MPC allotransplantation; however this observation was limited to the injection sites [42]. Zhang et al. reported similar results following the transplantation of allogeneic human UCBCs [43]. Oxidative stress, fusion inability, and some administration methodologies are the suspected causes of poor cell survival following transplantation. Intramuscular administration was the preferred method of cellular delivery in most of the clinical studies since many cell types are not able to cross the endothelial barriers of the skeletal muscle tissue. The migration of the injected cells was generally restricted to short distances from the injection site. The resultant large intramuscular pockets of cells subsequently lead to cell death due to inefficient nutrient supply in vivo. Currently, there is only one ongoing clinical trial on stem cell treatment of muscular dystrophies in the NIH database. In this study the researchers aim to treat DMD by human UCBC transplantation.

7. Myasthenia Gravis

MG is an Ach receptor antibody-mediated autoimmune neuromuscular disease. The pathogenesis of MG involves the hyperactivity of T lymphocytes, activation of complement system, deficiency of immunomodulation, and impairment of immune homeostasis.

MSC has got the potential to correct impaired immune homeostasis in diseases like MG because they express intercellular adhesion molecule (ICAM)-1, ICAM-2, lymphocyte function-associated antigen (LFA) 3, fibronectin, lamin, and collagen, which are involved in the process of immune reaction. Moreover MSCs can modulate the T lymphocyte function via direct physical contact through the cell adhesion molecules [44, 45]. However, there are a just a few experimental studies on the application of stem cells for the treatment of MG. The first study was published by Yu et al. [44]. In this study BMCs were applied intravenously to a mouse model of MG. The authors noted a significant decrease in the circulating levels of Ach receptor IgG antibody and a significant functional improvement. The same group injected human UCBCs to mouse model of MG in another experiment and obtained similar results [46]. Sheng et al. injected granulocyte macrophage colony stimulating factor (GM-CSF) to a mouse model of MG and noted a decreased immune response against the Ach receptors through the mobilization of tolerogenic precursor cells [47]. The results of these experiments were encouraging in terms of clinical application, and a phase I clinical trial is already on the way (ClinicalTrials.gov identifier: NCT00424489). The purpose of this study was stated as to assess the toxicity/feasibility of autologous hematopoietic stem cell transplantation for refractoryMG cases in the NIH database. Several other studies are required to assess the feasibility of stem cell therapy for MG, and the concerns about the safety of the stem cell application in case of MG should be further investigated.

8. Discussion and Conclusions

Stem cells are the regenerative reservoirs of the body. They are located in niches in the tissues and become active in case of tissue injury to replace the degenerated cells. There are two possible mechanisms for the therapeutic effects of the stem cells: differentiation into different cell types or paracrine secretion. Some neuromuscular diseases like MS, ALS, and MG have got an inflammatory component, and in this case anti-inflammatory and immunomodulatory effects of stem cells could provide a treatment. In case of muscular dystrophies, the mechanism is the degeneration of muscle fibers rather than autoimmune destruction so the differentiation capacity of the stem cells comes forward.

Currently there is a gap between animal and human applications of stem cells in neuromuscular disorders so as the promising results obtained in animal studies have not been reproduced in clinical trials yet. The reasons for this failure can be summarized as limited differentiation of stem cells into required cell types, short survival times of transplanted cells, and lack of widespread effect due to the limited migration of the cells away from the injection site. In case of neuromuscular diseases, the preferred way for the delivery of the cells should be direct injection to the target tissues since the biological barriers may limit the migration of stem cells to the target tissues thus decreasing the efficacy of the treatment. Nevertheless lack of long distance migration of the stem cells in the tissues is still a major obstacle in front of cellular therapies as mentioned above. Ideas like corrective gene delivery via stem cells are theoretically brilliant, however, despite some improvement, they have not reached the desired clinical effectiveness yet. One single common result of stem cell trials is the safety of the procedure. Therefore further research can safely be carried out to overcome the current obstacles. Moreover, introduction of new stem cell types such as iPS cells will help to overcome donor site and ethical issues and increase the availability of stem cells.

In conclusion, stem cells still hold a great promise for the treatment of traditionally untreatable diseases. Continuing research will enable the clinicians to understand the biological processes of the neuromuscular diseases better and allow the modification of current treatment protocols accordingly. However the expectations from clinical applications should be kept realistic, and more importantly best effort should be done to prevent the worsening of the pathologies by cellular treatments.