The Epithelium–Molecular Landscaping for an Interactive BarrierView this Special Issue
Review Article | Open Access
Akihiko Saito, Hiroyoshi Sato, Noriaki Iino, Tetsuro Takeda, "Molecular Mechanisms of Receptor-Mediated Endocytosis in the Renal Proximal Tubular Epithelium", BioMed Research International, vol. 2010, Article ID 403272, 7 pages, 2010. https://doi.org/10.1155/2010/403272
Molecular Mechanisms of Receptor-Mediated Endocytosis in the Renal Proximal Tubular Epithelium
Receptor-mediated endocytosis is a pivotal function of renal proximal tubule epithelial cells (PTECs) to reabsorb and metabolize substantial amounts of proteins and other substances in glomerular filtrates. The function accounts for the conservation of nutrients, including carrier-bound vitamins and trace elements, filtered by glomeruli. Impairment of the process results in a loss of such substances and development of proteinuria, an important clinical sign of kidney disease and a risk marker for cardiovascular disease. Megalin is a multiligand endocytic receptor expressed at clathrin-coated pits of PTEC, playing a central role in the process. Megalin cooperates with various membrane molecules and interacts with many intracellular adaptor proteins for endocytic trafficking. Megalin is also involved in signaling pathways in the cells. Megalin-mediated endocytic overload leads to damage of PTEC. Further studies are needed to elucidate the mechanism of megalin-mediated endocytosis and develop strategies for preventing the damage of PTEC.
Renal proximal tubular epithelial cells (PTECs) are involved in a variety of vital functions. Of these, receptor-mediated endocytosis is a pivotal function of the cells to reabsorb and metabolize proteins and other substances in glomerular filtrates. Megalin is a membrane receptor that plays a central role in the endocytic functions of PTEC. Megalin cooperates with various molecules in the cells, taking up ligands into the endocytic pathway to lysosomes, as well as mediating signal transduction. In this review, we focus on recent progress in the research on megalin and its associated molecules. We also discuss how impaired or overloaded endocytosis induces PTEC damage which is tightly associated with the onset of proteinuria and the development of chronic kidney disease (CKD).
2. Megalin: A Major Endocytic Receptor in PTEC
Megalin is a large (~600 kDa) glycoprotein member of the low-density lipoprotein (LDL) receptor family [1, 2] that is primarily expressed at clathrin-coated pits and partly at microvilli of PTEC (Figure 1) [3, 4]. Megalin contains a huge extracellular domain responsible for its multispecific properties. The domain consists of 4398 amino acids (in humans) and is made by three types of repeats which are characteristic of the LDL receptor family: (1) 36 cysteine-rich complement-type repeats organized in four clusters, (2) 16 growth factor repeats separated by 8 YWTD containing spacer regions involved in pH dependent release of ligands in endosomal compartments , and (3) a single epidermal growth factor-like repeat. The extracellular domain is followed by a single transmembrane segment and a cytoplasmic domain of 209 amino acids. The cytoplasmic tail contains two endocytic motifs (NPXY) mediating clustering into clathrin coated pits and an NPXY-like motif (NQNY) involved in apical sorting of the receptor  as well as other protein interaction motifs (SH3 and PDZ domains) and phosphorylation sites [1, 2]. The physiological potential of these regulatory motifs has not yet been fully understood.
Megalin plays a critical role in the reabsorption of glomerular-filtered substances including albumin and low-molecular-weight proteins. Also, megalin may take up proteins that are released by PTEC to the apical tubular space. Megalin knockout mice display low-molecular-weight proteinuria and albuminuria . Furthermore, patients with Donnai-Barrow and facio-oculo-acoustico-renal syndromes, caused by mutations in the megalin gene, show increased urinary excretion of albumin and low-molecular-weight proteins . In this process, meglin mediates the conservation of carrier bound vitamins and trace elements filtered by glomeruli, including vitamin D , vitamin A , vitamin , and iron . Megalin cooperates with a variety of molecules at the apical membranes and also in the cytoplasm of PTEC (Figure 1) as described in the next section.
3. Molecules Associated with Megalin’s Functions in PTEC
3.1. Cubilin-Amnionless Complex (CUBAM)
Cubilin is a 460-kDa peripheral glycoprotein, thus lacking transmembrane and intracellular segments, but anchored to the apical membranes in PTEC. It was originally identified as the receptor for intrinsic factor-vitamin [13, 14], and its gene defects are the causes of hereditary megaloblastic anaemia 1 or Imerslund-Gräsbeck syndrome (selective vitamin B12 malabsorption with proteinuria) . Cubilin is also involved in the absorption of various protein ligands present in glomerular filtrates, including albumin, transferrin, and vitamin D-binding protein . Cubilin is known to interact with megalin for its endocytic functions [12, 16]; however, it is bound more firmly by a protein called amnionless, forming a complex named CUBAM, to be translocated to the plasma membrane [17, 18]. Amnionless, a 38–50 kDa membrane protein with a single transmembrane domain, was initially identified as a component for the normal development of the trunk mesoderm derived from the middle streak . Its gene defects also cause hereditary megaloblastic anaemia . However, the role of amnionless in PTEC is not fully identified.
3.2. Na+/H+ Exchanger Isoform 3 (NHE3)
NHE3, the main NHE isoform in PTEC, mediates isotonic reabsorption of approximately two thirds of the filtered NaCl and water, the reabsorption of bicarbonate, and the secretion of ammonium . It also contributes to the reabsorption of filtered citrate, amino acids, and oligopeptides by providing H+ used for the H+-coupled cotransporters. Enhanced NHE3 activity is assumed to be a factor for increased Na+ reabsorption and the development of hypertension in diabetes. NHE3 was reported to interact with megalin in intermicrovillar clefts of PTEC [22, 23]. After endocytosis with megalin, NHE3 is postulated to utilize the outward transvesicular Na+ gradient of endocytic vesicles and early endosomes to drive inward movement of H+ and endosomal acidification, which is important for dissociating reabsorbed ligand proteins from megalin for further processing.
ClC-5 is a 746-amino acid protein originally assumed to belong to the voltage-gated chloride channel family , but more recent evidence suggests that it may function as an H+/Cl− exchanger . In kidney, ClC-5 is highly expressed in PTEC and and intercalated cells of collecting ducts . In PTEC, ClC-5 is located at the apical endosomes together with electrogenic V-type H+-ATPases , where it has a complementary function in endosomal acidification . The physiological relevance of ClC-5 in renal functions came into view when mutations in the CLCN5 gene were identified in patients with Dent’s disease, an X-linked renal tubular disorder . This disorder is characterized by low molecular weight proteinuria, hypercalciuria, nephrocalcinosis, nephrolithiasis, aminoaciduria, phosphaturia, glycosuria, and renal failure . The precise mechanism of this abnormality is not entirely clear but possibly results from defective acidification and/or reduced expression of megalin and cubilin in PTEC [29, 30].
3.4. Intracellular Adaptor Proteins
Various sorting and signaling proteins bind to megalin’s cytoplasmic tail such as JIP1 and JIP2, SEMCAP-1 (GIPC), ANKRA, Dab2, PDS-95, MegBP, and ARH [31–37]. ARH and Dab2 are components of the clathrin coat, and they bind to the first and third NPXY motif of megalin, respectively, through their PTB domains [33, 37]. ARH and Dab2 are known to interact with motor proteins as described below. Dab2 is also known to mediate signal transduction [38, 39].
4. Regulation of Megalin Expression
Cellular expression of megalin was found to be downregulated by the action of TGFß . We also found that megalin expression is upregulated in cultured PTEC by treatment with insulin or high-concentration glucose (17.5 mM), whereas it is downregulated by angiotnsin II . Furthermore, we demonstrated that there is competitive cross talk between anigotensin II type 1 receptor- and insulin-mediated signaling pathways in the regulation of megalin expression in the cells, suggesting a counter-balanced mechanism that regulates megalin expression and functions in PTEC .
Decreased megalin expression in PTEC has been found in the early diabetic stages in experimental animals [40, 42]. It is also suggested that the functions of megalin are impaired in patients in the early stages of diabetic nephropathy, since low-molecular-weight proteinuria are frequently observed in patients at these stages [43, 44]. Thus, the altered regulation of megalin expression and functions must be significantly responsible for the early development of proteinuria/albuminuria in diabetic patients. The mechanisms of the regulation remain to be further investigated.
5. Regulation of Megalin Transport by Motor Proteins
The mechanisms of intracellular transport of megalin are largely unknown. Reverse-direction molecular motor myosin VI was found to be linked to Dab2 and GIPC, which binds to the cytoplasmic tail of megalin, and is assumed to be involved in the endocytosis in PTEC . However, myosin VI knockout mice, used as an animal model for deafness, showed no apparent renal manifestation presenting proteinuria .
We recently identified that another motor protein, nonmuscle myosin heavy chain IIA (NMHC IIA), binds to Dab2 and is involved in megalin-mediated endocytosis . Genetic alterations of NMHC-IIA are known to cause inherited human diseases, known as MYH9 disorders, which are characterized by giant platelets, thrombocytopenia, and granulocyte inclusions [48, 49]. The spectrum of diseases due to mutations in the gene includes May-Hegglin anomaly, Sebastian syndrome, Fechtner syndrome, and Epstein syndrome [48–51]. It has been also reported that all of these disorders are related to development of kidney disease [50, 52]. The manifestation of kidney disease in MYH9 disorders indicates the importance of NMHC-IIA in maintaining normal kidney functions, which has been also verified by two recent genomewide scan analyses [53, 54].
Another megalin-binding adaptor protein ARH also associates with motor and centrosomal proteins and is involved in centrosome assembly and cytokinesis . The relevance of the adaptor protein’s association with such molecules in the regulation of megalin transport remains undetermined.
6. Overloaded Endocytosis-Induced PTEC Injury in CKD
Overloaded endocytosis in PTEC due to increased glomerular protein filtration has been postulated to be a cause of tubulointerstitial injury. Megalin is identified as the key molecule to initiate the pathogenic process . In diabetes, advanced glycation endproducts (AGEs) are generated in the circulation and involved in a variety of cellular damage . Megalin also mediates the endocyosis of glomerular-filtered AGE in PTEC [58, 59], which causes toxicity in the cells [60, 61]. In metabolic syndrome or dyslipidemia, free fatty acids are delivered to PTEC with the carrier proteins such as albumin or liver-type fatty acid binding protein . Metabolically overloaded PTECs are activated to express proinflammatory cytokines, such as MCP1 and TNFα, and lead to apoptosis  or epithelial-mesenchymal transition [63, 64].
7. Handling of Albumin in PTEC, Related to the Mechanism of Albuminuria
Albumin (~69 kDa) is the most abundant circulating protein, carrying a variety of substances in plasma. Glomerular albumin filtration is assumed to be 3–6 g/d in humans . Only negligible amounts of albumin are detected in urine, and the substantial remaining of glomerular-filtered albumin is reabsorbed in PTEC via endocytosis, mediated by megalin and CUBAM. Albuminuria is an important clinical sign of kidney disease such as diabetic nephropathy [66, 67] as well as a risk marker of cardiovascular disease (CVD) [68, 69]. Impaired endocytic functions of PTEC for albumin are relevant to the mechanisms of albuminuria.
After endocytosis, albumin is considered to be transferred to lysosomes for degradation to amino acids . On the contrary, the presence of a retrieval or transcytic pathway of albumin in PTEC is suggested . A recent analysis using neonatal Fc receptor knockout mice supports the retrieval pathway in PTEC where the receptor appears to play a critical role to reclaim albumin from the glomerular filtrates .
The association of albuminuria with the development of CVD may be related to the impairment of metabolic or synthetic functions of PTEC that may contribute to systemic vascular damage. For instance, vitamin D deficiency, which is caused by megalin dysfunction, is independently associated with increased cardiovascular mortality [73, 74]. Selenoprotein P, a major carrier of selenium, is taken up by megalin  and provides selenium for synthesizing glutathione peroxidase 3 (GPx3) in PTEC [76, 77]. GPx3 is secreted into the extracellular space from where it enters the blood and acts as antioxidant . Therefore, reduced uptake of selenoprotein P in PTEC due to impaired megalin function may result in decreased GPx3 synthesis by the cells and may be associated with the development of vascular diseases.
8. Megalin-Mediated Signaling
Biemesderfer and his colleagues identified that megalin undergoes regulated intramembrane proteolysis as some other membrane proteins such as those belonging to the Notch and amyloidal precursor protein families [79, 80]. They showed (1) that high levels of γ-secretase are expressed in the brush border and endocytic pathway of PTEC where it colocalizes with megalin, (2) that megalin is subjected to PKC-regulated, metalloprotease-mediated ectodomain shedding that produces a 35 to 40 kDa megalin COOH-terminal fragment (MCTF), and (3) that the MCTF is membrane bound and is constitutively processed by -secretase activity . They also found evidence suggesting that the COOH-terminal domain of megalin regulates megalin and NHE3 gene expression . These findings strongly indicate that megalin is not only involved in scavenging functions in PTEC but also participate in the signal transduction in the cells.
Megalin, an endocytic receptor, mediates the conservation of nutrients and carrier bound vitamins and trace elements in glomerular filtrates via interaction with various molecules in PTEC. Megalin also plays a critical role in the uptake of pathological substances or overloaded endocytosis that may lead to the cellular damage. Megalin-mediated signaling transduction may be also involved in the process. Further studies are needed to elucidate the molecular mechanism fully and develop strategies for preventing PTEC damage.
The authors thank Taeko Soma and Ryoko Niizuma for their help with preparing the manuscript.
- A. Saito, S. Pietromonaco, A. K.-C. Loo, and M. G. Farquhar, “Complete cloning and sequencing of rat gp330/“megalin,” a distinctive member of the low density lipoprotein receptor gene family,” Proceedings of the National Academy of Sciences of the United States of America, vol. 91, no. 21, pp. 9725–9729, 1994.
- G. Hjälm, E. Murray, G. Crumley et al., “Cloning and sequencing of human gp330, a -binding receptor with potential intracellular signaling properties,” European Journal of Biochemistry, vol. 239, no. 1, pp. 132–137, 1996.
- P. J. Verroust, R. Kozyraki, T. G. Hammond, S. K. Moestrup, and E. I. Christensen, “Physiopathologic role of cubilin and megalin,” Advances in Nephrology from the Necker Hospital, vol. 30, pp. 127–145, 2000.
- E. I. Christensen, P. J. Verroust, and R. Nielsen, “Receptor-mediated endocytosis in renal proximal tubule,” Pflügers Archiv European Journal of Physiology, vol. 458, no. 6, pp. 1039–1048, 2009.
- C. G. Davis, J. L. Goldstein, T. C. Sudhof, R. G. Anderson, D. W. Russell, and M. S. Brown, “Acid-dependent ligand dissociation and recycling of LDL receptor mediated by growth factor homology region,” Nature, vol. 326, no. 6115, pp. 760–765, 1987.
- T. Takeda, H. Yamazaki, and M. G. Farquhar, “Identification of an apical sorting determinant in the cytoplasmic tail of megalin,” American Journal of Physiology, vol. 284, no. 5, pp. C1105–C1113, 2003.
- J.-R. Leheste, B. Rolinski, H. Vorum et al., “Megalin knockout mice as an animal model of low molecular weight proteinuria,” American Journal of Pathology, vol. 155, no. 4, pp. 1361–1370, 1999.
- S. Kantarci, L. Al-Gazali, R. S. Hill et al., “Mutations in LRP2, which encodes the multiligand receptor megalin, cause Donnai-Barrow and facio-oculo-acoustico-renal syndromes,” Nature Genetics, vol. 39, no. 8, pp. 957–959, 2007.
- A. Nykjaer, J. C. Fyfe, R. Kozyraki et al., “Cubilin dysfunction causes abnormal metabolism of the steroid hormone 25(OH) vitamin D3,” Proceedings of the National Academy of Sciences of the United States of America, vol. 98, no. 24, pp. 13895–13900, 2001.
- E. I. Christensen, J. O. Moskaug, H. Vorum et al., “Evidence for an essential role of megalin in transepithelial transport of retinol,” Journal of the American Society of Nephrology, vol. 10, no. 4, pp. 685–695, 1999.
- S. K. Moestrup, H. Birn, P. B. Fischer et al., “Megalin-mediated endocytosis of transcobalamin-vitamin-B12 complexes suggests a role of the receptor in vitamin-B12 homeostasis,” Proceedings of the National Academy of Sciences of the United States of America, vol. 93, no. 16, pp. 8612–8617, 1996.
- R. Kozyraki, J. Fyfe, P. J. Verroust et al., “Megalin-dependent cubilin-mediated endocytosis is a major pathway for the apical uptake of transferrin in polarized epithelia,” Proceedings of the National Academy of Sciences of the United States of America, vol. 98, no. 22, pp. 12491–12496, 2001.
- B. Seetharam, J. S. Levine, M. Ramasamy, and D. H. Alpers, “Purification, properties, and immunochemical localization of a receptor for intrinsic factor-cobalamin complex in the rat kidney,” Journal of Biological Chemistry, vol. 263, no. 9, pp. 4443–4449, 1988.
- B. Seetharam, E. I. Christensen, S. K. Moestrup, T. G. Hammond, and P. J. Verroust, “Identification of rat yolk sac target protein of teratogenic antibodies, gp280, as intrinsic factor-cobalamin receptor,” Journal of Clinical Investigation, vol. 99, no. 10, pp. 2317–2322, 1997.
- M. Aminoff, J. E. Carter, R. B. Chadwick et al., “Mutations in CUBN, encoding the intrinsic factor-vitamin B12 receptor, cubilin, cause hereditary megaloblastic anaemia 1,” Nature Genetics, vol. 21, no. 3, pp. 309–313, 1999.
- R. R. Yammani, S. Seetharam, and B. Seetharam, “Identification and characterization of two distinct ligand binding regions of cubilin,” Journal of Biological Chemistry, vol. 276, no. 48, pp. 44777–44784, 2001.
- J. C. Fyfe, M. Madsen, P. Højrup et al., “The functional cobalamin (vitamin B12)-intrinsic factor receptor is a novel complex of cubilin and amnionless,” Blood, vol. 103, no. 5, pp. 1573–1579, 2004.
- G. Coudroy, J. Gburek, R. Kozyraki et al., “Contribution of cubilin and amnionless to processing and membrane targeting of cubilin-amnionless complex,” Journal of the American Society of Nephrology, vol. 16, no. 8, pp. 2330–2337, 2005.
- S. Kalantry, S. Manning, O. Haub et al., “The amnionless gene, essential for mouse gastrulation, encodes a visceral-endoderm-specific protein with an extracellular cysteine-rich domain,” Nature Genetics, vol. 27, no. 4, pp. 412–416, 2001.
- S. M. Tanner, M. Aminoff, F. A. Wright et al., “Amnionless, essential for mouse gastrulation, is mutated in recessive hereditary megaloblastic anemia,” Nature Genetics, vol. 33, no. 3, pp. 426–429, 2003.
- I. A. Bobulescu and O. W. Moe, “Luminal exchange in the proximal tubule,” Pflügers Archiv European Journal of Physiology, vol. 458, no. 1, pp. 5–21, 2009.
- D. Biemesderfer, T. Nagy, B. DeGray, and P. S. Aronson, “Specific association of megalin and the exchanger isoform NHE3 in the proximal tubule,” Journal of Biological Chemistry, vol. 274, no. 25, pp. 17518–17524, 1999.
- D. Biemesderfer, B. DeGray, and P. S. Aronson, “Active (9.6 s) and inactive (21 s) oligomers of NHE3 in microdomains of the renal brush border,” Journal of Biological Chemistry, vol. 276, no. 13, pp. 10161–10167, 2001.
- S. Uchida, “In vivo role of CLC chloride channels in the kidney,” American Journal of Physiology, vol. 279, no. 5, pp. F802–F808, 2000.
- O. Scheel, A. A. Zdebik, S. Lourdel, and T. J. Jentsch, “Voltage-dependent electrogenic chloride/proton exchange by endosomal CLC proteins,” Nature, vol. 436, no. 7049, pp. 424–427, 2005.
- T. J. Jentsch, “Chloride transport in the kidney: lessons from human disease and knockout mice,” Journal of the American Society of Nephrology, vol. 16, no. 6, pp. 1549–1561, 2005.
- W. Günther, A. Lüchow, F. Cluzeaud, A. Vandewalle, and T. J. Jentsch, “CIC-5, the chloride channel mutated in Dent's disease, colocalizes with the proton pump in endocytotically active kidney cells,” Proceedings of the National Academy of Sciences of the United States of America, vol. 95, no. 14, pp. 8075–8080, 1998.
- O. M. Wrong, A. G. W. Norden, and T. G. Feest, “Dent's disease; a familial proximal renal tubular syndrome with low-molecular-weight proteinuria, hypercalciuria, nephrocalcinosis, metabolic bone disease, progressive renal failure and a marked male predominance,” Quarterly Journal of Medicine, vol. 87, no. 8, pp. 473–493, 1994.
- E. I. Christensen, O. Devuyst, G. Dom et al., “Loss of chloride channel ClC-5 impairs endocytosis by defective trafficking of megalin and cubilin in kidney proximal tubules,” Proceedings of the National Academy of Sciences of the United States of America, vol. 100, no. 14, pp. 8472–8477, 2003.
- A. Tanuma, H. Sato, T. Takeda et al., “Functional characterization of a novel missense CLCN5 mutation causing alterations in proximal tubular endocytic machinery in Dent's disease,” Nephron Physiology, vol. 107, no. 4, pp. p87–p97, 2007.
- M. Gotthardt, M. Trommsdorff, M. F. Nevitt et al., “Interactions of the low density lipoprotein receptor gene family with cytosolic adaptor and scaffold proteins suggest diverse biological functions in cellular communication and signal transduction,” Journal of Biological Chemistry, vol. 275, no. 33, pp. 25616–25624, 2000.
- K. Rader, R. A. Orlando, X. Lou, and M. G. Farquhar, “Characterization of ANKRA, a novel ankyrin repeat protein that interacts with the cytoplasmic domain of megalin,” Journal of the American Society of Nephrology, vol. 11, no. 12, pp. 2167–2178, 2000.
- A. V. Oleinikov, J. Zhao, and S. P. Makker, “Cytosolic adaptor protein Dab2 is an intracellular ligand of endocytic receptor gp600/megalin,” Biochemical Journal, vol. 347, part 3, pp. 613–621, 2000.
- X. Lou, T. Mcquistan, R. A. Orlando, and M. G. Farquhar, “GAIP, GIPC and Gi3 are concentrated in endocytic compartments of proximal tubule cells: putative role in regulating megalin's function,” Journal of the American Society of Nephrology, vol. 13, no. 4, pp. 918–927, 2002.
- M. Larsson, G. Hjälm, A. M. Sakwe et al., “Selective interaction of megalin with postsynaptic density-95 (PSD-95)-like membrane-associated guanylate kinase (MAGUK) proteins,” Biochemical Journal, vol. 373, no. 2, pp. 381–391, 2003.
- H. H. Petersen, J. Hilpert, D. Militz et al., “Functional interaction of megalin with the megalin-binding protein (MegBP), a novel tetratrico peptide repeat-containing adaptor molecule,” Journal of Cell Science, vol. 116, no. 3, pp. 453–461, 2003.
- M. Nagai, T. Meerloo, T. Takeda, and M. G. Farquhar, “The adaptor protein ARH escorts megalin to and through endosomes,” Molecular Biology of the Cell, vol. 14, no. 12, pp. 4984–4996, 2003.
- B. A. Hocevar, A. Smine, X.-X. Xu, and P. H. Howe, “The adaptor molecule disabled-2 links the transforming growth factor receptors to the Smad pathway,” EMBO Journal, vol. 20, no. 11, pp. 2789–2801, 2001.
- C. Prunier, B. A. Hocevar, and P. H. Howe, “Wnt signaling: physiology and pathology,” Growth Factors, vol. 22, no. 3, pp. 141–150, 2004.
- L. M. Russo, E. del Re, D. Brown, and H. Y. Lin, “Evidence for a role of transforming growth factor (TGF)-1 in the induction of postglomerular albuminuria in diabetic nephropathy: amelioration by soluble TGF- type II receptor,” Diabetes, vol. 56, no. 2, pp. 380–388, 2007.
- M. Hosojima, H. Sato, K. Yamamoto et al., “Regulation of megalin expression in cultured proximal tubule cells by angiotensin II type 1A receptor—and insulin-mediated signaling cross talk,” Endocrinology, vol. 150, no. 2, pp. 871–878, 2009.
- A. Tojo, M. Onozato, H. Ha et al., “Reduced albumin reabsorption in the proximal tubule of early-stage diabetic rats,” Histochemistry and Cell Biology, vol. 116, no. 3, pp. 269–276, 2001.
- P. Pontuch, T. Jensen, T. Deckert, P. Ondrejka, and M. Mikulecky, “Urinary excretion of retinol-binding protein in type 1 (insulin-dependent) diabetic patients with microalbuminuria and clinical diabetic nephropathy,” Acta Diabetologica, vol. 28, no. 3-4, pp. 206–210, 1992.
- C.-Y. Hong, K. Hughes, K.-S. Chia, V. Ng, and S.-L. Ling, “Urinary α1-microglobulin as a marker of nephropathy in type 2 diabetic Asian subjects in Singapore,” Diabetes Care, vol. 26, no. 2, pp. 338–342, 2003.
- T. Hasson, “Myosin VI: two distinct roles in endocytosis,” Journal of Cell Science, vol. 116, no. 17, pp. 3453–3461, 2003.
- K. B. Avraham, T. Hasson, K. P. Steel et al., “The mouse Snell's waltzer deafness gene encodes an unconventional myosin required for structural integrity of inner ear hair cells,” Nature Genetics, vol. 11, no. 4, pp. 369–375, 1995.
- K. Hosaka, T. Takeda, N. Iino et al., “Megalin and nonmuscle myosin heavy chain IIA interact with the adaptor protein Disabled-2 in proximal tubule cells,” Kidney International, vol. 75, no. 12, pp. 1308–1315, 2009.
- M. J. Kelley, W. Jawien, T. L. Ortel, and J. F. Korczak, “Mutation of MYH9, encoding non-muscle myosin heavy chain A, in May-Hegglin anomaly,” Nature Genetics, vol. 26, no. 1, pp. 106–108, 2000.
- M. Seri, R. Cusano, S. Gangarossa et al., “Mutations in MYH9 result in the May-Hegglin anomaly, and Fechtner and Sebastian syndromes,” Nature Genetics, vol. 26, no. 1, pp. 103–105, 2000.
- K. E. Heath, A. Campos-Barros, A. Toren et al., “Nonmuscle myosin heavy chain IIA mutations define a spectrum of autosomal dominant macrothrombocytopenias: May-Hegglin anomaly and Fechtner, Sebastian, Epstein, and alport-like syndromes,” American Journal of Human Genetics, vol. 69, no. 5, pp. 1033–1045, 2001.
- C. Arrondel, N. Vodovar, B. Knebelmann et al., “Expression of the nonmuscle myosin heavy chain IIA in the human kidney and screening for MYH9 mutations in Epstein and Fechtner syndromes,” Journal of the American Society of Nephrology, vol. 13, no. 1, pp. 65–74, 2002.
- M. Seri, M. Savino, D. Bordo et al., “Epstein syndrome: another renal disorder with mutations in the nonmuscle myosin heavy chain 9 gene,” Human Genetics, vol. 110, no. 2, pp. 182–186, 2002.
- W. H. L. Kao, M. J. Klag, L. A. Meoni et al., “MYH9 is associated with nondiabetic end-stage renal disease in African Americans,” Nature Genetics, vol. 40, no. 10, pp. 1185–1192, 2008.
- J. B. Kopp, M. W. Smith, G. W. Nelson et al., “MYH9 is a major-effect risk gene for focal segmental glomerulosclerosis,” Nature Genetics, vol. 40, no. 10, pp. 1175–1184, 2008.
- S. Lehtonen, M. Shah, R. Nielsen et al., “The endocytic adaptor protein ARH associates with motor and centrosomal proteins and is involved in centrosome assembly and cytokinesis,” Molecular Biology of the Cell, vol. 19, no. 7, pp. 2949–2961, 2008.
- Y. Motoyoshi, T. Matsusaka, A. Saito et al., “Megalin contributes to the early injury of proximal tubule cells during nonselective proteinuria,” Kidney International, vol. 74, no. 10, pp. 1262–1269, 2008.
- S. Dronavalli, I. Duka, and G. L. Bakris, “The pathogenesis of diabetic nephropathy,” Nature Clinical Practice Endocrinology and Metabolism, vol. 4, no. 8, pp. 444–452, 2008.
- A. Saito, R. Nagai, A. Tanuma et al., “Role of megalin in endocytosis of advanced glycation end products: implications for a novel protein binding to both megalin and advanced glycation end products,” Journal of the American Society of Nephrology, vol. 14, no. 5, pp. 1123–1131, 2003.
- A. Saito, T. Takeda, K. Sato et al., “Significance of proximal tubular metabolism of advanced glycation end products in kidney diseases,” Annals of the New York Academy of Sciences, vol. 1043, pp. 637–643, 2005.
- K. Sebeková, R. Schinzel, H. Ling et al., “Advanced glycated albumin impairs protein degradation in the kidney proximal tubules cell line LLC-PK1,” Cellular and Molecular Biology, vol. 44, no. 7, pp. 1051–1060, 1998.
- P. Verbeke, M. Perichon, B. Friguet, and H. Bakala, “Inhibition of nitric oxide synthase activity by early and advanced glycation end products in cultured rabbit proximal tubular epithelial cells,” Biochimica et Biophysica Acta, vol. 1502, no. 3, pp. 481–494, 2000.
- Y. Oyama, T. Takeda, H. Hama et al., “Evidence for megalin-mediated proximal tubular uptake of L-FABP, a carrier of potentially nephrotoxic molecules,” Laboratory Investigation, vol. 85, no. 4, pp. 522–531, 2005.
- W. C. Burns, P. Kantharidis, and M. C. Thomas, “The role of tubular epithelial-mesenchymal transition in progressive kidney disease,” Cells Tissues Organs, vol. 185, no. 1–3, pp. 222–231, 2007.
- F. M. Strutz, “EMT and proteinuria as progression factors,” Kidney International, vol. 75, no. 5, pp. 475–481, 2009.
- M. Gekle, “Renal tubule albumin transport,” Annual Review of Physiology, vol. 67, pp. 573–594, 2005.
- G. C. Viberti, R. D. Hill, R. J. Jarrett, A. Argyropoulos, U. Mahmud, and H. Keen, “Microalbuminuria as a predictor of clinical nephropathy in insulin-dependent diabetes mellitus,” Lancet, vol. 1, no. 8287, pp. 1430–1432, 1982.
- C. E. Mogensen, “Microalbuminuria predicts clinical proteinuria and early mortality in maturity-onset diabetes,” New England Journal of Medicine, vol. 310, no. 6, pp. 356–360, 1984.
- H. C. Gerstein, J. F. E. Mann, Q. Yi et al., “Albuminuria and risk of cardiovascular events, death, and heart failure in diabetic and nondiabetic individuals,” Journal of the American Medical Association, vol. 286, no. 4, pp. 421–426, 2001.
- K. Wachtell, H. Ibsen, M. H. Olsen et al., “Albuminuria and cardiovascular risk in hypertensive patients with left ventricular hypertrophy: the LIFE study,” Annals of Internal Medicine, vol. 139, no. 11, pp. 901–906, 2003.
- A. B. Maunsbach, “Absorption of I125-labeled homologous albumin by rat kidney proximal tubule cells. A study of microperfused single proximal tubules by electron microscopic autoradiography and histochemistry,” Journal of Ultrasructure Research, vol. 15, no. 3-4, pp. 197–241, 1966.
- W. D. Comper, L. M. Hilliard, D. J. Nikolic-Paterson, and L. M. Russo, “Disease-dependent mechanisms of albuminuria,” American Journal of Physiology, vol. 295, no. 6, pp. F1589–F1600, 2008.
- M. Sarav, Y. Wang, B. K. Hack et al., “Renal FcRn reclaims albumin but facilitates elimination of IgG,” Journal of the American Society of Nephrology, vol. 20, no. 9, pp. 1941–1952, 2009.
- S. Pilz, W. März, B. Wellnitz et al., “Association of vitamin D deficiency with heart failure and sudden cardiac death in a large cross-sectional study of patients referred for coronary angiography,” Journal of Clinical Endocrinology and Metabolism, vol. 93, no. 10, pp. 3927–3935, 2008.
- H. Dobnig, S. Pilz, H. Scharnagl et al., “Independent association of low serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D levels with all-cause and cardiovascular mortality,” Archives of Internal Medicine, vol. 168, no. 12, pp. 1340–1349, 2008.
- G. E. Olson, V. P. Winfrey, K. E. Hill, and R. F. Burk, “Megalin mediates selenoprotein P uptake by kidney proximal tubule epithelial cells,” Journal of Biological Chemistry, vol. 283, no. 11, pp. 6854–6860, 2008.
- N. Avissar, D. B. Ornt, Y. Yagil et al., “Human kidney proximal tubules are the main source of plasma glutathione peroxidase,” American Journal of Physiology, vol. 266, no. 2, pp. C367–C375, 1994.
- R. L. Maser, B. S. Magenheimer, and J. P. Calvet, “Mouse plasma glutathione peroxidase. cDNA sequence analysis and renal proximal tubular expression and secretion,” Journal of Biological Chemistry, vol. 269, no. 43, pp. 27066–27073, 1994.
- J. C. Whitin, S. Bhamre, D. M. Tham, and H. J. Cohen, “Extracellular glutathione peroxidase is secreted basolaterally by human renal proximal tubule cells,” American Journal of Physiology, vol. 283, no. 1, pp. F20–F28, 2002.
- M. S. Brown, J. Ye, R. B. Rawson, and J. L. Goldstein, “Regulated intramembrane proteolysis: a control mechanism conserved from bacteria to humans,” Cell, vol. 100, no. 4, pp. 391–398, 2000.
- A. Louvi and S. Artavanis-Tsakonas, “Notch signalling in vertebrate neural development,” Nature Reviews Neuroscience, vol. 7, no. 2, pp. 93–102, 2006.
- Z. Zou, B. Chung, T. Nguyen, S. Mentone, B. Thomson, and D. Biemesderfer, “Linking receptor-mediated endocytosis and cell signaling: evidence for regulated intramembrane proteolysis of megalin in proximal tubule,” Journal of Biological Chemistry, vol. 279, no. 33, pp. 34302–34310, 2004.
- Y. Li, R. Cong, and D. Biemesderfer, “The COOH terminus of megalin regulates gene expression in opossum kidney proximal tubule cells,” American Journal of Physiology, vol. 295, no. 2, pp. C529–C537, 2008.
Copyright © 2010 Akihiko Saito et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.