Relevance of extracellular matrix, its receptors, and cell adhesion molecules in mammalian nephrogenesis

Engineering physical microenvironment for stem cell based regenerative medicine

Regenerative medicine has rapidly evolved over the past decade owing to its potential applications to improve human health. Targeted differentiations of stem cells promise to regenerate a variety of tissues and/or organs despite significant challenges. Recent studies have demonstrated the vital role of the physical microenvironment in regulating stem cell fate and improving differentiation efficiency. In this review, we summarize the main physical cues that are crucial for controlling stem cell differentiation. Recent advances in the technologies for the construction of physical microenvironment and their implications in controlling stem cell fate are also highlighted.

Illustration of extensive extracellular matrix at the epithelial-mesenchymal interface within the renal stem/progenitor cell niche

BACKGROUND

Stem/progenitor cells are promising candidates to treat diseased renal parenchyma. However, implanted stem/progenitor cells are exposed to a harmful atmosphere of degenerating parenchyma. To minimize hampering effects after an implantation investigations are in progress to administer these cells within an artificial polyester interstitum supporting survival. Learning from nature the renal stem/progenitor cell niche appears as a valuable model. At this site epithelial stem/progenitor cells within the collecting duct ampulla face mesenchymal stem/progenitor cells. Both cell types do not have close contact but are separated by a wide interstitium.

METHODS

To analyze extracellular matrix in this particular interstitium, special contrasting for transmission electron microscopy was performed. Kidneys of neonatal rabbits were fixed in solutions containing glutaraldehyde (GA) or in combination with cupromeronic blue, ruthenium red and tannic acid.

RESULTS

GA revealed a basal lamina at the ampulla and a bright but inconspicuously looking interstitial space. In contrast, GA containing cupromeronic blue exhibits numerous proteoglycan braces lining from the ampulla towards the interstitial space. GA containing ruthenium red or tannic acid demonstrates clouds of extracellular matrix protruding from the basal lamina of the ampulla to the surface of mesenchymal stem/progenitor cells.

CONCLUSIONS

The actual data show that the interstitium between epithelial and mesenchymal stem/progenitor cells contains much more and up to date unknown extracellular matrix than earlier observed by classical GA fixation.

Increased endogenous H2S generation by CBS, CSE, and 3MST gene therapy improves ex vivo renovascular relaxation in hyperhomocysteinemia

Hydrogen sulfide (H(2)S) has recently been identified as a regulator of various physiological events, including vasodilation, angiogenesis, antiapoptotic, and cellular signaling. Endogenously, H(2)S is produced as a metabolite of homocysteine (Hcy) by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3MST). Although Hcy is recognized as vascular risk factor at an elevated level [hyperhomocysteinemia (HHcy)] and contributes to vascular injury leading to renovascular dysfunction, the exact mechanism is unclear. The goal of the current study was to investigate whether conversion of Hcy to H(2)S improves renovascular function. Ex vivo renal artery culture with CBS, CSE, and 3MST triple gene therapy generated more H(2)S in the presence of Hcy, and these arteries were more responsive to endothelial-dependent vasodilation compared with nontransfected arteries treated with high Hcy. Cross section of triple gene-delivered renal arteries immunostaining suggested increased expression of CD31 and VEGF and diminished expression of the antiangiogenic factor endostatin. In vitro endothelial cell culture demonstrated increased mitophagy during high levels of Hcy and was mitigated by triple gene delivery. Also, dephosphorylated Akt and phosphorylated FoxO3 in HHcy were reversed by H(2)S or triple gene delivery. Upregulated matrix metalloproteinases-13 and downregulated tissue inhibitor of metalloproteinase-1 in HHcy were normalized by overexpression of triple genes. Together, these results suggest that H(2)S plays a key role in renovasculopathy during HHcy and is mediated through Akt/FoxO3 pathways. We conclude that conversion of Hcy to H(2)S by CBS, CSE, or 3MST triple gene therapy improves renovascular function in HHcy.

Expression of focal adhesion proteins in the developing rat kidney

Focal adhesions play a critical role as centers that transduce signals by cell-matrix interactions and regulate fundamental processes such as proliferation, migration, and differentiation. Focal adhesion kinase (FAK), paxillin, integrin-linked kinase (ILK), and hydrogen peroxide-inducible clone-5 (Hic-5) are major proteins that contribute to these events. In this study, we investigated the expression of focal adhesion proteins in the developing rat kidney. Western blotting analysis revealed that the protein levels of FAK, p-FAK(397), paxillin, p-paxillin(118), and Hic-5 were high in embryonic kidneys, while ILK expression persisted from the embryonic to the mature stage. Immunohistochemistry revealed that FAK, p-FAK(397), paxillin, and p-paxillin(118) were strongly expressed in condensed mesenchymal cells and the ureteric bud. They were detected in elongating tubules and immature glomerular cells in the nephrogenic zone. Hic-5 was predominantly expressed in mesenchymal cells as well as immature glomerular endothelial and mesangial cells, suggesting that Hic-5 might be involved in mesenchymal cell development. ILK expression was similar to that of FAK in the developmental stages. Interestingly, ILK was strongly expressed in podocytes in mature glomeruli. ILK might play a role in epithelial cell differentiation as well as kidney growth and morphogenesis. In conclusion, the temporospatially regulated expression of focal adhesion proteins during kidney development might play a role in morphogenesis and cell differentiation.

beta1-Integrin-collagen interaction suppresses FoxO3a by the coordination of Akt and PP2A

When cells attach to the extracellular matrix (ECM) a proliferation permissive signal is engaged. The mechanism involves activation of the integrin/PI3K/Akt signal pathway. FoxO3a is a transcriptional activator and inhibits cell proliferation via up-regulating the expression of the cell cycle inhibitor p27. Furthermore, it is known that activated Akt can suppress FoxO3a function. However, it is not known whether integrin interaction with the ECM regulates FoxO3a function. We examined whether the beta1-integrin-mediated signaling pathway promotes fibroblast proliferation via FoxO3a suppression. We found that when fibroblasts are attached to collagen, PTEN protein expression and activity are inhibited due to promotion of PTEN degradation. This decrease in PTEN function permits FoxO3a suppression via the PI3K/Akt pathway. In contrast, the inhibition of PI3K/Akt or reconstitution of PTEN restores FoxO3a expression on collagen. Furthermore, we found that the serine/threonine phosphatase PP2A also regulates FoxO3a. PP2A expression/activity is low when fibroblasts are attached to collagen, and PP2A overexpression augments FoxO3a levels. Thus the mechanism involves a coordinated decrease in PTEN and PP2A phosphatase activity and increase in PI3K/Akt activity. We show that beta1-integrin-ECM interaction decreases FoxO3a protein levels via caspase-3-mediated cleavage. Our novel finding indicates that during fibroblast interaction with ECM, activation of beta1-integrin/PI3K/Akt by inhibiting PTEN in combination with low PP2A phosphatase activity synergistically inhibits FoxO3a, promoting fibroblast proliferation.

Inhibition of ceramide-redox signaling pathway blocks glomerular injury in hyperhomocysteinemic rats

Ceramide-activated NAD(P)H oxidase has been reported to participate in homocysteine (Hcys)-induced abnormal metabolism of the extracellular matrix (ECM) in rat glomerular mesangial cells. However, it remains unknown whether this ceramide-redox signaling pathway contributes to glomerular injury induced by hyperhomocysteinemia (hHcys) in vivo. The present study was designed to address this question, defining the role of ceramide and activated NAD(P)H oxidase in the development of hHcys-induced glomerular injury. Uninephrectomized Sprague-Dawley rats were fed a folate-free diet for 8 weeks to produce hHcys and the de novo ceramide synthesis inhibitor myriocin or the NAD(P)H oxidase inhibitor apocynin was administrated. Rats with folate-free diet significantly increased plasma Hcys levels, renal ceramide levels, and NAD(P)H oxidase activity accompanied by marked glomerular injury. Treatment of rats with myriocin significantly reduced ceramide levels and improved glomerular injury, as shown by decreased urinary albumin excretion and reduced glomerular damage index. ECM components changed towards to normal levels with decreased tissue inhibitor of metalloproteinase-1 and increased matrix metalloproteinase-1 activity. NAD(P)H oxidase activity and Rac GTPase activity were reduced by 69 and 66%, respectively. In rats treated with apocynin, similar beneficial effects in protecting glomeruli from hHcys-induced injury were observed. These results support the view that de novo ceramide production is involved in Hcys-induced NAD(P)H oxidase activity in the kidney of hHcys rats and indicate the important role of ceramide-mediated redox signaling in hHcys-induced glomerular injury in rats.

Microscopic analysis of the cellular events during scatter factor/hepatocyte growth factor-induced epithelial tubulogenesis

Scatter factor/hepatocyte growth factor (SF/HGF), a large multifunctional polypeptide growth and motility factor, is known to play important roles during embryonic development, adult tissue growth and repair. In an established three-dimensional type I collagen model, SF/HGF induces Madin-Darby canine kidney (MDCK) epithelial cysts to form long, branching tubules (tubulogenesis). In addition, the composition of the surrounding extracellular matrix (ECM) has been shown to modulate SF/HGF-induced morphogenesis, where tubulogenesis was completely abrogated in Matrigel basement membrane. Many cellular events that occur during SF/HGF-mediated remodelling, and its modulation by the ECM, remain unclear. We have investigated these mechanisms through microscopic examination of the time-course of SF/HGF-induced responses in MDCK cysts cultured in type I collagen or Matrigel. We found that early responses to SF/HGF were matrix-independent. Changes included increased paracellular spacing between normally closely apposed lateral membranes, and the formation of filopodial processes, indicating a partial motile response. Cell-cell contact was maintained, with the persistence of cell junctions. Therefore, while one or a number of ECM components are preventing SF/HGF-primed cells from undergoing an invasive and/or migratory programme, non-permissive matrices are not preventing SF/HGF signalling to the cell. Later matrix-dependent responses, which occurred in type I collagen but not Matrigel, included the formation of basal protrusions that comprise two or more neighbouring cells, which extend to form nascent tubules. Modified polarity of cells comprising the basal protrusions was evident, with a marker for the apical membrane being found in the same region as adherens junctions and desmosomes, typically localized at lateral membranes. We propose a model for SF/HGF-induced tubulogenesis in which tubules form from basal protrusions of adjacent cells. This mechanism of in vitro tubule formation has many similarities to reported in vivo epithelial tubulogenesis.

Role of extracellular matrix in kidney development and repair

Extracellular matrix (ECM) molecules and their receptors exert a dynamic role in cell-matrix interactions during kidney development and repair processes. They provide a physical substratum for the spatial organization of the cells, but also regulate cell growth and proliferation by interacting with growth factors. In addition, they can regulate signal transduction pathways by binding to integrins or by modulating the activity of signaling molecules such as Wnts. ECM and ECM-related molecules control multiple (if not all) steps of kidney development, including ureteric bud branching morphogenesis, mesenchymal condensation, nephron formation, terminal differentiation of renal tubules, and glomerular basement membrane assembly. Their role still needs to be better documented in renal repair. The emergence of conditionally mutated mice for basement membrane components will provide a useful tool to demonstrate further the involvement of ECM and ECM-related proteins in development and repair.

Molecular mechanisms of epithelial morphogenesis

Epithelial morphogenesis comprises the various processes by which epithelia contribute to organ formation and body shape. These complex and diverse events play a central role in animal development and regeneration. Recently, the characterization of some of the molecular mechanisms involved in epithelial morphogenesis has provided an abundance of new information on the role and regulation of the cytoskeleton, cell-cell adhesion, and cell-matrix adhesion in these processes. In this review, we discuss our current understanding of the molecular mechanisms driving cell shape changes, cell intercalation, fusion of epithelia, ingression, egression, and cell migration. Our discussion is mostly focused on results from Drosophila and mammalian tissue culture but also draws on the insights gained from other organisms.

Homocysteine enhances TIMP-1 expression and cell proliferation associated with NADH oxidase in rat mesangial cells

BACKGROUND

Recent studies in our laboratory demonstrated that chronic hyperhomocysteinemia (hHcys) induced glomerular sclerosis. The mechanism mediating hHcys-induced glomerular damage remains unknown. The present study was designed to test a hypothesis that homocysteine (Hcys) increases the production by nicotinamide adenine dinucleotide (NADH) oxidase and thereby stimulates the formation of tissue inhibitor of metalloproteinase (TIMP-1) in rat mesangial cells, consequently leads to glomerulosclerosis.

METHODS

Rat mesangial cells were incubated with L-homocysteine (L-Hcys) to determine the effects of Hcys on cell proliferation and metabolism of extracellular matrix (ECM). Northern blot, Western blot, oligonucleotide transfection, measurements of NADH oxidase activity and levels, and cell proliferation assay were performed.

RESULTS

In cultured rat mesangial cells, treatment with L-Hcys (40 to 160 micromol/L) markedly increased the mRNA levels of TIMP-1 and Gp91 and led to accumulation of collagen I, which were accompanied by enhanced cell proliferation and NADH oxidase activity in mesangial cells. These Hcys-induced biochemical and functional changes were substantially blocked by a NADH oxidase inhibitor, diphenylene iodonium chloride (DPI) or a superoxide dismutase (SOD) mimetic, hydroxyl-tetramethylpiperidin-oxyl (TEMPOL). Moreover, blockade of NADH oxidase subunit, phox22, by its antisense oligodeoxynucleotide also eliminated the increase in NADH oxidase activity induced by L-Hcys.

CONCLUSION

These results indicate that Hcys-induced alterations of ECM metabolism in mesangial cells are associated with enhanced NADH oxidase activity and that oxidative stress-stimulated up-regulation of TIMP-1 may play an important role in the deposition of collagen or ECM elements in the glomeruli during hHcys.

Renal fibrosis: collagen composition and assembly regulates epithelial-mesenchymal transdifferentiation

Type IV collagen is a major component of basement membranes and it provides structural and functional support to various cell types. Type IV collagen exists in a highly complex suprastructure form and recent studies implicate that protomer (the trimeric building unit of type IV collagen) assembly is mediated by the NC1 domain present in the C-terminus of each collagen alpha-chain polypeptide. Here we show that type IV collagen contributes to the maintenance of the epithelial phenotype of proximal tubular epithelial cells, whereas type I collagen promotes epithelial-to-mesenchymal transdifferentiation (EMT). In addition, the recombinant human alpha1NC1 domain inhibits assembly of type IV collagen NC1 hexamers and potentially disrupts the deposition of type IV collagen, facilitating EMT in vitro. Inhibition of type IV collagen assembly by the alpha1NC1 domain up-regulates the production of transforming growth factor-beta1 in proximal tubular epithelial cells, an inducer of EMT. These results strongly suggest that basement membrane architecture is pivotal for the maintenance of epithelial phenotype and that changes in basement membrane architecture potentially lead to up-regulation of transforming growth factor-beta1, which contributes to EMT during renal fibrosis.

Collagen gel overlay induces two phases of apoptosis in MDCK cells

We previously demonstrated that collagen gel overlay induced cell remodeling to form lumen and apoptosis in Madin-Darby canine kidney cells. In the present study, we established that collagen gel overlay-induced apoptosis was initiated at areas exclusive of cell remodeling within 24 h (first phase) and extended into areas of cell remodeling within 48 h (second phase). Collagen gel overlay-induced apoptosis was accompanied by selective proteolysis of focal adhesion kinase (FAK), talin, p130cas, and c- src. Upon collagen gel overlay, FAK was initially degraded into a 90-kDa product during the first phase and subsequently into a 80-kDa product during the second phase. Collagen gel overlay-induced apoptosis of focal adhesion complex proteins and apoptosis of the first phase could be blocked only by a protease inhibitor cocktail. In addition, we found that both DEVD-fmk and ZVAD-fmk inhibited secondary proteolysis of FAK, but only ZVAD-fmk blocked collagen gel overlay-induced apoptosis of the second phase. Finally, collagen gel overlay-induced apoptosis and proteolysis of focal adhesion complex proteins were completely inhibited by the combination of protease inhibitor cocktail and ZVAD-fmk. Taken together, collagen gel overlay induces two phases of apoptosis; the first phase is dependent on proteolysis of focal adhesion complex proteins, and the second phase on activation of caspases.

Integrins in kidney development, function, and disease

Integrins are heterodimeric cell surface receptors that mediate heterophilic cell-cell interactions and interactions between cells and the extracellular matrix (Hynes RO. Cell 69: 11-25, 1991). As such, they are involved in morphogenetic processes during development, as well as in the maintenance of normal tissue architecture in fully developed organs. Integrins are now recognized to be a large family of receptors, and several different integrins have been demonstrated as being expressed in the developing and adult kidney (Korhonen M, Ylkanne J, Laitinen L, and Virtanen I. Development 122: 3537-3547, 1996; Rahilly MA and Fleming S. J Pathol 167: 327-334, 1992). This review will summarize present knowledge about integrin expression in the developing, normal, and diseased kidney and attempt to provide a hypothetical framework for understanding integrin function in the urogenital system. Since the last time this area was reviewed (Hamerski DA and Santoro S. Curr Opin Nephrol Hypertens 8: 9-14, 1999), there have been significant publications on the roles of integrins in kidney development and disease. At present, there are many more questions than answers, and integrins present an area where many novel and exciting findings will emerge in the coming years.