INVITED REVIEW
Integrins in kidney development, function, and disease

Jordan A. Kreidberg1 and Jordan M. Symons2

1 Department of Medicine, Children's Hospital, and Department of Pediatrics, Harvard Medical School, Boston, Massachusetts 02115; and 2 Division of Nephrology, Children's Hospital and Regional Medical Center, University of Washington School of Medicine, Seattle, Washington 98105


    ABSTRACT
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ABSTRACT
INTRODUCTION
INTEGRIN EXPRESSION DURING...
INTEGRIN EXPRESSION IN ADULT...
REFERENCES

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.

adhesion receptors; nephrogenesis; kidney disease


    INTRODUCTION
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ABSTRACT
INTRODUCTION
INTEGRIN EXPRESSION DURING...
INTEGRIN EXPRESSION IN ADULT...
REFERENCES

IN MAMMALS, more than 20 integrin heterodimers are expressed, each of which is comprised of a single alpha - and beta -subunit. In most cases, more than one type of alpha -subunit is found to heterodimerize with a particular beta -subunit; thus integrins are usually divided into subgroups on the basis of the beta -subunit. The group of integrins demonstrating the most widespread expression in the kidney are those that contain the beta 1-subunit; this is the subgroup of integrins that serves as the major family of receptors for the extracellular matrix (ECM) in mammalian tissues. The beta 1-integrins have been characterized as receptors for fibronectin, collagen, vitronectin, thrombospondin, and distinct isoforms of laminin. (The specificity of each integrin is denoted in the text.) Integrin function in tissues is inferred from in vitro studies demonstrating binding to different components of the ECM. For example, those expressed by mesenchymal cells that bind connective tissue components such as type I collagen or fibronectin are believed to act by anchoring mesenchymal cells within the surrounding ECM. Alternatively, integrins expressed on the basal surface of epithelial cells, which mainly bind laminin, are presumed to adhere cells to their laminin-rich basement membranes. This review will focus on the role of integrins themselves. A recent review in this journal has focused more generally on the role of the extracellular matrix and its receptors in the kidney (62).


    INTEGRIN EXPRESSION DURING KIDNEY DEVELOPMENT
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INTRODUCTION
INTEGRIN EXPRESSION DURING...
INTEGRIN EXPRESSION IN ADULT...
REFERENCES

A small number of studies have characterized integrin expression in the developing kidney (31, 47). Nearly all of these studies have utilized human fetal kidneys and have examined integrin expression in the nephrogenic zone of the developing cortex. Thus these studies do not encompass the initial period of metanephric induction and early branching morphogenesis, presumably because of the difficulty of obtaining human fetal tissue from these early time points. Furthermore, complete studies of integrin expression have not been performed in rodents, where such early embryos can easily be obtained. Therefore, although we have a good understanding of integrin expression during the differentiation of the nephron, it is also possible that distinct integrin-mediated interactions occur during early kidney development, a point to be addressed later in this review. This point is important because morphological studies on the development of kidney architecture have demonstrated a distinction between the early rounds of branching morphogenesis that contribute to the major and minor calyces and medullary papillae and subsequent dichotomous branching events that give rise to medullary-cortical collecting ducts (41, 42).

Expression and Function During Metanephric Induction

The development of the nephron within the nephrogenic zone of the cortex involves an initial condensation of renal stem cells around a derivative of the ureteric bud (4, 56). Continued branching of the ureteric bud gives rise to the collecting ducts. Branching of the derivatives of the ureteric bud is initially symmetrical, after which a dichotomous branching pattern gives rise to the complex architecture of the collecting system. The condensed stem cells form pretubular aggregates that then undergo a mesenchymal-to-epithelial transformation, leading to the formation of a simple epithelial tubule. The tubule then undergoes a complex pattern of differentiation, involving elongation, segmentation, and convolution to finally form a mature nephron. As each nephron matures, a connection is established between the end of the nascent distal tubule and a portion of a ureteric bud derivative that is itself maturing into a collecting duct. These stem cells also give rise to the stromal population present during nephrogenesis and thereafter (4). Rahilly and Fleming (47) detected expression of alpha 4beta 1-integrin, a receptor for fibronectin and vascular cell adhesion molecule, by undifferentiated cells along the periphery of the kidney. This population presumably included but was not restricted to renal stem cells. However, expression of alpha 4beta 1 in this region was not confirmed by Korhonen et al. (31) in their study of integrin expression during kidney development. Korhonen et al. and Rahilly and Fleming (47) also examined ureteric bud derivatives and maturing nephrons of the nephrogenic zone of human fetal kidneys for expression of different alpha -integrin subunits. Ureteric bud derivatives along the periphery of the cortex expressed abundant alpha 6beta 1- and smaller amounts of alpha 3beta 1-integrin. The alpha 6beta 1- and alpha 3beta 1-integrins were also expressed in the part of the pretubular aggregate that had undergone epithelialization, with alpha 3beta 1 mainly restricted to that part of the nascent tubule containing presumptive glomerular podocytes (also referred to as visceral epithelial cells) (31, 47). Other than the diffuse expression of alpha 4beta 1-integrin detected by Rahilly and Fleming (47), neither the stem cell population lining the periphery of the kidney nor the areas where stem cells condensed around the ureteric bud during pretubular aggregate formation demonstrated specific integrin expression before that of alpha 3beta 1 and alpha 6beta 1 noted above. However, it is possible that future studies will demonstrate integrin expression in this cell population.

One of the most striking roles for integrins in kidney development was revealed on targeting the alpha 8-integrin gene in mice (37). The alpha 8beta 1-integrin is expressed on the metanephric mesenchyme at sites where it is in contact with the ureteric bud (37). In most alpha 8-null embryos, extension of the ureteric bud ceases on contact with the metanephric mesenchyme, and no kidney is produced. For unknown reasons, in a fraction of null embryos kidney development progresses to varying extents and small kidneys are formed that in rare instances can actually maintain postnatal viability of null mice. The key ligand of alpha 8beta 1 in the kidney is not known with certainty. The alpha 8beta 1-integrin appears to be a receptor for fibronectin and osteopontin and probably for other as yet unidentified ligands (13, 37). Fibronectin-deficient embryos die before the onset of kidney development (18), but it is unlikely that a molecule with such widespread expression would also act as the exclusive ligand in a specific interaction between the ureteric bud and the metanephric mesenchyme. Additionally, because kidney development is normal in the osteopontin-mutant mouse (49), it is unlikely that osteopontin is the only relevant ligand. However, in experiments where anti-osteopontin antibodies were added to metanephric organ cultures, some inhibition of growth was observed (50). This provides one of several known examples where the results of genetic knockout and antibody inhibition experiments are not in agreement and leave open the possibility that interactions between alpha 8beta 1-integrin and osteopontin are crucial during kidney development

The knowledge that normal nephrogenesis requires an interaction between alpha 8beta 1-integrin and an unknown ligand adds to the number of ligand-receptor events previously demonstrated to be crucial for early kidney development. These include interactions among c-ret and glial-derived neurotropic factor (GDNF)/GDNF receptor-alpha , Wnt-4, and a presumed Frizzled receptor, and between BMP-7 and its receptor (reviewed in Ref. 34). Indeed, given the growing awareness that signals transduced by integrin-ECM interactions and growth factor-receptor interactions are often integrated within cells to produce physiological responses, it will be of great interest to eventually determine whether a growth factor such as GDNF signals coordinately with alpha 8beta 1-integrin.

Expression During Nephron Differentiation

Thus far, only expression of beta 1-integrins has been examined in detail during the development of individual nephrons. It is very likely that additional integrins are expressed in the developing kidney, particularly alpha vbeta 3 and/or alpha vbeta 5 in the developing vasculature. Whether these integrins might have functions unique to the kidney that are not observed during vasculogenesis and angiogenesis in other organs remains to be determined.

alpha 1beta 1. The alpha 1beta 1-integrin, which is a receptor for collagen and laminin, was found to be expressed in S-shaped tubules mainly by cells invading the glomerular cleft, i.e., those cells that will contribute to the capillary network within the glomerulus. In more mature glomeruli, alpha 1beta 1 continued to be restricted to the mesangial area within glomeruli (31, 47).

alpha 2beta 1. The alpha 2beta 1-integrin, a receptor for laminin and collagen, is expressed in the part of the S-shaped tubule that will contribute to proximal but not distal tubules (31, 47). Expression is also observed in endothelial cells within capillary loops of immature glomeruli (31, 47). In more mature kidneys, alpha 2beta 1-integrin is expressed by distal tubules and collecting ducts, as well as glomerular endothelial cells (31, 47).

alpha 3beta 1. The alpha 3beta 1-integrin was originally characterized as a promiscuous receptor, binding collagen, fibronectin, laminin, and entactin/nidogen (11, 14, 45, 64). More recent studies have demonstrated that although ECM components might be weak ligands, alpha 3beta 1 binds with much higher affinity to isoforms of laminin, including laminin-5 and laminin-10/11 (12, 30). Because little if any laminin-5 is present in the kidney, it is likely that either laminin-10 or -11, which contains the alpha 5 chain as part of the laminin heterotrimer, is the important ligand for this integrin in the kidney. Indeed, alpha 5-containing laminins are colocalized with sites of alpha 3beta 1-integrin expression within the kidney (63). As mentioned above, alpha 3beta 1 is expressed weakly by the ureteric bud and most highly in those cells of the early tubule that represent the presumptive podocytes (31, 47). Weaker staining is also seen in the cells of the forming Bowman's capsule. Proximal and distal tubule precursor cells do not express alpha 3beta 1, although in more mature kidneys expression is observed in distal tubules and collecting ducts (31, 47). In maturing glomeruli, alpha 3beta 1 is highly expressed by glomerular podocytes in a polarized pattern along the glomerular basement membrane (GBM) (31, 47).

Targeted mutations in mice have also been instructive about the importance of integrins in epithelial differentiation. Most notably, mice homozygous for a mutation in the alpha 3-integrin gene have several defects in kidney development (32). As discussed above, alpha 3beta 1 is the predominant integrin expressed along the basal surface of podocytes. Podocytes deficient in alpha 3beta 1-integrin appear unable to assemble mature foot processes, and, instead, cytoplasmic projections from the podocyte cell body are flattened against the GBM (32). Moreover, the GBM itself is fragmented, and along much of its length there appears to be a failure of fusion of the epithelial- and endothelial-derived components. Additionally, there appear to be fewer capillary loops in each glomerulus, and individual loops have a much wider diameter than usual, such that several blood cells are observed in histological sections through capillaries within glomeruli (32). This histological picture suggests a dynamic process during glomerulogenesis, where migration of podocytes around ingrowing capillaries, mediated by alpha 3beta 1-integrin, plays an essential role in stimulating capillary branching and maturation of the basement membrane.

The lack of formation of mature foot processes in alpha 3beta 1-integrin-deficient podocytes suggests that signals transduced by this integrin are essential for triggering the cytoskeletal rearrangements required to assemble and maintain foot process structure (see Fig. 1). Because alpha 3beta 1-deficient podocytes appear to remain adherent to the basement membrane along their entire length of contact, rather than simply dissociating from the basement membrane altogether, it seems the concept of integrins as simple adhesion receptors is oversimplified. Instead, it may be more appropriate to think of integrins as receptors transducing signals on contact with the ECM that elicit specific behavioral responses such as adhesion, migration, filopodial extension and, in the case of podocytes, foot process assembly. Importantly, all of these responses involve cytoskeletal rearrangement, and there is an emerging understanding of how integrin-ECM interactions affect cytoskeletal assembly through the downstream activation of Rho family GTPases (20).


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Fig. 1.   The possible involvement of integrins such as alpha 3beta 1 in podocyte differentiation. The alpha 3beta 1-integrin may be stimulated by a change in the composition of the basement membrane or may act to organize the basement membrane. As a consequence, signals are transduced that involve Rho family GTPases to result in the cytoskeletal reorganization required for foot process formation.

Wang et al. (63) have recently analyzed the role of alpha 3beta 1-integrin in cytoskeletal organization of collecting duct epithelial cells in monolayer culture. Past studies with other epithelial cell types such as keratinocytes had demonstrated that antibodies blocking function of alpha 3beta 1 caused cells to dissociate, with the concomitant loss of the cortical or submembrane cytoskeleton characteristic of epithelial cells (7, 8). Collecting duct epithelial cells prepared from kidneys deficient in alpha 3beta 1-integrin also were unable to assemble a cortical cytoskeleton and instead assembled actin stress fibers (63). In contrast to studies with blocking antibodies, alpha 3beta 1-integrin-deficient collecting duct cells retained cadherin-mediated cell-cell junctions. However, in alpha 3beta 1-integrin-deficient cells these cell-cell junctions were less well organized than in normal epithelial cells, and there was a decreased association of the cytoskeleton with cadherin-catenin complexes (63). These results indicate a role for alpha 3beta 1-integrin in organizing epithelial cortical cytoskeletons and also suggest that alpha 3beta 1 regulates the function of cadherins in epithelial cells.

alpha 6beta 1. Once the nephron becomes more fully differentiated, alpha 6beta 1-integrin continues to be expressed along both proximal and distal tubular basement membranes, as well as by collecting ducts, indicating that it is a major laminin receptor expressed by tubular epithelial cells (31, 47). Significantly, little if any alpha 6beta 4-integrin appears to be expressed during tubular differentiation, in contrast to epithelial cells of many other organs (31). The alpha 6beta 1-integrin is also expressed transiently along the GBM, possibly by both podocytes and endothelial cells (31, 47). In surprising contrast to alpha 3-mutant mice, newborn mice carrying a targeted mutation of the alpha 6-integrin gene appear to have normal kidneys (19).

A Role for Integrins in Branching Morphogenesis During Development of the Collecting System?

A role for the extracellular matrix in directing the pattern of branching was suggested by Bernfield and co-workers (5). They suggested that the composition of the basement membrane determined whether epithelial cells would proliferate (see Fig. 2). To the extent that this model might be valid, integrins are obvious candidates to transduce signals to cells on the basis of the composition of the surrounding matrix. Any signals transduced by integrins presumably act coordinately with growth factor signaling (57).


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Fig. 2.   A hypothetical role for integrins in branching morphogenesis. Left: integrins may sense mesenchyme-induced changes in the extracellular matrix adjacent to growing buds, including alterations in the composition of the basement membrane. Growth factors such as glial-derived neurotropic factor (GDNF) stimulate their cognate receptors such as c-RET. Integrins and growth factors then coordinately signal to stimulate cell proliferation (Prolif). Along the stalk of the tubule, growth factor is not present, and/or the basement membrane is not stimulatory to integrins, and proliferation is not signaled. Right: potential feedback loop involving alpha 8beta 1-integrin, expressed on the mesenchyme. On binding a ligand on the ureteric bud, alpha 8beta 1 may stimulate a signal, presently unknown, that maintains continued growth of the ureteric bud. Orange, ECM; blue, growth factor receptors; green, integrins; yellow, growth factors.

Kidneys of alpha 3beta 1-integrin-deficient newborn mice also have fewer collecting ducts within the papillary region of the medulla (32). This is suggestive of a decrease in branching morphogenesis during early kidney development, although this phenotype could also be due to either an overall decrease in proliferation or an increase in apoptosis of epithelial cells during development. Thus far, efforts to quantitatively compare the latter two possibilities have not shown significant differences between wild-type and mutant kidneys (J. Symons and J. Kreidberg, unpublished observations). Despite the appearance of fewer collecting ducts in alpha 3beta 1-deficient kidneys, epithelial cells in those ducts that are present appear to be normal, except for a thinning of their basement membranes (63). Two hypotheses that may explain the development of fewer collecting ducts in alpha 3beta 1-integrin-deficient kidneys are 1) a subset of collecting duct cells requires a unique function of alpha 3beta 1; or 2) there is a quantitative decrease in proliferation or branching in the absence of alpha 3beta 1-integrin. Thus far, no subset of collecting duct cells has been described that expresses alpha 3beta 1 to the exclusion of other integrins, a result more consistent with, but not proving, the latter possibility.

A further unexpected result was obtained on intercrossing alpha 3- and alpha 6-mutant mice (10). In alpha 3/alpha 6 double-null embryos examined at embryonic day 14, kidney development appears no different than in alpha 3-only null embryos (10). Therefore, if some integrin is providing redundant function that allows limited collecting duct development in alpha 3beta 1-deficient embryos, it is unlikely to be alpha 6beta 1 but might still be alpha 2beta 1. The genetic targeting of the alpha 2-integrin gene has not yet been reported; thus conclusions are yet be drawn about the relative importance of alpha 2beta 1-integrin in kidney development. Alternatively, it is possible that integrins simply are not crucial for the development and maintenance of epithelial tubular structures during early organ development, even though this would otherwise appear to be highly unlikely.

A study of alpha v-containing integrins in metanephric organ culture found expression of alpha vbeta 3-, alpha vbeta 5-, and alpha vbeta 6-integrins in metanephric tissue (61). Addition of alpha v-antisense oligonuleotides or anti-alpha v-blocking antibodies to the organ cultures resulted in dysgenic growth with decreased branching of the ureteric bud (61). Similar dysgenic effects result from the addition of antisense oligonucleotides to fibrillin, a ligand of alpha vbeta 3-integrin, suggesting this interaction as a crucial one during kidney development (28). However, the alpha v-integrin-knockout mouse has apparently normal kidneys (2), so the role of alpha v-containing integrins in kidney development remains unclear.

Two results from in vitro tissue culture systems suggest that integrins are indeed important for the early rounds of branching morphogenesis that occur during metanephric development, despite the failure of gene-targeting experiments to demonstrate this involvement. When function-blocking antibodies specific for alpha 6beta 1-integrin are added to kidney organ culture, tubulogenesis is inhibited (15). This is in marked contrast to the normal kidney development observed in alpha 6-integrin-knockout mice (19). As with other such discrepancies, it is possible that developing tissues are better able to compensate for missing proteins when they are deficient from the beginning of embryogenesis, rather than when their function is acutely blocked by the addition of antibodies, but the mechanistic basis for this difference is unknown. The alpha 6beta 1-blocking antibodies also inhibited branching in cultured submandibular gland, suggesting a generalized role in branching morphogenesis (26). It has not been possible to do similar experiments examining the role of other integrins, as potent function-blocking antibodies that cross-react with other rodent integrins besides alpha 6beta 1 are not available.

An additional model system for studying epithelial development involves culturing Madin-Darby canine kidney (MDCK) cells in three-dimensional collagen gels. MDCK cells are derived from canine medullary collecting ducts, and it was shown several years ago that they form branched tubular structures in three-dimensional gels on stimulation with hepatocyte growth factor (HGF) (36). The extent to which this system serves as a model for branching morphogenesis must be viewed with caution for the following reasons: 1) HGF- and c-met (HGF receptor)-knockout mice have normal kidney development; 2) branching appears to be random, as opposed to the specific patterns observed in vivo; and 3) the tubules that form in gels do not have the appearance of classic tubules in cross section, although there is some degree of apical-basolateral polarization present. Saelman et al. (55) made elegant use of this system to examine the role of alpha 2beta 1-integrin in tubule formation. They expressed antisense alpha 2-integrin RNA in MDCK cells to demonstrate that alpha 2beta 1-integrin was required for tubule formation in collagen gels (55). For this reason, as well as those discussed above, it will become especially important to observe the extent of kidney development in alpha 2beta 1-integrin-deficient mice, even though this may require conditional gene targeting if the alpha 2 knockout proves to be early embryonically lethal.


    INTEGRIN EXPRESSION IN ADULT KIDNEY AND THE ROLE OF INTEGRINS IN KIDNEY DISEASE
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Glomerulus

The alpha 3beta 1-integrin is the major ECM receptor expressed by podocytes along the GBM (31, 43). One would presume that this integrin should play an important role in adhering foot processes to the GBM. To our knowledge, ultrastructural studies have not been performed thus far to demonstrate whether alpha 3beta 1-integrin is only present where foot processes contact the GBM, or whether it is continuous along the entire basal aspect of the podocyte. This will be an important point for future studies, as cytoskeletal rearrangements that are modulated by interactions between integrins and the GBM probably play a crucial role in foot process maturation. Patey et al. (43) also observed expression of alpha vbeta 3-integrin on podocytes; expression of alpha v-integrins was not examined in other studies of kidney integrin expression (43). Very weak staining of alpha 6beta 1-integrin has also been observed on podocytes (31, 43, 47).

Integrins expressed on Bowman's capsule include alpha 1beta 1, alpha 3beta 1, alpha 6beta 1, and alpha vbeta 3 (31, 43, 47). The alpha 1beta 1-integrin appears to be the most prominent type expressed on mesangial cells; these cells also express lesser amounts of alpha 2beta 1, alpha 3beta 1, and alpha 6beta 1 (31, 43, 47). Finally, endothelial cells, both in the glomerulus and along major vessels, express alpha 3beta 1-, alpha 5beta 1-, alpha 6beta 1-, and alpha v-containing integrins (31, 43, 47). Unfortunately, there is little that can be said about the relative significance of these expression patterns. There is generally little understanding of the physiological significance of particular integrin repertoires, other than that this provides some indication of what might be the significant interactions with the ECM required of each cell type.

Congential Nephrotic Syndrome

In congenital nephrotic syndromes (CNS) there is a failure to form mature foot processes during glomerulogenesis, leading to severe proteinuria and the clinical manifestations of the nephrotic syndrome soon after birth, including severe edema and hypoalbuminemia. There are striking similarities between the phenotype of alpha 3-integrin-knockout mice (32) and CNS. In addition to the lack of foot processes, there are notable proximal tubule abnormalities, including the formation of cysts and the accumulation of cytoplasmic vesicles. In alpha 3beta 1-integrin-deficient kidneys, these proximal tubule lesions are thought to be secondary to glomerular dysfunction, as proximal tubules do not normally express alpha 3beta 1-integrin (31). Although these findings might have suggested alpha 3-integrin as a candidate gene responsible for some forms of CNS, thus far no genetic kidney disease has been mapped near the alpha 3-integrin human chromsomal locus. Indeed, mutations in the gene encoding the novel protein nephrin, expressed at slit diaphragms between podocyte foot processes, have been established as the etiology of the Finnish type of CNS, the best-described form of the disease (29, 54). Moreover, despite the similarities between CNS and the alpha 3-integrin knockout, loss of alpha 3beta 1-integrin appears to be a late event in glomerulosclerosis, and early nephrotic lesions even appear to show an increase in immunostaining for alpha 3beta 1-integrin (35). Therefore, if CNS and a deficiency in alpha 3beta 1-integrin affect a common pathway involved in foot process formation, it is most likely that CNS mutations affect proteins acting downsteam of alpha 3beta 1-integrin.

Glomerulonephritis and Chronic Renal Injury

Several studies have demonstrated an increased expression of integrins by glomerular and tubular cells during early phases of glomerulonephritis, by cells forming crescents, and subsequent decreased expression in end-stage lesions (3, 44, 59). Interestingly, alpha vbeta 3-integrin was expressed on podocytes in crescentic glomeruli, suggesting that it may play a role in the adhesion of crescentic cells (3, 59). Additionally, expression of alpha vbeta 5 is upregulated on tubular cells in glomerulonephritic kidneys (46).

Abnormalities of glomerular podocytes can be demonstrated in both acute glomerular injury and in chronic renal diseases. Foot process effacement, characterized by the collapse of the interdigitating components of podocytes that aid in the formation of a filtration barrier, is seen in glomerular diseases manifesting with proteinuria. Adler et al. (1) demonstrated that intact or F(ab')2 anti-beta 1 antibodies (Ab) had a greater effect than Fab fragments in an experimental system that measured permeability to albumin in isolated glomeruli, suggesting that integrin crosslinking is involved in maintaining the permeability barrier. As opposed to the developmental abnormality in congenital nephrotic syndrome, immune-mediated injury is thought to underlie these abnormal podocyte findings. An experimental model of complement-mediated injury to a glomerular epithelial cell line demonstrated reversible disruption of actin filaments that could explain the morphological changes seen in glomerular immune injury (60). Complement-mediated injury reversibly disrupts glomerular epithelial cell actin microfilaments and focal adhesions (60). Interestingly, despite their severed connection to the cytoskeleton, matrix-associated integrins were preserved in this experimental model. Given the role played by integrins in the regulation of cytoskeletal organization, their preservation may be related to the reversibility of this form of glomerular injury.

Mesangial cell proliferation characterizes numerous forms of acute glomerular injury; ordered mesangial remodeling is essential to the restoration of normal glomerular function. Progressive glomerular disease can manifest with ongoing mesangial cell proliferation and deposition of extracellular matrix components, leading to glomerulosclerosis and loss of renal function. As mediators of the interaction between cells and ECM, integrins may play a role in this delicate balance between ordered growth and excess proliferation. Normal mesangial cells express alpha 1beta 1- and alpha 5beta 1-integrins; this expression is increased in proliferative forms of glomerulonephritis (27, 33). Integrin overexpression may be related to elevated levels of glomerular transforming growth factor-beta , which has been detected in association with the upregulated beta 1-integrins (27, 33).

Chronic forms of renal injury are characterized histologically by ongoing glomerulosclerosis and advancing interstitial fibrosis with tubular atrophy. This pattern can be seen as the final common pathway for most forms of progressive renal disease, from glomerulonephritis to chronic rejection of transplanted renal allografts. A study of integrin expression in renal biopsies of patients with chronic renal injury demonstrated an increased distribution of alpha 5beta 1- and alpha v-integrins in areas with greater degrees of chronic histological damage (53). These integrins serve as fibronectin receptors, and this expression pattern is therefore consistent with the notion their increased expression serves to augment fibronectin assembly as part of an ongoing fibrotic process. It is also possible that a more complex regulatory circuit is operative, whereby intracellular signals activate integrin-mediated matrix assembly on the cell surface by virtue of a direct effect on the integrin itself, such as inducing a conformation change that improves its ability to bind ECM ligands. Much more needs to be learned about this "inside- out" signaling affecting integrin function (16) to determine whether this is a valid mechanism in glomerulonephritis and other kidney disease.

Proximal Tubules

The alpha 6beta 1 receptor appears to be the major laminin receptor expressed by proximal tubule epithelial cells (31, 43, 47). Other prominent laminin receptors, including alpha 2beta 1 and alpha 3beta 1, have not been detected on proximal tubules.

Distal Tubules

The alpha 2beta 1-, alpha 3beta 1-, and alpha 6beta 1-integrins all appear to be expressed by distal tubules (31, 43, 47). It remains unclear why distal tubules would require more diverse integrin expression than proximal tubules. It can be hypothesized that integrin repertoires affect such parameters as basement membrane composition, permeability, or the tensile strength of the interactions between cells and their respective basement membranes. If so, it is likely that these parameters are different in distinct types of tubules, thus the requirement for different integrins. Unfortunately, at this time these possibilities remain matters for speculation.

Collecting Ducts

Similarly to distal tubules, alpha 2beta 1-, alpha 3beta 1-, and alpha 6beta 1-integrins also appear to be expressed by collecting duct epithelial cells (31, 43, 47). Korhonen et al. (31) also detected weak staining of beta 4-integrins along collecting ducts, suggesting that some alpha 6 might be heterodimerized with beta 4- subunit, although this has not been confirmed in other studies (31).

Acute Tubular Injury

The major interest regarding the role of integrins in tubular injury centers on how modulations in integrin function and localization may be involved in the exfoliation of epithelial cells into the tubular lumen during acute renal failure. A study involving kidney cells in culture showed apical expression of the alpha 3-integrin subunit after oxidative stress (17). This led to the suggestion that apical expression of integrins may actively be involved in exfoliation by binding matrix components in tubular lumen. Zuk et al. (66) have recently published an exhaustive study of integrin expression in a postischemic-reperfusion model in rat kidneys. They observed that integrin expression changed from exclusively basal to basolateral during the first several hours after ischemic injury. In contrast to the earlier study, however, they failed to observe apical expression of beta 1-integrins, even after tubular lumen were filled with exfoliated material (66). A small amount of apical expression was observed 120 h after ischemic injury, but by this time tubular injury was already resolving. Zuk et al. present a model in which migration of integrins from a basal to lateral localization leads to progressive detachment from the basement membrane and exfoliation (66). As tubular cells regenerate, they pass through a stage where integrins are expressed on all membranes and subsequently become polarized on the basal membrane. An important caveat here is these studies have only examined expression of beta 1-integrins, leaving open the possibility that other integrins, such as alpha vbeta 3, are expressed apically or are otherwise involved in exfoliation.

The observation that integrin localization is altered after acute renal injury prompted studies to test whether peptides that contain the RGD (arginine-glycine-aspartic acid) sequence would ameliorate the degree of injury (21, 22). The RGD sequence is found at sites where a subset of integrins bind components of the extracellular matrix; these include alpha 5beta 1, alpha 8beta 1, and alpha vbeta 3. RGD sites are not thought to be involved in the interaction of integrins such as alpha 3beta 1 and alpha 6beta 1 with the basement membrane. Studies by Goligorsky et al. (22) and Noiri and co-workers (39, 40), using the ischemic rat kidney model, showed that adminstration of certain RGD peptides lessened the degree of tubular injury, as adjudged by the appearance of the urine sediment, the extent of tubular dilatation, and the creatinine clearance. An increase in the uptake of labeled RGD peptides by tubular epithelium after renal injury was also documented (52). The results of Zuk et al. (66), who failed to observe apical expression of beta 1-integrins, raise the question of whether RGD peptides affect the function of basolaterally localized integrins. As mentioned above, these studies have only detected beta 1-integrins, such that alpha vbeta 3-integrin remains a potential target of RGD peptides. Thus our understanding of the role of integrins in the pathogenesis of acute renal failure remains incomplete and provides a ripe area for further study.

A recent study by Noiri et al. (38) provides a possible mechanistic explanation for the action of RGD peptides (see Fig. 3). They compared renal injury in wild-type and the osteopontin-deficient mice that were mentioned previously. Osteopontin contains an RGD site and is a ligand for alpha 8beta 1- and alpha vbeta 3-integrins. Ischemia-related renal dysfunction and pathology were more severe in osteopontin-deficient mice (38). In vitro, osteopontin was protective when proximal tubular cells were exposed to hypoxic conditions, whereas osteopontin missing the RGD sequence was not protective (38). Cytoprotection during hypoxia could also be provided in vitro by RGD peptides (38). These results suggest that the RGD sequence is the crucial element within the osteopontin protein that stimulates processes which prevent cytotoxic injury. A possible mechanism for the action of osteopontin is suggested by the observation that osteopontin has been shown to suppress expression of an inducible nitric-oxide synthase (51, 58), an enzyme associated with renal injury after ischemia-reperfusion (6).


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Fig. 3.   A hypothetical role for integrins and arginine-glycine-aspartic acid (RGD)-containing proteins in tubular injury after ischemia-reperfusion. Top right: on hypoxic injury, basal polarization of integrins is lost and integrins are expressed on lateral membranes, where they are involved in migration and eventual exfoliation. Bottom right: if RGD-containing proteins are present, they are bound by lateral integrins and block them from mediating lateral migration and exfoliation.

In addition to their possible causative role in acute tubular damage, integrins may also be involved in mechanisms of repair after acute renal injury. After the exfoliation of tubular cells noted above, recovery of renal function requires a reconstitution of the normal tubular epithelium through proliferation of residual cells. In contradistinction to the proposal that altered integrin localization promotes renal damage through cell sloughing, integrins may also be necessary for the reparative processes that occur after acute tubular injury. Proximal tubular cells exposed to a free radical-generating system that simulates in vivo tubular injury had a diminished proliferative capacity in the presence of inhibitors to integrin function, including, ironically, glycine-RGD peptides similar to those used to limit renal injury due to sloughing of cells (65). Furthermore, injured cells were more likely to undergo apoptosis in the setting of integrin inhibition. Consequently, integrins may play multiple roles in the dynamic arc of acute tubular injury, exacerbating damage in its earliest phases and later promoting repair.

Diabetic Nephropathy

Alterations in integrin expression have been studied in diabetic nephropathy (DN). Similar to what is observed in glomerulonephritis as noted above, expression of integrins in the glomerulus with DN appears to increase during early damage, on epithelial, endothelial, and mesangial cells (25). One study showed this increase to continue during severe DN except for endothelial integrin expression, which returned to normal levels (25). In contrast, a study of the expression of alpha 3beta 1-integrin by glomerular cells in diabetic rats showed a decrease in expression of alpha 3beta 1 in short- and long-term diabetic animals (48). Much more work is required to determine whether abnormalities in integrin-mediated signal transduction, particularly as it relates to interactions of integrins with the GBM, is involved in DN.

Polycystic Kidney Disease

There has been limited examination of changes in integrin expression during the pathogenesis of polycystic kidney disease (PKD). Generation and progressive enlargement of tubular cysts in all forms of PKD are thought to stem from interplay among three basic processes: tubular cell hypertrophy, tubular fluid secretion, and abnormalities related to the tubular cell ECM. Given their status as mediators for reciprocal interactions between the ECM and epithelial cells, integrins can be hypothesized as having a causal role in cyst formation. Alternatively, impairment of integrin-basement membrane interactions could be downstream of more primary abnormalities that lead to cystogenesis. An examination of the expression of several beta l-integrins in PKD demonstrated irregular expression of alpha 2beta 1-, alpha 3beta 1-, and alpha 6beta 1-integrins in cystic epithelial cells (9). Expression of alpha 1beta 1-integrin was increased, also suggesting a possible role for this integrin in cyst formation (9). These changes occurred early in the process of cyst formation, indicating that a disturbance of interactions between integrins and tubular membranes may have a causal role in PKD. More work in this area will be needed to further elucidate a role for integrins in PKD.

Summary

It is now well established that there is widespread expression of integrins during kidney development and in adult kidneys. Gene-targeting experiments have been informative about locations where individual integrins fulfill a crucial function, most notably alpha 8beta 1 in interactions between the ureteric bud and the metanephric mesenchyme, and alpha 3beta 1 in glomerulogenesis. In contrast, gene knockouts of other widely expressed integrin subunits, such as alpha 6 and alpha v, led to seemingly normal kidneys, suggesting that many integrins may have redundant functions during nephrogenesis. Moreover, the study of how integrin-mediated signal transduction is involved in renal epithelial differentiation, metanephric induction, and mature kidney function is just beginning and promises to add importantly to our understanding of renal physiology.


    ACKNOWLEDGEMENTS

The authors thank Anna Zuk and Michael Goligorsky for helpful discussions.


    FOOTNOTES

Address for reprint requests and other correspondence: J. A. Kreidberg, Children's Hospital, 300 Longwood Ave., Boston, MA 02115 (E-mail: Kreidberg{at}hub.tch.harvard.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.


    REFERENCES
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REFERENCES

1.   Adler, S, Sharma R, Savin VJ, Abbi R, and Eng B. Alteration of glomerular permeability to macromolecules induced by cross-linking of beta 1 integrin receptors. Am J Pathol 149: 987-96, 1996[Abstract].

2.   Bader, BL, Rayburn H, Crowley D, and Hynes RO. Extensive vasculogenesis, angiogenesis, and organogenesis precede lethality in mice lacking all alpha v integrins. Cell 95: 507-519, 1998[ISI][Medline].

3.   Baraldi, A, Zambruno G, Furci L, Ballestri M, Tombesi A, Ottani D, Lucchi L, and Lusvarghi E. Beta 1 and beta 3 integrin upregulation in rapidly progressive glomerulonephritis. Nephrol Dial Transplant 10: 1155-1161, 1995[Abstract].

4.   Bard, JBL, McConnell JE, and Davies JA. Towards a genetic basis for kidney development. Mech Dev 48: 3-11, 1994[ISI][Medline].

5.   Bernfield, M, Banerjee SD, Koda JE, and Rapraeger AC. Remodeling of the basement membrane as a mechanism of morphogenetic tissue interaction. In: The Role of Extracellular Matrix in Development. New York: Liss, 1984, p. 545-572.

6.   Blantz, RC, Lortie M, Vallon V, Gabbai FB, Parmer RJ, and Thomson S. Activities of nitric oxide in normal physiology and uremia. Semin Nephrol 16: 144-150, 1996[ISI][Medline].

7.   Carter, WG, Kaur P, Gil SG, Gahr PJ, and Wayner EA. Distinct functions for integrins alpha 3 beta 1 in focal adhesions and alpha 6 beta 4/bullous pemphigoid antigen in a new stable anchoring contact (SAC) of keratinocytes: relation to hemidesmosomes. J Cell Biol 111: 3141-3154, 1990[Abstract].

8.   Carter, WG, Wayner EA, Bouchard TS, and Kaur P. The role of integrins alpha 2 beta 1 and alpha 3 beta 1 in cell-cell and cell-substrate adhesion of human epidermal cells. J Cell Biol 110: 1387-1404, 1990[Abstract].

9.   Daikha-Dahmane, F, Narcy F, Dommergues M, Lacoste M, Beziau A, and Gubler MC. Distribution of alpha-integrin subunits in fetal polycystic kidney diseases. Pediatr Nephrol 11: 267-273, 1997[ISI][Medline].

10.   De Arcangelis, A, Mark M, Kreidberg J, Sorokin L, and Georges-Labouesse E. Synergistic activities of alpha 3 and alpha 6 integrins are required during apical ectodermal ridge formation and organogenesis in the mouse. Development 126: 3957-3968, 1999[Abstract/Free Full Text].

11.   Dedhar, S, Jewell K, Rojiani M, and Gray V. The receptor for the basement membrane glycoprotein entactin is the integrin alpha 3/beta 1. J Biol Chem 267: 18908-18914, 1992[Abstract/Free Full Text].

12.   Delwel, GO, de MA, Hogervorst F, Jaspars LH, Fles DL, Kuikman I, Lindblom A, Paulsson M, Timpl R, and Sonnenberg A. Distinct and overlapping ligand specificities of the alpha 3A beta 1 and alpha 6A beta 1 integrins: recognition of laminin isoforms. Mol Biol Cell 5: 203-215, 1994[Abstract].

13.   Denda, S, Reichardt LF, and Muller U. Identification of osteopontin as a novel ligand for the integrin alpha8 beta1 and potential roles for this integrin-ligand interaction in kidney morphogenesis. Mol Biol Cell 9: 1425-1435, 1998[Abstract/Free Full Text].

14.   Elices, MJ, Urry LA, and Hemler ME. Receptor functions for the integrin VLA-3: fibronectin, collagen, and laminin binding are differentially influenced by Arg-Gly-Asp peptide and by divalent cations. J Cell Biol 112: 169-181, 1991[Abstract].

15.   Falk, M, Salmivirta K, Durbeej M, Larsson E, Ekblom M, Vestweber D, and Ekblom P. Integrin alpha 6B beta 1 is involved in kidney tubulogenesis in vitro. J Cell Sci 109: 2801-2810, 1996[Abstract/Free Full Text].

16.   Faull, RJ, and Ginsberg MH. Inside-out signaling through integrins. J Am Soc Nephrol 7: 1091-1097, 1996[Abstract].

17.   Gailit, J, Colflesh D, Rabiner I, Simone J, and Goligorsky MS. Redistribution and dysfunction of integrins in cultured renal epithelial cells exposed to oxidative stress. Am J Physiol Renal Fluid Electrolyte Physiol 264: F149-F157, 1993[Abstract/Free Full Text].

18.   George, EL, Georges LE, Patel KR, Rayburn H, and Hynes RO. Defects in mesoderm, neural tube and vascular development in mouse embryos lacking fibronectin. Development 119: 1079-1091, 1993[Abstract/Free Full Text].

19.   Georges, LE, Messaddeq N, Yehia G, Cadalbert L, Dierich A, and Le MM. Absence of integrin alpha 6 leads to epidermolysis bullosa and neonatal death in mice. Nature Genet 13: 370-373, 1996[ISI][Medline].

20.   Giancotti, FG. Integrin signaling: specificity and control of cell survival and cell cycle progression. Curr Opin Cell Biol 9: 691-700, 1997[ISI][Medline].

21.   Goligorsky, MS, Noiri E, Kessler H, and Romanov V. Therapeutic effect of arginine-glycine-aspartic acid peptides in acute renal injury. Clin Exp Pharmacol Physiol 25: 276-279, 1998[ISI][Medline].

22.   Goligorsky, MS, Noiri E, Kessler H, and Romanov V. Therapeutic potential of RGD peptides in acute renal injury. Kidney Int 51: 1487-1492, 1997[ISI][Medline].

23.   Hamerski, DA, and Santoro SA. Integrins and the kidney: biology and pathobiology. Curr Opin Nephrol Hypertens 8: 9-14, 1999[ISI][Medline].

24.   Hynes, RO. Integrins: versatility, modulation, and signaling in cell adhesion. Cell 69: 11-25, 1992[ISI][Medline].

25.   Jin, DK, Fish AJ, Wayner EA, Mauer M, Setty S, Tsilibary E, and Kim Y. Distribution of integrin subunits in human diabetic kidneys. J Am Soc Nephrol 7: 2636-2645, 1996[Abstract].

26.   Kadoya, Y, Kadoya K, Durbeej M, Holmvall K, Sorokin L, and Ekblom P. Antibodies against domain E3 of laminin-1 and integrin alpha 6 subunit perturb branching epithelial morphogenesis of submandibular gland, but by different modes. J Cell Biol 129: 521-534, 1995[Abstract].

27.   Kagami, S, Kuhara T, Yasutomo K, Okada K, Loster K, Reutter W, and Kuroda Y. Transforming growth factor-beta (TGF-beta) stimulates the expression of beta1 integrins and adhesion by rat mesangial cells. Exp Cell Res 229: 1-6, 1996[ISI][Medline].

28.   Kanwar, YS, Ota K, Yang Q, Kumar A, Wada J, Kashihara N, and Peterson DR. Isolation of rat fibrillin-1 cDNA and its relevance in metanephric development. Am J Physiol Renal Physiol 275: F710-F723, 1998[Abstract/Free Full Text].

29.   Kestila, M, Lenkkeri U, Mannikko M, Lamerdin J, McCready P, Putaala H, Ruotsalainen V, Morita T, Nissinen M, Herva R, Kashtan CE, Peltonen L, Holmberg C, Olsen A, and Tryggvason K. Positionally cloned gene for a novel glomerular protein-nephrin-is mutated in congenital nephrotic syndrome. Mol Cell 1: 575-582, 1998[ISI][Medline].

30.   Kikkawa, Y, Sanzen N, and Sekiguchi K. Isolation and characterization of laminin-10/11 secreted by human lung carcinoma cells. Laminin-10/11 mediates cell adhesion through integrin alpha3 beta1. J Biol Chem 273: 15854-15859, 1998[Abstract/Free Full Text].

31.   Korhonen, M, Ylanne J, Laitinen L, and Virtanen I. The alpha 1-alpha 6 subunits of integrins are characteristically expressed in distinct segments of developing and adult human nephron. J Cell Biol 111: 1245-1254, 1990[Abstract].

32.   Kreidberg, JA, Donovan MJ, Goldstein SL, Rennke H, Shepherd K, Jones RC, and Jaenisch R. Alpha 3 beta 1 integrin has a crucial role in kidney and lung organogenesis. Development 122: 3537-3547, 1996[Abstract/Free Full Text].

33.   Kuhara, T, Kagami S, and Kuroda Y. Expression of beta 1-integrins on activated mesangial cells in human glomerulonephritis. J Am Soc Nephrol 8: 1679-1687, 1997[Abstract].

34.   Lechner, MS, and Dressler GR. The molecular basis of embryonic kidney development. Mech Dev 62: 105-120, 1997[ISI][Medline].

35.   Ljungberg, P, Virtanen I, Holmberg C, and Jalanko H. Distribution of renal integrin receptors and their ligands in congenital nephrotic syndrome of the Finnish type. Virchows Arch 428: 333-346, 1996[ISI][Medline].

36.   Montesano, R, Schaller G, and Orci L. Induction of epithelial tubular morphogenesis in vitro by fibroblast-derived soluble factors. Cell 66: 697-711, 1991[ISI][Medline].

37.   Muller, U, Wang D, Denda S, Meneses JJ, Pedersen RA, and Reichardt LF. Integrin alpha8beta1 is critically important for epithelial-mesenchymal interactions during kidney morphogenesis. Cell 88: 603-613, 1997[ISI][Medline].

38.   Noiri, E, Dickman K, Miller F, Romanov G, Romanov VI, Shaw R, Chambers AF, Rittling SR, Denhardt DT, and Goligorsky MS. Reduced tolerance to acute renal ischemia in mice with a targeted disruption of the osteopontin gene. Kidney Int 56: 74-82, 1999[ISI][Medline].

39.   Noiri, E, Gailit J, Sheth D, Magazine H, Gurrath M, Muller G, Kessler H, and Goligorsky MS. Cyclic RGD peptides ameliorate ischemic acute renal failure in rats. Kidney Int 46: 1050-1058, 1994[ISI][Medline].

40.   Noiri, E, Romanov V, Forest T, Gailit J, DiBona GF, Miller F, Som P, Oster ZH, and Goligorsky MS. Pathophysiology of renal tubular obstruction: therapeutic role of synthetic RGD peptides in acute renal failure. Kidney Int 48: 1375-1385, 1995[ISI][Medline].

41.   Osathanondh, V, and Potter EL. Development of the human kidney as shown by microdissection II. Renal pelvis, calyces, and papillae. Arch Pathol 76: 53-65, 1963.

42.   Osathanondh, V, and Potter EL. Development of the human kidney as shown by microdissection III. Formation and interrelationship of collecting tubules and nephrons. Arch Pathol 76: 66-78, 1963.

43.   Patey, N, Halbwachs M, Droz D, Lesavre P, and Noel LH. Distribution of integrin subunits in normal human kidney. Cell Adhesion Commun 2: 159-167, 1994[ISI][Medline].

44.   Patey, N, Lesavre P, Halbwachs-Mecarelli L, and Noel LH. Adhesion molecules in human crescentic glomerulonephritis. J Pathol 179: 414-420, 1996[ISI][Medline].

45.   Pattaramalai, S, Skubitz KM, and Skubitz AP. A novel recognition site on laminin for the alpha 3 beta 1 integrin. Exp Cell Res 222: 281-290, 1996[ISI][Medline].

46.   Peruzzi, L, Trusolino L, Amore A, Gianoglio B, Cirina P, Basso G, Emancipator SN, Marchisio PC, and Coppo R. Tubulointerstitial responses in the progression of glomerular diseases: albuminuria modulates alpha v beta 5 integrin. Kidney Int 50: 1310-1320, 1996[ISI][Medline].

47.   Rahilly, MA, and Fleming S. Differential expression of integrin alpha chains by renal epithelial cells. J Pathol 167: 327-334, 1992[ISI][Medline].

48.   Regoli, M, and Bendayan M. Alterations in the expression of the alpha 3 beta 1 integrin in certain membrane domains of the glomerular epithelial cells (podocytes) in diabetes mellitus. Diabetologia 40: 15-22, 1997[ISI][Medline].

49.   Rittling, SR, Matsumoto HN, McKee MD, Nanci A, An XR, Novick KE, Kowalski AJ, Noda M, and Denhardt DT. Mice lacking osteopontin show normal development and bone structure but display altered osteoclast formation in vitro. J Bone Miner Res 13: 1101-1111, 1998[ISI][Medline].

50.   Rogers, SA, Padanilam BJ, Hruska KA, Giachelli CM, and Hammerman MR. Metanephric osteopontin regulates nephrogenesis in vitro. Am J Physiol Renal Physiol 272: F469-F476, 1997[Abstract/Free Full Text].

51.   Rollo, EE, Laskin DL, and Denhardt DT. Osteopontin inhibits nitric oxide production and cytotoxicity by activated RAW264.7 macrophages. J Leuk Biol 60: 397-404, 1996[Abstract].

52.   Romanov, V, Noiri E, Czerwinski G, Finsinger D, Kessler H, and Goligorsky MS. Two novel probes reveal tubular and vascular Arg-Gly-Asp (RGD) binding sites in the ischemic rat kidney. Kidney Int 52: 93-102, 1997[ISI][Medline].

53.   Roy-Chaudhury, P, Hillis G, McDonald S, Simpson JG, and Power DA. Importance of the tubulointerstitium in human glomerulonephritis. II. Distribution of integrin chains beta 1, alpha 1 to 6 and alpha V. Kidney Int 52: 103-110, 1997[ISI][Medline].

54.   Ruotsalainen, V, Ljungberg P, Wartiovaara J, Lenkkeri U, Kestila M, Jalanko H, Holmberg C, and Tryggvason K. Nephrin is specifically located at the slit diaphragm of glomerular podocytes. Proc Natl Acad Sci USA 96: 7962-7967, 1999[Abstract/Free Full Text].

55.   Saelman, EU, Keely PJ, and Santoro SA. Loss of MDCK cell alpha 2 beta 1 integrin expression results in reduced cyst formation, failure of hepatocyte growth factor/scatter factor-induced branching morphogenesis, and increased apoptosis. J Cell Sci 108: 3531-3540, 1995[Abstract/Free Full Text].

56.   Saxen, L. Organogenesis of the Kidney. Cambridge, UK: Cambridge Univ. Press, 1987.

57.   Schwartz, MA. Integrins, oncogenes, and anchorage independence. J Cell Biol 139: 575-578, 1997[Free Full Text].

58.   Scott, JA, Weir ML, Wilson SM, Xuan JW, Chambers AF, and McCormack DG. Osteopontin inhibits inducible nitric oxide synthase activity in rat vascular tissue. Am J Physiol Heart Circ Physiol 275: H2258-H2265, 1998[Abstract/Free Full Text].

59.   Shikata, K, Makino H, Morioka S, Kashitani T, Hirata K, Ota Z, Wada J, and Kanwar YS. Distribution of extracellular matrix receptors in various forms of glomerulonephritis. Am J Kidney Dis 25: 680-688, 1995[ISI][Medline].

60.   Topham, PS, Haydar SA, Kuphal R, Lightfoot JD, and Salant DJ. Complement-mediated injury reversibly disrupts glomerular epithelial cell actin microfilaments and focal adhesions. Kidney Int 55: 1763-1775, 1999[ISI][Medline].

61.   Wada, J, Kumar A, Liu Z, Ruoslahti E, Reichardt L, Marvaldi J, and Kanwar YS. Cloning of mouse integrin alphaV cDNA and role of the alphaV-related matrix receptors in metanephric development. J Cell Biol 132: 1161-1176, 1996[Abstract].

62.   Wallner, EI, Yang Q, Peterson DR, Wada J, and Kanwar YS. Relevance of extracellular matrix, its receptors, and cell adhesion molecules in mammalian nephrogenesis. Am J Physiol Renal Physiol 275: F467-F477, 1998[Abstract/Free Full Text].

63.   Wang, Z, Symons J, Goldstein S, McDonald A, Miner J, and Kreidberg JA. alpha 3beta 1 integrin regulates epithelial cytoskeletal organization. J Cell Sci 112: 2925-2935, 1999[Abstract/Free Full Text].

64.   Weitzman, JB, Pasqualini R, Takada Y, and Hemler ME. The function and distinctive regulation of the integrin VLA-3 in cell adhesion, spreading, and homotypic cell aggregation. J Biol Chem 268: 8651-8657, 1993[Abstract/Free Full Text].

65.   Wijesekera, DS, Zarama MJ, and Paller MS. Effects of integrins on proliferation and apoptosis of renal epithelial cells after acute injury. Kidney Int 52: 1511-1520, 1997[ISI][Medline].

66.   Zuk, A, Bonventre JV, Brown D, and Matlin KS. Polarity, integrin, and extracellular matrix dynamics in the postischemic rat kidney. Am J Physiol Cell Physiol 275: C711-C731, 1998[Abstract].


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