Morphogenesis of the glomerular filter: the synchronous assembly and maturation of two distinct extracellular matrices

Rethinking glomerular basement membrane thickening in diabetic nephropathy: adaptive or pathogenic?

Diabetic nephropathy (DN) is the leading cause of chronic kidney disease in the United States and is a major cause of cardiovascular disease and death. DN develops insidiously over a span of years before clinical manifestations, including microalbuminuria and declining glomerular filtration rate (GFR), are evident. During the clinically silent period, structural lesions develop, including glomerular basement membrane (GBM) thickening, mesangial expansion, and glomerulosclerosis. Once microalbuminuria is clinically apparent, structural lesions are often considerably advanced, and GFR decline may then proceed rapidly toward end-stage kidney disease. Given the current lack of sensitive biomarkers for detecting early DN, a shift in focus toward examining the cellular and molecular basis for the earliest structural change in DN, i.e., GBM thickening, may be warranted. Observed within one to two years following the onset of diabetes, GBM thickening precedes clinically evident albuminuria. In the mature glomerulus, the podocyte is likely key in modifying the GBM, synthesizing and assembling matrix components, both in physiological and pathological states. Podocytes also secrete matrix metalloproteinases, crucial mediators in extracellular matrix turnover. Studies have shown that the critical podocyte-GBM interface is disrupted in the diabetic milieu. Just as healthy podocytes are essential for maintaining the normal GBM structure and function, injured podocytes likely have a fundamental role in upsetting the balance between the GBM's synthetic and degradative pathways. This article will explore the biological significance of GBM thickening in DN by reviewing what is known about the GBM's formation, its maintenance during health, and its disruption in DN.

The Gne M712T mouse as a model for human glomerulopathy

Pathological glomerular hyposialylation has been implicated in certain unexplained glomerulopathies, including minimal change nephrosis, membranous glomerulonephritis, and IgA nephropathy. We studied our previously established mouse model carrying a homozygous mutation in the key enzyme of sialic acid biosynthesis, N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase. Mutant mice died before postnatal day 3 (P3) from severe glomerulopathy with podocyte effacement and segmental glomerular basement membrane splitting due to hyposialylation. Administration of the sialic acid precursor N-acetylmannosamine (ManNAc) led to improved sialylation and survival of mutant pups beyond P3. We determined the onset of the glomerulopathy in the embryonic stage. A lectin panel, distinguishing normally sialylated from hyposialylated glycans, used WGA, SNA, PNA, Jacalin, HPA, and VVA, indicating glomerular hyposialylation of predominantly O-linked glycoproteins in mutant mice. The glomerular glycoproteins nephrin and podocalyxin were hyposialylated in this unique murine model. ManNAc treatment appeared to ameliorate the hyposialylation status of mutant mice, indicated by a lectin histochemistry pattern similar to that of wild-type mice, with improved sialylation of both nephrin and podocalyxin, as well as reduced albuminuria compared with untreated mutant mice. These findings suggest application of our lectin panel for categorizing human kidney specimens based on glomerular sialylation status. Moreover, the partial restoration of glomerular architecture in ManNAc-treated mice highlights ManNAc as a potential treatment for humans affected with disorders of glomerular hyposialylation.

Lack of collagen XVIII long isoforms affects kidney podocytes, whereas the short form is needed in the proximal tubular basement membrane

Collagen XVIII is characterized by three variant N termini, an interrupted collagenous domain, and a C-terminal antiangiogenic domain known as endostatin. We studied here the roles of this collagen type and its variant isoforms in the mouse kidney. Collagen XVIII appeared to be in a polarized orientation in the tubular basement membranes (BMs), the endostatin domain embedded in the BM, and the N terminus residing at the BM-fibrillar matrix interface. In the case of the glomerular BM (GBM), collagen XVIII was expressed in different isoforms depending on the side of the GBM. The orientation appeared polarized here, too, both the endothelial promoter 1-derived short variant of collagen XVIII and the epithelial promoter 2-derived longer variants having their C-terminal endostatin domains embedded in the BM and the N termini at the respective BM-cell interfaces. In addition to loosening of the proximal tubular BM structure, the Col18a1(-/-) mice showed effacement of the glomerular podocyte foot processes, and microindentation studies showed changes in the mechanical properties of the glomeruli, the Col18a1(-/-) glomeruli being ∼30% softer than the wild-type. Analysis of promoter-specific knockouts (Col18a1(P1/P1) and Col18a1(P2/P2)) indicated that tubular BM loosening is due to a lack of the shortest isoform, whereas the glomerular podocyte effacement was due to a lack of the longer isoforms. We suggest that lack of collagen XVIII may also have disparate effects on kidney function in man, but considering the mild physiological findings in the mutant mice, such effects may manifest themselves only late in life or require other compounding molecular changes.

Integrin beta1-mediated matrix assembly and signaling are critical for the normal development and function of the kidney glomerulus

The human kidneys filter 180 l of blood every day via about 2.5 million glomeruli. The three layers of the glomerular filtration apparatus consist of fenestrated endothelium, specialized extracellular matrix known as the glomerular basement membrane (GBM) and the podocyte foot processes with their modified adherens junctions known as the slit diaphragm (SD). In this study we explored the contribution of podocyte beta1 integrin signaling for normal glomerular function. Mice with podocyte specific deletion of integrin beta1 (podocin-Cre beta1-fl/fl mice) are born normal but cannot complete postnatal renal development. They exhibit detectable proteinuria on day 1 and die within a week. The kidneys of podocin-Cre beta1-fl/fl mice exhibit normal glomerular endothelium but show severe GBM defects with multilaminations and splitting including podocyte foot process effacement. The integrin linked kinase (ILK) is a downstream mediator of integrin beta1 activity in epithelial cells. To further explore whether integrin beta1-mediated signaling facilitates proper glomerular filtration, we generated mice deficient of ILK in the podocytes (podocin-Cre ILK-fl/fl mice). These mice develop normally but exhibit postnatal proteinuria at birth and die within 15 weeks of age due to renal failure. Collectively, our studies demonstrate that podocyte beta1 integrin and ILK signaling is critical for postnatal development and function of the glomerular filtration apparatus.

Heparan sulfate analysis from diabetic rat glomeruli

One of the major complicating factors in insulin-dependent diabetes mellitus is nephropathy. Several investigators have linked heparan sulfate (HS) alterations in the glomerular basement membrane (GBM) with albuminuria as a marker of abnormal blood filtration and the subsequent progression to renal failure. In this study, we examined the fine structure of HS in the glomerulus and the GBM isolated from the kidneys of rats injected with streptozotocin. Using fluorophore-assisted carbohydrate electrophoresis, we obtained disaccharide composition analyses for HS. In a time course study, we observed that normal rat HS isolated from the GBM becomes more N-sulfated as the glomeruli mature over a period of 8 weeks. Diabetic rats injected with streptozotocin at the beginning of this period showed a reversal of this trend. Using a graded sieve technique, we found that two different sizes of glomeruli could be isolated from the rat kidneys and that there was a significant difference in the HS disaccharide content between these two pools of glomeruli. Only the larger sized glomeruli had less N-sulfation of HS as a result of insulin-dependent diabetes mellitus. This change in the fine structure of HS was localized to the GBM and was not associated with cell surface HS. We also generated oligosaccharides of HS that portray fine structural alterations in the diabetic rats indicative of a loss of the sulfation of N-acetylglucosamine.

Podocyte differentiation in the absence of endothelial cells as revealed in the zebrafish avascular mutant, cloche

The physiological functions of the zebrafish pronephros are blood plasma filtration and osmoregulation. The pronephric glomerulus is vascularized through a capillary network sprouting from the dorsal aorta. Vascularization of the glomerulus, visualized by flk-1 expression and alkaline phosphatase reactivity, involves the intimate association between podocytes and endothelial cells and the formation of an intervening glomerular basement membrane (GBM). Cell-cell interactions between podocytes and endothelial cells are thought to play an important role in glomerular angiogenesis. In order to determine whether endothelial cell-derived signals were required for podocyte differentiation, we employed in situ hybridization and electron microscopy to investigate glomerulogenesis in the zebrafish mutant cloche (clo), where endothelial cell development is blocked at an early stage. In clo mutants, glomerular epithelial cells expressing the podocyte specific marker wt1 display well-formed foot processes and are able to form a GBM, suggesting podocytes are able to morphologically differentiate in the absence of endothelia or endothelial-derived signals. The presence of irregular aggregates in the clo GBM as well as the apparent effacement of podocyte foot processes implies a role for endothelial cells in the maintenance of the mature glomerular filtration barrier.

Cell adhesion molecules and extracellular-matrix constituents in kidney development and disease

Functional analyses of cell-matrix interactions during kidney organogenesis have provided compelling evidence that extracellular-matrix glycoproteins and their receptors play instructive roles during kidney development. Two concepts are worthy of emphasis. First, matrix molecules appear to regulate signal transduction pathways, either by activating cell-surface receptors such as integrins directly or by modulating the activity of signaling molecules such as WNTs. Second, basement membranes are highly organized structures and have distinct molecular compositions, which are optimized for their diverse functions. The importance of these findings is highlighted by the fact that mutations affecting basement-membrane components lead to inherited forms of kidney disease.

Role of laminin isoforms in glomerular structure

Laminin along with collagen type IV, proteoglycans, and entactin are major components of basement membranes. Basement membrane components are synthesized at high levels during development. The formation of specialized basement membranes may play important roles in cell and tissue function by influencing cell proliferation, phenotype, migration and gene expression as well as tissue architecture. The growing diversity of laminin isoforms influences the formation of distinct basement membranes. Many of the laminin chains sequenced to date are expressed during glomerular development under strict temporal control. Also, some studies suggest that additional laminin chains exist and contribute to unique isoforms expressed within the renal glomerulus. This article will review the status of characterization of laminin isoforms expressed by glomerular cells, point out possible differences in isoforms expressed by different species, and discuss the implications of the complexity of glomerular laminins. In order to fully understand the nature of the glomerular laminins and their importance, information from studies of cells in culture, whole tissue, and those that use molecular and protein analysis must be integrated.