A molecular look at the glomerular barrier

Notch signaling in diabetic nephropathy

Notch signaling is an evolutionarily conserved cell-cell signaling system that controls the fate of cells during development. In this review, we will summarize the literature that notch signaling during development controls nephron number and segmentation and therefore could influence kidney disease susceptibility. We will also review the evidence that Notch is reactivated in adult-onset diabetic kidney disease where it promotes the development of nephropathy including glomerulopathy, tubulointerstitial fibrosis and possibly arteriopathy and inflammation. Finally, we will review the evidence that blockade of pathogenic Notch signaling alters the natural history of diabetic nephropathy and thus could represent a novel therapeutic approach to the management of diabetic kidney disease.

Deficient degradation of homotrimeric type I collagen, α1(I)3 glomerulopathy in oim mice

Col1a2-deficient (oim) mice synthesize homotrimeric type I collagen due to nonfunctional proα2(I) collagen chains. Our previous studies revealed a postnatal, progressive type I collagen glomerulopathy in this mouse model, but the mechanism of the sclerotic collagen accumulation within the renal mesangium remains unclear. The recent demonstration of the resistance of homotrimeric type I collagen to cleavage by matrix metalloproteinases (MMPs), led us to investigate the role of MMP-resistance in the glomerulosclerosis of Col1a2-deficient mice. We measured the pre- and post-translational expression of type I collagen and MMPs in glomeruli from heterozygous and homozygous animals. Both the heterotrimeric and homotrimeric isotypes of type I collagen were equally present in whole kidneys of heterozygous mice by immunohistochemistry and biochemical analysis, but the sclerotic glomerular collagen was at least 95-98% homotrimeric, suggesting homotrimeric type I collagen is the pathogenic isotype of type I collagen in glomerular disease. Although steady-state MMP and Col1a1 mRNA levels increased with the disease progression, we found these changes to be a secondary response to the deficient clearance of MMP-resistant homotrimers. Increased renal MMP expression was not sufficient to prevent homotrimeric type I collagen accumulation.

Preterm birth and the kidney: implications for long-term renal health

Although the majority of preterm neonates now survive infancy, there is emerging epidemiological evidence to demonstrate that individuals born preterm exhibit an elevated risk for the development of hypertension and renal impairment later in life, thus supporting the developmental origins of health and disease hypothesis. The increased risk may potentially be attributed to a negative impact of preterm birth on nephron endowment. Indeed, at the time when most preterm neonates are delivered, nephrogenesis in the kidney is still ongoing with the majority of nephrons normally formed during the third trimester of pregnancy. A number of clinical studies have provided evidence of altered renal function during the neonatal period, but to date there have been limited studies describing the consequences of preterm birth on kidney structure. Importantly, studies in the preterm baboon have shown that nephrogenesis is clearly ongoing following preterm birth; however, the presence of abnormal glomeruli (up to 18% in some cases) is of concern. Similar glomerular abnormalities have been described in autopsied preterm infants. Prenatal and postnatal factors such as exposure to certain medications, hyperoxia and intrauterine and/or extrauterine growth restriction are likely to have a significant influence on nephrogenesis and final nephron endowment. Further studies are required to determine the factors contributing to renal maldevelopment and to identify potential interventional strategies to maximize nephron endowment at the start of life, thereby optimizing long-term renal health for preterm individuals.

Alterations in fatty acid utilization and an impaired antioxidant defense mechanism are early events in podocyte injury: a proteomic analysis

Ultrastructural alterations of podocytes are closely associated with loss of glomerular filtration function. In the present study, we explored changes at the proteome level that paralleled the disturbances of podocyte architecture in the early stages of puromycin aminonucleoside (PA) nephrosis in vivo. Using two-dimensional fluorescence difference gel electrophoresis and vacuum matrix-assisted laser desorption/ionization mass spectrometry combined with postsource decay fragment ion analysis and high-energy collision-induced dissociation tandem mass spectrometry, 23 differentially expressed protein spots, corresponding to 16 glomerular proteins that are involved in various cellular functions, were unambiguously identified, and a subset was corroborated by Western blot analysis. The majority of these proteins were primarily related to fatty acid metabolism and redox regulation. Key enzymes of the mitochondrial beta-oxidation pathway and antioxidant enzymes were consistently down-regulated in PA nephrosis. These changes were paralleled by increased expression levels of CD36. PA treatment of murine podocytes in culture resembled these specific protein changes in vitro. In this cell system, the modulatory effects of albumin-bound fatty acids on the expression levels of Mn-superoxide dismutase in response to PA were demonstrated as well. Taken together, these results indicate that a disrupted fatty acid metabolism in concert with an impaired antioxidant defense mechanism in podocytes may play a role in the early stages of PA-induced lesions in podocytes.

Microalbuminuria and Proximal Tubule Remodeling in the Cardiometabolic Syndrome

Microalbuminuria is a simple screening test that is not only associated with an increased risk of progressive renal insufficiency, but also an increased risk of cardiovascular disease and stroke in the cardiometabolic syndrome. The role of oxidative stress, inflammation, and cellular-extracellular matrix remodeling fibrosis is very important, and the authors have previously observed that albuminuria is related, in part, to loss of the integrity of the glomerular filtration apparatus. The proximal tubule may play a more important role than previously thought, as it is estimated that in health this portion of the nephron reabsorbs 5-8 g of albumin that normally leaks through the glomerulus on a daily basis. Recently, the authors have made important preliminary observational findings regarding proximal tubule microvilli remodeling and oxidative stress, which may help to explain microalbuminuria. These observations suggest that albuminuria is associated with proximal tubule injury, as well as loss of integrity of the glomerular filtration barrier in association with obesity and insulin resistance.

TGF-β1 Regulates the PINCH-1–Integrin-Linked Kinase–α-Parvin Complex in Glomerular Cells

Glomerular damage is a major cause of renal failure. Recent studies suggest that a ternary protein complex that consists of PINCH-1, integrin-linked kinase, and alpha-parvin, cytoplasmic components of cell-extracellular matrix adhesions, plays pivotal roles in regulation of glomerular cell behavior. It is reported here that TGF-beta1, a key factor in the progression of glomerular failure, regulates the PINCH-1-integrin-linked kinase-alpha-parvin (PIP) complex formation in glomerular podocytes and mesangial cells. Treatment of podocytes with TGF-beta1 inhibited the PIP complex formation. Forced disruption of the PIP complex in podocytes activated p38 mitogen-activated protein kinase and promoted apoptosis. Importantly, inhibition of p38 mitogen-activated protein kinase, either with a chemical p38 inhibitor (SB202190) or with a dominant negative form of p38alpha, alleviates podocyte apoptosis that is induced by the disruption of the PIP complex. In contrast to an inhibitory role in podocytes, TGF-beta1 promotes the PIP complex formation in mesangial cells. Thus, TGF-beta1 regulates the PIP complex in a cell type-dependent manner. Because the PIP complex promotes glomerular mesangial matrix deposition and protects podocytes from apoptosis, the TGF-beta1-induced up- and downregulation of the PIP complex likely contribute to the pleiotropic effects of TGF-beta1 on different glomerular cell types and hence the progression of glomerular failure.

Organization of the pronephric filtration apparatus in zebrafish requires Nephrin, Podocin and the FERM domain protein Mosaic eyes

Podocytes are specialized cells of the kidney that form the blood filtration barrier in the kidney glomerulus. The barrier function of podocytes depends upon the development of specialized cell-cell adhesion complexes called slit-diaphragms that form between podocyte foot processes surrounding glomerular blood vessels. Failure of the slit-diaphragm to form results in leakage of high molecular weight proteins into the blood filtrate and urine, a condition called proteinuria. In this work, we test whether the zebrafish pronephros can be used as an assay system for the development of glomerular function with the goal of identifying novel components of the slit-diaphragm. We first characterized the function of the zebrafish homolog of Nephrin, the disease gene associated with the congenital nephritic syndrome of the Finnish type, and Podocin, the gene mutated in autosomal recessive steroid-resistant nephrotic syndrome. Zebrafish nephrin and podocin were specifically expressed in pronephric podocytes and required for the development of pronephric podocyte cell structure. Ultrastructurally, disruption of nephrin or podocin expression resulted in a loss of slit-diaphragms at 72 and 96 h post-fertilization and failure to form normal podocyte foot processes. We also find that expression of the band 4.1/FERM domain gene mosaic eyes in podocytes is required for proper formation of slit-diaphragm cell-cell junctions. A functional assay of glomerular filtration barrier revealed that absence of normal nephrin, podocin or mosaic eyes expression results in loss of glomerular filtration discrimination and aberrant passage of high molecular weight substances into the glomerular filtrate.

The number of podocyte and slit diaphragm is decreased in experimental diabetic nephropathy

PURPOSE

To determine the number of podocyte, slit diaphragms, slit diaphragm extensions and GBM thickness in diabetic nephropathy.

METHODS

Sixty "Rattus Wistar"of both sexes weighing 200-300 g were divided in two experimental groups: normal group 10 animals, and alloxan diabetic rats--50 animals. Alloxan was administered in a single IV dose of 42 mg/kg body weight. Body weight, water and food intake, diuresis, and blood and urine glucose were determined in both groups before alloxan injection and two weeks, six and twelve months after alloxan injection. Proteinuria was measured at 12 months in both groups. After 12 months animals were sacrificed, and the right kidney processed for electron microscopy.

RESULTS

Clear clinical and laboratory signs of severe diabetes were seen, in all alloxan-diabetic rats at all follow-up times. Glomerular basement membrane (GBM) thickening, podocyte number, and slit diaphragm number and extension were determined. GBM of all diabetic rats was significantly thicker (median=0.29 microm; semi-interquartile range=0.065 microm) than in the normal rats (0.23 microm; 0.035 microm). Diabetic rat podocyte number (8; 1), slit diaphragm number (4; 1), and slit diaphragm extension (0.021 microm; 0.00435 microm) were significantly lower than in normal rats (11; 1) and (7; 1.5), and (0.031 microm; 0.0058 microm). Diabetic rat proteinuria (0.060 mg/24 h; 0.037 mg/24 h) was higher than in normal rats (0.00185 mg/24 h; 0.00055 mg/24 h).

CONCLUSION

Experimental diabetes is associated with significant (p<0.05) changes in podocyte foot process, slit number, slit diaphragm extension, and GBM thickness.

Interaction between Rf-1 and Rf-4 quantitative trait loci increases susceptibility to renal damage in double congenic rats

BACKGROUND

Five quantitative trait loci (QTLs), Rf-1 to Rf-5, were found in Fawn-Hooded hypertensive (FHH) rats influencing susceptibility to renal damage. Previously, we found that single transfer of the Rf-1 QTL from FHH rats onto the renal-resistant August x Copenhagen Irish (ACI) strain caused a small increase in renal susceptibility. To investigate the separate role of the Rf-4 QTL and its interaction with Rf-1, we generated a single congenic strain carrying Rf-4 and a double congenic carrying both Rf-1 and Rf-4.

METHODS

Differences in renal susceptibility between ACI, Rf-1A, and Rf-4 single congenics and Rf-1A+4 double congenics were assessed using four different treatments: control (two-kidney), two-kidney with l-arginine analogue N-nitro-l-arginine methyl ester (L-NAME)-induced hypertension, unilateral nephrectomy, and unilateral nephrectomy + L-NAME. In separate experiments, renal blood flow (RBF) autoregulation was compared between two-kidney ACI and congenic rats.

RESULTS

Compared to ACI, Rf-1A rats developed more renal damage, while Rf-4 rats did not. The most severe renal damage was found in the Rf-1A+4 double congenic rats. Analysis of variance (ANOVA) demonstrated a significant interaction between the Rf-1A and Rf-4 QTLs. The magnitude of the interaction varied with the type and duration of the treatment. The RBF autoregulation was impaired in Rf-1A single and Rf-1A+4 double congenics, while in Rf-4 single congenics it was similar to that of ACI controls.

CONCLUSION

These findings indicate that the Rf-1 QTL directly influences renal susceptibility and autoregulation. In contrast, the Rf-4 QTL shows no direct effects, but significantly increases susceptibility to renal damage via an interaction with Rf-1.

Renal redox stress and remodeling in metabolic syndrome, type 2 diabetes mellitus, and diabetic nephropathy: paying homage to the podocyte

Type 2 diabetes mellitus has reached epidemic proportions and diabetic nephropathy is the leading cause of end-stage renal disease. The metabolic syndrome constitutes a milieu conducive to tissue redox stress. This loss of redox homeostasis contributes to renal remodeling and parallels the concurrent increased vascular redox stress associated with the cardiometabolic syndrome. The multiple metabolic toxicities, redox stress and endothelial dysfunction combine to weave the complicated mosaic fabric of diabetic glomerulosclerosis and diabetic nephropathy. A better understanding may provide both the clinician and researcher tools to unravel this complicated disease process. Cellular remodeling of podocyte foot processes in the Ren-2 transgenic rat model of tissue angiotensin II overexpression (TG(mREN-2)27) and the Zucker diabetic fatty model of type 2 diabetes mellitus have been observed in preliminary studies. Importantly, angiotensin II receptor blockers have been shown to abrogate these ultrastructural changes in the foot processes of the podocyte in preliminary studies. An integrated, global risk reduction, approach in therapy addressing the multiple metabolic abnormalities combined with attempts to reach therapeutic goals at an earlier stage could have a profound effect on the development and progressive nature to end-stage renal disease and ultimately renal replacement therapy.

Role of Fat1 in cell-cell contact formation of podocytes in puromycin aminonucleoside nephrosis and neonatal kidney

BACKGROUND

Fat1 is a member of the cadherin superfamily characterized by its 34 cadherin repeats in the extracellular domain. Fat1 was originally found as a component of the slit diaphragm of podocytes, but its function in podocytes remains obscure. To gain insight into its role in podocytes, we expanded our study of Fat1 expression to puromycin aminonucleoside (PAN) nephrosis, the neonatal kidney, and the primary podocyte culture, where slit diaphragms are absent or disappear.

METHODS

Expression of Fat1 was examined in isolated glomeruli of PAN nephrosis by the ribonuclease protection assay and Western blot analysis and in the neonatal kidney by in situ hybridization. Fat1 localization in glomeruli and in the primary culture was confirmed by immunofluorescence or immunoelectron microscopy.

RESULTS

In PAN nephrotic rats, glomerular expression of Fat1 increased rather than decreased at both transcript and protein levels in comparison with normal rats. Immunofluorescence microscopy revealed distinct staining for Fat1 along the glomerular capillary wall, where nephrin staining was weakened or disappeared. Immunoelectron microscopy demonstrated significant accumulation of immunogold particles for Fat1 at intercellular junctions newly formed between podocytes in the nephrosis. In the primary culture of podocytes, Fat1 was mainly localized at cell-cell contact sites and in tips of cellular processes. In the neonatal kidney, immature podocytes expressed Fat1 more intensely than mature podocytes as shown by in situ hybridization. Double-labeled immunostaining using anti-pan cadherin antibody revealed that Fat1 in podocytes colocalized with cadherin in immature glomeruli, indicating that junctional complexes of developing podocytes contain Fat1.

CONCLUSION

These findings suggest that Fat1 may be a fundamental component of intercellular junctions of podocytes, and may be involved in the initial step of cell contacts of podocytes.

Abnormal basement membrane in the inner ear and the kidney of the Mpv17-/- mouse strain: ultrastructural and immunohistochemical investigations

The loss of the function of the peroxisomal Mpv17-protein and associated imbalanced radical oxygen species (ROS) homeostasis leads to an early onset of focal segmental glomerulosclerosis and sensorineural deafness associated with severe degeneration of cochlear structures. An excessive enlargement of basal laminae of the stria vascularis capillaries and glomeruli indicates numerous changes in their molecular composition. The basement membrane (BM) of the glomeruli and the stria vascularis are simultaneously affected in early stages of the disease and the lamination, splitting of the membrane and formation of the "basket weaving" seen at the onset of the disease in the kidney are similar to the ultrastructural alterations characteristic for Alporta9s syndrome. The progressive alteration of the BMs is accompanied by irregularity in the distribution of the collagen IV subunits and by an accumulation of the laminin B2(gamma1) in the inner ear and B(beta1) in the kidney. Since Mpv17 protein contributes to ROS homeostasis, further studies are necessary to elucidate downstream signaling molecules activated by ROS. These studies explain the cellular responses to missing Mpv17-protein, such as accumulation of the extracellular matrix, degeneration, and apoptosis in the inner ear.

The cellular basis of albuminuria

The appearance of albumin in the urine has long been recognized as a cardinal feature of kidney disease and more recently has been shown to also be an independent cardiovascular risk factor associated with insulin resistance. Recent studies on rare human genetic variants, targeted gene disruption in mouse models and cultured glomerular cells in vitro have dramatically improved our understanding of the cellular and molecular basis of albuminuria. This review aims to summarize the current state of knowledge, to illustrate known mechanisms of proteinuria in disease states and to suggest a possible explanation for the link between albuminuria and insulin resistance.

CD151, the first member of the tetraspanin (TM4) superfamily detected on erythrocytes, is essential for the correct assembly of human basement membranes in kidney and skin

Tetraspanins are thought to facilitate the formation of multiprotein complexes at cell surfaces, but evidence illuminating the biologic importance of this role is sparse. Tetraspanin CD151 forms very stable laminin-binding complexes with integrins alpha3beta1 and alpha6beta1 in kidney and alpha3beta1 and alpha6beta4 in skin. It is encoded by a gene at the same position on chromosome 11p15.5 as the MER2 blood group gene. We show that CD151 expresses the MER2 blood group antigen and is located on erythrocytes. We examined CD151 in 3 MER2-negative patients (2 are sibs) of Indian Jewish origin with end-stage kidney disease. In addition to hereditary nephritis the sibs have sensorineural deafness, pretibial epidermolysis bullosa, and beta-thalassemia minor. The 3 patients are homozygous for a single nucleotide insertion (G383) in exon 5 of CD151, causing a frameshift and premature stop signal at codon 140. The resultant truncated protein would lack its integrin-binding domain. We conclude that CD151 is essential for the proper assembly of the glomerular and tubular basement membrane in kidney, has functional significance in the skin, is probably a component of the inner ear, and could play a role in erythropoiesis.