The podocyte: a major player in the development of diabetic nephropathy?

A translational model of chronic diabetic nephropathy in the Nile grass rat

Diabetic nephropathy (DN) is a major healthcare challenge for individuals with diabetes and associated with increased cardiovascular morbidity and mortality. The existing rodent models do not fully represent the complex course of the human disease. Hence, developing a translational model of diabetes that reproduces both the early and the advanced characteristics of DN and faithfully recapitulates the overall human pathology is an unmet need. Here, we introduce the Nile grass rat (NGR) as a novel model of DN and characterize key pathologies underlying DN. NGRs spontaneously developed insulin resistance, reactive hyperinsulinemia, and hyperglycemia. Diabetic NGRs evolved DN and the key histopathological aspects of the human advanced DN, including glomerular hypertrophy, infiltration of mononuclear cells, tubular dilatation, and atrophy. Enlargement of the glomerular tufts and the Bowman's capsule areas accompanied the expansion of the Bowman's space. Glomerular sclerosis, renal arteriolar hyalinosis, Kimmelsteil-Wilson nodular lesions, and protein cast formations in the kidneys of diabetic NGR occurred with DN. Diabetic kidneys displayed interstitial and glomerular fibrosis, key characteristics of late human pathology as well as thickening of the glomerular basement membrane and podocyte effacement. Signs of injury included glomerular lipid accumulation, significantly more apoptotic cells, and expression of KIM-1. Diabetic NGRs became hypertensive, a known risk factor for kidney dysfunction, and showed decreased glomerular filtration rate. Diabetic NGRs recapitulate the breadth of human DN pathology and reproduce the consequences of chronic kidney disease, including injury and loss of function of the kidney. Hence, NGR represents a robust model for studying DN-related complications and provides a new foundation for more detailed mechanistic studies of the genesis of nephropathy, and the development of new therapeutic approaches.

MiR-770-5p facilitates podocyte apoptosis and inflammation in diabetic nephropathy by targeting TIMP3

OBJECTIVE

Diabetic nephropathy (DN) is one of the most severe and frequent diabetic complications. MicroRNAs (miRNAs) have been reported to play a vital role in DN pathogenesis. The present study aimed to investigate the molecular mechanism of miR-770-5p in DN.

METHODS

Podocyte injury model was established by treating mouse podocytes with high glucose (HG, 33 mM) for 24 h. The levels of miR-770-5p and TIMP3 were examined in kidney tissues and podocytes using quantitative real-time PCR (qRT-PCR). Flow cytometry analysis was applied to detect apoptosis in podocytes. Western blot assay was used to measure the protein levels of B-cell lymphoma 2 (Bcl-2), Bcl-2 associated X (Bax) and tissue inhibitors of metalloproteinase 3 (TIMP3). Enzyme-linked immunosorbent assay (ELISA) was conducted to measure the levels of inflammatory factors. The interaction between miR-770-5p and TIMP3 was determined by MicroT-CDS and luciferase reporter assay.

RESULTS

MiR-770-5p was up-regulated and TIMP3 was down-regulated in DN kidney tissues and HG-stimulated podocytes. Depletion of miR-770-5p suppressed cell apoptosis and the release of pro-inflammatory factors in HG-treated podocytes. Additionally, TIMP3 was a target of miR-770-5p in HG-treated podocytes. TIMP3 inhibited cell apoptosis and inflammation in HG-treated podocytes. Moreover, TIMP3 knockdown alleviated the inhibitory effect of miR-770-5p silencing on podocyte apoptosis and inflammatory response.

CONCLUSION

Knockdown of miR-770-5p suppressed podocyte apoptosis and inflammatory response by targeting TIMP3 in HG-treated podocytes, indicating that miR-770-5p may be a potential therapeutic target for DN therapy.

Silencing of USP22 suppresses high glucose-induced apoptosis, ROS production and inflammation in podocytes

Ubiquitin-specific protease 22 (USP22) has been reported to mediate various cellular processes, including cell proliferation and apoptosis. However, its role in high glucose-induced podocytes and diabetic rats remains unknown. In the current study, podocytes were treated with different concentrations of d-glucose to establish a high glucose-induced injury model. Additionally, intravenous tail injection of rats with 65 mg kg(-1) of streptozotocin (STZ) was performed to establish a diabetic rat model. Our findings showed that the treatment of podocytes with high d-glucose significantly increased the USP22 expression level. Silencing of USP22 in podocytes attenuated high d-glucose-induced apoptosis and inflammatory responses, evidenced by increases in proliferation and MMP levels and decreases in the apoptotic rate, ROS production, the Bax/Bcl-2 ratio, caspase-3 expression and secretion of TNF-α, IL-1β, IL-6 and TGF-β1. In addition, podocytes with USP22 overexpression significantly enhanced the effect of high d-glucose-induced apoptosis and inflammatory responses. Similar to the protective effect of USP22 knockdown, resveratrol (RSV) depressed not only high d-glucose- and USP22 overexpression-induced cytotoxicity, but also the secretion of TNF-α, IL-1β, IL-6 and TGF-β1. Notably, silencing of USP22 in diabetic rats conferred a similar protective effect against high glucose-induced apoptosis and inflammation. Taken together, the findings of the present study have demonstrated for the first time that USP22 inhibition attenuates high glucose-induced podocyte injuries and inflammation.

PLK2 Plays an Essential Role in High D-Glucose-Induced Apoptosis, ROS Generation and Inflammation in Podocytes

Diabetic kidney disease (DKD) is a serious complication of hyperglycemia. Currently, there is no effective therapeutic intervention for DKD. In this study, we sought to provide a set of gene profile in diabetic kidneys. We identified 338 genes altered in diabetes-induced DKD glomeruli, and PLK2 exhibited the most dramatic change. Gene set enrichment analysis (GSEA) indicated multiple signaling pathways are involved DKD pathogenesis. Here, we investigated whether PLK2 contributes to podocyte dysfunction, a characteristic change in the development of DKD. High D-glucose (HDG) significantly increased PLK2 expression in mouse podocytes. Suppressing PLK2 attenuated HDG-induced apoptosis and inflammatory responses both in vitro and in vivo. NAC, an antioxidant reagent, rescued HDG and PLK2 overexpression-induced kidney injuries. In summary, we demonstrated that silencing PLK2 attenuates HDG-induced podocyte apoptosis and inflammation, which may serve as a future therapeutic target in DKD.

Glycogen synthase kinase‑3β is required for epithelial‑mesenchymal transition and barrier dysfunction in mouse podocytes under high glucose conditions

Epithelial-mesenchymal transition (EMT) is important for diabetic nephropathy (DN). Podocytes are specialized epithelial cells, which form a major component of the glomerular filtration barrier. Podocyte damage has been suggested to be the primary mechanism behind the albuminuria associated with DN. The present study aimed to determine the function of glycogen synthase kinase (GSK)-3β in EMT and barrier dysfunction of mouse podocytes exposed to high glucose (HG) conditions. Matured and differentiated podocytes were treated with normal glucose (NG), HG or NG + mannitol. Podocytes were also transfected with a small interfering RNA (siRNA) against GSK-3β or a scrambled siRNA, or were treated with lithium chloride (LiCl), a GSK-3β inhibitor, under NG or HG conditions. The expression levels of the epithelial cell markers, nephrin and podocin, and the myofibroblast cell markers, α-smooth muscle actin (SMA) and fibronectin, in podocytes by western blot analysis and immunofluorescence staining, respectively. The monolayer barrier function was assessed by albumin inflow. The phosphorylation and activity levels of GSK-3β were also quantified. It was observed that HG promotes EMT in podocytes, due to the increased levels of podocin and nephrin expression and the reduced α-SMA and fibronectin expression levels. HG also induced barrier dysfunction and increased the expression level of total GSK-3β, Try216-phosphorylated-GSK-3β and the GSK-3β activity in podocytes. Transfection of GSK-3β siRNA or treatment with LiCl reversed the HG-induced EMT and barrier dysfunction in podocytes. In conclusion, the present study determined that GSK-3β is required for EMT and barrier dysfunction in podocytes under HG conditions; therefore, GSK-3β may be a novel target for the treatment of DN.

Silencing of Histone Deacetylase 9 Expression in Podocytes Attenuates Kidney Injury in Diabetic Nephropathy

Podocyte dysfunction is important in the onset and development of diabetic nephropathy (DN). Histone deacetylases (HDACs) have been recently proved to play critical roles in the pathogenesis of DN. As one subtype of the class IIa HDACs, HDAC9 is capable to repress/de-repress their target genes in tumor, inflammation, atherosclerosis and metabolic diseases. In the present study, we investigate whether HDAC9 is involved in the pathophysiologic process of DN, especially the podocyte injury. Firstly, we explored the expression patterns and localization of HDAC9 and found that HDAC9 expression was significantly up-regulated in high glucose (HG)-treated mouse podocytes, as well as kidney tissues from diabetic db/db mice and patients with DN. Secondly, knockdown of HDAC9 in mouse podocytes significantly suppressed HG-induced reactive oxygen species (ROS) generation, cell apoptosis and inflammation through JAK2/STAT3 pathway and reduced the podocytes injury by decreasing the expression levels of Nephrin and Podocin. Moreover, in diabetic db/db mice, silencing of HDAC9 attenuated the glomerulosclerosis, inflammatory cytokine release, podocyte apoptosis and renal injury. Collectively, these data indicate that HDAC9 may be involved in the process of DN, especially podocyte injury. Our study suggest that inhibition of HDAC9 may have a therapeutic potential in DN treatment.

Management of diabetic nephropathy: Recent progress and future perspective

Diabetic nephropathy (DN), a leading cause of end-stage renal disease (ESRD) affecting ∼20-30% diabetics, is associated with increased cardiovascular mortality. The progression of kidney disease in patients with diabetes can take many years. It occurs as a result of interaction between both genetic and environmental factors in individuals with both type 1 and type 2 diabetes. Hyperglycaemia, hypertension, and genetic pre-disposition are the main risk factors besides elevated serum lipids, smoking habits, and the amount of dietary proteins. Interventions such as glycaemic control, blood pressure control and inhibition of the renin-angiotensin-aldosterone system have been shown to slow this progression. Despite the implementation of these strategies, the number of patients with diabetes that ultimately develop end-stage renal disease remains high. The treatment of DN, therefore, has posed a formidable challenge besides optimization of renin-angiotensin-aldosterone system blockade in patients with DN; additional investigation has focused on the potential of novel therapies that target various pathways upregulated by hyperglycaemia or other targets believed to promote the progression of DN such as oxidative stress, inflammation, endothelin system and vitamin D receptors. This review article addresses the pathogenesis and some of the well established principles regarding the progression and accepted management of DN, and also includes the perspectives of novel anti-DN agents and the future directions for the prevention of DN.

Mycophenolate mofetil and valsartan inhibit podocyte apoptosis in streptozotocin-induced diabetic rats

AIM

To investigate the effects of mycophenolate mofetil on apoptosis and the expression of Bax and Bcl-2 of podocytes in rats with diabetic nephropathy.

METHODS

A total of 40 male rats were randomly divided into two groups: healthy control group (NC, n = 8) and diabetic nephropathy group (DN, n = 32). The diabetic rat models were induced by streptozotocin, which was injected intraperitoneally. After the DN model was established successfully, the DN group was subdivided into four groups: (1) group treated with the dissolvent (DN), (2) group treated with mycophenolate mofetil (DN + M), (3) group treated with valsartan (DN + V) and (4) group treated with mycophenolate mofetil and valsartan (DN + MV). After 16 weeks of treatment, kidney and body weight, urinary protein level and serum glucose were measured. Histomorphology of renal tissue was observed by an optical microscope. Apoptosis of podocytes was determined by transferase-mediated dUTP nick-end labeling (TUNEL) test. The expression of nephrin and Wilms' tumor suppressor gene (WT1) were detected by immunohistochemistry and real-time PCR, and the protein expression of Bax and Bcl-2 were examined by Western blot.

RESULTS

The mRNA and protein expression of nephrin and WT1 were both downregulated. The apoptosis rate, the expression of Bax, caspase-3 activity and cleavage increased, while the expression of Bcl-2 decreased in the DN group compared with the NC group. However, they were all improved in the groups treated with either mycophenolate mofetil or valsartan.

CONCLUSION

Mycophenolate mofetil can protect renal function by increasing Bcl-2 expression and decreasing Bax expression and podocyte apoptosis rate, thereby reducing proteinuria.

Apelin retards the progression of diabetic nephropathy

Apelin and its receptor APJ have pleiotropic effects in mice and humans and play a protective role in cardiovascular diseases at least partially by inhibiting oxidative stress. Our objective was to study the effect of apelin on the progression of kidney disease in mice with established type 1 diabetes. Ove26 mice with type 1 diabetes received daily subcutaneous injections of apelin for 2 or 14 wk. APJ localizes in the glomeruli and blood vessels of kidneys. Renal APJ expression was reduced in diabetic mice but increased after treatment with apelin. Apelin treatment did not affect glycemia, body weight, or blood pressure in diabetic mice. Whole kidney and glomerular hypertrophy, as well as renal inflammation, including monocyte chemoattractant protein 1 and vascular cell adhesion molecule 1 expression, NF-κB activation, and monocyte infiltration, was inhibited after short and long treatment with apelin. Apelin administration significantly reduced albuminuria at 6 mo. Short treatment with apelin was sufficient to reverse the downregulation of the antioxidant enzyme catalase. Expression of angiotensin II and angiotensin type 1 receptor (AT1) in kidneys from diabetic mice treated was not affected by apelin. These findings show for the first time that apelin exerts a protective effect on the diabetic kidney. Short administration is sufficient to reduce kidney and glomerular hypertrophy as well as renal inflammation, but prolonged treatment is required to improve albuminuria. This effect was independent of the activation of the renin angiotensin system but correlated with upregulation of the antioxidant catalase. Apelin may represent a novel tool to treat diabetic nephropathy.

Relationship between urinary podocytes and kidney diseases

Podocyte loss is an important component of disease progression in glomerular diseases. To some extent, the loss of podocytes can predict the degree of damage and the advancement of renal disease. Detecting the loss of podocytes in the urine could be a valuable, noninvasive method for obtaining information about the activity of the disease or the disease type, allowing the early diagnosis of glomerular diseases. One of the most robust markers that has been successfully used for urinary podocyte diagnostics is podocalyxin (PDX). PDX is a sialoprotein that is expressed on podocytes and on a variety of nonrenal cells as well as on glomerular endothelial and parietal epithelial cells. Therefore, podocyte loss can be detected by the amount of PDX in the urine. The relationship between urinary podocytes and renal diseases is supported by the detection of podocytes in patients with immunoglobulin A (IgA) nephropathy, Henoch-Schönlein purpura nephritis, lupus nephritis, diabetic nephropathy, and focal segmental glomerulosclerosis. The use of technology for detecting podocytes in the urine would have broad implications for the evaluation of disease activity, the degree of dedifferentiation, and the possibility of regeneration.

Podocyte COX-2 exacerbates diabetic nephropathy by increasing podocyte (pro)renin receptor expression

Diabetic nephropathy (DN) increases podocyte cyclooxygenase-2 (COX-2) expression, and COX-2 inhibition reduces proteinuria and glomerular injury in animal models of diabetes. To investigate the role of podocyte COX-2 in development of diabetic nephropathy, we employed a streptozotocin model of diabetic mellitus in wild-type and transgenic mice expressing COX-2 selectively in podocytes. Progressive albuminuria developed only in diabetic COX-2 transgenic mice despite hyperglycemia, BP, and GFR being similar to those in wild-type mice. Transgenic mice also manifested significant foot-process effacement, moderate mesangial expansion, and segmental thickening of the glomerular basement membrane. In cultured podocytes overexpressing COX-2, high glucose induced cell injury and increased both expression of the pro(renin) receptor and activation of the renin-angiotensin system. Downregulation of the (pro)renin receptor attenuated the injury induced by high glucose. In vivo, podocyte pro(renin) receptor expression increased in diabetic COX-2-transgenic mice, and treatment with a COX-2 inhibitor abrogated the upregulation of (pro)renin receptor and reduced albuminuria, foot-process effacement, and mesangial matrix expansion. In summary, these results demonstrate that increased expression of podocyte COX-2 predisposes to diabetic glomerular injury and that the (pro)renin receptor may be one mediator for this increased susceptibility to injury.

TEM stereometric analyses of glomeruli in aging OVE26 transgenic diabetic mice

BACKGROUND/AIMS

Glomerular lesions in diabetic nephropathy (DN) have been studied in numerous murine diabetic models, but the critical feature of aging is often absent. Since histopathology indicates that in mice, DN glomerular lesions may just begin to develop at about 5 months of age, we utilized the long-lived OVE26 transgenic diabetic model for stereometric analyses of DN glomerulopathic aging.

METHODS

Albuminuria was determined by ELISA, and transmission electron microscopy stereometry was utilized exclusively to demonstrate changes in glomerular cell density and number, and alterations in the glomerular filtration barrier in OVE26 mice at 60, 150, and 450 days of age.

RESULTS

Compared to age-matched controls, albuminuria in diabetic mice is significant at 60 days. At 150 days, glomerular volume and mesangial, endothelial and total cell numbers, and podocyte effacement are significantly increased, while podocyte, endothelial, and total cell density are significantly decreased. Endothelial fenestrations are decreased, and glomerular basement membrane thickness is increased. At 450 days, stereometric alterations are exacerbated.

CONCLUSION

Our data indicate that in OVE26 mice, albuminuria precedes morphological glomerular lesions and could be due to early-onset hyperglycemia. Moreover, in this model, most DN glomerulopathic lesions occur relatively late in life, and it is possible that they may result from prolonged hyperglycemia-induced oxidative stress.

Expression profiles of podocytes exposed to high glucose reveal new insights into early diabetic glomerulopathy

Podocyte injury has been suggested to have a pivotal role in the pathogenesis of diabetic glomerulopathy. To glean insights into molecular mechanisms underlying diabetic podocyte injury, we generated temporal global gene transcript profiles of podocytes exposed to high glucose for a time interval of 1 or 2 weeks using microarrays. A number of genes were altered at both 1 and 2 weeks of glucose exposure compared with controls grown under normal glucose. These included extracellular matrix modulators, cell cycle regulators, extracellular transduction signals and membrane transport proteins. Novel genes that were altered at both 1 and 2 weeks of high-glucose exposure included neutrophil gelatinase-associated lipocalin (LCN2 or NGAL, decreased by 3.2-fold at 1 week and by 7.2-fold at 2 weeks), endothelial lipase (EL, increased by 3.6-fold at 1 week and 3.9-fold at 2 week) and UDP-glucuronosyltransferase 8 (UGT8, increased by 3.9-fold at 1 week and 5.0-fold at 2 weeks). To further validate these results, we used real-time PCR from independent podocyte cultures, immunohistochemistry in renal biopsies and immunoblotting on urine specimens from diabetic patients. A more detailed time course revealed changes in LCN2 and EL mRNA levels as early as 6 hours and in UGT8 mRNA level at 12 hours post high-glucose exposure. EL immunohistochemistry on human tissues showed markedly increased expression in glomeruli, and immunoblotting readily detected EL in a subset of urine samples from diabetic nephropathy patients. In addition to previously implicated roles of these genes in ischemic or oxidative stress, our results further support their importance in hyperglycemic podocyte stress and possibly diabetic glomerulopathy pathogenesis and diagnosis in humans.

Ameliorating effects of L-carnitine on diabetic podocyte injury

High glucose levels can change podocyte gene expression and subsequently induce podocyte damage through altered glucose metabolism. l-Carnitine is known to play a beneficial role in diabetes; however, there are no studies on the effects of l-carnitine on podocyte alteration under high glucose conditions. This study investigated whether l-carnitine can attenuate diabetic podocyte injury through the prevention of loss of slit diaphragm proteins. The l-carnitine treatment group showed increased glucose uptakes compared to the control group, suggesting that glucose utilization in the podocytes was increased by l-carnitine. l-Carnitine treatment also prevented decreased mRNA expressions of nephrin and podocin in the high glucose-stimulated podocytes. However, mRNA expressions of CD2AP and α-actinin-4 were not significantly changed by the high glucose conditions. When these data are taken together, l-carnitine can increase glucose uptake in podocytes under high glucose conditions, and its mechanism may be at least partly related to the up-regulation of nephrin and podocin. Our results help clarify the beneficial effects of l-carnitine in diabetic nephropathy.

The glomerulus--a view from the outside--the podocyte

In the past decade, podocyte research has been greatly aided by the development of powerful new molecular, cellular and animal tools, leading to elucidation of an increasing number of proteins involved in podocyte function and identification of mutated genes in hereditary glomerulopathies. Accumulating evidence indicates that podocyte disorders may not only underlie these hereditary glomerulopathies but also play crucial role in a broad spectrum of acquired glomerular diseases. Genetic susceptibility, environmental influence and systemic responses are all involved in the mediation of the pathogenesis of podocytopathies. Injured podocytes may predisopose to further injury of other podocytes and other adjacent/distant renal cells in a vicious cycle, leading to inexorable progression of glomerular injury. The classic view is that podocytes have a limited ability to proliferate in the normal mature kidney. However, recent research in rodents has provided suggestive evidence for podocyte regeneration resulting from differentiation of progenitor cells within Bowman's capsule.

ADAM10 is expressed in human podocytes and found in urinary vesicles of patients with glomerular kidney diseases

Background

The importance of the Notch signaling in the development of glomerular diseases has been recently described. Therefore we analyzed in podocytes the expression and activity of ADAM10, one important component of the Notch signaling complex.

Methods

By Western blot, immunofluorescence and immunohistochemistry analysis we characterized the expression of ADAM10 in human podocytes, human urine and human renal tissue.

Results

We present evidence, that differentiated human podocytes possessed increased amounts of mature ADAM10 and released elevated levels of L1 adhesion molecule, one well known substrate of ADAM10. By using specific siRNA and metalloproteinase inhibitors we demonstrate that ADAM10 is involved in the cleavage of L1 in human podocytes. Injury of podocytes enhanced the ADAM10 mediated cleavage of L1. In addition, we detected ADAM10 in urinary podocytes from patients with kidney diseases and in tissue sections of normal human kidney. Finally, we found elevated levels of ADAM10 in urinary vesicles of patients with glomerular kidney diseases.

Conclusions

The activity of ADAM10 in human podocytes may play an important role in the development of glomerular kidney diseases.

Mouse models of diabetic nephropathy

Diabetic nephropathy is a major cause of ESRD worldwide. Despite its prevalence, a lack of reliable animal models that mimic human disease has delayed the identification of specific factors that cause or predict diabetic nephropathy. The Animal Models of Diabetic Complications Consortium (AMDCC) was created in 2001 by the National Institutes of Health to develop and characterize models of diabetic nephropathy and other complications. This interim report and our online supplement detail the progress made toward that goal, specifically in the development and testing of murine models. Updates are provided on validation criteria for early and advanced diabetic nephropathy, phenotyping methods, the effect of background strain on nephropathy, current best models of diabetic nephropathy, negative models, and views of future directions. AMDCC investigators and other investigators in the field have yet to validate a complete murine model of human diabetic kidney disease. Nonetheless, the critical analysis of existing murine models substantially enhances our understanding of this disease process.

Podocyte loss in aging OVE26 diabetic mice

Recent studies show that podocyte nuclear density (N(V)) and numbers of renal podocytes per glomerulus (N) are altered in experimental and spontaneous diabetes mellitus. N(V) and N are generally reduced, and it has been hypothesized that these morphological changes may relate to the loss of glomerular permselectivity in diabetic nephropathy (DN). In the current study, OVE26 transgenic diabetic mice and age-matched (FVB) controls (60, 150, or 450 days) were fixed by vascular perfusion and renal cortical tissues were prepared for morphometric analyses. ImageJ software and point counting analyses were carried out on light and transmission electron micrographs to determine glomerular volume (V(G)), N(V), and N. As expected, mean V(G) in OVE26 mice increased substantially ( approximately 134%) over the course of the study and was significantly increased over FVB mice at all ages. At 60 days, N(V) and N were not statistically distinguishable in OVE26 and control mice, while at 150 days, N(V) was significantly reduced in diabetics but not N. In 450-day-old OVE26 animals, however, N(V) and N were both significantly decreased ( approximately 231% and approximately 99%, respectively) relative to age-matched FVB mice. These data suggest that in the OVE26 model of diabetes, significant podocyte loss occurs relatively late in the course of the disease. Moreover, it seems possible that these podocytic changes could play a role in sustaining the increased permeability of the blood-urine barrier in the later stages of diabetic renal decompensation.

Messenger RNA expression of podocyte-associated molecules in the urinary sediment of patients with diabetic nephropathy

BACKGROUND

Podocyte loss plays an important role in the pathogenesis of diabetic nephropathy. We hypothesize that messenger RNA expression of podocyte-associated molecules in urinary sediment may provide important clinical information in patients with diabetic nephropathy.

METHOD

We studied 21 patients with biopsy-proven diabetic nephropathy and 9 healthy controls. The mRNA expression of nephrin, podocin, synaptopodin, Wilms' tumor-1 (WT-1) and alpha-actinin-4 in urinary sediment were measured by real-time quantitative polymerase chain reaction. The degree of histological damage was quantified by morphometric analysis. Patients were then followed for an average of 25.63 +/- 10.76 months. The rate of glomerular filtration rate (GFR) decline was calculated by the least-square regression.

RESULTS

There were significant differences in nephrin, podocin, synaptopodin, alpha-actinin-4 (p < 0.01 for all comparisons) and WT-1 (p = 0.028) expression between patients and normal controls. Urinary nephrin expression was significantly correlated with proteinuria (r = 0.502, p = 0.020); urinary synaptopodin was significantly correlated with proteinuria (r = 0.585, p = 0.005), serum creatinine (r = 0.516, p = 0.017) and estimated GFR (r = -0.560, p = 0.008), and urinary WT-1 expression was significantly correlated with the degree of tubulointerstitial fibrosis (r = 0.558, p = 0.009). There was no significant correlation between GFR decline and urinary expression of target genes.

CONCLUSION

Urinary mRNA expressions of nephrin, podocin, synaptopodin, WT-1 and alpha-actinin-4 are higher in patients with diabetic nephropathy than in normal controls. Urinary nephrin and synaptopodin expressions are correlated with baseline clinical parameters such as proteinuria or renal function, while WT-1 expression is related to the degree of histological damage. Our results suggest that urinary mRNA expression of podocyte-associated molecules may be used for risk stratification of diabetic nephropathy.

New insights into structural patterns encountered in glomerulosclerosis

PURPOSE OF REVIEW

The term 'focal segmental glomerulosclerosis' covers a variety of diseases with different histopathological patterns. There is a need for clues to interpret histological findings in terms of etiology. Studies in transgenic animal models published in recent years have targeted the podocyte with respect to its impact on the development of glomerulosclerosis. Our aim was to survey those models in an attempt to discover correlations between histopathological patterns and pathogenic mechanisms.

RECENT FINDINGS

The most obvious conclusion to draw from recent studies is that virtually all forms of glomerulosclerosis start with a lesion or dysfunction of podocytes. In hereditary glomerular diseases and transgenic animal models, two patterns of glomerular degeneration may be distinguished. All diseases with late onset appear to follow the 'classic' pathway to focal segmental glomerulosclerosis, starting with an adhesion of the tuft to the Bowman's capsule and eventually leading to nephron degeneration. In contrast, those with early onset frequently exhibit changes that indicate a severe dysregulation of podocyte function resulting in diffuse global endocapillary damage (i.e. mesangial expansion and rarefaction of capillaries).

SUMMARY

Such insights derived from animal models might be useful in elucidating the mechanisms of multifactorial human diseases like diabetic glomerulopathy.

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.

Podocyte lesions in patients with obesity-related glomerulopathy

BACKGROUND

Obesity-related glomerulopathy is an important complication of obesity that shares some pathophysiological factors relevant to renal damage with diabetic nephropathy. It was recognized that podocyte lesions lead to proteinuria and glomerulosclerosis in patients with some proteinuric glomerular diseases, especially diabetic nephropathy. However, podocyte changes in patients with obesity-related glomerulopathy and the underlying mechanisms are unclear.

METHODS

Glomerular volume, podocyte number, and foot-process width were evaluated in 46 patients with biopsy-proven obesity-related glomerulopathy. Renal tissue from 10 kidney donors served as controls.

RESULTS

Glomerular volume was enlarged markedly in patients with obesity-related glomerulopathy (P < 0.001), with a corresponding decrease in podocyte density (P < 0.001) compared with controls. Foot-process width on the peripheral glomerular basement membrane was increased significantly, whereas mesangial volume fraction did not differ between patients with obesity-related glomerulopathy and controls. Degree of proteinuria was associated strongly with decreased podocyte density (P = 0.001), increased foot-process width (P = 0.001), and decreased podocyte number (P = 0.002). In addition, the endogenous creatinine clearance rate correlated with decreased podocyte number (P = 0.008). Interestingly, podocyte number and density correlated significantly with abnormalities in fasting glucose (P < 0.001) and insulin levels (P < 0.001) and Homeostasis Model Assessment of Insulin Resistance (P < 0.001).

CONCLUSION

Decreased podocyte density and number were observed in patients with obesity-related glomerulopathy, and changes in podocytes correlated with degree of proteinuria and renal function impairment in these patients. In addition to the enlargement in glomerular volume with consequential mechanical stretch, metabolic disorders also may contribute to the development of podocyte lesions in patients with obesity-related glomerulopathy.

Regulation of renal fatty acid and cholesterol metabolism, inflammation, and fibrosis in Akita and OVE26 mice with type 1 diabetes

In Akita and OVE26 mice, two genetic models of type 1 diabetes, diabetic nephropathy is characterized by mesangial expansion and loss of podocytes, resulting in glomerulosclerosis and proteinuria, and is associated with increased expression of profibrotic growth factors, proinflammatory cytokines, and increased oxidative stress. We have also found significant increases in renal triglyceride and cholesterol content. The increase in renal triglyceride content is associated with 1) increased expression of sterol regulatory element-binding protein (SREBP)-1c and carbohydrate response element-binding protein (ChREBP), which collectively results in increased fatty acid synthesis, 2) decreased expression of peroxisome proliferator-activated receptor (PPAR)-alpha and -delta, which results in decreased fatty acid oxidation, and 3) decreased expression of farnesoid X receptor (FXR) and small heterodimer partner (SHP). The increase in cholesterol content is associated with 1) increased expression of SREBP-2 and 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase, which results in increased cholesterol synthesis, and 2) decreased expression of liver X receptor (LXR)-alpha, LXR-beta, and ATP-binding cassette transporter-1, which results in decreased cholesterol efflux. Our results indicate that in type 1 diabetes, there is altered renal lipid metabolism favoring net accumulation of triglycerides and cholesterol, which are driven by increases in SREBP-1, ChREBP, and SREBP-2 and decreases in FXR, LXR-alpha, and LXR-beta, which may also play a role in the increased expression of profibrotic growth hormones, proinflammatory cytokines, and oxidative stress.

Altered vitamin D metabolism in type II diabetic mouse glomeruli may provide protection from diabetic nephropathy

The db/db mouse develops features of type II diabetes mellitus as the result of impaired signaling through its abnormal leptin receptor. In spite of accurate metabolic features of diabetes, renal disease manifestations in these mice are not as severe as in humans suggesting the presence of protective genes. There is a growing body of evidence in humans for the relevance of vitamin D in diabetes. Here we followed a large cohort of db/db mice and their non-diabetic db/+ littermates. Transcriptional profiling revealed significant upregulation of 23 genes involved in Ca2+ homeostasis and vitamin D metabolism in db/db glomeruli relative to db/+ glomeruli. Increased glomerular expression of vitamin D3 1alpha-hydroxylase, vitamin D binding protein, calbindins D9K and D28K, and calcyclin mRNA was confirmed by quantitative reverse transcription-polymerase chain reaction in 20-, 36-, and 52-week-old db/db glomeruli. Although vitamin D3 1alpha-hydroxylase protein was primarily expressed and upregulated in db/db renal tubules, it was also expressed in glomerular podocytes in vivo. Serum 1,25-dihydroxyvitamin D3 and urinary Ca2+ excretion were increased >3-fold in db/db mice compared to db/+ mice. Cultured glomerular podocytes had mRNA for vitamin D3 1alpha-hydroxylase, vitamin D receptor, and calbindin D28K, each of which was increased in high glucose conditions. High glucose also led to enhanced production of fibronectin and collagen IV protein, which was blocked by 1,25-dihydroxyvitamin D3. These results show that vitamin D metabolism is altered in db/db mice leading to metabolic and transcriptional effects. The podocyte is affected by paracrine and potentially autocrine effects of vitamin D, which may explain why db/db mice are resistant to progressive diabetic nephropathy.

Influence of C-peptide on early glomerular changes in diabetic mice

BACKGROUND

C-peptide has been shown to ameliorate diabetes-induced functional and structural renal changes in animal models as well as in patients with type 1 diabetes. This study aims to examine the molecular effects of C-peptide on early glomerular changes in a mouse model of type 1 diabetes.

METHODS

Fourteen days after induction of diabetes by streptozotocin (STZ), the animals received rat C-peptide for either 24 h or 7 days. Urinary albumin excretion was measured by ELISA. Glomerular mRNA expression of the transforming growth factor (TGF)-beta(1) and type IV collagen was quantified by real-time PCR. The effect of C-peptide on type IV collagen gene expression in cultured murine podocytes was also examined.

RESULTS

C-peptide decreased urinary albumin excretion from 0.29 to 0.18 microg/min (-40.7%, P < 0.01). The transcript level of (alpha3)IV collagen in glomeruli was up-regulated 2.2-fold in diabetic mice and was inhibited by 45-70% (P < 0.05) upon C-peptide treatment. C-peptide suppressed glomerular expression of TGF-beta(1) by 36.6% after 7 days (P < 0.05) but not 24 h after injection. In vitro studies using cultured podocytes revealed that C-peptide dose-dependently inhibited TGF-beta-induced up-regulation of type IV collagen. Moreover, both pertussis toxin (PTX) and a specific inhibitor for extracellular signal-regulated kinase (ERK) pathway reversed the inhibitory effect of C-peptide on TGF-beta. Finally, C-peptide was shown to up-regulate the activity of ERK in podocytes.

CONCLUSIONS

These findings indicate that C-peptide suppresses specific aspects of early glomerular changes in a mouse model of diabetes and that the effect is at least in part mediated via interaction with the TGF-beta signal in glomerular podocytes.

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.