Inducible overexpression of sFlt-1 in podocytes ameliorates glomerulopathy in diabetic mice

Mechanisms of diabetic complications

It is increasingly apparent that not only is a cure for the current worldwide diabetes epidemic required, but also for its major complications, affecting both small and large blood vessels. These complications occur in the majority of individuals with both type 1 and type 2 diabetes. Among the most prevalent microvascular complications are kidney disease, blindness, and amputations, with current therapies only slowing disease progression. Impaired kidney function, exhibited as a reduced glomerular filtration rate, is also a major risk factor for macrovascular complications, such as heart attacks and strokes. There have been a large number of new therapies tested in clinical trials for diabetic complications, with, in general, rather disappointing results. Indeed, it remains to be fully defined as to which pathways in diabetic complications are essentially protective rather than pathological, in terms of their effects on the underlying disease process. Furthermore, seemingly independent pathways are also showing significant interactions with each other to exacerbate pathology. Interestingly, some of these pathways may not only play key roles in complications but also in the development of diabetes per se. This review aims to comprehensively discuss the well validated, as well as putative mechanisms involved in the development of diabetic complications. In addition, new fields of research, which warrant further investigation as potential therapeutic targets of the future, will be highlighted.

Uncoupling of VEGF with endothelial NO as a potential mechanism for abnormal angiogenesis in the diabetic nephropathy

Abnormal angiogenesis is a well characterized complication in diabetic retinopathy and is now recognized as a feature of diabetic nephropathy. The primary growth factor driving the increased angiogenesis in diabetic retinopathy and nephropathy is vascular endothelial growth factor (VEGF). While VEGF is considered an important growth factor for maintaining glomerular capillary integrity and function, increased action of VEGF in diabetic renal disease may carry adverse consequences. Studies by our group suggest that the effects of VEGF are amplified in the setting of endothelial dysfunction and low nitric oxide (NO) levels, which are a common feature in the diabetic state. The lack of NO may amplify the effects of VEGF to induce inflammation (via effects on the macrophage) and may lead to dysregulation of the vasculature, exacerbating features of diabetic renal disease. In this review, we summarize how an "uncoupling" of the VEGF-NO axis may contribute to the pathology of the diabetic kidney.

VEGF and angiopoietins in diabetic glomerulopathy: how far for a new treatment?

Diabetic nephropathy (DN) is the major cause of end-stage renal disease in Western countries and its prevalence continues to increase (United States Renal Data System 2010, http://www.usrds.org/). Treatments currently utilised for DN provide only partial renoprotection, hence the need to identify new targets for therapeutic intervention. Metabolic and haemodynamic abnormalities have been implicated in the pathogenesis of DN, triggering the activation of intracellular signaling molecules that lead to the dysregulation of vascular growth factors and cytokines, such as vascular endothelial growth factor (VEGF) and angiopoietins, important players in the functional and structural regulation of the glomerular filtration barrier. This review focuses on the importance of VEGF-A and angiopoietins in kidney physiology and in the diabetic kidney, exploring their potential therapeutic role in the prevention and delay of diabetic glomerulopathy.

Vegfa protects the glomerular microvasculature in diabetes

Vascular endothelial growth factor A (VEGFA) expression is increased in glomeruli in the context of diabetes. Here, we tested the hypothesis that this upregulation of VEGFA protects the glomerular microvasculature in diabetes and that therefore inhibition of VEGFA will accelerate nephropathy. To determine the role of glomerular Vegfa in the development and progression of diabetic nephropathy, we used an inducible Cre-loxP gene-targeting system that enabled genetic deletion of Vegfa selectively from glomerular podocytes of wild-type or diabetic mice. Type 1 diabetes was induced in mice using streptozotocin (STZ). We then assessed the extent of glomerular dysfunction by measuring proteinuria, glomerular pathology, and glomerular cell apoptosis. Vegfa expression increased in podocytes in the STZ model of diabetes. After 7 weeks of diabetes, diabetic mice lacking Vegfa in podocytes exhibited significantly greater proteinuria with profound glomerular scarring and increased apoptosis compared with control mice with diabetes or Vegfa deletion without diabetes. Reduced local production of glomerular Vegfa in a mouse model of type 1 diabetes promotes endothelial injury accelerating the progression of glomerular injury. These results suggest that upregulation of VEGFA in diabetic kidneys protects the microvasculature from injury and that reduction of VEGFA in diabetes may be harmful.

The endothelium in diabetic nephropathy

The long-term complications of diabetes are characterized by pathologic changes in both the microvasculature and conduit vessels. Although the fenestrated glomerular endothelium classically has been viewed as providing little in the way of an impediment to macromolecular flow, increasing evidence illustrates that this is not the case. Rather, hyperglycemia-mediated endothelial injury may predispose to albuminuria in diabetes both through direct effects and through bidirectional communication with neighboring podocytes. Although neo-angiogenesis of the glomerular capillaries may be a feature of early diabetes, particularly in the experimental setting, loss of capillaries in the glomerulus and in the interstitium are key events that each correlate closely with declining glomerular filtration rate in patients with diabetic nephropathy. The hypoxic milieu that follows the microvascular rarefaction provides a potent stimulus for fibrogenesis, leading to the glomerulosclerosis and tubulointerstitial fibrosis that characterize advanced diabetic kidney disease. Given the pivotal role the endothelium plays in both the development and the progression of diabetic nephropathy we need effective strategies that prevent its loss or accelerate its regeneration. Such advances likely will lead not only to improved tissue oxygenation and reduced fibrosis, but also to improved long-term renal function.

Glomerular VEGF resistance induced by PKCδ/SHP-1 activation and contribution to diabetic nephropathy

This study characterizes the effect of glucose-induced activation of protein kinase Cδ (PKCδ) and Src homology-2 domain-containing phosphatase-1 (SHP-1) expression on vascular endothelial growth factor (VEGF) actions in glomerular podocytes in cultures and in glomeruli of diabetic rodents. Elevation of glucose levels induced PKCδ and p38 mitogen-activated protein kinase (p38 MAPK) to increase SHP-1 expression, increased podocyte apoptosis, and inhibited VEGF activation in podocytes and glomerular endothelial cells. The adverse effects of high glucose levels can be negated by molecular inhibitors of PKCδ, p38MAPK, and SHP-1 and only partially reduced by antioxidants and nuclear factor-κB (NF-κB) inhibitor. Increased PKCδ activation and SHP-1 expression correlated with loss of VEGF signaling and podocyte numbers in the glomeruli of diabetic rats and mice. In contrast, diabetic PKCδ-knockout (Prkcd(-/-)) mice did not exhibit activation of p38 MAPK and SHP-1 or inhibition of VEGF signaling in renal glomeruli. Functionally, diabetic Prkcd(-/-) mice had decreased expressions of TGFβ, VEGF, and extracellular matrix and less albuminuria than diabetic Prkcd(+/+) mice. Hyperglycemia and diabetes can cause glomerular podocyte apoptosis and endothelial dysfunction partly due to increased PKCδ/p38 MAPK activation and the expression of SHP-1 to cause VEGF resistance, independent of NF-κB activation.

Evaluation of placental growth factor and soluble Fms-like tyrosine kinase 1 as predictors of all-cause and cardiovascular mortality in patients with Type 1 diabetes with and without diabetic nephropathy

AIMS

Placental growth factor is a vascular endothelial growth factor involved in angiogenesis, vascular inflammation and plaque formation. Soluble Fms-like tyrosine kinase 1 is a decoy receptor for placental growth factor, reducing its activity. The aim of this study is to evaluate the predictive value of placental growth factor and soluble Fms-like tyrosine kinase 1 in relation to all-cause and cardiovascular mortality and decline in kidney function in Type 1 diabetes.

METHODS

This was a prospective, observational follow-up study with 8 (0-13) years [median (range)] of follow-up, including patients with Type 1 diabetes, of whom 458 had diabetic nephropathy [278 men; age 42 ± 11 years (mean ± sd), diabetes duration 28 ± 9 years, glomerular filtration rate 76 ± 33 ml min(-1) 1.73 m(-2) ] and 442 had long-standing normoalbuminuria (234 men; age 45 ± 12 years, diabetes duration 28 ± 10 years).

RESULTS

Placental growth factor and soluble Fms-like tyrosine kinase 1 levels measured at baseline were higher in patients with diabetic nephropathy compared with patients with long-standing normoalbuminuria [median (range)] 15 (4-131) vs. 11 (7-64) ng/l, (P < 0.001) and 86 (42-3462) vs. 77 (43-1557) ng/l (P < 0.001), respectively. In patients with diabetic nephropathy, high levels of placental growth factor predicted all-cause and cardiovascular mortality [hazard ratio 1.94 (1.16-3.24) and hazard ratio 2.91 (1.45-5.85)] after adjustment for sex, age, smoking, systolic blood pressure, HbA(1c) , cholesterol, glomerular filtration rate and previous cardiovascular disease. High levels of placental growth factor predicted increased risk of end-stage renal disease [hazard ratio 2.77 (1.47-5.14)], but covariate adjustments attenuated the association [hazard ratio 1.89 (0.91-3.95)]. Among patients with long-standing normoalbuminuria, placental growth factor levels predicted fatal and non-fatal cardiovascular events [hazard ratio 1.97 (1.03-3.76)], but not all-cause mortality. Baseline soluble Fms-like tyrosine kinase 1 levels did not predict outcome in either group after adjustment.

CONCLUSION

Placental growth factor is elevated in patients with Type 1 diabetes and diabetic nephropathy and predicts all-cause and cardiovascular mortality, but not deterioration of kidney function.

Vascular endothelial growth factor receptor 2 direct interaction with nephrin links VEGF-A signals to actin in kidney podocytes

The transmembrane protein nephrin is an essential component of slit diaphragms, the specialized cell junctions that link podocyte foot processes. Podocytes are epithelial cells that surround the glomerular capillaries in the kidney and are necessary for the organ-filtering function. Nephrin signaling complex transduces extracellular cues to the podocyte cytoskeleton and regulates podocyte shape and function. Vascular endothelial growth factor A (VEGF-A) is a required growth factor produced and secreted by podocytes. Accumulating evidence suggests a cross-talk between VEGF-A and nephrin signaling pathways. We previously showed that in vivo nephrin associates with VEGF receptor-2 (VEGFR2), the signaling receptor for VEGF-A. In the present work, we characterized the interaction between nephrin and VEGFR2 in cultured cells and in vitro. We demonstrate that nephrin-VEGFR2 interaction is direct using mass spectrometry, immunoprecipitation, GST-binding assays, and blot overlay experiments. This interaction occurs through VEGFR2 and nephrin cytoplasmic domains. Nephrin-VEGFR2 interaction is modulated by tyrosine phosphorylation of both cytoplasmic domains. Furthermore, the nephrin-VEGFR2 complex involves Nck and actin. VEGF-A signaling via this complex results in decreased cell size. We provide evidence that this multiprotein interaction occurs in cultured podocytes. We propose that the nephrin-VEGFR2 complex acts as a key mediator to transduce local VEGF-A signals to the podocyte actin cytoskeleton, regulating the foot process structure and glomerular filter integrity.

Relevance of VEGF and nephrin expression in glomerular diseases

The glomerular filtration barrier is affected in a large number of acquired and inherited diseases resulting in extensive leakage of plasma albumin and larger proteins, leading to nephrotic syndrome and end-stage renal disease. Unfortunately, the molecular mechanisms governing the development of the nephrotic syndrome remain poorly understood. Here, I give an overview of recent investigations that have focused on characterizing the interrelationships between the slit diaphragm components and podocytes-secreted VEGF, which have a significant role for maintaining the normal podocyte structure and the integrity of the filtering barrier.

Induction of podocyte-derived VEGF ameliorates podocyte injury and subsequent abnormal glomerular development caused by puromycin aminonucleoside

Our previous studies using puromycin aminonucleoside (PAN) established that podocyte damage leads to glomerular growth arrest during development and glomerulosclerosis later in life. This study examined the potential benefit of maintaining podocyte-derived VEGF in podocyte defense and survival after PAN injury using conditional transgenic podocytes and mice, in which human VEGF-A (hVEGF) transgene expression is controlled by tetracycline responsive element (TRE) promoter and reverse tetracycline transactivator (rtTA) in podocytes. In vitro experiments used primary cultured podocytes harvested from mice carrying podocin-rtTA and TRE-hVEGF transgenes, in which hVEGF can be induced selectively. Induction of VEGF in PAN-exposed podocytes resulted in preservation of intrinsic VEGF, α-actinin-4 and synaptopodin, antiapoptotic marker Bcl-xL/Bax, as well as attenuation in apoptotic marker cleaved/total caspase-3. In vivo, compared with genotype controls, PAN-sensitive neonatal mice with physiologically relevant levels of podocyte-derived VEGF showed significantly larger glomeruli. Furthermore, PAN-induced up-regulation of desmin, down-regulation of synaptopodin and nephrin, and disruption of glomerular morphology were significantly attenuated in VEGF-induced transgenic mice. Our data indicate that podocyte-derived VEGF provides self-preservation functions, which can rescue the cell after injury and preempt subsequent deterioration of the glomerulus in developing mice.

Podocyte vascular endothelial growth factor (Vegf₁₆₄) overexpression causes severe nodular glomerulosclerosis in a mouse model of type 1 diabetes

AIMS/HYPOTHESIS

The pathogenic role of excessive vascular endothelial growth factor (VEGF)-A in diabetic nephropathy has not been defined. We sought to test whether increased podocyte VEGF-A signalling determines the severity of diabetic glomerulopathy.

METHODS

Podocyte-specific, doxycycline-inducible Vegf₁₆₄ (the most abundant Vegfa isoform) overexpressing adult transgenic mice were made diabetic with low doses of streptozotocin and examined 12 weeks after onset of diabetes. We studied diabetic and non-diabetic transgenic mice fed a standard or doxycycline-containing diet. VEGF-A and albuminuria were measured by ELISA, creatinine was measured by HPLC, renal morphology was examined by light and electron microscopy, and gene expression was assessed by quantitative PCR, immunoblotting and immunohistochemistry.

RESULTS

Podocyte Vegf₁₆₄ overexpression in our mouse model of diabetes resulted in advanced diabetic glomerulopathy, characterised by Kimmelstiel-Wilson-like nodular glomerulosclerosis, microaneurysms, mesangiolysis, glomerular basement membrane thickening, podocyte effacement and massive proteinuria associated with hyperfiltration. It also led to increased VEGF receptor 2 and semaphorin3a levels, as well as nephrin and matrix metalloproteinase-2 downregulation, whereas circulating VEGF-A levels were similar to those in control diabetic mice.

CONCLUSIONS/INTERPRETATION

Collectively, these data demonstrate that increased podocyte Vegf₁₆₄ signalling dramatically worsens diabetic nephropathy in a streptozotocin-induced mouse model of diabetes, resulting in nodular glomerulosclerosis and massive proteinuria. This suggests that local rather than systemic VEGF-A levels determine the severity of diabetic nephropathy and that semaphorin3a signalling and matrix metalloproteinase-2 dysregulation are mechanistically involved in severe diabetic glomerulopathy.

Amelioration of renal alterations in obese type 2 diabetic mice by vasohibin-1, a negative feedback regulator of angiogenesis

The involvement of VEGF-A as well as the therapeutic efficacy of angiogenesis inhibitors in diabetic nephropathy have been reported. We recently reported the therapeutic effects of vasohibin-1 (VASH-1), an endogenous angiogenesis inhibitor, in a type 1 diabetic nephropathy model (Nasu T, Maeshima Y, Kinomura M, Hirokoshi-Kawahara K, Tanabe K, Sugiyama H, Sonoda H, Sato Y, Makino H. Diabetes 58: 2365-2375, 2009). In this study, we investigated the therapeutic efficacy of VASH-1 on renal alterations in obese mice with type 2 diabetes. Diabetic db/db mice received intravenous injections of adenoviral vectors encoding human VASH-1 (AdhVASH-1) and were euthanized 8 wk later. AdhVASH-1 treatment resulted in significant suppression of glomerular hypertrophy, glomerular hyperfiltration, albuminuria, increase in the CD31(+) glomerular endothelial area, F4/80(+) monocyte/macrophage infiltration, the accumulation of type IV collagen, and mesangial matrix. An increase in the renal levels of VEGF-A, VEGFR-2, transforming growth factor (TGF)-β1, and monocyte chemoattractant protein-1 in diabetic animals was significantly suppressed by AdhVASH-1 (immunoblotting). AdhVASH-1 treatment significantly recovered the loss and altered the distribution patterns of nephrin and zonula occludens (ZO)-1 and suppressed the increase in the number of fibroblast-specific protein-1 (FSP-1(+)) and desmin(+) podocytes in diabetic mice. In vitro, recombinant human VASH-1 (rhVASH-1) dose dependently suppressed the upregulation of VEGF induced by high ambient glucose (25 mM) in cultured mouse podocytes. In addition, rhVASH-1 significantly recovered the mRNA levels of nephrin and the protein levels of ZO-1 and P-cadherin and suppressed the increase in protein levels of desmin, FSP-1, Snail, and Slug in podocytes under high-glucose condition. Taken together, these results suggest the potential use of VASH-1 as a novel therapeutic agent in type 2 diabetic nephropathy mediated via antiangiogenic effects and maintenance of podocyte phenotype in association with antiproteinuric effects.

Antagonism of VEGF by genetically engineered dendritic cells is essential to induce antitumor immunity against malignant ascites

Malignant ascitis (MA) is a highly intractable and immunotherapy-resistant state of advanced gastrointestinal and ovarian cancers. Using a murine model of MA with CT26 colon cancer cells, we here determined that the imbalance between the VEGF-A/vascular permeability factor and its decoy receptor, soluble fms-like tryrosine kinase receptor-1 (sFLT-1), was a major cause of MA resistance to dendritic cell (DC)-based immunotherapy. We found that the ratio of VEGF-A/sFLT-1 was increased not only in murine but also in human MA, and F-gene-deleted recombinant Sendai virus (rSeV/dF)-mediated secretion of human sFLT-1 by DCs augmented not only the activity of DCs themselves, but also dramatically improved the survival of tumor-bearing animals associated with enhanced CTL activity and its infiltration to peritoneal tumors. These findings were not seen in immunodeficient mice, indicating that a VEGF-A/sFLT-1 imbalance is critical for determining the antitumor immune response by DC-vaccination therapy against MA.

Rho kinase inhibition protects kidneys from diabetic nephropathy without reducing blood pressure

Rho-associated kinases (ROCK) are activated in the kidney as well as in cultured cells of diabetic models and have been implicated in renal pathophysiology. To explore whether inhibition of ROCK is protective, we studied its role in a model of accelerated diabetic nephropathy where uninephrectomized rats were made diabetic by streptozotocin. After establishing diabetes, rats were treated with the ROCK inhibitor fasudil continuously or for the final 6 weeks of an 18-week experimental period. The results were compared to similar rats given losartan, an established treatment of clinical and experimental diabetic nephropathy, or a combination of both agents. Vehicle-treated diabetic and non-diabetic uninephrectomized rats served as controls. Diabetes resulted in a rapid development of albuminuria, higher glomerulosclerosis and interstitial fibrosis scores, lower glomerular filtration rates, and increased expression of several molecular markers of diabetic nephropathy. Eighteen weeks of fasudil treatment reduced renal ROCK activity, and ameliorated diabetes-induced structural changes in the kidney and expression of the molecular markers in association with a modest anti-proteinuric effect but no change in blood pressure. Late intervention with fasudil reduced glomerulosclerosis, but did not influence proteinuria. Most effects of fasudil were comparable to those of losartan, although losartan lowered blood pressure and further lowered proteinuria. The combination of both treatments was no different than losartan alone. Thus, ROCK inhibition protected the kidney from diabetic nephropathy even though it did not reduce the blood pressure.

Long-term blockade of vascular endothelial growth factor receptor-2 aggravates the diabetic renal dysfunction associated with inactivation of the Akt/eNOS-NO axis

BACKGROUND

Diabetic nephropathy is characterized by abnormal angiogenesis, and this is driven by several factors, including hyperglycaemia and ischaemia. We investigated the role of vascular endothelial growth factor receptor-2 (VEGFR-2) blockade and its effects on diabetic nephropathy.

METHODS

Male db/db and db/m mice received long-term treatment with dRK6, an arginine-rich anti-VEGF hexapeptide, for 12 weeks or short-term treatment for only the first 4 weeks, starting from 8 weeks of age.

RESULTS

The urinary albuminuria and VEGF excretion varied according to the duration of diabetes, and the urinary VEGF levels were strongly correlated with the levels of albuminuria. Diabetes increased the VEGFR-2 expression in the kidneys. At the end of the 12-week study, compared with the db/db control mice, the db/db mice with long-term dRK6 treatment, which selectively inhibited VEGFR-2, had more albuminuria, related to weak nephrin signalling and advanced renal phenotypes, which were associated with hypoxia-oxidative stress, and an increased number of apoptotic endothelial cells. Interestingly, these changes were related to a decrease in phospho-Akt/eNOS-NO bioavailability. On the in vitro study, dRK6 increased the number of apoptotic human umbilical vein endothelial cells (HUVECs) in the high glucose media by blocking phospho-Akt/eNOS-NO signalling, and this was related to the increased oxidative stress. The short-term inhibition of VEGFR-2 neither improved the albuminuria nor the renal phenotype induced by diabetes.

CONCLUSIONS

Long-term selective blockade of VEGFR-2 by dRK6 had deleterious renal effects, and this was associated with downregulation of the Akt/eNOS-NO axis in db/db mice. Short-term VEGFR-2 blockade did not improve the renal phenotypes and the albuminuria. These findings suggest that VEGF-A-VEGFR-2 inhibition, regardless of how long it may be, does not ameliorate diabetic nephropathy in type 2 diabetes.

Glomerular structure and function require paracrine, not autocrine, VEGF-VEGFR-2 signaling

VEGF is a potent vascular growth factor produced by podocytes in the developing and mature glomerulus. Specific deletion of VEGF from podocytes causes glomerular abnormalities including profound endothelial cell injury, suggesting that paracrine signaling is critical for maintaining the glomerular filtration barrier (GFB). However, it is not clear whether normal GFB function also requires autocrine VEGF signaling in podocytes. In this study, we sought to determine whether an autocrine VEGF-VEGFR-2 loop in podocytes contributes to the maintenance of the GFB in vivo. We found that induced, whole-body deletion of VEGFR-2 caused marked abnormalities in the kidney and also other tissues, including the heart and liver. By contrast, podocyte-specific deletion of the VEGFR-2 receptor had no effect on glomerular development or function even up to 6 months old. Unlike cell culture models, enhanced expression of VEGF by podocytes in vivo caused foot process fusion and alterations in slit diaphragm-associated proteins; however, inhibition of VEGFR-2 could not rescue this defect. Although VEGFR-2 was dispensable in the podocyte, glomerular endothelial cells depended on VEGFR-2 expression: postnatal deletion of the receptor resulted in global defects in the glomerular microvasculature. Taken together, our results provide strong evidence for dominant actions of a paracrine VEGF-VEGFR-2 signaling loop both in the developing and in the filtering glomerulus. VEGF produced by the podocyte regulates the structure and function of the adjacent endothelial cell.

Angiogenesis and chronic kidney disease

The number of patients requiring renal replacement therapy due to end-stage renal disease (ESRD) is increasing worldwide. The prevalence of chronic kidney disease (CKD), and the importance of CKD as a risk factor in development of ESRD and in complicating cardiovascular disease (CVD) have been confirmed. In recent years, the involvement of angiogenesis-related factors in the progression of CKD has been studied, and the potential therapeutic effects on CKD of modulating these factors have been identified. Vascular endothelial growth factor (VEGF)-A, a potent pro-angiogenic factor, is involved in the development of the kidney, in maintenance of the glomerular capillary structure and filtration barrier, and in the renal repair process after injury. VEGF-A is also involved in the development of early diabetic nephropathy, demonstrated by the therapeutic effects of anti-VEGF-A antibody. Angiopoietin (Ang)-1 induces the maturation of newly formed blood vessels, and the therapeutic effects of Ang-1 in diabetic nephropathy have been described. In experimental models of diabetic nephropathy, the therapeutic effects of angiogenesis inhibitors, including angiostatin, endostatin and tumstatin peptides, the isocoumarin NM-3, and vasohibin-1, have been reported.

Further analysis of the involvement of angiogenesis-related factors in the development of CKD is required. Determining the disease stage at which therapy is most effective and developing an effective drug delivery system targeting the kidney will be essential for pro-or anti-angiogenic strategies for patients with CKD.

Modulation of notch-1 signaling alleviates vascular endothelial growth factor-mediated diabetic nephropathy

OBJECTIVE

Disturbances in podocytes are typically associated with marked proteinuria, a hallmark of diabetic nephropathy. This study was conducted to investigate modulation of Notch-1 signaling in high glucose (HG)-stressed human podocytes and in a diabetic animal model.

RESEARCH DESIGN AND METHODS

Expression of the Notch signaling components was examined in HG-treated podocytes, human embryonic kidney cells (HEK293), and kidneys from diabetic animals by RT-qPCR, Western blot analysis, and immunohistochemical staining. The association between the Notch signaling, VEGF expression, and podocyte integrity was evaluated.

RESULTS

Notch-1 signaling was significantly activated in HG-cultured human podocytes and HEK293 cells and kidneys from diabetic animals. HG also augmented VEGF expression, decreasing nephrin expression and podocyte number-a critical event for the development of proteinuria in diabetic nephropathy. After use of pharmacological modulators or specific shRNA knockdown strategies, inhibition of Notch-1 signaling significantly abrogated VEGF activation and nephrin repression in HG-stressed cells and ameliorated proteinuria in the diabetic kidney.

CONCLUSIONS

Our findings suggest that upregulation of Notch-1 signaling in HG-treated renal podocytes induces VEGF expression and subsequent nephrin repression and apoptosis. Modulation of Notch-1 signaling may hold promise as a novel therapeutic strategy for the treatment of diabetic nephropathy.

Deletion of von Hippel-Lindau in glomerular podocytes results in glomerular basement membrane thickening, ectopic subepithelial deposition of collagen {alpha}1{alpha}2{alpha}1(IV), expression of neuroglobin, and proteinuria

Vascular endothelial growth factor, which is critical for blood vessel formation, is regulated by hypoxia inducible transcription factors (HIFs). A component of the E3 ubiquitin ligase complex, von Hippel-Lindau (VHL) facilitates oxygen-dependent polyubiquitination and proteasomal degradation of HIFalpha subunits. Hypothesizing that deletion of podocyte VHL would result in HIFalpha hyperstabilization, we crossed podocin promoter-Cre transgenic mice, which express Cre recombinase in podocytes beginning at the capillary loop stage of glomerular development, with floxed VHL mice. Vascular patterning and glomerular development appeared unaltered in progeny lacking podocyte VHL. However, urinalysis showed increased albumin excretion by 4 weeks when compared with wild-type littermates with several sever cases (>1000 microg/ml). Many glomerular ultrastructural changes were seen in mutants, including focal subendothelial delamination and widespread podocyte foot process broadening, and glomerular basement membranes (GBMs) were significantly thicker in 16-week-old mutants compared with controls. Moreover, immunoelectron microscopy showed ectopic deposition of collagen alpha1alpha2alpha1(IV) in GBM humps beneath podocytes. Significant increases in the number of Ki-67-positive mesangial cells were also found, but glomerular WT1 expression was significantly decreased, signifying podocyte death and/or de-differentiation. Indeed, expression profiling of mutant glomeruli suggested a negative regulatory feedback loop involving the HIFalpha prolyl hydroxylase, Egln3. In addition, the brain oxygen-binding protein, Neuroglobin, was induced in mutant podocytes. We conclude that podocyte VHL is required for normal maintenance of podocytes, GBM composition and ultrastructure, and glomerular barrier properties.

Podocyte-specific overexpression of GLUT1 surprisingly reduces mesangial matrix expansion in diabetic nephropathy in mice

Increased expression of the facilitative glucose transporter, GLUT1, leads to glomerulopathy that resembles diabetic nephropathy, whereas prevention of enhanced GLUT1 expression retards nephropathy. While many of the GLUT1-mediated effects are likely due to mesangial cell effects, we hypothesized that increased GLUT1 expression in podocytes also contributes to the progression of diabetic nephropathy. Therefore, we generated two podocyte-specific GLUT1 transgenic mouse lines (driven by a podocin promoter) on a db/m C57BLKS background. Progeny of the two founders were used to generate diabetic db/db and control db/m littermate mice. Immunoblots of glomerular lysates showed that transgenic mice had a 3.5-fold (line 1) and 2.1-fold (line 2) increase in GLUT1 content compared with wild-type mice. Both lines showed similar increases in fasting blood glucose and body weights at 24 wk of age compared with wild-type mice. Mesangial index (percent PAS-positive material in the mesangial tuft) increased 88% (line 1) and 75% (line 2) in the wild-type diabetic mice but only 48% (line 1) and 39% (line 2) in the diabetic transgenic mice (P < 0.05, transgenic vs. wild-type mice). This reduction in mesangial expansion was accompanied by a reduction in fibronectin accumulation, and vascular endothelial growth factor (VEGF) levels increased only half as much in the transgenic diabetic mice as in wild-type diabetic mice. Levels of nephrin, neph1, CD2AP, podocin, and GLUT4 were not significantly different in transgenic compared with wild-type mice. Taken together, increased podocyte GLUT1 expression in diabetic mice does not contribute to early diabetic nephropathy; surprisingly, it protects against mesangial expansion and fibronectin accumulation possibly by blunting podocyte VEGF increases.

Overexpression of VEGF-A in podocytes of adult mice causes glomerular disease

We sought to examine the pathogenic role of excessive VEGF-A expression in podocytes, since it has been reported that diabetic nephropathy and other glomerular diseases are associated with increased VEGF-A expression. The induction of podocyte-specific VEGF164 overexpression in adult transgenic mice led to proteinuria, glomerulomegaly, glomerular basement membrane thickening, mesangial expansion, loss of slit diaphragms, and podocyte effacement. When doxycycline-mediated VEGF164 was stopped, these abnormalities reversed. These findings were associated with reversible downregulation of metalloproteinase 9 and nephrin expression. Using transmission electron microscopy, we established that VEGF-A receptor-2 (VEGFR2) was expressed in podocytes and glomerular endothelial cells. We also found that VEGF164 induced VEGFR2 phosphorylation in podocytes. Further, we were able to co-immunoprecipitate VEGFR2 and nephrin using whole kidney lysates, confirming interaction in vivo. This implies that autocrine and paracrine VEGF-A signaling through VEGFR2 occurs in podocytes and may mediate the glomerular phenotype caused by VEGF164 overexpression. Thus, we suggest that podocyte VEGF164 overexpression in adult mice is sufficient to induce glomerular filtration barrier structural and functional abnormalities similar to those present in murine diabetic nephropathy.

Soluble Flt-1 gene therapy ameliorates albuminuria but accelerates tubulointerstitial injury in diabetic mice

VEGF is recognized as a major mediator in the development of diabetic nephropathy. Soluble Flt-1 (sFlt-1) is the endogenous inhibitor of VEGF, and recently genetic overexpression of sFlt-1 in the podocyte was shown to be protective in murine diabetic nephropathy. In this study, we performed a translational study to determine whether an intramuscular gene transfer of sFlt-1 can prevent the progression of renal disease in diabetic db/db mice. Adeno-associated virus-1 (AAV1) encoding human sFlt-1 in two different doses was intramuscularly administrated in db/db and wild-type mice. The sFlt-1-AAV1 treatment significantly increased serum sFlt-1 level at 4 and 8 wk. A dose that was developed in this study caused minimal abnormalities in normal mice but reduced albuminuria in diabetic db/db mice. In renal histology, sFlt-1 treatment at this dose had minimal effects on mesangial expansion in diabetic mice, whereas podocyte injury was significantly improved, at 8 wk. Unfortunately, tubulointerstitial injury was markedly exacerbated by sFlt-1 treatment in association with a reduction in endogenous VEGF expression and peritubular capillary loss. In conclusion, gene therapy with sFlt-1-AAV1 protects podocytes but accelerates tubulointerstitial injury in diabetic db/db mice. These data suggest systemic overexpression of sFlt-1 will not likely be useful for treating diabetic nephropathy.

Vasohibin-1, a negative feedback regulator of angiogenesis, ameliorates renal alterations in a mouse model of diabetic nephropathy

OBJECTIVE

The involvement of proangiogenic factors such as vascular endothelial growth factor as well as the therapeutic efficacy of angiogenesis inhibitors in early diabetic nephropathy has been reported. Vasohibin-1 (VASH-1) is a unique endogenous angiogenesis inhibitor that is induced in endothelial cells by proangiogenic factors. We investigated the therapeutic efficacy of VASH-1 in an early diabetic nephropathy model.

RESEARCH DESIGN AND METHODS

Streptozotocin- induced type 1 diabetic mice received intravenous injections of adenoviral vectors encoding VASH-1 (AdhVASH-1) or beta-gal (AdLacZ) every other week and were killed after 28 days.

RESULTS

Treatment with AdhVASH-1 resulted in sustained increase in the protein levels of VASH-1 in the liver and sera, in the absence of any inflammatory alterations. AdhVASH-1 treatment significantly suppressed renal hypertrophy, glomerular hypertrophy, glomerular hyperfiltration, albuminuria, increase of the CD31(+) glomerular endothelial area, F4/80(+) monocyte/macrophage infiltration, the accumulation of type IV collagen, and mesangial matrix compared with AdLacZ-treated diabetic mice. Increase in the renal levels of transforming growth factor-beta1, monocyte chemoattractant protein-1, and receptor for advanced glycation end products in diabetic animals was significantly suppressed by AdhVASH-1 (real-time PCR and immunoblot). VASH-1 significantly suppressed the increase of transforming growth factor-beta, monocyte chemoattractant protein-1, and receptor for advanced glycation end products, induced by high ambient glucose in cultured mouse mesangial cells. Increased phosphorylation of VEGFR2 was suppressed in AdVASH-1-treated diabetic animals and in cultured glomerular endothelial cells. Endogenous mouse VASH-1 was localized to the mesangial and endothelial area in glomeruli of diabetic mice.

CONCLUSIONS

These results suggest the potential therapeutic efficacy of VASH-1 in treating early diabetic nephropathy potentially mediated via glomerular endothelial and mesangial cells.

Update on the podocyte

PURPOSE OF REVIEW

The podocyte is a key cell in the selective filtering action of the glomerular capillary wall. Podocyte injury is of pathogenetic and prognostic significance in human glomerular disease; podocyte repair and regeneration are important therapeutic targets. This article will review recent advances in understanding the complex biology, structure and function of the podocyte and the related clinical implications.

RECENT FINDINGS

Recent studies of podocytes in vivo and in vitro are described, highlighting clinically relevant observations and those that help us to understand the ways in which we may harness nature's own mechanisms to repair and/or renew these specialized glomerular cells.

SUMMARY

Drugs that have beneficial effects on podocytes can improve our ability to treat important renal diseases, including diabetic nephropathy. Currently available agents can be harnessed in this way and the rapid progress in the study of podocytes is highlighting new therapeutic targets that can bring even more specificity.