Nef interaction with actin compromises human podocyte actin cytoskeletal integrity

The HIV-1 accessory protein Nef is considered to play an important role in the development of a podocyte phenotype in HIV-1 associated nephropathy. We hypothesized that Nef may be altering the podocyte phenotype both structurally and functionally. To elucidate the involved mechanisms, podocyte proteins interacting with Nef were identified using GST pull down assay and yeast two hybrid assay. The GST pull down assay on protein extracts made from stable colonies of conditionally immortalized human podocytes expressing Nef (Nef/CIHP) displayed a band at 45 kD, which was identified as actin by mass spectrometry. Yeast two hybrid assay identified the following Nef-interacting proteins: syntrophin, filamin B, syntaxin, translational elongation factor 1, and zyxin. The Nef-actin and Nef-zyxin interactions were confirmed by co-localization studies on Nef/CIHP stable cell lines. The co-localization studies also showed that Nef/CIHP stable cell lines had a decreased number of actin filaments (stress fibers), displayed formation of lamellipodia, and increased number of podocyte projections (filopodia). Nef/CIHP displayed an enhanced cortical F-actin score index (P<0.001) and thus indicated a reorganization of F-actin in the cortical regions. Microarray analysis showed that Nef enhanced the expression of Rac1, syndecan-4, Rif, and CDC42 and attenuated the expression of syndecan-3 and syntenin. In addition, Nef/CIHPs displayed a diminished sphingomyelinase (ASMase) activity. Functionally, Nef/CIHPs displayed diminished attachment and enhanced detachment to their substrate. These findings indicate that Nef interaction with actin compromises the podocyte cytoskeleton integrity.

Exogenous vascular endothelial growth factor supplementation can restore the podocyte barrier-forming capacity disrupted by sera of preeclamptic women

AIM

To determine whether exogenous vascular endothelial growth factor (VEGF) supplementation to sera from women with preeclampsia can restore the podocyte barrier-forming capacity disrupted after stimulation with sera from these women.

MATERIAL AND METHODS

Enzyme-linked immunosorbent assays (ELISAs) for sFlt-1 (soluble VEGF receptor 1) and VEGF were performed in the sera of preeclamptic and healthy pregnant women. VEGF-enriched preeclamptic serum was used to evaluate the capacity of such sera to restore the diminished podocyte barrier capacity secondary to stimulation with untreated preeclamptic sera.

RESULTS

Two groups, preeclampsia (n=15) and healthy (n=15) pregnant women, were established. Median serum levels (pg/mL) of sFlt-1 and VEGF were significantly different between groups: 3987 versus 1432 and 6 versus 15.6 (P<0.001). Altered resistance values of podocytes stimulated with sera from women with preeclampsia were recovered after exogenous VEGF supplementation.

CONCLUSION

The mean resistance value of cultured podocytes, an indicator of the integrity of the cytoskeleton and the capacity of these cells to form layers, is altered in women with preeclampsia. High levels of sFlt-1, an inhibitor of VEGF observed in high concentrations in the serum of these patients, could be the underlying cause of this alteration. We provide evidence that exogenous VEGF supplementation of the sera of these women could recover this indicator.

IQGAP1 interacts with components of the slit diaphragm complex in podocytes and is involved in podocyte migration and permeability in vitro

IQGAP1 is a scaffold protein that interacts with proteins of the cytoskeleton and the intercellular adhesion complex. In podocytes, IQGAP1 is associated with nephrin in the glomerular slit diaphragm (SD) complex, but its role remains ill-defined. In this work, we investigated the interaction of IQGAP1 with the cytoskeleton and SD proteins in podocytes in culture, and its role in podocyte migration and permeability. Expression, localization, and interactions between IQGAP1 and SD or cytoskeletal proteins were determined in cultured human podocytes by Western blot (WB), immunocytolocalization (IC), immunoprecipitation (IP), and In situ Proximity Ligation assay (IsPL). Involvement of IQGAP1 in migration and permeability was also assessed. IQGAP1 expression in normal kidney biopsies was studied by immunohistochemistry. IQGAP1 expression by podocytes increased during their in vitro differentiation. IC, IP, and IsPL experiments showed colocalizations and/or interactions between IQGAP1 and SD proteins (nephrin, MAGI-1, CD2AP, NCK 1/2, podocin), podocalyxin, and cytoskeletal proteins (α-actinin-4). IQGAP1 silencing decreased podocyte migration and increased the permeability of a podocyte layer. Immunohistochemistry on normal human kidney confirmed IQGAP1 expression in podocytes and distal tubular epithelial cells and also showed an expression in glomerular parietal epithelial cells. In summary, our results suggest that IQGAP1, through its interaction with components of SD and cytoskeletal proteins, is involved in podocyte barrier properties.

Current concepts of the podocyte in nephrotic syndrome

Nephrotic syndrome is a disorder of the glomerular filtration barrier, and central to the filtration mechanism of the glomerular filtration barrier is the podocyte. We are starting to better understand how this cell, with its unique architectural features, fulfils its exact filtration properties. The multiprotein complex between adjacent podocyte foot processes, the slit diaphragm, is essential to the control of the actin cytoskeleton and cell morphology. Many of the proteins within the slit diaphragm, including nephrin, podocin, transient receptor potential-6 channel, and α-actinin-4, have been identified via genetic studies of inherited nephrotic syndromes. Signaling from slit diaphragm proteins to the actin cytoskeleton is mediated via the Rho GTPases. These are thought to be involved in the control of podocyte motility, which has been postulated as a focus of proteinuric pathways. Nephrotic syndrome is currently treated with immunosuppressive therapy, with significant adverse effects. These therapies may work in nephrotic syndrome due to specific effects on the podocytes. This review aims to describe our current understanding of the cellular pathways and molecules within the podocyte relevant to nephrotic syndrome and its treatment. With our current knowledge of the cellular biology of the podocyte, there is much hope for targeted therapies for nephrotic syndromes.

Familial focal segmental glomerulosclerosis (FSGS)-linked α-actinin 4 (ACTN4) protein mutants lose ability to activate transcription by nuclear hormone receptors

Mutations in α-actinin 4 (ACTN4) are linked to familial forms of focal segmental glomerulosclerosis (FSGS), a kidney disease characterized by proteinuria due to podocyte injury. The mechanisms underlying ACTN4 mutant-associated FSGS are not completely understood. Although α-actinins are better known to cross-link actin filaments and modulate cytoskeletal organization, we have previously shown that ACTN4 interacts with transcription factors including estrogen receptor and MEF2s and potentiates their transcriptional activity. Nuclear receptors including retinoic acid receptor (RAR) have been proposed to play a protective role in podocytes. We show here that ACTN4 interacts with and enhances transcriptional activation by RARα. In addition, FSGS-linked ACTN4 mutants not only mislocalized to the cytoplasm, but also lost their ability to associate with nuclear receptors. Consequently, FSGS-linked ACTN4 mutants failed to potentiate transcriptional activation by nuclear hormone receptors in podocytes. In addition, overexpression of these mutants suppressed the transcriptional activity mediated by endogenous wild-type ACTN4 possibly by a cytoplasmic sequestration mechanism. Our data provide the first link between FSGS-linked ACTN4 mutants and transcriptional activation by nuclear receptor such as RARα and peroxisome proliferator-activated receptor γ.

Nicorandil as a novel therapy for advanced diabetic nephropathy in the eNOS-deficient mouse

Nicorandil is an orally available drug that can act as a nitric oxide donor, an antioxidant, and an ATP-dependent K channel activator. We hypothesized that it may have a beneficial role in treating diabetic nephropathy. We administered nicorandil to a model of advanced diabetic nephropathy (the streptozotocin-induced diabetes in mice lacking endothelial nitric oxide synthase, eNOSKO); controls included diabetic eNOS KO mice without nicorandil and nondiabetic eNOS KO mice treated with either nicorandil or vehicle. Mice were treated for 8 wk. Histology, blood pressure, and renal function were determined. Additional studies involved examining the effects of nicorandil on cultured human podocytes. Here, we found that nicorandil did not affect blood glucose levels, blood pressure, or systemic endothelial function, but significantly reduced proteinuria and glomerular injury (mesangiolysis and glomerulosclerosis). Nicorandil protected against podocyte loss and podocyte oxidative stress. Studies in cultured podocytes showed that nicorandil likely protects against glucose-mediated oxidant stress via the ATP-dependent K channel as opposed to its NO-stimulating effects. In conclusion, nicorandil may be beneficial in diabetic nephropathy by preserving podocyte function. We recommend clinical trials to determine whether nicorandil may benefit diabetic nephropathy or other conditions associated with podocyte dysfunction.

WT1 mutants reveal SRPK1 to be a downstream angiogenesis target by altering VEGF splicing

Angiogenesis is regulated by the balance of proangiogenic VEGF(165) and antiangiogenic VEGF(165)b splice isoforms. Mutations in WT1, the Wilms' tumor suppressor gene, suppress VEGF(165)b and cause abnormal gonadogenesis, renal failure, and Wilms' tumors. In WT1 mutant cells, reduced VEGF(165)b was due to lack of WT1-mediated transcriptional repression of the splicing-factor kinase SRPK1. WT1 bound to the SRPK1 promoter, and repressed expression through a specific WT1 binding site. In WT1 mutant cells SRPK1-mediated hyperphosphorylation of the oncogenic RNA binding protein SRSF1 regulated splicing of VEGF and rendered WT1 mutant cells proangiogenic. Altered VEGF splicing was reversed by wild-type WT1, knockdown of SRSF1, or SRPK1 and inhibition of SRPK1, which prevented in vitro and in vivo angiogenesis and associated tumor growth.

Effect of Different Carriers on in vitro Permeation of Meloxicam through Rat Skin

The ability of β-cyclodextrin, hydroxypropyl-β-cyclodextrin, polyvinyl pyrrolidone and urea to influence the percutaneous absorption of meloxicam through isolated rat skin was evaluated. Carrier complex were prepared by kneading method in 1:1 and 1:2 in molar ratios for β-cyclodextrin and hydroxypropyl-β-cyclodextrin and in 1:1, 1:3 and 1:5 in weight ratios for polyvinyl pyrrolidone and urea. The complexes were characterized by IR, DSC and evaluated for solubility, dissolution and skin permeability. The solubility, dissolution and permeability of meloxicam were enhanced by using the carriers. The influence of cyclodextrins, polyvinyl pyrrolidone and urea on in vitro permeation of meloxicam through rat skin was investigated by incorporation of prepared carrier complex in 1% carbopol gel. The prepared gel was evaluated for drug content, pH and viscosity and in vitro permeation. All the percutaneous parameters like flux (Jss), amount permeated (Q₆), diffusivity (D), permeability coefficient (K_p), partition coefficient (K) and release rate constant (k) were calculated statistically. In vitro permeation study showed the trend that the penetration flux and enhancement factor increases with increasing concentration of β-cyclodextrin and hydroxypropyl-β-cyclodextrin and then decrease dramatically in case of hydroxypropyl-β-cyclodextrin gel formulation with the increase to 1:2 ratio. Similar changes in pattern of permeation were also observed with polyvinyl pyrrolidone and urea carrier complex. These findings concluded that the carriers cyclodextrins, polyvinyl pyrrolidone and urea could be used as transdermal permeation enhancer in topical preparation of meloxicam.

Prorenin receptor is essential for normal podocyte structure and function

The prorenin receptor is an accessory subunit of the vacuolar H(+)-ATPase, suggesting that it has fundamental functions beyond activation of the local renin-angiotensin system. Podocytes express the prorenin receptor, but its function in these cells is unknown. Here, podocyte-specific, conditional, prorenin receptor-knockout mice died of kidney failure and severe proteinuria within 4 weeks of birth. The podocytes of these mice exhibited foot process effacement with reduced and altered localization of the slit-diaphragm proteins nephrin and podocin. Furthermore, the podocytes contained numerous autophagic vacuoles, confirmed by enhanced accumulation of microtubule-associated protein 1 light chain 3-positive intracellular vesicles. Ablation of the prorenin receptor selectively suppressed expression of the V(0) c-subunit of the vacuolar H(+)-ATPase in podocytes, resulting in deacidification of intracellular vesicles. In conclusion, the prorenin receptor is important for the maintenance of normal podocyte structure and function.

Transcription of nephrin-Neph3 gene pair is synergistically activated by WT1 and NF-κB and silenced by DNA methylation

BACKGROUND

Nephrin and Neph3 are homologous molecules expressed in the podocyte slit diaphragms that are essential for normal glomerular ultrafiltration. Nephrin and Neph3 genes form a bidirectional gene pair suggesting that they may share key features in their regulation. We investigated if nephrin and Neph3 genes have similar mechanisms in their transcriptional regulation focussing on transcription factor Wilms' tumour 1 (WT1) and nuclear factor-κB (NF-κB) and DNA methylation.

METHODS

Transcriptional regulation of nephrin and Neph3 by WT1 and NF-κB was analysed by overexpression studies, reporter gene assay and chromatin immunoprecipitation using A293 cells and cultured podocytes. The interaction between WT1 and NF-κB was studied by co-immunoprecipitation. The effect of NF-κB activator tumour necrosis factor-α (TNF-α) with or without NF-κB pathway inhibitor (BAY 11-7082) on nephrin and Neph3 messenger RNA (mRNA) expression and on cellular distribution of NF-κB was determined by quantitative polymerase chain reaction (PCR) and immunostaining, respectively. The role of DNA methylation in regulating nephrin and Neph3 genes was studied by demethylating agent (5-aza-2'-deoxycytidine) treatment and quantitative PCR.

RESULTS

WT1 and NF-κB interact with nephrin and Neph3 promoter and cooperatively regulate nephrin and Neph3. The cooperation was further supported by the physical interaction between WT1 and NF-κB. TNF-α increased nephrin and Neph3 mRNA expression and this effect was mediated by NF-κB. Furthermore, DNA methylation played a role in silencing nephrin and Neph3 expression in a cell-type and differentiation stage-dependent manner.

CONCLUSION

These results provide novel insights into the transcriptional regulation of nephrin and Neph3 genes and indicate that nephrin and Neph3 share the same mechanisms in their regulation.

Receptor-mediated endocytosis of α-galactosidase A in human podocytes in Fabry disease

Injury to the glomerular podocyte is a key mechanism in human glomerular disease and podocyte repair is an important therapeutic target. In Fabry disease, podocyte injury is caused by the intracellular accumulation of globotriaosylceramide. This study identifies in the human podocyte three endocytic receptors, mannose 6-phosphate/insulin-like growth II receptor, megalin, and sortilin and demonstrates their drug delivery capabilities for enzyme replacement therapy. Sortilin, a novel α-galactosidase A binding protein, reveals a predominant intracellular expression but also surface expression in the podocyte. The present study provides the rationale for the renal effect of treatment with α-galactosidase A and identifies potential pathways for future non-carbohydrate based drug delivery to the kidney podocyte and other potential affected organs.

Monocyte- and endothelial-derived microparticles induce an inflammatory phenotype in human podocytes

BACKGROUND/AIMS

Proteinuria is associated with cardiovascular and chronic kidney disease. Microparticles (MPs) are bioactive vesicles shed from activated cells and also linked to cardiovascular disease. MP-like structures have been identified in the glomerular basement membrane, urinary space and between the glomerular basement membrane and the podocyte. We hypothesised that circulating MPs may provide a link between vascular injury and kidney diseases by inducing podocyte phenotypic alterations, thus propagating glomerular dysfunction and proteinuria.

METHODS

Human umbilical vein endothelial cells and U937 monocytes were stimulated with TNF-α to produce MPs. These MPs were confirmed by electron microscopy, and added to differentiated podocyte monolayers to determine effects on podocyte albumin endocytosis and the production of soluble mediators.

RESULTS

Monocyte and endothelial MPs upregulated podocyte production of pro-inflammatory mediators monocyte chemoattractant protein-1 (p < 0.001) and interleukin-6 (p < 0.001). Only monocyte MPs upregulated podocyte secretion of VEGF (p < 0.001), known to regulate glomerular permeability. Endothelial MPs decreased podocyte albumin endocytosis by 13% compared to control cells (p < 0.01).

CONCLUSION

MPs alter endocytic functions of podocytes and induce secretion of pro-inflammatory cytokines, potentially leading to glomerular inflammation in vivo and the development of proteinuria. This study identifies a potential pathophysiological role for circulating MPs in the kidney through effects on the podocyte.

Formulation and Evaluation of Tramadol hydrochloride Rectal Suppositories

Rectal suppositories of tramadol hydrochloride were prepared using different bases and polymers like PEG, cocoa butter, agar and the effect of different additives on in vitro release of tramadol hydrochloride was studied. The agar-based suppositories were non-disintegrating/non-dissolving, whereas PEGs were disintegrating/dissolving and cocoa butter were melting suppositories. All the prepared suppositories were evaluated for various physical parameters like weight variation, drug content and hardness. The PEG and cocoa butter suppositories were evaluated for macromelting range, disintegration and liquefaction time. In vitro release study was performed by USP type I apparatus. The prepared suppositories were within the permissible range of all physical parameters. In vitro drug release was in the order of PEG>Agar>cocoa butter. Addition of PVP, HPMC in agar suppositories retards the release. The mechanism of drug release was diffusion controlled and follows first order kinetics. The results suggested that blends of PEG of low molecular weight (1000) with high molecular weight (4000 and 6000) in different percentage and agar in 10% w/w as base used to formulate rapid release suppositories. The sustained release suppositories can be prepared by addition of PVP, HPMC in agar-based suppositories and by use of cocoa butter as base.

An in vitro model of the glomerular capillary wall using electrospun collagen nanofibres in a bioartificial composite basement membrane

The filtering unit of the kidney, the glomerulus, contains capillaries whose walls function as a biological sieve, the glomerular filtration barrier. This comprises layers of two specialised cells, glomerular endothelial cells (GEnC) and podocytes, separated by a basement membrane. Glomerular filtration barrier function, and dysfunction in disease, remains incompletely understood, partly due to difficulties in studying the relevant cell types in vitro. We have addressed this by generation of unique conditionally immortalised human GEnC and podocytes. However, because the glomerular filtration barrier functions as a whole, it is necessary to develop three dimensional co-culture models to maximise the benefit of the availability of these cells. Here we have developed the first two tri-layer models of the glomerular capillary wall. The first is based on tissue culture inserts and provides evidence of cell-cell interaction via soluble mediators. In the second model the synthetic support of the tissue culture insert is replaced with a novel composite bioartificial membrane. This consists of a nanofibre membrane containing collagen I, electrospun directly onto a micro-photoelectroformed fine nickel supporting mesh. GEnC and podocytes grew in monolayers on either side of the insert support or the novel membrane to form a tri-layer model recapitulating the human glomerular capillary in vitro. These models will advance the study of both the physiology of normal glomerular filtration and of its disruption in glomerular disease.

Establishment of conditionally immortalized human glomerular mesangial cells in culture, with unique migratory properties

The aim of this study was to establish an immortalized human mesangial cell line similar to mesangial cells in vivo for use as a tool for understanding glomerular cell function. Mesangial cells were isolated from glomerular outgrowths from a normal human kidney, then retrovirally transfected with a temperature-sensitive SV40T antigen+human telomerase (hTERT). Mesangial cells exhibited features of compact cells with small bodies in a confluent monolayer at 33°C, but the cell shape changed to flat and stellate after 5 days in growth-restrictive conditions (37°C). Western blot and immunofluorescence analysis showed that podocyte markers (nephrin, CD2AP, podocin, Wilms' tumor-1) and an endothelial-specific molecule (VE-cadherin) were not detectable in this cell line, whereas markers characteristic of mesangial cells (α-SMA, fibronectin, and PDGFβ-R) were strongly expressed. In migration assays, a significant reduction in wound surface was observed in podocyte and endothelial cells as soon as 12 h (75 and 62%, respectively) and complete wound closure after 24 h. In contrast, no significant change was observed in mesangial cells after 12 h, and even after 48 h the wounds were not completely closed. Until now, conditionally immortalized podocyte and endothelial cell lines derived from mice and humans have been described, and this has greatly boosted research on glomerular physiology and pathology. We have established the first conditionally immortalized human glomerular mesangial cell line, which will be an important adjunct in studies of representative glomerular cells, as well as in coculture studies. Unexpectedly, mesangial cells' ability to migrate seems to be slower than for other glomerular cells, suggesting this line will demonstrate functional properties distinct from previously available mesangial cell cultures. This conditionally immortalized human mesangial cell line represents a new tool for the study of human mesangial cell biology in vitro.

Toll-like receptor 3 ligands induce CD80 expression in human podocytes via an NF-κB-dependent pathway

BACKGROUND

Recent studies suggest that CD80 (also known as B7.1) is expressed on podocytes in minimal-change disease (MCD) and may have a role in mediating proteinuria. CD80 expression is known to be induced by Toll-like receptor (TLR) ligands in dendritic cells. We therefore evaluated the ability of TLR to induce CD80 in human cultured podocytes.

METHODS

Conditionally immortalized human podocytes were evaluated for TLR expression. Based on high expression of TLR3, we evaluated the effect of polyinosinic-polycytidylic acid (polyIC), a TLR3 ligand, to induce CD80 expression in vitro.

RESULTS

TLR1-6 and 9 messenger RNA (mRNA) were expressed in podocytes. Among TLR ligands 1-9, CD80 mRNA expression was significantly induced by polyIC and lipopolysaccharide (TLR4 ligand) with the greatest stimulation by polyIC (6.8 ± 0.7 times at 6 h, P < 0.001 versus control). PolyIC induced increased expression of Cathepsin L, decreased synaptopodin expression and resulted in actin reorganization which suggested a similar injury pattern as observed with lipopolyssaccharide. PolyIC induced type I and type II interferon signaling, nuclear factor kappa B (NF-κB) activation and the induction of CD80 expression. Knockdown of CD80 protected against actin reorganization and reduced synaptopodin expression in response to polyIC. Dexamethasone, a corticosteroid commonly used to treat MCD, also blocked both basal and polyIC-stimulated CD80 expression, as did inhibition of NF-κB.

CONCLUSIONS

Activation of TLR3 on cultured human podocytes induces CD80 expression and phenotypic change via an NF-κB-dependent mechanism and is partially blocked by dexamethasone. These studies provide a mechanism by which viral infections may cause proteinuria.

Kindlin-2 regulates podocyte adhesion and fibronectin matrix deposition through interactions with phosphoinositides and integrins

Kindlin-2 is a FERM and PH domain-containing integrin-binding protein that is emerging as an important regulator of integrin activation. How kindlin-2 functions in integrin activation, however, is not known. We report here that kindlin-2 interacts with multiple phosphoinositides, preferentially with phosphatidylinositol 3,4,5-trisphosphate. Although integrin-binding is essential for focal adhesion localization of kindlin-2, phosphoinositide-binding is not required for this process. Using biologically and clinically relevant glomerular podocytes as a model system, we show that integrin activation and dependent processes are tightly regulated by kindlin-2: depletion of kindlin-2 reduced integrin activation, matrix adhesion and fibronectin matrix deposition, whereas overexpression of kindlin-2 promoted these processes. Furthermore, we provide evidence showing that kindlin-2 is involved in phosphoinositide-3-kinase-mediated regulation of podocyte-matrix adhesion and fibronectin matrix deposition. Mechanistically, kindlin-2 promotes integrin activation and integrin-dependent processes through interacting with both integrins and phosphoinositides. TGF-β1, a mediator of progressive glomerular failure, markedly increased the level of kindlin-2 and fibronectin matrix deposition, and the latter process was reversed by depletion of kindlin-2. Our results reveal important functions of kindlin-2 in the regulation of podocyte-matrix adhesion and matrix deposition and shed new light on the mechanism whereby kindlin-2 functions in these processes.

Low-level C-reactive protein levels exert cytoprotective actions on human podocytes

BACKGROUND

Albuminuria and elevated C-reactive protein (CRP) levels are common manifestations of many inflammatory diseases. Cardiovascular-based drugs, with secondary anti-inflammatory actions, such as angiotensin-converting enzyme-inhibitors are able to reduce both proteinuria and CRP levels, raising the question of whether CRP directly influences the processes that result in proteinuria. As proteinuria is thought to be induced as a result of podocyte dysfunction, we investigated whether there is a pathomechanistic link with CRP.

METHODS

Podocytes were analysed for evidence of endogenous CRP production in response to inflammatory agents. In addition, they were incubated in the presence of various concentrations of exogenous CRP and analysed for evidence of a response to treatment.

RESULTS

Our results demonstrated that inflammatory agents such as macrophage-conditioned medium and interleukin-1β induced the expression of CRP messenger RNA in podocytes. However, they were unable to induce CRP protein. Stimulation of podocytes with exogenous CRP demonstrated that 10 μg/mL CRP induced a low but significant level of interleukin-6 secretion. Tumour necrosis factor α, however, was not detected. CRP did up-regulate the expression of the slit diaphragm proteins nephrin and CD2AP, as well as the structural proteins ezrin and podocalyxin-like protein-1, proteins known to be involved in signalling via the phosphotidylinositol-3 (PI-3) kinase pathway. CRP exposure reduced caspase-3 enzyme activity and up-regulated the expression of the anti-apoptotic protein Bcl-2. In the presence of the PI-3 kinase inhibitor LY294002, the ability of CRP to suppress caspase-3 activity was significantly reduced.

CONCLUSIONS

Taken together, these data suggest that rather than inducing podocyte damage, CRP may be a survival factor for podocytes by maintaining their structural integrity and initiating a survival cascade, which may facilitate podocyte recovery from injury.

GSK3β inactivation in podocytes results in decreased phosphorylation of p70S6K accompanied by cytoskeletal rearrangements and inhibited motility

The inhibition of mTOR kinase after renal transplantation has been associated with podocyte injury and proteinuria; however, the signaling pathways regulating these effects are not well understood. We found that prolonged rapamycin treatment in podocytes leads to an increase in glycogen synthase kinase 3β (GSK3β) phosphorylation, resulting in inactivation of total GSK3β kinase activity. To investigate the cellular consequences of the inactivation of GSK3β, we used two inhibitors reducing kinase activity and studied the cross talk between GSK3 function and the Akt/mammalian target of rapamycin (mTOR) pathway. Both GSK3 inhibitors reduced the phosphorylation of the mTOR downstream target, p70(S6K), indicating that GSK3 inhibition in podocytes is able to cause similar effects as treatment with rapamycin. Moreover, GSK3 inhibition was accompanied by the reduced expression of slit diaphragm-associated proteins and resulted in an altered cytoskeletal structure and reduced motility of podocytes, suggesting that GSK3 kinase can modulate Akt/mTOR-dependent signaling in podocytes.

Podocytes as a target of insulin

Diabetic nephropathy (DN) presents with a gradual breakdown of the glomerular filtration barrier to protein, culminating in widespread glomerular damage and renal failure. The podocyte is the central cell of the glomerular filtration barrier, and possesses unique architectural and signaling properties guided by the expression of key podocyte specific proteins. How these cellular features are damaged by the diabetic milieu is unclear, but what is becoming increasingly clear is that damage to the podocyte is a central event in DN. Here we present accumulating evidence that insulin action itself is important in podocyte biology, and may be deranged in the pathomechanism of early DN. This introduces a rationale for therapeutic intervention to improve podocyte insulin sensitivity early in the presentation of DN.