Podocyte actin in health, disease and treatment

Update on the treatment of focal segmental glomerulosclerosis in renal transplantation

Focal segmental glomerulosclerosis (FSGS) represents one of the most severe glomerular diseases, with frequent progression to end-stage renal disease and a high rate of recurrence in renal allografts (30%-50%). Recurrent FSGS portends a negative outcome, with the hazard ratio of graft failure being two-fold higher then that of other glomerulonephritis. Two patterns of clinical presentations are observed: Early recurrence, which is characterized by massive proteinuria within hours to days after implantation of the renal graft, and late recurrence, which occurs several months or years after the transplantation. Many clinical conditions have been recognized as risk factors for recurrence, including younger age, rapid progression of the disease to end-stage renal disease on native kidneys, and loss of previous renal allografts due to recurrence. However, much less is known about the incidence and risk factors of the so-called “de novo” type of FSGS, for which sufferers are transplanted patients without disease on native kidneys; but, rapid development of allograft failure is frequently observed. Management of both forms is challenging, and none of the approaches proposed to date have been demonstrated as consistently beneficial or effective. In the present review we report an update on the available therapeutic strategies for FSGS in renal transplantation within the context of a critical overview of the current literature.

Secondary Focal Segmental Glomerulosclerosis: From Podocyte Injury to Glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a common cause of proteinuria and nephrotic syndrome leading to end stage renal disease (ESRD). There are two types of FSGS, primary (idiopathic) and secondary forms. Secondary FSGS shows less severe clinical features compared to those of the primary one. However, secondary FSGS has an important clinical significance because a variety of renal diseases progress to ESRD thorough the form of secondary FSGS. The defining feature of FSGS is proteinuria. The key event of FSGS is podocyte injury which is caused by multiple factors. Unanswered questions about how these factors act on podocytes to cause secondary FSGS are various and ill-defined. In this review, we provide brief overview and new insights into FSGS, podocyte injury, and their potential linkage suggesting clues to answer for treatment of the disease.

Podocyte Purinergic P2X4 Channels Are Mechanotransducers That Mediate Cytoskeletal Disorganization

Podocytes are specialized, highly differentiated epithelial cells in the kidney glomerulus that are exposed to glomerular capillary pressure and possible increases in mechanical load. The proteins sensing mechanical forces in podocytes are unconfirmed, but the classic transient receptor potential channel 6 (TRPC6) interacting with the MEC-2 homolog podocin may form a mechanosensitive ion channel complex in podocytes. Here, we observed that podocytes respond to mechanical stimulation with increased intracellular calcium concentrations and increased inward cation currents. However, TRPC6-deficient podocytes responded in a manner similar to that of control podocytes, and mechanically induced currents were unaffected by genetic inactivation of TRPC1/3/6 or administration of the broad-range TRPC blocker SKF-96365. Instead, mechanically induced currents were significantly decreased by the specific P2X purinoceptor 4 (P2X4) blocker 5-BDBD. Moreover, mechanical P2X4 channel activation depended on cholesterol and podocin and was inhibited by stabilization of the actin cytoskeleton. Because P2X4 channels are not intrinsically mechanosensitive, we investigated whether podocytes release ATP upon mechanical stimulation using a fluorometric approach. Indeed, mechanically induced ATP release from podocytes was observed. Furthermore, 5-BDBD attenuated mechanically induced reorganization of the actin cytoskeleton. Altogether, our findings reveal a TRPC channel-independent role of P2X4 channels as mechanotransducers in podocytes.

Schip1 is a novel podocyte foot process protein that mediates actin cytoskeleton rearrangements and forms a complex with Nherf2 and ezrin

Background

Podocyte foot process effacement accompanied by actin cytoskeleton rearrangements is a cardinal feature of many progressive human proteinuric diseases.

Results

By microarray profiling of mouse glomerulus, SCHIP1 emerged as one of the most highly enriched transcripts. We detected Schip1 protein in the kidney glomerulus, specifically in podocytes foot processes. Functionally, Schip1 inactivation in zebrafish by morpholino knock-down results in foot process disorganization and podocyte loss leading to proteinuria. In cultured podocytes Schip1 localizes to cortical actin-rich regions of lamellipodia, where it forms a complex with Nherf2 and ezrin, proteins known to participate in actin remodeling stimulated by PDGFβ signaling. Mechanistically, overexpression of Schip1 in vitro causes accumulation of cortical F-actin with dissolution of transversal stress fibers and promotes cell migration in response to PDGF-BB stimulation. Upon actin disassembly by latrunculin A treatment, Schip1 remains associated with the residual F-actin-containing structures, suggesting a functional connection with actin cytoskeleton possibly via its interaction partners. A similar assay with cytochalasin D points to stabilization of cortical actin cytoskeleton in Schip1 overexpressing cells by attenuation of actin depolymerisation.

Conclusions

Schip1 is a novel glomerular protein predominantly expressed in podocytes, necessary for the zebrafish pronephros development and function. Schip1 associates with the cortical actin cytoskeleton network and modulates its dynamics in response to PDGF signaling via interaction with the Nherf2/ezrin complex. Its implication in proteinuric diseases remains to be further investigated.

Inverted formin 2 regulates actin dynamics by antagonizing Rho/diaphanous-related formin signaling

Mutations in inverted formin 2 INF2 are a common cause of familial FSGS. INF2 interacts with diaphanous-related formins (mDia) and antagonizes mDia-mediated actin polymerization in response to active Rho signaling, suggesting that dysregulation of these pathways may mediate the development of INF2-related FSGS. However, the precise mechanisms by which INF2 regulates actin-dependent podocyte behavior remain largely unknown. Here, we investigated the possible role of INF2 in both lamellipodia-associated actin dynamics and actin-dependent slit diaphragm (SD) protein trafficking by manipulating the expression of INF2 and the activity of Rho/mDia signaling in cultured podocytes. Activation of mDia in the absence of INF2 led to defective formation of lamellipodia and abnormal SD trafficking. Effects of mutations disrupting the INF2-mDia interaction suggested the specificity of the mDia-antagonizing effect of INF2 in maintaining the lamellipodium. Furthermore, we found that SD trafficking requires INF2 interaction with lipid raft components. In summary, INF2 regulates lamellipodial actin dynamics and the trafficking of slit diaphragm proteins by opposing Rho/mDia-mediated actin polymerization. Thus, in podocytes, INF2 appears to be an important modulator of actin-dependent behaviors that are under the control of Rho/mDia signaling.

N-wasp is required for stabilization of podocyte foot processes

Alteration of cortical actin structures is the common final pathway leading to podocyte foot process effacement and proteinuria. The molecular mechanisms that safeguard podocyte foot process architecture and maintain the three-dimensional actin network remain elusive. Here, we demonstrate that neuronal Wiskott-Aldrich syndrome protein (N-WASP), which promotes actin nucleation, is required to stabilize podocyte foot processes. Mice lacking N-WASP specifically in podocytes were born with normal kidney function but developed significant proteinuria 3 weeks after birth, suggesting an important role for N-WASP in maintaining foot processes. In addition, inducing deletion of N-WASP in adult mice resulted in severe proteinuria and kidney failure. Electron microscopy showed an accumulation of electron-dense patches of actin and strikingly altered morphology of podocyte foot processes. Although basic actin-based processes such as cell migration were not affected, primary cultures of N-WASP-deficient podocytes revealed significant impairment of dynamic actin reorganization events, including the formation of circular dorsal ruffles. Taken together, our findings suggest that N-WASP-mediated actin nucleation of branched microfilament networks is specifically required for the maintenance of foot processes, presumably sustaining the mechanical resistance of the filtration barrier.

Prostaglandin E2 promotes cellular recovery from established nephrotoxic serum nephritis in mice, prosurvival, and regenerative effects on glomerular cells

We postulated that prostaglandin E2 (PGE2), which exhibits regulatory functions to control immune-mediated inflammation, fibrosis, oxidative stress, and tissue/cellular regeneration, has the potential to improve the course of nephritis. Therefore, the therapeutic potential of prostanoid on established nephritis in mice was evaluated focusing on its role on renal cellular recovery, with emphasis on its cytoprotecting and growth-promoting effects. Acute nephritis was induced in mice by single injection of nephrotoxic serum (NTS), followed by PGE2 administration with severity of nephritis evaluated over time. Mice injected with PGE2 recovered promptly with normalization of blood urea nitrogen and urine protein levels and histology. Recovery was observed with dosing of prostanoid at day 1, as well as day 4. With the use of selective EP1-4 receptor agonists, EP3 receptor has been identified as important in mediating beneficial effects of PGE2 in our system. PGE2 normalized glomerular cell losses during nephrotoxic serum-induced nephritis, restored synaptopodin distribution and F-actin filaments arrangement in glomeruli. In cell culture, PGE2 reduced nephrotoxim serum (NTS)-induced apoptosis of glomerular cells and promoted cell reproliferation after NTS-mediated injury. In conclusion, PGE2 treatment promotes resolution of glomerular inflammation. Consistent with this observation, the regenerative and cytoprotective effects of prostanoid on glomerular cells in culture were observed, suggesting that PGE2 may be beneficial in the treatment of glomerulonephritis.

Acute and sustained actions of hyperglycaemia on endothelial and glomerular barrier permeability

Microalbuminuria is an established marker of systemic endothelial dysfunction, which for patients with diabetes signals an increased risk of both diabetic nephropathy and cardiovascular complications. A better understanding of the pathogenesis of microalbuminuria is important in the quest of finding new approaches to treat patients with diabetes. Direct acute effects of episodes of hyperglycaemia (HG) could have implications for the microalbuminuria seen in early diabetes before renal structural alterations have started, especially in those patients with poor glycaemic control. This review summarizes the literature evidence that acute or sustained HG may lead to an increased vascular or glomerular permeability. Special focus is on glomerular barrier permeability. There is evidence in the literature that HG increases systemic capillary and glomerular barrier permeability within 20-30 min in vivo in rats and mice. Furthermore, exposure of monolayers of cultured endothelial cells to HG has been shown to increase monolayer permeability rapidly and transiently (during 60-100 min). Instant cellular changes following F-actin cytoskeleton rearrangements, which could be abrogated by Rho-kinase (ROCK) inhibition, are implicated. Data in this review also suggest that activation of protein kinase C, the polyol pathway, and an increased release of reactive oxygen species (ROS) and cytokines could contribute to the increase in barrier permeability induced by HG. Recent in vitro data from cultured podocyte monolayers also designates a role of insulin in acute podocyte F-actin remodelling, underpinning the complexity of the mechanisms leading to glomerular and endothelial barrier alterations in diabetes mellitus.

Focal segmental glomerulosclerosis after renal transplantation

Focal segmental glomerulosclerosis (FSGS) is a clinicopathologic syndrome of proteinuria, usually of nephrotic range, associated with focal and segmental sclerotic glomerular lesions. Therefore, FSGS is diagnosed by clinical features and histopathological examination of renal biopsy. The natural history of the condition varies, and although it may respond to treatment, FSGS is an important disease in the etiology of end-stage renal disease (ESRD). Furthermore, after kidney transplantation, approximately 30% of patients with FSGS develop recurrent FSGS. The risk factors for recurrence of FSGS include childhood onset and age <15 yr, rapid progression of the primary FSGS to ESRD, recurrence of FSGS in a previous allograft, diffuse mesangial hypercellularity in the native kidney, collapsing FSGS, and podocin gene mutation. In addition, after kidney transplantation, de novo FSGS also develops in approximately 10-20% of allografts, associated with a complication of hyperfiltration injury, chronic transplant glomerulopathy, and calcineurin inhibitor toxicity. FSGS is considered a podocyte disease, and the pathology is characterized by segmental FSGS lesion with glomerular epithelial hypercellularity. The pathological diagnosis of FSGS is based on the 2004 Columbia classification system. In the present minireview, we discuss the pathology of recurrence and de novo FSGS after kidney transplantation.

Alpha-actinin-4 and CLP36 protein deficiencies contribute to podocyte defects in multiple human glomerulopathies

Genetic alterations of α-actinin-4 can cause podocyte injury through multiple mechanisms. Although a mechanism involving gain-of-α-actinin-4 function was well described and is responsible for a dominantly inherited form of human focal segmental glomerulosclerosis (FSGS), evidence supporting mechanisms involving loss-of-α-actinin-4 function in human glomerular diseases remains elusive. Here we show that α-actinin-4 deficiency occurs in multiple human primary glomerulopathies including sporadic FSGS, minimal change disease, and IgA nephropathy. Furthermore, we identify a close correlation between the levels of α-actinin-4 and CLP36, which form a complex in normal podocytes, in human glomerular diseases. siRNA-mediated depletion of α-actinin-4 in human podocytes resulted in a marked reduction of the CLP36 level. Additionally, two FSGS-associated α-actinin-4 mutations (R310Q and Q348R) inhibited the complex formation between α-actinin-4 and CLP36. Inhibition of the α-actinin-4-CLP36 complex, like loss of α-actinin-4, markedly reduced the level of CLP36 in podocytes. Finally, reduction of the CLP36 level or disruption of the α-actinin-4-CLP36 complex significantly inhibited RhoA activity and generation of traction force in podocytes. Our studies reveal a critical role of the α-actinin-4-CLP36 complex in podocytes and provide an explanation as to how α-actinin-4 deficiency or mutations found in human patients could contribute to podocyte defects and glomerular failure through a loss-of-function mechanism.