CIN85/RukL is a novel binding partner of nephrin and podocin and mediates slit diaphragm turnover in podocytes

Roles of myosin 1e and the actin cytoskeleton in kidney functions and familial kidney disease

Mammalian kidneys are responsible for removing metabolic waste and maintaining fluid and electrolyte homeostasis via selective filtration. One of the proteins closely linked to selective renal filtration is myosin 1e (Myo1e), an actin-dependent molecular motor found in the specialized kidney epithelial cells involved in the assembly and maintenance of the renal filter. Point mutations in the gene encoding Myo1e, MYO1E, have been linked to familial kidney disease, and Myo1e knockout in mice leads to the disruption of selective filtration. In this review, we discuss the role of the actin cytoskeleton in renal filtration, the known and hypothesized functions of Myo1e, and the possible explanations for the impact of MYO1E mutations on renal function.

c-Cbl induced podocin ubiquitination contributes to the podocytes injury in diabetic nephropathy

The ubiquitination function in diabetic nephropathy (DN) has attracted much attention, but there is a lack of information on its ubiquitylome profile. To examine the differences in protein content and ubiquitination in the kidney between db/db mice and db/m mice, we deployed liquid chromatography-mass spectrometry (LC-MS/MS) to conduct analysis. We determined 145 sites in 86 upregulated modified proteins and 66 sites in 49 downregulated modified proteins at the ubiquitinated level. Moreover, 347 sites among the 319 modified proteins were present only in the db/db mouse kidneys, while 213 sites among the 199 modified proteins were present only in the db/m mouse kidneys. The subcellular localization study indicated that the cytoplasm had the highest proportion of ubiquitinated proteins (31.87%), followed by the nucleus (30.24%) and the plasma membrane (20.33%). The enrichment analysis revealed that the ubiquitinated proteins are mostly linked to tight junctions, oxidative phosphorylation, and thermogenesis. Podocin, as a typical protein of slit diaphragm, whose loss is a crucial cause of proteinuria in DN. Consistent with the results of ubiquitination omics, the K261R mutant of podocin induced the weakest ubiquitination compared with the K301R and K370R mutants. As an E3 ligase, c-Cbl binds to podocin, and the regulation of c-Cbl can impact the ubiquitination of podocin. In conclusion, in DN, podocin ubiquitination contributes to podocyte injury, and K261R is the most significant site. c-Cbl participates in podocin ubiquitination and may be a direct target for preserving the integrity of the slit diaphragm structure, hence reducing proteinuria in DN.

Natural Products Against Renal Fibrosis via Modulation of SUMOylation

Renal fibrosis is the common and final pathological process of kidney diseases. As a dynamic and reversible post-translational modification, SUMOylation and deSUMOylation of transcriptional factors and key mediators significantly affect the development of renal fibrosis. Recent advances suggest that SUMOylation functions as the promising intervening target against renal fibrosis, and natural products prevent renal fibrosis via modulating SUMOylation. Here, we introduce the mechanism of SUMOylation in renal fibrosis and therapeutic effects of natural products. This process starts by summarizing the key mediators and enzymes during SUMOylation and deSUMOylation and its regulation role in transcriptional factors and key mediators in renal fibrosis, then linking the mechanism findings of SUMOylation and natural products to develop novel therapeutic candidates for treating renal fibrosis, and concludes by commenting on promising therapeutic targets and candidate natural products in renal fibrosis via modulating SUMOylation, which highlights modulating SUMOylation as a promising strategy for natural products against renal fibrosis.

Podocyte Endocytosis in Regulating the Glomerular Filtration Barrier

Endocytosis is a mechanism that internalizes and recycles plasma membrane components and transmembrane receptors via vesicle formation, which is mediated by clathrin-dependent and clathrin-independent signaling pathways. Podocytes are specialized, terminally differentiated epithelial cells in the kidney, located on the outermost layer of the glomerulus. These cells play an important role in maintaining the integrity of the glomerular filtration barrier in conjunction with the adjacent basement membrane and endothelial cell layers within the glomerulus. An intact podocyte endocytic machinery appears to be necessary for maintaining podocyte function. De novo pathologic human genetic mutations and loss-of-function studies of critical podocyte endocytosis genes in genetically engineered mouse models suggest that this pathway contributes to the pathophysiology of development and progression of proteinuria in chronic kidney disease. Here, we review the mechanism of cellular endocytosis and its regulation in podocyte injury in the context of glomerular diseases. A thorough understanding of podocyte endocytosis may shed novel insights into its biological function in maintaining a functioning filter and offer potential targeted therapeutic strategies for proteinuric glomerular diseases.

Serum and Glucocorticoid-Inducible Kinase 3/Nedd4-2 Signaling Pathway Participates in Podocyte Injury by Regulating the Stability of Nephrin

Serum and glucocorticoid-inducible kinase 3 (SGK3) is involved in maintaining podocyte function by regulating the protein levels of podocin and CD2-associated protein. Nephrin is also one of the slit diaphragm proteins of podocytes, but whether SGK3 participates in podocyte injury by regulating the levels of nephrin remains unclear. In this study, we focused on whether SGK3 affects nephrin levels and the mechanisms involved in the same. In the kidneys of adriamycin (ADR)-induced podocyte injury mouse model, the protein levels of SGK3 and nephrin were significantly decreased. Furthermore, the expression of SGK3 was negatively correlated with the output of proteinuria, and positively correlated with the levels of nephrin. In ADR-treated conditionally immortalized mouse podocyte cells (MPCs), the protein levels of nephrin and SGK3 were inhibited, while the constitutive expression of SGK3 reversed the ADR-induced decline in nephrin protein levels. Furthermore, ADR treatment or SGK3 inactivation enhanced the ubiquitin-proteasome degradation of nephrin in MPCs, and dramatically activated downstream effector proteins of SGK3, neural precursor cells expressing developmentally downregulated protein 4 subtype 2 (Nedd4-2) and glycogen synthase kinase-3 β (GSK3β). Similarly, Nedd4-2 or GSK3β overexpression resulted in increased activity of Nedd4-2 or GSK3β, and significantly downregulated nephrin levels. Interestingly, ubiquitin-mediated protein degradation of nephrin was regulated by Nedd4-2, rather than by GSK3β. In summary, SGK3 inactivation downregulated the levels of nephrin by increasing Nedd4-2 and GSK3β activity in ADR-induced podocyte injury model; in particular, the SGK3/Nedd4-2 signaling pathway was found to be involved in ubiquitin-mediated proteasome degradation of nephrin.

The protective effect of beta-hydroxybutyric acid on renal glomerular epithelial cells in adriamycin-induced injury

Beta-hydroxybutyric acid (BHB) exerts a protective effect in experimental of kidney disease models. However, the mechanisms underlying this activity are not well defined. BHB stands out for its ability to inhibit the Nε-lysine acetylation of histone and non-histone proteins, which may affect cellular processes and protein functions. In adriamycin-injured murine glomerular podocytes, BHB ameliorates podocyte damage and preserves actin cytoskeleton integrity, reminiscent of the effect of MS275, a highly selective inhibitor of lysine deacetylase. Further research found that adriamycin causes the reduced acetylation of nephrin, WT-1, and GSK3β. This process is abrogated by the lysine deacetylase inhibitor or BHB, suggesting that the acetylation of these molecules regulates their activity. In contrast, anacardic acid, a selective inhibitor of acetyltransferase, decreases the acetylation of nephrin, WT-1, and GSK3β and mitigates the podocyte protective effects of BHB. Taken together, BHB attenuates adriamycin-elicited glomerular epithelial cell injury, at least in part, by inhibiting the deacetylation of the key molecules implicated in glomerular injury.

Metformin Protects against Podocyte Injury in Diabetic Kidney Disease

Metformin is the most commonly prescribed drug for treating type 2 diabetes mellitus (T2D). Its mechanisms of action have been under extensive investigation, revealing that it has multiple cellular targets, either direct or indirect ones, via which it regulates numerous cellular pathways. Diabetic kidney disease (DKD), the serious complication of T2D, develops in up to 50% of the individuals with T2D. Various mechanisms contribute to the development of DKD, including hyperglycaemia, dyslipidemia, oxidative stress, chronic low-grade inflammation, altered autophagic activity and insulin resistance, among others. Metformin has been shown to affect these pathways, and thus, it could slow down or prevent the progression of DKD. Despite several animal studies demonstrating the renoprotective effects of metformin, there is no concrete evidence in clinical settings. This review summarizes the renoprotective effects of metformin in experimental settings. Special emphasis is on the effects of metformin on podocytes, the glomerular epithelial cells that are central in maintaining the glomerular ultrafiltration function.

Tyrosine Phosphorylation of CD2AP Affects Stability of the Slit Diaphragm Complex

BACKGROUND

CD2-associated protein (CD2AP), a slit diaphragm-associated scaffolding protein involved in survival and regulation of the cytoskeleton in podocytes, is considered a "stabilizer" of the slit diaphragm complex that connects the slit diaphragm protein nephrin to the cytoskeleton of the cell. Tyrosine phosphorylation of slit diaphragm molecules can influence their surface expression, but it is unknown whether tyrosine phosphorylation events of CD2AP are also physiologically relevant to slit diaphragm stability.

METHODS

We used isoelectric focusing, western blot analysis, and immunofluorescence to investigate phosphorylation of CD2AP, and phospho-CD2AP antibodies and site-directed mutagenesis to define the specific phosphorylated tyrosine residues. We used cross-species rescue experiments in Cd2ap^KD zebrafish and in Drosophila cindr^RNAi mutants to define the physiologic relevance of CD2AP phosphorylation of the tyrosine residues.

RESULTS

We found that VEGF-A stimulation can induce a tyrosine phosphorylation response in CD2AP in podocytes, and that these phosphorylation events have an important effect on slit diaphragm protein localization and functionality in vivo. We demonstrated that tyrosine in position Y10 of the SH3-1 domain of CD2AP is indispensable for CD2AP function in vivo. We found that the binding affinity of nephrin to CD2AP is significantly enhanced in the absence of Y10; however, unexpectedly, this increased affinity leads not to stabilization but to functional impairment of the glomerular filtration barrier.

CONCLUSIONS

Our findings provide insight into CD2AP and its phosphorylation in the context of slit diaphragm functionality, and indicate a fine-tuned affinity balance of CD2AP and nephrin that is influenced by receptor tyrosine kinase stimulation.

MAGI-2 and scaffold proteins in glomerulopathy

In many cells and tissues, including the glomerular filtration barrier, scaffold proteins are critical in optimizing signal transduction by enhancing structural stability and functionality of their ligands. Recently, mutations in scaffold protein membrane-associated guanylate kinase inverted 2 (MAGI-2) encoding gene were identified among the etiology of steroid-resistant nephrotic syndrome. MAGI-2 interacts with core proteins of multiple pathways, such as transforming growth factor-β signaling, planar cell polarity pathway, and Wnt/β-catenin signaling in podocyte and slit diaphragm. Through the interaction with its ligand, MAGI-2 modulates the regulation of apoptosis, cytoskeletal reorganization, and glomerular development. This review aims to summarize recent findings on the role of MAGI-2 and some other scaffold proteins, such as nephrin and synaptopodin, in the underlying mechanisms of glomerulopathy.

C-terminal oligomerization of podocin mediates interallelic interactions

Interallelic interactions of membrane proteins are not taken into account while evaluating the pathogenicity of sequence variants in autosomal recessive disorders. Podocin, a membrane-anchored component of the slit diaphragm, is encoded by NPHS2, the major gene mutated in hereditary podocytopathies. We formerly showed that its R229Q variant is only pathogenic when trans-associated to specific 3' mutations and suggested the causal role of an abnormal C-terminal dimerization. Here we show by FRET analysis and size exclusion chromatography that podocin oligomerization occurs exclusively through the C-terminal tail (residues 283-382): principally through the first C-terminal helical region (H1, 283-313), which forms a coiled coil as shown by circular dichroism spectroscopy, and through the 332-348 region. We show the principal role of the oligomerization sites in mediating interallelic interactions: while the monomer-forming R286Tfs*17 podocin remains membranous irrespective of the coexpressed podocin variant identity, podocin variants with an intact H1 significantly influence each other's localization (r² = 0.68, P = 9.2 × 10^-32). The dominant negative effect resulting in intracellular retention of the pathogenic F344Lfs*4-R229Q heterooligomer occurs in parallel with a reduction in the FRET efficiency, suggesting the causal role of a conformational rearrangement. On the other hand, oligomerization can also promote the membrane localization: it can prevent the endocytosis of F344Lfs*4 or F344* podocin mutants induced by C-terminal truncation. In conclusion, C-terminal oligomerization of podocin can mediate both a dominant negative effect and interallelic complementation. Interallelic interactions of NPHS2 are not restricted to the R229Q variant and have to be considered in compound heterozygous individuals.

Nephrin Signaling in the Podocyte: An Updated View of Signal Regulation at the Slit Diaphragm and Beyond

Podocytes are a major component of the glomerular blood filtration barrier, and alterations to the morphology of their unique actin-based foot processes (FP) are a common feature of kidney disease. Adjacent FP are connected by a specialized intercellular junction known as the slit diaphragm (SD), which serves as the ultimate barrier to regulate passage of macromolecules from the blood. While the link between SD dysfunction and reduced filtration selectivity has been recognized for nearly 50 years, our understanding of the underlying molecular circuitry began only 20 years ago, sparked by the identification of NPHS1, encoding the transmembrane protein nephrin. Nephrin not only functions as the core component of the extracellular SD filtration network but also as a signaling scaffold via interactions at its short intracellular region. Phospho-regulation of several conserved tyrosine residues in this region influences signal transduction pathways which control podocyte cell adhesion, shape, and survival, and emerging studies highlight roles for nephrin phospho-dynamics in mechanotransduction and endocytosis. The following review aims to summarize the last 5 years of advancement in our knowledge of how signaling centered at nephrin directs SD barrier formation and function. We further provide insight on promising frontiers in podocyte biology, which have implications for SD signaling in the healthy and diseased kidney.

JNK is antagonized to ensure the correct number of interommatidial cells pattern the Drosophila retina

Apoptosis is crucial during the morphogenesis of most organs and tissues, and is utilized for tissues to achieve their proper size, shape and patterning. Many signaling pathways contribute to the precise regulation of apoptosis. Here we show that Jun N-terminal Kinase (JNK) activity contributes to the coordinated removal of interommatidial cells via apoptosis in the Drosophila pupal retina. This is consistent with previous findings that JNK activity promotes apoptosis in other epithelia. However, we found that JNK activity is repressed by Cindr (the CIN85 and CD2AP ortholog) in order to promote cell survival. Reducing the amount of Cindr resulted in ectopic cell death. Increased expression of the Drosophila JNK basket in the setting of reduced cindr expression was found to result in even more severe apoptosis, whilst ectopic death was found to be reduced if retinas were heterozygous for basket. Hence Cindr is required to properly restrict JNK-mediated apoptosis in the pupal eye, resulting in the correct number of interommatidial cells. A lack of precise control over developmental apoptosis can lead to improper tissue morphogenesis.

ShcA Adaptor Protein Promotes Nephrin Endocytosis and Is Upregulated in Proteinuric Nephropathies

Nephrin is a key structural component of the podocyte slit diaphragm, and proper expression of nephrin on the cell surface is critical to ensure integrity of the blood filtration barrier. Maintenance of nephrin within this unique cell junction has been proposed to require dynamic phosphorylation events and endocytic recycling, although the molecular mechanisms that control this interplay are poorly understood. Here, we investigated the possibility that the phosphotyrosine adaptor protein ShcA regulates nephrin turnover. Western blotting and immunostaining analysis confirmed that ShcA is expressed in podocytes. In immunoprecipitation and pulldown assays, ShcA, via its SH2 domain, was associated with several phosphorylated tyrosine residues on nephrin. Overexpression of ShcA promoted nephrin tyrosine phosphorylation and reduced nephrin signaling and cell surface expression in vitro In a rat model of reversible podocyte injury and proteinuria, phosphorylated nephrin temporally colocalized with endocytic structures coincident with upregulation of ShcA expression. In vivo biotinylation assays confirmed that nephrin expression decreased at the cell surface and correspondingly increased in the cytosol during the injury time course. Finally, immunostaining in kidney biopsy specimens demonstrated overexpression of ShcA in several human proteinuric kidney diseases compared with normal conditions. Our results suggest that increases in ShcA perturb nephrin phosphosignaling dynamics, leading to aberrant nephrin turnover and slit diaphragm disassembly.

A novel in vivo method to quantify slit diaphragm protein abundance in murine proteinuric kidney disease

Injury of the glomerular filter causes proteinuria by disrupting the sensitive interplay of the glomerular protein network. To date, studies of the expression and trafficking of glomerular proteins have been mostly limited to in vitro or histologic studies. Here, we report a novel in vivo biotinylation assay that allows the quantification of surface expression of glomerular proteins in mice. Kidneys were perfused in situ with biotin before harvest. Afterwards glomeruli were isolated and lyzed. The protein of interest was separated by immunoprecipitation and the amount of surface-expressed protein was quantified by Western blot analysis with streptavidin staining. As proof-of-concept, we examined the presence of nephrin in the slit diaphragm in two well-established murine models of proteinuric kidney disease: nephrotoxic nephritis and adriamycin nephropathy. In proteinuric animals, significantly less nephrin was detected in the slit diaphragm. When proteinuria decreased once again during the course of disease, the amount of surface nephrin returned to the baseline. Our present results suggest that our assay is a valuable tool to study the glomerular filter in proteinuric kidney diseases. Note that the assay is not limited to proteins expressed in the slit diaphragm, and all surface proteins that are accessible to biotin perfusion and immunoprecipitation qualify for this analysis.

Sorting Nexin 9 facilitates podocin endocytosis in the injured podocyte

The irreversibility of glomerulosclerotic changes depends on the degree of podocyte injury. We have previously demonstrated the endocytic translocation of podocin to the subcellular area in severely injured podocytes and found that this process is the primary disease trigger. Here we identified the protein sorting nexin 9 (SNX9) as a novel facilitator of podocin endocytosis in a yeast two-hybrid analysis. SNX9 is involved in clathrin-mediated endocytosis, actin rearrangement and vesicle transport regulation. Our results revealed and confirmed that SNX9 interacts with podocin exclusively through the Bin–Amphiphysin–Rvs (BAR) domain of SNX9. Immunofluorescence staining revealed the expression of SNX9 in response to podocyte adriamycin-induced injury both in vitro and in vivo. Finally, an analysis of human glomerular disease biopsy samples demonstrated strong SNX9 expression and co-localization with podocin in samples representative of severe podocyte injury, such as IgA nephropathy with poor prognosis, membranous nephropathy and focal segmental glomerulosclerosis. In conclusion, we identified SNX9 as a facilitator of podocin endocytosis in severe podocyte injury and demonstrated the expression of SNX9 in the podocytes of both nephropathy model mice and human patients with irreversible glomerular disease.

Endocytic Trafficking at the Mature Podocyte Slit Diaphragm

Endocytic trafficking couples cell signaling with the cytoskeletal dynamics by organizing a crosstalk between protein networks in different subcellular compartments. Proteins residing in the plasma membrane are internalized and transported as cargo in endocytic vesicles (i.e., endocytosis). Subsequently, cargo proteins can be delivered to lysosomes for degradation or recycled back to the plasma membrane. The slit diaphragm is a modified tight junction connecting foot processes of the glomerular epithelial cells, podocytes. Signaling at the slit diaphragm plays a critical role in the kidney while its dysfunction leads to glomerular protein loss (proteinuria), manifesting as nephrotic syndrome, a rare condition with an estimated incidence of 2-4 new cases per 100,000 each year. Relatively little is known about the role of endocytic trafficking in podocyte signaling and maintenance of the slit diaphragm integrity. This review will focus on the role of endocytic trafficking at the mature podocyte slit diaphragm.

CIN85 Deficiency Prevents Nephrin Endocytosis and Proteinuria in Diabetes

Diabetic nephropathy (DN) is the major cause of end-stage renal disease worldwide. Podocytes are important for glomerular filtration barrier function and maintenance of size selectivity in protein filtration in the kidney. Podocyte damage is the basis of many glomerular diseases characterized by loss of interdigitating foot processes and decreased expression of components of the slit diaphragm. Nephrin, a podocyte-specific protein, is the main component of the slit diaphragm. Loss of nephrin is observed in human and rodent models of diabetic kidney disease. The long isoform of CIN85 (RukL) is a binding partner of nephrin that mediates nephrin endocytosis via ubiquitination in podocytes. Here we demonstrate that the loss of nephrin expression and the onset of proteinuria in diabetic mice correlate with an increased accumulation of ubiquitinated proteins and expression of CIN85/RukL in podocytes. CIN85/RukL deficiency preserved nephrin surface expression on the slit diaphragm and reduced proteinuria in diabetic mice, whereas overexpression of CIN85 in zebrafish induced severe edema and disruption of the filtration barrier. Thus, CIN85/RukL is involved in endocytosis of nephrin in podocytes under diabetic conditions, causing podocyte depletion and promoting proteinuria. CIN85/RukL expression therefore shows potential to be a novel target for antiproteinuric therapy in diabetes.

Development of a Functional Glomerulus at the Organ Level on a Chip to Mimic Hypertensive Nephropathy

Glomerular hypertension is an important factor exacerbating glomerular diseases to end-stage renal diseases because, ultimately, it results in glomerular sclerosis (especially in hypertensive and diabetic nephropathy). The precise mechanism of glomerular sclerosis caused by glomerular hypertension is unclear, due partly to the absence of suitable in vitro or in vivo models capable of mimicking and regulating the complex mechanical forces and/or organ-level disease processes. We developed a “glomerulus-on-a-chip” (GC) microfluidic device. This device reconstitutes the glomerulus with organ-level glomerular functions to create a disease model-on-a chip that mimics hypertensive nephropathy in humans. It comprises two channels lined by closely opposed layers of glomerular endothelial cells and podocytes that experience fluid flow of physiological conditions to mimic the glomerular microenvironment in vivo. Our results revealed that glomerular mechanical forces have a crucial role in cellular cytoskeletal rearrangement as well as the damage to cells and their junctions that leads to increased glomerular leakage observed in hypertensive nephropathy. Results also showed that the GC could readily and flexibly meet the demands of a renal-disease model. The GC could provide drug screening and toxicology testing, and create potential new personalized and accurate therapeutic platforms for glomerular disease.

The ubiquitin ligase Ubr4 controls stability of podocin/MEC-2 supercomplexes

The PHB-domain protein podocin maintains the renal filtration barrier and its mutation is an important cause of hereditary nephrotic syndrome. Podocin and its Caenorhabditis elegans orthologue MEC-2 have emerged as key components of mechanosensitive membrane protein signalling complexes. Whereas podocin resides at a specialized cell junction at the podocyte slit diaphragm, MEC-2 is found in neurons required for touch sensitivity. Here, we show that the ubiquitin ligase Ubr4 is a key component of the podocin interactome purified both from cultured podocytes and native glomeruli. It colocalizes with podocin and regulates its stability. In C. elegans, this process is conserved. Here, Ubr4 is responsible for the degradation of mislocalized MEC-2 multimers. Ubiquitylomic analysis of mouse glomeruli revealed that podocin is ubiquitylated at two lysine residues. These sites were Ubr4-dependent and were conserved across species. Molecular dynamics simulations revealed that ubiquitylation of one site, K301, do not only target podocin/MEC-2 for proteasomal degradation, but may also affect stability and disassembly of the multimeric complex. We suggest that Ubr4 is a key regulator of podocyte foot process proteostasis.

The adaptor protein Cindr regulates JNK activity to maintain epithelial sheet integrity

Epithelia are essential barrier tissues that must be appropriately maintained for their correct function. To achieve this a plethora of protein interactions regulate epithelial cell number, structure and adhesion, and differentiation. Here we show that Cindr (the Drosophila Cin85 and Cd2ap ortholog) is required to maintain epithelial integrity. Reducing Cindr triggered cell delamination and movement. Most delaminating cells died. These behaviors were consistent with JNK activation previously associated with loss of epithelial integrity in response to ectopic oncogene activity. We confirmed a novel interaction between Cindr and Drosophila JNK (dJNK), which when perturbed caused inappropriate JNK signaling. Genetically reducing JNK signaling activity suppressed the effects of reducing Cindr. Furthermore, ectopic JNK signaling phenocopied loss of Cindr and was partially rescued by concomitant cindr over-expression. Thus, correct Cindr-dJNK stoichiometry is essential to maintain epithelial integrity and disturbing this balance may contribute to the pathogenesis of disease states, including cancer.

Podocyte endocytosis in the regulation of the glomerular filtration barrier

Severe defects in the glomerular filtration barrier result in nephrotic syndrome, which is characterized by massive proteinuria. The podocyte, a specialized epithelial cell with interdigitating foot processes separated by a slit diaphragm, plays a vital role in regulating the passage of proteins from the capillary lumen to Bowman's space. Recent findings suggest a critical role for endocytosis in podocyte biology as highlighted by genetic mouse models of disease and human genetic mutations that result in the loss of the integrity of the glomerular filtration barrier. In vitro podocyte studies have also unraveled a plethora of constituents that are differentially internalized to maintain homeostasis. These observations provide a framework and impetus for understanding the precise regulation of podocyte endocytic machinery in both health and disease.

Pharmacological targeting of actin-dependent dynamin oligomerization ameliorates chronic kidney disease in diverse animal models

Dysregulation of the actin cytoskeleton in podocytes represents a common pathway in the pathogenesis of proteinuria across a spectrum of chronic kidney diseases (CKD). The GTPase dynamin has been implicated in the maintenance of cellular architecture in podocytes through its direct interaction with actin. Furthermore, the propensity of dynamin to oligomerize into higher-order structures in an actin-dependent manner and to crosslink actin microfilaments into higher order structures have been correlated with increased actin polymerization and global organization of the actin cytoskeleton in the cell. We found that use of the small molecule Bis-T-23, which promotes actin-dependent dynamin oligomerization and thus increased actin polymerization in injured podocytes, was sufficient to improve renal health in diverse models of both transient kidney disease and of CKD. In particular, administration of Bis-T-23 in these renal disease models restored the normal ultrastructure of podocyte foot processes, lowered proteinuria, lowered collagen IV deposits in the mesangial matrix, diminished mesangial matrix expansion and extended lifespan. These results further establish that alterations in the actin cytoskeleton of kidney podocytes is a common hallmark of CKD, while also underscoring the significant regenerative potential of injured glomeruli and that targeting the oligomerization cycle of dynamin represents an attractive potential therapeutic target to treat CKD.

An update: the role of Nephrin inside and outside the kidney

Nephrin is a key molecule in podocytes to maintain normal slit diaphragm structure. Nephin interacts with many other podocyte and slit diaphragm protein and also mediates important cell signaling pathways in podocytes. Loss of nephrin during the development leads to the congenital nephrotic syndrome in children. Reduction of nephrin expression is often observed in adult kidney diseases including diabetic nephropathy and HIV-associated nephropathy. The critical role of nephrin has been confirmed by different animal models with nephrin knockout and knockdown. Recent studies demonstrate that knockdown of nephrin expression in adult mice aggravates the progression of unilateral nephrectomy and Adriamycin-induced kidney disease. In addition to its critical role in maintaining normal glomerular filtration unit in the kidney, nephrin is also expressed in other organs. However, the exact role of nephrin in kidney and extra-renal organs has not been well characterized. Future studies are required to determine whether nephrin could be developed as a drug target to treat patients with kidney disease.

Metformin Ameliorates Podocyte Damage by Restoring Renal Tissue Podocalyxin Expression in Type 2 Diabetic Rats

Podocalyxin (PCX) is a signature molecule of the glomerular podocyte and of maintaining integrity of filtration function of glomerulus. The aim of this study was to observe the effect of different doses of metformin on renal tissue PCX expression in type 2 diabetic rats and clarify its protection on glomerular podocytes. Type 2 diabetic Sprague-Dawley (SD) rats in which diabetes was induced by high-fat diet/streptozotocin (HFD-STZ) were treated with different doses of metformin (150, 300, and 500 mg/kg per day, resp.) for 8 weeks. Various biochemical parameters, kidney histopathology, and renal tissue PCX expression levels were examined. In type 2 diabetic rats, severe hyperglycemia and hyperlipidemia were developed. Urinary albumin and PCX were markedly increased. Diabetes induced significant alterations in renal glomerular structure. In addition, protein and mRNA expression of renal tissue PCX were highly decreased. However, treatment of rats with different doses of metformin restored all these changes to a varying degree. These results suggested that metformin can ameliorate glomerular podocyte damage in type 2 diabetic rats, which may be partly associated with its role in restoring PCX expression and inhibiting urinary excretion of PCX with dose dependence.

MAGI-2 scaffold protein is critical for kidney barrier function

MAGUK Inverted 2 (MAGI-2) is a PTEN-interacting scaffold protein implicated in cancer on the basis of rare, recurrent genomic translocations and deletions in various tumors. In the renal glomerulus, MAGI-2 is exclusively expressed in podocytes, specialized cells forming part of the glomerular filter, where it interacts with the slit diaphragm protein nephrin. To further explore MAGI-2 function, we generated Magi-2-KO mice through homologous recombination by targeting an exon common to all three alternative splice variants. Magi-2 null mice presented with progressive proteinuria as early as 2 wk postnatally, which coincided with loss of nephrin expression in the glomeruli. Magi-2-null kidneys revealed diffuse podocyte foot process effacement and focal podocyte hypertrophy by 3 wk of age, as well as progressive podocyte loss. By 5.5 wk, coinciding with a near-complete loss of podocytes, Magi-2-null mice developed diffuse glomerular extracapillary epithelial cell proliferations, and died of renal failure by 3 mo of age. As confirmed by immunohistochemical analysis, the proliferative cell populations in glomerular lesions were exclusively composed of activated parietal epithelial cells (PECs). Our results reveal that MAGI-2 is required for the integrity of the kidney filter and podocyte survival. Moreover, we demonstrate that PECs can be activated to form glomerular lesions resembling a noninflammatory glomerulopathy with extensive extracapillary proliferation, sometimes resembling crescents, following rapid and severe podocyte loss.

MAGI-2 is critical for the formation and maintenance of the glomerular filtration barrier in mouse kidney

Membrane-associated guanylate kinase inverted 2 (MAGI-2) is a tight junction protein in epithelial tissues. We previously reported the detailed expression patterns of MAGI-2 in mouse tissues, including kidney podocytes, based on results obtained from Venus knock-in mice for Magi2 locus. In the present study, homozygous deletion of the Magi2 gene in mice caused neonatal lethality, which was explained by podocyte morphological abnormalities and anuria. Immunohistological analysis showed that loss of MAGI-2 function induced a significant decrease in nephrin and dendrin at the slit diaphragm of the kidney, although other components of the slit diaphragm were unchanged. Furthermore, nuclear translocation of dendrin was observed in the podocytes of the MAGI-2-null mutants, along with enhanced expression of cathepsin L, which is reported to be critical for rearrangement of the actin cytoskeleton in podocytes. Expression analysis of the null mutants showed that loss of MAGI-2 function induces abnormal expression of various types of adhesion-related molecules. The present study is the first to demonstrate that MAGI-2 has a critical role in maintaining the functional structure of the slit diaphragm and that this molecule has an essential role in the functioning of the kidney filtration barrier.

SUMOylation determines turnover and localization of nephrin at the plasma membrane

Podocyte effacement and the reformation of foot processes and slit diaphragms can be induced within minutes experimentally. Therefore, it seems likely that the slit diaphragm proteins underlie orchestrated recycling mechanisms under the control of posttranslational modifiers. One of these modifiers, SUMO (small ubiquitin-like modifier), is an ubiquitin-like protein with a 20% corresponding identity to ubiquitin. Modification by SUMOs to proteins on lysine residues can block the ubiquitination of the same site leading to the stabilization of the target protein. Here we found in vitro and in vivo that nephrin is a substrate modified by SUMO proteins thereby increasing its steady-state level and expression at the plasma membrane. A conversion of lysines to arginines at positions 1114 and 1224 of the intracellular tail of murine nephrin led to decreased stability of nephrin, decreased expression at the plasma membrane, and decreased PI3K/AKT signaling. Furthermore, treatment of podocytes with the SUMOylation inhibitor ginkgolic acid led to reduced membrane expression of nephrin. Similarly, the conversion of lysine to arginine at position 1100 of human nephrin caused decreased stability and expression at the plasma membrane. As SUMOylation is a reversible process, our results suggest that SUMOylation participates in the tight orchestration of nephrin turnover at the slit diaphragm.

aPKCλ maintains the integrity of the glomerular slit diaphragm through trafficking of nephrin to the cell surface

The slit diaphragm (SD), the specialized intercellular junction between renal glomerular epithelial cells (podocytes), provides a selective-filtration barrier in renal glomeruli. Dysfunction of the SD results in glomerular diseases that are characterized by disappearance of SD components, such as nephrin, from the cell surface. Although the importance of endocytosis and degradation of SD components for the maintenance of SD integrity has been suggested, the dynamic nature of the turnover of intact cell-surface SD components remained unclear. Using isolated rat glomeruli we show that the turnover rates of cell-surface SD components are relatively high; they almost completely disappear from the cell surface within minutes. The exocytosis, but not endocytosis, of heterologously expressed nephrin requires the kinase activity of the cell polarity regulator atypical protein kinase C (aPKC). Consistently, we demonstrate that podocyte-specific deletion of aPKCλ resulted in a decrease of cell-surface localization of SD components, causing massive proteinuria. In conclusion, the regulation of SD turnover by aPKC is crucial for the maintenance of SD integrity and defects in aPKC signalling can lead to proteinuria. These findings not only reveal the pivotal importance of the dynamic turnover of cell-surface SD components but also suggest a novel pathophysiological basis in glomerular disease.

MicroRNA-29a promotion of nephrin acetylation ameliorates hyperglycemia-induced podocyte dysfunction

Podocyte dysfunction is a detrimental feature in diabetic nephropathy, with loss of nephrin integrity contributing to diabetic podocytopathy. MicroRNAs (miRs) reportedly modulate the hyperglycemia-induced perturbation of renal tissue homeostasis. This study investigated whether regulation of histone deacetylase (HDAC) actions and nephrin acetylation by miR-29 contributes to podocyte homeostasis and renal function in diabetic kidneys. Hyperglycemia accelerated podocyte injury and reduced nephrin, acetylated nephrin, and miR-29a levels in primary renal glomeruli from streptozotocin-induced diabetic mice. Diabetic miR-29a transgenic mice had better nephrin levels, podocyte viability, and renal function and less glomerular fibrosis and inflammation reaction compared with diabetic wild-type mice. Overexpression of miR-29a attenuated the promotion of HDAC4 signaling, nephrin ubiquitination, and urinary nephrin excretion associated with diabetes and restored nephrin acetylation. Knockdown of miR-29a by antisense oligonucleotides promoted HDAC4 action, nephrin loss, podocyte apoptosis, and proteinuria in nondiabetic mice. In vitro, interruption of HDAC4 signaling alleviated the high glucose-induced apoptosis and inhibition of nephrin acetylation in podocyte cultures. Furthermore, HDAC4 interference increased the acetylation status of histone H3 at lysine 9 (H3K9Ac), the enrichment of H3K9Ac in miR-29a proximal promoter, and miR-29a transcription in high glucose-stressed podocytes. In conclusion, hyperglycemia impairs miR-29a signaling to intensify HDAC4 actions that contribute to podocyte protein deacetylation and degradation as well as renal dysfunction. HDAC4, via epigenetic H3K9 hypoacetylation, reduces miR-29a transcription. The renoprotective effects of miR-29a in diabetes-induced loss of podocyte integrity and renal homeostasis highlights the importance of post-translational acetylation reactions in podocyte microenvironments. Increasing miR-29a action may protect against diabetic podocytopathy.

Signal transduction in podocytes--spotlight on receptor tyrosine kinases

The mammalian kidney filtration barrier is a complex multicellular, multicomponent structure that maintains homeostasis by regulating electrolytes, acid-base balance, and blood pressure (via maintenance of salt and water balance). To perform these multiple functions, podocytes--an important component of the filtration apparatus--must process a series of intercellular signals. Integrating these signals with diverse cellular responses enables a coordinated response to various conditions. Although mature podocytes are terminally differentiated and cannot proliferate, they are able to respond to growth factors. It is possible that the initial response of podocytes to growth factors is beneficial and protective, and might include the induction of hypertrophic cell growth. However, extended and/or uncontrolled growth factor signalling might be maladaptive and could result in the induction of apoptosis and podocyte loss. Growth factors signal via the activation of receptor tyrosine kinases (RTKs) on their target cells and around a quarter of the 58 RTK family members that are encoded in the human genome have been identified in podocytes. Pharmacological inhibitors of many RTKs exist and are currently used in experimental and clinical cancer therapy. The identification of pathological RTK-mediated signal transduction pathways in podocytes could provide a starting point for the development of novel therapies for glomerular disorders.

Planar cell polarity pathway regulates nephrin endocytosis in developing podocytes

The noncanonical Wnt/planar cell polarity (PCP) pathway controls a variety of cell behaviors such as polarized protrusive cell activity, directional cell movement, and oriented cell division and is crucial for the normal development of many tissues. Mutations in the PCP genes cause malformation in multiple organs. Recently, the PCP pathway was shown to control endocytosis of PCP and non-PCP proteins necessary for cell shape remodeling and formation of specific junctional protein complexes. During formation of the renal glomerulus, the glomerular capillary becomes enveloped by highly specialized epithelial cells, podocytes, that display unique architecture and are connected via specialized cell-cell junctions (slit diaphragms) that restrict passage of protein into the urine; podocyte differentiation requires active remodeling of cytoskeleton and junctional protein complexes. We report here that in cultured human podocytes, activation of the PCP pathway significantly stimulates endocytosis of the core slit diaphragm protein, nephrin, via a clathrin/β-arrestin-dependent endocytic route. In contrast, depletion of the PCP protein Vangl2 leads to an increase of nephrin at the cell surface; loss of Vangl2 functions in Looptail mice results in disturbed glomerular maturation. We propose that the PCP pathway contributes to podocyte development by regulating nephrin turnover during junctional remodeling as the cells differentiate.

The expression of podocyte-specific proteins in parietal epithelial cells is regulated by protein degradation

The role of parietal epithelial cells (PECs) in glomerular disease is unclear because they also express podocyte proteins under pathophysiological conditions. To help resolve this, we established a novel PEC isolation technique in rats and mice to investigate which regulatory mechanisms lead to podocyte protein expression in PECs. This pure pool of naive PECs was then compared with PECs in primary culture and immortalized PECs in permanent culture. The naive PECs expressed low levels of podocyte-specific mRNA. Accordingly, in crescentic glomerulonephritis, single PECs activated the podocin promoter in vivo. In primary culture, PECs expressed a distinct morphology from podocytes but with high transcript and protein levels of PEC markers. In contrast to naive PECs, cultured PECs also expressed podocyte proteins, and this correlated with reduced proteolytic activity but not with increased transcript levels. Activation of autophagy or proteasomal degradation decreased the levels of podocyte proteins in PECs, whereas inhibition of proteasomal degradation led to the stabilization of podocyte proteins in PECs. Thus, naive PECs express podocyte transcripts physiologically and these podocyte proteins are stable under pathological conditions through decreased proteolysis.

Regulation of podocalyxin expression in the kidney of streptozotocin-induced diabetic rats with Chinese herbs (Yishen capsule)

Background

Diabetic nephropathy is an emergent issue in China with increase in patients with type II diabetes. There are several successful Chinese herbal products for the treatment of patients with diabetic nephropathy in China. However, the mechanisms mediating the biological activity of these products are still unclear. Podocalyxin is a sialoprotein critical to maintaining integrity of filtration function of glomerulus.

Methods

By employing streptozotocin-induced diabetic rats and a Chinese herb formulation (Yishen capsule), we examined the regulation of podocalyxin expression in the kidney by Yishen capsule through immunofluorescent staining and reverse transcriptase polymerase chain reaction.

Results

After injection of STZ, there were significant increase in both blood glucose and urinary protein. Serum creatinine and BUN were also increased in rats with injection of STZ. Moreover, expression of podocalyxin in the glomerulus was gradually reduced after injection of STZ. There was also a loss of podocyte foot processes in the glomerular basement membrane. However, Yishen capsule or benazepril was able to restore the expression of podocalyxin and podocyte foot processes in the kidney. Although Yishen capsule could reduce urinary protein level, it has little effect on blood glucose level in the rats injected with STZ.

Conclusions

Yishen capsule could attenuate the loss of podocalyxin in the glomerulus of rats injected with STZ.

Signaling from the podocyte intercellular junction to the actin cytoskeleton

Observations of hereditary glomerular disease support the contention that podocyte intercellular junction proteins are essential for junction formation and maintenance. Genetic deletion of most of these podocyte intercellular junction proteins results in foot process effacement and proteinuria. This review focuses on the current understanding of molecular mechanisms by which podocyte intercellular junction proteins such as the nephrin-neph1-podocin-receptor complex coordinate cytoskeletal dynamics and thus intercellular junction formation, maintenance, and injury-dependent remodeling.

A novel domain regulating degradation of the glomerular slit diaphragm protein podocin in cell culture systems

Mutations in the gene NPHS2 are the most common cause of hereditary steroid-resistant nephrotic syndrome. Its gene product, the stomatin family member protein podocin represents a core component of the slit diaphragm, a unique structure that bridges the space between adjacent podocyte foot processes in the kidney glomerulus. Dislocation and misexpression of slit diaphragm components have been described in the pathogenesis of acquired and hereditary nephrotic syndrome. However, little is known about mechanisms regulating cellular trafficking and turnover of podocin. Here, we discover a three amino acids-comprising motif regulating intracellular localization of podocin in cell culture systems. Mutations of this motif led to markedly reduced degradation of podocin. These findings give novel insight into the molecular biology of the slit diaphragm protein podocin, enabling future research to establish the biological relevance of podocin turnover and localization.

Membrane trafficking in podocyte health and disease

Podocytes are highly specialized epithelial cells localized in the kidney glomerulus. The distinct cell signaling events and unique cytoskeletal architecture tailor podocytes to withstand changes in hydrostatic pressure during glomerular filtration. Alteration of glomerular filtration leads to kidney disease and frequently manifests with proteinuria. It has been increasingly recognized that cell signaling and cytoskeletal dynamics are coupled more tightly to membrane trafficking than previously thought. Membrane trafficking coordinates the cross-talk between protein networks and signaling cascades in a spatially and temporally organized fashion and may be viewed as a communication highway between the cell exterior and interior. Membrane trafficking involves transport of cargo from the plasma membrane to the cell interior (i.e., endocytosis) followed by cargo trafficking to lysosomes for degradation or to the plasma membrane for recycling. Yet, recent studies indicate that the conventional classification does not fully reflect the complex and versatile nature of membrane trafficking. While the increasing complexity of elaborate protein scaffolds and signaling cascades is being recognized in podocytes, the role of membrane trafficking is less well understood. This review will focus on the role of membrane trafficking in podocyte health and disease.

Interactions between Drosophila IgCAM adhesion receptors and cindr, the Cd2ap/Cin85 ortholog

BACKGROUND

Morphogenetic modeling of tissues requires coordinated regulation of adhesion. For its correct patterning, the Drosophila pupal eye requires several Immunoglobulin superfamily cell adhesion molecules (IgCAMs) and the adaptor protein Cindr. Orthologs of these proteins are essential components of specialized junctions of the vertebrate kidney; the Cindr ortholog Cd2ap is essential for the integrity of this structure.

RESULTS

Reducing Cindr during fly eye development led to incorrect distribution of the IgCAMs Roughest (Rst) and Hibris (Hbs). Both bound Cindr. Disrupting endocytosis similarly led to Rst and Hbs mis-localization; our data suggests an additional early requirement for endocytosis in regulating Hbs localization or stability. Finally, Rst and Hbs localized correctly only when in stable membrane complexes and we propose that Cindr anchors these to the cytoskeleton. This regulation likely does not extend to IgCAMs Kin of irre (Kirre) and Sticks and stones (Sns) in the pupal eye; neither interacted with Cindr in in vitro assays. Nonetheless, Kirre and Sns partially mis-localized when Cindr was reduced, possibly due to interactions with Rst/Hbs.

CONCLUSIONS

Our data suggests Cindr recapitulates both proposed functions of its mammalian orthologs Cd2ap and Cin85: targeting the IgCAMs Rst and Hbs for endocytosis and stabilizing these heterophilic IgCAM complexes.

Increased levels of the HER1 adaptor protein Rukl/CIN85 contribute to breast cancer malignancy

The adaptor protein regulator for ubiquitous kinase/c-Cbl-interacting protein of 85kDa (Ruk/CIN85) was found to modulate HER1/EGFR signaling and processes like cell adhesion and apoptosis. Although these features imply a role in carcinogenesis, it is so far unknown how and by which molecular mechanisms Ruk/CIN85 could affect a certain tumor phenotype. By analyzing samples from breast cancer patients, we found high levels of Ruk(l)/CIN85 especially in lymph node metastases from patients with invasive breast adenocarcinomas, suggesting that Ruk(l)/CIN85 contributes to malignancy. Expression of Ruk(l)/CIN85 in weakly invasive breast adenocarcinoma cells deficient of Ruk(l)/CIN85 indeed converted them into more malignant cells. In particular, Ruk(l)/CIN85 reduced the growth rate, decreased cell adhesion, enhanced anchorage-independent growth, increased motility in both transwell migration and wound healing assays as well as affected the response to epidermal growth factor. Thereby, Ruk(l)/CIN85 led to a more rapid and prolonged epidermal growth factor-dependent activation of Src, Akt and ERK1/2 and treatment with the Src inhibitor PP2 and the PI3K inhibitor LY294002 abolished the Ruk(l)/CIN85-dependent changes in cell motility. Together, this study indicates that high levels of Ruk(l)/CIN85 contribute to the conversion of breast adenocarcinoma cells into a more malignant phenotype via modulation of the Src/Akt pathway.

CD2AP/SHIP1 complex positively regulates plasmacytoid dendritic cell receptor signaling by inhibiting the E3 ubiquitin ligase Cbl

The human plasmacytoid dendritic cell (pDC) receptor BDCA2 forms a complex with the adaptor FcεR1γ to activate an ITAM-signaling cascade. BDCA2 receptor signaling negatively regulates the TLR7/9-mediated type 1 IFN responses in pDCs, which may play a key role in controlling self-DNA/RNA-induced autoimmunity. We report in this article that CD2-associated adaptor protein (CD2AP), which is highly expressed in human pDCs, positively regulates BDCA2/FcεR1γ receptor signaling. By immunoprecipitation and mass spectrometry analyses, we found that CD2AP bound to SHIP1. Knockdown of CD2AP or SHIP1 reduced the BDCA2/FcεR1γ-mediated ITAM signaling and blocked its inhibition of TLR9-mediated type 1 IFN production. Knockdown of CD2AP or SHIP1 also enhanced the ubiquitination and degradation of Syk and FcεR1γ that was mediated by the E3 ubiquitin ligase Cbl. This led us to discover that, upon BDCA2 cross-linking, the CD2AP/SHIP1 complex associated with Cbl and inhibited its E3 ubiquitin ligase activity. In human primary pDCs, cross-linking of the BDCA2/FcεR1γ complex induced the recruitment of the CD2AP/SHIP1/Cbl complex to the plasma membrane of pDCs, where it colocalized with the BDCA2/FcεR1γ complex. Therefore, CD2AP positively regulates BDCA2/FcεR1γ signaling by forming a complex with SHIP1 to inhibit the E3 ubiquitin ligase Cbl.

CD2AP regulates SUMOylation of CIN85 in podocytes

Podocytes are highly differentiated and polarized epithelial cells located on the visceral side of the glomerulus. They form an indispensable component of the glomerular filter, the slit diaphragm, formed by several transmembrane proteins and adaptor molecules. Disruption of the slit diaphragm can lead to massive proteinuria and nephrotic syndrome in mice and humans. CD2AP is an adaptor protein that is important for the maintenance of the slit diaphragm. Together with its paralogue, CIN85, CD2AP belongs to a family of adaptor proteins that are primarily described as being involved in endocytosis and downregulation of receptor tyrosine kinase activity. We have shown that full-length CIN85 is upregulated in podocytes in the absence of CD2AP, whereas in wild-type cells, full-length CIN85 is not detectable. In this study, we show that full-length CIN85 is postranslationally modified by SUMOylation in wild-type podocytes. We can demonstrate that CIN85 is SUMOylated by SUMO-1, -2, and -3 and that SUMOylation is enhanced in the presence of CD2AP. Conversion of lysine 598 to arginine completely abolishes SUMOylation and leads to increased binding of CIN85 to nephrin. Our results indicate a novel role for CD2AP in regulating posttranslational modification of CIN85.

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.

Decreases in podocin, CD2-associated protein (CD2AP) and tensin2 may be involved in albuminuria during septic acute renal failure

Podocytes have a peculiar structure constituting slit diaphragm (SD) and foot process (FP), and play essential roles in the glomerular filtration barrier. There is now ample evidence that SD- and FP-associated molecules, such as podocin and CD2-associated protein (CD2AP), are down-regulated during albuminuria of chronic kidney disease. However, it is still unclear whether these molecules are altered during acute renal failure (ARF) with albuminuria. Using lipopolysaccharide (LPS)-treated mice as a model of septic ARF, we provide evidence that the expression of SD- and FP-associated molecules becomes faint, along with albuminuria. In the LPS-treated mice, urinary albumin levels gradually increased, associated with the elevation of blood urea nitrogen levels, indicating the successful induction of albuminuria during septic ARF. In this pathological process, glomerular podocin expression became faint, especially at 36 hr post-LPS challenge (i.e., a peak of albuminuria). Likewise, LPS treatment led to a significant decrease in CD2AP, an anchorage between podocin and F-actin. With regard to this, tensin2 is a novel molecule that stabilizes F-actin extension. Interestingly, glomerular tensin2 expression levels were also decreased during the albuminuric phase, associated with losses of glomerular F-actin and synaptopodin under septic states. As a result, there were some lesions of podocytic FP effacement, as shown by electron microscopy. Based on these data, we emphasize the importance of concomitant decreases in podocin, CD2AP and tensin2 during septic ARF-associated proteinuria.

Tyrosine phosphorylation-dependent activation of TRPC6 regulated by PLC-γ1 and nephrin: effect of mutations associated with focal segmental glomerulosclerosis

The surface expression and channel activation of transient receptor potential canonical 6 (TRPC6) were regulated by tyrosine phosphorylation and resultant binding with stimulatory PLC-γ1 and inhibitory nephrin. Disease-causing mutations made the TRPC6s insensitive to nephrin suppression, suggesting that the cell-type–specific regulation of TRPC6 might be involved in the pathogenesis.

Transient receptor potential canonicals (TRPCs) play important roles in the regulation of intracellular calcium concentration. Mutations in the TRPC6 gene are found in patients with focal segmental glomerulosclerosis (FSGS), a proteinuric disease characterized by dysregulated function of renal glomerular epithelial cells (podocytes). There is as yet no clear picture for the activation mechanism of TRPC6 at the molecular basis, however, and the association between its channel activity and pathogenesis remains unclear. We demonstrate here that tyrosine phosphorylation of TRPC6 induces a complex formation with phospholipase C (PLC)-γ1, which is prerequisite for TRPC6 surface expression. Furthermore, nephrin, an adhesion protein between the foot processes of podocytes, binds to phosphorylated TRPC6 via its cytoplasmic domain, competitively inhibiting TRPC6–PLC-γ1 complex formation, TRPC6 surface localization, and TRPC6 activation. Importantly, FSGS-associated mutations render the mutated TRPC6s insensitive to nephrin suppression, thereby promoting their surface expression and channel activation. These results delineate the mechanism of TRPC6 activation regulated by tyrosine phosphorylation, and imply the cell type–specific regulation, which correlates the FSGS mutations with deregulated TRPC6 channel activity.