Neph1, a component of the kidney slit diaphragm, is tyrosine-phosphorylated by the Src family tyrosine kinase and modulates intracellular signaling by binding to Grb2

FilGAP controls cell-extracellular matrix adhesion and process formation of kidney podocytes

The function of kidney podocytes is closely associated with actin cytoskeleton regulated by Rho small GTPases. Loss of actin-driven cell adhesions and processes is connected to podocyte dysfunction, proteinuria, and kidney diseases. FilGAP, a GTPase-activating protein for Rho small GTPase Rac1, is abundantly expressed in kidney podocytes, and its gene is linked to diseases in a family with focal segmental glomerulosclerosis. In this study, we have studied the role of FilGAP in podocytes in vitro. Depletion of FilGAP in cultured podocytes induced loss of actin stress fibers and increased Rac1 activity. Conversely, forced expression of FilGAP increased stress fiber formation whereas Rac1 activation significantly reduced its formation. FilGAP localizes at the focal adhesion (FA), an integrin-based protein complex closely associated with stress fibers, that mediates cell-extracellular matrix (ECM) adhesion, and FilGAP depletion decreased FA formation and impaired attachment to the ECM. Moreover, in unique podocyte cell cultures capable of inducing the formation of highly organized processes including major processes and foot process-like projections, FilGAP depletion or Rac1 activation decreased the formation of these processes. The reduction of FAs and process formations in FilGAP-depleted podocyte cells was rescued by inhibition of Rac1 or P21-activated kinase 1 (PAK1), a downstream effector of Rac1, and PAK1 activation inhibited their formations. Thus, FilGAP contributes to both cell-ECM adhesion and process formation of podocytes by suppressing Rac1/PAK1 signaling.

Identification of Novel Genetic Risk Factors for Focal Segmental Glomerulosclerosis in Children: Results From the Chronic Kidney Disease in Children (CKiD) Cohort

RATIONALE & OBJECTIVE

Focal segmental glomerulosclerosis (FSGS) is a major cause of pediatric nephrotic syndrome, and African Americans exhibit an increased risk for developing FSGS compared with other populations. Predisposing genetic factors have previously been described in adults. Here we performed genomic screening of primary FSGS in a pediatric African American population.

STUDY DESIGN

Prospective cohort with case-control genetic association study design.

SETTING & PARTICIPANTS

140 African American children with chronic kidney disease from the Chronic Kidney Disease in Children (CKiD) cohort, including 32 cases with FSGS.

PREDICTORS

Over 680,000 common single-nucleotide polymorphisms (SNPs) were tested for association. We also ran a pathway enrichment analysis and a human leucocyte antigen (HLA)-focused association study.

OUTCOME

Primary biopsy-proven pediatric FSGS.

ANALYTICAL APPROACH

Multivariate logistic regression models.

RESULTS

The genome-wide association study revealed 169 SNPs from 14 independent loci significantly associated with FSGS (false discovery rate [FDR]<5%). We observed notable signals for genetic variants within the APOL1 (P=8.6×10^-7; OR, 25.8 [95% CI, 7.1-94.0]), ALMS1 (P=1.3×10^-7; 13.0% in FSGS cases vs 0% in controls), and FGFR4 (P=4.3×10^-6; OR, 24.8 [95% CI, 6.3-97.7]) genes, all of which had previously been associated with adult FSGS, kidney function, or chronic kidney disease. We also highlighted novel, functionally relevant genes, including GRB2 (which encodes a slit diaphragm protein promoting podocyte structure through actin polymerization) and ITGB1 (which is linked to renal injuries). Our results suggest a major role for immune responses and antigen presentation in pediatric FSGS through (1) associations with SNPs in PTPRJ (or CD148, P=3.5×10^-7), which plays a role in T-cell receptor signaling, (2) HLA-DRB1∗11:01 association (P=6.1×10^-3; OR, 4.5 [95% CI, 1.5-13.0]), and (3) signaling pathway enrichment (P=1.3×10^-6).

LIMITATIONS

Sample size and no independent replication cohort with genomic data readily available.

CONCLUSIONS

Our genetic study has identified functionally relevant risk factors and the importance of immune regulation for pediatric primary FSGS, which contributes to a better description of its molecular pathophysiological mechanisms.

Glycosphingolipid GM3 prevents albuminuria and podocytopathy induced by anti-nephrin antibody

Podocytopathy, which is characterized by injury to podocytes, frequently causes proteinuria or nephrotic syndrome. There is currently a paucity of effective therapeutic drugs to treat proteinuric kidney disease. Recent research suggests the possibility that glycosphingolipid GM3 maintains podocyte function by acting on various molecules including nephrin, but its mechanism of action remains unknown. Here, various analyses were performed to examine the potential relationship between GM3 and nephrin, and the function of GM3 in podocytes using podocytopathy mice, GM3 synthase gene knockout mice, and nephrin injury cells. Reduced amounts of GM3 and nephrin were observed in podocytopathy mice. Intriguingly, this reduction of GM3 and nephrin, as well as albuminuria, were inhibited by administration of valproic acid. However, when the same experiment was performed using GM3 synthase gene knockout mice, valproic acid administration did not inhibit albuminuria. Equivalent results were obtained in model cells. These findings indicate that GM3 acts with nephrin in a collaborative manner in the cell membrane. Taken together, elevated levels of GM3 stabilize nephrin, which is a key molecule of the slit diaphragm, by enhancing the environment of the cell membrane and preventing albuminuria. This study provides novel insight into new drug discovery, which may offer a new therapy for kidney disease with albuminuria.

Transmembrane protein KIRREL1 regulates Hippo signaling via a feedback loop and represents a therapeutic target in YAP/TAZ-active cancers

The Hippo tumor-suppressor pathway is frequently dysregulated in human cancers and represents a therapeutic target. However, strategies targeting the mammalian Hippo pathway are limited because of the lack of a well-established cell-surface regulator. Here, we show that transmembrane protein KIRREL1, by interacting with both SAV1 and LATS1/2, promotes LATS1/2 activation by MST1/2 (Hippo kinases), and LATS1/2 activation, in turn, inhibits activity of YAP/TAZ oncoproteins. Conversely, YAP/TAZ directly induce the expression of KIRREL1 in a TEAD1-4-dependent manner. Indeed, KIRREL1 expression positively correlates with canonical YAP/TAZ target gene expression in clinical tumor specimens and predicts poor prognosis. Moreover, transgenic expression of KIRREL1 effectively blocks tumorigenesis in a mouse intrahepatic cholangiocarcinoma model, indicating a tumor-suppressor role of KIRREL1. Hence, KIRREL1 constitutes a negative feedback mechanism regulating the Hippo pathway and serves as a cell-surface marker and potential drug target in cancers with YAP/TAZ dependency.

New Insights into the Treatment of Glomerular Diseases: When Mechanisms Become Vivid

Treatment for glomerular diseases has been extrapolated from the experience of other autoimmune disorders while the underlying pathogenic mechanisms were still not well understood. As the classification of glomerular diseases was based on patterns of juries instead of mechanisms, treatments were typically the art of try and error. With the advancement of molecular biology, the role of the immune agent in glomerular diseases is becoming more evident. The four-hit theory based on the discovery of gd-IgA1 gives a more transparent outline of the pathogenesis of IgA nephropathy (IgAN), and dysregulation of Treg plays a crucial role in the pathogenesis of minimal change disease (MCD). An epoch-making breakthrough is the discovery of PLA2R antibodies in the primary membranous nephropathy (pMN). This is the first biomarker applied for precision medicine in kidney disease. Understanding the immune system's role in glomerular diseases allows the use of various immunosuppressants or other novel treatments, such as complement inhibitors, to treat glomerular diseases more reasonable. In this era of advocating personalized medicine, it is inevitable to develop precision medicine with mechanism-based novel biomarkers and novel therapies in kidney disease.

Renal cell markers: lighthouses for managing renal diseases

Kidneys, one of the vital organs in our body, are responsible for maintaining whole body homeostasis. The complexity of renal function (e.g., filtration, reabsorption, fluid and electrolyte regulation, and urine production) demands diversity not only at the level of cell types but also in their overall distribution and structural framework within the kidney. To gain an in depth molecular-level understanding of the renal system, it is imperative to discern the components of kidney and the types of cells residing in each of the subregions. Recent developments in labeling, tracing, and imaging techniques have enabled us to mark, monitor, and identify these cells in vivo with high efficiency in a minimally invasive manner. In this review, we summarize different cell types, specific markers that are uniquely associated with those cell types, and their distribution in the kidney, which altogether make kidneys so special and different. Cellular sorting based on the presence of certain proteins on the cell surface allowed for the assignment of multiple markers for each cell type. However, different studies using different techniques have found contradictions in cell type-specific markers. Thus, the term "cell marker" might be imprecise and suboptimal, leading to uncertainty when interpreting the data. Therefore, we strongly believe that there is an unmet need to define the best cell markers for a cell type. Although the compendium of renal-selective marker proteins presented in this review is a resource that may be useful to researchers, we acknowledge that the list may not be necessarily exhaustive.

Molecular and structural basis of olfactory sensory neuron axon coalescence by Kirrel receptors

Projections from sensory neurons of olfactory systems coalesce into glomeruli in the brain. The Kirrel receptors are believed to homodimerize via their ectodomains and help separate sensory neuron axons into Kirrel2- or Kirrel3-expressing glomeruli. Here, we present the crystal structures of homodimeric Kirrel receptors and show that the closely related Kirrel2 and Kirrel3 have evolved specific sets of polar and hydrophobic interactions, respectively, disallowing heterodimerization while preserving homodimerization, likely resulting in proper segregation and coalescence of Kirrel-expressing axons into glomeruli. We show that the dimerization interface at the N-terminal immunoglobulin (IG) domains is necessary and sufficient to create homodimers and fail to find evidence for a secondary interaction site in Kirrel ectodomains. Furthermore, we show that abolishing dimerization of Kirrel3 in vivo leads to improper formation of glomeruli in the mouse accessory olfactory bulb as observed in Kirrel3-/- animals. Our results provide evidence for Kirrel3 homodimerization controlling axonal coalescence.

Phosphorylation of slit diaphragm proteins NEPHRIN and NEPH1 upon binding of HGF promotes podocyte repair

Phosphorylation (activation) and dephosphorylation (deactivation) of the slit diaphragm proteins NEPHRIN and NEPH1 are critical for maintaining the kidney epithelial podocyte actin cytoskeleton and, therefore, proper glomerular filtration. However, the mechanisms underlying these events remain largely unknown. Here we show that NEPHRIN and NEPH1 are novel receptor proteins for hepatocyte growth factor (HGF) and can be phosphorylated independently of the mesenchymal epithelial transition receptor in a ligand-dependent fashion through engagement of their extracellular domains by HGF. Furthermore, we demonstrate SH2 domain–containing protein tyrosine phosphatase-2–dependent dephosphorylation of these proteins. To establish HGF as a ligand, purified baculovirus-expressed NEPHRIN and NEPH1 recombinant proteins were used in surface plasma resonance binding experiments. We report high-affinity interactions of NEPHRIN and NEPH1 with HGF, although NEPHRIN binding was 20-fold higher than that of NEPH1. In addition, using molecular modeling we constructed peptides that were used to map specific HGF-binding regions in the extracellular domains of NEPHRIN and NEPH1. Finally, using an in vitro model of cultured podocytes and an ex vivo model of Drosophila nephrocytes, as well as chemically induced injury models, we demonstrated that HGF-induced phosphorylation of NEPHRIN and NEPH1 is centrally involved in podocyte repair. Taken together, this is the first study demonstrating a receptor-based function for NEPHRIN and NEPH1. This has important biological and clinical implications for the repair of injured podocytes and the maintenance of podocyte integrity.

Evolutionary conservation of intrinsically unstructured regions in slit-diaphragm proteins

Vertebrate kidneys contribute to homeostasis by regulating electrolyte, acid-base balance, removing toxic metabolites from blood, and preventing protein loss into the urine. Glomerular podocytes constitute the blood-urine barrier, and podocyte slit-diaphragm (SD), a modified tight junction, contributes to the glomerular permselectivity. Nephrin, KIRREL1, podocin, CD2AP, and TRPC6 are crucial members of the SD that interact with each other and contribute to the SD's structural and functional integrity. This study analyzed the distribution of these five essential SD proteins across the organisms for which the genome sequence is available. We found a diverse distribution of nephrin and KIRREL1 ranging from nematodes to higher vertebrates, whereas podocin, CD2AP, and TRPC6 are restricted to the vertebrates. Among invertebrates, nephrin and its orthologs consist of more immunoglobulin-3 domains, whereas in the vertebrates, CD80-like C2-set domains are predominant. In the case of KIRREL1 and its orthologs, more Ig domains were observed in invertebrates than vertebrates. Src Homology-3 (SH3) domain of CD2AP and SPFH domain of podocin are highly conserved among vertebrates. TRPC6 and its orthologs had conserved ankyrin repeats, TRP, and ion transport domains, except Chondrichthyes and Echinodermata, which do not possess the ankyrin repeats. Intrinsically unstructured regions (IURs) are conserved across the SD orthologs, suggesting IURs importance in the protein complexes that constitute the slit-diaphragm. For the first time, a study reports the evolutionary insights of vertebrate SD proteins and their invertebrate orthologs.

Autonomous Calcium Signaling in Human and Zebrafish Podocytes Controls Kidney Filtration Barrier Morphogenesis

BACKGROUND

Podocytes are critical to maintaining the glomerular filtration barrier, and mutations in nephrotic syndrome genes are known to affect podocyte calcium signaling. However, the role of calcium signaling during podocyte development remains unknown.

METHODS

We undertook live imaging of calcium signaling in developing podocytes, using zebrafish larvae and human kidney organoids. To evaluate calcium signaling during development and in response to channel blockers and genetic defects, the calcium biosensor GCaMP6s was expressed in zebrafish podocytes. We used electron microscopy to evaluate filtration barrier formation in zebrafish, and Fluo-4 to detect calcium signals in differentiating podocytes in human kidney organoids.

RESULTS

Immature zebrafish podocytes (2.5 days postfertilization) generated calcium transients that correlated with interactions with forming glomerular capillaries. Calcium transients persisted until 4 days postfertilization, and were absent after glomerular barrier formation was complete. We detected similar calcium transients in maturing human organoid glomeruli, suggesting a conserved mechanism. In both models, inhibitors of SERCA or IP3 receptor calcium-release channels blocked calcium transients in podocytes, whereas lanthanum was ineffective, indicating the calcium source is from intracellular podocyte endoplasmic-reticulum stores. Calcium transients were not affected by blocking heartbeat or by blocking development of endothelium or endoderm, and they persisted in isolated glomeruli, suggesting podocyte-autonomous calcium release. Inhibition of expression of phospholipase C-γ1, but not nephrin or phospholipase C-ε1, led to significantly decreased calcium activity. Finally, blocking calcium release affected glomerular shape and podocyte foot process formation, supporting the critical role of calcium signaling in glomerular morphogenesis.

CONCLUSIONS

These findings establish podocyte cell-autonomous calcium signaling as a prominent and evolutionarily conserved feature of podocyte differentiation and demonstrate its requirement for podocyte foot process formation.

The emerging role of cellular post-translational modifications in modulating growth and productivity of recombinant Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are one of the most commonly used host cell lines used for the production human therapeutic proteins. Much research over the past two decades has focussed on improving the growth, titre and cell specific productivity of CHO cells and in turn lowering the costs associated with production of recombinant proteins. CHO cell engineering has become of particular interest in recent years following the publication of the CHO cell genome and the availability of data relating to the proteome, transcriptome and metabolome of CHO cells. However, data relating to the cellular post-translational modification (PTMs) which can affect the functionality of CHO cellular proteins has only begun to be presented in recent years. PTMs are important to many cellular processes and can further alter proteins by increasing the complexity of proteins and their interactions. In this review, we describe the research presented from CHO cells to date related on three of the most important PTMs; glycosylation, phosphorylation and ubiquitination.

High-speed single-molecule imaging reveals signal transduction by induced transbilayer raft phases

Using single-molecule imaging with enhanced time resolutions down to 5 ms, we found that CD59 cluster rafts and GM1 cluster rafts were stably induced in the outer leaflet of the plasma membrane (PM), which triggered the activation of Lyn, H-Ras, and ERK and continually recruited Lyn and H-Ras right beneath them in the inner leaflet with dwell lifetimes <0.1 s. The detection was possible due to the enhanced time resolutions employed here. The recruitment depended on the PM cholesterol and saturated alkyl chains of Lyn and H-Ras, whereas it was blocked by the nonraftophilic transmembrane protein moiety and unsaturated alkyl chains linked to the inner-leaflet molecules. Because GM1 cluster rafts recruited Lyn and H-Ras as efficiently as CD59 cluster rafts, and because the protein moieties of Lyn and H-Ras were not required for the recruitment, we conclude that the transbilayer raft phases induced by the outer-leaflet stabilized rafts recruit lipid-anchored signaling molecules by lateral raft-lipid interactions and thus serve as a key signal transduction platform.

SHROOM3, the gene associated with chronic kidney disease, affects the podocyte structure

Chronic kidney disease is a public health burden and it remains unknown which genetic loci are associated with kidney function in the Japanese population, our genome-wide association study using the Biobank Japan dataset (excluding secondary kidney diseases, such as diabetes mellitus) clearly revealed that almost half of the top 50 single nucleotide polymorphisms associated with estimated glomerular filtration rate are located in the SHROOM3 gene, suggesting that SHROOM3 will be responsible for kidney function. Thus, to confirm this finding, supportive functional analyses were performed on Shroom3 in mice using fullerene-based siRNA delivery, which demonstrated that Shroom3 knockdown led to albuminuria and podocyte foot process effacement. The in vitro experiment shows that knockdown of Shroom3 caused defective formation of lamellipodia in podocyte, which would lead to the disruption of slit diaphragm. These results from the GWAS, in vivo and in vitro experiment were consistent with recent studies reporting that albuminuria leads to impairment of kidney function.

Regulation of the Actin Cytoskeleton in Podocytes

Podocytes are an integral part of the glomerular filtration barrier, a structure that prevents filtration of large proteins and macromolecules into the urine. Podocyte function is dependent on actin cytoskeleton regulation within the foot processes, structures that link podocytes to the glomerular basement membrane. Actin cytoskeleton dynamics in podocyte foot processes are complex and regulated by multiple proteins and other factors. There are two key signal integration and structural hubs within foot processes that regulate the actin cytoskeleton: the slit diaphragm and focal adhesions. Both modulate actin filament extension as well as foot process mobility. No matter what the initial cause, the final common pathway of podocyte damage is dysregulation of the actin cytoskeleton leading to foot process retraction and proteinuria. Disruption of the actin cytoskeleton can be due to acquired causes or to genetic mutations in key actin regulatory and signaling proteins. Here, we describe the major structural and signaling components that regulate the actin cytoskeleton in podocytes as well as acquired and genetic causes of actin dysregulation.

New insight into podocyte slit diaphragm, a therapeutic target of proteinuria

Dysfunction of slit diaphragm, a cell–cell junction of glomerular podocytes, is involved in the development of proteinuria in several glomerular diseases. Slit diaphragm should be a target of a novel therapy for proteinuria. Nephrin, NEPH1, P-cadherin, FAT, and ephrin-B1 were reported to be extracellular components forming a molecular sieve of the slit diaphragm. Several cytoplasmic proteins such as ZO-1, podocin, CD2AP, MAGI proteins and Par-complex molecules were identified as scaffold proteins linking the slit diaphragm to the cytoskeleton. In this article, new insights into these molecules and the pathogenic roles of the dysfunction of these molecules were introduced. The slit diaphragm functions not only as a barrier but also as a signaling platform transfer the signal to the inside of the cell. For maintaining the slit diaphragm function properly, the phosphorylation level of nephrin is strictly regulated. The recent studies on the signaling pathway from nephrin, NEPH1, and ephrin-B1 were reviewed. Although the mechanism regulating the function of the slit diaphragm had remained unclear, recent studies revealed TRPC6 and angiotensin II-regulating mechanisms play a critical role in regulating the barrier function of the slit diaphragm. In this review, recent investigations on the regulation of the slit diaphragm function were reviewed, and a strategy for the establishment of a novel therapy for proteinuria was proposed.

A Multi-layered Quantitative In Vivo Expression Atlas of the Podocyte Unravels Kidney Disease Candidate Genes

Damage to and loss of glomerular podocytes has been identified as the culprit lesion in progressive kidney diseases. Here, we combine mass spectrometry-based proteomics with mRNA sequencing, bioinformatics, and hypothesis-driven studies to provide a comprehensive and quantitative map of mammalian podocytes that identifies unanticipated signaling pathways. Comparison of the in vivo datasets with proteomics data from podocyte cell cultures showed a limited value of available cell culture models. Moreover, in vivo stable isotope labeling by amino acids uncovered surprisingly rapid synthesis of mitochondrial proteins under steady-state conditions that was perturbed under autophagy-deficient, disease-susceptible conditions. Integration of acquired omics dimensions suggested FARP1 as a candidate essential for podocyte function, which could be substantiated by genetic analysis in humans and knockdown experiments in zebrafish. This work exemplifies how the integration of multi-omics datasets can identify a framework of cell-type-specific features relevant for organ health and disease.

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Deep proteome and transcriptome analyses of native podocytes unravel druggable targets

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Static and dynamic proteomics uncover features of podocyte identity and proteostasis

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Candidate genes for nephrotic syndrome were predicted based on multi-omic integration

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FARP1 is a previously unreported candidate gene for human proteinuric kidney disease

Mutations in KIRREL1, a slit diaphragm component, cause steroid-resistant nephrotic syndrome

Steroid-resistant nephrotic syndrome is a frequent cause of chronic kidney disease almost inevitably progressing to end-stage renal disease. More than 58 monogenic causes of SRNS have been discovered and majority of known steroid-resistant nephrotic syndrome causing genes are predominantly expressed in glomerular podocytes, placing them at the center of disease pathogenesis. Herein, we describe two unrelated families with steroid-resistant nephrotic syndrome with homozygous mutations in the KIRREL1 gene. One mutation showed high frequency in the European population (minor allele frequency 0.0011) and this patient achieved complete remission following treatment, but later progressed to chronic kidney disease. We found that mutant KIRREL1 proteins failed to localize to the podocyte cell membrane, indicating defective trafficking and impaired podocytes function. Thus, the KIRREL1 gene product has an important role in modulating the integrity of the slit diaphragm and maintaining glomerular filtration function.

Proteinuric Kidney Diseases: A Podocyte's Slit Diaphragm and Cytoskeleton Approach

Proteinuric kidney diseases are a group of disorders with diverse pathological mechanisms associated with significant losses of protein in the urine. The glomerular filtration barrier (GFB), comprised of the three important layers, the fenestrated glomerular endothelium, the glomerular basement membrane (GBM), and the podocyte, dictates that disruption of any one of these structures should lead to proteinuric disease. Podocytes, in particular, have long been considered as the final gatekeeper of the GFB. This specialized visceral epithelial cell contains a complex framework of cytoskeletons forming foot processes and mediate important cell signaling to maintain podocyte health. In this review, we will focus on slit diaphragm proteins such as nephrin, podocin, TRPC6/5, as well as cytoskeletal proteins Rho/small GTPases and synaptopodin and their respective roles in participating in the pathogenesis of proteinuric kidney diseases. Furthermore, we will summarize the potential therapeutic options targeting the podocyte to treat this group of kidney diseases.

Candidate genes in gastric cancer identified by constructing a weighted gene co-expression network

Background

Gastric cancer (GC) is one of the most common cancers with high mortality globally. However, the molecular mechanisms of GC are unclear, and the prognosis of GC is poor. Therefore, it is important to explore the underlying mechanisms and screen for novel prognostic markers and treatment targets.

Methods

The genetic and clinical data of GC patients in The Cancer Genome Atlas (TCGA) was analyzed by weighted gene co-expression network analysis (WGCNA). Modules with clinical significance and preservation were distinguished, and gene ontology and pathway enrichment analysis were performed. Hub genes of these modules were validated in the TCGA dataset and another independent dataset from the Gene Expression Omnibus (GEO) database by t-test. Furthermore, the significance of these genes was confirmed via survival analysis.

Results

We found a preserved module consisting of 506 genes was associated with clinical traits including pathologic T stage and histologic grade. PDGFRB, COL8A1, EFEMP2, FBN1, EMILIN1, FSTL1 and KIRREL were identified as candidate genes in the module. Their expression levels were correlated with pathologic T stage and histologic grade, also affected overall survival of GC patients.

Conclusion

These candidate genes may be involved in proliferation and differentiation of GC cells. They may serve as novel prognostic markers and treatment targets. Moreover, most of them were first reported in GC and deserved further research.

Acute Kidney Injury and Progression of Diabetic Kidney Disease

Diabetic kidney disease, commonly termed diabetic nephropathy (DN), is the most common cause of end-stage kidney disease (ESKD) worldwide. The characteristic histopathology of DN includes glomerular basement membrane thickening, mesangial expansion, nodular glomerular sclerosis, and tubulointerstitial fibrosis. Diabetes is associated with a number of metabolic derangements, such as reactive oxygen species overproduction, hypoxic state, mitochondrial dysfunction, and inflammation. In the past few decades, our knowledge of DN has advanced considerably although much needs to be learned. The traditional paradigm of glomerulus-centered pathophysiology has expanded to the tubule-interstitium, the immune response and inflammation. Biomarkers of proximal tubule injury have been shown to correlate with DN progression, independent of traditional glomerular injury biomarkers such as albuminuria. In this review, we summarize mechanisms of increased susceptibility to acute kidney injury in diabetes mellitus and the roles played by many kidney cell types to facilitate maladaptive responses leading to chronic and end-stage kidney disease.

KIRREL is differentially expressed in adipose tissue from 'fertil+' and 'fertil-' cows: in vitro role in ovary?

We have previously shown that dairy cows carrying the 'fertil-' haplotype for one quantitative trait locus affecting female fertility located on the bovine chromosome three (QTL-F-Fert-BTA3) have a significantly lower conception rate and body weight after calving than cows carrying the 'fertil+' haplotype. Here, we compared by Tiling Array the expression of genes included in the QTL-F-Fert-BTA3 in 'fertil+' and 'fertil-' adipose tissue one week after calving when plasma non-esterified fatty acid concentrations were greater in 'fertil-' animals. We observed that thirty-one genes were overexpressed whereas twelve were under-expressed in 'fertil+' as compared to 'fertil-' cows (P < 0.05). By quantitative PCR and immunoblot we confirmed that adipose tissue KIRREL mRNA and protein were significantly greater expressed in 'fertil+' than in 'fertil-'. KIRREL mRNA is abundant in bovine kidney, adipose tissue, pituitary, and ovary and detectable in hypothalamus and mammary gland. Its expression (mRNA and protein) is greater in kidney of 'fertil+' than 'fertil-' cows (P < 0.05). KIRREL (mRNA and protein) is also present in the different ovarian cells with a greater expression in granulosa cells of 'fertil+' than 'fertil-' cows. In cultured granulosa cells, recombinant KIRREL halved steroid secretion in basal state (P < 0.05). It also decreased cell proliferation (P < 0.05) and in vitro oocyte maturation (P < 0.05). These results were associated to a rapid increase in MAPK1/3 and MAPK14 phosphorylation in granulosa cells and to a decrease in MAPK1/3 phosphorylation in oocyte. Thus, KIRREL could be a potential metabolic messenger linking body composition and fertility.

Identification of novel therapeutic target genes and pathway in pancreatic cancer by integrative analysis

BACKGROUND

Gene alterations are crucial to the molecular pathogenesis of pancreatic cancer. The present study was designed to identify the potential candidate genes in the pancreatic carcinogenesis.

METHODS

Gene Expression Omnibus database (GEO) datasets of pancreatic cancer tissue were retrieval and the differentially expressed genes (DEGs) from individual microarray data were merged. Gene Ontology (GO) enrichment analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, protein-protein interaction (PPI) networks, and gene coexpression analysis were performed.

RESULTS

Three GEO datasets, including 74 pancreatic cancer samples and 55 controls samples were selected. A total of 2325 DEGs were identified, including 1383 upregulated and 942 downregulated genes. The GO terms for molecular functions, biological processes, and cellular component were protein binding, small molecule metabolic process, and integral to membrane, respectively. The most significant pathway in KEGG analysis was metabolic pathways. PPI network analysis indicated that the significant hub genes including cytochrome P450, family 2, subfamily E, polypeptide 1 (CYP2E1), mitogen-activated protein kinase 3 (MAPK3), and phospholipase C, gamma 1 (PLCG1). Gene coexpression network analysis identified 4 major modules, and the potassium channel tetramerization domain containing 10 (KCTD10), kin of IRRE like (KIRREL), dipeptidyl-peptidase 10 (DPP10), and unc-80 homolog (UNC80) were the hub gene of each modules, respectively.

CONCLUSION

Our integrative analysis provides a comprehensive view of gene expression patterns associated with the pancreatic carcinogenesis.

Targeting Neph1 and ZO-1 protein-protein interaction in podocytes prevents podocyte injury and preserves glomerular filtration function

Targeting protein-protein interaction (PPI) is rapidly becoming an attractive alternative for drug development. While drug development commonly involves inhibiting a PPI, in this study, we show that stabilizing PPI may also be therapeutically beneficial. Junctional proteins Neph1 and ZO-1 and their interaction is an important determinant of the structural integrity of slit diaphragm, which is a critical component of kidney's filtration system. Since injury induces loss of this interaction, we hypothesized that strengthening this interaction may protect kidney's filtration barrier and preserve kidney function. In this study, Neph1-ZO-1 structural complex was screened for the presence of small druggable pockets formed from contributions from both proteins. One such pocket was identified and screened using a small molecule library. Isodesmosine (ISD) a rare naturally occurring amino acid and a biomarker for pulmonary arterial hypertension was selected as the best candidate and to establish the proof of concept, its ability to enhance Neph1-CD and ZO-1 binding was tested. Results from biochemical binding analysis showed that ISD enhanced Neph1 and ZO-1 interaction under in vitro and in vivo conditions. Importantly, ISD treated podocytes were resistant to injury-induced loss of transepithelial permeability. Finally, mouse and zebrafish studies show that ISD protects from injury-induced renal damage.

The crucial role of protein phosphorylation in cell signaling and its use as targeted therapy (Review)

Protein phosphorylation is an important cellular regulatory mechanism as many enzymes and receptors are activated/deactivated by phosphorylation and dephosphorylation events, by means of kinases and phosphatases. In particular, the protein kinases are responsible for cellular transduction signaling and their hyperactivity, malfunction or overexpression can be found in several diseases, mostly tumors. Therefore, it is evident that the use of kinase inhibitors can be valuable for the treatment of cancer. In this review, we discuss the mechanism of action of phosphorylation, with particular attention to the importance of phosphorylation under physiological and pathological conditions. We also discuss the possibility of using kinase inhibitors in the treatment of tumors.

Glomerulosclerosis Induced by Deficiency of Membrane-Associated Guanylate Kinase Inverted 2 in Kidney Podocytes

Membrane-associated guanylate kinase inverted 2 (MAGI-2) is a component of the slit diaphragm (SD) of glomerular podocytes. Here, we investigated the podocyte-specific function of MAGI-2 using newly generated podocyte-specific MAGI-2-knockout (MAGI-2-KO) mice. Compared with podocytes from wild-type mice, podocytes from MAGI-2-KO mice exhibited SD disruption, morphologic abnormalities of foot processes, and podocyte apoptosis leading to podocyte loss. These pathologic changes manifested as massive albuminuria by 8 weeks of age and glomerulosclerosis and significantly higher plasma creatinine levels at 12 weeks of age; all MAGI-2-KO mice died by 20 weeks of age. Loss of MAGI-2 in podocytes associated with decreased expression and nuclear translocation of dendrin, which is also a component of the SD complex. Dendrin translocates from the SD to the nucleus of injured podocytes, promoting apoptosis. Our coimmunoprecipitation and in vitro reconstitution studies showed that dendrin is phosphorylated by Fyn and dephosphorylated by PTP1B, and that Fyn-induced phosphorylation prevents Nedd4-2-mediated ubiquitination of dendrin. Under physiologic conditions in vivo, phosphorylated dendrin localized at the SDs; in the absence of MAGI-2, dephosphorylated dendrin accumulated in the nucleus. Furthermore, induction of experimental GN in rats led to the downregulation of MAGI-2 expression and the nuclear accumulation of dendrin in podocytes. In summary, MAGI-2 and Fyn protect dendrin from Nedd4-2-mediated ubiquitination and from nuclear translocation, thereby maintaining the physiologic homeostasis of podocytes, and the lack of MAGI-2 in podocytes results in FSGS.

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.

GLCCI1 is a novel component associated with the PI3K signaling pathway in podocyte foot processes

Podocyte foot processes are interdigitated to form the slit diaphragm and are crucial for the glomerular filtration barrier. Glucocorticoid-induced transcript 1 (GLCCI1) is transcriptionally regulated, but its signaling pathway in podocytes is unknown. The main objective of this study was to investigate the regulation of podocyte foot process proteins and to investigate the role of GLCCI1 in the phosphoinositide 3-kinase (PI3K) pathway using high glucose-induced podocytes and streptozotocin-induced diabetic rats. In podocytes and rat kidneys, GLCCI1 was found to be highly specific for the glomerulus and podocyte foot processes similar to other podocyte-specific proteins (nephrin, podocin, synatopodin and podocalyxin) based on reverse transcription-PCR, western blotting, immunofluorescence and immunoelectron microscopy analyses. In addition, the decrease in the GLCCI1 expression level under hyperglycemic conditions was restored by treatment with a PI3K inhibitor (wortmannin). Immunofluorescence analysis confirmed that GLCCI1 colocalized with nephrin and synaptopodin both in vivo and in vitro. Finally, immunoelectron microscopy data from streptozotocin-induced diabetic rats showed that GLCCI1 also localized in podocyte foot processes. Hence, GLCCI1 is a component of podocyte foot processes, and its expression appears to be regulated via the PI3K pathway.

Cell biology of mesangial cells: the third cell that maintains the glomerular capillary

The renal glomerulus consists of glomerular endothelial cells, podocytes, and mesangial cells, which cooperate with each other for glomerular filtration. We have produced monoclonal antibodies against glomerular cells in order to identify different types of glomerular cells. Among these antibodies, the E30 clone specifically recognizes the Thy1.1 molecule expressed on mesangial cells. An injection of this antibody into rats resulted in mesangial cell-specific injury within 15 min, and induced mesangial proliferative glomerulonephritis in a reproducible manner. We examined the role of mesangial cells in glomerular function using several experimental tools, including an E30-induced nephritis model, mesangial cell culture, and the deletion of specific genes. Herein, we describe the characterization of E30-induced nephritis, formation of the glomerular capillary network, mesangial matrix turnover, and intercellular signaling between glomerular cells. New molecules that are involved in a wide variety of mesangial cell functions are also introduced.

Kin of IRRE-like Protein 2 Is a Phosphorylated Glycoprotein That Regulates Basal Insulin Secretion

Direct interactions among pancreatic β-cells via cell surface proteins inhibit basal and enhance stimulated insulin secretion. Here, we functionally and biochemically characterized Kirrel2, an immunoglobulin superfamily protein with β-cell-specific expression in the pancreas. Our results show that Kirrel2 is a phosphorylated glycoprotein that co-localizes and interacts with the adherens junction proteins E-cadherin and β-catenin in MIN6 cells. We further demonstrate that the phosphosites Tyr(595-596) are functionally relevant for the regulation of Kirrel2 stability and localization. Analysis of the extracellular and intracellular domains of Kirrel2 revealed that it is cleaved and shed from MIN6 cells and that the remaining membrane spanning cytoplasmic domain is processed by γ-secretase complex. Kirrel2 knockdown with RNA interference in MIN6 cells and ablation of Kirrel2 from mice with genetic deletion resulted in increased basal insulin secretion from β-cells, with no immediate influence on stimulated insulin secretion, total insulin content, or whole body glucose metabolism. Our results show that in pancreatic β-cells Kirrel2 localizes to adherens junctions, is regulated by multiple post-translational events, including glycosylation, extracellular cleavage, and phosphorylation, and engages in the regulation of basal insulin secretion.

Proteomic analysis of the kidney filtration barrier--Problems and perspectives

Diseases of the glomerular filter of the kidney are a leading cause of end-stage renal failure. The kidney filter is localized within the renal glomeruli, small microvascular units that are responsible for ultrafiltration of about 180 liters of primary urine every day. The renal filter consists of three layers, fenestrated endothelial cells, glomerular basement membrane, and the podocytes, terminally differentiated, arborized epithelial cells. This review demonstrates the use of proteomics to generate insights into the regulation of the renal filtration barrier at a molecular level. The advantages and disadvantages of different glomerular purification methods are examined, and the technical limitations that have been significantly improved by in silico or biochemical approaches are presented. We also comment on phosphoproteomic studies that have generated considerable molecular-level understanding of the physiological regulation of the kidney filter. Lastly, we conclude with an analysis of urinary exosomes as a potential filter-derived resource for the noninvasive discovery of glomerular disease mechanisms.

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.

Comparative phosphoproteomic analysis of mammalian glomeruli reveals conserved podocin C-terminal phosphorylation as a determinant of slit diaphragm complex architecture

Glomerular biology is dependent on tightly controlled signal transduction networks that control phosphorylation of signaling proteins such as cytoskeletal regulators or slit diaphragm proteins of kidney podocytes. Cross-species comparison of phosphorylation events is a powerful mean to functionally prioritize and identify physiologically meaningful phosphorylation sites. Here, we present the result of phosphoproteomic analyses of cow and rat glomeruli to allow cross-species comparisons. We discovered several phosphorylation sites with potentially high biological relevance, e.g. tyrosine phosphorylation of the cytoskeletal regulator synaptopodin and the slit diaphragm protein neph-1 (Kirrel). Moreover, cross-species comparisons revealed conserved phosphorylation of the slit diaphragm protein nephrin on an acidic cluster at the intracellular terminus and conserved podocin phosphorylation on the very carboxyl terminus of the protein. We studied a highly conserved podocin phosphorylation site in greater detail and show that phosphorylation regulates affinity of the interaction with nephrin and CD2AP. Taken together, these results suggest that species comparisons of phosphoproteomic data may reveal regulatory principles in glomerular biology. All MS data have been deposited in the ProteomeXchange with identifier PXD001005 (http://proteomecentral.proteomexchange.org/dataset/PXD001005).

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.

Slit diaphragm protein Neph1 and its signaling: a novel therapeutic target for protection of podocytes against glomerular injury

Podocytes are specialized epithelial cells that are critical components of the glomerular filtration barrier, and their dysfunction leads to proteinuria and renal failure. Therefore, preserving podocyte function is therapeutically significant. In this study, we identified Neph1 signaling as a therapeutic target that upon inhibition prevented podocyte damage from a glomerular injury-inducing agent puromycin aminonucleoside (PAN). To specifically inhibit Neph1 signaling, we used a protein transduction approach, where the cytoplasmic domain of Neph1 (Neph1CD) tagged with a protein transduction domain trans-activator of transcription was transduced in cultured podocytes prior to treatment with PAN. The PAN-induced Neph1 phosphorylation was significantly reduced in Neph1CD-transduced cells; in addition, these cells were resistant to PAN-induced cytoskeletal damage. The biochemical analysis using subfractionation studies showed that unlike control cells Neph1 was retained in the lipid raft fractions in the transduced cells following treatment with PAN, indicating that transduction of Neph1CD in podocytes prevented PAN-induced mislocalization of Neph1. In accordance, the immunofluorescence analysis further suggested that Neph1CD-transduced cells had increased ability to retain endogenous Neph1 at the membrane in response to PAN-induced injury. Similar results were obtained when angiotensin was used as an injury-inducing agent. Consistent with these observations, maintaining high levels of Neph1 at the membrane using a podocyte cell line overexpressing chimeric Neph1 increased the ability of podocytes to resist PAN-induced injury and PAN-induced albumin leakage. Using a zebrafish in vivo PAN and adriamycin injury models, we further demonstrated the ability of transduced Neph1CD to preserve glomerular function. Collectively, these results support the conclusion that inhibiting Neph1 signaling is therapeutically significant in preventing podocyte damage from glomerular injury.

Src64B phosphorylates Dumbfounded and regulates slit diaphragm dynamics: Drosophila as a model to study nephropathies

Drosophila nephrocytes are functionally homologous to vertebrate kidney podocytes. Both share the presence of slit diaphragms that function as molecular filters during the process of blood and haemolymph ultrafiltration. The protein components of the slit diaphragm are likewise conserved between flies and humans, but the mechanisms that regulate slit diaphragm dynamics in response to injury or nutritional changes are still poorly characterised. Here, we show that Dumbfounded/Neph1, a key diaphragm constituent, is a target of the Src kinase Src64B. Loss of Src64B activity leads to a reduction in the number of diaphragms, and this effect is in part mediated by loss of Dumbfounded/Neph1 tyrosine phosphorylation. The phosphorylation of Duf by Src64B, in turn, regulates Duf association with the actin regulator Dock. We also find that diaphragm damage induced by administration of the drug puromycin aminonucleoside (PAN model) directly associates with Src64B hyperactivation, suggesting that diaphragm stability is controlled by Src-dependent phosphorylation of diaphragm components. Our findings indicate that the balance between diaphragm damage and repair is controlled by Src-dependent phosphorylation of diaphragm components, and point to Src family kinases as novel targets for the development of pharmacological therapies for the treatment of kidney diseases that affect the function of the glomerular filtration barrier.

Podocyte expression of nonmuscle myosin heavy chain-IIA decreases in idiopathic nephrotic syndrome, especially in focal segmental glomerulosclerosis

BACKGROUND

Previous studies have identified significant associations between the development of idiopathic focal segmental glomerulosclerosis (FSGS) and MYH9 encoding nonmuscle myosin heavy chain-IIA (NMMHC-IIA). However, these studies focused only on the linkage of MYH9 polymorphisms and development of FSGS. There have been no reports on pathological changes of NMMHC-IIA in human glomerular diseases. Here we report on the precise localization of NMMHC-IIA in podocytes and changes in NMMHC-IIA expression in pathological states in rats and humans.

METHODS

Immunocytochemical (immunofluorescence and immunoelectron microscopy) studies were performed to determine the precise localization of NMMHC-IIA. Expression levels of NMMHC-IIA were investigated in puromycin aminonucleoside (PAN)-treated rats; and expression levels of NMMHC-IIA and other podocyte-related proteins were investigated in glomeruli of patients with idiopathic FSGS and other heavy proteinuric glomerular diseases.

RESULTS

NMMHC-IIA was located primarily at the cell body and primary processes of podocytes; this localization is distinct from other podocyte-related molecules causing hereditary FSGS. In PAN-treated rat kidneys, expression levels of NMMHC-IIA in podocytes decreased. Immunohistochemical analysis revealed that expression levels of NMMHC-IIA markedly decreased in idiopathic nephrotic syndrome, especially FSGS, whereas it did not change in other chronic glomerulonephritis showing apparent proteinuria. Changes in NMMHC-IIA expression were observed in glomeruli where expression of nephrin and synaptopodin was maintained.

CONCLUSIONS

Considering previous genome-wide association studies and development of FSGS in patients with MYH9 mutations, the characteristic localization of NMMHC-IIA and the specific decrease in NMMHC-IIA expression in idiopathic nephrotic syndrome, especially FSGS, suggest the important role of NMMHC-IIA in the development of FSGS.

Identification and characterization of novel Kirrel isoform during myogenesis

Somatic cell fusion is an essential component of skeletal muscle development and growth and repair from injury. Additional cell types such as trophoblasts and osteoclasts also require somatic cell fusion events to perform their physiological functions. Currently we have rudimentary knowledge on molecular mechanisms regulating somatic cell fusion events in mammals. We therefore investigated during in vitro murine myogenesis a mammalian homolog, Kirrel, of the Drosophila Melanogaster genes Roughest (Rst) and Kin of Irre (Kirre) which regulate somatic muscle cell fusion during embryonic development. Our results demonstrate the presence of a novel murine Kirrel isoform containing a truncated cytoplasmic domain which we term Kirrel B. Protein expression levels of Kirrel B are inverse to the occurrence of cell fusion events during in vitro myogenesis which is in stark contrast to the expression profile of Rst and Kirre during myogenesis in Drosophila. Furthermore, chemical inhibition of cell fusion confirmed the inverse expression pattern of Kirrel B protein levels in relation to cell fusion events. The discovery of a novel Kirrel B protein isoform during myogenesis highlights the need for more thorough investigation of the similarities and potential differences between fly and mammals with regards to the muscle cell fusion process.

Claudins reign: The claudin/EMP/PMP22/γ channel protein family in C. elegans

The claudin family of integral membrane proteins was identified as the major protein component of the tight junctions in all vertebrates. Since their identification, claudins, and their associated pfam00822 superfamily of proteins have been implicated in a wide variety of cellular processes. Claudin homologs have been identified in invertebrates as well, including Drosophila and C. elegans. Recent studies demonstrate that the C. elegans claudins, clc-1-clc- 5, and similar proteins in the greater PMP22/EMP/claudin/voltage-gated calcium channel γ subunit family, including nsy-4, and vab-9, while highly divergent at a sequence level from each other and from the vertebrate claudins, in many cases play roles similar to those traditionally assigned to their vertebrate homologs. These include regulating cell adhesion and passage of small molecules through the paracellular space, channel activity, protein aggregation, sensitivity to pore-forming toxins, intercellular signaling, cell fate specification and dynamic changes in cell morphology. Study of claudin superfamily proteins in C. elegans should continue to provide clues as to how claudin family protein function has been adapted to perform diverse functions at specialized cell-cell contacts in metazoans.

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.

Glomerular Filtration Barrier Assembly: An insight

A glomerulus is the network of capillaries that resides in the Bowman's capsule that functions as a filtration unit of kidney. The glomerular function ensures that essential plasma proteins are retained in blood and the filtrate is passed on as urine. The glomerular filtration assembly is composed of three main cellular barriers that are critical for the ultrafiltration process, the fenestrated endothelium, glomerular basement membrane and highly specialized podocytes. The podocytes along with their specialized junctions "slit diaphragm" form the basic backbone of this filtration assembly. The presence of high amounts of protein in urine a condition commonly referred as proteinuria indicates a defective glomerular filtration barrier. Various glomerular disorders including Nephrotic syndrome are characterized by significant alteration in the structure of podocytes that is associated with prolonged increase in the glomerular permeability leading to heavy proteinuria. Recent identification of proteins that are specifically localized at the slit diaphragm whose mutations and knockouts are known to result in loss of renal function has significantly advanced our understanding of the molecular makeup of this filtration assembly. The present review is an effort to summarize the recent developments in this field and highlight our understanding of the glomerular filtration barrier assembly.

ZO-1 regulates Erk, Smad1/5/8, Smad2, and RhoA activities to modulate self-renewal and differentiation of mouse embryonic stem cells

ZO-1/Tjp1 is a cytosolic adaptor that links tight junction (TJ) transmembrane proteins to the actin cytoskeleton and has also been implicated in regulating cell proliferation and differentiation by interacting with transcriptional regulators and signaling proteins. To explore possible roles for ZO-1 in mouse embryonic stem cells (mESCs), we inactivated the ZO-1 locus by homologous recombination. The lack of ZO-1 was found to affect mESC self-renewal and differentiation in the presence of leukemia-inhibiting factor (LIF) and Bmp4 or following removal of the growth factors. Our data suggest that ZO-1 suppresses Stat3 and Smad1/5/8 activities and sustains extracellular-signal-regulated kinase (Erk) activity to promote mESC differentiation. Interestingly, Smad2, critical for human but not mESC self-renewal, was hyperactivated in ZO-1(-/-) mESCs and RhoA protein levels were concomitantly enhanced, suggesting attenuation of the noncanonical transforming growth factor β (Tgfβ)/Activin/Nodal pathway that mediates ubiquitination and degradation of RhoA via the TJ proteins Occludin, Par6, and Smurf1 and activation of the canonical Smad2-dependent pathway. Furthermore, Bmp4-induced differentiation of mESCs in the absence of LIF was suppressed in ZO-1(-/-) mESCs, but differentiation down the neural or cardiac lineages was not disturbed. These findings reveal novel roles for ZO-1 in mESC self-renewal, pluripotency, and differentiation by influencing several signaling networks that regulate these processes. Possible implications for the differing relevance of Smad2 in mESC and human ESC self-renewal and how ZO-1 may connect to the different pathways are discussed.

The adaptor protein Grb2 is not essential for the establishment of the glomerular filtration barrier

The kidney filtration barrier is formed by the combination of endothelial cells, basement membrane and epithelial cells called podocytes. These specialized actin-rich cells form long and dynamic protrusions, the foot processes, which surround glomerular capillaries and are connected by specialized intercellular junctions, the slit diaphragms. Failure to maintain the filtration barrier leads to massive proteinuria and nephrosis. A number of proteins reside in the slit diaphragm, notably the transmembrane proteins Nephrin and Neph1, which are both able to act as tyrosine phosphorylated scaffolds that recruit cytoplasmic effectors to initiate downstream signaling. While association between tyrosine-phosphorylated Neph1 and the SH2/SH3 adaptor Grb2 was shown in vitro to be sufficient to induce actin polymerization, in vivo evidence supporting this finding is still lacking. To test this hypothesis, we generated two independent mouse lines bearing a podocyte-specific constitutive inactivation of the Grb2 locus. Surprisingly, we show that mice lacking Grb2 in podocytes display normal renal ultra-structure and function, thus demonstrating that Grb2 is not required for the establishment of the glomerular filtration barrier in vivo. Moreover, our data indicate that Grb2 is not required to restore podocyte function following kidney injury. Therefore, although in vitro experiments suggested that Grb2 is important for the regulation of actin dynamics, our data clearly shows that its function is not essential in podocytes in vivo, thus suggesting that Grb2 rather plays a secondary role in this process.

Direct regulation of nephrin tyrosine phosphorylation by Nck adaptor proteins

The transmembrane protein nephrin is a key component of the kidney slit diaphragm that contributes to the morphology of podocyte foot processes through signaling to the underlying actin cytoskeleton. We have recently reported that tyrosine phosphorylation of the cytoplasmic tail of nephrin facilitates recruitment of Nck SH2/SH3 adaptor proteins and subsequent actin remodeling and that phosphorylation of the Nck binding sites on nephrin is decreased during podocyte injury. We now demonstrate that Nck directly modulates nephrin phosphorylation through formation of a signaling complex with the Src family kinase Fyn. The ability of Nck to enhance nephrin phosphorylation is compromised in the presence of a Src family kinase inhibitor and when the SH3 domains of Nck are mutated. Furthermore, induced loss of Nck expression in podocytes in vivo is associated with a rapid reduction in nephrin tyrosine phosphorylation. Our results suggest that Nck may facilitate dynamic signaling events at the slit diaphragm by promoting Fyn-dependent phosphorylation of nephrin, which may be important in the regulation of foot process morphology and response to podocyte injury.

SIRPα interacts with nephrin at the podocyte slit diaphragm

The slit diaphragm (SD) is an intercellular junction between renal glomerular epithelial cells (podocytes) that is essential for permselectivity in glomerular ultrafiltration. The SD components, nephrin and Neph1, assemble a signaling complex in a tyrosine phosphorylation dependent manner, and regulate the unique actin cytoskeleton of podocytes. Mutations in the NPHS1 gene that encodes nephrin cause congenital nephrotic syndrome (CNS), which is characterized by the loss of the SD and massive proteinuria. Recently, we have identified the expression of the transmembrane glycoprotein signal regulatory protein α (SIRPα) at the SD. In the present study, we analyzed the expression of SIRPα in developing kidneys, in kidneys from CNS patients and in proteinuric rat models. The possibility that SIRPα interacts with known SD proteins was also investigated. SIRPα was concentrated at the SD junction during the maturation of intercellular junctions. In the glomeruli of CNS patients carrying mutations in NPHS1, where SD formation is disrupted, the expression of SIRPα as well as Neph1 and nephrin was significantly decreased, indicating that SIRPα is closely associated with the nephrin complex. Indeed, SIRPα formed hetero-oligomers with nephrin in cultured cells and in glomeruli. Furthermore, the cytoplasmic domain of SIRPα was highly phosphorylated in normal glomeruli, and its phosphorylation was dramatically decreased upon podocyte injury in vivo. Thus, SIRPα interacts with nephrin at the SD, and its phosphorylation is dynamically regulated in proteinuric states. Our data provide new molecular insights into the phosphorylation events triggered by podocyte injury.

Progress in pathogenesis of proteinuria

Aims. Proteinuria not only is a sign of kidney damage, but also is involved in the progression of renal diseases as an independent pathologic factor. Clinically, glomerular proteinuria is most commonly observed, which relates to structural and functional anomalies in the glomerular filtration barrier. The aim of this paper was to describe the pathogenesis of glomerular proteinuria. Data Sources. Articles on glomerular proteinuria retrieved from Pubmed and MEDLINE in the recent 5 years were reviewed. Results. The new understanding of the roles of glomerular endothelial cells and the glomerular basement membrane (GBM) in the pathogenesis of glomerular proteinuria was gained. The close relationships of slit diaphragm (SD) molecules such as nephrin, podocin, CD2-associated protein (CD2AP), a-actinin-4, transient receptor potential cation channel 6 (TRPC6), Densin and membrane-associated guanylate kinase inverted 1 (MAGI-1), α3β1 integrin, WT1, phospholipase C epsilon-1 (PLCE1), Lmx1b, and MYH9, and mitochondrial disorders and circulating factors in the pathogenesis of glomerular proteinuria were also gradually discovered. Conclusion. Renal proteinuria is a manifestation of glomerular filtration barrier dysfunction. Not only glomerular endothelial cells and GBM, but also the glomerular podocytes and their SDs play an important role in the pathogenesis of glomerular proteinuria.

Solution structure analysis of cytoplasmic domain of podocyte protein Neph1 using small/wide angle x-ray scattering (SWAXS)

Neph1 is present in podocytes, where it plays a critical role in maintaining the filtration function of the glomerulus, in part through signaling events mediated by its cytoplasmic domain that are involved in actin cytoskeleton organization. To understand the function of this protein, a detailed knowledge of the structure of the Neph1 cytoplasmic domain (Neph1-CD) is required. In this study, the solution structure of this domain was determined by small/wide angle x-ray scattering (SWAXS). Analysis of Neph1-CD by SWAXS suggested that this protein adopts a global shape with a radius of gyration and a maximum linear dimension of 21.3 and 70 Å, respectively. These parameters and the data from circular dichroism experiments were used to construct a structural model of this protein. The His-ZO-1-PDZ1 (first PDZ domain of zonula occludens) domain that binds Neph1-CD was also analyzed by SWAXS, to confirm that it adopts a global structure similar to its crystal structure. We used the SWAXS intensity profile, the structural model of Neph1-CD, and the crystal structure of ZO-1-PDZ1 to construct a structural model of the Neph1-CD·ZO-1-PDZ1 complex. Mapping of the intermolecular interactions suggested that in addition to the C-terminal residues Thr-His-Val, residues Lys-761 and Tyr-762 in Neph1 are also critical for stabilizing the complex. Estimated intensity values from the SWAXS data and in vivo and in vitro pull-down experiments demonstrated loss of binding to ZO-1 when these residues were individually mutated to alanines. Our findings present a structural model that provides novel insights into the molecular structure and function of Neph1-CD.

Nephrin regulates lamellipodia formation by assembling a protein complex that includes Ship2, filamin and lamellipodin

Actin dynamics has emerged at the forefront of podocyte biology. Slit diaphragm junctional adhesion protein Nephrin is necessary for development of the podocyte morphology and transduces phosphorylation-dependent signals that regulate cytoskeletal dynamics. The present study extends our understanding of Nephrin function by showing in cultured podocytes that Nephrin activation induced actin dynamics is necessary for lamellipodia formation. Upon activation Nephrin recruits and regulates a protein complex that includes Ship2 (SH2 domain containing 5′ inositol phosphatase), Filamin and Lamellipodin, proteins important in regulation of actin and focal adhesion dynamics, as well as lamellipodia formation. Using the previously described CD16-Nephrin clustering system, Nephrin ligation or activation resulted in phosphorylation of the actin crosslinking protein Filamin in a p21 activated kinase dependent manner. Nephrin activation in cell culture results in formation of lamellipodia, a process that requires specialized actin dynamics at the leading edge of the cell along with focal adhesion turnover. In the CD16-Nephrin clustering model, Nephrin ligation resulted in abnormal morphology of actin tails in human podocytes when Ship2, Filamin or Lamellipodin were individually knocked down. We also observed decreased lamellipodia formation and cell migration in these knock down cells. These data provide evidence that Nephrin not only initiates actin polymerization but also assembles a protein complex that is necessary to regulate the architecture of the generated actin filament network and focal adhesion dynamics.

Downregulation of the antioxidant protein peroxiredoxin 2 contributes to angiotensin II-mediated podocyte apoptosis

Podocytes have a significant role in establishing selective permeability of the glomerular filtration barrier. Sustained renin–angiotensin–aldosterone system activation is crucial to the pathogenesis of podocyte injury, but the mechanisms by which angiotensin II modulates podocyte survival due to physiological or injurious stimuli remain unclear. Here, we used proteomic analysis to find new mediators of angiotensin II–induced podocyte injury. Antioxidant protein peroxiredoxin 2 expression was decreased in cultured podocytes stimulated with angiotensin II. Peroxiredoxin 2 was found to be expressed in podocytes in vivo, and its expression was decreased in the glomeruli of rats transgenic for angiotensin II type 1 receptors in a podocyte-specific manner, or in rats infused with angiotensin II. Downregulation of peroxiredoxin 2 in podocytes resulted in increased reactive oxygen species release, protein overoxidation, and inhibition of the Akt pathway. Both treatment with angiotensin II and downregulation of peroxiredoxin 2 expression led to apoptosis of podocytes. Thus, peroxiredoxin 2 is an important modulator of angiotensin II–induced podocyte injury.

Tyrosine kinase signaling in kidney glomerular podocytes

During the last decade, several key molecules have been identified as essential components for the filtration barrier function of kidney glomerular podocytes. Mutations in genes encoding these molecules severely impair the podocyte architecture in the affected patients, leading to the development of proteinuria. Extensive investigations have been performed on the function of these molecules, which highlights the importance of tyrosine kinase signaling in the podocytes. An Src family tyrosine kinase, Fyn, plays a major role in this signaling pathway. Here, we review the current understanding of this important signal transduction system and its role in the development and the maintenance of podocytes.