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

Advances in focal segmental glomerulosclerosis research: genetic causes to non-coding RNAs

Focal Segmental Glomerulosclerosis (FSGS) is a clinicopathological illness characterized by podocyte damage, impairing glomerular filtration, and substantial proteinuria, which often results in end-stage renal disease (ESRD). Divided into primary, secondary, genetic, and idiopathic categories, its diverse origin highlights the intricacy of its diagnosis and treatment. The existing dependence on immunosuppressive medicines highlights their side effects and inconsistent efficacy, underscoring the pressing necessity for innovative, focused treatments. Recent advancements in genomics and molecular biology have shown the significant involvement of genetic alterations, especially in podocyte-associated proteins, in the pathogenesis of FSGS. Identifying possible novel biomarkers for diagnosing FSGS and monitoring disease activity has revitalized interest in this condition. Recent data underscores the significance of non-coding RNAs, including microRNAs (miRNAs), circular RNAs (circRNAs), and long non-coding RNAs (lncRNAs), in the modulation of gene expression and podocyte functionality. Please check and confirm that the authors and their respective affiliations have been correctly identified and amend if necessary. Particular dysregulated miRNAs and circRNAs have demonstrated potential as biomarkers for early diagnosis and disease monitoring. Furthermore, understanding lncRNA-mediated pathways provides novel therapeutic targets. This review consolidates current progress in elucidating the genetic and molecular processes of FSGS, emphasizing biomarker identification and treatment innovation.

The vicious circle of chronic kidney disease and hypertriglyceridemia: What is first, the hen or the egg?

Chronic kidney disease (CKD) is documented to cause alterations in lipid metabolism, and this was considered a potent driver of increased cardiovascular risk. Among the diverse alteration of lipid traits in CKD, research endeavours have predominantly concentrated on low-density lipoproteins (LDL) in view of the potent pro-atherogenic role of these lipoprotein particles and the demonstration of protective cardiovascular effect of reducing LDL. However, few studies have focused on the metabolism of triglyceride-rich lipoproteins and even fewer on their role in causing kidney damage. Therefore, the comprehensive description of the impact of hypertriglyceridemia (HTG) in CKD pathophysiology remains largely undetermined. This reflects the difficulty of disentangling the independent role of triglycerides (TG) in the complex, bidirectional relationship between TG and kidney disease. Abnormal neutral lipid accumulation in the intrarenal vasculature and renal cells eventually due to HTG may also promote glomerular injury, throughout mechanisms including oxidative stress, mitochondrial dysfunction and proinflammatory responses. While epidemiological and experimental evidence suggests a potential role of TG in kidney damage, the causal mechanisms and their clinical relevance remain unclear, representing a significant area for future investigation. This review aims to highlight the intricate interplay between TG metabolism and kidney disease, shedding light on the mechanisms through which HTG may influence kidney functionality.

Impact of atopic dermatitis on renal dysfunction: insights from patient data and animal models

Introduction

Atopic dermatitis (AD) is a chronic inflammatory skin disease characterized by pruritus, immune dysregulation, and compromised skin barrier function. Although there are some reports that indicate a link between AD and chronic kidney disease (CKD), the prevalence and underlying mechanism of the association between AD and CKD are still unclear. We aimed to clarify the mechanism underlying the association between AD and CKD using an AD-like mouse model.

Methods

Human serum and urine samples from adults in the U.S. were analyzed using data from the National Health and Nutrition Examination Survey (NHANES). An AD-like mouse model was established by repeatedly applying 2,4-dinitrochlorobenzene to the backs and ears of the mice. Kidney inflammation and podocyte function were evaluated via PAS and H&E staining, immunofluorescence staining, and electron microscopy.

Results

We found that compared to healthy subjects in the NHANES cohort study, patients with AD had altered kidney function. AD-like model mice exhibited albuminuria and renal dysfunction one to three months after the induction of AD. In addition, there were remarkable decreases in triglyceride and very-low-density lipoprotein levels and increases in low-density lipoprotein and non-high-density lipoprotein levels in AD-like model mice. After histological staining of the kidneys of AD-like model mice, macrophage and neutrophil infiltration was detected, and the foot process effacement of podocytes was observed via electron microscopy. In addition, the gene expression of slit diaphragm- and podocyte-related proteins such as nephrin, podocin, and synaptopodin decreased, whereas the gene expression of inflammatory mediators such as S100A8 and S100A9 increased.

Discussion

Following improvements in skin inflammation, alleviation of albuminuria, renal dysfunction and dyslipidemia were observed. These findings suggest that AD-related cutaneous inflammation is associated with albuminuria and podocyte dysfunction.

Effectiveness of a Novel PLA2R1 Knock-In Rat Model in Repairing Renal Function Damage

Phospholipase A2 receptor 1 (PLA2R1) exists in many animals and plays an important role in membranous nephropathy. In this study, we aimed to evaluate a PLA2R1 knock-in rat model with repaired kidney function to study the molecular mechanisms of membranous nephropathy. We constructed the PLA2R1 knockout [PLA2R1(-)] model and PLA2R1 knock in [PLA2R1(+)] model in rats. Consistent complement C3 and IgA expression was confirmed through colocalization studies. Urinary biochemical indicators were performed using Automatic Biochemistry Analyzer. The complement C3, IgG, and Nephrin were detected by immunofluorescence assay. The expression levels of complement C3, IgA, and PLA2R1 were detected by western blot. The differential expression proteins (DEPs) between control and PLA2R1(+) models were detected by liquid chromatography with tandem mass spectrometry. The PLA2R1(-) model showed proteinuria, complement C3 aggregation, and IgA and IgG deposition in the glomerulus. Comparing with the PLA2R1(-) model, the PLA2R1(+) model, the deposition of complement C3 and IgA in the glomerulus did not completely disappear, and IgG expression weakened. Moreover, the absolute value of urinary protein was much lower in the PLA2R1(+) model than in the PLA2R1(-) model, and some of the humanized PLA2R1 gene fragments repaired some of the kidney functions. Humanized PLA2R1-insertion in rats can repair part of the renal function and reduce proteinuria, which will help in studying the molecular mechanisms of membranous nephropathy, as well as the entire membranous nephropathy-related system and complement activation signaling pathway.

Effectiveness of a Novel PLA2R1 Knock-in Middle Age Rat Model in Repairing Renal Function Damage

Phospholipase A2 receptor 1 (PLA2R1) exists important role in membranous nephropathy. In this study, we evaluate a PLA2R1 in a middle-aged rat model of renal function repair to further investigate the molecular mechanisms of membranous nephropathy. We analyzed the PLA2R1 knockout (KO) model and PLA2R1 knock in (KI) model in rats, extending the time to 85 weeks of age. Urinary biochemical indicators were detected using a fully automated biochemical analyzer. The complement C3, IgG, and Nephrin were detected using the immunofluorescence method. Western blot was used to detect the expression levels of complement C3, IgA and PLA2R1 in middle-aged models. The KO model continues to display glomerular proteinuria, complement C3 aggregation, and IgA and IgG deposition. Comparing with the KO model, the deposition of complement C3 and IgA in the glomerulus of the KI chimeric model still exists and IgG expression weakened. Inserting humanized PLA2R1 into rats can continuously repair partial renal function and reduce proteinuria, which will help investigate the pathogenesis of membranous nephropathy and complement activation signaling pathways.

Identification of Genes Associated with Familial Focal Segmental Glomerulosclerosis Through Transcriptomics and In Silico Analysis, Including RPL27, TUBB6, and PFDN5

Focal segmental glomerulosclerosis (FSGS) is a major cause of nephrotic syndrome and often leads to progressive kidney failure. Its varying clinical presentation suggests potential genetic diversity, requiring further molecular investigation. This study aims to elucidate some of the genetic and molecular mechanisms underlying FSGS. The study focuses on the use of bioinformatic analysis of gene expression data to identify genes associated with familial FSGS. A comprehensive in silico analysis was performed using the GSE99340 data set from Gene Expression Omnibus (GEO) comparing gene expression in glomerular and tubulointerstitial tissues from FSGS patients (n = 10) and Minimal Change Disease (MCD) patients (n = 8). These findings were validated using transcriptomics data obtained using RNA sequencing from FSGS (n = 3) and control samples (n = 3) from the UAE. Further validation was conducted using qRT-PCR on an independent FFPE cohort (FSGS, n = 6; MCD, n = 7) and saliva samples (FSGS, n = 3; Control, n = 7) from the UAE. Three genes (TUBB6, RPL27, and PFDN5) showed significant differential expression (p < 0.01) when comparing FSGS and MCD with healthy controls. These genes are associated with cell junction organization and synaptic pathways of the neuron, supporting the link between FSGS and the neural system. These genes can potentially be useful as diagnostic biomarkers for FSGS and to develop new treatment options.

Podocyte cell-specific Npr1 is required for blood pressure and renal homeostasis in male and female mice: role of sex-specific differences

Atrial and brain natriuretic peptides (ANP and BNP) bind to guanylyl cyclase A/natriuretic peptide receptor A (GC-A/NPRA), stimulating natriuresis and diuresis and reducing blood pressure (BP), but the role of ANP/NPRA signaling in podocytes (highly specialized epithelial cells covering the outer surfaces of renal glomerular capillaries) remains unclear. This study aimed to determine the effect of conditional deletion of podocyte-specific Npr1 (encoding NPRA) gene knockout (KO) in male and female mice. Tamoxifen-treated wild-type control (PD Npr1 f/f; WT), heterozygous (PD-Cre-Npr1 f/+; HT), and KO (PD-Cre-Npr1 f/-) mice were fed a normal-, low-, or high-salt diet for 4 wk. Podocytes isolated from HT and KO male and female mice showed complete absence of Npr1 mRNA and NPRA protein compared with WT mice. BP, plasma creatinine, plasma sodium, urinary protein, and albumin/creatinine ratio were significantly increased, whereas plasma total protein, albumin, creatinine clearance, and urinary sodium levels were significantly reduced in the HT and KO male and female mice compared with WT mice. These changes were significantly greater in males than in females. On a normal-salt diet, glomerular filtration rate was significantly decreased in PD Npr1 HT and KO male and female mice compared with WT mice. Immunofluorescence of podocin and synaptopodin was also significantly reduced in HT and KO mice compared with WT mice. These observations suggest that in podocytes, ANP/NPRA signaling may be crucial in the maintenance and regulation of glomerular filtration and BP and serve as a biomarker of renal function in a sex-dependent manner.NEW & NOTEWORTHY Our results demonstrate that the podocyte-specific deletion of Npr1 showed increased blood pressure (BP) and altered biomarkers of renal functions, with greater magnitudes in animals fed a high-salt diet in a sex-dependent manner. The results suggest a direct and sex-dependent effect of Npr1 ablation in podocytes on the regulation of BP and renal function and reveal that podocytes may be considered an important target for the ANP-BNP/NPRA/cGMP signaling cascade.

Exploring the therapeutic mechanism of Yuebi decoction on nephrotic syndrome based on network pharmacology and experimental study

OBJECTIVE

This study aimed to explore the material basis of YBD and its possible mechanisms against NS through network pharmacology, molecular docking, and in vivo experiment.

METHODS

Active ingredients and potential targets of YBD were obtained through TCMSP and SwissTargetPrediction. NS-related targets were obtained from GeneCards, PharmGKB, and OMIM databases. The herb-ingredient-target network and PPI network were constructed by Cytoscape 3.9.1 and STRING database. GO and KEGG analyses were performed by DAVID database and ClueGO plugin. The connection between main active ingredients and core targets were revealed by molecular docking. To ascertain the effects and molecular mechanisms of YBD, a rat model was established by PAN.

RESULTS

We collected 124 active ingredients, 269 drug targets, and 2089 disease targets. 119 overlapping were screened for subsequent analysis. PPI showed that AKT1, STAT3, TRPC6, CASP3, JUN, PPP3CA, IL6, PTGS2, VEGFA, and NFATC3 were potential therapeutic targets of YBD against NS. Through GO and KEGG analyses, it showed the therapeutic effect of YBD on NS was closely involved in the regulation of pathways related to podocyte injury, including AGE-RAGE signaling pathway in diabetic complications and MAPK signaling pathway. Five key bioactive ingredients of YBD had the good affinity with the core targets. the experiment confirmed the renoprotective effects of YBD through reducing podocyte injury. Furthermore, YBD could downregulate expressions of PPP3CA, STAT3, NFATC3, TRPC6, and AKT1 in rats.

CONCLUSIONS

YBD might be a potential drug in the treatment of NS, and the underlying mechanism is closely associated with the inhibition of podocyte injury.

Discovery of pyridazinone derivatives bearing tetrahydroimidazo[1,2-a]pyrazine scaffold as potent inhibitors of transient receptor potential canonical 5 to ameliorate hypertension-induced renal injury in rats

Transient receptor potential canonical 5 (TRPC5) is a calcium-permeable non-selective cation channel involved in various pathophysiological processes, including renal injury. Recently, GFB-887, an investigational pyridazinone TRPC5 inhibitor, demonstrated significant therapeutic potential in a Phase II clinical trial for focal segmental glomerulosclerosis (FSGS), a rare and severe form of chronic kidney disease (CKD). In the current study, based on the structure of GFB-887, we conducted extensive structural modification to explore novel TRPC5 inhibitors with desirable drug-like properties and robust nephroprotective efficacy. A series of pyridazinone derivatives featuring a novel tetrahydroimidazo[1,2-a]pyrazine scaffold were synthesized and their activities were evaluated in HEK-293 cells stably expressing TRPC5 using a fluorescence-based Ca2+ mobilization assay. Among these compounds, compound 12 is turned out to be a potent TRPC5 inhibitor with apparent affinity comparable to the parent compound GBF-887. Compound 12 is highly selective on TRPC4/5 over TRPC3/6/7 and hERG channels, along with acceptable pharmacokinetic properties and a favorable safety profile. More importantly, in a rat model of hypertension-induced renal injury, oral administration of compound 12 (10 mg/kg, BID) efficaciously reduced mean blood pressure, inhibited proteinuria, and protected podocyte damage. These findings further confirmed the potential of TRPC5 inhibitors on the CKD treatment and provided compound 12 to be a valuable tool for exploring TRPC4/5 pathophysiology.

Protein tyrosine phosphatase 1B is a regulator of alpha-actinin4 in the glomerular podocyte

Glomerular podocytes are instrumental for the barrier function of the kidney, and podocyte injury contributes to proteinuria and the deterioration of renal function. Protein tyrosine phosphatase 1B (PTP1B) is an established metabolic regulator, and the inactivation of this phosphatase mitigates podocyte injury. However, there is a paucity of data regarding the substrates that mediate PTP1B actions in podocytes. This study aims to uncover novel substrates of PTP1B in podocytes and validate a leading candidate. To this end, using substrate-trapping and mass spectroscopy, we identified putative substrates of this phosphatase and investigated the actin cross-linking cytoskeletal protein alpha-actinin4. PTP1B and alpha-actinin4 co-localized in murine and human glomeruli and transiently transfected E11 podocyte cells. Additionally, podocyte PTP1B deficiency in vivo and culture was associated with elevated tyrosine phosphorylation of alpha-actinin4. Conversely, reconstitution of the knockdown cells with PTP1B attenuated alpha-actinin4 tyrosine phosphorylation. We demonstrated co-association between alpha-actinin4 and the PTP1B substrate-trapping mutant, which was enhanced upon insulin stimulation and disrupted by vanadate, consistent with an enzyme-substrate interaction. Moreover, we identified alpha-actinin4 tandem tyrosine residues 486/487 as mediators of its interaction with PTP1B. Furthermore, knockdown studies in E11 cells suggest that PTP1B and alpha-actinin4 are modulators of podocyte motility. These observations indicate that PTP1B and alpha-actinin4 are likely interacting partners in a signaling node that modulates podocyte function. Targeting PTP1B and plausibly this one of its substrates may represent a new therapeutic approach for podocyte injury that warrants additional investigation.

Biomolecular condensates in kidney physiology and disease

The regulation and preservation of distinct intracellular and extracellular solute microenvironments is crucial for the maintenance of cellular homeostasis. In mammals, the kidneys control bodily salt and water homeostasis. Specifically, the urine-concentrating mechanism within the renal medulla causes fluctuations in extracellular osmolarity, which enables cells of the kidney to either conserve or eliminate water and electrolytes, depending on the balance between intake and loss. However, relatively little is known about the subcellular and molecular changes caused by such osmotic stresses. Advances have shown that many cells, including those of the kidney, rapidly (within seconds) and reversibly (within minutes) assemble membraneless, nano-to-microscale subcellular assemblies termed biomolecular condensates via the biophysical process of hyperosmotic phase separation (HOPS). Mechanistically, osmotic cell compression mediates changes in intracellular hydration, concentration and molecular crowding, rendering HOPS one of many related phase-separation phenomena. Osmotic stress causes numerous homo-multimeric proteins to condense, thereby affecting gene expression and cell survival. HOPS rapidly regulates specific cellular biochemical processes before appropriate protective or corrective action by broader stress response mechanisms can be initiated. Here, we broadly survey emerging evidence for, and the impact of, biomolecular condensates in nephrology, where initial concentration buffering by HOPS and its subsequent cellular escalation mechanisms are expected to have important implications for kidney physiology and disease.

Deletion of protein tyrosine phosphatase SHP-1 restores SUMOylation of podocin and reverses the progression of diabetic kidney disease

Both clinical and experimental data suggest that podocyte injury is involved in the onset and progression of diabetic kidney disease (DKD). Although the mechanisms underlying the development of podocyte loss are not completely understood, critical structural proteins such as podocin play a major role in podocyte survival and function. We have reported that the protein tyrosine phosphatase SHP-1 expression increased in podocytes of diabetic mice and glomeruli of patients with diabetes. However, the in vivo contribution of SHP-1 in podocytes is unknown. Conditional podocyte-specific SHP-1-deficient mice (Podo-SHP-1-/-) were generated to evaluate the impact of SHP-1 deletion at four weeks of age (early) prior to the onset of diabetes and after 20 weeks (late) of diabetes (DM; Ins2+/C96Y) on kidney function (albuminuria and glomerular filtration rate) and kidney pathology. Ablation of the SHP-1 gene specifically in podocytes prevented and even reversed the elevated albumin/creatinine ratio, glomerular filtration rate progression, mesangial cell expansion, glomerular hypertrophy, glomerular basement membrane thickening and podocyte foot process effacement induced by diabetes. Moreover, podocyte-specific deletion of SHP-1 at an early and late stage prevented diabetes-induced expression of collagen IV, fibronectin, transforming growth factor-β, transforming protein RhoA, and serine/threonine kinase ROCK1, whereas it restored nephrin, podocin and cation channel TRPC6 expression. Mass spectrometry analysis revealed that SHP-1 reduced SUMO2 post-translational modification of podocin while podocyte-specific deletion of SHP-1 preserved slit diaphragm protein complexes in the diabetic context. Thus, our data uncovered a new role of SHP-1 in the regulation of cytoskeleton dynamics and slit diaphragm protein expression/stability, and its inhibition preserved podocyte function preventing DKD progression.

Proteins Secreted by Lung Cancer Cells Induce the Onset of Proteinuria via Focal Adhesion Kinase Signaling in Mice

Paraneoplastic nephrotic syndrome (PNS) is a complication seen in cancer patients. Ultrastructural examination shows the accumulation of proteins and the presence of foot process (FP) effacement in the glomeruli of PNS patients. Previously, we reported that orthotopic xenografts of Lewis lung carcinoma 1 in C57BL/6 mice caused them to develop lung cancer with albuminuria. This implies that these mice can be used as a model of human disease and suggests that Lewis lung carcinoma 1 cell-secreted proteins (LCSePs) contain nephrotoxic molecules and cause inflammation in renal cells. As podocyte effacement was present in glomeruli in this model, such podocyte injury may be attributable to either soluble LCSeP or LCSeP deposits triggering pathological progression. LCSePs in conditioned media was concentrated for nephrotoxicity testing. Integrin-focal adhesion kinase (FAK) signaling and inflammatory responses were evaluated in podocytes either exposed to soluble LCSePs or seeded onto substrates with immobilized LCSePs. FAK phosphorylation and interleukin-6 expression were higher in podocytes attached to LCSePs substrates than in those exposed to soluble LCSePs. Notably, LCSeP-based haptotaxis gave rise to altered signaling in podocytes. When podocytes were stimulated by immobilized LCSePs, FAK accumulated at focal adhesions, synaptopodin dissociated from F-actin, and disrupting the interactions between synaptopodin and α-actinin was observed. When FAK was inhibited by PF-573228 in immobilized LCSePs, the association between synaptopodin and α-actinin was observed in the podocytes. The association of synaptopodin and α-actinin with F-actin allowed FP stretching, establishing a functional glomerular filtration barrier. Therefore, in this mouse model of lung cancer, FAK signaling prompts podocyte FP effacement and proteinuria, indicative of PNS.

The role of exercise in improving hyperlipidemia-renal injuries induced by a high-fat diet: a literature review

A diet that is high in sugar and fat is a precursor to various chronic diseases, especially hyperlipidemia. Patients with hyperlipidemia have increased levels of plasma free fatty acids and an ectopic accumulation of lipids. The kidney is one of the main organs affected by this disease and, recently, there have been more studies conducted on renal injury caused by hyperlipidemia. The main pathological mechanism is closely related to renal lipotoxicity. However, in different kidney cells, the reaction mechanism varies due to the different affinities of the lipid receptors. At present, it is believed that in addition to lipotoxicity, hyperlipidemia induced-renal injury is also closely related to oxidative stress, endoplasmic reticulum stress, and inflammatory reactions, which are the result of multiple factors. Exercise plays an important role in the prevention of various chronic diseases and recently emerging researches indicated its positive effects to renal injury caused by hyperlipidemia. However, there are few studies summarizing the effects of exercise on this disease and the specific mechanisms need to be further explored. This article summarizes the mechanisms of hyperlipidemia induced-renal injury at the cellular level and discusses the ways in which exercise may regulate it. The results provide theoretical support and novel approaches for identifying the intervention target to treat hyperlipidemia induced-renal injury.

Phosphorylation of PACSIN2 at S313 Regulates Podocyte Architecture in Coordination with N-WASP

Changes in the dynamic architecture of podocytes, the glomerular epithelial cells, lead to kidney dysfunction. Previous studies on protein kinase C and casein kinase 2 substrates in neurons 2 (PACSIN2), a known regulator of endocytosis and cytoskeletal organization, reveal a connection between PACSIN2 and kidney pathogenesis. Here, we show that the phosphorylation of PACSIN2 at serine 313 (S313) is increased in the glomeruli of rats with diabetic kidney disease. We found that phosphorylation at S313 is associated with kidney dysfunction and increased free fatty acids rather than with high glucose and diabetes alone. Phosphorylation of PACSIN2 emerged as a dynamic process that fine-tunes cell morphology and cytoskeletal arrangement, in cooperation with the regulator of the actin cytoskeleton, Neural Wiskott-Aldrich syndrome protein (N-WASP). PACSIN2 phosphorylation decreased N-WASP degradation while N-WASP inhibition triggered PACSIN2 phosphorylation at S313. Functionally, pS313-PACSIN2 regulated actin cytoskeleton rearrangement depending on the type of cell injury and the signaling pathways involved. Collectively, this study indicates that N-WASP induces phosphorylation of PACSIN2 at S313, which serves as a mechanism whereby cells regulate active actin-related processes. The dynamic phosphorylation of S313 is needed to regulate cytoskeletal reorganization.

Reorganization and Suppression of Store-Operated Calcium Entry in Podocytes of Type 2 Diabetic Rats

Type 2 diabetes mellitus (DM2) is a widespread metabolic disorder that results in podocyte damage and diabetic nephropathy. Previous studies demonstrated that TRPC6 channels play a pivotal role in podocyte function and their dysregulation is associated with development of different kidney diseases including nephropathy. Here, using single channel patch clamp technique, we demonstrated that non-selective cationic TRPC6 channels are sensitive to the Ca²⁺ store depletion in human podocyte cell line Ab8/13 and in freshly isolated rat glomerular podocytes. Ca²⁺ imaging indicated the involvement of ORAI and sodium-calcium exchanger in Ca²⁺ entry induced upon store depletion. In male rats fed a high-fat diet combined with a low-dose streptozotocin injection, which leads to DM2 development, we observed the reduction of a store-operated Ca²⁺ entry (SOCE) in rat glomerular podocytes. This was accompanied by a reorganization of store-operated Ca²⁺ influx such that TRPC6 channels lost their sensitivity to Ca²⁺ store depletion and ORAI-mediated Ca²⁺ entry was suppressed in TRPC6-independent manner. Altogether our data provide new insights into the mechanism of SOCE organization in podocytes in the norm and in pathology, which should be taken into account when developing pharmacological treatment of the early stages of diabetic nephropathy.

Complement C3 mediates podocyte injury through TLR4/NFΚB-P65 signaling during ischemia-reperfusion acute kidney injury and post-injury fibrosis

BACKGROUND

The aim of this study was to explore the mechanism of complement C3a mediating podocyte injury during ischemia-reperfusion acute kidney injury (IR-AKI) and post-injury fibrosis.

METHODS

Renal artery clamping was used to establish IR-AKI and post-injury fibrosis model. HE and Masson staining were performed to observe renal fibrosis. The protein abundance levels were measured along with inflammatory markers, renal complement C3. Podocytes were treated with C3a with or without Toll-like receptor 4(TLR4) inhibitor. The effects of TLR4 up-regulation by TLR4 plasmids were examined.

RESULTS

C3^-/- resulted in amelioration of renal dysfunction by reducing podocyte damage and renal fibrosis. Immunoblot with renal tissue homogenates from IR-AKI mice revealed that C3^-/- decreased TLR4/Nuclear Factor-κB (NFκB)-P65.

CONCLUSION

Our results indicate that modulating C3/TLR4/NFκB-P65 signaling pathway is a novel therapeutic target for the IR-AKI and post-injury fibrosis.

Identification of circular dorsal ruffles as signal platforms for the AKT pathway in glomerular podocytes

Circular dorsal ruffles (CDRs) are rounded membrane ruffles induced by growth factors to function as precursors of the large-scale endocytosis called macropinocytosis. In addition to their role in cellular uptake, recent research using cell line systems has shown that CDRs/macropinocytosis regulate the canonical AKT-mTORC1 growth factor signaling pathway. However, as CDRs have not been observed in tissues, their physiological relevance has remained unclear. Here, utilizing ultrahigh-resolution scanning electron microscopy, we first report that CDRs are expressed in glomerular podocytes ex vivo and in vivo, and we visually captured the transformation process to macropinocytosis. Moreover, through biochemical and imaging analyses, we show that AKT phosphorylation localized to CDRs upstream of mTORC1 activation in podocyte cell lines and isolated glomeruli. These results demonstrate the physiological role of CDRs as signal platforms for the AKT-mTORC1 pathway in glomerular podocytes at the tissue level. As mTORC1 plays critical roles in podocyte metabolism, and aberrant activation of mTORC1 triggers podocytopathies, our results strongly suggest that targeting CDR formation could represent a potential therapeutic approach for these diseases.

Mycophenolic acid directly protects podocytes by preserving the actin cytoskeleton and increasing cell survival

Mycophenolate Mofetil (MMF) has an established role as a therapeutic agent in childhood nephrotic syndrome. While other immunosuppressants have been shown to positively affect podocytes, direct effects of MMF on podocytes remain largely unknown. The present study examines the effects of MMF's active component Mycophenolic Acid (MPA) on the transcriptome of podocytes and investigates its biological significance. We performed transcriptomics in cultured murine podocytes exposed to MPA to generate hypotheses on podocyte-specific effects of MPA. Accordingly, we further analyzed biological MPA effects on actin cytoskeleton morphology after treatment with bovine serum albumin (BSA) by immunofluorescence staining, as well as on cell survival following exposure to TNF-α and cycloheximide by neutral red assay. MPA treatment significantly (adjusted p < 0.05) affected expression of 351 genes in podocytes. Gene Ontology term enrichment analysis particularly clustered terms related to actin and inflammation-related cell death. Indeed, quantification of the actin cytoskeleton of BSA treated podocytes revealed a significant increase of thickness and number of actin filaments after treatment with MPA. Further, MPA significantly reduced TNFα and cycloheximide induced cell death. MPA has a substantial effect on the transcriptome of podocytes in vitro, particularly including functional clusters related to non-immune cell dependent mechanisms. This may provide a molecular basis for direct beneficial effects of MPA on the structural integrity and survival of podocytes under pro-inflammatory conditions.

Netrin G1 Is a Novel Target Antigen in Primary Membranous Nephropathy

BACKGROUND

Primary membranous nephropathy (MN) is caused by circulating autoantibodies binding to antigens on the podocyte surface. PLA2R1 is the main target antigen in 70%-80% of cases, but the pathogenesis is unresolved in 10%-15% of patients.

METHODS

We used native western blotting to identify IgG4 autoantibodies, which bind an antigen endogenously expressed on podocyte membranes, in the serum of the index patient with MN. These IgG4 autoantibodies were used to immunoprecipitate the target antigen, and mass spectrometry was used to identify Netrin G1 (NTNG1). Using native western blot and ELISA, NTNG1 autoantibodies were analyzed in cohorts of 888 patients with MN or other glomerular diseases.

RESULTS

NTNG1 was identified as a novel target antigen in MN. It is a membrane protein expressed in healthy podocytes. Immunohistochemistry confirmed granular NTNG1 positivity in subepithelial glomerular immune deposits. In prospective and retrospective MN cohorts, we identified three patients with NTNG1-associated MN who showed IgG4-dominant circulating NTNG1 autoantibodies, enhanced NTNG1 expression in the kidney, and glomerular IgG4 deposits. No NTNG1 autoantibodies were identified in 561 PLA2R1 autoantibodies-positive patients, 27 THSD7A autoantibodies-positive patients, and 77 patients with other glomerular diseases. In two patients with available follow-up of 2 and 4 years, both NTNG1 autoantibodies and proteinuria persisted.

CONCLUSIONS

NTNG1 expands the repertoire of target antigens in patients with MN. The clinical role of NTNG1 autoantibodies remains to be defined.

Potential therapeutic effects of natural compounds targeting autophagy to alleviate podocyte injury in glomerular diseases

Podocyte injury is a common cause of proteinuric kidney diseases. Uncontrollable progressive podocyte loss accelerates glomerulosclerosis and increases the risk of end-stage renal disease. To date, owing to the complex pathological mechanism, effective therapies for podocyte injury have been limited. Accumulating evidence supports the indispensable role of autophagy in the maintenance of podocyte homeostasis. A variety of natural compounds and their derivatives have been found to regulate autophagy through multiple targets, including promotes nuclear transfer of transcription factor EB and lysosomal repair. Here, we reviewed the recent studies on the use of natural compounds and their derivatives as autophagy regulators and discussed their potential applications in ameliorating podocyte injury. Several known natural compounds with autophagy-regulatory properties, such as quercetin, silibinin, kaempferol, and artemisinin, and their medical uses were also discussed. This review will help in improving the understanding of the podocyte protective mechanism of natural compounds and promote their development for clinical use.

mTOR-Dependent Autophagy Regulates Slit Diaphragm Density in Podocyte-like Drosophila Nephrocytes

Both mTOR signaling and autophagy are important modulators of podocyte homeostasis, regeneration, and aging and have been implicated in glomerular diseases. However, the mechanistic role of these pathways for the glomerular filtration barrier remains poorly understood. We used Drosophila nephrocytes as an established podocyte model and found that inhibition of mTOR signaling resulted in increased spacing between slit diaphragms. Gain-of-function of mTOR signaling did not affect spacing, suggesting that additional cues limit the maximal slit diaphragm density. Interestingly, both activation and inhibition of mTOR signaling led to decreased nephrocyte function, indicating that a fine balance of signaling activity is needed for proper function. Furthermore, mTOR positively controlled cell size, survival, and the extent of the subcortical actin network. We also showed that basal autophagy in nephrocytes is required for survival and limits the expression of the sns (nephrin) but does not directly affect slit diaphragm formation or endocytic activity. However, using a genetic rescue approach, we demonstrated that excessive, mTOR-dependent autophagy is primarily responsible for slit diaphragm misspacing. In conclusion, we established this invertebrate podocyte model for mechanistic studies on the role of mTOR signaling and autophagy, and we discovered a direct mTOR/autophagy-dependent regulation of the slit diaphragm architecture.

How immunosuppressive drugs may directly target podocytes in glomerular diseases

Podocytes are the direct target of immunologic injury in many immune-mediated glomerular diseases, leading to proteinuria and subsequent kidney failure. Immunosuppressive agents such as steroids, calcineurin inhibitors, and rituximab are the commonly used treatment strategies in this context for their immunotherapeutic or anti-inflammatory properties. However, in recent years, studies have demonstrated that immunosuppressive agents can have a direct effect on podocytes, introducing the concept of the non-immunologic mechanism of kidney protection by immunomodulators. In this review, we focus on the mechanisms by which these agents may directly target the podocyte independent of their systemic effects and examine their clinical significance.

The Role of Anti-PLA2R and Anti-THSD7A Antibodies in the Pathogenesis and Diagnostics of Primary Membranous Nephropathy: A Review of Current Knowledge for Clinical Practice

Primary membranous nephropathy (PMN) is considered a major cause of nephrotic syndrome. The discovery of circulating autoantibodies directed against glomerular podocytes helped to classify them as autoimmune diseases. Over the past years, there has been an increasing significance of anti-Phospholipase A2 Receptor (anti-PLA₂R), which has been detected in 70–80% of PMN cases, and relevance of anti-Thrombospondin type I domain-containing 7A (anti-THSD7A) even though they are present in 2–5% of patients. The results of clinical and experimental studies indicate that these antibodies are pathogenic. It radically changed the diagnostic and therapeutic approach. Measurement of antibody titers in the serum seems to be a valuable tool for identifying PMN and for the assessment of disease activity. By monitoring pathogenic antibodies levels rather than proteinuria or reduced glomerular filtration rate (GFR) as an indicator of glomerular disease, physicians would easier divide patients into those with active and inactive PMN disease and decide about their therapy. The aim of this review is to evaluate scientific evidence about the role of autoantibodies, namely anti-PLA₂R and anti-THSD7A, as PMN biomarkers. The present manuscript focuses on PMN pathogenesis and key data of diagnosis, monitoring of the disease, and treatment strategies that are currently being used in clinical practice.

Inhibition of p38 MAPK decreases hyperglycemia-induced nephrin endocytosis and attenuates albuminuria

Abstract

Chronic hyperglycemia, as in diabetes mellitus, may cause glomerular damage with microalbuminuria as an early sign. Noteworthy, even acute hyperglycemia can increase glomerular permeability before structural damage of the glomerular filter can be detected. Despite intensive research, specific antiproteinuric therapy is not available so far. Thus, a deeper understanding of the molecular mechanisms of albuminuria is desirable. P38 MAPK signaling is involved in the development of hyperglycemia-induced albuminuria. However, the mechanism of increased p38 MAPK activity leading to increased permeability and albuminuria remained unclear. Recently, we demonstrated that acute hyperglycemia triggers endocytosis of nephrin, the key molecule of the slit diaphragm, and induces albuminuria. Here, we identify p38 MAPK as a pivotal regulator of hyperglycemia-induced nephrin endocytosis. Activated p38 MAPK phosphorylates the nephrin c-terminus at serine 1146, facilitating the interaction of PKCα with nephrin. PKCα phosphorylates nephrin at threonine residues 1120 and 1125, mediating the binding of β-arrestin2 to nephrin. β-arrestin2 triggers endocytosis of nephrin by coupling it to the endocytic machinery, leading to increased glomerular permeability. Pharmacological inhibition of p38 MAPK preserves nephrin surface expression and significantly attenuates albuminuria.

Key messages

Acute hyperglycemia triggers endocytosis of nephrin.

Activated p38 MAPK phosphorylates the nephrin c-terminus at serine 1146, facilitating the interaction of PKCα with nephrin.

PKCα phosphorylates nephrin at threonine residues 1120 and 1125, mediating the binding of β-arrestin2 to nephrin.

β-arrestin2 triggers endocytosis of nephrin by coupling it to the endocytic machinery, leading to a leaky glomerular filter.

Pharmacological inhibition of p38 MAPK preserves nephrin surface expression and significantly attenuates albuminuria under hyperglycemic conditions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00109-022-02184-5.

Recent Advances in Traditional Chinese Medicine for Treatment of Podocyte Injury

Podocyte is also called glomerular epithelial cell, which has been considered as the final gatekeeper of glomerular filtration barrier (GFB). As a major contributor to proteinuria, podocyte injury underlies a variety of glomerular diseases and becomes the challenge to patients and their families in general. At present, the therapeutic methods of podocyte injury mainly include angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, steroid and immunosuppressive medications. Nevertheless, the higher cost and side effects seriously disturb patients with podocyte injury. Promisingly, traditional Chinese medicine (TCM) has received an increasing amount of attention from different countries in the treatment of podocyte injury by invigorating spleen and kidney, clearing heat and eliminating dampness, as well enriching qi and activating blood. Therefore, we searched articles published in peer-reviewed English-language journals through Google Scholar, PubMed, Web of Science, and Science Direct. The protective effects of active ingredients, herbs, compound prescriptions, acupuncture and moxibustion for treatment of podocyte injury were further summarized and analyzed. Meanwhile, we discussed feasible directions for future development, and analyzed existing deficiencies and shortcomings of TCM in the treatment of podocyte injury. In conclusion, this paper shows that TCM treatments can serve as promising auxiliary therapeutic methods for the treatment of podocyte injury.

Discovery of pyridazinone analogs as potent transient receptor potential canonical channel 5 inhibitors

A deepening understanding of the relationship between transient receptor potential canonical channel 5 (TRPC5) and chronic kidney disease (CKD), has led to the emergence of several types of TRPC5 inhibitors displaying clear therapeutic effect. Herein, we report the synthesis and biological evaluation of a series of pyrroledione TRPC5 inhibitors, culminating in the discovery of compound 16g with subtype selectivity. Compared with GFB-8438, a potent TRPC5 inhibitor (Goldfinch Bio), compound 16g showed improved inhibition of TRPC5 and enhanced protective effect against protamine sulfates (PS)-induced podocyte injury in vitro. In addition, compound 16g did not induce cell death in primary cultured hepatocytes and immortalized podocytes in a preliminary toxicity assessment, indicating its utility as a potent and safe inhibitor for studying the function of TRPC5.

Sickle cell nephropathy: A review of novel biomarkers and their potential roles in early detection of renal involvement

Whether the underlying mutations are homozygous, heterozygous, or co-inherited with other hemoglobinopathies, sickle cell disease is known to afflict the kidneys, leading to the clinical entity known as sickle cell nephropathy (SCN). Although common, SCN remains diagnostically elusive. Conventional studies performed in the context of renal disorders often fail to detect early stage SCN. This makes the quest for early diagnosis and treatment more challenging, and it increases the burden of chronic kidney disease-related morbidity among patients. Novel diagnostic tools have been employed to overcome this limitation. In this study, we discuss various biomarkers of SCN, including those employed in clinical practice and others recently identified in experimental settings, such as markers of vascular injury, endothelial dysfunction, tubulo-glomerular damage, and oxidative stress. These include kidney injury molecule-1, monocyte chemoattractant protein-1, N-acetyl-B-D-glucosaminidase, ceruloplasmin, orosomucoid, nephrin, and cation channels, among others. Furthermore, we explore the potential of novel biomarkers for refining diagnostic and therapeutic approaches and describe some obstacles that still need to be overcome. We highlight the importance of a collaborative approach to standardize the use of promising new biomarkers. Finally, we outline the limitations of conventional markers of renal damage as extensions of the pathogenic process occurring at the level of the organ and its functional subunits, with a discussion of the expected pattern of clinical and biochemical progression among patients with SCN.

Rho GTPases in kidney physiology and diseases

Rho family GTPases are molecular switches best known for their pivotal role in dynamic regulation of the actin cytoskeleton, but also of cellular morphology, motility, adhesion and proliferation. The prototypic members of this family (RhoA, Rac1 and Cdc42) also contribute to the normal kidney function and play important roles in the structure and function of various kidney cells including tubular epithelial cells, mesangial cells and podocytes. The kidney's vital filtration function depends on the structural integrity of the glomerulus, the proximal portion of the nephron. Within the glomerulus, the architecturally actin-based cytoskeleton podocyte forms the final cellular barrier to filtration. The glomerulus appears as a highly dynamic signalling hub that is capable of integrating intracellular cues from its individual structural components. Dynamic regulation of the podocyte cytoskeleton is required for efficient barrier function of the kidney. As master regulators of actin cytoskeletal dynamics, Rho GTPases are therefore of critical importance for sustained kidney barrier function. Dysregulated activities of the Rho GTPases and of their effectors are implicated in the pathogenesis of both hereditary and idiopathic forms of kidney diseases. Diabetic nephropathy is a progressive kidney disease that is caused by injury to kidney glomeruli. High glucose activates RhoA/Rho-kinase in mesangial cells, leading to excessive extracellular matrix production (glomerulosclerosis). This RhoA/Rho-kinase pathway also seems involved in the post-transplant hypertension frequently observed during treatment with calcineurin inhibitors, whereas Rac1 activation was observed in post-transplant ischaemic acute kidney injury.

Scaffold polarity proteins Par3A and Par3B share redundant functions while Par3B acts independent of atypical protein kinase C/Par6 in podocytes to maintain the kidney filtration barrier

Glomerular diseases are a major cause for chronic kidney disorders. In most cases podocyte injury is causative for disease development. Cytoskeletal rearrangements and morphological changes are hallmark features of podocyte injury and result in dedifferentiation and loss of podocytes. Here, we establish a link between the Par3 polarity complex and actin regulators necessary to establish and maintain podocyte architecture by utilizing mouse and Drosophila models to characterize the functional role of Par3A and Par3B and its fly homologue Bazooka in vivo. Only simultaneous inactivation of both Par3 proteins caused a severe disease phenotype. Rescue experiments in Drosophila nephrocytes revealed atypical protein kinase C (aPKC)-Par6 dependent and independent effects. While Par3A primarily acts via aPKC-Par6, Par3B function was independent of Par6. Actin-associated synaptopodin protein levels were found to be significantly upregulated upon loss of Par3A/B in mouse podocytes. Tropomyosin2, which shares functional similarities with synaptopodin, was also elevated in Bazooka depleted nephrocytes. The simultaneous depletion of Bazooka and Tropomyosin2 resulted in a partial rescue of the Bazooka knockdown phenotype and prevented increased Rho1, a member of a GTPase protein family regulating the cytoskeleton. The latter contribute to the nephrocyte phenotype observed upon loss of Bazooka. Thus, we demonstrate that Par3 proteins share a high functional redundancy but also have specific functions. Par3A acts in an aPKC-Par6 dependent way and regulates RhoA-GTP levels, while Par3B exploits Par6 independent functions influencing synaptopodin localization. Hence, Par3A and Par3B link elements of polarity signaling and actin regulators to maintain podocyte architecture.

Dual sEH/COX-2 Inhibition Using PTUPB-A Promising Approach to Antiangiogenesis-Induced Nephrotoxicity

Kidney injury from antiangiogenic chemotherapy is a significant clinical challenge, and we currently lack the ability to effectively treat it with pharmacological agents. Thus, we set out to investigate whether simultaneous soluble epoxide hydrolase (sEH) and cyclooxygenase-2 (COX-2) inhibition using a dual sEH/COX-2 inhibitor PTUPB could be an effective strategy for treating antiangiogenic therapy-induced kidney damage. We used a multikinase inhibitor, sorafenib, which is known to cause serious renal side effects. The drug was administered to male Sprague-Dawley rats that were on a high-salt diet. Sorafenib was administered over the course of 56 days. The study included three experimental groups; 1) control group (naïve rats), 2) sorafenib group [rats treated with sorafenib only (20 mg/kg/day p.o.)], and 3) sorafenib + PTUPB group (rats treated with sorafenib only for the initial 28 days and subsequently coadministered PTUPB (10 mg/kg/day i.p.) from days 28 through 56). Blood pressure was measured every 2 weeks. After 28 days, sorafenib-treated rats developed hypertension (161 ± 4 mmHg). Over the remainder of the study, sorafenib treatment resulted in a further elevation in blood pressure through day 56 (200 ± 7 mmHg). PTUPB treatment attenuated the sorafenib-induced blood pressure elevation and by day 56, blood pressure was 159 ± 4 mmHg. Urine was collected every 2 weeks for biochemical analysis. After 28 days, sorafenib rats developed pronounced proteinuria (9.7 ± 0.2 P/C), which intensified significantly (35.8 ± 3.5 P/C) by the end of day 56 compared with control (2.6 ± 0.4 P/C). PTUPB mitigated sorafenib-induced proteinuria, and by day 56, it reduced proteinuria by 73%. Plasma and kidney tissues were collected on day 56. Kidney histopathology revealed intratubular cast formation, interstitial fibrosis, glomerular injury, and glomerular nephrin loss at day 56 in sorafenib-treated rats. PTUPB treatment reduced histological features by 30%-70% compared with the sorafenib-treated group and restored glomerular nephrin levels. Furthermore, PTUPB also acted on the glomerular permeability barrier by decreasing angiotensin-II-induced glomerular permeability to albumin. Finally, PTUPB improved in vitro the viability of human mesangial cells. Collectively, our data demonstrate the potential of using PTUPB or dual sEH/COX-2 inhibition as a therapeutic strategy against sorafenib-induced glomerular nephrotoxicity.

MRTF: Basic Biology and Role in Kidney Disease

A lesser known but crucially important downstream effect of Rho family GTPases is the regulation of gene expression. This major role is mediated via the cytoskeleton, the organization of which dictates the nucleocytoplasmic shuttling of a set of transcription factors. Central among these is myocardin-related transcription factor (MRTF), which upon actin polymerization translocates to the nucleus and binds to its cognate partner, serum response factor (SRF). The MRTF/SRF complex then drives a large cohort of genes involved in cytoskeleton remodeling, contractility, extracellular matrix organization and many other processes. Accordingly, MRTF, activated by a variety of mechanical and chemical stimuli, affects a plethora of functions with physiological and pathological relevance. These include cell motility, development, metabolism and thus metastasis formation, inflammatory responses and—predominantly-organ fibrosis. The aim of this review is twofold: to provide an up-to-date summary about the basic biology and regulation of this versatile transcriptional coactivator; and to highlight its principal involvement in the pathobiology of kidney disease. Acting through both direct transcriptional and epigenetic mechanisms, MRTF plays a key (yet not fully appreciated) role in the induction of a profibrotic epithelial phenotype (PEP) as well as in fibroblast-myofibroblast transition, prime pathomechanisms in chronic kidney disease and renal fibrosis.

Tacrolimus Protects Podocytes from Apoptosis via Downregulation of TRPC6 in Diabetic Nephropathy

Podocyte injury plays an important role in diabetic nephropathy (DN), and apoptosis is one of its mechanisms. The transient receptor potential channel 6 (TRPC6) is expressed in podocytes and mediates podocyte injury induced by high glucose levels. Tacrolimus is a novel immunosuppressive agent that is reported to play an important role in podocyte protection. The purpose of this study was to investigate the potential mechanism of podocyte protection by tacrolimus in a type 2 diabetic mellitus (T2DM) rat model and in immortalized mouse podocytes (MPC5). Transmission electron microcopy was used to evaluate renal injury morphology. After treatment with FK506, we measured 24-hour urinary albumin-to-creatinine ratios and creatinine clearance rates as well as major biochemical parameters such as glucose, insulin, serum creatinine, urea nitrogen, total cholesterol, triglycerides, alanine transaminase, and aspartate aminotransferase. Nephrin and TRPC6 protein expression and podocyte apoptotic rates in vivo and in vitro were measured using immunohistochemical staining, TUNEL assays, and flow cytometry, respectively. Western blot was used to measure expression of cleaved-caspase-3 and bax/bcl-2. Exposed to high glucose (HG), DM rats exhibited disrupted biochemical conditions and impaired podocyte structure. Decreased expression of nephrin and increased expression of TRPC6, cleaved-caspase-3, and bax/bcl-2 ratios were found in podocytes, along with higher apoptotic percentage, while tacrolimus intervention counteracted the effect of HG on podocytes. Our results suggest that tacrolimus protects podocytes during the progression of type 2 diabetic nephropathy, possibly ameliorating podocyte apoptosis by downregulating the expression of TRPC6.

Mitochondrial Dysfunction in Podocytes Caused by CRIF1 Deficiency Leads to Progressive Albuminuria and Glomerular Sclerosis in Mice

Recent studies have implicated mitochondrial disruption in podocyte dysfunction, which is a characteristic feature of primary and diabetic glomerular diseases. However, the mechanisms by which primary mitochondrial dysfunction in podocytes affects glomerular renal diseases are currently unknown. To investigate the role of mitochondrial oxidative phosphorylation (OxPhos) in podocyte dysfunction, glomerular function was examined in mice carrying a loss of function mutation of the gene encoding CR6-interacting factor-1 (CRIF1), which is essential for intramitochondrial production and the subsequent insertion of OxPhos polypeptides into the inner mitochondrial membrane. Homozygotic deficiency of CRIF1 in podocytes resulted in profound and progressive albuminuria from 3 weeks of age; the CRIF1-deficient mice also developed glomerular and tubulointerstitial lesions by 10 weeks of age. Furthermore, marked glomerular sclerosis and interstitial fibrosis were observed in homozygous CRIF1-deficient mice at 20 weeks of age. In cultured mouse podocytes, loss of CRIF1 resulted in OxPhos dysfunction and marked loss or abnormal aggregation of F-actin. These findings indicate that the OxPhos status determines the integrity of podocytes and their ability to maintain a tight barrier and control albuminuria. Analyses of the glomerular function of the podocyte-specific primary OxPhos dysfunction model mice demonstrate a link between podocyte mitochondrial dysfunction, progressive glomerular sclerosis, and tubulointerstitial diseases.

Role of Rho GTPase Interacting Proteins in Subcellular Compartments of Podocytes

The first step of urine formation is the selective filtration of the plasma into the urinary space at the kidney structure called the glomerulus. The filtration barrier of the glomerulus allows blood cells and large proteins such as albumin to be retained while eliminating the waste products of the body. The filtration barrier consists of three layers: fenestrated endothelial cells, glomerular basement membrane, and podocytes. Podocytes are specialized epithelial cells featured by numerous, actin-based projections called foot processes. Proteins on the foot process membrane are connected to the well-organized intracellular actin network. The Rho family of small GTPases (Rho GTPases) act as intracellular molecular switches. They tightly regulate actin dynamics and subsequent diverse cellular functions such as adhesion, migration, and spreading. Previous studies using podocyte-specific transgenic or knockout animal models have established that Rho GTPases are crucial for the podocyte health and barrier function. However, little attention has been paid regarding subcellular locations where distinct Rho GTPases contribute to specific functions. In the current review, we discuss cellular events involving the prototypical Rho GTPases (RhoA, Rac1, and Cdc42) in podocytes, with particular focus on the subcellular compartments where the signaling events occur. We also provide our synthesized views of the current understanding and propose future research directions.

Anti-apoptosis mechanism of triptolide based on network pharmacology in focal segmental glomerulosclerosis rats

Triptolide (TPL), the active component of Tripterygium wilfordii, exhibits anti-cancer and antioxidant functions. We aimed to explore the anti-apoptosis mechanism of TPL based on network pharmacology and in vivo and in vitro research validation using a rat model of focal segmental glomerulosclerosis (FSGS). The chemical structures and pharmacological activities of the compounds reported in T. wilfordii were determined and used to perform the network pharmacology analysis. The Traditional Chinese Medicine Systems Pharmacology Database (TCMSP) was then used to identify the network targets for 16 compounds from Tripterygium wilfordii. Our results showed that 47 overlapping genes obtained from the GeneCards and OMIM databases were involved in the occurrence and development of FSGS and used to construct the protein–protein interaction (PPI) network using the STRING database. Hub genes were identified via the MCODE plug-in of the Cytoscape software. IL4 was the target gene of TPL in FSGS and was mainly enriched in the cell apoptosis term and p53 signaling pathway, according to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses. TPL inhibited FSGS-induced cell apoptosis in rats and regulated IL4, nephrin, podocin, and p53 protein levels via using CCK8, TUNEL, and Western blot assays. The effects of IL4 overexpression, including inhibition of cell viability and promotion of apoptosis, were reversed by TPL. TPL treatment increased the expression of nephrin and podocin and decreased p53 expression in rat podocytes. In conclusion, TPL inhibited podocyte apoptosis by targeting IL4 to alleviate kidney injury in FSGS rats.

Altered glycosylation of IgG4 promotes lectin complement pathway activation in anti-PLA2R1-associated membranous nephropathy

Primary membranous nephropathy (pMN) is a leading cause of nephrotic syndrome in adults. In most cases, this autoimmune kidney disease is associated with autoantibodies against the M-type phospholipase A2 receptor (PLA2R1) expressed on kidney podocytes, but the mechanisms leading to glomerular damage remain elusive. Here, we developed a cell culture model using human podocytes and found that anti-PLA2R1-positive pMN patient sera or isolated IgG4, but not IgG4-depleted sera, induced proteolysis of the 2 essential podocyte proteins synaptopodin and NEPH1 in the presence of complement, resulting in perturbations of the podocyte cytoskeleton. Specific blockade of the lectin pathway prevented degradation of synaptopodin and NEPH1. Anti-PLA2R1 IgG4 directly bound mannose-binding lectin in a glycosylation-dependent manner. In a cohort of pMN patients, we identified increased levels of galactose-deficient IgG4, which correlated with anti-PLA2R1 titers and podocyte damage induced by patient sera. Assembly of the terminal C5b-9 complement complex and activation of the complement receptors C3aR1 or C5aR1 were required to induce proteolysis of synaptopodin and NEPH1 by 2 distinct proteolytic pathways mediated by cysteine and aspartic proteinases, respectively. Together, these results demonstrated a mechanism by which aberrantly glycosylated IgG4 activated the lectin pathway and induced podocyte injury in primary membranous nephropathy.

Generation of Monogenic Candidate Genes for Human Nephrotic Syndrome Using 3 Independent Approaches

INTRODUCTION

Steroid-resistant nephrotic syndrome (SRNS) is the second most common cause of chronic kidney disease during childhood. Identification of 63 monogenic human genes has delineated 12 distinct pathogenic pathways.

METHODS

Here, we generated 2 independent sets of nephrotic syndrome (NS) candidate genes to augment the discovery of additional monogenic causes based on whole-exome sequencing (WES) data from 1382 families with NS.

RESULTS

We first identified 63 known monogenic causes of NS in mice from public databases and scientific publications, and 12 of these genes overlapped with the 63 known human monogenic SRNS genes. Second, we used a set of 64 genes that are regulated by the transcription factor Wilms tumor 1 (WT1), which causes SRNS if mutated. Thirteen of these WT1-regulated genes overlapped with human or murine NS genes. Finally, we overlapped these lists of murine and WT1 candidate genes with our list of 120 candidate genes generated from WES in 1382 NS families, to identify novel candidate genes for monogenic human SRNS. Using this approach, we identified 7 overlapping genes, of which 3 genes were shared by all datasets, including SYNPO. We show that loss-of-function of SYNPO leads to decreased CDC42 activity and reduced podocyte migration rate, both of which are rescued by overexpression of wild-type complementary DNA (cDNA), but not by cDNA representing the patient mutation.

CONCLUSION

Thus, we identified 3 novel candidate genes for human SRNS using 3 independent, nonoverlapping hypotheses, and generated functional evidence for SYNPO as a novel potential monogenic cause of NS.

Mass spectrometry-based proteomic exploration of the small urinary extracellular vesicles in ANCA-associated vasculitis in comparison with total urine

ANCA-associated vasculitis (AAV) is a rare, but potentially severe autoimmune disease, even nowadays displaying increased mortality and morbidity. Finding early biomarkers of activity and prognosis is thus very important. Small extracellular vesicles (EVs) isolated from urine can be considered as a non-invasive source of biomarkers. We evaluated several protocols for urinary EV isolation. To eliminate contaminating non-vesicular proteins due to AAV associated proteinuria we used proteinase K treatment. We investigated the differences in proteomes of small EVs of patients with AAV compared to healthy controls by label-free LC-MS/MS. In parallel, we performed an analogous proteomic analysis of urine samples from identical patients. The study results showed significant differences and similarities in both EV and urine proteome, the latter one being highly affected by proteinuria. Using bioinformatics tools we explored differentially changed proteins and their related pathways with a focus on the pathophysiology of AAV. Our findings indicate significant regulation of Golgi enzymes, such as MAN1A1, which can be involved in T cell activation by N-glycans glycosylation and may thus play a key role in pathogenesis and diagnosis of AAV. SIGNIFICANCE: The present study explores for the first time the changes in proteomes of small extracellular vesicles and urine of patients with renal ANCA-associated vasculitis compared to healthy controls by label-free LC-MS/MS. Isolation of vesicles from proteinuric urine samples has been modified to minimize contamination by plasma proteins and to reduce co-isolation of extraluminal proteins. Differentially changed proteins and their related pathways with a role in the pathophysiology of AAV were described and discussed. The results could be helpful for the research of potential biomarkers in renal vasculitis associated with ANCA.

A different perspective on the filtration barrier after kidney stone formation: An immunohistochemical and biochemical study

The aim of this study is to investigate whether the filtration barrier is affected by experimental kidney stone formation. Thirty-two rats divided into 4 equally groups (n = 8) at random. Group I control; Group II 1% ethylene glycol; Group III 1% Ethylene glycol + 0.25% Ammonium chloride; Group IV 1% Ethylene glycol + 0.5% Ammonium chloride group. Tissues applied hematoxylin-eosin, periodic-acid-Schiff, Pizzolato's staining. Immunohistochemically stained with integrin α3β1, type IV collagen, laminin, nephrin, CD2-associated protein (CD2AP) and podocin to show the filtration barrier structure. The TUNEL method was used for apoptosis. The amount of calcium, magnesium, creatinine and uric acid in urine and blood samples, also urine microprotein determined. Stones were formed in all experimental groups. Urine calcium, creatinine, uric acid levels decreased, magnesium levels were not changed. No statistically significant change was observed in blood serum results and TUNEL analysis. Immunohistochemical results showed an increase in nephrin, podocin, CD2AP, laminin and a decrease in integrin α3β1 and type IV collagen. Consequently, there is an increase in the expression densities of the proteins incorporated in the structure to prevent loss of functionality in the cellular part supporting the structure against a weakening of the basement membrane structure in the glomerular structure in which urine is filtered.

Synaptopodin Is Dispensable for Normal Podocyte Homeostasis but Is Protective in the Context of Acute Podocyte Injury

BACKGROUND

Synaptopodin (Synpo) is an actin-associated protein in podocytes and dendritic spines. Many functions in regulating the actin cytoskeleton via RhoA and other pathways have been ascribed to Synpo, yet no pathogenic mutations in the SYNPO gene have been discovered in patients. Naturally occurring Synpo isoforms are known (Synpo-short and -long), and a novel truncated version (Synpo-T) is upregulated in podocytes from Synpo mutant mice. Synpo-T maintains some Synpo functions, which may prevent a podocyte phenotype from emerging in unchallenged mutant mice.

METHODS

Novel mouse models were generated to further investigate the functions of Synpo. In one, CRISPR/Cas9 deleted most of the Synpo gene, preventing production of any detectable Synpo protein. Two other mutant strains made truncated versions of the protein. Adriamycin injections were used to challenge the mice, and Synpo functions were investigated in primary cultured podocytes.

RESULTS

Mice that could not make detectable Synpo (Synpo ^-/- ) did not develop any kidney abnormalities up to 12 months of age. However, Synpo ^-/- mice were more susceptible to Adriamycin nephropathy. In cultured primary podocytes from mutant mice, the absence of Synpo caused loss of stress fibers, increased the number and size of focal adhesions, and impaired cell migration. Furthermore, loss of Synpo led to decreased RhoA activity and increased Rac1 activation.

CONCLUSIONS

In contrast to previous findings, podocytes can function normally in vivo in the absence of any Synpo isoform. Synpo plays a protective role in the context of podocyte injury through its involvement in actin reorganization and focal adhesion dynamics.

Usefulness of the cytokines expression of Th1/Th2/Th17 and urinary CD80 excretion in adult-onset minimal change disease

Background

Minimal change disease (MCD) is a common form of nephrotic syndrome in adults. However, the molecular mechanism underlying the pathogenesis of MCD remains incompletely understood. In this study, we aimed to investigate the role of the cytokines expression of Th1/Th2/Th17 and urinary CD80 excretion in adult-onset MCD patients.

Methods

The lymphocyte subsets, 34 cytokine levels of Th1/Th2/Th17, serum and urine concentrations of CD80, and expression of CD80 in glomeruli were analyzed in 28 cases (15 males and 13 females; average age: 34.1 years, age range: 18–56 years), including 10 patients with MCD in relapse, nine patients with MCD in remission and nine healthy controls.

Results

There was no significant difference of CD3⁺CD4⁺ cells proportion among patients with MCD in relapse, MCD in remission and healthy controls (P = 0.802). The cytokine levels of GM-CSF and tumor necrosis factor (TNF)-related activation-induced cytokine (TRANCE) in patients with MCD in relapse increased 1.5 times higher than those in remission. An evident increase in the excretion of urinary CD80 was found in patients with relapsed MCD compared with those in remission (598.4 ± 115.8 vs 81.78 ± 7.04 ng/g creatinine, P < 0.001) and healthy controls (598.4 ± 115.8 vs 67.44 ±  8.94  ng/g creatinine, P < 0.001). CD80 expression was observed in podocyte of MCD patient in relapse by immunofluorescence technique.

Conclusions

The cytokines GM-CSF and TRANCE are increased and the urinary CD80 levels are elevated in adult-onset MCD patients in relapse, indicating a disorder of Th1/Th2/Th17 balance and that the elevated excretion of CD80 may underlie the pathogenesis and development of adult-onset MCD.

Repeated Administration of Clinical Doses of Tramadol and Tapentadol Causes Hepato- and Nephrotoxic Effects in Wistar Rats

Tramadol and tapentadol are fully synthetic and extensively used analgesic opioids, presenting enhanced therapeutic and safety profiles as compared with their peers. However, reports of adverse reactions, intoxications and fatalities have been increasing. Information regarding the molecular, biochemical, and histological alterations underlying their toxicological potential is missing, particularly for tapentadol, owing to its more recent market authorization. Considering the paramount importance of liver and kidney for the metabolism and excretion of both opioids, these organs are especially susceptible to toxicological damage. In the present study, we aimed to characterize the putative hepatic and renal deleterious effects of repeated exposure to therapeutic doses of tramadol and tapentadol, using an in vivo animal model. Male Wistar rats were randomly divided into six experimental groups, composed of six animals each, which received daily single intraperitoneal injections of 10, 25 or 50 mg/kg tramadol or tapentadol (a low, standard analgesic dose, an intermediate dose and the maximum recommended daily dose, respectively). An additional control group was injected with normal saline. Following 14 consecutive days of administration, serum, urine and liver and kidney tissue samples were processed for biochemical, metabolic and histological analysis. Repeated administration of therapeutic doses of both opioids led to: (i) increased lipid and protein oxidation in liver and kidney, as well as to decreased total liver antioxidant capacity; (ii) decreased serum albumin, urea, butyrylcholinesterase and complement C3 and C4 levels, denoting liver synthesis impairment; (iii) elevated serum activity of liver enzymes, such as alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase and γ-glutamyl transpeptidase, as well as lipid profile alterations, also reflecting hepatobiliary commitment; (iv) derangement of iron metabolism, as shown through increases in serum iron, ferritin, haptoglobin and heme oxygenase-1 levels. In turn, elevated serum cystatin C, decreased urine creatinine output and increased urine microalbumin levels were detected upon exposure to tapentadol only, while increased serum amylase and urine N-acetyl-β-D-glucosaminidase activities were observed for both opioids. Collectively, these results are compatible with kidney injury. Changes were also found in the expression levels of liver- and kidney-specific toxicity biomarker genes, upon exposure to tramadol and tapentadol, correlating well with alterations in lipid profile, iron metabolism and glomerular and tubular function. Histopathological analysis evidenced sinusoidal dilatation, microsteatosis, mononuclear cell infiltrates, glomerular and tubular disorganization, and increased Bowman's spaces. Although some findings are more pronounced upon tapentadol exposure, our study shows that, when compared with acute exposure, prolonged administration of both opioids smooths the differences between their toxicological effects, and that these occur at lower doses within the therapeutic range.

oxLDL promotes podocyte migration by regulating CXCL16, ADAM10 and ACTN4

Nephrotic syndrome (NS) is one of the most common causes of chronic kidney disease in the pediatric population. Hyperlipidemia is one of the main features of NS. The present study investigated the role of CXC motif chemokine ligand 16 (CXCL16) and ADAM metallopeptidase domain 10 (ADAM10) in oxidized low-density lipoprotein (oxLDL)-stimualted podocytes and the underlying mechanisms. CXCL16 and ADAM10 expression levels in oxLDL-treated podocytes were measured via reverse transcription-quantitative PCR and western blotting. Cell migration assays were conducted to assess the migration of oxLDL-treated podocytes. CXCL16 or ADAM10 overexpression and knockdown assays were conducted. The results indicated that oxLDL stimulation increased ADAM10 and CXCL16 expression levels, and enhanced podocyte migration compared with the control group. Moreover, CXCL16 and ADAM10 overexpression significantly increased podocyte migration and the expression of actinin-α4 (ACTN4) compared with the control groups. By contrast, CXCL16 and ADAM10 knockdown significantly reduced podocyte migration and the expression of ACTN4 compared with the control groups. The results suggested that oxLDL promoted podocyte migration by regulating CXCL16 and ADAM10 expression, as well as by modulating the actin cytoskeleton. Therefore, CXCL16 and ADAM10 may serve as novel therapeutic targets for primary nephrotic syndrome in children.

Protein misfolding in endoplasmic reticulum stress with applications to renal diseases

Protein misfolding may be the result of a variety of different processes that disrupt the ability of a protein to form a thermodynamically stable tertiary structure that allows it to perform its proper function. In this chapter, we explore the nature of a protein's form that allows it to have a stable tertiary structure, and examine specific mutation that are known to occur in the coding regions of DNA that disrupt a protein's ability to be folded into a thermodynamically stable tertiary structure. We examine the consequences of these protein misfoldings in terms of the endoplasmic reticulum stress response and resulting unfolded protein response. These conditions are specifically related to renal diseases. Further, we explore novel therapeutics, pharmacological chaperones, that are being developed to alleviate the disease burden associated with protein misfolding caused by mutations. These interventions aim to stabilize protein folding intermediates and allow proper folding to occur as well as prevent protein aggregation and the resulting pathophysiological consequences.

Podocyte RhoGTPases: new therapeutic targets for nephrotic syndrome?

Podocytes, or glomerular epithelial cells, form the final layer in the glomerular capillary wall of the kidney. Along with the glomerular basement membrane and glomerular endothelial cells, they make up the glomerular filtration barrier which allows the passage of water and small molecules and, in healthy individuals, prevents the passage of albumin and other key proteins. The podocyte is a specialised and terminally differentiated cell with a specific cell morphology that is largely dependent on a highly dynamic underlying cytoskeletal network and that is essential for maintaining glomerular function and integrity in healthy kidneys. The RhoGTPases (RhoA, Rac1 and Cdc42), which act as molecular switches that regulate actin dynamics, are known to play a crucial role in maintaining the cytoskeletal and molecular integrity of the podocyte foot processes in a dynamic manner. Recently, novel protein interaction networks that regulate the RhoGTPases in the podocyte and that are altered by disease have been discovered. This review will discuss these networks and their potential as novel therapeutic targets in nephrotic syndrome. It will also discuss the evidence that they are direct targets for (a) steroids, the first-line agents for the treatment of nephrotic syndrome, and (b) certain kinase inhibitors used in cancer treatment, leading to nephrotoxicity.

Genetic Deletion of Emp2 Does Not Cause Proteinuric Kidney Disease in Mice

Nephrotic syndrome is one of the most common glomerular diseases in children and can be classified on the basis of steroid responsiveness. While multiple genetic causes have been discovered for steroid resistant nephrotic syndrome, the genetics of steroid sensitive nephrotic syndrome remains elusive. Mutations in Epithelial Membrane Protein 2 (EMP2), a member of the GAS3/PMP22 tetraspan family of proteins, were recently implicated as putative monogenic cause of steroid sensitive nephrotic syndrome. We investigated this hypothesis by developing Emp2 reporter and knockout mouse models. In lacZ reporter mice (engineered to drive expression of the enzyme β-galactosidase under the control of the endogenous murine Emp2 promoter), Emp2 promoter activity was not observed in podocytes but was particularly prominent in medium- and large-caliber arterial vessels in the kidney and other tissues where it localizes specifically in vascular smooth muscle cells (vSMCs) but not in the endothelium. Strong Emp2 expression was also found in non-vascular smooth muscle cells found in other organs like the stomach, bladder, and uterus. Global and podocyte-specific Emp2 knockout mice were viable and did not develop nephrotic syndrome showing no evidence of abnormal glomerular histology or ultrastructure. Altogether, our results do not support that loss of function of EMP2 represent a monogenic cause of proteinuric kidney disease. However, the expression pattern of Emp2 indicates that it may be relevant in smooth muscle function in various organs and tissues including the vasculature.