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

IL-17 in type II diabetes mellitus (T2DM) immunopathogenesis and complications; molecular approaches

Chronic inflammation has long been considered the characteristic feature of type II diabetes mellitus (T2DM) Immunopathogenesis. Pro-inflammatory cytokines are considered the central drivers of the inflammatory cascade leading to β-cell dysfunction and insulin resistance (IR), two major pathologic events contributing to T2DM. Analyzing the cytokine profile of T2DM patients has also introduced interleukin-17 (IL-17) as an upstream regulator of inflammation, regarding its role in inducing the nuclear factor-kappa B (NF-κB) pathway. In diabetic tissues, IL-17 induces the expression of inflammatory cytokines and chemokines. Hence, IL-17 can deteriorate insulin signaling and β-cell function by activating the JNK pathway and inducing infiltration of neutrophils into pancreatic islets, respectively. Additionally, higher levels of IL-17 expression in patients with diabetic complications compared to non-complicated individuals have also proposed a role for IL-17 in T2DM complications. Here, we highlight the role of IL-17 in the Immunopathogenesis of T2DM and corresponding pathways, recent advances in preclinical and clinical studies targeting IL-17 in T2DM, and corresponding challenges and possible solutions.

Minimal Change Disease: Pathogenetic Insights from Glomerular Proteomics

The mechanism underlying podocyte dysfunction in minimal change disease (MCD) remains unknown. This study aimed to shed light on the potential pathophysiology of MCD using glomerular proteomic analysis. Shotgun proteomics using label-free quantitative mass spectrometry was performed on formalin-fixed, paraffin-embedded (FFPE) renal biopsies from two groups of samples: control (CTR) and MCD. Glomeruli were excised from FFPE renal biopsies using laser capture microdissection (LCM), and a single-pot solid-phase-enhanced sample preparation (SP3) digestion method was used to improve yield and protein identifications. Principal component analysis (PCA) revealed a distinct separation between the CTR and MCD groups. Forty-eight proteins with different abundance between the two groups (p-value ≤ 0.05 and |FC| ≥ 1.5) were identified. These may represent differences in podocyte structure, as well as changes in endothelial or mesangial cells and extracellular matrix, and some were indeed found in several of these structures. However, most differentially expressed proteins were linked to the podocyte cytoskeleton and its dynamics. Some of these proteins are known to be involved in focal adhesion (NID1 and ITGA3) or slit diaphragm signaling (ANXA2, TJP1 and MYO1C), while others are structural components of the actin and microtubule cytoskeleton of podocytes (ACTR3 and NES). This study suggests the potential of mass spectrometry-based shotgun proteomic analysis with LCM glomeruli to yield valuable insights into the pathogenesis of podocytopathies like MCD. The most significantly dysregulated proteins in MCD could be attributable to cytoskeleton dysfunction or may be a compensatory response to cytoskeleton malfunction caused by various triggers.

Nephrotic Syndrome Throughout Childhood: Diagnosing Podocytopathies From the Womb to the Dorm

The etiologies of podocyte dysfunction that lead to pediatric nephrotic syndrome (NS) are vast and vary with age at presentation. The discovery of numerous novel genetic podocytopathies and the evolution of diagnostic technologies has transformed the investigation of steroid-resistant NS while simultaneously promoting the replacement of traditional morphology-based disease classifications with a mechanistic approach. Podocytopathies associated with primary and secondary steroid-resistant NS manifest as diffuse mesangial sclerosis, minimal change disease, focal segmental glomerulosclerosis, and collapsing glomerulopathy. Molecular testing, once an ancillary option, has become a vital component of the clinical investigation and when paired with kidney biopsy findings, provides data that can optimize treatment and prognosis. This review focuses on the causes including selected monogenic defects, clinical phenotypes, histopathologic findings, and age-appropriate differential diagnoses of nephrotic syndrome in the pediatric population with an emphasis on podocytopathies.

Prevalence of kidney health genetic variants in adults with sickle cell nephropathy

The pathophysiology and genetic risk for sickle cell disease (SCD)-related chronic kidney disease (CKD) are not well understood. In 70 adults with SCD-related CKD and without APOL1 inherited in a high-risk pattern, 24 (34%) had pathogenic variants in candidate genes using KidneySeq™. A moderate impact INF2 variant was observed in 20 (29%) patients and those with 3 versus 0-2 pathogenic or moderate impact glomerular genetic variants had higher albuminuria and lower estimated glomerular filtration rate (adjusted p ≤ 0.015). Using a panel of preselected genes implicated in kidney health, we observed several variants in people with sickle cell nephropathy.

Quantitative phenotyping of Nphs1 knockout mice as a prerequisite for gene replacement studies

Steroid-resistant nephrotic syndrome (SRNS) is the second most frequent cause of chronic kidney disease before the age of 25 yr. Nephrin, encoded by NPHS1, localizes to the slit diaphragm of glomerular podocytes and is the predominant structural component of the glomerular filtration barrier. Biallelic variants in NPHS1 can cause congenital nephrotic syndrome of the Finnish type, for which, to date, no causative therapy is available. Recently, adeno-associated virus (AAV) vectors targeting the glomerular podocyte have been assessed as a means for gene replacement therapy. Here, we established quantitative and reproducible phenotyping of a published, conditional Nphs1 knockout mouse model (Nphs1tm1.1Pgarg/J and Nphs2-Cre+) in preparation for a gene replacement study using AAV vectors. Nphs1 knockout mice (Nphs1fl/fl Nphs2-Cre+) exhibited 1) a median survival rate of 18 days (range: from 9 to 43 days; males: 16.5 days and females: 20 days); 2) an average foot process (FP) density of 1.0 FP/µm compared with 2.0 FP/µm in controls and a mean filtration slit density of 2.64 µm/µm2 compared with 4.36 µm/µm2 in controls; 3) a high number of proximal tubular microcysts; 4) the development of proteinuria within the first week of life as evidenced by urine albumin-to-creatinine ratios; and 5) significantly reduced levels of serum albumin and elevated blood urea nitrogen and creatinine levels. For none of these phenotypes, significant differences between sexes in Nphs1 knockout mice were observed. We quantitatively characterized five different phenotypic features of congenital nephrotic syndrome in Nphs1fl/fl Nphs2-Cre+ mice. Our results will facilitate future gene replacement therapy projects by allowing for sensitive detection of even subtle molecular effects.NEW & NOTEWORTHY To evaluate potential, even subtle molecular, therapeutic effects of gene replacement therapy (GRT) in a mouse model, prior rigorous quantifiable and reproducible disease phenotyping is necessary. Here, we, therefore, describe such a phenotyping effort in nephrin (Nphs1) knockout mice to establish the basis for GRT for congenital nephrotic syndrome. We believe that our findings set an important basis for upcoming/ongoing gene therapy approaches in the field of nephrology, especially for monogenic nephrotic syndrome.

Swollen Feet: Considering the Paradoxical Roles of Interleukins in Nephrotic Syndrome

Interleukins are a family of 40 bioactive peptides that act through cell surface receptors to induce a variety of intracellular responses. While interleukins are most commonly associated with destructive, pro-inflammatory signaling in cells, some also play a role in promoting cellular resilience and survival. This review will highlight recent evidence of the cytoprotective actions of the interleukin 1 receptor (IL-1R)- and common gamma chain receptor (IL-Rγc)-signaling cytokines in nephrotic syndrome (NS). NS results from the injury or loss of glomerular visceral epithelial cells (i.e., podocytes). Although the causes of podocyte dysfunction vary, it is clear that pro-inflammatory cytokines play a significant role in regulating the propagation, duration and severity of disease. Pro-inflammatory cytokines signaling through IL-1R and IL-Rγc have been shown to exert anti-apoptotic effects in podocytes through the phosphoinositol-3-kinase (PI-3K)/AKT pathway, highlighting the potential utility of IL-1R- and IL-Rγc-signaling interleukins for the treatment of podocytopathy in NS. The paradoxical role of interleukins as drivers and mitigators of podocyte injury is complex and ill-defined. Emerging evidence of the cytoprotective role of some interleukins in NS highlights the urgent need for a nuanced understanding of their pro-survival benefits and reveals their potential as podocyte-sparing therapeutics for NS.

Central role of podocytes in mediating cellular cross talk in glomerular health and disease

Podocytes are highly specialized epithelial cells that surround the capillaries of the glomeruli in the kidney. Together with the glomerular endothelial cells, these postmitotic cells are responsible for regulating filtrate from the circulating blood with their organized network of interdigitating foot processes that wrap around the glomerular basement membrane. Although podocyte injury and subsequent loss is the hallmark of many glomerular diseases, recent evidence suggests that the cell-cell communication between podocytes and other glomerular and nonglomerular cells is critical for the development and progression of kidney disease. In this review, we highlight these key cellular pathways of communication and how they might be a potential target for therapy in glomerular disease. We also postulate that podocytes might serve as a central hub for communication in the kidney under basal conditions and in response to cellular stress, which may have implications for the development and progression of glomerular diseases.

Ameliorative effects of Modified Huangqi Chifeng decoction on podocyte injury via autophagy mediated by PI3K/AKT/mTOR and AMPK/mTOR pathways

ETHNOPHARMACOLOGICAL RELEVANCE

Proteinuria is recognized as a risk factor for the exacerbation of chronic kidney disease. Modified Huangqi Chifeng decoction (MHCD) has distinct advantages in reducing proteinuria. Our previous experimental results have shown that MHCD can inhibit excessive autophagy. However, the specific mechanism by which MHCD regulates autophagy needs to be further explored.

AIM OF THE STUDY

In this study, in vivo and in vitro experiments were conducted to further clarify the protective mechanism of MHCD on the kidney and podocytes by regulating autophagy based on phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) and adenosine monophosphate-activated protein kinase (AMPK)/mTOR signaling pathways.

MATERIALS AND METHODS

By a single injection via the tail vein, Sprague-Dawley rats received Adriamycin (5 mg/kg) to establish a model of proteinuria nephropathy. They were divided into control, model, MHCD, 3-methyladenine (3 MA), 3 MA + MHCD, and telmisartan groups and were administered continuously for 6 weeks. The MHCD-containing serum was prepared, and a model of podocyte injury induced by Adriamycin (0.2 μg/mL) was established.

RESULTS

MHCD reduced the 24-h urine protein levels and relieved pathological kidney damage. During autophagy in the kidneys of rats with Adriamycin-induced nephropathy, the PI3K/AKT/mTOR signaling pathway is inhibited, while the AMPK/mTOR signaling pathway is activated. MHCD antagonized these effects, thereby inhibiting excessive autophagy. MHCD alleviated Adriamycin-induced podocyte autophagy, as demonstrated using Pik3r1 siRNA and an overexpression plasmid for Prkaa1/Prkaa2. Furthermore, MHCD could activate the PI3K/AKT/mTOR signaling pathway while suppressing the AMPK/mTOR signaling pathway.

CONCLUSIONS

This study demonstrated that MHCD can activate the interaction between the PI3K/AKT/mTOR and the AMPK/mTOR signaling pathways to maintain autophagy balance, inhibit excessive autophagy, and play a role in protecting the kidneys and podocytes.

Oxidative Stress Contributes to Slit Diaphragm Defects Caused by Disruption of Endocytosis

Introduction

Podocyte slit diaphragms are an important component of the glomerular filtration barrier. Podocyte injury frequently includes defects in slit diaphragms, and various mechanisms for these defects have been described, including altered endocytic trafficking of slit diaphragm proteins or oxidative stress. However, the potential relationship between endocytosis and oxidative stress in the context of slit diaphragm integrity has not been extensively considered.

Methods

To examine the potential relationships between endocytosis, oxidative stress, and slit diaphragm integrity, we induced genetic or pharmacological disruption of endocytosis in Drosophila nephrocytes (the insect orthologue of podocytes) and cultured human podocytes. We then employed immunofluorescence microscopy to analyze protein localization and levels, and to quantify signal from reactive oxygen species (ROS) dyes. Immunoprecipitation from podocyte cell lysates was used to examine effects on slit diaphragm protein complex formation (i.e., nephrin/podocin and nephrin/ZO-1).

Results

Disruption of endocytosis in nephrocytes and podocytes led to slit diaphragm defects, elevated levels of ROS (oxidative stress), and activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidant pathway. In nephrocytes with defective endocytosis, perturbation of Nrf2 signaling exacerbated slit diaphragm defects. Conversely, overexpression of Nrf2 target genes catalase or glucose-6-phosphate dehydrogenase (G6PD) significantly ameliorated slit diaphragm defects caused by disruption of endocytosis.

Conclusion

Oxidative stress is an important consequence of defective endocytosis and contributes to the defects in slit diaphragm integrity associated with disruption of endocytic trafficking.

Urinary nephrin-a potential marker of early glomerular injury: a systematic review and meta-analysis

BACKGROUND

Both early recognition of glomerular injury and diagnosis of renal injury remain important problems in clinical settings, and current diagnostic biomarkers have limitations. The aim of this review was to determine the diagnostic accuracy of urinary nephrin for detecting early glomerular injury.

METHODS

A search was conducted through electronic databases for all relevant studies published until January 31, 2022. The methodological quality was evaluated using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) tool. Pooled sensitivity, specificity, and other estimates of diagnostic accuracy were determined using a random effect model. The Summary Receiver Operating Characteristics (SROC) was used to pool the data and to estimate the area under the curve (AUC).

RESULTS

The meta-analysis included 15 studies involving 1587 participants. Overall, the pooled sensitivity of urinary nephrin for detecting glomerular injury was 0.86 (95% CI 0.83-0.89) and specificity was 0.73 (95% CI 0.70-0.76). The AUC-SROC to summarise the diagnostic accuracy was 0.90. As a predictor of preeclampsia, urinary nephrin showed a sensitivity of 0.78 (95% CI 0.71-0.84) and specificity of 0.79 (95% CI 0.75-0.82), and as a predictor of nephropathy the sensitivity was 0.90 (95% CI 0.87-0.93), and specificity was 0.62 (95% CI 0.56-0.67). A subgroup analysis using ELISA as a method of diagnosis showed a sensitivity of 0.89 (95% CI 0.86-0.92), and a specificity of 0.72 (95% CI 0.69-0.75).

CONCLUSION

Urinary nephrin may be a promising marker for the detection of early glomerular injury. ELISA assays appear to provide reasonable sensitivity and specificity. Once translated into clinical practice, urinary nephrin could provide an important addition to a panel of novel markers to help in the detection of acute and chronic renal injury.

Novel variants in CRB2 targeting the malfunction of slit diaphragm related to focal segmental glomerulosclerosis

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is a leading cause of steroid-resistant nephrotic syndrome (SRNS) that predominantly affects the podocytes. While mutations in genes causing pediatric SRNS have enhanced our understanding of FSGS, the disease's etiology remains complex and poorly understood.

METHODS

Whole exome sequencing (WES) was performed on a 9-year-old girl with SRNS associated with FSGS (SRNS-FSGS). We analyzed the expression of CRB2, slit diaphragm (SD)-associated proteins, and sphingosine 1-phosphate receptor 1 (S1PR1) in the proband and CRB2 knock-down podocytes.

RESULTS

In this study, we identified two novel compound heterozygous mutations in the Crumbs homolog 2 (CRB2) gene (c.2905delinsGCCACCTCGCGCTGGCTG, p.T969Afs*179 and c.3268C > G, p.R1090G) in a family with early-onset SRNS-FSGS. Our findings demonstrate that these CRB2 abnormalities were the underlying cause of SRNS-FSGS. CRB2 defects led to the dysfunction of podocyte SD-related proteins, including podocin, nephrin, and zonula occludens-1 (ZO-1), by reducing the phosphorylation level of S1PR1. Interestingly, the podocytic cytoskeleton remained unaffected, as demonstrated by normal expression and localization of synaptopodin. Our study also revealed a secondary decrease in CRB2 expression in idiopathic FSGS patients, indicating that CRB2 mutations may cause FSGS through a previously unknown mechanism involving SD-related proteins.

CONCLUSIONS

Overall, our findings shed new light on the pathogenesis of SRNS-FSGS and revealed that the novel pathogenic mutations in CRB2 contribute to the development of FSGS through a previously unknown mechanism involving SD-related proteins. A higher resolution version of the Graphical abstract is available as Supplementary information.

Recellularization of Acellular Xeno Kidney Scaffold: An In Vivo Method to Generate Bioartificial Kidney

A significant hurdle for kidney tissue engineering is reproducing the complex three-dimensional structure of the kidney. In our study, a stepwise approach of generating a reproducible Xeno kidney scaffold from a goat kidney is described, which can be implanted and recellularized by host cells. We have proposed a combination of sodium dodecyl sulfate and Triton-X-100-based protocol to generate a reproducible Xeno kidney scaffold, which was then analyzed by histology, DNA quantification, SEM, and renal angiography. Further, a small portion from the cortico-medullar region of the acellular scaffold was implanted in the rat's kidney subcapsular pocket for a period of 1 month, to check the recruitment of host cells into the scaffold. Post implantation, the extracellular matrix of the scaffold was well preserved and it did not induce any damage or inflammation in the native kidney. Implantation of the Xeno scaffold resulted in apparent early vascularization which helped in the recruitment of the host cells, which was characterized by histology, immunohistochemistry, and scanning electron microscopy. Implanted Xeno scaffold showed AQP-1, Nephrin, α-SMA, and VEGF expression in proximal tubules and renal glomerulus. Importantly, Ki-67 and WTAP-expressing cells were also observed near proximal tubules suggesting a high level of proliferation in the scaffold. Thus, showing the potential of Xeno kidney development that can be recellularized by the host cell to engineer into a functional kidney.

Research progress of natural active compounds on improving podocyte function to reduce proteinuria in diabetic kidney disease

Diabetic kidney disease (DKD) is a primary cause of end-stage renal disease. Proteinuria is a clinical indicator of the different stages of DKD, and podocyte injury is a major cause of proteinuria. Podocyte-specific proteins (PSPs) play important roles in the normal filtration of podocytes. Studies have shown that natural active compounds (NACs) can ameliorate proteinuria; however, the mechanism related to PSPs needs to be explored. In this study, the five stages of DKD related to proteinuria and the functions of PSPs are displayed separately. Mechanisms for ameliorating proteinuria and improving the PSPs of the 15 NACs are summarized. The in vitro and in vivo mechanistic research showed that five compounds, astragaloside IV, ligustrazine, berberine, emodin and resveratrol, exerted renal protective effects via AMPK signaling, icariin and berberine via TLR4 signaling, hirudin and baicalin via MAPK signaling, curcumin and baicalin via NF-κB signaling, and emodin via protein kinase RNA-like endoplasmic reticulum kinase signaling. The 13 PSPs were divided into five categories: actin cytoskeleton, basal domain, apical domain, slit diaphragm, and others. In conclusion, anti-inflammatory effects, anti-oxidative stress, and enhanced autophagy are the main mechanisms underlying the ameliorative effects of NACs. Podocyte apoptosis is mainly related to nephrin and podocin, which are the most studied slit diaphragm PSPs.

Radiation nephropathy: Mechanisms of injury and recovery in a murine model

BACKGROUND

Radiation nephropathy (RN) can be a severe late complication for patients treated with radiotherapy (RT) targeting abdominal and paraspinal tumors. Recent studies investigating the mechanisms of RT-mediated injury in the kidney have demonstrated that RT disrupts the cellular integrity of renal podocytes leading to cell death and loss of renal function.

AIM

To determine if RT-induced renal dysfunction is associated with alterations in podocyte and glomerular function, and whether RT-induced podocyte alterations were associated with changes in the glomerular basement membrane (GBM).

METHODS

C57BL/6 mice were treated with focal bilateral X-irradiation using a single dose (SD) of 4 Gy, 10 Gy, or 14 Gy or fractionated dosing (FD) of 5x6Gy or 24x2Gy. Then, 10-40 weeks after RT parameters of renal function were measured, along with glomerular filtration rate (GFR) and glomerular histology, as well as ultrastructural changes in GBM by transmission electron microscopy.

RESULTS

RT treatment resulted in persistent changes in renal function beginning at 10 weeks with little recovery up to 40 weeks post RT. Dose dependent changes were seen with increasing SD but no functional sparing was evident after FD. RT-induced loss of renal function was associated with expansion of the GBM and significant increases in foot process width, and associated with significant reduction in GFR, podocyte loss, and renal fibrosis.

CONCLUSION

For the first time, these data show that expansion of the GBM is one consequence of radiation injury, and disarrangement of the GBM might be associated with the death of podocytes. These data shed new light on the role podocyte injury and GBM in RT-induced renal dysfunction.

A Systematic Compilation of Human SH3 Domains: A Versatile Superfamily in Cellular Signaling

SRC homology 3 (SH3) domains are fundamental modules that enable the assembly of protein complexes through physical interactions with a pool of proline-rich/noncanonical motifs from partner proteins. They are widely studied modular building blocks across all five kingdoms of life and viruses, mediating various biological processes. The SH3 domains are also implicated in the development of human diseases, such as cancer, leukemia, osteoporosis, Alzheimer's disease, and various infections. A database search of the human proteome reveals the existence of 298 SH3 domains in 221 SH3 domain-containing proteins (SH3DCPs), ranging from 13 to 720 kilodaltons. A phylogenetic analysis of human SH3DCPs based on their multi-domain architecture seems to be the most practical way to classify them functionally, with regard to various physiological pathways. This review further summarizes the achievements made in the classification of SH3 domain functions, their binding specificity, and their significance for various diseases when exploiting SH3 protein modular interactions as drug targets.

Unveiling Angiotensin II and Losartan-Induced Gene Regulatory Networks Using Human Urine-Derived Podocytes

Podocytes are highly specialized cells that play a pivotal role in the blood filtration process in the glomeruli of the kidney, and their dysfunction leads to renal diseases. For this reason, the study and application of this cell type is of great importance in the field of regenerative medicine. Hypertension is mainly regulated by the renin-angiotensin-aldosterone system (RAAS), with its main mediator being angiotensin II (ANG II). Elevated ANG II levels lead to a pro-fibrotic, inflammatory, and hypertrophic milieu that induces apoptosis in podocytes. The activation of RAAS is critical for the pathogenesis of podocyte injury; as such, to prevent podocyte damage, patients with hypertension are administered drugs that modulate RAAS signaling. A prime example is the orally active, non-peptide, selective angiotensin-II-type I receptor (AGTR1) blocker losartan. Here, we demonstrate that SIX2-positive urine-derived renal progenitor cells (UdRPCs) and their immortalized counterpart (UM51-hTERT) can be directly differentiated into mature podocytes. These podocytes show activation of RAAS after stimulation with ANG II, resulting in ANG II-dependent upregulation of the expression of the angiotensin-II-type I receptor, AGTR1, and the downregulated expression of the angiotensin-II-type II receptor 2 (AGTR2). The stimulation of podocytes with losartan counteracts ANG II-dependent changes, resulting in a dependent favoring of the specific receptor from AGTR1 to AGTR2. Transcriptome analysis revealed 94 losartan-induced genes associated with diverse biological processes and pathways such as vascular smooth muscle contraction, the oxytocin signaling pathway, renin secretion, and ECM-receptor interaction. Co-stimulation with losartan and ANG II induced the exclusive expression of 106 genes associated with DNA methylation or demethylation, cell differentiation, the developmental process, response to muscle stretch, and calcium ion transmembrane transport. These findings highlight the usefulness of UdRPC-derived podocytes in studying the RAAS pathway and nephrotoxicity in various kidney diseases.

Deciphering the pharmacological mechanisms of Rostellularia procumbens (L) Nees. Extract alleviates adriamycin-induced nephropathy in vivo and in vitro

BACKGROUND

Rostellularia procumbens (L) Nees. is an effective traditional Chinese herbal medicine for the treatment of patients with chronic glomerulonephritis (CGN) in the clinic. However, the underlying molecular mechanisms need further elucidation.

PURPOSE

This study aims to investigate the renoprotective mechanisms of n-butanol extract from Rostellularia procumbens (L) Nees. (J-NE) in vivo and in vitro.

METHODS

The components of J-NE were analyzed by UPLC-MS/MS. In vivo, the nephropathy model was induced in mice by tail vein injection with adriamycin (10 mg·kg^-1), and mice were treated with vehicle or J-NE or benazepril by daily gavage. In vitro, MPC5 cells exposed to adriamycin (0.3 μg/ml) were treated with J-NE. The effects of J-NE inhibit podocyte apoptosis and protect against adriamycin-induced nephropathy were determined by Network pharmacology, RNA-seq, qPCR, ELISA, immunoblotting, flow cytometry, and TUNEL assay, according to the experimental protocols.

RESULT

The results showed that treatment significantly improved ADR-induced renal pathological changes, and the therapeutic mechanism of J-NE was related to the inhibition of podocyte apoptosis. Further molecular mechanism studies found that J-NE inhibited inflammation, increase the proteins expression levels of Nephrin and Podocin, reduce TRPC6 and Desmin expression levels and calcium ion levels in podocytes, and decrease the proteins expression levels of PI3K, p-PI3K, Akt and p-Akt to attenuated apoptosis. Furthermore, 38 compounds of J-NE were identified.

CONCLUSION

J-NE exerted the renoprotective effects by inhibiting podocyte apoptosis, which provides effective evidence for the treatment of J-NE targeting renal injury in CGN.

Loss of surface transport is a main cellular pathomechanism of CRB2 variants causing podocytopathies

Crumbs2 (CRB2) is a central component of the renal filtration barrier and part of the slit diaphragm, a unique cell contact formed by glomerular podocytes. Some CRB2 variants cause recessive inherited forms of steroid-resistant nephrotic syndrome. However, the disease-causing potential of numerous CRB2 variants remains unknown. Here, we report the establishment of a live-cell imaging-based assay, allowing a quantitative evaluation of the pathogenic potential of so far non-categorized CRB2 variants. Based on in silico data analysis and protein prediction software, putative disease-associated CRB2 missense variants were selected, expressed as CRB2-GFP fusion proteins, and analyzed in reporter cell lines with BFP-labeled plasma membrane. We found that in comparison with PM-localized WT, disease-associated CRB2 variants remained predominantly at the ER. Accumulation at the ER was also present for several non-characterized CRB2 variants and variants in which putative disulfide bridge-forming cysteines were replaced. Strikingly, WT CRB2 retained inside the ER in cells lacking protein disulfide isomerase A3, indicating that posttranslational modification, especially the formation of disulfide bridges, is a crucial step for the CRB2 PM transport.

Loss of S1P Lyase Expression in Human Podocytes Causes a Reduction in Nephrin Expression That Involves PKCδ Activation

Sphingosine 1-phosphate (S1P) lyase (SPL, Sgpl1) is an ER-associated enzyme that irreversibly degrades the bioactive lipid, S1P, and thereby regulates multiple cellular functions attributed to S1P. Biallelic mutations in the human Sglp1 gene lead to a severe form of a particular steroid-resistant nephrotic syndrome, suggesting that the SPL is critically involved in maintaining the glomerular ultrafiltration barrier, which is mainly built by glomerular podocytes. In this study, we have investigated the molecular effects of SPL knockdown (kd) in human podocytes to better understand the mechanism underlying nephrotic syndrome in patients. A stable SPL-kd cell line of human podocytes was generated by the lentiviral shRNA transduction method and was characterized for reduced SPL mRNA and protein levels and increased S1P levels. This cell line was further studied for changes in those podocyte-specific proteins that are known to regulate the ultrafiltration barrier. We show here that SPL-kd leads to the downregulation of the nephrin protein and mRNA expression, as well as the Wilms tumor suppressor gene 1 (WT1), which is a key transcription factor regulating nephrin expression. Mechanistically, SPL-kd resulted in increased total cellular protein kinase C (PKC) activity, while the stable downregulation of PKCδ revealed increased nephrin expression. Furthermore, the pro-inflammatory cytokine, interleukin 6 (IL-6), also reduced WT1 and nephrin expression. In addition, IL-6 caused increased PKCδ Thr⁵⁰⁵ phosphorylation, suggesting enzyme activation. Altogether, these data demonstrate that nephrin is a critical factor downregulated by the loss of SPL, which may directly cause podocyte foot process effacement as observed in mice and humans, leading to albuminuria, a hallmark of nephrotic syndrome. Furthermore, our in vitro data suggest that PKCδ could represent a new possible pharmacological target for the treatment of a nephrotic syndrome induced by SPL mutations.

Dynein-Mediated Trafficking: A New Mechanism of Diabetic Podocytopathy

Key Points

The expression of dynein is increased in human and rodent models of diabetic nephropathy (DN), eliciting a new dynein-driven pathogenesis. Uncontrolled dynein impairs the molecular sieve of kidney by remodeling the postendocytic triage and homeostasis of nephrin. The delineation of the dynein-driven pathogenesis promises a broad spectrum of new therapeutic targets for human DN.

Background

Diabetic nephropathy (DN) is characterized by increased endocytosis and degradation of nephrin, a protein that comprises the molecular sieve of the glomerular filtration barrier. While nephrin internalization has been found activated in diabetes-stressed podocytes, the postinternalization trafficking steps that lead to the eventual depletion of nephrin and the development of DN are unclear. Our work on an inherited podocytopathy uncovered that dysregulated dynein could compromise nephrin trafficking, leading us to test whether and how dynein mediates the pathogenesis of DN.

Methods

We analyzed the transcription of dynein components in public DN databases, using the Nephroseq platform. We verified altered dynein transcription in diabetic podocytopathy by quantitative PCR. Dynein-mediated trafficking and degradation of nephrin was investigated using an in vitro nephrin trafficking model and was demonstrated in a mouse model with streptozotocin (STZ)-induced DN and in human kidney biopsy sections.

Results

Our transcription analysis revealed increased expression of dynein in human DN and diabetic mouse kidney, correlated significantly with the severity of hyperglycemia and DN. In diabetic podocytopathy, we observed that dynein-mediated postendocytic sorting of nephrin was upregulated, resulting in accelerated nephrin degradation and disrupted nephrin recycling. In hyperglycemia-stressed podocytes, Dynll1 , one of the most upregulated dynein components, is required for the recruitment of dynein complex that mediates the postendocytic sorting of nephrin. This was corroborated by observing enhanced Dynll1-nephrin colocalization in podocytes of diabetic patients, as well as dynein-mediated trafficking and degradation of nephrin in STZ-induced diabetic mice with hyperglycemia. Knockdown of Dynll1 attenuated lysosomal degradation of nephrin and promoted its recycling, suggesting the essential role of Dynll1 in dynein-mediated mistrafficking.

Conclusions

Our studies show that hyperglycemia stimulates dynein-mediated trafficking of nephrin to lysosomes by inducing its expression. The decoding of dynein-driven pathogenesis of diabetic podocytopathy offers a spectrum of new dynein-related therapeutic targets for DN.

Formation and function of a highly specialised type of organelle in cardiac valve cells

Within a cell, vesicles play a crucial role in the transport of membrane material and proteins to a given target membrane, and thus regulate a variety of cellular functions. Vesicular transport occurs by means of, among others, endocytosis, where cargoes are taken up by the cell and are processed further upon vesicular trafficking, i.e. transported back to the plasma membrane via recycling endosomes or the degraded by fusion of the vesicles with lysosomes. During evolution, a variety of vesicles with individual functions arose, with some of them building up highly specialised subcellular compartments. In this study, we have analysed the biosynthesis of a new vesicular compartment present in the valve cells of Drosophila melanogaster. We show that the compartment is formed by invaginations of the plasma membrane and grows via re-routing of the recycling endosomal pathway. This is achieved by inactivation of other membrane-consuming pathways and a plasma membrane-like molecular signature of the compartment in these highly specialised heart cells.

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.

Loss of Calponin 2 causes age-progressive proteinuria in mice

Proteinuria is a major manifestation of kidney disease, reflecting injuries of glomerular podocytes. Actin cytoskeleton plays a pivotal role in stabilizing the foot processes of podocytes against the hydrostatic pressure of filtration. Calponin is an actin associated protein that regulates mechanical tension-related cytoskeleton functions and its role in podocytes has not been established. Here we studied the kidney phenotypes of calponin isoform 2 knockout (KO) mice. Urine samples were examined to quantify the ratio of albumin and creatinine. Kidney tissue samples were collected for histology and ultrastructural studies. A mouse podocyte cell line (E11) was used to study the expression and cellular localization of calponin 2. In comparison with wild-type (WT) controls, calponin 2 KO mice showed age-progressive high proteinuria and degeneration of renal glomeruli. High levels of calponin 2 are expressed in E11 podocytes and colocalized with actin stress fibers, tropomyosin and myosin IIA. Electron microscopy showed that aging calponin 2 KO mice had effacement of the podocyte foot processes and increased thickness of the glomerular basement membrane as compared to that of WT control. The findings demonstrate that deletion of calponin 2 aggravates age-progressive degeneration of the glomerular structure and function as filtration barrier. The critical role of calponin 2 in podocytes suggests a molecular target for understanding the pathogenesis of proteinuria and therapeutic development.

Complementary Nck1/2 Signaling in Podocytes Controls Actinin-4-Mediated Actin Organization, Adhesion, and Basement Membrane Composition

BACKGROUND

Maintenance of the kidney filtration barrier requires coordinated interactions between podocytes and the underlying glomerular basement membrane (GBM). GBM ligands bind podocyte integrins, which triggers actin-based signaling events critical for adhesion. Nck1/2 adaptors have emerged as essential regulators of podocyte cytoskeletal dynamics. However, the precise signaling mechanisms mediated by Nck1/2 adaptors in podocytes remain to be fully elucidated.

METHODS

We generated podocytes deficient in Nck1 and Nck2 and used transcriptomic approaches to profile expression differences. Proteomic techniques identified specific binding partners for Nck1 and Nck2 in podocytes. We used cultured podocytes and mice deficient in Nck1 and/or Nck2, along with podocyte injury models, to comprehensively verify our findings.

RESULTS

Compound loss of Nck1/2 altered expression of genes involved in actin binding, cell adhesion, and extracellular matrix composition. Accordingly, Nck1/2-deficient podocytes showed defects in actin organization and cell adhesion in vitro, with podocyte detachment and altered GBM morphology present in vivo. We identified distinct interactomes for Nck1 and Nck2 and uncovered a mechanism by which Nck1 and Nck2 cooperate to regulate actin bundling at focal adhesions via α actinin-4. Furthermore, loss of Nck1 or Nck2 resulted in increased matrix deposition in vivo, with more prominent defects in Nck2-deficient mice, consistent with enhanced susceptibility to podocyte injury.

CONCLUSION

These findings reveal distinct, yet complementary, roles for Nck proteins in regulating podocyte adhesion, controlling GBM composition, and sustaining filtration barrier integrity.

Selective endocytosis controls slit diaphragm maintenance and dynamics in Drosophila nephrocytes

The kidneys generate about 180 l of primary urine per day by filtration of plasma. An essential part of the filtration barrier is the slit diaphragm, a multiprotein complex containing nephrin as major component. Filter dysfunction typically manifests with proteinuria and mutations in endocytosis regulating genes were discovered as causes of proteinuria. However, it is unclear how endocytosis regulates the slit diaphragm and how the filtration barrier is maintained without either protein leakage or filter clogging. Here, we study nephrin dynamics in podocyte-like nephrocytes of Drosophila and show that selective endocytosis either by dynamin- or flotillin-mediated pathways regulates a stable yet highly dynamic architecture. Short-term manipulation of endocytic functions indicates that dynamin-mediated endocytosis of ectopic nephrin restricts slit diaphragm formation spatially while flotillin-mediated turnover of nephrin within the slit diaphragm is needed to maintain filter permeability by shedding of molecules bound to nephrin in endosomes. Since slit diaphragms cannot be studied in vitro and are poorly accessible in mouse models, this is the first analysis of their dynamics within the slit diaphragm multiprotein complex. Identification of the mechanisms of slit diaphragm maintenance will help to develop novel therapies for proteinuric renal diseases that are frequently limited to symptomatic treatment.

Decreased GM3 correlates with proteinuria in minimal change nephrotic syndrome and focal segmental glomerulosclerosis

BACKGROUND

Glycolipids on cell membrane rafts play various roles by interacting with glycoproteins. Recently, it was reported that the glycolipid GM3 is expressed in podocytes and may play a role in podocyte protection. In this report, we describe the correlation between changes in GM3 expression in glomeruli and proteinuria in minimal change nephrotic syndrome (MCNS) and focal segmental glomerulosclerosis (FSGS) patients.

METHODS

We performed a case-control study of the correlation between nephrin/GM3 expression levels and proteinuria in MCNS and FSGS patients who underwent renal biopsy at our institution between 2009 and 2014. Normal renal tissue sites were used from patients who had undergone nephrectomy at our institution and gave informed consent.

RESULTS

Both MCNS and FSGS had decreased GM3 and Nephrin expression compared with the normal (normal vs. MCNS, FSGS; all p < 0.01). Furthermore, in both MCNS and FSGS, GM3 expression was negatively correlated with proteinuria (MCNS: r = - 0.61, p < 0.01, FSGS: r = - 0.56, p < 0.05). However, nephrin expression had a trend to correlate with proteinuria in FSGS (MCNS: r = 0.19, p = 0.58, FSGS: r = - 0.48, p = 0.06). Furthermore, in a simple linear regression analysis, GM3 expression also correlated with proteinuric change after 12 months of treatment (MCNS: r = 0.40, p = 0.38, FSGS: r = 0. 68, p < 0.05).

CONCLUSION

We showed for the first time that decreased GM3 expression correlates with proteinuria in MCNS and FSGS patients. Further studies are needed on the podocyte-protective effects of GM3.

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.

Nephrin expression in human epidermal keratinocytes and its implication in poor wound closure

Nephrin is a type-1 transmembrane protein and a component of the slit diaphragm renal-filtration barrier. It has several functions in actin remodeling and cell-cell adhesion. Nephrin is principally located in the kidney glomerulus, but several studies have reported that nephrin is found in the pancreas, brain, and placenta. However, nephrin expression and its role in human skin have not yet been reported. First, using single-cell RNA sequencing, immunohistochemistry, and immuno-electron microscopy, nephrin expression was confirmed in human-skin epidermal keratinocytes. Nephrin expression colocalized with the expression of zonula occludens-1 in keratinocytes and was closely related to keratinocyte cell density, proliferation, and migration. High glucose treatment decreased nephrin expression and compromised keratinocyte cell migration without yes-associated protein nuclear entry. This reduced cell migration under high glucose conditions was improved in nephrin-overexpressing keratinocytes. Nephrin was highly expressed on the margins of re-epithelized epidermis based on in vivo mice and ex vivo human skin wound models. The results demonstrate that nephrin is expressed in human-skin keratinocytes and functions in cell adhesion, proliferation, and migration. In conclusion, this study suggests that nephrin may have a variety of physiological roles in human skin.

A critical role of the podocyte cytoskeleton in the pathogenesis of glomerular proteinuria and autoimmune podocytopathies

Selective glomerular filtration relies on the membrane separating the glomerular arterioles from the Bowman space. As a major component of the glomerular filtration barrier, podocytes form foot processes by the actin cytoskeleton, which dynamically adjusts in response to environmental changes to maintain filtration barrier integrity. The slit diaphragms bridge the filtration slits between neighboring foot processes and act as signaling hubs interacting with the actin cytoskeleton. Focal adhesions relay signals to regulate actin dynamics while allowing podocyte adherence to the basement membrane. Mutations in actin regulatory and signaling proteins may disrupt the actin cytoskeleton, resulting in foot process retraction, effacement, and proteinuria. Large-scale gene expression profiling platforms, transgenic animal models, and other in vivo gene delivery methods now enhance our understanding of the interactions among podocyte focal adhesions, slit diaphragms, and actin dynamics. In addition, our team found that at least 66% of idiopathic nephrotic syndrome (INS) children have podocyte autoantibodies, which was defined as a new disease subgroup-, autoimmune podocytopathies. This review outlines the pathophysiological mechanisms of podocyte cytoskeleton protein interactions in proteinuria and glomerular podocytopathy.

Nephrinuria and podocytopathies

The discovery of nephrin in 1998 has launched a new era in glomerular diseases research, emphasizing its crucial role in the structure and function of the glomerular filtration barrier. In the past 20 years, substantial advances have been made in understanding podocyte structure and function as well as the discovery of several podocyte-related proteins including nephrin. The glomerular filtration barrier is comprised of podocytes, the glomerular basement membrane and endothelial cells. Podocytes, with their specialized slit diaphragm, form the essential backbone of the glomerular filtration barrier. Nephrin is a crucial structural and functional feature of the slit diaphragm that prevents plasma protein, blood cell and macromolecule leakage into the urine. Podocyte damage results in nephrin release. Podocytopathies are kidney diseases in which podocyte damage drives proteinuria, i.e., nephrotic syndrome. Many kidney diseases involve podocytopathy including congenital nephrotic syndrome of Finnish type, diffuse mesangial sclerosis, minimal change disease, focal segmental glomerulosclerosis, collapsing glomerulonephropathy, diabetic nephropathy, lupus nephropathy, hypertensive nephropathy and preeclampsia. Recently, urinary nephrin measurement has become important in the early detection of podocytopathies. In this chapter, we elaborate the main structural and functional features of nephrin as a podocyte-specific protein, pathomechanisms of podocytopathies which result in nephrinuria, highlight the most commonly used methods for detecting urinary nephrin and investigate the diagnostic, prognostic and potential therapeutic relevance of urinary nephrin in primary and secondary proteinuric kidney diseases.

Podocyte specific deletion of PKM2 ameliorates LPS-induced podocyte injury through beta-catenin

BACKGROUND

Acute kidney injury (AKI) is associated with a severe decline in kidney function caused by abnormalities within the podocytes' glomerular matrix. Recently, AKI has been linked to alterations in glycolysis and the activity of glycolytic enzymes, including pyruvate kinase M2 (PKM2). However, the contribution of this enzyme to AKI remains largely unexplored.

METHODS

Cre-loxP technology was used to examine the effects of PKM2 specific deletion in podocytes on the activation status of key signaling pathways involved in the pathophysiology of AKI by lipopolysaccharides (LPS). In addition, we used lentiviral shRNA to generate murine podocytes deficient in PKM2 and investigated the molecular mechanisms mediating PKM2 actions in vitro.

RESULTS

Specific PKM2 deletion in podocytes ameliorated LPS-induced protein excretion and alleviated LPS-induced alterations in blood urea nitrogen and serum albumin levels. In addition, PKM2 deletion in podocytes alleviated LPS-induced structural and morphological alterations to the tubules and to the brush borders. At the molecular level, PKM2 deficiency in podocytes suppressed LPS-induced inflammation and apoptosis. In vitro, PKM2 knockdown in murine podocytes diminished LPS-induced apoptosis. These effects were concomitant with a reduction in LPS-induced activation of β-catenin and the loss of Wilms' Tumor 1 (WT1) and nephrin. Notably, the overexpression of a constitutively active mutant of β-catenin abolished the protective effect of PKM2 knockdown. Conversely, PKM2 knockdown cells reconstituted with the phosphotyrosine binding-deficient PKM2 mutant (K433E) recapitulated the effect of PKM2 depletion on LPS-induced apoptosis, β-catenin activation, and reduction in WT1 expression.

CONCLUSIONS

Taken together, our data demonstrates that PKM2 plays a key role in podocyte injury and suggests that targetting PKM2 in podocytes could serve as a promising therapeutic strategy for AKI.

TRIAL REGISTRATION

Not applicable. Video abstract.

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.

Empagliflozin Ameliorates Preeclampsia and Reduces Postpartum Susceptibility to Adriamycin in a Mouse Model Induced by Angiotensin Receptor Agonistic Autoantibodies

Preeclampsia (PE) is the leading cause of maternal and perinatal morbidity and mortality and also is a risk factor for cardiovascular and kidney disease later in life. PE is associated with oversecretion of autoantibodies against angiotensin II type 1 receptor (AT1-AA) by the placenta into the maternal circulation. Here, we sought to determine the therapeutic value of the sodium-glucose co-transporter 2 (SGLT2) inhibitor empagliflozin (EMPA) in mice with AT1-AA-induced preeclampsia. Pregnant mice were injected with AT1-AA at gestation day (GD) 13 and treated daily with EMPA until GD 19, at which point some of the maternal mice were sacrificed and assessed. The other maternal mice were labored on time and challenged with adriamycin (ADR) at 12 weeks postpartum; their offspring were assessed for fetal outcomes. We showed that EMPA treatment significantly relieved high systolic blood pressure and proteinuria and ameliorated kidney injury in PE mice without affecting fetal outcomes. EMPA also ameliorated podocyte injury and oxidative stress, reduced the expression of SGLT2 and activated the AMPK/SIRT1 signaling pathway in vivo and in vitro. Remarkably, EMPA treatment during pregnancy reduced ADR-induced kidney and podocyte injury postpartum. These findings suggest that EMPA could be a potential pharmacological agent for PE.

Sticks and Stones, a conserved cell surface ligand for the Type IIa RPTP Lar, regulates neural circuit wiring in Drosophila

Type IIa receptor-like protein tyrosine phosphatases (RPTPs) are essential for neural development. They have cell adhesion molecule (CAM)-like extracellular domains that interact with cell-surface ligands and coreceptors. We identified the immunoglobulin superfamily CAM Sticks and Stones (Sns) as a new partner for the Drosophila Type IIa RPTP Lar. Lar and Sns bind to each other in embryos and in vitro, and the human Sns ortholog, Nephrin, binds to human Type IIa RPTPs. Genetic analysis shows that Lar and Sns function together to regulate larval neuromuscular junction development, axon guidance in the mushroom body (MB), and innervation of the optic lobe (OL) medulla by R7 photoreceptors. In the neuromuscular system, Lar and Sns are both required in motor neurons, and may function as coreceptors. In the MB and OL, however, the relevant Lar-Sns interactions are in trans (between neurons), so Sns functions as a Lar ligand in these systems.

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

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

Sex Differences in Glomerular Protein Expression and Effects of Soy-Based Diet on Podocyte Signaling

Background:

Kidney disease is a major public health issue arising from loss of glomerular podocyte function, and there are considerable sex differences in its prognosis. Evidence suggests a renoprotective effect of estrogen and soy diet-derived phytoestrogens, although the molecular basis for this is poorly understood.

Objective:

Here, we aim to assess sex differences in expression of key proteins associated with podocyte survival and determine the effects of dietary soy on glomerular and podocyte signaling.

Methods:

Male and female FVB mice were fed control, low (1%), and high (20%) doses of isolated soy protein (ISP) in utero and until 100 days of age. Spot urine was collected to measure proteinuria and isolated glomeruli were used to quantify activated and total levels of nephrin, Akt, and ERK1/2. To investigate protective effects of specific soy phytoestrogens, cultured podocytes were treated with or without daidzein and subject to control or high glucose as a model of podocyte injury.

Results:

Nephrin and Akt were elevated at baseline in glomeruli from females compared to males. Both sexes that were fed 1% and 20% ISP displayed robust increases in total glomerular Akt compared to controls, and these effects were more prominent in females. A similar trend at both doses in both sexes was observed with activated Akt and total nephrin. Notably, males exclusively showed increased phosphorylation of nephrin and extracellular signal-regulated kinase (ERK) at the 1% ISP dose; however, no overt changes in urinary albumin excretion or podocin levels were observed, suggesting that the soy diets did not impair podocyte function. Finally, in cultured male and female podocytes, daidzein treatment suppressed high glucose-induced ERK activation.

Conclusions:

Together, our findings reveal a putative mechanism to explain the protective influence of sex on kidney disease progression, and they provide further evidence to support a beneficial role for dietary soy in preserving glomerular function.

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.

Mechanisms of Proteinuria in HIV

Proteinuria is common in the setting of HIV infection, and may reflect comorbid kidney disease, treatment-related nephrotoxicity, and HIV-related glomerular diseases. The mechanisms of podocyte and tubulointerstial injury in HIV-associated nephropathy (HIVAN) have been the subject of intense investigation over the past four decades. The pathologic contributions of viral gene expression, dysregulated innate immune signaling, and ancestry-driven genetic risk modifiers have been explored in sophisticated cellular and whole animal models of disease. These studies provide evidence that injury-induced podocyte dedifferentiation, hyperplasia, cytoskeletal dysregulation, and apoptosis may cause the loss of glomerular filtration barrier integrity and slit diaphragm performance that facilitates proteinuria and tuft collapse in HIVAN. Although the incidence of HIVAN has declined with the introduction of antiretroviral therapy, the collapsing FSGS lesion has been observed in the context of other viral infections and chronic autoimmune disorders, and with the use of interferon-based therapies in genetically susceptible populations. This highlights the fact that the lesion is not specific to HIVAN and that the role of the immune system in aggravating podocyte injury warrants further exploration. This review will summarize our progress in characterizing the molecular mechanisms of podocyte dysfunction in HIVAN and other forms of HIV-associated kidney disease.

PP2A protects podocytes against Adriamycin-induced injury and epithelial-to-mesenchymal transition via suppressing JIP4/p38-MAPK pathway

Protein phosphatase 2A (PP2A) is one of the major protein serine/threonine phosphatases (PPPs) with regulatory effects on several cellular processes, but its role and function in Adriamycin (ADR)-treated podocytes injury needs to be further explored. Mice podocytes were treated with ADR and PP2A inhibitor (okadaic acid, OA). After transfection, cell apoptosis was detected by flow cytometry. Expressions of podocytes injury-, apoptosis- and epithelial-to-mesenchymal transition (EMT)- and JNK-interacting protein 4/p38-Mitogen-Activated Protein Kinase (JIP4/p38-MAPK) pathway-related factors were measured using quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot as needed. Interaction between PP2A and JIP4/MAPK pathway was confirmed using co-immunoprecipitation (Co-Ip) assay. In podocytes, ADR inhibited PP2A, Nephrin and Wilms' tumor (WT) 1 expressions yet upregulated apoptosis and Desmin expression, and suppressing PP2A expressionenhanced the effects. PP2A overexpression reversed the effects of ADR on PP2A and podocyte injury-related factors expressions and apoptosis of podocytes. JIP4 was the candidate gene interacting with both PP2A and p38-MAPK pathway, and PP2A overexpression alleviated the effects of ADR on p38-MAPK pathway-related factors expressions. Additionally, in ADR-treated podocytes, PP2A suppression enhanced the effects of ADR, yet silencing of JIP4 reversed the effects of PP2A suppression on regulating p38-MAPK pathway-, apoptosis- and EMT-related factors expressions and apoptosis, with upregulations of B-cell lymphoma-2 (Bcl-2) and E-cadherin and down-regulations of Bcl-2 associated protein X (Bax), cleaved (C)-casapse-3, N-cadherin, Vimentin and Snail. PP2A protects ADR-treated podocytes against injury and EMT by suppressing JIP4/p38-MAPK pathway, showing their interaction in podocytes.

Role of actin cytoskeleton in podocytes

The selectivity of the glomerular filter is established by physical, chemical, and signaling interplay among its three core constituents: glomerular endothelial cells, the glomerular basement membrane, and podocytes. Functional impairment or injury of any of these three components can lead to proteinuria. Podocytes are injured in many forms of human and experimental glomerular disease, including minimal change disease, focal segmental glomerulosclerosis, and diabetes mellitus. One of the earliest signs of podocyte injury is loss of their distinct structure, which is driven by dysregulated dynamics of the actin cytoskeleton. The status of the actin cytoskeleton in podocytes depends on a set of actin binding proteins, nucleators and inhibitors of actin polymerization, and regulatory GTPases. Mutations that alter protein function in each category have been implicated in glomerular diseases in humans and animal models. In addition, a growing body of studies suggest that pharmacological modifications of the actin cytoskeleton have the potential to become novel therapeutics for podocyte-dependent chronic kidney diseases. This review presents an overview of the essential proteins that establish actin cytoskeleton in podocytes and studies demonstrating the feasibility of drugging actin cytoskeleton in kidney diseases.

In Vivo Distribution of Poly(ethylene glycol) Functionalized Iron Oxide Nanoclusters: An Ultrastructural Study

The in vivo distribution of 50 nm clusters of polyethylene glycol-conjugated superparamagnetic iron oxide nanoparticles (SPIONs-PEG) was conducted in this study. SPIONs-PEG were synthesized de novo, and their structure and paramagnetic behaviors were analyzed by specific methods (TEM, DLS, XRD, VSM). Wistar rats were treated with 10 mg Fe/kg body weight SPIONs-PEG and their organs and blood were examined at two intervals for short-term (15, 30, 60, 180 min) and long-term (6, 12, 24 h) exposure evaluation. Most exposed organs were investigated through light and transmission electron microscopy, and blood and urine samples were examined through fluorescence spectrophotometry. SPIONs-PEG clusters entered the bloodstream after intraperitoneal and intravenous administrations and ended up in the urine, with the highest clearance at 12 h. The skin and spleen were within normal histological parameters, while the liver, kidney, brain, and lungs showed signs of transient local anoxia or other transient pathological affections. This study shows that once internalized, the synthesized SPIONs-PEG disperse well through the bloodstream with minor to nil induced tissue damage, are biocompatible, have good clearance, and are suited for biomedical applications.

ShcA expression in podocytes is dispensable for glomerular development but its upregulation is associated with kidney disease

BACKGROUND

ShcA (SHC1) is a phosphotyrosine adaptor protein which plays broad signaling roles within the cell. Systemic loss of ShcA during embryogenesis is lethal, while its aberrant expression contributes to disease. We recently demonstrated that ShcA is highly expressed during glomerular development and that it is upregulated within podocytes in experimental kidney injury and chronic kidney disease. The objective of this study was to analyze the in vivo role of ShcA in podocytes.

METHODS

We selectively deleted all three isoforms of ShcA from mouse kidney podocytes using the Cre/lox system driven by the podocyte-specific podocin promoter (Nphs2). Immunostaining of kidney sections was used to confirm ShcA deletion in podocytes. Coomassie blue staining of protein gels was used to detect urinary albumin. Light and electron microscopy were used to assess glomerular morphology. Transcript levels of SHC1 in human renal disease were assessed using the Nephroseq database.

RESULTS

Mice lacking podocyte ShcA were born at the expected Mendelian frequency and did not display overt renal impairment or changes in podocyte architecture beyond one year of age. In parallel, we correlated increased ShcA mRNA expression in the human kidney with proteinuria and reduced glomerular filtration rate.

CONCLUSION

Our studies reveal that ShcA is dispensable for normal kidney function, but its upregulation is associated with disease.

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.

Recessive, gain-of-function toxicity in an APOL1 BAC transgenic mouse model mirrors human APOL1 kidney disease

People of recent sub-Saharan African ancestry develop kidney failure much more frequently than other groups. A large fraction of this disparity is due to two coding sequence variants in the APOL1 gene. Inheriting two copies of these APOL1 risk variants, known as G1 and G2, causes high rates of focal segmental glomerulosclerosis (FSGS), HIV-associated nephropathy and hypertension-associated end-stage kidney disease. Disease risk follows a recessive mode of inheritance, which is puzzling given the considerable data that G1 and G2 are toxic gain-of-function variants. We developed coisogenic bacterial artificial chromosome (BAC) transgenic mice harboring either the wild-type (G0), G1 or G2 forms of human APOL1. Expression of interferon gamma (IFN-γ) via plasmid tail vein injection results in upregulation of APOL1 protein levels together with robust induction of heavy proteinuria and glomerulosclerosis in G1/G1 and G2/G2 but not G0/G0 mice. The disease phenotype was greater in G2/G2 mice. Neither heterozygous (G1/G0 or G2/G0) risk variant mice nor hemizygous (G1/-, G2/-) mice had significant kidney injury in response to IFN-γ, although the heterozygous mice had a greater proteinuric response than the hemizygous mice, suggesting that the lack of significant disease in humans heterozygous for G1 or G2 is not due to G0 rescue of G1 or G2 toxicity. Studies using additional mice (multicopy G2 and a non-isogenic G0 mouse) supported the notion that disease is largely a function of the level of risk variant APOL1 expression. Together, these findings shed light on the recessive nature of APOL1-nephropathy and present an important model for future studies.

Slit diaphragm maintenance requires dynamic clathrin-mediated endocytosis facilitated by AP-2, Lap, Aux and Hsc70-4 in nephrocytes

Background

The Slit diaphragm (SD) is the key filtration structure in human glomerular kidney that is affected in many types of renal diseases. SD proteins are known to undergo endocytosis and recycling to maintain the integrity of the filtration structure. However, the key components of this pathway remain unclear.

Methods

Using the Drosophila nephrocyte as a genetic screen platform, we screened most genes involved in endocytosis and cell trafficking, and identified the key components of the cell trafficking pathway required for SD protein endocytosis and recycling.

Results

We discovered that the SD protein endocytosis and recycling pathway contains clathrin, dynamin, AP-2 complex, like-AP180 (Lap), auxilin and Hsc70-4 (the endocytosis part) followed by Rab11 and the exocyst complex (the recycling part). Disrupting any component in this pathway led to disrupted SD on the cell surface and intracellular accumulation of mislocalized SD proteins. We also showed the first in vivo evidence of trapped SD proteins in clathrin-coated pits at the plasma membrane when this pathway is disrupted.

Conclusions

All genes in this SD protein endocytosis and recycling pathway, as well as SD proteins themselves, are highly conserved from flies to humans. Thus, our results suggest that the SD proteins in human kidney undergo the same endocytosis and recycling pathway to maintain the filtration structure, and mutations in any genes in this pathway could lead to abnormal SD and renal diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00595-4.

Crumbs2 Is an Essential Slit Diaphragm Protein of the Renal Filtration Barrier

BACKGROUND

Crumbs2 is expressed at embryonic stages as well as in the retina, brain, and glomerular podocytes. Recent studies identified CRB2 mutations as a novel cause of steroid-resistant nephrotic syndrome (SRNS).

METHODS

To study the function of Crb2 at the renal filtration barrier, mice lacking Crb2 exclusively in podocytes were generated. Gene expression and histologic studies as well as transmission and scanning electron microscopy were used to analyze these Crb2^podKO knockout mice and their littermate controls. Furthermore, high-resolution expansion microscopy was used to investigate Crb2 distribution in murine glomeruli. For pull-down experiments, live cell imaging, and transcriptome analyses, cell lines were applied that inducibly express fluorescent protein-tagged CRB2 wild type and mutants.

RESULTS

Crb2^podKO mice developed proteinuria directly after birth that preceded a prominent development of disordered and effaced foot processes, upregulation of renal injury and inflammatory markers, and glomerulosclerosis. Pull-down assays revealed an interaction of CRB2 with Nephrin, mediated by their extracellular domains. Expansion microscopy showed that in mice glomeruli, Crb2 and Nephrin are organized in adjacent clusters. SRNS-associated CRB2 protein variants and a mutant that lacks a putative conserved O-glycosylation site were not transported to the cell surface. Instead, mutants accumulated in the ER, showed altered glycosylation pattern, and triggered an ER stress response.

CONCLUSIONS

Crb2 is an essential component of the podocyte's slit diaphragm, interacting with Nephrin. Loss of slit diaphragm targeting and increasing ER stress are pivotal factors for onset and progression of CRB2-related SRNS.

Biomolecular condensates in membrane receptor signaling

Clustering is a prominent feature of receptors at the plasma membrane (PM). It plays an important role in signaling. Liquid-liquid phase separation (LLPS) of proteins is emerging as a novel mechanism underlying the observed clustering. Receptors/transmembrane signaling proteins can be core components essential for LLPS (such as LAT or nephrin) or clients enriched at the phase-separated condensates (for example, at the postsynaptic density or at tight junctions). Condensate formation has been shown to regulate signaling in multiple ways, including by increasing protein binding avidity and by modulating the local biochemical environment. In moving forward, it is important to study protein LLPS at the PM of living cells, its interplay with other factors underlying receptor clustering, and its signaling and functional consequences.

Endoplasmic reticulum-associated degradation is required for nephrin maturation and kidney glomerular filtration function

Podocytes are key to the glomerular filtration barrier by forming a slit diaphragm between interdigitating foot processes; however, the molecular details and functional importance of protein folding and degradation in the ER remain unknown. Here, we show that the SEL1L-HRD1 protein complex of ER-associated degradation (ERAD) is required for slit diaphragm formation and glomerular filtration function. SEL1L-HRD1 ERAD is highly expressed in podocytes of both mouse and human kidneys. Mice with podocyte-specific Sel1L deficiency develop podocytopathy and severe congenital nephrotic syndrome with an impaired slit diaphragm shortly after weaning and die prematurely, with a median lifespan of approximately 3 months. We show mechanistically that nephrin, a type 1 membrane protein causally linked to congenital nephrotic syndrome, is an endogenous ERAD substrate. ERAD deficiency attenuated the maturation of nascent nephrin, leading to its retention in the ER. We also show that various autosomal-recessive nephrin disease mutants were highly unstable and broken down by SEL1L-HRD1 ERAD, which attenuated the pathogenicity of the mutants toward the WT allele. This study uncovers a critical role of SEL1L-HRD1 ERAD in glomerular filtration barrier function and provides insights into the pathogenesis associated with autosomal-recessive disease mutants.