Podocyte RhoGTPases: new therapeutic targets for nephrotic syndrome?

Xuebijing injection and its bioactive components alleviate nephrotic syndrome by inhibiting podocyte inflammatory injury

Xuebijing injection (XBJ) is widely used to treat nephrotic syndrome (NS) in clinic, but its bioactive components and therapeutic mechanism are still unclear. In this study, the bioactive components of XBJ were determined by ultra-performance liquid chromatography quadrupole time-of-flight tandem mass spectrometry (UPLC-Q-TOF/MS). The therapeutic effect of XBJ on NS was evaluated in BALB/c mice induced by adriamycin (ADR, 10 mg/kg) via a single tail vein. The protective effect of XBJ and its bioactive components on podocytes was demonstrated using mouse podocytes (MPC-5) induced by lipopolysaccharide (LPS, 4 μg/mL). The results show that 33 components of XBJ were identified. Furthermore, 12 bioactive components were detected in blood, including protocatechuic acid, salvianolic acid C, benzoyloxypaeoniflorin, danshensu, salvianolic acid A, salvianolic acid B, catechin, caffeic acid, galloylpaeoniflorin, oxypaeoniflorin, hydroxysafflor yellow A, rosmarinic acid. The relative content (%) of the bioactive components were 59.32, 16.01, 9.97, 9.73, 8.72, 8.31, 7.92, 6.54, 1.54, 1.30, 0.68 and 0.59 in this order. After XBJ treatment, the renal function, hyperlipidemia and renal pathological damage were improved in NS model mice. Moreover, the levels of nephrin and desmin which are functional proteins in podocytes were reversed, and the levels of pro-inflammatory factors were reduced by XBJ. Interestingly, protocatechuic acid and salvianolic acid C also showed good protective effects on podocyte function and reduced the level of inflammation in LPS-induced MPC-5. The study is the first time to elucidate the bioactive components of XBJ and its potential therapeutic mechanism for treating NS by protecting podocyte function.

Slowing the Progression of Diabetic Kidney Disease

Diabetes is the most frequent cause of kidney disease that progresses to end-stage renal disease worldwide, and diabetic kidney disease is significantly related to unfavorable cardiovascular outcomes. Since the 1990s, specific therapies have emerged and been approved to slow the progression of diabetic kidney disease, namely, renin-angiotensin-aldosterone system blockers (including angiotensin-converting enzyme inhibitors (ACEi) angiotensin receptor blockers (ARBs), the non-steroidal mineralocorticoid receptor antagonist (NS-MRA), finerenone, and sodium-glucose cotransporter-2 (SGLT2) inhibitors). Mechanistically, these different classes of agents bring different anti-inflammatory, anti-fibrotic, and complementary hemodynamic effects to patients with diabetic kidney disease such that they have additive benefits on slowing disease progression. Within the coming year, there will be data on renal outcomes using the glucagon-like peptide-1 receptor agonist, semaglutide. All the aforementioned medications have also been shown to improve cardiovascular outcomes. Thus, all three classes (maximally dosed ACEi or ARB, low-dose SGLT-2 inhibitors, and the NS-MRA, finerenone) form the "pillars of therapy" such that, when used together, they maximally slow diabetic kidney disease progression. Ongoing studies aim to expand these pillars with additional medications to potentially normalize the decline in kidney function and reduce associated cardiovascular mortality.

A YAP/TAZ-ARHGAP29-RhoA Signaling Axis Regulates Podocyte Protrusions and Integrin Adhesions

Glomerular disease due to podocyte malfunction is a major factor in the pathogenesis of chronic kidney disease. Identification of podocyte-specific signaling pathways is therefore a prerequisite to characterizing relevant disease pathways and developing novel treatment approaches. Here, we employed loss of function studies for EPB41L5 (Yurt) as a central podocyte gene to generate a cell type-specific disease model. Loss of Yurt in fly nephrocytes caused protein uptake and slit diaphragm defects. Transcriptomic and proteomic analysis of human EPB41L5 knockout podocytes demonstrated impaired mechanotransduction via the YAP/TAZ signaling pathway. Further analysis of specific inhibition of the YAP/TAZ-TEAD transcription factor complex by TEADi led to the identification of ARGHAP29 as an EPB41L5 and YAP/TAZ-dependently expressed podocyte RhoGAP. Knockdown of ARHGAP29 caused increased RhoA activation, defective lamellipodia formation, and increased maturation of integrin adhesion complexes, explaining similar phenotypes caused by loss of EPB41L5 and TEADi expression in podocytes. Detection of increased levels of ARHGAP29 in early disease stages of human glomerular disease implies a novel negative feedback loop for mechanotransductive RhoA-YAP/TAZ signaling in podocyte physiology and disease.

CdGAP maintains podocyte function and modulates focal adhesions in a Src kinase-dependent manner

Rho GTPases are regulators of the actin cytoskeleton and their activity is modulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchanging factors (GEFs). Glomerular podocytes have numerous actin-based projections called foot processes and their alteration is characteristic of proteinuric kidney diseases. We reported previously that Rac1 hyperactivation in podocytes causes proteinuria and glomerulosclerosis in mice. However, which GAP and GEF modulate Rac1 activity in podocytes remains unknown. Here, using a proximity-based ligation assay, we identified CdGAP (ARHGAP31) and β-PIX (ARHGEF7) as the major regulatory proteins interacting with Rac1 in human podocytes. CdGAP interacted with β-PIX through its basic region, and upon EGF stimulation, they both translocated to the plasma membrane in podocytes. CdGAP-depleted podocytes had altered cell motility and increased basal Rac1 and Cdc42 activities. When stimulated with EGF, CdGAP-depleted podocytes showed impaired β-PIX membrane-translocation and tyrosine phosphorylation, and reduced activities of Src kinase, focal adhesion kinase, and paxillin. Systemic and podocyte-specific CdGAP-knockout mice developed mild but significant proteinuria, which was exacerbated by Adriamycin. Collectively, these findings show that CdGAP contributes to maintain podocyte function and protect them from injury.

NUP133 Controls Nuclear Pore Assembly, Transcriptome Composition, and Cytoskeleton Regulation in Podocytes

Steroid-resistant nephrotic syndrome (SRNS) frequently leads to end-stage renal disease, ultimately requiring kidney replacement therapies. SRNS is often caused by hereditary monogenic mutations, specifically affecting specialized epithelial cells (podocytes) of the glomerular filtration barrier. Mutations in several components of the nuclear pore complex, including NUP133 and NUP107, have been recently identified to cause hereditary SRNS. However, underlying pathomechanisms, eliciting podocyte-specific manifestations of these nucleoporopathies, remained largely elusive. Here, we generated an in vitro model of NUP133-linked nucleoporopathies using CRISPR/Cas9-mediated genome editing in human podocytes. Transcriptome, nuclear pore assembly, and cytoskeleton regulation of NUP133 loss-of-function, mutant, and wild-type podocytes were analyzed. Loss of NUP133 translated into a disruption of the nuclear pore, alterations of the podocyte-specific transcriptome, and impaired cellular protrusion generation. Surprisingly, comparative analysis of the described SRNS-related NUP133 mutations revealed only mild defects. Am impaired protein interaction in the Y-complex and decrease of NUP133 protein levels might be the primary and unifying consequence of mutant variants, leading to a partial loss-of-function phenotype and disease manifestation in susceptible cell types, such as podocytes.

Proteomic and metabolomic profiling of urine uncovers immune responses in patients with COVID-19

The utility of the urinary proteome in infectious diseases remains unclear. Here, we analyzed the proteome and metabolome of urine and serum samples from patients with COVID-19 and healthy controls. Our data show that urinary proteins effectively classify COVID-19 by severity. We detect 197 cytokines and their receptors in urine, but only 124 in serum using TMT-based proteomics. The decrease in urinary ESCRT complex proteins correlates with active SARS-CoV-2 replication. The downregulation of urinary CXCL14 in severe COVID-19 cases positively correlates with blood lymphocyte counts. Integrative multiomics analysis suggests that innate immune activation and inflammation triggered renal injuries in patients with COVID-19. COVID-19-associated modulation of the urinary proteome offers unique insights into the pathogenesis of this disease. This study demonstrates the added value of including the urinary proteome in a suite of multiomics analytes in evaluating the immune pathobiology and clinical course of COVID-19 and, potentially, other infectious diseases.

Nuclear Receptor Interacting Protein-2 Mediates the Stabilization and Activation of β-Catenin During Podocyte Injury

Objective: Activation of β-catenin causes podocyte injury and proteinuria, but how β-catenin signalling is regulated during podocyte injury remains elusive. Nuclear receptor interacting protein 2 (NRIP2) modulates the Wnt pathway in colorectal cancer-initiating cells, but the role of NRIP2 in podocyte injury has not yet been investigated. We aimed to examine the interaction between NRIP2 and β-catenin signalling.

Materials and Methods: Knockdown or overexpression of NRIP2 and β-catenin and chemical treatments were performed in cultured human podocytes. Immunoprecipitation, immunoblotting and immunofluorescence assays were used to assess protein interactions and expression. Data from the GEO dataset and kidney tissues from patients with focal segmental glomerulosclerosis (FSGS) and surgical nephrectomy were examined. An adriamycin (ADR) nephropathy model was established in NRIP2 knockout mice.

Results: NRIP2 knockdown accelerated β-catenin degradation, which was reversed by MG132; specifically, NRIP2 bound β-catenin and stabilized it to prevent its degradation through the ubiquitin proteasomal pathway. Overexpression of NRIP2 led to β-catenin activation and Snail1 induction, and these effects were attenuated by β-catenin knockdown. NRIP2 knockdown blocked ADR-stimulated β-catenin activation. In ADR mice, genetic knockout of Nrip2 ameliorated podocyte injury and loss, glomerulosclerosis, and proteinuria by inhibiting β-catenin activation. Moreover, NRIP2 was significantly upregulated in podocytes of FSGS patients and colocalized with nuclear β-catenin.

Conclusion: These results established NRIP2 as a stabilizer of β-catenin activation through the ubiquitin proteasomal pathway in podocyte injury.

The Pathophysiologic Role of Gelsolin in Chronic Kidney Disease: Focus on Podocytes

Chronic kidney disease (CKD) is normally related to proteinuria, a common finding in a compromised glomerular filtration barrier (GFB). GFB is a structure composed of glomerular endothelial cells, the basement membrane, and the podocytes. CKD with podocyte damage may be associated with actin cytoskeleton reorganization, resulting in podocyte effacement. Gelsolin plays a critical role in several diseases, including cardiovascular diseases and cancer. Our current study aimed to determine the connection between gelsolin and podocyte, and thus the mechanism underlying podocyte injury in CKD. Experiments were carried out on Drosophila to demonstrate whether gelsolin had a physiological role in maintaining podocyte. Furthermore, the survival rate of gelsolin-knocked down Drosophila larvae was extensively reduced after AgNO₃ exposure. Secondly, the in vitro podocytes treated with puromycin aminonucleoside (PAN) enhanced the gelsolin protein expression, as well as small GTPase RhoA and Rac1, which also regulated actin dynamic expression incrementally with the PAN concentrations. Thirdly, we further demonstrated in vivo that GSN was highly expressed inside the glomeruli with mitochondrial dysfunction in a CKD mouse model. Our findings suggest that an excess of gelsolin may contribute to podocytes damage in glomeruli.

The effectiveness of cyclosporine A for patients with steroid-resistant nephrotic syndrome: A protocol for systematic review and meta-analysis

BACKGROUND

The purpose of this study is to determine the efficacy and safety of Cyclosporine A (CsA) for patients with steroid-resistant nephrotic syndrome (SRNS).

METHODS

This study will be designed following the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols statement guidelines. Studies are identified through systematic searches in November 2021 with no restrictions on date and time, and publication status using the following bibliographic databases: Embase, Medline, PubMed, Web of Science, Science Direct, and the Cochrane Library. The risk of bias of included studies is estimated by taking into consideration the characteristics including random sequence generation, allocation concealment, blinding of patients, blinding of outcome assessment, completeness of outcome data, selective reporting, and other bias by Cochrane Collaboration's tool. Data synthesis and analyses are performed using Stata version 10.0 software.

RESULTS

The results of this systematic review and meta-analysis will be published in a peer-reviewed journal.

CONCLUSION

CsA may be an effective and safe therapy for SRNS. However, additional randomized controlled studies are needed to thoroughly assess the role of CsA in the treatment of SRNS.

OPEN SCIENCE FRAMEWORK REGISTRATION NUMBER

10.17605/OSF.IO/P6YB9.

Rapamycin attenuates PLA2R activation-mediated podocyte apoptosis via the PI3K/AKT/mTOR pathway

Membranous nephropathy (MN) is the most common cause of nephrotic syndrome in adults without diabetes. Primary MN has been associated with circulating antibodies against native podocyte antigens, including phospholipase A2 receptor (PLA2R); however, precision therapy targeting the signaling cascade of PLA2R activation is lacking. Both PLA2R and the mammalian target of rapamycin (mTOR) exist in podocytes, but the interplay between these two proteins and their roles in MN warrants further exploration. This study aimed to investigate the crosstalk between PLA2R activation and mTOR signaling in a human podocyte cell line. We demonstrated that podocyte apoptosis was induced by Group IB secretory phospholipase A2 (sPLA2IB) in a concentration- and time-dependent manner via upregulation of phosphoinositide 3-kinase (PI3K), protein kinase B (AKT), and mTOR, and inhibited by rapamycin or LY294002. Furthermore, aberrant activation of the PI3K/AKT/mTOR pathway triggers both extrinsic (caspase-8 and caspase-3) and intrinsic (Bcl-2-associated X protein [BAX], B-cell lymphoma 2 [BCL-2], cytochrome c, caspase-9, and caspase-3) apoptotic cascades in podocytes. The therapeutic implications of our findings are that strategies to reduce PLA2R activation and PI3K/AKT/mTOR pathway inhibition in PLA2R-activated podocytes help protect podocytes from apoptosis. The therapeutic potential of rapamycin shown in this study provides cellular evidence supporting the repurposing of rapamycin for MN treatment.

The intersection of Mineralocorticoid Receptor (MR) activation and the FGF23 - Klotho cascade. A Duopoly that promotes renal and cardiovascular injury

The nexus of CKD and cardiovascular disease (CVD) amplifies the morbidity and mortality of CKD, emphasizing the need for defining and establishing therapeutic initiatives to modify and abrogate the progression of CKD and concomitant CV risks. In addition to the traditional CV risk factors, disturbances of mineral metabolism are specific risk factors that contribute to the excessive CV mortality in patients with CKD. These risk factors include dysregulations of circulating factors that modulate phosphate metabolism including fibroblast growth factor 23 (FGF23) and soluble Klotho. Reduced circulating levels and suppressed renal klotho expression may be associated with adverse outcomes in CKD patients. While elevated circulating concentrations or locally produced FGF23 in the strained heart exert pro-hypertrophic mechanisms on the myocardium, Klotho attenuates tissue fibrosis, progression of CKD, cardiomyopathy, endothelial dysfunction, vascular stiffness, and vascular calcification. Mineralocorticoid receptor (MR) activation in non-classical targets, mediated by aldosterone and other ligands, amplifies CVD in CKD. In concert, we detail how the interplay of elevated FGF23, activation of the MR, and concomitant reductions of circulating Klotho in CKD, may potentiate each other's deleterious effects on kidney and the heart, thereby contributing to the initiation and progression of kidney and cardiac functional deterioration, acting through multipronged albeit complementary mechanistic pathways.

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.

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.

Rho GTPase regulatory proteins in podocytes

The Rho family of small GTPases (Rho GTPases) are the master regulators of the actin cytoskeleton and consist of 22 members. Previous studies implicated dysregulation of Rho GTPases in podocytes in the pathogenesis of proteinuric glomerular diseases. Rho GTPases are primarily regulated by the three families of proteins; guanine nucleotide exchange factors (GEFs; 82 members), GTPase-activating proteins (GAPs; 69 members), and GDP dissociation inhibitors (GDIs; 3 members). Since the regulatory proteins far outnumber their substrate Rho GTPases and act in concert in a cell/context-dependent manner, the upstream regulatory mechanism directing Rho GTPases in podocytes is largely unknown. In this review, we summarize recent advances in the understanding of the role of Rho GTPase regulatory proteins in podocytes, including the known mutations of these proteins that cause proteinuria in humans. We also provide critical appraisal of the in vivo and in vitro studies and identify the knowledge gap in the field that will require further studies.