FHL2 mediates podocyte Rac1 activation and foot process effacement in hypertensive nephropathy

Circulating RAC1 contributed to steroid-sensitive nephrotic syndrome: Mendelian randomization, single-cell RNA-sequencing, proteomic, and experimental evidence

OBJECTIVES

The small GTPase Rac1 (RAC1) has been linked to podocyte disorders and steroid-sensitive nephrotic syndrome (SSNS). The aim of this study was to explore and validate the potential causal association between circulating RAC1 and SSNS.

METHODS

The association between circulating RAC1 and SSNS at both gene expression and proteomic levels was investigated using Mendelian randomization analysis, and further validated by single-cell RNA-sequencing, proteomic analysis, and experimental studies. The genetic instruments comprised cis-expression quantitative trait loci (cis-eQTLs) associated with RAC1 gene expression and protein QTLs correlated with plasma RAC1 protein levels. Causal associations were estimated utilizing the inverse variance weighted and MR-PRESSO methods. Validation of RAC1 expression was conducted through single-cell RNA-sequencing of peripheral blood mononuclear cells from patients with SSNS and healthy controls. Proteomic analysis was performed among patients with minimal change nephrotic syndrome. Experimental validation was conducted using a puromycin aminonucleoside (PAN)-induced nephrosis model.

RESULTS

Increased expression of RAC1 was associated with a higher risk of SSNS (gene expression level: odds ratio [OR], 1.53; 95% confidence interval [CI], 1.02-2.28; protein level: OR, 1.82; 95% CI, 1.05-3.17). The results of MR-PRESSO were consistent (gene expression level: OR, 1.49; 95% CI, 1.17-1.92; protein level: OR, 1.81; 95% CI, 1.16-2.85). Single-cell RNA sequencing and proteomic analysis confirmed elevated RAC1 expression in patients with SSNS compared to healthy controls. Experimental data further supported increased RAC1 expression in PAN-induced nephropathy.

CONCLUSIONS

Increased expression of RAC1 might be causally associated with SSNS, suggesting that targeting RAC1 might represent a potential therapeutic strategy for SSNS.

Renin-angiotensin system-mediated nitric oxide signaling in podocytes

Nitric oxide (NO) is widely recognized for its role in regulating renal function and blood pressure. However, the precise mechanisms by which NO affects renal epithelial cells remain understudied. Our previous research has shown that NO signaling in glomerular podocytes can be initiated by Angiotensin II (ANG II) but not by ATP. This study aims to elucidate the crucial interplay between the renin-angiotensin system (RAS) and NO production in podocytes. To conduct our research, we used cultured human podocytes and freshly isolated rat glomeruli. A variety of RAS peptides were used, alongside confocal microscopy, to detect NO production and NO/Ca2+ cross talk. Dynamic changes in the podocyte cytoskeleton, mediated by RAS-NO intracellular signaling, were observed using fluorescent labeling for F-actin and scanning probe microscopy. The experiments demonstrated that ANG II and ANG III generated high levels of NO by activating the angiotensin II type 2 receptor (AT2R). We did not detect functional MAS receptor presence in podocytes, and the moderate NO response to ANG 1-7 was also mediated through AT2R. Furthermore, NO production impacted intracellular Ca2+ signaling and correlated with an increase in podocyte volume and growth. Scanning probe experiments revealed that AT2R activation and the corresponding NO generation are responsible for the protrusion of podocyte lamellipodia. Taken together, our data indicate that AT2R activation enhances NO production in podocytes and subsequently mediates changes in Ca2+ signaling and podocyte volume dynamics. These mechanisms may play a significant role in both physiological and pathophysiological interactions between the RAS and podocytes.NEW & NOTEWORTHY The renin-angiotensin system plays a crucial role in the production of intracellular nitric oxide within podocytes. This mechanism operates through the activation of the angiotensin II type 2 receptor, leading to dynamic modifications in intracellular calcium levels and the actin filament network. This intricate process is vital for linking the activity of angiotensin receptors to podocyte function.

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

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

Effect of botanical drugs in improving symptoms of hypertensive nephropathy: Analysis of real-world data, retrospective cohort, network, and experimental assessment

Background/aim: Hypertensive nephropathy (HN) is a common complication of hypertension. Traditional Chinese medicine has long been used in the clinical treatment of Hypertensive nephropathy. However, botanical drug prescriptions have not been summarized. The purpose of this study is to develop a prescription for improving hypertensive nephropathy, explore the evidence related to clinical application of the prescription, and verify its molecular mechanism of action.

Methods: In this study, based on the electronic medical record data on Hypertensive nephropathy, the core botanical drugs and patients’ symptoms were mined using the hierarchical network extraction and fast unfolding algorithm, and the protein interaction network between botanical drugs and Hypertensive nephropathy was established. The K-nearest neighbors (KNN) model was used to analyze the clinical and biological characteristics of botanical drug compounds to determine the effective compounds. Hierarchical clustering was used to screen for effective botanical drugs. The clinical efficacy of botanical drugs was verified by a retrospective cohort. Animal experiments were performed at the target and pathway levels to analyze the mechanism.

Results: A total of 14 botanical drugs and five symptom communities were obtained from real-world clinical data. In total, 76 effective compounds were obtained using the K-nearest neighbors model, and seven botanical drugs were identified as Gao Shen Formula by hierarchical clustering. Compared with the classical model, the Area under the curve (AUC) value of the K-nearest neighbors model was the best; retrospective cohort verification showed that Gao Shen Formula reduced serum creatinine levels and Chronic kidney disease (CKD) stage [OR = 2.561, 95% CI (1.025–6.406), p < 0.05]. With respect to target and pathway enrichment, Gao Shen Formula acts on inflammatory factors such as TNF-α, IL-1β, and IL-6 and regulates the NF-κB signaling pathway and downstream glucose and lipid metabolic pathways.

Conclusion: In the retrospective cohort, we observed that the clinical application of Gao Shen Formula alleviates the decrease in renal function in patients with hypertensive nephropathy. It is speculated that Gao Shen Formula acts by reducing inflammatory reactions, inhibiting renal damage caused by excessive activation of the renin-angiotensin-aldosterone system, and regulating energy metabolism.

Insulin controls cytoskeleton reorganization and filtration barrier permeability via the PKGIα-Rac1-RhoA crosstalk in cultured rat podocytes

Podocyte foot processes are an important cellular layer of the glomerular barrier that regulates glomerular permeability. Insulin via the protein kinase G type Iα (PKGIα) signaling pathway regulates the balance between contractility and relaxation (permeability) of the podocyte barrier by regulation of the actin cytoskeleton. This mechanism was shown to be disrupted in diabetes. Rho family guanosine-5'-triphosphates (GTPases) are dynamic modulators of the actin cytoskeleton and expressed in cells that form the glomerular filtration barrier. Thus, changes in Rho GTPase activity may affect glomerular permeability to albumin. The present study showed that Rho family GTPases control podocyte migration and permeability. Moreover these processes are regulated by insulin in PKGIα-dependent manner. Modulation of the PKGI-dependent activity of Rac1 and RhoA GTPases with inhibitors or small-interfering RNA impair glomerular permeability to albumin. We also demonstrated this mechanism in obese, insulin-resistant Zucker rats. We propose that PKGIα-Rac1-RhoA crosstalk is necessary in proper organization of the podocyte cytoskeleton and consequently the stabilization of glomerular architecture and regulation of filtration barrier permeability.

Blood pressure management and renal protection: Revisiting hypertensive nephropathy

Hypertension has traditionally been the most common cardiovascular disease, and epidemiological studies suggest that the incidence continues to rise. Despite a plethora of antihypertensive agents, the management of blood pressure (BP) remains suboptimal. Addressing this issue is paramount to minimize hypertensive complications, including hypertensive nephropathy, a clinical entity whose definition has been challenged recently. Still, accumulating studies endorse poorly managed BP as an independent risk factor for both the onset of renal dysfunction and aggravation of baseline kidney disease. Nevertheless, current recommendations are not only discordant from one another but also offer inadequate evidence for the optimal BP control targets for renal protection, as since the cutoff values were primarily established on the premise of minimizing cardiovascular sequelae rather than kidney dysfunction. Although intense BP management was traditionally considered to compromise perfusion toward renal parenchyma, literature has gradually established that renal prognosis is more favorable as compared with the standard threshold. This review aims to elucidate the renal impact of poorly controlled hypertension, elaborate on contemporary clinical references for BP control, and propose future directions to improve the holistic care of hypertensive individuals.

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.