Podocyte Endocytosis in Regulating the Glomerular Filtration Barrier

Targeted Modulation of Mitochondrial Oxidative Stress Ameliorates 5-Fluorouracil-Induced Renal Injury in BALB/c Mice

Background: The present study reports the protective effect conferred by scavenging mitochondrial oxidative stress (mtOS) in 5-fluorouracil (5-FU)-induced renal injury. Methods: 5-FU renal toxicity model was created by administering 5-FU (12 mg/kg b.w. intraperitoneally [i.p.], for 4 days) to male BALB/c mice. The protective effect of mitochondria-targeted antioxidant (MTA), Mito-TEMPO coadministered at a dosage of 0.1 mg/kg b.w. i.p., was established in terms of levels/expressions of renal injury markers, histopathological alterations, oxidative DNA damage, proinflammatory markers, mtOS, mitochondrial dysfunction, and modulation of apoptotic proteins and apoptotic cell death. Results: A significant rise in the levels of serum urea, uric acid, and creatinine was noted after 5-FU administration to the animals. Immunohistochemical and ELISA findings demonstrated significant decrease in podocin and conversely a significant increase in neutrophil gelatinase-associated lipocalin (NGAL) expression after 5-FU challenge. The histopathological analysis further revealed Bowman's capsule dilation, glomerular condensation, and vacuolar degeneration. Mito-TEMPO treatment significantly lowered renal injury markers, reversed the expressions of podocin and NGAL to normal, and restored normal histoarchitecture of renal tissue. Mitochondrial reactive oxygen species (mtROS), mtLPO, activity of mitochondrial enzyme complexes, and mitochondrial antioxidant defense status were significantly improved in Mito-TEMPO protected group as compared to the 5-FU group. Further, significantly decreased expression of 8-OHdG, reduction in apoptotic cell death, and modulation of apoptotic proteins Bax, Bcl-2, and caspase-3 were noted in Mito-TEMPO protected group, indicating its protective effect against 5-FU-induced renal injury. Conclusion: The approach of targeting mtOS using MTA, Mito-TEMPO, may prove as safe adjuvant in alleviating renal toxicity during 5-FU chemotherapy.

The impact of social determinants of health on chronic kidney disease risk: evidence from the CHARLS study

Background

Empirical evidence regarding the relationship between social determinants of health (SDH) and renal outcomes remains limited. Consequently, the objective of this study was to investigate the potential association between SDH and the development of chronic kidney disease (CKD) across various levels.

Methods

Data were sourced from the 2011 China Health and Retirement Longitudinal Study (CHARLS), which included 6,290 Chinese participants aged 40 years and older. Among these participants, 4,115 underwent a follow-up assessment in the 2015 survey. The primary outcome measure was the incidence of CKD, operationally defined as a reduction in estimated glomerular filtration rate to <60 ml/min/1.73 m2. To analyze the association between varying levels of SDH and renal outcomes, a Cox proportional hazards regression model was employed.

Results

The findings indicate that, in comparison to individuals with a pension, higher education, and no need for family support, the risk of developing CKD increased by 43, 49, and 52%, respectively. Furthermore, the combination of requiring family support, being unmarried, and lacking medical insurance was associated with an elevated incidence of CKD. Utilizing the counting model of adverse SDH indicators, it was observed that when the number of adverse SDH was equal to or greater than four, there was a significant increase in the risk of CKD. The incidence density of CKD was found to rise in correlation with the severity of adverse SDH, with the incidence density in the adverse SDH group being 0.06 per person-year higher than that in the favorable SDH group. After adjusting for multiple variables, the hazard ratio (HR) for incident CKD was 2.47 [95% confidence interval (CI): 1.46-4.16] in the adverse SDH group compared to the favorable SDH group, a finding that persisted across various subgroups.

Conclusion

Research indicates that financial support, pensions, education, marital status, and health insurance significantly impact CKD risk. Higher income, pension coverage, education, marital stability, and insurance lower this risk. Evaluating adverse SDH indicators helps assess individual SDH levels and CKD risk, with four or more indicators suggesting high risk. Therefore, adverse SDH measures can predict CKD.

G protein-coupled receptor 107 deficiency promotes development of diabetic nephropathy

Diabetic nephropathy (DN) is characterized by glomerular basement membrane (GBM) thickening, primarily due to the abnormal accumulation of collagen type IV (COL4) in the extracellular matrix (ECM) of podocytes. Podocytes endocytosis is crucial for maintaining COL4 balance and GBM integrity. Previous studies have shown that G protein-coupled receptor 107 (GPR107) facilitates clathrin-dependent transferrin internalization and recycling in murine embryonic fibroblast cells. Therefore, the aim of the study is to investigate the role of GPR107 in regulating COL4 balance within the podocytes ECM and its potential as a therapeutic target for DN. Here, we found a significant decrease in GPR107 expression in renal tissues from DN patients and streptozocin (STZ)-induced DN mice. Furthermore, GPR107-deficient mice with STZ-induced DN exhibited more severe kidney damage, marked by increased GBM thickening and COL4 accumulation. In vitro, GPR107 deficiency under high-glucose conditions promoted COL4 accumulation in the ECM of podocytes due to increased COL4 production and decreased COL4 degradation. Mechanistically, we demonstrated that GPR107 contributes to angiotensin II receptor type 1 (AT1R) internalization through clathrin-mediated endocytosis (CME) in podocytes. Therefore, GPR107 deficiency impairs AT1R internalization, leading to increased membrane-bound AT1R. This, in turn, activates the AT1R/Ca2+ signaling pathway to promote phosphorylation of cAMP-response element-binding protein (CREB), ultimately enhancing COL4 synthesis and inhibiting the expression of matrix metalloproteinase 2 (MMP-2). These findings shed light on new functions of GPR107 in DN and offer new insights into a therapeutic target for DN.

Targeting endoplasmic reticulum stress: an innovative therapeutic strategy for podocyte-related kidney diseases

The endoplasmic reticulum (ER) is a vital organelle responsible for protein quality control, including the folding, modification, and transport of proteins. When misfolded or unfolded proteins accumulate in the ER, it triggers endoplasmic reticulum stress (ERS) and activates the unfolded protein response (UPR) to restore ER homeostasis. However, prolonged or excessive ERS can lead to apoptosis. The kidneys play a crucial role in maintaining physiological functions by excreting metabolic waste, regulating blood volume, balancing electrolytes and acid-base levels, and secreting various bioactive substances. Podocytes, epithelial cells situated outside the glomerular basement membrane, are essential for maintaining the structural integrity and permeability of the glomerular filtration barrier. Previous studies have shown that ERS in podocytes can contribute to the development of diseases such as glomerulonephritis, hereditary nephropathy, and diabetic kidney disease, potentially progressing to end-stage renal disease and causing patient mortality. As such, investigating ERS in podocytes has become a key area of focus in kidney disease research. This study examines recent advancements in understanding the effects of excessive ERS on podocytes across various kidney diseases, highlights the role of podocyte ERS in disease progression, and explores the potential therapeutic benefits of targeting the UPR to manage ERS in kidney diseases, thereby providing a scientific basis for clinical interventions.

Exploring the Functionality of the Krüppel-like Factors in Kidney Development, Metabolism, and Diseases

The kidneys contribute to the overall health of an organism by maintaining systemic homeostasis. This process involves various biological mechanisms, in which the Krüppel-like factors (KLFs), a family of transcription factors, are essential for regulating development, differentiation, proliferation, and cellular apoptosis. They also play a role in the metabolic regulation of essential nutrients, such as glucose and lipids. The dysregulation of these transcription factors is associated with the development of various pathologies, which can ultimately lead to renal fibrosis, severely compromising kidney function. In this context, the present article provides a comprehensive review of the existing literature, offering an enriching analysis of the findings related to the role of KLFs in nephrology, while also highlighting their potential therapeutic role in the treatment of renal diseases.

Discovery of a Small Molecule with an Inhibitory Role for RAB11

RAB11, a pivotal RabGTPase, regulates essential cellular processes such as endocytic recycling, exocytosis, and autophagy. The protein was implicated in various human diseases, including cancer, neurodegenerative disorders, viral infections, and podocytopathies. However, a small-molecular inhibitor is lacking. The complexity and workload associated with potential assays make conducting large-scale screening for RAB11 challenging. We employed a tiered approach for drug discovery, utilizing deep learning-based computational screening to preselect compounds targeting a specific pocket of RAB11 protein with experimental validation by an in vitro platform reflecting RAB11 activity through the exocytosis of GFP. Further validation included the exposure of Drosophila by drug feeding. In silico pre-screening identified 94 candidates, of which 9 were confirmed using our in vitro platform for Rab11 activity. Focusing on compounds with high potency, we assessed autophagy, which independently requires RAB11, and validated three of these compounds. We further analyzed the dose-response relationship, observing a biphasic, potentially hormetic effect. Two candidate compounds specifically caused a shift in Rab11 vesicles to the cell periphery, without significant impact on Rab5 or Rab7. Drosophila larvae exposed to another candidate compound with predicted oral bioavailability exhibited minimal toxicity, subcellular dispersal of endogenous Rab11, and a decrease in RAB11-dependent nephrocyte function, further supporting an inhibitory role. Taken together, the combination of computational screening and experimental validation allowed the identification of small molecules that modify the function of Rab11. This discovery may further open avenues for treating RAB11-associated disorders.

Roles of myosin 1e and the actin cytoskeleton in kidney functions and familial kidney disease

Mammalian kidneys are responsible for removing metabolic waste and maintaining fluid and electrolyte homeostasis via selective filtration. One of the proteins closely linked to selective renal filtration is myosin 1e (Myo1e), an actin-dependent molecular motor found in the specialized kidney epithelial cells involved in the assembly and maintenance of the renal filter. Point mutations in the gene encoding Myo1e, MYO1E, have been linked to familial kidney disease, and Myo1e knockout in mice leads to the disruption of selective filtration. In this review, we discuss the role of the actin cytoskeleton in renal filtration, the known and hypothesized functions of Myo1e, and the possible explanations for the impact of MYO1E mutations on renal function.

Enzyme-activatable kidney-targeted dendrimer-drug conjugate for efficient childhood nephrotic syndrome therapy

Rationale: Childhood nephrotic syndrome (NS) is a serious disease affecting the health and quality of life of children, which is characterized by a series of pathophysiological changes due to the increased permeability of the glomerular membrane to plasma proteins. Low renal drug distribution and inefficient cellular uptake, resulting from cellular dysfunctions of filtration and internalization, are the main barriers to drug treatment in childhood NS, leading to deterioration in nephropathy. However, efficient therapeutic methods against childhood NS are still lacking in clinic. Methods: This study found that γ-glutamyltransferase (GGT) was highly expressed in the glomeruli of childhood NS in juvenile rats. We proposed GGT as the receptor target of the kidney-targeted drug delivery system, and then designed a GGT enzyme-responsive dendrimer-drug conjugate (GSHPD) as a kidney-targeted drug delivery platform for treating childhood NS. This platform could overcome the physiological and cellular uptake barriers of the kidney through receptor-mediated transcytosis. Results: GSHPD was composed of glutathione-modified polyamidoamine dendrimers and conjugated with triptolide (TP). Once GSHPD was delivered to the glomerulus in nephropathy, the overexpressed GGT in the endothelial cells of the glomerular capillaries activated the γ-glutamyl transfer reactions of glutathione to generate positively charged primary amines. The resulting cationic conjugate rapidly underwent caveola-mediated endocytosis and exocytosis, augmenting its renal accumulation and cellular internalization. Active TP was gradually released by intracellular enzyme hydrolysis, enabling sustained therapeutic effects and resulting in significant recovery of renal physiological function (e.g., lowering the levels of urea nitrogen and serum creatinine, improving the levels of urinary creatinine and creatinine clearance rate, and inhibiting podocyte injury). Conclusion: The conjugate exhibited an excellent kidney-targeted distribution and a potent recovery of renal physiological function in NS of juvenile rats. This study presented a promising and active kidney-targeted drug delivery platform for efficient childhood nephropathy therapy.

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

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

Biophysical characterization and insights into the oligomeric nature of CD2-associated protein

INTRODUCTION

Glomerular podocytes are specialized epithelial cells localized to the blood-urine interface of the kidney. Podocyte slit-diaphragm (SD), a size-and-charge-selective junction, is instrumental in blood ultrafiltration and the formation of protein-free urine. The SD consists of macromolecular complexes of several proteins, such as nephrin, podocin, and CD2-associated protein (CD2AP). CD2AP is an adapter protein and is considered to be crucial for the integrity of SD. Mutations in the SD proteins cause nephrotic syndrome (NS), characterized by proteinuria. SD proteins' structural features must be elucidated to understand the mechanism of proteinuria in NS. In this study, we expressed, purified, and biophysically characterized heterologously expressed human CD2AP.

METHODS

Codon-optimized human CD2AP was expressed in E. coli Rosetta cells. The recombinant protein was induced with 1 mM IPTG and purified by Ni-NTA affinity chromatography. Analytical size-exclusion chromatography, blue native-PAGE, circular dichroism, and fluorescence spectroscopy were performed to decipher the oligomeric nature, secondary structural content, and tertiary packing of CD2AP.

RESULTS

Our analysis revealed that CD2AP adopts a predominantly disordered secondary structure despite exhibiting moderate tertiary packing, characterized by low helical and β-sheet content. CD2AP readily assembles into homo-oligomers, with octamers and tetramers constituting the primary population. Interestingly, the inherent flexibility of CD2AP's secondary structural elements appears resistant to thermal denaturation. Frameshift mutation (p.K579Efs*7) that leads to loss of the coiled-coil domain promotes aberrant oligomerization of CD2AP through SH3 domains.

CONCLUSION

We successfully expressed full-length human CD2AP in a heterologous system, wherein the secondary structure of CD2AP is predominantly disordered. CD2AP can form higher-order oligomers, and the significance of these oligomers and the impact of mutations in the context of size-selective permeability of SD needs further investigation.

Annexin A1 may contribute to the morphological changes in podocytes by mediating endocytic vesicle fusion and transport via promotion of SNARE assembly in idiopathic membranous nephropathy

BACKGROUND

Annexin A1 is a membrane-associated calcium-binding protein that participates in the progression of many diseases by facilitating vesicle aggregation. It has been documented that reducing vesicle formation alleviates podocyte injury and albuminuria in idiopathic membranous nephropathy (IMN). However, the role of Annexin A1 (ANXA1) in IMN is unknown.

METHODS

Electron microscopy was used to observe the numbers of vesicles in podocytes. The expression of ANXA1 in IMN was investigated by bioinformatics analysis. We validated the hub genes with the Nephroseq V5 online tool and microarray data from the GEO. Immunohistochemical staining and qPCR were performed to measure gene and protein expression.

RESULTS

The numbers of vesicles in IMN podocytes were significantly increased. Bioinformatics analysis showed that ANXA1, one of the differentially expressed genes, was upregulated in glomeruli from IMN patients. In the validation database and dataset, we confirmed that ANXA1 expression was upregulated in the glomeruli of IMN patients. We revealed that the increased expression of ANXA1 was negatively correlated with the glomerular filtration rate (GFR) and proteinuria. Moreover, ANXA1 was enriched in the biological process of vesicle fusion, in which the expression of SNAREs and the SNARE complex was increased. Finally, the expression of ANXA1 and genes related to SNAREs and the SNARE complex was upregulated in glomeruli from IMN patients according to immunohistochemical staining and qPCR.

CONCLUSION

We conclude that ANXA1 may mediate endocytic vesicle fusion and transport by promoting SNARE assembly, contributing to the morphological changes in podocytes and massive proteinuria in IMN.

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.

Ferroptosis: new insight into the mechanisms of diabetic nephropathy and retinopathy

Diabetic nephropathy (DN) and diabetic retinopathy (DR) are the most serious and common diabetes-associated complications. DN and DR are all highly prevalent and dangerous global diseases, but the underlying mechanism remains to be elucidated. Ferroptosis, a relatively recently described type of cell death, has been confirmed to be involved in the occurrence and development of various diabetic complications. The disturbance of cellular iron metabolism directly triggers ferroptosis, and abnormal iron metabolism is closely related to diabetes. However, the molecular mechanism underlying the role of ferroptosis in DN and DR is still unclear, and needs further study. In this review article, we summarize and evaluate the mechanism of ferroptosis and its role and progress in DN and DR, it provides new ideas for the diagnosis and treatment of DN and DR.

Induced pluripotent stem cells-podocytes promote repair in acute kidney injury is dependent on Mafb/CCR5/Nampt axis-mediated M2 macrophage polarization

Induced pluripotent stem cells (iPSCs) have been the focus of cellular therapy studies. The use of iPSCs in regenerative medicine is limited by their tumorigenic potential. This study sought to determine whether iPSCs-derived podocytes attenuate acute kidney injury (AKI) and the molecular mechanism. Inoculation of iPSCs-podocytes significantly promoted the repair of kidney injury in AKI mice, reduced the levels of kidney injury factors Scr, BUN, and urinary NAG, and alleviated the inflammatory response. Histological analysis revealed a significant increase in the number of M2 macrophages and a significant decrease in M1 macrophages in the kidney tissues. Subsequently, the genes and signaling pathways that may be associated with kidney injury repair in mice were analyzed by RNA-seq and bioinformatics prediction. The polarization of M2 macrophages was promoted by MAF bZIP transcription factor B (Mafb)-mediated activation of C-C motif chemokine receptor 5 (Ccr5) and nicotinamide phosphoribosyltransferase (Nampt) signaling pathway. Taken together, these results show that iPSCs-podocytes depend on Mafb to activate the Nampt signaling pathway through transcriptional activation of Ccr5, thereby promoting the repair of AKI caused by ischemia-reperfusion.