Enhanced podocyte vesicle transport in the nephrotic rat

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.

Effects of a novel ANLN E841K mutation associated with SRNS on podocytes and its mechanism

BACKGROUND

Steroid-resistant nephrotic syndrome (SRNS) is characterized by unrelieved proteinuria after an initial 4-8 weeks of glucocorticoid therapy. Genes in podocytes play an important role in causing SRNS.

OBJECTIVE

This study aimed to report a pathogenic mutation in SRNS patients and investigate its effects on podocytes, as well as the pathogenic mechanism.

METHODS

We screened out a novel mutation by using whole-exon sequencing in the SRNS cohort and verified it via Sanger sequencing. Conservative analysis and bioinformatic analysis were used to predict the pathogenicity of the mutation. In vitro, stable podocyte cell lines were constructed to detect the effect of the mutation on the function of the podocyte. Moreover, an in vivo mouse model of podocyte ANLN gene knockout (ANLNpodKO) was used to confirm clinical manifestations. Transcriptome analysis was performed to identify differential gene expression and related signaling pathways.

RESULTS

ANLN E841K was screened from three unrelated families. ANLN E841K occurred in the functional domain and was predicted to be harmful. The pathological type of A-II-1 renal biopsy was minimal change disease, and the expression of ANLN was decreased. Cells in the mutation group showed disordered cytoskeleton, faster cell migration, decreased adhesion, increased endocytosis, slower proliferation, increased apoptosis, and weakened interaction with CD2 association protein. ANLNpodKO mice exhibited more obvious proteinuria, more severe mesangial proliferation, glomerular atrophy, foot process fusion, and increased tissue apoptosis levels than ANLNflox/flox mice after tail vein injection of adriamycin. Upregulated differentially expressed genes in cells of the mutation group were mainly enriched in the PI3K-AKT pathway.

CONCLUSION

The novel mutation known as ANLN E841K affected the function of the ANLN protein by activating the PI3K/AKT/mTOR/apoptosis pathway, thus resulting in structural and functional changes in podocytes. Our study indicated that ANLN played a vital role in maintaining the normal function of podocytes. Video Abstract.

Urinary Podocyte Excretion Predicts Urinary Protein Selectivity and Renal Prognosis

Background

Urinary podocyte excretion is related to a reduction in glomerular podocyte numbers, glomerulosclerosis, and urinary protein selectivity. To elucidate the role of urinary podocytes in proteinuria and renal prognosis and to identify the factors that cause podocyte detachment, we examined urinary podocytes in 120 renal biopsy patients.

Methods

Podocytes were identified in urinary sediments stained with fluorescent-labeled anti-podocalyxin antibodies in ten high power fields. The amounts of protein bands, separated by SDS-polyacrylamide gel electrophoresis, were calculated using an image software program and the correlation with urinary podocytes was analyzed. Podocyte surface pores were observed using a low-vacuum scanning electron microscope. The renal prognosis, including induction of hemodialysis or 30% reduction in eGFR, was investigated.

Results

Urinary podocyte excretion showed a higher positive correlation with albumin excretion compared to IgG, prealbumin, and transferrin. There were no significant correlations between urinary podocyte count and low molecular weight proteins, including β2-microglobulin and α1-microglobulin. The number of podocyte surface pores was positively correlated with proteinuria, suggesting enhanced albumin transcytosis. The hemodynamic pressure on the glomerular capillary wall, including products of pulse pressure and pulse rate (water hammer pressure), was positively correlated with urinary podocyte excretion. Urinary podocyte excretion and Tamm–Horsfall protein (THP) were independent risk factors for renal prognosis but were not related to response to treatment.

Conclusion

Urinary podocyte excretion was correlated with urinary albumin excretion, indicating specific albumin transport by podocytes. Podocytes were detached from the glomerular capillaries by water hammer pressure and THP was involved in the renal prognosis.

Podocyte Endocytosis in Regulating the Glomerular Filtration Barrier

Endocytosis is a mechanism that internalizes and recycles plasma membrane components and transmembrane receptors via vesicle formation, which is mediated by clathrin-dependent and clathrin-independent signaling pathways. Podocytes are specialized, terminally differentiated epithelial cells in the kidney, located on the outermost layer of the glomerulus. These cells play an important role in maintaining the integrity of the glomerular filtration barrier in conjunction with the adjacent basement membrane and endothelial cell layers within the glomerulus. An intact podocyte endocytic machinery appears to be necessary for maintaining podocyte function. De novo pathologic human genetic mutations and loss-of-function studies of critical podocyte endocytosis genes in genetically engineered mouse models suggest that this pathway contributes to the pathophysiology of development and progression of proteinuria in chronic kidney disease. Here, we review the mechanism of cellular endocytosis and its regulation in podocyte injury in the context of glomerular diseases. A thorough understanding of podocyte endocytosis may shed novel insights into its biological function in maintaining a functioning filter and offer potential targeted therapeutic strategies for proteinuric glomerular diseases.

Decreased Podocyte Vesicle Transcytosis and Albuminuria in APC C-Terminal Deficiency Mice with Puromycin-Induced Nephrotic Syndrome

In minimal change nephrotic syndrome, podocyte vesicle transport is enhanced. Adenomatous polyposis coli (APC) anchors microtubules to cell membranes and plays an important role in vesicle transport. To clarify the role of APC in vesicle transport in podocytes, nephrotic syndrome was induced by puromycin amino nucleoside (PAN) injection in mice expressing APC1638T lacking the C-terminal of microtubule-binding site (APC1638T mouse); this was examined in renal tissue changes. The kidney size and glomerular area of APC1638T mice were reduced (p = 0.014); however, the number of podocytes was same between wild-type (WT) mice and APC1638T mice. The ultrastructure of podocyte foot process was normal by electron microscopy. When nephrotic syndrome was induced, the kidneys of WT+PAN mice became swollen with many hyaline casts, whereas these changes were inhibited in the kidneys of APC1638T+PAN mice. Electron microscopy showed foot process effacement in both groups; however, APC1638T+PAN mice had fewer vesicles in the basal area of podocytes than WT+PAN mice. Cytoplasmic dynein-1, a motor protein for vesicle transport, and α-tubulin were significantly reduced in APC1638T+PAN mice associated with suppressed urinary albumin excretion compared to WT+PAN mice. In conclusion, APC1638T mice showed reduced albuminuria associated with suppressed podocyte vesicle transport when minimal change nephrotic syndrome was induced.

Intracellular trafficking pathway of albumin in glomerular epithelial cells

The intracellular trafficking pathway of albumin in podocytes remains controversial. We therefore analysed albumin endocytosis through caveolae, subsequent transcytosis, and exocytosis. In Western blot and immunofluorescence analysis in vitro, methyl-beta-cyclodextrin (MBCD) treatment significantly decreased the expression of caveolin-1 and albumin in cultured human podocytes after incubation with albumin; additionally, MBCD interfered with albumin endocytosis through caveolae in the experiment using Transwell plates. In the immunofluorescence analysis, albumin was incubated with cultured human podocytes, and colocalisation analysis with organelles and cytoskeletons in the podocytes showed that albumin particles colocalised with caveolin-1 and Fc-receptor but not clathrin in endocytosis, colocalised with actin cytoskeleton but not microtubules in transcytosis, and colocalised with early endosomes and lysosomes but not proteasome, endoplasmic reticulum, or Golgi apparatus. In the electron microscopic analysis of podocytes in nephrotic syndrome model mice, gold-labelled albumin was shown as endocytosis, transcytosis, and exocytosis with caveolae. These results indicate the intracellular trafficking of albumin through podocytes. Albumin enters through caveolae with the Fc-receptor, moves along actin, and reaches the early endosome, where some of them are sorted for lysosomal degradation, and others are directly transported outside the cells through exocytosis. This intracellular pathway may be a new aetiological hypothesis for albuminuria.

Changes of Transporters and Drug-metabolizing Enzymes in Nephrotic Syndrome

BACKGROUND

Drug-metabolizing enzymes and transporters play key roles in drug disposition and drug interactions. The alterations of their expression will influence drug pharmacokinetics and pharmacodynamics. However, the changes in the expression of enzymes and transporters in the disease state are still unclear.

OBJECTIVE

Our study was to investigate the changes in the expression of main enzymes and drug transporters distributed in Adriamycin nephropathy rat liver, kidney, and intestine.

METHODS

An intravenous injection with a single dose of Adriamycin (6mg/kg) was made to establish Adriamycin nephropathy (AN) model and normal groups were injected with normal saline. Serum was collected for lipid metabolism, renal, and hepatic function measurement. The real-time PCR and western blot were applied to determine the mRNA and protein expression of drug enzymes and transporters.

RESULTS

In the kidney, a greater expression of Mdr1, Mrp2, Mrp4 Oat2 and Oct2 mRNA was found in AN rats as compared with control rats. In the liver, the expression of Bcrp mRNA was more doubled or tripled than control groups and downregulation of Mdr1, Mrp2, Mrp4 and Bsep gene expression was found in AN rats. Besides, we observed a downward trend of Cyp1a2, Cyp3a4 and Cyp2c9 mRNA levels in AN groups. In the duodenum, the expression of Mdr1 and Mrp3 mRNA level was decreased, while Bcrp and Mrp2 mRNA were increased.

CONCLUSION

The changes in drug-metabolizing enzymes and transporters expression in AN rats were clarified, which may be beneficial for understanding the altered pharmacokinetics and pharmacodynamics of clinical drugs and reduce unexpected clinical findings for nephropathy patients.

Dysregulated Dynein-Mediated Trafficking of Nephrin Causes INF2-related Podocytopathy

BACKGROUND

FSGS caused by mutations in INF2 is characterized by a podocytopathy with mistrafficked nephrin, an essential component of the slit diaphragm. Because INF2 is a formin-type actin nucleator, research has focused on its actin-regulating function, providing an important but incomplete insight into how these mutations lead to podocytopathy. A yeast two-hybridization screen identified the interaction between INF2 and the dynein transport complex, suggesting a newly recognized role of INF2 in regulating dynein-mediated vesicular trafficking in podocytes.

METHODS

Live cell and quantitative imaging, fluorescent and surface biotinylation-based trafficking assays in cultured podocytes, and a new puromycin aminoglycoside nephropathy model of INF2 transgenic mice were used to demonstrate altered dynein-mediated trafficking of nephrin in INF2 associated podocytopathy.

RESULTS

Pathogenic INF2 mutations disrupt an interaction of INF2 with dynein light chain 1, a key dynein component. The best-studied mutation, R218Q, diverts dynein-mediated postendocytic sorting of nephrin from recycling endosomes to lysosomes for degradation. Antagonizing dynein-mediated transport can rescue this effect. Augmented dynein-mediated trafficking and degradation of nephrin underlies puromycin aminoglycoside-induced podocytopathy and FSGS in vivo.

CONCLUSIONS

INF2 mutations enhance dynein-mediated trafficking of nephrin to proteolytic pathways, diminishing its recycling required for maintaining slit diaphragm integrity. The recognition that dysregulated dynein-mediated transport of nephrin in R218Q knockin podocytes opens an avenue for developing targeted therapy for INF2-mediated FSGS.

V-ATPase blockade reduces renal gluconeogenesis and improves insulin secretion in type 2 diabetic rats

Vacuolar H⁺-adenosine triphosphatase (V-ATPase) stimulates vesicular acidification that may activate cytoplasmic enzymes, hormone secretion and membrane recycling of transporters. We investigated the effect of blockade of V-ATPase by bafilomycin B1 on renal gluconeogenesis, mitochondrial enzymes, and insulin secretion in type 2 diabetic rats. Spontaneous type 2 diabetic Torii rats were treated with intraperitoneal injection of bafilomycin B1 for 1 week, and the kidneys were examined after 24 h of starvation in metabolic cages. The renal expression and activity of V-ATPase were increased in the brush border membrane of the proximal tubules in diabetic rats. The blockade of V-ATPase by bafilomycin B1 reduced renal V-ATPase activity and urinary ammonium in diabetic rats. Treatment with bafilomycin suppressed the enhanced renal gluconeogenesis enzymes and mitochondrial electron transport enzymes in type 2 diabetic rats and reduced the renal cytoplasmic glucose levels. The insulin index and pancreatic insulin granules were decreased in diabetic rats with increased V-ATPase expression in islet cells, and treatment with bafilomycin B1 reversed these changes and increased the insulin secretion index. Hepatosteatosis in type 2 diabetic rats was ameliorated by bafilomycin treatment. As a consequence, treatment with bafilomycin B1 significantly decreased the plasma glucose level after 24 h of starvation in diabetic rats. In conclusion, a V-ATPase inhibitor improved plasma glucose levels in type 2 diabetes by inhibiting renal mitochondrial gluconeogenesis and improving insulin secretion.

Overexpression of Myo1e promotes albumin endocytosis by mouse glomerular podocytes mediated by Dynamin

Background

As a fundamental process internalizing molecules from the plasma membrane, endocytosis plays a crucial role in podocyte biology. Our previous study has identified that overexpression of Myole may enhance podocyte endocytosis. However, its potential mechanism has been not well understand. Thus, we aimed to analyze whether albumin endocytosis by mouse glomerular podocytes is dependent on Myo1e expression. Also, we aimed to elucidate whether the underlying mechanism is mediated by Dynamin.

Methods

Firstly, mouse podocyte cells (MPC5) were treated with different concentrations of FITC-bovine serum albumin (BSA). The fluorescence intensity and cell viability were detected by flow cytometry and MTT assays, respectively. Afterwards, the optimal concentration of FITC-BSA was determined. Secondly, MPC5 cells were treated with Myole overexpression or knockdown. Cell morphology was observed under microscope. Immunofluorescence assay was used to determine the expression of F-actin. The protein expression of nephrin and podocin was detected by western blot. Flow cytometry was used to detect MPC5 cell apoptosis with annexin V. Finally, MPC5 cells were treated with Myole overexpression and/or Dynasore (a GTPase inhibitor of Dynamin). The fluorescence intensity was detected using flow cytometry assay.

Results

MPC5 endocytosis BSA was elevated with a concentration-dependent manner. MTT results showed that MPC5 cell viability was inhibited with a concentration-dependent manner. Myo1e overexpression promoted podocyte endocytic FITC-BSA, which was contrary to its knockdown. Under microscope, after inhibition of Myo1e, podocyte foot process fusion was observed. Myo1e overexpression promoted the expression of cytoskeleton F-actin and podocyte-specific molecules (nephrin and podocin) in podocyte endocytic FITC-BSA. Furthermore, we found that Myo1e promoted the apoptosis of podocytes. Dynasore attenuated the increase in endocytosis of FITC-BSA induced by Myo1e overexpression, suggesting that podocytes might mediate albumin endocytosis via Myo1e-Dynamin-Albumin.

Conclusion

Our findings revealed that overexpression of Myo1e promotes albumin endocytosis in mouse glomerular podocyte endocytic albumin mediated by Dynamin.

Mechanism Underlying Selective Albuminuria in Minimal Change Nephrotic Syndrome

As water and solutes are filtered through the slit membrane, it is an a priori concept that a slit membrane is an essential filtration barrier for proteins, including albumin. However, in cases of minimal change nephrotic syndrome, the number of slit membranes is reduced by the foot process effacement and tight junction-like cell adhesion. Furthermore, albumin endocytosis is enhanced in the podocytes under condition of minimal change disease, and albumin is selectively transported by the albumin receptor FcRn. Suppressing the endocytosis of albumin with anti-FcRn antibody decreases the urinary protein level. The expression of motor molecules, such as cytoplasmic dynein 1 and myosin IX, is increased in the podocytes under conditions of minimal change nephrotic syndrome, suggesting the enhanced transport of vesicles containing albumin. Podocyte vesicle transport may play an important role in the pathology of selective albuminuria in cases of nephrotic syndrome.