Effect of down-regulation of TRPC6 on the puromycin aminonucleoside-induced apoptosis of mouse podocytes

TRPC5 Channel Inhibition Protects Podocytes in Puromycin-Aminonucleoside Induced Nephrosis Models

Podocyte injury and the appearance of proteinuria are key features of several progressive kidney diseases. Genetic deletion or selective inhibition of TRPC5 channels with small-molecule inhibitors protects podocytes in rodent models of kidney disease, but less is known about the human relevance and translatability of TRPC5 inhibition. Here, we investigate the effect of TRPC5 inhibition in puromycin aminonucleoside (PAN)-treated rats, human iPSC-derived podocytes, and kidney organoids. We first established that systemic administration of the TRPC5 inhibitor AC1903 was sufficient to protect podocyte cytoskeletal proteins and suppress proteinuria in PAN-induced nephrosis rats, an established model of podocyte injury. TRPC5 current was recorded in the human iPSC-derived podocytes and was blocked by AC1903. PAN treatment caused podocyte injury in human iPSC-derived podocytes and kidney organoids. Inhibition of TRPC5 channels reversed the effects of PAN-induced injury in human podocytes in both 2D and 3D culture systems. Taken together, these results revealed the relevance of TRPC5 channel inhibition in puromycin-aminonucleoside induced nephrosis models, highlighting the potential of this therapeutic strategy for patients.

Losartan Protects Podocytes against High Glucose-induced Injury by Inhibiting B7-1 Expression

The role of B7-1 in podocyte injury has received increasing attention. The aim of this study was to investigate whether losartan protects podocytes of patients with diabetic kidney disease (DKD) by regulating B7-1 and the underlying mechanisms. Rats with streptozotocin-induced DKD were treated with losartan for 8 weeks. Biochemical changes in blood and urine were analyzed. Kidneys were isolated for electron microscopy, immunofluorescence, real-time quantitative PCR (RT-PCR), and Western blot analysis. Immortalized mouse podocyte cells were cultured in normal or high glucose medium in the presence or absence of losartan for 48 h, and then the cells were collected for immunofluorescence, PCR, Western blotting and monolayer permeability detection. The phosphatidylinositol 3-kinase (PI3K) 110α subunit and angiotensin II type 1 receptor (AT1R) plasmids were transfected into podocytes, respectively, and then Western blotting was performed to assess the expression of B7-1 protein. The results showed that losartan ameliorated podocyte structure and function in the rat model of DKD, and reduced the expression of B7-1 protein. Overexpression of PI3K 110α subunit in podocytes attenuated the inhibitory effect of losartan on B7-1 expression in high glucose-stimulated podocytes. The expression of B7-1 was significantly increased by overexpression of AT1R and significantly reduced by blocking PI3K 110α subunit. We conclude that losartan protects podocytes against high glucose-induced injury by inhibiting AT1R-mediated B7-1 expression. This effect is dependent on the AT1R-PI3K 110α subunit pathway.

[Research advances in the association between transient receptor potential cation channel 6 and kidney disease]

Transient receptor potential cation channel 6 (TRPC6) is a member of the transient receptor superfamily encoded by the TRPC6 gene and is widely expressed in tissues and organs of the human body, especially in the glomerular podocytes. TRPC6 interacts with various slit diaphragm (SD) proteins including podocin, nephrin, ACTN4, and CD2AP to maintain the normal structure and function of glomerular podocytes. Foot process fusion caused by podocyte damage due to various factors is the most important morphological change in kidney disease. This article reviews the biological function of TRPC6 and its effect on kidney disease.

Targeted inhibition of endoplasmic reticulum stress: New hope for renal fibrosis (Review)

Chronic kidney disease (CKD) has a very high mortality rate and remains a global health challenge. Inhibiting renal fibrosis is one of the most promising therapeutic strategies for CKD. Recent studies have indicated that endoplasmic reticulum stress (ERS) serves an active role in the development of acute and chronic kidney disease, especially with regards to renal fibrosis. In the current review, the authors summarize the latest understanding of the role of ERS during the onset of renal fibrosis. ERS promotes renal fibrosis through multiple signaling pathways, such as transforming growth factor-β, epithelial-mesenchymal transition and oxidative stress. In addition, ERS also causes podocyte damage, leading to increased proteinuria and the development of renal fibrosis in rat models. In conclusion, targeted inhibition of ERS may become a promising therapeutic strategy for renal fibrosis.

Cyclin-dependent kinase 2 protects podocytes from apoptosis

Loss of podocytes is an early feature of diabetic nephropathy (DN) and predicts its progression. We found that treatment of podocytes with sera from normoalbuminuric type 1 diabetes patients with high lipopolysaccharide (LPS) activity, known to predict progression of DN, downregulated CDK2 (cyclin-dependent kinase 2). LPS-treatment of mice also reduced CDK2 expression. LPS-induced downregulation of CDK2 was prevented in vitro and in vivo by inhibiting the Toll-like receptor (TLR) pathway using immunomodulatory agent GIT27. We also observed that CDK2 is downregulated in the glomeruli of obese Zucker rats before the onset of proteinuria. Knockdown of CDK2, or inhibiting its activity with roscovitine in podocytes increased apoptosis. CDK2 knockdown also reduced expression of PDK1, an activator of the cell survival kinase Akt, and reduced Akt phosphorylation. This suggests that CDK2 regulates the activity of the cell survival pathway via PDK1. Furthermore, PDK1 knockdown reduced the expression of CDK2 suggesting a regulatory loop between CDK2 and PDK1. Collectively, our data show that CDK2 protects podocytes from apoptosis and that reduced expression of CDK2 associates with the development of DN. Preventing downregulation of CDK2 by blocking the TLR pathway with GIT27 may provide a means to prevent podocyte apoptosis and progression of DN.

Developmental changes in the expression and function of TRPC6 channels related the F-actin organization during differentiation in podocytes

The transient receptor potential canonical (TRPC) 6 channel is an important ion channel located in podocytes, which plays an essential role in regulating calcium homeostasis of the cell signaling. Podocytes are specialized, terminally differentiated cells surrounding glomerular capillaries, and are the subject of keen interest because of their key roles in kidney development and disease. Here we wonder whether TRPC6 channels undergo developmental changes in the expression and function during the podocyte differentiation, and whether they contribute to the maturation of podocytes. Using morphological, immunohistochemical and electrophysiological techniques, we investigated the development of distribution and expression of TRPC6 in conditionally immortalized mouse podocyte cell line. Our results showed that the distribution of TRPC6 channels changed with the maturity of podocyte differentiation. The fluorescent intensity of TRPC6 on cell surface increased, which was accompanied by a corresponding increase in the density of current flowing through the channels. TRPC6 inhibition by TRPC6 siRNA or SKF-96365, a blocker or TRP cation channels, resulted in F-actin cytoskeleton disruption only on the developmental stage of podocytes. These results strongly support the conclusion that TPRC6 is an essential component of the slit diaphragm and is required for development of glomerulus.

Podocyte apoptosis is prevented by blocking the Toll-like receptor pathway

High serum lipopolysaccharide (LPS) activity in normoalbuminuric patients with type 1 diabetes (T1D) predicts the progression of diabetic nephropathy (DN), but the mechanisms behind this remain unclear. We observed that treatment of cultured human podocytes with sera from normoalbuminuric T1D patients with high LPS activity downregulated 3-phosphoinositide-dependent kinase-1 (PDK1), an activator of the Akt cell survival pathway, and induced apoptosis. Knockdown of PDK1 in cultured human podocytes inhibited antiapoptotic Akt pathway, stimulated proapoptotic p38 MAPK pathway, and increased apoptosis demonstrating an antiapoptotic role for PDK1 in podocytes. Interestingly, PDK1 was downregulated in the glomeruli of diabetic rats and patients with type 2 diabetes before the onset of proteinuria, further suggesting that reduced expression of PDK1 associates with podocyte injury and development of DN. Treatment of podocytes in vitro and mice in vivo with LPS reduced PDK1 expression and induced apoptosis, which were prevented by inhibiting the Toll-like receptor (TLR) signaling pathway with the immunomodulatory agent GIT27. Our data show that LPS downregulates the cell survival factor PDK1 and induces podocyte apoptosis, and that blocking the TLR pathway with GIT27 may provide a non-nephrotoxic means to prevent the progression of DN.

Dexamethasone Resisted Podocyte Injury via Stabilizing TRPC6 Expression and Distribution

TRPC6, a member of the canonical transient receptor potential channel (TRPC) subfamily, is an important cation selective ion channel on podocytes. Podocytes are highly differentiated cells located on the visceral face of glomerular basement membrane and featured by numerous foot processes, on which nephrin, podocin, and TRPC6 locate. Podocytes and the slit diaphragm (SD) between adjacent foot processes form a selective filtration barrier impermeable to proteins. TRPC6 is very critical for normal podocyte function. To investigate the function of TRPC6 in podocytes and its relation to proteinuria in kidney diseases, we over-expressed TRPC6 in podocytes by puromycin aminonucleoside (PAN) and observed the changes of foot processes, TRPC6 protein distribution, and mRNA expression. Accordingly, in this study, we further investigated the role of specific signaling mechanisms underlying the prosurvival effects of dexamethasone (DEX) on podocyte repair. Our results showed that podocytes processes of overexpressing TRPC6 were reduced remarkably. These changes could be rescued by DEX via blocking TRPC6 channel. Additionally, our results also showed an improvement in TRPC6 arrangement in the cells and decrease of mRNA expression and protein distribution. From these results, we therefore proposed that overexpression of TRPC6 in podocytes may be one of the fundamental changes relating to the dysfunction of the SD and proteinuria. DEX may be maintained the structure and function integrity of SD by blocking TRPC6 signal pathway and played an important role in mechanisms of anti-proteinuria.

Calcium entry via TRPC6 mediates albumin overload-induced endoplasmic reticulum stress and apoptosis in podocytes

Albumin, which is the most abundant component of urine proteins, exerts injurious effects on renal cells in chronic kidney diseases. However, the toxicity of albumin to podocytes is not well elucidated. Here, we show that a high concentration of albumin triggers intracellular calcium ([Ca(2+)](i)) increase through mechanisms involving the intracellular calcium store release and extracellular calcium influx in conditionally immortalized podocytes. The canonical transient receptor potential-6 (TRPC6) channel, which is associated with a subset of familial forms of focal segmental glomerulosclerosis (FSGS) and several acquired proteinuric kidney diseases, was shown to be one of the important Ca(2+) permeable ion channels in podocytes. Therefore we explored the role of TRPC6 on albumin-induced functional and structural changes in podocytes. It was found that albumin-induced increase in [Ca(2+)](i) was blocked by TRPC6 siRNA or SKF-96365, a blocker of TRP cation channels. Long-term albumin exposure caused an up-regulation of TRPC6 expression in podocytes, which was inhibited by TRPC6 siRNA. Additionally, the inhibition of TRPC6 prevented the F-actin cytoskeleton disruption that is induced by albumin overload. Moreover, albumin overload induced expression of the endoplasmic reticulum (ER) stress protein GRP78, led to caspase-12 activation and ultimately podocyte apoptosis, all of which were abolished by the knockdown of TRPC6 using TRPC6 siRNA. These results support the view that albumin overload may induce ER stress and the subsequent apoptosis in podocytes via TRPC6-mediated Ca(2+) entry.

Regulation of CD2-associated protein influences podocyte endoplasmic reticulum stress-mediated apoptosis induced by albumin overload

Proteinuria is an exacerbating factor of chronic kidney diseases, leading to glomerulosclerosis. However, the molecular mechanisms mediating protein overload-induced podocyte injury are poorly understood. Recent studies have shown that apoptosis mediated by endoplasmic reticulum (ER) stress participated in the progression of a variety of kidney diseases. In the present study, we investigated the role of CD2-associated protein (CD2AP) in protein overload-induced ER stress and subsequent podocyte apoptosis. Conditionally immortalized mouse podocytes were cultured in vitro and treated with different concentrations of bovine serum albumin (BSA). In addition, CD2AP eukaryotic expression vector or siRNA was transfected into podocytes before exposed to BSA. Albumin endocytosis and podocyte apoptosis were visualized by confocal microscopy. The subcellular organelles were observed by transmission electron microscopy. The expressions of GRP78, caspase-12 and CD2AP were detected by RT-PCR or Western blot analysis. It was found that albumin was endocytosed by podocytes in a time-dependent manner. Accumulation of albumin in podocytes induced ER stress and apoptosis in a concentration-dependent manner as indicated by upregulation of GRP78 and caspase-12. Meanwhile, the subcellular organelles were disrupted and the expression of CD2AP was downregulated by high concentration of albumin. Transfection of CD2AP eukaryotic expression vector into podocytes increased CD2AP expression, depressed GRP78 and caspase-12 expressions, and inhibited podocyte apoptosis. In contrast, transfection of CD2AP siRNA deteriorated the above changes induced by BSA. It is concluded protein overload induces podocyte apoptosis via ER stress and CD2AP may play a crucial role in albumin overload-induced ER stress and apoptosis in podocytes.

Epithelial-to-mesenchymal transition in podocytes mediated by activation of NADPH oxidase in hyperhomocysteinemia

The present study tested the hypothesis that hyperhomocysteinemia (hHcys) induces podocytes to undergo epithelial-to-mesenchymal transition (EMT) through the activation of NADPH oxidase (Nox). It was found that increased homocysteine (Hcys) level suppressed the expression of slit diaphragm-associated proteins, P-cadherin and zonula occludens-1 (ZO-1), in conditionally immortalized mouse podocytes, indicating the loss of their epithelial features. Meanwhile, Hcys remarkably increased the abundance of mesenchymal markers, such as fibroblast specific protein-1 (FSP-1) and α-smooth muscle actin (α-SMA). These phenotype changes in podocytes induced by Hcys were accompanied by enhanced superoxide (O⁻₂) production, which was substantially suppressed by inhibition of Nox activity. Functionally, Hcys significantly enhanced the permeability of the podocyte monolayer coupled with increased EMT, and this EMT-related increase in cell permeability could be restored by Nox inhibitors. In mice lacking gp91( phox ) (gp91(-/-)), an essential Nox subunit gene, hHcys-enhanced podocyte EMT and consequent glomerular injury were examined. In wild-type (gp91(+/+)) mice, hHcys induced by a folate-free diet markedly enhanced expression of mesenchymal markers (FSP-1 and α-SMA) but decreased expression of epithelial markers of podocytes in glomeruli, which were not observed in gp91(-/-) mouse glomeruli. Podocyte injury, glomerular sclerotic pathology, and marked albuminuria observed in gp91(+/+) mice with hHcys were all significantly attenuated in gp91(-/-) mice. These results suggest that hHcys induces EMT of podocytes through activation of Nox, which represents a novel mechanism of hHcys-associated podocyte injury.