Sustained activation of N-methyl-D-aspartate receptors in podoctyes leads to oxidative stress, mobilization of transient receptor potential canonical 6 channels, nuclear factor of activated T cells activation, and apoptotic cell death

The roles of TRPC6 in renal tubular disorders: a narrative review

The transient receptor potential canonical 6 (TRPC6) channel, a nonselective cation channel that allows the passage of Ca2+, plays an important role in renal diseases. TRPC6 is activated by Ca2+ influx, oxidative stress, and mechanical stress. Studies have shown that in addition to glomerular diseases, TRPC6 can contribute to renal tubular disorders, such as acute kidney injury, renal interstitial fibrosis, and renal cell carcinoma (RCC). However, the tubule-specific physiological functions of TRPC6 have not yet been elucidated. Its pathophysiological role in ischemia/reperfusion (I/R) injury is debatable. Thus, TRPC6 may have dual roles in I/R injury. TRPC6 induces renal fibrosis and immune cell infiltration in a unilateral ureteral obstruction (UUO) mouse model. Additionally, TRPC6 overexpression may modify G2 phase transition, thus altering the DNA damage checkpoint, which can cause genomic instability and RCC tumorigenesis and can control the proliferation of RCC cells. This review highlights the importance of TRPC6 in various conditions of the renal tubular system. To better understand certain renal disorders and ultimately identify new therapeutic targets to improve patient care, the pathophysiology of TRPC6 must be clarified.

Non-voltage-gated Ca2+ channel signaling in glomerular cells in kidney health and disease

Positioned at the head of the nephron, the renal corpuscle generates a plasma ultrafiltrate to initiate urine formation. Three major cell types within the renal corpuscle, the glomerular mesangial cells, podocytes, and glomerular capillary endothelial cells, communicate via endocrine- and paracrine-signaling mechanisms to maintain the structure and function of the glomerular capillary network and filtration barrier. Ca2+ signaling mediated by several distinct plasma membrane Ca2+ channels impacts the functions of all three cell types. The past two decades have witnessed pivotal advances in understanding of non-voltage-gated Ca2+ channel function and regulation in the renal corpuscle in health and renal disease. This review summarizes the current knowledge of the physiological and pathological impact of non-voltage-gated Ca2+ channel signaling in mesangial cells, podocytes and glomerular capillary endothelium. The main focus is on transient receptor potential and store-operated Ca2+ channels, but ionotropic N-methyl-d-aspartate receptors and purinergic receptors also are discussed. This update of Ca2+ channel functions and their cellular signaling cascades in the renal corpuscle is intended to inform the development of therapeutic strategies targeting these channels to treat kidney diseases, particularly diabetic nephropathy.

Mitochondrial Signaling, the Mechanisms of AKI-to-CKD Transition and Potential Treatment Targets

Acute kidney injury (AKI) is increasing in prevalence and causes a global health burden. AKI is associated with significant mortality and can subsequently develop into chronic kidney disease (CKD). The kidney is one of the most energy-demanding organs in the human body and has a role in active solute transport, maintenance of electrochemical gradients, and regulation of fluid balance. Renal proximal tubular cells (PTCs) are the primary segment to reabsorb and secrete various solutes and take part in AKI initiation. Mitochondria, which are enriched in PTCs, are the main source of adenosine triphosphate (ATP) in cells as generated through oxidative phosphorylation. Mitochondrial dysfunction may result in reactive oxygen species (ROS) production, impaired biogenesis, oxidative stress multiplication, and ultimately leading to cell death. Even though mitochondrial damage and malfunction have been observed in both human kidney disease and animal models of AKI and CKD, the mechanism of mitochondrial signaling in PTC for AKI-to-CKD transition remains unknown. We review the recent findings of the development of AKI-to-CKD transition with a focus on mitochondrial disorders in PTCs. We propose that mitochondrial signaling is a key mechanism of the progression of AKI to CKD and potential targeting for treatment.

The characteristics of extrachromosomal circular DNA in patients with end-stage renal disease

BACKGROUND

End-stage renal disease (ESRD) is the final stage of chronic kidney disease (CKD). In addition to the structurally intact chromosome genomic DNA, there is a double-stranded circular DNA called extrachromosomal circular DNA (eccDNA), which is thought to be involved in the epigenetic regulation of human disease. However, the features of eccDNA in ESRD patients are barely known. In this study, we identified eccDNA from ESRD patients and healthy people, as well as revealed the characteristics of eccDNA in patients with ESRD.

METHODS

Using the high-throughput Circle-Sequencing technique, we examined the eccDNA in peripheral blood mononuclear cells (PBMCs) from healthy people (NC) (n = 12) and ESRD patients (n = 16). We analyzed the length distribution, genome elements, and motifs feature of eccDNA in ESRD patients. Then, after identifying the specific eccDNA in ESRD patients, we explored the potential functions of the target genes of the specific eccDNA. Finally, we investigated the probable hub eccDNA using algorithms.

RESULTS

In total, 14,431 and 11,324 eccDNAs were found in the ESRD and NC groups, respectively, with sizes ranging from 0.01 kb to 60 kb at most. Additionally, the ESRD group had a greater distribution of eccDNA on chromosomes 4, 11, 13, and 20. In two groups, we also discovered several motifs of specific eccDNAs. Furthermore, we identified 13,715 specific eccDNAs in the ESRD group and 10,585 specific eccDNAs in the NC group, both of which were largely annotated as mRNA catalog. Pathway studies using Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) showed that the specific eccDNA in ESRD was markedly enriched in cell junction and communication pathways. Furthermore, we identified potentially 20 hub eccDNA-targeting genes from all ESRD-specific eccDNA-targeting genes. Also, we found that 39 eccDNA-targeting genes were associated with ESRD, and some of these eccDNAs may be related to the pathogenesis of ESRD.

CONCLUSIONS

Our findings revealed the characteristics of eccDNA in ESRD patients and discovered potentially hub and ESRD-relevant eccDNA-targeting genes, suggesting a novel probable mechanism of ESRD.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Store-operated Ca2+ channel signaling: Novel mechanism for podocyte injury in kidney disease

Studies over the last decade have markedly broadened our understanding of store-operated Ca2+ channels (SOCs) and their roles in kidney diseases and podocyte dysfunction. Podocytes are terminally differentiated glomerular visceral epithelial cells which are tightly attached to the glomerular capillary basement membrane. Podocytes and their unique foot processes (pedicels) constitute the outer layer of the glomerular filtration membrane and the final barrier preventing filtration of albumin and other plasma proteins. Diabetic nephropathy and other renal diseases are associated with podocyte injury and proteinuria. Recent evidence demonstrates a pivotal role of store-operated Ca2+ entry (SOCE) in maintaining structural and functional integrity of podocytes. This article reviews the current knowledge of SOCE and its contributions to podocyte physiology. Recent studies of the contributions of SOC dysfunction to podocyte injury in both cell culture and animal models are discussed, including work in our laboratory. Several downstream signaling pathways mediating SOC function in podocytes also are examined. Understanding the pivotal roles of SOC in podocyte health and disease is essential, as SOCE-activated signaling pathways are potential treatment targets for podocyte injury-related kidney diseases.

The Effects of TRPC6 Knockout in Animal Models of Kidney Disease

Diseases that induce a loss of renal function affect a substantial portion of the world's population and can range from a slight decline in the glomerular filtration rate or microalbuminuria to complete kidney failure. Kidney disorders can be acute or chronic, but any significant reduction in renal function is associated with increased all-cause morbidity and mortality, especially when the conditions become chronic. There is an urgent need for new therapeutic approaches to slow or halt the progression of kidney disease. One potential target of considerable interest is the canonical transient receptor potential-6 (TRPC6) channel. TRCP6 is a cationic channel with a significant permeability to Ca2+. It is expressed in several tissues, including in multiple cell types of the kidney in glomeruli, microvasculature, and tubules. Here, we will describe TRPC6 channels and their roles in signal transduction, with an emphasis on renal cells, and the studies implicating TRPC6 channels in the progression of inherited and acquired kidney diseases. We then describe studies using TRPC6 knockout mice and rats subjected to treatments that model human diseases, including nephrotic syndromes, diabetic nephropathy, autoimmune glomerulonephritis, and acute kidney injuries induced by renal ischemia and by obstruction of the urinary tract. TRPC6 knockout has been shown to reduce glomerular manifestations of disease in several of these models and reduces renal fibrosis caused by urinary tract obstruction. TRPC6 knockout has proven to be less effective at reducing diabetic nephropathy in mouse and rat models. We also summarize the implications of these studies for drug development.

Insights into the regulation of podocyte and glomerular function by lactate and its metabolic sensor G-protein-coupled receptor 81

Podocytes and their foot processes are an important cellular layer of the renal filtration barrier that is involved in regulating glomerular permeability. Disturbances of podocyte function play a central role in the development of proteinuria in diabetic nephropathy. The retraction and effacement of podocyte foot processes that form slit diaphragms are a common feature of proteinuria. Correlations between the retraction of foot processes and the development of proteinuria are not well understood. Unraveling peculiarities of podocyte energy metabolism notably under diabetic conditions will provide insights into the pathogenesis of diabetic nephropathy. Intracellular metabolism in the cortical area of podocytes is regulated by glycolysis, whereas energy balance in the central area is controlled by oxidative phosphorylation and glycolysis. High glucose concentrations were recently reported to force podocytes to switch from mitochondrial oxidative phosphorylation to glycolysis, resulting in lactic acidosis. In this review, we hypothesize that the lactate receptor G-protein-coupled receptor 81 (also known as hydroxycarboxylic acid receptor 81) may contribute to the control of podocyte function in both health and disease.

Gastrointestinal motility modulation by stress is associated with reduced smooth muscle contraction through specific transient receptor potential channel

Excessive stress response causes disability in social life. There are many diseases caused by stress, such as gastrointestinal motility disorders, depression, eating disorders, and cardiovascular diseases. Transient receptor potential (TRP) channels underlie non-selective cation currents and are downstream effectors of G protein-coupled receptors. Ca²⁺ influx is important for smooth muscle contraction, which is responsible for gastrointestinal motility. Little is known about the possible involvement of TRP channels in the gastrointestinal motility disorders due to stress. The purpose of this study was to measure the changes in gastrointestinal motility caused by stress and to elucidate the mechanism of these changes. The stress model used the water immersion restraint stress. Gastrointestinal motility, especially the ileum, was recorded responses to electric field stimulation (EFS) by isometric transducer. EFS-induced contraction was significantly reduced in the ileum of stressed mouse. Even under the conditions treated with atropine, EFS-induced contraction was significantly reduced in the ileum of stressed mouse. In addition, carbachol-induced, neurokinin A-induced, and substance P-induced contractions were all significantly reduced in the ileum of stressed mouse. Furthermore, the expression of TRPC3 was decreased in the ileum of stressed mouse. These results suggest that the gastrointestinal motility disorders due to stress is associated with specific non-selective cation channel.

NMDA receptor-mediated CaMKII/ERK activation contributes to renal fibrosis

Background

This study aimed to understand the mechanistic role of N-methyl-D-aspartate receptor (NMDAR) in acute fibrogenesis using models of in vivo ureter obstruction and in vitro TGF-β administration.

Methods

Acute renal fibrosis (RF) was induced in mice by unilateral ureteral obstruction (UUO). Histological changes were observed using Masson’s trichrome staining. The expression levels of NR1, which is the functional subunit of NMDAR, and fibrotic and epithelial-to-mesenchymal transition markers were measured by immunohistochemical and Western blot analysis. HK-2 cells were incubated with TGF-β, and NMDAR antagonist MK-801 and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) antagonist KN-93 were administered for pathway determination. Chronic RF was introduced by sublethal ischemia–reperfusion injury in mice, and NMDAR inhibitor dextromethorphan hydrobromide (DXM) was administered orally.

Results

The expression of NR1 was upregulated in obstructed kidneys, while NR1 knockdown significantly reduced both interstitial volume expansion and the changes in the expression of α-smooth muscle actin, S100A4, fibronectin, COL1A1, Snail, and E-cadherin in acute RF. TGF-β1 treatment increased the elongation phenotype of HK-2 cells and the expression of membrane-located NR1 and phosphorylated CaMKII and extracellular signal–regulated kinase (ERK). MK801 and KN93 reduced CaMKII and ERK phosphorylation levels, while MK801, but not KN93, reduced the membrane NR1 signal. The levels of phosphorylated CaMKII and ERK also increased in kidneys with obstruction but were decreased by NR1 knockdown. The 4-week administration of DXM preserved renal cortex volume in kidneys with moderate ischemic–reperfusion injury.

Conclusions

NMDAR participates in both acute and chronic renal fibrogenesis potentially via CaMKII-induced ERK activation.

Glutamate-Gated NMDA Receptors: Insights into the Function and Signaling in the Kidney

N-Methyl-d-aspartate receptor (NMDAR) is a glutamate-gated ionotropic receptor that intervenes in most of the excitatory synaptic transmission within the central nervous system (CNS). Aside from being broadly distributed in the CNS and having indispensable functions in the brain, NMDAR has predominant roles in many physiological and pathological processes in a wide range of non-neuronal cells and tissues. The present review outlines current knowledge and understanding of the physiological and pathophysiological functions of NMDAR in the kidney, an essential excretory and endocrine organ responsible for the whole-body homeostasis. The review also explores the recent findings regarding signaling pathways involved in NMDAR-mediated responses in the kidney. As established from diverse lines of research reviewed here, basal levels of receptor activation within the kidney are essential for the maintenance of healthy tubular and glomerular function, while a disproportionate activation can lead to a disruption of NMDAR's downstream signaling pathways and a myriad of pathophysiological consequences.

Transmembrane insertases and N-glycosylation critically determine synthesis, trafficking, and activity of the nonselective cation channel TRPC6

Transient receptor potential cation channel subfamily C member 6 (TRPC6) is a widely expressed ion channel. Gain-of-function mutations in the human TRPC6 channel cause autosomal-dominant focal segmental glomerulosclerosis, but the molecular components involved in disease development remain unclear. Here, we found that overexpression of gain-of-function TRPC6 channel variants is cytotoxic in cultured cells. Exploiting this phenotype in a genome-wide CRISPR/Cas screen for genes whose inactivation rescues cells from TRPC6-associated cytotoxicity, we identified several proteins essential for TRPC6 protein expression, including the endoplasmic reticulum (ER) membrane protein complex transmembrane insertase. We also identified transmembrane protein 208 (TMEM208), a putative component of a signal recognition particle-independent (SND) ER protein-targeting pathway, as being necessary for expression of TRPC6 and several other ion channels and transporters. TRPC6 expression was also diminished by loss of the previously uncharacterized WD repeat domain 83 opposite strand (WDR83OS), which interacted with both TRPC6 and TMEM208. Additionally enriched among the screen hits were genes involved in N-linked protein glycosylation. Deletion of the mannosyl (α-1,3-)-glycoprotein β-1,2-N-acetylglucosaminyltransferase (MGAT1), necessary for the generation of complex N-linked glycans, abrogated TRPC6 gain-of-function variant-mediated Ca2+ influx and extracellular signal-regulated kinase activation in HEK cells, but failed to diminish cytotoxicity in cultured podocytes. However, mutating the two TRPC6 N-glycosylation sites abrogated the cytotoxicity of mutant TRPC6 and reduced its surface expression. These results expand the targets of TMEM208-mediated ER translocation to include multipass transmembrane proteins and suggest that TRPC6 N-glycosylation plays multiple roles in modulating channel trafficking and activity.

Hydrogen sulfide inhibits Ca2+-induced mitochondrial permeability transition pore opening in type-1 diabetes

Hydrogen sulfide (H2S) attenuates N-methyl-d-aspartate receptor-R1 (NMDA-R1) and mitigates diabetic renal damage; however, the molecular mechanism is not well known. Whereas NMDA-R1 facilitates Ca2+ permeability, H2S is known to inhibit L-type Ca2+ channel. High Ca2+ activates cyclophilin D (CypD), a gatekeeper protein of mitochondrial permeability transition pore (MPTP), thus facilitating molecular exchange between matrix and cytoplasm causing oxidative outburst and cell death. We tested the hypothesis of whether NMDA-R1 mediates Ca2+ influx causing CypD activation and MPTP opening leading to oxidative stress and renal injury in diabetes. We also tested whether H2S treatment blocks Ca2+ channel and thus inhibits CypD and MPTP opening to prevent renal damage. C57BL/6J and Akita (C57BL/6J-Ins2Akita) mice were treated without or with H2S donor GYY4137 (0.25 mg·kg-1·day-1 ip) for 8 wk. In vitro studies were performed using mouse glomerular endothelial cells. Results indicated that low levels of H2S and increased expression of NMDA-R1 in diabetes induced Ca2+ permeability, which was ameliorated by H2S treatment. We observed cytosolic Ca2+ influx in hyperglycemic (HG) condition along with mitochondrial-CypD activation, increased MPTP opening, and oxidative outburst, which were mitigated with H2S treatment. Renal injury biomarker KIM-1 was upregulated in HG conditions and normalized following H2S treatment. Inhibition of NMDA-R1 by pharmacological blocker MK-801 revealed similar results. We conclude that NMDA-R1-mediated Ca2+ influx in diabetes induces MPTP opening via CypD activation leading to increased oxidative stress and renal injury, and H2S protects diabetic kidney from injury by blocking mitochondrial Ca2+ permeability through NMDA-R1 pathway.

Expression of Glutamate Receptor Subtype 3 Is Epigenetically Regulated in Podocytes under Diabetic Conditions

Background

Recent studies suggest a role of epigenetics in the pathogenesis of diabetic kidney disease. However, epigenetic changes occurring specifically in kidney cells is poorly understood.

Methods

To examine the epigenetic regulation of genes in podocytes under diabetic conditions, we performed DNA methylation and transcriptomic profiling in podocytes exposed to high glucose conditions.

Results

Comparative analysis of genes with DNA methylation changes and correspondingly altered mRNA expression identified 337 hypomethylated genes with increased mRNA expression and only 2 hypermethyated genes (ESX1 and GRIA3) with decreased mRNA expression. Glutamate ionotropic receptor AMPA type subunit 3 (GRIA3) belongs to the ionotropic class of glutamate receptors that mediate fast excitatory synaptic transmission in the central nervous system. As podocytes have glutamate-containing vesicles and various glutamate receptors mediate important biological effects in podocytes, we further examined GRIA3 expression and its function in podocytes. Real-time PCR and western blots confirmed the suppression of GRIA3 expression in podocytes under high glucose conditions, which were abolished in the presence of a DNA methyltransferase inhibitor. Sites of DNA hypermethylation were also confirmed by bisulfite sequencing of the GRIA3 promoter region. GRIA3 mRNA and protein expression was suppressed in diabetic kidneys of human and mouse models, and knockdown of GRIA3 exacerbated high glucose-induced apoptosis in cultured podocytes.

Conclusion

These results indicate that decreased GRIA3 expression in podocytes in diabetic condition heightens podocyte apoptosis and loss.

Reactive oxygen species released from astrocytes treated with amyloid beta oligomers elicit neuronal calcium signals that decrease phospho-Ser727-STAT3 nuclear content

The transcription factor STAT3 has a crucial role in the development and maintenance of the nervous system. In this work, we treated astrocytes with oligomers of the amyloid beta peptide (AβOs), which display potent synaptotoxic activity, and studied the effects of mediators released by AβOs-treated astrocytes on the nuclear location of neuronal serine-727-phosphorylated STAT3 (pSerSTAT3). Treatment of mixed neuron-astrocyte cultures with 0.5µMAβOs induced in neurons a significant decrease of nuclear pSerSTAT3, but not of phosphotyrosine-705 STAT3, the other form of STAT3 phosphorylation. This decrease did not occur in astrocyte-poor neuronal cultures revealing a pivotal role for astrocytes in this response. To test if mediators released by astrocytes in response to AβOs induce pSerSTAT3 nuclear depletion, we used conditioned medium derived from AβOs-treated astrocyte cultures. Treatment of astrocyte-poor neuronal cultures with this medium caused pSerSTAT3 nuclear depletion but did not modify overall STAT3 levels. Extracellular catalase prevented the pSerSTAT3 nuclear depletion caused by astrocyte-conditioned medium, indicating that reactive oxygen species (ROS) mediate this response. This conditioned medium also increased neuronal oxidative tone, leading to a ryanodine-sensitive intracellular calcium signal that proved to be essential for pSerSTAT3 nuclear depletion. In addition, this depletion decreased BCL2 and Survivin transcription and significantly increased BAX/BCL2 ratio. This is the first description that ROS generated by AβOs-treated astrocytes and neuronal calcium signals jointly regulate pSerSTAT3 nuclear distribution in neurons. We propose that astrocytes release ROS in response to AβOs, which by increasing neuronal oxidative tone, generate calcium signals that cause pSerSTAT3 nuclear depletion and loss of STAT3 protective transcriptional activity.

TRPC5 Does Not Cause or Aggravate Glomerular Disease

Transient receptor potential channel 5 (TRPC5) is highly expressed in brain and kidney and mediates calcium influx and promotes cell migration. In the kidney, loss of TRPC5 function has been reported to benefit kidney filter dynamics by balancing podocyte cytoskeletal remodeling. However, in vivo gain-in-function studies of TRPC5 with respect to kidney function have not been reported. To address this gap, we developed two transgenic mouse models on the C57BL/6 background by overexpressing either wild-type TRPC5 or a TRPC5 ion-pore mutant. Compared with nontransgenic controls, neither transgenic model exhibited an increase in proteinuria at 8 months of age or a difference in LPS-induced albuminuria. Moreover, activation of TRPC5 by Englerin A did not stimulate proteinuria, and inhibition of TRPC5 by ML204 did not significantly lower the level of LPS-induced proteinuria in any group. Collectively, these data suggest that the overexpression or activation of the TRPC5 ion channel does not cause kidney barrier injury or aggravate such injury under pathologic conditions.

TRPC6 expression in neurons is differentially regulated by NR2A- and NR2B-containing NMDA receptors

The expression of transient receptor potential canonical 6 (TRPC6) in central nervous system (CNS) is important for neuronal functions and certain neural disorders. However, the regulatory mechanism of TRPC6 expression in neurons is still obscure. In this study, we show that TRPC6 expression in the primary cultured cortical neurons is bidirectionally regulated by glutamate. Activation of NR2A-containing NMDARs induces TRPC6 transcription through a calcineurin-dependent pathway. In contrast, activation of NR2B-containing NMDARs causes TRPC6 degradation through calpain. Thus, TRPC6 expression in neurons is regulated by glutamate in a bidirectional manner that is dependent on NR2A and NR2B.

TRPC3 Channels in Cardiac Fibrosis

Cardiac stiffness, caused by interstitial fibrosis due to deposition of extracellular matrix proteins, is thought as a major clinical outcome of heart failure with preserved ejection fraction (HFpEF). Canonical transient receptor potential (TRPC) subfamily proteins are components of Ca²⁺-permeable non-selective cation channels activated by receptor stimulation and mechanical stress, and have been attracted attention as a key mediator of maladaptive cardiac remodeling. How TRPC-mediated local Ca²⁺ influx encodes a specific signal to induce maladaptive cardiac remodeling has been long obscure, but our recent studies suggest a pathophysiological significance of channel activity-independent function of TRPC proteins for amplifying redox signaling in heart. This review introduces the current understanding of the physiological and pathophysiological roles of TRPCs, especially focuses on the role of TRPC3 as a positive regulator of reactive oxygen species (PRROS) in heart. We have revealed that TRPC3 stabilizes NADPH oxidase 2 (Nox2), a membrane-bound reactive oxygen species (ROS)-generating enzyme, by forming stable protein complex with Nox2, which leads to amplification of mechanical stress-induced ROS signaling in cardiomyocytes, resulting in induction of fibrotic responses in cardiomyocytes and cardiac fibroblasts. Thus, the TRPC3 function as PRROS will offer a new therapeutic strategy for the prevention or treatment of HFpEF.

NMDAR antagonists for the treatment of diabetes mellitus-Current status and future directions

Diabetes mellitus is characterized by chronically elevated blood glucose levels accelerated by a progressive decline of insulin-producing β-cells in the pancreatic islets. Although medications are available to transiently adjust blood glucose to normal levels, the effects of current drugs are limited when it comes to preservation of a critical mass of functional β-cells to sustainably maintain normoglycemia. In this review, we recapitulate recent evidence on the role of pancreatic N-methyl-D-aspartate receptors (NMDARs) in β-cell physiology, and summarize effects of morphinan-based NMDAR antagonists that are beneficial for insulin secretion, glucose tolerance and islet cell survival. We further discuss NMDAR-mediated molecular pathways relevant for neuronal cell survival, which may also be important for the preservation of β-cell function and mass. Finally, we summarize the literature for evidence on the role of NMDARs in the development of diabetic long-term complications, and highlight beneficial pharmacologic aspects of NMDAR antagonists in diabetic nephropathy, retinopathy as well as neuropathy.

Estradiol mitigates ischemia reperfusion-induced acute renal failure through NMDA receptor antagonism in rats

In the present study, we investigated possible involvement of N-methyl-D-aspartate receptors (NMDAR) in estradiol mediated protection against ischemia reperfusion (I/R)-induced acute renal failure (ARF) in rats. Bilateral renal ischemia of 40 min followed by reperfusion for 24 h induced ARF in male wistar rats. Quantification of serum creatinine, creatinine clearance (CrCl), blood urea nitrogen (BUN), uric acid, potassium, fractional excretion of sodium (Fe_Na), and urinary microproteins was done to assess I/R-induced renal damage in rats. Oxidative stress in kidneys was measured in terms of myeloperoxidase activity, thiobarbituric acid reactive substances, superoxide anion generation, and reduced glutathione levels. Hematoxylin & eosin and periodic acid Schiff stains were used to reveal structural changes in renal tissues. Estradiol benzoate (0.5 and 1.0 mg/kg, intraperitoneally, i.p.) was administered 1 h prior to I/R in rats. In separate groups, rats were treated with NMDAR agonists, glutamic acid (200 mg/kg, i.p.), and spermidine (20 mg/kg, i.p.) before administration of estradiol. Marked increase in serum creatinine, BUN, uric acid, serum potassium, Fe_Na, microproteinuria, and reduction in CrCl demonstrated I/R-induced ARF in rats. Treatment with estradiol mitigated I/R-induced changes in serum/urine parameters. Moreover, estrogen attenuated oxidative stress and structural changes in renal tissues. Prior administration of glutamic acid and spermidine abolished estradiol mediated renoprotection in rats. These results indicate the involvement of NMDAR in estradiol mediated renoprotective effect. In conclusion, we suggest that NMDAR antagonism serves as one of the mechanisms in estradiol-mediated protection against I/R-induced ARF in rats.

Nicotine Induces Podocyte Apoptosis through Increasing Oxidative Stress

Background

Cigarette smoking plays an important role in the progression of chronic kidney disease (CKD). Nicotine, one of the major components of cigarette smoking, has been demonstrated to increase proliferation of renal mesangial cells. In this study, we examined the effect of nicotine on podocyte injury.

Methods

To determine the expression of nicotinic acetylcholine receptors (nAChR subunits) in podocytes, cDNAs and cell lysate of cultured human podocytes were used for the expression of nAChR mRNAs and proteins, respectively; and mouse renal cortical sections were subjected to immunofluorescant staining. We also studied the effect of nicotine on podocyte nephrin expression, reactive oxygen species (ROS) generation (via DCFDA loading followed by fluorometric analysis), proliferation, and apoptosis (morphologic assays). We evaluated the effect of nicotine on podocyte downstream signaling including phosphorylation of ERK1/2, JNK, and p38 and established causal relationships by using respective inhibitors. We used nAChR antagonists to confirm the role of nicotine on podocyte injury.

Results

Human podocytes displayed robust mRNA and protein expression of nAChR in vitro studies. In vivo studies, mice renal cortical sections revealed co-localization of nAChRs along with synaptopodin. In vitro studies, nephrin expression in podocyte was decreased by nicotine. Nicotine stimulated podocyte ROS generation; nonetheless, antioxidants such as N-acetyl cysteine (NAC) and TEMPOL (superoxide dismutase mimetic agent) inhibited this effect of nicotine. Nicotine did not modulate proliferation but promoted apoptosis in podocytes. Nicotine enhanced podocyte phosphorylation of ERK1/2, JNK, and p38, and their specific inhibitors attenuated nicotine-induced apoptosis. nAChR antagonists significantly suppressed the effects of nicotine on podocyte.

Conclusions

Nicotine induces podocyte apoptosis through ROS generation and associated downstream MAPKs signaling. The present study provides insight into molecular mechanisms involved in smoking associated progression of chronic kidney disease.

NMDA Receptors as Potential Therapeutic Targets in Diabetic Nephropathy: Increased Renal NMDA Receptor Subunit Expression in Akita Mice and Reduced Nephropathy Following Sustained Treatment With Memantine or MK-801

N-methyl-d-aspartate (NMDA) receptors are expressed throughout the kidney, and the abundance of these receptors and some of their endogenous agonists are increased in diabetes. Moreover, sustained activation of podocyte NMDA receptors induces Ca(2+) influx, oxidative stress, loss of slit diaphragm proteins, and apoptosis. We observed that NMDA receptor subunits and their transcripts are increased in podocytes and mesangial cells cultured in elevated glucose compared with controls. A similar increase in NMDA subunits, especially NR1, NR2A, and NR2C, was observed in glomeruli and tubules of Akita mice. Sustained continuous treatment with the strong NMDA receptor antagonist dizocilpine (MK-801) for 28 days starting at 8 weeks of age reduced 24-h albumin excretion and mesangial matrix expansion and improved glomerular ultrastructure in Akita mice. MK-801 did not alleviate reduced Akita mouse body weight and had no effect on kidney histology or ultrastructure in DBA/2J controls. The structurally dissimilar NMDA antagonist memantine also reduced diabetic nephropathy, although it was less effective than MK-801. Inhibition of NMDA receptors may represent a valid therapeutic approach to reduce renal complications of diabetes, and it is possible to develop well-tolerated agents with minimal central nervous system effects. Two such agents, memantine and dextromethorphan, are already in widespread clinical use.

20-Hydroxyeicosatetraenoic Acid (20-HETE) Modulates Canonical Transient Receptor Potential-6 (TRPC6) Channels in Podocytes

The arachidonic acid metabolite 20-hydroxyeicosatetraenoic acid (20-HETE) regulates renal function, including changes in glomerular function evoked during tubuloglomerular feedback (TGF). This study describes the cellular actions of 20-HETE on cultured podocytes, assessed by whole-cell recordings from cultured podocytes combined with pharmacological and cell-biological manipulations of cells. Bath superfusion of 20-HETE activates cationic currents that are blocked by the pan-TRP blocker SKF-96365 and by 50 μM La³⁺, and which are attenuated after siRNA knockdown of TRPC6 subunits. Similar currents are evoked by a membrane-permeable analog of diacylgycerol (OAG), but OAG does not occlude responses to maximally-activating concentrations of 20-HETE (20 μM). Exposure to 20-HETE also increased steady-state surface abundance of TRPC6 subunits in podocytes as assessed by cell-surface biotinylation assays, and increased cytosolic concentrations of reactive oxygen species (ROS). TRPC6 activation by 20-HETE was eliminated in cells pretreated with TEMPOL, a membrane-permeable superoxide dismutase mimic. Activation of TRPC6 by 20-HETE was also blocked when whole-cell recording pipettes contained GDP-βS, indicating a role for either small or heterotrimeric G proteins in the transduction cascade. Responses to 20-HETE were eliminated by siRNA knockdown of podocin, a protein that organizes NADPH oxidase complexes with TRPC6 subunits in this cell type. In summary, modulation of ionic channels in podocytes may contribute to glomerular actions of 20-HETE.

NMDA receptors participate in the progression of diabetic kidney disease by decreasing Cdc42-GTP activation in podocytes

Podocytes play important roles in the progression of diabetic kidney disease (DKD) and these roles are closely associated with cytoskeletal actin dynamics. N-Methyl-d-aspartate receptors (NMDARs), which consist of two functional NR1 subunits and two regulatory NR2 subunits, are widely expressed in the brain but are also found in podocytes. Here, we found increased NR1 expression in two diabetic mouse models and in podocytes incubated in high glucose (HG). In diabetic mice, knockdown of NR1 using lentivirus carrying NR1-shRNA ameliorated the pathological features associated with DKD, and reversed the decreased expression of synaptopodin and Wilms' tumour-1. In podocytes incubated with HG, NR1 was secreted from the endoplasmic reticulum and this was blocked by bisindolylmaleimide I. NR1 knockdown decreased the cell shape remodelling, cell collapse, bovine serum albumin permeability, and migration induced by HG. After HG incubation, levels of cell division control protein 42 (Cdc42) and its active form increased, and a significantly higher Cdc42-GTP level, increased Cdc42 translocation onto the leading edges, and lower migration ability were found in podocytes with NR1 knockdown. Increases in the number and length of filopodia were found in podocytes with NR1 knockdown but these were abolished by Cdc42-GTP blockade with ML141. In conclusion, the activation of NMDARs plays an important role in DKD by reducing Cdc42-GTP activation. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Curcumin alleviates ischemia reperfusion-induced acute kidney injury through NMDA receptor antagonism in rats

OBJECTIVE

The present study investigated the role of N-methyl-d-aspartate (NMDA) receptors in curcumin-mediated renoprotection against ischemia reperfusion (I/R)-induced acute kidney injury (AKI) in rats.

METHODS

Rats were subjected to bilateral renal I/R (40 min I, 24 hours R) to induce AKI. Kidney injury was assessed by measuring creatinine clearance, blood urea nitrogen, plasma uric acid, potassium level, fractional excretion of sodium, and macroproteinuria. Oxidative stress in renal tissues was assessed by measuring myeloperoxidase activity, thiobarbituric acid reactive substances, superoxide anion generation, and reduced glutathione content. Hematoxylin & eosin staining was done to assess histological changes in renal tissues. Curcumin (30 and 60 mg/kg) was administered one hour before subjecting rats to AKI. In separate groups, NMDA receptor agonists, glutamic acid (200 mg/kg), and spermidine (20 mg/kg) were administered prior to curcumin treatment in rats followed by AKI.

RESULTS

I/R-induced AKI was demonstrated by significant change in plasma and urine parameters along with marked increase in oxidative stress and histological changes in renal tissues that were aggravated with pretreatment of glutamic acid and spermidine in rats. Administration of curcumin resulted in significant protection against AKI. However, glutamic acid and spermidine pretreatments prevented curcumin-mediated renoprotection.

CONCLUSION

It is concluded that NMDA receptor antagonism significantly contributes towards curcumin-mediated protection against I/R-induced AKI.

Second Messenger-Operated Calcium Entry Through TRPC6

Canonical transient receptor potential 6 (TRPC6) proteins assemble into heteromultimeric structures forming non-selective cation channels. In addition, many TRPC6-interacting proteins have been identified like some enzymes, channels, pumps, cytoskeleton-associated proteins, immunophilins, or cholesterol-binding proteins, indicating that TRPC6 are engaged into macromolecular complexes. Depending on the cell type and the experimental conditions used, TRPC6 activity has been reported to be controlled by diverse modalities. For instance, the second messenger diacylglycerol, store-depletion, the plant extract hyperforin or H2O2 have all been shown to trigger the opening of TRPC6 channels. A well-characterized consequence of TRPC6 activation is the elevation of the cytosolic concentration of Ca(2+). This latter response can reflect the entry of Ca(2+) through open TRPC6 channels but it can also be due to the Na(+)/Ca(2+) exchanger (operating in its reverse mode) or voltage-gated Ca(2+) channels (recruited in response to a TRPC6-mediated depolarization). Although TRPC6 controls a diverse array of biological functions in many tissues and cell types, its pathophysiological functions are far from being fully understood. This chapter covers some key features of TRPC6, with a special emphasis on their biological significance in kidney and blood cells.

The Na+/K+-ATPase and the amyloid-beta peptide aβ1-40 control the cellular distribution, abundance and activity of TRPC6 channels

The Na(+)/K(+)-ATPase interacts with the non-selective cation channels TRPC6 but the functional consequences of this association are unknown. Experiments performed with HEK cells over-expressing TRPC6 channels showed that inhibiting the activity of the Na(+)/K(+)-ATPase with ouabain reduced the amount of TRPC6 proteins and depressed Ca(2+) entry through TRPC6. This effect, not mimicked by membrane depolarization with KCl, was abolished by sucrose and bafilomycin-A, and was partially sensitive to the intracellular Ca(2+) chelator BAPTA/AM. Biotinylation and subcellular fractionation experiments showed that ouabain caused a multifaceted redistribution of TRPC6 to the plasma membrane and to an endo/lysosomal compartment where they were degraded. The amyloid beta peptide Aβ(1-40), another inhibitor of the Na(+)/K(+)-ATPase, but not the shorter peptide Aβ1-16, reduced TRPC6 protein levels and depressed TRPC6-mediated responses. In cortical neurons from embryonic mice, ouabain, veratridine (an opener of voltage-gated Na(+) channel), and Aβ(1-40) reduced TRPC6-mediated Ca(2+) responses whereas Aβ(1-16) was ineffective. Furthermore, when Aβ(1-40) was co-added together with zinc acetate it could no longer control TRPC6 activity. Altogether, this work shows the existence of a functional coupling between the Na(+)/K(+)-ATPase and TRPC6. It also suggests that the abundance, distribution and activity of TRPC6 can be regulated by cardiotonic steroids like ouabain and the naturally occurring peptide Aβ(1-40) which underlines the pathophysiological significance of these processes.

Podocytes: recent biomolecular developments

Podocytes are postmitotic renal glomerular cells with multiple ramifications that extend from the cell body. Processes departing from a podocyte interdigitate with corresponding projections from neighboring cells and form an intricate web that enwraps the glomerular capillary completely. Podocyte processes are interconnected by the slit diaphragm, an adhesion junction mostly formed by Ig-like molecules, cadherins/protocadherins, ephrin/eph, and neurexin molecules organized in an assembly that resembles synaptic junctions. Podocyte failure is primarily or secondarily implicated in all forms of proteinuric glomerular diseases, as confirmed by the morphological changes of their elaborate cell architecture detectable by electron microscopy. Importantly, mutations of podocyte proteins are responsible for the most severe forms of congenital nephrotic syndrome. In the last 15 years, progressive technological advances have aided the study of podocyte biology and pathology, confirming the relevance of podocyte molecules and signaling pathways for the function of the glomerular filter. This review will examine the most important and newest discoveries in the field, which is rapidly evolving, hopefully leading to a detailed knowledge of this fascinating cell and to the development of specific therapeutic options for proteinuric diseases.

Prevention of PKG1α oxidation augments cardioprotection in the stressed heart

The cGMP-dependent protein kinase-1α (PKG1α) transduces NO and natriuretic peptide signaling; therefore, PKG1α activation can benefit the failing heart. Disease modifiers such as oxidative stress may depress the efficacy of PKG1α pathway activation and underlie variable clinical results. PKG1α can also be directly oxidized, forming a disulfide bond between homodimer subunits at cysteine 42 to enhance oxidant-stimulated vasorelaxation; however, the impact of PKG1α oxidation on myocardial regulation is unknown. Here, we demonstrated that PKG1α is oxidized in both patients with heart disease and in rodent disease models. Moreover, this oxidation contributed to adverse heart remodeling following sustained pressure overload or Gq agonist stimulation. Compared with control hearts and myocytes, those expressing a redox-dead protein (PKG1α(C42S)) better adapted to cardiac stresses at functional, histological, and molecular levels. Redox-dependent changes in PKG1α altered intracellular translocation, with the activated, oxidized form solely located in the cytosol, whereas reduced PKG1α(C42S) translocated to and remained at the outer plasma membrane. This altered PKG1α localization enhanced suppression of transient receptor potential channel 6 (TRPC6), thereby potentiating antihypertrophic signaling. Together, these results demonstrate that myocardial PKG1α oxidation prevents a beneficial response to pathological stress, may explain variable responses to PKG1α pathway stimulation in heart disease, and indicate that maintaining PKG1α in its reduced form may optimize its intrinsic cardioprotective properties.

Glutamate receptors in the kidney

l-Glutamate (l-Glu) plays an essential role in the central nervous system (CNS) as an excitatory neurotransmitter, and exerts its effects by acting on a large number of ionotropic and metabotropic receptors. These receptors are also expressed in several peripheral tissues, including the kidney. This review summarizes the general properties of ionotropic and metabotropic l-Glu receptors, focusing on N-methyl-d-aspartate (NMDA) and Group 1 metabotropic glutamate receptors (mGluRs). NMDA receptors are expressed in the renal cortex and medulla, and appear to play a role in the regulation of renal blood flow, glomerular filtration, proximal tubule reabsorption and urine concentration within medullary collecting ducts. Sustained activation of NMDA receptors induces Ca(2+) influx and oxidative stress, which can lead to glomerulosclerosis, for example in hyperhomocysteinemia. Group 1 mGluRs are expressed in podocytes and probably in other cell types. Mice in which these receptors are knocked out gradually develop albuminuria and glomerulosclerosis. Several endogenous agonists of l-Glu receptors, which include sulfur-containing amino acids derived from l-homocysteine, and quinolinic acid (QA), as well as the co-agonists glycine and d-serine, are present in the circulation at concentrations capable of robustly activating ionotropic and metabotropic l-Glu receptors. These endogenous agonists may also be secreted from renal parenchymal cells, or from cells that have migrated into the kidney, by exocytosis or by transporters such as system x(-)(c), or by transporters involved in ammonia secretion. l-Glu receptors may be useful targets for drug therapy, and many selective orally-active compounds exist for investigation of these receptors as potential drug targets for various kidney diseases.

The potential of targeting NMDA receptors outside the CNS

INTRODUCTION

NMDA receptor (NMDAR) is an ionotropic glutamate receptor with a high permeability to calcium and a unique feature of controlling numerous calcium-dependent processes. Apart from being widely distributed in the CNS, the presence of NMDAR and its potential significance in a variety of non-neuronal cells and tissues has become an interesting research topic.

AREAS COVERED

The current review summarizes prevailing knowledge on the role of NMDARs in the kidney, bone and parathyroid gland, three main organs responsible for calcium homeostasis, as well as in the heart, an organ whose function is highly dependable on balanced intracellular calcium concentrations. The review also examines studies that have advanced our understanding of the therapeutic potential of NMDAR agonists and antagonists in renal, cardiovascular and bone pathologies.

EXPERT OPINION

NMDARs have a preeminent role in many physiological and pathological processes outside the CNS. In certain organs and/or disease conditions, activating the NMDAR leads to beneficial effects for the target organ, whereas in other diseases cell signaling downstream of NMDAR activation can exacerbate their pathology. Therefore, targeting NMDARs therapeutically is rather intricate work, and surely requires more extensive investigation in order to properly tune up the diverse NMDAR's actions translating them into beneficial cellular responses.

Angiotensin II has acute effects on TRPC6 channels in podocytes of freshly isolated glomeruli

A key role for podocytes in the pathogenesis of proteinuric renal diseases has been established. Angiotensin II causes depolarization and increased intracellular calcium concentration in podocytes; members of the cation TRPC channels family, particularly TRPC6, are proposed as proteins responsible for calcium flux. Angiotensin II evokes calcium transient through TRPC channels and mutations in the gene encoding the TRPC6 channel result in the development of focal segmental glomerulosclerosis. Here we examined the effects of angiotensin II on intracellular calcium ion levels and endogenous channels in intact podocytes of freshly isolated decapsulated mouse glomeruli. An ion channel with distinct TRPC6 properties was identified in wild type, but was absent in TRPC6 knockout mice. Single channel electrophysiological analysis found that angiotensin II acutely activated native TRPC-like channels in both podocytes of freshly isolated glomeruli and TRPC6 channels transiently overexpressed in CHO cells; the effect was mediated by changes in the channel open probability. Angiotensin II evoked intracellular calcium transients in the wild type podocytes, which was blunted in TRPC6 knockout glomeruli. Pan-TRPC inhibitors gadolinium and SKF 96365 reduced the response in wild type glomerular epithelial cells, whereas the transient in TRPC6 knockout animals was not affected. Thus, angiotensin II-dependent activation of TRPC6 channels in podocytes may have a significant role in the development of kidney diseases.

Angiotensin II activation of TRPC6 channels in rat podocytes requires generation of reactive oxygen species

Angiotensin II (AII) plays a major role in the progression of chronic kidney diseases. Podocytes are essential components of the ultrafiltration apparatus, and are targets for AII signaling. AII has been shown to increase generation of reactive oxygen species (ROS) in podocytes. Canonical transient receptor potential-6 (TRPC6) channels stimulate Ca(2+) influx in podocytes, and have been implicated in glomerular disease. We observed that AII increased cationic currents in rat podocytes in an isolated glomerulus preparation in which podocytes are still attached to the underlying capillary. This effect was completely blocked by SKF-96365, by micromolar La(3+) , and by siRNA knockdown of TRPC6, indicating that TRPC6 is the primary source of Ca(2+) influx mobilized by endogenously expressed angiotensin II receptors in these cells. These responses were also blocked by the AT1R antagonist losartan, the phospholipase C inhibitor D-609, and by inhibition of G protein signaling. The pan-protein kinase C inhibitor chelerythrine had no effect. Importantly, pretreating podocytes with the ROS quencher manganese (III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP) eliminated AII activation of TRPC6. Significant reductions of AII effects on podocyte TRPC6 were also observed after pretreatment with NADPH oxidase inhibitors apocynin or diphenylene iodonium (DPI). These data suggest that ROS production permits activation of TRPC6 channels by G protein and PLC-dependent cascades initiated by AII acting on AT1Rs in podocytes. This pathway also provides a basis whereby two forms of cellular stress-oxidative stress and Ca(2+) overload-converge on common pathways relevant to disease.

ATP acting through P2Y receptors causes activation of podocyte TRPC6 channels: role of podocin and reactive oxygen species

Extracellular ATP may contribute to Ca(2+) signaling in podocytes during tubuloglomerular feedback (TGF) and possibly as a result of local tissue damage. TRPC6 channels are Ca(2+)-permeable cationic channels that have been implicated in the pathophysiology of podocyte diseases. Here we show using whole cell recordings that ATP evokes robust activation of TRPC6 channels in mouse podocyte cell lines and in rat podocytes attached to glomerular capillaries in ex vivo glomerular explants. The EC50 for ATP is ~10 μM and is maximal at 100 μM, and currents were blocked by the P2 antagonist suramin. In terms of maximal currents that can be evoked, ATP is the strongest activator of podocyte TRPC6 that we have characterized to date. Smaller currents were observed in response to ADP, UTP, and UDP. ATP-evoked currents in podocytes were abolished by TRPC6 knockdown and by pretreatment with 10 μM SKF-96365 or 50 μM La(3+). ATP effects were also abolished by inhibiting G protein signaling and by the PLC/PLA2 inhibitor D-609. ATP effects on TRPC6 were also suppressed by knockdown of the slit diaphragm scaffolding protein podocin, and also by tempol, a membrane-permeable quencher of reactive oxygen species. Modulation of podocyte TRPC6 channels, especially in foot processes, could provide a mechanism for regulation of glomerular function by extracellular nucleotides, possibly leading to changes in permeation through slit diaphragms. These results raise the possibility that sustained ATP signaling could contribute to foot process effacement, Ca(2+)-dependent changes in gene expression, and/or detachment of podocytes.

Lovastatin inhibits human B lymphoma cell proliferation by reducing intracellular ROS and TRPC6 expression

Clinical evidence suggests that statins reduce cancer incidence and mortality. However, there is lack of in vitro data to show the mechanism by which statins can reduce the malignancies of cancer cells. We used a human B lymphoma Daudi cells as a model and found that lovastatin inhibited, whereas exogenous cholesterol (Cho) stimulated, proliferation cell cycle progression in control Daudi cells, but not in the cells when transient receptor potential canonical 6 (TRPC6) channel was knocked down. Lovastatin decreased, whereas Cho increased, the levels of intracellular reactive oxygen species (ROS) respectively by decreasing or increasing the expression of p47-phox and gp91-phox (NOX2). Reducing intracellular ROS with either a mimetic superoxide dismutase (TEMPOL) or an NADPH oxidase inhibitor (apocynin) inhibited cell proliferation, particularly in Cho-treated cells. The effects of TEMPOL or apocynin were mimicked by inhibition of TRPC6 with SKF-96365. Lovastatin decreased TRPC6 expression and activity via a Cho-dependent mechanism, whereas Cho increased TRPC6 expression and activity via an ROS-dependent mechanism. Consistent with the fact that TRPC6 is a Ca(2+)-permeable channel, lovastatin decreased, but Cho increased, intracellular Ca(2+) also via ROS. These data suggest that lovastatin inhibits malignant B cell proliferation by reducing membrane Cho, intracellular ROS, TRPC6 expression and activity, and intracellular Ca(2+).

The podocyte power-plant disaster and its contribution to glomerulopathy

Proper podocyte function within the glomerulus demands a high and continuous energy supply that is mainly derived from the respiratory chain of the inner mitochondrial membrane. Dysregulations in the metabolic homeostasis of podocytes may result in podocyte damage and glomerular disease. This article highlights the current knowledge about podocyte energy supply by the respiratory chain. We review the regulation of mitochondrial oxidative metabolism with regard to podocytopathy and discuss the latest understanding of different mitochondrial dysfunctions of the podocyte in diabetic nephropathy and focal segmental glomerulosclerosis (FSGS). We discuss genetic forms of mitochondriopathy of the podocyte and end with recent knowledge about crosstalk between NADH and NADPH and potential therapeutic options for podocyte mitochondriopathy. We aim to raise awareness for the complex and interesting mechanisms of podocyte damage by impaired energy supply that, despite of novel findings in recent years, is poorly understood so far.

Role of NADPH oxidase-mediated reactive oxygen species in podocyte injury

Proteinuria is an independent risk factor for end-stage renal disease (ESRD) (Shankland, 2006). Recent studies highlighted the mechanisms of podocyte injury and implications for potential treatment strategies in proteinuric kidney diseases (Zhang et al., 2012). Reactive oxygen species (ROS) are cellular signals which are closely associated with the development and progression of glomerular sclerosis. NADPH oxidase is a district enzymatic source of cellular ROS production and prominently expressed in podocytes (Zhang et al., 2010). In the last decade, it has become evident that NADPH oxidase-derived ROS overproduction is a key trigger of podocyte injury, such as renin-angiotensin-aldosterone system activation (Whaley-Connell et al., 2006), epithelial-to-mesenchymal transition (Zhang et al., 2011), and inflammatory priming (Abais et al., 2013). This review focuses on the mechanism of NADPH oxidase-mediated ROS in podocyte injury under different pathophysiological conditions. In addition, we also reviewed the therapeutic perspectives of NADPH oxidase in kidney diseases related to podocyte injury.

NOX2 interacts with podocyte TRPC6 channels and contributes to their activation by diacylglycerol: essential role of podocin in formation of this complex

Canonical transient receptor potential-6 (TRPC6) channels have been implicated in the pathophysiology of glomerular diseases. TRPC6 channels are typically activated by diacylglycerol (DAG) during PLC-dependent transduction cascades. TRPC6 channels can also be activated by reactive oxygen species (ROS). We previously showed that podocin is required for DAG analogs to produce robust activation of TRPC6 channels in podocytes. Here we show that endogenous TRPC6 channels in immortalized podocytes reciprocally coimmunoprecipitate with the catalytic subunit of the NADPH oxidase NOX2 (gp91(phox)). The NOX2-TRPC6 interaction was not detected in cells stably expressing a short hairpin RNA targeting podocin, although NOX2 and TRPC6 were present at normal levels. Application of a membrane-permeable DAG analog [1-oleoyl-2-acetyl-sn-glycerol (OAG)] increased generation of ROS in podocytes, but this effect was not detected in podocin knockdown cells. OAG also increased steady-state surface expression of the NOX2 regulatory subunit p47(phox). In whole cell recordings, TRPC6 activation by OAG was reduced in podocytes pretreated with the NOX2 inhibitor apocynin, by the pan-NOX inhibitor diphenylene iodonium, and by tempol, a ROS quencher. Cholesterol depletion and disruption of lipid rafts by methyl-β-cyclodextrin reduced activation of podocyte TRPC6 channels by OAG and also eliminated the NOX2-TRPC6 interaction as assessed by coimmunoprecipitation. These data suggest that active NOX2 assembles with TRPC6 at podocin-organized sterol-rich raft domains and becomes catalytically active in response to DAG. The localized production of ROS contributes to TRPC6 activation by chemical stimuli such as DAG. Podocin appears to be necessary for assembly of the NOX2-TRPC6 complex in lipid rafts.

Opposing effects of podocin on the gating of podocyte TRPC6 channels evoked by membrane stretch or diacylglycerol

Gain-of-function mutations in the transient receptor potential (TRP) cation channel subfamily C member 6 (TRPC6) gene and mutations in the NPHS2 gene encoding podocin result in nephrotic syndromes. The purpose of this study was to determine the functional significance of biochemical interactions between these proteins. We observed that gating of TRPC6 channels in podocytes is markedly mechanosensitive and can be activated by hyposmotic stretch or indentation of the plasma membrane. Stretch activation of cationic currents was blocked by small interfering RNA knockdown of TRPC6, as well as by SKF-96365 or micromolar La(3+). Stretch activation of podocyte TRPC6 persisted in the presence of inhibitors of phospholipase C (U-73122) and phospholipase A2 (ONO-RS-082). Robust stretch responses also persisted when recording electrodes contained guanosine 5'-O-(2-thiodiphosphate) at concentrations that completely suppressed responses to ANG II. Stretch responses were enhanced by cytochalasin D but were abolished by the peptide GsMTx4, suggesting that forces are transmitted to the channels through the plasma membrane. Podocin and TRPC6 interact at their respective COOH termini. Knockdown of podocin markedly increased stretch-evoked activation of TRPC6 but nearly abolished TRPC6 activation evoked by a diacylglycerol analog. These data suggest that podocin acts as a switch to determine the preferred mode of TRPC6 activation. They also suggest that podocin deficiencies will result in Ca(2+) overload in foot processes, as with gain-of-function mutations in the TRPC6 gene. Finally, they suggest that mechanical activation of TRP family channels and the preferred mode of TRP channel activation may depend on whether members of the stomatin/prohibitin family of hairpin loop proteins are present.

High glucose-induced apoptosis in cultured podocytes involves TRPC6-dependent calcium entry via the RhoA/ROCK pathway

Increasing evidence indicates that podocyte apoptosis is a key event in the development of diabetic nephrology. However, the underlying mechanism of this apoptosis remains poorly understood. In this study, we report that high levels of glucose enhanced the expression of TRPC6 and TRPC6-dependent Ca(2+) influx, but glucose levels did not affect TRPC1 and TRPC5 expression. TRPC6 knockdown by siRNA interference attenuated the observed increase in glucose-induced podocyte apoptosis. High glucose levels also increased the generation of ROS; inhibition of ROS activity by N-acetyl-l-cysteine attenuated the high glucose-induced increase in TRPC6 expression and Ca(2+) influx. Exogenous treatment with H2O2 mimicked the high glucose response, resulting in an increase in TRPC6 expression and Ca(2+) influx. Taken together, these data suggest that high glucose levels induce ROS, thereby mediating TRPC6 expression and Ca(2+) influx. Because RhoA activity is increased following TRPC6 activation, we investigated whether TRPC6 is involved in high glucose-induced apoptosis via the RhoA/ROCK pathway. We report that high glucose levels produced an increase in RhoA activity, and this effect was abolished by the knockdown of TRPC6. Moreover, inhibition of the RhoA/ROCK pathway by a ROCK inhibitor, Y27632, also attenuated high glucose-induced apoptosis. We conclude that TRPC6 is involved in high glucose-induced podocyte apoptosis through the RhoA/ROCK pathway.

High glucose induces podocyte apoptosis by stimulating TRPC6 via elevation of reactive oxygen species

Podocyte number is significantly reduced in diabetic patients and animal models, but the mechanism remains unclear. In the present study, we found that high glucose induced apoptosis in control podocytes which express transient receptor potential canonical 6 (TRPC6) channels, but not in TRPC6 knockdown podocytes in which TRPC6 was knocked down by TRPC6 silencing short hairpin RNA (shRNA). This effect was reproduced by treatment of podocytes with the reactive oxygen species (ROS), hydrogen peroxide (H2O2). Single-channel data from cell-attached, patch-clamp experiments showed that both high glucose and H2O2 activated the TRPC6 channel in control podocytes, but not in TRPC6 knockdown podocytes. Confocal microscopy showed that high glucose elevated ROS in podocytes and that H2O2 reduced the membrane potential of podocytes and elevated intracellular Ca(2+) via activation of TRPC6. Since intracellular Ca(2+) overload induces apoptosis, H2O2-induced apoptosis may result from TRPC6-mediated elevation of intracellular Ca(2+). These data together suggest that high glucose induces apoptosis in podocytes by stimulating TRPC6 via elevation of ROS.