Knockout of TRPC6 promotes insulin resistance and exacerbates glomerular injury in Akita mice

Lipid nephrotoxicity mediated by HIF-1α activation accelerates tubular injury in diabetic nephropathy

This study is intended to explore the effect of hypoxia-inducible factor-1α (HIF-1α) activation on lipid accumulation in the diabetic kidney. A type 1 diabetic rat model was established by STZ intraperitoneal injection. Cobalt chloride (CoCl2) and YC-1 were used as the HIF-1α activator and antagonist, respectively. CoCl2 treatment significantly increased HIF-1α expression, accelerated lipid deposition, and accelerated tubular injury in diabetic kidneys. In vitro, CoCl2 effectively stabilized HIF-1α and increased its transportation from the cytoplasm to the nucleus, which was accompanied by significantly increased lipid accumulation in HK-2 cells. Furthermore, results obtained in vivo showed that HIF-1α protein expression in the renal tubules of diabetic rats was significantly downregulated by YC-1 treatment. Meanwhile, lipid accumulation in the tubules of the DM + YC-1 group was markedly decreased in comparison to the DM + DMSO group. Accordingly, PAS staining revealed that the pathological injury caused to the tubular epithelial cells was alleviated by YC-1 treatment. Furthermore, the blood glucose level, urine albumin creatinine ratio, and NAG creatinine ratio in the DM + YC-1 group were significantly decreased compared to the DM + DMSO group. Moreover, the protein expression levels of transforming growth factor β1 (TGF-β1) and connective tissue growth factor (CTGF) in diabetic kidneys were decreased by YC-1 treatment. Our findings demonstrate that the activation of HIF-1α contributed to interstitial injury in a rat model of diabetic nephropathy and that the underlying mechanism involved the induction of lipid accumulation.

Recent advancement of sonogenetics: A promising noninvasive cellular manipulation by ultrasound

Recent advancements in biomedical research have underscored the importance of noninvasive cellular manipulation techniques. Sonogenetics, a method that uses genetic engineering to produce ultrasound-sensitive proteins in target cells, is gaining prominence along with optogenetics, electrogenetics, and magnetogenetics. Upon stimulation with ultrasound, these proteins trigger a cascade of cellular activities and functions. Unlike traditional ultrasound modalities, sonogenetics offers enhanced spatial selectivity, improving precision and safety in disease treatment. This technology broadens the scope of non-surgical interventions across a wide range of clinical research and therapeutic applications, including neuromodulation, oncologic treatments, stem cell therapy, and beyond. Although current literature predominantly emphasizes ultrasonic neuromodulation, this review offers a comprehensive exploration of sonogenetics. We discuss ultrasound properties, the specific ultrasound-sensitive proteins employed in sonogenetics, and the technique's potential in managing conditions such as neurological disorders, cancer, and ophthalmic diseases, and in stem cell therapies. Our objective is to stimulate fresh perspectives for further research in this promising field.

Small molecules targeting canonical transient receptor potential channels: an update

Transient receptor potential canonical (TRPC) channels belong to an important class of non-selective cation channels. This channel family consists of multiple members that widely participate in various physiological and pathological processes. Previous studies have uncovered the intricate regulation of these channels, as well as the spatial arrangement of TRPCs and the binding sites for various small molecule compounds. Multiple small molecules have been identified as selective agonists or inhibitors targeting different subtypes of TRPC, including potential preclinical drug candidates. This review covers recent advancements in the understanding of TRPC regulation and structure and the discovery of TRPC small molecules over the past few years, with the aim of facilitating research on TRPCs and small-molecule drug discovery.

The activation of spliced X-box binding protein 1 by isorhynchophylline therapy improves diabetic encephalopathy

The primary symptom of diabetic encephalopathy (DE), a kind of central diabetic neuropathy caused by diabetes mellitus (DM), is cognitive impairment. In addition, the tetracyclic oxindole alkaloid isorhynchophylline (IRN) helps lessen cognitive impairment. However, it is still unclear how IRN affects DM and DE and what mechanisms are involved. The effectiveness of IRN on brain insulin resistance was carefully examined in this work, both in vitro and in vivo. We found that IRN accelerates spliced form of X-box binding protein 1 (sXBP1) translocation into the nucleus under high glucose conditions in vitro. IRN also facilitates the nuclear association of pCREB with sXBP1 and the binding of regulatory subunits of phosphatidylinositol 3-kinase (PI3K) p85α or p85β with XBP1 to restore high glucose impairment. Also, IRN treatment improves high glucose-mediated impairment of insulin signaling, endoplasmic reticulum stress, and pyroptosis/apoptosis by depending on sXBP1 in vitro. In vivo studies suggested that IRN attenuates cognitive impairment, ameliorating peripheral insulin resistance, activating insulin signaling, inactivating activating transcription factor 6 (ATF6) and C/EBP homology protein (CHOP), and mitigating pyroptosis/apoptosis by stimulation of sXBP1 nuclear translocation in the brain. In summary, these data indicate that IRN contributes to maintaining insulin homeostasis by activating sXBP1 in the brain. Thus, IRN is a potent antidiabetic agent as well as an sXBP1 activator that has promising potential for the prevention or treatment of DE.

A Putative Role for TRPC6 in Immune-Mediated Kidney Injury

Excessive activation of the immune system is the cause of a wide variety of renal diseases. However, the pathogenic mechanisms underlying the aberrant activation of the immune system in the kidneys often remain unknown. TRPC6, a member of the Ca2+-permeant family of TRPC channels, is important in glomerular epithelial cells or podocytes for the process of glomerular filtration. In addition, TRPC6 plays a crucial role in the development of kidney injuries by inducing podocyte injury. However, an increasing number of studies suggest that TRPC6 is also responsible for tightly regulating the immune cell functions. It remains elusive whether the role of TRPC6 in the immune system and the pathogenesis of renal inflammation are intertwined. In this review, we present an overview of the current knowledge of how TRPC6 coordinates the immune cell functions and propose the hypothesis that TRPC6 might play a pivotal role in the development of kidney injury via its role in the immune system.

Podocyte protease activated receptor 1 stimulation in mice produces focal segmental glomerulosclerosis mirroring human disease signaling events

About 30% of patients who have a kidney transplant with underlying nephrotic syndrome (NS) experience rapid relapse of disease in their new graft. This is speculated to be due to a host-derived circulating factor acting on podocytes, the target cells in the kidney, leading to focal segmental glomerulosclerosis (FSGS). Our previous work suggests that podocyte membrane protease receptor 1 (PAR-1) is activated by a circulating factor in relapsing FSGS. Here, the role of PAR-1 was studied in human podocytes in vitro, and using a mouse model with developmental or inducible expression of podocyte-specific constitutively active PAR-1, and using biopsies from patients with nephrotic syndrome. In vitro podocyte PAR-1 activation caused a pro-migratory phenotype with phosphorylation of the kinase JNK, VASP protein and docking protein Paxillin. This signaling was mirrored in podocytes exposed to patient relapse-derived NS plasma and in patient disease biopsies. Both developmental and inducible activation of transgenic PAR-1 (NPHS2 Cre PAR-1Active+/-) caused early severe nephrotic syndrome, FSGS, kidney failure and, in the developmental model, premature death. We found that the non-selective cation channel protein TRPC6 could be a key modulator of PAR-1 signaling and TRPC6 knockout in our mouse model significantly improved proteinuria and extended lifespan. Thus, our work implicates podocyte PAR-1 activation as a key initiator of human NS circulating factor and that the PAR-1 signaling effects were partly modulated through TRPC6.

Limonin, a natural ERK2 agonist, protects against ischemic acute kidney injury

Acute kidney injury (AKI) is a refractory clinical syndrome with limited effective treatments. Amid AKI, activation of the extracellular signal-regulated kinase (ERK) cascade plays a critical role in promoting kidney repair and regeneration. However, a mature ERK agonist in treating kidney disease remains lacking. This study identified limonin, a member of the class of compounds known as furanolactones, as a natural ERK2 activator. Employing a multidisciplinary approach, we systemically dissected how limonin mitigates AKI. Compared to vehicles, pretreatment of limonin significantly preserved kidney functions after ischemic AKI. We revealed that ERK2 is a significant protein linked to the limonin's active binding sites through structural analysis. The molecular docking study showed a high binding affinity between limonin and ERK2, which was confirmed by the cellular thermal shift assay and microscale thermophoresis. Mechanistically, we further validated that limonin promoted tubular cell proliferation and reduced cell apoptosis after AKI by activating ERK signaling pathway in vivo. In vitro and ex vivo, blockade of ERK abolished limonin's capacity of preventing tubular cell death under hypoxia stress. Our results indicated that limonin is a novel ERK2 activator with strong translational potential in preventing or mitigating AKI.

Macrophage inflammatory protein-1β as a novel therapeutic target for renal protection in diabetic kidney disease

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease worldwide and the prevalence of DKD has increased over recent decades. Inflammation is involved in the development and progression of DKD. In this study, we explored the potential role of macrophage inflammatory protein-1β (MIP-1β) in DKD. Clinical non-diabetic subjects and DKD patients with different levels of urine albumin-to-creatinine ratio (ACR) were enrolled in the study. Lepr^db/^db mice and MIP-1β knockout mice were also used as mouse models for DKD. We found that serum MIP-1β levels were elevated in the DKD patients, especially those with ACRs that were less than or equal to 300, suggesting that MIP-1β is activated in clinical DKD. The administration of anti-MIP-1β antibodies attenuated DKD severity in the Lepr^db/^db mice, which also showed reduced glomerular hypertrophy and podocyte injury, as well as decreased inflammation and fibrosis, suggesting that MIP-1β plays a role in the development of DKD. The MIP-1β knockout mice showed improved renal function and decreased renal glomerulosclerosis and fibrosis in DKD. Furthermore, podocytes from the MIP-1β knockout mice showed less high glucose-induced inflammation and fibrosis compared to those from wild-type mice. In conclusion, the inhibition or deletion of MIP-1β protected podocytes, modulated renal inflammation, and ameliorated experimental DKD, suggesting that novel anti-MIP-1β strategies could potentially be used to treat DKD.

Transient receptor potential channel 6 knockdown prevents high glucose-induced Müller cell pyroptosis

BACKGROUND

Transient receptor potential channel 6 (TRPC6) is reported to be involved in the pathogenesis of diabetic complications, but its role in diabetic retinopathy (DR) remains unknown. The aim of our study was to determine the role and mechanism of TRPC6 in DR.

METHODS

High glucose was used to construct a DR cell model using rat retinal Müller cells (rMC-1). Intracellular Ca²⁺, reactive oxygen species (ROS) and cell pyroptosis were evaluated by flow cytometry. Protein levels of NLRP3, pro-caspase-1, active caspase-1, gasdermin D (GSDMD), GSDMD-N, TRPC6 and H3K27ac were detected by Western blot. mRNA levels of EP300 and TRPC6 were analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR). Levels of IL-1β and IL-18 were estimated by enzyme linked immunosorbent assay (ELISA). The interaction between EP300 and TRPC6 was validated by a chromatin immunoprecipitation assay.

RESULTS

The knockdown of TRPC6 reduced inflammation and cell pyroptosis in HG induced rMC-1 cells, whereas overexpression of TRPC6 had the opposite effects. The inhibition of ROS and NLRP3 reversed TRPC6-mediated cell pyroptosis in the DR cell model. In addition, EP300 increased the expression of H3K27ac and TRPC6 to promote cell pyroptosis, which was suppressed by the knockdown of TRPC6.

CONCLUSIONS

Our study revealed a novel EP300/H3K27ac/TRPC6 signaling pathway that may contribute to HG induced Müller cell pyroptosis. TRPC6 played a novel role in Müller cell pyroptosis triggered by HG, and may be a potential target for DR treatment in the future.

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.

Inhibition of TRPC6 suppressed TGFβ-induced fibroblast-myofibroblast transdifferentiation in renal interstitial NRK-49F cells

Renal fibrosis is a global health concern with limited curative treatment. Canonical transient receptor potential channel 6 (TRPC6), a nonselective cation channel, has been shown to regulate the renal fibrosis in murine models. However, the molecular mechanism is unclear. Fibroblast-myofibroblast transdifferentiation is one of the critical steps in the progression of renal fibrosis. In the present study, we demonstrate that transforming growth factor (TGF)-β1 exposure significantly increases the TRPC6 expression in renal interstitial fibroblast NRK-49F cells. Pharmacological inhibition of TRPC6 and knockdown of Trpc6 by siRNA alleviate TGF-β1-increased expression levels of α-smooth muscle actin (α-SMA) and collagen I, two key markers of myofibroblasts. Although direct activation of TRPC6 by 1-oleoyl-2-acetyl-sn-glycerol (OAG) does not affect the expression of α-SMA and collagen I, OAG potentiates TGF-β1-induced fibroblast-myofibroblast transdifferentiation. Further study demonstrates that TGF-β1 exposure increases the phosphorylation level of p38 and Yes-associated protein (YAP) translocation into the nuclei. Inhibition of p38 and YAP decreases TGF-β1-enhanced TRPC6 and α-SMA expression. In conclusion, we demonstrate that TRPC6 is a key regulator of TGF-β1-induced fibroblast-myofibroblast transdifferentiation and provides the mechanism of how TGF-β1 exposure regulates TRPC6 expression in NRK-49F fibroblasts.

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.

Long noncoding RNA SNHG5 promotes podocyte injury via the microRNA-26a-5p/TRPC6 pathway in diabetic nephropathy

Podocyte injury is a characteristic pathological hallmark of diabetic nephropathy (DN). However, the exact mechanism of podocyte injury in DN is incompletely understood. This study was conducted using db/db mice and immortalized mouse podocytes. High-throughput sequencing was used to identify the differentially expressed long noncoding RNAs in kidney of db/db mice. The lentiviral shRNA directed against long noncoding RNA small nucleolar RNA host gene 5 (SNHG5) or microRNA-26a-5p (miR-26a-5p) agomir was used to treat db/db mice to regulate the SNHG5/miR-26a-5p pathway. Here, we found that the expression of transient receptor potential canonical type 6 (TRPC6) was significantly increased in injured podocytes under the condition of DN, which was associated with markedly decreased miR-26a-5p. We determined that miR-26a-5p overexpression ameliorated podocyte injury in DN via binding to 3'-UTR of Trpc6, as evidenced by the markedly reduced activity of luciferase reporters by miR-26a-5p mimic. Then, the upregulated SNHG5 in podocytes and kidney in DN was identified, and it was proved to sponge to miR-26a-5p directly using luciferase activity, RNA immunoprecipitation, and RNA pull-down assay. Knockdown of SNHG5 attenuated podocyte injury in vitro, accompanied by an increased expression of miR-26a-5p and decreased expression of TRPC6, demonstrating that SNHG5 promoted podocyte injury by controlling the miR-26a-5p/TRPC6 pathway. Moreover, knockdown of SNHG5 protects against podocyte injury and progression of DN in vivo. In conclusion, SNHG5 promotes podocyte injury via the miR-26a-5p/TRPC6 pathway in DN. Our findings provide novel insights into the pathophysiology of podocyte injury and a potential new therapeutic strategy for DN.

Gain-of-function, focal segmental glomerulosclerosis Trpc6 mutation minimally affects susceptibility to renal injury in several mouse models

Mutations in TRPC6 are a cause of autosomal dominant focal segmental glomerulosclerosis in humans. Many of these mutations are known to have a gain-of-function effect on the non-specific cation channel function of TRPC6. In vitro studies have suggested these mutations affect several signaling pathways, but in vivo studies have largely compared wild-type and Trpc6-deficient rodents. We developed mice carrying a gain-of-function Trpc6 mutation encoding an E896K amino acid change, corresponding to a known FSGS mutation in TRPC6. Homozygous mutant Trpc6 animals have no appreciable renal pathology, and do not develop albuminuria until very advanced age. The Trpc6E896K mutation does not impart susceptibility to PAN nephrosis. The animals show a slight delay in recovery from the albumin overload model. In response to chronic angiotensin II infusion, Trpc6E896K/E896K mice have slightly greater albuminuria initially compared to wild-type animals, an effect that is lost at later time points, and a statistically non-significant trend toward more glomerular injury. This phenotype is nearly opposite to that of Trpc6-deficient animals previously described. The Trpc6 mutation does not appreciably impact renal interstitial fibrosis in response to either angiotensin II infusion, or folate-induced kidney injury. TRPC6 protein and TRPC6-agonist induced calcium influx could not be detected in glomeruli. In sum, these findings suggest that a gain-of-function Trpc6 mutation confers only a mild susceptibility to glomerular injury in the mouse.

Transcriptional regulatory networks of circulating immune cells in type 1 diabetes: A community knowledgebase

Investigator-generated transcriptomic datasets interrogating circulating immune cell (CIC) gene expression in clinical type 1 diabetes (T1D) have underappreciated re-use value. Here, we repurposed these datasets to create an open science environment for the generation of hypotheses around CIC signaling pathways whose gain or loss of function contributes to T1D pathogenesis. We firstly computed sets of genes that were preferentially induced or repressed in T1D CICs and validated these against community benchmarks. We then inferred and validated signaling node networks regulating expression of these gene sets, as well as differentially expressed genes in the original underlying T1D case:control datasets. In a set of three use cases, we demonstrated how informed integration of these networks with complementary digital resources supports substantive, actionable hypotheses around signaling pathway dysfunction in T1D CICs. Finally, we developed a federated, cloud-based web resource that exposes the entire data matrix for unrestricted access and re-use by the research community.

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Re-use of transcriptomic type 1 diabetes (T1D) circulating immune cells (CICs) datasets

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We generated transcriptional regulatory networks for T1D CICs

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Use cases generate substantive hypotheses around signaling pathway dysfunction in T1D CICs

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Networks are freely accessible on the web for re-use by the research community

Cortex Mori Radicis attenuates streptozotocin-induced diabetic renal injury in mice via regulation of transient receptor potential canonical channel 6

OBJECTIVE

Cortex Mori Radicis (CMR) has been reported to possess anti-pyretic, anti-convulsant, anti-allergic, anti-inflammatory, and anti-diabetic effects. In this study, we aimed to investigate the effect of CMR on streptozotocin (STZ)-induced diabetic renal injury in mice and explore the underlying mechanism.

METHODS

Mice were gavaged with different doses of CMR for continuous 7 days. Then, STZ (50 mg/kg) was applied to induce renal injury associated with type 1 diabetes. Firstly, blood glucose levels and metabolic parameters were evaluated, including weight, food intake, and excrement. HE and PAS staining were performed to present renal histological changes. Renal inflammation, fibrosis, and oxidative stress were assayed by real time PCR and ELISA, separately. Additionally, podocyte-related markers, such as nephrin and wilms' tumor-1 (WT-1) were detected by immunohistochemical staining and Western blot separately. Lastly, expression of transient receptor potential canonical channel 6 (TRPC6) and activation of MAPK signaling pathways were assayed.

RESULTS

CMR pretreatment significantly lowered the blood glucose levels, suppressed renal inflammation, fibrosis and oxidative stress, and relieved renal pathological injury, accompanying the inhibition of nephrin and WT-1 expression in STZ-induced diabetic mice. Moreover, CMR decreased the expression of TRPC6 and suppressed phosphorylation of ERK, but not P38 MAPK and JNK. Notably, the application of hyperforin, a specific activator of TRPC6, significantly abrogated the hypoglycemic effect of CMR and reversed the suppression of CMR on TRPC6 expression and ERK activation in the diabetic mice.

CONCLUSION

Our findings indicated that CMR attenuated early renal injury in STZ-induced diabetic mice through inhibiting ERK signaling via regulation of TRPC6, which suggests that CMR can be considered as a promising candidate for the management of diabetes-related renal complications.

Pharmacology of TRPC Channels and Its Potential in Cardiovascular and Metabolic Medicine

Transient receptor potential canonical (TRPC) proteins assemble to form homo- or heterotetrameric, nonselective cation channels permeable to K⁺, Na⁺, and Ca²⁺. TRPC channels are thought to act as complex integrators of physical and chemical environmental stimuli. Although the understanding of essential physiological roles of TRPC channels is incomplete, their implication in various pathological mechanisms and conditions of the nervous system, kidneys, and cardiovascular system in combination with the lack of major adverse effects of TRPC knockout or TRPC channel inhibition is driving the search of TRPC channel modulators as potential therapeutics. Here, we review the most promising small-molecule TRPC channel modulators, the understanding of their mode of action, and their potential in the study and treatment of cardiovascular and metabolic disease.

Deletion of heterogeneous nuclear ribonucleoprotein F in renal tubules downregulates SGLT2 expression and attenuates hyperfiltration and kidney injury in a mouse model of diabetes

AIMS/HYPOTHESIS

We previously reported that renal tubule-specific deletion of heterogeneous nuclear ribonucleoprotein F (Hnrnpf) results in upregulation of renal angiotensinogen (Agt) and downregulation of sodium-glucose co-transporter 2 (Sglt2) in Hnrnpf^RT knockout (KO) mice. Non-diabetic Hnrnpf^RT KO mice develop hypertension, renal interstitial fibrosis and glycosuria with no renoprotective effect from downregulated Sglt2 expression. Here, we investigated the effect of renal tubular Hnrnpf deletion on hyperfiltration and kidney injury in Akita mice, a model of type 1 diabetes.

METHODS

Akita Hnrnpf^RT KO mice were generated through crossbreeding tubule-specific (Pax8)-Cre mice with Akita floxed-Hnrnpf mice on a C57BL/6 background. Male non-diabetic control (Ctrl), Akita, and Akita Hnrnpf^RT KO mice were studied up to the age of 24 weeks (n = 8/group).

RESULTS

Akita mice exhibited elevated systolic blood pressure as compared with Ctrl mice, which was significantly higher in Akita Hnrnpf^RT KO mice than Akita mice. Compared with Akita mice, Akita Hnrnpf^RT KO mice had lower blood glucose levels with increased urinary glucose excretion. Akita mice developed kidney hypertrophy, glomerular hyperfiltration (increased glomerular filtration rate), glomerulomegaly, mesangial expansion, podocyte foot process effacement, thickened glomerular basement membranes, renal interstitial fibrosis and increased albuminuria. These abnormalities were attenuated in Akita Hnrnpf^RT KO mice. Treatment of Akita Hnrnpf^RT KO mice with a selective A1 adenosine receptor inhibitor resulted in an increase in glomerular filtration rate. Renal Agt expression was elevated in Akita mice and further increased in Akita Hnrnpf^RT KO mice. In contrast, Sglt2 expression was increased in Akita and decreased in Akita Hnrnpf^RT KO mice.

CONCLUSIONS/INTERPRETATION

The renoprotective effect of Sglt2 downregulation overcomes the renal injurious effect of Agt when these opposing factors coexist under diabetic conditions, at least partly via the activation of tubuloglomerular feedback.

Blockade of the natriuretic peptide clearance receptor attenuates proteinuria in a mouse model of focal segmental glomerulosclerosis

Glomerular podocytes play a key role in proteinuric diseases. Accumulating evidence suggests that cGMP signaling has podocyte protective effects. The major source of cGMP generation in podocytes is natriuretic peptides. The natriuretic peptide clearance receptor (NPRC) binds and degrades natriuretic peptides. As a result, NPRC inhibits natriuretic peptide-induced cGMP generation. To enhance cGMP generation in podocytes, we blocked natriuretic peptide clearance using the specific NPRC ligand ANP(4-23). We then studied the effects of NPRC blockade in both cultured podocytes and in a mouse transgenic (TG) model of focal segmental glomerulosclerosis (FSGS) created in our laboratory. In this model, a single dose of the podocyte toxin puromycin aminonucleoside (PAN) causes robust albuminuria in TG mice, but only mild disease in non-TG animals. We found that natriuretic peptides protected cultured podocytes from PAN-induced apoptosis, and that ANP(4-23) enhanced natriuretic peptide-induced cGMP generation in vivo. PAN-induced heavy proteinuria in vehicle-treated TG mice, and this increase in albuminuria was reduced by treatment with ANP(4-23). Treatment with ANP(4-23) also reduced the number of mice with glomerular injury and enhanced urinary cGMP excretion, but these differences were not statistically significant. Systolic BP was similar in vehicle and ANP(4-23)-treated mice. These data suggest that: 1. Pharmacologic blockade of NPRC may be useful for treating glomerular diseases such as FSGS, and 2. Treatment outcomes might be improved by optimizing NPRC blockade to inhibit natriuretic peptide clearance more effectively.

Advances in TRP channel drug discovery: from target validation to clinical studies

Transient receptor potential (TRP) channels are multifunctional signalling molecules with many roles in sensory perception and cellular physiology. Therefore, it is not surprising that TRP channels have been implicated in numerous diseases, including hereditary disorders caused by defects in genes encoding TRP channels (TRP channelopathies). Most TRP channels are located at the cell surface, which makes them generally accessible drug targets. Early drug discovery efforts to target TRP channels focused on pain, but as our knowledge of TRP channels and their role in health and disease has grown, these efforts have expanded into new clinical indications, ranging from respiratory disorders through neurological and psychiatric diseases to diabetes and cancer. In this Review, we discuss recent findings in TRP channel structural biology that can affect both drug development and clinical indications. We also discuss the clinical promise of novel TRP channel modulators, aimed at both established and emerging targets. Last, we address the challenges that these compounds may face in clinical practice, including the need for carefully targeted approaches to minimize potential side-effects due to the multifunctional roles of TRP channels.

Insulin resistance in glomerular podocytes: Potential mechanisms of induction

Glomerular podocytes are a target for the actions of insulin. Accumulating evidence indicates that exposure to nutrient overload induces insulin resistance in these cells, manifested by abolition of the stimulatory effect of insulin on glucose uptake. Numerous recent studies have investigated potential mechanisms of the induction of insulin resistance in podocytes. High glucose concentrations stimulated reactive oxygen species production through NADPH oxidase activation, decreased adenosine monophosphate-activated protein kinase (AMPK) phosphorylation, and reduced deacetylase sirtuin 1 (SIRT1) protein levels and activity. Calcium signaling involving transient receptor potential cation channel C, member 6 (TRPC6) also was demonstrated to play an essential role in the regulation of insulin-dependent signaling and glucose uptake in podocytes. Furthermore, podocytes exposed to diabetic environment, with elevated insulin levels become insulin resistant as a result of degradation of insulin receptor (IR), resulting in attenuation of insulin signaling responsiveness. Also elevated levels of palmitic acid appear to be an important factor and contributor to podocytes insulin resistance. This review summarizes cellular and molecular alterations that contribute to the development of insulin resistance in glomerular podocytes.

A Rare Autosomal Dominant Variant in Regulator of Calcineurin Type 1 (RCAN1) Gene Confers Enhanced Calcineurin Activity and May Cause FSGS

BACKGROUND

Podocyte dysfunction is the main pathologic mechanism driving the development of FSGS and other morphologic types of steroid-resistant nephrotic syndrome (SRNS). Despite significant progress, the genetic causes of most cases of SRNS have yet to be identified.

METHODS

Whole-genome sequencing was performed on 320 individuals from 201 families with familial and sporadic NS/FSGS with no pathogenic mutations in any known NS/FSGS genes.

RESULTS

Two variants in the gene encoding regulator of calcineurin type 1 (RCAN1) segregate with disease in two families with autosomal dominant FSGS/SRNS. In vitro, loss of RCAN1 reduced human podocyte viability due to increased calcineurin activity. Cells expressing mutant RCAN1 displayed increased calcineurin activity and NFAT activation that resulted in increased susceptibility to apoptosis compared with wild-type RCAN1. Treatment with GSK-3 inhibitors ameliorated this elevated calcineurin activity, suggesting the mutation alters the balance of RCAN1 regulation by GSK-3β, resulting in dysregulated calcineurin activity and apoptosis.

CONCLUSIONS

These data suggest mutations in RCAN1 can cause autosomal dominant FSGS. Despite the widespread use of calcineurin inhibitors in the treatment of NS, genetic mutations in a direct regulator of calcineurin have not been implicated in the etiology of NS/FSGS before this report. The findings highlight the therapeutic potential of targeting RCAN1 regulatory molecules, such as GSK-3β, in the treatment of FSGS.

Canonical transient receptor potential channels and their modulators: biology, pharmacology and therapeutic potentials

Canonical transient receptor potential channels (TRPCs) are nonselective, high calcium permeability cationic channels. The TRPCs family includes TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, and TRPC7. These channels are widely expressed in the cardiovascular and nervous systems and exist in many other human tissues and cell types, playing several crucial roles in the human physiological and pathological processes. Hence, the emergence of TRPCs modulators can help investigate these channels’ applications in health and disease. It is worth noting that the TRPCs subfamilies have structural and functional similarities, which presents a significant difficulty in screening and discovering of TRPCs modulators. In the past few years, only a limited number of selective modulators of TRPCs were detected; thus, additional research on more potent and more selective TRPCs modulators is needed. The present review focuses on the striking desired therapeutic effects of TRPCs modulators, which provides intel on the structural modification of TRPCs modulators and further pharmacological research. Importantly, TRPCs modulators can significantly facilitate future studies of TRPCs and TRPCs related diseases.

Critical Role for AMPK in Metabolic Disease-Induced Chronic Kidney Disease

Chronic kidney disease (CKD) is prevalent in 9.1% of the global population and is a significant public health problem associated with increased morbidity and mortality. CKD is associated with highly prevalent physiological and metabolic disturbances such as hypertension, obesity, insulin resistance, cardiovascular disease, and aging, which are also risk factors for CKD pathogenesis and progression. Podocytes and proximal tubular cells of the kidney strongly express AMP-activated protein kinase (AMPK). AMPK plays essential roles in glucose and lipid metabolism, cell survival, growth, and inflammation. Thus, metabolic disease-induced renal diseases like obesity-related and diabetic chronic kidney disease demonstrate dysregulated AMPK in the kidney. Activating AMPK ameliorates the pathological and phenotypical features of both diseases. As a metabolic sensor, AMPK regulates active tubular transport and helps renal cells to survive low energy states. AMPK also exerts a key role in mitochondrial homeostasis and is known to regulate autophagy in mammalian cells. While the nutrient-sensing role of AMPK is critical in determining the fate of renal cells, the role of AMPK in kidney autophagy and mitochondrial quality control leading to pathology in metabolic disease-related CKD is not very clear and needs further investigation. This review highlights the crucial role of AMPK in renal cell dysfunction associated with metabolic diseases and aims to expand therapeutic strategies by understanding the molecular and cellular processes underlying CKD.

Transient Receptor Potential Canonical (TRPC) Channels: Then and Now

Twenty-five years ago, the first mammalian Transient Receptor Potential Canonical (TRPC) channel was cloned, opening the vast horizon of the TRPC field. Today, we know that there are seven TRPC channels (TRPC1-7). TRPCs exhibit the highest protein sequence similarity to the Drosophila melanogaster TRP channels. Similar to Drosophila TRPs, TRPCs are localized to the plasma membrane and are activated in a G-protein-coupled receptor-phospholipase C-dependent manner. TRPCs may also be stimulated in a store-operated manner, via receptor tyrosine kinases, or by lysophospholipids, hypoosmotic solutions, and mechanical stimuli. Activated TRPCs allow the influx of Ca2+ and monovalent alkali cations into the cytosol of cells, leading to cell depolarization and rising intracellular Ca2+ concentration. TRPCs are involved in the continually growing number of cell functions. Furthermore, mutations in the TRPC6 gene are associated with hereditary diseases, such as focal segmental glomerulosclerosis. The most important recent breakthrough in TRPC research was the solving of cryo-EM structures of TRPC3, TRPC4, TRPC5, and TRPC6. These structural data shed light on the molecular mechanisms underlying TRPCs' functional properties and propelled the development of new modulators of the channels. This review provides a historical overview of the major advances in the TRPC field focusing on the role of gene knockouts and pharmacological tools.

Tacrolimus ameliorates tubulointerstitial inflammation in diabetic nephropathy via inhibiting the NFATc1/TRPC6 pathway

Tubulointerstitial inflammation is crucial for the progression of diabetic nephropathy (DN), and tubular cells act as a driving force in the inflammatory cascade. Emerging data suggested that tacrolimus (TAC) ameliorates podocyte injury and macrophage infiltration in streptozotocin (STZ) mice. However, the effect of TAC on tubulointerstitial inflammation remains unknown. We found that albuminuria and tubulointerstitial damage improved in db/db mice treated with TAC. Macrophage infiltration and expression of IL‐6, TNF‐α, fibronectin, collagen 1 and cleaved caspase 3 were inhibited as well. In addition, the expression of nuclear factor of activated T cell 1 (NFATc1) and transient receptor potential channel 6 (TRPC6) was up‐regulated in the kidneys of DN patients and correlated with tubular injury and inflammation. The expression of NFATc1 and TRPC6 also increased in the kidneys of db/db mice and HK‐2 cells with high glucose (HG), while TAC inhibited these effects. HG‐induced inflammatory markers and apoptosis were reversed by TAC and NFATc1 siRNA in HK‐2 cells, which was abolished by TRPC6 plasmid. Furthermore, HG‐induced TRPC6 expression was inhibited by NFATc1 siRNA, while NFATc1 nuclear translocation was inhibited by TAC, but was restored by TRPC6 plasmid in HK‐2 cells under HG conditions. These findings suggest that TAC ameliorates tubulointerstitial inflammation in DN through NFATc1/TRPC6 feedback loop.

Comparison of the effects of insulin and SGLT2 inhibitor on the Renal Renin-Angiotensin system in type 1 diabetes mice

AIMS

SGLT2 inhibitors have been proposed as an adjunct to insulin therapy for glycemic control in type 1 diabetes (T1D) patients. However, concern has been raised due to an increase in renin-angiotensin-system (RAS) activity reported in a clinical trial in which an SGLT2 inhibitor was added while insulin dose was reduced in T1D patients. We previously reported that insulin inhibits intrarenal angiotensinogen (Agt) gene transcription and RAS activation. We hypothesized that insulin, rather than SGLT2 inhibition might regulate the intrarenal RAS.

METHODS

We compared RAS activity in non-diabetic wild type mice, Akita mice (T1D model) and Akita mice treated with insulin or the SGLT2 inhibitor canagliflozin.

RESULTS

Treatment of Akita mice with insulin or canagliflozin produced similar reductions in blood glucose, whereas insulin, but not canagliflozin, reduced elevated systolic blood pressure. Akita mice exhibited increased renal Agt mRNA/protein expression, which was attenuated by insulin, but not by canagliflozin. Furthermore, insulin was more effective than canagliflozin in lowering kidney weight and albuminuria.

CONCLUSIONS

Insulin, but not canagliflozin, lowers intrarenal RAS activity in Akita mice. Our findings can be of potential clinical importance, especially for T1D patients who are not on RAS inhibitors at the time of adding SGLT2 inhibitors.

Role of Transient Receptor Potential Canonical Channel 6 (TRPC6) in Diabetic Kidney Disease by Regulating Podocyte Actin Cytoskeleton Rearrangement

Podocyte injury is an important pathogenesis step causing proteinuric kidney diseases such as diabetic kidney disease (DKD). Actin cytoskeleton rearrangement in podocyte induced by multiple pathogenic factors is believed to be the key process resulting in glomerular injury. Many studies have recently shown that transient receptor potential canonical channel 6 (TRPC6) in podocyte plays a critical role in the development and progression of proteinuric kidney disease by regulating its actin cytoskeleton rearrangement. This review is aimed at summarizing the role of TRPC6 on DKD by regulating the podocyte actin cytoskeleton rearrangement, thereby help further broaden our views and understanding on the mechanism of DKD and provide a theoretic basis for exploring new therapeutic targets for DKD patients.

Caspase-3 Promotes Diabetic Kidney Disease Through Gasdermin E-Mediated Progression to Secondary Necrosis During Apoptosis

BACKGROUND

Apoptosis has been repeatedly linked with diabetic kidney disease (DKD), which is a programmed cell death mediated by effector caspases-3, 6 and 7, targeting >600 substrates. However, the pathophysiologic correlations of this process remain obscure. As a putative tumor suppressor, gasdermin E (GSDME) was recently reported to be cleaved by caspase-3 to produce a GSDME-N fragment which targets the plasma membrane to switch apoptosis to secondary necrosis. However, it remains elusive whether GSDME is involved in the regulation of DKD.

METHODS

To evaluate the therapeutic potential of caspase-3 inhibition in DKD, we administered caspase-3 inhibitor Z-DEVD-FMK to STZ-induced diabetic mice for eight weeks. Albuminuria, renal function, pathological changes, and indicators of secondary necrosis and fibrosis were evaluated. In vitro, human tubule epithelial cells (HK-2 cells) were subjected to high-glucose treatment. Secondary necrosis was determined by LDH release, GSDME cleavage, and morphological feature under confocal microscopy. Z-DEVD-FMK and GSDME inhibition by shRNA were administered to suppress the cleavage and expression of GSDME. Flow cytometry, cytotoxicity assay and immunoblot were used to assess cell death and fibrogenesis.

RESULTS

Caspase-3 inhibition by Z-DEVD-FMK ameliorated albuminuria, renal function, and tubulointerstitial fibrosis in diabetic mice. The nephroprotection mediated by Z-DEVD-FMK was potentially associated with inhibition of GSDME. In vitro, molecular and morphological features of secondary necrosis were observed in glucose-stressed HK-2 cells, evidenced by active GSDME cleavage, ballooning of the cell membrane, and release of cellular contents. Here we showed that caspase-3 inhibition prevented GSDME activation and cell death in glucose-treated tubular cells. Specifically, knocking down GSDME directly inhibited secondary necrosis and fibrogenesis.

CONCLUSION

These data suggest GSDME-dependent secondary necrosis plays a crucial role in renal injury, and provides a new insight into the pathogenesis of DKD and a promising target for its treatment.

TRPC Channels in Proteinuric Kidney Diseases

Over a decade ago, mutations in the gene encoding TRPC6 (transient receptor potential cation channel, subfamily C, member 6) were linked to development of familial forms of nephrosis. Since this discovery, TRPC6 has been implicated in the pathophysiology of non-genetic forms of kidney disease including focal segmental glomerulosclerosis (FSGS), diabetic nephropathy, immune-mediated kidney diseases, and renal fibrosis. On the basis of these findings, TRPC6 has become an important target for the development of therapeutic agents to treat diverse kidney diseases. Although TRPC6 has been a major focus for drug discovery, more recent studies suggest that other TRPC family members play a role in the pathogenesis of glomerular disease processes and chronic kidney disease (CKD). This review highlights the data implicating TRPC6 and other TRPC family members in both genetic and non-genetic forms of kidney disease, focusing on TRPC3, TRPC5, and TRPC6 in a cell type (glomerular podocytes) that plays a key role in proteinuric kidney diseases.

Small Fluorescein Arsenical Hairpin-Based Förster Resonance Energy Transfer Analysis Reveals Changes in Amino- to Carboxyl-Terminal Interactions upon OAG Activation of Classical Transient Receptor Potential 6

Although the overall structure of many classical transient receptor potential proteins (TRPC), including human and murine TRPC6, were recently resolved by cryoelectron microscopy analysis, structural changes during channel activation by 1-oleoyl-1-acetyl-sn-glycerol (OAG), the membrane-permeable analog of diacylglycerol, were not defined. Moreover, data on carboxyl- and amino-terminal interactions were not provided, as the amino-terminal regions of murine and human TRPC6 were not resolved. Therefore, we employed a Förster resonance energy transfer (FRET) approach using a small fluorescein arsenical hairpin (FlAsH) targeted to a short tetracysteine sequence at the unresolved amino-terminus and cerulean, a cyan fluorescent protein, as a tag at the carboxyl-terminus of the murine TRPC6 protein. After OAG as well as GSK-1702934A activation, FRET efficiency was simultaneously and significantly reduced, indicating a decreased interaction between the amino to carboxyl termini in the functional tagged murine TRPC6 tetramer (TRPC6 WT) heterologously expressed in human embryonic kidney 293T cells. There was a significant reduction in the FRET signal obtained from analysis of murine TRPC6 FRET constructs with homologous amino-terminal mutations (M131T, G108S) that had been identified in human patients with inherited focal segmental glomerulosclerosis, a condition that can lead to end-stage renal disease. A novel, designed loss-of-function TRPC6 mutation (N109A) in the amino-terminus in close proximity to the carboxyl-terminus produced similar FRET ratios. SIGNIFICANCE STATEMENT: Our data show for the first time that FlAsH-tagging of ion channels is a promising tool to study conformational changes after channel opening and may significantly advance the analysis of ion channel activation as well as their mutants involved in channelopathies.

TRPC6 inactivation does not protect against diabetic kidney disease in streptozotocin (STZ)-treated Sprague-Dawley rats

Canonical transient receptor potential-6 (TRPC6) channels have been implicated in the progression of several forms of kidney disease (1). While there is strong evidence that glomerular TRPC6 channels are dysregulated in diabetic nephropathy (DN), there is no consensus as to whether deletion or inactivation of TRPC6 is protective in animal models of DN. A previous study in Dahl salt-sensitive rats suggests that TRPC6 knockout has a modest protective effect in streptozotocin (STZ)-induced DN (2). In the present study, we examined whether inactivation of TRPC6 channels by CRISPR/Cas9 editing (Trpc6 del/del rats) affects progression of STZ-induced DN in Sprague-Dawley rats. Wild-type littermates (Trpc6 wt/wt rats) were used as controls. We observed that a single injection of STZ resulted in severe hyperglycemia that was sustained over a 10-week period, accompanied by a marked reduction in circulating C-peptide, dyslipidemia, and failure to gain weight compared to vehicle-treated animals. Those effects were equally severe in Trpc6 wt/wt and Trpc6 del/del rats. STZ treatment resulted in increased urine albumin excretion at 4, 8, and 10 weeks after injection, and this effect was equally severe in Trpc6 wt/wt and Trpc6 del/del rats. TRPC6 inactivation had no effect on blood urea nitrogen (BUN), plasma creatinine concentration, urine nephrin excretion, or kidney weight:body weight ratio measured 10 weeks after STZ injection. STZ treatment evoked modest and equivalent mesangial expansion in Trpc6 wt/wt and Trpc6 del/del rats. In summary, we observed no protective effect of TRPC6 inactivation on STZ-induced DN in rats on the Sprague-Dawley background.

Charting a TRP to Novel Therapeutic Destinations for Kidney Diseases

Ion channels are critical to kidney function, and their dysregulation leads to several distinct kidney diseases. Of the diversity of ion channels in kidney cells, the transient receptor potential (TRP) superfamily of proteins plays important and varied roles in both maintaining homeostasis as well as in causing disease. Recent work showed that TRPC5 blockers could successfully protect critical components of the kidney filter both in vitro and in vivo, thus revealing TRPC5 as a tractable therapeutic target for focal and segmental glomerulosclerosis (FSGS), a common cause of kidney failure. Human genetics point to three additional TRP channels as plausible therapeutic targets: TRPC6 in FSGS, PKD2 in polycystic kidney disease, and TRPM6 in familial hypomagnesemia with secondary hypocalcemia (HSH). We conclude that targeting TRP channels could pave the way for much needed therapies for kidney diseases.

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.

TRPC6 in diabetic kidney disease: good guy or bad guy?

An association of altered activity of transient receptor potential cation channel C6 (TRPC6) with the development of various diabetic complications was previously reported. In this issue, Wang et al. tested the contribution of TRPC6 in type 1 diabetes setting by breeding Akita mice with TRPC6 knockout mice. The authors revealed that the knockout of TRPC6 decreased proteinuria and attenuated tubule injury but promoted mesangial expansion in Akita mice. Therefore, TRPC6 has both beneficial and detrimental effects on glomeruli function.

Role of TRPC6 in Progression of Diabetic Kidney Disease

PURPOSE OF REVIEW

The underlining goal of this review is to offer a concise, detailed look into current knowledge surrounding transient receptor potential canonical channel 6 (TRPC6) in the progression of diabetic kidney disease (DKD).

RECENT FINDINGS

Mutations and over-activation in TRPC6 channel activity lead to the development of glomeruli injury. Angiotensin II, reactive oxygen species, and other factors in the setting of DKD stimulate drastic increases in calcium influx through the TRPC6 channel, causing podocyte hypertrophy and foot process effacement. Loss of the podocytes further promote deterioration of the glomerular filtration barrier and play a major role in the development of both albuminuria and the renal injury in DKD. Recent genetic manipulation with TRPC6 channels in various rodent models provide additional knowledge about the role of TRPC6 in DKD and are reviewed here. The TRPC6 channel has a pronounced role in the progression of DKD, with deviations in activity yielding detrimental outcomes. The benefits of targeting TRPC6 or its upstream or downstream signaling pathways in DKD are prominent.

TRPC channels: Regulation, dysregulation and contributions to chronic kidney disease

Mutations in the gene encoding canonical transient receptor potential-6 (TRPC6) channels result in severe nephrotic syndromes that typically lead to end-stage renal disease. Many but not all of these mutations result in a gain in the function of the resulting channel protein. Since those observations were first made, substantial work has supported the hypothesis that TRPC6 channels can also contribute to progression of acquired (non-genetic) glomerular diseases, including primary and secondary FSGS, glomerulosclerosis during autoimmune glomerulonephritis, and possibly in type-1 diabetes. Their regulation has been extensively studied, especially in podocytes, but also in mesangial cells and other cell types present in the kidney. More recent evidence has implicated TRPC6 in renal fibrosis and tubulointerstitial disease caused by urinary obstruction. Consequently TRPC6 is being extensively investigated as a target for drug discovery. Other TRPC family members are present in kidney. TRPC6 can form a functional heteromultimer with TRPC3, and it has been suggested that TRPC5 may also play a role in glomerular disease progression, although the evidence on this is contradictory. Here we review literature on the expression and regulation of TRPC6, TRPC3 and TRPC5 in various cell types of the vertebrate kidney, the evidence that these channels are dysregulated in disease models, and research showing that knock-out or pharmacological inhibition of these channels can reduce the severity of kidney disease. We also summarize several areas that remain controversial, and some of the large gaps of knowledge concerning the fundamental role of these proteins in regulation of renal function.

Ion channels and transporters in diabetic kidney disease

Type 1 and 2 diabetes mellitus are major medical epidemics affecting millions of patients worldwide. Diabetes mellitus is the leading cause of diabetic kidney disease (DKD), which is the most common cause of end-stage renal disease (ESRD). DKD is associated with significant changes in renal hemodynamics and electrolyte transport. Alterations in renal ion transport triggered by pathophysiological conditions in diabetes can exacerbate hypertension, accelerate renal injury, and are integral to the development of DKD. Renal ion transporters and electrolyte homeostasis play a fundamental role in functional changes and injury to the kidney during DKD. With the large number of ion transporters involved in DKD, understanding the roles of individual transporters as well as the complex cascades through which they interact is essential in the development of effective treatments for patients suffering from this disease. This chapter aims to gather current knowledge of the major renal ion transporters with altered expression and activity under diabetic conditions, and provide a comprehensive overview of their interactions and collective functions in DKD.