Identification and localization of TRPC channels in the rat kidney

Calcium signalling and transport in the kidney

The kidney plays a pivotal role in regulating calcium levels within the body. Approximately 98% of the filtered calcium is reabsorbed in the nephron, and this process is tightly controlled to maintain calcium homeostasis, which is required to facilitate optimal bone mineralization, preserve serum calcium levels within a narrow range, and support intracellular signalling mechanisms. The maintenance of these functions is attributed to a delicate balance achieved by various calcium channels, transporters, and calcium-binding proteins in renal cells. Perturbation of this balance due to deficiency or dysfunction of calcium channels and calcium-binding proteins can lead to severe complications. For example, polycystic kidney disease is linked to aberrant calcium transport and signalling. Furthermore, dysregulation of calcium levels can promote the formation of kidney stones. This Review provides an updated description of the key aspects of calcium handling in the kidney, focusing on the function of various calcium channels and the physiological stimuli that control these channels or are communicated through them. A discussion of the role of calcium as an intracellular second messenger and the pathophysiology of renal calcium dysregulation, as well as a summary of gaps in knowledge and future prospects, are also included.

Hydrogen sulfide induces Ca2+ influx in the principal cells of rat cortical collecting ducts

Hydrogen sulfide (H2S) acts as a gas-signaling agent in various tissues. Although it has been reported that endogenous enzymes that generate H2S are expressed abundantly in the kidney, few reports examine cellular responses to H2S in renal tubular epithelial cells. In this study, we investigated the effects of NaHS, an H2S donor, and l-cysteine, a substrate for H2S production, on the principal cells of rat cortical collecting ducts (CCDs). NaHS increased the intracellular Ca2+ concentration ([Ca2+]i) in the principal cells. The removal of extracellular Ca2+ largely attenuated the [Ca2+]i response. The TRPV4 channel blocker significantly inhibited the effect of NaHS. Extracellular administration of l-cysteine also elicited a rise in [Ca2+]i. Prior treatment of CCDs with AOAA, an inhibitor of H2S production enzyme, l-cysteine-induced [Ca2+]i response was significantly reduced. These results suggest that not only exogenous H2S but also endogenously produced H2S triggers the extracellular influx pathway of Ca2+ in the principal cells of rat CCDs.

New insights in the renal distribution profile of TRPC3 - Of mice and men

Several reports previously investigated the Transient Receptor Potential Canonical subfamily channel 3 (TRPC3) in the kidney. However, most of the conclusions are based on animal samples or cell cultures leaving the door open for human tissue investigations. Moreover, results often disagreed among investigators. Histological description is lacking since most of these studies focused on functional aspects. Nevertheless, the same reports highlighted the potential key-role of TRPC3 in renal disorders. Hence, our interest to investigate the localization of TRPC3 in human kidneys. For this purpose, both healthy mouse and human kidney samples that were originated from tumor nephrectomies have been prepared for immunohistochemical staining using a knockout-validated antibody. A blocking peptide was used to confirm antibody specificity. A normalized weighted diaminobenzidine (DAB) area score between 0 and 3 comparable to a pixelwise H-score was established and employed for semiquantitative analysis. Altogether, our results suggest that glomeruli only express little TRPC3 compared to several segments of the tubular system. Cortical and medullary proximal tubules are stained, although intracortical differences in staining exist in mice. Intermediate tubules, however, are only weakly stained. The distal tubule was studied in three localizations and staining was marked although slightly varying throughout the different subsegments. Finally, the collecting duct was also immunolabeled in both human and mouse tissue. We therefore provide evidence that TRPC3 is expressed in various localizations of both human and mouse samples. We verify results of previous studies and propose until now undescribed localizations of TRPC3 in the mouse but especially and of greater interest in the human kidney. We thereby not only support the translational concept of the TRPC3 channel as key-player in physiology and pathophysiology of the human kidney but also present new potential targets to functional analysis.

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.

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.

TRPC Channels in the Physiology and Pathophysiology of the Renal Tubular System: What Do We Know?

The study of transient receptor potential (TRP) channels has dramatically increased during the past few years. TRP channels function as sensors and effectors in the cellular adaptation to environmental changes. Here, we review literature investigating the physiological and pathophysiological roles of TRPC channels in the renal tubular system with a focus on TRPC3 and TRPC6. TRPC3 plays a key role in Ca²⁺ homeostasis and is involved in transcellular Ca²⁺ reabsorption in the proximal tubule and the collecting duct. TRPC3 also conveys the osmosensitivity of principal cells of the collecting duct and is implicated in vasopressin-induced membrane translocation of AQP-2. Autosomal dominant polycystic kidney disease (ADPKD) can often be attributed to mutations of the PKD2 gene. TRPC3 is supposed to have a detrimental role in ADPKD-like conditions. The tubule-specific physiological functions of TRPC6 have not yet been entirely elucidated. Its pathophysiological role in ischemia-reperfusion injuries is a subject of debate. However, TRPC6 seems to be involved in tumorigenesis of renal cell carcinoma. In summary, TRPC channels are relevant in multiples conditions of the renal tubular system. There is a need to further elucidate their pathophysiology to better understand certain renal disorders and ultimately create new therapeutic targets to improve patient care.

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.

Hypercalciuria switches Ca2+ signaling in proximal tubular cells, induces oxidative damage to promote calcium nephrolithiasis

Proximal tubule (PT) transports most of the renal Ca2+, which was usually described as paracellular (passive). We found a regulated Ca2+ entry pathway in PT cells via the apical transient receptor potential canonical 3 (TRPC3) channel, which initiates transcellular Ca2+ transport. Although TRPC3 knockout (-/-) mice were mildly hypercalciuric and displayed luminal calcium phosphate (CaP) crystals at Loop of Henle (LOH), no CaP + calcium oxalate (CaOx) mixed urine crystals were spotted, which are mostly found in calcium nephrolithiasis (CaNL). Thus, we used oral calcium gluconate (CaG; 2%) to raise the PT luminal [Ca2+]o further in TRPC3 -/- mice for developing such mixed stones to understand the mechanistic role of PT-Ca2+ signaling in CaNL. Expectedly, CaG-treated mice urine samples presented with numerous mixed crystals with remains of PT cells, which were pronounced in TRPC3 -/- mice, indicating PT cell damage. Notably, PT cells from CaG-treated groups switched their mode of Ca2+ entry from receptor-operated to store-operated pathway with a sustained rise in intracellular [Ca2+] ([Ca2+]i), indicating the stagnation in PT Ca2+ transport. Moreover, those PT cells from CaG-treated groups demonstrated an upregulation of calcification, inflammation, fibrotic, oxidative stress, and apoptotic genes; effects of which were more robust in TRPC3 ablated condition. Furthermore, kidneys from CaG-treated groups exhibited fibrosis, tubular injury and calcifications with significant reactive oxygen species generation in the urine, thus, indicating in vivo CaNL. Taken together, excess PT luminal Ca2+ due to escalation of hypercalciuria in TRPC3 ablated mice induced surplus CaP crystal formation and caused stagnation of PT [Ca2+]i, invoking PT cell injury, hence mixed stone formation.

Renal cell markers: lighthouses for managing renal diseases

Kidneys, one of the vital organs in our body, are responsible for maintaining whole body homeostasis. The complexity of renal function (e.g., filtration, reabsorption, fluid and electrolyte regulation, and urine production) demands diversity not only at the level of cell types but also in their overall distribution and structural framework within the kidney. To gain an in depth molecular-level understanding of the renal system, it is imperative to discern the components of kidney and the types of cells residing in each of the subregions. Recent developments in labeling, tracing, and imaging techniques have enabled us to mark, monitor, and identify these cells in vivo with high efficiency in a minimally invasive manner. In this review, we summarize different cell types, specific markers that are uniquely associated with those cell types, and their distribution in the kidney, which altogether make kidneys so special and different. Cellular sorting based on the presence of certain proteins on the cell surface allowed for the assignment of multiple markers for each cell type. However, different studies using different techniques have found contradictions in cell type-specific markers. Thus, the term "cell marker" might be imprecise and suboptimal, leading to uncertainty when interpreting the data. Therefore, we strongly believe that there is an unmet need to define the best cell markers for a cell type. Although the compendium of renal-selective marker proteins presented in this review is a resource that may be useful to researchers, we acknowledge that the list may not be necessarily exhaustive.

Reno-Protective Effect of GLP-1 Receptor Agonists in Type1 Diabetes: Dual Action on TRPC6 and NADPH Oxidases

Diabetic kidney disease (DKD), a serious diabetic complication, results in podocyte loss and proteinuria through NADPH oxidases (NOX)-mediated ROS production. DUOX1 and 2 are NOX enzymes that require calcium for their activation which enters renal cells through the pivotal TRPC channels. Hypoglycemic drugs such as liraglutide can interfere with this deleterious mechanism imparting reno-protection. Herein, we aim to investigate the reno-protective effect of GLP1 receptor agonist (GLP1-RA), via its effect on TRPC6 and NADPH oxidases. To achieve our aim, control or STZ-induced T1DM Sprague-Dawley rats were used. Rats were treated with liraglutide, metformin, or their combination. Functional, histological, and molecular parameters of the kidneys were assessed. Our results show that treatment with liraglutide, metformin or their combination ameliorates DKD by rectifying renal function tests and protecting against fibrosis paralleled by restored mRNA levels of nephrin, DUOX1 and 2, and reduced ROS production. Treatment with liraglutide reduces TRPC6 expression, while metformin treatment shows no effect. Furthermore, TRPC6 was found to be directly interacting with nephrin, and indirectly interacting with DUOX1, DUOX2 and GLP1-R. Our findings suggest that treatment with liraglutide may prevent the progression of diabetic nephropathy by modulating the crosstalk between TRPC6 and NADPH oxidases.

Investigation of the effect of hyperthyroidism on endoplasmic reticulum stress and tran- sient receptor potential canonical 1 channel in the kidney

Background/aim

Hyperthyroidism is associated with results in increased glomerular filtration rate as well as increased renin-angio- tensin-aldosterone activation. The disturbance of Ca2+ homeostasis in the endoplasmic reticulum (ER) is associated with many diseases, including diabetic nephropathy and hyperthyroidism. Transient receptor potential canonical 1 (TRPC1) channel is the first cloned TRPC family protein. Although it is expressed in many places in the kidney, its function is uncertain. TRPC1 is involved in regulating Ca2+ homeostasis, and its upregulation increases ER Ca2+ level, activates the unfolded protein response, which leads to cellular damage in the kidney. This study investigated the role of TRPC1 in the kidneys of hyperthyroid rats in terms of ER stress markers that are gluco- se-regulated protein 78 (GRP78), activating transcription factor 6 (ATF6), (protein kinase R (PKR)-like endoplasmic reticulum kinase) (PERK), Inositol-requiring enzyme 1 (IRE1).

Materials and methods

Twenty male rats were assigned into control and hyperthyroid groups (n = 10). Hyperthyroidism was induced by adding 12 mg/L thyroxine into the drinking water of rats for 4 weeks. The serum-free T3 and T4 (fT3, fT4), TSH, blood urea nitrogen (BUN), and creatinine levels were measured. The histochemical analysis of kidney sections for morphological changes and also im- munohistochemical and western blot analysis of kidney sections were performed for GRP78, ATF6, PERK, IRE1, TRPC1 antibodies.

Results

TSH, BUN, and creatinine levels decreased while fT3 and fT4 levels increased in the hyperthyroid rat. The morphologic analy- sis resulted in the capillary basal membrane thickening in glomeruli and also western blot, and immunohistochemical results showed an increase in TRPC1, GRP78, and ATF6 in the hyperthyroid rat (p < 0.05).

Conclusion

In conclusion, in our study, we showed for the first time that the relationship between ER stress and TRPC1, and their increased expression caused renal damage in hyperthyroid rats.

Polymodal roles of TRPC3 channel in the kidney

TRPC3 is a Ca²⁺-permeable cation channel commonly activated by the G-protein coupled receptors (GPCR) and mechanical distortion of the plasma membrane. TRPC3-mediated Ca²⁺ influx has been implicated in a variety of signaling processes in both excitable and non-excitable cells. Kidneys play a commanding role in maintaining whole-body homeostasis and setting blood pressure. TRPC3 is expressed abundantly in the renal vasculature and in epithelial cells, where it is well positioned to mediate signaling and transport functions in response to GPCR-dependent endocrine stimuli. In addition, TRPC3 could be activated by mechanical forces resulting from dynamic changes in the renal tubule fluid flow and osmolarity. This review critically analyzes the available published evidence of the physiological roles of TRPC3 in different parts of the kidney and describes the pathophysiological ramifications of TRPC3 ablation. We also speculate how this evidence could be further translated into the clinic.

Protective effects of dapagliflozin against oxidative stress-induced cell injury in human proximal tubular cells

Elevated reactive oxygen species (ROS) in type 2 diabetes cause cellular damage in many organs. Recently, the new class of glucose-lowering agents, SGLT-2 inhibitors, have been shown to reduce the risk of developing diabetic complications; however, the mechanisms of such beneficial effect are largely unknown. Here we aimed to investigate the effects of dapagliflozin on cell proliferation and cell death under oxidative stress conditions and explore its underlying mechanisms. Human proximal tubular cells (HK-2) were used. Cell growth and death were monitored by cell counting, water-soluble tetrazolium-1 (WST-1) and lactate dehydrogenase (LDH) assays, and flow cytometry. The cytosolic and mitochondrial (ROS) production was measured using fluorescent probes (H2DCFDA and MitoSOX) under normal and oxidative stress conditions mimicked by addition of H₂O₂. Intracellular Ca²⁺ dynamics was monitored by FlexStation 3 using cell-permeable Ca²⁺ dye Fura-PE3/AM. Dapagliflozin (0.1–10 μM) had no effect on HK-2 cell proliferation under normal conditions, but an inhibitory effect was seen at an extreme high concentration (100 μM). However, dapagliflozin at 0.1 to 5 μM showed remarkable protective effects against H₂O₂-induced cell injury via increasing the viable cell number at phase G0/G1. The elevated cytosolic and mitochondrial ROS under oxidative stress was significantly decreased by dapagliflozin. Dapagliflozin increased the basal intracellular [Ca²⁺]_i in proximal tubular cells, but did not affect calcium release from endoplasmic reticulum and store-operated Ca²⁺ entry. The H₂O₂-sensitive TRPM2 channel seemed to be involved in the Ca²⁺ dynamics regulated by dapagliflozin. However, dapagliflozin had no direct effects on ORAI1, ORAI3, TRPC4 and TRPC5 channels. Our results suggest that dapagliflozin shows anti-oxidative properties by reducing cytosolic and mitochondrial ROS production and altering Ca²⁺ dynamics, and thus exerts its protective effects against cell damage under oxidative stress environment.

Store-operated calcium entry: Pivotal roles in renal physiology and pathophysiology

Research conducted over the last two decades has dramatically advanced the understanding of store-operated calcium channels (SOCC) and their impact on renal function. Kidneys contain many types of cells, including those specialized for glomerular filtration (fenestrated capillary endothelium, podocytes), water and solute transport (tubular epithelium), and regulation of glomerular filtration and renal blood flow (vascular smooth muscle cells, mesangial cells). The highly integrated function of these myriad cells effects renal control of blood pressure, extracellular fluid volume and osmolality, electrolyte balance, and acid-base homeostasis. Many of these cells are regulated by Ca²⁺ signaling. Recent evidence demonstrates that SOCCs are major Ca²⁺ entry portals in several renal cell types. SOCC is activated by depletion of Ca²⁺ stores in the sarco/endoplasmic reticulum, which communicates with plasma membrane SOCC via the Ca²⁺ sensor Stromal Interaction Molecule 1 (STIM1). Orai1 is recognized as the main pore-forming subunit of SOCC in the plasma membrane. Orai proteins alone can form highly Ca²⁺ selective SOCC channels. Also, members of the Transient Receptor Potential Canonical (TRPC) channel family are proposed to form heteromeric complexes with Orai1 subunits, forming SOCC with low Ca²⁺ selectivity. Recently, Ca²⁺ entry through SOCC, known as store-operated Ca²⁺ entry (SOCE), was identified in glomerular mesangial cells, tubular epithelium, and renovascular smooth muscle cells. The physiological and pathological relevance and the characterization of SOCC complexes in those cells are still unclear. In this review, we summarize the current knowledge of SOCC and their roles in renal glomerular, tubular and vascular cells, including studies from our laboratory, emphasizing SOCE regulation of fibrotic protein deposition. Understanding the diverse roles of SOCE in different renal cell types is essential, as SOCC and its signaling pathways are emerging targets for treatment of SOCE-related diseases.

Electrolyte handling in the isolated perfused rat kidney: demonstration of vasopressin V2-receptor-dependent calcium reabsorption

BACKGROUND

The most profound effect of vasopressin on the kidney is to increase water reabsorption through V₂-receptor (V₂R) stimulation, but there are also data suggesting effects on calcium transport. To address this issue, we have established an isolated perfused kidney model with accurate pressure control, to directly study the effects of V₂R stimulation on kidney function, isolated from systemic effects.

METHODS

The role of V₂R in renal calcium handling was studied in isolated rat kidneys using a new pressure control system that uses a calibration curve to compensate for the internal pressure drop up to the tip of the perfusion cannula.

RESULTS

Kidneys subjected to V₂R stimulation using desmopressin (DDAVP) displayed stable osmolality and calcium reabsorption throughout the experiment, whereas kidneys not administered DDAVP exhibited a simultaneous fall in urine osmolality and calcium reabsorption. Epithelial sodium channel (ENaC) inhibition using amiloride resulted in a marked increase in potassium reabsorption along with decreased sodium reabsorption.

CONCLUSIONS

A stable isolated perfused kidney model with computer-controlled pressure regulation was developed, which retained key physiological functions. The preparation responds to pharmacological inhibition of ENaC channels and activation of V₂R. Using the model, the dynamic effects of V₂R stimulation on calcium handling and urine osmolality could be visualised. The study thereby provides evidence for a stimulatory role of V₂R in renal calcium reabsorption.

Nephrotoxicity and genotoxicity of silver nanoparticles in juvenile rats and possible mechanisms of action

Because of their widespread use and potential adverse effects in young developing organism, this study focused on the nephrotoxicity and genotoxicity of chronic low-dose exposure to silver nanoparticles (AgNPs) in 32 14-day-old male Wistar rats, randomly divided into three groups receiving AgNP solution (3 mg/kg body weight) intraperitoneally for one, two, or three weeks and the untreated control group (eight animals per group). When the rats were eight weeks old, blood creatinine and urine microalbumin were tested, followed by haematoxylin and eosin (H&E) staining. Proteinuria was found in the animals treated with AgNP for three weeks, and H&E staining revealed pathological changes in the kidney sections of this group. DNA damage was detected with the alkaline comet assay in the groups treated for two and three weeks. All results indicate that chronic exposure, even at a low dose, may affect animal health. The main culprit might be increased and time-dependent reactive oxygen species (ROS) production. Highly reactive ROS could cause a major structural damage to proteins and DNA, change the expression of ion channel proteins, and trigger inflammation. The findings of our in vivo experiment raise concern about nephrotoxic and genotoxic effects of silver nanoparticles in young organisms and call for further investigation of nanoparticle properties that can be modified to minimise the risks.

RNA-seq analysis reveals TRPC genes to impact an unexpected number of metabolic and regulatory pathways

The seven-member transient receptor potential canonical genes (TRPC1-7) encode cation channels linked to several human diseases. There is little understanding of the participation of each TRPC in each pathology, considering functional redundancy. Also, most of the inhibitors available are not specific. Thus, we developed mice that lack all of the TRPCs and performed a transcriptome analysis in eight tissues. The aim of this research was to address the impact of the absence of all TRPC channels on gene expression. We obtained a total of 4305 differentially expressed genes (DEGs) in at least one tissue where spleen showed the highest number of DEGs (1371). Just 21 genes were modified in all the tissues. Performing a pathway enrichment analysis, we found that many important signaling pathways were modified in more than one tissue, including PI3K (phosphatidylinositol 3-kinase/protein kinase-B) signaling pathway, cytokine-cytokine receptor interaction, extracellular matrix (ECM)-receptor interaction and circadian rhythms. We describe for the first time the changes at the transcriptome level due to the lack of all TRPC proteins in a mouse model and provide a starting point to understand the function of TRPC channels and their possible roles in pathologies.

TRPC channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

TRPC3 determines osmosensitive [Ca2+]i signaling in the collecting duct and contributes to urinary concentration

It is well-established that the kidney collecting duct (CD) plays a central role in regulation of systemic water homeostasis. Aquaporin 2 (AQP2)-dependent water reabsorption in the CD critically depends on the arginine vasopressin (AVP) antidiuretic input and the presence of a favorable osmotic gradient at the apical plasma membrane with tubular lumen being hypotonic compared to the cytosol. This osmotic difference creates a mechanical force leading to an increase in [Ca2+]i in CD cells. The significance of the osmosensitive [Ca2+]i signaling for renal water transport and urinary concentration remain unknown. To examine molecular mechanism and physiological relevance of osmosensitivity in the CD, we implemented simultaneous direct measurements of [Ca2+]i dynamics and the rate of cell swelling as a readout of the AQP2-dependent water reabsorption in freshly isolated split-opened CDs of wild type and genetically manipulated animals and combined this with immunofluorescent detection of AVP-induced AQP2 trafficking and assessment of systemic water balance. We identified the critical role of the Ca2+-permeable TRPC3 channel in osmosensitivity and water permeability in the CD. We further demonstrated that TRPC3 -/- mice exhibit impaired urinary concentration, larger urinary volume and a greater weight loss in response to water deprivation despite increased AVP levels and AQP2 abundance. TRPC3 deletion interfered with AQP2 translocation to the plasma membrane in response to water deprivation. In summary, we provide compelling multicomponent evidence in support of a critical contribution of TRPC3 in the CD for osmosensitivity and renal water handling.

Identification and classification of epithelial cells in nephron segments by actin cytoskeleton patterns

The basic functional unit in a kidney is the nephron, which is a long and morphologically segmented tubule. The nephron begins with a cluster of capillaries called glomerulus through which the blood is filtered into the Bowman's space. The filtrate flows through the nephron segments. During this flow, electrolytes and solutes are reabsorbed by channels and transport systems into the capillaries wrapped around the nephron. Many questions related to renal function focus on identifying the sites of expression of these systems. In this study, we mapped whole kidney sections by confocal microscopic imaging of fluorescent phalloidin, which binds to actin filaments. In tile scans (composed of hundreds of images) of these sections, the cortex and the medullary regions (outer and inner stripes of the outer medulla, and inner medulla) could be easily identified by their cytoskeletal patterns. At a higher resolution, we identified distinct features of the actin cytoskeleton in the apical, basal, and lateral borders of the cells. These features could be used to identify segments of a nephron (the proximal tubule, thin and thick segments of Henle's loop, and distal tubule), the collecting duct system, the papillary ducts in the papilla, and the urothelium that covers the pelvis. To verify our findings, we used additional markers, including aquaporin isoforms, cytokeratin 8‐18, and WGA lectin. This study highlights the power of high‐resolution confocal microscopy for identifying specific cell types using the simple probe of F‐actin‐binding phalloidin.

NHE8 attenuates Ca2+ influx into NRK cells and the proximal tubule epithelium

To garner insights into the renal regulation of Ca2+ homeostasis, we performed an mRNA microarray on kidneys from mice treated with the Ca2+-sensing receptor (CaSR) agonist cinacalcet. This revealed decreased gene expression of Na+/H+ exchanger isoform 8 (NHE8) in response to CaSR activation. These results were confirmed by quantitative real-time PCR. Moreover, administration of vitamin D also decreased NHE8 mRNA expression. In contrast, renal NHE8 protein expression from the same samples was increased. To examine the role of NHE8 in transmembrane Ca2+ fluxes, we used the normal rat kidney (NRK) cell line. Cell surface biotinylation and confocal immunofluorescence microscopy demonstrated NHE8 apical expression. Functional experiments found 5-( N-ethyl- N-isopropyl)amiloride (EIPA)-inhibitable NHE activity in NRK cells at concentrations minimally attenuating NHE1 activity in AP-1 cells. To determine how NHE8 might regulate Ca2+ balance, we measured changes in intracellular Ca2+ uptake by live cell Ca2+ imaging with the fluorophore Fura-2 AM. Inhibition of NHE8 with EIPA or by removing extracellular Na+-enhanced Ca2+ influx into NRK cells. Ca2+ influx was mediated by a voltage-dependent Ca2+ channel rather than directly via NHE8. NRK cells express Cav1.3 and display verapamil-sensitive Ca2+ influx and NHE8 inhibition-augmented Ca2+ influx via a voltage-dependent Ca2+ channel. Finally, proximal tubules perused ex vivo demonstrated increased Ca2+ influx in the presence of luminal EIPA at a concentration that would inhibit NHE8. The results of the present study are consistent with NHE8 regulating Ca2+ uptake into the proximal tubule epithelium.

WNK1 promotes renal tumor progression by activating TRPC6-NFAT pathway

Deregulation of Ca²⁺ signaling has been regarded as one of the key features of cancer progression. Lysine-deficient protein kinase 1 (WNK1), a major regulator of renal ion transport, regulates Ca²⁺ signaling through stimulating the phosphatidylinositol 4-kinase IIIα (PI4KIIIα) to activate Gαq-coupled receptor/PLC-β signaling. However, the contribution of WNK1-mediated Ca²⁺ signaling in the development of clear-cell renal-cell carcinoma (ccRCC) is yet unknown. We found that the canonical transient receptor potential channel (TRPC)6 was widely expressed in ccRCC tissues and functioned as a primary Ca²⁺ influx mechanism. We further identified that the expressions of WNK1, PI4KIIIα, TRPC6, and the nuclear factor of activated T cells cytoplasmic 1 (NFATc1) were elevated in the tumor tissues compared with the adjacent normal tissues. WNK1 expression was directly associated with the nuclear grade of ccRCC tissues. Functional experiments showed that WNK1 activated TRPC6-mediated Ca²⁺ influx and current by stimulating PI4KIIIα. Notably, the inhibition of WNK1-mediated TRPC6 activation and its downstream substrate calcineurin attenuated NFATc1 activation and the subsequent migration and proliferation of ccRCC. These findings revealed a novel perspective of WNK1 signaling in targeting the TRPC6-NFATc1 pathway as a therapeutic potential for renal-cell carcinoma.-Kim, J.-H., Hwang, K.-H., Eom, M., Kim, M., Park, E. Y., Jeong, Y., Park, K.-S., Cha, S.-K. WNK1 promotes renal tumor progression by activating TRPC6-NFAT pathway.

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.

Localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) α, β, γ in the three major salivary glands in situ of mice and their response to β-adrenoceptor stimulation

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K), which is composed of three isozymes (α, β and γ), catalyzes the production of phosphatidylinositol bisphosphate (PIP2). This phospholipid functions in membrane trafficking, as an anchor for actin cytoskeletons and as a regulator of intramembranous channels/transporters. It is also a precursor of such second messengers as diacylglycerol, inositol triphosphate and phosphatidylinositol (3,4,5)-triphosphate. In the present study, the expression and localization of endogenous PIP5Ks were examined in the three major salivary glands of young adult mice in situ. In western blotting of normal control glands, immunoreactive bands for individual PIP5Ks were detectable, with the highest density in the parotid gland and the weakest density in the submandibular gland. In immuno-light microscopy under non-stimulated condition, weak immunoreactivity for PIP5Kα was confined to the apical plasmalemma in parotid, but not sublingual or submandibular, acinar cells. Immunoreactivity for PIP5Kβ was weak to moderate and confined to ductal cells but not acinar cells, whereas that for PIP5Kγ was selectively and intensely detected in myoepithelial cells but not acinar cells, and it was weak in ductal cells in the three glands. In western blot of the parotid gland stimulated by isoproterenol, a β-adrenoceptor agonist, no changes were seen in the intensity of immunoreactive bands for any of the PIP5Ks. In contrast, in immuno-light microscopy, the apical immunoreactivity for PIP5Kα in parotid acinar cells was transiently and distinctly increased after the stimulation. The increased immunoreactivity was ultrastructurally localized on most apical microvilli and along contiguous plasma membrane, where membranous invaginations of various shapes and small vesicles were frequently found. It was thus suggested that PIP5Kα is involved in post-exocytotic membrane dynamics via microvillous membranes. The present finding further suggests that each of the three isoforms of PIP5K functions through its product PIP2 discretely in different cells of the glands to regulate saliva secretion.

A model of calcium transport and regulation in the proximal tubule

The objective of this study was to examine theoretically how Ca²⁺ reabsorption in the proximal tubule (PT) is modulated by Na⁺ and water fluxes, parathyroid hormone (PTH), Na⁺-glucose cotransporter (SGLT2) inhibitors, and acetazolamide. We expanded a previously published mathematical model of water and solute transport in the rat PT (Layton AT, Vallon V, Edwards A. Am J Physiol Renal Physiol 308: F1343–F1357, 2015) that did not include Ca²⁺. Our results indicate that Ca²⁺ reabsorption in the PT is primarily driven by the transepithelial Ca²⁺ concentration gradient that stems from water reabsorption, which is itself coupled to Na⁺ reabsorption. Simulated variations in permeability or transporter activity elicit opposite changes in paracellular and transcellular Ca²⁺ fluxes, whereas a simulated decrease in filtration rate lowers both fluxes. The model predicts that PTH-mediated inhibition of the apical Na⁺/H⁺ exchanger NHE3 reduces Na⁺ and Ca²⁺ transport to a similar extent. It also suggests that acetazolamide- and SGLT2 inhibitor-induced calciuria at least partly stems from reduced Ca²⁺ reabsorption in the PT. In addition, backleak of phosphate (PO₄) across tight junctions is predicted to reduce net PO₄ reabsorption by ~20% under normal conditions. When transcellular PO₄ transport is substantially reduced by PTH, paracellular PO₄ flux is reversed and contributes significantly to PO₄ reabsorption. Furthermore, PTH is predicted to exert an indirect impact on PO₄ reabsorption via its inhibitory action on NHE3. This model thus provides greater insight into the mechanisms that modulate Ca²⁺ and PO₄ reabsorption in the PT.

ORAI channels are critical for receptor-mediated endocytosis of albumin

Impaired albumin reabsorption by proximal tubular epithelial cells (PTECs) has been highlighted in diabetic nephropathy (DN), but little is known about the underlying molecular mechanisms. Here we find that ORAI1-3, are preferentially expressed in PTECs and downregulated in patients with DN. Hyperglycemia or blockade of insulin signaling reduces the expression of ORAI1-3. Inhibition of ORAI channels by BTP2 and diethylstilbestrol or silencing of ORAI expression impairs albumin uptake. Transgenic mice expressing a dominant-negative Orai1 mutant (E108Q) increases albuminuria, and in vivo injection of BTP2 exacerbates albuminuria in streptozotocin-induced and Akita diabetic mice. The albumin endocytosis is Ca²⁺-dependent and accompanied by ORAI1 internalization. Amnionless (AMN) associates with ORAIs and forms STIM/ORAI/AMN complexes after Ca²⁺ store depletion. STIM1/ORAI1 colocalizes with clathrin, but not with caveolin, at the apical membrane of PTECs, which determines clathrin-mediated endocytosis. These findings provide insights into the mechanisms of protein reabsorption and potential targets for treating diabetic proteinuria.

Patients with diabetic nephropathy suffer from impaired albumin reabsorption by proximal tubular epithelial cells. Here authors use diabetic and transgenic mouse models and in vitro models to show the cause for this lies in the down regulation and internalization of the ion channels, ORAI1-3.

Therapeutic Effects of FK506 on IgA Nephropathy Rat

BACKGROUND/AIMS

FK506 is an immunosuppressive drug and a calcineurin inhibitor that has been widely used in kidney disease in recent years. FK506 shows a wide range of biological and pharmaceutical effects; however, the mechanism of its anti- proliferative effect has not been well elucidated. An IgA nephropathy (IgAN) model was used to generate a mesangial cell proliferation model. This study aims to examine the effect of FK506 on IgAN rats and the underlying mechanisms.

METHODS

Hematuria, proteinuria and renal function were measured. To observe the pathological conditions, we performed HE (hematoxylin - eosin) and PAS (periodic acid - schiff) staining. Transcription and protein expression levels were detected by qRT - PCR (quantitative real-time polymerase chain reaction) and Wb (western blotting). The location and semi-quantitative expression levels of TRPCs, CaN (Calcineurin) and α-SMA were examined by IHC (Immunohistochemical staining).

RESULTS

We found that FK506 could improve hematuria, proteinuria and renal function, especially in the HF (high-dose FK506) groups. Renal pathological changes were ameliorated in the treatment groups. FK506 could significantly decrease TRPCs, CaN, phosphorylation of ERK1/2 and α-SMA expression.

CONCLUSION

Taken together, these results suggest that the therapeutic effect of FK506 on IgAN might be partially associated with the down-regulated expression of TRPC channels, CaN and phosphorylation of ERK1/2.

Role of adaptor protein p66Shc in renal pathologies

p66Shc is one of the three adaptor proteins encoded by the Shc1 gene, which are expressed in many organs, including the kidney. Recent studies shed new light on several key questions concerning the signaling mechanisms mediated by p66Shc. The central goal of this review article is to summarize recent findings on p66Shc and the role it plays in kidney physiology and pathology. This article provides a review of the various mechanisms whereby p66Shc has been shown to function within the kidney through a wide range of actions. The mitochondrial and cytoplasmic signaling of p66Shc, as it relates to production of reactive oxygen species (ROS) and renal pathologies, is further discussed.

Effects of phospho- and calciotropic hormones on electrolyte transport in the proximal tubule

Calcium and phosphate are critical for a myriad of physiological and cellular processes within the organism. Consequently, plasma levels of calcium and phosphate are tightly regulated. This occurs through the combined effects of the phospho- and calciotropic hormones, parathyroid hormone (PTH), active vitamin D ₃, and fibroblast growth factor 23 (FGF23). The organs central to this are the kidneys, intestine, and bone. In the kidney, the proximal tubule reabsorbs the majority of filtered calcium and phosphate, which amounts to more than 60% and 90%, respectively. The basic molecular mechanisms responsible for phosphate reclamation are well described, and emerging work is delineating the molecular identity of the paracellular shunt wherein calcium permeates the proximal tubular epithelium. Significant experimental work has delineated the molecular effects of PTH and FGF23 on these processes as well as their regulation of active vitamin D ₃ synthesis in this nephron segment. The integrative effects of both phospho- and calciotropic hormones on proximal tubular solute transport and subsequently whole body calcium-phosphate balance thus have been further complicated. Here, we first review the molecular mechanisms of calcium and phosphate reabsorption from the proximal tubule and how they are influenced by the phospho- and calciotropic hormones acting on this segment and then consider the implications on both renal calcium and phosphate handling as well as whole body mineral balance.

TRPC Channels in Health and Disease

This chapter offers a brief introduction of the functions of TRPC channels in non-neuronal systems. We focus on three major organs of which the research on TRPC channels have been most focused on: kidney, heart, and lung. The chapter highlights on cellular functions and signaling pathways mediated by TRPC channels. It also summarizes several inherited diseases in humans that are related to or caused by TRPC channel mutations and malfunction. A better understanding of TRPC channels functions and the importance of TRPC channels in health and disease should lead to new insights and discovery of new therapeutic approaches for intractable disease.

Increased glomerular filtration rate and impaired contractile function of mesangial cells in TRPC6 knockout mice

The present study was conducted to determine if TRPC6 regulates glomerular filtration rate (GFR) and the contractile function of glomerular mesangial cells (MCs). GFR was assessed in conscious TRPC6 wild type and knockout mice, and in anesthetized rats with and without in vivo knockdown of TRPC6 in kidneys. We found that GFR was significantly greater, and serum creatinine level was significantly lower in TRPC6 deficient mice. Consistently, local knockdown of TRPC6 in kidney using TRPC6 specific shRNA construct significantly attenuated Ang II-induced GFR decline in rats. Furthermore, Ang II-stimulated contraction and Ca²⁺ entry were significantly suppressed in primary MCs isolated from TRPC6 deficient mice, and the Ca²⁺ response could be rescued by re-introducing TRPC6. Moreover, inhibition of reverse mode of Na⁺-Ca²⁺ exchange by KB-R7943 significantly reduced Ca²⁺ entry response in TRPC6-expressing, but not in TRPC6-knocked down MCs. Ca²⁺ entry response was also significantly attenuated in Na⁺ free solution. Single knockdown of TRPC6 and TRPC1 resulted in a comparable suppression on Ca²⁺ entry with double knockdown of both. These results suggest that TRPC6 may regulate GFR by modulating MC contractile function through multiple Ca²⁺ signaling pathways.

Melamine induces Ca2+-sensing receptor activation and elicits apoptosis in proximal tubular cells

Melamine causes renal tubular cell injury through inflammation, fibrosis, and apoptosis. Although melamine affects the rise in intracellular Ca²⁺ concentration ([Ca²⁺]_i)_, reactive oxygen species (ROS) production, and proapoptotic pathway activation, the mechanism of upstream Ca²⁺ signaling is unknown. Because melamine has some structural similarities with l-amino acids, which endogenously activate Ca²⁺-sensing receptors (CSR), we examined the effect of melamine on CSR-induced Ca²⁺ signaling and apoptotic cell death. We show here that melamine activates CSR, causing a sustained Ca²⁺ entry in the renal epithelial cell line, LLC-PK1. Moreover, such CSR stimulation resulted in a rise in [Ca²⁺]_i, leading to enhanced ROS production. Furthermore, melamine-induced elevated [Ca²⁺]_i and ROS production caused a dose-dependent increase in apoptotic (by DAPI staining, DNA laddering, and annexin V assay) and necrotic (propidium iodide staining) cell death. Upon examining the downstream mechanism, we found that transforming growth factor β1 (TGF-β1), which increases extracellular matrix genes and proapoptotic signaling, was also upregulated at lower doses of melamine, which could be due to an early event inducing apoptosis. Additionally, cells exposed to melamine displayed a rise in pERK activation and lactate dehydrogenase release resulting in cytotoxicity. These results offer a novel insight into the molecular mechanisms by which melamine exerts its effect on CSR, causing a sustained elevation of [Ca²⁺]_i, leading to ROS generation, fibronectin production, proapoptotic pathway activation, and renal cell damage. Together, these results thus suggest that melamine-induced apoptosis and/or necrosis may subsequently result in acute kidney injury and promote kidney stone formation.

Characterization of flow-regulated cortical collecting duct endothelin-1 production

Collecting duct (CD) endothelin-1 (ET-1) is an autocrine inhibitor of Na⁺ and water reabsorption. Salt or water loading increases CD ET-1 production; this is likely due, at least in part, to increased tubule fluid flow. The mechanisms by which flow stimulates CD ET-1 production are incompletely understood. In particular, flow induction of cortical CD (CCD) and inner medullary CD (IMCD) ET-1 synthesis may occur via different mechanisms. Since flow-mediated ET-1 production in IMCD has been more extensively characterized than in the CCD, this study was undertaken to further examine putative signaling pathways involved in flow-stimulated CCD ET-1 production. The CD cell line, mpkCCDcl4, was exposed to static or flow (2 dyne/cm² for 2 h) conditions and ET-1/GAPDH mRNA levels were assessed. Intracellular Ca²⁺, Ca²⁺-stimulated Ca²⁺ release, calcineurin, and protein kinase c α/β isoforms were all involved in the ET-1 flow response. TRPC6, but not other CD-expressed TRP channels (TRPC3, 4, and 5, or TRPV4) played a role in the ET-1 flow response. Purinergic signaling pathways and cilia were not involved in the ET-1 flow response. Based on these and previously published findings, we present a comparison of flow-stimulated CD ET-1 production between CCD and IMCD We suggest that flow-stimulated CCD ET-1 production may be more involved in responding to Na⁺ delivery, while IMCD ET-1 production may be more responsive to water and solute delivery; the responsible pathways for mediating these effects in the two regions of the CD appear to be substantially distinct from one another.

The renal TRPV4 channel is essential for adaptation to increased dietary potassium

To maintain potassium homeostasis, kidneys exert flow-dependent potassium secretion to facilitate kaliuresis in response to elevated dietary potassium intake. This process involves stimulation of calcium-activated large conductance maxi-K (BK) channels in the distal nephron, namely the connecting tubule and the collecting duct. Recent evidence suggests that the TRPV4 channel is a critical determinant of flow-dependent intracellular calcium elevations in these segments of the renal tubule. Here, we demonstrate that elevated dietary potassium intake (five percent potassium) increases renal TRPV4 mRNA and protein levels in an aldosterone-dependent manner and causes redistribution of the channel to the apical plasma membrane in native collecting duct cells. This, in turn, leads to augmented TRPV4-mediated flow-dependent calcium ion responses in freshly isolated split-opened collecting ducts from mice fed the high potassium diet. Genetic TRPV4 ablation greatly diminished BK channel activity in collecting duct cells pointing to a reduced capacity to excrete potassium. Consistently, elevated potassium intake induced hyperkalemia in TRPV4 knockout mice due to deficient renal potassium excretion. Thus, regulation of TRPV4 activity in the distal nephron by dietary potassium is an indispensable component of whole body potassium balance.

Hydrogen peroxide suppresses TRPM4 trafficking to the apical membrane in mouse cortical collecting duct principal cells

A Ca²⁺-activated nonselective cation channel (NSC_Ca) is found in principal cells of the mouse cortical collecting duct (CCD). However, the molecular identity of this channel remains unclear. We used mpkCCD_c14 cells, a mouse CCD principal cell line, to determine whether NSC_Ca represents the transient receptor potential (TRP) channel, the melastatin subfamily 4 (TRPM4). A Ca²⁺-sensitive single-channel current was observed in inside-out patches excised from the apical membrane of mpkCCD_c14 cells. Like TRPM4 channels found in other cell types, this channel has an equal permeability for Na⁺ and K⁺ and has a linear current-voltage relationship with a slope conductance of ~23 pS. The channel was inhibited by a specific TRPM4 inhibitor, 9-phenanthrol. Moreover, the frequency of observing this channel was dramatically decreased in TRPM4 knockdown mpkCCD_c14 cells. Unlike those previously reported in other cell types, the TRPM4 in mpkCCD_c14 cells was unable to be activated by hydrogen peroxide (H₂O₂). Conversely, after treatment with H₂O₂, TRPM4 density in the apical membrane of mpkCCD_c14 cells was significantly decreased. The channel in intact cell-attached patches was activated by ionomycin (a Ca²⁺ ionophore), but not by ATP (a purinergic P₂ receptor agonist). These data suggest that the NSC_Ca current previously described in CCD principal cells is actually carried through TRPM4 channels. However, the physiological role of this channel in the CCD remains to be further determined.

Canonical Transient Receptor Potential 6 Channel: A New Target of Reactive Oxygen Species in Renal Physiology and Pathology

SIGNIFICANCE

Regulation of Ca2+ signaling cascade by reactive oxygen species (ROS) is becoming increasingly evident and this regulation represents a key mechanism for control of many fundamental cellular functions. Canonical transient receptor potential (TRPC) 6, a member of Ca2+-conductive channel in the TRPC family, is widely expressed in kidney cells, including glomerular mesangial cells, podocytes, tubular epithelial cells, and vascular myocytes in renal microvasculature. Both overproduction of ROS and dysfunction of TRPC6 channel are involved in renal injury in animal models and human subjects. Although regulation of TRPC channel function by ROS has been well described in other tissues and cell types, such as vascular smooth muscle, this important cell regulatory mechanism has not been fully reviewed in kidney cells. Recent Advances: Accumulating evidence has shown that TRPC6 is a redox-sensitive channel, and modulation of TRPC6 Ca2+ signaling by altering TRPC6 protein expression or TRPC6 channel activity in kidney cells is a downstream mechanism by which ROS induce renal damage.

CRITICAL ISSUES

This review highlights how recent studies analyzing function and expression of TRPC6 channels in the kidney and their response to ROS improve our mechanistic understanding of oxidative stress-related kidney diseases.

FUTURE DIRECTIONS

Although it is evident that ROS regulate TRPC6-mediated Ca2+ signaling in several types of kidney cells, further study is needed to identify the underlying molecular mechanism. We hope that the newly identified ROS/TRPC6 pathway will pave the way to new, promising therapeutic strategies to target kidney diseases such as diabetic nephropathy. Antioxid. Redox Signal. 25, 732-748.

Calcium-permeable ion channels in the kidney

Calcium ions (Ca(2+)) are crucial for a variety of cellular functions. The extracellular and intracellular Ca(2+) concentrations are thus tightly regulated to maintain Ca(2+) homeostasis. The kidney, one of the major organs of the excretory system, regulates Ca(2+) homeostasis by filtration and reabsorption. Approximately 60% of the Ca(2+) in plasma is filtered, and 99% of that is reabsorbed by the kidney tubules. Ca(2+) is also a critical signaling molecule in kidney development, in all kidney cellular functions, and in the emergence of kidney diseases. Recently, studies using genetic and molecular biological approaches have identified several Ca(2+)-permeable ion channel families as important regulators of Ca(2+) homeostasis in kidney. These ion channel families include transient receptor potential channels (TRP), voltage-gated calcium channels, and others. In this review, we provide a brief and systematic summary of the expression, function, and pathological contribution for each of these Ca(2+)-permeable ion channels. Moreover, we discuss their potential as future therapeutic targets.

Ways of calcium reabsorption in the kidney

The role of the kidney in calcium homeostasis has been reshaped from a classic view in which the kidney was regulated by systemic calcitropic hormones such as vitamin D3 or parathyroid hormone to an organ actively taking part in the regulation of calcium handling. With the identification of the intrinsic renal calcium-sensing receptor feedback system, the regulation of paracellular calcium transport involving claudins, and new paracrine regulators such as klotho, the kidney has emerged as a crucial modulator not only of calciuria but also of calcium homeostasis. This review summarizes recent molecular and endocrine contributors to renal calcium handling and highlights the tight link between calcium and sodium reabsorption in the kidney.

Targeting the podocyte cytoskeleton: from pathogenesis to therapy in proteinuric kidney disease

Glomerular injury often incites a progression to chronic kidney disease, which affects millions of patients worldwide. Despite our current understanding of this disease's pathogenesis, there is still a lack of therapy available to curtail its progression. However, exciting new data strongly suggest the podocyte-an actin-rich, terminally differentiated epithelial cell that lines the outside of the glomerular filtration barrier-as a therapeutic target. The importance of podocytes in the pathogenesis of human nephrotic syndrome is best characterized by identification of genetic mutations, many of which regulate the actin cytoskeleton. The intricate regulation of the podocyte actin cytoskeleton is fundamental in preserving an intact glomerular filtration barrier, and this knowledge has inspired new research targeting actin-regulating proteins in these cells. This review will shed light on recent findings, which have furthered our understanding of the molecular mechanisms regulating podocyte actin dynamics, as well as discoveries that have therapeutic implications in the treatment of proteinuric kidney disease.

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.

Vasopressin regulates renal calcium excretion in humans

Antidiuretic hormone or arginine vasopressin (AVP) increases water reabsorption in the collecting ducts of the kidney. Three decades ago, experimental models have shown that AVP may increase calcium reabsorption in rat kidney. The objective of this study was to assess whether AVP modulates renal calcium excretion in humans. We analyzed calcium, potassium, and sodium fractional excretion in eight patients affected by insipidus diabetes (nephrogenic or central) under acute vasopressin receptor agonist action and in 10 patients undergoing oral water load test affected or not by inappropriate antidiuretic hormone secretion (SIADH). Synthetic V2 receptor agonist (dDAVP) reduced significantly calcium fractional excretion from 1.71% to 0.58% (P < 0.05) in patients with central diabetes insipidus. In patients with nephrogenic diabetes insipidus (resistant to AVP), calcium fractional excretion did not change significantly after injection (0.48–0.68%, P = NS). In normal subjects undergoing oral water load test, calcium fractional excretion increased significantly from 1.02% to 2.54% (P < 0.05). Patients affected by SIADH had a high calcium fractional excretion at baseline that remained stable during test from 3.30% to 3.33% (P = NS), possibly resulting from a reduced calcium absorption in renal proximal tubule. In both groups, there was a significant correlation between urine output and calcium renal excretion. In humans, dDAVP decreases calcium fractional excretion in the short term. Conversely, water intake, which lowers AVP concentration, increases calcium fractional excretion. The correlation between urine output and calcium excretion suggests that AVP-related antidiuresis increases calcium reabsorption in collecting ducts.

Role of renal TRP channels in physiology and pathology

Kidneys critically contribute to the maintenance of whole-body homeostasis by governing water and electrolyte balance, controlling extracellular fluid volume, plasma osmolality, and blood pressure. Renal function is regulated by numerous systemic endocrine and local mechanical stimuli. Kidneys possess a complex network of membrane receptors, transporters, and ion channels which allows responding to this wide array of signaling inputs in an integrative manner. Transient receptor potential (TRP) channel family members with diverse modes of activation, varied permeation properties, and capability to integrate multiple downstream signals are pivotal molecular determinants of renal function all along the nephron. This review summarizes experimental data on the role of TRP channels in a healthy mammalian kidney and discusses their involvement in renal pathologies.

Evidence of a Role for Fibroblast Transient Receptor Potential Canonical 3 Ca2+ Channel in Renal Fibrosis

Transient receptor potential canonical (TRPC) Ca(2+)-permeant channels, especially TRPC3, are increasingly implicated in cardiorenal diseases. We studied the possible role of fibroblast TRPC3 in the development of renal fibrosis. In vitro, a macromolecular complex formed by TRPC1/TRPC3/TRPC6 existed in isolated cultured rat renal fibroblasts. However, specific blockade of TRPC3 with the pharmacologic inhibitor pyr3 was sufficient to inhibit both angiotensin II- and 1-oleoyl-2-acetyl-sn-glycerol-induced Ca(2+) entry in these cells, which was detected by fura-2 Ca(2+) imaging. TRPC3 blockade or Ca(2+) removal inhibited fibroblast proliferation and myofibroblast differentiation by suppressing the phosphorylation of extracellular signal-regulated kinase (ERK1/2). In addition, pyr3 inhibited fibrosis and inflammation-associated markers in a noncytotoxic manner. Furthermore, TRPC3 knockdown by siRNA confirmed these pharmacologic findings. In adult male Wistar rats or wild-type mice subjected to unilateral ureteral obstruction, TRPC3 expression increased in the fibroblasts of obstructed kidneys and was associated with increased Ca(2+) entry, ERK1/2 phosphorylation, and fibroblast proliferation. Both TRPC3 blockade in rats and TRPC3 knockout in mice inhibited ERK1/2 phosphorylation and fibroblast activation as well as myofibroblast differentiation and extracellular matrix remodeling in obstructed kidneys, thus ameliorating tubulointerstitial damage and renal fibrosis. In conclusion, TRPC3 channels are present in renal fibroblasts and control fibroblast proliferation, differentiation, and activation through Ca(2+)-mediated ERK signaling. TRPC3 channels might constitute important therapeutic targets for improving renal remodeling in kidney disease.

TRPC6 channel as an emerging determinant of the podocyte injury susceptibility in kidney diseases

Podocytes (terminally differentiated epithelial cells of the glomeruli) play a key role in the maintenance of glomerular structure and permeability and in the incipiency of various renal abnormalities. Injury to podocytes is considered a major contributor to the development of kidney disease as their loss causes proteinuria and progressive glomerulosclerosis. The physiological function of podocytes is critically dependent on proper intracellular calcium handling; excessive calcium influx in these cells may result in the effacement of foot processes, apoptosis, and subsequent glomeruli damage. One of the key proteins responsible for calcium flux in the podocytes is transient receptor potential cation channel, subfamily C, member 6 (TRPC6); a gain-of-function mutation in TRPC6 has been associated with the onset of the familial forms of focal segmental glomerulosclerosis (FSGS). Recent data also revealed a critical role of this channel in the onset of diabetic nephropathy. Therefore, major efforts of the research community have been recently dedicated to unraveling the TRPC6-dependent effects in the initiation of podocyte injury. This mini-review focuses on the TRPC6 channel in podocytes and colligates recent data in an attempt to shed some light on the mechanisms underlying the pathogenesis of TRPC6-mediated glomeruli damage and its potential role as a therapeutic target for the treatment of chronic kidney diseases.

Flow regulation of endothelin-1 production in the inner medullary collecting duct

Collecting duct-derived endothelin (ET)-1 is an autocrine inhibitor of Na(+) and water reabsorption; its deficiency causes hypertension and water retention. Extracellular fluid volume expansion increases collecting duct ET-1, thereby promoting natriuresis and diuresis; however, how this coupling between volume expansion and collecting duct ET-1 occurs is incompletely understood. One possibility is that volume expansion increases tubular fluid flow. To investigate this, cultured IMCD3 cells were subjected to static or flow conditions. Exposure to a shear stress of 2 dyn/cm(2) for 2 h increased ET-1 mRNA content by ∼2.3-fold. Absence of perfusate Ca(2+), chelation of intracellular Ca(2+), or inhibition of Ca(2+) signaling (calmodulin, Ca(2+)/calmodulin-dependent kinase, calcineurin, PKC, or phospholipase C) prevented the flow response. Evaluation of possible flow-activated Ca(2+) entry pathways revealed no role for transient receptor potential (TRP)C3, TRPC6, and TRPV4; however, cells with TRPP2 (polycystin-2) knockdown had no ET-1 flow response. Flow increased intracellular Ca(2+) was blunted in TRPP2 knockdown cells. Nonspecific blockade of P2 receptors, as well as specific inhibition of P2X7 and P2Y2 receptors, prevented the ET-1 flow response. The ET-1 flow response was not affected by inhibition of either epithelial Na(+) channels or the mitochondrial Na(+)/Ca(2+) exchanger. Taken together, these findings provide evidence that in IMCD3 cells, flow, via polycystin-2 and P2 receptors, engages Ca(2+)-dependent signaling pathways that stimulate ET-1 synthesis.

Recombinant human erythropoietin pretreatment attenuates acute renal tubular injury against ischemia-reperfusion by restoring transient receptor potential channel-6 expression and function in collecting ducts

OBJECTIVE

Acute renal tubular injury is a serious complication in the postoperative period, which is associated with high mortality and increased ICU stay. We aimed to demonstrate the protective effect of rhEPO against acute tubular injury induced by ischemia-reperfusion and to explore the mechanism of canonical transient receptor potential channel-6.

DESIGN

Randomized laboratory animal study.

SETTINGS

Animal research laboratory.

INTERVENTIONS

Male Sprague-Dawley rats were randomly divided into three groups: the sham group, the control group, and the rhEPO group. Experimental acute tubular injury was established in rats by bilateral renal arterial occlusion for 30 minutes followed by reperfusion.

MEASUREMENTS AND MAIN RESULTS

Blood samples were obtained for cystatin-C and neutrophil gelatinase-associated lipocalin measurements by enzyme-linked immunosorbance assays. Seventy-two hours after reperfusion, urine samples were collected for osmolality and fractional excretion of sodium (%) assays on a chemistry analyzer. Kidneys were harvested at 24, 48, and 72 hours after reperfusion. Transient receptor potential channel-6, aquaporin-2, and Na,K-ATPase expression in collecting ducts were studied by immunofluorescence and Western blot. Coimmunoprecipitations were also performed to identify the possible signalplex relation between transient receptor potential channel-6 and aquaporin-2 or Na,K-ATPase channels. RhEPO pretreatment significantly inhibited serum cystatin-C (2 hr: 453 ± 64 μg/L vs 337 ± 28 μg/L, p < 0.01), serum neutrophil gelatinase-associated lipocalin (72 hr: 1,175 ± 107 ng/L vs 1,737 ± 402 ng/L, p < 0.05), and urinary fractional excretion of sodium (%) increase (0.9 ± 0.1 vs 2.2 ± 0.8, p < 0.05) and alleviated the decrease of urinary osmolality (1,293 ± 101 mosmol/kg H2O vs 767 ± 91 mosmol/kg H2O, p < 0.05) induced by ischemia-reperfusion injury. Meanwhile, recombinant human erythropoietin greatly improved the ischemia-reperfusion-induced attenuation of transient receptor potential channel-6 expression (48 hr: 42% ± 2% vs 67% ± 2% and 72 hr: 55% ± 2% vs 66% ± 2%), as well as aquaporin-2 and Na,K-ATPase expression in collecting ducts. Transient receptor potential channel-6 functionally interacted with Na,K-ATPase but not aquaporin-2.

CONCLUSIONS

Recombinant human erythropoietin pretreatment at the dose of 5,000 IU/kg potently prevented ischemia-reperfusion-induced acute tubular injury, which might be partly attributed to the restoring the effect of transient receptor potential channel-6 expression and collecting duct function.

Therapeutic target for nephrotic syndrome: Identification of novel slit diaphragm associated molecules

The slit diaphragm bridging the neighboring foot processes functions as a final barrier of glomerular capillary wall for preventing the leak of plasma proteins into primary urine. It is now accepted that the dysfunction of the sit diaphragm contributes to the development of proteinuria in several glomerular diseases. Nephrin, a gene product of NPHS1, a gene for a congenital nephrotic syndrome of Finnish type, constitutes an extracellular domain of the slit diaphragm. Podocin was identified as a gene product of NPHS2, a gene for a familial steroid-resistant nephrotic syndrome of French. Podocin binds the cytoplasmic domain of nephrin. After then, CD2 associated protein, NEPH1 and transient receptor potential-6 were also found as crucial molecules of the slit diaphragm. In order to explore other novel molecules contributing to the development of proteinuria, we performed a subtraction hybridization assay with a normal rat glomerular RNA and a glomerular RNA of rats with a puromycin aminonucleoside nephropathy, a mimic of a human minimal change type nephrotic syndrome. Then we have found that synaptic vesicle protein 2B, ephrin-B1 and neurexin were already downregulated at the early stage of puromycin aminonucleoside nephropathy, and that these molecules were localized close to nephrin. It is conceivable that these molecules are the slit diaphragm associated molecules, which participate in the regulation of the barrier function. These molecules could be targets to establish a novel therapy for nephrotic syndrome.

TRPC6 single nucleotide polymorphisms and progression of idiopathic membranous nephropathy

BACKGROUND

Activating mutations in the Transient Receptor Potential channel C6 (TRPC6) cause autosomal dominant focal segmental glomerular sclerosis (FSGS). TRPC6 expression is upregulated in renal biopsies of patients with idiopathic membranous glomerulopathy (iMN) and animal models thereof. In iMN, disease progression is characterized by glomerulosclerosis. In addition, a context-dependent TRPC6 overexpression was recently suggested in complement-mediated podocyte injury in e.g. iMN. Hence, we hypothesized that genetic variants in TRPC6 might affect susceptibility to development or progression of iMN.

METHODS & RESULTS

Genomic DNA was isolated from blood samples of 101 iMN patients and 292 controls. By direct sequencing of the entire TRPC6 gene, 13 single nucleotide polymorphisms (SNPs) were identified in the iMN cohort, two of which were causing an amino acid substitution (rs3802829; Pro15Ser and rs36111323, Ala404Val). No statistically significant differences in genotypes or allele frequencies between patients and controls were observed. Clinical outcome in patients was determined (remission n = 26, renal failure n = 46, persistent proteinuria n = 29, follow-up median 80 months {range 51-166}). The 13 identified SNPs showed no association with remission or renal failure. There were no differences in genotypes or allele frequencies between patients in remission and progressors.

CONCLUSIONS

Our data suggest that TRPC6 polymorphisms do not affect susceptibility to iMN, or clinical outcome in iMN.

Emerging urinary markers of renal injury in obstructive nephropathy

The effects of obstruction on renal function are the consequence of many factors that profoundly alter all components of glomerular function. Besides the acute effects on glomerular filtration rate and tubule function, a chronic obstruction induces tubular and interstitial injury that results from the activation of different pathways. The progression of tubulointerstitial injury leads to chronic renal damage characterized by tubular atrophy, inflammatory cell infiltration, and interstitial fibrosis. Obstructive nephropathy is an evolving disease in which the renal damage continues even after relief of the obstruction. In particular, it has been demonstrated that the time of relief is the most important factor in predicting long-term renal function deterioration. In this setting, the EGF/MCP-1 ratio, urinary NGAL, and urinary KIM-1 are useful early biomarkers of progressive renal damage and could have a potential role in predicting the long-term renal outcome. This minireview summarizes the role of these emerging urinary biomarkers of obstructive nephropathy based on the current understanding of the pathophysiology of renal injury.