Vasopressin-induced membrane trafficking of TRPC3 and AQP2 channels in cells of the rat renal collecting duct

Sex-specific Stone-forming Phenotype in Mice During Hypercalciuria/Urine Alkalinization

Sex differences in kidney stone formation are well known. Females generally have slightly acidic blood and higher urine pH when compared with males, which makes them more vulnerable to calcium stone formation, yet the mechanism is still unclear. We aimed to examine the role of sex in stone formation during hypercalciuria and urine alkalinization through acetazolamide and calcium gluconate supplementation, respectively, for 4 weeks in wild-type (WT) and moderately hypercalciuric [TRPC3 knockout [KO](-/-)] male and female mice. Our goal was to develop calcium phosphate (CaP) and CaP+ calcium oxalate mixed stones in our animal model to understand the underlying sex-based mechanism of calcium nephrolithiasis. Our results from the analyses of mice urine, serum, and kidney tissues show that female mice (WT and KO) produce more urinary CaP crystals, higher [Ca2+], and pH in urine compared to their male counterparts. We identified a sex-based relationship of stone-forming phenotypes (types of stones) in our mice model following urine alkalization/calcium supplementation, and our findings suggest that female mice are more susceptible to CaP stones under those conditions. Calcification and fibrotic and inflammatory markers were elevated in treated female mice compared with their male counterparts, and more so in TRPC3 KO mice compared with their WT counterparts. Together these findings contribute to a mechanistic understanding of sex-influenced CaP and mixed stone formation that can be used as a basis for determining the factors in sex-related clinical studies.

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.

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.

TRP Channels as Molecular Targets to Relieve Endocrine-Related Diseases

Transient receptor potential (TRP) channels are polymodal channels capable of sensing environmental stimuli, which are widely expressed on the plasma membrane of cells and play an essential role in the physiological or pathological processes of cells as sensors. TRPs often form functional homo- or heterotetramers that act as cation channels to flow Na⁺ and Ca²⁺, change membrane potential and [Ca²⁺]_i (cytosolic [Ca²⁺]), and change protein expression levels, channel attributes, and regulatory factors. Under normal circumstances, various TRP channels respond to intracellular and extracellular stimuli such as temperature, pH, osmotic pressure, chemicals, cytokines, and cell damage and depletion of Ca²⁺ reserves. As cation transport channels and physical and chemical stimulation receptors, TRPs play an important role in regulating secretion, interfering with cell proliferation, and affecting neural activity in these glands and their adenocarcinoma cells. Many studies have proved that TRPs are widely distributed in the pancreas, adrenal gland, and other glands. This article reviews the specific regulatory mechanisms of various TRP channels in some common glands (pancreas, salivary gland, lacrimal gland, adrenal gland, mammary gland, gallbladder, and sweat gland).

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.

Renal vascular TRP channels

Members of the transient receptor potential (TRP) channels that are expressed in the kidney have gained prominence in recent years following discoveries of their role in maintaining the integrity of the filtration barrier, regulating tubular reabsorption of Ca²⁺ and Mg²⁺, and sensing osmotic stimuli. Furthermore, evidence has linked mutations in TRP channels to kidney disease pathophysiological mechanisms, including focal segmental glomerulosclerosis, disturbances in Mg²⁺ homeostasis, and polycystic kidney disease. Several subtypes of TRP channels are expressed in the renal vasculature, from preglomerular arteries and arterioles to the descending vasa recta. Although investigations on the physiological and pathological significance of renal vascular TRP channels are sparse, studies on isolated vessels and cells have suggested their involvement in renal vasoregulation. Renal blood flow (RBF) is an essential determinant of kidney function, including glomerular filtration, water and solute reabsorption, and waste product excretion. Functional alterations in ion channels that are expressed in the endothelium and smooth muscle of renal vessels can modulate renal vascular resistance, arterial pressure, and RBF. Hence, renal vascular TRP channels are potential therapeutic targets for the treatment of kidney disease. This review summarizes the current knowledge of TRP channel expression in renal vasculature and their role in controlling kidney function in health and disease.

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TRP channels are widely distributed in mammalian kidneys in glomerular, tubular, and vascular cells.

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TRPC and TRPV channels are functionally expressed in afferent arterioles.

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TRPC4 may regulate Ca²⁺ signaling in the descending vasa recta.

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Smooth muscle, endothelial, and pericyte TRP channels may participate in signal transduction mechanisms.

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TRP channels underlie renal autoregulation and regional kidney perfusion in health and disease.

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.

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.

STIM1-dependent membrane insertion of heteromeric TRPC1-TRPC4 channels in response to muscarinic receptor stimulation

Muscarinic receptor stimulation results in activation of nonselective cation (NSC) channels in guinea pig adrenal medullary (AM) cells. The biophysical and pharmacological properties of the NSC channel suggest the involvement of heteromeric channels of TRPC1 with TRPC4 or TRPC5. This possibility was explored in PC12 cells and guinea pig AM cells. Proximity ligation assay (PLA) revealed that when exogenously expressed in PC12 cells, TRPC1 forms a heteromeric channel with TRPC4, but not with TRPC5, in a STIM1-dependent manner. The heteromeric TRPC1-TRPC4 channel was also observed in AM cells and trafficked to the cell periphery in response to muscarine stimulation. To explore whether heteromeric channels are inserted into the cell membrane, tags were attached to the extracellular domains of TRPC1 and TRPC4. PLA products developed between the tags in cells stimulated by muscarine, but not in resting cells, indicating that muscarinic stimulation results in the membrane insertion of channels. This membrane insertion required expression of full-length STIM1. We conclude that muscarinic receptor stimulation results in the insertion of heteromeric TRPC1-TRPC4 channels into the cell membrane in PC12 cells and guinea pig AM cells.

Evidence for a regulated Ca2+ entry in proximal tubular cells and its implication in calcium stone formation

Calcium phosphate (CaP) crystals, which begin to form in the early segments of the loop of Henle (LOH), are known to act as precursors for calcium stone formation. The proximal tubule (PT), which is just upstream of the LOH and is a major site for Ca²⁺ reabsorption, could be a regulator of such CaP crystal formation. However, PT Ca²⁺ reabsorption is mostly described as being paracellular. Here, we show the existence of a regulated transcellular Ca²⁺ entry pathway in luminal membrane PT cells induced by Ca²⁺-sensing receptor (CSR, also known as CASR)-mediated activation of transient receptor potential canonical 3 (TRPC3) channels. In support of this idea, we found that both CSR and TRPC3 are physically and functionally coupled at the luminal membrane of PT cells. More importantly, TRPC3-deficient mice presented with a deficiency in PT Ca²⁺ entry/transport, elevated urinary [Ca²⁺], microcalcifications in LOH and urine microcrystals formations. Taken together, these data suggest that a signaling complex comprising CSR and TRPC3 exists in the PT and can mediate transcellular Ca²⁺ transport, which could be critical in maintaining the PT luminal [Ca²⁺] to mitigate formation of the CaP crystals in LOH and subsequent formation of calcium stones.

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.

Canonical transient receptor potential 3 channels in atrial fibrillation

The pathogenesis of atrial fibrillation (AF) is largely dependent on structural remodeling and electrical reconfiguration, which in turn drive localized fibrosis. Canonical transient receptor potential 3 (TRPC3) channel is indispensable regulator of fibrosis development, promoting fibroblasts to transition into myofibroblasts via intracellular Ca²⁺ overload. TRPC3 is a non-voltage gated, non-selective cation channel that regulates the permeability of the cell to Ca²⁺. When subjected to various external physical and chemical stimuli, such as angiotensin II (AngII), mechanical stretch, hypoxia, or oxidative stress, TRPC3 coordinates with downstream signal transduction pathways to alter gene expression and thereby regulate a number of distinct pathological patterns and mechanisms. This review will focus on how TRPC3 affects AF pathogenesis by exploring the underlying mechanisms governing fibrosis associated with particular signaling proteins, ultimately highlighting the characteristics of TPRC3 that mark it as a novel therapeutic target for AF alleviation.

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.

Physiological and pathophysiological role of transient receptor potential canonical channels in cardiac myocytes

Transient receptor potential canonical (TRPC) channels constitute a family of seven Ca²⁺ permeable ion channels, named TRPC1 to 7. These channels are abundantly expressed in the mammalian heart, yet mechanisms underlying activation of TRPC channels and their precise role in cardiac physiology remain poorly understood. In this review, we perused original literature regarding TRPC channels in cardiomyocytes. We first reviewed studies on TRPC channel assembly and sub-cellular localization across multiple species and cell types. Our review indicates that TRPC localization in cardiac cells is still a topic of controversy. We then examined common molecular biology tools used to infer on location and physiological roles of TRPC channels in the heart. We subsequently reviewed pharmacological tools used to modulate TRPC activity in both cardiac and non-cardiac cells. Suggested physiological roles in the heart include modulation of heart rate and sensing of mechanical strain. We examined studies on the contribution of TRPC to cardiac pathophysiology, mainly hypertrophic signaling. Several TRPC channels, particularly TRPC1, 3 and 6 were proposed to play a crucial role in hypertrophic signaling. Finally, we discussed gaps in our understanding of the location and physiological role of TRPC channels in cardiomyocytes. Closing these gaps will be crucial to gain a full understanding of the role of TRPC channels in cardiac pathophysiology and to further explore these channels as targets for treatments for cardiac diseases, in particular, hypertrophy.

Interactions of the Mechanosensitive Channels with Extracellular Matrix, Integrins, and Cytoskeletal Network in Osmosensation

Life is maintained in a sea water-like internal environment. The homeostasis of this environment is dependent on osmosensory system translation of hydromineral information into osmotic regulatory machinery at system, tissue and cell levels. In the osmosensation, hydromineral information can be converted into cellular reactions through osmoreceptors, which changes thirst and drinking, secretion of antidiuretic vasopressin (VP), reabsorption of water and salt in the kidneys at systemic level as well as cellular metabolic activity and survival status at tissue level. The key feature of osmosensation is the activation of mechanoreceptors or mechanosensors, particularly transient receptor potential vallinoid (TRPV) and canonical (TRPC) family channels, which increases cytosolic Ca²⁺ levels, activates osmosensory cells including VP neurons and triggers a series of secondary reactions. TRPV channels are sensitive to both hyperosmotic and hyposmotic stimuli while TRPC channels are more sensitive to hyposmotic challenge in neurons. The activation of TRP channels relies on changes in cell volume, membrane stretch and cytoskeletal reorganization as well as hydration status of extracellular matrix (ECM) and activity of integrins. Different families of TRP channels could be activated differently in response to hyperosmotic and hyposmotic stimuli in different spatiotemporal orders, leading to differential reactions of osmosensory cells. Together, they constitute the osmosensory machinery. The activation of this osmoreceptor complex is also associated with the activity of other osmolarity-regulating organelles, such as water channel protein aquaporins, Na-K-2Cl cotransporters, volume-sensitive anion channels, sodium pump and purinergic receptors in addition to intercellular interactions, typically astrocytic neuronal interactions. In this article, we review our current understandings of the composition of osmoreceptors and the processes of osmosensation.

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.

Molecular mechanisms regulating aquaporin-2 in kidney collecting duct

The kidney collecting duct is an important renal tubular segment for regulation of body water homeostasis and urine concentration. Water reabsorption in the collecting duct principal cells is controlled by vasopressin, a peptide hormone that induces the osmotic water transport across the collecting duct epithelia through regulation of water channel proteins aquaporin-2 (AQP2) and aquaporin-3 (AQP3). In particular, vasopressin induces both intracellular translocation of AQP2-bearing vesicles to the apical plasma membrane and transcription of the Aqp2 gene to increase AQP2 protein abundance. The signaling pathways, including AQP2 phosphorylation, RhoA phosphorylation, intracellular calcium mobilization, and actin depolymerization, play a key role in the translocation of AQP2. This review summarizes recent data demonstrating the regulation of AQP2 as the underlying molecular mechanism for the homeostasis of water balance in the body.

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.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

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.

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.

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.

Activation of endothelial transient receptor potential C3 channel is required for small conductance calcium-activated potassium channel activation and sustained endothelial hyperpolarization and vasodilation of cerebral artery

Background

Transient receptor potential C3 (TRPC3) has been demonstrated to be involved in the regulation of vascular tone through endothelial cell (EC) hyperpolarization and endothelium‐dependent hyperpolarization–mediated vasodilation. However, the mechanism by which TRPC3 regulates these processes remains unresolved. We tested the hypothesis that endothelial receptor stimulation triggers rapid TRPC3 trafficking to the plasma membrane, where it provides the source of Ca²⁺ influx for small conductance calcium‐activated K⁺ (SK_Ca) channel activation and sustained EC hyperpolarization.

Methods and Results

Pressurized artery studies were performed with isolated mouse posterior cerebral artery. Treatment with a selective TRPC3 blocker (Pyr3) produced significant attenuation of endothelium‐dependent hyperpolarization–mediated vasodilation and endothelial Ca²⁺ response (EC‐specific Ca²⁺ biosensor) to intraluminal ATP. Pyr3 treatment also resulted in a reduced ATP‐stimulated global Ca²⁺ and Ca²⁺ influx in primary cultures of cerebral endothelial cells. Patch‐clamp studies with freshly isolated cerebral ECs demonstrated 2 components of EC hyperpolarization and K⁺ current activation in response to ATP. The early phase was dependent on intermediate conductance calcium‐activated K⁺ channel activation, whereas the later sustained phase relied on SK_Ca channel activation. The SK_Ca channel–dependent phase was completely blocked with TRPC3 channel inhibition or in ECs of TRPC3 knockout mice and correlated with increased trafficking of TRPC3 (but not SK_Ca channel) to the plasma membrane.

Conclusions

We propose that TRPC3 dynamically regulates SK_Ca channel activation through receptor‐dependent trafficking to the plasma membrane, where it provides the source of Ca²⁺ influx for sustained SK_Ca channel activation, EC hyperpolarization, and endothelium‐dependent hyperpolarization–mediated vasodilation.

Direct visualization and quantitative analysis of water diffusion in complex biological tissues using CARS microscopy

To date, it has not been possible to measure microscopic diffusive water movements in epithelia and in the interstitial space of complex tissues and organs. Diffusive water movements are essential for life because they convey physiologically important small molecules, e.g. nutrients and signaling ligands throughout the extracellular space of complex tissues. Here we report the development of a novel method for the direct observation and quantitative analysis of water diffusion dynamics in a biologically organized tissue using Coherent Anti-Stokes Raman Scattering (CARS) microscopy. Using a computer simulation model to analyze the CARS O-H bond vibration data during H₂O/D₂O exchange in a 3D epithelial cyst, we succeeded in measuring the diffusive water permeability of the individual luminal and basolateral water pathways and also their response to hormonal stimulation. Our technique will be applicable to the measurement of diffusive water movements in other structurally complex and medically important tissues and organs.

Discrete control of TRPV4 channel function in the distal nephron by protein kinases A and C

We have recently documented that the Ca(2+)-permeable TRPV4 channel, which is abundantly expressed in distal nephron cells, mediates cellular Ca(2+) responses to elevated luminal flow. In this study, we combined Fura-2-based [Ca(2+)]i imaging with immunofluorescence microscopy in isolated split-opened distal nephrons of C57BL/6 mice to probe the molecular determinants of TRPV4 activity and subcellular distribution. We found that activation of the PKC pathway with phorbol 12-myristate 13-acetate significantly increased [Ca(2+)]i responses to flow without affecting the subcellular distribution of TRPV4. Inhibition of PKC with bisindolylmaleimide I diminished cellular responses to elevated flow. In contrast, activation of the PKA pathway with forskolin did not affect TRPV4-mediated [Ca(2+)]i responses to flow but markedly shifted the subcellular distribution of the channel toward the apical membrane. These actions were blocked with the specific PKA inhibitor H-89. Concomitant activation of the PKA and PKC cascades additively enhanced the amplitude of flow-induced [Ca(2+)]i responses and greatly increased basal [Ca(2+)]i levels, indicating constitutive TRPV4 activation. This effect was precluded by the selective TRPV4 antagonist HC-067047. Therefore, the functional status of the TRPV4 channel in the distal nephron is regulated by two distinct signaling pathways. Although the PKA-dependent cascade promotes TRPV4 trafficking and translocation to the apical membrane, the PKC-dependent pathway increases the activity of the channel on the plasma membrane.

TRPV4 dysfunction promotes renal cystogenesis in autosomal recessive polycystic kidney disease

The molecular mechanism of cyst formation and expansion in autosomal recessive polycystic kidney disease (ARPKD) is poorly understood, but impaired mechanosensitivity to tubular flow and dysfunctional calcium signaling are important contributors. The activity of the mechanosensitive Ca(2+)-permeable TRPV4 channel underlies flow-dependent Ca(2+) signaling in murine collecting duct (CD) cells, suggesting that this channel may contribute to cystogenesis in ARPKD. Here, we developed a method to isolate CD-derived cysts and studied TRPV4 function in these cysts laid open as monolayers and in nondilated split-open CDs in a rat model of ARPKD. In freshly isolated CD-derived cyst monolayers, we observed markedly impaired TRPV4 activity, abnormal subcellular localization of the channel, disrupted TRPV4 glycosylation, decreased basal [Ca(2+)]i, and loss of flow-mediated [Ca(2+)]i signaling. In contrast, nondilated CDs of these rats exhibited functional TRPV4 with largely preserved mechanosensitive properties. Long-term systemic augmentation of TRPV4 activity with a selective TRPV4 activator significantly attenuated the renal manifestations of ARPKD in a time-dependent manner. At the cellular level, selective activation of TRPV4 restored mechanosensitive Ca(2+) signaling as well as the function and subcellular distribution of TRPV4. In conclusion, the functional status of TRPV4, which underlies mechanosensitive Ca(2+) signaling in CD cells, inversely correlates with renal cystogenesis in ARPKD. Augmenting TRPV4 activity may have therapeutic potential in ARPKD.

Mechanisms and roles of muscarinic activation in guinea-pig adrenal medullary cells

Muscarinic receptors are expressed in the adrenal medullary (AM) cells of various mammals, but their physiological roles are controversial. Therefore, the ionic mechanism for muscarinic receptor-mediated depolarization and the role of muscarinic receptors in neuronal transmission were investigated in dissociated guinea-pig AM cells and in the perfused guinea-pig adrenal gland. Bath application of muscarine induced an inward current at -60 mV. This inward current was partially suppressed by quinine with an IC(50) of 6.1 μM. The quinine-insensitive component of muscarine-induced currents changed the polarity at -78 mV and was inhibited by bupivacaine, a TWIK-related acid-sensitive K(+) (TASK) channel inhibitor. Conversely, the current-voltage relationship for the bupivacaine-insensitive component of muscarine currents showed a reversal potential of -5 mV and a negative slope below -40 mV. External application of La(3+) had a double action on muscarine currents of both enhancement and suppression. Immunoblotting and immunocytochemistry revealed expression of TASK1 channels and cononical transient receptor potential channels 1, 4, 5, and 7 in guinea-pig AM cells. Retrograde application of atropine reversibly suppressed transsynaptically evoked catecholamine secretion from the adrenal gland. The results indicate that muscarinic receptor stimulation in guinea-pig AM cells induces depolarization through inhibition of TASK channels and activation of nonselective cation channels and that muscarinic receptors are involved in neuronal transmission from the splanchnic nerve.

AT1a receptor signaling is required for basal and water deprivation-induced urine concentration in AT1a receptor-deficient mice

It is well recognized that ANG II interacts with arginine vasopressin (AVP) to regulate water reabsorption and urine concentration in the kidney. The present study used ANG II type 1a (AT(1a)) receptor-deficient (Agtr1a(-/-)) mice to test the hypothesis that AT(1a) receptor signaling is required for basal and water deprivation-induced urine concentration in the renal medulla. Eight groups of wild-type (WT) and Agtr1a(-/-) mice were treated with or without 24-h water deprivation and 1-desamino-8-d-AVP (DDAVP; 100 ng/h ip) for 2 wk or with losartan (10 mg/kg ip) during water deprivation. Under basal conditions, Agtr1a(-/-) mice had lower systolic blood pressure (P < 0.01), greater than threefold higher 24-h urine excretion (WT mice: 1.3 ± 0.1 ml vs. Agtr1a(-/-) mice: 5.9 ± 0.7 ml, P < 0.01), and markedly decreased urine osmolality (WT mice: 1,834 ± 86 mosM/kg vs. Agtr1a(-/-) mice: 843 ± 170 mosM/kg, P < 0.01), without significant changes in 24-h urinary Na(+) excretion. These responses in Agtr1a(-/-) mice were associated with lower basal plasma AVP (WT mice: 105 ± 8 pg/ml vs. Agtr1a(-/-) mice: 67 ± 6 pg/ml, P < 0.01) and decreases in total lysate and membrane aquaporin-2 (AQP2; 48.6 ± 7% of WT mice, P < 0.001) and adenylyl cyclase isoform III (55.6 ± 8% of WT mice, P < 0.01) proteins. Although 24-h water deprivation increased plasma AVP to the same levels in both strains, 24-h urine excretion was still higher, whereas urine osmolality remained lower, in Agtr1a(-/-) mice (P < 0.01). Water deprivation increased total lysate AQP2 proteins in the inner medulla but had no effect on adenylyl cyclase III, phosphorylated MAPK ERK1/2, and membrane AQP2 proteins in Agtr1a(-/-) mice. Furthermore, infusion of DDAVP for 2 wk was unable to correct the urine-concentrating defects in Agtr1a(-/-) mice. These results demonstrate that AT(1a) receptor-mediated ANG II signaling is required to maintain tonic AVP release and regulate V(2) receptor-mediated responses to water deprivation in the inner medulla.

Activation of transient receptor potential canonical 3 (TRPC3)-mediated Ca2+ entry by A1 adenosine receptor in cardiomyocytes disturbs atrioventricular conduction

Although the activation of the A(1)-subtype of the adenosine receptors (A(1)AR) is arrhythmogenic in the developing heart, little is known about the underlying downstream mechanisms. The aim of this study was to determine to what extent the transient receptor potential canonical (TRPC) channel 3, functioning as receptor-operated channel (ROC), contributes to the A(1)AR-induced conduction disturbances. Using embryonic atrial and ventricular myocytes obtained from 4-day-old chick embryos, we found that the specific activation of A(1)AR by CCPA induced sarcolemmal Ca(2+) entry. However, A(1)AR stimulation did not induce Ca(2+) release from the sarcoplasmic reticulum. Specific blockade of TRPC3 activity by Pyr3, by a dominant negative of TRPC3 construct, or inhibition of phospholipase Cs and PKCs strongly inhibited the A(1)AR-enhanced Ca(2+) entry. Ca(2+) entry through TRPC3 was activated by the 1,2-diacylglycerol (DAG) analog OAG via PKC-independent and -dependent mechanisms in atrial and ventricular myocytes, respectively. In parallel, inhibition of the atypical PKCζ by myristoylated PKCζ pseudosubstrate inhibitor significantly decreased the A(1)AR-enhanced Ca(2+) entry in both types of myocytes. Additionally, electrocardiography showed that inhibition of TRPC3 channel suppressed transient A(1)AR-induced conduction disturbances in the embryonic heart. Our data showing that A(1)AR activation subtly mediates a proarrhythmic Ca(2+) entry through TRPC3-encoded ROC by stimulating the phospholipase C/DAG/PKC cascade provide evidence for a novel pathway whereby Ca(2+) entry and cardiac function are altered. Thus, the A(1)AR-TRPC3 axis may represent a potential therapeutic target.

Williams-Beuren syndrome hypercalcemia: is TRPC3 a novel mediator in calcium homeostasis?

Williams-Beuren syndrome (WBS) is a neurodevelopmental disorder associated with hypercalcemia of unknown origin. This syndrome results from the deletion of contiguous genes on chromosome 7, including the general transcription factor IIi gene. The general transcription factor IIi gene encodes TFII-I, which suppresses cell-surface accumulation of transient receptor potential C3 (TRPC3) channels, involved in calcium transport in lymphocytes. We describe the case of a patient with WBS with hypercalcemia associated with abnormal TRPC3 expression. Analysis of peripheral lymphocytes revealed a sharp increase in TRPC3 expression, compared with control patients. To investigate the potential role of TRPC3 in calcium homeostasis, we performed specific immunostaining on the intestine and the kidney, major calcium-regulating tissues. We provide the first demonstration that TRPC3 is expressed in normal digestive epithelium and renal tubules in control patients, and overexpressed in the intestine in the patient with WBS. Taken together, these data suggest that calcium metabolism abnormalities observed in WBS may be attributable to TFII-I haploinsufficiency and subsequent TRPC3 overexpression, thereby increasing both digestive and renal calcium absorption. This original observation prompts further investigation of TRPC3 as a novel actor of calcium homeostasis.

Functional interaction between AQP2 and TRPV4 in renal cells

We have previously demonstrated that renal cortical collecting duct cells (RCCD(1)), responded to hypotonic stress with a rapid activation of regulatory volume decrease (RVD) mechanisms. This process requires the presence of the water channel AQP2 and calcium influx, opening the question about the molecular identity of this calcium entry path. Since the calcium permeable nonselective cation channel TRPV4 plays a crucial role in the response to mechanical and osmotic perturbations in a wide range of cell types, the aim of this work was to test the hypothesis that the increase in intracellular calcium concentration and the subsequent rapid RVD, only observed in the presence of AQP2, could be due to a specific activation of TRPV4. We evaluated the expression and function of TRPV4 channels and their contribution to RVD in WT-RCCD(1) (not expressing aquaporins) and in AQP2-RCCD(1) (transfected with AQP2) cells. Our results demonstrated that both cell lines endogenously express functional TRPV4, however, a large activation of the channel by hypotonicity only occurs in cells that express AQP2. Blocking of TRPV4 by ruthenium red abolished calcium influx as well as RVD, identifying TRPV4 as a necessary component in volume regulation. Even more, this process is dependent on the translocation of TRPV4 to the plasma membrane. Our data provide evidence of a novel association between TRPV4 and AQP2 that is involved in the activation of TRPV4 by hypotonicity and regulation of cellular response to the osmotic stress, suggesting that both proteins are assembled in a signaling complex that responds to anisosmotic conditions.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Enhanced store-operated Ca²+ entry and TRPC channel expression in pulmonary arteries of monocrotaline-induced pulmonary hypertensive rats

Pulmonary hypertension (PH) is associated with profound vascular remodeling and alterations in Ca(2+) homeostasis in pulmonary arterial smooth muscle cells (PASMCs). Previous studies show that canonical transient receptor potential (TRPC) genes are upregulated and store-operated Ca(2+) entry (SOCE) is augmented in PASMCs of chronic hypoxic rats and patients of pulmonary arterial hypertension (PAH). Here we further examine the involvement of TRPC and SOCE in PH with a widely used rat model of monocrotaline (MCT)-induced PAH. Rats developed severe PAH, right ventricular hypertrophy, and significant increase in store-operated TRPC1 and TRPC4 mRNA and protein in endothelium-denuded pulmonary arteries (PAs) 3 wk after MCT injection. Contraction of PA and Ca(2+) influx in PASMC evoked by store depletion using cyclopiazonic acid (CPA) were enhanced dramatically, consistent with augmented SOCE in the MCT-treated group. The time course of increase in CPA-induced contraction corresponded to that of TRPC1 expression. Endothelin-1 (ET-1)-induced vasoconstriction was also potentiated in PAs of MCT-treated rats. The response was partially inhibited by SOCE blockers, including Gd(3+), La(3+), and SKF-96365, as well as the general TRPC inhibitor BTP-2, suggesting that TRPC-dependent SOCE was involved. Moreover, the ET-1-induced contraction and Ca(2+) response in the MCT group were more susceptible to the inhibition caused by the various SOCE blockers. Hence, our study shows that MCT-induced PAH is associated with increased TRPC expression and SOCE, which are involved in the enhanced vascular reactivity to ET-1, and support the hypothesis that TRPC-dependent SOCE is an important pathway for the development of PH.

Purinergic activation of Ca2+-permeable TRPV4 channels is essential for mechano-sensitivity in the aldosterone-sensitive distal nephron

Mechanical forces are known to induce increases of [Ca²⁺]_i in the aldosterone-sensitive distal nephron (ASDN) cells to regulate epithelial transport. At the same time, mechanical stress stimulates ATP release from ASDN cells. In this study, we combined ratiometric Fura-2 based monitoring of [Ca²⁺]_i in freshly isolated split-opened ASDN with targeted deletion of P2Y2 and TRPV4 in mice to probe a role for purinergic signaling in mediating mechano-sensitive responses in ASDN cells. ATP application causes a reproducible transient Ca²⁺ peak followed by a sustained plateau. Individual cells of the cortical collecting duct (CCD) and the connecting tubule (CNT) respond to purinergic stimulation with comparative elevations of [Ca²⁺]_i. Furthermore, ATP-induced Ca²⁺-responses are nearly identical in both principal (AQP2-positive) and intercalated (AQP2-negative) cells as was confirmed using immunohistochemistry in split-opened ASDN. UTP application produces elevations of [Ca²⁺]_i similar to that observed with ATP suggesting a dominant role of P2Y2-like receptors in generation of [Ca²⁺]_i response. Indeed, genetic deletion of P2Y2 receptors decreases the magnitude of ATP-induced and UTP-induced Ca²⁺ responses by more than 70% and 90%, respectively. Both intracellular and extracellular sources of Ca²⁺ appeared to contribute to the generation of ATP-induced Ca²⁺ response in ASDN cells. Importantly, flow- and hypotonic-induced Ca²⁺ elevations are markedly blunted in P2Y2 −/− mice. We further demonstrated that activation of mechano-sensitive TRPV4 channel plays a major role in the sustained [Ca²⁺]_i elevation during purinergic stimulation. Consistent with this, ATP-induced Ca²⁺ plateau are dramatically attenuated in TRV4 −/− mice. Inhibition of TRPC channels with 10 µM BTP2 also decreased ATP-induced Ca²⁺ plateau whilst to a lower degree than that observed with TRPV4 inhibition/genetic deletion. We conclude that stimulation of purinergic signaling by mechanical stimuli leads to activation of TRPV4 and, to a lesser extent, TRPCs channels, and this is an important component of mechano-sensitive response of the ASDN.

Regulation of TRP signalling by ion channel translocation between cell compartments

The TRP (transient receptor potential) family of ion channels is a heterogeneous family of calcium permeable cation channels that is subdivided into seven subfamilies: TRPC ("Canonical"), TRPV ("Vanilloid"), TRPM ("Melastatin"), TRPA ("Ankyrin"), TRPN ("NOMPC"), TRPP ("Polycystin"), and TRPML ("Mucolipin"). TRP-mediated ion currents across the cell membrane are determined by the single channel conductance, by the fraction of activated channels, and by the total amount of TRP channels present at the plasma membrane. In many cases, the amount of TRP channels at the plasma membrane is altered in response to physiological stimuli by translocation of channels to and from the plasma membrane. Regulated translocation has been described for channels of the TRPC, TRPV, TRPM, and TRPA family and is achieved by vesicular transport of these channels along cellular exocytosis and endocytosis pathways. This review summarizes the stimuli and signalling cascades involved in the translocation of TRP channels and highlights interactions of TRP channels with proteins of the endocytosis and exocytosis machineries.

Mechanisms of vasopressin-induced intracellular Ca2+ oscillations in rat inner medullary collecting duct

Arginine vasopressin (AVP) causes increase in intracellular Ca(2+) concentration with an oscillatory pattern. Ca(2+) mobilization is required for AVP-stimulated apical exocytosis in inner medullary collecting duct (IMCD). The mechanistic basis of these Ca(2+) oscillations was investigated by confocal fluorescence microscopy and flash photolysis of caged molecules in perfused IMCD. Photorelease of caged cAMP and direct activation of ryanodine receptors (RyRs) by photorelease of caged cyclic ADP-ribose (cADPR) both mimicked the AVP-induced Ca(2+) oscillations. Preincubation of IMCD with 100 μM 8-bromo-cADPR (a competitive inhibitor of cADPR) delayed the onset and attenuated the magnitude of AVP-induced Ca(2+) oscillations. These observations indicate that the cADPR/RyR pathway is capable of supporting Ca(2+) oscillations and endogenous cADPR plays a major role in the AVP-induced Ca(2+) oscillations in IMCD. In contrast, photorelease of caged inositol 1,4,5-trisphosphate (IP(3)) induced Ca(2+) release but did not maintain sustained Ca(2+) oscillations. Removal of extracellular Ca(2+) halted ongoing AVP-mediated Ca(2+) oscillation, suggesting that it requires extracellular Ca(2+) entry. AVP-induced Ca(2+) oscillation was unaffected by nifedipine. Intracellular Ca(2+) store depletion induced by 20 μM thapsigargin in Ca(2+)-free medium triggered store-operated Ca(2+) entry (SOCE) in IMCD, which was attenuated by 1 μM GdCl(3) and 50 μM SKF-96365. After incubation of IMCD with 1 nM AVP in Ca(2+)-free medium, application of extracellular Ca(2+) also triggered Ca(2+) influx, which was sensitive to GdCl(3) and SKF-96365. In summary, our observations are consistent with the notion that AVP-induced Ca(2+) oscillations in IMCD are mediated by the interplay of Ca(2+) release from RyRs and a Ca(2+) influx mechanism involving nonselective cation channels that resembles SOCE.

TRPC6 channels and their binding partners in podocytes: role in glomerular filtration and pathophysiology

Loss or dysfunction of podocytes is a major cause of glomerular kidney disease. Several genetic forms of glomerular disease are caused by mutations in genes that encode structural elements of the slit diaphragm or the underlying cytoskeleton of podocyte foot processes. The recent discovery that gain-of-function mutations in Ca(2+)-permeable canonical transient receptor potential-6 channels (TRPC6) underlie a subset of familial forms of focal segmental glomerulosclerosis (FSGS) has focused attention on the basic cellular physiology of podocytes. Several recent studies have examined the role of Ca(2+) dynamics in normal podocyte function and their possible contributions to glomerular disease. This review summarizes the properties of TRPC6 and related channels, focusing on their permeation and gating properties, the nature of mutations associated with familial FSGS, and the role of TRPC channels in podocyte cell biology as well as in glomerular pathophysiology. TRPC6 interacts with several proteins in podocytes, including essential slit diaphragm proteins and mechanosensitive large-conductance Ca(2+)-activated K(+) channels. The signaling dynamics controlling ion channel function and localization in podocytes appear to be quite complex.

Role of TRPC3 channels in ATP-induced Ca2+ signaling in principal cells of the inner medullary collecting duct

The transient receptor potential channel TRPC3 is exclusively expressed in the apical membrane of principal cells of the collecting duct (CD) both in vivo and in the mouse CD cell line IMCD-3. Previous studies revealed that ATP-induced apical-to-basolateral transepithelial Ca(2+) flux across IMCD-3 monolayers is increased by overexpression of TRPC3 and attenuated by a dominant negative TRPC3 construct, suggesting that Ca(2+) entry across the apical membrane occurs via TRPC3 channels. To test this hypothesis, we selectively measured the Ca(2+) permeability of the apical membrane of fura-2-loaded IMCD-3 cells using the Mn(2+) quench technique. Mn(2+) influx across the apical membrane was increased 12- to 16-fold by apical ATP and was blocked by the pyrazole derivative BTP2, a known inhibitor of TRPC3 channels, with an IC(50) value <100 nM. In contrast, Mn(2+) influx was only increased approximately 2-fold by basolateral ATP. Mn(2+) influx was also activated by apical, but not basolateral, 1-stearoyl-2-acetyl-sn-glycerol (SAG), a known activator of TRPC3 channels. Apical ATP- and SAG-induced Mn(2+) influx was increased by overexpression of TRPC3 and completely blocked by expression of the dominant negative TRPC3 construct. Mn(2+) influx was also stimulated approximately 2-fold by thapsigargin applied to either the apical or basolateral side. Thapsigargin-induced flux was blocked by BTP2 but was unaffected by overexpression of TRPC3 or by dominant negative TRPC3. Apical ATP, but not basolateral ATP, increased transepithelial (45)Ca(2+) flux. These results demonstrate that the apical membrane of IMCD-3 cells has two distinct Ca(2+) influx pathways: 1) a store-operated channel activated by thapsigargin and basolateral ATP and 2) TRPC3 channels activated by apical ATP. Only activation of TRPC3 leads to net transepithelial apical-to-basolateral Ca(2+) flux. Furthermore, these results demonstrate that native TRPC3 is not a store-operated channel in IMCD-3 cells.

Role of cAMP/PKA signaling cascade in vasopressin-induced trafficking of TRPC3 channels in principal cells of the collecting duct

Transient receptor potential channels TRPC3 and TRPC6 are expressed in principal cells of the collecting duct (CD) along with the water channel aquaporin-2 (AQP2) both in vivo and in the cultured mouse CD cell line IMCD-3. The channels are primarily localized to intracellular vesicles, but upon stimulation with the antidiuretic hormone arginine vasopressin (AVP), TRPC3 and AQP2 translocate to the apical membrane. In the present study, the effect of various activators and inhibitors of the adenylyl cyclase (AC)/cAMP/PKA signaling cascade on channel trafficking was examined using immunohistochemical techniques and by biotinylation of surface membrane proteins. Both in vivo in rat kidney and in IMCD-3 cells, translocation of AQP2 and TRPC3 (but not TRPC6) was stimulated by [deamino-Cys(1), d-Arg(8)]-vasopressin (dDAVP), a specific V2-receptor agonist, and blocked by [adamantaneacetyl(1), O-Et-d-Tyr(2), Val(4), aminobutyryl(6), Arg(8,9)]-vasopressin (AEAVP), a specific V2-receptor antagonist. In IMCD-3 cells, translocation of TRPC3 and AQP2 was activated by forskolin, a direct activator of AC, or by dibutyryl-cAMP, a membrane-permeable cAMP analog. AVP-, dDAVP-, and forskolin-induced translocation in IMCD-3 cells was blocked by SQ22536 and H89, specific inhibitors of AC and PKA, respectively. Translocation stimulated by dibutyryl-cAMP was unaffected by AEAVP but could be blocked by H89. AVP- and forskolin-induced translocation of TRPC3 in IMCD-3 cells was also blocked by two additional inhibitors of PKA, specifically Rp-cAMPS and the myristoylated inhibitor of PKA (m-PKI). Quantification of TRPC3 membrane insertion in IMCD-3 cells under each assay condition using a surface membrane biotinylation assay, confirmed the translocation results observed by immunofluorescence. Importantly, AVP-induced translocation of TRPC3 as estimated by biotinylation was blocked on average 95.2 +/- 1.0% by H89, Rp-cAMPS, or m-PKI. Taken together, these results demonstrate that AVP stimulation of V2 receptors in principal cells of the CD causes translocation of TRPC3 to the apical membrane via stimulation of the AC/cAMP/PKA signaling cascade.

Mechanoregulation of BK channel activity in the mammalian cortical collecting duct: role of protein kinases A and C

Flow-stimulated net K secretion (J(K)) in the cortical collecting duct (CCD) is mediated by an iberiotoxin (IBX)-sensitive BK channel, and requires an increase in intracellular Ca2+ concentration ([Ca2+](i)). The alpha-subunit of the reconstituted BK channel is phosphorylated by PKA and PKC. To test whether the BK channel in the native CCD is regulated by these kinases, J(K) and net Na absorption (J(Na)) were measured at slow (approximately 1) and fast (approximately 5 nl x min(-1) x mm(-1)) flow rates in rabbit CCDs microperfused in the presence of mPKI, an inhibitor of PKA; calphostin C, which inhibits diacylglycerol binding proteins, including PKC; or bisindolylmaleimide (BIM) and Gö6976, inhibitors of classic and novel PKC isoforms, added to luminal (L) and/or basolateral (B) solutions. L but not B mPKI increased J(K) in CCDs perfused at a slow flow rate; a subsequent increase in flow rate augmented J(K) modestly. B mPKI alone or with L inhibitor abolished flow stimulation of J(K). Similarly, L calphostin C increased J(K) in CCDs perfused at slow flow rates, as did calphostin C in both L and B solutions. The observation that IBX inhibited the L mPKI- and calphostin C-mediated increases in J(K) at slow flow rates implicated the BK channel in this K flux, a notion suggested by patch-clamp analysis of principal cells. The kinase inhibited by calphostin C was not PKC as L and/or B BIM and Gö6976 failed to enhance J(K) at the slow flow rate. However, addition of these PKC inhibitors to the B solution alone or with L inhibitor blocked flow stimulation of J(K). Interpretation of these results in light of the effects of these inhibitors on the flow-induced elevation of [Ca2+](i) suggests that the principal cell apical BK channel is tonically inhibited by PKA and that flow stimulation of J(K) in the CCD is PKA and PKC dependent. The specific targets of the kinases remain to be identified.

A voltage-dependent Ca2+ influx pathway regulates the Ca2+-dependent Cl(-) conductance of renal IMCD-3 cells

We have previously shown that the membrane conductance of mIMCD-3 cells at a holding potential of 0 mV is dominated by a Ca2+-dependent Cl(-) current (I(CLCA)). Here we report that I(CLCA) activity is also voltage dependent and that this dependence on voltage is linked to the opening of a novel Al3+-sensitive, voltage-dependent, Ca2+ influx pathway. Using whole-cell patch-clamp recordings at a physiological holding potential (-60 mV), ICLCA was found to be inactive and resting currents were predominantly K+ selective. However, membrane depolarization to 0 mV resulted in a slow, sigmoidal, activation of ICLCA (T(0.5) approximately 500 s), while repolarization in turn resulted in a monoexponential decay in I(CLCA) (T (0.5) approximately 100 s). The activation of I(CLCA) by depolarization was reduced by lowering extracellular Ca2+ and completely inhibited by buffering cytosolic Ca2+ with EGTA, suggesting a role for Ca2+ influx in the activation of I(CLCA). However, raising bulk cytosolic Ca2+ at -60 mV did not produce sustained I(CLCA) activity. Therefore I(CLCA) is dependent on both an increase in intracellular Ca2+ and depolarization to be active. We further show that membrane depolarization is coupled to opening of a Ca2+ influx pathway that displays equal permeability to Ca2+ and Ba2+ ions and that is blocked by extracellular Al3+ and La3+. Furthermore, Al3+ completely and reversibly inhibited depolarization-induced activation of ICLCA, thereby directly linking Ca2+ influx to activation of I(CLCA). We speculate that during sustained membrane depolarization, calcium influx activates ICLCA which functions to modulate NaCl transport across the apical membrane of IMCD cells.

The role of transient receptor potential channels in kidney disease

The transient receptor potential (TRP) superfamily consists, in mammals, of six protein subfamilies, TRPC, TRPM, TRPV, TRPA, TRPML and TRPP. TRPs are cation channels involved in many physiological processes and in the pathogenesis of various disorders. In the kidney, TRP channels are expressed along the nephron, and a role for some of these channels in renal function has been proposed. TRPC3 is thought to facilitate calcium ion influx into the principal cells of the collecting duct in response to vasopressin. TRPM3 and TRPV4 might be osmosensors, whereas the TRPP1/TRPP2 complex could function as a mechanosensor in the cilia of renal epithelial cells. A number of kidney diseases have also been linked to dysfunctional activity of TRPs. TRPC6 dysfunction has been associated with the onset of focal segmental glomerosclerosis; TRPP2 dysfunction is linked to autosomal-dominant polycystic kidney disease, TRPM6 mutations underlie hypomagnesemia with secondary hypocalcemia, and TRPV1 dysfunction is implicated in renal hypertension. A link between TRPC1 dysfunction and diabetic nephropathy has also been suggested in an animal model. Animal studies have implicated a role for TRPV5 in idiopathic hypercalciuria and vitamin D-dependent rickets, although these observations have not been confirmed in patients. This Review focuses on the role of renal TRP channels in health and disease.

TRPC3 activation by erythropoietin is modulated by TRPC6

Regulation of intracellular calcium ([Ca(2+)](i)) by erythropoietin (Epo) is an essential part of signaling pathways controlling proliferation and differentiation of erythroid progenitors, but regulatory mechanisms are largely unknown. TRPC3 and the homologous TRPC6 are two members of the transient receptor potential channel (TRPC) superfamily that are expressed on normal human erythroid precursors. Here we show that TRPC3 expression increases but TRPC6 decreases during erythroid differentiation. This is associated with a significantly greater increase in [Ca(2+)](i) in response to Epo stimulation, suggesting that the ratio of TRPC3/TRPC6 is physiologically important. In HEK 293T cells heterologously expressing TRPC and erythropoietin receptor (Epo-R), Epo stimulated an increase in [Ca(2+)](i) through TRPC3 but not TRPC6. Replacement of the C terminus of TRPC3 with the TRPC6 C terminus (TRPC3-C6C) resulted in loss of activation by Epo. In contrast, substitution of the C terminus of TRPC6 with that of TRPC3 (TRPC6-C3C) resulted in an increase in [Ca(2+)](i) in response to Epo. Substitution of the N termini had no effect. Domains in the TRPC3 C terminus between amino acids 671 and 746 are critical for the response to Epo. Epo-R and phospholipase Cgamma associated with TRPC3, and these interactions were significantly reduced with TRPC6 and TRPC3-C6C chimeras. TRPC3 and TRPC6 form heterotetramers. Coexpression of TRPC6 or C3/C6 chimeras with TRPC3 and Epo-R inhibited the Epo-stimulated increase in [Ca(2+)](i). In a heterologous expression system, Epo stimulation increased cell surface expression of TRPC3, which was inhibited by TRPC6. However, in primary erythroblasts, an increase in TRPC3 cell surface expression was not observed in erythroblasts in which Epo stimulated an increase in [Ca(2+)](i), demonstrating that increased membrane insertion of TRPC3 is not required. These data demonstrate that TRPC6 regulates TRPC3 activation by Epo. Endogenously, regulation of TRPC3 by TRPC6 may primarily be through modulation of signaling mechanisms, including reduced interaction of TRPC6 with phospholipase Cgamma and Epo-R.

Controlled aquaporin-2 expression in the hypertonic environment

The corticomedullary osmolality gradient is the driving force for water reabsorption occurring in the kidney. In the collecting duct, this gradient allows luminal water to move across aquaporin (AQP) water channels, thereby increasing urine concentration. However, this same gradient exposes renal cells to great osmotic challenges. These cells must constantly adapt to fluctuations of environmental osmolality that challenge cell volume and incite functional change. This implies profound alterations of cell phenotype regarding water permeability. AQP2 is an essential component of the urine concentration mechanism whose controlled expression dictates apical water permeability of collecting duct principal cells. This review focuses on changes of AQP2 abundance and trafficking in hypertonicity-challenged cells. Intracellular mechanisms governing these events are discussed and the biological relevance of altered AQP2 expression by hypertonicity is outlined.

Physiology and pathophysiology of canonical transient receptor potential channels

The existence of a mammalian family of TRPC ion channels, direct homologues of TRP, the visual transduction channel of flies, was discovered during 1995-1996 as a consequence of research into the mechanism by which the stimulation of the receptor-Gq-phospholipase Cbeta signaling pathway leads to sustained increases in intracellular calcium. Mammalian TRPs, TRPCs, turned out to be nonselective, calcium-permeable cation channels, which cause both a collapse of the cell's membrane potential and entry of calcium. The family comprises 7 members and is widely expressed. Many cells and tissues express between 3 and 4 of the 7 TRPCs. Despite their recent discovery, a wealth of information has accumulated, showing that TRPCs have widespread roles in almost all cells studied, including cells from excitable and nonexcitable tissues, such as the nervous and cardiovascular systems, the kidney and the liver, and cells from endothelia, epithelia, and the bone marrow compartment. Disruption of TRPC function is at the root of some familial diseases. More often, TRPCs are contributing risk factors in complex diseases. The present article reviews what has been uncovered about physiological roles of mammalian TRPC channels since the time of their discovery. This analysis reveals TRPCs as major and unsuspected gates of Ca(2+) entry that contribute, depending on context, to activation of transcription factors, apoptosis, vascular contractility, platelet activation, and cardiac hypertrophy, as well as to normal and abnormal cell proliferation. TRPCs emerge as targets for a thus far nonexistent field of pharmacological intervention that may ameliorate complex diseases.