Transient receptor potential vanilloid type 1 channels act as mechanoreceptors and cause substance P release and sensory activation in rat kidneys

Neuroimmune interplay in kidney health and disease: Role of renal nerves

Renal nerves and their role in physiology and disease have been a topic of increasing interest in the past few decades. Renal inflammation contributes to many cardiorenal disease conditions, including hypertension, chronic kidney disease, and polycystic kidney disease. Much is known about the role of renal sympathetic nerves in physiology - they contribute to the regulation of sodium reabsorption, renin release, and renal vascular resistance. In contrast, far less is known about afferent, or "sensory," renal nerves, which convey signals from the kidney to the brain. While much remains unknown about these nerves in the context of normal physiology, even less is known about their contribution to disease states. Furthermore, it has become apparent that the crosstalk between renal nerves and the immune system may augment or modulate disease. Research from other fields, especially pain research, has provided critical insight into neuroimmune crosstalk. Sympathetic renal nerve activity may increase immune cell recruitment, but far less work has been done investigating the interplay between afferent renal nerves and the immune system. Evidence from other fields suggests that inflammation may augment afferent renal nerve activity. Furthermore, these nerves may exacerbate renal inflammation through the release of afferent-specific neurotransmitters.

Mechanosensitive ion channel Piezo1 mediates mechanical ventilation-exacerbated ARDS-associated pulmonary fibrosis

INTRODUCTION

Pulmonary fibrosis is a major cause of the poor prognosis of acute respiratory distress syndrome (ARDS). While mechanical ventilation (MV) is an indispensable life-saving intervention for ARDS, it may cause the remodeling process in lung epithelial cells to become disorganized and exacerbate ARDS-associated pulmonary fibrosis. Piezo1 is a mechanosensitive ion channel that is known to play a role in regulating diverse physiological processes, but whether Piezo1 is necessary for MV-exacerbated ARDS-associated pulmonary fibrosis remains unknown.

OBJECTIVES

This study aimed to explore the role of Piezo1 in MV-exacerbated ARDS-associated pulmonary fibrosis.

METHODS

Human lung epithelial cells were stimulated with hydrochloric acid (HCl) followed by mechanical stretch for 48 h. A two-hitmodel of MV afteracidaspiration-inducedlunginjuryin mice was used. Mice were sacrificed after 14 days of MV. Pharmacological inhibition and knockout of Piezo1 were used to delineate the role of Piezo1 in MV-exacerbated ARDS-associated pulmonary fibrosis. In some experiments, ATP or the ATP-hydrolyzing enzyme apyrase was administered.

RESULTS

The stimulation of human lung epithelial cells to HCl resulted in phenotypes of epithelial-mesenchymal transition (EMT), which were enhanced by mechanical stretching. MV exacerbated pulmonary fibrosis in mice exposed to HCl. Pharmacologicalinhibitionorknockout of Piezo1 attenuated the MV-exacerbated EMT process and lung fibrosis in vivo and in vitro. Mechanistically, the observed effects were mediated by Piezo1-dependent Ca2+ influx and ATP release in lung epithelial cells.

CONCLUSIONS

Our findings identify a key role for Piezo1 in MV-exacerbated ARDS-associated pulmonary fibrosis that is mediated by increased ATP release in lung epithelial cells. Inhibiting Piezo1 may constitute a novelstrategyfor the treatment of MV-exacerbated ARDS-associated pulmonary fibrosis.

Meta-Analysis of Mechano-Sensitive Ion Channels in Human Hearts: Chamber- and Disease-Preferential mRNA Expression

The cardiac cell mechanical environment changes on a beat-by-beat basis as well as in the course of various cardiac diseases. Cells sense and respond to mechanical cues via specialized mechano-sensors initiating adaptive signaling cascades. With the aim of revealing new candidates underlying mechano-transduction relevant to cardiac diseases, we investigated mechano-sensitive ion channels (MSC) in human hearts for their chamber- and disease-preferential mRNA expression. Based on a meta-analysis of RNA sequencing studies, we compared the mRNA expression levels of MSC in human atrial and ventricular tissue samples from transplant donor hearts (no cardiac disease), and from patients in sinus rhythm (underlying diseases: heart failure, coronary artery disease, heart valve disease) or with atrial fibrillation. Our results suggest that a number of MSC genes are expressed chamber preferentially, e.g., CHRNE in the atria (compared to the ventricles), TRPV4 in the right atrium (compared to the left atrium), CACNA1B and KCNMB1 in the left atrium (compared to the right atrium), as well as KCNK2 and KCNJ2 in ventricles (compared to the atria). Furthermore, 15 MSC genes are differentially expressed in cardiac disease, out of which SCN9A (lower expressed in heart failure compared to donor tissue) and KCNQ5 (lower expressed in atrial fibrillation compared to sinus rhythm) show a more than twofold difference, indicative of possible functional relevance. Thus, we provide an overview of cardiac MSC mRNA expression in the four cardiac chambers from patients with different cardiac diseases. We suggest that the observed differences in MSC mRNA expression may identify candidates involved in altered mechano-transduction in the respective diseases.

Periglomerular afferent innervation of the mouse renal cortex

Recent studies using a novel method for targeted ablation of afferent renal nerves have demonstrated their importance in the development and maintenance of some animal models of hypertension. However, relatively little is known about the anatomy of renal afferent nerves distal to the renal pelvis. Here, we investigated the anatomical relationship between renal glomeruli and afferent axons identified based on transient receptor potential vanilloid 1 channel (TRPV1) lineage or calcitonin gene related peptide (CGRP) immunolabeling. Analysis of over 6,000 (10,000 was accurate prior to the removal of the TH data during the review process) glomeruli from wildtype C57BL/6J mice and transgenic mice expressing tdTomato in TRPV1 lineage cells indicated that approximately half of all glomeruli sampled were closely apposed to tdTomato+ or CGRP+ afferent axons. Glomeruli were categorized as superficial, midcortical, or juxtamedullary based on their depth within the cortex. Juxtamedullary glomeruli were more likely to be closely apposed by afferent axon subtypes than more superficial glomeruli. High-resolution imaging of thick, cleared renal slices and subsequent distance transformations revealed that CGRP+ axons closely apposed to glomeruli were often found within 2 microns of nephrin+ labeling of glomerular podocytes. Furthermore, imaging of thick slices suggested that CGRP+ axon bundles can closely appose multiple glomeruli that share the same interlobular artery. Based on their expression of CGRP or tdTomato, prevalence near glomeruli, proximity to glomerular structures, and close apposition to multiple glomeruli within a module, we hypothesize that periglomerular afferent axons may function as mechanoreceptors monitoring glomerular pressure. These anatomical findings highlight the importance of further studies investigating the physiological role of periglomerular afferent axons in neural control of renal function in health and disease.

Capsaicin Decreases Kidney Iron Deposits and Increases Hepcidin Levels in Diabetic Rats with Iron Overload: A Preliminary Study

Iron overload (IOL) increases the risk of diabetes mellitus (DM). Capsaicin (CAP), an agonist of transient receptor potential vanilloid-1 (TRPV1), reduces the effects of IOL. We evaluated the effects of chronic CAP administration on hepcidin expression, kidney iron deposits, and urinary biomarkers in a male Wistar rat model with IOL and DM (DM-IOL). IOL was induced with oral administration of iron for 12 weeks and DM was induced with streptozotocin. Four groups were studied: Healthy, DM, DM-IOL, and DM-IOL + CAP (1 mg·kg^-1·day^-1 for 12 weeks). Iron deposits were visualized with Perls tissue staining and a colorimetric assay. Serum hepcidin levels were measured with an enzyme-linked immunosorbent assay. Kidney biomarkers were assayed in 24 h urine samples. In the DM-IOL + CAP group, the total area of iron deposits and the total iron content in kidneys were smaller than those observed in both untreated DM groups. CAP administration significantly increased hepcidin levels in the DM-IOL group. Urinary levels of albumin, cystatin C, and beta-2-microglobulin were similar in all three experimental groups. In conclusion, we showed that in a DM-IOL animal model, CAP reduced renal iron deposits and increased the level of circulating hepcidin.

Ion channels as a therapeutic target for renal fibrosis

Renal ion channel transport and electrolyte disturbances play an important role in the process of functional impairment and fibrosis in the kidney. It is well known that there are limited effective drugs for the treatment of renal fibrosis, and since a large number of ion channels are involved in the renal fibrosis process, understanding the mechanisms of ion channel transport and the complex network of signaling cascades between them is essential to identify potential therapeutic approaches to slow down renal fibrosis. This review summarizes the current work of ion channels in renal fibrosis. We pay close attention to the effect of cystic fibrosis transmembrane conductance regulator (CFTR), transmembrane Member 16A (TMEM16A) and other Cl− channel mediated signaling pathways and ion concentrations on fibrosis, as well as the various complex mechanisms for the action of Ca2+ handling channels including Ca2+-release-activated Ca2+ channel (CRAC), purinergic receptor, and transient receptor potential (TRP) channels. Furthermore, we also focus on the contribution of Na+ transport such as epithelial sodium channel (ENaC), Na+, K+-ATPase, Na+-H+ exchangers, and K+ channels like Ca2+-activated K+ channels, voltage-dependent K+ channel, ATP-sensitive K+ channels on renal fibrosis. Proposed potential therapeutic approaches through further dissection of these mechanisms may provide new therapeutic opportunities to reduce the burden of chronic kidney disease.

Sympathoexcitatory responses to renal chemosensitive stimuli are exaggerated at nighttime in rats

The circadian cycle impacts sympathetic nerve activity (SNA), cardiovascular hemodynamics, and renal function. Activation of renal sensory nerves by chemosensory and mechanosensory stimuli reflexively changes efferent SNA and arterial blood pressure (ABP) to maintain homeostasis. However, it is unclear to what extent circadian cycle influences reflex SNA and ABP responses to renal sensory stimuli. Renal, splanchnic, and lumbar SNA and ABP responses to intrarenal arterial infusion of bradykinin or capsaicin and elevated renal pelvic pressure were measured in male and female Sprague-Dawley rats during nighttime (wakeful/active phase) and daytime (inactive phase). Intrarenal arterial bradykinin infusion significantly increased efferent renal SNA, splanchnic SNA, and ABP but not lumbar SNA. Responses were greater during nighttime versus daytime. Similarly, intrarenal arterial capsaicin infusion significantly increased renal SNA and splanchnic SNA, and responses were again greater during nighttime. Elevated renal pelvic pressure increased renal SNA and splanchnic SNA; however, responses did not differ between daytime and nighttime. Finally, afferent renal nerve activity responses to bradykinin were not different between daytime and nighttime. Thus, renal chemokines elicit greater sympathoexcitatory responses at nighttime that cannot be attributed to differences in afferent renal nerve activity. Collectively, these data suggest that the circadian cycle alters the excitability of central autonomic networks to alter baseline SNA and ABP as well as the magnitude of visceral reflexes.NEW & NOTEWORTHY The current study discovers that the circadian cycle influences sympathetic and hemodynamic responses to activation of renal chemosensitive sensory fibers. Sympathetic responses to intrarenal bradykinin or capsaicin infusion were exaggerated during nighttime (active period), but mechanosensitive responses to elevated renal pelvic pressure were not. Importantly, renal afferent nerve responses were not different between nighttime and daytime. These data suggest that the circadian cycle modulates sympathetic responses to visceral afferent activation.

Calcium influx through TRPV4 channels involve in hyperosmotic stress-induced epithelial-mesenchymal transition in tubular epithelial cells

The epithelial-mesenchymal transition (EMT) is a biological process that occurs in the pathogenesis of kidney diseases in which injured tubular epithelial cells transform into myofibroblasts. We previously showed that mannitol-mediated hyperosmotic stress induces EMT of tubular epithelial cells. Although Ca²⁺ signaling is essential for the induction of EMT in tubular epithelial cells, the role of specific calcium channels is unknown. In this study, we assessed the transient receptor potential vanilloid 4 (TRPV4)-mediated Ca²⁺ influx in the hyperosmolarity-induced EMT. The Fluo-4 assay was used to examine the effect of hyperosmotic stress on the intracellular Ca²⁺ level of normal rat kidney (NRK)-52E cells. Expression of a mesenchymal marker α-smooth muscle actin (α-SMA) and an epithelial marker E-cadherin was also observed by fluorescence microscopy. The hyperosmotic stress caused a transient increase in intracellular Ca²⁺ concentration as well as a decrease in E-cadherin and an increase in α-SMA expressions in tubular epithelial cells, indicating the induction of EMT. A TRPV4 channel antagonist inhibited hyperosmotic stress-induced Ca²⁺ influx and the EMT, whereas, a TRPV4 channel agonist increased Ca²⁺ influx and EMT induction in tubular epithelial cells without the hyperosmotic stress. These findings suggest that Ca²⁺ influx through TRPV4 channels contributes to the hyperosmotic stress-induced EMT of tubular epithelial cells.

Impact of anesthesia and sex on sympathetic efferent and hemodynamic responses to renal chemo- and mechanosensitive stimuli

Activation of renal sensory nerves by chemo- and mechanosensitive stimuli produces changes in efferent sympathetic nerve activity (SNA) and arterial blood pressure (ABP). Anesthesia and sex influence autonomic function and cardiovascular hemodynamics, but it is unclear to what extent anesthesia and sex impact SNA and ABP responses to renal sensory stimuli. We measured renal, splanchnic, and lumbar SNA and ABP in male and female Sprague-Dawley rats during contralateral renal infusion of capsaicin and bradykinin or during elevation in renal pelvic pressure. Responses were evaluated with a decerebrate preparation or Inactin, urethane, or isoflurane anesthesia. Intrarenal arterial infusion of capsaicin (0.1-30.0 μM) increased renal SNA, splanchnic SNA, or ABP but decreased lumbar SNA in the Inactin group. Intrarenal arterial infusion of bradykinin (0.1-30.0 μM) increased renal SNA, splanchnic SNA, and ABP but decreased lumbar SNA in the Inactin group. Elevated renal pelvic pressure (0-20 mmHg, 30 s) significantly increased renal SNA and splanchnic SNA but not lumbar SNA in the Inactin group. In marked contrast, SNA and ABP responses to every renal stimulus were severely blunted in the urethane and decerebrate groups and absent in the isoflurane group. In the Inactin group, the magnitude of SNA responses to chemo- and mechanosensory stimuli were not different between male and female rats. Thus, chemo- and mechanosensitive stimuli produce differential changes in renal, splanchnic, and lumbar SNA. Experimentally, future investigations should consider Inactin anesthesia to examine sympathetic and hemodynamic responses to renal sensory stimuli.NEW & NOTEWORTHY The findings highlight the impact of anesthesia, and to a lesser extent sex, on sympathetic efferent and hemodynamic responses to chemosensory and mechanosensory renal stimuli. Sympathetic nerve activity (SNA) and arterial blood pressure (ABP) responses were present in Inactin-anesthetized rats but largely absent in decerebrate, isoflurane, or urethane preparations. Renal chemosensory stimuli differentially changed SNA: renal and splanchnic SNA increased, but lumbar SNA decreased. Future investigations should consider Inactin anesthesia to study SNA and hemodynamic responses to renal sensory nerve activation.

TRPV1 Hyperfunction Contributes to Renal Inflammation in Oxalate Nephropathy

Inflammation worsens oxalate nephropathy by exacerbating tubular damage. The transient receptor potential vanilloid 1 (TRPV1) channel is present in kidney and has a polymodal sensing ability. Here, we tested whether TRPV1 plays a role in hyperoxaluria-induced renal inflammation. In TRPV1-expressed proximal tubular cells LLC-PK₁, oxalate could induce cell damage in a time- and dose-dependent manner; this was associated with increased arachidonate 12-lipoxygenase (ALOX12) expression and synthesis of endovanilloid 12(S)-hydroxyeicosatetraenoic acid for TRPV1 activation. Inhibition of ALOX12 or TRPV1 attenuated oxalate-mediated cell damage. We further showed that increases in intracellular Ca²⁺ and protein kinase C α activation are downstream of TRPV1 for NADPH oxidase 4 upregulation and reactive oxygen species formation. These trigger tubular cell inflammation via increased NLR family pyrin domain-containing 3 expression, caspase-1 activation, and interleukin (IL)-1β release, and were alleviated by TRPV1 inhibition. Male hyperoxaluric rats demonstrated urinary supersaturation, tubular damage, and oxidative stress in a time-dependent manner. Chronic TRPV1 inhibition did not affect hyperoxaluria and urinary supersaturation, but markedly reduced tubular damage and calcium oxalate crystal deposition by lowering oxidative stress and inflammatory signaling. Taking all these results together, we conclude that TRPV1 hyperfunction contributes to oxalate-induced renal inflammation. Blunting TRPV1 function attenuates hyperoxaluric nephropathy.

NK1 antagonists attenuate tau phosphorylation after blast and repeated concussive injury

Exposure to repeated concussive traumatic brain injury (TBI) and to blast-induced TBI has been associated with the potential development of the neurodegenerative condition known as chronic traumatic encephalopathy (CTE). CTE is characterized by the accumulation of hyperphosphorylated tau protein, with the resultant tau tangles thought to initiate the cognitive and behavioral manifestations that appear as the condition progresses. However, the mechanisms linking concussive and blast TBI with tau hyperphosphorylation are unknown. Here we show that single moderate TBI, repeated concussive TBI and blast-induced mild TBI all result in hyperphosphorylation of tau via a substance P mediated mechanism. Post-injury administration of a substance P, NK1 receptor antagonist attenuated the injury-induced phosphorylation of tau by modulating the activity of several key kinases including Akt, ERK1/2 and JNK, and was associated with improvement in neurological outcome. We also demonstrate that inhibition of the TRPV1 mechanoreceptor, which is linked to substance P release, attenuated injury-associated tau hyperphosphorylation, but only when it was administered prior to injury. Our results demonstrate that TBI-mediated stimulation of brain mechanoreceptors is associated with substance P release and consequent tau hyperphosphorylation, with administration of an NK1 receptor antagonist attenuating tau phosphorylation and associated neurological deficits. NK1 antagonists may thus represent a pharmacological approach to attenuate the potential development of CTE following concussive and blast TBI.

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.

Structure, kinetic properties and biological function of mechanosensitive Piezo channels

Mechanotransduction couples mechanical stimulation with ion flux, which is critical for normal biological processes involved in neuronal cell development, pain sensation, and red blood cell volume regulation. Although they are key mechanotransducers, mechanosensitive ion channels in mammals have remained difficult to identify. In 2010, Coste and colleagues revealed a novel family of mechanically activated cation channels in eukaryotes, consisting of Piezo1 and Piezo2 channels. These have been proposed as the long-sought-after mechanosensitive cation channels in mammals. Piezo1 and Piezo2 exhibit a unique propeller-shaped architecture and have been implicated in mechanotransduction in various critical processes, including touch sensation, balance, and cardiovascular regulation. Furthermore, several mutations in Piezo channels have been shown to cause multiple hereditary human disorders, such as autosomal recessive congenital lymphatic dysplasia. Notably, mutations that cause dehydrated hereditary xerocytosis alter the rate of Piezo channel inactivation, indicating the critical role of their kinetics in normal physiology. Given the importance of Piezo channels in understanding the mechanotransduction process, this review focuses on their structural details, kinetic properties and potential function as mechanosensors. We also briefly review the hereditary diseases caused by mutations in Piezo genes, which is key for understanding the function of these proteins.

Impact of anesthesia, sex, and circadian cycle on renal afferent nerve sensitivity

Elevated renal afferent nerve (ARNA) activity or dysfunctional reno-renal reflexes via altered ARNA sensitivity contribute to hypertension and chronic kidney disease. These nerves contain mechano- and chemosensitive fibers that respond to ischemia, changes in intrarenal pressures, and chemokines. Most studies have utilized various anesthetized preparations and exclusively male animals to characterize ARNA responses. Therefore, this study assessed the impact of anesthesia, sex, and circadian period on ARNA responses and sensitivity. Multifiber ARNA recordings were performed in male and female Sprague-Dawley rats (250-400 g) and compared across decerebrate versus Inactin, isoflurane, and urethane anesthesia groups. Intrarenal artery infusion of capsaicin (0.1-50.0 μM, 0.05 mL) produced concentration-dependent increases in ARNA; however, the ARNA sensitivity was significantly greater in decerebrate versus Inactin, isoflurane, and urethane groups. Increases in renal pelvic pressure (0-30 mmHg, 30 s) produced pressure-dependent increases in ARNA; however, ARNA sensitivity was again greater in decerebrate and Inactin groups versus isoflurane and urethane. Acute renal artery occlusion (30 s) increased ARNA, but responses did not differ across groups. Analysis of ARNA responses to increased pelvic pressure between male and female rats revealed significant sex differences only in isoflurane and urethane groups. ARNA responses to intrarenal capsaicin infusion were significantly blunted at nighttime versus daytime; however, ARNA responses to increased pelvic pressure or renal artery occlusion were not different between daytime and nighttime. These results demonstrate that ARNA sensitivity is greatest in decerebrate and Inactin-anesthetized groups but was not consistently influenced by sex.NEW & NOTEWORTHY We determined the impact of anesthesia, sex, and circadian cycle on renal afferent nerve (ARNA) sensitivity to chemical and mechanical stimuli. ARNA sensitivity to renal capsaicin infusion was greatest in decerebrate > Inactin > urethane or isoflurane groups. Elevated renal pelvic pressure significantly increased ARNA; decerebrate and Inactin groups exhibited the greatest ARNA sensitivity. Sex differences in renal afferent responses were not consistently observed. Circadian cycle altered chemosensory but not mechanosensory responses.

The involvement of renal afferents in the maintenance of cardiorenal diseases

Elevated sympathetic vasomotor activity is a common feature of cardiorenal diseases. Therefore, the sympathetic nervous system is an important therapeutic target, particularly the fibers innervating the kidneys. In fact, renal denervation has been applied clinically and shown promising results in patients with hypertension and chronic kidney disease. However, the underlying mechanisms involved in the cardiorenal protection induced by renal denervation have not yet been fully clarified. This mini-review highlights historical and recent aspects related to the role of renal sensory fibers in the control of cardiorenal function under normal conditions and in experimental models of cardiovascular disease. Results have demonstrated that alterations in renal sensory function participate in the maintenance of elevated sympathetic vasomotor activity and cardiorenal changes; as such, renal sensory fibers may be a potential therapeutic target for the treatment of cardiorenal diseases. Although it has not yet been applied in clinical practice, selective afferent renal denervation may be promising, since such an approach maintains efferent activity and can provide more refined control of renal function compared with total renal denervation. However, more studies are needed to understand the mechanisms by which renal afferents partially contribute to such changes, in addition to the need to evaluate the safety and advantages of the approach for application in the clinical practice.

TRPV1 Hyperfunction Involved in Uremic Toxin Indoxyl Sulfate-Mediated Renal Tubular Damage

Indoxyl sulfate (IS) is accumulated during severe renal insufficiency and known for its nephrotoxic properties. Transient receptor potential vanilloid 1 (TRPV1) is present in the kidney and acts as a renal sensor. However, the mechanism underlying IS-mediated renal tubular damage in view of TRPV1 is lacking. Here, we demonstrated that TRPV1 was expressed in tubular cells of Lilly Laboratories cell-porcine kidney 1 (LLC-PK₁) and Madin-Darby canine kidney cells (MDCK). IS treatment in both cells exhibited tubular damage with increased LDH release and reduced cell viability in dose- and time-dependent manners. MDCK, however, was more vulnerable to IS. We, therefore, investigated MDCK cells to explore a more detailed mechanism. Interestingly, IS-induced tubular damage was markedly attenuated in the presence of selective TRPV1 blockers. IS showed no effect on TRPV1 expression but significantly increased arachidonate 12-lipoxygenase (ALOX12) protein, mRNA expression, and 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE) amounts in a dose-dependent manner, indicating that the ALOX12/12(S)-HETE pathway induced TRPV1 hyperfunction in IS-mediated tubulotoxicity. Blockade of ALOX12 by cinnamyl-3,4-dihydroxy-α-cyanocinnamate or baicalein attenuated the effects of IS. Since aryl hydrocarbon receptor (AhR) activation after IS binding is crucial in mediating cell death, here, we found that the AhR blockade not only ameliorated tubular damage but also attenuated ALOX12 expression and 12(S)-HETE production caused by IS. The uremic toxic adsorbent AST-120, however, showed little effect on ALOX12 and 12(S)-HETE, as well as IS-induced cell damage. These results clearly indicated that IS activated AhR and then upregulated ALOX12, and this induced endovanilloid 12(S)-HETE synthesis and contributed to TRPV1 hyperfunction in IS-treated tubular cells. Further study on TRPV1 may attenuate kidney susceptibility to the functional loss of end-stage kidney disease via IS.

Ablation of TRPV1 Elevates Nocturnal Blood Pressure in Western Diet-fed Mice

Background: This study tested the hypothesis that genetically ablation of transient receptor potential vanilloid type 1 (TRPV1) exacerbates impairment of baroreflex in mice fed a western diet (WD) and leads to distinct diurnal and nocturnal blood pressure patterns.

Methods: TRPV1 gene knockout (TRPV1^-/-) and wild-type (WT) mice were given a WD or normal diet (CON) for 4 months.

Results: Capsaicin, a selective TRPV1 agonist, increased ipsilateral afferent renal nerve activity in WT but not TRPV1^-/- mice. The sensitivity of renal sympathetic nerve activity and heart rate responses to baroreflex were reduced in TRPV1^-/--CON and WT-WD and further decreased in TRPV1^-/--WD compared to the WT-CON group. Urinary norepinephrine and serum insulin and leptin at day and night were increased in WT-WD and TRPV1^-/--WD, with further elevation at night in TRPV1^-/--WD. WD intake increased leptin, IL-6, and TNF-α in adipose tissue, and TNF-α antagonist III, R-7050, decreased leptin in TRPV1^-/--WD. The urinary albumin level was higher in TRPV1^-/--WD than WT-WD. Blood pressure was not dif-ferent during daytime among all groups, but increased at night in the TRPV1^-/--WD group compared with other groups.

Conclusions: TRPV1 ablation leads to elevated nocturnal but not diurnal blood pressure, which is probably attributed to fur-ther enhancement of sympathetic drives at night.

Activation of TRPV1 channel antagonizes diabetic nephropathy through inhibiting endoplasmic reticulum-mitochondria contact in podocytes

The impairment of podocyte protein filtration function caused by excessive mitochondrial calcium intake is a critical feature of diabetic nephropathy (DN). Ca²⁺ channel transient receptor potential cation channel subfamily V member 1 (TRPV1) has been reported to protect against ischemia-reperfusion induced acute renal injury, but there is no report about its role in DN. Here, we report that dietary capsaicin potently inhibits and reverses chronic renal structural and functional damages in db/db or streptozotocin (STZ)-induced diabetic mice in a TRPV1-dependent manner. Activation of TRPV1 by capsaicin alleviated hyperglycemia-induced mitochondrial dysfunction in podocytes, accompanied by reduced mitochondria-associated membranes (MAMs) formation and fewer Ca²⁺ transport from endoplasmic reticulum (ER) to mitochondria. Mechanistically, TRPV1-mediated transient Ca²⁺ influx activated 5' AMP-activated protein kinase (AMPK) that reduced the transcription of Fundc1, a key molecule participating in MAMs formation. Inhibition of AMPK or overexpression of Fundc1 obviously blocked the inhibitory effect of capsaicin on MAMs formation and functional decline in podocytes. These findings emphasize the critical role of mitochondrial Ca²⁺ homeostasis in the maintenance of normal renal function and suggest an effective intervention method to counteract DN.

Endothelial Transient Receptor Potential Channels and Vascular Remodeling: Extracellular Ca2 + Entry for Angiogenesis, Arteriogenesis and Vasculogenesis

Vasculogenesis, angiogenesis and arteriogenesis represent three crucial mechanisms involved in the formation and maintenance of the vascular network in embryonal and post-natal life. It has long been known that endothelial Ca²⁺ signals are key players in vascular remodeling; indeed, multiple pro-angiogenic factors, including vascular endothelial growth factor, regulate endothelial cell fate through an increase in intracellular Ca²⁺ concentration. Transient Receptor Potential (TRP) channel consist in a superfamily of non-selective cation channels that are widely expressed within vascular endothelial cells. In addition, TRP channels are present in the two main endothelial progenitor cell (EPC) populations, i.e., myeloid angiogenic cells (MACs) and endothelial colony forming cells (ECFCs). TRP channels are polymodal channels that can assemble in homo- and heteromeric complexes and may be sensitive to both pro-angiogenic cues and subtle changes in local microenvironment. These features render TRP channels the most versatile Ca²⁺ entry pathway in vascular endothelial cells and in EPCs. Herein, we describe how endothelial TRP channels stimulate vascular remodeling by promoting angiogenesis, arteriogenesis and vasculogenesis through the integration of multiple environmental, e.g., extracellular growth factors and chemokines, and intracellular, e.g., reactive oxygen species, a decrease in Mg²⁺ levels, or hypercholesterolemia, stimuli. In addition, we illustrate how endothelial TRP channels induce neovascularization in response to synthetic agonists and small molecule drugs. We focus the attention on TRPC1, TRPC3, TRPC4, TRPC5, TRPC6, TRPV1, TRPV4, TRPM2, TRPM4, TRPM7, TRPA1, that were shown to be involved in angiogenesis, arteriogenesis and vasculogenesis. Finally, we discuss the role of endothelial TRP channels in aberrant tumor vascularization by focusing on TRPC1, TRPC3, TRPV2, TRPV4, TRPM8, and TRPA1. These observations suggest that endothelial TRP channels represent potential therapeutic targets in multiple disorders featured by abnormal vascularization, including cancer, ischemic disorders, retinal degeneration and neurodegeneration.

Is Aberrant Reno-Renal Reflex Control of Blood Pressure a Contributor to Chronic Intermittent Hypoxia-Induced Hypertension?

Renal sensory nerves are important in the regulation of body fluid and electrolyte homeostasis, and blood pressure. Activation of renal mechanoreceptor afferents triggers a negative feedback reno-renal reflex that leads to the inhibition of sympathetic nervous outflow. Conversely, activation of renal chemoreceptor afferents elicits reflex sympathoexcitation. Dysregulation of reno-renal reflexes by suppression of the inhibitory reflex and/or activation of the excitatory reflex impairs blood pressure control, predisposing to hypertension. Obstructive sleep apnoea syndrome (OSAS) is causally related to hypertension. Renal denervation in patients with OSAS or in experimental models of chronic intermittent hypoxia (CIH), a cardinal feature of OSAS due to recurrent apnoeas (pauses in breathing), results in a decrease in circulating norepinephrine levels and attenuation of hypertension. The mechanism of the beneficial effect of renal denervation on blood pressure control in models of CIH and OSAS is not fully understood, since renal denervation interrupts renal afferent signaling to the brain and sympathetic efferent signals to the kidneys. Herein, we consider the currently proposed mechanisms involved in the development of hypertension in CIH disease models with a focus on oxidative and inflammatory mediators in the kidneys and their potential influence on renal afferent control of blood pressure, with wider consideration of the evidence available from a variety of hypertension models. We draw focus to the potential contribution of aberrant renal afferent signaling in the development, maintenance and progression of high blood pressure, which may have relevance to CIH-induced hypertension.

Selective ablation of TRPV1 by intrathecal injection of resiniferatoxin in rats increases renal sympathoexcitatory responses and salt sensitivity

This study tested the hypothesis that selective ablation of transient receptor potential vanilloid type 1 (TRPV1)-positive nerve fibers by intrathecal injection of resiniferatoxin (RTX) enhances renal sympathoexcitatory responses and salt sensitivity. Intrathecal injection of RTX (1.8 μg/kg) to the levels of lower thoracic and upper lumbar spinal cord (T8-L3) increased mean arterial pressure (MAP) in rats fed a normal (NS, 1% NaCl) or high-sodium (HS, 8% NaCl) diet for 4 weeks compared to vehicle-treated rats (NS: 121 ± 2 vs. 111 ± 2; HS: 154 ± 2 vs. 134 ± 2 mm Hg, both P < 0.05), with a greater increase in HS compared to NS rats (9 ± 1% vs. 15 ± 1%, P < 0.05). TRPV1 contents were decreased in T8-L3 segments of spinal dorsal horn but not in corresponding dorsal root ganglia and the kidney following RTX treatment (P < 0.05). Selective activation of GABA-A receptors with intrathecal T8-L3 segment-injection of muscimol (3 nmol/kg) decreased renal sympathetic nerve activity and increased urinary excretion in both NS and HS rats, with a greater effect in RTX-treated compared to vehicle-treated rats (P < 0.05). Chronic activation of GABA-A receptors with muscimol (50 mg/kg/day × 2, p.o.) abolished RTX treatment-induced pressor effects in NS and HS rats. GAD65/67, a GABA synthetase, in the spinal cord was downregulated and tyrosine hydroxylase in the kidney upregulated in NS or HS rats treated with RTX (P < 0.05). Thus, selective ablation of TRPV1-positive central terminals of sensory neurons plays a prohypertensive role possibly via inhibition of spinal GABA system especially with HS intake, suggesting that activation of TRPV1 in central terminals of sensory neurons may convey an antihypertensive effect.

Human podocytes express functional thermosensitive TRPV channels

BACKGROUND AND PURPOSE

Heat-sensitive transient receptor potential vanilloid (TRPV) channels are expressed in various epithelial tissues regulating, among else, barrier functions. Their expression is well established in the distal nephron; however, we have no data about their presence in podocytes. As podocytes are indispensable in the formation of the glomerular filtration barrier, we investigated the presence and function of Ca²⁺ -permeable TRPV1-4 channels in human podocyte cultures.

EXPERIMENTAL APPROACH

Expression of TRPV1-4 channels was investigated at protein (immunocytochemistry, Western blot) and mRNA (Q-PCR) level in a conditionally immortalized human podocyte cell line. Channel function was assessed by measuring intracellular Ca²⁺ concentration using Flou-4 Ca²⁺ -indicator dye and patch clamp electrophysiology upon applying various activators and inhibitors.

KEY RESULTS

Thermosensitive TRP channels were expressed in podocytes. The TRPV1-specific agonists capsaicin and resiniferatoxin did not affect the intracellular Ca²⁺ concentration. Cannabidiol, an activator of TRPV2 and TRPV4 channels, induced moderate Ca²⁺ -influxes, inhibited by both tranilast and HC067047, blockers of TRPV2 and TRPV4 channels respectively. The TRPV4-specific agonists GSK1016790A and 4α-phorbol 12,13-didecanoate induced robust Ca²⁺ -signals which were abolished by HC067047. Non-specific agonists of TRPV3 channels induced marked Ca²⁺ transients. However, TRPV3 channel blockers, ruthenium red and isopentenyl diphosphate only partly inhibited the responses and TRPV3 silencing was ineffective suggesting remarkable off-target effects of the compounds.

CONCLUSION AND IMPLICATIONS

Our results indicate the functional presence of TRPV4 and other thermosensitive TRPV channels in human podocytes and raise the possibility of their involvement in the regulation of glomerular filtration barrier.

The innervation of the kidney in renal injury and inflammation: a cause and consequence of deranged cardiovascular control

Extensive investigations have revealed that renal sympathetic nerves regulate renin secretion, tubular fluid reabsorption and renal haemodynamics which can impact on cardiovascular homoeostasis normally and in pathophysiological states. The significance of the renal afferent innervation and its role in determining the autonomic control of the cardiovascular system is uncertain. The transduction pathways at the renal afferent nerves have been shown to require pro-inflammatory mediators and TRPV1 channels. Reno-renal reflexes have been described, both inhibitory and excitatory, demonstrating that a neural link exists between kidneys and may determine the distribution of excretory and haemodynamic function between the two kidneys. The impact of renal afferent nerve activity on basal and reflex regulation of global sympathetic drive remains opaque. There is clinical and experimental evidence that in states of chronic kidney disease and renal injury, there is infiltration of T-helper cells with a sympatho-excitation and blunting of the high- and low-pressure baroreceptor reflexes regulating renal sympathetic nerve activity. The baroreceptor deficits are renal nerve-dependent as the dysregulation can be relieved by renal denervation. There is also experimental evidence that in obese states, there is a sympatho-excitation and disrupted baroreflex regulation of renal sympathetic nerve activity which is mediated by the renal innervation. This body of information provides an important basis for directing greater attention to the role of renal injury/inflammation causing an inappropriate activation of the renal afferent nerves as an important initiator of aberrant autonomic cardiovascular control.

Neurogenic inflammation after traumatic brain injury and its potentiation of classical inflammation

Background

The neuroinflammatory response following traumatic brain injury (TBI) is known to be a key secondary injury factor that can drive ongoing neuronal injury. Despite this, treatments that have targeted aspects of the inflammatory pathway have not shown significant efficacy in clinical trials.

Main body

We suggest that this may be because classical inflammation only represents part of the story, with activation of neurogenic inflammation potentially one of the key initiating inflammatory events following TBI. Indeed, evidence suggests that the transient receptor potential cation channels (TRP channels), TRPV1 and TRPA1, are polymodal receptors that are activated by a variety of stimuli associated with TBI, including mechanical shear stress, leading to the release of neuropeptides such as substance P (SP). SP augments many aspects of the classical inflammatory response via activation of microglia and astrocytes, degranulation of mast cells, and promoting leukocyte migration. Furthermore, SP may initiate the earliest changes seen in blood-brain barrier (BBB) permeability, namely the increased transcellular transport of plasma proteins via activation of caveolae. This is in line with reports that alterations in transcellular transport are seen first following TBI, prior to decreases in expression of tight-junction proteins such as claudin-5 and occludin. Indeed, the receptor for SP, the tachykinin NK1 receptor, is found in caveolae and its activation following TBI may allow influx of albumin and other plasma proteins which directly augment the inflammatory response by activating astrocytes and microglia.

Conclusions

As such, the neurogenic inflammatory response can exacerbate classical inflammation via a positive feedback loop, with classical inflammatory mediators such as bradykinin and prostaglandins then further stimulating TRP receptors. Accordingly, complete inhibition of neuroinflammation following TBI may require the inhibition of both classical and neurogenic inflammatory pathways.

TRP Channels as Therapeutic Targets in Diabetes and Obesity

During the last three to four decades the prevalence of obesity and diabetes mellitus has greatly increased worldwide, including in the United States. Both the short- and long-term forecasts predict serious consequences for the near future, and encourage the development of solutions for the prevention and management of obesity and diabetes mellitus. Transient receptor potential (TRP) channels were identified in tissues and organs important for the control of whole body metabolism. A variety of TRP channels has been shown to play a role in the regulation of hormone release, energy expenditure, pancreatic function, and neurotransmitter release in control, obese and/or diabetic conditions. Moreover, dietary supplementation of natural ligands of TRP channels has been shown to have potential beneficial effects in obese and diabetic conditions. These findings raised the interest and likelihood for potential drug development. In this mini-review, we discuss possibilities for better management of obesity and diabetes mellitus based on TRP-dependent mechanisms.

Complex reinnervation pattern after unilateral renal denervation in rats

Renal denervation (DNX) is a treatment for resistant arterial hypertension. Efferent sympathetic nerves regrow, but reinnervation by renal afferent nerves has only recently been shown in the renal pelvis of rats after unilateral DNX. We examined intrarenal perivascular afferent and sympathetic efferent nerves after unilateral surgical DNX. Tyrosine hydroxylase (TH), CGRP, and smooth muscle actin were identified in kidney sections from 12 Sprague-Dawley rats, to distinguish afferents, efferents, and vasculature. DNX kidneys and nondenervated kidneys were examined 1, 4, and 12 wk after DNX. Tissue levels of CGRP and norepinephrine (NE) were measured with ELISA and mass spectrometry, respectively. DNX decreased TH and CGRP labeling by 90% and 95%, respectively (P < 0.05) within 1 wk. After 12 wk TH and CGRP labeling returned to baseline with a shift toward afferent innervation (P < 0.05). Nondenervated kidneys showed a doubling of both labels within 12 wk (P < 0.05). CGRP content decreased by 72% [3.2 ± 0.3 vs. 0.9 ± 0.2 ng/gkidney; P < 0.05] and NA by 78% [1.1 ± 0.1 vs. 0.2 ± 0.1 pmol/mgkidney; P < 0.05] 1 wk after DNX. After 12 wk, CGRP, but not NE, content in DNX kidneys was fully recovered, with no changes in the nondenervated kidneys. The use of phenol in the DNX procedure did not influence this result. We found morphological reinnervation and transmitter recovery of afferents within 12 wk after DNX. Despite morphological evidence of sympathetic regrowth, NE content did not fully recover. These results suggest a long-term net surplus of afferent influence on the DNX kidney may be contributing to the blood pressure lowering effect of DNX.

Blockade of the N-Methyl-D-Aspartate Glutamate Receptor Ameliorates Lipopolysaccharide-Induced Renal Insufficiency

N-methyl-D-aspartate (NMDA) receptor activation in rat kidney reduces renal perfusion and ultrafiltration. Hypoperfusion-induced ischemia is the most frequent cause of functional insufficiency in the endotoxemic kidney. Here, we used non-hypotensive rat model of lipopolysaccharide-induced endotoxemia to examine whether NMDA receptor hyperfunction contributes to acute kidney injury. Lipopolysaccharide-induced renal damage via increased enzymuria and hemodynamic impairments were ameliorated by co-treatment with the NMDA receptor blocker, MK-801. The NMDA receptor NR1 subunit in the rat kidney mainly co-localized with serine racemase, an enzyme responsible for synthesizing the NMDA receptor co-agonist, D-serine. The NMDA receptor hyperfunction in lipopolysaccharide-treated kidneys was demonstrated by NR1 and serine racemase upregulation, particularly in renal tubules, and by increased D-serine levels. Lipopolysaccharide also induced cell damage in cultured tubular cell lines and primary rat proximal tubular cells. This damage was mitigated by MK-801 and by small interfering RNA targeting NR1. Lipopolysaccharide increased cytokine release in tubular cell lines via toll-like receptor 4. The release of interleukin-1β from these cells are the most abundant. An interleukin-1 receptor antagonist not only attenuated cell death but also abolished lipopolysaccharide-induced NR1 and serine racemase upregulation and increases in D-serine secretion, suggesting that interleukin-1β-mediated NMDA receptor hyperfunction participates in lipopolysaccharide-induced tubular damage. The results of this study indicate NMDA receptor hyperfunction via cytokine effect participates in lipopolysaccharide-induced renal insufficiency. Blockade of NMDA receptors may represent a promising therapeutic strategy for the treatment of sepsis-associated renal failure.

H2O2 generated by NADPH oxidase 4 contributes to transient receptor potential vanilloid 1 channel-mediated mechanosensation in the rat kidney

The presence of NADPH oxidase (Nox) in the kidney, especially Nox4, results in H2O2 production, which regulates Na(+) excretion and urine formation. Redox-sensitive transient receptor potential vanilloid 1 channels (TRPV1s) are distributed in mechanosensory fibers of the renal pelvis and monitor changes in intrapelvic pressure (IPP) during urine formation. The present study tested whether H2O2 derived from Nox4 affects TRPV1 function in renal sensory responses. Perfusion of H2O2 into the renal pelvis dose dependently increased afferent renal nerve activity and substance P (SP) release. These responses were attenuated by cotreatment with catalase or TRPV1 blockers. In single unit recordings, H2O2 activated afferent renal nerve activity in response to rising IPP but not high salt. Western blots revealed that Nox2 (gp91(phox)) and Nox4 are both present in the rat kidney, but Nox4 is abundant in the renal pelvis and originates from dorsal root ganglia. This distribution was associated with expression of the Nox4 regulators p22(phox) and polymerase δ-interacting protein 2. Coimmunoprecipitation experiments showed that IPP increases polymerase δ-interacting protein 2 association with Nox4 or p22(phox) in the renal pelvis. Interestingly, immunofluorescence labeling demonstrated that Nox4 colocalizes with TRPV1 in sensory fibers of the renal pelvis, indicating that H2O2 generated from Nox4 may affect TRPV1 activity. Stepwise increases in IPP and saline loading resulted in H2O2 and SP release, sensory activation, diuresis, and natriuresis. These effects, however, were remarkably attenuated by Nox inhibition. Overall, these results suggest that Nox4-positive fibers liberate H2O2 after mechanostimulation, thereby contributing to a renal sensory nerve-mediated diuretic/natriuretic response.

An updated review of mechanotransduction in skin disorders: transcriptional regulators, ion channels, and microRNAs

INTRODUCTION

The skin is constantly exposed and responds to a wide range of biomechanical cues. The mechanobiology of skin has already been known and applied by clinicians long before the fundamental molecular mechanisms of mechanotransduction are elucidated.

MATERIALS AND METHODS

Despite increasing knowledge on the mediators of biomechanical signaling such as mitogen-associated protein kinases, Rho GTPases or FAK-ERK pathways, the key elements of mechano-responses transcription factors, and mechano-sensors remain unclear. Recently, canonical biochemical components of Hippo and Wnt signaling pathway YAP and β-catenin were found to exhibit undefined mechanical sensitivity. Mechanical forces were identified to be the dominant regulators of YAP/TAZ activity in a multicellular context. Furthermore, different voltage or ligand sensitive ion channels in the cell membrane exhibited their mechanical sensitivity as mechano-sensors. Additionally, a large number of microRNAs have been confirmed to regulate cellular behavior and contribute to various skin disorders under mechanical stimuli. Mechanosensitive (MS) microRNAs could not only be activated by distinct mechanical force pattern, but also responsively target MS sensors such as e-cadherin and cytoskeleton constituent RhoA.

CONCLUSION

Thus, a comprehensive understanding of this regulatory network of cutaneous mechanotransduction will facilitate the development of novel approaches to wound healing, hypertrophic scar formation, skin regeneration, and the progression or initiation of skin diseases.

Physiological and pathological functions of mechanosensitive ion channels

Rapid sensation of mechanical stimuli is often mediated by mechanosensitve ion channels. Their opening results from conformational changes induced by mechanical forces. It leads to membrane permeation of selected ions and thereby to electrical signaling. Newly identified mechanosensitive ion channels are emerging at an astonishing rate, including some that are traditionally assigned for completely different functions. In this review, we first provide a brief overview of ion channels that are known to play a role in mechanosensation. Next, we focus on three representative ones, including the transient receptor potential channel V4 (TRPV4), Kv1.1 voltage-gated potassium (Kv) channel, and Piezo channels. Their structures, biophysical properties, expression and targeting patterns, and physiological functions are highlighted. The potential role of their mechanosensation in related diseases is further discussed. In sum, mechanosensation appears to be achieved in a variety of ways by different proteins and plays a fundamental role in the function of various organs under normal and abnormal conditions.

Different responses of galanin and calcitonin gene-related peptide to capsaicin stimulation on dorsal root ganglion neurons in vitro

Both galanin (Gal) and calcitonin gene-related peptide (CGRP) are sensory neuropeptides which expressed in dorsal root ganglion (DRG) neurons and are involved in nociceptive processing. Capsaicin (CAP) influences nociceptive processing via influencing the expression of sensory neuropeptides in primary sensory neurons. However, little is known about the alterations of Gal and CGRP expression at the same condition stimulated by CAP. In the present study, primary cultured DRG neurons were used to determine the different responses of Gal and CGRP to CAP stimulation. DRG neurons were cultured for 48 hours and then exposed to CAP (2 μmol/L), capsazepine (CPZ) (2 μmol/L) plus CAP (2 μmol/L), or extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor PD98059 (10 μmol/L) plus CAP (2 μmol/L) for an additional 24hours. The DRG neurons were continuously exposed to culture media as a control. After that, the levels of Gal mRNA and CGRP mRNA of DRG neurons were determined using real time-PCR analysis. Gal and CGRP expression in situ was detected by an immunofluorescent labeling technique. The levels of phosphorylated-ERK1/2 (pERK1/2) protein were detected using a Western blot assay. The results showed that CAP evoked increases of Gal and its mRNA and decreases of CGRP and its mRNA in DRG neurons. Administration of either CPZ or PD98059 blocked the effects of CAP. These data indicate that Gal and CGRP shared different responses to CAP stimulation. Gal and CGRP may have different effects in nociceptive processing during neurogenic inflammation.

Transient receptor potential vanilloid 1 (TRPV1), TRPV4, and the kidney

Recent preclinical data indicate that activators of transient receptor potential channels of the vanilloid receptor subtype 1 (TRPV1) may improve the outcome of ischaemic acute kidney injury (AKI). The underlying mechanisms are unclear, but may involve TRPV1 channels in dorsal root ganglion neurones that innervate the kidney. Recent data identified TRPV4, together with TRPV1, to serve as major calcium influx channels in endothelial cells. In these cells, gating of individual TRPV4 channels within a four-channel cluster provides elementary calcium influx (calcium sparklets) to open calcium-activated potassium channels and promote vasodilation. The TRPV receptors can also form heteromers that exhibit unique conductance and gating properties, further increasing their spatio-functional diversity. This review summarizes data on electrophysiological properties of TRPV1/4 and their modulation by endogenous channel agonists such as 20-HETE, phospholipase C and phosphatidylinositide 3-kinase (PI3 kinase). We review important roles of TRPV1 and TRPV4 in kidney physiology and renal ischaemia reperfusion injury; further studies are warranted to address renoprotective mechanism of vanilloid receptors in ischaemic AKI including the role of the capsaicin receptor TRPV1 in primary sensory nerves and/or endothelium. Particular attention should be paid to understand the kidneys' ability to respond to ischaemic stimuli after catheter-based renal denervation therapy in man, whereas the discovery of novel pharmacological TRPV modulators may be a successful strategy for better treatment of acute or chronic kidney failure.

Angiotensinergic innervation of the kidney: present knowledge and its significance

Intrarenal neurotransmission implies the co-release of neuropeptides at the neuro-effector junction with direct influence on parameters of kidney function. The presence of an angiotensin (Ang) II-containing phenotype in catecholaminergic postganglionic and sensory fibers of the kidney, based on immunocytological investigations, has only recently been reported. These angiotensinergic fibers display a distinct morphology and intrarenal distribution, suggesting anatomical and functional subspecialization linked to neuronal Ang II-expression. This review discusses the present knowledge concerning these fibers, and their significance for renal physiology and the pathogenesis of hypertension in light of established mechanisms. The data suggest a new role of Ang II as a co-transmitter stimulating renal target cells or modulating nerve traffic from or to the kidney. Neuronal Ang II is likely to be an independent source of intrarenal Ang II. Further physiological experimentation will have to explore the role of the angiotensinergic renal innervation and integrate it into existing concepts.

N-octanoyl-dopamine is an agonist at the capsaicin receptor TRPV1 and mitigates ischemia-induced [corrected] acute kidney injury in rat

Since stimulation of transient receptor potential channels of the vanilloid receptor subtype 1 (TRPV1) mitigates acute kidney injury (AKI) and endogenous N-acyl dopamine derivatives are able to activate TRPV1, we tested if synthetic N-octanoyl-dopamine (NOD) activates TRPV1 and if it improves AKI. These properties of NOD and its intrinsic anti-inflammatory character were compared with those of dopamine (DA). TRPV1 activation and anti-inflammatory properties of NOD and DA were tested using primary cell cultures in vitro. The influence of NOD and DA on AKI was tested in a prospective, randomized, controlled animal study with 42 inbred male Lewis rats (LEW, RT1), treated intravenously with equimolar concentrations of DA or NOD one hour before the onset of warm ischemia and immediately before clamp release. NOD, but not DA, activates TRPV1 channels in isolated dorsal root ganglion neurons (DRG) that innervate several tissues including kidney. In TNFα stimulated proximal tubular epithelial cells, inhibition of NFκB and subsequent inhibition of VCAM1 expression by NOD was significantly stronger than by DA. NOD improved renal function compared to DA and saline controls. Histology revealed protective effects of NOD on tubular epithelium at day 5 and a reduced number of monocytes in renal tissue of DA and NOD treated rats. Our data demonstrate that NOD but not DA activates TRPV1 and that NOD has superior anti-inflammatory properties in vitro. Although NOD mitigates deterioration in renal function after AKI, further studies are required to assess to what extend this is causally related to TRPV1 activation and/or desensitization.

Role of transient receptor potential vanilloid 1 in inflammation and autoimmune diseases

Transient receptor potential vanilloid 1 (TRPV1), a non-selective cation channel, is a receptor activated by high temperatures and chemical agonists such as the vanilloids and protons. Because of these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurons. TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for controlling pain pharmacologically because it is a point where many proalgesic pathways converge and it is upregulated and sensitized by inflammation and injury. However, whether TRPV1 agonists promote or inhibit inflammation remains unclear. We recently demonstrated that SA13353 (1-[2-(1-adamantyl)ethyl]-1-pentyl-3-[3-(4-pyridyl)propyl]urea), a novel TRPV1 agonist, inhibits tumor necrosis factor-a production by the activation of capsaicin-sensitive afferent neurons and reduces the severity of symptoms in kidney injury, lung inflammation, arthritis, and encephalomyelitis. These results suggest that TRPV1 agonists may act as anti-inflammatories in certain inflammatory and autoimmune conditions in vivo. Given the potential deleterious effects of inhibiting the population of channels with a protective function, caution should be taken in the use of potent TRPV1 antagonists as a general strategy to treat inflammation. Further studies are required to clarify the role of TRPV1 and neuropeptides, which are released because of TRPV1 activation in inflammation and autoimmune diseases.

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.

Inhibition of renin release by arachidonic acid metabolites, 12(s)-HPETE and 12-HETE: role of TRPV1 channels

We test the hypothesis that 12-hydroperoxyeicosatetraenoic acid (12(s)-HPETE) and 12-hydroxyeicosatetraenoic acid (12-HETE) perfused into the renal pelvis increase afferent renal nerve activity (ARNA) and suppress renin release in rats fed a low-salt (LS) diet via activation of the transient receptor potential vanilloid type 1 (TRPV1) expressed in renal sensory nerves. 12(s)-HPETE or 12-HETE given into the left renal pelvis dose-dependently increased ARNA, which was abolished by AMG9810, a selective TRPV1 antagonist, or by RP67580, a selective neurokinin 1 receptor antagonist, in normal salt or LS-treated rats. 12(s)-HPETE, 12-HETE, or substance P perfused into the left renal pelvis suppressed plasma angiotensin I (Ang I) levels in LS rats, which was abolished by AMG9810 or attenuated by ipsilateral renal denervation (RD). 12(s)-HPETE or 12-HETE increased release of substance P and calcitonin gene-related peptide from the ipsilateral kidney, which was abolished by AMG9810 but not RP67580, RD, or RP67580 plus RD. Immunofluorescence staining showed that TRPV1-positive nerve fibers located in the renal cortex, medulla, and pelvis, and that the sympathetic nerve marker, neuropeptide Y, but not neurokinin 1 receptors expressed in the juxtaglomerular region colocalized with renin. Thus, our data show that 12(s)-HPETE and 12-HETE enhance ARNA and substance P/calcitonin gene-related peptide release but suppress renin activity in LS rats, and these effects are abolished when TRPV1 is blocked. These results indicate that TRPV1 mediates 12(s)-HPETE and 12-HETE action in the kidney in such a way that dysfunction in TRPV1 may lead to disintegrated regulation of renin and renal function.

TRPV channels and vascular function

Transient receptor potential channels, of the vanilloid subtype (TRPV), act as sensory mediators, being activated by endogenous ligands, heat, mechanical and osmotic stress. Within the vasculature, TRPV channels are expressed in smooth muscle cells, endothelial cells, as well as in peri-vascular nerves. Their varied distribution and polymodal activation properties make them ideally suited to a role in modulating vascular function, perceiving and responding to local environmental changes. In endothelial cells, TRPV1 is activated by endocannabinoids, TRPV3 by dietary agonists and TRPV4 by shear stress, epoxyeicosatrienoic acids (EETs) and downstream of Gq-coupled receptor activation. Upon activation, these channels contribute to vasodilation via nitric oxide, prostacyclin and intermediate/small conductance potassium channel-dependent pathways. In smooth muscle, TRPV4 is activated by endothelial-derived EETs, leading to large conductance potassium channel activation and smooth muscle hyperpolarization. Conversely, smooth muscle TRPV2 channels contribute to global calcium entry and may aid constriction. TRPV1 and TRPV4 are expressed in sensory nerves and can cause vasodilation through calcitonin gene-related peptide and substance P release as well as mediating vascular function via the baroreceptor reflex (TRPV1) or via increasing sympathetic outflow during osmotic stress (TRPV4). Thus, TRPV channels play important roles in the regulation of normal and pathological cellular function in the vasculature.

TRPV1-related modulation of spinal neuronal activity and behavior in a rat model of osteoarthritic pain

The TRPV1 receptor functions as a molecular integrator, and blockade of this receptor modulates enhanced somatosensitivity across several animal models of pathological pain, including models of osteoarthritic (OA) pain. In order to further characterize the contributions of TRPV1 to OA-related pain, we investigated the systemic effects of a selective TRPV1 receptor antagonist, A-889425, on grip force behavior, and on the evoked and spontaneous firing of spinal wide dynamic range (WDR) and nociceptive specific (NS) neurons in the monoiodoacetate (MIA) model of OA. Administration of A-889425 (10-300 μmol/kg, p.o.) alleviated grip force impairment in OA rats 3 weeks after the MIA injection. Also at 3 weeks post-MIA injection, the responses of WDR and NS neurons to 300 g von Frey hair stimulation of the knee joint were significantly reduced by A-889425 administration (10 and 30 μmol/kg, i.v.) in OA, but not sham-OA rats. Spontaneous firing of WDR neurons was elevated in the OA rats compared to sham-OA rats and may reflect ongoing discomfort in the OA animal. In addition to an effect on mechanotransmission, systemic administration of A-889425 reduced the elevated spontaneous firing of WDR neurons in OA rats but did not alter spontaneous firing in sham rats. The present data demonstrate that blockade of TRPV1 receptors modulates the firing of two important classes of spinal nociceptive neurons in a rat model of OA. The effect of A-889425 on neuronal responses to intense mechanical stimulation of the knee and on the spontaneous firing of WDR neurons adds to the growing appreciation for the role of TRPV1 receptors in pathological mechanotransmission and possibly non-evoked discomfort, respectively.

Mechanosensitive transient receptor potential vanilloid type 1 channels contribute to vascular remodeling of rat fistula veins

OBJECTIVE

We previously showed that matrix metalloproteinases (MMPs) contribute to tremendous blood flow-induced venous wall thickening during the maturation of an arteriovenous fistula (AVF). However, how veins in the fistula sense a dramatic change in the blood flow remains unknown. Because mechanosensitive transient receptor potential vanilloid channels (TRPVs) are present in the endothelium, we examined whether the Ca2+-permeable TRPVs play a role in remodeling of fistula veins.

METHODS

The fistula veins were generated at femoral AVF of Wistar rats. Changes in the hemodynamics and the width and internal radius of the iliac vein were studied at 3, 7, 14, and 28 days, then the iliac vein was removed and examined for changes in wall thickness and protein or mRNA expression by immunofluorecent stain, Western blot, or real time PCR. Changes in MMP2 activity was examined by gelatin zymography. Two ligatures were performed in iliac vein to prevent venodilatation to confirm the effect of dramatic changes in hemodynamics on TRPV expression. The specific role of TRPV was studied in another group of fistula veins given with capsazepine via a subcutaneous mini-osmotic pump for 28 days.

RESULTS

The fistula veins demonstrated high flow/wall shear stress (WSS), wall thickening, and venodilatation compared with control veins. The WSS increase was positively correlated with upregulation of TRPV1, but not TRPV4. Narrowing fistula veins prevented TRPV1 upregulation, indicating that high flow directly upregulates TRPV1. We examined the underlying signaling components and found that enhanced Ca2+/calmodulin-dependent protein kinase II (CaMK II) activity upregulated endothelial nitric oxide synthase (eNOS) and downregulated arginase I in the fistula veins. These changes were reversed by a CaMK II inhibitor. The relative levels of eNOS and arginase I activity consequently augmented NO formation, which coincided with an increase in MMP2 activity. Chronic inhibition of TRPV1 in the fistula veins by capsazepine showed no effect on high flow and TRPV1 expression, but markedly attenuated WSS, which was concomitantly associated with attenuation of CaMK II activity, NO-dependent MMP2 activation, and remodeling.

CONCLUSION

These findings indicate that TRPV1 is essential in the remodeling of AVFs and that WSS leads to TRPV1 upregulation, which then enhances remodeling, therefore, inhibition of TRPV1 pathway may prolong the lifespan of an AVF by decreasing WSS and vein wall remodeling.

Effects of a high-salt diet on TRPV-1-dependent renal nerve activity in Dahl salt-sensitive rats

OBJECTIVE

To test the hypothesis that transient receptor potential vanilloid type 1 channel (TRPV1)-mediated increases in afferent renal nerve activity (ARNA) and release of substance P (SP) and calcitonin gene-related peptide (CGRP) from the renal pelvis are suppressed in Dahl salt-sensitive (DS), but not -resistant (DR), rats fed a high-salt (HS) diet.

METHODS AND RESULTS

Male DS and DR rats were given a HS or low-salt (LS) diet for 3 weeks. Perfusion of capsaicin (CAP, 10(-6)M), a selective TRPV1 agonist, into the left renal pelvis increased ipsilateral ARNA in all groups, but with a smaller magnitude in DS-HS compared to other groups. CAP increased contralateral urine flow in all groups except DS-HS rats. CAP-induced release of SP and CGRP from the renal pelvis was less in DS-HS compared to other groups. Western blot showed that TRPV1 expression in the kidney decreased while expression of neurokinin 1 receptors increased in DS-HS compared to other groups.

CONCLUSION

TRPV1-mediated increases in ARNA and release of SP and CGRP in the renal pelvis are impaired in DS rats fed a HS diet, which can likely be attributed to suppressed TRPV1 expression in the kidney and contributes to increased salt sensitivity.

TRPV1 splice variants: structure and function

The capsaicin receptor (TRPV1) is a non-selective cation channel predominantly expressed in specialized sensory neurons that detect painful stimuli. Although its many functional roles continue to be revealed, it has been confirmed to play a critical role in the perception of peripheral inflammatory hyperalgesia and pain. TRPV1 not only is sensitized and/or activated under a wide range of conditions including inflammation and nerve injury but also undergoes changes in expressed levels in response to these same pathologic conditions. Just as our understanding of the structural requirements of TRPV1 activation has grown, there is evidence that TRPV1 forms heteromeric channel complexes. This review is focused on the structural and functional consequence of TRPV1 splice variants: VR.5'sv, TRPV1b/beta and TRPV1var. Through their co-expression and formation of heteromeric complexes with TRPV1, they have been shown to modulate TRPV1 activation. Moreover, TRPV1 splice variant subunits may also contribute unique properties of activation such as the detection of hypertonic conditions.

Ablation of transient receptor potential vanilloid 1 abolishes endothelin-induced increases in afferent renal nerve activity: mechanisms and functional significance

Endothelin 1 (ET-1) and its receptors, ETA and ETB, play important roles in regulating renal function and blood pressure, and these components are expressed in sensory nerves. Activation of transient receptor potential vanilloid (TRPV) 1 channels expressed in sensory nerves innervating the renal pelvis enhances afferent renal nerve activity (ARNA), diuresis, and natriuresis. We tested the hypothesis that ET-1 increases ARNA via activation of ETB, whereas ETA counterbalances ETB in wild-type (WT) but not TRPV1-null mutant mice. ET-1 alone or with BQ123, an ETA antagonist, perfused into the left renal pelvis increased ipsilateral ARNA in WT but not in TRPV1-null mutant mice, and ARNA increases were greater in the latter. [Ala1, 3,11,15]-endothelin 1, an ETB agonist, increased ARNA that was greater than that induced by ET-1 in WT mice only. [Ala1, 3,11,15]-endothelin 1-induced increases in ARNA were abolished by chelerythrine, a protein kinase C inhibitor, but not by H89, a protein kinase A inhibitor. Chelerythrine, H89, and BQ788, an ETB antagonist, did not affect ARNA triggered by capsaicin in WT mice. Substance P release from the renal pelvis was increased by [Ala1, 3,11,15]-endothelin 1 in WT mice only, and the increase was abolished by chelerythrine but not by H89. Chelerythrine, H89, and BQ788 did not affect capsaicin-induced substance P release. Our data show that ET1 increases ARNA via activation of ETB, whereas ETA counterbalances ETB in WT but not in TRPV1-null mutant mice, suggesting that TRPV1 mediates ETB-dependent increases in ARNA, diuresis, and natriuresis possibly via the protein kinase C pathway.

Mechanosensitive TRP channels in cardiovascular pathophysiology

Transient receptor potential (TRP) proteins constitute a large non-voltage-gated cation channel superfamily, activated polymodally by various physicochemical stimuli, and are implicated in a variety of cellular functions. Known activators for TRP include not only chemical stimuli such as receptor stimulation, increased acidity and pungent/cooling agents, but temperature change and various forms of mechanical stimuli such as osmotic stress, membrane stretch, and shear force. Recent investigations have revealed that at least ten mammalian TRPs exhibit mechanosensitivity (TRPC1, 5, 6; TRPV1, 2, 4; TRPM3, 7; TRPA1; TRPP2), but the mechanisms underlying it appear considerably divergent and complex. The proposed mechanisms are associated with lipid bilayer mechanics, specialized force-transducing structures, biochemical reactions, membrane trafficking and transcriptional regulation. Many of mechanosensitive (MS)-TRP channel likely undergo multiple regulations via these mechanisms. In the cardiovascular system in which hemodynamic forces constantly operate, the impact of mechanical stress may be particularly significant. Extensive morphological and functional studies have indicated that several MS-TRP channels are expressed in cardiac muscle, vascular smooth muscle, endothelium and vasosensory neurons, each differentially contributing to cardiovascular (CV) functions. To further complexity, the recent evidence suggests that mechanical stress may synergize with neurohormonal mechanisms thereby amplifying otherwise marginal responses. Furthermore, the currently available data suggest that MS-TRP channels may be involved in CV pathophysiology such as cardiac arrhythmia, cardiac hypertrophy/myopathy, hypertension and aneurysms. This review will overview currently known mechanisms for mechanical activation/modulation of TRPs and possible connections of MS-TRP channels to CV disorders.