Ablation of TRPV1 Abolishes Salicylate-Induced Sympathetic Activity Suppression and Exacerbates Salicylate-Induced Renal Dysfunction in Diet-Induced Obesity

Sodium salicylate (SA), a cyclooxygenase inhibitor, has been shown to increase insulin sensitivity and to suppress inflammation in obese patients and animal models. Transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel expressed in afferent nerve fibers. Cyclooxygenase-derived prostaglandins are involved in the activation and sensitization of TRPV1. This study tested whether the metabolic and renal effects of SA were mediated by the TRPV1 channel. Wild-type (WT) and TRPV1-/- mice were fed a Western diet (WD) for 4 months and received SA infusion (120mg/kg/day) or vehicle for the last 4 weeks of WD feeding. SA treatment significantly increased blood pressure in WD-fed TRPV1-/- mice (p < 0.05) but not in WD-fed WT mice. Similarly, SA impaired renal blood flow in TRPV1-/- mice (p < 0.05) but not in WT mice. SA improved insulin and glucose tolerance in both WT and TRPV1-/- mice on WD (all p < 0.05). In addition, SA reduced renal p65 and urinary prostaglandin E2, prostaglandin F1α, and interleukin-6 in both WT and TRPV1-/- mice (all p < 0.05). SA decreased urine noradrenaline levels, increased afferent renal nerve activity, and improved baroreflex sensitivity in WT mice (all p < 0.05) but not in TRPV1-/- mice. Importantly, SA increased serum creatinine and urine kidney injury molecule-1 levels and decreased the glomerular filtration rate in obese WT mice (all p < 0.05), and these detrimental effects were significantly exacerbated in obese TRPV1-/- mice (all p < 0.05). Lastly, SA treatment increased urine albumin levels in TRPV1-/- mice (p < 0.05) but not in WT mice. Taken together, SA-elicited metabolic benefits and anti-inflammatory effects are independent of TRPV1, while SA-induced sympathetic suppression is dependent on TRPV1 channels. SA-induced renal dysfunction is dependent on intact TRPV1 channels. These findings suggest that SA needs to be cautiously used in patients with obesity or diabetes, as SA-induced renal dysfunction may be exacerbated due to impaired TRPV1 in obese and diabetic patients.

Transient receptor potential vanilloid 4 channel participates in mouse ventricular electrical activity

The TRPV4 channel is a calcium-permeable channel (PCa/PNa ∼ 10). Its expression has been reported in ventricular myocytes, where it is involved in several cardiac pathological mechanisms. In this study, we investigated the implication of TRPV4 in ventricular electrical activity. Left ventricular myocytes were isolated from trpv4+/+ and trpv4-/- mice. TRPV4 membrane expression and its colocalization with L-type calcium channels (Cav1.2) was confirmed using Western blot biotinylation, immunoprecipitation, and immunostaining experiments. Then, electrocardiograms (ECGs) and patch-clamp recordings showed shortened QTc and action potential (AP) duration in trpv4-/- compared with trpv4+/+ mice. Thus, TRPV4 activator GSK1016790A produced a transient and dose-dependent increase in AP duration at 90% of repolarization (APD90) in trpv4+/+ but not in trpv4-/- myocytes or when combined with TRPV4 inhibitor GSK2193874 (100 nM). Hence, GSK1016790A increased calcium transient (CaT) amplitude in trpv4+/+ but not in trpv4-/- myocytes, suggesting that TRPV4 carries an inward Ca2+ current in myocytes. Conversely, TRPV4 inhibitor GSK2193874 (100 nM) alone reduced APD90 in trpv4+/+ but not in trpv4-/- myocytes, suggesting that TRPV4 prolongs AP duration in basal condition. Finally, introducing TRPV4 parameters in a mathematical model predicted the development of an inward TRPV4 current during repolarization that increases AP duration and CaT amplitude, in accord with what was found experimentally. This study shows for the first time that TRPV4 modulates AP and QTc durations. It would be interesting to evaluate whether TRPV4 could be involved in long QT-mediated ventricular arrhythmias.NEW & NOTEWORTHY Transient receptor potential vanilloid 4 (TRPV4) is expressed at the membrane of mouse ventricular myocytes and colocalizes with non-T-tubular L-type calcium channels. Deletion of trpv4 gene in mice results in shortened QT interval on electrocardiogram and reduced action potential duration of ventricular myocytes. Pharmacological activation of TRPV4 channel leads to increased action potential duration and increased calcium transient amplitude in trpv4-/- but not in trpv4-/- ventricular myocytes. To the contrary, TRPV4 channel pharmacological inhibition reduces action potential duration in trpv4+/+ but not in trpv4-/- myocytes. Integration of TRPV4 channel in a computational model of mouse action potential shows that the channel carries an inward current contributing to slowing down action potential repolarization and to increase calcium transient amplitude, similarly to what is observed experimentally. This study highlights for the first time the involvement of TRPV4 channel in ventricular electrical activity.

Genetic Deletion of TrpV1 and TrpA1 Does Not Alter Avoidance of or Patterns of Brainstem Activation to Citric Acid in Mice

Exposure of the oral cavity to acidic solutions evokes not only a sensation of sour, but also of sharp or tangy. Acidic substances potentially stimulate both taste buds and acid-sensitive mucosal free nerve endings. Mice lacking taste function (P2X2/P2X3 double-KO mice) refuse acidic solutions similar to wildtype (WT) mice and intraoral infusion of acidic solutions in these KO animals evokes substantial c-Fos activity within orosensory trigeminal nuclei as well as of the nucleus of the solitary tract (nTS) (Stratford, Thompson, et al. 2017). This residual acid-evoked, non-taste activity includes areas that receive inputs from trigeminal and glossopharyngeal peptidergic (CGRP-containing) nerve fibers that express TrpA1 and TrpV1 both of which are activated by low pH. We compared avoidance responses in WT and TrpA1/V1 double-KO (TRPA1/V1Dbl-/-) mice in brief-access behavioral assay (lickometer) to 1, 3, 10, and 30 mM citric acid, along with 100 µM SC45647 and H2O. Both WT and TRPA1/V1Dbl-/- show similar avoidance, including to higher concentrations of citric acid (10 and 30 mM; pH 2.62 and pH 2.36, respectively), indicating that neither TrpA1 nor TrpV1 is necessary for the acid-avoidance behavior in animals with an intact taste system. Similarly, induction of c-Fos in the nTS and dorsomedial spinal trigeminal nucleus was similar in the WT and TRPA1/V1Dbl-/- animals. Taken together these results suggest non-TrpV1 and non-TrpA1 receptors underlie the residual responses to acids in mice lacking taste function.

TRPV4 Antagonism Prevents Mechanically Induced Myotonia

OBJECTIVE

Myotonia is caused by involuntary firing of skeletal muscle action potentials and causes debilitating stiffness. Current treatments are insufficiently efficacious and associated with side effects. Myotonia can be triggered by voluntary movement (electrically induced myotonia) or percussion (mechanically induced myotonia). Whether distinct molecular mechanisms underlie these triggers is unknown. Our goal was to identify ion channels involved in mechanically induced myotonia and to evaluate block of the channels involved as a novel approach to therapy.

METHODS

We developed a novel system to enable study of mechanically induced myotonia using both genetic and pharmacologic mouse models of myotonia congenita. We extended ex vivo studies of excitability to in vivo studies of muscle stiffness.

RESULTS

As previous work suggests activation of transient receptor potential vanilloid 4 (TRPV4) channels by mechanical stimuli in muscle, we examined the role of this cation channel. Mechanically induced myotonia was markedly suppressed in TRPV4-null muscles and in muscles treated with TRPV4 small molecule antagonists. The suppression of mechanically induced myotonia occurred without altering intrinsic muscle excitability, such that myotonia triggered by firing of action potentials (electrically induced myotonia) was unaffected. When injected intraperitoneally, TRPV4 antagonists lessened the severity of myotonia in vivo by approximately 80%.

INTERPRETATION

These data demonstrate that there are distinct molecular mechanisms triggering electrically induced and mechanically induced myotonia. Our data indicates that activation of TRPV4 during muscle contraction plays an important role in triggering myotonia in vivo. Elimination of mechanically induced myotonia by TRPV4 inhibition offers a new approach to treating myotonia. ANN NEUROL 2020;88:297-308.

TRPV4 channel opening mediates pressure-induced pancreatitis initiated by Piezo1 activation

Elevated pressure in the pancreatic gland is the central cause of pancreatitis following abdominal trauma, surgery, endoscopic retrograde cholangiopancreatography, and gallstones. In the pancreas, excessive intracellular calcium causes mitochondrial dysfunction, premature zymogen activation, and necrosis, ultimately leading to pancreatitis. Although stimulation of the mechanically activated, calcium-permeable ion channel Piezo1 in the pancreatic acinar cell is the initial step in pressure-induced pancreatitis, activation of Piezo1 produces only transient elevation in intracellular calcium that is insufficient to cause pancreatitis. Therefore, how pressure produces a prolonged calcium elevation necessary to induce pancreatitis is unknown. We demonstrate that Piezo1 activation in pancreatic acinar cells caused a prolonged elevation in intracellular calcium levels, mitochondrial depolarization, intracellular trypsin activation, and cell death. Notably, these effects were dependent on the degree and duration of force applied to the cell. Low or transient force was insufficient to activate these pathological changes, whereas higher and prolonged application of force triggered sustained elevation in intracellular calcium, leading to enzyme activation and cell death. All of these pathological events were rescued in acinar cells treated with a Piezo1 antagonist and in acinar cells from mice with genetic deletion of Piezo1. We discovered that Piezo1 stimulation triggered transient receptor potential vanilloid subfamily 4 (TRPV4) channel opening, which was responsible for the sustained elevation in intracellular calcium that caused intracellular organelle dysfunction. Moreover, TRPV4 gene-KO mice were protected from Piezo1 agonist- and pressure-induced pancreatitis. These studies unveil a calcium signaling pathway in which a Piezo1-induced TRPV4 channel opening causes pancreatitis.

TRPV1 in experimental autoimmune prostatitis

BACKGROUND

Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) is a disorder that is characterized by persistent pelvic pain in men of any age. Although several studies suggest that the transient receptor potential vanilloid 1 (TRPV1) channel is involved in various pathways of chronic pain, the TRPV1 channel has not been implicated in chronic pelvic pain associated with CP/CPPS.

METHODS

Male C57BL/6J (B6) and TRPV1 knockout (TRPV1 KO) mice (5-7 weeks old) were used to study the development of pelvic allodynia in a murine model of CP/CPPS called experimental autoimmune prostatitis (EAP). The prostate lobes, dorsal root ganglia (DRG), and spinal cord were excised at day 20. The prostate lobes were assessed for inflammation, TRPV1 expression, and mast cell activity. DRG and spinal cord, between the L6-S4 regions, were analyzed to determine the levels of phosphorylated ERK1/2 (p-ERK 1/2). To examine the therapeutic potential of TRPV1, B6 mice with EAP received intraurethral infusion of a TRPV1 antagonist at day 20 (repeated every 2 days) and pelvic pain was evaluated at days 20, 25, 30, and 35.

RESULTS

Our data showed that B6 mice with EAP developed pelvic tactile allodynia at days 7, 14, and 20. In contrast, TRPV1 KO mice with EAP do not develop pelvic tactile allodynia at any time point. Although we observed no change in the levels of TRPV1 protein expression in the prostate from B6 mice with EAP, there was evidence of significant inflammation and elevated mast cell activation. Interestingly, the prostate from TRPV1 KO mice with EAP showed a lack of mast cell activation despite evidence of prostate inflammation. Next, we observed a significant increase of p-ERK1/2 in the DRG and spinal cord from B6 mice with EAP; however, p-ERK1/2 expression was unaltered in TRPV1 KO mice with EAP. Finally, we confirmed that intraurethral administration of a TRPV1 antagonist peptide reduced pelvic tactile allodynia in B6 mice with EAP after day 20.

CONCLUSIONS

We demonstrated that in a murine model of CP/CPPS, the TRPV1 channel is key to persistent pelvic tactile allodynia and blocking TRPV1 in the prostate may be a promising strategy to quell chronic pelvic pain.

Incidence and Clinical Features of TRPV4-Linked Axonal Neuropathies in a USA Cohort of Charcot-Marie-Tooth Disease Type 2

Mutations in TRPV4 are linked to a group of clinically distinct, but also overlapping axonal neuropathies, including Charcot-Marie-Tooth disease type 2C (CMT2C), scapuloperoneal spinal muscular atrophy, and congenital distal spinal muscular atrophy. The incidence of TRPV4-linked cases ranges from 0 to 7% in overall axonal neuropathy cohorts from European countries and Australia. However, the data from other areas remain largely unknown. In this study, we screened for TRPV4 mutations in a well-characterized USA cohort of 62 unrelated CMT2 patients without mutations in MFN2, GARS, NEFL, and GDAP1. All 15 coding exons of TRPV4 were analyzed by Sanger-sequencing. Clinical features of TRPV4-linked patients were compared with those lacking TRPV4 mutations. We identified two TRPV4 mutations in two patients. A TRPV4-R316C was identified in a patient with family history, while a TRPV4-R269C in an apparently sporadic case. Marked clinical variations were observed in the patients with TRPV4 mutations. Our data suggest that TRPV4-linked CMT2C accounts for a sizable fraction in this USA cohort of CMT2; it has a wide phenotypic spectrum, and vocal cord paralysis, scapular weakness and wasting, skeletal dysplasia, and hearing loss are suggestive signs for TRPV4-linked CMT2C.

Targeting TRPV1 to relieve motion sickness symptoms in mice by electroacupuncture and gene deletion

Motion sickness (MS) is an acute disorder that occurs in healthy individuals worldwide regardless of gender, age, or ethnicity. Our study used a mouse model to rule out the effects of any psychological factors related to MS and EA. Subjects were randomly separated into four groups, namely the control group (Con), motion sickness inducing group (MS), mentioning sickness inducing with electroacupuncture treatment group (EA) and motion sickness inducing only in TRPV1 knockout mice group (TRPV1-/-). The consumption of kaolin, a non-nutrient substance, was measured as a behavior observed response of an emetic reflex in a murine model. This behavior is referred to as pica behavior. Our results showed that pica behavior was observed in the MS group. Moreover, kaolin consumption in the EA group decreased to the average baseline of the control group. A similar result was observed in TRPV1 null mice. We also observed an increase of TRPV1 and related molecules in the thalamus, hypothalamic and brain stem after MS stimulation and a significant decrease in the EA and TRPV1 null groups. This is the first study to demonstrate that TRPV1 pathways are possibly associated with mechanisms of MS, and can be attended through EA or TRPV1 genetic manipulation.

Decreased sensory nerve excitation and bone pain associated with mouse Lewis lung cancer in TRPV1-deficient mice

Bone pain is one of the most common and life-limiting complications of cancer metastasis to bone. Although the mechanism of bone pain still remains poorly understood, bone pain is evoked as a consequence of sensitization and excitation of sensory nerves (SNs) innervating bone by noxious stimuli produced in the microenvironment of bone metastases. We showed that bone is innervated by calcitonin gene-related protein (CGRP)⁺ SNs extending from dorsal root ganglia (DRG), the cell body of SNs, in mice. Mice intratibially injected with Lewis lung cancer (LLC) cells showed progressive bone pain evaluated by mechanical allodynia and flinching with increased CGRP⁺ SNs in bone and augmented SN excitation in DRG as indicated by elevated numbers of pERK- and pCREB-immunoreactive neurons. Immunohistochemical examination of LLC-injected bone revealed that the tumor microenvironment is acidic. Bafilomycin A1, a selective inhibitor of H⁺ secretion from vacuolar proton pump, significantly alleviated bone pain, indicating that the acidic microenvironment contributes to bone pain. We then determined whether the transient receptor potential vanilloid 1 (TRPV1), a major acid-sensing nociceptor predominantly expressed on SNs, plays a role in bone pain by intratibially injecting LLC cells in TRPV1-deficient mice. Bone pain and SN excitation in the DRG and spinal dorsal horn were significantly decreased in TRPV1 ^-/- mice compared with wild-type mice. Our results suggest that TRPV1 activation on SNs innervating bone by the acidic cancer microenvironment in bone contributes to SN activation and bone pain. Targeting acid-activated TRPV1 is a potential therapeutic approach to cancer-induced bone pain.

Activation of temperature-sensitive TRPV1-like receptors in ARC POMC neurons reduces food intake

Proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC) respond to numerous hormonal and neural signals, resulting in changes in food intake. Here, we demonstrate that ARC POMC neurons express capsaicin-sensitive transient receptor potential vanilloid 1 receptor (TRPV1)-like receptors. To show expression of TRPV1-like receptors in ARC POMC neurons, we use single-cell reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, electrophysiology, TRPV1 knock-out (KO), and TRPV1-Cre knock-in mice. A small elevation of temperature in the physiological range is enough to depolarize ARC POMC neurons. This depolarization is blocked by the TRPV1 receptor antagonist and by Trpv1 gene knockdown. Capsaicin-induced activation reduces food intake that is abolished by a melanocortin receptor antagonist. To selectively stimulate TRPV1-like receptor-expressing ARC POMC neurons in the ARC, we generate an adeno-associated virus serotype 5 (AAV5) carrying a Cre-dependent channelrhodopsin-2 (ChR2)–enhanced yellow fluorescent protein (eYFP) expression cassette under the control of the two neuronal POMC enhancers (nPEs). Optogenetic stimulation of TRPV1-like receptor-expressing POMC neurons decreases food intake. Hypothalamic temperature is rapidly elevated and reaches to approximately 39 °C during treadmill running. This elevation is associated with a reduction in food intake. Knockdown of the Trpv1 gene exclusively in ARC POMC neurons blocks the feeding inhibition produced by increased hypothalamic temperature. Taken together, our findings identify a melanocortinergic circuit that links acute elevations in hypothalamic temperature with acute reductions in food intake.

Intense exercise acutely decreases appetite and subsequent food intake. As exercise is accompanied by increased body temperature, we hypothesized that a rise in body temperature during exercise plays a role in reducing food intake. The hypothalamic neurons are major components of the neural circuits that control feeding in response to hormones and neural signals. Among hypothalamic neurons, those that express proopiomelanocortin (POMC) in the arcuate nucleus of the hypothalamus are important in controlling food intake. In this study, we found that these POMC-expressing neurons express TRPV1-like thermoreceptors that are activated by an increase in temperature within the physiological range in mice. We also showed that an increase in body temperature during exercise is directly sensed by these POMC-expressing neurons through activation of the TRPV1-like receptors. Hence, this study provides a novel perspective on the cellular mechanisms underlying energy balance: body temperature reduces food intake via TRPV1-like receptors in POMC-expressing neurons in the arcuate nucleus of the hypothalamus.

The use of flow cytometry to examine calcium signalling by TRPV1 in mixed cell populations

Flow cytometric analysis of calcium mobilisation has been in use for many years in the study of specific receptor engagement or isolated cell:cell communication. However, calcium mobilisation/signaling is key to many cell functions including apoptosis, mobility and immune responses. Here we combine multiplex surface staining of whole spleen with Indo-1 AM to visualise calcium mobilisation and examine calcium signaling in a mixed immune cell culture over time. We demonstrate responses to a TRPV1 agonist in distinct cell subtypes without the need for cell separation. Multi parameter staining alongside Indo-1 AM to demonstrate calcium mobilization allows the study of real time calcium signaling in a complex environment.

TRPV1 deletion impaired fracture healing and inhibited osteoclast and osteoblast differentiation

Fracture healing, in which osteoclasts and osteoblasts play important roles, has drawn much clinical attention. Osteoclast deficiency or decreased osteoblast activity will impair fracture healing. TRPV1 is a member of the Ca2+ permeable cation channel subfamily, and pharmacological inhibition of TRPV1 prevents ovariectomy-induced bone loss, which makes TRPV1 a potential target for osteoporosis. However, whether long term TRPV1 inhibition or TRPV1 deletion will affect the fracture healing process is unclear. In this study, we found that the wild-type mice showed a well-remodeled fracture callus, whereas TRPV1 knockout mice still had an obvious fracture gap with unresorbed soft-callus 4 weeks post-fracture. The number of osteoclasts was reduced in the TRPV1 knockout fracture callus, and osteoclast formation and resorption activity were also impaired in vitro. TRPV1 deletion decreased the calcium oscillation frequency and peak cytoplasmic concentration in osteoclast precursors, subsequently reducing the expression and nuclear translocation of NFATc1 and downregulating DC-stamp, cathepsin K, and ATP6V. In addition, TRPV1 deletion caused reduced mRNA and protein expression of Runx2 and ALP in bone marrow stromal cells (BMSCs) and reduced calcium deposition in vitro. Our results suggest that TRPV1 deletion impairs fracture healing, and inhibited osteoclastogenesis and osteogenesis.

Transient receptor potential vanilloid 1-immunoreactive signals in murine enteric glial cells

AIM

To investigate the possible involvement of transient receptor potential vanilloid 1 (TRPV1) in maturation of enteric glial cells (EGCs).

METHODS

Immunohistochemical and immunocytochemical techniques were used to analyze EGC markers in myenteric plexus (MP) as well as cultured MP cells and EGCs using TRPV1 knockout (KO) mice.

RESULTS

We detected TRPV1-immunoreactive signals in EGC in the MP of wild-type (WT) but not KO mice. Expression of glial fibrillary acidic protein (GFAP) immunoreactive signals was lower at postnatal day (PD) 6 in KO mice, though the difference was not clear at PD 13 and PD 21. When MP cells were isolated and cultured from isolated longitudinal muscle-MP preparation from WT and KO mice, the yield of KO EGC was lower than that of WT EGC, while the yield of KO and WT smooth muscle cells showed no difference. Addition of BCTC, a TRPV1 antagonist, to enriched EGC culture resulted in a decrease in the protein ratio of GFAP to S100B, another EGC/astrocyte-specific marker.

CONCLUSION

These results address the possibility that TRPV1 may be involved in the maturation of EGC, though further studies are necessary to validate this possibility.

Modulation of TRPV4 by diverse mechanisms

Transient receptor potential ion channels (TRP) are a superfamily of non-selective ion channels which are opened in response to a diverse range of stimuli. The TRP vanilloid 4 (TRPV4) ion channel is opened in response to heat, mechanical stimuli, hypo-osmolarity and arachidonic acid metabolites. However, recently TRPV4 has been identified as an ion channel that is modulated by, and opened by intracellular signalling cascades from other receptors and signalling pathways. Although TRPV4 knockout mice show relatively mild phenotypes, some mutations in TRPV4 cause severe developmental abnormalities, such as the skeletal dyplasia and arthropathy. Regulated TRPV4 function is also essential for healthy cardiovascular system function as a potent agonist compromises endothelial cell function, leading to vascular collapse. A better understanding of the signalling mechanisms that modulate TRPV4 function is necessary to understand its physiological roles. Post translational modification of TRPV4 by kinases and other signalling molecules can modulate TRPV4 opening in response to stimuli such as mechanical and hyposmolarity and there is an emerging area of research implicating TRPV4 as a transducer of these signals as opposed to a direct sensor of the stimuli. Due to its wide expression profile, TRPV4 is implicated in multiple pathophysiological states. TRPV4 contributes to the sensation of pain due to hypo-osmotic stimuli and inflammatory mechanical hyperalsgesia, where TRPV4 sensitizaton by intracellular signalling leads to pain behaviors in mice. In the vasculature, TRPV4 is a regulator of vessel tone and is implicated in hypertension and diabetes due to endothelial dysfunction. TRPV4 is a key regulator of epithelial and endothelial barrier function and signalling to and opening of TRPV4 can disrupt these critical protective barriers. In respiratory function, TRPV4 is involved in cystic fibrosis, cilary beat frequency, bronchoconstriction, chronic obstructive pulmonary disease, pulmonary hypertension, acute lung injury, acute respiratory distress syndrome and cough.In this review we highlight how modulation of TRPV4 opening is a vital signalling component in a range of tissues and why understanding of TRPV4 regulation in the body may lead to novel therapeutic approaches to treating a range of disease states.

Role of the ion channel, transient receptor potential cation channel subfamily V member 1 (TRPV1), in allergic asthma

BACKGROUND

Asthma prevalence has increased world-wide especially in children; thus there is a need to develop new therapies that are safe and effective especially for patients with severe/refractory asthma. CD4(+) T cells are thought to play a central role in disease pathogenesis and associated symptoms. Recently, TRPV1 has been demonstrated to regulate the activation and inflammatory properties of CD4(+) cells. The aim of these experiments was to demonstrate the importance of CD4(+) T cells and the role of TRPV1 in an asthma model using a clinically ready TRPV1 inhibitor (XEN-D0501) and genetically modified (GM) animals.

METHODS

Mice (wild type, CD4 (-/-) or TRPV1 (-/-)) and rats were sensitised with antigen (HDM or OVA) and subsequently topically challenged with the same antigen. Key features associated with an allergic asthma type phenotype were measured: lung function (airway hyperreactivity [AHR] and late asthmatic response [LAR]), allergic status (IgE levels) and airway inflammation.

RESULTS

CD4(+) T cells play a central role in both disease model systems with all the asthma-like features attenuated. Targeting TRPV1 using either GM mice or a pharmacological inhibitor tended to decrease IgE levels, airway inflammation and lung function changes.

CONCLUSION

Our data suggests the involvement of TRPV1 in allergic asthma and thus we feel this target merits further investigation.

CPEB3 Deficiency Elevates TRPV1 Expression in Dorsal Root Ganglia Neurons to Potentiate Thermosensation

Cytoplasmic polyadenylation element binding protein 3 (CPEB3) is a sequence-specific RNA-binding protein that downregulates translation of multiple plasticity-related proteins (PRPs) at the glutamatergic synapses. Activity-induced synthesis of PRPs maintains long-lasting synaptic changes that are critical for memory consolidation and chronic pain manifestation. CPEB3-knockout (KO) mice show aberrant hippocampus-related plasticity and memory, so we investigated whether CPEB3 might have a role in nociception-associated plasticity. CPEB3 is widely expressed in the brain and peripheral afferent sensory neurons. CPEB3-KO mice with normal mechanosensation showed hypersensitivity to noxious heat. In the complete Freund's adjuvant (CFA)-induced inflammatory pain model, CPEB3-KO animals showed normal thermal hyperalgesia and transiently enhanced mechanical hyperalgesia. Translation of transient receptor potential vanilloid 1 (TRPV1) RNA was suppressed by CPEB3 in dorsal root ganglia (DRG), whereas CFA-induced inflammation reversed this inhibition. Moreover, CPEB3/TRPV1 double-KO mice behaved like TRPV1-KO mice, with severely impaired thermosensation and thermal hyperalgesia. An enhanced thermal response was recapitulated in non-inflamed but not inflamed conditional-KO mice, with cpeb3 gene ablated mostly but not completely, in small-diameter nociceptive DRG neurons. CPEB3-regulated translation of TRPV1 RNA may play a role in fine-tuning thermal sensitivity of nociceptors.

Electroacupuncture at ST25 inhibits jejunal motility: Role of sympathetic pathways and TRPV1

AIM: To investigate whether electroacupuncture (EA) at ST25 affects jejunal motility in vivo and if so, whether a sympathetic pathway is involved.

METHODS: Jejunal motility was assessed using a manometric balloon placed in the jejunum approximately about 3-5 cm away from the suspensory ligament of the duodenum in anesthetized animals. The effects of EA at ST25 were measured in male Sprague-Dawley rats, some of which were treated with propranolol or clenbuterol (EA intensities: 1, 3, 5, 7, and 9 mA), and in male transient receptor potential vanilloid-1 (TRPV1) (capsaicin receptor) knockout mice (EA intensities: 1, 2, and 4 mA).

RESULTS: Anesthetized rats exhibited three types of fasting jejunal motor patterns (types A, B, and C), and only type C rats responded to EA stimulation. In type C rats, EA at ST25 significantly suppressed the motor activity of the jejunum in an intensity-dependent manner. The inhibitory effect of EA was weakened by propranolol (β adrenoceptor antagonist) and disappeared with clenbuterol (β adrenoceptor agonist) induced inhibition of motility, suggesting that the effect of EA on motility is mediated via a sympathetic pathway. Compared with wild-type mice, EA at ST25 was less effective in TRPV1 knockout mice, suggesting that this multi-modal sensor channel participates in the mechanism.

CONCLUSION: EA at ST25 was found to inhibit jejunal motility in an intensity-dependent manner, via a mechanism in which sympathetic nerves and TRPV1 receptors play an important role.

[Cartilage metabolism and mechanobioscience.]

Joint cartilage is under exposure to repetitive mechanical stress. It is well known fact that the mechanical stress is a major cause of joint disorders such as osteoarthritis. However the molecular mechanisms of cartilage under mechanical stress remain unclear. In recent years, some notable mechanosensor or signal cascades relating to mechanotransduction had been identified, which might be promising targets for treatment of joint disorders.

TRPV1 Channels and Gastric Vagal Afferent Signalling in Lean and High Fat Diet Induced Obese Mice

Aim

Within the gastrointestinal tract vagal afferents play a role in control of food intake and satiety signalling. Activation of mechanosensitive gastric vagal afferents induces satiety. However, gastric vagal afferent responses to mechanical stretch are reduced in high fat diet mice. Transient receptor potential vanilloid 1 channels (TRPV1) are expressed in vagal afferents and knockout of TRPV1 reduces gastro-oesophageal vagal afferent responses to stretch. We aimed to determine the role of TRPV1 on gastric vagal afferent mechanosensitivity and food intake in lean and HFD-induced obese mice.

Methods

TRPV1+/+ and -/- mice were fed either a standard laboratory diet or high fat diet for 20wks. Gastric emptying of a solid meal and gastric vagal afferent mechanosensitivity was determined.

Results

Gastric emptying was delayed in high fat diet mice but there was no difference between TRPV1+/+ and -/- mice on either diet. TRPV1 mRNA expression in whole nodose ganglia of TRPV1+/+ mice was similar in both dietary groups. The TRPV1 agonist N-oleoyldopamine potentiated the response of tension receptors in standard laboratory diet but not high fat diet mice. Food intake was greater in the standard laboratory diet TRPV1-/- compared to TRPV1+/+ mice. This was associated with reduced response of tension receptors to stretch in standard laboratory diet TRPV1-/- mice. Tension receptor responses to stretch were decreased in high fat diet compared to standard laboratory diet TRPV1+/+ mice; an effect not observed in TRPV1-/- mice. Disruption of TRPV1 had no effect on the response of mucosal receptors to mucosal stroking in mice on either diet.

Conclusion

TRPV1 channels selectively modulate gastric vagal afferent tension receptor mechanosensitivity and may mediate the reduction in gastric vagal afferent mechanosensitivity in high fat diet-induced obesity.

Ca(v)3.2 calcium channels: the key protagonist in the supraspinal effect of paracetamol

To exert its analgesic action, paracetamol requires complex metabolism to produce a brain-specific lipoamino acid compound, AM404, which targets central transient receptor potential vanilloid receptors (TRPV1). Lipoamino acids are also known to induce analgesia through T-type calcium-channel inhibition (Ca(v)3.2). In this study we show that the antinociceptive effect of paracetamol in mice is lost when supraspinal Ca(v)3.2 channels are inhibited. Therefore, we hypothesized a relationship between supraspinal Ca(v)3.2 and TRPV1, via AM404, which mediates the analgesic effect of paracetamol. AM404 is able to activate TRPV1 and weakly inhibits Ca(v)3.2. Interestingly, activation of TRPV1 induces a strong inhibition of Ca(v)3.2 current. Supporting this, intracerebroventricular administration of AM404 or capsaicin produces antinociception that is lost in Ca(v)3.2(-/-) mice. Our study, for the first time, (1) provides a molecular mechanism for the supraspinal antinociceptive effect of paracetamol; (2) identifies the relationship between TRPV1 and the Ca(v)3.2 channel; and (3) suggests supraspinal Ca(v)3.2 inhibition as a potential pharmacological strategy to alleviate pain.

TRPV5: a Ca(2+) channel for the fine-tuning of Ca(2+) reabsorption

TRPV5 is one of the two channels in the TRPV family that exhibit high selectivity to Ca(2+) ions. TRPV5 mediates Ca(2+) influx into cells as the first step to transport Ca(2+) across epithelia. The specialized distribution in the distal tubule of the kidney positions TRPV5 as a key player in Ca(2+) reabsorption. The responsiveness in expression and/or activity of TRPV5 to hormones such as 1,25-dihydroxyvitamin D3, parathyroid hormone, estrogen, and testosterone makes TRPV5 suitable for its role in the fine-tuning of Ca(2+) reabsorption. This role is further optimized by the modulation of TRPV5 trafficking and activity via its binding partners; co-expressed proteins; tubular factors such as calbindin-D28k, calmodulin, klotho, uromodulin, and plasmin; extracellular and intracellular factors such as proton, Mg(2+), Ca(2+), and phosphatidylinositol-4,5-bisphosphate; and fluid flow. These regulations allow TRPV5 to adjust its overall activity in response to the body's demand for Ca(2+) and to prevent kidney stone formation. A point mutation in mouse Trpv5 gene leads to hypercalciuria similar to Trpv5 knockout mice, suggesting a possible role of TRPV5 in hypercalciuric disorders in humans. In addition, the single nucleotide polymorphisms in Trpv5 gene prevalently present in African descents may contribute to the efficient renal Ca(2+) reabsorption among African descendants. TRPV5 represents a potential therapeutic target for disorders with altered Ca(2+) homeostasis.

TRPV2

Transient receptor potential vanilloid type 2, TRPV2, is a calcium-permeable cation channel belonging to the TRPV channel family. This channel is activated by heat (>52 °C), various ligands, and mechanical stresses. In most of the cells, a large portion of TRPV2 is located in the endoplasmic reticulum under unstimulated conditions. Upon stimulation of the cells with phosphatidylinositol 3-kinase-activating ligands, TRPV2 is translocated to the plasma membrane and functions as a cation channel. Mechanical stress may also induce translocation of TRPV2 to the plasma membrane. The expression of TRPV2 is high in some types of cells including neurons, neuroendocrine cells, immune cells involved in innate immunity, and certain types of cancer cells. TRPV2 may modulate various cellular functions in these cells.

TRPV6 channels

TRPV6 (former synonyms ECAC2, CaT1, CaT-like) displays several specific features which makes it unique among the members of the mammalian Trp gene family (1) TRPV6 (and its closest relative, TRPV5) are the only highly Ca(2+)-selective channels of the entire TRP superfamily (Peng et al. 1999; Wissenbach et al. 2001; Voets et al. 2004). (2) Translation of Trpv6 initiates at a non-AUG codon, at ACG, located upstream of the annotated AUG, which is not used for initiation (Fecher-Trost et al. 2013). The ACG codon is nevertheless decoded by methionine. Not only a very rare event in eukaryotic biology, the full-length TRPV6 protein existing in vivo comprises an amino terminus extended by 40 amino acid residues compared to the annotated truncated TRPV6 protein which has been used in most studies on TRPV6 channel activity so far. (In the following numbering occurs according to this full-length protein, with the numbers of the so far annotated truncated protein in brackets). (3) Only in humans a coupled polymorphism of Trpv6 exists causing three amino acid exchanges and resulting in an ancestral Trpv6 haplotype and a so-called derived Trpv6 haplotype (Wissenbach et al. 2001). The ancestral allele encodes the amino acid residues C197(157), M418(378) and M721(681) and the derived alleles R197(157), V418(378) and T721(681). The ancestral haplotype is found in all species, the derived Trpv6 haplotype has only been identified in humans, and its frequency increases with the distance to the African continent. Apparently the Trpv6 gene has been a strong target for selection in humans, and its derived variant is one of the few examples showing consistently differences to the orthologues genes of other primates (Akey et al. 2004, 2006; Stajich and Hahn 2005; Hughes et al. 2008). (4) The Trpv6 gene expression is significantly upregulated in several human malignancies including the most common cancers, prostate and breast cancer (Wissenbach et al. 2001; Zhuang et al. 2002; Fixemer et al. 2003; Bolanz et al. 2008). (5) Male mice lacking functional TRPV6 channels are hypo-/infertile making TRPV6 one of the very few channels essential for male fertility (Weissgerber et al. 2011, 2012).

TRPV4 deficiency causes sexual dimorphism in bone metabolism and osteoporotic fracture risk

We explored the role of transient receptor potential vanilloid 4 (TRPV4) in murine bone metabolism and association of TRPV4 gene variants with fractures in humans. Urinary and histomorphometrical analyses demonstrated reduced osteoclast activity and numbers in male Trpv4(-/-) mice, which was confirmed in bone marrow-derived osteoclast cultures. Osteoblasts and bone formation as shown by serum procollagen type 1 amino-terminal propeptide and histomorphometry, including osteoid surface, osteoblast and osteocyte numbers were not affected in vivo. Nevertheless, osteoblast differentiation was enhanced in Trpv4(-/-) bone marrow cultures. Cortical and trabecular bone mass was 20% increased in male Trpv4(-/-) mice, compared to sex-matched wild type (Trpv4(+/+)) mice. However, at the same time intracortical porosity was increased and bone matrix mineralization was reduced. Together, these lead to a maximum load, stiffness and work to failure of the femoral bone, which were not different compared to Trpv4(+/+) mice, while the bone material was less resistant to stress and less elastic. The differential impacts on these determinants of bone strength were likely responsible for the lack of any changes in whole bone strength in the Trpv4(-/-) mice. None of these skeletal parameters were affected in female Trpv4(-/-) mice. The T-allele of rs1861809 SNP in the TRPV4 locus was associated with a 30% increased risk (95% CI: 1.1-1.6; p=0.013) for non-vertebral fracture risk in men, but not in women, in the Rotterdam Study. Meta-analyses with the population-based LASA study confirmed the association with non-vertebral fractures in men. This was lost when the non-population-based studies Mr. OS and UFO were included. In conclusion, TRPV4 is a male-specific regulator of bone metabolism, a determinant of bone strength, and a potential risk predictor for fractures through regulation of bone matrix mineralization and intra-cortical porosity. This identifies TRPV4 as a unique sexually dimorphic therapeutic and/or diagnostic candidate for osteoporosis.

Osteoblastic differentiation enhances expression of TRPV4 that is required for calcium oscillation induced by mechanical force

Mechanical stress is known to alter bone mass and the loss of force stimuli leads to reduction of bone mass. However, molecules involved in this phenomenon are incompletely understood. As mechanical force would affect signaling events in cells, we focused on a calcium channel, TRPV4 regarding its role in the effects of force stimuli on calcium in osteoblasts. TRPV4 expression levels were enhanced upon differentiation of osteoblasts in culture. We found that BMP-2 treatment enhanced TRPV4 gene expression in a dose dependent manner. BMP-2 effects on TRPV4 expression were suppressed by inhibitors for transcription and new protein synthesis. In these osteoblasts, a TRPV4-selective agonist, 4α-PDD, enhanced calcium signaling and the effects of 4α-PDD were enhanced in differentiated osteoblasts compared to the control cells. Fluid flow, as a mechanical stimulation, induced intracellular calcium oscillation in wild type osteoblasts. In contrast, TRPV4 deficiency suppressed calcium oscillation significantly even when the cells were subjected to fluid flow. These data suggest that TRPV4 is involved in the flow-induced calcium signaling in osteoblasts.

Chronic alteration in phosphatidylinositol 4,5-biphosphate levels regulates capsaicin and mustard oil responses

There is an agreement that acute (in minutes) hydrolysis and accumulation of phosphatidylinositol 4,5-bisphosphate (PIP(2) ) modulate TRPV1 and TRPA1 activities. Because inflammation results in PIP(2) depletion, persisting for long periods (hours to days) in pain models and in the clinic, we examined whether chronic depletion and accumulation of PIP(2) affect capsaicin (CAP) and mustard oil (MO) responses. In addition, we wanted to evaluate whether the effects of PIP(2) depend on TRPV1 and TRPA1 coexpression and whether the PIP(2) actions vary in expression cells vs. sensory neurons. Chronic PIP(2) production was stimulated by overexpression of phosphatidylinositol-4-phosphate-5-kinase, and PIP(2) -specific phospholipid 5'-phosphatase was selected to reduce plasma membrane levels of PIP(2) . Our results demonstrate that CAP (100 nM) responses and receptor tachyphylaxis are not significantly influenced by chronic changes in PIP(2) levels in wild-type (WT) or TRPA1 null-mutant sensory neurons as well as CHO cells expressing TRPV1 alone or with TRPA1. However, low concentrations of CAP (20 nM) produced a higher response after PIP(2) depletion in cells containing TRPV1 alone but not TRPV1 together with TRPA1. MO (25 μM) responses were also not affected by PIP(2) in WT sensory neurons and cells coexpressing TRPA1 and TRPV1. In contrast, PIP(2) reduction leads to pronounced tachyphylaxis to MO in cells with both channels. Chronic effect of PIP(2) on TRPA1 activity depends on presence of the TRPV1 channel and cell type (CHO vs. sensory neurons). In summary, chronic alterations in PIP(2) levels regulate magnitude of CAP and MO responses as well as MO tachyphylaxis. This regulation depends on coexpression profile of TRPA1 and TRPV1 and cell type.

Intestinal epithelial stem/progenitor cells are controlled by mucosal afferent nerves

Background

The maintenance of the intestinal epithelium is of great importance for the survival of the organism. A possible nervous control of epithelial cell renewal was studied in rats and mice.

Methods

Mucosal afferent nerves were stimulated by exposing the intestinal mucosa to capsaicin (1.6 mM), which stimulates intestinal external axons. Epithelial cell renewal was investigated in the jejunum by measuring intestinal thymidine kinase (TK) activity, intestinal ³H-thymidine incorporation into DNA, and the number of crypt cells labeled with BrdU. The influence of the external gut innervation was minimized by severing the periarterial nerves.

Principal Findings

Luminal capsaicin increased all the studied variables, an effect nervously mediated to judge from inhibitory effects on TK activity or ³H-thymidine incorporation into DNA by exposing the mucosa to lidocaine (a local anesthetic) or by giving four different neurotransmitter receptor antagonists i.v. (muscarinic, nicotinic, neurokinin1 (NK1) or calcitonin gene related peptide (CGRP) receptors). After degeneration of the intestinal external nerves capsaicin did not increase TK activity, suggesting the involvement of an axon reflex. Intra-arterial infusion of Substance P (SP) or CGRP increased intestinal TK activity, a response abolished by muscarinic receptor blockade. Immunohistochemistry suggested presence of M3 and M5 muscarinic receptors on the intestinal stem/progenitor cells. We propose that the stem/progenitor cells are controlled by cholinergic nerves, which, in turn, are influenced by mucosal afferent neuron(s) releasing acetylcholine and/or SP and/or CGRP. In mice lacking the capsaicin receptor, thymidine incorporation into DNA and number of crypt cells labeled with BrdU was lower than in wild type animals suggesting that nerves are important also in the absence of luminal capsaicin, a conclusion also supported by the observation that atropine lowered thymidine incorporation into DNA by 60% in control rat segments.

Conclusion

Enteric nerves are of importance in maintaining the intestinal epithelial barrier.

Trpv6 mediates intestinal calcium absorption during calcium restriction and contributes to bone homeostasis

Energy-dependent intestinal calcium absorption is important for the maintenance of calcium and bone homeostasis, especially when dietary calcium supply is restricted. The active form of vitamin D, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], is a crucial regulator of this process and increases the expression of the transient receptor potential vanilloid 6 (Trpv6) calcium channel that mediates calcium transfer across the intestinal apical membrane. Genetic inactivation of Trpv6 in mice (Trpv6(-/-)) showed, however, that TRPV6 is redundant for intestinal calcium absorption when dietary calcium content is normal/high and passive diffusion likely contributes to maintain normal serum calcium levels. On the other hand, Trpv6 inactivation impaired the increase in intestinal calcium transport following calcium restriction, however without resulting in hypocalcemia. A possible explanation is that normocalcemia is maintained at the expense of bone homeostasis, a hypothesis investigated in this study. In this study, we thoroughly analyzed the bone phenotype of Trpv6(-/-) mice receiving a normal (approximately 1%) or low (approximately 0.02%) calcium diet from weaning onwards using micro-computed tomography, histomorphometry and serum parameters. When dietary supply of calcium is normal, Trpv6 inactivation did not affect growth plate morphology, bone mass and remodeling parameters in young adult or aging mice. Restricting dietary calcium had no effect on serum calcium levels and resulted in a comparable reduction in bone mass accrual in Trpv6(+/+) and Trpv6(-/-) mice (-35% and 45% respectively). This decrease in bone mass was associated with a similar increase in bone resorption, whereas serum osteocalcin levels and the amount of unmineralized bone matrix were only significantly increased in Trpv6(-/-) mice. Taken together, our findings indicate that TRPV6 contributes to intestinal calcium transport when dietary calcium supply is limited and in this condition indirectly regulates bone formation and/or mineralization.

Hydrogen peroxide is a novel mediator of inflammatory hyperalgesia, acting via transient receptor potential vanilloid 1-dependent and independent mechanisms

Inflammatory diseases associated with pain are often difficult to treat in the clinic due to insufficient understanding of the nociceptive pathways involved. Recently, there has been considerable interest in the role of reactive oxygen species (ROS) in inflammatory disease, but little is known of the role of hydrogen peroxide (H(2)O(2)) in hyperalgesia. In the present study, intraplantar injection of H(2)O(2)-induced a significant dose- and time-dependent mechanical and thermal hyperalgesia in the mouse hind paw, with increased c-fos activity observed in the dorsal horn of the spinal cord. H(2)O(2) also induced significant nociceptive behavior such as increased paw licking and decreased body liftings. H(2)O(2) levels were significantly raised in the carrageenan-induced hind paw inflammation model, showing that this ROS is produced endogenously in a model of inflammation. Moreover, superoxide dismutase and catalase significantly reduced carrageenan-induced mechanical and thermal hyperalgesia, providing evidence of a functionally significant endogenous role. Thermal, but not mechanical, hyperalgesia in response to H(2)O(2) (i.pl.) was longer lasting in TRPV1 wild type mice compared to TRPV1 knockouts. It is unlikely that downstream lipid peroxidation was increased by H(2)O(2). In conclusion, we demonstrate a notable effect of H(2)O(2) in mediating inflammatory hyperalgesia, thus highlighting H(2)O(2) removal as a novel therapeutic target for anti-hyperalgesic drugs in the clinic.

Active intestinal calcium transport in the absence of transient receptor potential vanilloid type 6 and calbindin-D9k

To study the role of the epithelial calcium channel transient receptor potential vanilloid type 6 (TRPV6) and the calcium-binding protein calbindin-D9k in intestinal calcium absorption, TRPV6 knockout (KO), calbindin-D9k KO, and TRPV6/calbindin-D(9k) double-KO (DKO) mice were generated. TRPV6 KO, calbindin-D9k KO, and TRPV6/calbindin-D9k DKO mice have serum calcium levels similar to those of wild-type (WT) mice ( approximately 10 mg Ca2+/dl). In the TRPV6 KO and the DKO mice, however, there is a 1.8-fold increase in serum PTH levels (P < 0.05 compared with WT). Active intestinal calcium transport was measured using the everted gut sac method. Under low dietary calcium conditions there was a 4.1-, 2.9-, and 3.9-fold increase in calcium transport in the duodenum of WT, TRPV6 KO, and calbindin-D9k KO mice, respectively (n = 8-22 per group; P > 0.1, WT vs. calbindin-D9k KO, and P < 0.05, WT vs. TRPV6 KO on the low-calcium diet). Duodenal calcium transport was increased 2.1-fold in the TRPV6/calbindin-D9k DKO mice fed the low-calcium diet (P < 0.05, WT vs. DKO). Active calcium transport was not stimulated by low dietary calcium in the ileum of the WT or KO mice. 1,25-Dihydroxyvitamin D3 administration to vitamin D-deficient null mutant and WT mice also resulted in a significant increase in duodenal calcium transport (1.4- to 2.0-fold, P < 0.05 compared with vitamin D-deficient mice). This study provides evidence for the first time using null mutant mice that significant active intestinal calcium transport occurs in the absence of TRPV6 and calbindin-D9k, thus challenging the dogma that TRPV6 and calbindin-D9k are essential for vitamin D-induced active intestinal calcium transport.

Methanandamide activation of a novel current in mouse trigeminal ganglion sensory neurons in vitro

Anandamide is an endogenous agonist for cannabinoid receptors and produces analgesia by acting at these receptors in several sites in the brain and peripheral nervous system. Anandamide is also an agonist at the TRPV1 receptor, a protein that serves as an important integrator of noxious stimuli in sensory neurons. Although anandamide actions at CB1 and TRPV1 receptors can explain many of its effects on sensory neurons, some apparently CB1- and TRPV1-independent effects of anandamide have been reported. To explore possible mechanisms underlying these effects we examined the actions of the stable anandamide analog methanandamide on the membrane properties of trigeminal ganglion neurons from mice with TRPV1 deleted. We found that methanandamide and anandamide activate a novel current in a subpopulation of small trigeminal ganglion neurons. Methanandamide activated the current (EC(50) 2 microM) more potently than it activates TRPV1 under the same conditions. The methanandamide-activated current reverses at 0 mV and does not inactivate at positive potentials but declines rapidly at negative membrane potentials. Activation of the current is not mediated via cannabinoid receptors and does not appear to involve G proteins. The phytocannabinoid Delta(9)-tetrahydrocannabinol, the endocannabinoid-related molecules N-arachidonoyl dopamine and N-arachidonoyl glycine and the non-specific TRPV channel activator 2-aminoethoxydiphenyl borate do not mimic the effects of methanandamide. The molecular identity of the current remains to be established, but we have identified a potential new effector for endocannabinoids in sensory neurons, and activation of this current may underlie some of the previously reported CB1 and TRPV1-independent effects of these compounds.

Thermoregulation: channels that are cool to the core

TRP channels are molecular sensors that endow animals with the ability to sense external temperatures. Recent studies have provided strong evidence that at least one TRP channel, TRPV1, is also essential for the maintenance of internal body temperature.

Hydroxy-alpha-sanshool activates TRPV1 and TRPA1 in sensory neurons

Sanshools are major active ingredients of Zanthoxylum piperitum and are used as food additives in East Asia. Sanshools cause irritant, tingling and sometimes paresthetic sensations on the tongue. However, the molecular mechanism underlying the pungent or tingling sensation induced by sanshools is not known. Because many transient receptor potential (TRP) channels are responsible for the sensations induced by various spices and food additives, we expressed 17 TRP channels in human embryonic kidney (HEK) cells and investigated their activation by hydroxy-alpha-sanshool (HalphaSS) or hydroxy-beta-sanshool (HbetaSS) isolated from Zanthoxylum piperitum. It was found that HalphaSS, but not HbetaSS, depolarized sensory neurons with concomitant firing of action potentials and evoked inward currents. Among 17 TRP channels expressed in HEK cells, HalphaSS caused Ca(2+) influx in cells transfected with TRPV1 or TRPA1, and evoked robust inward currents in cells transfected with TRPV1 or TRPA1. In primary cultured sensory neurons, HalphaSS induced inward currents and Ca(2+) influx in a capsazepine-dependent manner. Moreover, HalphaSS-induced currents and Ca(2+) influx were greatly diminished in TRPV1(-/-) mice. HalphaSS evoked licking behavior when injected into a single hind paw of wild-type mice, but this was much reduced in TRPV1-deficient mice. These results indicate that TRPV1 and TRPA1 are molecular targets of HalphaSS in sensory neurons. We conclude that the activations of TRPV1 and TRPA1 by HalphaSS explain its unique pungent, tingling sensation.

Nonthermal activation of transient receptor potential vanilloid-1 channels in abdominal viscera tonically inhibits autonomic cold-defense effectors

An involvement of the transient receptor potential vanilloid (TRPV) 1 channel in the regulation of body temperature (T(b)) has not been established decisively. To provide decisive evidence for such an involvement and determine its mechanisms were the aims of the present study. We synthesized a new TRPV1 antagonist, AMG0347 [(E)-N-(7-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl)-3-(2-(piperidin-1-yl)-6-(trifluoromethyl)pyridin-3-yl)acrylamide], and characterized it in vitro. We then found that this drug is the most potent TRPV1 antagonist known to increase T(b) of rats and mice and showed (by using knock-out mice) that the entire hyperthermic effect of AMG0347 is TRPV1 dependent. AMG0347-induced hyperthermia was brought about by one or both of the two major autonomic cold-defense effector mechanisms (tail-skin vasoconstriction and/or thermogenesis), but it did not involve warmth-seeking behavior. The magnitude of the hyperthermic response depended on neither T(b) nor tail-skin temperature at the time of AMG0347 administration, thus indicating that AMG0347-induced hyperthermia results from blockade of tonic TRPV1 activation by nonthermal factors. AMG0347 was no more effective in causing hyperthermia when administered into the brain (intracerebroventricularly) or spinal cord (intrathecally) than when given systemically (intravenously), which indicates a peripheral site of action. We then established that localized intra-abdominal desensitization of TRPV1 channels with intraperitoneal resiniferatoxin blocks the T(b) response to systemic AMG0347; the extent of desensitization was determined by using a comprehensive battery of functional tests. We conclude that tonic activation of TRPV1 channels in the abdominal viscera by yet unidentified nonthermal factors inhibits skin vasoconstriction and thermogenesis, thus having a suppressive effect on T(b).

Bone cancer increases transient receptor potential vanilloid subfamily 1 expression within distinct subpopulations of dorsal root ganglion neurons

Bone cancer pain has a strong impact on the quality of life of patients but is difficult to treat. Therefore, the mechanisms of bone cancer pain require elucidation for the purpose of development of new therapeutics. A recent study showed that activation of transient receptor potential vanilloid subfamily 1 (TRPV1) was involved in bone cancer pain. In this study, we re-evaluated the analgesic effects of pharmacological blockade of TRPV1 using the potent TRPV1 antagonist 5-iodoresiniferatoxin (I-RTX) and examined whether bone cancer can change TRPV1 expression and distribution in the primary sensory neurons in a mouse model of bone cancer pain. Implantation of osteosarcoma into the femur induced ongoing and movement-evoked bone cancer-related pain behaviors. These behaviors were significantly reduced by i.p. administration of I-RTX, compared with vehicle. Western blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses revealed that TRPV1 level was significantly increased in dorsal root ganglions (DRGs) ipsilateral to sarcoma implantation. Immunohistochemical analysis showed that implantation of osteosarcoma induced not only an increase in the percentage of TRPV1-positive neurons among DRG neurons (24.3+/-1.3% in sham mice and 31.2+/-1.3% in mice with osteosarcoma implantation, P<0.05) but also an overall shift in the distribution of area of profiles to the right. Colocalization study showed that the percentages of colocalization of TRPV1 with neurofilament 200 kD (NF200) and calcitonin gene-related peptide (CGRP) but not isolectin B4 (IB4) among DRG neurons in mice with osteosarcoma implantation were increased compared with those in sham mice (from 0.8+/-0.1% to 2.1+/-0.3% for TRPV1 and NF200 and from 21.1+/-1.3% to 26.5+/-0.2% for TRPV1 and CGRP). In conclusion, TRPV1 activation plays a critical role in the generation of bone cancer pain, and bone cancer increases TRPV1 expression within distinct subpopulation of DRG neurons. These findings may lead to novel strategies for the treatment of bone cancer pain.

TRPV4 initiates the acute calcium-dependent permeability increase during ventilator-induced lung injury in isolated mouse lungs

We have previously implicated calcium entry through stretch-activated cation channels in initiating the acute pulmonary vascular permeability increase in response to high peak inflation pressure (PIP) ventilation. However, the molecular identity of the channel is not known. We hypothesized that the transient receptor potential vanilloid-4 (TRPV4) channel may initiate this acute permeability increase because endothelial calcium entry through TRPV4 channels occurs in response to hypotonic mechanical stress, heat, and P-450 epoxygenase metabolites of arachidonic acid. Therefore, permeability was assessed by measuring the filtration coefficient (K(f)) in isolated perfused lungs of C57BL/6 mice after 30-min ventilation periods of 9, 25, and 35 cmH(2)O PIP at both 35 degrees C and 40 degrees C. Ventilation with 35 cmH(2)O PIP increased K(f) by 2.2-fold at 35 degrees C and 3.3-fold at 40 degrees C compared with baseline, but K(f) increased significantly with time at 40 degrees C with 9 cmH(2)O PIP. Pretreatment with inhibitors of TRPV4 (ruthenium red), arachidonic acid production (methanandamide), or P-450 epoxygenases (miconazole) prevented the increases in K(f). In TRPV4(-/-) knockout mice, the high PIP ventilation protocol did not increase K(f) at either temperature. We have also found that lung distention caused Ca(2+) entry in isolated mouse lungs, as measured by ratiometric fluorescence microscopy, which was absent in TRPV4(-/-) and ruthenium red-treated lungs. Alveolar and perivascular edema was significantly reduced in TRPV4(-/-) lungs. We conclude that rapid calcium entry through TRPV4 channels is a major determinant of the acute vascular permeability increase in lungs following high PIP ventilation.

Bladder afferent sensitivity in wild-type and TRPV1 knockout mice

Understanding bladder afferent pathways may reveal novel targets for therapy of lower urinary tract disorders such as overactive bladder syndrome and cystitis. Several potential candidate molecules have been postulated as playing a significant role in bladder function. One such candidate is the transient receptor potential vanilloid 1 (TRPV1) ion channel. Mice lacking the TRPV1 channel have altered micturition thresholds suggesting that TRPV1 channels may play a role in the detection of bladder filling. The aim of this study was therefore to investigate the role of TRPV1 receptors in controlling bladder afferent sensitivity in the mouse using pharmacological receptor blockade and genetic deletion of the channel. Multiunit afferent activity was recorded in vitro from bladder afferents taken from wild-type (TRPV+/+) mice and knockout (TRPV1-/-) mice. In wild-type preparations, ramp distension of the bladder to a maximal pressure of 40 mmHg produced a graded increase in afferent activity. Bath application of the TRPV1 antagonist capsazepine (10 mum) caused a significant attenuation of afferent discharge in TRPV1+/+ mice. Afferent responses to distension were significantly attenuated in TRPV1-/- mice in which sensitivity to intravesical hydrochloric acid (50 mm) and capsaicin (10 microm) were also blunted. Altered mechanosensitivity occurred in the absence of any changes in the pressure-volume relationship during filling indicating that this was not secondary to a change in bladder compliance. Single-unit analysis was used to classify individual afferents into low-threshold and high-threshold fibres. Low threshold afferent responses were attenuated in TRPV1-/- mice compared to the TRPV1+/+ littermates while surprisingly high threshold afferent sensitivity was unchanged. While TRPV1 channels are not considered to be mechanically gated, the present study demonstrates a clear role for TRPV1 in the excitability of particularly low threshold bladder afferents. This suggests that TRPV1 may play an important role in normal bladder function.

Effects of capsaicin on VGSCs in TRPV1-/- mice

Two different mechanisms by which capsaicin blocks voltage-gated sodium channels (VGSCs) were found by using knockout mice for the transient receptor potential V1 (TRPV1(-/-)). Similar with cultured rat trigeminal ganglion (TG) neurons, the amplitude of tetrodotoxin-resistant (TTX-R) sodium current was reduced 85% by 1 muM capsaicin in capsaicin sensitive neurons, while only 6% was blocked in capsaicin insensitive neurons of TRPV1(+/+) mice. The selective effect of low concentration capsaicin on VGSCs was reversed in TRPV1(-/-) mice, which suggested that this effect was dependent on TRPV1 receptor. The blockage effect of high concentration capsaicin on VGSCs in TRPV1(-/-) mice was the same as that in capsaicin insensitive neurons of rats and TRPV1(+/+) mice. It is noted that non-selective effect of capsaicin on VGSCs shares many similarities with local anesthetics. That is, firstly, both blockages are concentration-dependent and revisable. Secondly, being accompanied with the reduction of amplitude, voltage-dependent inactivation curve shifts to hyperpolarizing direction without a shift of activation curve. Thirdly, use-dependent blocks are induced at high stimulus frequency.

Sensitization to heat through G-protein-coupled receptor pathways in the isolated sciatic mouse nerve

We have previously shown that isolated rat sciatic nerve axons express capsaicin, heat and proton sensitivity and respond to stimulation with a Ca(++)-dependent and graded calcitonin gene related peptide (CGRP) release. There is morphological evidence for stimulated vesicular exocytosis and for capsaicin receptor, transient receptor potential vanilloid type-1 (TRPV1, formerly VR1) translocation in the axolemma of unmyelinated nerve fibres. In sensory nerve terminals CGRP release in response to noxious heat can be sensitized by activation of G-protein-coupled receptors and related protein kinases. We present evidence that also in isolated mouse sciatic nerve axons the intracellular protein kinase A (PKA)- and C (PKC)-dependent transduction pathways modulate heat-induced (45 degrees C) CGRP release. This is demonstrated using the direct activators, forskolin and phorbol 12-myristate 13-acetate (PMA), as well as prostaglandin E2 (PGE(2)) and bradykinin acting through G-protein-coupled receptors. Inhibition at rest of protein kinases A or C left heat-induced CGRP release unchanged. In TRPV1 knockout animals no sensitization to heat was observed using a combined stimulation by prostaglandin E2 and bradykinin. To a surprising degree, peripheral nerve axons resemble peripheral sensory terminals in their common properties of sensory and signal transduction.

Effects of body temperature on neural activity in the hippocampus: regulation of resting membrane potentials by transient receptor potential vanilloid 4

Physiological body temperature is an important determinant for neural functions, and it is well established that changes in temperature have dynamic influences on hippocampal neural activities. However, the detailed molecular mechanisms have never been clarified. Here, we show that hippocampal neurons express functional transient receptor potential vanilloid 4 (TRPV4), one of the thermosensitive TRP (transient receptor potential) channels, and that TRPV4 is constitutively active at physiological temperature. Activation of TRPV4 at 37 degrees C depolarized the resting membrane potential in hippocampal neurons by allowing cation influx, which was observed in wild-type (WT) neurons, but not in TRPV4-deficient (TRPV4KO) cells, although dendritic morphology, synaptic marker clustering, and synaptic currents were indistinguishable between the two genotypes. Furthermore, current injection studies revealed that TRPV4KO neurons required larger depolarization to evoke firing, equivalent to WT neurons, indicating that TRPV4 is a key regulator for hippocampal neural excitabilities. We conclude that TRPV4 is activated by physiological temperature in hippocampal neurons and thereby controls their excitability.

Activation of transient receptor potential vanilloid type-1 channel prevents adipogenesis and obesity

We tested the hypothesis that activation of transient receptor potential vanilloid type-1 (TRPV1) by capsaicin prevents adipogenesis. TRPV1 channels in 3T3-L1-preadipocytes and visceral adipose tissue from mice and humans were detected by immunoblotting and quantitative real-time RT-PCR. The effect of TRPV1 on cytosolic calcium was determined fluorometrically in 3T3-L1-preadipocytes and in human visceral fat tissue. Adipogenesis in stimulated 3T3-L1-preadipocytes was determined by oil red O-staining of intracellular lipid droplets, triglyceride levels, expression of peroxisome proliferator-activated receptor-gamma, and expression of fatty acid synthase. Long-term feeding experiments were undertaken in wild-type mice and TRPV1 knockout mice. We detected TRPV1 channels in 3T3-L1-preadipocytes and visceral adipose tissue from mice and humans. In vitro, the TRPV1 agonist capsaicin dose-dependently induced calcium influx and prevented the adipogenesis in stimulated 3T3-L1-preadipocytes. RNA interference knockdown of TRPV1 in 3T3-L1-preadipocytes attenuated capsaicin-induced calcium influx, and adipogenesis in stimulated 3T3-L1-preadipocytes was no longer prevented. During regular adipogenesis TRPV1 channels were downregulated which was accompanied by a significant and time-dependent reduction of calcium influx. Compared with lean counterparts in visceral adipose tissue from obese db/db and ob/ob mice, and from obese human male subjects we observed a reduced TRVP1 expression. The reduced TRPV1 expression in visceral adipose tissue from obese humans was accompanied by reduced capsaicin-induced calcium influx. The oral administration of capsaicin for 120 days prevented obesity in male wild type mice but not in TRPV1 knockout mice assigned to high fat diet. We conclude that the activation of TRPV1 channels by capsaicin prevented adipogenesis and obesity.

Reduced anxiety, conditioned fear, and hippocampal long-term potentiation in transient receptor potential vanilloid type 1 receptor-deficient mice

The transient receptor potential vanilloid type 1 channel (TRPV1) (formerly called vanilloid receptor VR1) is known for its key role of functions in sensory nerves such as perception of inflammatory and thermal pain. Much less is known about the physiological significance of the TRPV1 expression in the brain. Here we demonstrate that TRPV1 knock-out mice (TRPV1-KO) show less anxiety-related behavior in the light-dark test and in the elevated plus maze than their wild-type littermates with no differences in locomotion. Furthermore, TRPV1-KO mice showed less freezing to a tone after auditory fear conditioning and stress sensitization. This reduction of conditioned and sensitized fear could not be explained by alterations in nociception. Also, tone perception per se was unaffected, as revealed by determination of auditory thresholds through auditory brainstem responses and distortion-product otoacoustic emissions. TRPV1-KO showed also less contextual fear if assessed 1 d or 1 month after strong conditioning protocols. These impairments in hippocampus-dependent learning were mirrored by a decrease in long-term potentiation in the Schaffer collateral-commissural pathway to CA1 hippocampal neurons. Our data provide first evidence for fear-promoting effects of TRPV1 with respect to both innate and conditioned fear and for a decisive role of this receptor in synaptic plasticity.

TRPV4 as a flow sensor in flow-dependent K+ secretion from the cortical collecting duct

The transient receptor vanilloid-4 (TRPV4) is a mechanosensitive, swell-activated cation channel that is abundant in the renal distal tubules. Immunolocalization studies, however, present conflicting data as to whether TRPV4 is expressed along the apical and/or basolateral membranes. To disclose the role of TRPV4 in flow-dependent K+ secretion in distal tubules in vivo, urinary K+ excretion and net transports of K+ and Na+ in the cortical collecting duct (CCD) were measured with an in vitro microperfusion technique in TRPV4 +/+ and TRPV4 −/− mice. Both net K+ secretion and Na+ reabsorption were flow dependently increased in the CCDs isolated from TRPV4 +/+mice, which were significantly enhanced by a luminal application of 50 μM 4α-phorbol-12,13-didecanoate (4αPDD), an agonist of TRPV4. No flow dependence of net K+ and Na+ transports or effects of 4αPDD on CCDs were observed in TRPV4 −/− mice. A basolateral application of 4αPDD had little effect on these ion transports in the TRPV4 +/+ CCDs, while the luminal application did. Urinary K+ excretion was significantly smaller in TRPV4 −/− than in TRPV4 +/+ mice when urine production was stimulated by a venous application of furosemide. These observations suggested an essential role of the TRPV4 channels in the luminal or basolateral membrane as flow sensors in the mechanism underlying the flow-dependent K+ secretion in mouse CCDs.

Mouse colon sensory neurons detect extracellular acidosis via TRPV1

Extracellular acidification contributes to pain by activating or modulating nociceptor activity. To evaluate acidic signaling from the colon, we characterized acid-elicited currents in thoracolumbar (TL) and lumbosacral (LS) dorsal root ganglion (DRG) neurons identified by content of a fluorescent dye (DiI) previously injected into the colon wall. In 13% of unidentified LS DRG neurons (not labeled with DiI) and 69% of LS colon neurons labeled with DiI, protons activated a sustained current that was significantly and reversibly attenuated by the transient receptor potential vanilloid receptor 1 (TRPV1) antagonist capsazepine. In contrast, 63% of unidentified LS DRG neurons and 4% of LS colon neurons exhibited transient amiloride-sensitive acid-sensing ion channel (ASIC) currents. The peak current density of acid-elicited currents was significantly reduced in colon sensory neurons from TRPV1-null mice, supporting predominant expression of TRPV1 in LS colon sensory neurons, which was also confirmed immunohistochemically. Similar to LS colon DRG neurons, acid-elicited currents in TL colon DRG neurons were mediated predominantly by TRPV1. However, the pH producing half-activation of responses significantly differed between TL and LS colon DRG neurons. The properties of acid-elicited currents in colon DRG neurons suggest differential contributions of ASICs and TRPV1 to colon sensation and likely nociception.

Role of transient receptor potential vanilloid 1 receptors in endotoxin-induced airway inflammation in the mouse

Airways are densely innervated by capsaicin-sensitive sensory neurons expressing transient receptor potential vanilloid 1 (TRPV1) receptors/ion channels, which play an important regulatory role in inflammatory processes via the release of sensory neuropeptides. The aim of the present study was to investigate the role of TRPV1 receptors in endotoxin-induced airway inflammation and consequent bronchial hyperreactivity with functional, morphological, and biochemical techniques using receptor gene-deficient mice. Inflammation was evoked by intranasal administration of Escherichia coli lipopolysaccharide (60 microl, 167 microg/ml) in TRPV1 knockout (TRPV1(-/-)) mice and their wild-type counterparts (TRPV1(+/+)) 24 h before measurement. Airway reactivity was assessed by unrestrained whole body plethysmography, and its quantitative indicator, enhanced pause (Penh), was calculated after inhalation of the bronchoconstrictor carbachol. Histological examination and spectrophotometric myeloperoxidase measurement was performed from the lung. Somatostatin concentration was measured in the lung and plasma with radioimmunoassay. Bronchial hyperreactivity, histological lesions (perivascular/peribronchial edema, neutrophil/macrophage infiltration, goblet cell hyperplasia), and myeloperoxidase activity were significantly greater in TRPV(-/-) mice. Inflammation markedly elevated lung and plasma somatostatin concentrations in TRPV1(+/+) but not TRPV1(-/-) animals. In TRPV1(-/-) mice, exogenous administration of somatostatin-14 (4 x 100 microg/kg ip) diminished inflammation and hyperreactivity. Furthermore, in wild-type mice, antagonizing somatostatin receptors by cyclo-somatostatin (4 x 250 microg/kg ip) increased these parameters. This study provides the first evidence for a novel counterregulatory mechanism during endotoxin-induced airway inflammation, which is mediated by somatostatin released from sensory nerve terminals in response to activation of TRPV1 receptors of the lung. It reaches the systemic circulation and inhibits inflammation and consequent bronchial hyperreactivity.

Impaired vasodilation in response to perivascular nerve stimulation in mesenteric arteries of TRPV1-null mutant mice

BACKGROUND

The role of the transient receptor potential vanilloid type 1 (TRPV1) channels expressed in perivascular sensory nerves in the regulation of vascular reactivity is largely unknown. This study was designed to test the hypothesis that vasodilation induced by electrical field stimulation (EFS) of perivascular sensory nerves is mediated by the TRPV1 via release of sensory neurotransmitters in wild-type (WT) mice, and this effect is abolished in gene-targeted TRPV1-null mutant (TRPV1(-/-)) mice.

METHODS

Isolated mesenteric resistance arteries from WT and TRPV1(-/-)) mice were perfused and pretreated with guanethedine and atropine to block sympathetic and parasympathetic nerve activity, respectively. After precontracting with phenylephrine, changes of vascular diameters induced by EFS were monitored in the absence or presence of the TRPV1 receptor antagonist capsazepine; the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP8-37; or the substance P (SP) receptor antagonist, RP67580.

RESULTS

EFS-induced vasodilation was significantly reduced in arteries in TRPV1(-/-)) mice when compared to that of WT mice. Capsazepine and CGRP8-37 attenuated vasodilation induced by EFS in WT but not TRPV1(-/-)) mice. In contrast, RP67580 had no effect on the EFS-induced vasodilation in WT or TRPV1(-/-)) mice. The release of CGRP in the face of EFS challenge was significantly increased in both WT and TRPV1(-/-)) arteries, which was attenuated by capsazepine in WT but not TRPV1(-/-)) arteries. Exogenous CGRP caused dose-dependent vasodilation to a similar degree in WT and TRPV1(-/-)) arteries.

CONCLUSIONS

Our data show that in WT mice transmural stimulation of perivascular sensory nerves activates the TRPV1, leading to CGRP release from sensory nerve endings; and blockade of CGRP, but not SP, receptors abolishes TRPV1-mediated vasodilation during EFS. All these effects are impaired in TRPV1(-/-)) mice, indicating that TRPV1 plays a key role in modulating perivascular sensory nerve-mediated vasodilation.

Vitamin D and calcium receptors: links to hypercalciuria

PURPOSE OF REVIEW

In idiopathic hypercalciuria, patients have increased intestinal Ca absorption and decreased renal Ca reabsorption, with either elevated or normal serum levels of 1,25-dihydroxyvitamin D. As 1,25-dihydroxyvitamin D exerts its biologic effects through interactions with the vitamin D receptor, we examine the actions of this receptor and 1,25-dihydroxyvitamin D in animals with genetic hypercalciuria.

RECENT FINDINGS

In genetic hypercalciuric stone-forming rats intestinal calcium transport is increased and renal calcium reabsorption is reduced, yet serum 1,25-dihydroxyvitamin D levels are normal. Elevated intestinal and kidney vitamin D receptors suggest that increased tissue 1,25-dihydroxyvitamin D-vitamin D receptor complexes enhance 1,25-dihydroxyvitamin D actions on intestine and kidney, and vitamin D-dependent over-expression of renal calcium-sensing receptor alone can decrease tubule calcium reabsorption. In TRPV5-knockout mice, ablation of the renal calcium-influx channel decreases tubular calcium reabsorption, and secondary elevations in 1,25-dihydroxyvitamin D increase intestinal calcium transport.

SUMMARY

1,25-Dihydroxyvitamin D or vitamin D receptor may change intestinal and renal epithelial calcium transport simultaneously or calcium-transport changes across renal epithelia may be primary with a vitamin D-mediated secondary increase in intestinal transport. The extent of homology between the animal models and human idiopathic hypercalciuria remains to be determined.

The novel vitamin D analog ZK191784 as an intestine-specific vitamin D antagonist

Vitamin D [1,25(OH)2D3] plays a crucial role in Ca2+ homeostasis by stimulating Ca2+ (re)absorption and bone turnover. The 1,25(OH)2D3 analog ZK191784 was recently developed to dissociate the therapeutic immunomodulatory activity from the hypercalcemic side effects of 1,25(OH)2D3 and contains a structurally modified side chain characterized by a 22,23-double bond, 24R-hydroxy group, 25-cyclopropyl ring, and 5-butyloxazole unit. We investigated the effect of ZK191784 on Ca2+ homeostasis and the regulation of Ca2+ transport proteins in wild-type (WT) mice and mice lacking the renal epithelial Ca2+ channel TRPV5 (TRPV5-/-). The latter display hypercalciuria, hypervitaminosis D, increased intestinal expression of the epithelial Ca2+ channel TRPV6, the Ca2+-binding protein calbindin-D(9K), and intestinal Ca2+ hyperabsorption. ZK191784 normalized the Ca2+ hyperabsorption and the expression of intestinal Ca2+ transport proteins in TRPV5-/- mice. Furthermore, the compound decreased intestinal Ca2+ absorption in WT mice and reduced 1,25(OH)2D3-dependent 45Ca2+ uptake by Caco-2 cells, substantiating a 1,25(OH)2D3-antagonistic action of ZK191784 in the intestine. ZK191784 increased renal TRPV5 and calbindin-D(28K) expression and decreased urine Ca2+ excretion in WT mice. Both 1,25(OH)2D3 and ZK191784 enhanced transcellular Ca2+ transport in primary cultures of rabbit connecting tubules and cortical collecting ducts, indicating a 1,25(OH)2D3-agonistic effect in kidney. ZK191784 enhanced bone TRPV6 mRNA levels and 1,25(OH)2D3 as well as ZK191784 stimulated secretion of the bone formation marker osteocalcin in rat osteosarcoma cells, albeit to a different extent. In conclusion, ZK191784 is a synthetic 1,25(OH)2D3 ligand displaying a unique tissue-specific profile when administered in vivo. Because ZK191784 acts as an intestine-specific 1,25(OH)2D3 antagonist, this compound will be associated with less hypercalcemic side effects compared with the 1,25(OH)2D3 analogs currently used in clinical practice.