Regulation of TRP channels: a voltage–lipid connection

The synaptic lipidome in health and disease

Adequate homeostasis of lipid, protein and carbohydrate metabolism is essential for cells to perform highly specific tasks in our organism, and the brain, with its uniquely high energetic requirements, posesses singular characteristics. Some of these are related to its extraordinary dotation of synapses, the specialized subcelluar structures where signal transmission between neurons occurs in the central nervous system. The post-synaptic compartment of excitatory synapses, the dendritic spine, harbors key molecules involved in neurotransmission tightly packed within a minute volume of a few femtoliters. The spine is further compartmentalized into nanodomains that facilitate the execution of temporo-spatially separate functions in the synapse. Lipids play important roles in this structural and functional compartmentalization and in mechanisms that impact on synaptic transmission. This review analyzes the structural and dynamic processes involving lipids at the synapse, highlighting the importance of their homeostatic balance for the physiology of this complex and highly specialized structure, and underscoring the pathologies associated with disbalances of lipid metabolism, particularly in the perinatal and late adulthood periods of life. Although small variations of the lipid profile in the brain take place throughout the adult lifespan, the pathophysiological consequences are clinically manifested mostly during late adulthood. Disturbances in lipid homeostasis in the perinatal period leads to alterations during nervous system development, while in late adulthood they favor the occurrence of neurodegenerative diseases.

Critical contributions of pre-S1 shoulder and distal TRP box in DAG-activated TRPC6 channel by PIP2 regulation

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ or PIP₂) regulates the activities of numerous membrane proteins, including diacylglycerol(DAG)-activated TRPC3/6/7 channels. Although PIP₂ binding is known to support DAG-activated TRP channel activity, its binding site remains unknown. We screened for PIP₂ binding sites within TRPC6 channels through extensive mutagenesis. Using voltage-sensitive phosphatase (DrVSP), we found that Arg437 and Lys442, located in the channel’s pre-S1 domain/shoulder, are crucial for interaction with PIP₂. To gain structural insights, we conducted computer protein–ligand docking simulations with the pre-S1 domain/shoulder of TRPC6 channels. Further, the functional significance of PIP₂ binding to the pre-S1 shoulder was assessed for receptor-operated channel functions, cross-reactivity to DAG activation, and the kinetic model simulation. These results revealed that basic residues in the pre-S1 domain/shoulder play a central role in the regulation of PIP₂-dependent gating. In addition, neutralizing mutation of K771 in the distal TRP box reversed the effect of PIP₂ depletion from inhibiting to potentiating channel activity. A similar effect was seen in TRPV1 channels, which suggests that TRPC6 possesses a common but robust polarity switch mediating the PIP₂-dependent effect. Overall, these mutagenesis studies reveal functional and structural insights for how basic residues and channel segments in TRP channels are controlled through phosphoinositides recognition.

Examining the role of transient receptor potential canonical 5 (TRPC5) in osteoarthritis

Introduction

Osteo-arthritis (OA) involves joint degradation and usually pain; with mechanisms poorly understood and few treatment options. There is evidence that the transient receptor potential canonical 5 (TRPC5) mRNA expression is reduced in OA patients’ synovia. Here we examine the profile of TRPC5 in DRG and involvement in murine models of OA.

Design

TRPC5 KO mice were subjected to partial meniscectomy (PMNX) or injected with monoiodoacetate (MIA) and pain-related behaviours were determined. Knee joint pathological scores were analysed and gene expression changes in ipsilateral synovium and dorsal root ganglia (DRG) determined. c-Fos protein expression in the ipsilateral dorsal horn of the spinal cord was quantified.

Results

TRPC5 KO mice developed a discrete enhanced pain-related phenotype. In the MIA model, the pain-related phenotype correlated with c-Fos expression in the dorsal horn and increased expression of nerve injury markers ATF3, CSF1 and galanin in the ipsilateral DRG. There were negligible differences in the joint pathology between WT and TRPC5 KO mice, however detailed gene expression analysis determined increased expression of the mast cell marker CD117 as well as extracellular matrix remodelling proteinases MMP2, MMP13 and ADAMTS4 in MIA-treated TRPC5 KO mice. TRPC5 expression was defined to sensory subpopulations in DRG.

Conclusions

Deletion of TRPC5 receptor signalling is associated with exacerbation of pain-like behaviour in OA which correlates with increased expression of enzymes involved in extracellular remodelling, inflammatory cells in the synovium and increased neuronal activation and injury in DRG. Together, these results identify a modulating role for TRPC5 in OA-induced pain-like behaviours.

Antinociception mechanisms of action of cannabinoid-based medicine: an overview for anesthesiologists and pain physicians

Cannabinoid-based medications possess unique multimodal analgesic mechanisms of action, modulating diverse pain targets. Cannabinoids are classified based on their origin into three categories: endocannabinoids (present endogenously in human tissues), phytocannabinoids (plant derived) and synthetic cannabinoids (pharmaceutical). Cannabinoids exert an analgesic effect, peculiarly in hyperalgesia, neuropathic pain and inflammatory states. Endocannabinoids are released on demand from postsynaptic terminals and travels retrograde to stimulate cannabinoids receptors on presynaptic terminals, inhibiting the release of excitatory neurotransmitters. Cannabinoids (endogenous and phytocannabinoids) produce analgesia by interacting with cannabinoids receptors type 1 and 2 (CB1 and CB2), as well as putative non-CB1/CB2 receptors; G protein-coupled receptor 55, and transient receptor potential vanilloid type-1. Moreover, they modulate multiple peripheral, spinal and supraspinal nociception pathways. Cannabinoids-opioids cross-modulation and synergy contribute significantly to tolerance and antinociceptive effects of cannabinoids. This narrative review evaluates cannabinoids' diverse mechanisms of action as it pertains to nociception modulation relevant to the practice of anesthesiologists and pain medicine physicians.

Physiology and Pathophysiology of Itch

Itch is a topic to which everyone can relate. The physiological roles of itch are increasingly understood and appreciated. The pathophysiological consequences of itch impact quality of life as much as pain. These dynamics have led to increasingly deep dives into the mechanisms that underlie and contribute to the sensation of itch. When the prior review on the physiology of itching was published in this journal in 1941, itch was a black box of interest to a small number of neuroscientists and dermatologists. Itch is now appreciated as a complex and colorful Rubik's cube. Acute and chronic itch are being carefully scratched apart and reassembled by puzzle solvers across the biomedical spectrum. New mediators are being identified. Mechanisms blur boundaries of the circuitry that blend neuroscience and immunology. Measures involve psychophysics and behavioral psychology. The efforts associated with these approaches are positively impacting the care of itchy patients. There is now the potential to markedly alleviate chronic itch, a condition that does not end life, but often ruins it. We review the itch field and provide a current understanding of the pathophysiology of itch. Itch is a disease, not only a symptom of disease.

Gastric vagal afferent neuropathy following experimental spinal cord injury

Dramatic impairment of gastrointestinal (GI) function accompanies high-thoracic spinal cord injury (T3-SCI). The vagus nerve contains mechano- and chemosensory fibers as well as the motor fibers necessary for the central nervous system (CNS) control of GI reflexes. Cell bodies for the vagal afferent fibers are located within the nodose gangla (NG) and the majority of vagal afferent axons are unmyelinated C fibers that are sensitive to capsaicin through activation of transient receptor potential vanilloid-1 (TRPV1) channels. Vagal afferent fibers also express receptors for GI hormones, including cholecystokinin (CCK). Previously, T3-SCI provokes a transient GI inflammatory response as well as a reduction of both gastric emptying and centrally-mediated vagal responses to GI peptides, including CCK. TRPV1 channels and CCK-A receptors (CCKar) expressed in vagal afferents are upregulated in models of visceral inflammation. The present study investigated whether T3-SCI attenuates peripheral vagal afferent sensitivity through plasticity of TRPV1 and CCK receptors. Vagal afferent response to graded mechanical stimulation of the stomach was significantly attenuated by T3-SCI at 3-day and 3-week recovery. Immunocytochemical labeling for CCKar and TRPV1 demonstrated expression on dissociated gastric-projecting NG neurons. Quantitative assessment of mRNA expression by qRT-PCR revealed significant elevation of CCKar and TRPV1 in the whole NG following T3-SCI in 3-day recovery, but levels returned to normal after 3-weeks. Three days after injury, systemic administration of CCK-8 s showed a significantly diminished gastric vagal afferent response in T3-SCI rats compared to control rats while systemic capsaicin infusion revealed a significant elevation of vagal response in T3-SCI vs control rats. These findings demonstrate that T3-SCI provokes peripheral remodeling and prolonged alterations in the response of vagal afferent fibers to the physiological signals associated with digestion.

Depletion of plasma membrane-associated phosphoinositides mimics inhibition of TRPM7 channels by cytosolic Mg2+, spermine, and pH

Transient receptor potential cation channel subfamily M member 7 (TRPM7) is an ion channel/protein kinase belonging to the TRP melastatin and eEF2 kinase families. Under physiological conditions, most native TRPM7 channels are inhibited by cytoplasmic Mg²⁺, protons, and polyamines. Currents through these channels (I_TRPM7) are robustly potentiated when the cell interior is exchanged with low Mg²⁺-containing buffers. I_TRPM7 is also potentiated by phosphatidyl inositol bisphosphate (PI(4,5)P₂) and suppressed by its hydrolysis. Here we characterized internal Mg²⁺- and pH-mediated inhibition of TRPM7 channels in HEK293 cells overexpressing WT voltage-sensing phospholipid phosphatase (VSP) or its catalytically inactive variant VSP-C363S. VSP-mediated depletion of membrane phosphoinositides significantly increased channel sensitivity to Mg²⁺ and pH. Proton concentrations that were too low to inhibit I_TRPM7 when the VSP-C363S variant was expressed (pH 8.2) became inhibitory in WT VSP-expressing cells. At pH 6.5, protons inhibited I_TRPM7 both in WT and VSP C363S-expressing cells but with a faster time course in the WT VSP-expressing cells. Inhibition by 150 μm Mg²⁺ was also significantly faster in the WT VSP-expressing cells. Cellular PI(4,5)P₂ depletion increased the sensitivity of TRPM7 channels to the inhibitor 2-aminoethyl diphenyl borinate, which acidifies the cytosol. Single substitutions at Ser-1107 of TRPM7, reducing its sensitivity to Mg²⁺, also decreased its inhibition by spermine and acidic pH. Furthermore, these channel variants were markedly less sensitive to VSP-mediated PI(4,5)P₂ depletion than the WT. We conclude that the internal Mg²⁺-, polyamine-, and pH-mediated inhibition of TRPM7 channels is not direct but, rather, reflects electrostatic screening and resultant disruption of PI(4,5)P₂-channel interactions.

TRP Channels as Novel Targets for Endogenous Ligands: Focus on Endocannabinoids and Nociceptive Signalling

Background:

Chronic pain is a significant clinical problem and a very complex pathophysiological phenomenon. There is growing evidence that targeting the endocannabinoid system may be a useful approach to pain alleviation. Classically, the system includes G protein-coupled receptors of the CB1 and CB2 subtypes and their endogenous ligands. More recently, several subtypes of the large superfamily of cation TRP channels have been coined as “ionotropic cannabinoid receptors”, thus highlighting their role in cannabinoid signalling. Thus, the aim of this review was to explore the intimate connection between several “painful” TRP channels, endocannabinoids and nociceptive signalling.

Methods:

Research literature on this topic was critically reviewed allowing us not only summarize the existing evidence in this area of research, but also propose several possible cellular mechanisms linking nociceptive and cannabinoid signaling with TRP channels.

Results:

We begin with an overview of physiology of the endocannabinoid system and its major components, namely CB1 and CB2 G protein-coupled receptors, their two most studied endogenous ligands, anandamide and 2-AG, and several enzymes involved in endocannabinoid biosynthesis and degradation. The role of different endocannabinoids in the regulation of synaptic transmission is then discussed in detail. The connection between the endocannabinoid system and several TRP channels, especially TRPV1-4, TRPA1 and TRPM8, is then explored, while highlighting the role of these same channels in pain signalling.

Conclusion:

There is increasing evidence implicating several TRP subtypes not only as an integral part of the endocannabinoid system, but also as promising molecular targets for pain alleviation with the use of endo- and phytocannabinoids, especially when the function of these channels is upregulated under inflammatory conditions.

Hippocampus and Hypothalamus RNA-sequencing of WFS1-deficient Mice

Wolfram syndrome is caused by mutations in the WFS1 gene. WFS1 protein dysfunction results in a range of neuroendocrine syndromes and is mostly characterized by juvenile-onset diabetes mellitus and optic atrophy. WFS1 has been shown to participate in membrane trafficking, protein processing and Ca²⁺ homeostasis in the endoplasmic reticulum. Aim of the present study was to find the transcriptomic changes influenced by WFS1 in the hypothalamus and hippocampus using RNA-sequencing. The WFS1-deficient mice were used as a model system to analyze the changes in transcriptional networks. The number of differentially expressed genes between hypothalami of WFS1-deficient (Wfs1KO) and wild-type (WT) mice was 43 and between hippocampi 311 with False Discovery Rate (FDR) <0.05. Avpr1a and Avpr1b were significantly upregulated in the hypothalamus and hippocampus of Wfs1KO mice respectively. Trpm8 was the most upregulated gene in the hippocampus of Wfs1KO mice. The functional analysis revealed significant enrichment of networks and pathways associated with protein synthesis, cell-to-cell signaling and interaction, molecular transport, metabolic disease and nervous system development and function. In conclusion, the transcriptomic profiles of WFS1-deficient hypothalamus and hippocampus do indicate the activation of degenerative molecular pathways causing the clinical occurrences typical to Wolfram syndrome.

Direct Anandamide Activation of TRPV1 Produces Divergent Calcium and Current Responses

In the brainstem nucleus of the solitary tract (NTS), primary vagal afferent neurons express the transient receptor potential vanilloid subfamily member 1 (TRPV1) at their central terminals where it contributes to quantal forms of glutamate release. The endogenous membrane lipid anandamide (AEA) is a putative TRPV1 agonist in the brain, yet the extent to which AEA activation of TRPV1 has a neurophysiological consequence is not well established. We investigated the ability of AEA to activate TRPV1 in vagal afferent neurons in comparison to capsaicin (CAP). Using ratiometric calcium imaging and whole-cell patch clamp recordings we confirmed that AEA excitatory activity requires TRPV1, binds competitively at the CAP binding site, and has low relative affinity. While AEA-induced increases in peak cytosolic calcium were similar to CAP, AEA-induced membrane currents were significantly smaller. Removal of bath calcium increased the AEA current with no change in peak CAP currents revealing a calcium sensitive difference in specific ligand activation of TRPV1. Both CAP- and AEA-activated TRPV1 currents maintained identical reversal potentials, arguing against a major difference in ion selectivity to resolve the AEA differences in signaling. In contrast with CAP, AEA did not alter spontaneous glutamate release at NTS synapses. We conclude: (1) AEA activation of TRPV1 is markedly different from CAP and produces different magnitudes of calcium influx from whole-cell current; and (2) exogenous AEA does not alter spontaneous glutamate release onto NTS neurons. As such, AEA may convey modulatory changes to calcium-dependent processes, but does not directly facilitate glutamate release.

Analysis of metabolic effects of menthol on WFS1-deficient mice

In this study, we investigated the physiological regulation of energy metabolism in wild‐type (WT) and WFS1‐deficient (Wfs1KO) mice by measuring the effects of menthol treatment on the O₂ consumption, CO₂ production, rectal body temperature, and heat production. The basal metabolism and behavior was different between these genotypes as well as TRP family gene expressions. Wfs1KO mice had a shorter life span and weighed less than WT mice. The food and water intake of Wfs1KO mice was lower as well as the body temperature when compared to their WT littermates. Furthermore, Wfs1KO mice had higher basal O₂ consumption, and CO₂ and heat production than WT mice. In addition, Wfs1KO mice showed a higher response to menthol administration in comparison to WT mice. The strongest menthol effect was seen on different physiological measures 12 h after oral administration. The highest metabolic response of Wfs1KO mice was seen at the menthol dose of 10 mg/kg. Menthol increased O₂ consumption, and CO₂ and heat production in Wfs1KO mice when compared to their WT littermates. In addition, the expression of Trpm8 gene was increased. In conclusion, our results show that the Wfs1KO mice develop a metabolic phenotype characterized with several physiological dysfunctions.

Reprint of "Mechanisms of lipid regulation and lipid gating in TRPC channels"

TRPC proteins form cation channels that integrate and relay cellular signals by mechanisms involving lipid recognition and lipid-dependent gating. The lipohilic/amphiphilic molecules that function as cellular activators or modulators of TRPC proteins span a wide range of chemical structures. In this context, cellular redox balance is likely linked to the lipid recognition/gating features of TRPC channels. Both classical ligand-protein interactions as well as indirect and promiscuous sensory mechanisms have been proposed. Some of the recognition processes are suggested to involve ancillary lipid-binding scaffolds or regulators as well as dynamic protein-protein interactions determined by bilayer architecture. A complex interplay of protein-protein and protein-lipid interactions is likely to govern the gating and/or plasma membrane recruitment of TRPC channels, thereby providing a distinguished platform for signal integration and coincident signal detection. Both the primary molecular event(s) of lipid recognition by TRPC channels as well as the transformation of these events into distinct gating movements is poorly understood at the molecular level, and it remains elusive whether lipid sensing in TRPCs is conferred to a distinct sensor domain. Recent structural information on the molecular action of lipophilic activators in distantly related members of the TRP superfamily encourages speculations on TRPC gating mechanisms involved in lipid recognition/gating. This review aims to provide an update on the current understanding of the lipid-dependent control of TRPC channels with focus on the TRPC lipid sensing, signal-integration hub and a short discussion of potential links to redox signaling.

Mechanisms of lipid regulation and lipid gating in TRPC channels

TRPC proteins form cation channels that integrate and relay cellular signals by mechanisms involving lipid recognition and lipid-dependent gating. The lipohilic/amphiphilic molecules that function as cellular activators or modulators of TRPC proteins span a wide range of chemical structures. In this context, cellular redox balance is likely linked to the lipid recognition/gating features of TRPC channels. Both classical ligand-protein interactions as well as indirect and promiscuous sensory mechanisms have been proposed. Some of the recognition processes are suggested to involve ancillary lipid-binding scaffolds or regulators as well as dynamic protein-protein interactions determined by bilayer architecture. A complex interplay of protein-protein and protein-lipid interactions is likely to govern the gating and/or plasma membrane recruitment of TRPC channels, thereby providing a distinguished platform for signal integration and coincident signal detection. Both the primary molecular event(s) of lipid recognition by TRPC channels as well as the transformation of these events into distinct gating movements is poorly understood at the molecular level, and it remains elusive whether lipid sensing in TRPCs is conferred to a distinct sensor domain. Recent structural information on the molecular action of lipophilic activators in distantly related members of the TRP superfamily encourages speculations on TRPC gating mechanisms involved in lipid recognition/gating. This review aims to provide an update on the current understanding of the lipid-dependent control of TRPC channels with focus on the TRPC lipid sensing, signal-integration hub and a short discussion of potential links to redox signaling.

Regulation of Mg2+ Reabsorption and Transient Receptor Potential Melastatin Type 6 Activity by cAMP Signaling

The transient receptor potential melastatin type 6 (TRPM6) epithelial Mg(2+) channels participate in transcellular Mg(2+) transport in the kidney and intestine. Previous reports suggested a hormonal cAMP-dependent regulation of Mg(2+) reabsorption in the kidney. The molecular details of this process are, however, unknown. Adenylate cyclase 3 (Adcy3) has been shown to colocalize with the Na(+)/Cl(-) cotransporter, a marker of the distal convoluted segment of the kidney, the principal site of TRPM6 expression. Given the critical role of TRPM6 in Mg(2+) reabsorption, an inducible kidney-specific Adcy3 deletion mouse model was characterized for blood and urinary electrolyte disturbances under a normal--and low--Mg(2+) diet. Increased urinary Mg(2+) wasting and Trpm6 mRNA levels were observed in the urine and kidney of Adcy3-deleted animals compared with wild-type controls. Serum Mg(2+) concentration was significantly lower in Adcy3-deleted animals at day 7 on the low Mg(2+) diet. Using patch clamp electrophysiology, cell surface biotinylation, and total internal reflection fluorescence live cell imaging of transfected HEK293 cells, we demonstrated that cAMP signaling rapidly potentiates TRPM6 activity by promoting TRPM6 accumulation at the plasma membrane and increasing its single-channel conductance. Comparison of electrophysiological data from cells expressing the phosphorylation-deficient S1252A or phosphomimetic S1252D TRPM6 mutants suggests that phosphorylation at this intracellular residue participates in the observed stimulation of channel activity. Altogether, these data support a physiologically relevant magnesiotropic role of cAMP signaling in the kidney by a direct stimulatory action of protein kinase A on the plasma membrane trafficking and function of TRPM6 ion channels.

Transient receptor potential ion channels in primary sensory neurons as targets for novel analgesics

The last decade has witnessed an explosion in novel findings relating to the molecules involved in mediating the sensation of pain in humans. Transient receptor potential (TRP) ion channels emerged as the greatest group of molecules involved in the transduction of various physical stimuli into neuronal signals in primary sensory neurons, as well as, in the development of pain. Here, we review the role of TRP ion channels in primary sensory neurons in the development of pain associated with peripheral pathologies and possible strategies to translate preclinical data into the development of effective new analgesics. Based on available evidence, we argue that nociception-related TRP channels on primary sensory neurons provide highly valuable targets for the development of novel analgesics and that, in order to reduce possible undesirable side effects, novel analgesics should prevent the translocation from the cytoplasm to the cell membrane and the sensitization of the channels rather than blocking the channel pore or binding sites for exogenous or endogenous activators.

Transient receptor potential channels as drug targets: from the science of basic research to the art of medicine

The large Trp gene family encodes transient receptor potential (TRP) proteins that form novel cation-selective ion channels. In mammals, 28 Trp channel genes have been identified. TRP proteins exhibit diverse permeation and gating properties and are involved in a plethora of physiologic functions with a strong impact on cellular sensing and signaling pathways. Indeed, mutations in human genes encoding TRP channels, the so-called "TRP channelopathies," are responsible for a number of hereditary diseases that affect the musculoskeletal, cardiovascular, genitourinary, and nervous systems. This review gives an overview of the functional properties of mammalian TRP channels, describes their roles in acquired and hereditary diseases, and discusses their potential as drug targets for therapeutic intervention.

Targeting Pain-evoking Transient Receptor Potential Channels for the Treatment of Pain

Chronic pain affects billions of lives globally and is a major public health problem in the United States. However, pain management is still a challenging task due to a lack of understanding of the fundamental mechanisms of pain. In the past decades transient receptor potential (TRP) channels have been identified as molecular sensors of tissue damage and inflammation. Activation/sensitization of TRP channels in peripheral nociceptors produces neurogenic inflammation and contributes to both somatic and visceral pain. Pharmacological and genetic studies have affirmed the role of TRP channels in multiple forms of inflammatory and neuropathic pain. Thus pain-evoking TRP channels emerge as promising therapeutic targets for a wide variety of pain and inflammatory conditions.

TRPC5

Human canonical transient receptor potential channel 5 (TRPC5) has been cloned from the Xq23 region on chromosome X as a suspect in nonsyndromic mental retardation. TRPC5 is a Ca(2+)-permeable cation channel predominantly expressed in the CNS, including the hippocampus, cerebellum, amygdala, sensory neurons, and retina. It also shows more restricted expression in the periphery, notably in the kidney and cardiovascular system. Homotetrameric TRPC5 channels are primarily activated by receptors coupled to Gq and phospholipase C and/or Gi proteins, but TRPC5 channels may also gate in a store-dependent manner, which requires other partner proteins such TRPC1, STIM1, and Orai1. There is an impressive array of other activators of TRPC5 channels, such as nitric oxide, lysophospholipids, sphingosine-1-phosphate, reduced thioredoxin, protons, lanthanides, and calcium, and many can cause its direct activation. Moreover, TRPC5 shows constitutive activity, and it is responsive to membrane stretch and cold. Thus, TRPC5 channels have significant potential for synergistic activation and may serve as an important focal point in Ca(2+) signalling and electrogenesis. Moreover, TRPC5 functions in partnership with about 60 proteins, including TRPC1, TRPC4, calmodulin, IP3 receptors, NHERF, NCS-1, junctate, stathmin 2, Ca(2+)-binding protein 1, caveolin, and SESTD1, while its desensitisation is mediated by both protein kinases A and C. TRPC5 has a distinct voltage dependence shared only with its closest relative, TRPC4. Its unique N-shaped activation curve underlined by intracellular Mg(2+) block seems to be perfectly "shaped" to trigger action potential discharge, but not to grossly interfere with the action potential shape. The range of biological functions of TRPC5 channels is also impressive, from neurotransmission to control of axon guidance and vascular smooth muscle cell migration and contractility. Recent studies of Trpc5 gene knockouts begin to uncover its roles in fear, anxiety, seizures, and cold sensing.

Retinoids activate the irritant receptor TRPV1 and produce sensory hypersensitivity

Retinoids are structurally related derivatives of vitamin A and are required for normal vision as well as cell proliferation and differentiation. Clinically, retinoids are effective in treating many skin disorders and cancers. Application of retinoids evokes substantial irritating side effects, including pain and inflammation; however, the precise mechanisms accounting for the sensory hypersensitivity are not understood. Here we show that both naturally occurring and synthetic retinoids activate recombinant or native transient receptor potential channel vanilloid subtype 1 (TRPV1), an irritant receptor for capsaicin, the pungent ingredient of chili peppers. In vivo, retinoids produced pain-related behaviors that were either eliminated or significantly reduced by genetic or pharmacological inhibition of TRPV1 function. These findings identify TRPV1 as an ionotropic receptor for retinoids and provide cellular and molecular insights into retinoid-evoked hypersensitivity. These findings also suggest that selective TRPV1 antagonists are potential therapeutic drugs for treating retinoid-induced sensory hypersensitivity.

Polyester modification of the mammalian TRPM8 channel protein: implications for structure and function

The TRPM8 ion channel is expressed in sensory neurons and is responsible for sensing environmental cues, such as cold temperatures and chemical compounds, including menthol and icilin. The channel functional activity is regulated by various physical and chemical factors and is likely to be preconditioned by its molecular composition. Our studies indicate that the TRPM8 channel forms a structural-functional complex with the polyester poly-(R)-3-hydroxybutyrate (PHB). We identified by mass spectrometry a number of PHB-modified peptides in the N terminus of the TRPM8 protein and in its extracellular S3-S4 linker. Removal of PHB by enzymatic hydrolysis and site-directed mutagenesis of both the serine residues that serve as covalent anchors for PHB and adjacent hydrophobic residues that interact with the methyl groups of the polymer resulted in significant inhibition of TRPM8 channel activity. We conclude that the TRPM8 channel undergoes posttranslational modification by PHB and that this modification is required for its normal function.

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.

Functional TRPV1 expression in human corneal fibroblasts

Corneal wound healing in mice subsequent to an alkali burn results in dysregulated inflammation and opacification. Transient receptor potential vanilloid subtype 1 (TRPV1) channel activation in all tissue layers by endogenous ligands contributes to this sight compromising outcome since in TRPV1 knockout mice wound healing results instead in tissue transparency restoration. However, it is not known if primary human stromal fibroblasts exhibit such expression even though functional TRPV1 expression is evident in an immortalized human corneal epithelial cell line. In primary human corneal fibroblasts (HCF), TRPV1 gene expression and localization were identified based on the results of quantitative RT-PCR and immunocytochemistry, respectively. Western blot analysis identified a 100 kD protein corresponding to TRPV1 protein expression in a positive control. Single-cell fluorescence imaging detected in fura2-AM loaded cells Ca(2+) transients that rose 1.8-fold above the baseline induced by a selective TRPV1 agonist, capsaicin (CAP), which were blocked by a TRPV1 antagonist, capsazepine (CPZ) or exposure to a Ca(2+) free medium. The whole-cell mode of the planar patch-clamp technique identified TRPV1-induced currents that rose 1.76-fold between -60 and +130 mV. CAP-induced time dependent changes in the phosphorylation status of mitogen activated protein kinase (MAPK) signaling mediators that led to a 2.5-fold increase in IL-6 release after 24 h. This rise did not occur either in TRPV1 siRNA gene silenced cells or during exposure to SB203580 (10 μM), a selective p38 MAPK inhibitor. Taken together, identification of functional TRPV1 expression in HCF suggests that in vivo its activation by injury contributes to corneal opacification and inflammation during wound healing. These undesirable effects may result in part from increases in IL-6 expression mediated by p-p38 MAPK signaling.

Natural and synthetic modulators of SK (K(ca)2) potassium channels inhibit magnesium-dependent activity of the kinase-coupled cation channel TRPM7

BACKGROUND AND PURPOSE

Transient receptor potential cation channel subfamily M member 7 (TRPM7) is a bifunctional protein comprising a TRP ion channel segment linked to an α-type protein kinase domain. TRPM7 is essential for proliferation and cell growth. Up-regulation of TRPM7 function is involved in anoxic neuronal death, cardiac fibrosis and tumour cell proliferation. The goal of this work was to identify non-toxic inhibitors of the TRPM7 channel and to assess the effect of blocking endogenous TRPM7 currents on the phenotype of living cells.

EXPERIMENTAL APPROACH

We developed an aequorin bioluminescence-based assay of TRPM7 channel activity and performed a hypothesis-driven screen for inhibitors of the channel. The candidates identified were further assessed electrophysiologically and in cell biological experiments.

KEY RESULTS

TRPM7 currents were inhibited by modulators of small conductance Ca²⁺ -activated K⁺ channels (K(Ca)2.1-2.3; SK) channels, including the antimalarial plant alkaloid quinine, CyPPA, dequalinium, NS8593, SKA31 and UCL 1684. The most potent compound NS8593 (IC₅₀ 1.6 µM) specifically targeted TRPM7 as compared with other TRP channels, interfered with Mg²⁺ -dependent regulation of TRPM7 channel and inhibited the motility of cultured cells. NS8593 exhibited full and reversible block of native TRPM7-like currents in HEK 293 cells, freshly isolated smooth muscle cells, primary podocytes and ventricular myocytes.

CONCLUSIONS AND IMPLICATIONS

This study reveals a tight overlap in the pharmacological profiles of TRPM7 and K(Ca)2.1-2.3 channels. NS8593 acts as a negative gating modulator of TRPM7 and is well-suited to study functional features and cellular roles of endogenous TRPM7.

Ion channels and transporters in lymphocyte function and immunity

Lymphocyte function is regulated by a network of ion channels and transporters in the plasma membrane of B and T cells. These proteins modulate the cytoplasmic concentrations of diverse cations, such as calcium, magnesium and zinc ions, which function as second messengers to regulate crucial lymphocyte effector functions, including cytokine production, differentiation and cytotoxicity. The repertoire of ion-conducting proteins includes calcium release-activated calcium (CRAC) channels, P2X receptors, transient receptor potential (TRP) channels, potassium channels, chloride channels and magnesium and zinc transporters. This Review discusses the roles of ion conduction pathways in lymphocyte function and immunity.

Ionic channels as targets for drug design: a review on computational methods

Ion channels are involved in a broad range of physiological and pathological processes. The implications of ion channels in a variety of diseases, including diabetes, epilepsy, hypertension, cancer and even chronic pain, have signaled them as pivotal drug targets. Thus far, drugs targeting ion channels were developed without detailed knowledge of the molecular interactions between the lead compounds and the target channels. In recent years, however, the emergence of high-resolution structures for a plethora of ion channels paves the way for computer-assisted drug design. Currently, available functional and structural data provide an attractive platform to generate models that combine substrate-based and protein-based approaches. In silico approaches include homology modeling, quantitative structure-activity relationships, virtual ligand screening, similarity and pharmacophore searching, data mining, and data analysis tools. These strategies have been frequently used in the discovery and optimization of novel molecules with enhanced affinity and specificity for the selected therapeutic targets. In this review we summarize recent applications of in silico methods that are being used for the development of ion channel drugs.

Shuffling the cards in signal transduction: Calcium, arachidonic acid and mechanosensitivity

Cell signaling is a very complex network of biochemical reactions triggered by a huge number of stimuli coming from the external medium. The function of any single signaling component depends not only on its own structure but also on its connections with other biomolecules. During prokaryotic-eukaryotic transition, the rearrangement of cell organization in terms of diffusional compartmentalization exerts a deep change in cell signaling functional potentiality. In this review I briefly introduce an intriguing ancient relationship between pathways involved in cell responses to chemical agonists (growth factors, nutrients, hormones) as well as to mechanical forces (stretch, osmotic changes). Some biomolecules (ion channels and enzymes) act as “hubs”, thanks to their ability to be directly or indirectly chemically/mechanically co-regulated. In particular calcium signaling machinery and arachidonic acid metabolism are very ancient networks, already present before eukaryotic appearance. A number of molecular “hubs”, including phospholipase A2 and some calcium channels, appear tightly interconnected in a cross regulation leading to the cellular response to chemical and mechanical stimulations.

Synergistic role of TRPV1 and TRPA1 in pancreatic pain and inflammation

BACKGROUND & AIMS

The transient receptor potential (TRP) channels TRPV1 and TRPA1 have each been associated with regulation of efferent properties of primary afferent neurons that initiate neurogenic inflammation and are required for the development of inflammatory hyperalgesia. To evaluate the role of these channels in producing pain during pancreatic inflammation, we studied pancreatic nodose ganglion (NG) and dorsal root ganglion (DRG) sensory neurons (identified by content of retrograde tracer) and behavioral outcomes in a mouse model of acute pancreatitis.

METHODS

Pancreatic inflammation was induced by 8 hourly injections of cerulein (50 μg/kg). The extent of inflammation, pancreatic neuron TRP channel expression and function and excitability, and pain-related behaviors were evaluated over the course of the following week.

RESULTS

Histology and myeloperoxidase activity confirmed pancreatic inflammation that was associated with increased excitability and messenger RNA expression of the TRP channels in NG and DRG pancreatic neurons. Calcium imaging of pancreatic NG and DRG neurons from mice given cerulein revealed increased responses to TRP agonists. TRPV1 and TRPA1 antagonists attenuated cerulein-induced pain behaviors and pancreatic inflammation; they had a synergistic effect.

CONCLUSIONS

Pancreatic inflammation significantly increased the expression and functional properties of TRPV1 and TRPA1, as well as the excitability of pancreatic sensory neurons in vagal and spinal pathways. TRP channel antagonists acted synergistically to reverse pancreatic inflammation and associated pain behaviors; reagents that target interactions between these channels might be developed to reduce pain in patients with acute pancreatitis.

Voltage- and temperature-dependent activation of TRPV3 channels is potentiated by receptor-mediated PI(4,5)P2 hydrolysis

TRPV3 is a thermosensitive channel that is robustly expressed in skin keratinocytes and activated by innocuous thermal heating, membrane depolarization, and chemical agonists such as 2-aminoethyoxy diphenylborinate, carvacrol, and camphor. TRPV3 modulates sensory thermotransduction, hair growth, and susceptibility to dermatitis in rodents, but the molecular mechanisms responsible for controlling TRPV3 channel activity in keratinocytes remain elusive. We show here that receptor-mediated breakdown of the membrane lipid phosphatidylinositol (4,5) bisphosphate (PI(4,5)P₂) regulates the activity of both native TRPV3 channels in primary human skin keratinocytes and expressed TRPV3 in a HEK-293–derived cell line stably expressing muscarinic M₁-type acetylcholine receptors. Stimulation of PI(4,5)P₂ hydrolysis or pharmacological inhibition of PI 4 kinase to block PI(4,5)P₂ synthesis potentiates TRPV3 currents by causing a negative shift in the voltage dependence of channel opening, increasing the proportion of voltage-independent current and causing thermal activation to occur at cooler temperatures. The activity of single TRPV3 channels in excised patches is potentiated by PI(4,5)P₂ depletion and selectively decreased by PI(4,5)P₂ compared with related phosphatidylinositol phosphates. Neutralizing mutations of basic residues in the TRP domain abrogate the effect of PI(4,5)P₂ on channel function, suggesting that PI(4,5)P₂ directly interacts with a specific protein motif to reduce TRPV3 channel open probability. PI(4,5)P₂-dependent modulation of TRPV3 activity represents an attractive mechanism for acute regulation of keratinocyte signaling cascades that control cell proliferation and the release of autocrine and paracrine factors.

TRPM channels in the vasculature

Recent studies show that mammalian melastatin TRPM nonselective cation channels (TRPM1-8), members of the largest and most diverse TRP subfamily, are widely expressed in the endothelium and vascular smooth muscles. When activated, these channels similarly to other TRPs permit the entry of sodium, calcium and magnesium, thus causing membrane depolarisation. Although membrane depolarisation reduces the driving force for calcium entry via TRPMs as well as other pathways for calcium entry, in smooth muscle myocytes expressing voltage-gated Ca(2+) channels the predominant functional effect is an increase in intracellular Ca(2+) concentration and myocyte contraction. This review focuses on several best documented aspects of vascular functions of TRPMs, including the role of TRPM2 in oxidant stress, regulation of endothelial permeability and cell death, the connection between TRPM4 and myogenic response, significance of TRPM7 for magnesium homeostasis, vessel injury and hypertension, and emerging evidence that the cold and menthol receptor TRPM8 is involved in the regulation of vascular tone.

TRP channels and their implications in metabolic diseases

The transient receptor potential (TRP) channel superfamily is composed of 28 nonselective cation channels that are ubiquitously expressed in many cell types and have considerable functional diversity. Although changes in TRP channel expression and function have been reported in cardiovascular disease and renal disorders, the pathogenic roles of TRP channels in metabolic diseases have not been systemically reviewed. In this review, we summarised the distribution of TRP channels in several metabolic tissues and discussed their roles in mediating and regulating various physiological and pathophysiological metabolic processes and diseases including diabetes, obesity, dyslipidaemia, metabolic syndrome, atherosclerosis, metabolic bone diseases and electrolyte disturbances. This review provides new insight into the involvement of TRP channels in the pathogenesis of metabolic disorders and implicates these channels as potential therapeutic targets for the management of metabolic diseases.

Pharmacological inhibition of intracellular calcium release blocks acid-induced bone resorption

In vivo chronic metabolic acidosis induces net Ca2+ efflux from bone, and incubation of neonatal mouse calvariae in medium simulating physiological metabolic acidosis induces bone resorption. It appears that activation of the proton (H+) receptor OGR1 in the osteoblast leads to an increase in intracellular Ca2+, which is associated with an increase in cyclooxygenase 2 (COX2) and PGE2-induced receptor activator of NF-κB ligand (RANKL) and H+-induced osteoclastic bone resorption. To support this hypothesis, we tested whether intracellular Ca2+ signaling was integral to H+-induced bone resorption by determining whether 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) and 2-aminoethoxydiphenyl borate (2-APB), inhibitors of inositol trisphosphate-mediated Ca2+ signaling, would block H+-induced bone resorption in cultured neonatal calvariae and, if so, would do so by inhibiting H+-induced stimulation of COX2 and RANKL in osteoblastic cells. We found that H+-induced bone resorption is significantly inhibited by TMB-8 and 2-APB. Both compounds also inhibit H+-induced stimulation of COX2 protein in calvariae and COX2 mRNA and protein levels in primary osteoblasts. H+-induced stimulation of RANKL in calvarial cultures, as well as primary cells, is also completely inhibited by TMB-8 and 2-APB. These results support the hypothesis that H+ stimulation of net Ca2+ efflux from bone, mediated by COX2- and subsequent PGE2-induced RANKL production, is initiated in the osteoblast via activation of Ca2+ signaling.

Mechanisms of the sensory effects of tacrolimus on the skin

BACKGROUND

Tacrolimus is an immunosuppressant drug currently used for the treatment of atopic dermatitis and pruritus. This topical therapy is effective and safe, but transient burning, stinging and itch are frequently reported.

OBJECTIVES

To understand the mechanisms underlying these burning sensations.

METHODS

We examined the impact of tacrolimus on substance P (SP) release in an in vitro model of cutaneous neurogenic inflammation. Because phosphorylation of TRPV1 (transient receptor potential subtype vanilloid 1) plays a role in the induction of pain, we investigated whether tacrolimus regulates the phosphorylation state of TRPV1. Finally, we used a macropatch to evaluate the impact of tacrolimus on voltage-gated calcium currents of sensory neurons.

RESULTS

Tacrolimus was able to induce initial SP release by extracellular calcium influx and inhibited SP release induced by capsaicin after 1, 24 and 72 h of pretreatment. Analysis of TRPV1 phosphorylation by Western blot confirmed the capacity of tacrolimus to favour phosphorylation. An electrophysiological study showed inhibitory effects on calcium currents.

CONCLUSIONS

The efficacy of tacrolimus in pruritus, as well as the sensory side-effects, could be explained by a direct effect on neurons through an effect on calcineurin, possibly by a desensitization of TRPV1 and calcium currents through the PIP(2) regulation pathway.

Inhibitory odorant signaling in Mammalian olfactory receptor neurons

Odorants inhibit as well as excite olfactory receptor neurons (ORNs) in many species of animals. Cyclic nucleotide-dependent activation of canonical mammalian ORNs is well established but it is still unclear how odorants inhibit these cells. Here we further implicate phosphoinositide-3-kinase (PI3K), an indispensable element of PI signaling in many cellular processes, in olfactory transduction in rodent ORNs. We show that odorants rapidly and transiently activate PI3K in the olfactory cilia and in the olfactory epithelium in vitro. We implicate known G-protein-coupled isoforms of PI3K and show that they modulate not only the magnitude but also the onset kinetics of the electrophysiological response of ORNs to complex odorants. Finally, we show that the ability of a single odorant to inhibit another can be PI3K dependent. Our collective results provide compelling support for the idea that PI3K-dependent signaling mediates inhibitory odorant input to mammalian ORNs and at least in part contributes to the mixture suppression typically seen in the response of ORNs to complex natural odorants.

Regulation of the putative TRPV1t salt taste receptor by phosphatidylinositol 4,5-bisphosphate

Regulation of the putative amiloride and benzamil (Bz)-insensitive TRPV1t salt taste receptor by phosphatidylinositol 4,5-bisphosphate (PIP(2)) was studied by monitoring chorda tympani (CT) taste nerve responses to 0.1 M NaCl solutions containing Bz (5 x 10(-6) M; a specific ENaC blocker) and resiniferatoxin (RTX; 0-10 x 10(-6) M; a specific TRPV1 agonist) in Sprague-Dawley rats and in wildtype (WT) and TRPV1 knockout (KO) mice. In rats and WT mice, RTX elicited a biphasic effect on the NaCl + Bz CT response, increasing the CT response between 0.25 x 10(-6) and 1 x 10(-6) M. At concentrations >1 x 10(-6) M, RTX inhibited the CT response. An increase in PIP(2) by topical lingual application of U73122 (a phospholipase C blocker) or diC8-PIP(2) (a short chain synthetic PIP(2)) inhibited the control NaCl + Bz CT response and decreased its sensitivity to RTX. A decrease in PIP(2) by topical lingual application of phenylarsine oxide (a phosphoinositide 4 kinase blocker) enhanced the control NaCl + Bz CT response, increased its sensitivity to RTX stimulation, and inhibited the desensitization of the CT response at RTX concentrations >1 x 10(-6) M. The ENaC-dependent NaCl CT responses were not altered by changes in PIP(2). An increase in PIP(2) enhanced CT responses to sweet (0.3 M sucrose) and bitter (0.01 M quinine) stimuli. RTX produced the same increase in the Bz-insensitive Na(+) response when present in salt solutions containing 0.1 M NaCl + Bz, 0.1 M monosodium glutamate + Bz, 0.1 M NaCl + Bz + 0.005 M SC45647, or 0.1 M NaCl + Bz + 0.01 M quinine. No effect of RTX was observed on CT responses in WT mice and rats in the presence of the TRPV1 blocker N-(3-methoxyphenyl)-4-chlorocinnamide (1 x 10(-6) M) or in TRPV1 KO mice. We conclude that PIP(2) is a common intracellular effector for sweet, bitter, umami, and TRPV1t-dependent salt taste, although in the last case, PIP(2) seems to directly regulate the taste receptor protein itself, i.e., the TRPV1 ion channel or its taste receptor variant, TRPV1t.

Regulation of the novel Mg2+ transporter transient receptor potential melastatin 7 (TRPM7) cation channel by bradykinin in vascular smooth muscle cells

BACKGROUND

Transient receptor potential melastatin 7 (TRPM7) channels have been identified in the vasculature. However, their regulation and function remain unclear.

METHODS

Here, we tested the hypothesis that bradykinin and its second messenger cAMP upregulate TRPM7, which stimulates activation of annexin-1 (TRPM7 substrate) and increases transmembrane Mg2+ transport and vascular smooth muscle cell (VSMC) migration. Human and rat VSMCs were studied. TRPM7 phosphorylation was assessed by immunoprecipitation:immunoblotting using antiphospho-serine/threonine and anti-TRPM7 antibodies. [Mg2+]i was measured by mag-fura-2. TRPM7 was downregulated by small interfering RNA and 2-aminoethoxydiphenyl borate. Annexin-1 activity was assessed by cytosol-to-membrane translocation. Cell migration and invasion, functional responses to bradykinin, were assessed in transwell chambers.

RESULTS

Bradykinin increased expression of TRPM7 and annexin-1. TRPM7 was rapidly (5 min) phosphorylated on serine/threonine but not on tyrosine residues by bradykinin. [Mg2+]i was increased in bradykinin-stimulated cells (0.53 versus 0.68 mmol/l, basal versus bradykinin, P < 0.01). Annexin-1 activation was increased by bradykinin and inhibited by 2-aminoethoxydiphenyl borate. Although Hoe 140 (B2 receptor antagonist), U-73122 (phospholipase C inhibitor), 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (c-Src inhibitor) and chelerythrine (protein kinase C inhibitor) blocked bradykinin actions, dibutyryl-c-AMP was without effect. In small interfering RNA-transfected and in 2-aminoethoxydiphenyl borate-treated cells, bradykinin-induced Mg2+ influx and VSMC migration were reduced.

CONCLUSION

Our results demonstrate that bradykinin regulates TRPM7 and its downstream target annexin-1 through phospholipase C-dependent, protein kinase C-dependent and c-Src-dependent and cAMP-independent pathways; effects are mediated through bradykinin type 2 receptor; and bradykinin regulates VSMC [Mg2+]i and migration through TRPM7. These data identify TRPM7/annexin-1/Mg2+ as a novel pathway in bradykinin signaling.

TRPC5 channels undergo changes in gating properties during the activation-deactivation cycle

TRPC5 are non-specific cation channels activated through phospholipase C-dependent pathways, although the precise gating mechanism is unknown. TRPC5 current-voltage relationships (I-Vs) change systematically during the activation-deactivation cycle, shifting between outwardly rectifying and doubly rectifying shapes. Since several TRP family members exhibit voltage-dependent properties, we investigated whether the various I-V relationships were due to changes in gating. Using patch-clamp recordings of rat TRPC5 transfected HEK293 cells, we found that TRPC5 currents had distinct biophysical characteristics correlated with individual I-V shapes, a phenomenon we call 'phases.' At rest, channels were closed at most potentials, although strong depolarizations (>+80 mV) stimulated small outward currents (Phase 0). For 10-15 sec after activation, voltage steps evoked small inward and large outward currents with time- and voltage-dependent kinetics (Phase 1, outwardly-rectifying I-Vs). At maximal inward amplitude, currents were voltage-independent at all potentials (Phase 2, doubly-rectifying I-Vs owing to Mg2+ block). During desensitization (Phase 3), currents reverted to a Phase 1-like voltage-dependence. La3+ ions potentiated inward TRPC5 currents by promoting a reversible transition from Phase 3 to Phase 2. Single channel recordings revealed asymmetric conductance properties with values of approximately 40 pS at negative potentials and approximately 130 pS at >+60 mV. Mutation of D633, a cytoplasmic residue that mediates Mg2+ block, decreased channel activity at negative potentials during Phase 2. We conclude that TRPC5 gating properties can switch reversibly between voltage-dependent and voltage-independent states. The modulation of phase transitions by external agents such as La3+ and EBP50, a scaffolding protein, may constitute a novel mechanism for regulation of channel activity.

Isoform-specific inhibition of TRPC4 channel by phosphatidylinositol 4,5-bisphosphate

Full-length transient receptor potential (TRP) cation channel TRPC4alpha and shorter TRPC4beta lacking 84 amino acids in the cytosolic C terminus are expressed in smooth muscle and endothelial cells where they regulate membrane potential and Ca(2+) influx. In common with other "classical" TRPCs, TRPC4 is activated by G(q)/phospholipase C-coupled receptors, but the underlying mechanism remains elusive. Little is also known about any isoform-specific channel regulation. Here we show that TRPC4alpha but not TRPC4beta was strongly inhibited by intracellularly applied phosphatidylinositol 4,5-bisphosphate (PIP(2)). In contrast, several other phosphoinositides (PI), including PI(3,4)P(2), PI(3,5)P(2), and PI(3,4,5)P(3), had no effect or even potentiated TRPC4alpha indicating that PIP(2) inhibits TRPC4alpha in a highly selective manner. We show that PIP(2) binds to the C terminus of TRPC4alpha but not that of TRPC4beta in vitro. Its inhibitory action was dependent on the association of TRPC4alpha with actin cytoskeleton as it was prevented by cytochalasin D treatment or by the deletion of the C-terminal PDZ-binding motif (Thr-Thr-Arg-Leu) that links TRPC4 to F-actin through the sodium-hydrogen exchanger regulatory factor and ezrin. PIP(2) breakdown appears to be a required step in TRPC4alpha channel activation as PIP(2) depletion alone was insufficient for channel opening, which additionally required Ca(2+) and pertussis toxin-sensitive G(i/o) proteins. Thus, TRPC4 channels integrate a variety of G-protein-dependent stimuli, including a PIP(2)/cytoskeleton dependence reminiscent of the TRPC4-like muscarinic agonist-activated cation channels in ileal myocytes.

Transient receptor potential melastatin 6 and 7 channels, magnesium transport, and vascular biology: implications in hypertension

Magnesium, an essential intracellular cation, is critically involved in many biochemical reactions involved in the regulation of vascular tone and integrity. Decreased magnesium concentration has been implicated in altered vascular reactivity, endothelial dysfunction, vascular inflammation, and structural remodeling, processes important in vascular changes and target organ damage associated with hypertension. Until recently, very little was known about mechanisms regulating cellular magnesium homeostasis, and processes controlling transmembrane magnesium transport had been demonstrated only at the functional level. Two cation channels of the transient receptor potential melastatin (TRPM) cation channel family have now been identified as magnesium transporters, TRPM6 and TRPM7. These unique proteins, termed chanzymes because they possess a channel and a kinase domain, are differentially expressed, with TRPM6 being found primarily in epithelial cells and TRPM7 occurring ubiquitously. Vascular TRPM7 is modulated by vasoactive agents, pressure, stretch, and osmotic changes and may be a novel mechanotransducer. In addition to its magnesium transporter function, TRPM7 has been implicated as a signaling kinase involved in vascular smooth muscle cell growth, apoptosis, adhesion, contraction, cytoskeletal organization, and migration, important processes involved in vascular remodeling associated with hypertension and other vascular diseases. Emerging evidence suggests that vascular TRPM7 function may be altered in hypertension. This review discusses the importance of magnesium in vascular biology and implications in hypertension and highlights the transport systems, particularly TRPM6 and TRPM7, which may play a role in the control of vascular magnesium homeostasis. Since the recent identification and characterization of Mg2+-selective transporters, there has been enormous interest in the field. However, there is still a paucity of information, and much research is needed to clarify the exact mechanisms of magnesium regulation in the cardiovascular system and the implications of aberrant transmembrane magnesium transport in the pathogenesis of hypertension and other vascular diseases.

H bonding at the helix-bundle crossing controls gating in Kir potassium channels

Specific stimuli such as intracellular H⁺ and phosphoinositides (e.g., PIP₂) gate inwardly rectifying potassium (Kir) channels by controlling the reversible transition between the closed and open states. This gating mechanism underlies many aspects of Kir channel physiology and pathophysiology; however, its structural basis is not well understood. Here, we demonstrate that H⁺ and PIP₂ use a conserved gating mechanism defined by similar structural changes in the transmembrane (TM) helices and the selectivity filter. Our data support a model in which the gating motion of the TM helices is controlled by an intrasubunit hydrogen bond between TM1 and TM2 at the helix-bundle crossing, and we show that this defines a common gating motif in the Kir channel superfamily. Furthermore, we show that this proposed H-bonding interaction determines Kir channel pH sensitivity, pH and PIP₂ gating kinetics, as well as a K⁺-dependent inactivation process at the selectivity filter and therefore many of the key regulatory mechanisms of Kir channel physiology.

The TRP channel superfamily: insights into how structure, protein-lipid interactions and localization influence function

TRP (transient receptor potential) cationic channels are key molecules that are involved in a variety of diverse biological processes ranging from fertility to osmosensation and nociception. Increasing our knowledge of these channels will help us to understand a range of physiological and pathogenic processes, as well as highlighting potential therapeutic drug targets. The founding members of the TRP family, Drosophila TRP and TRPL (TRP-like) proteins, were identified within the last two decades and there has been a subsequent explosion in the number and type of TRP channel described. Although information is accumulating as to the function of some of the TRP channels, the activation and inactivation mechanisms, structure, and interacting proteins of many, if not most, are awaiting elucidation. The Cell and Molecular Biology of TRP Channels Meeting held at the University of Bath included speakers working on a number of the different subfamilies of TRP channels and provided a basis for highlighting both similarities and differences between these groups. As the TRP channels mediate diverse functions, this meeting also brought together an audience with wide-ranging research interests, including biochemistry, cell biology, physiology and neuroscience, and inspired lively discussion on the issues reviewed herein.

Dissection of the components for PIP2 activation and thermosensation in TRP channels

Phosphatidylinositol 4,5-bisphosphate (PIP2) plays a central role in the activation of several transient receptor potential (TRP) channels. The role of PIP2 on temperature gating of thermoTRP channels has not been explored in detail, and the process of temperature activation is largely unexplained. In this work, we have exchanged different segments of the C-terminal region between cold-sensitive (TRPM8) and heat-sensitive (TRPV1) channels, trying to understand the role of the segment in PIP2 and temperature activation. A chimera in which the proximal part of the C-terminal of TRPV1 replaces an equivalent section of TRPM8 C-terminal is activated by PIP2 and confers the phenotype of heat activation. PIP2, but not temperature sensitivity, disappears when positively charged residues contained in the exchanged region are neutralized. Shortening the exchanged segment to a length of 11 aa produces voltage-dependent and temperature-insensitive channels. Our findings suggest the existence of different activation domains for temperature, PIP2, and voltage. We provide an interpretation for channel-PIP2 interaction using a full-atom molecular model of TRPV1 and PIP2 docking analysis.