The TRPM2 channel: A thermo-sensitive metabolic sensor

Living organisms continually experience changes in ambient temperature. To detect such temperature changes for adaptive behavioral responses, we evolved the ability to sense temperature. Thermosensitive transient receptor potential (TRP) channels, so-called thermo-TRPs, are involved in many physiologic functions in diverse organisms and constitute important temperature sensors. One of the important roles of thermo-TRPs is detecting ambient temperature in sensory neurons. Importantly, the functional expression of thermo-TRPs is observed not only in sensory neurons but also in tissues and cells that are not exposed to drastic temperature changes, indicating that thermo-TRPs are involved in many physiologic functions within the body's normal temperature range. Among such thermo-TRPs, this review focuses on one thermo-sensitive metabolic sensor in particular, TRPM2, and summarizes recent progress to clarify the regulatory mechanisms and physiologic functions of TRPM2 at body temperature under various metabolic states.     

The Thermosensitive Potassium Channel TREK-1 Contributes to Coolness-Evoked Responses of Grueneberg Ganglion Neurons

Neurons of the Grueneberg ganglion (GG) residing in the vestibule of the murine nose are activated by cool ambient temperatures. Activation of thermosensory neurons is usually mediated by thermosensitive ion channels of the transient receptor potential (TRP) family. However, there is no evidence for the expression of thermo-TRPs in the GG, suggesting that GG neurons utilize distinct mechanisms for their responsiveness to cool temperatures. In search for proteins that render GG neurons responsive to coolness, we have investigated whether TREK/TRAAK channels may play a role; in heterologous expression systems, these potassium channels have been previously found to close upon exposure to coolness, leading to a membrane depolarization. The results of the present study indicate that the thermosensitive potassium channel TREK-1 is expressed in those GG neurons that are responsive to cool temperatures. Studies analyzing TREK-deficient mice revealed that coolness-evoked responses of GG neurons were clearly attenuated in these animals compared with wild-type conspecifics. These data suggest that TREK-1 channels significantly contribute to the responsiveness of GG neurons to cool temperatures, further supporting the concept that TREK channels serve as thermoreceptors in sensory cells. Moreover, the present findings provide the first evidence of how thermosensory GG neurons are activated by given temperature stimuli in the absence of thermo-TRPs.     

TRP channels and analgesia

Since cloning and characterizing the first nociceptive ion channel Transient Receptor Potential (TRP) Vanilloid 1 (TRPV1), other TRP channels involved in nociception have been cloned and characterized, which include TRP Vanilloid 2 (TRPV2), TRP Vanilloid 3 (TRPV3), TRP Vanilloid 4 (TRPV4), TRP Ankyrin 1 (TRPA1) and TRP Melastatin 8 (TRPM8), more recently TRP Canonical 1, 5, 6 (TRPC1, 5, 6), TRP Melastatin 2 (TRPM2) and TRP Melastatin 3 (TRPM3). These channels are predominantly expressed in C and Aδ nociceptors and transmit noxious thermal, mechanical and chemical sensitivities. TRP channels are modulated by pro-inflammatory mediators, neuropeptides and cytokines. Significant advances have been made targeting these receptors either by antagonists or agonists to treat painful conditions. In this review, we will discuss TRP channels as targets for next generation analgesics and the side effects that may ensue as a result of blocking/activating these receptors, because they are also involved in physiological functions such as release of vasoactive neuropeptides and regulation of vascular tone, maintenance of the body temperature, gastrointestinal motility, urinary bladder control, etc.     

TRP channels in thermosensation

The ability to sense external temperature is assumed by somatosensory neurons, in which temperature information is converted to neural activity by afferent input to the central nervous system. Somatosensory neurons consist of various populations with specialized gene expression, including thermosensitive transient receptor potential ion channels (thermo-TRPs). Thermo-TRPs are responsible for thermal transduction at the peripheral ends of somatosensory neurons and over a wide range of temperatures. In this review, we focus on several thermo-TRPs expressed in sensory neurons: TRPV1, TRPV4, TRPM2, TRPM3, TRPM8, TRPC5, and TRPA1. TRPV3, TRPV4, and TRPC5 expressed in non-neuronal cells that are also involved in somatosensation are also discussed, whereas TRPM2 and TRPM8 are involved in thermosensation in the brain.     

TRP channels and pain

Since the molecular identification of the capsaicin receptor, now known as TRPV1, transient receptor potential (TRP) channels have occupied an important place in the understanding of sensory nerve function in the context of pain. Several TRP channels exhibit sensitivity to substances previously known to cause pain or pain-like sensations; these include cinnamaldehyde, menthol, gingerol, and icillin. Many TRP channels also exhibit significant sensitivity to increases or decreases in temperature. Some TRP channels are sensitized in vitro by the activation of other receptors such that these channels may be activated by processes, such as inflammation that result in pain. TRP channels are suggested to be involved in processes as diverse as sensory neuron activation events, neurotransmitter release and action in the spinal cord, and release of inflammatory mediators. These functions strongly suggest that specific and selective inhibition of TRP channel activity will be of use in alleviating pain.     

TRP channels and pain

Since the molecular identification of the capsaicin receptor, now known as TRPV1, transient receptor potential (TRP) channels have occupied an important place in the understanding of sensory nerve function in the context of pain. Several TRP channels exhibit sensitivity to substances previously known to cause pain or pain-like sensations; these include cinnamaldehyde, menthol, gingerol, and icillin. Many TRP channels also exhibit significant sensitivity to increases or decreases in temperature. Some TRP channels are sensitized in vitro by the activation of other receptors such that these channels may be activated by processes, such as inflammation that result in pain. TRP channels are suggested to be involved in processes as diverse as sensory neuron activation events, neurotransmitter release and action in the spinal cord, and release of inflammatory mediators. These functions strongly suggest that specific and selective inhibition of TRP channel activity will be of use in alleviating pain.     

The Anoctamin Family Channel Subdued Mediates Thermal Nociception in Drosophila

Calcium-permeable and thermosensitive transient receptor potential (TRP) channels mediate the nociceptive transduction of noxious temperature in Drosophila nociceptors. However, the underlying molecular mechanisms are not completely understood. Here we find that Subdued, a calcium-activated chloride channel of the Drosophila anoctamin family, functions in conjunction with the thermo-TRPs in thermal nociception. Genetic analysis with deletion and the RNAi-mediated reduction of subdued show that subdued is required for thermal nociception in nociceptors. Further genetic analysis of subdued mutant and thermo-TRP mutants show that they interact functionally in thermal nociception. We find that Subdued expressed in heterologous cells mediates a strong chloride conductance in the presence of both heat and calcium ions. Therefore, our analysis suggests that Subdued channels may amplify the nociceptive neuronal firing that is initiated by thermo-TRP channels in response to thermal stimuli.     

Mechanotransduction by TRP Channels: General Concepts and Specific Role in the Vasculature

Transient receptor potential (TRP) ion channel superfamily is involved in sensing and transmission of a broad variety of external or internal stimuli, including but not limited to mechanical stress. Based on homology analysis, genetic and molecular studies have recently identified TRP channels in different tissues, comprising blood vessels. In invertebrates, many TRP channels including five TRPV channels identified in Caenorhabditis elegans and two in Drosophila have been implicated in mechanosensory behaviors as molecular basis of volume regulation, hearing and touch sensitivity. Consistently, in mammals many TRP family members such as TRPC1, TRPC3, TRPC6, TRPM4, TRPM7, TRPN1, TRPA1, TRPY1, TRPP1, TRPP2, and notably, TRPV1, TPRV2 as well as TRPV4 have been reported to be involved in mechanotransduction. This review summarizes recent and at times controversial findings on the role and regulation of TRP channels in mechanotransduction. Specifically, we highlight the relevance of TRPV channels in vascular regulation and focus on TRPV4 in the vascular system of the lung, which is constantly exposed to a unique combination of circumferential and longitudinal strains. In light of our observation in intact pulmonary microvessels that mechanical stress induced Ca²⁺ signaling in endothelial cells is closely related to TRPV4 activity, we postulate that TRPV4 plays a critical role in lung vascular mechanotransduction. The progress in this rapidly expanding field may allow for the identification of new molecular targets and the development of new therapeutic approaches in a number of intractable diseases related to mechanical stress.     

TRP channels in mechanosensation: direct or indirect activation?

Ion channels of the transient receptor potential (TRP) superfamily are involved in a wide variety of neural signalling processes, most prominently in sensory receptor cells. They are essential for mechanosensation in systems ranging from fruitfly hearing, to nematode touch, to mouse mechanical pain. However, it is unclear in many instances whether a TRP channel directly transduces the mechanical stimulus or is part of a downstream signalling pathway. Here, we propose criteria for establishing direct mechanical activation of ion channels and review these criteria in a number of mechanosensory systems in which TRP channels are involved.     

Proteins modulating TRP channel function

TRP channels are involved in different signaling cascades; TRP channels can be activated via hormones and neurotransmitter in a receptor/G-protein-mediated manner or by osmotic, thermic or mechanic stimuli. The overall functional role of TRP channels within these processes of hormonal cellular control, nociception or cellular calcium homeostasis is still unclear, as these complex processes often involve macromolecular structures. Whereas the integration of Drosophila TRP in the phototransduction process is becoming clear, the understanding of the participation of mammalian TRP channels in signal transduction complexes is only beginning. TRP channels have been demonstrated to interact with PDZ domain proteins, and both scaffold and regulatory function have been shown for INAD, the PDZ domain protein of the Drosophila phototransduction complex. In mammalian cells, the interaction of NHERF and TRPC4 has been shown and it is anticipated that NHERF may abolish the apparent store-dependent regulation of TRPC4 and TRPC5. Whereas TRP channels and PDZ domain proteins form permanent heterodimeric proteins, the interaction of calcium-binding proteins is dependent on the calcium concentration and is, therefore, dynamic. The prototype of calcium-binding protein used for experiments is calmodulin; whether or not calmodulin is also the natural interaction partner of TRP channels is an open question.     

Mechanism of Ca(2+)-dependent desensitization in TRP channels

The 30+ members of the family of TRP channels are diverse in their physiological roles, yet the mechanisms that regulate their gating may be conserved. In particular, all TRP channels show an activity-dependent inhibition which is mediated by Ca(2+). The mechanism by which Ca(2+) inhibits TRP channels is currently a matter of intense debate, with Ca(2+)-regulated kinases, phosphatases, phospholipases and calmodulin all proposed to be involved. In this review, we will discuss different mechanisms for Ca(2+)-dependent desensitization in TRP channels. We will conclude with a model that focuses on Ca(2+)-dependent activation of phospholipase C and Ca(2+) binding to calmodulin and propose that the phospholipase C and calmodulin pathways are structurally and functionally coupled.     

Comparing ion conductance recordings of synthetic lipid bilayers with cell membranes containing TRP channels

In this article we compare electrical conductance events from single channel recordings of three TRP channel proteins (TRPA1, TRPM2 and TRPM8) expressed in human embryonic kidney cells with channel events recorded on synthetic lipid membranes close to melting transitions. Ion channels from the TRP family are involved in a variety of sensory processes including thermo- and mechano-reception. Synthetic lipid membranes close to phase transitions display channel-like events that respond to stimuli related to changes in intensive thermodynamic variables such as pressure and temperature. TRP channel activity is characterized by typical patterns of current events dependent on the type of protein expressed. Synthetic lipid bilayers show a wide spectrum of electrical phenomena that are considered typical for the activity of protein ion channels. We find unitary currents, burst behavior, flickering, multistep-conductances, and spikes behavior in both preparations. Moreover, we report conductances and lifetimes for lipid channels as described for protein channels. Non-linear and asymmetric current–voltage relationships are seen in both systems. Without further knowledge of the recording conditions, no easy decision can be made whether short current traces originate from a channel protein or from a pure lipid membrane.     

TRPs in bladder diseases

This review attempts to provide an overview of the current knowledge of TRP proteins and their possible role in bladder function and disease. At present, there are 28 transient receptor potential (TRP) channels (subdivided into 7 categories or families) which are involved in a number of functions [G.A. Hicks, TRP channels as therapeutic targets: hot property, or time to cool down? Neurogastroenterology and Motility 18, (2006) 590–594., J.D. Levine, N. Alessandri-Haber, TRP channels: targets for the relief of pain, Biochimica et Biophysica Acta 1772, (2007) 989–1003.]. Of those belonging to the group 1 subfamily, a number of TRPV, TRPM and TRPA proteins associated with osmoregulation, thermal, chemical and mechanical signaling mechanisms have been shown to be expressed within the lower urinary tract. Though the biological role of many of these channels in urinary bladder function still remains elusive, TRPV1 is by far the best characterized and is thought to be involved in a number of bladder disorders [A. Szallasi, P.M. Blumberg, Vanilloid (Capsaicin) Receptors and Mechanisms, Pharmacological Reviews 51, (1999) 150–221., I. Nagy, P. Santha, G. Jansco, L. Urban, The role of the vanilloid (capsaicin) receptor (TRPV1) in physiology and pathology, European Journal of Pharmacology 500, (2004) 351–369.].     

TRPs in bladder diseases

This review attempts to provide an overview of the current knowledge of TRP proteins and their possible role in bladder function and disease. At present, there are 28 transient receptor potential (TRP) channels (subdivided into 7 categories or families) which are involved in a number of functions [G.A. Hicks, TRP channels as therapeutic targets: hot property, or time to cool down? Neurogastroenterology and Motility 18, (2006) 590-594., J.D. Levine, N. Alessandri-Haber, TRP channels: targets for the relief of pain, Biochimica et Biophysica Acta 1772, (2007) 989-1003.]. Of those belonging to the group 1 subfamily, a number of TRPV, TRPM and TRPA proteins associated with osmoregulation, thermal, chemical and mechanical signaling mechanisms have been shown to be expressed within the lower urinary tract. Though the biological role of many of these channels in urinary bladder function still remains elusive, TRPV1 is by far the best characterized and is thought to be involved in a number of bladder disorders [A. Szallasi, P.M. Blumberg, Vanilloid (Capsaicin) Receptors and Mechanisms, Pharmacological Reviews 51, (1999) 150-221., I. Nagy, P. Santha, G. Jansco, L. Urban, The role of the vanilloid (capsaicin) receptor (TRPV1) in physiology and pathology, European Journal of Pharmacology 500, (2004) 351-369.].     

Integration of phosphoinositide- and calmodulin-mediated regulation of TRPC6

Multiple TRP channels are regulated by phosphoinositides (PIs). However, it is not known whether PIs bind directly to TRP channels. Furthermore, the mechanisms through which PIs regulate TRP channels are obscure. To analyze the role of PI/TRP interactions, we used a biochemical approach, focusing on TRPC6. TRPC6 bound directly to PIs, and with highest potency to phosphatidylinositol 3,4,5-trisphosphate (PIP(3)). We found that PIP(3) binding disrupted the association of calmodulin (CaM) with TRPC6. We identified the PIP(3)-binding site and found that mutations that increased or decreased the affinity of the PIP(3)/TRPC6 interaction enhanced or reduced the TRPC6-dependent current, respectively. PI-mediated disruption of CaM binding appears to be a theme that applies to other TRP channels, such as TRPV1, as well as to the voltage-gated channels KCNQ1 and Ca(v)1.2. We propose that regulation of CaM binding by PIs provides a mode for integration of channel regulation by Ca(2+) and PIs.     

Intragastric administration of allyl isothiocyanate increases carbohydrate oxidation via TRPV1 but not TRPA1 in mice

The transient receptor potential (TRP) channel family is composed of a wide variety of cation-permeable channels activated polymodally by various stimuli and is implicated in a variety of cellular functions. Recent investigations have revealed that activation of TRP channels is involved not only in nociception and thermosensation but also in thermoregulation and energy metabolism. We investigated the effect of intragastric administration of TRP channel agonists on changes in energy substrate utilization of mice. Intragastric administration of allyl isothiocyanate (AITC; a typical TRPA1 agonist) markedly increased carbohydrate oxidation but did not affect oxygen consumption. To examine whether TRP channels mediate this increase in carbohydrate oxidation, we used TRPA1 and TRPV1 knockout (KO) mice. Intragastric administration of AITC increased carbohydrate oxidation in TRPA1 KO mice but not in TRPV1 KO mice. Furthermore, AITC dose-dependently increased intracellular calcium ion concentration in cells expressing TRPV1. These findings suggest that AITC might activate TRPV1 and that AITC increased carbohydrate oxidation via TRPV1.     

Mechanism of Ca2+-dependent desensitization in TRP channels

The 30+ members of the family of TRP channels are diverse in their physiological roles, yet the mechanisms that regulate their gating may be conserved. In particular, all TRP channels show an activity-dependent inhibition which is mediated by Ca²⁺. The mechanism by which Ca²⁺ inhibits TRP channels is currently a matter of intense debate, with Ca²⁺-regulated kinases, phosphatases, phospholipases, and calmodulin all proposed to be involved. In this review, we will discuss different mechanisms for Ca²⁺-dependent desensitization in TRP channels. We will conclude with a model that focuses on Ca²⁺-dependent activation of phospholipase C and Ca²⁺ binding to calmodulin and propose that the phospholipase C and calmodulin pathways are structurally and functionally coupled.     

Calcium store contents control the expression of TRPC1, TRPC3 and TRPV6 proteins in LNCaP prostate cancer cell line

The mammalian homologues of the Drosophila transient receptor potential (TRP) represent a superfamily of ion channels involved in Ca(2+) homeostasis. Several members of this family are activated either by a depletion of the internal stores of Ca(2+) or by stimulation of G protein-coupled receptors. In androgen responsive prostate cancer cell line LNCaP, TRPC1, TRPC4 and/or TRPV6 have been reported to function as store-operated channels (SOCs) while TRPC3 might be involved in the response to agonist stimulation, possibly through the induction of diacylglycerol production by phospholipase C. However, the control of expression of these TRP proteins is largely unknown. In the present study, we have investigated if the expression of the TRP proteins possibly involved in the capacitative influx of calcium is influenced by the contents of Ca(2+) in the endoplasmic reticulum. Using real-time PCR and Western blot techniques, we show that the expression of TRPC1, TRPC3 and TRPV6 proteins increases after a prolonged (24-48 h) depletion of the stores with thapsigargin. The upregulation of TRPC1 and TRPC3 depends on the store contents level and involves the activation of the Ca(2+)/calmodulin/calcineurin/NFAT pathway. Functionally, cells overexpressing TRPC1, TRPC3 and TRPV6 channels after a prolonged depletion of the stores showed an increased [Ca(2+)](i) response to alpha-adrenergic stimulation. However, the store-operated entry of calcium was unchanged. The isolated overexpression of TRPV6 (without overexpression of TRPC1 and TRPC3) did not produce this increased response to agonists, therefore suggesting that TRPC1 and/or TRPC3 proteins are responsible for the response to alpha-adrenergic stimulation but that TRPC1, TPRC3 and TRPV6 proteins, expressed alone or concomitantly, are not sufficient for SOC formation.     

Non-selective cation channels, transient receptor potential channels and ischemic stroke

Several pathways to neural cell death are involved in ischemic stroke, and all require monovalent or divalent cation influx, implicating non-selective cation (NC) channels. NC channels are also likely to be involved in the dysfunction of vascular endothelial cells that leads to formation of edema following cerebral ischemia. Two newly described NC channels have emerged as potential participants in ischemic stroke, the acid sensing ion channel (ASIC), and the sulfonylurea receptor-1 (SUR1)-regulated NC(Ca-ATP) channel. Non-specific blockers of NC channels, including pinokalant (LOE 908 MS) and rimonabant (SR141716A), have beneficial effects in rodent models of ischemic stroke. Evidence is accumulating that NC channels formed by members of the transient receptor potential (TRP) family are also up-regulated in ischemic stroke and may play a direct role in calcium-mediated neuronal death. The nascent field of NC channels, including TRP channels, in ischemic stroke is poised to provide novel mechanistic insights and therapeutic strategies for this often devastating human condition.