|Transient receptor potential cation channel, subfamily M, member 8|
|Symbols||; LTRPC6; TRPP8|
|External IDs||IUPHAR: ChEMBL: GeneCards:|
|RNA expression pattern|
The TRPM8 channel is a homotetramer, composed of four identical subunits with a transmembrane domain with six helices (S1-S6). The first four, S1-S4, act as the voltage sensor and allow binding of menthol, icilin and similar channel agonists. S5 and S6 and a connecting loop, also part of the structure, make up the pore, a non-selective cation channel which consists of a highly conserved hydrophobic region, A range of diverse components are required for the high level of specificity in responding to result in ion flow to cold and menthol stimuli.
TRPM8 is an ion channel, upon activation it allows the entry of Na+ (sodium) and Ca2+ (calcium) ions to the cell that leads to depolarization and the generation of an action potential. The signal is conducted from primary afferents (type C- and A-delta) eventually leading to the sensation of cold and cold pain.
The TRPM8 protein is expressed in sensory neurons, and it is activated by cold temperatures and cooling agents, such as menthol and icilin whereas WS-12 and CPS-369 are the most selective agonist of TRPM8.
TRPM8 is also expressed in the prostate, lungs, and bladder where its function is not well understood.
Role in the nervous system
The transient receptor potential channel (TRP) superfamily, which includes the menthol (TRPM8) and capsaicin receptors (TRPV1), serve a variety of functions in the peripheral and central nervous systems. In the peripheral nervous system, TRPs respond to stimuli from temperature, pressure, inflammatory agents, and receptor activation. Central nervous system roles of the receptors include neurite outgrowth, receptor signaling, and excitoxic cell death resulting from noxious stimuli.
McKemy et al., 2002 provided some of the first evidence for existence of a cold-activated receptor throughout the mammalian somatosensory system. Using calcium imaging and patch clamp based approaches, they showed a response in dorsal root ganglion (DRG) neurons that exposure to cold, 20 °C or cooler, lead to a response in calcium influx. This receptor was shown to respond to both cold temperatures, menthol, and similar now-known agonists of the TRPM8 receptor. It works in conjunction with the TRPV1 receptor to maintain a feasible threshold temperature range in which our cells are comfortable and our perception of these stimuli occurs at the spinal cord and brain, which integrate signals from different fibers of varying sensitivity to temperature. Application of menthol to skin or mucus membranes results directly in membrane depolarization, followed by calcium influx via Voltage-dependent calcium channels, providing evidence for the role of TRPM8 and other TRP receptors to mediate our sensory interaction with the environment in response to cold in the same way as in response to menthol.
In contrast to the TRPV1 (capsaicin) receptor, which is potentiated by low pH, acidic conditions were shown to inhibit the TRPM8 Ca2+ response to menthol and icilin (an agonist of the menthol receptor). It is hypothesized the TRPV1 and TRPM8 receptors act together in response to inflammatory conditions: TRPV1, by proton action, increases the burning sensation of pain, while the acidity inhibits TRPM8 to block the more pleasant sensation of coolness in more dire instances of pain.
Numerous studies have been published investigating the effect of L-menthol application as a model for TRPM8-sensitization. The primary consensus finding is that TRPM8 sensitization increases the sensation of cold pain, also known as cold hyperalgesia. An experiment was done in a double-blind two-way crossover study by applying 40% L-menthol to the forearm, using ethanol as a control. Activation of the TRPM8-receptor channel (the primary menthol receptor channel) resulted in increased sensitization to the menthol stimulus. To investigate the mechanisms of this sensitization, Wasner et al., 2004, performed A fiber conduction blockade of the superficial radial nerve in another group of subjects. This ended up reducing the menthol-induced sensation of cold and hyperalgesia because blocking A fiber conduction resulted in inhibition of a class of Group C nerve fiber nociceptors needed to transduce the sensation of pain. They concluded menthol sensitizes cold-sensitive peripheral C nociceptors and activates cold-specific A delta fibers.
As is common in response to many other sensory stimuli, much experimental evidence exists for the desensitization of human response of TRPM8 receptors to menthol. Testing involving administration of menthol and nicotine-containing cigarettes non-smokers, which induced what they classified as an irritant response, after initial sensitization, showed a declining response in subjects over time, lending itself to the incidence of desensitization. Ethanol, with similar irritant and desensitization properties, was used to control for nicotine, to distinguish it from menthol-induced response. The menthol receptor was seen to sensitize or desensitize based on cellular conditions, and menthol produces increased activity in Ca2+-voltage gated channels that is not seen in ethanol, cyclohexanol and other irritant controls, suggestive of a specific molecular receptor. Dessirier et al., 2001, also claim the cross-desensitization of menthol receptors can occur by unknown molecular mechanisms, though they hypothesize the importance of Ca2+ in reducing cell excitability in a way similar to that in the capsaicin receptor.
Cliff et al., 1994, performed a study to discover more about the properties of the menthol receptor and whether menthol had the ability to cross-desensitize with other chemical irritant receptors. Capsaicin was known to cross-desensitize with other irritant agonists, where the same information was not known about menthol. The study involved subjects swishing either menthol or capsaicin for an extended time at regular intevals. There were three significant conclusions about cross-desensitizing: 1) Both chemicals self-desensitize, 2) menthol receptors can desensitize in response to capsaicin, and, most novelly, 3) capsaicin receptors are desensitized in response to menthol.
In a search for compounds that activated the TRPM8 cold receptor, compounds that produce a cooling-sensation were sought out from the fragrance industries. Of 70 relevant compounds, the following 10 produced the associated [Ca2+]-increase response in mTRPM8-transfected HEK293 cells used to identify agonists. Experimentally identified and commonly utilized agonists of the menthol receptor include linalool, geraniol, hydroxy-citronellal, WS-3, WS-23, Frescolat MGA, Frescolat ML, PMD 38, Coolact P and Cooling Agent 10.
BCTC, thio-BCTC, and capsazepine were identified as antagonists of the TRPM8 receptor. These antagonists physically block the receptor for cold and menthol, by binding to the S1-S4 voltage-sensing domain, preventing response.
Cold-patches have traditionally been used to induce analgesia or relief in pain which is caused as result of traumatic injuries. The underlying mechanism of cold-induced analgesia remained obscure until the discovery of TRPM8.
One research group has reported that TRPM8 is activated by chemical cooling agents (such as menthol) or when ambient temperatures drop below approximately 26 °C, suggesting that it mediates the detection of cold thermal stimuli by primary afferent sensory neurons of afferent nerve fibers.
Three independent research groups have reported that mice lacking functional TRPM8 gene expression are severely impaired in their ability to detect cold temperatures. Remarkably, these animals are deficient in many diverse aspects of cold signaling, including cool and noxious cold perception, injury-evoked sensitization to cold, and cooling-induced analgesia. These animals provide a great deal of insight into the molecular signaling pathways that participate in the detection of cold and painful stimuli. Many research groups, both in universities and pharmaceutical companies, are now actively involved in looking for selective TRPM8 ligands to be used as new generation of neuropathic analgesic drugs.
Interestingly, low concentrations of TRPM8 agonists such as menthol (or icilin) found to be antihyperalgesic in certain conditions, whereas high concentrations of menthol caused both cold and mechanical hyperalgesia in healthy volunteers.
TRPM8 knockout mice not only indicated that TRPM8 is required for cold sensation but also revealed that TRPM8 mediates both cold and mechanical allodynia in rodent models of neuropathic pain. Furthermore, recently it was shown that TRPM8 antagonists are effective in reversing established pain in neuropathic and visceral pain models.
TRPM8 upregulation in bladder tissues correlates with pain in patients with painful bladder syndromes. Furthermore, TRPM8 is upregulated in many prostate cancer cell lines and Dendreon/Genentech are pursuing an agonist approach to induce apoptosis and prostate cancer cell death.
Role in cancer
TRPM8 channels may be a target for treating prostate cancer. TRPM8 is an androgen dependent Ca2+ channel necessary for prostate cancer cells to survive and grow. Immunfluorescence showed expression of the TRPM8 protein in the ER and plasma membrane of the androgen-responsive LNCaP cell line. TRPM8 was expressed in androgen-insensitive cells, but it was not shown to be needed for their survival. By knockout of TRPM8 with siRNAs targeting TRPM8 mRNAs, the necessity of the TRPM8 receptor was shown in the androgen-dependent cancer cells. This has useful implications in terms of gene therapy, as there are so few treatment options for men with prostate cancer. As an androgen-regulated protein whose function is lost as cancer develops in cells, the TRPM8 protein seems to be especially critical in regulating calcium levels and has recently been proposed as the focus of new drugs used to treat prostate cancer.
- Andersen HH, Møller G, Winter R, Eskelund P, Arendt-Nielsen L (Aug 2013). "A review of topical high-concentration L-menthol as a translational model of cold allodynia and hyperalgesia". European Journal of Pain. doi:10.1002/j.1532-2149.2013.00380.x. PMID 23963768.
- Clapham DE, Julius D, Montell C, Schultz G (December 2005). "International Union of Pharmacology. XLIX. Nomenclature and structure-function relationships of transient receptor potential channels". Pharmacological reviews 57 (4): 427–50. doi:10.1124/pr.57.4.6. PMID 16382100.
- Pedretti A, Marconi C, Bettinelli I, Vistoli G (May 2009). "Comparative modeling of the quaternary structure for the human TRPM8 channel and analysis of its binding features". Biochim. Biophys. Acta 1788 (5): 973–82. doi:10.1016/j.bbamem.2009.02.007. PMID 19230823.
- Sherkheli M.A. et al. (2007). "Selective TRPM8 agonists: a novel group of neurophathic analgesics". FEBS Journal 274 (s1): 232. doi:10.1111/j.0014-2956.2007.05861_4.x.
- Sherkheli M.A., G G, Ak VE, Jf D, H H (October 2008). "Menthol derivative WS-12 selectively activates transient receptor potential melastatin-8 (TRPM8) ion channels". Pak J Pharm Sci 21 (4): 370–8. PMID 18930858.
- Moran MM, Xu H, Clapham DE (June 2004). "TRP ion channels in the nervous system". Curr. Opin. Neurobiol. 14 (3): 362–9. doi:10.1016/j.conb.2004.05.003. PMID 15194117.
- McKemy DD, Neuhausser WM, Julius D (March 2002). "Identification of a cold receptor reveals a general role for TRP channels in thermosensation". Nature 416 (6876): 52–8. doi:10.1038/nature719. PMID 11882888.
- Behrendt HJ, Germann T, Gillen C, Hatt H, Jostock R (February 2004). "Characterization of the mouse cold-menthol receptor TRPM8 and vanilloid receptor type-1 VR1 using a fluorometric imaging plate reader (FLIPR) assay". Br. J. Pharmacol. 141 (4): 737–45. doi:10.1038/sj.bjp.0705652. PMC 1574235. PMID 14757700.
- Wasner G, Schattschneider J, Binder A, Baron R (May 2004). "Topical menthol--a human model for cold pain by activation and sensitization of C nociceptors". Brain : a journal of neurology 127 (Pt 5): 1159–71. doi:10.1093/brain/awh134. PMID 14985268.
- Dessirier JM, O'Mahony M, Carstens E (May 2001). "Oral irritant properties of menthol: sensitizing and desensitizing effects of repeated application and cross-desensitization to nicotine". Physiol. Behav. 73 (1-2): 25–36. doi:10.1016/S0031-9384(01)00431-0. PMID 11399291.
- Abe J, Hosokawa H, Sawada Y, Matsumura K, Kobayashi S (2006). "Ca2+-dependent PKC activation mediates menthol-induced desensitization of transient receptor potential M8". Neurosci. Lett. 397 (1-2): 140–4. doi:10.1016/j.neulet.2005.12.005. PMID 16380208.
- Cliff MA, Green BG (March 1996). "Sensitization and desensitization to capsaicin and menthol in the oral cavity: interactions and individual differences". Physiol. Behav. 59 (3): 487–94. doi:10.1016/0031-9384(95)02089-6. PMID 8700951.
- Bautista DM, Siemens J, Glazer JM, Tsuruda PR, Basbaum AI, Stucky CL, Jordt SE, Julius D (2007). "The menthol receptor TRPM8 is the principal detector of environmental cold.". Nature 448 (7150): 204–208. doi:10.1038/nature05910. PMID 17538622.
- Daniels RL, McKemy DD (2007). "Mice left out in the cold: commentary on the phenotype of TRPM8-nulls". Mol Pain 3 (1): 23. doi:10.1186/1744-8069-3-23. PMC 1988789. PMID 17705869.
- Proudfoot CJ, Garry EM, Cottrell DF, Rosie R, Anderson H, Robertson DC, Fleetwood-Walker SM, Mitchell R (August 2006). "Analgesia mediated by the TRPM8 cold receptor in chronic neuropathic pain". Current biology : CB 16 (16): 1591–605. doi:10.1016/j.cub.2006.07.061. PMID 16920620.
- Colburn RW, Lubin ML, Stone DJ Jr, Wang Y, Lawrence D, D'Andrea MR, Brandt MR, Liu Y, Flores CM, Qin N (May 2007). "Attenuated cold sensitivity in TRPM8 null mice". Neuron 54 (3): 379–86. doi:10.1016/j.neuron.2007.04.017. PMID 17481392.
- Lashinger ES, Steiginga MS, Hieble JP, Leon LA, Gardner SD, Nagilla R, Davenport EA, Hoffman BE, Laping NJ, Su X (September 2008). "AMTB, a TRPM8 channel blocker: evidence in rats for activity in overactive bladder and painful bladder syndrome". American journal of physiology. Renal physiology 295 (3): F803–10. doi:10.1152/ajprenal.90269.2008. PMID 18562636.
- Mukerji G, Yiangou Y, Corcoran SL, Selmer IS, Smith GD, Benham CD, Bountra C, Agarwal SK, Anand P (2006). "Cool and menthol receptor TRPM8 in human urinary bladder disorders and clinical correlations". BMC urology 6: 6. doi:10.1186/1471-2490-6-6. PMC 1420318. PMID 16519806.
- "Dendreon: Targeting Cancer, Transforming Lives". Dendreon Corporation. 2005-09-21. Retrieved 2008-10-31.[dead link]
- Zhang L, Barritt GJ (November 2004). "Evidence that TRPM8 is an androgen-dependent Ca2+ channel required for the survival of prostate cancer cells". Cancer Res. 64 (22): 8365–73. doi:10.1158/0008-5472.CAN-04-2146. PMID 15548706.
- Clapham DE, Julius D, Montell C, Schultz G (2006). "International Union of Pharmacology. XLIX. Nomenclature and structure-function relationships of transient receptor potential channels.". Pharmacol. Rev. 57 (4): 427–50. doi:10.1124/pr.57.4.6. PMID 16382100.
- Voets T, Owsianik G, Nilius B (2007). "TRPM8.". Handb Exp Pharmacol 179 (179): 329–44. doi:10.1007/978-3-540-34891-7_20. PMID 17217067.