TRPM8

Transient receptor potential cation channel subfamily M member 8 (TRPM8), also known as the cold and menthol receptor 1 (CMR1), is a is_associated_with::protein that in humans is encoded by the TRPM8 is_associated_with::gene.

Structure
The TRPM8 channel is a is_associated_with::homotetramer, composed of four identical subunits with a is_associated_with::transmembrane domain with six helices (S1-S6). The first four, S1-S4, act as the is_associated_with::voltage sensor and allow binding of is_associated_with::menthol, is_associated_with::icilin and similar channel is_associated_with::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 is_associated_with::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.

Function
TRPM8 is an is_associated_with::ion channel, upon activation it allows the entry of Na+ (is_associated_with::sodium) and Ca2+ (is_associated_with::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 is_associated_with::menthol and is_associated_with::icilin whereas WS-12 and CPS-369 are the most selective agonist of TRPM8.

TRPM8 is also expressed in the is_associated_with::prostate, lungs, and bladder where its function is not well understood.

Role in the nervous system
The is_associated_with::transient receptor potential channel (TRP) superfamily, which includes the menthol (TRPM8) and is_associated_with::capsaicin receptors (TRPV1), serve a variety of functions in the peripheral and is_associated_with::central nervous systems. In the peripheral nervous system, TRPs respond to stimuli from is_associated_with::temperature, is_associated_with::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 is_associated_with::calcium imaging and is_associated_with::patch clamp based approaches, they showed a response in is_associated_with::dorsal root ganglion (DRG) is_associated_with::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 is_associated_with::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 is_associated_with::depolarization, followed by calcium influx via is_associated_with::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.

pH-sensitivity
In contrast to the is_associated_with::TRPV1 (is_associated_with::capsaicin) receptor, which is potentiated by low pH, is_associated_with::acidic conditions were shown to inhibit the TRPM8 Ca2+ response to is_associated_with::menthol and is_associated_with::icilin (an is_associated_with::agonist of the menthol receptor). It is hypothesized the is_associated_with::TRPV1 and TRPM8 receptors act together in response to inflammatory conditions: is_associated_with::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.

Sensitization
Numerous studies have been published investigating the effect of L-menthol application as a model for TRPM8-sensitization. The primary is_associated_with::consensus finding is that TRPM8 sensitization increases the sensation of cold pain, also known as cold is_associated_with::hyperalgesia. An experiment was done in a is_associated_with::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 is_associated_with::Group C nerve fiber is_associated_with::nociceptors needed to transduce the sensation of pain. They concluded menthol sensitizes cold-sensitive peripheral C nociceptors and activates cold-specific A delta fibers.

Desensitization
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 is_associated_with::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. is_associated_with::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, is_associated_with::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 is_associated_with::capsaicin receptor.

Mutagenesis of is_associated_with::protein kinase C phoshorylation sites in TRPM8 (wild type serines and threonines replaced by alanine in mutants) reduces the desensitizing response.

Cross-desensitization
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. is_associated_with::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 is_associated_with::capsaicin, and, most novelly, 3) capsaicin receptors are desensitized in response to menthol.

Agonists
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 is_associated_with::linalool, is_associated_with::geraniol, hydroxy-is_associated_with::citronellal, is_associated_with::WS-3, is_associated_with::WS-23, is_associated_with::Frescolat MGA, is_associated_with::Frescolat ML, is_associated_with::PMD 38, is_associated_with::Coolact P and is_associated_with::Cooling Agent 10.

Antagonists
BCTC, is_associated_with::thio-BCTC, and is_associated_with::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.

Clinical significance
Cold-patches have traditionally been used to induce is_associated_with::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 is_associated_with::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 is_associated_with::afferent nerve fibers.

Three independent research groups have reported that mice lacking functional TRPM8 gene expression are severely impaired in their ability to detect is_associated_with::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 is_associated_with::ligands to be used as new generation of is_associated_with::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 is_associated_with::knockout mice not only indicated that TRPM8 is required for cold sensation but also revealed that TRPM8 mediates both cold and mechanical is_associated_with::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 is_associated_with::prostate cancer. TRPM8 is an is_associated_with::androgen dependent Ca2+ channel necessary for is_associated_with::prostate cancer cells to survive and grow. Immunfluorescence showed expression of the TRPM8 protein in the ER and plasma membrane of the androgen-responsive is_associated_with::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 is_associated_with::siRNAs targeting TRPM8 is_associated_with::mRNAs, the necessity of the TRPM8 receptor was shown in the androgen-dependent cancer cells. This has useful implications in terms of is_associated_with::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.