Transient receptor potential cation channel subfamily M (melastatin) member 8 (TRPM8), also known as the cold and menthol receptor 1 (CMR1), is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

that in humans is encoded by the ''TRPM8''

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

. The TRPM8 channel is the primary molecular transducer of cold somatosensation in humans. In addition, mints can desensitize a region through the activation of TRPM8 receptors (the 'cold'/menthol receptor).

Structure

The TRPM8 channel is a homotetramer, composed of four identical subunits with a transmembrane domain with six helices (S1–6). The first four, S1–4, act as the voltage sensor and allow binding of

menthol Menthol is an organic compound, specifically a Monoterpene, monoterpenoid, that occurs naturally in the oils of several plants in the Mentha, mint family, such as Mentha arvensis, corn mint and peppermint. It is a white or clear waxy crystallin ...

, icilin and similar channel

agonists An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agon ...

. 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 In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

region. A range of diverse components are required for the high level of specificity in response to cold and menthol stimuli which eventually lead to ion flow through the protein channel.

Function

TRPM8 is an

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...

: upon activation, it allows the entry of Na⁺ and Ca²⁺ ions into the cell, which 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

and icilin whereas WS-12 and CPS-369 are the most selective agonists of TRPM8. TRPM8 is also expressed in the

prostate The prostate is an male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found in all male mammals. It differs between species anatomically, chemica ...

, 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 Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) (, rarely ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a potent Irritation, irritant for Mammal, mammals, including humans, and produces ...

receptors ( TRPV1), serve a variety of functions in the

peripheral A peripheral device, or simply peripheral, is an auxiliary hardware device that a computer uses to transfer information externally. A peripheral is a hardware component that is accessible to and controlled by a computer but is not a core compo ...

and

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

s. In the peripheral nervous system, TRPs respond to stimuli from

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

, inflammatory agents, and

receptor Receptor may refer to: * Sensory receptor, in physiology, any neurite structure that, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and respond ...

activation. Central nervous system roles of the receptors include neurite outgrowth, receptor signaling, and excitotoxic 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 The patch clamp technique is a laboratory technique in electrophysiology used to study ionic currents in individual Cell isolation, isolated living cells, tissue sections, or patches of cell membrane. The technique is especially useful in the st ...

based approaches, they showed a response in

dorsal root ganglion A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the do ...

(DRG)

neurons A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

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 mucous membranes results directly in membrane

depolarization In biology, depolarization or hypopolarization is a change within a cell (biology), cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolar ...

, 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.

Properties

pH-sensitivity

In contrast to the TRPV1 (

) receptor, which is potentiated by low pH,

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

ic conditions were shown to inhibit the TRPM8 Ca²⁺ response to

and icilin (an

agonist An agonist is a chemical that activates a Receptor (biochemistry), receptor to produce a biological response. Receptors are Cell (biology), cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an R ...

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.

Sensitization

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 Hyperalgesia ( or ; ''hyper'' from Greek ὑπέρ (''huper'') 'over' + ''-algesia'' from Greek ἄλγος (algos) 'pain') is an abnormally increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves and ...

. An experiment was done in a

double-blind In a blind or blinded experiment, information which may influence the participants of the experiment is withheld until after the experiment is complete. Good blinding can reduce or eliminate experimental biases that arise from a participants' expec ...

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 Group C nerve fibers are one of three classes of nerve fiber in the central nervous system (CNS) and peripheral nervous system (PNS). The Group C fibers are unmyelinated and have a small diameter and low conduction velocity, whereas Groups A ...

nociceptor A nociceptor (; ) is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sensation of pain to direct attention to the body part, ...

s 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

nicotine Nicotine is a natural product, naturally produced alkaloid in the nightshade family of plants (most predominantly in tobacco and ''Duboisia hopwoodii'') and is widely used recreational drug use, recreationally as a stimulant and anxiolytic. As ...

-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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

, with similar irritant and desensitization properties, was used as a 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

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

Caryophyllene Caryophyllene (), more formally (−)-β-caryophyllene (BCP), is a natural bicyclic sesquiterpene that occurs widely in nature. Caryophyllene is notable for having a cyclobutane ring, as well as a ''trans''-double bond in a 9-membered ring, both ...

inhibits TRPM8, which helps mammals to improve cold tolerance at low ambient temperatures.

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.

Capsaicin Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) (, rarely ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a potent Irritation, irritant for Mammal, mammals, including humans, and produces ...

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 intervals. 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 sensitized in response to menthol.

Ligands

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 a2+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, icilin, WS-12/ Acoltremon, Frescolat MGA, Frescolat ML, PMD 38, Coolact P, M8-Ag and Cooling Agent 10. Traditionally used agonists include

and borneol.

Antagonists

BCTC, thio-BCTC, capsazepine and M8-An were identified as

antagonist An antagonist is a character in a story who is presented as the main enemy or rival of the protagonist and is often depicted as a villain.voltage-sensing domain, preventing response. * AMG-333 * RQ-00434739 * RQ-00203078 * PF-05105679 cas: 398583-31-7 * M8 B * AMTB * 5-benzyloxytryptamine * Anandamide * N-Arachidonoyl dopamine *

Tetrahydrocannabinol Tetrahydrocannabinol (THC) is a cannabinoid found in cannabis. It is the principal psychoactive constituent of ''Cannabis'' and one of at least 113 total cannabinoids identified on the plant. Although the chemical formula for THC (C21H30O2) de ...

* Tetrahydrocannabinolic acid *

Cannabidiol Cannabidiol (CBD) is a phytocannabinoid, one of 113 identified cannabinoids in ''Cannabis'', along with tetrahydrocannabinol (THC), and accounts for up to 40% of the plant's extract. Medically, it is an anticonvulsant used to treat multiple f ...

* Cannabidiolic acid * Cannabigerol *

Tetrahydrocannabivarin Tetrahydrocannabivarin (THCV, THV, O-4394, GWP42004) is a Homologous series, homologue of tetrahydrocannabinol (THC) having a propyl (3-carbon) side chain instead of pentyl (5-carbon), making it non-psychoactive in lower doses. It has been shown ...

* Tetrahydrocannabivarin Acid * Cannabidivarin * Cannabigerolic acid * Cannabigerovarin * Cannabichromene * Cannabinol

Clinical significance

Cold-patches have traditionally been used to induce

analgesia Pain management is an aspect of medicine and health care involving relief of pain (pain relief, analgesia, pain control) in various dimensions, from acute and simple to chronic and challenging. Most physicians and other health professionals ...

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

) or when ambient temperatures drop below approximately , suggesting that it mediates the detection of cold thermal stimuli by primary afferent sensory neurons of

afferent nerve fiber Afferent nerve fibers are axons (nerve fibers) of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections ''arrive'' at a particular brain region. In the peripheral nerv ...

s. Three independent research groups have reported that mice lacking functional TRPM8 gene expression are severely impaired in their ability to detect

cold Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjectivity, subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute t ...

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 In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ...

to be used as new generation of neuropathic analgesic drugs. 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 A knockout mouse, or knock-out mouse, is a genetically modified mouse (''Mus musculus'') in which researchers have inactivated, or " knocked out", an existing gene by replacing it or disrupting it with an artificial piece of DNA. They are importan ...

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 Prostate cancer is the neoplasm, uncontrolled growth of cells in the prostate, a gland in the male reproductive system below the bladder. Abnormal growth of the prostate tissue is usually detected through Screening (medicine), screening tests, ...

. TRPM8 is an

androgen An androgen (from Greek ''andr-'', the stem of the word meaning ) is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes ...

dependent Ca²⁺ channel necessary for prostate cancer cells to survive and grow. Immunofluorescence 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

mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...

s, the necessity of the TRPM8 receptor was shown in the androgen-dependent cancer cells. This has useful implications in terms of

gene therapy Gene therapy is Health technology, medical technology that aims to produce a therapeutic effect through the manipulation of gene expression or through altering the biological properties of living cells. The first attempt at modifying human DNA ...

, 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.

References

External links

* {{Transient receptor potential channel modulators Ion channels