Transient receptor potential channel

Transient receptor potential channels (TRP channels) are a group of ion channels located mostly on the plasma membrane of numerous animal cell types. Most of these are grouped into two broad groups: Group 1 includes TRPC ( "C" for canonical), TRPV ("V" for vanilloid), TRPVL ("VL" for vanilloid-like), TRPM ("M" for melastatin), TRPS ("S" for soromelastatin), TRPN ("N" for mechanoreceptor potential C), and TRPA ("A" for ankyrin). Group 2 consists of TRPP ("P" for polycystic) and TRPML ("ML" for mucolipin). Other less-well categorized TRP channels exist, including yeast channels and a number of Group 1 and Group 2 channels present in non-animals. Many of these channels mediate a variety of sensations such as pain, temperature, different kinds of taste, pressure, and vision. In the body, some TRP channels are thought to behave like microscopic thermometers and used in animals to sense hot or cold. Some TRP channels are activated by molecules found in spices like garlic (allicin), chili pepper (capsaicin), wasabi (allyl isothiocyanate); others are activated by menthol, camphor, peppermint, and cooling agents; yet others are activated by molecules found in cannabis (i.e., THC, CBD and CBN) or stevia. Some act as sensors of osmotic pressure, volume, stretch, and vibration. Most of the channels are activated or inhibited by signaling lipids and contribute to a family of lipid-gated ion channels. These ion channels have a relatively non-selective permeability to cations, including sodium, calcium and magnesium. TRP channels were initially discovered in the so-called "transient receptor potential" mutant (trp-mutant) strain of the fruit fly Drosophila, hence their name (see History of Drosophila TRP channels below). Later, TRP channels were found in vertebrates where they are ubiquitously expressed in many cell types and tissues. TRP channels are tetrameric, with each protomer composed of 6 membrane-spanning helices with intracellular N- and C-termini. Mammalian TRP channels are activated and regulated by a wide variety of stimuli and are expressed throughout the body.

Families

In the animal TRP superfamily there are currently 9 proposed families split into two groups, each family containing a number of subfamilies. Group one consists of TRPC, TRPV, TRPVL, TRPA, TRPM, TRPS, and TRPN, while group two contains TRPP and TRPML. There is an additional family labeled TRPY that is not always included in either of these groups. All of these sub-families are similar in that they are molecular sensing, non-selective cation channels that have six transmembrane segments, however, each sub-family is unique and shares little structural homology with one another. This uniqueness gives rise to the various sensory perception and regulation functions that TRP channels have throughout the body. Group one and group two vary in that both TRPP and TRPML of group two have a much longer extracellular loop between the S1 and S2 transmembrane segments. Another differentiating characteristic is that all the group one sub-families either contain an N-terminal intracellular ankyrin repeat sequence, a C-terminal TRP domain sequence, or both—whereas both group two sub-families have neither. Below are members of the sub-families and a brief description of each:

TRPA

TRPA, A for "ankyrin", is named for the large amount of ankyrin repeats found near the N-terminus. TRPA is primarily found in afferent nociceptive nerve fibers and is associated with the amplification of pain signaling as well as cold pain hypersensitivity. These channels have been shown to be both mechanical receptors for pain and chemosensors activated by various chemical species, including isothiocyanates (pungent chemicals in substances such as mustard oil and wasabi), cannabinoids, general and local analgesics, and cinnamaldehyde. While TRPA1 is expressed in a wide variety of animals, a variety of other TRPA channels exist outside of vertebrates. TRPA5, painless, pyrexia, and waterwitch are distinct phylogenetic branches within the TRPA clade, and are only evidenced to be expressed in crustaceans and insects, while HsTRPA arose as a Hymenoptera-specific duplication of waterwitch. Like TRPA1 and other TRP channels, these function as ion channels in a number of sensory systems. TRPA- or TRPA1-like channels also exists in a variety of species as a phylogenetically distinct clade, but these are less well understood.

TRPC

TRPC, C for "canonical", is named for being the most closely related to Drosophila TRP, the namesake of TRP channels. The phylogeny of TRPC channels has not been resolved in detail, but they are present across animal taxa. There are actually only six TRPC channels expressed in humans because TRPC2 is found to be expressed solely in mice and is considered a pseudo-gene in humans; this is partly due to the role of TRPC2 in detecting pheromones, which mice have an increased ability compared to humans. Mutations in TRPC channels have been associated with respiratory diseases along with focal segmental glomerulosclerosis in the kidneys. All TRPC channels are activated either by phospholipase C (PLC) or diacyglycerol (DAG).

TRPML

TRPML, ML for "mucolipin", gets its name from the neurodevelopmental disorder mucolipidosis IV. Mucolipidosis IV was first discovered in 1974 by E.R. Berman who noticed abnormalities in the eyes of an infant. These abnormalities soon became associated with mutations to the MCOLN1 gene which encodes for the TRPML1 ion channel. TRPML is still not highly characterized. The three known vertebrate copies are restricted to jawed vertebrates, with some exceptions (e.g. Xenopus tropicalis).

TRPM

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