1.A.4 The Transient Receptor Potential Ca2+ Channel (TRP-CC) Family
TRP (transient receptor potential) channels represent a superfamily of cation channels conserved from worms to humans (Vennekens et al. 2012). They comprise seven subfamilies (TRPC, TRPV, TRPM, TRPN, TRPA, TRPP, and TRPML) that can be consider to be in two classes, group I (TRPC/V/M/N/A) and TRPML (TRP MucoLipin) and TRPP (TRP Polycystin) making up group II (Fine et al. 2019). According to Latorre et al. (2009), TRP channels can be grouped into seven subfamilies based on their amino acid sequence homology: (1) the canonical or classic TRPs, (2) the vanilloid receptor TRPs, (3) the melastatin or long TRPs, (4) ankyrin (whose only member is the transmembrane protein 1 [TRPA1]), (5) TRPN after the nonmechanoreceptor potential C (nonpC), and the more distant cousins, the (6) polycystins and (7) mucolipins. Members of the VIC (1.A.1), RIR-CaC (2.A.3) and TRP-CC (1.A.4). Families have similar transmembrane domain structures, but very different cytosolic doman structures (Mio et al. 2008). Because of their role as cellular sensors, polymodal activation and gating properties, many TRP channels are activated by a variety of different stimuli and function as signal integrators (Latorre et al., 2009; Montell, 2005; Ramsey et al., 2006). These mammalian proteins have been tabulated revealing their accepted designations, activators and inhibitors, putative interacting proteins and proposed functions (Clapham, 2007). The founding members of the TRP superfamily are the TRPC (TRP canonical) channels, which can be activated following the stimulation of phospholipase C and/or depletion of internal calcium stores (Montell, 2005). TRPC channels regulate nicotine-dependent behavior (Feng et al., 2006). The intrinsic assembly domains that assure tetrameric TRP channel formation have been reviewed (Schindl and Romanin 2007). They also function by depolarising the membrane potential, which triggers the activation of voltage-gated Ca2+ channels. Structural differences in the extracellular portions, transmembrane domains and the cytoplasmic domains of TRPC channels have been reviewed (Guo and Chen 2019). Cryo-EM has led to an explosion of TRP structures in the last few years (Cao 2020). These structures have confirmed that TRP channels assemble as tetramers and resemble voltage-gated ion channels in their overall architecture. However, each TRP subtype is endowed with a unique set of soluble domains that may confer diverse regulatory mechanisms. TRP channel structures have revealed sites and mechanisms of action of numerous synthetic and natural compounds, as well as those for endogenous ligands such as lipids, Ca2+, and calmodulin (Cao 2020).
Alonso-Carbajo et al. 2017 reviewed the functions of these proteins in human vascular smooth muscle and cardiac striated muscle. The mammalian TRP superfamily includes at least 22 genes grouped into three major subfamilies based on sequence homology: TRPV (vanilloid), TRPC (canonical), and TRPM (melastatin). Three additional subfamilies (the 'distant TRPs'), TRPP (polycystin), TRPML (mucolipin), and TRPA bring the total number of TRP-related proteins to around 30. TRP proteins are six transmembrane-domain polypeptide subunits, and four subunits assemble in the plasma membrane to form functional channels. All TRP channels are cation permeable, and most are not selective for monovalent or divalent ions. However, TRPV5 and TRPV6, display specificity for Ca2+ ions, and TRPM4 and TRPM5 are highly selective for monovalent cations and impermeant to Ca2+. TRP channels are activated by stimuli including changes in pressure, temperature, osmolarity, and intracellular Ca2+. Fatty acids and receptor-dependent vasoconstrictor agonists also activate vascular TRP channels. Most channels assemble from four identical TRP subunits, but when multiple TRP subunits are coexpressed, heteromeric channels can form (Alonso-Carbajo et al. 2017). Over 75 structures of these proteins have been solved by cryo-EM (Madej and Ziegler 2018). The reveal a lack of an apparent general mechanism underlying channel opening and closing. Similarly, the structures reveal a surprising diversity in which chemical ligands bind TRP channels (Hilton et al. 2019).
The mammalian TRPM gene family can be subdivided into distinct categories of cation channels that are either highly permeable for Ca2+ (TRPM3/6/7), nonselective (TRPM2/8), or Ca2+ impermeable (TRPM4/5). TRPM6/7 are fused to alpha-kinase domains, whereas TRPM2 is linked to an ADP-ribose phosphohydrolase (Nudix domain). Phylogenetic evidence suggests that Nudix-linked channels represent an ancestral type of TRPM that is present in various phyla, ranging from protists to humans (Mederos y Schnitzler et al., 2008). The pore-forming segments of invertebrate TRPM2-like proteins display high sequence similarity to those of Ca2+-selective TRPMs. Restoration of only two 'ancient' pore residues in human TRPM2 (Q981E/P983Y) increased (4-fold) its permeability for Ca2+. Conversely, introduction of a 'modern' sequence motif into mouse TRPM7 (E1047Q/Y1049P) resulted in the loss of Ca2+ permeation and a linear TRPM2-like current-voltage relationship (Mederos y Schnitzler et al., 2008).
Volatile anaesthetics (VAs) are the most widely used compounds to induce reversible loss of consciousness and maintain general anaesthesia during surgical interventions. VAs depress central nervous system functions mainly through modulation of ion channels in the neuronal membrane, including 2-pore-domain K+ channels, GABA, NMDA receptors and nociceptive and thermosensitive TRP channels expressed in the peripheral nervous system, including TRPV1, TRPA1, TRPM3 and TRPM8 (Kelemen et al. 2020).
The TRP-CC family includes a variety of channel/sensors that respond to temperature, touch, pain, osmolarity, pheromones, taste, and other stimuli (Clapham, 2003). It has also been called the store-operated calcium channel (SOC) family. These proteins are the prinicipal components in mechanosensitive channels in vertebrate hair cells (TRPA1; 1.A.4.6.1) and stretch-activated channels in various vertebrate cell types (TRPC1; 1.A.4.1.3) (Barritt and Rychkov, 2005). TRPA1 and TRPC1 may use different mechanisms of activation. (a) The functional TRPA1 channel is probably a tetramer that is composed of four identical TRPA1 polypeptide chains or a mixture of TRPA1 and another channel polypeptide. Each TRPA1 polypeptide has 17 ankyrin repeats at the cytoplasmic amino terminus. It is proposed that these are coupled to motor proteins or other regulatory proteins on the cytoplasmic face of the plasma membrane (Barritt and Rychkov, 2005). In response to the deflection of the mechanosensitive cilia bundle induced by sound, tension on the ankyrin repeat domains or changes in protein-protein interactions are altered and the channel opens to admit Ca2+ and other cations. (b) The functional TRPC1 channel is probably a tetramer that is composed of four identical TRPC1 polypeptides or a mixture of TRPC1 polypeptides and another polypeptide. Although each TRPC1 polypeptide contains 3 or 4 ankyrin domains at the N terminus, it is proposed that these are not directly involved in channel gating. In response to a stimulus, such as stretching of the membrane by an increase in the volume of the cell, the channel opens and admits Ca2+. It is possible that release of Ca2+ from the endoplasmic reticulum that is induced by thapsigargin also acts as a stimulus, which alters cell volume and therefore can activate TRPC1 through changes in tension of the phospholipid bilayer. The activation of TRP channels by polyunsaturated fatty acids has been examined, and residues involved have been identified (Riehle et al. 2018).
Prototypical members of the TRP-CC family include the Drosophila retinal proteins TRP and TRPL (Montell and Rubin, 1989; Hardie and Minke, 1993). The 81 aas integral membrane INAF-B protein forms a complex with TRP channels, and they stabilize each other (Cheng and Nash, 2007). SOC members of the family mediate the entry of extracellular Ca2+ into cells in response to depletion of intracellular Ca2+ stores (Clapham, 1996) and agonist stimulated production of inositol-1,4,5 trisphosphate (IP3). One member of the TRP-CC family, mammalian Htrp3, has been shown to form a tight complex with the IP3 receptor (TC #1.A.3.2.1). This interaction is apparently required for IP3 to stimulate Ca2+ release via Htrp3. The vanilloid receptor subtype 1 (VR1), which is the receptor for capsaicin (the 'hot' ingredient in chili peppers) and serves as a heat-activated ion channel in the pain pathway (Caterina et al., 1997), is also a member of this family, and is activated by cannabinoids (i.e., anandamide) and certain inflammatory metabolites of arachidonate such as prostaglandin E2 (Olah et al., 2001). The stretch-inhibitable non-selective cation channel (SIC) is identical to the vanilloid receptor throughout all of its first 700 residues, but it exhibits a different sequence in its last 100 residues. VR1 and SIC transport monovalent cations as well as Ca2+. VR1 is about 10x more permeable to Ca2+ than to monovalent ions. Ca2+ overload probably causes cell death after chronic exposure to capsaicin (McCleskey and Gold, 1999).
The proteins of the TRP-CC family exhibit the same topological organization with a probable KscA-type 3-dimensional structure (Dodier et al., 2004; Dohke et al., 2004). They consist of about 700-800 (VR1, SIC or ECaC) or 1300 (TRP proteins) amino acyl residues with six transmembrane spanners (TMSs) as well as a short hydrophobic 'loop' region between TMSs 5 and 6. This loop region may dip into the membrane and contribute to the ion permeation pathway (Hardie and Minke, 1993). An aspartate residue in the P-loop may form a ring of negative charges that modulate pore properties including ion selectivity and inhibitory characteristics (García-Martínez et al., 2000). VR1 forms homotetramers. In these respects, members of the TRP-CC family resemble those of the VIC family. When one member of the TRP-CC family, the IGF-regulated Ca2+ channel of Mus musculus (TC #1.A.4.2.4), was PSI-BLASTED, it retrieved a partial sequence of a Zea mays K+ channel protein (887 aas; gbY07632) that is clearly a member of the VIC family. The two homologous protein segments of 150 residues were 28% identical, 42% similar with a PSI-BLAST score (without iterations) of 2e6. This observation further suggests a common origin for certain domains in the TRP-CC and VIC families.
All members of the vanilloid family of TRP channels (TRPV) possess an N-terminal ankyrin repeat domain (ARD), which regulates calcium uptake and homeostasis. It is essential for channel assembly and regulation. The 1.7 Å crystal structure of the TRPV6-ARD revealed conserved structural elements unique to the ARDs of TRPV proteins. First, a large twist between the fourth and fifth repeats is induced by residues conserved in all TRPV ARDs. Second, the third finger loop is the most variable region in sequence, length and conformation. In TRPV6, a number of putative regulatory phosphorylation sites map to the base of this third finger. The TRPV6-ARD does not assemble as a tetramer and is monomeric in solution (Phelps et al., 2008). Voltage sensing in thermo-TRP channels has been reviewed by Brauchi et al. (Brauchi and Orio, 2011).
The transient receptor potential (TRP) family of ion channels participate in many signaling pathways. TRPV1 functions as a molecular integrator of noxious stimuli, including heat, low pH, and chemical ligands. The 19-A structure of TRPV1 determined by using single-particle electron cryomicroscopy exhibits fourfold symmetry and comprises two distinct regions: a large open basket-like domain, likely corresponding to the cytoplasmic N- and C-terminal portions, and a more compact domain, corresponding to the transmembrane portion (Moiseenkova et al., 2008). The assignment of transmembrane and cytoplasmic regions was supported by fitting crystal structures of the structurally homologous Kv1.2 channel and isolated TRPV1 ankyrin repeats into the TRPV1 structure.
Most local anaesthetics used clinically are relatively hydrophobic molecules that gain access to their blocking site on the sodium channel by diffusing into or through the cell membrane. These anaesthetics block sodium channels and the excitability of neurons. Binshtok et al. (2007) tested the possibility that the excitability of primary sensory nociceptor (pain-sensing) neurons could be blocked by introducing the charged, membrane-impermeant lidocaine derivative QX-314 through the pore of the noxious-heat-sensitive TRPV1 channel (TC #1.A.4.2.1). They found that charged sodium-channel blockers can be targeted into nociceptors by the application of TRPV1 agonists to produce a pain-specific local anaesthesia. QX-314 applied externally had no effect on the activity of sodium channels in small sensory neurons when applied alone, but when applied in the presence of the TRPV1 agonist capsaicin, QX-314 blocked sodium channels and inhibited excitability (Binshtok et al., 2007).
The amino termini of TRP-CC proteins normally contain a proline-rich region and one or more ankyrin domains. VR1, for example, exhibits three such repeat domains in its amino terminal hydrophilic segment (432 amino acids). It also has a hydrophilic C-terminus that lacks recognizable motifs. The sequence similarity between VR1 and other TRP-CC family proteins is within and adjacent to the sixth TMS, including the hydrophobic 'loop' region. Unlike other TRP-CC family members, VR1 is not a SOC. Mammals appear to have multiple VR1 homologues.
One member of the TRP-CC family, TRP-PLIK (1862 aas; AF346629), has been implicated in the regulation of cell division. It has an N-terminal TRP-CC-like sequence and a C-terminal protein kinase-like sequence. It was shown to autophosphorylate and exhibits an ATP phosphorylation-dependent, non-selective, Ca2+-permeable, outward rectifying conductance (Runnels et al., 2001). Another long homologue, Melastatin, is associated with melanocytic tumor progression whereas another homologue, MTR1, is associated with Beckwith-Wiedemann syndrome and a predisposition for neoplasia. Each of these proteins may be present in the cell as several splice variants.
The rabbit kidney epithelial Ca2+ channel, ECaC, is a Ca2+-selective cation channel with monovalent cation transport activity sensitive to strong inhibition by low concentrations of Ca2+ or Mg2+. ECaC is >100 x more permeable to Ca2+ than Na+. Mutation of D542 to alanine (D542A) (not present in the TRP-CC homologue) abolishes Ca2+ permeation and divalent cation inhibition of monovalent cation permeation. The mutation does not inhibit the latter transport activity. The D542K mutation generates a nonfunctional channel. Thus, a single residue determines the characteristic cation selectivity of ECaC.
The ability to detect variations in humidity is critical for many animals. Birds, reptiles and insects all show preferences for specific humidities that influence their mating, reproduction and geographic distribution. Because of their large surface area to volume ratio, insects are particularly sensitive to humidity, and its detection can influence their survival. Two types of hygroreceptors exist in insects: one responds to an increase (moist receptor) and the other to a reduction (dry receptor) in humidity. Although previous data indicated that mechanosensation might contribute to hygrosensation, the cellular basis of hygrosensation and the genes involved in detecting humidity remain unknown. To understand better the molecular bases of humidity sensing,(Liu et al., 2007b) investigated several genes encoding channels associated with mechanosensation, thermosensing or water transport. They identified two Drosophila melanogaster transient receptor potential channels needed for sensing humidity: CG31284, named water witch (wtrw), which is required to detect moist air, and nanchung (nan), which is involved in detecting dry air. Neurons associated with specialized sensory hairs in the third segment of the antenna express these channels. Neurons expressing wtrw and nan project to central nervous system regions associated with mechanosensation (Liu et al., 2007b). The six TRP channels of dinoflagelates do not appear to be mechanoreceptors but rather are components of a mechanotransduction signaling pathway and may be activated via a PLC-dependent mechanism (Lindström et al. 2017).
TRP channels are calcium-permeable nonselective cation channels with six TMS domains and a putative pore loop between TMSs 5 and 6 (Hu et al., 2012). About 28 mammalian TRP channels have been identified, with different numbers of splicing variants for each channel gene. TRP channels have been classified into six different subgroups, including TRPV (1-6), TRPM (1-8), TRPC (1-7), TRPA1, TRPP (1-3), and TRPML (1-3), according to their sequence similarities. In general, TRP channels are involved in calcium handling (e.g., intracellular calcium mobilization and calcium reabsorption) and a broad range of sensory modalities, including pain, temperature, taste, etc. TRP channelopathies are part of important mechanisms in a variety of diseases such as neurodegenerative disorders, diabetes mellitus, inflammatory bowel diseases, epilepsy, cancer, etc. Several members of the TRP family, TRPV1-4, TRPM8, and TRPA1, also called 'ThermoTRPs,' are involved in the detection of temperature changes, thus acting as the molecular thermometers of our body. They are also polymodal nociceptors that integrate painful stimuli such as noxious temperatures and chemical insults. For example, the TRPV1 channel mediates thermal hyperalgesia and pain induced by capsaicin and acid. TRPA1 is a nociceptor that integrates many noxious environmental stimuli including oxidants and electrophilic agents. Gene deletion animals have been created to study the role of TRP channels in pain and nociception; involvement of TRPV1, TRPV3, TRPV4, and TRPA1 in nociception has been confirmed (Hu et al., 2012).
A class of ion channels that belongs to the transient receptor potential (TRP) superfamily and is present in specialized neurons are temperature detectors. These channels are classified into subfamilies, namely canonical (TRPC), melastatin (TRPM), ankyrin (TRPA), and vanilloid (TRPV). Some of these channels are activated by heat (TRPM2/4/5, TRPV1-4), while others by cold (TRPA1, TRPC5, and TRPM8) (Baez et al. 2014). These channels resemble voltage-dependent K+ channels, with their subunits containing six transmembrane segments that form tetramers. Thermal TRP channels are polymodal receptors that can be activated by temperature, voltage, pH, lipids, and agonists. Their high temperature sensitivity is due to a large enthalpy change ( approximately 100 kcal/mol), which is about five times the enthalpy change in voltage-dependent gating. Pi-helices in TRP channels probably function in gating (Zubcevic and Lee 2019).
TRPV cation channels are polymodal sensors involved in a variety of physiological processes. TRPV2 is regulated by temperature, ligands such as probenecid and cannabinoids, and lipids. It may play a role in somatosensation, osmosensation and innate immunity. Zubcevic et al. 2016 presented the atomic model of rabbit TRPV2 in its putative desensitized state, as determined by cryo-EM at 4 A resolution. TMS6 (S6), which is involved in gate opening, adopts a conformation different from the one observed in TRPV1. Structural comparisons of TRPV1 and TRPV2 indicate that a rotation of the ankyrin-repeat domain is coupled to pore opening via the TRP domain, and this pore opening can be modulated by rearrangements in the secondary structure of S6.
Plasma membrane ion channels, and in particular TRPC channels, need a specific membrane environment and association with scaffolding, signaling, and cytoskeleton proteins in order to play their important functional roles. TRPC proteins are incorporated into macromolecular complexes including Ca2+ signaling proteins and proteins involved in vesicle trafficking, cytoskeletal interactions, and scaffolding. Association of TRPC with calmodulin (CaM), IP3R, PMCA, Gq/11, RhoA, and a variety of scaffolding proteins has been demonstrated. The interactions between TRPC channels and adaptor proteins determines their modes of regulation as well as their cellular localizations and functions. Adaptor proteins are involved in assembling Ca2+signaling complexes, in the correct sub-cellular localization of protein partners, and in the regulation of TRPC channelosome.The S4 - S5 linker is the gear box of TRP channel gating, and many pathogenic mutations occur in this region (Hofmann et al. 2017). High resolution structures are known for TRPV1, TRPV2, TRPV6, TRPA1, and TRPP2 (Hofmann et al. 2017).
Mechanosensory transduction for senses such as proprioception, touch, balance, acceleration, hearing and pain relies on mechanotransduction channels, which convert mechanical stimuli into electrical signals in specialized sensory cells. There are two major models. One is the membrane-tension model: force applied to the membrane generates a change in membrane tension that is sufficient to gate the channel, as in bacterial MscL channels (TC# 1.A.22) and certain eukaryotic potassium channels (TC# 1.A.1). The other is the tether model: force is transmitted via a tether to gate the channel. The transient receptor potential (TRP) channel NOMPC is important for mechanosensation-related behaviours such as locomotion, touch and sound sensation across different species including Caenorhabditis elegans, Drosophila and zebrafish. NOMPC is the founding member of the TRPN subfamily, and is thought to be gated by tethering of its ankyrin repeat domain to microtubules of the cytoskeleton (Jin et al. 2017).
The generalized transport reaction catalyzed by TRP-CC family members is:
Ca2+ (out) ⇌ Ca2+ (in)
C+ and Ca2+ (out) ⇌ C+ and Ca2+ (in).
Hu H, Bandell M, Grandl J, Petrus M. (2012) http://www.ncbi.nlm.nih.gov/pubmed?term=22593966
Transient receptor potential (TRP) protein. Assembles in vivo as a homomultimeric channel, not as a heteromeric channel with TrpL as the subunit (Katz et al. 2013).
TRP protein of Drosophila melanogaster (P19334)
Trp-2 channel; controls nicotine-dependent behavior (Xiao and Xu 2009). The TRPC orthologues TRP-1 and -2 genetically complement the loss of syndecan by suppressing neuronal guidance and locomotory defects related to increases in neuronal calcium levels. The widespread and conserved syndecan-TRPC axis therefore fine tunes cytoskeletal organization and cell behavior (Gopal et al. 2015).
Trp-2 of Caenorhabditis elegans
TRP channel homologue, Trp1, of 766 aas and 6 - 9 TMSs. Contains Ankyrin - PKD1 - TrpC channel domains. Exhibits properties of mammalian signal transduction Trp channels (Arias-Darraz et al. 2015).
TRP channel homologue of Chlamydomonas reinhardtii (Chlamydomonas smithii)
TrpC4 of 977aas. In epidermal keratinocytes, a syndecan-TRPC4 complex controls adhesion, adherens junction composition, and early differentiation in vivo and in vitro (Gopal et al. 2015). Constitutively active TRPC1/C4-dependent background Ca2+ entry fine-tunes Ca2+ cycling in beating adult cardiomyocytes. Double TRPC1/C4-gene inactivation protects against development of maladaptive cardiac remodelling without altering cardiac or extracardiac functions contributing to this pathogenesis (Camacho Londoño et al. 2015). A cryo-EM structure of TRPC4 in its unliganded (apo) state has beeen solved to an overall resolution of 3.3 A. It reveals a unique architecture with a long pore loop stabilized by a disulfide bond. Beyond the shared tetrameric six-transmembrane fold, the TRPC4 structure deviates from other TRP channels with a unique N-terminal cytosolic domain which forms extensive aromatic contacts with the TRP and the C-terminal domains (Duan et al. 2018).
TrpC4 of Homo sapiens
Transient receptor potential ion channel protein, TRP6, OF 2341 aas and 6 - 9 TMSs.
TRP6 OF Chlamydomonas reinhardtii (Chlamydomonas smithii)
Flagellar associated calcium channel protein of 1,729 aas, FAP148 (Wheeler and Brownlee 2008).
FAP148 of Chlamydomonas reinhardtii
Transient potential protein-gamma, Trpγ, of 1128 aas and 10 TMSs. A light-sensitive cation/calcium channel that is required for inositide-mediated Ca2+ entry in the retina during phospholipase C (PLC)-mediated phototransduction. It forms a regulated cation channel when heteromultimerized with TrpL (Xu et al. 2000).
TrpL of Drosophila melanogaster (Fruit fly)
TRP7 receptor-activated capacitative Ca2+ entry channel
TRP7 of Mus musculus (Q9WVC5)
TRPC1 store-operated Ca2+ channel (Liu et al., 2003) (activated by the metabotropic [G- protein-dependent] glutamate receptor, mGluR1) (Kim et al., 2003) (controls salivary gland fluid secretion in mice (Liu et al., 2007a). Constitutively active TRPC1/C4-dependent background Ca2+ entry fine-tunes Ca2+ cycling in beating adult cardiomyocytes. Double TRPC1/C4-gene inactivation protects against development of maladaptive cardiac remodelling without altering cardiac or extracardiac functions contributing to this pathogenesis (Camacho Londoño et al. 2015). Regulated by drebrin (DBN1; 649 aas; Q16643) (Pabon et al. 2017). TRPC1 null mutations exacerbate memory loss and apoptosis induced by amyloid-beta (Li et al. 2018). Pulsed focused ultrasound (pFUS) acoustic radiation forces mechanically activate a Na+-containing TRPC1 (TC# 1.A.4..1.3) channel generating current upstream of voltage-gated Ca2+ channels (VGCC) rather than directly opening VGCC (Burks et al. 2019).
TRPC1 of Homo sapiens (P48995)
TRPC3 store-operated non-selective cation channel (activated by thapsigargin and 2 acyl glycerol; forms a heteromeric channel with TrpC1, TC #1.A.4.1.3) (Liu et al., 2005). A structural model of the TRPC3 permeation pathway based on a sodium channel (TC# 1.A.1.14.5) with a localized selectivity filter and an occluding gate with evidence for allosteric coupling between the gate and the selectivity filter has been proposed (Ko et al. 2009; Lichtenegger et al. 2013). The channel may have a large internal chamber surrounded by signal sensing antennas (Mio et al. 2007). TRPC channels are involved in store-operated calcium entry and calcium homeostasis, and they are implicated in human diseases such as neurodegenerative disease, cardiac hypertrophy, and spinocerebellar ataxia (Fan et al. 2018). The structure in a lipid-occupied, closed state has been solved at 3.3 Å resolution. TRPC3 has four elbow-like membrane reentrant helices prior to the first transmembrane helix. The TRP helix is perpendicular to, and thus disengaged from, the pore-lining S6, suggesting a different gating mechanism from other TRP subfamily channels. The third transmembrane helix S3 is remarkably long, shaping a unique transmembrane domain, and constituting an extracellular domain that may serve as a sensor of external stimuli. Fan et al. 2018 identified two lipid binding sites, one being sandwiched between the pre-S1 elbow and the S4-S5 linker, and the other being close to the ion-conducting pore, where the conserved LWF motif of the TRPC family is located. The cytoplasmic domain allosterically modulates channel gating (Sierra-Valdez et al. 2018). This channel may be present in mitochondria (Parrasia et al. 2019).
TRPC3 of Homo sapiens (Q13507)
Transient receptor potential canonical-6, TRPC6, a non-selective cation channel that is directly activated by diacylglycerol (DAG (Szabó et al. 2015). Mutation causes a particularly aggressive form of familial focal segmental glomerulosclerosis (Winn et al., 2005; Mukerji et al., 2007). Tang et al. 2018 presented the structure of the human TRPC6 homotetramer in complex with a high-affinity inhibitor, BTDM, solved by single-particle cryo-electron microscopy to 3.8 Å resolution. The structure shows a two-layer architecture in which the bell-shaped cytosolic layer holds the transmembrane layer. Extensive inter-subunit interactions of cytosolic domains, including the N-terminal ankyrin repeats and the C-terminal coiled-coil, contribute to the tetramer assembly. The high-affinity inhibitor BTDM wedges between the S5-S6 pore domain and voltage sensor-like domain to inhibit channel opening (Tang et al. 2018). TRPC6 may regulate the glomerular filtration rate by modulating mesangial cell contractile function through multiple Ca2+ signaling pathways (Li et al. 2017).
TRPC6 of Homo sapiens (Q9Y210)
Sperm TRP-3 (SPE-41) Ca2+-permeable channel. Translocated from vesicles to the plasma membrane upon sperm activation in a process dependent on the 4TMS SPE-38 protein (8.A.36.1.1) (Singaravelu et al., 2012) during sperm-egg interactions leading to fertilization (Xu et al., 2003).
TRP-3 of Caenorhabditis elegans (AAQ22724)
Short transient receptor channel 5 (TrpC5 or Htrp5) (transports Ca2+ and Sr2+ in the presence of Orai1 and STIM1 (TC# 1.A.52.1.1) (Ma et al., 2008). It is a cold-transducer in the peripheral nervous system (Zimmermann et al., 2011). A small-molecule inhibitor suppresses progressive kidney disease in rats (Zhou et al. 2017). ORAI and TRP, and the transmembrane Ca2+ sensors, stromal interaction molecules (STIMs), are involved in thrombosis and thrombo-inflammation in platelets and immune cells. Disregulated store-operated Ca2+ (SOCE) fluxes in platelets and immune cells are responsible, and the potential of SOCE inhibition as a therapeutic option to prevent or treat arterial thrombosis as well as thrombo-inflammatory disease states such as ischemic stroke have been considered (Mammadova-Bach et al. 2019).
TrpC5 of Homo sapiens (Q9UL62)
TrpL (Trp-like), isoform A (1124 aas). A light-sensitive calcium channel that is required for inositide-mediated Ca2+ entry in the retina during phospholipase C (PLC)-mediated phototransduction (Lan et al. 1998; Chyb et al. 1999). It is required for vision in the dark and in dim light. and binds calmodulin. Trp and TrpL act together in the light response (Bähner et al. 2002). TrpL assembles in vivo as a homo-multimeric channe, not as a hetero-meric channels as reported previously (Katz et al. 2013).
TrpL of Drosophila melanogaster (P48994)
Trp-1 isoform channel; controls nicotne-dependent behavior (Xiao and Xu 2009). TRPC orthologues TRP-1 and -2 genetically complement the loss of syndecan by suppressing neuronal guidance and locomotory defects related to increases in neuronal calcium levels. The widespread and conserved syndecan-TRPC axis therefore fine tunes cytoskeletal organization and cell behavior (Gopal et al. 2015).
Trp-1 of Caenorhabditis elegans
TRP cation-slective channel homologue of 1177 aas
TRP channel homologue of Chlamydomonas reinhardtii (Chlamydomonas smithii)
TRP channel homologue of 962 aas
TRP channel homologue of Oxytricha trifallax
TRP channel homologue of 1486 aas
TRP channel homologue of Volvox carteri
Vanilloid receptor subtype 1 (VR1 or TRPV1) (noxious, heat-sensitive [opens with increasing temperatures; e.g., >42°C]; also sensitive to acidic pH and voltage and inflamation; serves as the receptor for the alkaloid irritant, capsaicin, for resiniferatoxin and for endo-cannabinoids (Murillo-Rodriguez et al. 2017). It is regulated by bradykinin and prostaglandin E2) (contains a C-terminal region, adjacent to the channel gate, that determines the coupling of stimulus sensing and channel opening (Garcia-Sanz et al., 2007; Matta and Ahern, 2007). Activated and sensitized by local anesthetics in sensory neurons (Leffler et al., 2008). A bivalent tarantula toxin activates the capsaicin receptor (TRPV1) by targeting the outer pore domain (Bohlen et al., 2010). Single-channel properties of TRPV1 are modulated by phosphorylation (Studer and McNaughton, 2010). TRPV1 mediates an itch associated response (Kim et al., 2011). The thermosensitive TRP channel pore turret is part of the temperature activation apparatus (Yang et al., 2010). Modular thermal sensors in temperature-gated transient receptor potential (TRP) channels have been identified (Yao et al., 2011). TRPV1 opening is associated with major structural rearrangements in the outer pore, including the pore helix and selectivity filter, as well as pronounced dilation of a hydrophobic constriction at the lower gate, suggesting a dual gating mechanism (Cao et al. 2013). Allosteric coupling between upper and lower gates may account for modulation exhibited by TRPV1 and other TRP channels (Liao et al. 2013). Regulates longevity and metabolism by neuropeptides in mice (Riera et al. 2014). The pore of TRPV1 contains the structural elements sufficient for activation by noxious heat (Zhang et al. 2017). In bull sperm, TRPV1 functions in the regulation of motility and the acrosome reaction (Kumar et al. 2019). The dynamics of water in the transmembrane pore of TRPV1 have been studied (Trofimov et al. 2019). TRPV1 - 6 channel subunits do not combine arbitrarily. With the exception of TRPV5 and TRPV6, TRPV channel subunits preferentially assemble into homomeric complexes (Hellwig et al. 2005).
VR1 of Rattus norvegicus
TRPV5 epithelial Ca2+ channel (ECaC1) (forms homo- and heterotetrameric channels with TRPV6; requires the S100A10-annexin 2 complex for routing to the plasma membrane) (Hoenderop et al., 2003; van de Graaf et al., 2003). The kidney maintains whole body calcium homoeostasis due to the reabsorption of Ca2+ filtered by the kidney glomerulus. TRPV5 regulates urinary Ca2+ excretion by mediating active Ca2+ reabsorption in the distal convoluted tubule of the kidney. The histidine kinase, nucleoside diphosphate kinase B (NDPK-B), activates TRPV5 channel activity and Ca2+ flux, and this activation requires histidine 711 in the carboxy terminal tail of TRPV5. In addition, the histidine phosphatase, protein histidine phosphatase 1 (PHPT1), inhibits NDPK-B activated TRPV5 (Cai et al. 2014). TRPV5 also transports cadmium (Cd2+). The L530R mutation is associated with recurrent kidney stones (Wang et al. 2017). May be stabilized by Mucin-1 (Muc1; P15941) (Al-Bataineh et al. 2017). TRPV5 inhibitors have been identified (Hughes et al. 2019).
TRPV5 of Homo sapiens (NP_062815)
TRPV6 epithelial Ca2+ channel (ECaC2) (forms homo- and heterotetrameric channels with TRPV5; requires the S100A10-annexin 2 complex for routing to the plasma membrane) (Hoenderop et al., 2003; van de Graaf et al., 2003). Epithelial TrpV6, but not TrpV5, is inhibited by the regulator of G-protein signaling 2 (RGS2; Q9JHX0; 211 aas) by direct binding (Schoeber et al., 2006). Cyclophilin B is an accessory activating protein (Stumpf et al., 2008). The crystal structure of rat TRPV6 at 3.25 A resolution revealed shared and unique features compared with other TRP channels (Saotome et al. 2016). Intracellular domains engage in extensive interactions to form an intracellular 'skirt' involved in allosteric modulation. In the K+ channel-like transmembrane domain, Ca2+ selectivity is determined by direct coordination of Ca2+ by a ring of aspartate side chains in the selectivity filter (Saotome et al. 2016). Replacing Gly-516 within the cytosolic S4-S5 linker (conserved in all TRP channel proteins) by ser forces the channels into an open conformation, thereby enhancing constitutive Ca2+ entry and preventing inactivation (Hofmann et al. 2016). Tetrameric ion channels have either swapped or non-swapped arrangements of the S1-S4 and pore domains. Singh et al. 2017 showed that mutations in the transmembrane domain can result in conversion from a domain-swapped to the non-swapped fold. These results raise the possibility that a single ion channel subtype can fold into either arrangement in vivo, affecting its function in normal or disease states. Cryo-EM structures of human TRPV6 in the open and closed states shows that the channel selectivity filter adopts similar conformations in both states, consistent with its explicit role in ion permeation. The iris-like channel opening is accompanied by an alpha-to-pi-helical transition in the pore-lining transmembrane helix S6 at an alanine hinge just below the selectivity filter. As a result of this transition, the S6 helices bend and rotate, exposing different residues to the ion channel pore in the open and closed states (McGoldrick et al. 2017). TRPV6 is an epithelial Ca2+-selective channel associated with transient neonatal hyperparathyroidism (TNHP), an autosomal-recessive disease caused by TRPV6 mutations that affect maternal-fetal calcium transport (Suzuki et al. 2018). TRPV6 mediates calcium uptake in epithelia, and its expression increases in numerous types of cancer while inhibitors suppress tumor growth. Singh et al. 2018 presented crystal and cryo-EM structures of human and rat TRPV6 bound to 2-aminoethoxydiphenyl borate (2-APB), a TRPV6 inhibitor and modulator of numerous TRP channels. 2-APB binds to TRPV6 in a pocket formed by the cytoplasmic half of the S1-S4 transmembrane helix bundle. 2-APB induces TRPV6 channel closure by modulating protein-lipid interactions. The 2-APB binding site may be present in other members of vanilloid subfamily TRP channels. The crystal structure has been determined (PubMed: 30299652 and Yelshanskaya et al. 2020).
TRPV6 of Homo sapiens (NP_071858)
TrpV1 of 839 aas. Ligand-activated non-selective calcium permeant cation channel involved in detection of noxious chemical and thermal stimuli. TRPV1 channels are present in odontoblasts, suggesting that odontoblasts may directly respond to noxious stimuli such as a thermal-heat stimulus (Okumura et al. 2005). It may mediate proton influx and be involved in intracellular acidosis in nociceptive neurons. It is also involved in mediating inflammatory pain and hyperalgesia (Benemei et al. 2015). The 3.4 Å resolution structure shows that the overall fold is the same as for voltage-gated ion channels (TC# 1.A.1) (Liao et al. 2013). Capsaicin-induced apoptosis in Glioma cells is mediated by TRPV1 (Amantini et al. 2007). Capsaicin binds to a pocket formed by the channel's TMSs, where it takes a ""tail-up, head-down"" configuration. Binding is mediated by both hydrogen bonds and van der Waals interactions. Upon binding, capsaicin stabilizes the open state of TRPV1 by ""pull-and-contact"" with the S4-S5 linker (Yang and Zheng 2017). Several protein kinases, including PKD1 (protein kinase D1), Cdk5 (cyclin-dependent kinase 5) and LIMK (LIM- motif containing kinase) regulate TRPV1 and inflammatory thermal hyperalgesia (Zhang and Wang 2017). TrpV1 and TrpA1 are inflammatory mediators causing cutaneous chronic itch in several diseases (Xie and Li 2018). The locations and characteristics of volatile general anesthetic binding sites in the transmembrane domain of TRPV1 have been examined (Jorgensen and Domene 2018). The TRPV1 ion channel is a neuronal sensor that plays an important role in nociception and neuropathic as well as inflammatory pain. In clinical trials, hyperthermia and thermo-hypoaesthesia are major side effects of TRPV1 antagonists (Damann et al. 2020). The TRPV1 ion channel is a polymodal sensor integrating stimuli from molecular modulators with temperature, pH and transmembrane potential. Temperature dependent gating may constitute the molecular basis for its role in heat sensation and body temperature regulation. Damann et al. 2020 characterized the prototypic smallmolecule TRPV1 inhibitors GRT12360V and GRTE16523.
TrpV1 of Homo sapiens
Epithelial calcium channel 2, ECaC2; TrpV6 of 719 aas and 6 TMSs. It displays all structural features typical for mammalian ECaCs including three ankyrin repeats, six transmembrane domains, and a putative pore region between TM V and TM VI (Qiu and Hogstrand 2004).
ECaC2 of Takifugu rubripes (Japanese pufferfish) (Fugu rubripes)
Stretch-inhibitable non-selective cation channel, SIC
SIC of Rattus norvegicus
Vitamin D-responsive, apical, epithelial Ca2+ channel, ECaC
ECaC of Oryctolagus cuniculus
Insulin-like growth factor I-regulated Ca2+ channel
IGF-regulated Ca2+ channel of Mus musculus
Vanilloid receptor-related, osmotically activated channel, VR-OAC (also called TRPV4 and Trp12); required for bladder voiding in mice (Gevaert et al., 2007). Regulated by Pacsin3 via its SH3 domain which affects its subcellular localization and inhibits its activity in a stimulus-specific fashion (D'hoedt et al., 2008). Responsible for autosomal dominant brachyolmia (Rock et al., 2008). Multiple gating mechanisms have been demonstrated for TRPV4 (Loukin et al., 2010). TRPV4 Ca2+ signalling regulates endothelial vascular function (Sonkusare et al., 2012) and adipose oxidative metabolism, inflammation and energy homeostasis (Ye et al. 2012). H2O2 induces Ca2+ influx into microvascular endothelial cells via TrpV4 (Suresh et al. 2015). TrpV4 orthologs are volume-sensors, rather than osmo-sensors (Toft-Bertelsen et al. 2017) that mediate fluid secretion by the ciliary body. They are important for vertebrate vision by providing nutritive support to the cornea and lens, and by maintaining intraocular pressure (Jo et al. 2016). Interacts with the A-kinase anchor protein 5 (AKAP5 or AKAP79 of 427 aas; TC# 8.A.28.1.6; P24588) (Mack and Fischer 2017). Mutations in TRPV4 are associated with accelerated chondrogenic differentiation of dental pulp stem cells (Nonaka et al. 2019). The homolog in Cynops pyrrhogaster (85% identical) is inhibited by RN1734 and may play a role in the sperm acrosome reaction (Kon et al. 2019).
VR-OAC of Rattus norvegicus
Intestinal endocyte Ca2+ (Sr2+; Ba2+) entry channel, CaT1. Excision of the Trpv6 gene leads to severe defects in epididymal Ca2+ absorption and male fertility as does the single D541A pore mutation (Weissgerber et al., 2012).
CaT1 of Rattus norvegicus
The noxious heat (>52°C)-sensitive vanilloid-like receptor cation selective channel, TRPV2. Ca2+-dependent desensitization of TRPV2 channels is mediated by hydrolysis of phosphatidylinositol 4,5-bisphosphate (Mercado et al., 2010). Deleting the first N-terminal 74 residues preceding the ankyrin repeat domain (ARD) shows a key role for this region in targeting the protein to the membrane. Co-translational insertion of the membrane-embedded region occurs with the TM1-TM4 and TM5-TM6 regions assembling as independent folding domains. ARD is not required for TM domain insertion into the membrane (Doñate-Macian et al. 2015). The TRPV2 structure has been solved at 4 Å resolution by cryoEM (Zubcevic et al. 2016). Formation of a physical complex between mouse TRPV2 (GRC) and the mouse RGA protein promotes cell surface expression of TRPV2 (Stokes et al. 2005).
TRPV2 of Homo sapiens
The temperature (heat; >39°C)-sensitive, capsaicin-insensitive receptor cation-selective channel, TRPV3 or TRL3 (may form heterooligomers with VR1 (TRPV1; TC #1.A.4.2.1)). Incensole acetate, an incense component, elicits psychoactivity by activating TRPV3 channels in the brain (Moussaieff et al., 2008). TRPV3 is activated by synthetic small-molecule chemicals and natural compounds from plants as well as warm temperatures. Its function is regulated by a variety of physiological factors including extracellular divalent cations and acidic pH, intracellular ATP, membrane voltage, and arachidonic acid. It shows a broad expression pattern in both neuronal and non-neuronal tissues including epidermal keratinocytes, epithelial cells in the gut, endothelial cells in blood vessels, and neurons in dorsal root ganglia and the CNS. TRPV3 null mice exhibit abnormal hair morphogenesis and compromised skin barrier function, and it may play critical roles in inflammatory skin disorders, itch, and pain sensation (Luo and Hu 2014). TRPV3 gating involves large rearrangements at the cytoplasmic inter-protomer interface, and this motion triggers coupling between cytoplasmic and transmembrane domains, priming the channel for opening (Zubcevic et al. 2019). Mutations in TRPV3 cause painful focal plantar keratoderma (Peters et al. 2020).
TRPV3 of Homo sapiens
Olfactory, mechanosensitive channel. Forms a complex with Stim1 and Orai1 (TC# 1.A.52.1.1) which is required for SOC currents (Cheng et al., 2008) (most similar to 1.A.4.8.1, but both are most closely related to 1.A.4.2). Serves as a chemo-, osmo- and touch sensation receptor (Xiao and Xu 2009).
Olfactory channel of Caenorhabditis elegans
The Nanchung (Nan) hearing ion channel; mediates hypo-osmotically activated Ca2+ influx in chordotonal neurons of insects (Kim et al., 2003). Nanchung is the "dry" humidity receptor, one of two hygrosensation receptors. These two transient receptor potential channels are needed for sensing humidity. The other is Water witch (Wtrw), involved in detecting moist air. Neurons associated with specialized sensory hairs in the third segment of the antenna express these channels, and neurons expressing Wtrw and Nan project to central nervous system regions associated with mechanosensation. Construction of the hygrosensing system with opposing receptors may allow an organism to very sensitively detect changes in environmental humidity (Liu et al. 2007). Two commercial insecticides, pymetrozine and pyrifluquinazon, target the heteromeric TRPV ion channel complex which is specifically expressed in the chordotonal organ neurons in Drosophila species and may play roles in male-specific behavior (Mao et al. 2018).
Nan of Drosophila melanogaster (833 aas; Q9VUD5)
TrpV-type Osm-2 (OSM2) chemo-, osmo- and touch sensation receptor channel (Xiao and Xu 2009).
Osm-2 of Caenorhabditis elegans
TRP channel homologue of 1240 aas
TRP channel homologue of Ectocarpus siliculosus
TRP channel homologue of 1724 aas
TRP channel homologue of Ectocarpus siliculosus (Brown alga)
Vacuolar, voltage-dependent cation-selective, Ca2+-activated channel, YVC1. (Yeast vacuolar conductance protein 1; also called TrpY1; Yor088w) (Chang et al., 2009). Activated by stretch to release vacuolar Ca2+ into the cytoplasm upon osmotic upshock (Zhou et al. 2005). (Also activated by glucose, indole and other aromatic compounds (Haynes et al., 2008; Groppi et al. 2011)). Glutathione activates by reversible glutathionylation of specific cysteyl residues in YVC1 (Chandel et al. 2016). Channel activity is activated by cytoplasmic Ca2+ and inhibited by vacuolar lumen Ca2+, and two residues, D401 and D405, are involved in Ca2+ sensing in the lumen (Amini et al. 2018).
YVC1 (Yor088w) of Saccharomyces cerevisiae (Q12324)
Yvc1 or TrpY2 of 678 aas and 9 apparent TMSs. It has the same mechanosenstivity as does the S. cereviseae ortholog (Zhou et al. 2005). 45% identical to the latter protein.
Yvc1 of Kluyveromyces lactis
Yvc-1, Yvc1 or TrpY3 of 676 aas and 9 apparent TMSs. It has the same mechanosensitive properties of the S. cerevisiae ortholog with TC# 1.A.4.4.1 (Zhou et al. 2005). 57% identical to the latter protein.
TrpY3 of Candida albicans
Mg2+-selective channel/kinase-1; Mg2+-ATP-regulated divalent cation channel, LTRPC7, TRPM7, or TRP-PLIK, of 1862 aas. Bradykinin regulates TRPM7 and its downstream target annexin-1 through a phospholipase C-dependent, protein kinase C-dependent and c-Src-dependent pathway that is cAMP-independent; effects are mediated through the bradykinin type 2 receptor (Callera et al. 2009). TRPM7 is a Mg2+ sensor and transducer of signaling pathways during stressful environmental conditions. Its kinase can act on its own in chromatin remodeling processes, but TRPM6's kinase activity regulates intracellular trafficking of TRPM7 and TRPM7-dependent cell growth (Cabezas-Bratesco et al. 2015). Syndecans (proteoglycans) regulate TRPC channels to control cytosolic calcium equilibria and consequent cell behavior. In fibroblasts, ligand interactions with heparan sulfate of syndecan-4 recruit cytoplasmic protein kinase C to target serine714 of TRPC7 with subsequent control of the cytoskeleton and the myofibroblast phenotype (Gopal et al. 2015). May be associated with melanocytic tumors. Phenanthrenes, naltriben derivatives, are stimulatory agonist of the TRPM7 channel (Liu et al. 2016). Transient receptor potential melastatin 7, TRP7, ion channels regulate magnesium homeostasis in vascular smooth muscle cells, and its activity is positively regulated by aldosterone and angiotensin II (He et al. 2005).
Channel-kinase-1 (LTRPC7) of Homo sapiens
TrpCC family member, Gon2. Required for initiation and continuation of postembryonic mitotic cell division of gonadal cells Z1 and Z4. Zygotic expression is necessary for hermaphrodite fertility. Probably a cation channel that functions together with Gem1 (TC#2.A.1.13.22) (Kemp et al. 2009).
Gon-2 of Caenorhabditis elegans
Transient receptor potential cation channel subfamily M member 8, TrpM8, the primary cold and menthol receptor in humans. The structure has been solved for the collared flycatcher at 4.1 Å resolution (6BPQ_A - D (Yin et al. 2017).
TrpM8 of Ficedula albicollis (collared flycatcher)
TrpM4 of 1213 aas and 6 TMSs. Calcium-activated non selective cation channel that mediates membrane depolarization. While it is activated by increases in intracellular Ca2+, it is impermeable to it. It does mediate transport of monovalent cations (Na+ > K+ > Cs+ > Li+), leading to depolarize the membrane. It thereby plays a central role in the function of cardiomyocytes, neurons from entorhinal cortex, dorsal root and vomeronasal neurons, endocrine pancreas cells, kidney epithelial cells, cochlea hair cells etc. It also participates in T-cell activation by modulating Ca2+ oscillations after T lymphocyte activation (Demion et al. 2007). The structure has been determined by cryo EM both with and without ATP (Guo et al. 2017). It consists of multiple transmembrane and cytosolic domains, which assemble into a three-tiered architecture. The N-terminal nucleotide-binding domain and the C-terminal coiled-coil participate in the tetrameric assembly of the channel; ATP binds at the nucleotide-binding domain to inhibit channel activity. TRPM4 has an exceptionally wide filter although it is only permeable to monovalent cations; filter residue Gln973 is essential in defining monovalent selectivity. The S1-S4 domain and the post-S6 TRP domain form the central gating apparatus that probably houses the Ca2+- and PtdIns(4,5)P2-binding sites (Guo et al. 2017). TRPM4 currents are activated by micromolar concentrations of cytoplasmic Ca2+and progressively desensitized. Zhang et al. 2005 showed that desensitization can be explained by a loss of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) from the channels.
TRPM4 of Mus musculus
TRPM8 of the collared flycatcher of 1103 aas. It is 83% identical to the human ortholog. Its structure has been determined to ~4.1 Å resolution by cryo EM (Yin et al. 2018). The structure reveals a three-layered architecture. The amino-terminal domain with a fold distinct among known TRP structures, together with the carboxyl-terminal region, forms a large two-layered cytosolic ring that extensively interacts with the transmembrane channel layer. The structure suggests that the menthol-binding site is located within the voltage-sensor-like domain and thus provides a structural glimpse of the design principle of the molecular transducer for cold and menthol sensation (Yin et al. 2018).
TRP8 of Ficedula albicollis (Collared flycatcher) (Muscicapa albicollis)
Melastatin 1 or transient receptor potential melastatin-1 (TRPM1; LTRPC1, MLSN, MLSN1) (a non-selective, Ca2+-permeable cation channel, implicated in cell death (Wilkinson et al., 2008). Required for dim light vision. Purified TRPM1 is mostly dimeric. The three-dimensional structure of TRPM1 dimers is characterized by a small putative transmembrane domain and a larger domain with a hollow cavity (Agosto et al. 2014). Since dimers are not likely to be functional ion channels, the authors suggested that additional partner subunits participate in forming the transduction channel required for dim light vision and the ON pathway. The N-terminal region of TRPM1 (residues L242 to E344) regulates activity by direct interaction by the S100A1 calcium-binding protein (TC# 8.A.81) (Jirku et al. 2016). TRPM1 is required for synaptic transmission between photoreceptors and the ON subtype of bipolar cells (Agosto et al. 2018).
Melastatin 1 of Homo sapiens
MLSN1- and TRP-related MTR1 (TrpM5; LTRPC5) of 1165 aas and 6 TMSs. Associated with the Beckman-Wiedemann Syndrum and causes a predisposition for neoplasia (Prawitt et al. 2000). Involved in taste to bitter, sweet and umami, but not absolutely required for some of these. Thus, Trpm5-dependent and Trpm5-independent pathways underlie bitter, sweet, and umami tastes (Damak et al. 2006). Voltage-modulated Ca2+-activated, monovalent cation (Na+, K+, Cs+) channel (VCAM) that mediates transient membrane depolarization. It is blocked by extracellular acidification but activated by arachidonic acid (Prawitt et al. 2003).
MTR1 of Homo sapiens
Intracellular Ca2+-activated nonselective monovalent cation (Na+ and K+) channel (non-permeable to Ca2+), TRPM4b. Forms a protein-protein interaction with the TRPC3 channel and suppresses store-operated Ca+ entry (Park et al., 2008). Contributes to the mammalian atrial action potential (Simard et al. 2013). TRPM4 is widely expressed and is associated with a variety of cardiovascular disorders. Autzen et al. 2018 presented two structures of full-length human TRPM4 embedded in lipid nanodiscs at ~3-angstrom resolution, as determined by single-particle cryo-electron microscopy. These structures, with and without calcium bound, reveal the general architecture for this major subfamily of TRP channels and a well-defined calcium-binding site within the intracellular side of the S1-S4 domain. The structures correspond to two distinct closed states. Calcium binding induces conformational changes that likely prime the channel for voltage-dependent opening (Autzen et al. 2018). TRPM4 functions as a limiting factor for antigen evoked calcium rise in connective tissue type mast cells, and concurrent translocation of TRPM4 into the plasma membrane is part of this mechanism (Rixecker et al. 2016). Gain-of-function mutations in the TRPM4 activation gate cause progressive symmetric erythrokeratoderma (Wang et al. 2018).
TRPM4b of Homo sapiens
ADP-ribose/NAD/pyrimidine nucleotide-gated Ca2+ permeable, cation nonselective, long transient receptor potential channel-2, LTRPC2; Melastatin 2; TRPM2 (ATP inhibitable). The 3-D structure resembles a swollen bell shaped structure (Maruyama et al., 2007). It can be converted to an anion-selective channel by introducing a lysyl residue in TMS 6 (Kuhn et al., 2007). It transports Ca2+ and Mg2+ with equal facility (Xia et al., 2008). Four Ca2+ ions activate TRPM2 channels by binding in deep crevices near the pore but intracellularly of the gate (Csanády and Törocsik, 2009). Protons also regulate activity (Starkus et al., 2010). It is present in the plasma membrane and lysosomes, and plays a role in ROS-induced inflammatory processes and cell death. Melastatin is required for innate immunity against Listeria monocytogenes (Knowles et al., 2011). It functions in pathogen-evoked phagocyte activation, postischemic neuronal apoptosis, and glucose-evoked insulin secretion, by linking these cellular responses to oxidative stress (Tóth and Csanády, 2012). Pore collapse upon prolonged stimulation underlies irreversible inactivation (Tóth and Csanády 2012). TRPM2 is preferentially expressed in cells of the myeloid lineage and modulates signaling pathways converging into NF-kB but does not seem to play a major role in myeloid leukemogenesis. Its loss does not augment the cytotoxicity of standard AML chemotherapeutic agents (Haladyna et al. 2016). TrpM2, expressed in hypothalamic neurons in the brain is a thermosensitive, redox-sensitive channel, required for thermoregulation. It regulates body temperature, limiting fever and driving hypothermia (Song et al. 2016). Tseng et al. 2016 suggested a mechanistic link between TRPM2-mediated Ca2+ influx and p47 phox signaling to induce excess ROS production and TXNIP-mediated NLRP3 inflammasome activation under high gllucose in Type 2 diabetes Mellitus. The cryoEM strcuture reveals a C-terminal NUDT9 homology (NUDT9H) domain responsible for binding ADP-ribose(ADPR) (Wang et al. 2018). Both ADPR and Ca2+ are required for TRPM2 activation, and structures with ADPR and Ca2+ show both intra- and inter-subunit interactions with the N-terminal TRPM homology region (MHR1/2/3) in the apo state, but undergoing conformational changes upon ADPR binding, resulting in rotation of MHR1/2 and disruption of the inter-subunit interaction. Ca2+ binding further engages transmembrane helices and the conserved TRP helix to cause conformational changes at the MHR arm and the lower gating pore to potentiate channel opening (Wang et al. 2018). Consecutive structural rearrangements and channel activation are induced by binding of ADPR in two indispensable locations, and the binding of Ca2+ in the transmembrane domain (Huang et al. 2019). An N-terminal TRPC2 splice variant of 213 aas inhibits calcium influx (Chu et al. 2005). An antogonists of channel function has been identified (Cruz-Torres et al. 2020).
LTRPC2 of Homo sapiens
Transient receptor potential cation channel subfamily, member 3, TRPM3. It is subject to muscarinic receptor activation. An alternative ion permeation pathway in TRPM3 allows large inward currents upon hyperpolarization, independently of the central pore. Four residues in S4 (W982, R985, D988 and G991) are determinants of the properties of the alternative ion permeation pathway (Held et al. 2018). TRPM3 is a thermosensitive TRP channel, playing a central role in noxious heat sensation. Volitile anesthetics (VAs) inhibit TRPM3-mediated transmembrane currents. Chloroform, halothane, isoflurane and sevoflurane inhibited both the agonist-induced (pregnenolone sulfate, CIM0216) and heat-activated Ca2+ signals and transmembrane currents in a concentration dependent way in cells overexpressing recombinant TRPM3 (Kelemen et al. 2020). Among the tested VAs, halothane was the most potent blocker (IC50=0.52+/-0.05 mM). VAs exerted their effects on native TRPM3 channels expressed in sensory neurons of the dorsal root ganglia. While VAs activated certain sensory neurons independently of TRPM3, they strongly and reversibly inhibited the agonist-induced TRPM3 activity.
TrpM3 of Homo sapiens (Q9HCF6)
Cold-sensitive (opens with decreasing temperatures; e.g., <22°C) and menthol-sensitive cation-selective channel, transient receptor potential melastatin 8 (TRPM8). TRPM8 is activated by low temperatures and cooling agents such as menthol. It underlies the cold-induced excitation of sensory neurons. Its gating is regulated by voltage and lysophospholipids which induce prolonged channel opening (Vanden Abeele et al., 2006; Bautista et al., 2007; Matta and Ahern, 2007). It can be converted to an anion-selective channel by introducing a lysyl residue in TMS 6 (Kuhn et al., 2007). Gating of TRPM8 channels is activated by cold and chemical agonists in planar lipid bilayers (Zakharian et al., 2010). Residues involved in intra- and intersubunit interactions have been identified, and their link with channel activity, sensitivity to icilin, menthol and cold, and their impact on channel oligomerization have been measured (Bidaux et al. 2015). Targeting the small isoform of TRPM8 may be useful to fight prostate cancer (Bidaux et al. 2016). The human isoform is 83% identical to the TRPM8 of the collared flycatcher (TC# 1.A.4.5.13), the structure of which has been characterized to 4.1 Å resolution (Yin et al. 2018). Activation of TRPM8 by cooling compounds relies on allosteric actions of agonist and the membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP2). The cryoEM structures of TRPM8 in complex with the synthetic cooling compound icilin, PIP2, and Ca2+, as well as in complex with the menthol analog WS-12 and PIP2 revealed the binding sites for cooling agonists and PIP2 in TRPM8. PIP2 binds to TRPM8 in two different modes, which illustrate the mechanism of allosteric coupling between PIP2 and agonists.
4TMS-TRPM8 isoforms form functional channels in the ER and participate in regulation of the steady-state Ca2+ concentration in mitochondria and the ER. 4TMS-TRPM8 isoforms are ER Ca2+ release channels (Bidaux et al. 2018). Human PIRT (TC# 8.A.64) attenuates human TPRM8 conductance, unlike mouse PIRT, which enhances mouse TRPM8 conductance (Hilton et al. 2018). PIRT and the TRPM8 S1–S4 domain interact with a 1:1 binding stoichiometry, suggesting that a functional tetrameric TRPM8 channel has four PIRT-binding sites (Hilton et al. 2018). TRPM8 has been implicated in nociception and pain and is regarded as an attractive target for the pharmacological treatment of neuropathic pain syndromes. A series of analogues of N,N'-dibenzyl tryptamine 1, a potent TRPM8 antagonist, were made and studied. Molecular modeling studies identified the putative binding mode of these antagonists, suggesting that they could influence an interaction network between the S1-4 transmembrane segments and the TRP domains of the channel subunits (Bertamino et al. 2018). Cold sensitivity is due to nonconserved residues located within the pore loop (residues 526 - 556) (Pertusa et al. 2018). Diver et al. 2019 have presented cryo-EM structures of TRPM8 in ligand-free, antagonist- or calcium-bound forms, revealing how robust conformational changes give rise to two non-conducting states, closed and desensitized. A malleable ligand-binding pocket accommodates drugs of diverse chemical structures, and delineates the ion permeation pathway, including the contribution of lipids to pore architecture. Direct calcium binding mediates stimulus-evoked desensitization. Large rearrangements within the S4-S5 linker reposition the S1-S4 and pore domains relative to the TRP helix, suggesting a model for modulation of TRPM8 and possibly other TRP channels (Diver et al. 2019).
TRPM8 of Homo sapiens
The intestinal/renal Mg2+ absorption Mg2+ influx channel, Melastatin6 or TRPM6 (5x higher affinity for Mg2+ than Ca2+; regulated by internal Mg2+) (Voets et al., 2004). TRPM6 and its closest homologue TRPM7 (also a Mg2+-permeable cation channel) assemble to form a functional heterooligomeric channel (Chubanov et al., 2004). Mutations in TRPM6 promotes hypomagnesemia with secondary hypocalcemia (Chubanov et al., 2007). TRPM6 and the closely related TRPM7 are large channel-kinase proteins (Li et al., 2007; Schmitz et al., 2007). TRPM7 also transports protons competitively with Mg2+ and Ca2+ (Numata and Okada, 2008). Intracellular ATP regulates TRPM6 channel activity via its α-kinase domain independently of α-kinase activity (Thébault et al., 2008). Also plays a role in Zn2+ homeostasis and Zn2+- mediated neuronal injury (Inoue et al., 2010). The protein is cleaved to release a chromatin-modifying kinase (Krapivinsky et al. 2014). TRPM7 is a Mg2+ sensor and transducer of signaling pathways under stressful environmental conditions. Its kinase can act on its own in chromatin remodeling processes, but TRPM6's kinase activity regulates intracellular trafficking of TRPM7 and TRPM7-dependent cell growth (Cabezas-Bratesco et al. 2015). Residues involved in cation selectivity have been identified (Topala et al. 2007). eviewed by Schäffers et al. 2018. Calmodulin (CaM) and S100A1 share the same binding domain at the TRPM6 N-terminus although the ligand-binding mechanisms may be different (Zouharova et al. 2019). TRPM7 activation potentiates store-operated Ca2+ entry (SOCE) in enamel cells but requires ORAI (Souza Bomfim et al. 2020).
TRPM6 of Homo sapiens (NP_060132)
TRPM7 of Homo sapiens (TC #1.A.4.5.1)
Transient receptor potential cation channel TrpM
rpM of Drosophila melanogaster
Cold-activated cation channel in nociceptive sensory neurons, ANKTM1, with lower activation temperature (in the noxious cold range) than TRPM8 (TC #1.A.4.5.7) (Story et al., 2003). Also called TRPA1 (Acc #AAS78661) which translates sound into electric signals in the ear. It sits at the tips of cilia in the inner ear and allows passage of K+ and Ca2+ into the cell. Vibrations in the hair cause the channel to open and close. The frequency of the sound waves generate an electrical signal of the same frequency (Jordt et al., 2004). (Shows 25% identity with α-latrotoxin precursor (TC #1.C.18.104.22.168) in its N-terminal half.) TRPA1 is a polyunsaturated fatty acid sensor in mammals, but not in flies and fish (Motter and Ahern, 2012). TRPA1 is regulated by its N-terminal ankyrin repeat domain (Zayats et al., 2012).
ANKTM1 of Mus musculus (Q8BLA8)
Warm-activated thermosensory cation channel of insects, ThermoTRPV, ANKTM1 or TrpA1 (Viswanath et al., 2003). It is required to control activity during the warm part of the day (Roessingh et al. 2015). The TrpA1(A) transcript spliced with exon10b (TrpA1(A)10b) that is present in a subset of midgut enteroendocrine cells (EECs) is critical for uracil-dependent defecation of microorganisms (Du et al. 2016).
ANKTM1 of Drosophila melanogaster (1197 aas; Q7Z020)
The nociceptive neuron TRPA1 (Trp-ankyrin 1) senses peripheral damage by transmitting pain signals (activated by cold temperatures, pungent compounds and environmental irritants). Noxious compounds also activate through covalent modification of cysteyl residues (Macpherson et al., 2007). TRPA1 is an excitatory, nonselective cation channel implicated in somatosensory function, pain, and neurogenic inflammation. Through covalent modification of cysteine and lysine residues, TRPA1 can be activated by electrophilic compounds, including active ingredients of pungent natural products (e.g., allyl isothiocyanate), environmental irritants (e.g., acrolein), and endogenous ligands (4-hydroxynonenal) (Chen et al., 2008). General anesthetics activate TRPA1 nociceptive ion channels to enhance pain and inflammation (Matta et al., 2008; Leffler et al., 2011). TMS5 is a critical molecular determinant of menthol sensitivity (Xiao et al., 2008) and a variety of inhibitors which are analgesics. Another class of inhibitors are in the thiadiazole structural class of compounds, and they bind to the TRPA1 ankyrin repeat 6 (Tseng et al. 2018). Inhibitors are potential analgesics. The majority of TRPA1 inhibitors interact with the S5 transmembrane helices, forming part of the pore region of the channel. TRPA1 is a component of the nociceptive response to CO2 (Wang et al., 2010). TRPA1 is a polyunsaturated fatty acid sensor in mammals but not in flies and fish (Motter and Ahern, 2012). It is regulated by its N-terminal ankyrin repeat domain (Zayats et al., 2012). Mutations in TrpA1 cause alterred pain perception (Kremeyer et al. 2010). The hop compound, eudesmol, an oxygenated sesquiterpene, activates the channel (Ohara et al. 2015). These channels regulate heat and cold perception, mechanosensitivity, hearing, inflammation, pain, circadian rhythms, chemoreception, and other processes (Laursen et al. 2014). TRPA1 is a polymodal ion channel sensitive to temperature and chemical stimuli, but its resposes are species specific (Laursen et al. 2015). A probable binding site for general anesthetics has been identified (Ton et al. 2017), and specific residues involved in binding of the anesthetic, propofol, are known (Woll et al. 2017). TrpV1 and TrpA1 are inflammatory mediators causing cutaneous chronic itch in several diseases (Xie and Li 2018). TRPA1 is specifically activated by natural products including allyl isothiocyanate (mustard oil), cinnamaldehyde (cinnamon), allicin (garlic) and trans-anethole in Fennel Oil (FO) (Memon et al. 2019). Mutations in TRPA1 result in insensitivity to pain promoting algogens such as capsaicin, acid, and allyl isothiocyanate (AITC), have been documented (Eigenbrod et al. 2019). TRPA1 transduces noxious chemical stimuli into nociceptor electrical excitation and neuropeptide release, leading to pain and neurogenic inflammation. It is regulated by the membrane environment. Startek et al. 2019 found that mouse TRPA1 localizes to cholesterol-rich domains, and that cholesterol depletion decreases channel sensitivity to chemical agonists. Two structural motifs in TMSs 2 and 4 are involved in cholesterol interactions that are necessary for normal agonist sensitivity and plasma membrane localization. TRPA1 is an irritant sensor and a therapeutic target for treating pain, itch, and respiratory diseases. It can be activated by electrophilic compounds such as allyl isothiocyanate (AITC). A class of piperidine carboxamides (PIPCs) are potent noncovalent agonists (Chernov-Rogan et al. 2019). Saikosaponins are channel antogonists (Lee et al. 2019).
TRPA1 of Homo sapiens (O75762)
Thermosensitive TPR channel TRPA1 (TrpA-1) of 1211 aas. Detects a temperature drop promoting increased longevity. This requires TPRA1-mediated Ca2+ influx and activation of protein kinase C. Human TRPA1 (TC# 1.A.4.6.3) can functionally substitute for worm TRPA-1 in promoting longevity (Xiao et al. 2013). Also mediates touch sensation.
TRPA1 of Caenorhabditis elegans
Water witch (Wtrw) of 986 aas, the "moist" humidity receptor, one of two hygrosensation receptors. These two transient receptor potential channels are needed for sensing humidity. The other is Nanchung (Nan), involved in detecting dry air. Neurons associated with specialized sensory hairs in the third segment of the antenna express these channels, and neurons expressing Wtrw and Nan project to central nervous system regions associated with mechanosensation. Construction of the hygrosensing system with opposing receptors may allow an organism to very sensitively detect changes in environmental humidity (Liu et al. 2007).
WtrW of Drosophila melanogaster
TRP ankyrin 1 (TRPA1 of 1188 aas). It is a homotetrameric, non-selective, cation channel with multiple ankyrin repeats at the N-terminus. The systems from insects to birds are heat activatable, and this activation is dependent on an extracellular Ca2+ binding site near the vestibule surface. Neutralization of acidic amino acids by extracellular Ca2+ seems to be important for heat-evoked activation (Kurganov et al. 2017).
TRPA1 of Anolis carolinensis (Green anole) (American chameleon)
The sensory ion channel in tactile bristles of insects, NompC. The atomic structure of Drosophila NOMPC has been determined by single-particle electron cryo-microscopy. Structural analyses suggested that the ankyrin repeat domain (29 repeats) of NOMPC resembles a helical spring, suggesting its role of linking mechanical displacement of the cytoskeleton to the opening of the channel (Jin et al. 2017). Compression of the ankyrin chains imparts a rotational torque on the TRP domain, which may result in channel opening (Argudo et al. 2019).
NompC of Drosophila melanogaster (1619 aas; AAF59842)
The pore forming subunit, Trp-4, a mechanosensitive cation/Ca2+ channel. Present in ciliated mechanosensitive neurons; Activation and latency occur in the microsecond range. trp-4 mutations alter ion selectivity (Kang et al., 2010; Xiao and Xu 2009).
Trp-4 of Caenorhabditis elegans (Q9GRV5)
TRP-like ion channel PKD2 (Polycystic kidney disease-related ion channel 2). Regulates cytoplasmic calcium ion concentrations (Ma et al. 2011).
Pkd2 of Schizosaccharomyces pombe
Flavin carrier protein 2, Flc2p. May be responsible for the transport of FAD (and heme) into the endoplasmic reticulum lumen, where FAD may be required for oxidative protein folding involved in disulfide bridge formation (Protchenko et al. 2006).
Flc2 of Saccharomyces cerevisiae
Trp-like channel protein
TRP-like channel protein of Schizosaccharomyes pombe (O94543)