J Am Soc Nephrol 15: 1690–1699, 2004 The Transient Receptor Potential Superfamily of Ion Channels CHOU-LONG HUANG Division of Nephrology, Department of Medicine, and Charles and Jane Pak Center for Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical Center, Dallas, Texas Abstract. The transient receptor potential (TRP) superfamily of and have diverse functions, ranging from thermal, tactile, taste, proteins is cation-selective ion channels with six predicted osmolar, and fluid flow sensing to transepithelial Ca2ϩ and transmembrane segments and intracellularly localized amino Mg2ϩ transport. Mutations of TRP proteins produce many and carboxyl termini. Members of the TRP superfamily are renal diseases, including Mg2ϩ wasting, hypocalcemia, and identified on the basis of amino acid sequence and structural polycystic kidney diseases. This review focuses on recent similarity and are classified into TRPC, TRPV, TRPM, TRPP, advances in the understanding of their functions. TRPN, and TRPML subfamilies. TRP channels are widespread The first transient receptor potential (TRP) protein was discov- TRPC, TRPV, and TRPM subfamilies. More recent classifica- ered in studies that examined Drosophila phototransduction tion has expanded TRP superfamily to include three additional, (1). The photoreceptor cells of Drosophila exhibit sustained more distantly related subfamilies, TRPP, TRPML, and TRPN receptor potentials in response to continuous light exposure. (Figure 1). Structurally, all of these TRP channels have six The ionic basis for the sustained receptor potentials is influx of predicted transmembrane (TM) segments and N-terminal and Ca2ϩ from the extracellular space. Cosens and Manning (1) C-terminal cytoplasmic tails similar to topologies of voltage- reported in 1969 that one group of mutant flies exhibits TRP gated Kϩ,Naϩ, and Ca2ϩ channels; cyclic nucleotide-gated upon continuous light exposure and named it trp, for transient channels; and hyperpolarization-activated channels (Figure 2). receptor potential. The trp gene was cloned in 1989 (2) and The fourth TM segment of TRP channels lacks the complete subsequently shown to encode a Ca2ϩ-permeable cation chan- set of positively charged residues necessary for voltage sensing nel (3). Since then, many channels that bear sequence and in many voltage-gated channels. The six TM polypeptide sub- structural similarities to the Drosophila TRP have been cloned units of TRP channels likely assemble as tetramers to form from flies, worms, and mammals. Together, they form the TRP cation-permeable pores. Across the entire superfamily, the superfamily. amino acid sequence identity is only ~20%. The similarity Ion channels (e.g., voltage-gated Kϩ channels) are typically between subfamilies is limited primarily to the transmembrane identified by their modes of activation and/or ion selectivity. segments. Within each TRP subfamily, amino acid sequence Each family or superfamily of ion channels of similar activa- similarity is much higher and extends along the entire polypep- tion and selectivity consists of multiple members of channel tide. The N-terminal cytoplasmic region of some subfamilies proteins with amino acid sequence homology. Unlike most ion contains several ankyrin-binding repeats (Figure 2). Ankyrin- channel families, the TRP superfamily of ion channels are binding repeats are 33-residue motifs that mediate cytoskeletal identified on the basis of homology only. The mode of activa- anchoring or protein–protein interaction. Some subfamilies tion and selectivity for TRP channels are disparate. Some TRP contain a conserved stretch of 25 amino acids, called the TRP channels are activated by ligands, whereas others are regulated domain, or a kinase or phosphatase domain in the C-terminal by physical stimuli (e.g., heat) or yet-unknown mechanisms. cytoplasmic region. All TRP channels are cation selective, but the selectivity ratio 2ϩ ϩ for Ca versus the monovalent cation Na (PCa/PNa) varies Ͼ Ͻ Molecular Identification and Characterization widely, ranging from 100:1, to 1 to 10:1, to 0.05:1. The of TRP Channels lack of common identifying features has in part contributed to The TRPC Subfamily the confusing nomenclature of TRP channels in the literature. The TRPC (for canonical TRP) subfamily is composed of A recent consensus report proposed a unified nomenclature proteins that are most highly related to Drosophila TRP. It for the TRP superfamily (4). It classified TRP channels into consists of seven mammalian members, TRPC1 to 7 (5–10). Members of the TRPC subfamily contain three to four ankyrin repeats in the N-terminus and a conserved TRP domain in the Correspondence to Dr. Chou-Long Huang, Department of Medicine, UT Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-8856. Phone: 214- C-terminus (Figure 2). Human TRPC subfamily can be divided 648-8627; Fax: 214-648-2071; E-mail: [email protected] into three subgroups on the basis of amino acid homology: 1046-6673/1507-1690 TRPC1, TRPC4/5, and TRPC3/6/7. TRPC2 is a pseudogene in Journal of the American Society of Nephrology human but seems to encode an expressed protein in rat and Copyright © 2004 by the American Society of Nephrology mouse (8–10). TRPC4 and TRPC5 share ~65% homology. DOI: 10.1097/01.ASN.0000129115.69395.65 TRPC3, TRPC6, and TRPC7 share ~70 to 80% homology. J Am Soc Nephrol 15: 1690–1699, 2004 TRP Channels 1691 Members of the TRPC subfamily are highly expressed in central nervous system and to lesser degrees in many periph- eral tissues, including kidney. The selectivity ratio for Ca2ϩ ϩ versus Na (PCa/PNa) for TRPC channels ranges from 0.5 to 10:1. TRPC channels are involved in Ca2ϩ entry in response to activation of phospholipase C (PLC) by membrane receptors and thus play important roles in the regulation of intracellular Ca2ϩ concentration by hormones and growth factors (see Func- tions of TRP Channels). The TRPV Subfamily The TRPV subfamily is named after the first mammalian member of the subfamily, vanilloid receptor 1 (VR1). It con- tains six mammalian members, TRPV1 to 6. TRPV1 (VR1) was isolated by expression cloning using capsaicin, a vanilloid compound derived from “hot” pepper, as a binding ligand (11). TRPV2 to 4 were isolated by searching for expressed sequence tags (EST) with amino acid homology to TRPV1 (12–16). TRPV1 to 4 share 40 to 50% amino acid homology, are 2ϩ Ca -permeable nonselective cation channels (PCa/PNa ~3 to 10:1), and have steep temperature sensitivity. TRPV5 and TRPV6 were isolated by expression cloning of proteins that mediate Ca2ϩ transport in kidney and intestine, respectively (17,18). They are the most highly Ca2ϩ-selective TRP channels Ͼ (PCa/PNa 100:1). Overall, TRPV subfamily channels contain three to four ankyrin repeats in the N-terminus and a TRP domain in the C-terminus (Figure 2). TRPV channels are also present in invertebrates. Invertebrate TRPV proteins include C. elegans OSM-9 and OCR (for OSM-9/capsaicin receptor re- lated) (19,20). OSM-9 is more closely related to TRPV5/6 than TRPV1 to 4 at the amino acid level. The TRPM Subfamily The TRPM subfamily is named after the founding member, Figure 1. Classification of the transient receptor potential (TRP) melastatin. Melastatin, TRPM1, is a tumor suppressor protein superfamily. isolated in a screen for genes whose level of expression (in- versely) correlated with the severity of metastatic potential of a melanoma cell line (21). There are eight mammalian mem- bers in the TRPM subfamily. TRPM subfamily channels lack ankyrin repeats in the N-terminus but contain the TRP domain in the C-terminus (Figure 2). The C-terminus of TRPM pro- teins is considerably longer than the corresponding region of other TRP. The C-terminus of several members of TRPM contains enzyme domains. These TRPM proteins are thus called “chanzymes.” TRPM2 (initially called TRPC7 and LTRPC2) is a Ca2ϩ- permeable channel that contains a C-terminal ADP-ribose py- rophosphatase domain (22–25). ADP-ribose pyrophosphatase catalyzes the hydrolysis of nucleoside diphosphate derivatives. The domain in TRPM2, however, is an ineffective hydrolase but binds ADP-ribose and NAD (23–25). ADP-ribose and NAD directly activate TRPM2 to allow Ca2ϩ influx. ATP counteracts the NAD-induced activation. TRPM2 is also reg- ␣ ulated by H2O2 and TNF- (26,27). Thus, TRPM2 may be Figure 2. Domain organization of TRP channels. The TRP domain is important in sensing oxidative stress and in linking apoptosis a highly conserved 25–amino acid region. The region identified as with the metabolism of ADP-ribose and NAD. ϩ “TRP box” is nearly invariant. X denotes any amino acid. TRPM3 is a Ca2 -permeable, nonselective cation channel 1692 Journal of the American Society of Nephrology J Am Soc Nephrol 15: 1690–1699, 2004 whose activity is increased by hypotonicity (28). TRPM4 and Functions of TRP Channels TRPM5 are the only TRP channels that are permeable to The TRP superfamily comprises a large number of ion 2ϩ Ͻ monovalent cations but not Ca (PCa/PNa 0.05:1) (29,30). channels with diverse functions. The division into subfamilies ϩ However, they are activated by intracellular Ca2 (29,30). on the basis of amino acid sequence and structural similarity ϩ Activation of TRPM4 and TRPM5 by intracellular Ca2 leads does not provide functional classification for TRP proteins. For to membrane depolarization. TRPM6 and TRPM7 both contain example, members of the TRPV subfamily are involved in a C-terminal protein kinase domain (31–36). The role of these thermal and nociceptive sensing as well as transporting Ca2ϩ in kinase domains in the channel function of TRPM6 and TRPM7 epithelial tissues. Moreover, thermal sensing is not restricted to is not known. TRPM8 is an outward rectifying channel that can members of the TRPV subfamily. TRPM8 also functions in be activated by cold (37,38). It was isolated by expression thermal sensing. In this section, TRP channels are discussed on cloning of a menthol receptor from sensory neurons.