Transient Receptor Potential Channels and Intracellular Signaling

Geoffrey E. Woodard*; Stewart O. Sage†; Juan A. Rosado‡    * Metabolic Diseases Branch, NIDDK, National Institutes of Health, Bethesda, Maryland
† Department of Physiology, Development, and Neuroscience, University of Cambridge, Cambridge, United Kingdom
‡ Department of Physiology, University of Extremadura, Cáceres, Spain

I Introduction

The transient receptor potential (TRP) proteins are six transmembrane domain-containing subunits that form cation-selective ion channels. The TRP superfamily includes at least 22 related channels that play an important role in a number of cellular functions ranging from sensory transduction (including invertebrate vision, temperature, pain, and gustatory and osmolarity detection) to development. The first member of the TRP superfamily was identified as a protein involved in phototransduction in Drosophila. The transient receptor potential (trp) gene was named on the basis of the transient, rather than sustained, response to light in mutant flies. From the beginning a relationship between TRP proteins and ionic currents across the membrane was suggested since trp mutants displayed a defect in light-induced Ca2+ influx, which together with the predicted structure of TRP and the related protein, TRPL, raised the possibility that these proteins were Ca2+ influx channels (Montell et al., 2002a).

TRP proteins are present in yeast, Drosophila, Caenorhabditis elegans, and mammals. TRP channels are widely expressed in both excitable and nonexcitable cells, and TRP-related channels have been proposed as candidates to mediate Ca2+ entry. Although all TRP proteins form cation channels these differ significantly in their selectivity and activation mechanisms, although most members of the TRP superfamily share significant sequence homology.

TRP-related channels can be grouped into three subfamilies: those most closely related to TRP (TRPC, TRPV, and TRPM), two subfamilies that are more distantly related to TRP (TRPP and TRPML), and a less related TRPN group that is expressed in flies and worms and includes the mechanosensory channel NOMPC (Montell et al., 2002b). The TRPC subfamily encompases the mammalian proteins that display the greatest similarity to Drosophila TRP, sharing between 32 and 47% amino acid homology over the N-terminal 800 amino acids, including three or four ankyrin repeats, the six transmembrane domains, and a highly conserved 25 amino acid sequence known as the TRP box. TRPV proteins also include three or four ankyrin domains but lack the TRP box, and TRPM proteins contain a TRP box, but no ankyrin repeats (Montell et al., 2002b).

Most TRP channels are nonselective for monovalent and divalent cations with Ca2+ to Na+ permeability ratios (PCa/PNa) of ≤10. Exceptions are TRPM4 and TRPM5, which are selective for monovalent cations, and the Ca2+-selective TRPV5 and TRPV6, which have a PCa/PNa > 100. In contrast to voltage-gated channels, TRP channels lack the voltage sensitivity of the 24 membrane-spanning CaV or NaV families. Therefore, TRP channel opening induces membrane depolarization from the resting membrane potential (about −70 mV in most mammalian cells) to around 0 mV while increasing cytosolic Ca2+ and/or Na+ concentrations (Clapham et al., 2003). This article presents an overview of the structure and molecular relationships among the TRP channels and their role in physiological cell processes.

II TRP Channel Superfamily

A TRP Channel Structure and Functional Features

All members of the TRPC family are believed to share a common architecture. The analysis of the structure of TRP proteins reveals that these proteins contain six transmembrane domains, with cytoplasmic N- and C-termini, and a pore region between the transmembrane domains 5 and 6 (Hoenderop et al., 2003). However, in comparison with other ionic channels little is known about the architecture of the TRP channels or the structural organization of the pore region, responsible for the selectivity of the channels. The N-terminus contains three to four ankyrin repeats, a predicted coiled coil region, and a putative caveolin-binding domain. On the other hand, the C-terminus includes the TRP signature motif (EWKFAR), a proline-rich motif, the calmodulin/inositol 1,4,5-trisphosphate (IP3) receptor-binding (CIRB) domain, and a predicted coiled-coil region. TRPC4 and TRPC5 also contain a PDZ-binding motif in the C-terminus (Dohke et al., 2004; Montell, 2001; Vannier et al., 1998; Vazquez et al., 2004).

The ankyrin repeat is one of the most frequently observed amino acid motifs. This protein–protein interaction module is involved in a number of cellular functions. However, unlike other binding domains, ankyrin repeats do not recognize a conserved sequence or structure, but in contrast can bind to a variety of domains making it difficult to predict possible binding partners for the TRPC ankyrin repeats (Vazquez et al., 2004). The ankyrin repeats appear to be required for correct location of TRPC3 (Wedel et al., 2003) and TRPC6 (Hofmann et al., 2002) in the plasma membrane. However, the requirement of all ankyrin repeats for functional expression of TRP channels deserves further investigation, since for some TRP channels, such as the TRPC1, a splice variant missing certain ankyrin repeats (TRPC1A) forms a Ca2+-permeable cation channel activated by depletion of intracellular Ca2+ stores (Zitt et al., 1996).

The cytoplasmic N- and C-termini also contain a coiled-coil motif, which consists of several heptad repeats folding into an α-helix, which, by association with other α-helices, forms a supercoil. Coiled-coil motifs are commonly involved in protein oligomerization (Woolfson, 2005), therefore these domains might contribute to homo- and heteromerization of TRP channels or the association of TRP proteins with other coiled-coil motif-containing proteins (Vazquez et al., 2004). Consistent with this, coiled-coil motifs in the N-terminus of TRPC1 have been shown to homodimerize (Engelke et al., 2002). In TRPC1, calmodulin binding to the coiled-coil domain has been demonstrated and the deletion of this domain resulted in diminished Ca2+-dependent inactivation of store-operated Ca2+ entry (Singh et al., 2002).

As previously mentioned, TRP proteins contain a caveolin-binding domain in the N-termini, close to the first transmembrane domain, which allows them to associate with specific plasma membrane microdomains called caveolae. The association of TRPC1 and TRPC3 with caveolins has been demonstrated (Brazer et al., 2003; Lockwich et al., 2001) and deletion of this motif in TRPC1 results in the loss of targeting to the plasma membrane.

There is great variability in the permeation properties of different members of the TRP superfamily. There are channels, such as TRPV1, that are rather nonselective for mono- and divalent cations (Caterina et al., 1997), TRP channels, including TRPM4 and TRPM5, permeable to monovalent cations and impermeable to Ca2+ (Hofmann et al., 2003; Launay et al., 2002), and TRP channels that show a high selectivity for Ca2+ over monovalent cations, including TRPV5 and TRPV6 (Vennekens et al., 2000; Yue et al., 2001). This variability contrasts with most other families of ion channels, where the differences in pore permeability within one family are generally small. Although functional studies have been published of all TRPCs and TRPVs and of most TRPMs (Clapham et al., 2001; Montell et al., 2002a), reporting that these proteins operate as cation channels, a systematic analysis of the pore region of TRP channels has not yet been carried out. It is generally accepted that demonstrations that specific mutations in the putative pore domain of the candidate channel protein alter the pore properties, such as ion selectivity and conductance, provides compelling...