Cytoplasmic ankyrin repeats of transient receptor potential A1 (TRPA1) dictate sensitivity to thermal and chemical stimuli

Abstract

Transient receptor potential (TRP) channels are polymodal signal detectors that respond to a wide array of physical and chemical stimuli, making them important components of sensory systems in both vertebrate and invertebrate organisms. Mammalian TRPA1 channels are activated by chemically reactive irritants, whereas snake and Drosophila TRPA1 orthologs are preferentially activated by heat. By comparing human and rattlesnake TRPA1 channels, we have identified two portable heat-sensitive modules within the ankyrin repeat-rich aminoterminal cytoplasmic domain of the snake ortholog. Chimeric channel studies further demonstrate that sensitivity to chemical stimuli and modulation by intracellular calcium also localize to the N-terminal ankyrin repeat-rich domain, identifying this region as an integrator of diverse physiological signals that regulate sensory neuron excitability. These findings provide a framework for understanding how restricted changes in TRPA1 sequence account for evolution of physiologically diverse channels, also identifying portable modules that specify thermosensitivity.chemosensation | pain | thermosensation | calcium modulation | somatosensation P rimary afferent (somatosensory) neurons detect a range of physical and chemical stimuli, including temperature, pressure, and noxious irritants (1). The transient receptor potential (TRP) channel family has been shown to play a predominant role in these processes, particularly in regard to thermosensitivity and chemosensitivity (2-5). TRPA1, otherwise known as the "wasabi receptor," plays a key role in somatosensation in evolutionarily diverse phyla, including vertebrate and invertebrate species. Mammalian TRPA1 is expressed by primary afferent sensory neurons of the pain pathway, where it functions as a sensor of environmental and endogenous chemical irritants, such as allyl isothiocyanate (AITC), acrolein, and 4-hydroxynonena, and contributes to cellular mechanisms underlying inflammatory pain (6-9).TRPA1 channels show species-specific functional variation to suit their physiological roles. For example, snakes are unique among vertebrates in that their TRPA1 channels are heat-sensitive, which some species (rattlesnakes, boas, and pythons) have exploited to detect infrared radiation (10). Similarly, insect TRPA1 channels are heat-sensitive and contribute to thermal avoidance behaviors (11)(12)(13)(14). In these cases, however, thermosensitivity comes at the expense of chemosensitivity, such that AITC and other chemical irritants still activate these channels but with reduced potency compared with mammalian orthologs (10). Despite clear physiological differences between snake and mammalian TRPA1 channels, they share significant amino acid identity (56%), providing a unique opportunity to exploit sequence comparisons and domain swaps to pinpoint structural elements associated with stimulus detection and/or gating. Delineating elements that contribute to TRPA1 function will provide insights into the evolutionary process whereby structural changes lead t...

Unfolding of a Temperature-Sensitive Domain Controls Voltage-Gated Channel Activation

“…Buried hydrophilic residues in coiled-coil cores carry a well-established energetic penalty with respect to quaternary architecture stability (Lupas and Gruber, 2005). Although TRPA1 ankyrin repeats have been implicated in temperature responses (Cordero-Morales et al, 2011; Jabba et al, 2014), it is striking that they form a cage around the CTD (Figure 7B). Hence, many of the reported ankyrin repeat domain effects may be due to modulation of CTD transitions.…”

Structure of Thermally Activated TRP Channels

“…Extensive and elegant studies have been employed to determine their “molecular temperature switch” (Brauchi, Orta, Salazar, Rosenmann, & Latorre, 2006; Cordero-Morales, Gracheva, & Julius, 2011; Grandl et al, 2008, 2010; Kim, Patapoutian, & Grandl, 2013; Yang, Cui, Wang, & Zheng, 2010; Yao, Liu, & Qin, 2011); however, a temperature-dependent detector may not exist at a specific site since temperature affects the global conformation of these large membrane protein complexes (Clapham & Miller, 2011; Liu, Hui, & Qin, 2003). High-resolution structural information for these channels will greatly accelerate understanding of how the entire protein operates as a thermosensor.…”

“…The molecular mechanisms by which TRPA1 detects temperature from warm to cold remain to be determined. However, a recent chimeric analysis showed that the TRPA1 ankyrin repeats and N-terminal flexible linker are important for detecting and transducing different stimuli (Cordero-Morales et al, 2011). …”

Evolutionary dynamics of metazoan TRP channels

“…The numbers of ARs present in each channel are different: 14 to 15 in TRPA, 3 to 4 in TRPC, 29 in TRPN, and 4 to 6 in TRPV channels. ARs appear to be necessary for interactions with ligands and protein partners and for temperature sensitivity [11,17,24]. A highly conserved domain of 23-25 amino acids (TRP domain) is present, which extends from the C-terminus to the transmembrane domains, in TRPC, TRPM, TRPN, and TRPV channels [41,72].…”