Structural basis of temperature sensation by the TRP channel TRPV3
Published on 10.21.2019 in Nature Structural & Molecular Biology
Appu K. Singh, Luke L. McGoldrick, Lusine Demirkhanyan, Merfilius Leslie, Eleonora Zakharian and Alexander I. Sobolevsky
In mammals and other animals, temperature perception is mediated by primary afferent neurons, however, the identity of the molecular sensors of temperature remained enigmatic until it was found that a member of the transient receptor potential (TRP) superfamily of ion channels, TRPV1, acts as a thermoreceptor1. This discovery resulted in the rapid identification of ten other members of the TRP channel superfamily, collectively described as the thermo-TRPs. Thermo-TRPs exhibit unusually high temperature coefficient (Q10) values compared to non-temperature sensitive ion channels2-4, allowing them to open and close in response to changes in temperature within a physiologically relevant range. Thermo-TRPs are polymodal by nature and respond synergistically to distinct chemical stimuli, membrane potential, and temperature5.
There are four thermo-TRPs in the vanilloid subfamily of TRP channels: TRPV1 and TRPV2 are activated by noxious heat (>43°C), while TRPV3 and TRPV4 can respond to warm temperatures (< 33°C)6-9. The non-vanilloid TRP channels TRPM810,11 and TRPC512 open at moderately cold (< 24°C) temperatures, while TRPA113,14 is activated by noxious cold (< 17°C). Furthermore, TRPM2-TRPM515-18 also contribute to temperature sensation with variable temperature thresholds for activation. The genetic ablation of individual thermo-TRPs did not substantially perturb the thermo-phenotype of mice19-23, suggesting that thermo-TRPs have partially overlapping temperature thresholds in vivo. TRPV1, TRPA1 and TRPM3 triple knockout mice, however, lacked the pain response to noxious heat24, emphasizing the importance of functionally redundant thermo-TRPs in temperature and pain perception. As a result of their physiological roles, the thermo-TRPs are promising analgesic drug targets.
TRPV3 is predominantly expressed in skin keratinocytes and mediates warm and pain sensation6,9. Temperature-dependent activation of TRPV3 is use-dependent and hysteretic; its initial activation requires a higher temperature threshold (>50°C), while any subsequent activation can occur at a relatively lower temperature (~33°C)25. Previously solved structures of TRPV3 have provided an insight into its architecture and ligand-dependent gating26,27. However, how thermo-TRPs, including TRPV3, respond to changes in temperature at the molecular level remained largely unknown28-30. To answer this question, we captured TRPV3 in different conformational states using heat. Our structures inform the molecular basis of temperature sensation by TRPV3 and thermo-TRPs in general. We have structurally identified two conformational steps of TRPV3’s heat activation29. During the first step, sensitization, the lipid occupancy in binding sites 1 and 2 is reduced, and the S1-S4 and pore domains move closer together. As S6 moves closer to S4, it undergoes an a-to-p helical transition and bends over the S4-S5 linker. While the ion channel pore remains hydrophobically sealed, the a-to-p transition results in the ~100° rotation of the C-terminal half of S6, which exposes different residues to the channel pore. Concurrently, S6 becomes two helical turns longer while the TRP helix becomes two helical turns shorter.
The second step of TRPV3’s activation, pore opening, is accompanied by removal of the lipids from the binding sites 1 and 2 and further movement of the S1-S4 and pore domains closer to each other. Additionally, the S6 helices splay away from the pore axis, the TRP helices tilt towards the S4-S5 linker, and the linker domain undergoes conformational changes, which are stabilized by the unlatching of the C-terminus from the inter-subunit interfaces. Grossly, the conformational rearrangements accompanying pore opening are more dramatic, when compared to those that occur during sensitization, and include shortening of the ion channel and rotation of the intracellular skirt domain.
To isolate the channel’s transition from the sensitized to open state, we used TRPV3Y564A, which appears sensitized even at 4 °C. This mutant exhibits a high open probability in the 22-42 °C temperature range and has a low Q10 value. This suggests that the transition from the sensitized to open state is weakly temperature-dependent. Correspondingly, we hypothesize that the strong temperature dependence and high Q10 of TRPV3WT originate from the closed-to-sensitized state transition. Our hypothetical two-step temperature-induced activation mechanism of TRPV3 is reminiscent of the activation mechanisms of TRPM8 and TRPV1, which exhibit two contrasting Q10 values as well as entropy and enthalpy changes during transitional and stabilized channel gating31-35. Because the closed-to-sensitized state transition is accompanied by conformational changes that take place only in the transmembrane domain, its components – the S1-S4 domain, the pore domain and the TRP helix – may contribute to a TRPV3 temperature sensor. However, gating of the ion channel, which culminates in the ion conductance through the TRPV3 pore, is a multistep process that requires both sensitization and pore opening. For this reason, a change in any of the activation steps may alter the apparent response of the channel29. Correspondingly, the linker domain and the C-terminus, which regulate the sensitized-to-open state transition, are not necessarily responsible for temperature sensing but contribute to heat activation and appear important for temperature-dependent gating, in addition to the transmembrane domain5,34,36-44.
What are the physical bases of the strong temperature dependence of the closed-to-sensitized state transition? The laws of thermodynamics dictate that this transition must involve an unusually large change in enthalpy between the closed and sensitized states29,30,45,46. Since the transition between these two states involves relatively small conformational changes, the presumed large changes in enthalpy can be accounted for by significant changes in heat capacity39,40,45,47. In proteins, the heat capacity changes appear to result mainly from the hydration of residues; exposure of hydrophobic residues to the aqueous environment is associated with a positive change in heat capacity, while that of polar or charged residues is associated with a negative change. During the TRPV3 sensitization step, the change in heat capacity could, therefore, arise from different exposure of the transmembrane domain elements to the membrane versus solvent in the closed and sensitized states. Indeed, the 100° axial rotation of the bottom of S6, its elongation and shortening of the TRP helix during sensitization provide ~8 residues per subunit of the TRPV3 tetramer that change their hydrophobic environment. Analogous conformational changes in the homologous region of TRPV1 were proposed to contribute to TRPV1 activation via the altered wetting of the pore and of the four peripheral cavities located between each S6 and S4-S5 linker48.
TRPV3-like conformational rearrangements were also observed during TRPV6 gating, which is not temperature-dependent49. On the other hand, TRPV6 does not show such strong gating-associated changes in annular lipid occupancy as does TRPV3, highlighting a possibility of the important role of lipids in TRPV3 thermosensitivity. Since lipid bilayers undergo temperature-dependent phase transitions, melting of annular lipids surrounding the transmembrane domain of TRPV3 with increasing temperature could result in an energy sufficient for its thermal activation29. Correspondingly, the large change in enthalpy between the closed and sensitized states of TRPV3 may originate, in part, from different interactions of the surrounding lipids with the altered S1-S4-pore domain interfaces. Cholesterol molecules, which strongly modulate fluidity of lipid membranes, may represent some of these annular lipids. In fact, the shape of the cholesterol molecule resembles the shape of putative lipids in TRPV3 structure, very different from a typical two-tail appearance of phospholipids. Further supporting the important role of lipids in TRPV3 thermosensitivity, the Y564A mutation in the putative lipid binding site 2 renders TRPV3Y564A channels weakly temperature-sensitive. Additional research is required to better understand the role of lipids, hydrophobic interactions and residue solvation in the temperature-dependent gating of thermo-TRPs and our structures provide a foundation for such studies.
This work was done in collaboration with Dr. Zakharian group from University of Illinois College of Medicine Peoria.
1. Caterina, M.J. et al. The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389, 816-24 (1997).
2. Patapoutian, A., Peier, A.M., Story, G.M. & Viswanath, V. Thermotrp channels and beyond: Mechanisms of temperature sensation (vol 4, pg 529, 2003). Nature Reviews Neuroscience 4, 691-691 (2003).
3. Jordt, S.E., McKemy, D.D. & Julius, D. Lessons from peppers and peppermint: the molecular logic of thermosensation. Current Opinion in Neurobiology 13, 487-492 (2003).
4. Clapham, D.E. TRP channels as cellular sensors. Nature 426, 517-524 (2003).
5. Voets, T. et al. The principle of temperature-dependent gating in cold- and heat-sensitive TRP channels. Nature 430, 748-754 (2004).
6. Peier, A.M. et al. A heat-sensitive TRP channel expressed in keratinocytes. Science 296, 2046-9 (2002).
7. Guler, A.D. et al. Heat-evoked activation of the ion channel, TRPV4. J Neurosci 22, 6408-14 (2002).
8. Caterina, M.J., Rosen, T.A., Tominaga, M., Brake, A.J. & Julius, D. A capsaicin-receptor homologue with a high threshold for noxious heat. Nature 398, 436-41 (1999).
9. Smith, G.D. et al. TRPV3 is a temperature-sensitive vanilloid receptor-like protein. Nature 418, 186-190 (2002).
10. Bautista, D.M. et al. The menthol receptor TRPM8 is the principal detector of environmental cold. Nature 448, 204-208 (2007).
11. Dhaka, A. et al. TRPM8 is required for cold sensation in mice. Neuron 54, 371-378 (2007).
12. Zimmermann, K. et al. Transient receptor potential cation channel, subfamily C, member 5 (TRPC5) is a cold-transducer in the peripheral nervous system. Proceedings of the National Academy of Sciences of the United States of America 108, 18114-18119 (2011).
13. Kwan, K.Y. et al. TRPA1 contributes to cold, mechanical, and chemical Nociception but is not essential for hair-cell transduction. Neuron 50, 277-289 (2006).
14. Karashima, Y. et al. TRPA1 acts as a cold sensor in vitro and in vivo. Proceedings of the National Academy of Sciences of the United States of America 106, 1273-1278 (2009).
15. Talavera, K. et al. Heat activation of TRPM5 underlies thermal sensitivity of sweet taste. Nature 438, 1022-5 (2005).
16. Vriens, J. et al. TRPM3 Is a Nociceptor Channel Involved in the Detection of Noxious Heat. Neuron 70, 482-494 (2011).
17. Song, K. et al. The TRPM2 channel is a hypothalamic heat sensor that limits fever and can drive hypothermia. Science 353, 1393-1398 (2016).
18. Tan, C.H. & McNaughton, P.A. The TRPM2 ion channel is required for sensitivity to warmth. Nature 536, 460-+ (2016).
19. Caterina, M.J. et al. Impaired nociception and pain sensation in mice lacking the capsaicin receptor. Science 288, 306-13 (2000).
20. Davis, J.B. et al. Vanilloid receptor-1 is essential for inflammatory thermal hyperalgesia. Nature 405, 183-187 (2000).
21. Moqrich, A. et al. Impaired thermosensation in mice lacking TRPV3, a heat and camphor sensor in the skin. Science 307, 1468-72 (2005).
22. Huang, S.M., Li, X.X., Yu, Y.Y., Wang, J. & Caterina, M.J. TRPV3 and TRPV4 ion channels are not major contributors to mouse heat sensation. Molecular Pain 7(2011).
23. Park, U. et al. TRP vanilloid 2 knock-out mice are susceptible to perinatal lethality but display normal thermal and mechanical nociception. J Neurosci 31, 11425-36 (2011).
24. Vandewauw, I. et al. A TRP channel trio mediates acute noxious heat sensing. Nature 555, 662-+ (2018).
25. Liu, B.Y., Yao, J., Zhu, M.X. & Qin, F. Hysteresis of gating underlines sensitization of TRPV3 channels. Journal of General Physiology 138, 509-520 (2011).
26. Zubcevic, L. et al. Conformational ensemble of the human TRPV3 ion channel. Nat Commun 9, 4773 (2018).
27. Singh, A.K., McGoldrick, L.L. & Sobolevsky, A.I. Structure and gating mechanism of the transient receptor potential channel TRPV3. Nat Struct Mol Biol 25, 805-813 (2018).
28. Diaz-Franulic, I., Poblete, H., Mino-Galaz, G., Gonzalez, C. & Latorre, R. Allosterism and Structure in Thermally Activated Transient Receptor Potential Channels. Annual Review of Biophysics, Vol 45 45, 371-398 (2016).
29. Feng, Q. Temperature Sensing by Thermal TRP Channels: Thermodynamic Basis and Molecular Insights. Thermal Sensors 74, 19-50 (2014).
30. Voets, T. Quantifying and Modeling the Temperature-Dependent Gating of TRP Channels. Reviews of Physiology, Biochemistry and Pharmacology, Vol 162 162, 91-119 (2012).
31. Brauchi, S., Orio, P. & Latorre, R. Clues to understanding cold sensation: thermodynamics and electrophysiological analysis of the cold receptor TRPM8. Proceedings of the National Academy of Sciences of the United States of America 101, 15494-9 (2004).
32. Zakharian, E., Cao, C. & Rohacs, T. Gating of transient receptor potential melastatin 8 (TRPM8) channels activated by cold and chemical agonists in planar lipid bilayers. The Journal of neuroscience : the official journal of the Society for Neuroscience 30, 12526-34 (2010).
33. Sun, X. & Zakharian, E. Regulation of the temperature-dependent activation of transient receptor potential vanilloid 1 (TRPV1) by phospholipids in planar lipid bilayers. J Biol Chem 290, 4741-7 (2015).
34. Brauchi, S., Orio, P. & Latorre, R. Clues to understanding cold sensation: thermodynamics and electrophysiological analysis of the cold receptor TRPM8. Proc Natl Acad Sci U S A 101, 15494-9 (2004).
35. Zakharian, E., Cao, C. & Rohacs, T. Gating of Transient Receptor Potential Melastatin 8 (TRPM8) Channels Activated by Cold and Chemical Agonists in Planar Lipid Bilayers. Journal of Neuroscience 30, 12526-12534 (2010).
36. Vlachova, V. et al. Functional role of C-terminal cytoplasmic tail of rat vanilloid receptor 1. J Neurosci 23, 1340-50 (2003).
37. Yao, J., Liu, B. & Qin, F. Modular thermal sensors in temperature-gated transient receptor potential (TRP) channels. Proc Natl Acad Sci U S A 108, 11109-14 (2011).
38. Liu, B. & Qin, F. Single-residue molecular switch for high-temperature dependence of vanilloid receptor TRPV3. Proc Natl Acad Sci U S A 114, 1589-1594 (2017).
39. Grandl, J. et al. Temperature-induced opening of TRPV1 ion channel is stabilized by the pore domain. Nature Neuroscience 13, 708-714 (2010).
40. Grandl, J. et al. Pore region of TRPV3 ion channel is specifically required for heat activation. Nature Neuroscience 11, 1007-1013 (2008).
41. Brauchi, S., Orta, G., Salazar, M., Rosenmann, E. & Latorre, R. A hot-sensing cold receptor: C-terminal domain determines thermosensation in transient receptor potential channels. J Neurosci 26, 4835-40 (2006).
42. Zhang, F. et al. Heat activation is intrinsic to the pore domain of TRPV1. Proceedings of the National Academy of Sciences of the United States of America 115, E317-E324 (2018).
43. Gregorio-Teruel, L. et al. The Integrity of the TRP Domain Is Pivotal for Correct TRPV1 Channel Gating. Biophysical Journal 109, 529-541 (2015).
44. Kim, S.E., Patapoutian, A. & Grandl, J. Single Residues in the Outer Pore of TRPV1 and TRPV3 Have Temperature-Dependent Conformations. Plos One 8(2013).
45. Clapham, D.E. & Miller, C. A thermodynamic framework for understanding temperature sensing by transient receptor potential (TRP) channels. Proc Natl Acad Sci U S A 108, 19492-7 (2011).
46. Latorre, R., Zaelzer, C. & Brauchi, S. Structure-functional intimacies of transient receptor potential channels. Q Rev Biophys 42, 201-46 (2009).
47. Chowdhury, S., Jarecki, B.W. & Chanda, B. A molecular framework for temperature-dependent gating of ion channels. Cell 158, 1148-1158 (2014).
48. Kasimova, M.A., Lindahl, E. & Delemotte, L. Determining the molecular basis of voltage sensitivity in membrane proteins. Journal of General Physiology 150, 1444-1458 (2018).
49. McGoldrick, L.L. et al. Opening of the human epithelial calcium channel TRPV6. Nature 553, 233-237 (2018).