Closing in on the heat-activation mechanisms of TRPV channels.

Abstract

The sensation of heat is mediated by cation-permeable ion channels of the thermoTRP kind. Among these, some of the principal heat-activated ion channels are formed by the TRPV1–4 members of the TRPV subfamily (Latorre et al. 2009). The mechanism by which these membrane proteins convert thermal energy to a channel-opening conformational change is still being intensely investigated and for the most part is not well understood. The availability of high-resolution structures provided by the cryo-EM revolution has made it easier to propose structure–function hypotheses to study heat activation.However, high quality functional studies based in electrophysiology are still the benchmark to prove mechanisms for ion channels. Quantifying the contribution of a channel region to the heat-absorbing step is a difficult task. So far, a good experimental approximation is the estimation of the activation enthalpy associated with the opening of the channel, measured from the steepness of the channel open probability as a function of temperature. In previous seminal work (Yao et al. 2011), the laboratory of Feng Qin identified a so-called membrane proximal domain (MPD), located between the first transmembrane domain and the N-terminal ankyrin repeat domains in all TRPV channels. The authors produced evidence which suggests that the MPD is a main determinant of the activation enthalpy in TRPV1–4 thermoTRP channels. Importantly, the suggestion that this region is involved in a heat-absorbing conformational change was supported by careful measurement of the apparent enthalpy. In the present work, Liu and Qin (2021) further dissect the contribution of the components of the MPD and of individual amino acid residues within the MPD to the apparent enthalpy of activation in TRPV1 and TRPV2 channels. These important new results begin to paint a detailed picture of the fine-tuning involved in channel activation by temperature. TRPV2 channels are the thermoTRP channels with the highest enthalpy of activation –more than double the activation enthalpy of TRPV1 – and are also activated by very high temperatures (>50°C compared to >40°C for TRPV1). Interestingly, the activation of TRPV2 is also different from TRPV1 in that it shows marked hysteresis. TRPV2 channels activate steeply after initial temperature stimuli that do not activate a large fraction of current. Regardless of these differences, Liu and Qin make use of a unique heat-activation technique that relies on rapid infrared laser activation of currents and of chimeric constructions in which parts of the MPD region are swapped between both types of channels to successfully identify a helix–turn–helix motif comprising 10 residues in TRPV2 and 11 residues in TRPV1 that is part of the MPD. Transplantation of this motif between the two channels is enough to produce chimaeras that activate with the enthalpy of activation of the donor channel. Further mutagenesis experiments identified single residues that are implicated in the high temperature dependence of activation of TRPV2. Of particular interest is a serine present in TRPV1 and absent in TRPV2 and TRPV3, another high temperature-activated channel. Reintroduction of the serine in the 10-residue motif of TRPV2 reduced the apparent enthalpy of activation to values comparable to TRPV1. The fact that a single serine is capable of such a large effect suggests that this serine drastically modulates the local structure of the motif or its local interactions or both. According to cryo-EM structures of TRPV2 (Zubcevic et al. 2016), this helix–turn–helix motif is poised to interact with the S2–S3 intracellular loop and the functionally important TRP domain helix. What is the function of the helix–turn– helix in the complete gating cycle of thermoTRP channels? One possibility is that it is part of a protein domain involved in heat absorption, a heat sensor of sorts. As discussed by the authors, this possibility is unlikely due to its small size and the fact that it is generally thought that a large number of interactions are needed to produce high temperature sensitivity. Another possible function for this motif is as a coupling region, capable of transmitting a heat-induced conformational change initiated in other regions of the channel to the activation gate formed by the S6 transmembrane segments. Careful functional experiments such as the ones reported in the present work as well as detailed structural determination of activated and deactivated states will be needed in the future in order to fully understand heat activation in TRPV channels. Meanwhile, these experiments remind us of the immense usefulness of careful electrophysiology.