1.A.4.5.7
Cold-sensitive (opens with decreasing temperatures; e.g., <22°C) and menthol-sensitive cation-selective channel, transient receptor potential melastatin 8 (TRPM8). TRPM8 is activated by low temperatures and cooling agents such as menthol. It underlies the cold-induced excitation of sensory neurons. Its gating is regulated by voltage and lysophospholipids which induce prolonged channel opening (Vanden Abeele et al., 2006; Bautista et al., 2007; Matta and Ahern, 2007). It can be converted to an anion-selective channel by introducing a lysyl residue in TMS 6 (Kuhn et al., 2007). Gating of TRPM8 channels is activated by cold and chemical agonists in planar lipid bilayers (Zakharian et al., 2010). Residues involved in intra- and intersubunit interactions have been identified, and their link with channel activity, sensitivity to icilin, menthol and cold, and their impact on channel oligomerization have been measured (Bidaux et al. 2015).  Targeting the small isoform of TRPM8 may be useful to fight prostate cancer (Bidaux et al. 2016). The human isoform is 83% identical to the TRPM8 of the collared flycatcher (TC# 1.A.4.5.13), the structure of which has been characterized to 4.1 Å resolution (Yin et al. 2018). Activation of TRPM8 by cooling compounds relies on allosteric actions of agonist and the membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP₂). The cryoEM structures of TRPM8 in complex with the synthetic cooling compound icilin, PIP₂, and Ca²⁺, as well as in complex with the menthol analog WS-12 and PIP₂ revealed the binding sites for cooling agonists and PIP₂ in TRPM8. PIP₂ binds to TRPM8 in two different modes, which illustrate the mechanism of allosteric coupling between PIP₂ and agonists.
     4TMS-TRPM8 isoforms form functional channels in the ER and participate in regulation of the steady-state Ca²⁺ concentration in mitochondria and the ER. 4TMS-TRPM8 isoforms are ER Ca²⁺ release channels (Bidaux et al. 2018). Human PIRT (TC# 8.A.64) attenuates human TPRM8 conductance, unlike mouse PIRT, which enhances mouse TRPM8 conductance (Hilton et al. 2018). PIRT and the TRPM8 S1–S4 domain interact with a 1:1 binding stoichiometry, suggesting that a functional tetrameric TRPM8 channel has four PIRT-binding sites (Hilton et al. 2018). TRPM8 has been implicated in nociception and pain and is regarded as an attractive target for the pharmacological treatment of neuropathic pain syndromes. A series of analogues of N,N'-dibenzyl tryptamine 1, a potent TRPM8 antagonist, were made and studied. Molecular modeling studies identified the putative binding mode of these antagonists, suggesting that they could influence an interaction network between the S1-4 transmembrane segments and the TRP domains of the channel subunits (Bertamino et al. 2018). Cold sensitivity is due to nonconserved residues located within the pore loop (residues 526 - 556) (Pertusa et al. 2018). Diver et al. 2019 have presented cryo-EM structures of TRPM8 in ligand-free, antagonist- or calcium-bound forms, revealing how robust conformational changes give rise to two non-conducting states, closed and desensitized. A malleable ligand-binding pocket accommodates drugs of diverse chemical structures, and delineates the ion permeation pathway, including the contribution of lipids to pore architecture. Direct calcium binding mediates stimulus-evoked desensitization. Large rearrangements within the S4-S5 linker reposition the S1-S4 and pore domains relative to the TRP helix, suggesting a model for modulation of TRPM8 and possibly other TRP channels (Diver et al. 2019). Menthol binding induces wide-spread conformational rearrangements within the transmembrane domains. A temporal sequence of conformational changes in the S6 bundle crossing and the selectivity filter leading to channel activation was estimated (Xu et al. 2020). Yin et al. 2022 presented cryo-electron microscopy structures of mouse TRPM8 in closed, intermediate, and open states along the ligand- and PIP₂-dependent gating pathways. The results uncover two discrete agonist sites, state-dependent rearrangements in the gate positions, and a disordered-to-ordered transition of the gate-forming S6 - elucidating the molecular basis of chemically induced cool sensation in mammals.