Research ArticleRegulation of transient receptor potential channels of melastatin type 8 (TRPM8): Effect of cAMP, cannabinoid CB1 receptors and endovanilloids
Introduction
Transient receptor potential (TRP) channels are the vanguard of animal sensory systems, by responding to temperature, touch, osmolarity, pheromones, taste and other physical and chemical stimuli. Their role, however, is broader than classical sensory transduction. TRP channels are not selective for Ca2+ and are made of six transmembrane polypeptide subunits that are believed to assemble as tetramers to form cation-permeable pores. A number of mammalian TRPs are gated in response to thermal stimuli as well as to chemical imitators of burning and cooling sensations, i.e. capsaicin and menthol, respectively. Both TRP of vanilloid type 1 (TRPV1) and TRP of melastatin type 8 (TRPM8) are Ca2+ permeable, non-specific cation channels abundantly expressed in subpopulations of primary afferent neurons. TRPV1 is activated by heat (> 43 °C) and protons (decreased pH), as well as by endogenous ligands (endovanilloids) such as N-arachidonoyldopamine, anandamide and lipoxygenase products of arachidonic acid, and is involved in the transduction of inflammatory and thermal pain [1], [2]. Both diacylglycerols (DAGs) and cAMP regulate the activity of some TRP channels by activating protein kinase C and A (PKC and PKA), respectively, and hence by catalyzing TRP channel Ser/Thr phosphorylation [3]. TRPV1 is also inhibited by phosphatidylinositol-4,5-bisphosphate (PIP2), which also can be considered as an endogenous ligand. Its sensitivity to heat and chemicals is enhanced by bradykinin and other mediators that act via PLCβ to hydrolyze PIP2 and cause DAG formation. DAGs can further activate TRPV1 via PKC [4], [5], [6]. On the other hand, TRPM8 is activated by cold (< 25 °C) and agents like menthol (an active ingredient of peppermint) and several other chilling compounds such as eucalyptol, and icilin (a synthetic compound), which induce a cool sensation [7], [8], [9]. In heterologous expression systems, TRPM8, unlike TRPV1, is activated by PIP2, which therefore can be considered as an endogenous TRPM8 agonist. Hydrolysis of PIP2 to DAGs by PLCβ downregulates TRPM8, which is then further inhibited by DAGs via PKC activation [10], [11].
TRPM8 not only senses temperature at periphery, but also mediates synaptic transmission at the first sensory synapse between dorsal root ganglion (DRG) and dorsal horn (DH) neurons in the spinal cord [12], [13]. However, TRPV1 and TRPM8 are expressed also in nervous and non-nervous tissues that are not subjected to temperatures changes, thus suggesting other important functional roles beyond that temperature sensors. For example, TRPM8 is upregulated in prostate cancer and in other non-prostate primary human tumors (breast, colon, lung and skin) whereas in the corresponding normal tissue is undetectable [14]. The two best known cooling compounds known to activate TRPM8, i.e. the synthetic icilin and menthol, appear to do so through seemingly different mechanisms. Analogous to the putative capsaicin binding site for TRPV1 [15], residues in the transmembrane TM2–TM3 linker region of TRPM8 are involved in icilin sensitivity [16], but not in sensitivity to cold or menthol [17]. Icilin is structurally distinct from menthol, and its activation of TRPM8 is strictly dependent on the presence of extracellular calcium [16], whereas the activation by menthol is not and depends on residues on the TM2 segment and the C-terminus [17].
We previously showed that an increase in the intracellular concentration of cAMP, with subsequent activation of protein kinase A (PKA), leads to an enhancement of anandamide and capsaicin activity on TRPV1 overexpressed in HEK-293 cells [18] and that activation of co-expressed cannabinoid CB1 receptors, partly by decreasing PIP2 levels, also sensitizes TRPV1 to the action of its ligands [19]. Conversely, indirect evidence exists in sensory neurons for TRPM8 desensitization by PKA [20]. Therefore, we investigated here the effects of PKA activation and CB1 stimulation on TRPM8 ligand sensitivity. Furthermore, we studied the effect on TRPM8 of a series of endogenous lipids and pharmacological tools previously shown to influence TRPV1 activity.
Section snippets
Compounds
Icilin and (−)-Menthol were purchased from Sigma Aldrich; Capsaicin, capsazepine, N-arachidonoyl-dopamine (NADA) and resiniferatoxin (RTX) from Alexis Biochemicals; 5-iodo-RTX (IRTX) from Tocris; SR141716A and SR144528 were a kind gift of Sanofi-Aventis; PEA, anandamide, 6-Iodo-Arvanil [21], 6-Iodo-Olvanil [21], 6-I-nordihydrocapsaicin [22] and 6-Iodo-phenylacetylrinvanil [23] were synthesized as described in the corresponding references. HU-210 was a kind gift of Prof. R. Mechoulam, Hebrew
Effect of 8-Br-cAMP and forskolin on TRPM8 activation by icilin
As previously shown [7], [9], icilin dose-dependently elevated intracellular Ca2+ in TRPM8-HEK-293 cells but not in non-transfected cells (Fig. 1 and data not shown). The EC50 of icilin was 0.19 ± 0.03 μM, similar to that previously reported by others [7], [9], [24], [25]. Pre-incubation (5 min), and then continued incubation, of TRPM8-HEK-293 cells with either 8-Br-cAMP (100 μM) or forskolin (10 μM) caused a right-ward shift in the icilin dose–response curve (Fig. 1A). The EC50 was 0.50 ± 0.04 μM
Discussion
Of the several members of the TRP family of ion channels described to date, TRPM8 and TRPV1 receptors act each as a counterpart of the other because of their sensitivity to opposing thermal stimuli, i.e. cold (< 25 °C) and heat (> 42 °C), respectively, and to natural products, i.e. capsaicin and resiniferatoxin on the one hand and menthol and eucalyptol, on the other hand. Accordingly, it has been shown that these two proteins are co-expressed to some extent in DRG [31], dental [32] and
Acknowledgments
We thank Sven-Eric Jordt and David Julius, Department Cellular and Molecular Pharmacology, University of California, San Francisco, for the gift of the rat TRPM8 cDNA construct. This work was partly supported by a Volkswagenstiftung grant to VDM.
References (45)
- et al.
Lipids as regulators of the activity of transient receptor potential type V1 (TRPV1) channels
Life Sci.
(2005) - et al.
A TRP channel that senses cold stimuli and menthol
Cell
(2002) - et al.
Molecular basis for species-specific sensitivity to “hot” chili peppers
Cell
(2002) - et al.
The super-cooling agent icilin reveals a mechanism of coincidence detection by a temperature-sensitive TRP channel
Neuron
(2004) - et al.
Prospects for prostate cancer imaging and therapy using high-affinity TRPM8 activators
Cell Calcium
(2007) - et al.
Ruthenium red as a capsaicin antagonist
Life Sci.
(1991) - et al.
The diversity in the vanilloid (TRPV) receptor family of ion channels
Trends Pharmacol. Sci.
(2002) - et al.
The activity of anandamide at vanilloid VR1 receptors requires facilitated transport across the cell membrane and is limited by intracellular metabolism
J. Biol. Chem.
(2001) - et al.
Noxious heat receptors present in cold-sensory cells in rats
Neurosci. Lett.
(2004) - et al.
Functional expression of thermo-transient receptor potential channels in dental primary afferent neurons: implication for tooth pain
J. Biol. Chem.
(2006)
Cooling inhibits capsaicin-induced currents in cultured rat dorsal root ganglion neurones
Neurosci. Lett.
More than cool: promiscuous relationships of menthol and other sensory compounds
Mol. Cell. Neurosci.
Cool (TRPM8) and hot (TRPV1) receptors in the bladder and male genital tract
J. Urol.
Molecular determinants of vanilloid sensitivity in TRPV1
J. Biol. Chem.
The capsaicin receptor: a heat-activated ion channel in the pain pathway
Nature
Endovanilloids. Putative endogenous ligands of transient receptor potential vanilloid 1 channels
Eur. J. Biochem.
Bradykinin and nerve growth factor release the capsaicin receptor from PtdIns(4,5)P2-mediated inhibition
Nature
Induction of vanilloid receptor channel activity by protein kinase C
Nature
Protein kinase C activation potentiates gating of the vanilloid receptor VR1 by capsaicin, protons, heat and anandamide
J. Physiol.
Identification of a cold receptor reveals a general role for TRP channels in thermosensation
Nature
Characterization of the mouse cold-menthol receptor TRPM8 and vanilloid receptor type-1 VR1 using a fluorometric imaging plate reader (FLIPR) assay
Br. J. Pharmacol.
Functional control of cold- and menthol-sensitive TRPM8 ion channels by phosphatidylinositol 4,5-bisphosphate
J. Neurosci.
Cited by (133)
The neglected role of endocannabinoid actions at TRPC channels in ataxia
2022, Neuroscience and Biobehavioral ReviewsTargeting Endocannabinoid System in Epilepsy: For Good or for Bad
2022, NeuroscienceInhibition of TRPM8 function by prostacyclin receptor agonists requires coupling to Gq/11 proteins
2023, British Journal of PharmacologyOverview: Chronic Pain and Cannabis-Based Medicines
2023, PharmacopsychiatryTherapeutic potential of TRPM8 channels in cancer treatment
2023, Frontiers in Pharmacology