Systematic and quantitative mRNA expression analysis of TRP channel genes at the single trigeminal and dorsal root ganglion level in mouse

Abstract Background Somatosensory nerve fibres arising from cell bodies within the trigeminal ganglia (TG) in the head and from a string of dorsal root ganglia (DRG) located lateral to the spinal cord convey endogenous and environmental stimuli to the central nervous system. Although several members of the transient receptor potential (TRP) superfamily of cation channels have been implicated in somatosensation, the expression levels of TRP channel genes in the individual sensory ganglia have never been systematically studied. Results Here, we used quantitative real-time PCR to analyse and compare mRNA expression of all TRP channels in TG and individual DRGs from 27 anatomically defined segments of the spinal cord of the mouse. At the mRNA level, 17 of the 28 TRP channel genes, TRPA1, TRPC1, TRPC3, TRPC4, TRPC5, TRPM2, TRPM3, TRPM4, TRPM5, TRPM6, TRPM7, TRPM8, TRPV1, TRPV2, TRPV4, TRPML1 and TRPP2, were detectable in every tested ganglion. Notably, four TRP channels, TRPC4, TRPM4, TRPM8 and TRPV1, showed statistically significant variation in mRNA levels between DRGs from different segments, suggesting ganglion-specific regulation of TRP channel gene expression. These ganglion-to-ganglion differences in TRP channel transcript levels may contribute to the variability in sensory responses in functional studies. Conclusions We developed, compared and refined techniques to quantitatively analyse the relative mRNA expression of all TRP channel genes at the single ganglion level. This study also provides for the first time a comparative mRNA distribution profile in TG and DRG along the entire vertebral column for the mammalian TRP channel family.</p

De rol van TRP kanalen in het voelen van pijnlijke hitte

Although several members of the transient receptor potential (TRP) superfamily of cation channels have been implicated in somatosensation, the expression levels of TRP channel genes in the individual sensory ganglia have never been systematically studied. In the first part of this thesis, we developed, compared, and refined techniques to quantitatively analyse the relative mRNA expression of all TRP channels in trigeminal and individual dorsal root ganglia from 27 anatomically defined segments of the spinal cord of the mouse. At the mRNA level, 17 of the 28 TRP channel genes, TRPA1, TRPC1, TRPC3, TRPC4, TRPC5, TRPM2, TRPM3, TRPM4, TRPM5, TRPM6, TRPM7, TRPM8, TRPV1, TRPV2, TRPV4, TRPML1, and TRPP2, were detectable in every tested ganglion. Notably, four TRP channels, TRPC4, TRPM4, TRPM8, and TRPV1, showed statistically significant variation in mRNA levels between dorsal root ganglia from different segments, suggesting ganglion-specific regulation of TRP channel gene expression. This study provides for the first time a comparative mRNA distribution profile in TG and DRG along the entire vertebral column for the mammalian TRP channel family. Furthermore, although it is well-known that acute heat sensing in mammals is performed by a subset of sensory neurons expressing the heat sensor TRPV1, genetic ablation of TRPV1 or other candidate heat sensors, such as TRPM3, only resulted in small deficits in noxious heat sensing. So, the molecular basis of noxious heat sensing remained largely unresolved. In the second part of this thesis, we demonstrate that acute noxious heat sensing is mediated by a set of three TRP ion channels. We compared the sensitivity to heat in wild type mice and mice deficient for TRPV1, TRPM3, and TRPA1 in combined knockouts. We found that all double knockout mice retain robust heat responses at the cellular and behavioural level. In contrast, Trpa1-/-/Trpm3-/-/Trpv1-/- mice lack heat responses in isolated sensory neurons, and show a striking deficit in the heat sensitivity of intact C and Aδ nerve fibres and in the nociceptive responses to painfully hot temperatures. Reintroducing any of the three TRP channels into triple knockout sensory neurons restores heat responsiveness. Together, these data indicate that TRPA1, TRPV1, and TRPM3 have critical but redundant roles in acute noxious heat sensing. This redundancy could be a fail-safe mechanism allowing heat avoidance, even when the function of one or two molecular heat sensors is compromised.status: publishe

Heat is absolute, cold is relative

In vivo imaging of the spinal cord provides insights into the coding of skin temperature. Intriguingly, while heat-responsive dorsal horn neurons encode absolute temperatures, cold-responsive neurons report relative drops.status: publishe

Reply to: Heat detection by the TRPM2 ion channel

status: publishe

VAMP7 regulates constitutive membrane incorporation of the cold-activated channel TRPM8

The cation channel TRPM8 plays a central role in the somatosensory system, as a key sensor of innocuously cold temperatures and cooling agents. Although increased functional expression of TRPM8 has been implicated in various forms of pathological cold hypersensitivity, little is known about the cellular and molecular mechanisms that determine TRPM8 abundance at the plasma membrane. Here we demonstrate constitutive transport of TRPM8 towards the plasma membrane in atypical, non-acidic transport vesicles that contain lysosomal-associated membrane protein 1 (LAMP1), and provide evidence that vesicle-associated membrane protein 7 (VAMP7) mediates fusion of these vesicles with the plasma membrane. In line herewith, VAMP7-deficient mice exhibit reduced functional expression of TRPM8 in sensory neurons and concomitant deficits in cold avoidance and icilin-induced cold hypersensitivity. Our results uncover a cellular pathway that controls functional plasma membrane incorporation of a temperature-sensitive TRP channel, and thus regulates thermosensitivity in vivo.status: publishe

VAMP7 regulates constitutive membrane incorporation of the cold-activated channel TRPM8.

International audienceThe cation channel TRPM8 plays a central role in the somatosensory system, as a key sensor of innocuously cold temperatures and cooling agents. Although increased functional expression of TRPM8 has been implicated in various forms of pathological cold hypersensitivity, little is known about the cellular and molecular mechanisms that determine TRPM8 abundance at the plasma membrane. Here we demonstrate constitutive transport of TRPM8 towards the plasma membrane in atypical, non-acidic transport vesicles that contain lysosomal-associated membrane protein 1 (LAMP1), and provide evidence that vesicle-associated membrane protein 7 (VAMP7) mediates fusion of these vesicles with the plasma membrane. In line herewith, VAMP7-deficient mice exhibit reduced functional expression of TRPM8 in sensory neurons and concomitant deficits in cold avoidance and icilin-induced cold hypersensitivity. Our results uncover a cellular pathway that controls functional plasma membrane incorporation of a temperature-sensitive TRP channel, and thus regulates thermosensitivity in vivo