- Open Access
© Drew and Wood; licensee BioMed Central Ltd. 2005
- Received: 06 February 2005
- Accepted: 15 February 2005
- Published: 15 February 2005
- Mechanical Stimulus
- Touch Receptor Neuron
- Mechanosensory System
- Touch Avoidance
- Mammalian Hair Cell
Mechanosensation plays a pivotal role in many aspects of pain pathology, yet the mammalian molecular transduction apparatus responsible for this sensory modality remains unknown. In January's edition of Nature Neuroscience, O'Hagan, Chalfie and Goodman  have provided direct electrophysiological evidence that somatic mechanotransduction in C. elegans is mediated by a complex of proteins previously identified in genetic screens for impaired touch sensation. Are the homologues of these proteins important for pain sensation in mammals? Perhaps surprisingly, the balance of evidence suggests that other proteins are better candidate noxious mechanosensors in mammals.
Many forms of pain, be it in acute, inflammatory or disease-related conditions, are triggered by mechanical stimuli. However, in mammals there is very little understanding of the molecular transduction process that converts mechanical stimuli into a change in membrane excitability. Studying mechanosensation in mammals is hampered by the diffuse and inaccessible distribution of nerve terminals in the periphery. The few studies of receptor potentials, made using extracellular recordings (mainly from Pacinian corpuscles of the cat's mesentery), do however suggest that mechanical stimuli depolarise termini by directly gating cationic channels .
To record from body touch receptors, O'Hagan et al used transgenic animals in which these cells were labelled with GFP. Using immobilised worms, the authors released the internal hydrostatic pressure away from the recording site and then exposed the cell bodies of posterior, lateral receptor neurons. Then patch-clamp recordings were made from the cell body while the mechanosensitive neurite was stimulated with a glass probe applied to the body wall. The authors observed that both the application and withdrawal of mechanical stimuli evoked rapidly adapting inward currents, whose amplitude was proportional to the magnitude of the stimulus. Consistent with the currents being mediated by members of the DEG/ENaC family, they were carried by sodium ions and blocked by amiloride. Next, given the extensive genetic analysis of mechanosensation in this species the investigators were able to extend their work by studying receptor currents in a range of mutant animals. Firstly, it was shown that null mutations in MEC-4, MEC-2 and MEC-6 abolished MRCs, suggesting that these 3 proteins (which physically interact) are essential for channel gating. An important control experiment was to show that voltage-gated currents in these mutants were normal. Subsequently, it was found that other (behaviourally less severe) mutations in MEC-4 and MEC-10 greatly reduced MRC amplitude and significantly altered the current-voltage relationship of MRCs. Hence, this is the first direct demonstration that MEC-4 and MEC-10 form the mechanotransducing ion channel in C. elegans. Finally, the group analysed MRCs in nematodes with a mutation in MEC-7, a β-tubulin required for formation of touch cell specific 15-protofilament microtubules, which had been hypothesised to be intracellular "anchors" required for channel gating. Interestingly, despite a large decrease in their amplitude and threshold, MRCs were not abolished in these mutants suggesting that MEC-7 is not an absolute requirement for channel gating.
This study represents a confirmation of the key aspects of a long-standing model of mechanotransduction. However, the relationship between this system and those in operation in mammalian somatic mechanosensation remains unclear. In mammals there are 9 identified DEG/ENac channels, which form two subfamilies; the epithelial sodium channels (ENaCα, β, γ and δ) and the acid sensing ion channels (ASIC1-4, and the closely related intestinal sodium channel, INaC). ENaCα, β and γ together form a constitutively active channel principally associated with non-neuronal tissues. β and γ ENaC do appear to be expressed in DRG neurons  but, as yet, their function there has not been studied. However, much interest was aroused in ASICs as potential mechanosensors because they are highly expressed in sensory neurons and 2 isoforms (ASIC3 and 1b) are almost exclusively expressed in these cells. Currently, the only known activator of these channels is external acidification, which gates 4 of the 6 known splice variants when they are expressed alone (interestingly MEC-4 and MEC-10 are not gated by protons) . However, it has been suggested that if localised in a mechanotransduction complex analogous to that found in C. elegans, ASICs might mediate mammalian mechanosensation . To test this hypothesis Michael Welsh and Gary Lewin collaborated in generating null mutants of ASIC1, 2 and 3 and assessing their somatosensory phenotypes using the skin-nerve preparation. In stark contrast to the dramatic effects of null mutations in MEC-4 and MEC-10, ablation of these genes had minor effects on mechanosensory responses. The first study found an approximate halving of the suprathreshold firing rates of rapidly adapting low threshold mechanoreceptors (LTMs) in ASIC2 nulls and a minor decrease in slowly adapting LTMs whilst the responses of all other fibre types were unchanged from wild type values . In ASIC3 knockouts, rapidly adapting LTMs had an increased sensitivity to mechanical stimuli whereas Aδ-mechanonociceptors showed a decrease in responsiveness  and in ASIC1 null mutants cutaneous mechanosensation was unchanged from wild-type levels . Whilst the analysis of double and triple knockouts would be worthwhile given the possibility that the remaining subunits functionally compensate for the missing ones in null mutants (although their expression was unchanged at the transcriptional level), the phenotypes of these animals is not consistent with ASICs being major transducers of mechanical stimuli in mammalian sensory nerves. Moreover, in an analysis of a separate line of ASIC2 nulls, no alteration in the sensitivity of rapidly adapting LTMs was found  and no group has reported mechanical gating of ASICs. Although mechanical gating of ion channels that are mechanosensitive in situ may be difficult using in vitro systems, different subpopulations of cultured DRG neurons are known to display distinct mechanically activated cationic currents  and these currents are unchanged in ASIC2 and/or 3 null mutants . These data therefore suggest that other ion channels act as the primary mechanotransducers in mammals.
Whilst research on body touch receptors in C. elegans focussed attention on DEG/ENaC channels, genetic screens of other mechanosensory systems, particularly in Drosophila, have also revealed major roles for TRP channels in mechanosensation. In fruit flies, TRP-like channels NOMPC  and Nanchung  have been strongly implicated as mechanotransduction channels in Type I mechanosensors required for touch and hearing, respectively. In Drosophila larvae, Painless, a TRPV-like protein, is expressed in nociceptor-like cells and mutants have defective responses to noxious thermal and mechanical stimuli . Also, in C. elegans OSM-9 is required for nose touch avoidance . Research in mammalian systems has now produced evidence suggesting TRPA1 may be the transduction channel in hair cells  whilst TRPC1 has recently been shown to be directly mechanosensitive . With regard to noxious mechanosensation, TRPV4 knockouts were found to have behavioural deficits in response to tail pressure , although this channel seems to be expressed at much higher levels in keratinocytes than in sensory neurons. Given that a number of TRP channels are already known to be central to thermosensation and inflammatory function in nociceptors, members of this family represent interesting candidates for mammalian noxious (and innocuous) mechanosensors.
In conclusion, the primary candidates for the role of mammalian mechanotransducers are members of the TRP and DEG/ENaC ion channel families, both of which are remarkably functionally diverse. However, the evidence supporting a function for any particular channel in mammalian mechanotransduction is much weaker than in invertebrate systems. Interestingly, the diversity of DEG/ENaC channels in C. elegans (28 homologues) in comparison to mammals (mice have 8) is striking, and the observation that mechanosensitive channels in nematodes form a distinct subgroup that all contain a specific extracellular regulatory domain  makes extrapolation of the C. elegans results to mammals less certain. Related to the diversity of putative mechanosensory ion channels is the issue of diversity in cellular systems that mediate mechanosensation. Despite similarities, the phylogenetic relationship between mammalian hair cells and primary somatosensory neurons and the analogous cells types in invertebrates is poorly established. Also, the extent to which chemically mediated mechanosensation functions in certain systems, potentially including some forms of mechanically induced pain, is currently unclear (for example see Ref. 24). Thus, much remains to be learnt regarding the molecular basis of mechanotransduction and when this is achieved, it should be possible to determine the evolutionary relationships of multiple mechanosensory systems. In addition, identification of the molecular basis of noxious mechanosensation should provide exciting new analgesic drug targets.
- O'Hagan R, Chalfie M, Goodman MB: The MEC-4 DEG/ENaC channel of Caenorhabditis elegans touch receptor neurons transduces mechanical signals. Nat Neurosci 2005, 8: 43–50. 10.1038/nn1362PubMedView ArticleGoogle Scholar
- Loewenstein WR, Skalak R: Mechanical transmission in a Pacinian corpuscle. An analysis and a theory. J Physiol (Lond) 1966, 182: 346–378.View ArticleGoogle Scholar
- Ernstrom GG, Chalfie M: Genetics of sensory mechanotransduction. Annu Rev Genet 2002, 36: 411–453. 10.1146/annurev.genet.36.061802.101708PubMedView ArticleGoogle Scholar
- Tavernarakis N, Driscoll M: Molecular modeling of mechanotransduction in the nematode Caenorhabditis elegans . Annu Rev Physiol 1997, 59: 659–689. 10.1146/annurev.physiol.59.1.659PubMedView ArticleGoogle Scholar
- Chelur DS, Ernstrom GG, Goodman MB, Yao CA, Chen L, O' Hagan R, Chalfie M: The mechano-sensory protein MEC-6 is a subunit of the C. elegans touch-cell degenerin channel. Nature 2002, 420: 669–673. 10.1038/nature01205PubMedView ArticleGoogle Scholar
- Goodman MB, Ernstrom GG, Chelur DS, O'Hagan R, Yao CA, Chalfie M: MEC-2 regulates C. elegans DEG/ENaC channels needed for mechanosensation. Nature 2002, 415: 1039–1042. 10.1038/4151039aPubMedView ArticleGoogle Scholar
- Drummond HA, Abboud FM, Welsh MJ: Localization of beta and gamma subunits of ENaC in sensory nerve endings in the rat foot pad. Brain Res 2000, 884: 1–12. 10.1016/S0006-8993(00)02831-6PubMedView ArticleGoogle Scholar
- Waldmann R, Lazdunski M: H + -gated cation channels: neuronal acid sensors in the NaC/DEG family of ion channels. Curr Opin Neurobiol 1998, 8: 418–424. 10.1016/S0959-4388(98)80070-6PubMedView ArticleGoogle Scholar
- Welsh MJ, Price MP, Xie J: Biochemical basis of touch perception: Mechanosensory function of DEG/ENaC channels. J Biol Chem 2001, 277: 2369–2372. 10.1074/jbc.R100060200PubMedView ArticleGoogle Scholar
- Price MP, Lewin GR, McIlwrath SL, Cheng C, Xie J, Heppenstall PA, Stucky CL, Mannsfeldt AG, Brennan TJ, Drummond HA, Qiao J, Benson CJ, Tarr DE, Hrstka RF, Yang B, Williamson RA, Welsh MJ: The mammalian sodium channel BNC1 is required for normal touch sensation. Nature 2000, 407: 1007–1011. 10.1038/35039512PubMedView ArticleGoogle Scholar
- Price MP, McIlwrath SL, Xie J, Cheng C, Qiao J, Tarr DE, Sluka KA, Brennan TJ, Lewin GR, Welsh MJ: The DRASIC cation channel contributes to the detection of cutaneous touch and acid stimuli in mice. Neuron 2001, 32: 1071–1083. 10.1016/S0896-6273(01)00547-5PubMedView ArticleGoogle Scholar
- Page AJ, Brierley SM, Martin CM, Martinez-Salgado C, Wemmie JA, Brennan TJ, Symonds E, Omari T, Lewin GR, Welsh MJ, Blackshaw LA: The ion channel ASIC1 contributes to visceral but not cutaneous mechanoreceptor function. Gastroenterology 2004, 127: 1739–1747.PubMedView ArticleGoogle Scholar
- Roza C, Puel JL, Kress M, Baron A, Diochot S, Lazdunski M, Waldmann R: Knockout of the ASIC2 channel in mice does not impair cutaneous mechanosensation, visceral mechanonociception and hearing. J Physiol 2004, 558: 659–669. 10.1113/jphysiol.2004.066001PubMed CentralPubMedView ArticleGoogle Scholar
- Drew LJ, Wood JN, Cesare P: Distinct mechanosensitive properties of capsaicin-sensitive and -insensitive sensory neurons. J Neurosci 2002, 22: RC228.PubMedGoogle Scholar
- Drew LJ, Rohrer DK, Price MP, Blaver K, Cockayne DA, Cesare P, Wood JN: ASIC2 and ASIC3 Do Not Contribute to Mechanically Activated Currents in Mammalian Sensory Neurons. J Physiol 2004, 556: 691–710. 10.1113/jphysiol.2003.058693PubMed CentralPubMedView ArticleGoogle Scholar
- Walker RG, Willingham AT, Zuker CS: A Drosophila mechanosensory transduction channel. Science 2000, 287: 2229–2234. 10.1126/science.287.5461.2229PubMedView ArticleGoogle Scholar
- Kim J, Chung YD, Park DY, Choi S, Shin DW, Soh H, Lee HW, Son W, Yim J, Park CS, Kernan MJ, Kim C: A TRPV family ion channel required for hearing in Drosophila . Nature 2003, 424: 81–84. 10.1038/nature01733PubMedView ArticleGoogle Scholar
- Tracey WD Jr, Wilson RI, Laurent G, Benzer S: painless, a Drosophila gene essential for nociception. Cell 2003, 113: 261–273. 10.1016/S0092-8674(03)00272-1PubMedView ArticleGoogle Scholar
- Colbert HA, Smith TL, Bargmann CI: OSM-9, a novel protein with structural similarity to channels, is required for olfaction, mechanosensation, and olfactory adaptation in Caenorhabditis elegans . J Neurosci 1997, 17: 8259–8269.PubMedGoogle Scholar
- Corey DP, Garcia-Anoveros J, Holt JR, Kwan KY, Lin SY, Vollrath MA, Amalfitano A, Cheung EL, Derfler BH, Duggan A, Geleoc GS, Gray PA, Hoffman MP, Rehm HL, Tamasauskas D, Zhang DS: TRPA1 is a candidate for the mechanosensitive transduction channel of vertebrate hair cells. Nature 2004, 432: 723–730. 10.1038/nature03066PubMedView ArticleGoogle Scholar
- Maroto R, Raso A, Wood TG, Kurosky A, Martinac B, Hamill OP: TRPC1 forms the stretch-activated cation channel in vertebrate cells. Nat Cell Biol 2005, 7: 179–185. 10.1038/ncb1218PubMedView ArticleGoogle Scholar
- Suzuki M, Mizuno A, Kodaira K, Imai M: Impaired pressure sensation in mice lacking TRPV4. J Biol Chem 2003, 278: 22664–22668. 10.1074/jbc.M302561200PubMedView ArticleGoogle Scholar
- Goodman MB, Schwarz EM: Transducing touch in Caenorhabditis elegans . Annu Rev Physiol 2002, 65: 429–452. 10.1146/annurev.physiol.65.092101.142659PubMedView ArticleGoogle Scholar
- Vlaskovska M, Kasakov L, Rong W, Bodin P, Bardini M, Cockayne DA, Ford AP, Burnstock G: P2X 3 knock-out mice reveal a major sensory role for urothelially released ATP. J Neurosci 2001, 21: 5670–5677.PubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.