Seeing Touch: Moving Closer to the Worm Mechanotransduction Complex

All organisms sense their immediate environment using specialized mechanoreceptors, cells that respond to external mechanical forces. These exquisitely sensitive cells underlie osmosensation, touch, hearing, balance and proprioception. All mechanoreceptors that have been studied in depth respond to external forces by opening transduction channels — mechanically gated ion channels. Although large-scale genetic screens have revealed an assortment of genes essential for mechanosensation in worms [1] and flies [2], definitive molecular identification of these transduction channels has largely eluded us. The earliest and most complete description of genes required for mechanosensation exploited the nematode Caenorhabditis elegans. To identify worms deficient in their responses to gentle touch, Chalfie and Sulston [1] devised a screen involving a simple prod with an eyebrow hair. They and others have found over 450 mutant worm lines defective in their response to such a stimulus, defining 18 genes [3]. Over the past two decades, most of the genes responsible for these mechanosensory (mec) mutants have been identified, including several that encode candidate subunits of a hypothetical transduction complex. Of particular interest are mec-4 and mec-10, two genes that code for degenerins (DEGs), ion channels related to the vertebrate epithelial sodium channels (ENaCs). Worms require mec-4 and mec-10 to generate neuronal responses to gentle touch stimuli [4–6], and their protein products MEC-4 and MEC-10 have been proposed to form part of the transduction channel in touch neurons [7]. Proving that a channel is directly opened by a mechanical force is a difficult task, however, and is even more challenging in C. elegans. Accessing neurons in nematodes requires rupture of the cuticle, and the high internal pressure causes cells to explode out the incision. To date, investigators have failed in attempts to demonstrate mechanically activated transduction currents in touch neurons from C. elegans. In a recent study, Suzuki et al. [8] have come much closer to proving that MEC-4 and MEC-10 are part of the C. elegans touch-cell transduction channel. To avoid the difficulties associated with direct electrical recording from C. elegans neurons, they used transgenic nematodes expressing a genetically encoded Ca2+ indicator — a ‘cameleon’ — and monitored intracellular Ca2+ changes in response to gentle touch in intact animals. Their results clearly show that mec-4 and mec-10 are essential for the immediate response of a neuron to gentle touch sensation, not just for basic neuronal function. Cameleons are intramolecular probes which exploit the phenomenon of fluorescence resonance energy transfer (FRET) [9]. The cameleon used by Suzuki et al. [8] consisted of yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP) moieties linked by calmodulin and a calmodulin-binding peptide. When Ca2+ enters a cell, it binds to the calmodulin moiety and brings the CFP closer to YFP, enhancing the FRET between the two fluorophores (Figure 1A). A change in the ratio of the fluorescent signals in the cell directly reflects a change in intracellular Ca2+. Because the cameleon used by Suzuki et al. [8] was under the control of the mec-4 promoter, intracellular Ca2+ could be monitored specifically in the six touch neurons that normally express mec-4 (Figure 1B). Suzuki et al. [8] monitored cameleon fluorescence while subjecting the worm to several modes of gentle touch. They found that mec-4-expressing neurons preferentially responded to a moving stimulus (buzzes along the cuticle) or a fast poke, but were less responsive to sustained pressure (presses in one spot). Following a fast poke, Ca2+ increased in the touch neurons within 50 milliseconds of the stimulus. By contrast, nematodes lacking mec-4 and mec-10 showed no Ca2+ increase in response to touch stimuli. Cells from mec-4 and mec-10 null nematodes showed a normal Ca2+ response to K+-induced depolarization, however, indicating that the membrane properties of these cells were normal. These data show that MEC-4 and MEC-10 are responsible for an event that occurs within 50 milliseconds of a mechanical stimulus, perhaps activation of the transduction channel itself. Suzuki et al. [8] also obtained data implicating an Ltype calcium channel, EGL-19, in the Ca2+ transients observed in response to gentle touch in C. elegans. Because nematodes lack voltage-dependent Na+ channels [10], the most likely role for these Ca2+ channels is in propagation of the signal from the distal site of transduction to the synapse. Recently, Shin et al. [11] demonstrated that the T-type calcium channel Cav3.2 is required for proper function of a class of mammalian skin mechanoreceptors. By contrast, null mutations in the only nematode gene for a T-type Ca2+ channel, cca1, do not affect touch sensitivity [8]. The difference between nematodes and mammals in the type of Ca2+ channels activated during mechanical stimulation may Dispatch Current Biology, Vol. 13, R967–R969, December 16, 2003, ©2003 Elsevier Science Ltd. All rights reserved. DOI 10.1016/j.cub.2003.11.045