Members of the DEG/ENaC ion channel subunit superfamily (degenerin/epithelial Na+ channel) encode amiloride-sensitive sodium ion channels important in strikingly diverse biological functions such as touch sensation, proprioception, taste, learning and memory. In humans, ENaCs reabsorb Na+ in kidney, colon and lung epithelia to maintain body salt and water homeostasis. In C. elegans, degenerins (named so because certain mutations in these genes induce neuronal degeneration) are implicated in mechanotransduction. The best studied of these are
mec-4 and
mec-10, which are co-expressed in the six touch cell neurons to form a heteromeric ion channel specifically required for a response to gentle touch stimuli. The DEG/ENaC superfamily consists of 21 C. elegans, 30 Drosophila and at least 9 mammalian genes. We recently isolated three probable null C. elegans degenerins: C24G7.2, T28F2.7 and ZK770.1, obtained by ethyl methanesulfonate (EMS) mutagenesis and PCR-based library screening. Two sensory systems respond to specific touches in the head region (Kaplan, 1993, P.N.A.S.). One of these is a backing response to head-on collision with an obstacle in the forward path. Laser ablation studies identified three classes of sensory neurons required in parallel for the response to head-on collision: ASH, FLP and OLQ. A second head touch mechanosensory system is a head withdrawal in response to light touch on the side of the head. Two classes of mechanosensory neurons, OLQ and IL1, sense gentle touch to the side of the head. We previously reported data from analysis of degenerin GFP fusions indicating that ZK770.1 is expressed in ASH, OLQ and IL1 neurons and the C24G7.2 is expressed in ASH neurons (Tavernarakis et al., IWM 662). Body touch, nose touch, and head touch assays on mutant ZK770.1 worms revealed head touch insensitivity, an interesting finding for two reasons. First, this is the first direct evidence of involvement of a C. elegans degenerin in a second touch modality. Second, since nose touch appears grossly unaffected by ZK770.1 deletion, this degenerin might be functionally redundant with C24G7.2 or with
deg-1, which is also expressed in ASH. We are assaying C24G7.2 and constructing double mutants to investigate further. Our genetic and behavioral analysis will set the stage for electrophysiological description of differences between nose touch and body-touch transducing channels. Another interesting observation is that C24G7.2 and T28F2.7 are expressed in some male tail cells that may participate in touch-sensitive components of mating behaviors. We are packing these lines off to our expert collaborators (Sternberg and Barr groups) for some fun and analysis.