We are using C. elegans as a model system to study osmosensation and mechanosensation in the nervous system. When the animals encounter high osmolarity, a mechanical stimulus or volatile repellent, they reverse direction to avoid the stimulus. Laser ablation studies have demonstrated that the ASH neurons are primarily responsible for these responses (CGC82; WBG 10(1):89; CGC2309 and CGC1374). The molecular mechanism of osmosensation or mechanosensation is unclear, but studies from other organisms have suggested that the receptors may be mechanically gated channels which respond to membrane stretch. DEG-1 and UNC-8, members of the degenerin family, were thought to be likely candidates for the ASH receptors based on their expression pattern and sequences. However, the
deg-1(
u506u550);
unc-8(
n491n1193) double null mutant animals are normal in their ASH-mediated responses (Hart, Kass, Shapiro and Kaplan, in prep). Still unidentified degenerins or unrelated proteins may act as sensory receptors in ASH.
osm-10 is specifically required for osmosensation (WBG 10(3):167). Wild type (N2) animals cross an osmotic barrier only 0-5% of the time, whereas
osm-10 (
n1602) animals cross the barrier 90-95% of the time.
osm-10(
n1602) animals are normal in their responses to other stimuli mediated by ASH.
osm-10 encodes a novel protein of 419 amino acids which is expressed in the ASH and ASI sensory neurons in the head, as well as PHA and PHB in the tail (Hart, Kass, Shapiro and Kaplan, in prep). OSM-10 protein contains 38 putative serine/threonine phosphorylation sites and is localized to the cytoplasm, suggesting that it may be involved in intracellular signal transduction.
osm-10(
n1602) is a recessive, genetic null allele, but not a molecular null. It changes an E to a K codon in a putative tyrosine phosphorylation site. The mutant OSM-10 expression is normal by western blot and immunohistochemical analysis. To identify additional proteins involved in osmosensation, we are cloning genes which interact geneticly with
osm-10 (
n1602)III. Two enhancers of
osm-10,
eos-1 (
nu288) IV and
eos-2(
nu268) III, have been identified. They are nonallelic noncomplementers of
osm-10(
n1602), suggesting that their gene products may act together in a common signal transduction pathway. Neither
eos-1 nor
eos-2 interact geneticly with
deg-1;
unc-8. Currently, we are screening for other genes involved in osmosensation, more alleles of
osm-10, and more nonallelic noncomplementers of
osm-10. In the latter screen, 1,873 haploid genomes have been screened, and 9 mutant strains have been identified. In collaboration with Dr. M. Vidal and Dr. M. Walhout, we are also identifying the physical interactors of OSM-10 in a yeast two-hybrid system. To assess the molecular basis of modality coding and mechanosensation in this circuit, we are also screening for additional nose touch defective (Not) mutants. Out of 8,710 worms screened, 15 mutants have been isolated, two of which are modality specific. Mapping and the further characterization of the mutations is underway. We hope that the characterization of these genes will lead us to a better understanding of how multicellular organisms sense different stimuli and how polymodal neurons differentiate between multiple sensory modalities.