Worm Breeder's Gazette 15(5): 41 (February 1, 1999)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Novel functions of the Ras-MAPK signal transduction pathway in neurons of C. elegans

Takaaki Hirotsu1, Satoshi Saeki 2, Yuichi Iino1

1 Molecular Genetics Research Laboratory, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
2 Tokyo Research Laboratories, Kyowahakkokogyo Co. Ltd., Machida, Japan

The Ras-mediated signal transduction pathways are known to play roles in vulval induction and several other developmental events in C. elegans. However, the function of Ras in the nervous system, which is currently the focus of attention in mammals, is not known in this organism. We have noticed that mutants affected in the pathway have altered chemotactic response, and report here our results indicating that the pathway functions in neurons involved in olfaction and in locomotion.

Chemotaxis to volatile odorants is mediated by two pairs of chemosensory neurons in C. elegans : AWA and AWC. The gain-of-function mutant of let-60ras, let-60 (n1046gf), showed a severe defect in chemotaxis to isoamylalcohol, which is sensed by AWC. This phenotype was suppressed by loss-of-function mutations in ksr-1, mek-2 or mpk-1, which function downstream of let-60 in vulval induction. Similar phenotypes were observed in the response to butanone and benzaldehyde, which are also sensed by AWC. These results indicate that LET-60Ras is involved in chemotactic response mediated by the AWC neurons, and that the MAPK pathway functions downstream of Ras. Whereas the let-60(gf) mutant showed normal response to odorants sensed by the AWA neurons, the let-60 (n2021lf) and lin-45 (sy96lf) mutants showed moderate but significant defects in chemotaxis to both AWA-sensed and AWC-sensed odorants.

It is an important question whether the pathway is involved in neural development or the function of mature nerve cells. To answer this question, we expressed let-60 under the control of the heat-shock promoter hsp16-2. N2 animals in which the let-60 (n1046gf) transgene was expressed during embryogenesis showed normal response to isoamylalcohol. In contrast, chemotaxis to isoamylalcohol decreased significantly when let-60(gf) was expressed in adults. Similarly, expression of let-60(+) in let-60(lf) adults partially restored the response to isoamylalcohol (which is sensed by AWC) and diacetyl(which is sensed by AWA). These results suggest that the Ras-MAPK pathway works in mature nerve cells and is not involved in neural development. Further, we placed let-60(n1046gf) under the control of the gcy-10 promoter, which drives expression in AWC, AWB and I1. Response to isoamylalcohol, but not to diacetyl, was affected in N2 animals transgenic for this construct. Similarly, gcy10::let-60(+) restored response to isoamylalcohol, but not to diacetyl, to the let-60(lf) mutant. Thus the proper functioning of the Ras-mediated signaling pathway in the AWC chemosensory neurons appears to be necessary for AWC-mediated chemotaxis.

Loss-of-function mutants affected in this pathway also exhibited another interesting phenotype. A large fraction (25%`60%) of the sem-5(n2019lf), let-60(lf) and lin-45(lf) mutants continuously circled on blank chemotaxis plates, as evident from the spiral tracks they left on the plates. This is not a general defect in locomotion, because these mutants are capable of normal locomotion and chemotaxis if water-soluble attractant NaCl, to which they can respond, is presented. The phenotype of let-60(lf) was rescued by the induced expression of let-60(+) during embryogenesis, or by the neuron-specific expression using the H20 promoter. These results suggest that the circular locomotion is due to the defects in neural development. gcy10::let-60(+) did not rescue the locomotion defect of the let-60(lf) animals.