C. elegans can sense and memorize various environmental stimuli and modify its behavioral response. Thermotaxis is a behavioral response to temperature and provides a good model system to study thermosensation and neural plasticity. To identify new molecular components involved in thermotaxis, we have isolated many thermotaxis-defective mutants (1).
ttx-5(
nj2) and
ttx-5(
nj6) mutants showed athermotactic (non-temperature-responsive) phenotype. These mutants also showed defective chemotactic responses to NaCl and odorants. We found that
ttx-5(
nj2) mutation did not complement with
eat-4 null mutation,
eat-4(
ky5).
eat-4 mutation has already been isolated as eating defective mutant (2, 3), and the
eat-4 mutants are defective in sensory behaviors, such as thermotaxis (4). The athermotactic abnormality of
ttx-5 mutants was rescued by genomic fragment including
eat-4 gene, and
ttx-5 mutants contained point mutations in
eat-4 gene. These results suggest that athermotactic abnormality of
ttx-5 mutants is caused by defective function of
eat-4 gene. The
eat-4 gene encodes a homologue of mammalian VGLUT (Vesicular Glutamate Transporter) (2). Since mammalian VGLUT is known to transport glutamate into synaptic vesicles and to be localized in presynaptic vesicles, C. elegans EAT-4 protein is thought to act also as a glutamate transporter. Glutamate is one of the major neurotransmitters and plays important role in neural system such as sensation, learning, and memory. However, the in vivo function of VGLUT is not well understood. The GFP-tagged
eat-4 transgene was expressed in almost all sensory neurons and many interneurons, including essential neurons for thermotaxis such as thermosensory neurons AFD and AWC, and their downstream inter neurons AIZ and RIA. To elucidate the essential neurons responsible for athermotactic phenotype of
eat-4 mutants, we are trying to determine the cells where EAT-4 functions by using the expression of
eat-4 cDNA under the control of various promoters. We found that expression of
eat-4 cDNA in sensory neurons of
eat-4(
ky5) mutants partially rescued athermotactic phenotype of
eat-4(
ky5) and strongly rescued defective chemotactic responses to NaCl. These results suggest that EAT-4 functions at least in sensory neurons for normal thermotaxis and chemotaxis, and that sensory signaling is regulated by EAT-4 dependent glutamate-mediated neurotransmission. (1) Okumura et. al., 2001, INWM abstract. (2) Lee et. al., 1999, J Neurosci 19, 159. (3) Avery, 1993, Genetics 133, 897. (4) I.M., unpublished.