C. elegans can sense and memorize temperature, and change its temperature response, depending on the past experience. For example, after cultivation with sufficient food at 20 degree, C. elegans migrates to its cultivation temperature (i.e. 20 degree) and then moves isothermally on a thermal gradient without food. This behavior is called thermotaxis, and several mutants defective in thermotaxis have been isolated. Of these,
tax-4 and
tax-2 show athermotactic phenotype, and both
tax-4 and
tax-2 gene were found to encode two different subunits of cyclic nucleotide-gated cation channel (CNG channel), which functions in several sensory neurons including AFD thermosensory neurons. The TAX-4/TAX-2 CNG channel shows higher affinity to cGMP than cAMP, suggesting that cGMP acts as a second messenger in thermosensory signal transduction. Thermotaxis must also require execution of some form of associative learning between temperature and food. Recently, our laboratory has been isolating mutants designated aho (abnormal hunger orientation), which are likely to be defective in mechanism of associative learning between temperature and food (see abstract by Mohri et al). The completion of C. elegans genome project makes us possible to systematically analyze the function and development of the nervous system by reverse genetic approach. Among 19,000 genes that exist in the C. elegans genome, about 1,700 genes are thought to function in the nervous system. In order to dissect the molecular mechanism of thermotaxis, we have started to construct TMP/UV-induced deletion mutants for the genes that likely function in neurons required for thermotaxis. Since (1) cGMP could be a second messenger in thermosensation and (2) associative learning in thermotaxis can be established only in a few hours (0.5 ~ 4 hr), which implicate the importance of change in neuronal activity, we focus on genes for (1) molecules related to cGMP signal transduction and (2) ion channels required for modulation of neuronal activity. For the first aim, we plan to knockout genes for guanylyl cyclase (GC), cyclic nucleotide phosphodiesterase (PDE), and cGMP dependent protein kinase (PKG). In C. elegans , there are at least 33 genes for GC, and at least two of these genes,
gcy-8 and
gcy-12, are expressed in AFD thermosensory neurons. Also, there are several genes for PDE and a single gene for PKG. For the second aim, we are paying attention to the following genes; 6 genes for voltage-gated chloride channel, and 11 genes for transient receptor potential (TRP) ion channel. To date, we screened about 2,400,000 genomes and obtained deletion mutants for GCY-12, a chloride channel, and a TRP ion channel. We are currently analyzing behavioral phenotype of these deletion mutants.