Worm Breeder's Gazette 15(4): 28 (October 1, 1998)

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.

GFP markers illuminating functional abnormalities in neurons required for thermotaxis

Hiroyuki Sasakura1, Atsushi Kuhara1, Hidetoshi Komatsu1, Ikue Mori2

1 Laboratory of Molecular Neurobiology, Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
2 Laboratory of Molecular Neurobiology, Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan, PRESTO, JST

        C. elegans can sense a range of temperatures and migrates toward
the temperature at which it was cultivated with food for several hours
on a temperature gradient. Laser ablation experiments revealed that
thermosensory neuron AFD and its downstream interneurons, AIY and AIZ,
are critical for this thermotaxis behavior (1). To date, several genes
required for thermotaxis have been identified. The tax-4 and tax-2 genes
encoding alpha and beta subunits of cyclic nucleotide-gated channel,
respectively, are expressed in several sensory neurons including AFD,
and both tax-4 and tax-2 mutants exhibit athermotactic
(non-temperature-responsive) phenotype (2,3). The ttx-3 gene encodes a
LIM homeodomain protein that is specifically expressed in AIY, and ttx-3
mutants show cryophilic (cold-seeking) phenotype (4). 
        Like ttx-3 mutants, ttx-1 and ttx-2 mutants are cryophilic and
are nearly, if not completely, normal in other behaviors such as
chemotaxis. Since AFD and AIY are likely to be involved specifically in
thermotaxis, we tried to observe any defects in AFD and AIY neurons of
ttx-1 and ttx-2 mutants, by introducing AFD-specific marker gcy-8::GFP 
and AIY-specific ttx-3::GFP. The same constructs were also introduced to
ttx-3, tax-2 and tax-4 mutants for comparison. We found that the
expression of gcy-8:GFP in AFD was significantly down-regulated in
ttx-1, tax-2 and tax-4 mutants. Noticeably, we have also observed that
the microvillus-like structures at sensory ending of AFD were often
abnormal in ttx-1 mutants, which is consistent with the previous report
based on the EM analysis (5). By contrast, ttx-2 and ttx-3 mutants
expressed gcy-8:GFP  at the same level as wild type animals. The
expressions of ttx-3::GFP in AIY of ttx-1, ttx-2, tax-2 and tax-4
mutants were all indistinguishable when compared with that of wild type,
but as already reported, it was much reduced in ttx-3 mutants, probably
due to self-regulation (4). 
        We are intrigued by the low expression of gcy-8::GFP in AFD of
tax-2 and tax-4 mutants, since we think it highly unlikely that a cyclic
nucleotide-gated channel plays a direct role in transcription of gcy-8 
encoding a guanylyl cyclase (6). Thus, the present study suggests that
the ttx-1 gene functions in AFD, but does not necessarily support the
possibility that it encodes a transcription regulator over other
possibilities. We are now testing the model that disruption of a gene
participating in signal transduction pathways for thermotaxis in AFD,
AIY or AIZ, leads to down-regulations of other genes involved in the
same or related signal transduction cascades. If so, appropriate GFP
markers will then be used not only to identify the cellular focus, but
also to help estimate the function of the genes affected in
thermotaxis-defective mutants.   
        We thank Barbara Wedel and David Garbers for gcy-8::GFP ; Oliver
Hobert for ttx-3::GFP; Kazuyoshi Ishii, Megumi Shirakawa and Yasumi
Ohshima for ttx strains. We are indebted to Yasumi Ohshima for help in
his laboratory at the initial stage of this work.

(1) Mori and Ohshima, 1995, Nature 376, 344-348. (2) Komatsu et al.,
1996, Neuron 17, 707-718. 
(3) Coburn and Bargmann, 1996, Neuron 17, 695-706. (4) Hobert et al.,
1997, Neuron 19, 345-357. 
(5) Perkins et al., 1986, Dev. Biol. 117, 456-487. (6) Yu et al., 1997,
PNAS 94, 3384-3387.