Worm Breeder's Gazette 14(4): 68 (October 1, 1996)

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.

ttx-3 is a LIM-homeobox gene and controls AIY function in the thermotactic pathway

Oliver Hobert1, Ikue Mori2, Yukiko Yamashita2, Hidehiro Honda2, Yasumi Ohshima2, Yanxia Liu1, Gary Ruvkun1

1 Department of Molecular Biology, Massachusetts General Hospital, Department of Genetics, Harvard Medical School, Boston, MA 02114
2 Department of Biology, Faculty of Science, Kyushu University, Fukuoka 812-81, Japan

Worms have the capacity to sense and store temperature information such
that wild-type animals move to approximately their cultivation
temperature when placed on a temperature gradient. ttx-3 mutant animals
were isolated based on their defective thermotactic behaviour; they move
independently of the cultivation temperature to colder temperatures.They
do not exhibit defects in chemo- or odortaxis.

We mapped the ttx-3 mutation to the lin-14, sma-5 interval on the X
chromosome. Doing GENEFINDER and BLAST searches on the corresponding
physical map we noticed the presence of a LIM homeobox (LHX) gene in
this region (on cosmid C40H5). We injected that cosmid and a subclone
bearing only the LHX gene including 3.1 kB upstream region into ttx-3
and found both arrays to rescue the thermotactic defect. Sequencing the
ttx-3(ks5) allele revealed a G to A mutation in a conserved splice donor
site between exon 5 and exon 6, both of which encode the second LIM
domain. Because of the presence of a stop codon after the aberrant
splicing site, this mutation is predicted to delete half of the protein
including its homeodomain. Such a deletion is predicted to cause a null
phenotype which is confirmed by the observation that a ttx-3(ks5) /
chromosomal deficiency heterozygote has the same phenotype as a
ttx-3(ks5) homozygote.

We cloned the cDNA of ttx-3 and by aligning its predicted protein
product to other LHX proteins we found it to be the ortholog of
Drosophila APTEROUS and vertebrate LH2/LHX2 proteins, both of which have
been shown to be expressed and function in nervous system development
(Lundgren et al., 1995, Xu et al., 1993).

Next, we constructed a ttx-3 promoter fusion to GFP to monitor ttx-3
expression in wildtype and ttx-3 mutant animals. In wildtype, we found
ttx-3-GFP  to be expressed exclusively in the AIY interneurons of adult
animals. In embryos we found expression in additional, as yet
unidentified cells. Laser ablation of AIY phenocopies the cryophilic
behaviour of the ttx-3 mutation, consistent with a  ttx-3  function in
the AIY neurons.

Using ttx-3-GFP to reveal AIY process morphology, we analysed the
neuroanatomy of AIY in ttx-3 mutant animals. We found AIY to be
generated; however,we observed multiple axon morphology defects in adult
animals. The outgrowth of additional axons represents the most penetrant
phenotype but prematurely terminated processes and misguided axons were
also apparent.

We conclude that ttx-3 is not necessary for the generation of AIY but
required for its functional specification. The TTX-3 LIM homeodomain
protein might regulate the expression of downstream target genes which
mediate neural connectivity, signaling or axonal outgrowth in a single
interneuron of the thermotaxis sensation and memory pathway.