The LIM homeobox (Lhx) gene
ttx-3 is required for correct thermotactic behavior (1,2). In
ttx-3 mutants, the AIY interneuron, a component of the thermoregulatory neural circuit, is structurally and functionally defective.
ttx-3::gfp reporter gene fusions are expressed in AIY. We have set out to test what cellular role the
ttx-3 gene plays and present here our analysis of the execution of the AIY cell fate in
ttx-3 mutants. Previously we have shown that in
ttx-3 mutants maintenance of postembryonic
ttx-3 expression is lost in AIY due to autoregulation. Using GFP reporter gene constructs, we have now found that expression of other AIY cell fate markers including a 7-TM receptor,
sra-11, a homeobox gene,
ceh-23, and a secreted protein, C36B7.7 (kindly provided by T. Ishihara) is also downregulated in AIY in
ttx-3 mutants. A GFP fusion to the octopamine/serotonin receptor
ser-2 (kindly provided by T. Niacaris), which we identified as being expressed in AIY as well, is also downregulated. Additionally, AIY loses its cholinergic phenotype in
ttx-3 mutants as detected by VAchT antibody staining (antibodies kindly provided by J.Duerr). These results suggest that AIY fails to differentiate correctly in
ttx-3 mutants. So far it is unclear whether
ttx-3 directly controls the expression of the genes described above or functions through intermediary transcription factors. We are addressing this question by delineating and comparing the
ttx-3 -dependent regulatory elements in the distinct AIY cell fate markers. Previously it was shown that mutations in the Lhx gene
lim-4 lead to a switch of the fate of the AWB sensory neuron to AWC (3). Although many if not all aspects of the correct fate of AIY are lost in
ttx-3 mutants, our preliminary tests for a cell fate switch of AIY into any of its lineal, structural or functional homologs suggest that AIY has not taken over the identity of AIM, AIZ, ASE or AWC. In contrast to AWB in
lim-4 and AIY in
ttx-3 mutants, the DVB motor neuron maintains its correct fate in animals mutant for the Lhx gene
lim-6 (O. H., unpubl.). Hence, in Lhx mutants the identity of a neuron may be maintained, lost or switched. References: (1) Hobert et al., 1997, Neuron 19, 345 (2) Mori and Oshima, 1995, Nature 376, 344 (3) Sagasti et al., 1999, Genes Dev 13, 1794