Seven members of the Lim- Homeobox gene family have been shown to exist in the worm. All of them are expressed in a non- overlapping pattern in head neurons, but some members (
mec-3,
lin-11 and
lim-7) show additional expression in non- neuronal tissues (Hobert et al.;WBG 14(5):67). Using a GFP reporter construct,
lim-6, a gene that belongs to the LMX-class of LIM homeobox genes, was shown to be expressed in the chemosensory neuron ASG and the odorsensory neuron AWA from late embryo stage throughout the whole life of the animal. In males, additional reporter gene expression was observed in the male specific CP motorneurons. We now found that in dauers, which were either induced by starvation or mutations in
daf-2 or
daf-7, two main changes in reporter gene expression can be observed. First,
lim-6::GFP expression is downregulated in ASG and second
lim-6::GFP expression is strongly upregulated in neurons of the ventral and dorsal nerve cord. These changes seem to be induced by neither pheromone nor absence of food alone, which might indicate that either both environmental inputs are needed together or that this change is a secondary consequense of the dauer entry.
daf-2 (
e1372) mutant animals that were grown under the permissive temperature until adulthood and then shifted to the non-permissive temperature also turn on
lim-6 reporter gene expression in the ventral nerve cord, suggesting that contineous
daf-2 is activity is required to repress
lim-6 in these neurons. In this regard it is also interesting to note that the vertebrate homolog of LIM-6, LMX-1, binds and transactivate the insulin promoter. To determine whether
lim-6 functions in the dauer developmental program and/or other processes, we are taking several approaches: A deletion in the
lim-6 gene has just been identified in a PCR deletion library screen in cooperation with Nemapharm. In addition to studies of this mutant strain we are performing behavioral assays with strains carrying an additional copy of an integrated
lim-6::VP16 construct, which presumably will lead to a constitutive activation of
lim-6 . Using
lim-6::GFP simply as a tool,we are also interested in the following aspects of ASG function: 1. What factors act in ASG to determine
lim-6 expression? 2. What factors are required for the determination of ASG cell fate and axonal pathfinding? We conducted several mutant screens ( two EMS, one UV; total of ca. 11000 haploid genomes screened) and isolated mutants which affect either
lim-6::GFP expression or ASG pathfinding as visualized with the
lim-6::GFP reporter gene. We are characterizing these mutants; it seems obvious so far, that those genes affecting ASG pathfinding are also affecting the pathfinding of many other neurons.