Worm Breeder's Gazette 15(5): 31 (February 1, 1999)

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.

The lim-6 LIM homeobox gene regulates GABAergic neuron differentiation, dauer arrest and uterine morphogenesis

Oliver Hobert, Kristin Tessmar, Gary Ruvkun

Mass.General Hospital, Harvard Medical School, Boston, MA02114

lim-6 is the worm ortholog of the mammalian lmx-1 LIM homeobox gene and represents one of seven LIM homeobox genes in C.elegans. A lim-6 reporter gene fusion that contains all exons, introns and 2kB of upstream regulatory regions is exclusively and strongly expressed in the ASG and AWA sensory neurons and in the male CP motorneurons. However, a reporter gene construct that contains a total of 4 kB of upstream sequences including all exons and introns is not expressed in ASG, AWA and CP but is strongly expressed in the GABAeric neurons DVB, AVL, RIS, RME/R, the ASEL chemosensory neuron, the PVT and RIGL/R neurons, in the excretory gland cell and in the developing uterus. This larger GFP reporter gene construct contains all the regulatory regions that are sufficient to rescue the mutant phenotype of lim-6 (see below) and its sites of expression correlate with the sites where lim-6 mutant animals display defective phenotypes.

A lim-6 deletion allele, nr2073, was kindly provided by NemaPharm, Inc.; it contains a 1.7 kb deletion which takes out the second LIM domain and the homeodomain and thus presumably represent a loss-of-function allele. lim-6(nr2073) animals are constipated and display strong expulsion defects in the defecation cycle. These defects are presumably due to lim-6 expression and action in AVL and DVB since laser ablation of both neurons causes a similar defect (McIntire et al., 1993) and since we can observe axonal morphology and sprouting defects of the AVL and DVB motorneurons in the lim-6 mutant animals. We also observed that expression of unc-25/GAD, the rate limiting enzyme of GABA synthesis (Jin et al., 1999), is partially dependent on lim-6 gene activity in RIS, AVL and DVB. lim-6 mutant animals display a strong daf-c phenotype in conjunction with daf-7(e1372); this defect can be phenocopied by unc-25(e156), but not by unc-30(e191), arguing that non-D-type GABAergic neurons are involved in regulating dauer arrest and that lim-6 presumably is required for GABA mediated control of dauer arrest.

In the developing uterus, lim-6 is transiently expressed in the uterine toroid cells and in the uv2 and uv3 cells. lim-6 mutant animals have a largely reduced brood size and most of their progeny hatches within the parent. In lim-6 mutant animals the uterine toroid cells do not separate and the lumen is clogged.

It is intriguing to note the similarities between the expression and mutant phenotypes of the LIM homeobox genes ttx-3, lin-11 and lim-6: All genes are expressed in postmitotic neurons and their expression is maintained throughout adulthood (Hobert et al., 1997, 1998). In each case, the LIM homeobox gene is neither required for the generation of the respective neurons that express the LIM homeobox gene nor for their initial axonal outgrowth. However, in the respective mutant animals these neurons display rather subtle neural sprouting defects, which might be indicative of signaling defects of the respective neuron. Possibly, it is a common theme of LIM homeobox genes to affect synaptic connectivity or synaptic signaling and as a secondary consequence axonal sprouting.

References: McIntire et al.(1993) Nature 364, 344-7; Jin et al.(1999) J.Neurosci.19, 539-49; Hobert et al.(1998) J.Neurosci.18, 2084-96; Hobert et al.(1997) Neuron 19, 345-57