Worm Breeder's Gazette 14(4): 67 (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.
Dept. of Genetics, Harvard Medical School and Dept. of Molecular Biology, Massachusetts General Hospital, Wellman 8, Boston, MA, 02114.
LIM-homeobox (LHX) genes, which encode proteins with two Zn-finger-like LIM domains and a DNA-binding homeodomain, have been shown to play roles in nervous system development of both invertebrate and vertebrate organisms. Originally, three LIM-homeobox genes were characterised in C.elegans: lin-11 and mec-3 (founding members of the LIM-homeobox family) were identified genetically (Freyd et al., 1990, Way et al., 1988), while the ceh-14 LHX gene was identified in a sequence based screen for homeobox genes (B!rglin et al., 1989). By doing lots of GENEFINDER and BLAST searches, we found that the C.elegans genome project has sequenced 4 more LHX genes, located on cosmids ZC64, C40H5, K03E6 and C04F1, which we termed CeLIM-4,-5,-6 and -7, respectively. We are interested in characterising the function of these genes by expression pattern analysis, and phenotypic analysis of animals bearing mutations in these genes. CeLIM-4 is located on the X chromosome right next to ceh-18. We constructed a CeLIM-4-GFP translational fusion and found CeLIM-4 to be expressed exclusively in several pairs of head neurons of which at least one is an amphid sensory neuron. We are in the process of identifying these cells. CeLIM-4 maps close to the let-43 gene; we are trying to rescue this mutant with CeLIM-4 but think it is unlikely that a mutant head neuron-specific transcription factor would cause lethality. In collaboration with R.Plasterk we have started to search for Tc1-insertions. CeLIM-5 is the C.elegans ortholog of the Drosophila apterous and vertebrate LH2/LHX2-genes. We found CeLIM-5 to be encoded by the ttx-3 gene (see accompanying abstract by Hobert et al.). In adults, CeLIM-5/ttx-3 is exclusively expressed in the AIY interneuron. CeLIM-6 is located on the X chromosome (next to unc-1) and is most closely related to the Lmx-subclass of LHX genes. Like CeLIM-4 and CeLIM-5, CeLIM-6 also shows a neuronal specific expression pattern: A promoter fusion to GFP demonstrates CeLIM-6 to be expressed most strongly in two pairs of head sensory neurons. which we preliminarily identified as the ASG and AWA sensory neurons. In collaboration with R.Plasterk we have started to search for Tc1-insertion for this gene as well. CeLIM-7 maps on chromsome I close to several let-mutations (which we are trying to rescue) and is the ortholog of the islet-class of vertebrate LHX genes. We are in the process of subcloning its promoter to determine its expression pattern. The function of the vertebrate islet-1 gene in motorneuron generation (Pfaff et al., 1996) suggests the possibility of a neuronal expression pattern for this gene as well. We constructed a new lin-11-GFP fusion containing 10 kB of lin-11 promoter sequence and preliminary expression analysis reveals a similar expression pattern similar to a lin-11-lacZ expression pattern reported by Freyd & Horvitz (several pairs of head neurons, including one pair of sensory neurons, VC neurons, vulval cells). We will use this construct to monitor axon morphology of these neurons in the lin-11 mutant background. Preliminary analysis with a different, smaller lin-11-GFP construct generously provided by Sawa & Horvitz revealed VC process morphology defects. So far, for every known subclass of vertebrate LHX genes a C.elegans homolog has been identified. Given this observation and the number of genes that have been estimated to be sequenced already by the GSC (~80%), we don't expect (m)any more LHX genes to exist in the worm. The overlapping expression patterns of several vertebrate LHX genes in the nervous system has led to the proposal of a "LIM-code", in which distinct combinationsof LHX gene expression specify different motor neuron fates (Tsuchida et al., 1994). Although our expression pattern analysis is not complete yet, we note that so far there is no significant overlap of C.elegans LHX gene expression. If this holds true, it would suggest that C. elegans LHX genes may specify the simpler nervous system of the worm in manner that is distinct from the postulated combinatorial LHX interactions that may specify the vertebrate nervous system.