Worm Breeder's Gazette 14(5): 67 (February 1, 1997)
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 Molecular Biology Massachusetts General Hospital Wellman 8 Boston, MA, 02114.
LIM homeobox genes play an important role in various developmental processes in multicellular organisms. The worm genome sequencing project has uncovered 4 novel LIM homeobox genes, 3 of which we had previously reported to be specifically expressed in the nervous system (Hobert et al.,WBG14(4): 67). The last remaining LIM homeobox gene, which we provisionally termed LIM-7, is located on chromosome I (cosmid C04F1). This gene shows a significant similarity to the islet-subclass of LIM homeobox genes and represents the first member of this subclass in C.elegans. In vertebrates, the islet-subclass of LIM homeobox genes consists of several members, most of which are expressesed in the central nervous system. In order to determine the expression pattern of LIM-7 we first cloned its cDNA by PCR to confirm the predicted exon/intron structure of the gene and then constructed a GFP-fusion gene containing the first two exons and 5'-upstream regulatory sequences of LIM-7. Stable transgenic lines expressing this reporter gene reveal a dynamic expression pattern of LIM-7: In late embryos and early larval stages, LIM-7 is exclusively expressed in a few head neurons, including an amphid sensory neuron and head motor neurons. Neuronal expression fades late in larval development. Starting with the onset of gonadogenesis in late larval stages, LIM-7 is strongly expressed in gonadal sheath cells.Expression in the somatic gonad perdures throughout adulthood and represents the only tissue in which LIM-7 is expressed in adults. ceh-18, a POU homeobox gene, is also expressed in the gonadal sheath cells (Greenstein et al., 1994, Genes & Development 8: 1935-1948). As exemplified by the genetic and physical interaction of the POU protein unc-86 and LIM protein mec-3 in touch cells and vertebrate Pit-1 and P-LIM in the pituitary, ceh-18 and LIM-7 might also represent POU-LIM combinatorial partners functioning in gonadal sheath cells. We are planning to test this hypothesis using protein interaction assays. ceh-18 does not affect the expression of LIM-7 since in ceh-18 mutant animals LIM-7-GFP is still expressed in the gonad. Looking for a candidate mutant phenotype of LIM-7, we noticed that Ann Rose's group has rescued the let-532 mutant with a pool of three overlapping cosmids, one of which contains the LIM-7 coding sequence (McDowell et al., Acedb). We are in the process of determining whether let-532 corresponds to LIM-7. Taken together with previous studies on LIM homeobox genes, our expression analysis of novel LIM homeobox genes revealed that all members of this class of homeobox genes are expressed in the nervous system: mec-3, LIM-4, ttx-3 and LIM-6 are expressed exclusively in a distinct subset of neurons, while lin-11, ceh-14 and LIM-7 are expressed in neuronal and non-neuronal cells (Way and Chalfie, 1989, Genes & Dev. 3: 1823-1833; Burglin, pers.comm.; Hobert et al., WBG14(4): 67; Freyd et al., WBG 11(5): 52). Furthermore, the expression pattern of LIM homeobox genes does not seem to overlap in any neuronal cells. LIM homeobox genes might thus represent neuronal cell-type specific determinants. The temporal expression pattern of the neural specific LIM homeobox genes as well as the functional analysis of mec-3 (Way and Chalfie, 1989) and ttx-3 (Hobert et al., WBG14(4): 68) further suggests that these genes are involved late in neuronal differentiation.