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.

An islet-class LIM homeobox gene expressed in head neurons and in the gonad

Oliver Hobert, Yanxia Liu, Gary Ruvkun

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.