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.

LIM-homeobox genes in the worm

Oliver Hobert, Yanxia Liu, Gary Ruvkun

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

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

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.