Anterior to the expression domains of HOX complex genes, several phyla employ conserved sets of genes for head and brain formation. We determined a function for
ceh-2, the C. elegans homolog of the ems/EMX homeobox genes. Loss of function mutations in the fly ems gene as well as their vertebrate Emx counterparts lead, among other defects, to reduction or loss of large brain regions.
ceh-2 expression is confined to a few epithelial cells, muscle cells and neurons in the anterior pharynx. We isolated a deletion mutant,
ceh-2(
ch4), that lacks most exons and the homeodomain and is therefore likely to be a null. From the
ceh-2 expressing cells, neurons M3 and NSM have known functions that we tested in the mutant. NSMs seem to function normally in a locomotion assay (Sawin, 1996), whereas we see an almost complete loss of M3 function. The inhibitory pulses of the M3 neurons that are seen in electropharyngeograms (EPG) during pharynx muscle contraction phase speed up relaxation of the muscle, which is necessary for effective bacterial trapping (Raizen and Avery, 1994). Most
ceh-2(
ch4) EPGs completely lack P phase transients in electropharyngeograms. The resulting slight starvation retards larval but not embryonic development by a few hours. Both phenotypes are fully rescued with a genomic clone that contains only the
ceh-2 gene.
ceh-2::gfp expression in
ceh-2 (
ch4) reveals that the shape of all cells is perfectly normal, suggesting that
ceh-2 is necessary for the final differentiation of M3 (and that it is not needed to maintain its own exppression). We are interested to find downstream genes and regulators of
ceh-2. The Na/Pi cotransporter
eat-4 is expressed in M3 and other neurons and has the same loss-of-function phenotype (Lee et al., 1999).
eat-4::lacZ expression in M3, but not in other neurons, seems to be reduced in
ceh-2(
ch4) (a strongly staining neuron in the vicinity of M3 makes the analysis difficult). We are attempting to rescue
ceh-2(
ch4) by expressing
eat-4 under the control of the
ceh-2 promoter to see whether reduced
eat-4 expression is the reason for loss of M3 function in
ceh-2(
ch4), or whether
ceh-2 might regulate other genes as well. We are also attempting ectopic rescue by expressing
ceh-2 in M2 from the
unc-17 promoter, which may give much information about the mode of action of
ceh-2. Furthermore, we are analysing
myo-2 expression in
ceh-2(
ch4) to see whether
ceh-2 has a recognizable function in the
m2 muscles.