ceh-6 is a member of the POU-III family of homeobox
genes, whose members include the human genes SCIP, Brn-1, Brn-2 and Brn-4.
Anti-ceh-6 antibodies and a ceh-6-lacZ transgene showed that the gene is
expressed in 8 bilaterally symmetric neurons in the `brain': RMDVL and
MDVR, and RMDDL/R (head motor neurons), AUAL/R (sensory circuitry), AVHL/R
(egg-laying circuitry). Further staining was observed in the excretory
cell, which is necessary for osmoregulation; the SABVL/R neurons; the Pn.a
cells in the ventral nerve cord of early
larval stages; and the Y, K, F, B, U rectal cells. This expression pattern in a variety of ectodermal and neuronal tissues is similar to that observed for the mammalian homologues of this gene. T. Bürglin with co-authors obtained a Tc1 transposon insertion in ceh-6. Using PCR, T. Bürglin with co-authors identified a strain, where the loss of Tc1 caused a 1.3kb deletion which removes part of the POU-specific domain. 80% of the animals die as embryos at the two-fold stage; the embryos rupture in the rectal area. Since ceh-6 is expressed in the rectal cells, hence these cells are not differentiated properly (or are transformed) and may not express necessary cell surface adhesion molecules. Those animals which hatch, have malformed rectal structures. However, the reason why the larval animals die is most likely due to a defect in the excretory cell, since the animals develop vacuoles throughout the body, indicative of improper osmoregulation.
ceh-20 is the homologue of the human PBX genes and Drosophila extradenticle. ceh-20 shares a highly conserved domain upstream of the homeodomain with exd and PBX, the PBC domain. Antisera against this protein have been generated. Staining is observed in the posterior half of mid-gastrulation embryos. During larval stages, all the cells in the ventral cord and the retrovesicular ganglion are staining. Weaker staining is seen in the hypodermis and in muscles. Notably the brain, the pharynx and most cells in the tail ganglion do not stain. Recently, mutations in this gene have been isolated in Mike Stern's laboratory (Yale), and consistent with the expression pattern, weak alleles show defects in the central body region, while strong alleles are lethal.
ceh-2. Affinity purified antibodies against a ceh-2 peptide show nuclear staining in only 5 to 6 cells at the anterior of the embryo just before pharyngeal morphogenesis. In later stages, staining in specific cells in the anterior section of the pharynx was observed.
ceh-13 is clustered with other homeobox genes, just as are homologous genes in HOM-C and Hox clusters. Strong evidences exist for it to be the orthologue of the Drosophila gene labial and for the evolutionary conservation of its local environment (D. Schaller et al. NAR Vol. 18, No. 3, p.2033-2036, 1990). ceh-13 is expressed earlier than the other genes in the cluster. ceh-13 is activated in response to an A-P cellular polarity and, depending on cell lineage, later translates this polarity into the regulation of specific target genes.
Analysis of CEH-13 protein expression throughout embryonic development
by means of antibody staining and reporter gene expression yielded a very
interesting expression pattern (C. Wittmann, in prep.). Both assays detect
nuclear CEH-13 in the Ep cell of 26-cell stage embryos. In 28-cell stage
embryos, additional staining appears in the posterior sisters of the last
AB division. Expression in later stages is compatible with the interpretation
that ceh-13 function is predominantly located to the anterior domain of
late embryos. Finally a reproducible staining of the ventral nerve cord
by the ceh-13 antibody is another thought provoking aspect of ceh-13 expression.
Throughout the reporter gene expression studies it became obvious, that a 9 kb fragment, including 8 kb of enhancer sequences the first exon, the first intron and part of the second exon is enough to confer wild type ceh-13 expression pattern to reporter genes in early developement. So starting with this well described reporter gene construct, I will further characterize the ceh-13 enhancer, in order to define minimal sequences that are necessary for the establishment of either domain of ceh-13 expression.
By rescue analysis and allele sequencing it was found lim-5 corresponds to the ttx-3 gene, a gene identified genetically based on defects in thermotactic behaviour (Mori and Ohshima, 1995; Hobert et al., submitted). Unlike other thermotaxis-defective mutants, the ttx-3 mutation specifically affects thermotaxis and not chemo- or odortaxis. The ttx-3 mutation decouples the dauer developmental program from its temperature dependence. Thus, the ttx-3 dependent thermosensory system not only couples to a motor output but also to a neuroendocrine signaling pathway.
It is possible that TTX-3 is part of an autoregulatory loop that regulates the initial expression of downstream target genes involved in neural signaling as well as later modulates downstream gene expression in behavioral plasticity.
Bürglin, T.R., and Ruvkun, G.: New motif in PBX genes. Nature Genet. 1, 319-320 (1992).
Bürglin, T.R.: A C. elegans prospero homologue defines a novel domain. Trends Biochem Sci. 19, 70-71 (1994).
Gehring, W.J., Affolter, M., and Bürglin, T.R. :Homeodomain proteins. Ann. Rev. Biochem. 63, 487-526 (1994).
Bürglin, T.R.: A comprehensive classification of homeobox genes. In: A guidebook to homeobox genes (D. Duboule, ed.), pp. 25-71, Oxford University Press, 1994.
Schaller et al. NAR Vol. 18, No. 3, p.2033-2036,