The C. elegans hypodermis is a model system for studying aspects of epithelial morphogenesis such as epiboly and ventral closure. Mutations in the genes
vab-1,
vab-2 and
vab-3 cause defects in morphogenesis of head region hypodermis.
vab-3 mutations affect the C. elegans Pax-6 locus, which is required for many aspects of head region development (Chisholm and Horvitz, Nature 377: 52, 1995). We have recently begun an analysis of
vab-1 and
vab-2. Fifteen mutant alleles of
vab-1 are known to exist. All fifteen alleles cause variably abnormal head and tail morphogenesis, and embryonic and larval lethality. Based on the penetrance of the lethality we have classified six alleles as strong, three as intermediate and six (including the canonical allele
e2) as weak. The embryonic lethality caused by
vab-1 alleles appears to result specifically from defects in ventral closure of the hypodermis.
vab-1 embryos develop normally until the comma stage, and hypodermal nuclei such as those of
hyp4,
hyp6,
hyp7 and the V and P cells are all present in initially normal positions. About 10% of
vab-1 embryos do not develop beyond the comma stage; internal cells progressively ooze out from the ventral midline, eventually resulting in arrest. Other
vab-1 embryos develop to later stages in embryogenesis with deformed heads or tails, and frequently rupture either in the ventral preanal region or the ventral head. Thus, some
vab-1 embryos appear to be defective in the ventral closure of the hypodermis. The deficiencies ccDf4 and maDf4 both uncover the
vab-1 locus. ccDf4/maDf4 trans-heterozygous embryos show a similar range of embryonic phenotypes to strong
vab-1 mutants, indicating that the strong
vab-1 alleles may be null mutations and that the null phenotype of
vab-1 is a variable defect in hypodermal morphogenesis. The basis for this striking variability is still unclear. We mapped
vab-1 close to the right to
hlh-1 and rescued the
vab-1 phenotypes by transformation with a cosmid from this region (M03A1). One gene predicted from the genomic sequence of this cosmid encodes a receptor protein tyrosine kinase (RPTK) of the Eph subfamily; sequence analysis of
vab-1 mutant alleles (see below) showed that this gene is
vab-1. Eph RPTKs have previously been isolated from vertebrates and form the largest subfamily of receptor tyrosine kinases. Eph RPTK signaling has been implicated in many aspects of vertebrate development, including topographic mapping of axonal projections and hindbrain segmentation. The predicted VAB-1 protein contains all the hallmarks of Eph RPTKs, including an extracellular cysteine-rich domain, two fibronectin type III repeats, and an intracellular tyrosine kinase domain. However, VAB-1 does not appear to be an ortholog of any particular vertebrate gene. We have found the molecular lesions produced by nine of the fifteen
vab-1 alleles. Two intermediate alleles cause missense alterations in the extracellular domain. Two weak alleles, including the venerable
e2, cause missense alterations in the kinase domain. Three strong alleles bear deletions of exons encoding parts of the extracellular domain, consistent with their genetic behavior as nulls. The allele
e118 bears a deletion of two kinase subdomains yet causes a weak mutant phenotype, suggesting that kinase activity may not be necessary for some of VAB-1's functions (a similar situation has been described for the mouse receptor Nuk). We are currently identifying the cells in which
vab-1 is expressed using reporter constructs. We are also using genetic and molecular approaches to identify other components of the
vab-1 signaling pathway.