Worm Breeder's Gazette 14(5): 66 (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.

vab-1 encodes an Eph family Receptor Tyrosine Kinase that functions in hypodermal morphogenesis

Sean George, Andrew Chisholm

Department of Biology, Sinsheimer Labs, UC Santa Cruz, CA 95064

        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