GSK-3 is a highly conserved serine-threonine protein kinase that is implicated in a variety of signaling pathways in other systems. We have begun to investigate the role in early C. elegans embryogenesis of a GSK-3 homolog tentatively named SGK-1 (for shaggy, GSK-3 related). Our data suggest that SGK-1 is involved in the Wnt/Mom pathway for P2-EMS signaling as well as the Notch/GLP-1 pathway for P2-ABp signaling. At the 4-cell stage of C.elegans embryogenesis the P2 cell induces its sister cell EMS to divide asymmetrically and to produce intestinal cells. This polarizing induction involves parallel inputs from a Wnt signal, MOM-2, and a C.elegans gene (
apr-1) related to the human APC gene (implicated in colon cancer). We find that in
sgk-1(RNAi) embryos, the signaling cell, P2, produces intestinal cells ectopically via a genetic program that depends on both
mom-2 and
apr-1 function. Interestingly our genetic analysis suggests that
sgk-1 lies upstream of
mom-5, a Frizzled related gene and a potential receptor for the MOM-2 signal. This genetic position distinguishes
sgk-1 from
src-1 which has a very similar extra gut phenotype (see abstract by Jennifer Hogan). We have also examined the source of extra pharynx produced in
sgk-1(RNAi) embryos. In normal development the GLP-1 (Notch related) receptor is required for the induction of the anterior half of the pharynx. We have found that all of the extra pharynx formed in
sgk-1(RNAi) embryos requires
glp-1(+) activity, suggesting that this extra pharynx results from a failure to downregulate
glp-1 function in the absence of SGK-1 activity. Consistent with this model we find that
sgk-1(RNAi) can suppress the pharyngeal induction defect of a
glp-1 hypomorph,
glp-1(
e2142). We are now trying to identify
sgk-1 mutants using PCR based deletion screening. We have incorporated an Egl mutation so that secondary screens can be done based on the predicted RNAi phenotype. We have found one candidate
ne217 that is now recovered as a heterozygous strain. This mutant has a maternal effect excess intestine and pharynx phenotype very similar to
sgk-1(RNAi), but is this mutant an allele of
sgk-1 or just a new extra-gut mutant?? We are now beginning to look for the SGK-1 targets.