An insulin receptor-like signaling pathway regulates C. elegans dauer formation and longevity. Mutations in the insulin receptor-like gene
daf-2 or in the PI3K homolog
age-1 cause constitutive arrest at the dauer stage and an increased lifespan. Mutations in the Fork head transcription factor DAF-16 completely suppress the dauer arrest and longevity phenotypes of
daf-2 and
age-1 mutants. We screened for suppressors of
age-1 in order to identify molecules that couple DAF-2 and AGE-1 to DAF-16. At least two novel suppressors of
age-1,
mg144 and
mg142, were isolated among a total of 10 independent suppressors from a screen of 3800 haploid genomes.
mg144 was shown to be an allele of the C. elegans Akt/PKB homolog
akt-1 by linkage analysis and reversion. An activating mutation in
akt-1 is a good candidate to be a suppressor of AGE-1 PI3K loss of function mutations because activation of mammalian Akt/PKB kinase is coupled to PI3K activity; the phospholipid products generated by PI3K are required for Akt/PKB activation. Akt/PKB has been implicated in cellular responses to insulin and PI3K signaling such as glucose transport, glycogen synthesis, and apoptosis. Our results demonstrate that Akt/PKB has transcriptional outputs (see below). Interestingly, the
akt-1(
mg144) mutation suppresses the dauer constitutive phenotype of
age-1 but not
daf-2 . We believe that
akt-1(
mg144) represents a bifurcation in the signaling pathway downstream of the DAF-2 insulin receptor-like protein. Also,
akt-1(
mg144) does not suppress the longevity phenotype of
age-1 animals suggesting that different outputs from
age-1 could function to regulate dauer formation and lifespan. We also analyzed inactivation of Akt/PKB signaling. Another homolog of Akt/PKB, which we have named
akt-2, was identified. Inactivation of either
akt-1 or
akt-2 alone by RNA mediated interference in N2 does not appear to have a phenotype. However simultaneous inactivation of
akt-1 and
akt-2 in the same animal results in a strong Daf-c phenotype. Consistent with their redundant roles in dauer formation, AKT-1/GFP and AKT-2/GFP have overlapping expression patterns in both the nervous system and in target tissues which are remodeled during dauer formation.
daf-16 null mutations are epistatic to
akt-1(RNAi);
akt-2(RNAi), which places
akt-1 and
akt-2 in the insulin receptor-like signaling pathway and demonstrates that the DAF-16 transcription factor is a major output of Akt/PKB signaling in C. elegans dauer formation.
mg142 is a semi-dominant suppressor of
age-1 that maps to the left arm of LGX. Like
akt-1(
mg144),
mg142 does not efficiently suppress the Daf-c phenotype of
daf-2 or the aging phenotype of
age-1. We are currently refining the map position of
mg142 and sequencing candidate genes in order to identify the molecular basis of the
mg142 mutation.