During larval development of the C. elegans hermaphrodite, the uterine and vulval epithelia undergo coordinated morphogenesis to produce a functional connection. The end result is that the uterine and vulval lumens are separated only by the cytoplasmic extension of the utse (uterine seam cell), which is thin enough to be broken when the first egg is laid. The utse is generated by the uterine p cells, which in turn are induced in response to
lin-12 -mediated signaling by the AC. Presenilins have been implicated in Alzheimer’s disease pathogenesis and LIN-12/Notch signaling during development. C. elegans has the
sel-12 and
hop-1 genes, with approximately 50% and 33% identity to mammalian presenilins, respectively, plus the divergent family member
spe-4 . During a screen for Egl mutants, we isolated a mutant called
ty11 in which the uterus and vulva were separated by thick tissue rather than the thin laminar process of the utse. We found that
ty11 was a missense mutation in
sel-12 resulting in a premature stop codon. While
hop-1 mutants had no apparent defect in uterine development, the uterine morphogenesis defect was more severe in
sel-12;
hop-1 than in
sel-12 mutants, implying biological redundancy between the two genes. Using
lin-11::lacZ expression studies and cell lineage analysis, we found that
sel-12 mutants had defects in uterine p cell fate induction. This could account for the uterine-vulval connection defects observed. But
sel-12 mutants also have abnormal vulval morphogenesis. So which tissue is the culprit in making the animals unable to lay eggs? We found that expression of wild-type
sel-12 in the presumptive p cells of
sel-12 mutant animals conferred significant rescue of the egg-laying defect, suggesting that
sel-12 function in the p cells is critical. In these experiments,
sel-12 was expressed under the control of the
egl-13/cog-2 promoter, which drives expression in the uterus but not in the vulva. The
egl-13/cog-2 gene (which encodes a SOX domain transcription factor) appears to function downstream of
sel-12 in the uterus and its expression is dramatically reduced in
sel-12 mutants. Given this, the rescue conferred by the
egl-13/cog-2::
sel-12 transcriptional fusion seems quite remarkable. We hypothesize that a positive feedback loop is initiated in which the small amounts of wild-type SEL-12 initially produced increase throughput through the
egl-13/cog-2 promoter, generating more SEL-12 protein, etc. Surprisingly, the presence of an integrated array containing multiple copies of the
egl-13/cog-2::GFP transcriptional fusion blocked the rescue of the
sel-12 egg-laying defect by
egl-13/cog-2::
sel-12 in animals rescued for the marker phenotype. We are currently performing experiments to test the hypothesis that this is due to competition for regulatory proteins that bind to the
egl-13/cog-2 promoter.