Asymmetric cell division (ACD) is a fundamental mechanism of developmental cell fate specification and adult tissue homeostasis. In C. elegans, the Wnt/beta-catenin asymmetry (WβA) pathway regulates ACDs throughout embryonic and larval development. Under control of Wnt ligand-induced polarity, the transcription factor TCF/POP-1 functions with the coactivator beta-catenin/SYS-1 to activate gene expression in the signaled cell or, in absence of the coactivator, to repress Wnt target genes in the nascent unsignaled daughter cell. To date, a broad investigation of Groucho function in WβA is lacking and the function of the short Groucho AES homolog, lsy-22 has only been evaluated in C. elegans neuronal cell fate decisions. Further, there is conflicting evidence showing TCF utilizing Groucho-mediated repression may be either aided or repressed by addition of AES subfamily of Groucho proteins. Here we demonstrate a genetic interaction between Groucho repressors and TCF/POP-1 in ACDs in the somatic gonad, the seam hypodermal stem cell lineage and the early embryo. Specifically, in the somatic gonad lineage, the signaled cell fate increases after individual and double Groucho loss of function, representing the first demonstration of Groucho function in wild-type WβA ACD. Further, WβA target gene misexpression occurs at a higher rate than DTC fate changes, suggesting derepression generates an intermediate cell fate. In seam cell ACD, loss of Groucho unc-37 or Groucho-like lsy-22 in a pop-1(RNAi) hypomorphic background enhances a pop-1 seam cell expansion and target gene misregulation. Moreover, while POP-1 depletion in lsy-22 null mutants yielded an expected increase in seam cells we observed a surprising seam cell decrease in the unc-37 null subjected to POP-1 depletion. This phenotype may be due to UNC-37 regulation of pop-1 expression in this tissue since we find misregulation of POP-1 in unc-37 mutants. Lastly, Groucho functions in embryonic endoderm development since we observe ectopic endoderm target gene expression in lsy-22(ot244) heterozygotes and unc-37(tm4649) heterozygotes subjected to intermediate levels of hda-1(RNAi). Together, these data indicate Groucho repressor modulation of cell fate via regulation of POP-1/TCF repression is widespread in asymmetric cell fate decisions and suggests a novel role of LSY-22 as a bona fide TCF repressor. As AES Grouchos are well-conserved, our model of combinatorial TCF repression by both Gro/TLE and AES warrants further investigation.
hecd-1 Modulates Notch Activity in Caenorhabditis elegans