Both intrinsic and extrinsic mechanisms polarize asymmetrically dividing cells. Many asymmetrically distributed intracellular molecules have been described that segregate fate to daughter cells during mitosis, and in C. elegans, Wnt signaling has also been shown to polarize asymmetrically dividing cells. However, it is not known whether cell interactions are involved in distributing intracellular factors, or the mechanism by which Wnts polarize asymmetrically dividing cells. We have found a potential link between the asymmetrically distributed protein HAM-1, that segregates fate in certain neuroblast divisions, and Wnt signaling. We are focusing on the asymmetric divisions that generate the HSN and PHB neurons. In this lineage, an HSN/PHB neuroblast divides asymmetrically to generate an anterior daughter cell that dies and a posterior daughter cell, the HSN/PHB precursor. This precursor then divides to produce the HSN and PHB neurons. In
ham-1 mutants, the HSN/PHB neuroblast frequently divides symmetrically producing two HSN/PHB precursors. The HAM-1 protein is asymmetrically localized to the posterior periphery in the HSN neuroblast and segregated to the HSN/PHB precursor upon division. In a yeast two-hybrid screen to identify proteins that interact with HAM-1, a dishevelled ( dsh ) homolog, C34F11.9, was isolated. DSH is a cytoplasmic protein, that upon Wnt stimulation becomes hyperphosphorylated and recruited to the cell membrane. In C. elegans, Wnt signaling orients the polarity of several asymmetrically dividing cells. The interaction of DSH with HAM-1, an asymmetrically localized protein, suggests that Wnt signaling may direct the asymmetric distribution of intracellular proteins. Although we were unable to generate an RNAi phenotype with C34F11.9, RNAi with a second dsh homolog, C27A2.6, resulted in a Ham-1 phenotype in the HSN/PHB lineage. Overexpression of C27A2.6 at high concentrations is lethal, while at lower concentrations we observe a weakly penetrant Ham-1 phenotype in the HSN/PHB lineage. A dsh ::GFP transcriptional fusion is strongly expressed in embryos at stages coincident with HAM-1 expression. Experiments are in progress to isolate mutations in C27A2.6 and to determine if dsh is required for the localization of HAM-1.