C-elegans MOM-5/Frizzled functions in MOM-2/Wnt-independent cell polarity and is localized asymmetrically prior to cell division

C. elegans embryonic cells have a common anterior/ posterior (a/p) polarity that is apparent in the localization of the transcription factor POP-1 [1, 2]. The level of nuclear POP-1 remains high in the anterior daughters of dividing cells but is lowered in the posterior daughters [2, 3]. To generate POP-1 asymmetry, most early embryonic cells require contact with signaling cells that express the ligand MOM-2/Wnt [4, 5]; the point of cell contact specifies the daughter with low nuclear POP-1 [6, 7]. In contrast, slightly older embryonic cells that have no apparent prior exposure to Wnt signaling can generate POP-1 asymmetry, provided these cells express MOM-5/Frizzled [7]. We show here that MOM-5::GFP is enriched at the posterior pole of cells prior to division and that a similar asymmetry is observed in cultured cells with no apparent prior exposure to Wnt signaling. While depleting these latter cells of MOM-5/Frizzled causes both daughter cells to have high levels of POP-1 [7], we show that both daughter cells have low levels of POP-1 in embryos with atypically high levels of MOM-5::GFP. These results suggest that MOM-5/Frizzled asymmetry leads to POP-1 asymmetry. In later embryogenesis, we find that MOM-5::GFP localizes to the leading edges of epidermal cells during ventral enclosure. These localization patterns suggest a parallel between MOM-5/ Frizzled and the roles of Drosophila Frizzled in planar polarity and dorsal enclosure.