Which signaling molecule helps distinguish MS and E lineages in EMS blastomere?
EMS blastomere division into MS mesoderm and E endoderm requires polarized Wnt signaling from P2 integrating cell polarity. P2 secretes MOM-2 Wnt, MOM-5 Frizzled, MES-1 immunoglobulin transmembrane signals activating Wnt/beta-catenin asymmetry pathway: WRM-1 beta-catenin, LIT-1 Nemo-like kinase and KIN-19 phosphorylate POP-1 TCF, exporting it from E nucleus via WRM-1 complex and reducing nuclear level. Low POP-1 plus high SKN-1 allows MED-1/2 and END-1/3 expression specifying endoderm in E, while high POP-1 in MS represses endoderm and promotes mesoderm genes tbx-35.
Ref: Thorpe et al., Cell 1997: MOM-2 Wnt signaling distinguishes MS and E fates downstream of EMS.