Egf signaling directs neoblast repopulation by regulating asymmetric cell division in planarians

The goal of this experiment is to identify target genes of Egf signaling which regulate recovery of the neoblast population following exposure to a sublethal dose of radiation. A large population of highly proliferative stem cells (neoblasts) is required for physiological tissue homeostasis and regeneration after injury in planarians. Recent studies indicate that survival of a few neoblasts after sublethal irradiation results in the clonal expansion of the surviving stem cells and the eventual restoration of tissue homeostasis and regenerative capacity. Many genes are known to be required for the normal function of neoblasts, but the precise mechanisms regulating the population dynamics of these adult pluripotent stem cells remain largely unknown. By coupling RNAi screening and sublethal irradiation, we uncovered a central role for Epidermal Growth Factor (EGF) signaling during in vivo neoblast expansion mediated by Smed-egfr-3 (egfr-3) and its putative ligand Smed-neuregulin-7 (nrg-7). Furthermore, the EGFR-3 protein localizes asymmetrically on the cytoplasmic membrane of neoblasts and the ratio of asymmetric to symmetric cell divisions decreases significantly in egfr-3(RNAi) worms. Our results not only provide the first molecular evidence of asymmetric stem cell divisions in planarian, but also demonstrate that EGF signaling likely functions as an essential regulator of neoblast clonal expansion.