P67. Conversion of Drosophila neuroepithelial stem cells into neuroblasts occurs during prolonged G1 phase, whose timing is regulated by Notch signaling

Abstract

Postembryonic neuroepithelial stem cells (NESCs) in the outer proliferation center (OPC) of the Drosophila optic lobe initially divide symmetrically to expand the pool of stem or progenitor cells, and are consequently converted into postembryonic neuroblasts (pNBs). pNBs generate both progenitor cells (pNBs) and differentiating cells (GMCs; ganglion mother cells) by asymmetric division, the latter of which then generate two neurons. Thus far the mechanism underlying the conversion of NESCs into pNBs has been unknown. We have shown that the NESCs dividing symmetrically at the border between NESCs and pNBs generate two NESCs, one of which is then converted into pNB during G1 phase by losing epithelial features including belted adherence junction (AJ) and high level of DE-cadherin expression at the AJ, which is followed by upregulation of pan-neural gene Asense. We found that Notch signaling is activated just before the start of conversion and rapidly downregulated upon completion of the conversion, which is marked by high level of Asense expression. Thus, we examined the role of Notch signaling in the conversion of NESCs by MARCM clonal analysis and found that Notch signaling regulates the timing of conversion through initiating morphological changes and upregulation of Asense in the NESCs.