A novel proneural function of Asense, integrated with of L'sc and Notch, promotes the Neuroepithelial to Neuroblast transition

In the developing Drosophila optic lobe, neuroepithelial (NE) cells are transformed progressively into neurogenic progenitors called neuroblasts (NBs). The progenitors undergoing this transition are identified by the expression of the Acheate Scute Complex (AS-C) factor Lethal of Scute (Lsc). Here we found that Asense (Ase), another AS-C factor, presents a peak of expression in the cells neighboring those transition Lsc expressing cells. This peak of Ase identifies a new transition step and it is necessary and sufficient to promote the NE to NB transition. Thus, our data provide the first direct evidence for a proneural role of Ase in CNS neurogenesis. Furthermore, we found that the peak of Ase is induced in a non-cell autonomous manner by Lsc through the activation of Notch signaling in the adjacent cells. This suggests that the two classic proneural activities, promoting neurogenesis and Notch signaling, have been split between Ase and Lsc. Thus, our data fit with a model in which the key proneural role of Ase is integrated with Notch and Lsc activities, facilitating the progressive transformation of NE cells into NBs. SUMMARY STATEMENTThe switch of progenitor cells towards neuron production is crucial for proper brain development. The transcription factor Asense promotes this transition through a mechanism integrated with timing and neurogenic signals.