Scarecrow, a homolog of mammalian Nkx2.1, regulates the temporal progression and glial differentiation of medulla neuroblasts for the optic lobe development in Drosophila melanogaster

Abstract

The Drosophila medulla is the largest structure in the adult visual nervous system. It contains a vast population of interneurons comprising over 80 different cell types generated by medulla neuroblasts (NBs). Aging NBs express a series of temporal transcription factors (tTFs) that contribute to neuronal diversity. Recent studies have shown that an NK-2 homeobox gene scarecrow (scro) works as a tTF covering from middle to late temporal windows; however, how its expression is established over multiple windows and what roles it plays in individual windows remain largely elusive. We found the lack of scro expression in the middle of the Dichaete (D) domain, implying its expression is not in a continuum as previously eluded. Overexpression and knockdown assays of scro or other tTFs using various tTF-Gal4 drivers further revealed distinctive roles played by Scro at each window, including the last one. The oldest NBs positive for Tll and Scro found at the most proximal region of the developing medulla field attained expression of Gcm (the master factor of glial differentiation) and Nerfin-1 (a suppressor of dedifferentiation via Notch suppression), which in turn led to NB-to-glia differentiation. Downregulation of either gcm or nerfin-1 resulted in the formation of ectopic NBs at the expense of glial cells. Moreover, scro-knockdown led to a loss of Gcm, Nerfin-1, and Prospero expression, misregulation of Notch expression, formation of ectopic NBs, and a substantial reduction in glial cell population, suggesting that Scro acts upstream of Gcm and Nerfin-1. The chromatin-immunoprecipitation (ChIP) assays support that Scro regulates the expression of gcm, nerfin-1, and pros, as well as several tTFs, the expression of which overlaps with Scro. In summary, this study not only verified previously suggested roles of Scro but also uncovered novel features of this gene in various temporal windows, including the promotion of NB-to-glial transition and discontinuous expression within the D domain.