Genetic and Molecular Studies of Neurogenesis

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Abstract The large human cerebral cortex is thought to arise from a specialized population of outer radial glia (oRG) neural stem cells in the outer subventricular zone (OSVZ). These stem cells divide to produce intermediate neural progenitors (INPs) that themselves divide to produce 8-12 neurons. In contrast, most non-primate neural stem cells produce 1 or 2 neurons with each division, resulting in a smaller brain. Thus, mouse and fish provide a relatively poor model for primate brain expansion, and direct study of primate fetal brain tissue is problematic. Over the past few years, our lab and others have developed a Drosophila model for understanding oRG-like stem cell lineages. We discovered a population of Drosophila brain neural stem cells (called type II neuroblasts; T2NBs) that generate INPs which each generate 8-12 neurons, similar to primate oRG stem cells. Now we can use the power and rapidity of Drosophila genetics to characterize the role of T2NBs in brain development, and suggest testable hypotheses for human brain organoid research. We previously discovered a series of transcription factors (TFs) and RNA-binding proteins that are sequentially expressed in T2NBs over the five days of larval life; these are excellent candidates for specifying the “temporal identity” of neurons in the lineage. We also discovered a series of TTFs that were sequentially expressed in each INP as it produced its lineage, and showed one of them (Eyeless, Ey; Pax6) was a validated functional temporal factor that specified late-born neuron identity. But many open questions remained: What are the neurons born from each temporal window in the T2NB lineage? Do the candidate temporal factors in T2NBs actually specify neuronal identity? Most TTFs are transiently expressed in progenitors (neuroblasts and INPs) but lack adult expression; how are TTF-specified neuronal identities consolidated and maintained in the adult fly? Are there TTF target genes that act to maintain neuronal identity? And lastly, how is T2NB temporal identity and INP temporal identity integrated to specify unique neuronal subtypes? These questions will be addressed in Aims 1-4, respectively. Our findings will provide insight into the role of INPs in expanding brain size in Drosophila, and suggest focused hypotheses to test for conserved mechanisms in primate tissue. Furthermore, our results may shed light on the development of the central complex, a conserved insect brain region used for celestial navigation of flies, bees, and butterflies.