View Program - 63rd Annual Drosophila Research Conference

The central nervous system of D. melanogaster is composed of numerous morphologically different neurons and glia which originate from a limited number of type II neural stem cells (NSC). Such neural diversity poses the important question of how these few NSCs give rise to a variety of neurons and glia; furthermore, NSCs’ failure to diversify in the brain results in severe brain pathologies and behavioral defects. D. melanogaster is a widely used model to test neuronal differentiation mechanisms and transcription-mediated development of neural circuits. Type II (TII) NSCs divide asymmetrically beginning in embryogenesis and continue to divide into late larval stages. Upon division, TII NSCs self-renew and produce an intermediate neural progenitor (INP) which will also divide asymmetrically into multiple progenitors that eventually compose most of the adult’s central complex.
Recently, TII NSCs were found to express transcription factors in a temporal manner to produce neuronal diversity. Among these temporal factors, the ecdysone receptor (EcR) is essential because it initiates the early-to-late transcription factor transition, and its production takes place 56 hours after larvae hatch (ALH). Seven-up (svp) activity is required from 0-36 hours ALH for induction of EcR transcription, but svp is downregulated at 36 hours; therefore, the 20 hour time gap between svp disappearance and EcR appearance suggests an additional regulatory mechanism is responsible for EcR induction and NSC temporal cascade transition from the early to late stage in larvae. Previous studies have shown these molecular transitions are linked to intrinsic and extrinsic cues which play important roles in the larval NSC transcription factor cascade. Cell division is an internal process essential for neuroblast development, and may trigger an intrinsic process via the cell cycle to establish neural diversity and activate transitions in the temporal cascade. We hypothesize that if the cell cycle is blocked, then transcription factors inducing later-stage development of neuroblasts will be absent while early-stage transcription factors will continue to be expressed into L2/L3 stages. To test this, two kinds of cell cycle disruptors are used: first, a cytokinesis cycle delay signal (CDK-1RNAi) which inhibits the formation of a protein kinase subunit controlling important aspects of cell cycle progression; and second, pavarotti downregulation (PavRNAi) which inhibits transcription of microtubule motor proteins in spindle apparatus formation. Cell cycle disruption at both 0 and 48 hours ALH, which are critical time points for proper EcR induction, showed lack of EcR in the NSC at 56 hours. These results support our hypothesis in that cell cycle disruption leads to persistent early-stage transcription factor expression into late larval developmental.

Intrinsic and Extrinsic Cues Regulate the Early-to-Late Transition of Transcription Factors in Drosophila Type II Neuroblast