Krüppel Regulates Cell Cycle Exit and Limits Adult Neurogenesis of Mushroom Body Neural Progenitors in Drosophila

In many organisms, including Drosophila and humans, neural progenitors exit the cell cycle and are eliminated by the end of development, thereby restricting adult neurogenesis to specific brain regions. Here, we identify the evolutionarily conserved transcription factor Kruppel (Kr) as a lineage-specific regulator of cell cycle exit and elimination of mushroom body neuroblasts (MBNBs), which generate the learning and memory centre of the Drosophila brain, a structure functionally analogous to the mammalian hippocampus. Neuroblast-specific Kr RNAi and the Irregular facet (KrIf-1) mutation prolong MBNB lifespan, enabling continued neurogenesis in the adult brain. Although Kr is expressed only at low levels in postembryonic MBNBs, its pupal stage-specific depletion or misexpression is sufficient to cause MBNB retention, revealing a previously unrecognised postembryonic function distinct from its established role in embryonic neurogenesis. Mechanistically, persistent MBNBs maintain expression of the early temporal factor IGF2 mRNA-binding protein (Imp) and fail to fully induce the late temporal factors Syncrip (Syp) and Eip93F (E93). Co-depletion of Imp suppresses MBNB retention caused by Kr depletion, demonstrating that Imp is a key downstream effector of Kr. In parallel, Kruppel homolog 1 (Kr-h1), another Kr family transcription factor and a well-established mediator of hormone-responsive transcription, functionally antagonises Kr by suppressing E93 expression: Kr-h1 knockdown partially rescues the Kr depletion phenotype, whereas Kr-h1 overexpression drives tumour-like neuroblast overgrowth. Together, our findings establish Kr as an MBNB-specific coordinator that integrates intrinsic temporal programmes with extrinsic signalling pathways to coordinate neural stem cell termination and neuronal fate transitions, with potential parallels in other organisms.