Date Published: December 13, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Di Wu, Litao Wu, Huanping An, Hongcun Bao, Pengfei Guo, Bei Zhang, Huimei Zheng, Fan Zhang, Wanzhong Ge, Yu Cai, Yongmei Xi, Xiaohang Yang.
By the end of neurogenesis in Drosophila pupal brain neuroblasts (NBs), nuclear Prospero (Pros) triggers cell cycle exit and terminates NB lifespan. Here, we reveal that in larval brain NBs, an intrinsic mechanism facilitates import and export of Pros across the nuclear envelope via a Ran‐mediated nucleocytoplasmic transport system. In rangap mutants, the export of Pros from the nucleus to cytoplasm is impaired and the nucleocytoplasmic transport of Pros becomes one‐way traffic, causing an early accumulation of Pros in the nuclei of the larval central brain NBs. This nuclear Pros retention initiates NB cell cycle exit and leads to a premature decrease of total NB numbers. Our data indicate that RanGAP plays a crucial role in this intrinsic mechanism that controls NB lifespan during neurogenesis. Our study may provide insights into understanding the lifespan of neural stem cells during neurogenesis in other organisms.
In Drosophila, neurogenesis occurs in two sequential steps. Neuroblasts (NBs), also known as neural stem cells, first form from the neuroectoderm in embryos and undergo a series of asymmetric divisions to produce self‐renewed NBs and ganglion mother cells (GMCs). The GMCs then divide terminally to generate neurons or glial cells to build the larval central nervous system (CNS). Once embryogenesis is completed, most abdominal NBs are eliminated through programmed cell death, whereas the cephalic and thoracic NBs enter mitotic quiescence at the embryo‐larval transition (Prokop & Technau, 1991; Tsuji, Hasegawa, & Isshiki, 2008). After the hatching of the larvae, most of the remaining NBs are reactivated and resume asymmetric divisions in the late first‐instar or early second‐instar larval stages to contribute ∼90% of cells in the adult CNS (Sousa‐Nunes, Cheng, & Gould, 2010; Srinivasan et al., 1998).
Pros is a homeodomain transcription factor expressed in NBs and contains both a nuclear localization signal (NLS) and a nuclear export signal (NES; Demidenko et al., 2001; Vaessin et al., 1991). Our data suggest that an intrinsic mechanism maintains Pros shuttling across the nuclear envelope in the larval brain NBs. Mutations in rangap disrupt this mechanism and lead to the impaired export of Pros, causing early accumulation of Pros in the nucleus and premature NB cell cycle exit. Our Caspase 3 staining and P35 expression data support the conclusion that apoptosis did not play a role in rangap NB missing phenotype.
X. Y., Y. X., and D. W. designed the experiments and wrote the manuscript. D. W. and L. W. conducted the RanGAP experiments. P. G. and B. Z. generated the rangap9 mutant allele by CRISPR/Cas9. H. B, H. Z, and F. Z. took part in biochemical and genetic experiments. H. A., W. G., and Y. C. gave many constructive suggestions for the project and valuable inputs for this manuscript.