A network of Notch-dependent and -independent her genes controls neural stem and progenitor cells in the zebrafish thalamic proliferation zone

Neural proliferation zones mediate brain growth, and employ Delta/Notch signaling and HES/HER transcription factors to balance neural stem cell (NSC) maintenance and generation of progenitors and neurons. We investigated Notch-dependency and function of her genes in the thalamic proliferation zone of developing zebrafish larvae. Nine Notch-dependent genes, her2, her4.1-5, her12, her15.1-2 , and two Notch-independent genes, her6, her9 , are differentially expressed, and define distinct NSC and progenitor populations. her6 prominently executes patterning information to maintain NSCs and the zona limitans intrathalamica Shh signaling activity. her6, her9 double mutants reveal that Notch-independent her genes predominantly regulate NSC maintenance and transition into the progenitor pool. Surprisingly, combined deletion of all Notch-dependent her genes does not affect NSCs or progenitor formation. Combined genetic manipulation of up to eleven Notch-dependent and -independent her genes revealed that Notch-dependent her genes may regulate progenitor progression into neurogenesis, but not progenitor generation itself. The her gene network is partially redundant, with Notch-independent her genes better substituting for loss of Notch-dependent genes than vice versa. Together, her gene regulatory feedback loops and crossregulation contribute to the observed robustness of NSC maintenance.