Definition of the characteristic neurogenesis pattern in the neural plate by the Hes1 orthologue gene, her6, during early zebrafish development

During vertebrate embryonic development, a distinctive, spotty neurogenesis pattern emerges in the early neural plate, which represents proneural clusters. The determination of this pattern depends on the interaction between proneural genes and bHLH-O-type transcription factor (TF) genes, Hes/her, which suppress neurogenesis. In this study, we focused on the mouse Hes1 orthologue, her6, to understand the mechanism that controls neurodevelopmental patterns in the developing brain in zebrafish (Danio rerio). We first assessed the expression pattern of her6 in the neural plate, observing that it is consistently expressed in the entire forebrain throughout somitogenesis, including her9 expression within it. Meanwhile, the expression patterns of her6 changed dynamically in the hindbrain, in contrast to the Notch-independent her genes. The expression pattern was not significantly affected by forced NICD expression and DAPT treatment at the bud stage, showing that her6 expression is Notch-independent in the neural plate at this stage. To analyze the roles of her6, we disrupted her6 using the CRISPR/Cas9 method. The mutants thus obtained showed a deformed midbrain-hindbrain region and failed to grow to adulthood. At the bud stage, ectopic expression of neurogenesis-related genes was observed in her6 mutants in specific regions of the neural plate, where neurogenesis does not occur and which are considered neural progenitor pools (NPPs) in wild-type embryos. Of note, no other Notch-independent her genes are known to be expressed in these NPP regions. In contrast, the expression of regionalization genes in the forebrain and hindbrain was not affected in her6 mutants. These findings suggest that her6 defines the primary neurogenesis pattern in the neural plate, together with other Notch-independent her genes.