nr2e1 influences exploratory behavior but is not necessary for terminal hormone differentiation in the zebrafish (Danio rerio) pituitary

Congenital hypopituitarism is characterized by multiple hormone deficiencies due to mutations in transcription factors involved in pituitary embryogenesis. Stem cells, which by definition can each give rise to a progenitor and an undifferentiated cell by asymmetric division, are present in the pituitary gland and are important during periods of high metabolic demand in different phases of life. In previous studies, the accumulation of the stem cell markers Sox2 and Nr2e1 was observed in the Ames mouse, which harbors a mutation in Prop1. Sox2 is the consensus stem cell marker in the pituitary gland, while the role of Nr2e1 in the pituitary development has not been characterized although it is essential for neural stem cell maintenance and neogenesis in the brain and its loss of function causes pathological aggression and lack of maternal instinct in mice. In this project, the zebrafish animal model was used to characterize the role of nr2e1, to confirm whether this gene can be involved in the pituitary terminal differentiation, and to determine the effects of this gene on animal behavior. The zebrafish is a particularly appropriate model for use in this project because it is easy to maintain, is economical, and has a rapid metabolism and growth rate. In the present study, we created 2 zebrafish models by knocking out prop1 and nr2e1 using the CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) genome-editing technique. This technique enables highly specific gene/reading frame interruption and/or base substitution in the genome, with low cellular toxicity and high heritability. Zebrafish with homozygous nr2e1 mutations develop and reproduce similarly to wild-type zebrafish, but present a more exploratory behavioral pattern compared to wild-type and heterozygous zebrafish. Based on immunofluorescence, Sox2 expression was higher in the mutant zebrafish than in the wild type and was not co-localized with Nr2e1 expression. Hormone expression did not differ between wild-type and mutant zebrafish. We conclude that nr2e1 is not crucial in the terminal differentiation of the hormone-forming pituitary gland; however, it induces a distinct behavioral phenotype at the larval stage. Analyses of zebrafish harboring a prop1 mutation are ongoing owing to issues with the establishment of the lineage (AU)