The isl2a transcription factor regulates pituitary development in zebrafish.

BACKGROUND: ISL LIM homeobox 2, also known as insulin gene enhancer protein ISL-2 ( ISL2 ), is a transcription factor gene that participates in a wide range of developmental events. However, the role of ISL2 in the hypothalamus-pituitary-thyroid axis is largely unknown. In the present study, we characterized the expression patterns of ISL2 and revealed its regulative role during embryogenesis using zebrafish.

METHODS: We used the CRISPR/Cas9 system to successfully establish homozygous ISL2 -orthologue ( isl2a and isl2b ) knockout zebrafish. Moreover, we utilized these knockout zebrafish to analyze the pituitary and thyroid phenotypes in vivo . For further molecular characterization, in situ hybridization and immunofluorescence were performed.

RESULTS: The isl2a mutant zebrafish presented with thyroid hypoplasia, reduced whole-body levels of thyroid hormones, increased early mortality, gender imbalance, and morphological retardation during maturity. Additionally, thyrotropes, a pituitary cell type, was notably decreased during development. Importantly, the transcriptional levels of pituitary-thyroid axis hormones-encoding genes, such as tshba , cga , and tg , were significantly decreased in isl2a mutants. Finally, the thyroid dysplasia in isl2a mutant larvae may be attributed to a reduction in proliferation rather than changes in apoptosis.

CONCLUSIONS: In summary, isl2a regulates the transcriptional levels of marker genes in hypothalamus-pituitary-thyroid axis, and isl2a knockout causing low thyroid hormone levels in zebrafish. Thus, isl2a identified by the present study, is a novel regulator for pituitary cell differentiation in zebrafish, resulting in thyroid gland hypoplasia and phenotypes of hypothyroidism.