AUTHOR=Yan Chen-Yan , Wu Feng-Yao , Sun Feng , Fang Ya , Zhang Rui-Jia , Zhang Chang-Run , Zhang Cao-Xu , Wang Zheng , Yang Rui-Meng , Yang Liu , Dong Mei , Zhang Qian-Yue , Ye Xiao-Ping , Song Huai-Dong , Zhao Shuang-Xia 

TITLE=The isl2a transcription factor regulates pituitary development in zebrafish

JOURNAL=Frontiers in Endocrinology

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2023.920548

DOI=10.3389/fendo.2023.920548

ISSN=1664-2392

ABSTRACT=<sec><title>Background</title><p>ISL LIM homeobox 2, also known as insulin gene enhancer protein ISL-2 (<italic>ISL2</italic>), is a transcription factor gene that participates in a wide range of developmental events. However, the role of <italic>ISL2</italic> in the hypothalamus-pituitary-thyroid axis is largely unknown. In the present study, we characterized the expression patterns of <italic>ISL2</italic> and revealed its regulative role during embryogenesis using zebrafish.</p></sec><sec><title>Methods</title><p>We used the CRISPR/Cas9 system to successfully establish homozygous <italic>ISL2</italic>-orthologue (<italic>isl2a</italic> and <italic>isl2b</italic>) knockout zebrafish. Moreover, we utilized these knockout zebrafish to analyze the pituitary and thyroid phenotypes <italic>in vivo</italic>. For further molecular characterization, <italic>in situ</italic> hybridization and immunofluorescence were performed.</p></sec><sec><title>Results</title><p>The <italic>isl2a</italic> 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 <italic>tshba</italic>, <italic>cga</italic>, and <italic>tg</italic>, were significantly decreased in <italic>isl2a</italic> mutants. Finally, the thyroid dysplasia in <italic>isl2a</italic> mutant larvae may be attributed to a reduction in proliferation rather than changes in apoptosis.</p></sec><sec><title>Conclusions</title><p>In summary, <italic>isl2a</italic> regulates the transcriptional levels of marker genes in hypothalamus-pituitary-thyroid axis, and <italic>isl2a</italic> knockout causing low thyroid hormone levels in zebrafish. Thus, <italic>isl2a</italic> identified by the present study, is a novel regulator for pituitary cell differentiation in zebrafish, resulting in thyroid gland hypoplasia and phenotypes of hypothyroidism.</p></sec>