Apelin signaling controls neuroendocrine homeostasis via Atoh8-dependent transcriptional regulation

Apelin is a pleiotropic peptide hormone with established roles in cardiovascular function, angiogenesis, and metabolism, but its contribution to neuroendocrine development and function remains poorly understood. Here, using genetic loss-of-function and inducible gain-of-function approaches in zebrafish, we show that Apelin signaling is dosage-sensitive and required for normal development, reproductive capacity, and physiological homeostasis. Both apelin deficiency and apelin overexpression disrupted embryonic development and reduced adult lifespan, while exhibiting markedly reduced fertility in both sexes. Transcriptomic profiling revealed broad changes in neuroendocrine, hormonal, stress-response, and inflammatory genes with young adult mutant expression profiles resembling those of aged wild-type fish. Both loss and excess of Apelin also disrupted hypothalamic-pituitary-thyroid (HPT) axis development, reducing hypothalamic thyrotropin-releasing hormone-expressing cells, pituitary gonadotropic cell populations, and thyroid gland volume. Transcriptomic analyses identified atoh8, encoding a basic helix-loop-helix transcription factor, as a candidate effector of Apelin signaling. Apelin expression increased atoh8 expression during early embryogenesis, whereas atoh8 mRNA partially rescued developmental defects in apelin mutants, including hypothalamic and pituitary markers, and thyroid volume. Conversely, atoh8 crispants displayed thyroid and pituitary abnormalities, supporting a role for Atoh8 in neuroendocrine development. Together, these findings establish Apelin signaling as a dosage-sensitive regulator of zebrafish development and tissue homeostasis and identify Atoh8 as a functional downstream effector that partially mediates Apelin-dependent HPT axis formation. This work links peptide hormone signaling to neuroendocrine development and provides a framework for investigating its broader contributions to vertebrate physiology, reproduction and aging.