Catch-Up Growth in Zebrafish Embryo Requires Neural Crest Cells Sustained by Irs1 Signaling.

Most animals display retarded growth in adverse conditions; however, upon the removal of unfavorable factors, they often show quick growth restoration, which is known as "catch-up" growth. In zebrafish embryos, hypoxia causes growth arrest, but subsequent reoxygenation induces catch-up growth. Here, we report the role of insulin receptor substrate (Irs)1-mediated insulin/insulinlike growth factor signaling (IIS) and the involvement of stem cells in catch-up growth in reoxygenated zebrafish embryos. Disturbed irs1 expression attenuated IIS, resulting in greater inhibition in catch-up growth than in normal growth and forced IIS activation‒restored catch-up growth. The irs1 knockdown induced noticeable cell death in neural crest cells (NCCs; multipotent stem cells) under hypoxia, and the pharmacological/genetic ablation of NCCs hindered catch-up growth. Furthermore, inhibition of the apoptotic pathway by pan-caspase inhibition or forced activation of Akt signaling in irs1 knocked-down embryos blocked NCC cell death and rescued catch-up growth. Our data indicate that this multipotent stem cell is indispensable for embryonic catch-up growth and that Irs1-mediated IIS is a prerequisite for its survival under severe adverse environments such as prolonged hypoxia.