SIK3-HDAC4-Warts Axis Functions as a Gatekeeper of Neural Stem Cell Reactivation in Drosophila

A delicate balance between the quiescent and proliferative states of neural stem cells (NSCs) is important for neurogenesis and homeostasis. Histone deacetylase 4 (HDAC4) variants are associated with neurodevelopmental disorders, however, its role in early brain development remains elusive. In this study, we demonstrate that Drosophila HDAC4 plays a crucial role in neural stem cells (NSCs) reactivation and brain development. Depletion of HDAC4 results in notable defects in NSC reactivation, while its overexpression leads to premature reactivation. HDAC4 is SUMOylated at Lys902, which enhances its protein stability by preventing HDAC4 from undergoing ubiquitin-proteasome-mediated degradation. Moreover, phosphorylation of HDAC4 by salt-inducible kinase 3 (SIK3), an AMPK-related kinase, allows cytoplasmic localization of HDAC4 and enhances the association between HDAC4 and Warts, a core kinase of the Hippo pathway. This HDAC4-Wts association inhibits Warts activity, and in turn, the inactivation of the Hippo pathway, triggering NSC reactivation. Finally, genetic epistasis experiments support the SIK3-HDAC4-Warts axis during NSC reactivation. In conclusion, our findings identify HDAC4 as a molecular switch that integrates SUMOylation, ubiquitination, and the Hippo pathway to govern NSC reactivation.