Circadian per2 regulates the zebrafish central neuronal Mauthner cell axon regeneration via carboxypeptidase E

Central nervous system (CNS) injuries often result in permanent neurological impairments or fatal outcomes. While circadian regulation is a key pathway affecting the regenerative capacity of peripheral neurons, the role of the intrinsic circadian clock in CNS axon regeneration remains elusive. Using a zebrafish larval model with disrupted circadian rhythms, we observed significant inhibition of CNS axon regeneration in a single Mauthner cell (M cell). Mutations or inhibition of the core circadian clock gene per2 in zebrafish larvae severely impaired axon regeneration and the acoustically evoked escape movements. Time-of-day-dependent transcriptomic analysis revealed that per2 regulates genes involved in regeneration-related pathways, such as mitochondrial metabolism and Wnt signaling transduction. Furthermore, quantitative RT- PCR and Western blot experiments indicated that the neurotrophic factor carboxypeptidase E (Cpe) exhibits robust circadian rhythmicity in wild type but is significantly downregulated in per2-/- mutants. Cpe deficiency significantly suppressed axonal regeneration, whereas its overexpression enhanced regenerative capacity, thereby rescuing the axonal regeneration impairment in per2-/- mutants. Overall, our findings demonstrate the critical role of a stable circadian rhythm system and per2 in CNS regeneration, as well as the potential function of Cpe in promoting axon regeneration.