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 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, reverse transcription quantitative 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.