The circadian clock time tunes axonal regeneration
Peripheral nervous system injuries lead to long-term neurological disability due to limited axonal regenerative ability. Injury-dependent and more recently injury-independent physiological mechanisms have provided important molecular insight into regenerative mechanisms. However, whether common molecular denominators underpinning both injury-dependent and independent biological processes exist remains unclear. We initially performed a comparative analysis of recently generated transcriptomic datasets associated with the regenerative ability of sciatic dorsal root ganglia (DRG). Surprisingly, circadian rhythms were identified as a the most significantly enriched biological process associated with regenerative capability. We demonstrate that DRG neurons possess an endogenous circadian clock with a 24h oscillations of circadian genes and that their regenerative ability displays a diurnal oscillation in a mouse model of sciatic nerve injury. Consistently, transcriptomic analysis of DRG neurons showed a significant time-of-day dependent enrichment for processes associated with neuronal development and axonal growth, for regeneration and circadian associated genes, including the core clock genes Bmal1 and Clock. Indeed, DRG-specific ablation of the non-redundant clock gene Bmal1showed that it is required for regenerative gene expression, neuronal intrinsic circadian axonal regeneration and target reinnervation. Lastly, Lithium, a chrono-active compound, enhanced nerve regeneration, in wildtype but not in clock genes Bmal1 and Cry1/2-deficient mice. Together, these data demonstrate that daily rhythms and the circadian clock fine-tune the regenerative response of DRG neurons, and they advocate for the use of chrono-active strategies in time-day dependent modulation of nerve repair. Overall design: L4-6 dorsal root ganglia were extracted 72hr after a sciatic nerve injury at either Zeitgeber (ZT)8 or ZT20 followed by neuronal cell enrichment  that was processed for RNA-seq