Huntingtin and Celsr3 Orchestrate Spontaneous Long-Distance Spinal Cord Regeneration

Spontaneous spinal cord regeneration is an endogenous process operational in diverse vertebrate and select mammalian species. Yet despite resulting in functional regeneration, axonal regrowth dynamics as well as underlying molecular programs are largely unknown. Live-cell imaging following laser axotomy of individual zebrafish spinal cord M-cell axons reveals a time-sensitive multistep process. From a genetic screen we uncover previously unrecognized roles for two evolutionary conserved genes, celsr3 and huntingtin (htt) in spontaneous spinal cord regeneration. Specifically, we find that htt is critical to initiate a transcriptional injury response in regenerating M-cells, enabling their axon to traverse the injury site. Once axons cross the injury site celsr3 functions to increase growth rates, while both celsr3 and htt function to sustain long-range axonal growth. Moreover, we show that htt is dispensable for optic nerve and peripheral motor neuron regeneration, and that reducing soma-to-injury site distance in htt mutant M-cell axons partially restores regeneration, demonstrating a selective role in long-range CNS axon regeneration. Consistent with a transport-based mechanism, genetic knockdown of htt-associated axonal transport proteins recapitulates the htt mutant regeneration phenotype. Combined, our results provide compelling evidence for a time and distance-sensitive multistep molecular framework in which celsr3 dependent growth rate modulation and htt dependent axonal transport promote spontaneous, long-range spinal cord regeneration.