Diapause induces functional axonal regeneration after necrotic insult in C. elegans

Many neurons are unable to regenerate after damage. The ability to regenerate after an insult depends on life stage, neuronal subtype, intrinsic and extrinsic factors. C. elegans is a powerful model to test the genetic and environmental factors that affect axonal regeneration after damage, since its axons can regenerate after neuronal insult. Here we demonstrate that diapause promotes the complete morphological regeneration of truncated touch receptor neuron (TRN) axons expressing a neurotoxic MEC-4(d) DEG/ENaC channel. Truncated axons of different lengths were repaired during diapause and we observed potent axonal regrowth from somas alone. Complete morphological regeneration depends on DLK-1 but neuronal sprouting and outgrowth is DLK-1 independent. We show that TRN regeneration is fully functional since animals regain their ability to respond to mechanical stimulation. Thus, diapause induced regeneration provides a simple model of complete axonal regeneration which will greatly facilitate the study of environmental and genetic factors affecting the rate at which neurons die.Author summary: Diapause entry and hibernation have the striking ability to protect the nervous system from diverse types of damage. Here we show that the diapausing dauer larvae of C. elegans regenerate broken mechanosensory neurons that were damaged by the hyperactivation of degenerins (MEC-4d) or by axotomy during diapause. This regeneration is complete and functional, rendering neurons capable of responding to touch after three days in diapause. Genetic inactivation of the insulin receptor DAF-2 promotes regeneration of mec-4d axons in non-dauer animals. Overexpression of the downstream transcription factor DAF-16 promotes neuronal protection in mec-4d neurons while loss of daf-16 accelerates mec-4d induced degeneration. Temperature sensitive activation of DAF-2 during diapause induces the loss of axonal integrity. This indicates that the insulin signaling pathway is an important underlying factor in regeneration. Additionally, we show that complete morphological regeneration depends on DLK-1, a conserved protein required for axonal repair. In this work we introduce a simple model of complete axonal regeneration, which will greatly facilitate the study of environmental and genetic factors affecting neurodegeneration and constitute an advantage in studying axonal regrowth.