Injury-free priming: induction of Primary Afferent Collateral Sprouting in uninjured sensory neurons in vivo primes them for enhanced axon outgrowth in vitro.

Prior “conditioning” nerve lesions can prime DRG neurons for enhanced axon regeneration. Here, we tested the hypothesis that adult DRG neurons can be primed for axon elongation in vitro without axonal injury by prior induction of Primary Afferent Collateral Sprouting (PACS) in vivo. Thoracic cutaneous nerves (T9, T10, T12, T13 but not T11) were transected to create zones of denervated skin. Neurons from the uninjured T11 DRG underwent PACS within the skin, as demonstrated by the expansion of its zones responsive to pinch up to 14 days. At 7 or 14 days after induction of collateral sprouting, DRG neurons were dissociated and cultured for 18 hours in defined media lacking neurotrophins and growth factors. Neurons from the uninjured T11 DRG had longer mean neurite lengths than neurons from naïve DRG. A larger proportion of neurons from the uninjured T11 DRG showed an elongating or arborizing phenotype than neurons from naïve DRG. Transcriptomic analysis of the uninjured T11 DRG and denervated/reinnervated skin reveal regulation of receptor/ligand systems and regulators of growth during collateral sprouting. For example, the glial cell-derived neurotrophic family ligands Artemin and Persephin were upregulated in denervated skin after 7 and/or 14 days. We suggest that extracellular cues in denervated skin modify the intrinsic growth program of uninjured DRG neurons that enhances their ability to elongate or arborize even after explantation. Collectively, these data confirm that induction of collateral sprouting does not induce an injury response yet primes many of these uninjured neurons for in vitro axon growth.Competing Interest StatementThe authors have declared no competing interest.View Full Text