Novel candidate intrinsic and extrinsic regulators of regeneration after axonal injury

Injured neurons in the peripheral nervous system (PNS) can regenerate long-distances and re-establish function, while in the central nervous system (CNS) they do not. Anatomical and cellular signaling discrepancies set up different post-traumatic landscapes between the PNS and CNS, therefore residing neurons respond distinctively to axonal injury. A clear example is when the lesioned peripheral branches of dorsal root ganglia (DRG) regenerate spontaneously, while the lesioned central branches do not. Understanding why such differences occur is the underlying theme of the work presented here. The regenerative capacity of the lesioned PNS is known to be transcription-dependent. Recently, the histone acetyl-transferase p300/CBP-associated factor (PCAF) was discovered to enhance the axonal regenerative program of the DRG as well as prime growth of injured central DRG axons. Although PCAF was found to be an essential player in the transcriptional regulation of a handful of key regeneration-associated genes (RAGs), the precise mechanism of how PCAF enhances axonal regeneration remained elusive. We identified PCAF-dependent RAGs (PCAF-RAGs) through analysis of the DRG transcriptome (RNA-seq) from wildtype and PCAF knockout mice after a peripheral regenerative lesion. Transcription factor binding site analysis predicted potential co-regulators, Specificity protein 1 (Sp1) and myc-associated zinc finger protein (MAZ). Knockdown and overexpression experiments showed that Sp1 and MAZ can enhance neurite outgrowth in vitro, dependent upon the presence of PCAF. More importantly, pharmaceutical activation of PCAF allowed for enhanced neurite outgrowth of cultured DRG and systemic administration of the PCAF activator promoted increased regeneration of the PNS. Anatomically, the periphery subjects its neurons to mechanical forces that may influence its repair outcome. These mechanical cues are missing from the CNS. Therefore, we explored if and how sensory neurons can respond regeneratively to mechanical stimulation. An in-house built bioreactor applied cyclic mechanical tension to PNS- or CNS-located DRG-nerve explants. Neurite length analysis showed that stretch can induce outgrowth of medium to large diameter DRG, partially through the expression of activating transcription factor 3 (ATF3). Our work shows that either a regenerative injury or mechanical stimulation regulate specific outgrowth-related transcriptional machinery. Better understanding of such mechanisms will allow recapitulation of regenerative programs in non-regenerating neuronal populations, in hopes of promoting functional regeneration and recovery