Aryl hydrocarbon receptor restricts axon regeneration of DRG neurons in response to injury

Injured neurons sense environmental cues to balance neural protection and axon regeneration, but the mechanisms are unclear. Here, we unveil aryl hydrocarbon receptor (AhR), a ligand-activated bHLH-PAS transcription factor, as molecular sensor and key regulator of acute stress response at the expense of axon regeneration. We demonstrate responsiveness of DRG sensory neurons to ligand-mediated AhR signaling, which functions to inhibit axon regeneration. Ahr deletion mimics the conditioning lesion in priming DRG to initiate axonogenesis gene programs; upon peripheral axotomy, Ahr ablation suppresses inflammation and stress signaling while augmenting pro-growth pathways. Moreover, comparative transcriptomics revealed signaling interactions between AhR and HIF-1a, two structurally related bHLH-PAS a units that share the dimerization partner Arnt/HIF-1ß. Functional assays showed that the growth advantage of AhR-deficient DRG neurons requires HIF-1a; but in the absence of Arnt, DRG neurons can still mount a regenerative response. We further unveil a link between bHLH-PAS transcription factors and DNA hydroxymethylation in response to peripheral axotomy, while neuronal single cell RNA-seq analysis revealed a link of the AhR regulon to RNA polymerase III regulation and integrated stress response (ISR). Altogether, AhR activation favors stress coping and inflammation at the expense of axon regeneration; targeting AhR can enhance nerve repair. Overall design: Ahr cKO (Ahr fl/fl Thy1CRE-ERT2/EYFP) or control (Ahr f/f or f/+) were injected with tamoxifen (once daily, 100 mg/kg i.p. for 5 consecutive days) at 28 weeks of age. Naive L4-L6 DRGs were isolated from n=3 mice per group (DRGs from each mouse represents 1 sample) at 5 weeks post-tamoxifen treament. For the injury group, sciatic nerve crush lesion was performed on n=3 mice per group at 5 weeks post-tamoxifen treament and L4-L6 DRGs were isolated 1 day later. DRGs were dissociated between ZT2 to 4 (ZT=0 when the light turns on in the mouse room). RNA was collected using QIAGEN RNeasy plus mini kit.