G9a inhibits CREB-triggered expression of mu opioid receptor in primary sensory neurons following peripheral nerve injury

Abstract

Neuropathic pain, a distressing and debilitating disorder, is still poorly managed in clinic. Opioids, like morphine, remain the mainstay of prescribed medications in the treatment of this disorder, but their analgesic effects are highly unsatisfactory in part due to nerve injury-induced reduction of opioid receptors in the first-order sensory neurons of dorsal root ganglia. G9a is a repressor of gene expression. We found that nerve injury-induced increases in G9a and its catalyzed repressive marker H3K9m2 are responsible for epigenetic silencing of Oprm1, Oprk1, and Oprd1 genes in the injured dorsal root ganglia. Blocking these increases rescued dorsal root ganglia Oprm1, Oprk1, and Oprd1 gene expression and morphine or loperamide analgesia and prevented the development of morphine or loperamide-induced analgesic tolerance under neuropathic pain conditions. Conversely, mimicking these increases reduced the expression of three opioid receptors and promoted the mu opioid receptor-gated release of primary afferent neurotransmitters. Mechanistically, nerve injury-induced increases in the binding activity of G9a and H3K9me2 to the Oprm1 gene were associated with the reduced binding of cyclic AMP response element binding protein to the Oprm1 gene. These findings suggest that G9a participates in the nerve injury-induced reduction of the Oprm1 gene likely through G9a-triggered blockage in the access of cyclic AMP response element binding protein to this gene.

The transcription factor C/EBPβ in the dorsal root ganglion contributes to peripheral nerve trauma–induced nociceptive hypersensitivity

“…6, G and H). In agreement with the previous reports (11, 12), Ehmt2 knockout increased the basal abundance of MOR and Kv1.2 proteins in cultured DRG neurons transfected with AAV5-EGFP (Fig. 6, G and H).…”

“…6, G and H). Combined with previous reports (11, 12), our findings indicate that C/EBPβ-triggered Ehmt2 gene activation participates in epigenetic silencing of Oprm1 and Kcna2 genes in the ipsilateral DRG neurons under neuropathic pain conditions.…”

“…The abundance of the transcription factor C/EBPβ, like that of other transcription factors (such as myeloid zinc finger 1) and non–transcription factor proteins (such as G9a) (8, 11, 12), can be regulated after peripheral nerve injury. We demonstrated that CCI led to an increase of C/EBPβ at both mRNA and protein levels in the ipsilateral DRG, suggesting activation of Cebpb gene transcription under neuropathic pain conditions.…”

“…Blocking the nerve injury–induced G9a increase in the ipsilateral DRG rescues the abundance of μ, κ, and Δ opioid receptors in the DRG; improves morphine analgesia; and impairs neuropathic pain (11–13). Conversely, overexpression of G9a in the DRG decreases the abundance of these three opioid receptors in the DRG and promotes the μ opioid receptor (MOR)–gated release of primary afferent neurotransmitters (12). These lines of evidence suggest that G9a is an endogenous instigator of neuropathic pain.…”

Nerve injury–induced epigenetic silencing of opioid receptors controlled by DNMT3a in primary afferent neurons

“…The thermal test as described below was performed before and 20 minutes after s.c. morphine or loperamide injection. Analgesic tolerance was induced in rats as previously described [18;46]. Briefly, morphine (10 mg/kg) was injected s.c. twice daily for 6 consecutive days in naïve rats or twice daily for 4 consecutive days starting on day 7 post-SNL.…”