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

Abstract

Opioids are the gold standard for pharmacological treatment of neuropathic pain, but their analgesic effects are unsatisfactory in part due to nerve injury-induced downregulation of opioid receptors in dorsal root ganglia (DRG) neurons. How nerve injury drives such downregulation remains elusive. DNA methyltransferase-(DNMT-) triggered DNA methylation represses gene expression. We show here that blocking the nerve injury-induced increase in DRG DNMT3a (a de novo DNMT) rescued the expression of Oprm1 and Oprk1 mRNAs and their respective encoding mu opioid receptor (MOR) and kappa opioid receptor (KOR) proteins in the injured DRG. Blocking this increase also prevented the nerve injury-induced increase in DNA methylation in the promoter and 5′-untranslated region of the Oprm1 gene in the injured DRG, restored morphine or loperamide (a peripheral acting MOR preferring agonist) analgesic effects, and attenuated the development of their analgesic tolerance under neuropathic pain conditions. Mimicking this increase reduced the expression of Oprm1 and Oprk1 mRNAs and their coding MOR and KOR in DRG and augmented MOR-gated neurotransmitter release from the primary afferents. Mechanistically, DNMT3a regulation of Oprm1 gene expression required the methyl-CpG-binding protein 1, MBD1, as MBD1 knockout resulted in the decreased binding of DNMT3a to the Oprm1 gene promoter and blocked the DNMT3a-triggered repression of Oprm1 gene expression in DRG neurons. These data suggest that DNMT3a is required for nerve injury-induced and MBD1-mediated epigenetic silencing of the MOR and KOR in the injured DRG. DNMT3a inhibition may serve as a promising adjuvant therapy for opioid use in neuropathic pain management.

DNMT3a‐triggered downregulation of K2p1.1 gene in primary sensory neurons contributes to paclitaxel‐induced neuropathic pain

“…All procedures used were approved by the Animal Care and Use Committee at the Rutgers New Jersey Medical School and consistent with the ethical guidelines of the US NIH and the IASP. The group sizes were based on previous reports …”

“…After the lamina was removed, the L3‐4 DRGs were exposed. The siRNA (1 μL/DRG, 40 μM) or viral solution (1 μL/DRG, 10 12 particles/ml) was injected into the left L3 and L4 DRGs with a glass micropipette . TurboFect in vivo transfection reagent (Thermo Scientific Inc.) was used as a delivery vehicle …”

“…Mechanical and heat testing, conditioned place preference (CPP) were performed as previously described …”

“…The siRNA (1 μL/DRG, 40 μM) or viral solution (1 μL/DRG, 10 12 particles/ml) was injected into the left L3 and L4 DRGs with a glass micropipette. 23,24 TurboFect in vivo transfection reagent (Thermo Scientific Inc.) was used as a delivery vehicle. [25][26][27] Paclitaxel-induced neuropathic pain model Briefly, paclitaxel (4 mg/kg; dissolved in the vehicle) or vehicle (a mixture of ethanol and Cremophor EL(1:1)) was injected intraperitoneally (i.p.)…”

“…Mechanical and heat testing, conditioned place preference (CPP) were performed as previously described. 23,24,29,30 For mechanical testing, two different von Frey filaments (0.17 and 0.4 g) were applied the plantar surface of both hind paws. Each stimulation was repeated 10 times.…”