Nerve Injury Diminishes Opioid Analgesia through Lysine Methyltransferase-mediated Transcriptional Repression of μ-Opioid Receptors in Primary Sensory Neurons

Abstract

The -opioid receptor (MOR, encoded by Oprm1) agonists are the mainstay analgesics for treating moderate to severe pain. Nerve injury causes down-regulation of MORs in the dorsal root ganglion (DRG) and diminishes the opioid effect on neuropathic pain. However, the epigenetic mechanisms underlying the diminished MOR expression caused by nerve injury are not clear. G9a (encoded by Ehmt2), a histone 3 at lysine 9 methyltransferase, is a key chromatin regulator responsible for gene silencing. In this study, we determined the role of G9a in diminished MOR expression and opioid analgesic effects in animal models of neuropathic pain. We found that nerve injury in rats induced a long-lasting reduction in the expression level of MORs in the DRG but not in the spinal cord. Nerve injury consistently increased the enrichment of the G9a product histone 3 at lysine 9 dimethylation in the promoter of Oprm1 in the DRG. G9a inhibition or siRNA knockdown fully reversed MOR expression in the injured DRG and potentiated the morphine effect on pain hypersensitivity induced by nerve injury. In mice lacking Ehmt2 in DRG neurons, nerve injury failed to reduce the expression level of MORs and the morphine effect. In addition, G9a inhibition or Ehmt2 knockout in DRG neurons normalized nerve injury-induced reduction in the inhibitory effect of the opioid on synaptic glutamate release from primary afferent nerves. Our findings indicate that G9a contributes critically to transcriptional repression of MORs in primary sensory neurons in neuropathic pain. G9a inhibitors may be used to enhance the opioid analgesic effect in the treatment of chronic neuropathic pain.Chronic neuropathic pain resulting from damage to the peripheral or central nervous system causes agonizing suffering and reduced quality of life. Neuropathic pain is often resistant to conventional analgesic treatments and remains a major therapeutic challenge. Opioid drugs such as morphine produce their therapeutic effects through binding to the -opioid receptors (MORs, 3 encoded by Oprm1) (1, 2) and are widely used to treat moderate to severe pain. In patients with neuropathic pain, however, the analgesic potency of MOR agonists is reduced (3, 4). The opioid effects are also diminished in animal models of neuropathic pain (5-7). MORs expressed at primary sensory neurons and their central terminals in the spinal dorsal horn are essential for the analgesic effects of opioids (8 -10). Peripheral nerve injury reduces the expression level of MORs in the dorsal root ganglion (DRG), contributing to the loss of opioid analgesic efficacy in neuropathic pain (6,11,12). However, the epigenetic mechanisms by which nerve injury leads to diminished MOR expression in the DRG remain unclear.Transcriptional homeostasis is largely maintained by the dynamic balance between positive and negative regulation of gene transcription. Gene expression is critically controlled by chromatin structure and the modification status of histone tails (13-15). DNA methylation and histone modifications are two major ...

G9a participates in nerve injury-induced Kcna2 downregulation in primary sensory neurons

“…Given that G9a is co-localized with Kcna2 in the DRG neurons, it is very likely that G9a participates in the mechanisms underlying neuropathic pain development by downregulating Kcna2 in the axotomized DRG. However, other potential mechanisms of G9a involvement in neuropathic pain cannot be ruled out, because G9a regulates the expression of other genes in the axotomized DRG33.…”

Contribution of the Suppressor of Variegation 3-9 Homolog 1 in Dorsal Root Ganglia and Spinal Cord Dorsal Horn to Nerve Injury–induced Nociceptive Hypersensitivity

“…SUV39H1 likely contributes to nerve injury-induced nociceptive hypersensitivity through epigenetic silencing of MOR in the injured DRG. Nerve injury-induced DRG MOR downregulation is also restored by the inhibition of DRG histone deacetylase or G9a 38;43;44 , suggesting multiple epigenetic mechanisms by which the MOR gene is silenced in the injured DRG after peripheral nerve injury. Compensation by other mechanisms may not occur, as blocking each mechanism can restore MOR expression in the injured DRG.…”

Transmission pathways and mediators as the basis for clinical pharmacology of pain

“…Importantly, in response to nerve injury (spinal nerve ligation), the expression of both μ-receptor mRNA and mu receptor-related proteins decreased [56]. More recently, a group has managed to characterize and describe these changes [57]. This study observed that down-regulation of mu opioid receptors occurs in the dorsal root ganglion, but not the spinal cord [57].…”

“…More recently, a group has managed to characterize and describe these changes [57]. This study observed that down-regulation of mu opioid receptors occurs in the dorsal root ganglion, but not the spinal cord [57]. The authors of this study also used mice with a knockout Ehmt2 (a histone methyltransferase) and observed that the knockout mice did not experience nerve-injury related resistance to opioid therapy [57].…”