Threshold effect of G9a/Glp on peripheral nerve injury-induced hypersensitivity

Abstract

BackgroundPrevious studies disclosed the pivotal role of methyltransferase complex G9a/Glp in the pathogenesis of neuropathic hypersensitivity induced by peripheral nerve injury. We observed that higher dose of G9a inhibitor improved nociceptive behavior, but the lower dose worsened pain. The aim of this study is to extensively observe the differential effect of various dosages of G9a/Glp inhibitors on nerve injury-induced allodynia.Materials and methodsAfter approval by the institutional ethical committee on pain research in conscious animals, C57BL/6 mice were used for measuring nociceptive behavior evoked with von Frey filaments after spared nerve injury. G9a/Glp inhibitor BIX01294 or UNC0638 was injected through the pre-buried intrathecal catheter. The dose–response curves of behavioral changes were depicted when inhibitors were administered once in bolus at the 14th day post spared nerve injury. Withdrawal behaviors were compared during the 49 days’ observation window after spared nerve injury with various dosages of inhibitors injected intrathecally for 14 days.ResultsDose–behavior curves of a single bolus of both BIX01294 and UNC0638 displayed a “V”-shaped responses of allodynia withdrawal from lower through higher dose when measured at the 14th day post spared nerve injury. A threshold dose of 10.0 µg for BIX01294 and 80.0 µg for UNC0638 significantly worsened allodynia. However, daily bolus intrathecal injection for 14 days of both inhibitors lower or higher than these threshold doses prominently improved nociceptive behavior, producing contrasting results. On the same animal, threshold dose followed by a lower or higher dose with a 14 days’ interval also showed contrast effect on nociceptive behavior, and a lower or higher dose to threshold dose sequence of inhibitor administration was vice versa.ConclusionsMethyltransferase complex G9a/Glp has a threshold role in mediating peripheral nerve injury-induced hypersensitivity at its low level versus high level through inhibiting and facilitating the nociceptive behavior, respectively.

Threshold Inhibition of Methyltransferase G9a/Glp Exacerbates Neuropathic Hypersensitivity through Mediating GRIN2B Methylation

“…2C). These results suggested intrathecal G9a/Glp threshold inhibition resulted in pain facilitation independent of treatment time points, consistent with our previous observation (25).…”

“…Although G9a and Glp expression was observed to increase in the spinal dorsal horn after nerve injury, however, how threshold inhibition of G9a/Glp further worsened allodynia following SNI still unknown. Thus, a threshold dose of BIX 01294 10 µg and UNC 0638 80 µg were intrathecal injected with doses selected according to our previous dose-response curve from behavioral experiments (25). As shown in Figure 2A, a 3-day BIX01294 (10 µg) intrathecal injection daily in bolus at either 1-week or 5-week post nerve injury further facilitated SNI-induced mechanical allodynia, which effect lasted several days and gradually recovered (Fig.…”

“…Recently, we extensively observed the nerve injury induced nociceptive behaviors changes following sequential doses of G9a/Glp inhibitors BIX 01294 and UNC 0638, and the results suggested G9a/Glp might have a dual role in mediating peripheral nerve injury induced hypersensitivity at its low level versus high level via inhibiting and facilitating the nociceptive behavior, respectively. Theoretically, inhibition of elevated G9a/Glp to a level just as higher than a so called "threshold point" may further increase pain sensitivity (25). Other than inhibitors dose difference, pain condition, transcription inhibitory targets, as well as specific nerve locations also contribute to such dual pain regulation effect of G9a/Glp, expanding our understanding of the epigenetic plastic regulation underlying neuropathic pain.…”

“…Moreover, several studies have suggested a similar pain facilitation effect of G9a via epigenetic silencing K+ channel genes in dorsal root ganglion (DRG) neurons to promote chronic pain development after nerve injury (21,22) or diminishing DRG neurons µ-opioid receptor gene (MOR) expression to decrease opioid analgesic efficacy during neuropathic pain treatment (23,24). However, our recent work suggested threshold inhibition of G9a/Glp, paradoxically, further worsened allodynia following SNI with mechanism still unknown (25). Thus, in this study, we investigated whether G9a related DNA methylation and histone modification is involved in the transcription alteration of NR2B following peripheral nerve injury and subsequently contributes to pain facilitation of G9a threshold inhibition.…”

K+ Channels in Primary Afferents and Their Role in Nerve Injury-Induced Pain

“…These findings implicate G9a in the transcriptional silencing associated with neuroplasticity in neuropathic pain. While there are some reports of the efficacy of small molecule G9a inhibitors in rodent models of neuropathic pain (Wang et al, 2017;Liang et al, 2019) any therapeutic potential of these substances is yet to be explored.…”