Increased miR-132-3p expression is associated with chronic neuropathic pain

Highlight

• Pain and inflammation up-regulates miR-132-3p in humans

• SNI causes spinal and DRG miR-132-3p up-regulation in rats

• Spinal miR-132-3p antagonism induces long-lasting anti-hyperalgesia

• Spinal miR-132-3p mimicry induces thermal and mechanical hyperalgesia

Abstract

Alterations in the neuro-immune balance play a major role in the pathophysiology of chronic neuropathic pain. MicroRNAs (miRNA) can regulate both immune and neuronal processes and may function as master switches in chronic pain development and maintenance. We set out to analyze the role of miR-132-3p, first in patients with peripheral neuropathies and second in an animal model of neuropathic pain. We initially determined miR-132-3p expression by measuring its levels in white blood cells (WBC) of 30 patients and 30 healthy controls and next in sural nerve biopsies of 81 patients with painful or painless inflammatory or non-inflammatory neuropathies based on clinical diagnosis. We found a 2.6 fold increase in miR-132-3p expression in WBC of neuropathy patients compared to healthy controls (p < 0.001). MiR-132-3p expression was also slightly up-regulated in sural nerve biopsies from neuropathy patients suffering from neuropathic pain compared to those without pain (1.2 fold; p < 0.001).

These promising findings were investigated further in an animal model of neuropathic pain, the spared nerve injury model (SNI). For this purpose miR-132-3p expression levels were measured in dorsal root ganglia and spinal cord of rats. Subsequently, miR-132-3p expression was pharmacologically modulated with miRNA antagonists or mimetics, and evoked pain and pain aversion were assessed.

Spinal miR-132-3p levels were highest 10 days after SNI, a time when persistent allodynia was established (p < 0.05). Spinal administration of miR-132-3p antagonists via intrathecal (i.t.) catheters dose dependently reversed mechanical allodyina (p < 0.001) and eliminated pain behavior in the place escape avoidance paradigm (p < 0.001). Intrathecal administration of miR-132-3p mimetic dose-dependently induced pain behavior in naïve rats (p < 0.001). Taken together these results indicate a pro-nociceptive effect of miR-132-3p in chronic neuropathic pain.

Introduction

Neuropathic pain is characteristically severe and persistent and may greatly impair health related quality of life by additionally inducing anxiety, depression, and cognitive impairment (Breivik et al., 2006). There is ample evidence for a potential role of the immune system and particularly of pro- and anti-inflammatory mediators in the pathophysiology of neuropathic pain (Kuner, 2010, McMahon and Malcangio, 2009). Peripheral neuropathies of the same etiology can either be painful or painless (Üçeyler et al., 2007). The mechanism for this discrepancy is unknown.

In recent years, non-coding RNAs have been studied in normal cellular functioning as well as in pathological processes (Huttenhofer and Schattner, 2006, Mattick, 2004). Micro-RNAs (miRNAs) are a family of non-coding RNAs that post-transcriptionally regulate gene expression by inhibiting mRNA translation or inhibiting mRNA and protein degradation (Mattick, 2004). Various diseases, including neuropathic pain disorders, appear to possess unique miRNA expression signatures. Recent reports on modulation of miRNA function in both neuronal and immune processes predict the therapeutic potential of manipulating miRNAs in diseases affecting the immune system and the brain (O'Connor et al., 2012, Soreq and Wolf, 2011). miRNAs that communicate between the nervous and immune system have been termed “neurimmiRs” and primarily target transcription factors or other regulatory genes, which enable simultaneous cross-communication between neural and immune compartments (Soreq and Wolf, 2011). Thus, miRNAs possibly control cellular pathways in multiple systems and act as “master-switches” (Soreq and Wolf, 2011).

Aberrant expression of several miRNAs has been reported throughout many peripheral and central nervous system loci associated with pain perception (Aldrich et al., 2009, Bai et al., 2007, Imai et al., 2011, Kusuda et al., 2011, von Schack et al., 2011). First reports describing characteristic miRNA expression profiles in blood or cerebrospinal fluid of patients with distinct pain conditions are starting to emerge (Andersen et al., 2016, Beyer et al., 2015, Bjersing et al., 2013, Orlova et al., 2011), however evidence linking specific miRNA expression profiles to specific pain disorders is still insufficient.

miR-132 is abundantly expressed in the brain and is emerging as a regulator of cognition, neuronal plasticity, and memory. It can regulate synapse structure and function (Bredy et al., 2011, Miller et al., 2012, Schratt, 2009, Soreq and Wolf, 2011). Hippocampal miR-132 mediates stress-induced cognitive deficits through suppression of acetylcholinesterase (Haramati et al., 2011) and miR-132 has recently been implicated in neuropathic pain after chronic constriction injury (CCI) (Arai et al., 2013). Similarly, spinal cord miR-132 is now proposed as a mediator of neuropathic pain following spared nerve injury (SNI) (Zhang et al., 2015). However, direct links between pain and miR-132 expression levels in human and/or animal models of neuropathic pain still remain elusive.

The current studies evaluated blood and sural nerve miR-132-3p, a splice variant of miR-132, expression in patients suffering from chronic neuropathic pain accompanying peripheral neuropathy and analyzed the role of miR-132-3p in pain behavior in an animal model of neuropathic pain.