Neuron–microglia interaction by purinergic signaling in neuropathic pain following neurodegeneration

Highlights

• Spinal microglia increase purinergic receptors following nerve damage.

• Microglial purinergic receptors are crucial for neuropathic pain.

• Hyperexcitability in dorsal horn neurons involves microglial purinergic signaling.

• Microglial purinergic receptors might be promising targets for treating neuropathic pain.

Abstract

Neuropathic pain, a chronic pain condition following nerve damage and degeneration, involves aberrant excitability in the dorsal horn of the spinal cord. A growing body of evidence has shown that the aberrant excitability might not be a consequence merely of changes in neurons, but rather of multiple alterations in glial cells, such as microglia, the immune cells of the central nervous system. Extracellular nucleotides play an important role in neuron–microglia communication through purinergic P2X and P2Y receptors expressed in microglia. Importantly, inhibiting the function or expression of these microglial molecules suppresses aberrant excitability of dorsal horn neurons and neuropathic pain, suggesting a crucial role for microglial purinergic signaling in mechanisms of neuropathic pain. Here, we describe recent advances in the understanding of neuron–microglia interactions by purinergic signaling in neuropathic pain following neurodegeneration.

This article is part of the Special Issue entitled ‘Purines in Neurodegeneration and Neuroregeneration’.

Introduction

We experience acute pain in response to noxious stimuli, which plays an important role as an early warning device that alerts us to the presence of damaging stimuli. Even if tissue damage is unavoidable and leads to heightened pain sensitivity in the inflamed and surrounding tissue (inflammatory pain), pain still has physiological significance. For example, pain might assist in wound repair, because contact with the damaged area is often minimized until healing has occurred. However, neuropathic pain, which occurs after nerve damage and degeneration induced by infection, autoimmune disease, or traumatic nerve injury, is a type of pathological pain that does not resolve even after the overt tissue damage has healed. This chronic pain provides no known physiological advantage, because it does not signal tissue damage. Symptoms of neuropathic pain are spontaneous pain, hyperalgesia (increased pain perception of noxious stimuli), and tactile allodynia (pain hypersensitivity to normally innocuous stimuli). Neuropathic pain, especially allodynia, is frequently resistant to currently available drugs when administered at doses that do not produce significant side effects. We are now beginning to understand that neuropathic pain is not just a symptom of disease, but is a consequence of disordered functioning of the nervous system (Beggs et al., 2012, Costigan et al., 2009).

Accumulating evidence demonstrating how peripheral nerve damage creates neuropathic pain has indicated that molecular and cellular alterations in primary sensory neurons and in the spinal dorsal horn (SDH) after peripheral nerve injury (PNI) have important roles in the pathogenesis of neuropathic pain (Scholz and Woolf, 2002, Woolf and Mannion, 1999, Woolf and Salter, 2000). A rapidly growing body of evidence indicates that spinal glial cells, in particular microglia, play a critical role in the pathogenesis of neuropathic pain. Extracellular nucleotides play an important role in neuron–glia communication through purinergic P2X and P2Y receptors (P2XR and P2YR, respectively). Microglia, which are thought to be residential macrophages in the central nervous system (CNS), express P2XRs and P2YRs, mainly P2X4R and P2X7R, as well as P2Y2R, P2Y6R, and P2Y12R (Inoue, 2006). Results from animal models of neuropathic pain have shown that microglial purinergic signaling via P2X/YRs is crucial for pathologically modulating pain processing in the SDH after PNI and for PNI-induced pain hypersensitivity (Beggs et al., 2012, Tsuda et al., 2005, Tsuda et al., 2013c). In this review article, we describe recent advances in our understanding of neuron–microglia interactions by purinergic signaling in neuropathic pain following neurodegeneration.

Microglial cells are known as resident tissue macrophages in the CNS and constitute 5–10% of total cells in the adult CNS. Accumulating evidence indicates that the origin of microglia might be primitive macrophages in the yolk sac. Fate mapping has revealed that microglia arise from early yolk sac-derived precursors that leave the yolk sac on E8.5–E9.0, migrating to the neuroectoderm via the primitive blood stream (Ginhoux et al., 2010). The precursors have erythro-myeloid potential (Kierdorf et al., 2013). Microglial generation is dependent on the transcription factors PU.1 and interferon regulatory factor 8 (IRF8) (Kierdorf et al., 2013), as well as interleukin-34 (IL-34) (Greter et al., 2012, Wang et al., 2012). However, microglial generation is independent of Myb (Schulz et al., 2012), which is essential for bone marrow-derived macrophages (Hashimoto et al., 2013, Yona et al., 2013). The yolk-sac-derived microglia presumably remain throughout life and might be maintained by self-renewal in the healthy CNS with little contribution from bone-marrow-derived monocytes/macrophages (Ajami et al., 2007).

In the adult, microglia have small cell bodies bearing branched and motile processes, which might monitor the local environment in the CNS (Davalos et al., 2005, Nimmerjahn et al., 2005). Microglia rapidly respond to a wide range of stimuli that threaten physiological homeostasis, including PNI. In a growing body of literature, it is evident that PNI leads to dramatic activation of microglia in the SDH. This response is commonly observed among various models of neuropathic pain. The morphological features of microglial activation include cell body hypertrophy with thickened and retracted processes, increased cell number, and increased staining of microglial markers, such as CD11b and ionized calcium-binding adapter molecule-1 (Iba1).

A neuronally derived signaling molecule that might be important for microglial activation remains to be determined, but several candidates have been reported. These include monocyte chemoattractant protein-1 (MCP-1 or CCL2) and metalloproteinase-9 (MMP-9), whose expressions are markedly increased in dorsal root ganglion neurons after PNI (Kawasaki et al., 2008a, Tanaka et al., 2004, Thacker et al., 2009, White et al., 2007, Zhang and De Koninck, 2006). Mice lacking chemotactic cytokine receptor 2, a receptor for MCP-1, or MMP-9-deficient mice show reduced microglia activation caused by PNI (Kawasaki et al., 2008a, Zhang et al., 2007). Conversely, intrathecal administration of MCP-1 or MMP-9 into normal rats resulted in microglial activation (Kawasaki et al., 2008a, Thacker et al., 2009). Substrates of MMP-9 for microglial activation remain unclear, but fractalkine, IL-1β, and tumor necrosis factor-α (TNFα) could be potential candidates (Suter et al., 2007).

The number of microglia in the SDH is markedly increased after PNI (Tsuda et al., 2013a). This might be associated with proliferation of resident microglia. Indeed, PNI induced an early and transient increase in the number of microglia positive to proliferation markers, such as bromodeoxyuridine (BrdU), a thymidine analog incorporated into DNA during the S phase of cell cycle (Echeverry et al., 2008, Gehrmann and Banati, 1995, Liu et al., 2000, Narita et al., 2006, Suter et al., 2007, Zhang et al., 2007). The proliferation activity of microglia peaks around 2 days after PNI and then declines to basal levels (Echeverry et al., 2008, Gehrmann and Banati, 1995). Conversely, it was shown that bone marrow-derived cells injected intravenously into lethally irradiated recipient mice infiltrate the SDH parenchyma ipsilateral to the PNI and are positive for Iba1 and display microglia-like morphology. However, the ability of bone marrow-derived cells to migrate into the parenchyma of the CNS, including the spinal cord, remains controversial as a result of experimental manipulations, such as irradiation (which could influence the blood-spinal cord barrier) and exogenously injected donor cells (Ajami et al., 2007).