Chemokines, neuronal-glial interactions, and central processing of neuropathic pain

Abstract

Millions of people worldwide suffer from neuropathic pain as a result of damage to or dysfunction of the nervous system under various disease conditions. Development of effective therapeutic strategies requires a better understanding of molecular and cellular mechanisms underlying the pathogenesis of neuropathic pain. It has been increasingly recognized that spinal cord glial cells such as microglia and astrocytes play a critical role in the induction and maintenance of neuropathic pain by releasing powerful neuromodulators such as proinflammatory cytokines and chemokines. Recent evidence reveals chemokines as new players in pain control. In this article, we review evidence for chemokine modulation of pain via neuronal-glial interactions by focusing on the central role of two chemokines, CX3CL1 (fractalkine) and CCL2 (MCP-1), because they differentially regulate neuronal-glial interactions. Release of CX3CL1 from neurons is ideal to mediate neuronal-to-microglial signaling, since the sole receptor of this chemokine, CX3CR1, is expressed in spinal microglia and activation of the receptor leads to phosphorylation of p38 MAP kinase in microglia. Although CCL2 was implicated in neuronal-to-microglial signaling, a recent study shows a novel role of CCL2 in astroglial-to-neuronal signaling after nerve injury. In particular, CCL2 rapidly induces central sensitization by increasing the activity of NMDA receptors in dorsal horn neurons. Insights into the role of chemokines in neuronal-glial interactions after nerve injury will identify new targets for therapeutic intervention of neuropathic pain.

Figure 1. Spinal nerve ligation (SNL) induces a substantial upregulation of CX3CR1 in spinal cord microglial cells

(a) Upregulation of CX3CR1 in the ipsilateral side of the spinal cord (L5), 3 days after ligation of the L5 spinal nerve. (b–d) Double staining reveals a complete co-localization of CX3CR1 with the microglial marker OX-42 in the dorsal horn. Scales, 100 μm. The images are re-produced from a paper by Zhuang et al., 2007 (Brain, Behavior, and Immunity) with permission.

Figure 2. Neuronal-microglial interactions: schematic showing how chemokines (CX3CL1, CCL2) produced from primary sensory neurons after nerve injury activate microglia in the spinal cord for the generation of neuropathic pain

Spinal microglial cells express the chemokine receptors CX3CR1 and CCR2. Activation of these receptors induces p38 phosphorylation in microglia, leading to the production of proinflammatory cytokines and growth factor, and subsequent sensitization of dorsal horn neurons (central sensitization) and neuropathic pain facilitation. Nerve injury also produces the protease cathepsin S (CatS) from microglia, which can cleave CX3CL1 from the cell surface of neurons (primary sensory and dorsal horn neurons) and astrocytes, leading to further activation of microglia.

Figure 3. Spinal nerve ligation (SNL) induces upregulation of CCL2 in spinal cord astrocytes

(a, b) Upregulation of CCL2 in the ipsilateral side of the spinal cord, 3 days after ligation of the spinal nerve. (c) Double staining reveals a co-localization of CCL2 (red) with astrocyte marker GFAP (green) in the superficial dorsal horn. (d) Double staining of CCR2 (green) with neuronal marker NeuN (red) in the dorsal horn of naïve animals. Spinal cords were obtained from CCR2-GFP mice. Scales, 50 μm. The images are re-produced from a paper by Gao et al., 2009 (J Neurosci) with permission.

Figure 4. Perfusion of spinal cord slices with CCL2 (100 ng/ml) enhances NMDA-induced current (a) and induce rapid phosphorylation of ERK (pERK) in superficial dorsal horn neurons (b)

Five minutes after CCL2 stimulation, the spinal cord slices were fixed and processed for pERK immunostaining. Scale, 100 μm. The data are re-produced from a paper by Gao et al., 2009 (J Neurosci) with permission.

Figure 5. Astroglial-neuronal interactions: schematic showing how chemokine (CCL2) produced from spinal cord astrocytes directly activates dorsal horn neurons in the spinal cord for the generation of neuropathic pain

Nerve injury produces TNF-α from microglia, which activates JNK in astrocytes. JNK activation in turn produces CCL2 in astrocytes. Activation of CCR2 in spinal cord neurons by CCL2 produces central sensitization via ERK-mediated activation of NMDA receptors, leading to an enhanced neuropathic pain states.

Figure 6. Strategies of targeting chemokine signaling for reducing neuropathic pain

In addition to direct targeting the chemokines with neutralizing antibodies and the chemokine receptors with antagonists, indirect targeting the synthesis of chemokines with inhibitors of NF-κB and MAPKs and cleavage of chemokines with proteases inhibitors may offer new opportunities for improving efficacy.