Expression of CCR2 in Both Resident and Bone Marrow-Derived Microglia Plays a Critical Role in Neuropathic Pain

Abstract

Neuropathic pain resulting from damage to or dysfunction of peripheral nerves is not well understood and difficult to treat. Although CNS hyperexcitability is a critical component, recent findings challenge the neuron-centric view of neuropathic pain etiology and pathology. Indeed, glial cells were shown to play an active role in the initiation and maintenance of pain hypersensitivity. However, the origins of these cells and the triggers that induce their activation have yet to be elucidated. Here we show that, after peripheral nerve injury induced by a partial ligation on the sciatic nerve, in addition to activation of microglia resident to the CNS, hematogenous macrophage/monocyte infiltrate the spinal cord, proliferate, and differentiate into microglia. Signaling from chemokine monocyte chemoattractant protein-1 (MCP-1, CCL2) to its receptor CCR2 is critical in the spinal microglial activation. Indeed, intrathecal injection of MCP-1 caused activation of microglia in wild-type but not in CCR2-deficient mice. Furthermore, treatment with an MCP-1 neutralizing antibody prevented bone marrow-derived microglia (BMDM) infiltration into the spinal cord after nerve injury. In addition, using selective knock-out of CCR2 in resident microglia or BMDM, we found that, although total CCR2 knock-out mice did not develop microglial activation or mechanical allodynia, CCR2 expression in either resident microglia or BMDM is sufficient for the development of mechanical allodynia. Thus, to effectively relieve neuropathic pain, both CNS resident microglia and blood-borne macrophages need to be targeted. These findings also open the door for a novel therapeutic strategy: to take advantage of the natural ability of bone marrow-derived cells to infiltrate selectively affected CNS regions by using these cells as vehicle for targeted drug delivery to inhibit hypersensitivity and chronic pain.

Temporal Kinetics of Microgliosis in the Spinal Dorsal Horn after Peripheral Nerve Injury in Rodents

“…This implies that microglia could proliferate multiple times during the first 3 d. Alternatively, it is also possible that microgliosis could be due to migration of SDH microglia from neighboring spinal segments (e.g., rostral L3 and caudal L5 segments) because microglia show chemotaxis. 31,32) However, infiltration of circulating monocytes in the SDH, 12) which is reported to express Iba1, does not contribute to the PNI-induced SDH microgliosis, since recent studies using bone marrow chimeric mice subjected to mild irradiation, parabiosis mice and doubletransgenic mice (enable distinct visualization of resident microglia and circulating monocytes) demonstrated no evidence for the involvement of circulating monocytes. 4,10,11) Numerous microglia induced by PNI gradually returned to pre-PNI levels, the time-course of which is similar to a recovery of PNI-induced pain hypersensitivity.…”

“…Since PNI-induced microgliosis in the SDH was recorded in 1970s 5) and the rodent models of neuropathic pain were established in 1990s, [6][7][8][9] studies have investigated the mechanism for microgliosis in the SDH after PNI. Two possible mechanisms (proliferation of resident microglia 10,11) and infiltration of bone marrow-derived monocytes 12) ) have been considered, but it is now thought that local expansion of resident microglia by proliferation is the primary cellular basis for SDH microgliosis after PNI. 4,10,11) However, in stark contrast to the characterization of temporal and spatial changes in the immunostaining levels of microglial markers (CD11b or ionized calcium-binding adapter molecule 1 (Iba1)) in the SDH that have so far been extensively studied, 4) the temporal kinetics of microglial proliferation after PNI have yet to be fully characterized.…”

IRF8 is a transcriptional determinant for microglial motility

“…In fact, following peripheral nerve injury, reactive microglia engulf myelinated axons with their processes in the spinal dorsal horn in a manner that is dependent on P2Y 12 R signals [27]. Furthermore, microglial chemotaxis-related genes are upregulated in the spinal cord and are required for the generation of neuropathic pain [28][29][30][31]. However, whether microglial motility itself correlates with the degree of pain hypersensitivity remains unclear.…”

CCR2 chemokine receptor signaling mediates pain in experimental osteoarthritis

“…All these cell types may contribute to pain signaling (5,19,20). MCP-1 can act on all of these cell types (12) and has been shown to promote macrophage infiltration into the spinal cord and DRG following nerve injury (21)(22)(23). Therefore, we examined changes in the DRG macrophage population following DMM surgery to assess whether these cells were infiltrating the innervating DRG as a result of the up-regulation of MCP-1.…”