Spinal Microgliosis Due to Resident Microglial Proliferation Is Required for Pain Hypersensitivity after Peripheral Nerve Injury

Abstract

Summary Peripheral nerve injury causes neuropathic pain accompanied by remarkable microgliosis in the spinal cord dorsal horn. However, it is still debated whether infiltrated monocytes contribute to injury-induced expansion of the microglial population. Here we found that spinal microgliosis predominantly results from local proliferation of resident microglia but not from infiltrating monocytes after spinal nerve transection (SNT), using two genetic mouse models (CCR2RFP/+:CX3CR1GFP/+ and CX3CR1creER/+:R26tdTomato/+ mice) as well as specific staining of microglia and macrophages. Pharmacological inhibition of SNT-induced microglial proliferation correlated with attenuated neuropathic pain hypersensitivities. Microglial proliferation is partially controlled by purinergic and fractalkine signaling, as CX3CR1−/− and P2Y12−/− mice show reduced spinal microglial proliferation and neuropathic pain. These results suggest that local microglial proliferation is the sole source of spinal microgliosis, which represents a potential therapeutic target for neuropathic pain management.

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

“…4) The time course of the PNIinduced SDH microgliosis in mice was revealed to be that microglia number increased from day 3, peaked at 1 week and declined later, which is almost similar to the temporal pattern of microgliosis reported in previous studies. 11,25,26) Under such conditions, we clearly showed that a proliferation burst of SDH microglia occurred during the early phase (the first 3 d after PNI) but not during the later phase (at least until 2 weeks post-PNI). The early phase proliferation is consistent with several previous papers, 11,13) but the later phase is controversial.…”

“…11,25,26) Under such conditions, we clearly showed that a proliferation burst of SDH microglia occurred during the early phase (the first 3 d after PNI) but not during the later phase (at least until 2 weeks post-PNI). The early phase proliferation is consistent with several previous papers, 11,13) but the later phase is controversial. It has previously been described that newly generated SDH microglia in response to PNI in rats continued to divide for at least 14 d after the injury.…”

“…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. While recent studies reported an involvement of colony-stimulating factor 1 (CSF1) expressed in injured dorsal root ganglion (DRG) neurons in the PNI-induced proliferation, 4,13,14) the expression of CSF1 in DRG neurons (and also its receptor CSF1R in microglia) is upregulated for a few weeks after PNI 13,14) when microglial proliferation has already terminated.…”

“…Since PNI-induced microgliosis in the SDH was recorded in 1970s 5) and the rodent models of neuropathic pain were established in 1990s, 6-9) 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.…”

“…While recent studies reported an involvement of colony-stimulating factor 1 (CSF1) expressed in injured dorsal root ganglion (DRG) neurons in the PNI-induced proliferation, 4,13,14) the expression of CSF1 in DRG neurons (and also its receptor CSF1R in microglia) is upregulated for a few weeks after PNI 13,14) when microglial proliferation has already terminated. 11,15) Thus, determining the kinetics of microglial proliferation after PNI would provide an important clue to identify a molecule(s) for the PNI-induced proliferation of SDH microglia. Because a recent study has demonstrated that interrupting spinal microgliosis suppresses PNI-induced mechanical pain hypersensitivity and provided evidence that it is a crucial step in neuropathic pain, 11) the kinetics of microglial proliferation after PNI would also provide valuable information for developing a new therapeutic strategy.…”