Injured sensory neuron–derived CSF1 induces microglial proliferation and DAP12-dependent pain

Abstract

SUMMARYAlthough microglia are implicated in nerve injury-induced neuropathic pain, how injured sensory neurons engage microglia is unclear. Here we demonstrate that peripheral nerve injury induces de novo expression of colony-stimulating factor 1 (CSF1) in injured sensory neurons. The CSF1 is transported to the spinal cord where it targets the microglial CSF1 receptor (CSF1R). Cre-mediated sensory neuron deletion of Csf1 completely prevented nerve injury-induced mechanical hypersensitivity and reduced microglia activation and proliferation. In contrast, intrathecal injection of CSF1 induces mechanical hypersensitivity and microglial proliferation. Nerve injury also upregulated CSF1 in motoneurons, where it is required for ventral horn microglial activation and proliferation. Downstream of CSF1R, we found that the microglial membrane adapter protein DAP12 is required for both nerve injury- and intrathecal CSF1-induced upregulation of pain-related microglial genes and the ensuing pain, but not for microglia proliferation. Thus, both CSF1 and DAP12 are potential targets for the pharmacotherapy of neuropathic pain.

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

“…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.…”

“…The mechanism underlying such a temporally restricted robust proliferation of microglia remains to be determined, but one candidate could be CSF1 derived from injured DRG neurons. 13,14) Indeed, CSF1 mRNA is induced day 1 in DRG neurons after PNI, which seems to temporally coincide with proliferation of SDH microglia. Furthermore, a conditional knockout of CSF1 in DRG neurons suppressed the PNI-induced microglial proliferation.…”

“…Furthermore, a conditional knockout of CSF1 in DRG neurons suppressed the PNI-induced microglial proliferation. 13) Conversely, intrathecal administration of CSF1 to normal mice induces proliferation of microglia. 13,14) These data suggest a necessity and sufficiency of CSF1 for microglial proliferation.…”

“…13) Conversely, intrathecal administration of CSF1 to normal mice induces proliferation of microglia. 13,14) These data suggest a necessity and sufficiency of CSF1 for microglial proliferation. However, it should be noted that upregulation of CSF1 in DRG neurons and its receptor CSF1R expressed in microglia are not only observed at day 1 after PNI but also persist for a few weeks later 13,14) when microglial proliferation has already terminated as shown in this study and others.…”

“…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.…”