Journal Article

GPR34 in spinal microglia exacerbates neuropathic pain in mice

Konishi, Hiroyuki, Kano, Kuniyuki, Kiyama, Hiroshi, Kobayashi, Hiroki, Hibi, Hideharu, Sayo, Akira, Kobayashi, Masaaki, Aoki, Junken

2019

Overview

Background Neuropathic pain is caused by sensory nerve injury, but effective treatments are currently lacking. Microglia are activated in the spinal dorsal horn after sensory nerve injury and contribute to neuropathic pain. Accordingly, molecules expressed by these cells are considered potential targets for therapeutic strategies. Our previous gene screening study using a mouse model of motor nerve injury showed that the G-protein-coupled receptor 34 gene (GPR34) is induced by nerve injury. Because GPR34 is now considered a microglia-enriched gene, we explored the possibility that it might be involved in microglial activation in the dorsal horn in a mouse model of neuropathic pain. Methods mRNA expression of GPR34 and pro-inflammatory molecules was determined by quantitative real-time PCR in wild-type and GPR34-deficient mice with L4 spinal nerve injury. In situ hybridization was used to identify GPR34 expression in microglia, and immunohistochemistry with the microglial marker Iba1 was performed to examine microglial numbers and morphology. Mechanical sensitivity was evaluated by the von Frey hair test. Liquid chromatography–tandem mass spectrometry quantified expression of the ligand for GPR34, lysophosphatidylserine (LysoPS), in the dorsal horn, and a GPR34 antagonist was intrathecally administrated to examine the effect of inhibiting LysoPS-GPR34 signaling on mechanical sensitivity. Results GPR34 was predominantly expressed by microglia in the dorsal horn after L4 nerve injury. There were no histological differences in microglial numbers or morphology between WT and GPR34-deficient mice. However, nerve injury-induced pro-inflammatory cytokine expression levels in microglia and pain behaviors were significantly attenuated in GPR34-deficient mice. Furthermore, the intrathecal administration of the GPR34 antagonist reduced neuropathic pain. Conclusions Inhibition of GPR34-mediated signal by GPR34 gene deletion reduced nerve injury-induced neuropathic pain by suppressing pro-inflammatory responses of microglia without affecting their morphology. Therefore, the suppression of GPR34 activity may have therapeutic potential for alleviating neuropathic pain.

• Publisher: BioMed Central,BioMed Central Ltd,Springer Nature B.V,BMC
• Subject: Allodynia / Analysis of Variance / Animals / Biomedical and Life Sciences / Biomedicine / Calcium-Binding Proteins - metabolism / Care and treatment / Causes of / Chronic pain / Clonal deletion / Cytokines / Cytokines - genetics / Cytokines - metabolism / Development and progression / Disease / Disease Models, Animal / Dorsal horn / Experiments / G protein-coupled receptors / G proteins / Gene deletion / Gene expression / Gene Expression Regulation - genetics / GPCR / GPR34 / Hybridization / Immunohistochemistry / Immunology / Inflammation / Interferon Regulatory Factors - metabolism / Kinases / Laboratory animals / Ligands / Lipids / Liquid chromatography / Lymphoma / Lysophospholipids - therapeutic use / LysoPS / Male / Mass spectroscopy / Mice / Mice, Inbred C57BL / Mice, Transgenic / Microfilament Proteins - metabolism / Microglia / Microglia - metabolism / Morphology / Nervous system diseases / Neuralgia / Neuralgia - drug therapy / Neuralgia - metabolism / Neuralgia - pathology / Neurobiology / Neurodegeneration / Neuroinflammation / Neurology / Neuroprotective agents / Neurosciences / Nitric Oxide Synthase Type II - genetics / Nitric Oxide Synthase Type II - metabolism / Pain / Pain Measurement / Pain Threshold - physiology / Physiological aspects / Protein Kinase C - metabolism / Proteins / Receptors, Lysophospholipid - antagonists & inhibitors / Receptors, Lysophospholipid - genetics / Receptors, Lysophospholipid - metabolism / RNA, Messenger - metabolism / Rodents / Roles / Sensory neurons / Spinal Cord - pathology / Spinal nerves / Studies / Testing / Time Factors / Tumor necrosis factor-TNF