Neuropathic pain gene-expression signatures in spinal microglia following nerve injury via near single-cell transcriptome analysis

Abstract

Microglia are resident immune cells responsible for maintaining homeostasis and sensing neuronal injury in the central nervous system (CNS). Microglial activation in response to peripheral nerve injury is known to be important in inducing pain hypersensitivities. Activated microglia is accompanied by changes in hypertrophic morphology, rate of proliferation, and alterations in gene expression. Large-scale screening to identify pain-related genes on tissue level such as in dorsal root ganglion (DRG) and spinal cord has revealed significantly altered expression of a number of genes. However, most of these genes are neuronal not microglial, indicating that whole tissue analysis may have limitations in finding targets from minor fraction of whole tissue transcriptome. In the present study, I adopted high resolution transcriptome assay from individually collected pools of 10 spinal microglia cells to identify changes in expression levels and cell-to-cell variation of activated microglial genes in peripheral nerve injury model. Differentially expressed genes (DEGs) analysis revealed that miR-29c was a critical factor for microglia activation and the development of neuropathic pain at post-operative day 1 (POD1). According to gene ontology analysis, early POD1 microglia exhibited a very distinct expression profile compared to late POD7 microglia, possibly leading to the transition from initiation to maintenance of neuropathic pain. Genes related to sensing function were influenced at POD1 while expressions of genes induced by signaling pathways were more predominant at POD7. Variation analysis revealed that 56 genes in group C genes showed decreased variance in injury with concomitant increase in variance in sham. In fact, only the group C genes showed tight clustering of the four experimental conditions. The 56 microglial genes, including Gria1, the gene encoding AMPA receptor subunit GluA1, potentially linked to the maintenance of neuropathic pain. Gria1 may regulate TNFα release via changing permeability of the AMPA receptor to Ca2+ influx. This study provides insights into spinal microglial biology and reveals novel microglial targets for the treatment of neuropathic pain.