Activation Of Spinal Microglia In Lumbar Disc Degeneration:Phenomenon And Mechanism

Introduction:Lumbar disc degeneration has long been regarded as a major contributor to the common back pain.To date,however,understanding of neurological changes that followed disc degeneration is limited.Microglia,the resident immune cells in the central nervous system,play an important role in mediating neuropathic pain.Early studies observed that disc degeneration may activate microglia in the spinal cord in mouse,suggesting that microglial activation may be an important pathway through which disc degeneration leads to back pain.Using a mouse model of lumbar disc degeneration,the current study aims to characterize changes of spinal microglia in disc degeneration and to determine the role of colony-stimulating factor 1(CSF1),a key regulator of myeloid lineage cells,in microglial activation.Methods:Thirty-eight CX3CR1GFP/+ mice(male,2-3 months old)were used in experimental and control groups.Under anesthesia,a posterolateral retroperitoneal approach was used to expose the L3/4 disc.The disc was punctured using a needle to induce disc injury in the experimental group but remained untouched in the control group.Three or four mice from each group were sacrificed at baseline,and each week after the surgery up to 4 weeks.The L3/4 disc,lumbar spinal cord,and L3 dorsal-root ganglions(DRG)were obtained for histological examinations and immunofluorescence analysis.Spinal cord microglia were visualized and images collected with confocal microscopy.Morphometric measurements,including somatic volume,process length,and ramification degree were analyzed for microglia in the dorsal horn using the Imaris program.Expression of CSF1 in DRG sensory neurons and CSF1-receptor(CSF1R)in L3 spinal cord were examined using immunostaining.Unpaired t tests were used to compare the number and morphological measurements of microglia at each week between the experimental and control groups.Results:Histological studies revealed that the punctured disc underwent progressive disc degeneration,including nucleus bulging,annulus loosening and disc space narrowing.In disc injury mice,microglia in the spinal cord were accumulated in the superficial layers of the dorsal horn.The number of microglia was significantly higher in experimental group than that in the control group at postoperative week 1,2,3 and 4(p<0.05 for all).Moreover,microglia exhibited larger soma size,shorter processes distance,less ramification,and increased cell number density(p<0.05 for all)in disc injury mice.In experimental mice,CSF1 was up-regulated in the sensory neurons of L3 DRG,so did CSF1R in spinal microglia.Conclusions:In mice,disc puncture and consequent disc degeneration led to microglial accumulation and activation in the lumbar spinal cord.Disc degeneration likely activates spinal microglia through the CSF1/CSF1R pathway to contribute to back pain.