Mechanisms of nerve injury-induced spinal cord microglia activation and neuropathic pain via dorsal root ganglia satellite glial cells

Abstract

Neuropathic pain is caused by damage to or dysfuction of the nervous system. It is a pathological symptom that transcends the beneficial function of pain to protect the body from harmful stimuli or environment and to help with healing. Neuropathic pain has the following clinical characteristics: pain sensitization following normally non-painful, often repetitive, stimulation (allodynia)

exaggerated reaction to normally painful stimuli (hyperalgesia)

and stimulus-independent uncommon sensations associated with tingling or burning (spontaneous pain). These symptoms produce severe pain, negatively affect social and economic activities, and cause the onset of mental illnesses such as depression. Classically, many studies have focused on the hyper-excitatory mechanism of sensory neurons and the central sensitization of pain nerve circuits to understand the pathogenesis of neuropathic pain. Based on these classical perspectives, research on the mechanism of pain hypersensitivity and the development of analgesic drugs has progressed. However, due to a lack of understanding of the precise molecular mechanisms underlying this condition, there remains no cure or drug that effectively controls neuropathic pain. It has been reported that microglia in the spinal cord dorsal horn and satellite glial cell (SGC) in the dorsal root ganglion are activated during the onset of neuropathic pain caused by peripheral nerve injury. The activation of these glial cells plays an important role in the development of pain hypersensitivity, and inhibiting the activation of glial cells in the spinal cord and ganglia using inhibitors such as minocycline and fluorocitrate alleviates neuropathic pain. However, the molecular mechanisms underlying spinal cord microglial and satellite glial activation in ganglia and the relationship between two glia after peripheral nerve injury remain unknown. Therefore, to study the mechanisms of activation of glial cells and to investigate the relationship between glial cells, I developed IkkB conditional knockout mice in which IKK/NF-kB-dependent proinflammatory satellite glial activation is abrogated. I induced neuropathic pain in conditional knockout and wild-type control mice by cutting the L5 spinal nerve, and performed pain behavior tests, immunohistochemistry, and various molecular experimental techniques to elucidate the activation mechanisms of spinal cord microglia and SGCs. Compared to control mice, nerve injury-induced SGC activation was markedly reduced and macrophage infiltration into nerve-injured ganglia was dramatically decreased in conditional knockout mice. In addition, the activation of spinal cord microglia was markedly reduced, and compared with control mice, allodynic and hyperalgesic symptoms were alleviated in conditional knockout mice. However, nerve-recruited macrophages did not affect spinal cord microglial activation and neuropathic pain, suggesting a causal effect of SGC activation on spinal cord microglia activation. In an effort to elucidate the molecular mechanism, I searched for genes that are differentially regulated in conditional knockout ganglia compared to control ganglia after nerve injury using microarray analysis and identified the St3gal2 gene. This St3gal2 gene encodes a protein that produces ganglioside GT1b and GD1a in neurons, and its expression is increased in injured sensory neurons of control mice but suppressed in conditional knockout mice. Following nerve injury, St3gal2 expression increase accompanied aberrant GT1b and GD1a production in injured neuron

GT1b is transported to the spinal cord along the central axon of sensory neurons. In the spinal cord, GT1b acts as an endogenous ligand of toll-like receptor 2, which is specifically expressed on microglia, to activate microglial cells, resulting in pain hypersensitivity. In conclusion, peripheral nerve injury primarily activates the SGC of ganglion through the IKK/NF-kB signaling pathway and induces aberrant GT1b expression in injured sensory neurons. GT1b produced by damaged sensory neurons may induce pain sensitization by secondarily activating spinal cord microglial cells. These findings suggest that the interaction between SGCs and sensory neurons precedes injured neurons through microglial communication in the spinal cord. Thus, inhibiting the activation of satellite glial cells or the production and action of ganglioside GT1b from damaged sensory neurons may serve as a new therapeutic strategy for neuropathic pain.