Repopulated spinal cord microglia exhibit a unique transcriptome and contribute to pain resolution

Targeting C1q prevents microglia-mediated synaptic removal in neuropathic pain

Activation of spinal microglia following peripheral nerve injury is a central component of neuropathic pain pathology. While the contributions of microglia-mediated immune and neurotrophic signalling have been well-characterized, the phagocytic and synaptic pruning roles of microglia in neuropathic pain remain less understood. Here, we show that peripheral nerve injury induces microglial engulfment of dorsal horn synapses, leading to a preferential loss of inhibitory synapses and a shift in the balance between inhibitory and excitatory synapse density. This synapse removal is dependent on the microglial complement-mediated synapse pruning pathway, as mice deficient in complement C3 and C4 do not exhibit synapse elimination. Furthermore, pharmacological inhibition of the complement protein C1q prevents dorsal horn inhibitory synapse loss and attenuates neuropathic pain. Therefore, these results demonstrate that the complement pathway promotes persistent pain hypersensitivity via microglia-mediated engulfment of dorsal horn synapses in the spinal cord, revealing C1q as a therapeutic target in neuropathic pain.

How microglia contribute to the induction and maintenance of neuropathic pain

Neuropathic pain is a debilitating condition caused by damage to the nervous system that results in changes along the pain pathway that lead to persistence of the pain sensation. Unremitting pain conditions are associated with maladaptive plasticity, disruption of neuronal activity that favours excitation over inhibition, and engagement of immune cells. The substantial progress made over the last two decades in the neuroimmune interaction research area points to a mechanistic role of spinal cord microglia, which are resident immune cells of the CNS. Microglia respond to and modulate neuronal activity during establishment and persistence of neuropathic pain states, and microglia-neuron pathways provide targets that can be exploited to attenuate abnormal neuronal activity and provide pain relief.

Fidgetin-like 2 knockdown increases acute neuroinflammation and improves recovery in a rat model of spinal cord injury

Spinal cord injury (SCI) can cause permanent dysfunction proceeding from multifaceted neuroinflammatory processes that contribute to damage and repair. Fidgetin-like 2 (FL2), a microtubule-severing enzyme that negatively regulates axon growth, microglial functions, and wound healing, has emerged as a potential therapeutic target for central nervous system injuries and neuroinflammation. To test the hypothesis that FL2 knockdown increases acute neuroinflammation and improves recovery after SCI, we examined the effects of nanoparticle-encapsulated FL2 siRNA treatment after a moderate contusion SCI in rats. SCI significantly increased FL2 expression in the lesion site and rostral to the lesion 1 day post-injury (dpi). A single treatment of FL2 siRNA after injury led to modestly improved locomotor recovery consistent with the preservation of corticospinal tract function, accompanied by reduced inflammation and increased presence of oligodendrocytes. In determining the acute effects of treatment, RNA sequencing and gene set enrichment analyses revealed that FL2 siRNA modulates early cellular responses, including chemokine signaling, both pro- and anti-inflammatory immune reactions, and neurotransmitter signaling pathways at 1, 4, and 7 dpi. Follow-up analyses at 4 dpi using dual in situ hybridization and immunohistochemistry demonstrated that SCI increased FL2 mRNA and that FL2 was colocalized with microglia/macrophages. FL2 downregulation resulted in a marked accumulation of microglia at the lesion site, accompanied by increased inflammatory markers (IL-1β, TGF-β1, and CD68). The results suggest SCI induces an increase in FL2 expression that undermines acute inflammatory responses as well as spinal cord integrity and growth. Overall, our study suggests that targeting FL2 holds promise as a therapeutic strategy for treating SCI.

TAM receptors mediate the Fpr2-driven pain resolution and fibrinolysis after nerve injury

Nerve injury causes neuropathic pain and multilevel nerve barrier disruption. Nerve barriers consist of perineurial, endothelial and myelin barriers. So far, it is unclear whether resealing nerve barriers fosters pain resolution and recovery. To this end, we analysed the nerve barrier property portfolio, pain behaviour battery and lipidomics for precursors of specialized pro-resolving meditators (SPMs) and their receptors in chronic constriction injury of the rat sciatic nerve to identify targets for pain resolution by resealing the selected nerve barriers. Of the three nerve barriers-perineurium, capillaries and myelin-only capillary tightness specifically against larger molecules, such as fibrinogen, recuperated with pain resolution. Fibrinogen immunoreactivity was elevated in rats not only at the time of neuropathic pain but also in nerve biopsies from patients with (but not without) painful polyneuropathy, indicating that sealing of the vascular barrier might be a novel approach in pain treatment. Hydroxyeicosatetraenoic acid (15R-HETE), a precursor of aspirin-triggered lipoxin A4, was specifically upregulated at the beginning of pain resolution. Repeated local application of resolvin D1-laden nanoparticles or Fpr2 agonists sex-independently resulted in accelerated pain resolution and fibrinogen removal. Clearing macrophages (Cd206) were boosted and fibrinolytic pathways (Plat) were induced, while inflammation (Tnfα) and inflammasomes (Nlrp3) were unaffected by this treatment. Blocking TAM receptors (Tyro3, Axl and Mer) and tyrosine kinase receptors linking haemostasis and inflammation completely inhibited all the effects. In summary, nanoparticles can be used as transporters for fleeting lipids, such as SPMs, and therefore expand the array of possible therapeutic agents. Thus, the Fpr2-Cd206-TAM receptor axis may be a suitable target for strengthening the capillary barrier, removing endoneurial fibrinogen and boosting pain resolution in patients with chronic neuropathic pain.

Sex differences in mechanisms of pain hypersensitivity

The introduction of sex-as-a-biological-variable policies at funding agencies around the world has led to an explosion of very recent observations of sex differences in the biology underlying pain. This review considers evidence of sexually dimorphic mechanisms mediating pain hypersensitivity, derived from modern assays of persistent pain in rodent animal models. Three well-studied findings are described in detail: the male-specific role of spinal cord microglia, the female-specific role of calcitonin gene-related peptide (CGRP), and the female-specific role of prolactin and its receptor. Other findings of sex-specific molecular involvement in pain are subjected to pathway analyses and reveal at least one novel hypothesis: that females may preferentially use Th1 and males Th2 T cell activity to mediate chronic pain.

Immune drivers of physiological and pathological pain

Physiological pain serves as a warning of exposure to danger and prompts us to withdraw from noxious stimuli to prevent tissue damage. Pain can also alert us of an infection or organ dysfunction and aids in locating such malfunction. However, there are instances where pain is purely pathological, such as unresolved pain following an inflammation or injury to the nervous system, and this can be debilitating and persistent. We now appreciate that immune cells are integral to both physiological and pathological pain, and that pain, in consequence, is not strictly a neuronal phenomenon. Here, we discuss recent findings on how immune cells in the skin, nerve, dorsal root ganglia, and spinal cord interact with somatosensory neurons to mediate pain. We also discuss how both innate and adaptive immune cells, by releasing various ligands and mediators, contribute to the initiation, modulation, persistence, or resolution of various modalities of pain. Finally, we propose that the neuroimmune axis is an attractive target for pain treatment, but the challenges in objectively quantifying pain preclinically, variable sex differences in pain presentation, as well as adverse outcomes associated with immune system modulation, all need to be considered in the development of immunotherapies against pain.