Mechanisms underlying mechanical sensitization induced by complement C5a: the roles of macrophages, TRPV1, and calcitonin gene-related peptide receptors

Abstract

The complement system significantly contributes to the development of inflammatory and neuropathic pain, but the underlying mechanisms are poorly understood. Recently, we identified the signaling pathway responsible for thermal hypersensitivity induced by the complement system component C5a. Here, we examine the mechanisms of another important action of C5a, induction of mechanical hypersensitivity. We found that intraplantar injection of C5a produced a dosedependent mechanical sensitization, and that this effect was blocked by chemogenetic ablation of macrophages in both male and female mice. Knockout (KO) of TRPV1 or pretreatment with the TRPV1 antagonists, AMG9810 or 5'-iodoresiniferatoxin (5'-IRTX), significantly reduced C5ainduced mechanical sensitization. Notably, local administration of 5'-IRTX 90 minutes after C5a injection resulted in a slow, but complete, reversal of mechanical sensitization, indicating that TRPV1 activity was required for maintaining C5a-induced mechanical hypersensitivity. This slow reversal suggests that neurogenic inflammation and neuropeptide release may be involved. Indeed, pretreatment with a calcitonin gene related peptide (CGRP) receptor antagonist (but not an antagonist of the neurokinin 1 receptor), prevented C5a-induced mechanical sensitization. Furthermore, intraplantar injection of CGRP produced significant mechanical sensitization in both wild-type (WT) and TRPV1 KO mice. Taken together, these findings suggest that C5a produces mechanical sensitization by initiating macrophage-to-sensory-neuron signaling cascade that involves activation of TRPV1 and CGRP receptor as critical steps in this process.

Cutaneous Neuroimmune Interactions in Peripheral Neuropathic Pain States

“…Complement system component C5a induces macrophage-dependent thermal and mechanical sensitization, involving the release of NGF and CGRP, respectively (116,117). In both studies, the macrophagederived neuropeptides activated TRPV1+ nociceptors, eliciting hyperalgesia (116,117). TRPV4 is implicated in both acute and chronic itch.…”

“…This interaction may represent a peripheral mechanism of chronic/neuropathic pain signalling and therefore provides a potential pharmacological target. Further, skin macrophages are key participants in both thermal and mechanical hyperalgesia mediated by the complement system (116,117), a key player of the innate immune system characterized by its contribution to inflammatory and neuropathic pain (118,119). Complement system component C5a induces macrophage-dependent thermal and mechanical sensitization, involving the release of NGF and CGRP, respectively (116,117).…”

“…Further, skin macrophages are key participants in both thermal and mechanical hyperalgesia mediated by the complement system (116,117), a key player of the innate immune system characterized by its contribution to inflammatory and neuropathic pain (118,119). Complement system component C5a induces macrophage-dependent thermal and mechanical sensitization, involving the release of NGF and CGRP, respectively (116,117). In both studies, the macrophagederived neuropeptides activated TRPV1+ nociceptors, eliciting hyperalgesia (116,117).…”

Identification of two natural coumarin enantiomers for selective inhibition of TRPV2 channels

“…Patch pipettes were prepared from borosilicate glass and fire‐polished to resistance about ∼4.0 MΩ. For whole‐cell recordings of TRPV2 and TRPM8 currents expressing in HEK 293 cells, both bath and pipette solutions contained 130 mM NaCl, 0.2 mM EDTA, and 3 mM HEPES, pH 7.2‐7.4, adjusted with NaOH 35,36 . Membrane potential was held at 0 mV, and currents were elicited by a protocol consisting of a 400‐ms step to +80 mV followed by a 400‐ms step to −80 mV at 1‐s intervals.…”

The complement cascade in the regulation of neuroinflammation, nociceptive sensitization, and pain

“…Up to this point, the only 'a' fragments produced are C4a and C2a, both of which have essentially no activity (16). Formation of the C3 convertase is a critical transition to terminal pathway signaling and the generation of highly active C3a and C5a components, both of which are powerful drivers of neuroinflammation and pain (Figure 1) (17)(18)(19)(20)(21).…”