Angiotensin II Triggers Peripheral Macrophage-to-Sensory Neuron Redox Crosstalk to Elicit Pain

Abstract

Injury, inflammation, and nerve damage initiate a wide variety of cellular and molecular processes that culminate in hyperexcitation of sensory nerves, which underlies chronic inflammatory and neuropathic pain. Using behavioral readouts of pain hypersensitivity induced by angiotensin II (Ang II) injection into mouse hindpaws, our study shows that activation of the type 2 Ang II receptor (AT2R) and the cell-damage-sensing ion channel TRPA1 are required for peripheral mechanical pain sensitization induced by Ang II in male and female mice. However, we show that AT2R is not expressed in mouse and human dorsal root ganglia (DRG) sensory neurons. Instead, expression/activation of AT2R on peripheral/skin macrophages (MΦs) constitutes a critical trigger of mouse and human DRG sensory neuron excitation. Ang II-induced peripheral mechanical pain hypersensitivity can be attenuated by chemogenetic depletion of peripheral MΦs. Furthermore, AT2R activation in MΦs triggers production of reactive oxygen/nitrogen species, which -activate TRPA1 on mouse and human DRG sensory neurons via cysteine modification of the channel. Our study thus identifies a translatable immune cell-to-sensory neuron signaling crosstalk underlying peripheral nociceptor sensitization. This form of cell-to-cell signaling represents a critical peripheral mechanism for chronic pain and thus identifies multiple druggable analgesic targets. Pain is a widespread health problem that is undermanaged by currently available analgesics. Findings from a recent clinical trial on a type II angiotensin II receptor (AT2R) antagonist showed effective analgesia for neuropathic pain. AT2R antagonists have been shown to reduce neuropathy-, inflammation- and bone cancer-associated pain in rodents. We report that activation of AT2R in macrophages (MΦs) that infiltrate the site of injury, but not in sensory neurons, triggers an intercellular redox communication with sensory neurons via activation of the cell damage/pain-sensing ion channel TRPA1. This MΦ-to-sensory neuron crosstalk results in peripheral pain sensitization. Our findings provide an evidence-based mechanism underlying the analgesic action of AT2R antagonists, which could accelerate the development of efficacious non-opioid analgesic drugs for multiple pain conditions.

Cutaneous inflammation differentially regulates the expression and function of Angiotensin‐II types 1 and 2 receptors in rat primary sensory neurons

“…Additionally, recent evidence shows that macrophages express AT2R which upon activation by Ang‐II release reactive oxygen/nitrogen species. These alter the function of TRPA1 receptors in sensory neurons (Shepherd et al ). Thus, macrophages recruited early during cutaneous inflammation may contribute to inflammatory pain via potentiation of the local renin–angiotensin system and also by sensitizing TRPA1 + terminals.…”

Transcriptomic analysis of native versus cultured human and mouse dorsal root ganglia focused on pharmacological targets

“…An interesting observation emerging from our work is that some macrophages are apparently present in DRG cultures and this macrophage phenotype is inflammatory in nature. An emerging literature describes DRG resident macrophages as key players in development of many chronic pain states, including neuropathic pain [23; 27; 39; 51; 61]. In future studies it may be possible to manipulate these macrophages to interact with DRG neurons in culture to push this neuropathic pain phenotype further toward the generation of spontaneous activity.…”

“…Traditionally, investigators have studied rodent nociceptors in vitro as dissociated cell cultures prepared from DRG or TG. More recently, investigators have also started to study DRG nociceptors from human organ donors and surgical patients [14; 39; 47; 50; 51; 59; 70]. This creates a “clinical bridge” for advancing mechanisms or therapeutics from rodents toward the clinic.…”

Anti‐hypersensitive effect of angiotensin (1‐7) on streptozotocin‐induced diabetic neuropathic pain in mice

“…administration of Ang II causes hypersensitivity (Shepherd, Copits, et al, ). While it was suggested that the induction of neuropathic pain involves the activation of AT2 receptors in DRGs (Smith et al, ), other reports indicate that it involves AT2 receptors on peripheral macrophages (Shepherd, Copits, et al, ; Shepherd, Mickle, et al, ). As the action site of Ang II is still debated and requires further studies, we have investigated pain transmission from the perspective of the spinal Ang system.…”