Fractalkine/CX3CR1 Contributes to Endometriosis-Induced Neuropathic Pain and Mechanical Hypersensitivity in Rats.

Zhiming Liu,Yaqiong Sun,Sisi Chen,Chunping Qiu,Jie Jiang,Wenzhi Li,Jun-Ming Zhang

doi:10.3389/fncel.2018.00495

Abstract

Pain is the most severe and common symptom of endometriosis. Its underlying pathogenetic mechanism is poorly understood. Nerve sensitization is a particular research challenge, due to the limitations of general endometriosis models and sampling nerve tissue from patients. The chemokine fractalkine (FKN) has been demonstrated to play a key role in various forms of neuropathic pain, while its role in endometriotic pain is unknown. Our study was designed to explore the function of FKN in the development and maintenance of peripheral hyperalgesia and central sensitization in endometriosis using a novel endometriosis animal model developed in our laboratory. After modeling, behavioral tests were carried out and the optimal time for molecular changes was obtained. We extracted ectopic tissues and L4–6 spinal cords to detect peripheral and central roles for FKN, respectively. To assess morphologic characteristics of endometriosis-like lesions—as well as expression and location of FKN/CX3CR1—we performed H&E staining, immunostaining, and western blotting analyses. Furthermore, inhibition of FKN expression in the spinal cord was achieved by intrathecal administration of an FKN-neutralizing antibody to demonstrate its function. Our results showed that implanted autologous uterine tissue around the sciatic nerve induced endometriosis-like lesions and produced mechanical hyperalgesia and allodynia. FKN was highly expressed on macrophages, whereas its receptor CX3CR1 was overexpressed in the myelin sheath of sciatic nerve fibers. Overexpressed FKN was also observed in neurons. CX3CR1/pp38-MAPK was upregulated in activated microglia in the spinal dorsal horn. Intrathecal administration of FKN-neutralizing antibody not only reversed the established mechanical hyperalgesia and allodynia, but also inhibited the expression of CX3CR1/pp38-MAPK in activated microglia, which was essential for the persistence of central sensitization. We concluded that the FKN/CX3CR1 signaling pathway might be one of the mechanisms of peripheral hyperalgesia in endometriosis, which requires further studies. Spinal FKN is important for the development and maintenance of central sensitization in endometriosis, and it may further serve as a novel therapeutic target to relieve persistent pain associated with endometriosis.

Highlights

Endometriosis is a common chronic inflammatory gynecologic disorder that is characterized by the growth of endometrial epithelial and stromal cells outside the uterus
Given that increased levels of FKN were found in the peritoneal fluid of women with endometriosis (Ahn et al, 2015; still without a known role in endometriotic pain), we planned to explore the functional roles of FKN in contributing to the development and maintenance of peripheral hyperalgesia and central sensitization in endometriosis using a novel rat model of sciatic endometriosis that we originally described in a previous publication (Chen et al, 2016)
Since the sciatic nerve arising from the L4–6 dorsal root ganglia (DRG)—as well as secondary pain neurons routed through the dorsal horn of the spinal cord—we evaluated the expression of FKN/CX3CR1 in the dorsal horn to investigate its role in central sensitization

Summary

Introduction

Endometriosis is a common chronic inflammatory gynecologic disorder that is characterized by the growth of endometrial epithelial and stromal cells outside the uterus. Macrophages are the major source of pro-inflammatory cytokines/chemokines, including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and monocyte chemoattractant protein 1 (MCP1; Cao et al, 2004; Sikora et al, 2018). The majority of these molecules were demonstrated to be involved in the pain pathogenesis (Mita et al, 2014; Neziri et al, 2014). Prolonged exposure of sensory neurons to inflammatory mediators would lead to central sensitization (Arnold et al, 2012) and cause estrogenindependent pain (Woolf and Salter, 2000), as observed in some patients with endometriosis and persistent pelvic pain (Neziri et al, 2010; As-Sanie et al, 2013; Brawn et al, 2014)

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Results

Conclusion