Suppression of TLR4-MyD88 signaling pathway attenuated chronic mechanical pain in a rat model of endometriosis

Wenliang Su,Xiuhua Zhang,Chao Ma,Yuguang Huang,Lulu Ma,Danning Wu,Jiawen Yu,Huan Cui

doi:10.1186/s12974-020-02066-y

Abstract

BackgroundAs a classic innate immunity pathway, Toll-like receptor 4 (TLR4) signaling has been intensively investigated for its function of pathogen recognition. The receptor is located not only on immune cells but also on sensory neurons and spinal glia. Recent studies revealed the involvement of neuronal TLR4 in different types of pain. However, the specific role of TLR4 signaling in the pain symptom of endometriosis (EM) remains obscure.MethodsThe rat endometriosis model was established by transplanting uterine horn tissue into gastrocnemius. Western blotting and/or immunofluorescent staining were applied to detect high mobility group box 1 (HMGB1), TLR4, myeloid differentiation factor-88 adaptor protein (MyD88), and nuclear factor kappa-B-p65 (NF-κB-p65) expression, as well as the activation of astrocyte and microglia. The antagonist of TLR4 (LPS-RS-Ultra, LRU) and MyD88 homodimerization inhibitory peptide (MIP) were intrathecally administrated to assess the behavioral effects of blocking TLR4 signaling on endometriosis-related pain.ResultsMechanical hyperalgesia was observed at the graft site, while HMGB1 was upregulated in the implanted uterine tissue, dorsal root ganglion (DRG), and spinal dorsal horn (SDH). Compared with sham group, upregulated TLR4, MyD88, and phosphorylated NF-κB-p65 were detected in the DRG and SDH in EM rats. The activation of astrocytes and microglia in the SDH was also confirmed in EM rats. Intrathecal application of LRU and MIP alleviated mechanical pain on the graft site of EM rats, with decreased phosphorylation of NF-κB-p65 in the DRG and reduced activation of glia in the SDH.ConclusionsHMGB1-TLR4-MyD88 signaling pathway in the DRG and SDH may involve in endometriosis-related hyperpathia. Blockade of TLR4 and MyD88 might serve as a potential treatment for pain in endometriosis.

Highlights

Endometriosis (EM), defined as the emergence of endometrial tissue outside the uterus, is a common and costly estrogen-dependent disorder, affecting about 176 million women worldwide [1]
Blocking Toll-like receptor 4 (TLR4)-myeloid differentiation factor-88 adaptor protein (MyD88) signaling pathway suppressed the activation of Transcription factors of the nuclear factor κB (NF-κB) in dorsal root ganglion (DRG) and activation of glia in spinal dorsal horn (SDH) under EM To investigate the mechanism of pain alleviation by LRU and MyD88 homodimerization inhibitory peptide (MIP), we examined the expression of phosphorylated NF-κB p65 in sensory neurons
Treatment with TLR4 antagonist (LRU) or MyD88 homodimerization inhibitory peptide (MIP) alleviated the hyperpathia in EM, and reversed the phosphorylation of NF-κB p65 in the DRG and the activation of astrocytes and microglia in the SDH. These results suggested that the TLR4-MyD88 signaling pathway in the DRG and SDH, which might be activated by proinflammatory High mobility group box 1 (HMGB1), was involved in EM-induced pain

Summary

Introduction

Endometriosis (EM), defined as the emergence of endometrial tissue outside the uterus, is a common and costly estrogen-dependent disorder, affecting about 176 million women worldwide [1]. The hallmark symptom of EM is chronic pain, mostly occurring around the ectopic uterine tissue [2, 3]. Identifying the pathogenesis of pain in EM and developing effective treatments remain necessary. HMGB1 served as a potential indicator of fibrosis in ectopic uterine tissue [12], which had been identified as a promising and admissible biomarker in plasma for EM. Sensory neurons in the DRG, especially the nociceptors, could be facilitated by HMGB1 in an advanced glycosylation end product-specific receptor (RAGE) and transient receptor potential vanilloid-1 (TRPV1) dependent manner [11, 13, 14]. As a classic innate immunity pathway, Toll-like receptor 4 (TLR4) signaling has been intensively investigated for its function of pathogen recognition. The specific role of TLR4 signaling in the pain symptom of endometriosis (EM) remains obscure

Objectives

Methods

Results

Discussion

Conclusion