Sensory nerve-derived neuropeptides accelerate the development and fibrogenesis of endometriosis.

STUDY QUESTION: Do sensory nerves play any role in the development of endometriosis?

SUMMARY ANSWER: Sensory nerves participate in all major steps (epithelial-mesenchymal transition (EMT), fibroblast-to-myofibroblast transdifferentiation (FMT) and smooth muscle metaplasia (SMM)) in the development and fibrogenesis of endometriotic lesions.

WHAT IS KNOWN ALREADY: Endometriotic lesions are known to be hyperinnervated due to neurogenesis resulting from neutrophins secreted by endometriotic lesions and possibly platelets. These neutrophins seem to preferentially favour production of sensory neurons at the expense of sympathetic neurons.

STUDY DESIGN, SIZE, DURATION: Three independent, yet complementary, prospective, randomized mouse experimentations were conducted. A total of 143 female Balb/C mice and 24 female immunodeficient nude Balb/C mice were used. The mice were sacrificed 2 or 4 weeks after the induction of endometriosis.

PARTICIPANTS/MATERIALS, SETTING, METHODS: In Experiment 1, 21 mice were randomly divided into three groups of equal size for sympathetic denervation, sensory denervation and controls. Denervation was carried out chemically. In Experiment 2, 24 nude mice were randomly divided into three equal-sized groups: the BEFORE and AFTER groups that respectively received surgical denervation 3 days before or after the induction of endometriosis by subcutaneous grafting of human endometriotic tissues, and the Control group that received a sham surgery without denervation 3 days before induction. For Experiments 1 and 2, all mice were sacrificed two weeks after induction of endometriosis. In Experiment 3, substance P (SP) and aprepitant, a potent and selective neurokinin 1 receptor (NK1R) antagonist, were used to activate and inhibit the NK1R signalling pathway, respectively. A total of 32 mice were randomly divided into four groups of equal sizes: control (CTL), SP, Before-Induction and After-Induction. One day before the induction of endometriosis, mice in CTL, SP and Before-Induction groups were infused with sterile saline, SP and aprepitant, respectively, via Alzet osmotic pumps. Two weeks after the induction, the After-induction group was infused with aprepitant in similar fashion. All mice were sacrificed four weeks after the induction of endometriosis. In all three experiments, the bodyweight and hotplate latency were evaluated before induction and sacrifice. In addition, all lesions were excised, weighed and processed for quantification and immunohistochemistry analysis of markers for EMT, FMT and SMM, and the extent of fibrosis was evaluated by Masson trichrome staining.

MAIN RESULTS AND THE ROLE OF CHANCE: In Experiment 1, chemical denervation of sympathetic and sensory nerves reduced the lesion weight by 43.2% (±23.1%) and 68.7% (±20.3%), respectively, as compared with controls. In particular, sensory denervation led to significantly greater reduction in lesion weight than sympathetic denervation. Sensory denervation also resulted in significantly improved hyperalgesia as compared with controls. In contrast, sympathetic denervation yielded only transient improvement in hyperalgesia. Both sympathetic and sensory denervation resulted in lower immunoreactivity against markers of proliferation and fibrosis, especially sensory denervation.In Experiment 2, surgical denervation before or after induction of endometriosis also decelerated the development of endometriosis, as manifested by significantly reduced lesion weight and extent of lesional fibrosis, along with improved hyperalgesia.In Experiment 3, NK1R activation by SP infusion accelerated lesional development, as evidenced by significantly increased lesional weight, more thorough progression of EMT, FMT, SMM, exaggerated lesional fibrosis and deteriorated hyperalgesia. In contrast, NK1R antagonism decelerated lesional development and improved hyperalgesia.

LARGE SCALE DATA: N/A.

LIMITATIONS, REASONS FOR CAUTION: This study is limited by the use of histologic and immunohistochemistry analyses only and the lack of molecular data.

WIDER IMPLICATIONS OF THE FINDINGS: Since sensory nerves are known to be important in wound healing and fibrogenesis, our findings also give more credence to the notion that endometriotic lesions are wounds undergoing repeated tissue injury and repair. As such, sensory nerves or the NK1R signalling pathway in particular may be potential targets for intervention.

STUDY FUNDING/COMPETING INTEREST(S): This research was supported by Grants 81471434 (SWG), 81530040 (SWG), 81771553 (SWG), 81671436 (XSL) and 81871144 (XSL) from the National Natural Science Foundation of China and an Excellence in Centres of Clinical Medicine grant (2017ZZ01016) from the Science and Technology Commission of Shanghai Municipality. None of the authors have anything to disclose.