Estrogen induced changes in uterine brain-derived neurotrophic factor and its receptors

Study question: Are brain-derived neurotrophic factor (BDNF) and its receptors, NTRK2, NGFR and SORT1, regulated by ovarian steroids in the uterus?

Summary answer: BDNF and its low affinity receptor, nerve growth factor receptor (NGFR), are regulated by estradiol in the uterus.

What is known already: Recent studies have revealed a central role for neurotrophins in placental development, endometrial stem cell neurogenesis, endometrial carcinoma and endometriosis. Complex signaling pathways involving BDNF and its receptors are regulated by ovarian hormones in the brain, however their expression and regulation in the uterus is poorly defined.

Study design, size, duration: This experimental animal study involved a total of 80 mice.

Participants/materials, setting, methods: Female C57BL/6 mice (n = 50) were monitored daily for estrous cycle stage, and uterine horns were collected. A second group of mice (n = 30) were ovariectomized and given estradiol, progesterone, estradiol + progesterone, or saline for 4 days. Uterine expression of BDNF and its receptors were quantified by real-time PCR and western blot, and localized using immunohistochemistry.

Main results and the role of chance: During the estrous cycle, expression of BDNF, NTRK2 and SORT1 remained constant, while NGFR declined 11-fold from pro-estrus through to diestrus (P = 0.005). In ovariectomized mice, estradiol treatment increased uterine expression of mature BDNF greater than 6-fold (P = 0.013, 25 kDa; P = 0.003, 27 kDa), pro-BDNF 5-fold (P = 0.041, 37 kDa band; P = 0.046, 40 kDa band), and NGFR 5-fold (P < 0.001) when compared with other treatments. NTRK2 and SORT1 were unaffected by ovarian hormones. NGFR was primarily localized in epithelial cells in mice in diestrus or in ovariectomized mice treated with progesterone (P ≤ 0.001; P ≤ 0.001, respectively). In contrast, NGFR switched to a stromal localization in ovariectomized mice administered estradiol (P = 0.002).

Limitations, reasons for caution: This study was performed in one only species.

Wider implications of the findings: Results of this study demonstrate the uterine regulation of BDNF and NGFR by estradiol, and highlight the striking difference between hormone exposure during the estrous cycle and daily estradiol exposure after ovariectomy on neurotrophin expression in the uterus. The results also show the spatial regulation of NGFR in the uterus in response to ovarian hormones. Sustained estrogen exposure, as seen in estrogen-dependent disease, may alter the delicate neurotrophin balance and inappropriately activate potent BDNF-NTRK2 pathways which are capable of contributing to endometrial pathology.

Study funding/competing interests: This study was supported by the Canadian Institutes of Health Research (CIHR) (W.G.F.), a NSERC Discovery Grant (W.G.F.), and a Vanier Canada Graduate Scholarship-CIHR (J.M.W.). J.M.W. is a member of the CIHR sponsored Reproduction and Early Development in Health training program. The authors declare no conflicts of interest.