The effects of immunization against bone morphogenetic protein-15 and growth differentiation factor-9 on ovarian function in the mare
Date
2017
Authors
Davis, Kelli, author
Bruemmer, Jason, advisor
Eckery, Douglas, committee member
Bouma, Gerrit, committee member
Pinedo, Pablo, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The Bureau of Land Management (BLM) claims the current wild horse and burro population is nearly three times what the rangeland can support. Unmanaged, the horse population doubles every four years, which is detrimental for wild horses, wildlife, and rangeland. The BLM is investigating means of population control for these horses. Currently, no contraceptive vaccine exists capable of inducing permanent sterility in mares. This project serves as the first half of a two-year study investigating the effect of vaccination against Bone Morphogenetic Protein 15 (BMP-15) and Growth Differentiation Factor 9 (GDF-9) on follicular growth and ovulation rates in mares. In mice, rats, sheep, cattle, humans, and deer, these growth-factors have been shown to be critical regulators of follicular development and ovulation rate. Mutations involving expression of either BMP-15 or GDF-9 either increase ovulation rates or induce sterility in investigated species, indicating their importance in fertility and their potential value as a target for contraceptive use. Since the role of these growth factors has been proven to be critical for normal follicular development in other species, we hypothesize that vaccination against BMP-15 and/or GDF-9 will prevent ovulation and/or accelerate the depletion of the oocyte reserve in the horse. For this project, 30 mares were randomly assigned to one of three groups (n=10/group). Mares were vaccinated with either BMP-15 or GDF-9 peptides conjugated to keyhole limpet hemocyanin (KLH) and Seppic Montanideā¢ Pet Gel A polymeric adjuvant, or a control of phosphate buffered saline and adjuvant. The horses received a primary vaccination and three booster injections at weeks 0, 6, 12, and 18. Mares were evaluated three days a week during the breeding season for follicular size and date of ovulation via transrectal ultrasonography. Abnormal ovulations and follicular development were noted. Estrous behavior and sexual receptiveness to a stallion were evaluated using a six-point teasing scale during a rail teasing technique with a stallion three times a week. In order to determine individual antibody responses to the immunizations, blood samples were collected every two weeks, with sera from the samples used for enzyme-linked immunosorbent assay (ELISA). Evaluation of antibody responses demonstrated the majority of BMP-15 treatment mares elicited a consistently high response to the BMP-15 vaccine. However, only two mares in the GDF-9 treatment group demonstrated a consistently high antibody response to the GDF-9 treatment. No difference in ovulation rate (P=0.66) was noted in the GDF-9 group when compared to controls (10.8 and 10.0 ovulations respectively). However, the number of ovulations in the BMP-15 group was decreased (P=0.02; 4.9 ovulations) compared to the control group. Both treatment groups demonstrated differences in the average size of ovulatory follicles (P<0.001) when compared to controls. On average the last recorded size of ovulatory follicles prior to ovulation measured 21.3 mm, 27.8mm, and 35.7mm in the BMP-15, GDF-9 and control groups respectively. Upon evaluation of teasing records, the both the BMP-15 and GDF-9 vaccinated mares displayed estrus behavior less frequently than controls following the second vaccination (P=0.05 and 0.03 respectively). This indicates altered estrous behavior in both BMP-15 and GDF-9 vaccinated mares. In the first year of this study, vaccination against BMP-15 successfully altered ovarian function in the mare by decreasing the ovulation rate and the size of ovulatory follicles. This altered ovarian function was also indicated by an alteration in estrous cycle behaviors in BMP-15 treated mares. Although GDF-9 did not appear to alter ovulation rate, the decrease in average size of follicles at ovulation and altered estrous behavior indicates further research is required to determine if greater effects are observed in subsequent years. Overall, altered ovarian function in both the BMP-15 and GDF-9 groups shows promise that vaccination against these growth factors could potentially serve as a contraceptive for use in controlling wild horse populations.