Follicular-fluid proteomics during equine follicle development

The complex composition of the follicular fluid (FF), the intimate proximity to the oocyte, and the continual changes in their composition have a major effect on folliculogenesis and oogenesis. To date, the profiling of FF proteomes during follicle selection, development, and ovulation has not been comprehensively investigated. Therefore, a shotgun proteomics approach and bioinformatics analyses were used to profile the proteomes of equine FF harvested in vivo from follicles at the following development stages: predeviation (18-20 mm), deviation (22-25 mm), postdeviation (26-29 mm), preovulatory (30-35 mm), and impending ovulation. A total of 294 proteins were detected in FF (FDR <1%), corresponding to 65 common proteins and 124, 142, 167, 132, and 142 proteins in the predeviation, deviation, postdeviation, preovulatory, and impending ovulation groups, respectively. The higher expression of properdin and several other proteins belonging to the complement system during the deviation time and ovulation suggested their contribution in the selection of the future dominant follicle and ovulation. Apolipoprotein A-1 and antithrombin-III appeared to be important throughout folliculogenesis. The "complement and coagulation cascades" was the major KEGG pathway across all stages of follicle development. The significant expression of several proteins belonging to the serine-type endopeptidase indicated their likely contribution to follicle and oocyte development. Our data provide an extensive description and functional analyses of the equine FF proteome during follicle selection, development, and ovulation. This information will help improve understanding of the ovarian function and ovulatory dysfunctions and might serve as a reference for future biomarker discovery for oocyte quality assessment.