Recombinant fetuin-B protein maintains high fertilization rate in cumulus cell-free mouse oocytes

STUDY QUESTION: Does fetuin-B inhibit premature zona pellucida (ZP) hardening in mouse oocytes in vitro and thus increase IVF rate? SUMMARY ANSWER: Supplementation of oocyte in vitro maturation (IVM) media with recombinant mouse fetuin-B (rmFetuB) increased fertilization rate without affecting mouse embryo development into blastocysts. WHAT IS KNOWN ALREADY: Mice deficient in fetuin-B are infertile owing to premature ZP hardening. Premature ZP hardening also occurs during oocyte IVM leading to decreased fertilization rate. STUDY DESIGN, SIZE, DURATION: We fertilized batches of 20-30 mouse metaphase II (Mll) stage oocytes from C57BL/6 mice with fresh sperm, and studied early embryo development until blastocyst hatching. PARTICIPANTS/MATERIALS, SETTING, METHODS: Oocytes were maintained with or without rmFetuB during IVM and IVF. Exogenous rmFetuB was added to media prior to oocyte isolation. ZP hardening was quantified by chymotrypsin digestion timing and by counting attached sperm. MAIN RESULTS AND THE ROLE OF CHANCE: In the absence of cumulus cells, rmFetuB dose-dependently inhibited ZP hardening and increased IVF rate (P = 0.039). Fetuin-B at ≥0.03 mg/ml also inhibited physiological, fertilization-triggered ZP hardening (indicated by increased sperm binding, P = 0.0002), without increasing embryo death. Exogenous rmFetuB increased IVF rate for up to 5 hours of IVM (P = 0.02 at 1 hour, P = 0.01 at 5 hours of IVM). LIMITATIONS, REASONS FOR CAUTION: Mll stage oocytes in this study were isolated from the ampullae of fetuin-B expressing mice. Thus, oocytes were protected against premature ZP hardening by endogenous fetuin-B. In humans and livestock, oocytes are usually isolated by follicle puncture before ovulation. In this situation, the deprivation of endogenous fetuin-B would occur earlier and the effect of exogenous fetuin-B in the IVF medium may be even more pronounced. Fertilization-triggered ZP hardening is essential for embryo development but in this study the effect of fetuin-B supplementation was only studied to blastocyst stage. Any influence of added fetuin-B on later embryo development after transplantation remains to be determined. WIDER IMPLICATIONS OF THE FINDINGS: The astacin-type protease ovastacin triggers definitive ZP hardening by cleaving the zona pellucida protein 2. Animal sera are known to inhibit premature ZP hardening. The addition of rFetuB to the culture medium of oocytes could increase IVF rates by the inhibition of premature ZP hardening. In this regard, the results could be useful for clinical activity. LARGE SCALE DATA: None. STUDY FUNDING/COMPETING INTERESTS: The research was supported by a grant from Deutsche Forschungsgemeinschaft and by the START program of the Medical Faculty of RWTH Aachen University. The authors ED, JF and WJD are named inventors on a patent application of RWTH Aachen University covering the use of fetuin-B in ovary and oocyte culture.