JUNO protein coated beads: A potential tool to predict bovine sperm fertilizing ability.

Considerable variation in fertility exists between bulls in AI centres, despite passing minimum post-thaw quality control checks. The development of a reliable in vitro test to predict bull fertility could enable the identification and selection of high fertility bulls, without the need to resort to test inseminations. An in-depth knowledge of the molecular basis of fertilization is a prerequisite to the development of such a test or combination of tests. To date, JUNO is the only oocyte plasma membrane receptor described to be involved in gamete binding for which the partner in the sperm, IZUMO1, is known. Despite the fact that this interaction appears to be conserved among mammals, it has not been confirmed yet in some species including cattle. Furthermore, an association between binding and fertility has not been tested. Here, we propose a sperm-binding assay based on magnetic sepharose beads coated with bovine recombinant JUNO protein (B) to study sperm binding. Bull sperm bound specifically to B demonstrating that the JUNO-IZUMO1 interaction is conserved in cattle. Moreover, the assay was able to distinguish between epididymal and ejaculated sperm. Lastly, the number of sperm cells bound to B was significantly lower for frozen-thawed sperm from bulls of low vs high field fertility. In conclusion, our findings document a novel valid sperm-binding assay to predict mammalian sperm function and to investigate the role of specific proteins involved in gamete recognition and fusion.