Effect of Mare Age on Transcript Abundance of Connexins-37 and -43, Zona Pellucida Proteins, and Sperm Binding

Zona pellucida (ZP) proteins are important for fertilization and sperm binding and are closely associated with cumulus cells. Communication between cumulus and oocytes is facilitated by intracellular membrane channels composed of connexins. The extent aging impacts potential differences in fertilization and reductions in fertility is not well understood. This study characterized age-related differences in transcript abundance of ZP proteins and connexins in cells from ovarian follicles. Additionally, differences in sperm binding to oocytes from old and young mares was evaluated. For experiment 1, oocytes, corona radiata, cumulus, and granulosa cells were collected from mares classified as young (4-12 years) or old (> 20 years). Transcript abundance was evaluated for connexins -37 (GJA4) and -43 (GJA1); zona pellucida glycoproteins 1, 2, 3, and 4 (ZP1, ZP2, ZP3, ZP4); Tubulin (TUBA1A), and equine chorionic gonadotropin β. For experiment 2, oocytes that failed to cleave following intracytoplasmic sperm injection (ICSI) were stored in salt solution for up to 4 years and used for sperm binding assays. Transcript abundance for GJA1 was decreased in oocytes, corona radiata, and granulosa cells while GJA4 was decreased in cumulus cells from old compared to young mares. Additionally, ZP1 tended to be decreased in corona radiata and cumulus cells from old mares. Oocytes from old mares tended to bind less spermatozoa compared young mares. Oocytes that failed to cleave following ICSI can be used for sperm binding studies for up to 2 years without losses in sperm binding. Our findings suggest that maternal age may contribute to changes in cellular communication and the ZP that could impact sperm binding.