Identification of proteins in the equine embryonic capsule

An acellular embryonic capsule envelops equine conceptuses between day 6 and day 23 after ovulation. As all of the factors mediating embryo-mother signalling must pass through the capsule, it acts like a 'mailbox'. Therefore, we have started to map the proteins in this special extracellular matrix at the interface between mother and embryo. In the present study, one- and two-dimensional gel electrophoresis were used to examine a range of proteins. Use of western blotting identified three specific proteins in the capsules of equine conceptuses recovered on day 16 after ovulation: insulin-like growth factor binding protein 3 (IGFBP-3), a 19 kDa uterine lipocalin (P19) and heparin-binding epidermal growth factor (HB-EGF). Western blotting of two-dimensional SDS-PAGE gels revealed the isoelectric points (pI values) of these proteins: IGFBP-3 was detected as the non-glycosylated 32 kDa form with two isoforms at about pI value 5.8; P19 had a pI value of 9.1; and several isoforms of HB-EGF were detected with molecular masses of approximately 28 kDa and a pI value range of 5.8-6.2. The origin of HB-EGF is not known, but IGFBP-3 is embryonic and P19 is maternal in origin and is thought to be a transport protein. In addition to playing a protective role, and probably also contributing to the mobility of the young conceptus within the uterus, the capsule may be thought of as the extracellular matrix of the embryo, which modulates the complex embryo-maternal signalling processes that take place during early pregnancy in mares.