During the third week of pregnancy, the equine conceptus is enclosed within a capsule, the glycan composition of which changes at around day 16 (ovulationZday 0) when the conceptus becomes immobilized (fixed) in the uterine lumen. Our objective was to characterize the process of fixation by identifying changes in major capsule-associated proteins. Individual equine conceptuses (nZ55) were collected transcervically by uterine lavage between days 13.5 and 26.5. Major proteins extracted from capsules were compared with those in fluids from the uterus and yolk sac by SDS-PAGE. Until day 14, a major capsule-associated protein that migrated at w10 kDa was identified by N-terminal sequencing as equine b2 microglobulin (b2M). During fixation, b2M in the capsule underwent limited proteolysis to an w8 kDa form lacking nine amino acids from the N terminus, and was subsequently degraded. Expression of b2M mRNA was detected in the yolk-sac wall tissues and endometrium between days 13.5 and 17.5. During this period, b2M in the capsule was evidently not part of a complex with major histocompatibility complex class 1 heavy a chain bands because these were undetectable in the capsule and uterine lavage. Uterocalin (p19) was detected in uterine lavage and capsule throughout fixation, but in yolk-sac fluid only before fixation. These studies indicate that intact b2M is a major protein associated with the embryonic capsule before fixation, after which it undergoes limited proteolysis to a truncated w8 kDa form that remains in the capsule after the conceptus is immobilized.

A critical period of early gestation in the mare involves the immobilization (fixation) of the encapsulated conceptus at around days 16-17. We compared the major proteins in the normal equine embryonic capsule and endometrial secretions around the period of fixation with those from pregnancies in the process of termination induced by administration of an analogue of prostaglandin F(2 alpha) (PGF(2 alpha)). Uterocalin and beta(2)-microglobulin (beta(2)M) associated with the embryonic capsule were proteolytically converted to smaller forms during the fixation period. These conversions were similar in conceptuses from control and treated mares. A 17 kDa cationic protein identified as a secretory phospholipase A2 (sPLA2) type IIA was detected bound to normal capsules but increased substantially in response to PGF(2 alpha). Two forms of uteroglobin were distinguished by partial amino acid sequences of approximately 6 kDa bands in flush fluids from normal pregnant uteri. After administration of PGF(2 alpha) one immunoreactive form of uteroglobin was preferentially increased. These studies demonstrate that failure of pregnancy in this model is associated with an increase in secretory phospholipase in the capsule and a change in the forms of uteroglobin in the uterine secretions.

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