Desialylation of core type 1 O-glycan in the equine embryonic capsule coincides with immobilization of the conceptus in the uterus

During the second and third weeks of pregnancy, the equine conceptus expands rapidly while it is enclosed within a glycan capsule. Around day 16 of gestation, the conceptus loses its mobility in the uterus by a process termed ‘fixation’, coinciding with various changes in the capsule. Here, we compared the structure of the carbohydrate moieties expressed by the capsule during pre- and post-fixation periods. The glycan structures were studied by chemical analyses in combination with mass spectrometry. Capsule material from conceptuses collected before fixation (days 13–16) was observed to carry a sialylated core type 1 O-linked glycan, Neu5Ac-(2→3)-Gal-(1→3)-GalNAc-(1→Ser/Thr. By comparison, analysis of post-fixation capsules (days 17–19) revealed a desialylated core type 1, Gal-(1→3)-GalNAc-(1→Ser/Thr. The equine embryonic capsule also furnished 4-substituted GlcNAc, 4-substituted Glc and 2,3,4,6-tetrasubstituted Glc residues, the concentrations of which did not change between pre- and post-fixation stages. The loss of sialic acid from the sialylated core type 1 in the capsule appears to be directly related to successful fixation of the conceptus, and thus critical to the continuance of pregnancy in horses.