Mucin‐like glycoproteins in the equine embryonic capsule

Abstract

The equine embryonic capsule replaces the zona pellucida and envelopes the conceptus during the second and third weeks of pregnancy. Although this capsule was described more than 100 years ago, its molecular structure has not been characterized. Here we present evidence that the glycoprotein(s) of the equine capsule resembles those of the mucin glycoprotein family. The resistance of the capsule to chemical and enzymatic solubilization was confirmed, and, as in mucins, protein constituted only 35-40% of its total dry mass. Determination of the sugar composition of the capsule using colorimetric assays and high-performance anion-exchange chromatography also showed it to have mucin-like characteristics. Gal, GalNAc, sulfated sugars, and sialic acid make up a high proportion of the capsular carbohydrate, while GlcNAc, Glc, and Man are minor components. These findings were verified using lectin histochemical staining of frozen sections of conceptuses. The results of amino acid analysis were also consistent with the proposal that the capsular glycoproteins belong to the mucin family. Removal of the covalently bound carbohydrate by beta-elimination under reducing conditions demonstrated that the capsule is O-glycosylated mainly on threonine residues. Affinity chromatography on jacalin-agarose confirmed that, like mucins, the capsular glycoproteins are heavily O-glycosylated. SDS-PAGE analysis revealed a prominent 21-kDa band, specific to the capsule, in preparations solubilized by trypsin but not by other proteases. Characterization of its constituent glycoprotein(s) should be helpful in elucidating the role of the capsule (and analogous blastocyst coverings in other species) during early pregnancy.

Oviductins possess chitinase‐ and mucin‐like domains: A lead in the search for the biological function of these oviduct‐specific ZP‐associating glycoproteins

“…The binding of acrosome-intact sperm cells to the mouse egg was also shown to be mediated in part by the interaction of 0-linked oligosaccharides (approximately 3,900 Da) of the ZP 3 molecule and a lectin-like protein of 56 kDa (Cheng et al, 1994). Mucin-like glycoproteins were reported to be constituents of the protective equine embryonic capsule (Oriol et al, 1993) and neozona of the rabbit (Shapiro et al, 1974) and the apical cell surface of the mouse uterine epithelium was also shown to express protective, proteolysis resistant, WGA-binding, mucin-type glycoproteins (Valdizan et al, 1992).…”

Fetomaternal interactions and influences during equine pregnancy

“…Owing to its negative electrostatic charge and unusual glycocalyx configuration (Oriol et al, 1993b), the outer surface of the capsule is very sticky towards other proteins (Fig. 2).…”

Production of Calcium Maintenance Factor Stanniocalcin-1 (STC1) by the Equine Endometrium During the Early Pregnant Period

“…Between days 11 and 16 of pregnancy, the conceptus rapidly expands its volume, but keeps its spherical shape surrounded by a unique, smooth structure called a "capsule" [6,7] that emerges beneath the zona pellucida on days 6-8 of pregnancy. The capsule is an acellular mucin-like glycoprotein [8] that is structurally strong and is considered essential for the survival of the conceptus and continuation of pregnancy [9,10]. Together with peristaltic myometrial contractions, the capsule is likely to give motility to the conceptus, although the precise stimulus for conceptus migration is unknown.…”