Transmembrane Mucin Expression and Function in Embryo Implantation and Placentation.

Transmembrane mucins (TMs) are extremely large, complex glycoproteins that line the apical surfaces of simple epithelia including those of the female reproductive tract. TMs provide a physical barrier consistent with their role as part of the innate immune system. This barrier function must be overcome in the context of embryo implantation to permit blastocyst attachment. Three major TMs have been identified in uterine epithelia of multiple species: MUC1, MUC4, and MUC16. MUC1 has been found in all species studied to date, whereas expression of MUC4 and MUC16 have been less well studied and may be species specific. The strategies for removing mucins to permit embryo attachment also vary in a species-specific way and include both hormonal suppression of TM gene expression and membrane clearance via cell surface proteases. Studies emerging from the cancer literature indicate that TMs can modulate a surprisingly wide variety of signal transduction processes. Furthermore, various cell surface proteins have been identified that bind either the oligosaccharide or protein motifs of TMs suggesting that these molecules may support cell attachment in some contexts, including trophoblast interactions with cells of the immune system. The intimate association of TMs at sites of embryo-maternal interaction and the varied functions these complex molecules can play make them key players in embryo implantation and placentation processes.