Developmental regulation of a pregnancy-specific oligosaccharide structure, NeuAcα2,6GalNAcβ1,4GlcNAc, on select members of the rat placental prolactin family

Successful pregnancy is dependent upon an array of signaling proteins secreted by the trophoblast cells of the placenta. Among these is a group of proteins related to pituitary prolactin, known as the prolactin/growth hormone family. These proteins are expressed at specific times during gestation and synthesized in distinct trophoblast cell types in the rat placenta. We report here that select members of this family, prolactin-like protein (PLP-A), PLP-B, PLP-C, decidual/trophoblast PRP, and placental lactogen I variant, only which are expressed in the spongiotrophoblast, late in rat placental development bear Asn-linked oligosaccharides terminating with NeuAcα2,6GalNAcβ1,4GlcNAcβ-R. This reflects the concurrent expression of these prolactin/growth hormone family members with the peptide-specific β1,4Gal-NAc-transferase and an α2,6-sialyltransferase, which can add sialic acid to terminal β1,4-linked GalNAc. We have determined that at least one of the prolactin-like proteins, PLP-A, is recognized by the protein-specific GalNAc-transferase. The presence of NeuAcα2,6GalNAc-β1,4GlcNAcβ-R on only a limited number of glycoproteins synthesized by the spongiotrophoblasts between mid gestation and birth reflects the need for both the GalNAc- transferase and the peptide recognition determinant for efficient addition of GalNAc. Thus, expression of the GalNAc-transferase and specific members of the prolactin/growth hormone family is developmentally regulated in the rat placenta, suggesting a physiological role for the terminal NeuAcα2,6GalNAcβ1,4Glc-NAcβ-R sequence on Asn-linked oligosaccharides of these proteins.