Alterations in the Glycosylation of Secreted Thyrotropin during Ontogenesis

We have examined the carbohydrate structure of thyrotropin (TSH) secreted in vitro by pituitaries from prenatal, perinatal, and mature rats using concanavalin A (ConA)-agarose chromatography and anion-exchange high performance liquid chromatography (HPLC). [3H]Glucosamine-labeled TSH was immuno-precipitated and treated with either Pronase to generate glycopeptides or a mixture of endo-beta-N-acetyl-glucosaminidase F and peptide:N-glycosidase F to release oligosaccharides. The percentage of secreted TSH glycopeptides not bound to ConA was greater in mature animals (47 +/- 3%) than in either prenatal (29 +/- 3%) or perinatal animals (29 +/- 6%), suggesting more multiantennary oligosaccharides in the older animals. These structural changes were characterized further by performing anion-exchange HPLC on released oligosaccharides. Secreted TSH from prenatal rats predominantly contained oligosaccharides with 1 and 2 negative charges, while TSH from mature rats contained these structures as well as 15% with 3 negative charges. In addition, the ratio of sialylated to sulfated oligosaccharides was greater in mature compared to prenatal animals for species with 1 negative charge (1.9-fold) as well as for species with 2 negative charges (4.3-fold). We also correlated the structural alterations noted by ConA analysis with anion-exchange HPLC. Oligosaccharides that bound to ConA and were eluted with alpha-methylglucoside, when analyzed by anion-exchange HPLC, consisted of species with 1 and 2 negative charges, whereas oligosaccharides that were unbound to ConA were predominantly species with three negative charges. Together, these data suggest that with maturation of the hypothalamic-pituitary-thyroid axis secreted TSH contains more negatively charged multiantennary oligosaccharides with increased sialylation and decreased sulfation.