Up-regulation of the α2,6-sialyltransferase messenger ribonucleic acid increases glycoconjugates containing α2,6-linked sialic acid residues in granulosa cells during follicular atresia of porcine ovaries

The sugar chains in cellular glycoconjugates have many biological functions. Extensive morphological development and remodeling occur in the ovary of female animals. This caused us to study glycobiological characteristics of ovarian cells, particularly granulosa cells that undergo apoptosis during follicular atresia. The lectin Sambucus sieboldiana agglutinin (SSA) specific for Sia alpha 2,6Gal/GalNAc showed positive staining for granulosa cells only in atretic follicles of porcine ovaries by lectin histochemistry. Lectin blot analysis for SSA demonstrated specific glycoproteins only in atretic follicles. Furthermore, we performed analysis of backbone structures of SSA-positive glycans carried by granulosa cell glycoproteins increased during atresia by glycosidase treatment. Most of these structures were Sia alpha 2,6Gal beta 1,4GlcNAc on complex-type N-glycans, suggesting that only ST6Gal I of four distinct alpha 2,6-sialyltransferases catalyzes alpha 2,6-sialic acid transfer in most of the increased glycoproteins of granulosa cells during follicular atresia. Reverse transcription-polymerase chain reaction analysis demonstrated that the expression of ST6Gal I mRNA was up-regulated in granulosa cells during atresia. These results suggested that the alteration of glycoconjugates by ST6Gal I in granulosa cells during atresia is involved in some processes of ovarian follicular atresia and granulosa cell apoptosis.