Genetically altered mice with different sialyltransferase deficiencies show tissue-specific alterations in sialylation and sialic acid 9-O-acetylation

Abstract

Glycan chains on glycoconjugates traversing the Golgi apparatus are often terminated by sialic acid residues, which can also be 9-O-acetylated. This process involves competition between multiple Golgi enzymes. Expression levels of Golgi enzyme mRNAs do not always correlate with enzyme activity, which in turn cannot accurately predict glycan sequences found on cell surfaces. Here we examine the cell type-specific expression of terminal glycans in tissues of normal mice in comparison with animals deficient in ST6Gal-I (transfers α2-6-linked sialic acid to Galβ1-4GlcNAc) or ST3Gal-I (transfers α2-3-linked sialic acid to Galβ1-3GalNAc). Tissues of ST6Gal-I null mice showed minimal binding of an α2-6-sialic acid-specific lectin, indicating that no other enzyme generates Siaα2-6Galβ1-4GlcNAc and that Siaα2-6GalNAc (sialyl-Tn) is rare in mice. However, exposed Galβ1-4GlcNAc termini were only moderately increased, indicating that these can be partially capped by other enzymes. Indeed, Galα1-3Galβ1-4GlcNAc and Fucα1-2Galβ1-4GlcNAc termini were enhanced in some tissues. Many tissues of ST3Gal-I null animals showed increases in Galβ1-3GalNAc termini, and some increases in poly-N-acetyllactosamines. However, overall expression of α2-3-linked sialic acid was selectively reduced only in a few instances, indicating that other ST3Gal enzymes can generate this linkage in most tissues. Highly selective losses of 9-O-acetylation of sialic acid residues were also observed, with ST6Gal-I deficiency causing loss on endothelium and ST3Gal-I deficiency giving a marked decrease on CD4+ lymphocytes. These data demonstrate selective regulation of sialylation and 9-O-acetylation, point to cell types with potential physiological defects in null animals, and show in vivo evidence for competition between Golgi enzymes.