Two urea transporters, UT-A1 and UT-A3, are expressed in the kidney terminal inner medullary collecting duct (IMCD) and are important for the production of concentrated urine. UT-A1, as the largest isoform of all UT-A urea transporters, has gained much attention and been extensively studied; however, the role and the regulation of UT-A3 are less explored. In this study, we investigated UT-A3 regulation by glycosylation modification. A site-directed mutagenesis verified a single glycosylation site in UT-A3 at Asn279. Loss of the glycosylation reduced forskolin-stimulated UT-A3 cell membrane expression and urea transport activity. UT-A3 has two glycosylation forms, 45 and 65 kDa. Using sugar-specific binding lectins, the UT-A3 glycosylation profile was examined. The 45-kDa form was pulled down by lectin concanavalin A (Con A) and Galant husnivalis lectin (GNL), indicating an immature glycan with a high amount of mannose (Man), whereas the 65-kDa form is a mature glycan composed of acetylglucosamine (GlcNAc) and poly-N-acetyllactosame (poly-LacNAc) that was pulled down by wheat germ agglutinin (WGA) and tomato lectin, respectively. Interestingly, the mature form of UT-A3 glycan contains significant amounts of sialic acid. We explored the enzymes responsible for directing UT-A3 sialylation. Sialyltransferase ST6GalI, but not ST3GalIV, catabolizes UT-A3 α2,6-sialylation. Activation of protein kinase C (PKC) by PDB treatment promoted UT-A3 glycan sialylation and membrane surface expression. The PKC inhibitor chelerythrine blocks ST6GalI-induced UT-A3 sialylation. Increased sialylation by ST6GalI increased UT-A3 protein stability and urea transport activity. Collectively, our study reveals a novel mechanism of UT-A3 regulation by ST6GalI-mediated sialylation modification that may play an important role in kidney urea reabsorption and the urinary concentrating mechanism.

Despite the prominence of carbohydrate-specific antibodies in human sera, data on their emergence and antigen specificities are limited. Whereas maternal IgG are transferred prenatally to the fetal circulation, IgM present in cord blood originate from fetal B lymphocytes. Considering the limited exposure of the fetus to foreign antigens, we assessed the repertoire of carbohydrate-specific antibodies in human cord blood and matched maternal blood samples using glycan arrays. Carbohydrate-specific IgM was absent in cord blood, whereas low cord blood IgG reactivity to glycans was detectable. Comparing IgG reactivities of matched pairs, we observed a general lack of correlation in the antigen specificity of IgG from cord blood and maternal blood due to a selective exclusion of most carbohydrate-specific IgG from maternofetal transfer. Given the importance of intestinal bacteria in inducing carbohydrate-specific antibodies, we analyzed global antibody specificities toward commensal bacteria. Similar IgG reactivities to specific Bacteroides species were detected in matched cord and maternal blood samples, thus pointing to an efficient maternal transfer of anti-microbial IgG. Due to the observed selectivity in maternofetal IgG transfer, the lack of fetal antibodies to carbohydrate epitopes is only partially compensated by maternal IgG, thus resulting in a weak response to carbohydrate antigens in neonates.