REGULATION OF HUMAN SALIVARY MUCIN GLYCOSYLATION

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The obvious mechanical contributions of human salivary mucins, by virtue of glycans densely packed around apoprotein cores, to protection and lubrication of oral cavity soft tissues have long been recognized. More recent studies demonstrated the additional ability of salivary mucins to modulate the biological environment of the oral cavity by direct interaction with oral bacteria. Binding of glycan structures on MG2 (MUC7), the human low molecular weight salivary mucin, to specific bacterial receptors have been implicated. SAalpha2,3Ga1Beta1, 3GalNAC- chains, along with two other Core 1 type chains, Fucalpha1,2Ga1Beta1,3GalNAc- and their common precursor. Ga1Beta1,3GalNAC-, comprise the major proportion of glycan chains on MG2. Two major isoforms of MG2, MG2a and MG2b, have been documented; these are putatively generated by differential expression of the sialylated- and fucosylated-Core 1 oligosaccharides. As a first step in understanding the molecular and cellular factors that dictate glycan composition in human salivary mucins, the biosynthesis of the sialylated- and fucosylated-Core 1 oligosaccharides on MG2 will be examined in detail. The first step will be the molecular characterization, by analysis of cDNA and genomic sequences, of the cognate glycosyltranferases, the alpha2,3-sialyltransferase and the alpha1,2-fucosyltransferase. Molecular probes generated as a result of these studies will be used to examine differential synthesis of the Core 1 oligosaccharides. The possibility that the MG2 isoforms result from different sub-populations of secretory cells expressing different levels of the sialytransferase and fucosyltransferase will be addressed by the in situ hybridization approach. Furthermore, within each cell that expresses both the fucosy- and sialytransferases, potential competition between these enzymes for the common substrate may dictate the final ratio of sialylated and fucosylated structures. This possibility, as well as the influence of apomucin primary structure on the differential glycosylation, will be examined by means of transfection and expression of apomucin and glycosyltransferase sequences. Finally, efforts will be made to identify cell lines of salivary gland origin that retain regulated mucin expression. These lines will be ultimately useful in the molecular dissection of pathways that regulate apomucin and glycosyltransferase expression in salivary glands.