Joseph Lau, PhD
The central thrusts of the research program are: 1) to elucidate the functional contribution of sialic acid epitopes in normal and malignant processes, and 2) to elucidate the molecular pathways that regulate the attachment of sialic acids. The current main, on-going project is to examine the contribution of the sialyltransferase ST6Gal-I in innate immune responses such as inflammation. There are a number of corollary projects: a) to examine the interaction between sialyltransferases and how this interaction influences the final outcome of sialyl-glycan structures; b) to assess the biologic role of sialyl epitopes and cognate sialyltransferases in development of immune functions; c) To assess the influence of sialic acids on tumor cell surfaces in anti-tumor immunity.
ST6Gal-I in Inflammatory Response
We have generated a mutant mouse, Siat1DP1, in which the promoter utilized to regulate ST6Gal-I expression during inflammatory has been abrogated. We showed that Siat1DP1 mouse, as well as the systemically ST6Gal-I-null mouse, exhibit significantly greater neutrophilic acute inflammation. Moreover, both ST6Gal-I deficient mice strains displayed more severe allergic airway eosinophilic inflammation than corresponding C56BL/6 control animals. We observed that a significantly greater pool of myeloid cells exists within the bone marrow of ST6Gal-I deficient animals than in the marrow of wild-type cohorts. This and other observations led us to hypothesize that altered myelopoiesis may be a significant factor in the overly robust inflammatory response in ST6Gal-I deficiency, and that ST6Gal-I serves as a normal regulator for the maintenance of myeloid cell numbers. This research is the first step towards delineating the role of sialyltransferases and cognate sialic acid structures in inflammation and myelopoiesis.
Interaction of Sialyltransferases in Specification of Sialyl-glycan Structures
The ten to twelve distinct sialic acid linkages that are documented in human and in mouse are synthesized by sialyltransferases encoded on 18 distinct genes. Gal(b1,4)GlcNAc-R structure, present on O-linked and N-linked glycans, are ubiquitous features on most mammalian cell surfaces, and can be modified by a number of sialyltransferases. ST6Gal sialyltransferase elaborates the a2,6-sialyl linkage to generate the Sia(a2,6)Gal(b1,4)GlcNAc-R structure that serves as ligand for a number of cell surface receptors, most notably CD22, an accessory molecule to the B cell receptor complex. The Sia(a2,3)Gal(b1,4)GlcNAc-R structure, in contrast, is synthesized by 2 or more a2,3-sialyltransferases, ST3Gal-3 and ST3Gal-4. The Sia(a2,3)Gal(b1,4)GlcNAc-R structure serves as precursors to ligands recognized by the Selectin family of carbohydrate binding receptors. Each sialyltransferase is encoded on a distinct gene and has unique developmental and cellular patterns of expression. In order to understand the contribution of the individual sialyltransferases in the specification of functionally divergent sialyl-glycans, we are embarking on the detailed characterization of these enzymes. This project, performed in collaboration with Dr. Khushi Matta from the Biophysics Department, is divided into three parts.
We are generating soluble, recombinant ST6Gal and ST3Gal sialyltransferases to test the specificity of these enzymes towards an exhaustive array of chemically synthesized acceptor compounds.
In order to assess the interplay between these enzymes within the context of a cell, we have undertaken generation of cultured cell lines expressing varying ratios of the cognate sialyltransferases. We will assess the ability of these sialyltransferases to compete for shared acceptor substrates.
In order to assess the interplay amongst these enzymes within the context of a whole animal, we will generate mutant mice by transgenic means that over-expresses these sialyltransferases. The consequence of deregulated expression on overall sialyl-glycan composition (as well as function) will be assessed. We have recently obtained a mouse line constitutively over-expressing the ST6Gal sialyltransferase. Phenotypic analysis on this mutant mouse will commence shortly.