Novel Developmental, Cellular and Biochemical Functions of Fucosylated Glycans in Mammals.

Abstract

Cell surface glycans are branched structures composed of linear and branched chains of monosaccharides that include fucose. Fucose has important biological functions during ontogeny and cellular differentiation, suggested by pathological phenotypes observed in different strains of mice, including mice with a null mutation in the 3’, 5’-epimerase/4’-reductase locus (FX-/- mice). FX-/- mice are conditionally deficient in all fucosylated glycans, and exhibit server thymic atrophy. This phenotype is cell autonomous and characterized by a fucose-dependent complete deficiency of mature T cell with loss of early thymic progenitors. Interestingly, Notch1 signaling deficient mouse models have been reported with the similar phenotypes, and there are reports that O-fucosylation is required in some in vitro experiments for Notch1 signaling. These results strongly suggested that lack of Notch1 signaling in FX-/- progenitors accounts for the thymic atrophy phenotype. By introducing the Notch1 intracellular domain into fucose-deficient FX-/- bone marrow-derived lymphoid progenitors, the T cell developmental defect characteristic of FX-/- lymphoid progenitors is rescued and thus implicated a fucose-dependent requirement for Notch1 signaling in this process. In vitro, OP9 cells that are bearing Notch ligand of Delta-like1 (Dll-1), Delta-like 4 (Dll-4) or Jagged2, instruct cells to assume a T lymphoid differentiation identity, whereas while OP9 cells bearing Jagged1 or Dll3 do not. However, in fucose-deficient experimental situation, FX-/- bone marrow cells fail to assume a T lineage identity when co-cultured with OP9-Dll1, OP9-Dll4 or OP9-Jagged2, and fail to initiate Notch1 signaling events. These results indicate that fucosylation is required for Notch1 signaling-dependent T cell differentiation. In effort to define the molecular mechanisms that account for fucosylation-controlled Notch1 signaling activation, I conclude from a series of experiments that (1) fucosylation controls the strength of binding between Notch1 and its ligand Dll4, and (2) fucosylation controls Notch1 receptor density at the surface of an E2a/Pbx1 immortalized bone marrow progenitor cell line. These results indicate that fucosylation controls Notch1 signaling strength and thus regulates the development of T lymphocytes. My studies have revealed novel biochemical, cellular and developmental functions of fucosylation in the development and signaling pathways characteristic of the mammalian lymphoid lineage.