Aberrant Type-2-Lactosaminoglycan Synthesis Severely Impairs Thrombopoiesis

Platelet recovery following myelosuppressive/myeloablative chemotherapy is crucial to avoid bleeding complications of cancer treatment. Platelets are produced by bone marrow megakaryocytes (MKs), which develop and mature from hematopoietic stem cells (HSC). Mature MKs interact with sinusoidal bone marrow endothelial cells to form transendothelial pseudopods called proplatelets from which platelets are released into the bloodstream. Platelet survival is dependent on correct glycan expression. We here investigate the role of Type-2-Lactosaminoglycans (Type-2-LacNAc) in platelet production. beta1,4Galactosyltransferase 1 (beta4GalT1) is a major enzyme involved in Type-2-LacNAc synthesis which adds Galactose (Gal) to terminal N-Acetylglucosamine (GlcNAc) to form beta1,4Gal-GlcNAc (Type-2-LacNAc).beta4GalT1 deficient mice die in uthero between E15.5 and E16.5. A small percentage of beta4GalT1^-/- mice survive until adulthood and they have severe macrothrombocytopenia but normal platelet clearance. Lethally irradiated wild type mice transplanted with beta4GalT1 deficient fetal liver cells failed to produce circulating beta4GalT1 deficient platelets, in marked contrast to beta4GalT1 deficient white blood cells, despite beta4GalT1 deficient MKs have been detected in the bone marrow of transplanted mice. beta4GalT1 deficient fetal liver MKs poorly produce proplatelets in vitro, following their normal maturation and differentiation, as judged by number, morphology, ploidy and expression of main surface glycoproteins. Our data strongly support the notion that glycosylation mediated by beta4GalT1 is crucial for platelet production in vitro and in vivo and demonstrate for the first time a role for post-translational glycan modification in platelet production.