No more sugar-coating it: mucins and platelet biology

In this issue of Blood, Kudo et al demonstrate a critical role for mucin-type O-glycans in terminal differentiation of megakaryocytes and platelet release, using conditional knockout mouse for the catalytic enzyme for O-glycosylation. This article provides a model for understanding what role such near ubiquitous modifiers may have in terminal hematopoietic differentiation.¹ 

For platelet production, stem cells differentiate into immature megakaryocytes that then further differentiate into mature megakaryocytes with unique features, including their large size and polyploidism.²  Terminally differentiated mature megakaryocytes undergo thrombopoiesis with the release of proplatelets and platelets. These complex processes of megakaryopoiesis and thrombopoiesis are unique, but their molecular mechanisms are only partially understood. A better understanding of these terminal steps may allow the generation of sufficient platelets to treat clinically relevant thrombocytopenia. Partial success has been achieved in generating platelets from ex vivo–generated megakaryocytes from a variety of sources including hematopoietic stem cells, fetal liver cells, embryonic stem cells, induced pluripotent stem cells, NF-E2–transduced fibroblasts, and preadipocytes.^3-7  One common problem with each of these approaches is that the number of platelets released per terminal megakaryocyte is low.

One area that has been underappreciated during megakaryopoiesis and thrombopoiesis involves pathways that modify both intracellular proteins and surface receptors. Kudo et al have focused on the role of O-glycosylation in these processes. Glycosylation has been well known as a common modification of many proteins resulting in a greater diversity of protein biologies.⁸  There are two main types of glycosylation: O-linked and N-linked. Accumulating evidence has shown a critical role for mucin-type O-glycosylation in protein stability, processing, and function.⁸  Abnormal mucin-type O-glycosylation has reportedly been associated with pathologic mechanisms.⁸  Mucin-type O-glycosylation initially begins by addition of an N-acetylgalactosamine (GalNac) sugar to Ser or Thr residues.⁸  Although several different glycan structures can be produced by extension of different sugar chains after the initial addition of GalNac, the most common extension is the core 1 or T-antigen structure.⁸  This extension is catalyzed by C1galt1. Kudo et al have generated conditional knockout mice with a C1galt1 gene deletion restricted to bone marrow cells (Mx1-C1 mice) since complete knockout of C1galt1 is lethal in mice. These Mx1-C1 mice had normal megakaryocyte maturation but little proplatelet formation and decreased platelet production. White and red cell counts were normal. Mx-1 mice had giant platelets, and this may be related to the fact that glycoprotein (GP) Ibα is a major O-glycoprotein, and this glycosylation is important for GPIbα stability, and consequently, important for the stability of the GPIb/IX/V receptor complex. The macroglycopeptide domain with its O-glycosylation sites in GPIbα is associated with regulating GPIbα stability. Deficiency of this receptor in turn is known to result in quantitative and qualitative abnormalities of the resultant platelets.⁹  Thus, the authors have naturally put a great deal of focus on this pathway. Their studies ascribe a major portion of the observed phenotype to this pathway, and this appears to be appropriate, given the known GPIb/IX/V pathobiology in Bernard-Soulier syndrome and that Mx1-C1 mice have characteristics similar to that syndrome. Clearly, glycosylation of other surface receptors and perhaps other proteins may contribute to the observed defect in thrombopoiesis and may have been missed. The findings of Kudo et al set the stage for further research on the mechanism by which GPIbα O-glycosylation results in its instability, contributing to thrombopoiesis and platelet function. It also shows the value of studying the role(s) of both O-type and even N-type glycosylation of other surface receptors in hematopoiesis, megakaryopoiesis, thrombopoiesis, and platelet biology. The technologies to carry out the genomic, proteomic and glycomic studies to screen candidate modifications on hematopoietic differentiation are set. The time is at hand to look at such modifiers in bone marrow biology.