Circulatory Glycan-Modifying Enzymes As Systemic Regulators In Blood Cell Development

Regulating blood cell production is crucial to meeting physiologic demands and to maintaining hematopoietic equilibrium. Elucidating the systemic factors and how they contribute to the interaction of hematopoietic progenitors with their bone marrow microenvironments is crucial to the mechanistic understanding of hematopoietic regulation and development of novel therapeutic strategies. Glycosyltransferases are enzymes that normally reside within the intracellular secretory apparatus to assemble glycans on nascent proteins and lipids in transit, but they are also present in abundance in the extracellular spaces, especially in systemic circulation. One such blood-borne enzyme is the ST6Gal-1 sialyltransferase that mediates the attachment of sialic acids in a6-linkages to glycoproteins. By virtue of their presence on cell surfaces, sialyl-glycans, which are conserved structures in mammals, occupy the critical interphase between cells and their external environment. We hypothesize that remodeling of these cell surface glycans can drastically modulate the communication of systemic cues among marrow hematopoietic cells. A mouse model genetically modified for low circulating ST6Gal-1 has increased numbers of hematopoietic progenitors, increased ability to produce inflammatory cells upon acute challenge, and faster recovery from cyclophosphamide-induced myelosuppression. In ex vivo cultures, addition of ST6Gal-1 strikingly delays expansion and differentiation of murine LSK (Lin^neg sca-1^pos cKit^pos) and human CD34^pos cells. We developed a subcutaneously implantable B16-F10 model genetically modified to overexpress circulatory ST6Gal-1 (B16^st6gal1). Mice bearing B16^st6gal1 had >10-fold elevation in circulating ST6Gal-1 levels, accompanied by >50% reduction in marrow granulocyte and B cell populations. B16^st6gal1 bearing mice also had 2-fold increase in the number of Lin^neg cells, suggesting a blockade in development of progenitors into end-effector cells. Murine LSK cell surfaces are decorated with a6-sialic acids but they do not endogenously express ST6Gal-1. Bone chimeras show that the a6-sialic acids on LSK surfaces require the distally synthesized ST6Gal-1 enzyme in circulation. Taken together, our data indicates the glycan-modifying enzyme, the ST6Gal-1 in circulation, is a potent systemic regulator of hematopoiesis by remodeling of cell surface glycan architecture of marrow hematopoietic stem and progenitors. (Funded by NIH Program of Excellence in Glycosciences award P01HL107146 and NIH R01AI56082)