Biosynthesis and Processing of Platelet Glycoproteins in Megakaryocytes

Extracellular matrix receptors and the differentiation of human megakaryocytes in vitro

We investigated the expression and functions of extracellular matrix receptors (or integrins) in the course of the differentiation of human megakaryocytes (Mks) leading to the formation of platelets. Integrins beta1 or Very Late Antigens (VLA) are specialized transmembrane receptors allowing the attachment of the cells to collagen (VLA-2), fibronectin (VLA-4 and -5) and laminin (VLA-6). A proportion of committed megakaryocytic progenitor cells (CFU-MK) adhere to fibronectin but not to collagen or laminin. The early immature Mks are retained on fibronectin (30%) and laminin (12%) but not on collagen whereas large mature Mks are still adherent to fibronectin and laminin and also acquired the capacity to adhere to collagen. The expression of the different VLA in the maturation of Mks correlates well with their adhesive properties. Hence, VLA-2 is not expressed on immature Mks but is present on the mature polyploid cells. VLA-4 is detected only on immature Mks which do not seem to bear VLA-5, while this last integrin appears on late Mks. VLA-6 showed a broad distribution from the early to late stages of Mks differentiation. Integrins beta3 of the cytoadhesin family are represented by alphaIIb beta3 that is the receptor for fibrinogen and alphaV beta3 which mediates adhesion to vitronectin. AlphaIIb beta3 is present on the CFU-MK and highly expressed throughout the Mks maturation stages while alphaV beta3 expression is much lower and seems to be detected only on the late Mks. The regulation of the expression of these receptors by cytokines and their respective roles in the maturation of Mks and the final production of platelets, are discussed. The development of efficient culture systems of human Mks in the presence of the recently cloned thrombopoietin will undoubtedly help to shed more light on the molecular mechanisms of their interactions via integrins with the BM microenvironment.