Extracellular matrix components
One of the most abundant components of the bone marrow space, besides cells, is a variety of extracellular matrix components. On selective binding, this environment, in combination with soluble cytokines, regulates haemopoietic progenitor proliferation and differentiation. Haemopietic progenitors develop as adherent cells in contact with extracellular matrix components in the bone marrow until they are released as non-adherent cells into the circulating blood.
Our lab has pursued for long time the hypothesis that the interaction of megakaryocytes with bone marrow extracellular matrix components contributes to the regulation of megakaryocyte function. We have demonstrated that some collagen type can support proplatelet formation, while type I collagen is the only extracellular matrix environment that inhibit this process. Our recent evidence indicates that these differences may be ascribed to peculiar structural properties of the collagens, as well as to differences in receptor engagement. By Atomic Force Microscopy we have documented that the tensile strength of fibrils in type I collagen structure is a fundamental requirement to regulate cytoskeleton contractility of human megakaryocytes through activation of the Rho-ROCK pathway and MLC-2 phosphorylation.
Recently, we have extended this study in vivo and we have demonstrated that, among bone marrow extracellular matrix components, fibronectin, type IV collagen and laminin are the most abundant around bone marrow sinusoids and constitute a peri-cellular matrix surrounding megakaryocytes. Most importantly, we have shown, for the first time, that megakaryocytes express components of the basement membrane and that these molecules contribute to the regulation of megakaryocyte development and bone marrow extracellular matrix homeostasis both in vitro and in vivo. Overall, we have deciphered the extracellular matrix component composition of the megakaryocyte environment and demonstrated that these cells express basement membrane proteins in close proximity to sinusoidal endothelial cells. Importantly, we also showed that extracellular matrix components differently modulate megakaryocyte development in vitro, reflecting the importance of their spatial localization and cell interactions in vivo. Finally, we demonstrated that the production of endogenous extracellular matrix components is not related to the physiological production of platelets in vivo but significantly boosted concomitantly to the regeneration of bone marrow environment following myelosuppression.
Extracellular matrix component Receptor
Cells use a wide spectrum of proteins and mechanisms to recognize their environment. Evidence demonstrates that extracellular matrix components receptors can be used by Mks to control the site of platelet formation and release. Whether and how extracellular matrix component receptor activity is regulated during thrombopoiesis in vivo is not known. To progress towards a better understanding of these critical mechanisms, significantly improved knowledge of the physical, cellular and biochemical interactions in the bone marrow environment is needed. Integrins are the major human receptors for cell adhesion on extracellular matrix components, however a variety of other interaction mechanisms are possible. Besides integrin alpha2beta1 and GPVI, expression and function of other collagen receptors on human megakaryocytes are unknown. Discoidin domain receptors (DDR1 and DDR2) are tyrosine-kinase collagen receptors that are stimulated by fibrillar and basement membrane collagens and mediate cell adhesion and migration in different tissues. We recently discovered that DDR1 is expressed by both human megakaryocytes and platelets. DDR1 is activated upon megakaryocyte adhesion on fibrillar type I collagen and regulates megakaryocyte Syk-mediated migration through activation of the tyrosine phosphatase SHP1. Altogether, these data point out that DDR1 may represent an important new regulator of megakaryocyte function.