Extracellular matrix nano-mechanics determines megakaryocyte function

Cell contact with extracellular matrix proteins (ECMs) leads to activation of specific biochemical signaling pathways and to cyto-skeletal modifications that regulate processes such as cell differentiation, migration and apoptosis. Mechanical properties of ECMs play an important role in determining cells behavior during these processes. In the bone marrow, ECMs concur to the generation of cues that are important for hemopoietic stem cells maturation and differentiation. Sites around the endosteal bone and vascular districts have been proposed as critical niches for stem cell differentiation into megakaryocytes (Mks). In this scenario, fibrillar type I collagen seems to be a key regulator of platelet release, as in vitro adhesion of Mks to this protein inhibits platelet release through the generation of a bulk cell contraction mediated by mechano-sensitive proteins, such as fibronectin, Rho-GTPase and myosin, that lead to cell spreading overtime. In this work we have used a chemical modified collagen that completely override in vitro collagen ligand pathways in directing Mks response in term of cell spreading, migration, platelet release and fibronectin assembly. This different behavior seems to be related to the different nano-mechanical properties of modified collagen with respect to native protein. In particular, N-acetylation of lysine side chains of collagen blocks the formation of banded fibrils and self-aggregation leading to differences in the in vitro sopramolecular organization. Atomic force microscopy analysis of Mks interaction with collagens clearly demonstrated that absence of fibrils, despite similar integrin engagement, and different mechanical properties of these proteins, regulate Mks behaviour and fate. New insights into signaling pathways and in mechano-sensing systems of cells need to be addressed but nanoscale mechanical properties of ECMs seem to have an important role in regulating megakaryocyte behavior in vitro and probably in vivo.