Matrix stiffness controls megakaryocyte adhesion, fibronectin fibrillogenesis and proplatelet formation through Itgβ3.

Megakaryocytes are the precursor cells of platelets, located in the bone marrow. Once mature, they extend elongated projections named proplatelets through sinusoid vessels, emerging from the marrow stroma into the circulating blood. Not all signals from the microenvironment that regulate proplatelet formation are understood, particularly those from the bone marrow biomechanics. We sought here to investigate how megakaryocytes perceive and adapt to modifications of the stiffness of their environment. The bone marrow is one of the softest tissue of the body while its rigidification resulting from excess fibronectin and other matrix protein deposition occur upon myelofibrosis. Here we have shown that mouse megakaryocytes are able to detect the stiffness of a fibronectin-coated substrate and adapt their morphology accordingly. Using a polydimethylsiloxane (PDMS) substrate with stiffness varying from physiological to pathological marrow, we found that a stiff matrix favors spreading, intracellular contractility and fibronectin fibrils assembly at the expense of proplatelet formation. Itgb3 but not Itgb1 is required for stiffness sensing, whereas both integrins are involved in fibrils assembly. Conversely, soft substrates promote proplatelet formation in an Itgb3-dependent manner, consistent with the ex vivo decrease in proplatelet formation and the in vivo decrease in platelet number in Itgb3-deficient mice. Our findings demonstrate the importance of environmental stiffness for megakaryocyte functions with potential physiopathological implications during pathologies that deregulates fibronectin deposition and modulates stiffness in the marrow.