Differentiation of Hematopoietic Stem Cell Modulated by Actomyosin Forces

Abstract

Specificity of fate decisions during stem cell differentiation appears determined in part by biophysical processes that include cellular contractility and matrix elasticity, and we had previously demonstrated that human mesenchymal stem cells (MSCs) specify lineage based on these cues [Engler et al., Cell 2006]. Here, we show the importance of actomyosin force as a central node in regulation of cell division, membrane stability and matrix-elasticity sensing during human hematopoietic stem cell (HSC) differentiation. Myosin is required for the survival of proliferating myeloid progenitors, while long-term primitive HSCs are resistant to myosin inhibition. In contrast, inhibition of cellular contractility facilitates megakaryocyte (MK) maturation by polyploidization and fragmentation into functional platelets in vivo (median fragmentation threshold at ∼1mN/m in vitro). In addition, differences in local tissue stiffness - such as exists between cortical bone and marrow - likely contribute to HSC differentiation because in vitro adhesion to stiff (34kPa) collagenous matrices tends to inhibit MK maturation whenever myosin is inhibited, while soft (0.3kPa) matrices with lower amount of collagens tend to facilitate this process. Quantitative mass spectrometry and confocal microscopy indicate both global remodeling of cytoskeletal proteomes and extensive lamin network formation during MK maturation, providing MKs in bone marrow with an ideal structure to shed platelets into permeating capillaries. Together, these data show that cell relaxation and soft matrices maintain primitiveness of HSCs and drive MK differentiation, whereas the opposite physical cues support HSC differentiation into myeloid progenitors.