Importance of environmental stiffness for megakaryocyte differentiation and proplatelet formation

Mechanical forces direct stem cell behaviour in development and regeneration

Stem cells and their local microenvironment, or niche, communicate through mechanical cues to regulate cell fate and cell behaviour and to guide developmental processes. During embryonic development, mechanical forces are involved in patterning and organogenesis. The physical environment of pluripotent stem cells regulates their self-renewal and differentiation. Mechanical and physical cues are also important in adult tissues, where adult stem cells require physical interactions with the extracellular matrix to maintain their potency. In vitro, synthetic models of the stem cell niche can be used to precisely control and manipulate the biophysical and biochemical properties of the stem cell microenvironment and to examine how the mode and magnitude of mechanical cues, such as matrix stiffness or applied forces, direct stem cell differentiation and function. Fundamental insights into the mechanobiology of stem cells also inform the design of artificial niches to support stem cells for regenerative therapies.

In vitro generation of platelets: Where do we stand?

Millions of platelets, specialized cells that participate in haemostatic and inflammatory functions, are transfused each year worldwide, but their supply is limited. Platelets are produced by megakaryocytes by extending proplatelets, directly into the bloodstream. Bone marrow structure and extracellular matrix composition together with soluble factors (e.g. Thrombopoietin) are key regulators of megakaryopoiesis by supporting cell differentiation and platelet release. Despite this knowledge, the scarcity of clinical cures for life threatening platelet diseases is in a large part due to limited insight into the mechanisms that control the developmental process of megakaryocytes and the mechanisms that govern the production of platelets within the bone marrow. To overcome these limitations, functional human tissue models have been developed and studied to extrapolate ex vivo outcomes for new insight on bone marrow functions in vivo. There are many challenges that these models must overcome, from faithfully mimicking the physiological composition and functions of bone marrow, to the collection of the platelets generated and validation of their viability and function for human use. The overall goal is to identify innovative instruments to study mechanisms of platelet release, diseases related to platelet production and new therapeutic targets starting from human progenitor cells.

A new path to platelet production through matrix sensing

Megakaryocytes (MK) in the bone marrow (BM) are immersed in a network of extracellular matrix components that regulates platelet release into the circulation. Combining biological and bioengineering approaches, we found that the activation of transient receptor potential cation channel subfamily V member 4 (TRPV4), a mechano-sensitive ion channel, is induced upon MK adhesion on softer matrices. This response promoted platelet production by triggering a cascade of events that lead to calcium influx, β1 integrin activation and internalization, and Akt phosphorylation, responses not found on stiffer matrices. Lysyl oxidase (LOX) is a physiological modulator of BM matrix stiffness via collagen crosslinking. In vivo inhibition of LOX and consequent matrix softening lead to TRPV4 activation cascade and increased platelet levels. At the same time, in vitro proplatelet formation was reduced on a recombinant enzyme-mediated stiffer collagen. These results suggest a novel mechanism by which MKs, through TRPV4, sense extracellular matrix environmental rigidity and release platelets accordingly.

The role of the extracellular matrix in primary myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm that arises from clonal proliferation of hematopoietic stem cells and leads to progressive bone marrow (BM) fibrosis. While cellular mutations involved in the development of PMF have been heavily investigated, noteworthy is the important role the extracellular matrix (ECM) plays in the progression of BM fibrosis. This review surveys ECM proteins contributors of PMF, and highlights how better understanding of the control of the ECM within the BM niche may lead to combined therapeutic options in PMF.