Megakaryocytes Contribute To The Establishment Of The Bone Marrow Environment By Expressing Extracellular Matrix proteins
Authors: Malara Alessandro , Gruppi Cristian , Currao Manuela , Balduini Alessandra 
Published: October 2019

Abstract

Introduction
the bone marrow microenvironment consists of various types of cells and their secreted extracellular matrix components that surround capillary-venous sinusoids, and plays a key role in the regulation of hematopoiesis. In general, extracellular matrix components interact with each other to form a structural framework that supports tissue organization and positional cues that regulate cellular processes. Megakaryocytes are rare cells in the bone marrow and, besides platelet release, growing evidences attribute new functions to these cells in the generation and maintenance of the bone marrow cell niche. Recent evidences, by our group, demonstrated that megakaryocytes are involved in matrix deposition and remodeling, as demonstrated by their role in fibronectin fibrillogenesis and the expression of matrix cross-linking enzymes, such as factor XIIIa, essential in the dynamic of megakaryocyte-matrix component interactions. Interestingly, individual extracellular matrix components were demonstrated to play a role in the regulation of megakaryocytes development in vitro. Fibronectin was shown to regulate megakaryocyte maturation and proplatelet extension, while type III and type IV collagens were demonstrated to support proplatelet formation in vitro. In contrast, type I collagen is an important physiological inhibitor of platelet release in vitro. However, little is known about the exact localization as well as function of these matrix components in vivo.

Results
in this work we have analyzed the spatial distribution of megakaryocytes and extracellular matrix components by immunofluorescence in murine femur sections. We found that megakaryocytes were predominantly located in the femur diaphysis with only 20% of megakaryocytes within 50μm from the endosteal surface and more than 80% of megakaryocytes located less than 50 μm from a sinusoid. Correlation between megakaryocyte distance from sinusoids and dimension suggested a gradient of maturing megakaryocytes towards the vascular niche. Next, we deciphered bone marrow extracellular matrix component composition by western blotting and mapped the location in situ of different collagens (I, III, IV, VI) and glycoproteins (fibronectin, laminin). We found that all these proteins were differently located in the endosteal and sinusoidal districts supporting the concept that regulation of hemopoiesis, in the bone marrow, may also depend from matrix distribution. Further, we showed, for the first time, that megakaryocytes were surrounded by a pericellular matrix mainly composed of fibronectin, laminin and type IV collagen. Interestingly, these three proteins were also demonstrated to promote thrombopoietin-dependent megakaryocyte differentiation in in vitro cultures of bone marrow hemopoietic progenitor cells. Finally, fibronectin, laminin and type IV collagen were also demonstrated to be expressed and synthesized by differentiated megakaryocytes in vitro as demonstrated by PCR and western blotting analysis. Most importantly, megakaryocyte expression of these extracellular matrix components was up-regulated in vivo during bone marrow reconstitution upon drug induced myelosuppression and, at a lesser extent, thrombocytopenia.

Conclusions
all together these results suggested that megakaryocytes are important extracellular matrix component-producing bone marrow cells and that released extracellular matrix components support megakaryopoiesis and concur to the generation of bone marrow niches.

Disclosures:
No relevant conflicts of interest to declare.