Key Points Natural silk protein sponge and vascular tubes reproduce human bone marrow niche environments for functional platelet generation ex vivo. Programmable bioengineered model for the investigation and therapeutic targeting of altered platelet formation.

Summary. Background: Megakaryocytes release platelets from the tips of cytoplasmic extensions, called proplatelets. In humans, the regulation of this process is still poorly characterized. Objective: To analyse the regulation of proplatelet formation by megakaryocyte adhesion to extracellular adhesive proteins through different membrane receptors. Methods: Human megakaryocytes were obtained by differentiation of cord blood-derived CD34 + cells, and proplatelet formation was evaluated by phase contrast and fluorescence microscopy. Results: We found that human megakaryocytes extended proplatelets in a time-dependent manner. Adhesion to fibrinogen, fibronectin or von Willebrand factor (VWF) anticipated the development of proplatelets, but dramatically limited both amplitude and duration of the process. Type I, but not type III or type IV, collagen totally suppressed proplatelet extension, and this effect was overcome by the myosin IIA antagonist blebbistatin. Integrin aIIbb3 was essential for megakaryocyte spreading on fibrinogen or VWF, but was not required for proplatelet formation. In contrast, proplatelet formation was prevented by blockade of GPIb-IX-V, or upon cleavage of GPIba by the metalloproteinase mocarhagin. Membraneassociated VWF was detected exclusively on proplateletforming megakaryocytes, but not on round mature cells that do not extend proplatelets. Conclusions: Our findings show that proplatelet formation in human megakaryocytes undergoes a complex spatio-temporal regulation orchestrated by adhesive proteins, GPIb-IX-V and myosin IIA.

Megakaryocytes associate with the bone marrow vasculature where they convert their cytoplasm into proplatelets that protrude through the vascular endothelium into the lumen and release platelets. The extracellular matrix (ECM) microenvironment plays a critical role in regulating these processes. In this work we demonstrate that, among bone marrow ECM components, fibronectin, type IV collagen and laminin are the most abundant around bone marrow sinusoids and constitute a peri-cellular matrix surrounding megakaryocytes. Most importantly, we report, for the first time, that megakaryocytes express components of the basement membrane and that these molecules contribute to the regulation of megakaryocyte development and bone marrow ECM homeostasis both in vitro and in vivo. In vitro, fibronectin induced a three-fold increase in the proliferation rate of mouse hematopoietic stem cells leading to higher megakaryocyte output with respect to cells treated only with thrombopoietin or other matrices. However, megakaryocyte ploidy level in fibronectin-treated cultures was significantly reduced. Stimulation with type IV collagen resulted in a 1.4-fold increase in megakaryocyte output, while all tested matrices supported proplatelet formation to a similar extent in megakaryocytes derived from fetal liver progenitor cells. In vivo, megakaryocyte expression of fibronectin and basement membrane components was up-regulated during bone marrow reconstitution upon 5-fluorouracil induced myelosuppression, while only type IV collagen resulted up-regulated upon induced thrombocytopenia. In conclusion, this work demonstrates that ECM components impact megakaryocyte behavior differently during their differentiation and highlights a new role for megakaryocyte as ECM-producing cells for the establishment of cell niches during bone marrow regeneration.

The mechanisms by which megakaryocytes (MKs) differentiate and release platelets into the circulation are not well understood. However, growing evidence indicates that a complex regulatory mechanism involving MK-matrix interactions may contribute to the quiescent or permissive microenvironment related to platelet release within bone marrow. To address this hypothesis, in this study we demonstrate that human MKs express and synthesize cellular fibronectin (cFN) and transglutaminase factor XIII-A (FXIII-A). We proposed that these 2 molecules are involved in a new regulatory mechanism of MK-type I collagen interaction in the osteoblastic niche. In particular, we demonstrate that MK adhesion to type I collagen promotes MK spreading and inhibits pro-platelet formation through the release and relocation to the plasma membrane of cFN. This regulatory mechanism is dependent on the engagement of FN receptors at the MK plasma membrane and on transglutaminase FXIII-A activity. Consistently, the same mechanism regulated the assembly of plasma FN (pFN) by adherent MKs to type I collagen. In conclusion, our data extend the knowledge of the mechanisms that regulate MK-matrix interactions within the bone marrow environment and could serve as an important step for inquiring into the origins of diseases such as myelofibrosis and congenital thrombocytopenias that are still poorly understood. (Blood. 2011;117(8):2476-2483) IntroductionHemopoietic stem cells reside in bone marrow-specialized niches that dictate how they differentiate, proliferate, mature, and enter the peripheral circulation. [1][2][3][4] Megakaryocyte (MK) maturation and platelet generation are consequent to MK migration from the osteoblastic to the vascular niche, where MKs extend pro-platelets and newly generated platelets are released into the bloodstream. 5,6 The characteristics of the microenvironment surrounding MKs play an important role in the regulation of platelet production within the bone marrow. 3,7 In particular, the interaction of MKs with different extracellular matrices (ECMs) that fill the bone marrow spaces seems to orchestrate their maturation in specific sites. 8 It has been demonstrated that interactions of primary human MKs with matrices thought to fill the vascular niche, such as fibrinogen or von Willebrand factor, are able to sustain MK maturation and pro-platelets, whereas type I collagen totally suppresses these events and prevents premature platelet release in the osteoblastic niche. 7,9 The negative regulation of pro-platelets by type I collagen is mediated by the interaction with the integrin ␣2␤1 and involves the Rho/ROCK pathway. 10,11 However, the exact sequence of events that determines the interaction of MKs with the ECM, and therefore their regulation, is not completely understood. 12 Recent studies 13 have demonstrated that the encounter between a cell and an adhesive substrate involves an initial passive interaction characterized by cell adhesion and spreading, followed by an active stage that involves actin polymerization and...

To cite this article: Balduini A, Malara A, Pecci A, Badalucco S, Bozzi V, Pallotta I, Noris P, Torti M, Balduini CL. Proplatelet formation in heterozygous Bernard-Soulier syndrome type Bolzano. J Thromb Haemost 2009; 7: 478À84.Summary. Background: Although mutations of GPIba are among the most frequent causes of inherited platelet disorders, the mechanisms for the onset of thrombocytopenia and platelet macrocytosis are still poorly defined. Objective: In this work we analyzed in vitro megakaryocyte differentiation and proplatelet formation in six subjects heterozygous for the Ala156Val mutation in the GPIba (Bolzano mutation). Methods: Human megakaryocytes were obtained by differentiation of patient cord blood-derived CD34 + cells and peripheral blood-derived CD45 + cells. Proplatelet formation was evaluated by phase contrast and fluorescence microscopy. Results: Megakaryocyte differentiation from both cord blood (one patient) and peripheral blood (five patients) was comparable to controls. However, proplatelet formation was reduced by about 50% with respect to controls. An identical defect of proplatelet formation was observed when megakaryocytes were plated on fibrinogen, von Willebrand factor or grown in suspension. Morphological evaluation of proplatelet formation revealed an increased size of proplatelet tips, which was consistent with the increased diameters of patientsÕ blood platelets. Moreover, atubulin distribution within proplatelets was severely deranged. Conclusions: Megakaryocytes from patients carrying a Bolzano allele of GPIba display both quantitative and qualitative abnormalities of proplatelet formation in vitro. These results suggest that a defect of platelet formation contributes to macrothrombocytopenia associated to the Bolzano mutation, and indicate a key role for GPIba in proplatelet formation.

Extracellular matrix (ECM) components initiate crucial biochemical and biomechanical cues that are required for bone marrow homeostasis. In our research, we prove that a peri-cellular matrix composed primarily of type III and type IV collagens, and fibronectin surrounds human megakaryocytes in the bone marrow. The data we collected support the hypothesis that bone marrow megakaryocytes possess a complete mechanism to synthesize the ECM components, and that thrombopoietin is a pivotal regulator of this new function inducing transforming growth factor-b1 (TGF-b1) release and consequent activation of the downstream pathways, both in vitro and in vivo. This activation results in a dose dependent increase of ECM component synthesis by megakaryocytes, which is reverted upon incubation with JAK and TGF-b1 receptor specific inhibitors. These data are pivotal for understanding the central role of megakaryocytes in creating their own regulatory niche within the bone marrow environment. STEM CELLS 2016;34:1123-1133 SIGNIFICANCE STATEMENTBone marrow is a finely organized organ formed by extracellular matrix (ECM) components and various cell types. Increasing evidences are highlightening the importance of the ECM components in the regulation of different aspects of cell behavior. In this regard, we recently demonstrated that mouse megakaryocytes, the bone marrow resident platelet progenitors, are able to produce ECM components which in turn regulate their function. Here we extend this observation by describing the mechanism by which human and mouse megakaryocytes produce those ECM components in vitro and in vivo.

BackgroundThe interaction of megakaryocytes with matrix proteins of the osteoblastic and vascular niche is essential for megakaryocyte maturation and proplatelet formation. Fibrinogen is present in the vascular niche and the fibrinogen receptor αIIbβ3 is abundantly expressed on megakaryocytes, however the role of the interaction between fibrinogen and αIIbβ3 in proplatelet formation in humans is not yet fully understood. We have recently reported a novel congenital macrothrombocytopenia associated with a heterozygous mutation of the β3 subunit of αIIbβ3. The origin of thrombocytopenia in this condition remains unclear and this may represent an interesting natural model to get further insight into the role of the megakaryocyte fibrinogen receptor in megakaryopoiesis.Methodology/Principal FindingsPatients' peripheral blood CD45+ cells in culture were differentiated into primary megakaryocytes and their maturation, spreading on different extracellular matrix proteins, and proplatelet formation were analyzed. Megakaryocyte maturation was normal but proplatelet formation was severely impaired, with tips decreased in number and larger in size than those of controls. Moreover, megakaryocyte spreading on fibrinogen was abnormal, with 50% of spread cells showing disordered actin distribution and more evident focal adhesion points than stress fibres. Integrin αIIbβ3 expression was reduced but the receptor was constitutively activated and a sustained, and substrate-independent, activation of proteins of the outside-in signalling was observed. In addition, platelet maturation from preplatelets was impaired.Conclusions/SignificanceOur data show that constitutive activation of αIIbβ3-mediated outside-in signalling in human megakaryocytes negatively influences proplatelet formation, leading to macrothombocytopenia.

SUMMARY Megakaryocytes are rare cells found in the bone marrow, responsible for the everyday production and release of millions of platelets into the bloodstream. Since the discovery and cloning, in 1994, of their principal humoral factor, thrombopoietin, and its receptor c-Mpl, many efforts have been directed to define the mechanisms underlying an efficient platelet production. However, more recently different studies have pointed out new roles for megakaryocytes as regulators of bone marrow homeostasis and physiology. In this review we discuss the interaction and the reciprocal regulation of megakaryocytes with the different cellular and extracellular components of the bone marrow environment. Finally, we provide evidence that these processes may concur to the reconstitution of the bone marrow environment after injury and their deregulation may lead to the development of a series of inherited or acquired pathologies.