The secret life of a megakaryocyte: emerging roles in bone marrow homeostasis control

The Role of Megakaryocyte Assessment in Bone Marrow Cytology

Despite the progress made in recent years in hematological diagnostics, cytological assessment of bone marrow, including the assessment of megakaryocytes, is still an important element of the diagnostic process. It is one of the most accurate methods for detecting even minor abnormalities, which is often superior to histopathological examination in this respect. Although, according to current recommendations, megakaryocytes are not included in the total percentage formula of the myelogram, the assessment of platelet function, morphological features, and quantitative disorders allows for the direction of further diagnostic process. In this article, we present the principles of assessing megakaryocytes in a bone marrow aspirate smear. We also characterize normal megakaryocytes at various stages of megakaryopoiesis, atypical megakaryocytes in pathological and physiological conditions, and cells with a similar morphology to megakaryocytes.

Distinct protein patterns related to postnatal development in small for gestational age preterm infants

BACKGROUND

Preterm infants, especially those born small for gestational age (SGA), are at risk of short-term and long-term health complications. Characterization of changes in circulating proteins postnatally in preterm infants may provide valuable fundamental insights into this population. Here, we investigated postnatal developmental patterns in preterm infants and explored protein signatures that deviate between SGA infants and appropriate for gestational age (AGA) infants using a mass spectrometry (MS)-based proteomics workflow.

METHODS

Longitudinal serum samples obtained at postnatal days 0, 3, 7, 14, and 28 from 67 preterm infants were analyzed using unbiased MS-based proteomics.

RESULTS

314 out of 833 quantified serum proteins change postnatally, including previously described age-related changes in immunoglobulins, hemoglobin subunits, and new developmental patterns, e.g. apolipoproteins (APOA4) and terminal complement cascade (C9) proteins. Limited differences between SGA and AGA infants were found at birth while longitudinal monitoring revealed 69 deviating proteins, including insulin-sensitizing hormone adiponectin, platelet proteins, and 24 proteins with an annotated function in the immune response.

CONCLUSIONS

This study shows the potential of MS-based serum profiling in defining circulating protein trajectories in the preterm infant population and its ability to identify longitudinal alterations in protein levels associated with SGA.

IMPACT

Postnatal changes of circulating proteins in preterm infants have not fully been elucidated but may contribute to development of health complications. Mass spectrometry-based analysis is an attractive approach to study circulating proteins in preterm infants with limited material. Longitudinal plasma profiling reveals postnatal developmental-related patterns in preterm infants (314/833 proteins) including previously described changes, but also previously unreported proteins. Longitudinal monitoring revealed an immune response signature between SGA and AGA infants. This study highlights the importance of taking postnatal changes into account for translational studies in preterm infants.

In Vitro Assessment of Injectable Bone Marrow Aspirate Concentrates Compared to Injectable Platelet-Rich Fibrin

BACKGROUND

Injectable platelet-rich fibrin (iPRF), a liquid form of PRF that is prepared from peripheral blood without anticoagulants, promotes tissue wound healing and regeneration. The present study focused on iPRF-like bone marrow aspirate concentrate (iBMAC) prepared without anticoagulant, and the regenerative potential of iPRF and iBMAC was compared in vitro.

METHODS

iPRF and iBMAC were prepared from the same New Zealand white rabbits. The cytocompatibility and regenerative potential of each concentrate were evaluated using primary rabbit gingival fibroblasts and osteoblasts.

RESULTS

Both gingival fibroblasts and osteoblasts treated with each concentrate exhibited excellent cell viability. Interestingly, compared to cells treated with iPRF, cells treated with iBMAC demonstrated significantly greater migration potential. Furthermore, higher mRNA levels of transforming growth factor-β (TGF-β), vascular endothelial growth factor (VEGF), and collagen I (COL1) were observed in gingival fibroblasts treated with iBMAC than in those treated with iPRF. Compared with osteoblasts treated with iPRF, osteoblasts treated with iBMAC exhibited greater differentiation potential, as indicated by increased osteocalcin (OCN) expression and mineralization capability.

CONCLUSION

The results of the in vitro study suggest that, compared with iPRF, iBMAC may promote wound healing and bone regeneration more effectively. However, further preclinical and clinical studies are needed to confirm the regenerative potential of iBMAC in the body.

Transcription factor 3 is dysregulated in megakaryocytes in myelofibrosis

Transcription factor 3 (TCF3) is a DNA transcription factor that modulates megakaryocyte development. Although abnormal TCF3 expression has been identified in a range of hematological malignancies, to date, it has not been investigated in myelofibrosis (MF). MF is a Philadelphia-negative myeloproliferative neoplasm (MPN) that can arise de novo or progress from essential thrombocythemia [ET] and polycythemia vera [PV] and where dysfunctional megakaryocytes have a role in driving the fibrotic progression. We aimed to examine whether TCF3 is dysregulated in megakaryocytes in MPN, and specifically in MF. We first assessed TCF3 protein expression in megakaryocytes using an immunohistochemical approach analyses and showed that TCF3 was reduced in MF compared with ET and PV. Further, the TCF3-negative megakaryocytes were primarily located near trabecular bone and had the typical "MF-like" morphology as described by the WHO. Genomic analysis of isolated megakaryocytes showed three mutations, all predicted to result in a loss of function, in patients with MF; none were seen in megakaryocytes isolated from ET or PV marrow samples. We then progressed to transcriptomic sequencing of platelets which showed loss of TCF3 in MF. These proteomic, genomic and transcriptomic analyses appear to indicate that TCF3 is downregulated in megakaryocytes in MF. This infers aberrations in megakaryopoiesis occur in this progressive phase of MPN. Further exploration of this pathway could provide insights into TCF3 and the evolution of fibrosis and potentially lead to new preventative therapeutic targets.

Hematopoietic and leukemic stem cells homeostasis: the role of bone marrow niche

The bone marrow microenvironment (BMM) has highly specialized anatomical characteristics that provide a sanctuary place for hematopoietic stem cells (HSCs) that allow appropriate proliferation, maintenance, and self-renewal capacity. Several cell types contribute to the constitution and function of the bone marrow niche. Interestingly, uncovering the secrets of BMM and its interaction with HSCs in health paved the road for research aiming at better understanding the concept of leukemic stem cells (LSCs) and their altered niche. In fact, they share many signals that are responsible for interactions between LSCs and the bone marrow niche, due to several biological similarities between LSCs and HSCs. On the other hand, LSCs differ from HSCs in their abnormal activation of important signaling pathways that regulate survival, proliferation, drug resistance, invasion, and spread. Targeting these altered niches can help in better treatment choices for hematological malignancies and bone marrow disorders in general and acute myeloid leukemia (AML) in particular. Moreover, targeting those niches may help in decreasing the emergence of drug resistance and lower the relapse rate. In this article, the authors reviewed the most recent literature on bone marrow niches and their relations with either normal HSCs and AML cells/LSC, by focusing on pathogenetic and therapeutic implications.

Megakaryocyte-induced contraction of plasma clots: cellular mechanisms and structural mechanobiology

ABSTRACT

Nonmuscle cell contractility is an essential feature underlying diverse cellular processes such as motility, morphogenesis, division and genome replication, intracellular transport, and secretion. Blood clot contraction is a well-studied process driven by contracting platelets. Megakaryocytes (MKs), which are the precursors to platelets, can be found in bone marrow and lungs. Although they express many of the same proteins and structures found in platelets, little is known about their ability to engage with extracellular proteins such as fibrin and contract. Here, we have measured the ability of MKs to compress plasma clots. Megakaryocytes derived from human induced pluripotent stem cells (iPSCs) were suspended in human platelet-free blood plasma and stimulated with thrombin. Using real-time macroscale optical tracking, confocal microscopy, and biomechanical measurements, we found that activated iPSC-derived MKs (iMKs) caused macroscopic volumetric clot shrinkage, as well as densification and stiffening of the fibrin network via fibrin-attached plasma membrane protrusions undergoing extension-retraction cycles that cause shortening and bending of fibrin fibers. Contraction induced by iMKs involved 2 kinetic phases with distinct rates and durations. It was suppressed by inhibitors of nonmuscle myosin IIA, actin polymerization, and integrin αIIbβ3-fibrin interactions, indicating that the molecular mechanisms of iMK contractility were similar or identical to those in activated platelets. Our findings provide new insights into MK biomechanics and suggest that iMKs can be used as a model system to study platelet contractility. Physiologically, the ability of MKs to contract plasma clots may play a role in the mechanical remodeling of intravascular blood clots and thrombi.

Glucocorticoids regulate thrombopoiesis by remodeling the megakaryocyte transcriptome

BACKGROUND

Glucocorticoids are widely known for their immunomodulatory action. Their synthetic analogs are used to treat several autoimmune diseases, including immune thrombocytopenia. However, their efficacy and mechanisms of action in immune thrombocytopenia are not fully understood.

OBJECTIVES

To investigate the mechanism of glucocorticoid actions on platelet production.

METHODS

The actions of glucocorticoids on platelet production were studied combining in vivo, ex vivo and in vitro approaches.

RESULTS

Dexamethasone reduced bleeding in mice and rapidly increased circulating young platelet counts. In vitro glucocorticoid treatment stimulated proplatelet formation by megakaryocytes and platelet-like particle release. This effect was blocked by glucocorticoid receptor antagonist RU486, indicating a glucocorticoid receptor-dependent mechanism. Genome-wide analysis revealed that dexamethasone regulates the expression of >1000 genes related to numerous cellular functions, including predominant cytoplasm and cytoskeleton reorganization. Dexamethasone and other glucocorticoids induced the expression of Gda (the gene encoding guanine deaminase), which has been reported to have a role in dendrite development. Inhibition of guanine deaminase enzymatic activity blocked dexamethasone stimulation of proplatelet formation, implicating a critical role for this enzyme in glucocorticoid-mediated platelet production.

CONCLUSION

Our findings identify glucocorticoids as new regulators of thrombopoiesis.

The hematopoietic microenvironment: a network of niches for the development of all blood cell lineages

Blood cell formation (hematopoiesis) takes place mainly in the bone marrow, within the hematopoietic microenvironment, composed of a number of different cell types and their molecular products that together shape spatially organized and highly specialized microstructures called hematopoietic niches. From the earliest developmental stages and throughout the myeloid and lymphoid lineage differentiation pathways, hematopoietic niches play a crucial role in the preservation of cellular integrity and the regulation of proliferation and differentiation rates. Current evidence suggests that each blood cell lineage develops under specific, discrete niches that support committed progenitor and precursor cells and potentially cooperate with transcriptional programs determining the gradual lineage commitment and specification. This review aims to discuss recent advances on the cellular identity and structural organization of lymphoid, granulocytic, monocytic, megakaryocytic, and erythroid niches throughout the hematopoietic microenvironment and the mechanisms by which they interconnect and regulate viability, maintenance, maturation, and function of the developing blood cells.

Mean platelet volume and mortality in patients with alcohol use disorder

Several recent studies have pointed out the relationship of platelet size with increased mortality or adverse clinical course. Most studies show that increased mean platelet volume (MPV) may be associated with a deleterious outcome in different settings such as sepsis or neoplasia, whereas other researchers have found the opposite. In inflammatory conditions there is an altered secretion of several cytokines, some of them exerting a marked influence on platelet biogenesis and/or on platelet activation and aggregation. Alcohol use disorder is a chronic situation characterized by a protracted low-grade inflammation. In this study we analyze the relationship between proinflammatory cytokines and MPV and their relationships with mortality in patients with alcohol abuse. We determined serum levels of tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-8 and routine laboratory variables among 184 patients with alcohol use disorder admitted to our hospital and followed-up for a median of 42 months. We found that MPV was inversely related to TNF-α (ρ=-0.34), and directly to IL-8 (ρ=0.32, p<0.001 in both cases) and to IL-6 (ρ=0.15; p = 0.046). Reduced MPV was related both with short-term (<6 months) and long-term mortality. Conclusion: These results suggest that inflammatory cytokines are strongly related to MPV. A low MPV is associated with a poor prognosis among patients with alcohol use disorder.

Megakaryocyte Secreted Factors Regulate Bone Marrow Niche Cells During Skeletal Homeostasis, Aging, and Disease

The bone marrow microenvironment contains a diverse array of cell types under extensive regulatory control and provides for a novel and complex mechanism for bone regulation. Megakaryocytes (MKs) are one such cell type that potentially acts as a master regulator of the bone marrow microenvironment due to its effects on hematopoiesis, osteoblastogenesis, and osteoclastogenesis. While several of these processes are induced/inhibited through MK secreted factors, others are primarily regulated by direct cell-cell contact. Notably, the regulatory effects that MKs exert on these different cell populations has been found to change with aging and disease states. Overall, MKs are a critical component of the bone marrow that should be considered when examining regulation of the skeletal microenvironment. An increased understanding of the role of MKs in these physiological processes may provide insight into novel therapies that can be used to target specific pathways important in hematopoietic and skeletal disorders.

Inside-to-outside and back to the future of megakaryopoiesis

A State of the Art lecture titled "Megakaryocytes and different thrombopoietic environments" was presented at the ISTH Congress in 2022. Circulating platelets are specialized cells produced by megakaryocytes. Leading studies point to the bone marrow niche as the core of hematopoietic stem cell differentiation, revealing interesting and complex environmental factors for consideration. Megakaryocytes take cues from the physiochemical bone marrow microenvironment, which includes cell-cell interactions, contact with extracellular matrix components, and flow generated by blood circulation in the sinusoidal lumen. Germinal and acquired mutations in hematopoietic stem cells may manifest in altered megakaryocyte maturation, proliferation, and platelet production. Diseased megakaryopoiesis may also cause modifications of the entire hematopoietic niche, highlighting the central role of megakaryocytes in the control of physiologic bone marrow homeostasis. Tissue-engineering approaches have been developed to translate knowledge from in vivo (inside) to functional mimics of native tissue ex vivo (outside). Reproducing the thrombopoietic environment is instrumental to gain new insight into its activity and answering the growing demand for human platelets for fundamental studies and clinical applications. In this review, we discuss the major achievements on this topic, and finally, we summarize relevant new data presented during the 2022 ISTH Congress that pave the road to the future of megakaryopoiesis.

Megakaryocytes form linear podosomes devoid of digestive properties to remodel medullar matrix

Bone marrow megakaryocytes (MKs) undergo a maturation involving contacts with the microenvironment before extending proplatelets through sinusoids to deliver platelets in the bloodstream. We demonstrated that MKs assemble linear F-actin-enriched podosomes on collagen I fibers. Microscopy analysis evidenced an inverse correlation between the number of dot-like versus linear podosomes over time. Confocal videomicroscopy confirmed that they derived from each-other. This dynamics was dependent on myosin IIA. Importantly, MKs progenitors expressed the Tks4/5 adaptors, displayed a strong gelatinolytic ability and did not form linear podosomes. While maturing, MKs lost Tks expression together with digestive ability. However, those MKs were still able to remodel the matrix by exerting traction on collagen I fibers through a collaboration between GPVI, ß1 integrin and linear podosomes. Our data demonstrated that a change in structure and composition of podosomes accounted for the shift of function during megakaryopoiesis. These data highlight the fact that members of the invadosome family could correspond to different maturation status of the same entity, to adapt to functional responses required by differentiation stages of the cell that bears them.

Bioinformatics of Differentially Expressed Genes in Phorbol 12-Myristate 13-Acetate-Induced Megakaryocytic Differentiation of K562 Cells by Microarray Analysis

Megakaryocytes are large hematopoietic cells present in the bone marrow cavity, comprising less than 0.1% of all bone marrow cells. Despite their small number, megakaryocytes play important roles in blood coagulation, inflammatory responses, and platelet production. However, little is known about changes in gene expression during megakaryocyte maturation. Here we identified the genes whose expression was changed during K562 leukemia cell differentiation into megakaryocytes using an Affymetrix GeneChip microarray to determine the multifunctionality of megakaryocytes. K562 cells were differentiated into mature megakaryocytes by treatment for 7 days with phorbol 12-myristate 13-acetate, and a microarray was performed using RNA obtained from both types of cells. The expression of 44,629 genes was compared between K562 cells and mature megakaryocytes, and 954 differentially expressed genes (DEGs) were selected based on a p-value < 0.05 and a fold change >2. The DEGs was further functionally classified using five major megakaryocyte function-associated clusters—inflammatory response, angiogenesis, cell migration, extracellular matrix, and secretion. Furthermore, interaction analysis based on the STRING database was used to generate interactions between the proteins translated from the DEGs. This study provides information on the bioinformatics of the DEGs in mature megakaryocytes after K562 cell differentiation.

Regenerative Potential of Solid Bone Marrow Aspirate Concentrate Compared to Platelet-Rich Fibrin

Platelet-rich fibrin (PRF) prepared from venous blood is used in the clinic to improve soft tissue wound healing. Nevertheless, arterial blood or bone marrow aspirate might also be a candidate for the source of PRF-like concentrates. The purpose of the present study was to investigate blood/bone marrow aspirate concentrates obtained from arterial blood, venous blood, and bone marrow aspirate to determine its respective regenerative potential in vitro. Arterial blood-derived PRF (Ar-PRF), venous blood-derived PRF (Ve-PRF), and solid bone marrow aspirate concentrate (sBMAC) were prepared from New Zealand white rabbits. Each clot was evaluated for its cytocompatibility and regenerative potential on primary rabbit gingival fibroblasts and osteoblasts. Both gingival fibroblasts and osteoblasts treated with each concentrate showed excellent viability. Interestingly, the sBMAC-treated cells demonstrated significantly greater migratory potential than the other treatment groups. Furthermore, higher mRNA levels of transforming growth factor-β (TGF-β), vascular endothelial growth factor (VEGF) and collagen I (COL1) in gingival fibroblasts were observed in sBMAC group compared with Ar-PRF and Ve-PRF groups. Greater osteoblast differentiation potential, including higher osteocalcin (OCN) expression and mineralization potential, was found in osteoblasts treated with sBMAC. However, minor differences between the behaviors of cells treated with Ar-PRF and Ve-PRF were observed. In conclusion, sBMAC might be a new candidate for promoting wound healing and bone regeneration. Further preclinical and clinical experiments are necessary to prove the regenerative potential of sBMAC in the body.

Megakaryocyte Diversity in Ontogeny, Functions and Cell-Cell Interactions

Hematopoietic stem cells (HSCs) rely on local interactions in the bone marrow (BM) microenvironment with stromal cells and other hematopoietic cells that facilitate their survival and proliferation, and also regulate their functions. HSCs and multipotent progenitor cells differentiate into lineage-specific progenitors that generate all blood and immune cells. Megakaryocytes (Mks) are hematopoietic cells responsible for producing blood platelets, which are essential for normal hemostasis and blood coagulation. Although the most prominent function of Mks is platelet production (thrombopoiesis), other increasingly recognized functions include HSC maintenance and host immune response. However, whether and how these diverse programs are executed by different Mk subpopulations remains poorly understood. This Perspective summarizes our current understanding of diversity in ontogeny, functions and cell-cell interactions. Cumulative evidence suggests that BM microenvironment dysfunction, partly caused by mutated Mks, can induce or alter the progression of a variety of hematologic malignancies, including myeloproliferative neoplasms (MPNs) and other disorders associated with tissue scarring (fibrosis). Therefore, as an example of the heterogeneous functions of Mks in malignant hematopoiesis, we will discuss the role of Mks in the onset and progression of BM fibrosis. In this regard, abnormal interactions between of Mks and other immune cells might directly contribute to fibrotic diseases. Overall, further understanding of megakaryopoiesis and how Mks interact with HSCs and immune cells has potential clinical implications for stem cell transplantation and other therapies for hematologic malignancies, as well as for treatments to stimulate platelet production and prevent thrombocytopenia.

Megakaryocyte-stromal cell interactions: effect on megakaryocyte proliferation, proplatelet production, and survival

Bone marrow stromal cells provide a proper environment for the development of hematologic lineages. The incorporation of different stromal cells to in vitro culture systems would be an attractive model to study megakaryopoiesis and thrombopoiesis. Our objective was to evaluate the participation of different types of stromal cells on in vitro megakaryopoiesis, thrombopoiesis and megakaryocyte (MK) survival. CD34-positive progenitors from umbilical cord blood were differentiated into MK precursors and then co-cultured with umbilical vein endothelial cells (HUVEC), bone marrow mesenchymal stem cells (MSCs), skin fibroblasts (SF) (all human) or mouse fibroblast cell line (L929). The number of MKs (CD61-positive cells) was increased in the presence of HUVEC and SF while L929 cells decreased total and mature MK count. Concerning thrombopoiesis, HUVEC increased proplatelet (PP)-producing MKs, while MSCs, L929 and SF had the opposite effect (immunofluorescence staining and microscopic analysis).  MK survival was enhanced in MSC and SF co-cultures, as assessed by evaluation of pyknotic nuclei. However, HUVEC and L929 did not prevent apoptosis of MKs. Reciprocally, the cloning efficiency of MSCs was decreased in the presence of MKs, while the ability of stromal cells (either MSCs or SF) to produce the extracellular matrix proteins type III collagen, fibronectin, dermatan sulfate, heparan sulfate and P4HB was preserved. These data indicate that each stromal cell type performs distinctive functions, which differentially modulate MK growth and platelet production, and, at the same time, that MKs also modify stromal cells behavior. Overall, our results highlight the relevance of considering the influence of stromal cells in MK research as well as the close interplay of different cell types within the bone marrow milieu.

Megakaryocyte-driven changes in bone health: lessons from mouse models of myelofibrosis and related disorders

Over the years, numerous studies demonstrated reciprocal communications between processes of bone marrow hematopoiesis and bone remodeling. Megakaryocytes, rare bone marrow cells responsible for platelet production, were demonstrated to be involved in bone homeostasis. Myelofibrosis, characterized by an increase in pleomorphic megakaryocytes in the bone marrow, commonly leads to the development of osteosclerosis. In vivo, an increase in megakaryocyte number was shown to result in osteosclerosis in GATA-1^low, NF-E2^-/-, TPO^high, Mpll^f/f;PF4cre, Lnk^-/-, Mpig6b^-/-, Mpig6b^fl/fl;Gp1ba-Cr^+/KI, Pt-vWD mouse models. In vitro, megakaryocytes stimulate osteoblast proliferation and have variable effects on osteoclast proliferation and activity through soluble factors and direct cell-cell communications. Intriguingly, new studies revealed that the ability of megakaryocytes to communicate with bone cells is affected by the age and sex of animals. This mini-review summarises changes seen in bone architecture and bone cell function in mouse models with an elevated number of megakaryocytes and the effects megakaryocytes have on osteoblasts and osteoclasts in vitro, and discusses potential molecular players that can mediate these effects.

Craniofacial Bone Tissue Engineering: Current Approaches and Potential Therapy

Craniofacial bone defects can result from various disorders, including congenital malformations, tumor resection, infection, severe trauma, and accidents. Successfully regenerating cranial defects is an integral step to restore craniofacial function. However, challenges managing and controlling new bone tissue formation remain. Current advances in tissue engineering and regenerative medicine use innovative techniques to address these challenges. The use of biomaterials, stromal cells, and growth factors have demonstrated promising outcomes in vitro and in vivo. Natural and synthetic bone grafts combined with Mesenchymal Stromal Cells (MSCs) and growth factors have shown encouraging results in regenerating critical-size cranial defects. One of prevalent growth factors is Bone Morphogenetic Protein-2 (BMP-2). BMP-2 is defined as a gold standard growth factor that enhances new bone formation in vitro and in vivo. Recently, emerging evidence suggested that Megakaryocytes (MKs), induced by Thrombopoietin (TPO), show an increase in osteoblast proliferation in vitro and bone mass in vivo. Furthermore, a co-culture study shows mature MKs enhance MSC survival rate while maintaining their phenotype. Therefore, MKs can provide an insight as a potential therapy offering a safe and effective approach to regenerating critical-size cranial defects.

Role of β1 integrin in thrombocytopoiesis

Thrombocytopoiesis is a complex process beginning at the level of hematopoietic stem cells, which ultimately generate megakaryocytes, large marrow cells with a distinctive morphology, and then, through a process of terminal maturation, megakaryocytes shed thousands of platelets into the circulation. This process is controlled by intrinsic and extrinsic factors. Emerging data indicate that an important intrinsic control on the late stages of thrombopoiesis is exerted by integrins, a family of transmembrane receptors composed of one α and one β subunit. One β subunit expressed by megakaryocytes is the β1 integrin, the role of which in the regulation of platelet formation is beginning to be clarified. Here, we review recent data indicating that activation of β1 integrin by outside-in and inside-out signaling regulates the interaction of megakaryocytes with the endosteal niche, which triggers their maturation, while its inactivation by galactosylation determines the migration of these cells to the perivascular niche, where they complete their terminal maturation and release platelets in the bloodstream. Furthermore, β1 integrin mediates the activation of transforming growth factor β (TGF-β), a protein produced by megakaryocytes that may act in an autocrine fashion to halt their maturation and affect the composition of their surrounding extracellular matrix. These findings suggest that β1 integrin could be a therapeutic target for inherited and acquired disorders of platelet production.

Myeloproliferative Disorders and its Effect on Bone Homeostasis: The Role of Megakaryocytes

Myeloproliferative Neoplasms (MPNs) are a heterogeneous group of chronic hematological diseases that arise from the clonal expansion of abnormal hematopoietic stem cells, of which Polycythemia Vera (PV), Essential Thrombocythemia (ET), and Primary Myelofibrosis (PMF) have been extensively reviewed in context of clonal expansion, fibrosis and other phenotypes. Here, we review current knowledge on the influence of different forms of MPN on bone health. Studies implicated various degrees of effect of different forms of MPN on bone density, and on osteoblast proliferation and differentiation, using murine models and human data. The majority of studies show that bone volume is generally increased in PMF patients, whereas it is slightly decreased or not altered in ET and PV patients, although possible differences between male and female phenotypes were not fully explored in most MPN forms. Osteosclerosis seen in PMF patients is a serious complication that can lead to bone marrow failure, and the loss of bone reported in some ET and PV patients can lead to osteoporotic fractures. Some MPN forms are associated with increased number of megakaryocytes (MKs), and several of the MK-associated factors in MPN are known to affect bone development. Here, we review known mechanisms involved in these processes, with focus on the role of MKs and secreted factors. Understanding MPN-associated changes in bone health could improve early intervention and treatment of this side effect of the pathology.

GPIbα is the driving force of hepatic thrombopoietin generation

Thrombopoietin (TPO), a glycoprotein hormone produced predominantly in the liver, plays important roles in the hematopoietic stem cell (HSC) niche, and is essential for megakaryopoiesis and platelet generation. Long-standing understanding proposes that TPO is constitutively produced by hepatocytes, and levels are fine-tuned through platelet and megakaryocyte internalization/degradation via the c-Mpl receptor. However, in immune thrombocytopenia (ITP) and several other diseases, TPO levels are inconsistent with this theory. Recent studies showed that platelets, besides their TPO clearance, can induce TPO production in the liver. Our group also accidentally discovered that platelet glycoprotein (GP) Ibα is required for platelet-mediated TPO generation, which is underscored in both GPIbα-/- mice and patients with Bernard-Soulier syndrome. This review will introduce platelet versatilities and several new findings in hemostasis and platelet consumption but focus on its roles in TPO regulation. The implications of these new discoveries in hematopoiesis and the HSC niche, particularly in ITP, will be discussed.

Adipocyte Fatty Acid Transfer Supports Megakaryocyte Maturation

Megakaryocytes (MKs) come from a complex process of hematopoietic progenitor maturation within the bone marrow that gives rise to de novo circulating platelets. Bone marrow microenvironment contains a large number of adipocytes with a still ill-defined role. This study aims to analyze the influence of adipocytes and increased medullar adiposity in megakaryopoiesis. An in vivo increased medullar adiposity in mice caused by high-fat-diet-induced obesity is associated to an enhanced MK maturation and proplatelet formation. In vitro co-culture of adipocytes with bone marrow hematopoietic progenitors shows that delipidation of adipocytes directly supports MK maturation by enhancing polyploidization, amplifying the demarcation membrane system, and accelerating proplatelet formation. This direct crosstalk between adipocytes and MKs occurs through adipocyte fatty acid transfer to MKs involving CD36 to reinforce megakaryocytic maturation. Thus, these findings unveil an influence of adiposity on MK homeostasis based on a dialogue between adipocytes and MKs.

Bone Marrow Aspirate Matrix: A Convenient Ally in Regenerative Medicine

The rise in musculoskeletal disorders has prompted medical experts to devise novel effective alternatives to treat complicated orthopedic conditions. The ever-expanding field of regenerative medicine has allowed researchers to appreciate the therapeutic value of bone marrow-derived biological products, such as the bone marrow aspirate (BMA) clot, a potent orthobiologic which has often been dismissed and regarded as a technical complication. Numerous in vitro and in vivo studies have contributed to the expansion of medical knowledge, revealing optimistic results concerning the application of autologous bone marrow towards various impactful disorders. The bone marrow accommodates a diverse family of cell populations and a rich secretome; therefore, autologous BMA-derived products such as the “BMA Matrix”, may represent a safe and viable approach, able to reduce the costs and some drawbacks linked to the expansion of bone marrow. BMA provides —it eliminates many hurdles associated with its preparation, especially in regards to regulatory compliance. The BMA Matrix represents a suitable alternative, indicated for the enhancement of tissue repair mechanisms by modulating inflammation and acting as a natural biological scaffold as well as a reservoir of cytokines and growth factors that support cell activity. Although promising, more clinical studies are warranted in order to further clarify the efficacy of this strategy.

Active hematopoiesis triggers exosomal release of PRDX2 that promotes osteoclast formation

Hematopoietic disorders, particularly hemolytic anemias, commonly lead to bone loss. We have previously reported that actively proliferating cancer cells stimulate osteoclastogenesis from late precursors in a RANKL-independent manner. We theorized that cancer cells exploit the physiological role of bone resorption to support expanding hematopoietic bone marrow and examined if hematopoietic cells can trigger osteoclastogenesis. Using phlebotomy-induced acute anemia in mice, we found strong correlation between augmented erythropoiesis and increased osteoclastogenesis. Conditioned medium (CM) from K562 erythroleukemia cells and primary mouse erythroblasts stimulated osteoclastogenesis when added to RANKL-primed precursors from mouse bone marrow or RAW264.7 cells. Using immunoblotting and mass spectrometry, PRDX2 was identified as a factor produced by erythroid cells in vitro and in vivo. PRDX2 was detected in K562-derived exosomes, and inhibiting exosomal release significantly decreased the osteoclastogenic capacity of K562 CM. Recombinant PRDX2 induced osteoclast formation from RANKL-primed primary or RAW 264.7 precursors to levels comparable to achieved with continuous RANKL treatment. Thus, increased bone marrow erythropoiesis secondary to anemia leads to upregulation of PRDX2, which is released in the exosomes and acts to induce osteoclast formation. Increased bone resorption by the osteoclasts expands bone marrow cavity, which likely plays a supporting role to increase blood cell production.

Lung megakaryocytes display distinct transcriptional and phenotypic properties

Megakaryocytes (MKs) are responsible for platelet biogenesis, which is believed to occur canonically in adult bone marrow (BM) and in the fetal liver during development. However, emerging evidence highlights the lung as a previously underappreciated residence for MKs that may contribute significantly to circulating platelet mass. Although a diversity of cells specific to the BM is known to promote the maturation and trafficking of MKs, little investigation into the impact of the lung niche on the development and function of MKs has been done. Here, we describe the application of single-cell RNA sequencing, coupled with histological, ploidy, and flow cytometric analyses, to profile primary MKs derived from syngeneic mouse lung and hematopoietic tissues. Transcriptional profiling demonstrated that lung MKs have a unique signature distinct from their hematopoietic counterparts, with lung MKs displaying enrichment for maturation markers, potentially indicating a propensity for more efficient platelet production. Reciprocally, fetal lung MKs also showed the robust expression of cytokines and growth factors that are known to promote lung development. Lastly, lung MKs possess an enrichment profile skewed toward roles in immunity and inflammation. These findings highlight the existence of a lung-specific MK phenotype and support the notion that the lung plays an independent role in the development and functional maturation of MKs. The immune phenotype displayed by lung MKs also introduces their potential role in microbial surveillance and antigen presentation.

Bone marrow aspirate clot: A feasible orthobiologic

Musculoskeletal disorders are one of the major health burdens and a leading source of disability worldwide, affecting both juvenile and elderly populations either as a consequence of ageing or extrinsic factors such as physical injuries. This condition often involves a group of locomotor structures such as the bones, joints and muscles and may therefore cause significant economic and emotional impact. Some pharmacological and non-pharmacological treatments have been considered as potential solutions, however, these alternatives have provided quite limited efficacy due to the short-term effect on pain management and inability to restore damaged tissue. The emergence of novel therapeutic alternatives such as the application of orthobiologics, particularly bone marrow aspirate (BMA) clot, have bestowed medical experts with considerable optimism as evidenced by the significant results found in numerous studies addressed in this manuscript. Although other products have been proposed for the treatment of musculoskeletal injuries, the peculiar interest in BMA, fibrin clot and associated fibrinolytic mechanisms continues to expand. BMA is a rich source of various cellular and molecular components which have demonstrated positive effects on tissue regeneration in many in vitro and in vivo models of musculoskeletal injuries. In addition to being able to undergo self-renewal and differentiation, the hematopoietic and mesenchymal stem cells present in this orthobiologic elicit key immunomodulatory and paracrine roles in inflammatory responses in tissue injury and drive the coagulation cascade towards tissue repair via different mechanisms. Although promising, these complex regenerative mechanisms have not yet been fully elucidated.

Mechanisms of platelet release: in vivo studies and in vitro modeling

Mechanisms related to platelet release in the context of the bone marrow niche are not completely known. In this review we discuss what has been discovered about four critical aspects of this process: 1) the bone marrow niche organization, 2) the role of the extracellular matrix components, 3) the mechanisms by which megakaryocytes release platelets and 4) the novel approaches to mimic the bone marrow environment and produce platelets ex vivo.

Intravital imaging of megakaryocytes

The dynamics of platelet formation could only be investigated since the development of two-photon microscopy in combination with suitable fluorescent labeling strategies. In this review paper, we give an overview of recent advances in fluorescence imaging of the bone marrow that have contributed to our understanding of platelet biogenesis during the last decade. We make a brief survey through the perspectives and limitations of today's intravital imaging, but also discuss complementary methods that may help to piece together the puzzle of megakaryopoiesis and platelet formation.

Defective interaction of mutant calreticulin and SOCE in megakaryocytes from patients with myeloproliferative neoplasms

Approximately one-fourth of patients with essential thrombocythemia or primary myelofibrosis carry a somatic mutation of the calreticulin gene (CALR), the gene encoding for calreticulin. A 52-bp deletion (type I mutation) and a 5-bp insertion (type II mutation) are the most frequent genetic lesions. The mechanism(s) by which a CALR mutation leads to a myeloproliferative phenotype has been clarified only in part. We studied the interaction between calreticulin and store-operated calcium (Ca2+) entry (SOCE) machinery in megakaryocytes (Mks) from healthy individuals and from patients with CALR-mutated myeloproliferative neoplasms (MPNs). In Mks from healthy subjects, binding of recombinant human thrombopoietin to c-Mpl induced the activation of signal transducer and activator of transcription 5, AKT, and extracellular signal-regulated kinase 1/2, determining inositol triphosphate-dependent Ca2+ release from the endoplasmic reticulum (ER). This resulted in the dissociation of the ER protein 57 (ERp57)-mediated complex between calreticulin and stromal interaction molecule 1 (STIM1), a protein of the SOCE machinery that leads to Ca2+ mobilization. In Mks from patients with CALR-mutated MPNs, defective interactions between mutant calreticulin, ERp57, and STIM1 activated SOCE and generated spontaneous cytosolic Ca2+ flows. In turn, this resulted in abnormal Mk proliferation that was reverted using a specific SOCE inhibitor. In summary, the abnormal SOCE regulation of Ca2+ flows in Mks contributes to the pathophysiology of CALR-mutated MPNs. In perspective, SOCE may represent a new therapeutic target to counteract Mk proliferation and its clinical consequences in MPNs.

Inorganic Polyphosphates As Storage for and Generator of Metabolic Energy in the Extracellular Matrix

Inorganic polyphosphates (polyP) consist of linear chains of orthophosphate residues, linked by high-energy phosphoanhydride bonds. They are evolutionarily old biopolymers that are present from bacteria to man. No other molecule concentrates as much (bio)chemically usable energy as polyP. However, the function and metabolism of this long-neglected polymer are scarcely known, especially in higher eukaryotes. In recent years, interest in polyP experienced a renaissance, beginning with the discovery of polyP as phosphate source in bone mineralization. Later, two discoveries placed polyP into the focus of regenerative medicine applications. First, polyP shows morphogenetic activity, i.e., induces cell differentiation via gene induction, and, second, acts as an energy storage and donor in the extracellular space. Studies on acidocalcisomes and mitochondria provided first insights into the enzymatic basis of eukaryotic polyP formation. In addition, a concerted action of alkaline phosphatase and adenylate kinase proved crucial for ADP/ATP generation from polyP. PolyP added extracellularly to mammalian cells resulted in a 3-fold increase of ATP. The importance and mechanism of this phosphotransfer reaction for energy-consuming processes in the extracellular matrix are discussed. This review aims to give a critical overview about the formation and function of this unique polymer that is capable of storing (bio)chemically useful energy.

Dengue viruses infect human megakaryocytes, with probable clinical consequences

One of the most important clinical signs of dengue virus infection is the reduction of white blood cells and platelets in human peripheral blood (leukopenia and thrombocytopenia, respectively), which may significantly impair the clearance of dengue virus by the immune system. The cause of thrombocytopenia and leukopenia during dengue infection is still unknown, but may be related to severe suppression of bone marrow populations including hematopoietic stem cells and megakaryocytes, the progenitors of white blood cells and platelets respectively. Here, we explored the possibility that bone marrow suppression, including ablation of megakaryocyte populations, is caused by dengue virus infection of megakaryocytes. We used three different models to measure dengue virus infection and replication: in vitro, in a human megakaryocyte cell line with viral receptors, ex vivo, in primary human megakaryocytes, and in vivo, in humanized mice. All three systems support dengue virus infection and replication, including virus strains from serotypes 1, 2, and 3, and clinical signs, in vivo; all assays showed viral RNA and/or infectious viruses 7–14 days post-infection. Although we saw no significant decrease in cell viability in vitro, there was significant depletion of mature megakaryocytes in vivo. We conclude that megakaryocytes can produce dengue viruses in the bone marrow niche, and a reduction of cell numbers may affect bone marrow homeostasis.

Dengue virus is the most common cause of viral hemorrhagic fever in humans. Over half of the world’s population lives in an at risk area for dengue virus infection, and this number will continue to grow as climate change allows the mosquito vectors of dengue virus to expand their breeding ranges to more temperate climates. Currently, there are no specific treatments for dengue virus infection. Understanding how dengue virus causes hemorrhagic fever could inform the development of these much needed treatments. Populations of important immune system mediators, such as white blood cells and platelets, are significantly dysregulated during dengue virus infection. These cells originate in the bone marrow, which experiences significant suppression, including a complete ablation of megakaryocytes (platelet progenitor cells), during DENV infection. Here, we add to the knowledge on how dengue virus induces bone marrow suppression by investigating whether dengue virus infects human megakaryocytes. We discovered that dengue virus infects human megakaryocytes in vitro, ex vivo, and in vivo models of dengue virus infection; however, dengue virus infection does not appear to directly affect viability of human megakaryocytes. Future studies will investigate whether infected megakaryocytes are still able to perform their functions of producing platelets and maintaining bone marrow homeostasis.

Mimicking megakaryopoiesis in vitro using biomaterials: Recent advances and future opportunities

Presently donor-derived platelets used in the clinic are associated with concerns about adequate availability, expense, risk of bacterial contamination and complications due to immunological reaction. To prevail over our dependence on transfusion of donor-derived platelets, efforts are being made to generate them in vitro. Development of biomaterials that support or mimic bone marrow niche micro-environmental cues could improve the in vitro production of platelets from megakaryocytes (MKs) derived from various stem cell sources. In spite of significant advances in the production of MKs from various stem cell sources using 2D as well as 3D culture approaches in vitro and the development of biomaterials-based platelet systems, yield and quality of these platelets remains unsuitable for clinical use. Thus, in vitro production of clinically useful platelets on a large scale remains an unmet target to date. This review summarizes the most frequently used 2D and 3D approaches to generate MKs and platelets in vitro, emphasizing the importance of mimicking in vivo micro-environment. Further, this review proposes the use of interpenetrating network (IPN) biomaterial-based approach as a promising strategy for improving the generation of MK and platelets in sufficient numbers in vitro. STATEMENT OF SIGNIFICANCE: Thrombocytopenia is one of the major global health and socio-economic problems. Transfusion with donor-derived platelets (PLTs) is the only effective treatment for this condition. However, this approach is limited by factors like short shelf-life of PLTs, PLT activation, alloimmunization, risk of bacterial contamination, infection etc. In vitro generated MKs and PLTs derived from non-donor-dependent sources may help to overcome the platelet transfusion concerns. Here we have reviewed various 2D and 3D strategies used for in vitro generation of MKs and PLTs, with special emphasis on various biomaterial platforms and different physico/chemical cues being used for the purpose. We have also proposed a biomaterial-based approach of using interpenetrating network (IPN) for generating clinically relevant numbers of MKs and PLTs.

Megakaryocytes participate in the occurrence of bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is characterized by the remodeling of fibrotic tissue and collagen deposition, which mainly results from aberrant fibroblasts proliferation and trans-differentiation to myofibroblasts. Patients with chronic myelogenous leukemia, myeloproliferative disorder, and scleroderma with pulmonary fibrosis complications show megakaryocyte infiltration in the lung. In this study, we demonstrated that the number of CD41⁺ megakaryocytes increased in bleomycin (BLM)-induced lung fibrosis tissues through the Chemokine (CXCmotif) ligand 12/Chemokine receptor 4 (CXCL12/CXCR4) axis. Pharmacological inhibition of the CXCL12/CXCR4 axis with WZ811 prevented migration of CD41⁺ megakaryocytes induced by BLM-injured lung tissue ex vivo and in vivo. In addition, WZ811 significantly attenuated lung fibrosis after BLM challenge. Moreover, megakaryocytes directly promoted fibroblast proliferation and trans-differentiation to myofibroblasts. We conclude that thrombopoietin (TPO) activated megakaryocytes through transforming growth factor β (TGF-β) pathway to promote fibroblast proliferation and trans-differentiation to myofibroblasts, which is abolished by treatment with selective TGF-βR-1/ALK5 inhibitors. Therefore, CD41⁺ megakaryocytes migrate to injured lung tissue partially through the CXCL12/CXCR4 axis to promote the proliferation and trans-differentiation of fibroblasts through direct contact and the TGF-β1 pathway.

EDA fibronectin-TLR4 axis sustains megakaryocyte expansion and inflammation in bone marrow fibrosis

The fibronectin EDA isoform (EDA FN) is instrumental in fibrogenesis but, to date, its expression and function in bone marrow (BM) fibrosis have not been explored. We found that mice constitutively expressing the EDA domain (EIIIA+/+), but not EDA knockout mice, are more prone to develop BM fibrosis upon treatment with the thrombopoietin (TPO) mimetic romiplostim (TPOhigh). Mechanistically, EDA FN binds to TLR4 and sustains progenitor cell proliferation and megakaryopoiesis in a TPO-independent fashion, inducing LPS-like responses, such as NF-κB activation and release of profibrotic IL-6. Pharmacological inhibition of TLR4 or TLR4 deletion in TPOhigh mice abrogated Mk hyperplasia, BM fibrosis, IL-6 release, extramedullary hematopoiesis, and splenomegaly. Finally, developing a novel ELISA assay, we analyzed samples from patients affected by primary myelofibrosis (PMF), a well-known pathological situation caused by altered TPO signaling, and found that the EDA FN is increased in plasma and BM biopsies of PMF patients as compared with healthy controls, correlating with fibrotic phase.

miR-15a-5p regulates expression of multiple proteins in the megakaryocyte GPVI signaling pathway

Essentials The action of microRNAs (miRs) in human megakaryocyte signaling is largely unknown. Cord blood-derived human megakaryocytes (MKs) were used to test the function of candidate miRs. miR-15a-5p negatively regulated MK GPVI-mediated αIIbβ3 activation and α-granule release. miR-15a-5p acts as a potential "master-miR" regulating genes in the MK GPVI signaling pathway. SUMMARY: Background Megakaryocytes (MKs) invest their progeny platelets with proteins and RNAs. MicroRNAs (miRs), which inhibit mRNA translation into protein, are abundantly expressed in MKs and platelets. Although platelet miRs have been associated with platelet reactivity and disease, there is a paucity of information on the function of miRs in human MKs. Objective To identify MK miRs that regulate the GPVI signaling pathway in the MK-platelet lineage. Methods Candidate miRs associated with GPVI-mediated platelet aggregation were tested for functionality in cultured MKs derived from cord blood. Results An unbiased, transcriptome-wide screen in 154 healthy donors identified platelet miR-15a-5p as significantly negatively associated with CRP-induced platelet aggregation. Platelet agonist dose-response curves demonstrated activation of αIIbβ3 in suspensions of cord blood-derived cultured MKs. Overexpression and knockdown of miR-15a-5p in these MKs reduced and enhanced, respectively, CRP-induced αIIbβ3 activation but did not alter thrombin or ADP stimulation. FYN, SRGN, FCER1G, MYLK. and PRKCQ, genes involved in GPVI signaling, were identified as miR-15a-5p targets and were inhibited or de-repressed in MKs with miR-15a-5p overexpression or inhibition, respectively. Lentiviral overexpression of miR-15a-5p also inhibited GPVI-FcRγ-mediated phosphorylation of Syk and PLCγ2, GPVI downstream signaling molecules, but effects of miR-15a-5p on αIIbβ3 activation did not extend to other ITAM-signaling receptors (FcγRIIa and CLEC-2). Conclusion Cord blood-derived MKs are a useful human system for studying the functional effects of candidate platelet genes. miR-15a-5p is a potential "master-miR" for specifically regulating GPVI-mediated MK-platelet signaling. Targeting miR-15a-5p may have therapeutic potential in hemostasis and thrombosis.

Megakaryocyte Contribution to Bone Marrow Fibrosis: many Arrows in the Quiver

In Primary Myelofibrosis (PMF), megakaryocyte dysplasia/hyperplasia determines the release of inflammatory cytokines that, in turn, stimulate stromal cells and induce bone marrow fibrosis. The pathogenic mechanism and the cells responsible for progression to bone marrow fibrosis in PMF are not completely understood. This review article aims to provide an overview of the crucial role of megakaryocytes in myelofibrosis by discussing the role and the altered secretion of megakaryocyte-derived soluble factors, enzymes and extracellular matrices that are known to induce bone marrow fibrosis.

On the Way to in vitro Platelet Production

The severely decreased platelet counts (10–30. 10³ platelets/μL) frequently observed in patients undergoing chemotherapy, radiation treatment, or organ transplantation are associated with life-threatening increased bleeding risks. To circumvent these risks, platelet transfusion remains the treatment of choice, despite some limitations which include a limited shelf-life, storage-related deterioration, the development of alloantibodies in recipients and the transmission of infectious diseases. A sustained demand has evolved in recent years for controlled blood products, free of infectious, inflammatory, and immune risks. As a consequence, the challenge for blood centers in the near future will be to ensure an adequate supply of blood platelets, which calls for a reassessment of our transfusion models. To meet this challenge, many laboratories are now turning their research efforts toward the in vitro and customized production of blood platelets. In recent years, there has been a major enthusiasm for the cultured platelet production, as illustrated by the number of reviews that have appeared in recent years. The focus of the present review is to critically asses the arguments put forward in support of the culture of platelets for transfusion purposes. In light of this, we will recapitulate the main advances in this quickly evolving field, while noting the technical limitations to overcome to make cultured platelet a transfusional alternative.

On the way to in vitro platelet production

The severely decreased platelet counts (10-30.10³ platelets/μL) frequently observed in patients undergoing chemotherapy, radiation treatment or organ transplantation are associated with life-threatening increased bleeding risks. To circumvent these risks, platelet transfusion remains the treatment of choice, despite some limitations which include a limited shelf-life, storage-related deterioration, the development of alloantibodies in recipients and the transmission of infectious diseases. A sustained demand has evolved in recent years for controlled blood products, free of infectious, inflammatory and immune risks. As a consequence, the challenge for blood centers in the near future will be to ensure an adequate supply of blood platelets, which calls for a reassessment of our transfusion models. To meet this challenge, many laboratories are now turning their research efforts towards the in vitro and customized production of blood platelets.

Clinicopathological study of 9 cases of megakaryocytes in pleural and peritoneal fluids

To systemically analyze megakaryocytes in pleural and peritoneal fluids and their clinical significance. We retrospectively examined 10,846 pleural, peritoneal, and pericardial fluid samples obtained from 3 hospitals over a 20-year period. Megakaryocytes were observed in the pleural fluid samples from 7 patients and peritoneal fluid samples from 2 patients, and the incidence was 0.83%. The clinical diagnoses of these 9 patients included myeloproliferative disorders, trauma, and tumors. The serous effusions in all 9 patients were bloody, and the megakaryocytes could be associated with trauma, bone marrow pollution, extramedullary hematopoiesis, or cancer. Additionally, differentiating between megakaryocytes and tumor cells or nuclear mesothelial cells in the pleural fluid is difficult. Therefore, megakaryocytes should be carefully observed and differentiated in pleural and peritoneal fluids because they can be confused with other cells in the clinic. Altogether, the megakaryocytes in the pleural and peritoneal fluids were mainly associated with contamination in the bone marrow or extramedullary hematopoiesis.

Association between gene polymorphisms and clinical features in idiopathic thrombocytopenic purpura patients

: Immune thrombocytopenic purpura (ITP) is an autoimmune disease in which increased platelet destruction and thrombocytopenia are diagnostic features. In fact, the exact pathogenesis of this disease is still unknown, but genetic changes can be a potential factor in the development of ITP. In this study, the relationship between polymorphisms with platelet destruction has been studied, which leads to decreased platelet count. Relevant literature was identified by a PubMed search (2000-2016) of English language papers using the terms 'ITP', 'polymorphism,' and 'immune system'. The majority of genetic changes (polymorphisms) occur in immune system genes, including interferon (IFN)-γ gene. These changes lead to the dysfunction of immune system and production of pathogenic antibodies against platelet surface glycoproteins such as glycoprotein IIb/IIIa, which eventually result in the destruction of platelets and increasing disease severity. In addition, IFN-γ as well as factors and cytokines involved in megakaryopoiesis, including stem cell factor and interleukin-3 (IL-3), leads to the differentiation of megakaryocytes and platelet release. Considering the fact that IFN-γ is a factor of inflammation and thrombocytopenia, coexistence of this cytokine with thrombopoietin, stem cell factor, and IL-3 results in megakaryocytes differentiation and platelet production, which can be effective to reduce disease severity and increase the platelet counts.

Sub-Cellular Localization of Metalloproteinases in Megakaryocytes

Metalloproteinases (MMPs) are zinc-dependent endopeptidases that play essential roles as the mediator of matrix degradation and remodeling during organogenesis, wound healing and angiogenesis. Although MMPs were originally identified as matrixin proteases that act in the extracellular matrix, more recent research has identified members of the MMP family in unusual locations within the cells, exerting distinct functions in addition to their established role as extracellular proteases. During thrombopoiesis, megakaryocytes (Mks) sort MMPs to nascent platelets through pseudopodial-like structure known as proplatelets. Previous studies identified gelatinases, MMP-2 and MMP-9, as a novel regulator system of Mks and the platelet function. In this work we have exploited a sensitive immunoassay to detect and quantify multiple MMP proteins and their localization, in conditioned medium and sub-cellular fractions of primary human CD34⁺-derived Mks. We provide evidence that Mks express other MMPs in addition to gelatinases MMP-2 and MMP-9, peculiar isoforms of MMP-9 and MMPs with a novel nuclear compartmentalization.

Multi-channel silk sponge mimicking bone marrow vascular niche for platelet production

In the bone marrow, the interaction of progenitor cells with the vasculature is fundamental for the release of blood cells into circulation. Silk fibroin, derived from Bombyx mori silkworm cocoons, is a promising protein biomaterial for bone marrow tissue engineering, because of its tunable architecture and mechanical properties, the capacity to incorporate labile compounds without loss of bioactivity and the demonstrated ability to support blood cell formation without premature activation. In this study, we fabricated a custom perfusion chamber to contain a multi-channel lyophilized silk sponge mimicking the vascular network in the bone marrow niche. The perfusion system consisted in an inlet and an outlet and 2 splitters that allowed funneling flow in each single channel of the silk sponge. Computational Fluid Dynamic analysis demonstrated that this design permitted confined flow inside the vascular channels. The silk channeled sponge supported efficient platelet release from megakaryocytes (Mks). After seeding, the Mks localized along SDF-1α functionalized vascular channels in the sponge. Perfusion of the channels allowed the recovery of functional platelets as demonstrated by increased PAC-1 binding upon thrombin stimulation. Further, increasing the number of channels in the silk sponge resulted in a proportional increase in the numbers of platelets recovered, suggesting applicability to scale-up for platelet production. In conclusion, we have developed a scalable system consisting of a multi-channeled silk sponge incorporated in a perfusion chamber that can provide useful technology for functional platelet production ex vivo.

Identification of megakaryocytes as a target of advanced glycation end products in diabetic complications in bone marrow

AIMS

To define the possible effect of diabetic conditions on megakaryocytes, the long-know precursors of platelets and lately characterized modulator of hematopoietic stem quiescence-activation transition.

METHODS

Megakaryoblastic MEG-01 cell culture and TPO/SCF/IL-3-induced differentiation of human umbilical blood mononuclear cells toward megakaryocytes were used to test effects of glycated bovine serum albumin (BSA-AGEs). The ob/ob mice and streptozotocin-treated mice were used as models of hyperglycemia. MTT was used to measure cell proliferation, FACS for surface marker and cell cycle, and RT-qPCR for the expression of interested genes. Megakaryocytes at different stages in marrow smear were checked under microscope.

RESULTS

When added in MEG-01 cultures at 200 μg/ml, BSA-AGEs increased proliferation of cells and enhanced mRNA expression of RAGE, VEGFα and PF4 in the cells. None of cell cycle distribution, PMA-induced platelet-like particles production, expression of GATA1/NF-E2/PU-1/IL-6/OPG/PDGF in MEG-01 cells nor TPO/SCF/IL-3 induced umbilical cord blood cells differentiation into megakaryocyte was affected by BSA-AGEs. In the ob/ob diabetic mice, MKs percentages in marrow cells and platelets in peripheral blood were significantly increased compared with control mice. In streptozotocin-induced diabetic mice, however, MKs percentage in marrow cells was decreased though peripheral platelet counts were not altered. Gene expression assay showed that the change in MKs in these two diabetic conditions might be explained by the alteration of GATA1 and NF-E2 expression, respectively.

CONCLUSIONS

Diabetic condition in animals might exert its influence on hematopoiesis via megakaryocytes-the newly identified modulator of hematopoietic stem cells in bone marrow.

Three-Dimensional Tissue Models for Studying Ex Vivo Megakaryocytopoiesis and Platelet Production

Three-dimensional (3D) tissue cultures in vitro enable a more physiological reconstruction of native tissues and organs. The bone marrow environment, structure and composition regulate megakaryocyte function and platelet production. Here, we describe the use of silk fibroin protein biomaterials to assemble 3D scaffolds mimicking the bone marrow niche architecture and extracellular matrix composition to support platelet release from human megakaryocytes. Additionally, we also propose the use of hyaluronan hydrogels, functionalized with extracellular matrix components, to reproduce the 3D matrix structure of the bone marrow environment for studying human megakaryocyte function.

Effects of vascular endothelial growth factors and their receptors on megakaryocytes and platelets and related diseases

It is well known that vascular endothelial growth factors (VEGFs) and their receptors (vascular endothelial growth factor receptors, VEGFRs) are expressed in different tissues, and VEGF-VEGFR loops regulate a wide range of responses, including metabolic homeostasis, cell proliferation, migration and tubuleogenesis. As ligands, VEGFs act on three structurally related VEGFRs (VEGFR1, VEGFR2 and VEGFR3 [also termed FLT1, KDR and FLT4, respectively]) that deliver downstream signals. Haematopoietic stem cells (HSCs), megakaryocytic cell lines, cultured megakaryocytes (MKs), primary MKs and abnormal MKs express and secrete VEGFs. During the development from HSCs to MKs, VEGFR1, VEGFR2 and VEGFR3 are expressed at different developmental stages, respectively, and re-expressed, e.g., VEGFR2, and play different roles in commitment, differentiation, proliferation, survival and polyplodization of HSCs/MKs via autocrine, paracrine and/or even intracrine loops. Moreover, VEGFs and their receptors are abnormally expressed in MK-related diseases, including myeloproliferative neoplasms, myelodysplastic syndromes and acute megakaryocytic leukaemia (a rare subtype of acute myeloid leukaemia), and they lead to the disordered proliferation/differentiation of bone marrow cells and angiogenesis, indicating that they are closely related to these diseases. Thus, targeting VEGF-VEGFR loops may be of potential therapeutic value.

Thrombopoiesis is spatially regulated by the bone marrow vasculature

In mammals, megakaryocytes (MKs) in the bone marrow (BM) produce blood platelets, required for hemostasis and thrombosis. MKs originate from hematopoietic stem cells and are thought to migrate from an endosteal niche towards the vascular sinusoids during their maturation. Through imaging of MKs in the intact BM, here we show that MKs can be found within the entire BM, without a bias towards bone-distant regions. By combining in vivo two-photon microscopy and in situ light-sheet fluorescence microscopy with computational simulations, we reveal surprisingly slow MK migration, limited intervascular space, and a vessel-biased MK pool. These data challenge the current thrombopoiesis model of MK migration and support a modified model, where MKs at sinusoids are replenished by sinusoidal precursors rather than cells from a distant periostic niche. As MKs do not need to migrate to reach the vessel, therapies to increase MK numbers might be sufficient to raise platelet counts.Megakaryocyte maturation is thought to occur as the cells migrate from a vessel-distant (endosteal) niche to the vessel within the bone. Here, the authors show that megakaryocytes represent largely sessile cells in close contact with the vasculature and homogeneously distributed in the bone marrow.