Blood platelet production and morphology. Thromb Res. 2012;129:241-244. [PMID: 22226434 DOI: 10.1016/j.thromres.2011.11.042]

Proteomic screening identifies PF4/Cxcl4 as a critical driver of myelofibrosis

Despite increased understanding of the genomic landscape of Myeloproliferative Neoplasms (MPNs), the pathological mechanisms underlying abnormal megakaryocyte (Mk)-stromal crosstalk and fibrotic progression in MPNs remain unclear. We conducted mass spectrometry-based proteomics on mice with Romiplostim-dependent myelofibrosis to reveal alterations in signaling pathways and protein changes in Mks, platelets, and bone marrow (BM) cells. The chemokine Platelet Factor 4 (PF4)/Cxcl4 was up-regulated in all proteomes and increased in plasma and BM fluids of fibrotic mice. High TPO concentrations sustained in vitro PF4 synthesis and secretion in cultured Mks, while Ruxolitinib restrains the abnormal PF4 expression in vivo. We discovered that PF4 is rapidly internalized by stromal cells through surface glycosaminoglycans (GAGs) to promote myofibroblast differentiation. Cxcl4 gene silencing in Mks mitigated the profibrotic phenotype of stromal cells in TPO-saturated co-culture conditions. Consistently, extensive stromal PF4 uptake and altered GAGs deposition were detected in Romiplostim-treated, JAK2V617F mice and BM biopsies of MPN patients. BM PF4 levels and Mk/platelet CXCL4 expression were elevated in patients, exclusively in overt fibrosis. Finally, pharmacological inhibition of GAGs ameliorated in vivo fibrosis in Romiplostim-treated mice. Thus, our findings highlight the critical role of PF4 in the fibrosis progression of MPNs and substantiate the potential therapeutic strategy of neutralizing PF4-GAGs interaction.

Reactive oxygen species downregulate dystroglycans in the megakaryocytes of rats with arterial hypertension

Hypertension is a multifactorial disease characterized by vascular and renal dysfunction, cardiovascular remodeling, inflammation, and fibrosis, all of which are associated with oxidative stress. We previously demonstrated cellular reactive oxygen species (ROS) imbalances may impact the structural and biochemical functions of blood cells and reported downregulation of β-dystroglycan (β-Dg) and overexpression of the epithelial sodium channel (ENaC) contribute to the pathophysiology of hypertension. In this study, we aimed to determine the expression of dystroglycans (Dg) and ENaC in platelet progenitors (megakaryocytes) and their surrounding niches. Thin sections of bone marrow from 5- and 28-week-old spontaneous hypertensive rats (SHR) were compared to age-matched normotensive rats (WKY). Cytometry and immunohistochemical assays demonstrated an oxidative environment in SHR bone marrow, characterized by high levels of myeloperoxidase and 3-nitrotyrosine and downregulation of peroxiredoxin II. In addition, transmission electron micrography and confocal microscopy revealed morphological changes in platelets and Mgks from SHR rats, including swollen mitochondria. Quantitative qRT-PCR assays confirmed downregulation of Dg mRNA and immunohistochemistry and western-blotting validated low expression of β-Dg, mainly in the phosphorylated form, in Mgks from 28-week-old SHR rats. Moreover, we observed a progressive increase in β-1 integrin expression in Mgks and extracellular matrix proteins in Mgk niches in SHR rats compared to WKY controls. These results indicate accumulation of ROS promotes oxidative stress within the bone marrow environment and detrimentally affects cellular homeostasis in hypertensive individuals.

The Application of Morphogo in the Detection of Megakaryocytes from Bone Marrow Digital Images with Convolutional Neural Networks

The evaluation of megakaryocytes is an important part of the work up on bone marrow smear examination. It has significance in the differential diagnosis, therapeutic efficacy assessment, and predication of prognosis of many hematologic diseases. The process of manual identification of megakaryocytes are tedious and lack of reproducibility; therefore, a reliable method of automated megakaryocytic identification is urgently needed. Three hundred and thirty-three bone marrow aspirate smears were digitized by Morphogo system. Pathologists annotated megakaryocytes on the digital images of marrow smears are applied to construct a large dataset for testing the system's predictive performance. Subsequently, we obtained megakaryocyte count and classification for each sample by different methods (system-automated analysis, system-assisted analysis, and microscopic examination) to study the correlation between different counting and classification methods. Morphogo system localized cells likely to be megakaryocytes on digital smears, which were later annotated by pathologists and the system, respectively. The system showed outstanding performance in identifying megakaryocytes in bone marrow smears with high sensitivity (96.57%) and specificity (89.71%). The overall correlation between the different methods was confirmed the high consistency (r ≥ 0.7218, R² ≥ 0.5211) with microscopic examination in classifying megakaryocytes. Morphogo system was proved as a reliable screen tool for analyzing megakaryocytes. The application of Morphogo system shows promises to advance the automation and standardization of bone marrow smear examination.

A Novel Antithrombocytopenia Agent, Rhizoma cibotii, Promotes Megakaryopoiesis and Thrombopoiesis through the PI3K/AKT, MEK/ERK, and JAK2/STAT3 Signaling Pathways

BACKGROUND

Cibotii rhizoma (CR) is a famous traditional Chinese medicine (TCM) used to treat bleeding, rheumatism, lumbago, etc. However, its therapeutic effects and mechanism against thrombocytopenia are still unknown so far. In the study, we investigated the effects of aqueous extracts of Cibotii rhizoma (AECRs) against thrombocytopenia and its molecular mechanism.

METHODS

Giemsa staining, phalloidin staining, and flow cytometry were performed to measure the effect of AECRs on the megakaryocyte differentiation in K562 and Meg-01 cells. A radiation-induced thrombocytopenia mouse model was constructed to assess the therapeutic actions of AECRs on thrombocytopenia. Network pharmacology and experimental verification were carried out to clarify its mechanism against thrombocytopenia.

RESULTS

AECRs promoted megakaryocyte differentiation in K562 and Meg-01 cells and accelerated platelet recovery and megakaryopoiesis with no systemic toxicity in radiation-induced thrombocytopenia mice. The PI3K/AKT, MEK/ERK, and JAK2/STAT3 signaling pathways contributed to AECR-induced megakaryocyte differentiation. The suppression of the above signaling pathways by their inhibitors blocked AERC-induced megakaryocyte differentiation.

CONCLUSIONS

AECRs can promote megakaryopoiesis and thrombopoiesis through activating PI3K/AKT, MEK/ERK, and JAK2/STAT3 signaling pathways, which has the potential to treat radiation-induced thrombocytopenia in the clinic.

A Comparative Study Between Porcine Peritoneum and Pericardium as Cardiovascular Material

Decellularized porcine pericardium has many applications in the cardiovascular field for its excellent properties. The peritoneum is a single-layer bio-dialysis membrane with many similarities and differences in physical characteristics, biochemical composition, and structure to the pericardium. The limited available literature suggests that, similar to the pericardium, the peritoneum has good application potential in the field of cardiovascular substitute materials. This research focused on comparing the differences between decellularized peritoneum and decellularized pericardium in microstructure, biochemical composition, mechanical properties, hemocompatibility, in vitro enzymatic degradation, in vitro calcification, cytocompatibility, and other vital indicators. The peritoneum was consistent with pericardium in terms of fibrous structure, hemocompatibility, in vitro calcification, and cytocompatibility. The peritoneal elastic fiber content (219 μg/mg) was significantly higher than that of the pericardium (66 μg/mg), resulting in two to three times higher maximum load (21.1 N) and burst pressure (1309 mmHg), and better performance than the pericardium in terms of in vitro resistance to enzymatic degradation. In the cardiovascular field, decellularized peritoneum can be used as vascular substitute material. Impact statement There are many similarities between the embryonic origin and morphological structure of the porcine peritoneum and the porcine pericardium, but little research has been done on the use of the porcine peritoneum as a biomaterial. In this compared research, we showed that porcine peritoneum had better resistance to enzymatic degradation, better stretching, and more suitable burst pressure for being used as vascular substitute material. This research is the first to describe the structural composition of porcine peritoneum and its advantageous properties as a cardiovascular material.

Bone marrow microenvironment of MPN cells

In this chapter, we will discuss the current knowledge concerning the alterations of the cellular components in the bone marrow niche in Myeloproliferative Neoplasms (MPNs), highlighting the central role of the megakaryocytes in MPN progression, and the extracellular matrix components characterizing the fibrotic bone marrow.

Megakaryocyte Cytoskeletal Proteins in Platelet Biogenesis and Diseases

Thrombopoiesis governs the formation of blood platelets in bone marrow by converting megakaryocytes into long, branched proplatelets on which individual platelets are assembled. The megakaryocyte cytoskeleton responds to multiple microenvironmental cues, including chemical and mechanical stimuli, sustaining the platelet shedding. During the megakaryocyte's life cycle, cytoskeletal networks organize cell shape and content, connect them physically and biochemically to the bone marrow vascular niche, and enable the release of platelets into the bloodstream. While the basic building blocks of the cytoskeleton have been studied extensively, new sets of cytoskeleton regulators have emerged as critical components of the dynamic protein network that supports platelet production. Understanding how the interaction of individual molecules of the cytoskeleton governs megakaryocyte behavior is essential to improve knowledge of platelet biogenesis and develop new therapeutic strategies for inherited thrombocytopenias caused by alterations in the cytoskeletal genes.

A brief review of the biology of megakaryocytes and platelets and their role in thrombosis associated with particulate air pollution

Air pollution is a worldwide public health issue and it is associated with millions of premature deaths due to cancer, thrombosis, and pulmonary and cardiovascular diseases. Thrombosis is the excessive clotting that blocks a blood vessel, and its etiology is multifactorial. In recent years, growing evidence has linked air pollution, especially particulate matter (PM) and metals, to the development of thrombosis. PM and metals induce lung and systemic inflammation and oxidative stress that are frequent mechanisms in thrombosis. Platelets are important effectors of physiological hemostasis and pathological thrombosis. They are responsible for the formation of the initial plug and are important in the cellular model of coagulation. Therefore, any changes in their morphology or function or an increase in activation could be extremely relevant in thrombosis. Megakaryocytes (MKs) in the bone marrow and in the lungs are the precursor cells of platelets, and the latter is the first organ injured by air pollution. There is substantial evidence of the effect that PM and metals have on platelets, but there is almost no research about the effect of PM and metals on MKs. It is very likely that the alterations produced by air pollution originate in these cells. In this article, we review the biology of MKs and platelets and their role in particulate air pollution-related thrombosis to emphasize the need for further research in this field.

Induction of differentiation of human stem cells ex vivo: Toward large-scale platelet production

Platelet transfusion is one of the most reliable strategies to cure patients suffering from thrombocytopenia or platelet dysfunction. With the increasing demand for transfusion, however, there is an undersupply of donors to provide the platelet source. Thus, scientists have sought to design methods for deriving clinical-scale platelets ex vivo. Although there has been considerable success ex vivo in the generation of transformative platelets produced by human stem cells (SCs), the platelet yields achieved using these strategies have not been adequate for clinical application. In this review, we provide an overview of the developmental process of megakaryocytes and the production of platelets in vivo and ex vivo, recapitulate the key advances in the production of SC-derived platelets using several SC sources, and discuss some strategies that apply three-dimensional bioreactor devices and biochemical factors synergistically to improve the generation of large-scale platelets for use in future biomedical and clinical settings.

Focal Adhesion Kinases in Platelet Function and Thrombosis

The focal adhesion kinase family includes 2 homolog members, FAK and Pyk2 (proline-rich tyrosine kinase 2), primarily known for their roles in nucleated cells as regulators of cytoskeletal dynamics and cell adhesion. FAK and Pyk2 are also expressed in megakaryocytes and platelets and are activated by soluble agonists and on adhesion to the subendothelial matrix. Despite high sequence homology and similar molecular organization, FAK and Pyk2 play different roles in platelet function. Whereas FAK serves mostly as a traditional focal adhesion kinase activated downstream of integrins, Pyk2 coordinates multiple signals from different receptors. FAK, but not Pyk2, is involved in megakaryocyte maturation and platelet production. In circulating platelets, FAK is recruited by integrin αIIbβ3 to regulate hemostasis, whereas it plays minimal roles in thrombosis. By contrast, Pyk2 is implicated in platelet activation and is an important regulator of thrombosis. The direct activation of Pyk2 by calcium ions provides a connection between GPCRs (G-protein coupled receptors) and Src family kinases. In this review, we provide the comprehensive overview of >20 years of investigations on the role and regulation of focal adhesion kinases in blood platelets, highlighting common and distinctive features of FAK and Pyk2 in hemostasis and thrombosis.

Mechanisms of thrombocytopenia in platelet-type von Willebrand disease

Platelet-type von Willebrand disease is an inherited platelet disorder characterized by thrombocytopenia with large platelets caused by gain-of-function variants in GP1BA leading to enhanced GPIbα-von Willebrand factor (vWF) interaction. GPIbα and vWF play a role in megakaryocytopoiesis, thus we aimed to investigate megakaryocyte differentiation and proplatelet-formation in platelet-type von Willebrand disease using megakaryocytes from a patient carrying the Met239Val variant and from mice carrying the Gly233Val variant. Platelet-type von Willebrand disease megakaryocytes bound vWF at an early differentiation stage and generated proplatelets with a decreased number of enlarged tips compared to control megakaryocytes. Moreover, they formed proplatelets upon contact with collagen, differently from normal megakaryocytes. Similarly, collagen triggered megakaryocytes showed defective activation of the RhoA-MLC2 axis, which prevents proplatelet formation, and increased phosphorylation of Lyn, which acts as a negative regulator of GPVI signaling, thus preventing ectopic proplatelet-formation on collagen. Consistently, human and murine bone marrow contained an increased number of extravascular platelets compared to controls. In addition, platelet survival of mutant mice was shortened compared to control mice, and the administration of desmopressin, raising circulating vWF, caused a marked drop in platelet count. Taken together, these results show for the first time that thrombocytopenia in platelet-type von Willebrand disease is due to the combination of different pathogenic mechanisms, i.e. the formation of a reduced number of platelets by megakaryocytes, the ectopic release of platelets in the bone marrow, and the increased clearance of platelet/vWF complexes.

Learning by counting blood platelets in population studies: survey and perspective a long way after Bizzozero

Platelet count represents a useful tool in clinical practice to discriminate individuals at higher risk of bleeding. Less obvious is the role of platelet count variability within the normal range of distribution in shaping the individual's disease risk profile. Epidemiological studies have shown that platelet count in the adult general population is associated with a number of health outcomes related to hemostasis and thrombosis. However, recent studies are suggesting a possible role of this platelet index also as an independent risk factor. In this review of adult population studies, we will first focus on known genetic and non-genetic determinants of platelet number variability. Next, we will evaluate platelet count as a marker and/or a predictor of disease risk and its interaction with other risk factors. We will then discuss the role of platelet count variability within the normal distribution range as a contribution to disease and mortality risk. The possibility of considering platelet count as a simple, inexpensive indicator of increased risk of disease and death in general populations could open new opportunities to investigate novel platelet pathophysiological roles as well as therapeutic opportunities. Future studies should also consider platelet count, not only platelet function, as a modulator of disease and mortality risk.

SEMA4D-heparin Complexes Immobilized on Titanium Surfaces Have Anticoagulant, Cell-Migration-Promoting, and Immunoregulatory Effects

Soluble semaphorin 4D (SEMA4D) is a 120 kDa transmembrane protein, which belongs to the semaphorin family of axon guidance molecules that act primarily axonal repellents. SEMA4D elicits its migration-promoting and immunomodulatory effects through activation of PLXNB1 and CD72, respectively. In this study, SEMA4D combined with heparin were adsorbed onto cationic surfaces. The biocompatibility evaluation results indicated that the SEMA4D-heparin-modified surfaces displayed less platelet adhesion and activation, prolonged activated partial thromboplastin time (APTT), prothrombin time (PT) and thrombin time (TT) and reduced fibrinogen gamma chain (FGG) exposure and fibrinogen adhesion. Additionally, endothelial cells (ECs) showed improved adhesion density and proliferation activity on the SEMA4D-heparin-modified surfaces. Chemotactic and haptotaxis assays indicated a highly guided migration for ECs on the modified surfaces. The immunological tests revealed that the SEMA4D-heparin complexes had a positive immunomodulatory effect on macrophages and promoted macrophages polarization into M2 phenotypes. Overall, the results suggested that the SEMA4D-heparin complexes can be a potential therapeutic agent to promote tissue healing and accelerate in situ endothelialization with minimal side effects and positive immunomodulatory effect.

The role of bone marrow microenvironment in platelet production and their implications for the treatment of thrombocytopenic diseases

OBJECTIVES

Impaired platelet production has been found to be an important pathological mechanism of thrombocytopenia in many diseases. Platelet generation is a complex process that mainly occurs in the bone marrow, and thus is closely regulated by the bone marrow microenvironment. This review attempts to summarize the most current knowledge referring the role of bone marrow microenvironment in the regulation of platelet production.

METHODS

The effects of multiple microenvironment ingredients in regulating megakaryopoiesis and thrombocytopoiesis have been discussed. Abnormalities of these components in thrombocytopenic diseases are also described.

DISCUSSIONS

Thrombocytopenia is a common clinical manifestation of a variety of diseases. The functional importance of platelets has driven the developments of a broad range of studies. Platelet generation mainly occurs within the bone marrow, where the cells, soluble factors, and extracellular matrix proteins collaboratively form a complex regulatory network, directing megakaryocytic proliferation and differentiation. Alteration in any part of the regulating network may result in defective platelet formation, and eventually lead to thrombocytopenia. A variety of thrombocytopenic diseases have been found to be related with the disregulated bone marrow microenvironment. Identification of the variations of these niche ingredients in certain diseases has facilitated the developments of multiple therapeutic regimes. Further studies that can combine these niche factors with their downstream regulatory factors will be beneficial for developing more effective therapies.

CONCLUSIONS

Further definition of the role of bone marrow microenvironment in platelet generation may deepen our understanding of the underlying mechanisms as well as provide new therapeutic targets for thrombocytopenic diseases.

X-linked thrombocytopenia with thalassemia displays bone marrow reticulin fibrosis and enhanced angiogenesis: comparisons with primary myelofibrosis

X-linked thrombocytopenia with thalassemia (XLTT) is caused by the mutation 216R > Q in exon 4 of the GATA1 gene. Male hemizygous patients display macrothrombocytopenia, splenomegaly, and a β-thalassemia trait. We describe two XLTT families where three males were initially misdiagnosed as having primary myelofibrosis (PMF) and all five investigated males showed mild-moderate bone marrow (BM) reticulin fibrosis. Comparative investigations were performed on blood samples and BM biopsies from males with XLTT, PMF patients and healthy controls. Like PMF, XLTT presented with high BM microvessel density, low GATA1 protein levels in megakaryocytes, and elevated blood CD34+ cell counts. But unlike PMF, the BM microvessel pericyte coverage was low in XLTT, and no collagen fibrosis was found. Further, as evaluated by immunohistochemistry, expressions of the growth factors VEGF, AGGF1, and CTGF were low in XLTT megakaryocytes and microvessels but high in PMF. Thus, although the reticulin fibrosis in XLTT might simulate PMF, opposing stromal and megakaryocyte features may facilitate differential diagnosis. Additional comparisons between these disorders may increase the understanding of mechanisms behind BM fibrosis in relation to pathological megakaryopoiesis.

Are Platelets Cells? And if Yes, are They Immune Cells?

Small fragments circulating in the blood were formally identified by the end of the nineteenth century, and it was suggested that they assisted coagulation via interactions with vessel endothelia. Wright, at the beginning of the twentieth century, identified their bone-marrow origin. For long, platelets have been considered sticky assistants of hemostasis and pollutants of blood or tissue samples; they were just cell fragments. As such, however, they were acknowledged as immunizing (to specific HPA and HLA markers): the platelet’s dark face. The enlightened face showed that besides hemostasis, platelets contained factors involved in healing. As early as 1930s, platelets entered the arsenal of medicines were transfused, and were soon manipulated to become a kind of glue to repair damaged tissues. Some gladly categorized platelets as cells but they were certainly not fully licensed as such for cell physiologists. Actually, platelets possess almost every characteristic of cells, apart from being capable of organizing their genes: they have neither a nucleus nor genes. This view prevailed until it became evident that platelets play a role in homeostasis and interact with cells other than with vascular endothelial cells; then began the era of physiological and also pathological inflammation. Platelets have now entered the field of immunity as inflammatory cells. Does assistance to immune cells itself suffice to license a cell as an “immune cell”? Platelets prove capable of sensing different types of signals and organizing an appropriate response. Many cells can do that. However, platelets can use a complete signalosome (apart from the last transcription step, though it is likely that this step can be circumvented by retrotranscribing RNA messages). The question has also arisen as to whether platelets can present antigen via their abundantly expressed MHC class I molecules. In combination, these properties argue in favor of allowing platelets the title of immune cells.

Megakaryopoiesis and platelet production: insight into hematopoietic stem cell proliferation and differentiation

Hematopoietic stem cells (HSCs) undergo successive lineage commitment steps to generate megakaryocytes (MKs) in a process referred to as megakaryopoiesis. MKs undergo a unique differentiation process involving endomitosis to eventually produce platelets. Many transcription factors participate in the regulation of this complex progress. Chemokines and other factors in the microenvironment where megakaryopoiesis and platelet production occur play vital roles in the regulation of HSC lineage commitment and MK maturation; among these factors, thrombopoietin (TPO) is the most important. Endomitosis is a vital process of MK maturation, and granules that are formed in MKs are important for platelet function. Proplatelets are firstly generated from mature MKs and then become platelets. The proplatelet production process was verified by novel studies that revealed that the mechanism is partially regulated by the invaginated membrane system (IMS), microtubules and Rho GTPases. The extracellular matrices (ECMs) and shear stress also affect and regulate the process while the mature MKs migrate from the marrow to the sub-endothelium region near the venous sinusoids leading to the release of platelets into the circulation. This review describes the entire process of megakaryopoiesis in detail, illustrates both the transcriptional and microenvironmental regulation of MKs and provides insight into platelet biogenesis.

Inherited thrombocytopenias frequently diagnosed in adults

The diagnosis of inherited thrombocytopenias is difficult, for many reasons. First, as they are all rare diseases, they are little known by clinicians, who therefore tend to suspect the most common forms. Second, making a definite diagnosis often requires complex laboratory techniques that are available in only a few centers. Finally, half of the patients have forms that have not yet been described. As a consequence, many patients with inherited thrombocytopenias are misdiagnosed with immune thrombocytopenia, and are at risk of receiving futile treatments. Misdiagnosis is particularly frequent in patients whose low platelet count is discovered in adult life, because, in these cases, even the inherited origin of thrombocytopenia may be missed. Making the correct diagnosis promptly is important, as we recently learned that some forms of inherited thrombocytopenia predispose to other illnesses, such as leukemia or kidney failure, and affected subjects therefore require close surveillance and, if necessary, prompt treatments. Moreover, medical treatment can increase platelet counts in specific disorders, and affected subjects can therefore receive drugs instead of platelet transfusions when selective surgery is required. In this review, we will discuss how to suspect, diagnose and manage inherited thrombocytopenias, with particular attention to the forms that frequently present in adults. Moreover, we describe four recently identified disorders that belong to this group of disorders that are often diagnosed in adults: MYH9-related disease, monoallelic Bernard-Soulier syndrome, ANKRD26-related thrombocytopenia, and familial platelet disorder with predisposition to acute leukemia.

Transcription factors in late megakaryopoiesis and related platelet disorders

Cell type-specific transcription factors regulate the repertoire of genes expressed in a cell and thereby determine its phenotype. The differentiation of megakaryocytes, the platelet progenitors, from hematopoietic stem cells is a well-known process that can be mimicked in culture. However, the efficient formation of platelets in culture remains a challenge. Platelet formation is a complicated process including megakaryocyte maturation, platelet assembly and platelet shedding. We hypothesize that a better understanding of the transcriptional regulation of this process will allow us to influence it such that sufficient numbers of platelets can be produced for clinical applications. After an introduction to gene regulation and platelet formation, this review summarizes the current knowledge of the regulation of platelet formation by the transcription factors EVI1, GATA1, FLI1, NFE2, RUNX1, SRF and its co-factor MKL1, and TAL1. Also covered is how some platelet disorders including myeloproliferative neoplasms, result from disturbances of the transcriptional regulation. These disorders give us invaluable insights into the crucial role these transcription factors play in platelet formation. Finally, there is discussion of how a better understanding of these processes will be needed to allow for efficient production of platelets in vitro.