Megakaryocytes and beyond: the birth of platelets

Compound K Promotes Megakaryocytic Differentiation by NLRP3 Inflammasome Activation

Platelets are essential blood components that maintain hemostasis, prevent excessive bleeding, and facilitate wound healing. Reduced platelet counts are implicated in various diseases, including leukemia, hepatitis, cancer, and Alzheimer's disease. Enhancing megakaryocytic differentiation is a promising strategy to increase platelet production. Compound K (CK), a major bioactive metabolite of ginsenosides from Panax ginseng, has demonstrated anti-cancer and neuroprotective properties. In this study, we investigated the effects of CK on megakaryocytic differentiation and apoptosis in chronic myeloid leukemia (CML) cell lines K562 and Meg-01. CK treatment significantly upregulated the mRNA expression of key megakaryocytic differentiation markers, including CD61, CD41, and CD42a, and promoted the formation of large, multinucleated cells in K562 cells. Additionally, flow cytometry analysis revealed that CK at 5 µM induced apoptosis, a critical process in thrombocytopoiesis, in both K562 and Meg-01 cells. RT2 Profiler PCR array analysis further identified a marked increase in the expression of genes associated with the activation of the NLRP3 inflammasome in CK-treated K562 and Meg-01 cells. This study is the first to demonstrate that CK promotes megakaryocytic differentiation and apoptosis through the activation of the ERK/EGR1 and NLRP3 inflammasome pathways. These findings suggest that CK may enhance platelet production, indicating its potential as a therapeutic candidate for platelet-related disorders and other associated diseases.

Unleashing the biomimetic targeting potential of platelet-derived nanocarriers on atherosclerosis

Atherosclerosis, the primary mechanism underlying the development of many cardiovascular illnesses, continues to be one of the leading causes of mortality worldwide. Platelet (PLT), which are essential for maintaining body homeostasis, have been strongly linked to the onset of atherosclerosis at various stages due to their inherent tendency to bind to atherosclerotic lesions and show an affinity for plaques. Therefore, mimicking PLT's innate adhesive features may be necessary to effectively target plaques. PLT-derived nanocarriers have emerged as a promising biomimetic targeting strategy for treating atherosclerosis due to their numerous advantages. These advantages include excellent biocompatibility, minimal macrophage phagocytosis, prolonged circulation time, targeting capability for impaired vascular sites, and suitability as carriers for anti-atherosclerotic drugs. Herein, we discuss the role of PLT in atherogenesis and propose the design of nanocarriers based on PLT-membrane coating and PLT-derived vesicles. These nanocarriers can target multiple biological elements relevant to plaque development. The review also emphasizes the current challenges and future research directions for the effective utilization of PLT-derived nanocarriers in treating atherosclerosis.

Recent advances in molecular mechanisms of skin wound healing and its treatments

The skin, being a multifaceted organ, performs a pivotal function in the complicated wound-healing procedure, which encompasses the triggering of several cellular entities and signaling cascades. Aberrations in the typical healing process of wounds may result in atypical scar development and the establishment of a persistent condition, rendering patients more vulnerable to infections. Chronic burns and wounds have a detrimental effect on the overall quality of life of patients, resulting in higher levels of physical discomfort and socio-economic complexities. The occurrence and frequency of prolonged wounds are on the rise as a result of aging people, hence contributing to escalated expenditures within the healthcare system. The clinical evaluation and treatment of chronic wounds continue to pose challenges despite the advancement of different therapeutic approaches. This is mainly owing to the prolonged treatment duration and intricate processes involved in wound healing. Many conventional methods, such as the administration of growth factors, the use of wound dressings, and the application of skin grafts, are used to ease the process of wound healing across diverse wound types. Nevertheless, these therapeutic approaches may only be practical for some wounds, highlighting the need to advance alternative treatment modalities. Novel wound care technologies, such as nanotherapeutics, stem cell treatment, and 3D bioprinting, aim to improve therapeutic efficacy, prioritize skin regeneration, and minimize adverse effects. This review provides an updated overview of recent advancements in chronic wound healing and therapeutic management using innovative approaches.

Exploring the Role of Platelets in Virus-Induced Inflammatory Demyelinating Disease and Myocarditis

Theiler's murine encephalomyelitis virus (TMEV) infection has been used as a mouse model for two virus-induced organ-specific immune-mediated diseases. TMEV-induced demyelinating disease (TMEV-IDD) in the central nervous system (CNS) is a chronic inflammatory disease with viral persistence and an animal model of multiple sclerosis (MS) in humans. TMEV infection can also cause acute myocarditis with viral replication and immune cell infiltration in the heart, leading to cardiac fibrosis. Since platelets have been reported to modulate immune responses, we aimed to determine the role of platelets in TMEV infection. In transcriptome analyses of platelets, distinct sets of immune-related genes, including major histocompatibility complex (MHC) class I, were up- or downregulated in TMEV-infected mice at different time points. We depleted platelets from TMEV-infected mice by injecting them with platelet-specific antibodies. The platelet-depleted mice had significantly fewer viral antigen-positive cells in the CNS. Platelet depletion reduced the severities of TMEV-IDD and myocarditis, although the pathology scores did not reach statistical significance. Immunologically, the platelet-depleted mice had an increase in interferon (IFN)-γ production with a higher anti-TMEV IgG2a/IgG1 ratio. Thus, platelets may play roles in TMEV infection, such as gene expression, viral clearance, and anti-viral antibody isotype responses.

Platelets and the Atherosclerotic Process: An Overview of New Markers of Platelet Activation and Reactivity, and Their Implications in Primary and Secondary Prevention

The key role played by platelets in the atherosclerosis physiopathology, especially in the acute setting, is ascertained: they are the main actors during thrombus formation and, thus, one of the major investigated elements related to atherothrombotic process involving coronary arteries. Platelets have been studied from different points of view, according with the technology advances and the improvement in the hemostasis knowledge achieved in the last years. Morphology and reactivity constitute the first aspects investigated related to platelets with a significant body of evidence published linking a number of their values and markers to coronary artery disease and cardiovascular events. Recently, the impact of genetics on platelet activation has been explored with promising findings as additional instrument for patient risk stratification; however, this deserves further confirmations. Moreover, the interplay between immune system and platelets has been partially elucidated in the last years, providing intriguing elements that will be basic components for future research to better understand platelet regulation and improve cardiovascular outcome of patients.

Platelets at the Vessel Wall in Non-Thrombotic Disease

Platelets are small, anucleate entities that bud from megakaryocytes in the bone marrow. Among circulating cells, platelets are the most abundant cell, traditionally involved in regulating the balance between thrombosis (the terminal event of platelet activation) and hemostasis (a protective response to tissue injury). Although platelets lack the precise cellular control offered by nucleate cells, they are in fact very dynamic cells, enriched in preformed RNA that allows them the capability of de novo protein synthesis which alters the platelet phenotype and responses in physiological and pathological events. Antiplatelet medications have significantly reduced the morbidity and mortality for patients afflicted with thrombotic diseases, including stroke and myocardial infarction. However, it has become apparent in the last few years that platelets play a critical role beyond thrombosis and hemostasis. For example, platelet-derived proteins by constitutive and regulated exocytosis can be found in the plasma and may educate distant tissue including blood vessels. First, platelets are enriched in inflammatory and anti-inflammatory molecules that may regulate vascular remodeling. Second, platelet-derived microparticles released into the circulation can be acquired by vascular endothelial cells through the process of endocytosis. Third, platelets are highly enriched in mitochondria that may contribute to the local reactive oxygen species pool and remodel phospholipids in the plasma membrane of blood vessels. Lastly, platelets are enriched in proteins and phosphoproteins which can be secreted independent of stimulation by surface receptor agonists in conditions of disturbed blood flow. This so-called biomechanical platelet activation occurs in regions of pathologically narrowed (atherosclerotic) or dilated (aneurysmal) vessels. Emerging evidence suggests platelets may regulate the process of angiogenesis and blood flow to tumors as well as education of distant organs for the purposes of allograft health following transplantation. This review will illustrate the potential of platelets to remodel blood vessels in various diseases with a focus on the aforementioned mechanisms.

Functional analysis and expression profile of human platelets infected by EBV in vitro

Platelet activation is commonly detected after infection by multiple viruses such as human immunodeficiency virus (HIV), H1N1 influenza, Hepatitis C virus (HCV), Ebola virus (EBV), and Dengue virus (DENV). Non-coding RNAs (ncRNAs) constitute the majority of the human transcribed genome, but the biology of platelet ncRNAs is largely unexplored. In this study, we performed microarray profiling to characterize the expression profile of human platelets infected with EBV in vitro after 2 h. A total of 187 long non-coding RNAs (lncRNAs) displayed differences, of which 114 were upregulated and 73 were downregulated; 78 microRNAs (miRNAs) showed differences, including 73 upregulated and 5 downregulated; 808 mRNAs displayed differences, among which 367 were upregulated and 441 were downregulated. Gene ontology (GO) analysis mostly related to G protein-coupled receptor signaling pathway, detection of chemical stimulus involved in sensory perception of smell and regulation of transcription by RNA polymerase II. Pathway analysis showed that the differentially expressed genes were mainly enriched in cell metabolism and immune-related response. A ceRNA network was established based on predicting regulatory pairs in differentially expressed genes, in which hsa-miR-6877-3p had the highest regulatory capability (degree = 31), FAM230A was the lncRNA with the highest regulatory capability (degree = 28). According to the EBV related miRNA regulation network, it revealed that ebv-miR-BART19-3p had the most target genes and BRWD1, FAM126B, TFRC and JMY were the genes most regulated by EBV-related miRNAs. After overlapping the three networks, we found that the EIFAK2 gene was strongly correlated with autologous ncRNAs, including hsa-miR-1972, hsa-miR-504-3p and hsa-miR-6825-5p, as well as with EBV ncRNAs, including EBER1, EBER2, miR-BART7-3p and miR-BART16. The present study contributes to a better understanding of the expression profiling of ncRNAs and their functions in platelets activated by EBV in vitro, and paves the way to further study on platelet function.

Cell-derived membrane biomimetic nanocarriers for targeted therapy of pulmonary disease

Pulmonary diseases are currently one of the major threats of human health, especially considering the recent COVID-19 pandemic. However, the current treatments are facing the challenges like insufficient local drug concentrations, the fast lung clearance and risks to induce unexpected inflammation. Cell-derived membrane biomimetic nanocarriers are recently emerged delivery strategy, showing advantages of long circulation time, excellent biocompatibility and immune escape ability. In this review, applications of using cell-derived membrane biomimetic nanocarriers from diverse cell sources for the targeted therapy of pulmonary disease were summarized. In addition, improvements of the cell-derived membrane biomimetic nanocarriers for augmented therapeutic ability against different kinds of pulmonary diseases were introduced. This review is expected to provide a general guideline for the potential applications of cell-derived membrane biomimetic nanocarriers to treat pulmonary diseases.

Platelets as Key Factors in Inflammation: Focus on CD40L/CD40

Platelets are anucleate cytoplasmic fragments derived from the fragmentation of medullary megakaryocytes. Activated platelets adhere to the damaged endothelium by means of glycoproteins on their surface, forming the platelet plug. Activated platelets can also secrete the contents of their granules, notably the growth factors contained in the α-granules, which are involved in platelet aggregation and maintain endothelial activation, but also contribute to vascular repair and angiogenesis. Platelets also have a major inflammatory and immune function in antibacterial defence, essentially through their Toll-like Receptors (TLRs) and Sialic acid-binding immunoglobulin-type lectin (SIGLEC). Platelet activation also contributes to the extensive release of anti- or pro-inflammatory mediators such as IL-1β, RANTES (Regulated on Activation, Normal T Expressed and Secreted) or CD154, also known as the CD40-ligand. Platelets are involved in the direct activation of immune cells, polynuclear neutrophils (PNNs) and dendritic cells via the CD40L/CD40 complex. As a general rule, all of the studies presented in this review show that platelets are capable of covering most of the stages of inflammation, primarily through the CD40L/CD40 interaction, thus confirming their own role in this pathophysiological condition.

Diagnostic value of platelet parameters and megakaryocyte formula of bone marrow in idiopathic thrombocytopenic purpura

The parameters of the megakaryocyte formula and peripheral blood platelet indices were studied in 237 people with diagnoses of idiopathic thrombocytopenic purpura (ITP) and myelodysplastic syndrome (MDS). A correlation analysis was performed between megakaryocyte subpopulations and platelet counts. The threshold values for MPV, PCT and PDW were determined by ROC analysis with the construction of the ROC curve, the calculation of the area under the curve (AUC) and the cutoff threshold. The obtained values make it possible to make a differential diagnosis between ITP and MDS by platelet parameters of peripheral blood without examining the bone marrow.

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.

Evaluation of angiogenic signaling molecules associated with reactive thrombocytosis in an iron-deficient rat model

BACKGROUND

Iron deficiency anemia (IDA)-induced reactive thrombocytosis can occur in children and adults. The underlying mechanism for this phenomenon is indeterminate. Traditional cytokines such as thrombopoietin (TPO), interleukin-6 (IL-6), and IL-11 involved in megakaryopoiesis have not been shown to be the cause. Recent studies suggest that growth factors and signaling molecules involved with angiogenesis influence the proliferation and differentiation of megakaryocytes.

METHODS

We investigated the possible association between angiogenic cytokines with reactive thrombocytosis due to IDA in an iron-deficient (ID) rat model. Complete blood count, iron panels, and TPO levels were measured at baseline and 5 weeks later in both control (C) and ID rats. Angiogenic cytokines were evaluated in the bone marrow in all rats.

RESULTS

We successfully induced IDA in our rats by phlebotomy and reduced iron diet. We did not find an increase of TPO in ID rats. A review of the bone marrow showed an increase in the number of megakaryocytes, vascular structures, as well as increased intensity of stain for vascular endothelial growth factor (VEGF), and CXC chemokine receptor 4 (CXCR4) in rats with IDA compared to controls.

CONCLUSIONS

Our results of histological bone marrow data suggest an important role for angiogenesis in the development of IDA-induced thrombocytosis.

IMPACT

Thrombocytosis is common with IDA in both children and adults, but the mechanism is unclear. We confirmed that TPO is not the major driver of iron deficiency-associated thrombocytosis. We confirmed the increase in the number of megakaryocytes in the bone marrow despite stable TPO levels. We provided evidence supporting an important role of angiogenesis in megakaryocytopoiesis/thrombopoiesis with increased vascular structures and angiogenic cytokines in the bone marrow of iron-deficient rats. The demonstration that angiogenesis may play an important role in secondary thrombocytosis could lead to a new approach in treating symptomatic reactive thrombocytosis by targeting angiogenesis.

Association of human breast cancer CD44-/CD24- cells with delayed distant metastasis

Tumor metastasis remains the main cause of breast cancer-related deaths, especially delayed breast cancer distant metastasis. The current study assessed the frequency of CD44^-/CD24^- breast cancer cells in 576 tissue specimens for associations with clinicopathological features and metastasis and investigated the underlying molecular mechanisms. The results indicated that higher frequency (≥19.5%) of CD44^-/CD24^- cells was associated with delayed postoperative breast cancer metastasis. Furthermore, CD44^-/CD24^-triple negative breast cancer (TNBC) cells spontaneously converted into CD44⁺/CD24^-cancer stem cells (CSCs) with properties similar to CD44⁺/CD24^-CSCs from primary human breast cancer cells and parental TNBC cells in terms of stemness marker expression, self-renewal, differentiation, tumorigenicity, and lung metastasis in vitro and in NOD/SCID mice. RNA sequencing identified several differentially expressed genes (DEGs) in newly converted CSCs and RHBDL2, one of the DEGs, expression was upregulated. More importantly, RHBDL2 silencing inhibited the YAP1/USP31/NF-κB signaling and attenuated spontaneous CD44^-/CD24^- cell conversion into CSCs and their mammosphere formation. These findings suggest that the frequency of CD44^-/CD24^- tumor cells and RHBDL2 may be valuable for prognosis of delayed breast cancer metastasis, particularly for TNBC.

Structural characterization of anticoagulant and antithrombotic polysaccharides isolated from Caesalpinia ferrea stem barks

This study aimed to isolate, characterize chemical-structurally and evaluate the effects of polysaccharides from Caesalpinia (Libidibia) ferrea stem barks in the haemostatic system. The deproteinated-polysaccharide extract (PE-Cf) after being fractionated by ion exchange chromatography-DEAE-cellulose resulted in three fractions (FI, FII, FIII) containing total carbohydrates (14.3-38%), including uronic acid (5-16%), and polyphenols (0.94-1.7 mg/g GAE). The polysaccharide fractions presented polydisperse profile in polyacrylamide gel electrophoresis (detected by Stains-All) and molecular masses (9.5 × 10⁴ Da-1.5 × 10⁵ Da) identified by gel permeation chromatography. FT-IR showed absorption bands (1630 cm^-1, 1396-1331 cm^-1), indicative of uronic acid, and a band at 1071 cm^-1, typical of COO^- groups of galacturonic acid. The NMR spectra of C. ferrea polysaccharides revealed a central core composed mainly by 5-linked α-Araf and minority components as α-Rhap and α-GalAp. UV spectra of fractions revealed discrete shoulders at 269-275 nm, characteristic of polyphenolic compounds. In vitro, polysaccharides inhibited the intrinsic and/or common coagulation pathway (aPTT test) (2.0-3.7 fold) and the platelet aggregation induced by 3 μM adenosine diphosphate (25-48%) and 5 μg/mL collagen (24%), but not that induced by arachidonic acid. In vivo, the polysaccharides inhibited (36-69%) venous thrombosis induced by hypercoagulability and stasis, showing discrete hemorrhagic effect. In conclusion, the polysaccharides of C. ferrea barks, containing arabinose, galactose, rhamnose and uronic acid, possess anticoagulant, antiplatelet and antithrombotic properties of low hemorrhagic risk, suggesting potential applicability in thromboembolic disorders.

Shared and Distinctive Ultrastructural Abnormalities Expressed by Megakaryocytes in Bone Marrow and Spleen From Patients With Myelofibrosis

Numerous studies have documented ultrastructural abnormalities in malignant megakaryocytes from bone marrow (BM) of myelofibrosis patients but the morphology of these cells in spleen, an important extramedullary site in this disease, was not investigated as yet. By transmission-electron microscopy, we compared the ultrastructural features of megakaryocytes from BM and spleen of myelofibrosis patients and healthy controls. The number of megakaryocytes was markedly increased in both BM and spleen. However, while most of BM megakaryocytes are immature, those from spleen appear mature with well-developed demarcation membrane systems (DMS) and platelet territories and are surrounded by platelets. In BM megakaryocytes, paucity of DMS is associated with plasma (thick with protrusions) and nuclear (dilated with large pores) membrane abnormalities and presence of numerous glycosomes, suggesting a skewed metabolism toward insoluble polyglucosan accumulation. By contrast, the membranes of the megakaryocytes from the spleen were normal but these cells show mitochondria with reduced crests, suggesting deficient aerobic energy-metabolism. These distinctive morphological features suggest that malignant megakaryocytes from BM and spleen express distinctive metabolic impairments that may play different roles in the pathogenesis of myelofibrosis.

Role of platelet biomarkers in inflammatory response

Beyond hemostasis, thrombosis and wound healing, it is becoming increasingly clear that platelets play an integral role in inflammatory response and immune regulation. Platelets recognize pathogenic microorganisms and secrete various immunoregulatory cytokines and chemokines, thus facilitating a variety of immune effects and regulatory functions. In this review, we discuss recent advances in signaling of platelet activation-related biomarkers in inflammatory settings and application prospects to apply for disease diagnosis and treatment.

Phospho-inositide-dependent kinase 1 regulates signal dependent translation in megakaryocytes and platelets

BACKGROUND

Regulated protein synthesis is essential for megakaryocyte (MK) and platelet functions, including platelet production and activation. PDK1 (phosphoinositide-dependent kinase 1) regulates platelet functional responses and has been associated with circulating platelet counts. Whether PDK1 also directly regulates protein synthetic responses in MKs and platelets, and platelet production by MKs, remains unknown.

OBJECTIVE

To determine if PDK1 regulates protein synthesis in MKs and platelets.

METHODS

Pharmacologic PDK1 inhibitors (BX-795) and mice where PDK1 was selectively ablated in MKs and platelets (PDK1-/- ) were used. PDK1 signaling in MKs and platelets (human and murine) were assessed by immunoblots. Activation-dependent translation initiation and protein synthesis in MKs and platelets was assessed by probing for dissociation of eIF4E from 4EBP1, and using m7-GTP pulldowns and S35 methionine incorporation assays. Proplatelet formation by MKs, synthesis of Bcl-3 and MARCKs protein, and clot retraction were employed for functional assays.

RESULTS

Inhibiting or ablating PDK1 in MKs and platelets abolished the phosphorylation of 4EBP1 and eIF4E by preventing activation of the PI3K and MAPK pathways. Inhibiting PDK1 also prevented dissociation of eIF4E from 4EBP1, decreased binding of eIF4E to m7GTP (required for translation initiation), and significantly reduced de novo protein synthesis. Inhibiting PDK1 reduced proplatelet formation by human MKs and blocked MARCKs protein synthesis. In both human and murine platelets, PDK1 controlled Bcl-3 synthesis. Inhibition of PDK1 led to complete failure of clot retraction in vitro.

CONCLUSIONS

PDK1 is a previously unidentified translational regulator in MKs and platelets, controlling protein synthetic responses, proplatelet formation, and clot retraction.

Deficiency of platelet adhesion molecule CD226 causes megakaryocyte development and platelet hyperactivity

This study used constitutive CD226 gene knockout (KO) mice as a model to investigate the functions and mechanisms of CD226 in megakaryocyte (MK) maturation and platelet activation. Although CD226 deficiency did not cause MK polyploidization or platelet granule abnormalities, increased MK counts were detected in the femora bone marrow (BM) and spleen of CD226 KO mice. Particularly, CD226 KO mice have a more extensive membrane system in MKs and platelets than wild-type (WT) mice. We also demonstrated that CD226 KO mice displayed increased platelet counts, shortened bleeding time, and enhanced platelet aggregation. CD226 KO platelets had an increased mature platelet ratio compared to the control platelets. In addition, the observed reduction in bleeding time may be due to decreased nitric oxide (NO) production in the platelets. Platelet-specific CD226-deficient mice showed similar increased MK counts, shortened bleeding time, enhanced platelet aggregation, and decreased NO production in platelets. Furthermore, we performed middle cerebral artery occlusion-reperfusion surgery on WT and CD226 KO mice to explore the potential effect of CD226 on acute ischemia-reperfusion injury; the results revealed that CD226 deficiency led to significantly increased infarct area. Thus, CD226 is a promising candidate for the treatment of thrombotic disorders.

New insights into cytoskeletal remodeling during platelet production

The past decade has brought unprecedented advances in our understanding of megakaryocyte (MK) biology and platelet production, processes that are strongly dependent on the cytoskeleton. Facilitated by technological innovations, such as new high-resolution imaging techniques (in vitro and in vivo) and lineage-specific gene knockout and reporter mouse strains, we are now able to visualize and characterize the molecular machinery required for MK development and proplatelet formation in live mice. Whole genome and RNA sequencing analysis of patients with rare platelet disorders, combined with targeted genetic interventions in mice, has led to the identification and characterization of numerous new genes important for MK development. Many of the genes important for proplatelet formation code for proteins that control cytoskeletal dynamics in cells, such as Rho GTPases and their downstream targets. In this review, we discuss how the final stages of MK development are controlled by the cellular cytoskeletons, and we compare changes in MK biology observed in patients and mice with mutations in cytoskeleton regulatory genes.

Aspirin eugenol ester inhibits agonist-induced platelet aggregation in vitro by regulating PI3K/Akt, MAPK and Sirt 1/CD40L pathways

Aspirin eugenol ester (AEE) was a promising drug candidate for treating inflammation, pain and fever and preventing cardiovascular diseases with fewer side effects than its precursors. Previous researches indicated that AEE could markedly inhibit agonist-induced platelet aggregation in vitro and ex vivo, however, the anti-platelet aggregation mechanisms of AEE remain to be defined. Here, AEE in vitro effects on agonist-induced granule-secretion, intercellular Ca²⁺ mobilization and thromboxane A₂ (TXA₂) generation were examined. Vasodilator-stimulated phosphoprotein (VASP), mitogen-activated protein kinase (MAPK), Akt, Sirt 1 and CD40L expressions were also studied. In agonist-activated platelets in vitro, AEE markedly attenuated granule secretion markers (P-selectin expression and ATP release), intercellular Ca²⁺ mobilization and thromboxane B₂ (TXB₂) formation. AEE also attenuated CD40L activation, suppressed extracellular-signal-regulated protein kinase 2 (ERK2), c-Jun N-terminal kinase 1 (JNK1) and Akt phosphorylation, and recovered Sirt1 expression, but the activation of p38, VASP^Ser157 and VASP^Ser239, and the levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) were not affected by AEE. Overall, this study demonstrates that AEE inhibits agonist-induced platelet aggregation in vitro by regulating PI3K/Akt, MAPK and Sirt 1/CD40L pathways.

Role of p53 and transcription-independent p53-induced apoptosis in shear-stimulated megakaryocytic maturation, particle generation, and platelet biogenesis

Megakaryocytes (Mks) derive from hematopoietic stem and progenitor cells (HSPCs) in the bone marrow and develop into large, polyploid cells that eventually give rise to platelets. As Mks mature, they migrate from the bone marrow niche into the vasculature, where they are exposed to shear forces from blood flow, releasing Mk particles (platelet-like particles (PLPs), pro/preplatelets (PPTs), and Mk microparticles (MkMPs)) into circulation. We have previously shown that transcription factor p53 is important in Mk maturation, and that physiological levels of shear promote Mk particle generation and platelet biogenesis. Here we examine the role of p53 in the Mk shear-stress response. We show that p53 is acetylated in response to shear in both immature and mature Mks, and that decreased expression of deacetylase HDAC1, and increased expression of the acetyltransferases p300 and PCAF might be responsible for these changes. We also examined the hypothesis that p53 might be involved in the shear-induced Caspase 3 activation, phosphatidylserine (PS) externalization, and increased biogenesis of PLPs, PPTs, and MkMPs. We show that p53 is involved in all these shear-induced processes. We show that in response to shear, acetyl-p53 binds Bax, cytochrome c is released from mitochondria, and Caspase 9 is activated. We also show that shear-stimulated Caspase 9 activation and Mk particle biogenesis depend on transcription-independent p53-induced apoptosis (TIPA), but PS externalization is not. This is the first report to show that shear flow stimulates TIPA and that Caspase 9 activation and Mk-particle biogenesis are directly modulated by TIPA.

Platelet communication with the vascular wall: role of platelet-derived microparticles and non-coding RNAs

Platelets play an important role in vascular homeostasis through their interaction with circulating blood cells as well as the vascular wall. Platelet-mediated communication with other cells can take the form of direct cell-cell interactions via membrane receptors or indirectly through the release of different soluble factors stored in their granules as well as through the release of microparticles. The latter carry different proteins and RNAs which are transferred to the target cells. The aim of this review is to discuss the role of platelet-derived factors, adhesion molecules as well as RNAs as mediators of the cross-talk between platelets and the vessel wall.

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.

A Combination of Immune Checkpoint Inhibition with Metronomic Chemotherapy as a Way of Targeting Therapy-Resistant Cancer Cells

Therapeutic resistance remains a major obstacle in treating many cancers, particularly in advanced stages. It is likely that cytotoxic lymphocytes (CTLs) have the potential to eliminate therapy-resistant cancer cells. However, their effectiveness may be limited either by the immunosuppressive tumor microenvironment, or by immune cell death induced by cytotoxic treatments. High-frequency low-dose (also known as metronomic) chemotherapy can help improve the activity of CTLs by providing sufficient stimulation for cytotoxic immune cells without excessive depletion. Additionally, therapy-induced removal of tumor cells that compete for shared nutrients may also facilitate tumor infiltration by CTLs, further improving prognosis. Metronomic chemotherapy can also decrease the number of immunosuppressive cells in the tumor microenvironment, including regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs). Immune checkpoint inhibition can further augment anti-tumor immune responses by maintaining T cells in an activated state. Combining immune checkpoint inhibition with metronomic administration of chemotherapeutic drugs may create a synergistic effect that augments anti-tumor immune responses and clears metabolic competition. This would allow immune-mediated elimination of therapy-resistant cancer cells, an effect that may be unattainable by using either therapeutic modality alone.

Bleeding and thrombosis in chronic ventricular assist device therapy: focus on platelets

PURPOSE OF REVIEW

Left ventricular assist devices (LVADs) have markedly improved the survival for patients with advanced heart failure but are plagued with significant morbidity, including pump thrombosis and bleeding. Better understanding of the platelet, and its role in the balance of bleeding and thrombosis, stands to impact the frequency and treatment of these significant complications.

RECENT FINDINGS

In patients with LVADs, there is little consistency linking traditional biomarkers of platelet activation and clinical events. A number of innovative methods of assessing platelet functionality, including shedding of platelet receptors and formation of microparticle complexes as well as measuring mitochondrial membrane potentials, exist and appear to be clinically relevant. Acquired von Willebrand syndrome, while not explaining all bleeding events, is a central feature of mechanical support and offers a target for innovative therapies.

SUMMARY

Although the platelet is only one component of impacting thrombosis and bleeding in patients supported with LVADs, it plays a central role in mediating these two opposing forces. Innovations in understanding platelet physiology as well as manipulating genomic and receptor interactions for an individual patient will be critical if we are to decrease these serious adverse events in the future.

The Plant Hormone Abscisic Acid Is a Prosurvival Factor in Human and Murine Megakaryocytes

Abscisic acid (ABA) is a phytohormone involved in pivotal physiological functions in higher plants. Recently, ABA has been proven to be also secreted and active in mammals, where it stimulates the activity of innate immune cells, mesenchymal and hematopoietic stem cells, and insulin-releasing pancreatic β cells through a signaling pathway involving the second messenger cyclic ADP-ribose (cADPR). In addition to behaving like an animal hormone, ABA also holds promise as a nutraceutical plant-derived compound in humans. Many biological functions of ABA in mammals are mediated by its binding to the LANCL-2 receptor protein. A putative binding of ABA to GRP78, a key regulator of endoplasmic reticulum stress, has also been proposed. Here we investigated the role of exogenous ABA in modulating thrombopoiesis, the process of platelet generation. Our results demonstrate that expression of both LANCL-2 and GRP78 is up-regulated during hematopoietic stem cell differentiation into mature megakaryocytes (Mks). Functional ABA receptors exist in mature Mks because ABA induces an intracellular Ca²⁺ increase ([Ca²⁺] _i ) through PKA activation and subsequent cADPR generation. In vitro exposure of human or murine hematopoietic progenitor cells to 10 μm ABA does not increase recombinant thrombopoietin (rTpo)-dependent Mk differentiation or platelet release. However, under conditions of cell stress induced by rTpo and serum deprivation, ABA stimulates, in a PKA- and cADPR-dependent fashion, the mitogen-activated kinase ERK 1/2, resulting in the modulation of lymphoma 2 (Bcl-2) family members, increased Mk survival, and higher rates of platelet production. In conclusion, we demonstrate that ABA is a prosurvival factor for Mks in a Tpo-independent manner.

Diagnostic value of platelet indices and bone marrow megakaryocytic parameters in immune thrombocytopenic purpura

Platelet indices could mirror megakaryopoietic activity in immune thrombocytopenic purpura (ITP), but its specificity and sensitivity need to be studied. The diagnostic performance of platelet indices was analyzed by receiver-operating characteristic curves, and the probability of true positive (sensitivity) and true negative (specificity) in predicting ITP, myelodysplasia, or controls was determined. Mean platelet volume (MPV) was higher, whereas plateletcrit (PCT) was significantly lower in ITP than in myelodysplasia and controls. The platelet distribution width in ITP patients was lower than in myelodysplasia, but higher than in controls. Increased megakaryocytes were only observed in ITP. A strong positive correlation was found between MPV and quantities of granular megakaryocytes, whereas a negative relationship existed between MPV and platelet-form megakaryocytes. In receiver-operating characteristic analysis, MPV and PCT gave a sensitivity of 70.3% (89.8%) and specificity of 74.8% (84.7%) at a cutoff of 9.35 (0.085) in diagnosis of ITP. Combined parallel test of MPV and PCT increased the sensitivity to 97.5 with 64.1% specificity, whereas series test increased the specificity to 94.7 with 62.7% sensitivity. Our results suggest that MPV, PCT, and platelet distribution width represent megakaryopoietic activity in bone marrow and may be reliable markers in ITP diagnosis.

Normal Wound Healing and Tumor Angiogenesis as a Game of Competitive Inhibition

Both normal wound healing and tumor angiogenesis are mitigated by the sequential, carefully orchestrated release of growth stimulators and inhibitors. These regulators are released from platelet clots formed at the sites of activated endothelium in a temporally and spatially controlled manner, and the order of their release depends on their affinity to glycosaminoglycans (GAG) such as heparan sulfate (HS) within the extracellular matrix, and platelet open canallicular system. The formation of vessel sprouts, triggered by angiogenesis regulating factors with lowest affinities for heparan sulfate (e.g. VEGF), is followed by vessel-stabilizing PDGF-B or bFGF with medium affinity for HS, and by inhibitors such as PF-4 and TSP-1 with the highest affinities for HS. The invasive wound-like edge of growing tumors has an overabundance of angiogenesis stimulators, and we propose that their abundance out-competes angiogenesis inhibitors, effectively preventing inhibition of angiogenesis and vessel maturation. We evaluate this hypothesis using an experimentally motivated agent-based model, and propose a general theoretical framework for understanding mechanistic similarities and differences between the processes of normal wound healing and pathological angiogenesis from the point of view of competitive inhibition.

Pathophysiological Significance of Store-Operated Calcium Entry in Megakaryocyte Function: Opening New Paths for Understanding the Role of Calcium in Thrombopoiesis

Store-Operated Calcium Entry (SOCE) is a universal calcium (Ca²⁺) influx mechanism expressed by several different cell types. It is now known that Stromal Interaction Molecule (STIM), the Ca²⁺ sensor of the intracellular compartments, together with Orai and Transient Receptor Potential Canonical (TRPC), the subunits of Ca²⁺ permeable channels on the plasma membrane, cooperate in regulating multiple cellular functions as diverse as proliferation, differentiation, migration, gene expression, and many others, depending on the cell type. In particular, a growing body of evidences suggests that a tight control of SOCE expression and function is achieved by megakaryocytes along their route from hematopoietic stem cells to platelet production. This review attempts to provide an overview about the SOCE dynamics in megakaryocyte development, with a focus on most recent findings related to its involvement in physiological and pathological thrombopoiesis.

Is there a treatment protocol in which platelet-rich plasma is effective?

AIM

We aimed to reveal whether there are prospective suggestions for effective and standard platelet-rich plasma applications.

METHODS

We searched for clinical trials and traced all the references of incorporated documents.

RESULTS

In literature, there was no study indicating which disease is treated by which mechanism of action, how much dose and content are prepared and applied, when the treatment is applied and how many cures are applied.

CONCLUSION

Guides introducing which concentrations of PRP are used for which diseases are to be prepared immediately by a committee which is comprised of primarily orthopedists, clinical pharmacologists and toxicologists.

Megakaryocytic Maturation in Response to Shear Flow Is Mediated by the Activator Protein 1 (AP-1) Transcription Factor via Mitogen-activated Protein Kinase (MAPK) Mechanotransduction

Megakaryocytes (MKs) are exposed to shear flow as they migrate from the bone marrow hematopoietic compartment into circulation to release pro/preplatelets into circulating blood. Shear forces promote DNA synthesis, polyploidization, and maturation in MKs, and platelet biogenesis. To investigate mechanisms underlying these MK responses to shear, we carried out transcriptional analysis on immature and mature stem cell-derived MKs exposed to physiological shear. In immature (day (d)9) MKs, shear exposure up-regulated genes related to growth and MK maturation, whereas in mature (d12) MKs, it up-regulated genes involved in apoptosis and intracellular transport. Following shear-flow exposure, six activator protein 1 (AP-1) transcripts (ATF4,JUNB,JUN,FOSB,FOS, andJUND) were up-regulated at d9 and two AP-1 proteins (JunD and c-Fos) were up-regulated both at d9 and d12. We show that mitogen-activated protein kinase (MAPK) signaling is linked to both the shear stress response and AP-1 up-regulation. c-Jun N-terminal kinase (JNK) phosphorylation increased significantly following shear stimulation, whereas JNK inhibition reduced shear-induced JunD expression. Although p38 phosphorylation did not increase following shear flow, its inhibition reduced shear-induced JunD and c-Fos expression. JNK inhibition reduced fibrinogen binding and P-selectin expression of d12 platelet-like particles (PLPs), whereas p38 inhibition reduced fibrinogen binding of d12 PLPs. AP-1 expression correlated with increased MK DNA synthesis and polyploidization, which might explain the observed impact of shear on MKs. To summarize, we show that MK exposure to shear forces results in JNK activation, AP-1 up-regulation, and downstream transcriptional changes that promote maturation of immature MKs and platelet biogenesis in mature MKs.

Megakaryocyte- and megakaryocyte precursor-related gene therapies

Hematopoietic stem cells (HSCs) can be safely collected from the body, genetically modified, and re-infused into a patient with the goal to express the transgene product for an individual's lifetime. Hematologic defects that can be corrected with an allogeneic bone marrow transplant can theoretically also be treated with gene replacement therapy. Because some genetic disorders affect distinct cell lineages, researchers are utilizing HSC gene transfer techniques using lineage-specific endogenous gene promoters to confine transgene expression to individual cell types (eg, ITGA2B for inherited platelet defects). HSCs appear to be an ideal target for platelet gene therapy because they can differentiate into megakaryocytes which are capable of forming several thousand anucleate platelets that circulate within blood vessels to establish hemostasis by repairing vascular injury. Platelets play an essential role in other biological processes (immune response, angiogenesis) as well as diseased states (atherosclerosis, cancer, thrombosis). Thus, recent advances in genetic manipulation of megakaryocytes could lead to new and improved therapies for treating a variety of disorders. In summary, genetic manipulation of megakaryocytes has progressed to the point where clinically relevant strategies are being developed for human trials for genetic disorders affecting platelets. Nevertheless, challenges still need to be overcome to perfect this field; therefore, strategies to increase the safety and benefit of megakaryocyte gene therapy will be discussed.

Characterization of the platelet transcriptome by RNA sequencing in patients with acute myocardial infarction

Transcripts in platelets are largely produced in precursor megakaryocytes but remain physiologically active as platelets translate RNAs and regulate protein/RNA levels. Recent studies using transcriptome sequencing (RNA-seq) characterized the platelet transcriptome in limited number of non-diseased individuals. Here, we expand upon these RNA-seq studies by completing RNA-seq in platelets from 32 patients with acute myocardial infarction (MI). Our goals were to characterize the platelet transcriptome using a population of patients with acute MI and relate gene expression to platelet aggregation measures and ST-segment elevation MI (STEMI) (n = 16) vs. non-STEMI (NSTEMI) (n = 16) subtypes. Similar to other studies, we detected 9565 expressed transcripts, including several known platelet-enriched markers (e.g. PPBP, OST4). Our RNA-seq data strongly correlated with independently ascertained platelet expression data and showed enrichment for platelet-related pathways (e.g. wound response, hemostasis, and platelet activation), as well as actin-related and post-transcriptional processes. Several transcripts displayed suggestively higher (FBXL4, ECHDC3, KCNE1, TAOK2, AURKB, ERG, and FKBP5) and lower (MIAT, PVRL3, and PZP) expression in STEMI platelets compared to NSTEMI. We also identified transcripts correlated with platelet aggregation to TRAP (ATP6V1G2, SLC2A3), collagen (CEACAM1, ITGA2), and ADP (PDGFB, PDGFC, ST3GAL6). Our study adds to current platelet gene expression resources by providing transcriptome-wide analyses in platelets isolated from patients with acute MI. In concert with prior studies, we identify various genes for further study in regards to platelet function and acute MI. Future platelet RNA-seq studies examining more diverse sets of healthy and diseased samples will add to our understanding of platelet thrombotic and non-thrombotic functions.

Translational approaches to functional platelet production ex vivo

Platelets, which are released by megakaryocytes, play key roles in haemostasis, angiogenesis, immunity, tissue regeneration and wound healing. The scarcity of clinical cures for life threatening platelet diseases is in a large part due to limited insight into the mechanisms that control the developmental process of megakaryocytes and the mechanisms that govern the production of platelets within the bone marrow. To overcome these limitations, functional human tissue models have been developed and studied to extrapolate ex vivo outcomes for new insight on bone marrow functions in vivo. There are many challenges that these models must overcome, from faithfully mimicking the physiological composition and functions of bone marrow, to the collection of the platelets generated and validation of their viability and function for human use. The overall goal is to identify innovative instruments to study mechanisms of platelet release, diseases related to platelet production and new therapeutic targets starting from human progenitor cells.

The role of platelets in inflammation

There is growing recognition of the critical role of platelets in inflammation and immune responses. Recent studies have indicated that antiplatelet medications may reduce mortality from infections and sepsis, which suggests possible clinical relevance of modifying platelet responses to inflammation. Platelets release numerous inflammatory mediators that have no known role in haemostasis. Many of these mediators modify leukocyte and endothelial responses to a range of different inflammatory stimuli. Additionally, platelets form aggregates with leukocytes and form bridges between leukocytes and endothelium, largely mediated by platelet P-selectin. Through their interactions with monocytes, neutrophils, lymphocytes and the endothelium, platelets are therefore important coordinators of inflammation and both innate and adaptive immune responses.

Antiplatelet drugs in patients with enhanced platelet turnover: biomarkers versus platelet function testing

Platelets are key players in atherothrombosis. Antiplatelet therapy comprising aspirin alone or with P2Y12-inhibitors are effective for prevention of atherothrombotic complications. However, there is interindividual variability in the response to antiplatelet drugs, leaving some patients at increased risk of recurrent atherothrombotic events. Several risk factors associated with high on-treatment platelet reactivity (HTPR), including elevated platelet turnover, have been identified. Platelet turnover is adequately estimated from the fraction of reticulated platelets. Reticulated platelets are young platelets, characterised by residual messenger RNA. They are larger, haemostatically more active and there is evidence that platelet turnover is a causal and prognostic factor in atherothrombotic disease. Whether platelet turnover per se represents a key factor in pathogenesis, progression and prognosis of atherothrombotic diseases (with focus on acute coronary syndromes) or whether it merely facilitates insufficient platelet inhibition will be discussed in this state-of-the art review.

Differential changes in platelet reactivity induced by acute physical compared to persistent mental stress

Platelets are important in hemostasis, but also contain adhesion molecules, pro-inflammatory and immune-modulatory compounds, as well as most of the serotonin outside the central nervous system. Dysbalance in the serotonin pathways is involved in the pathogenesis of depressive symptoms. Thus, changes in platelet aggregation and content of bioactive compounds are of interest when investigating physiological stress-related mental processes as well as stress-related psychiatric diseases such as depression. In the present study, a characterization of platelet reactivity in acute physical and persistent mental stress was performed (aggregation, serotonin and serotonin 2A-receptor, P-selectin, CD40 ligand, matrix metalloproteinase-2 and -9 (MMP-2 and -9), platelet/endothelial adhesion molecule-1 (PECAM-1), intercellular adhesion molecule-1 (ICAM-1), β-thromboglobulin (β-TG) and platelet factor 4 (PF-4). Acute physical stress increased platelet aggregability while leaving platelet content of bioactive compounds unchanged. Persistent mental stress led to changes in platelet content of bioactive compounds and serotonin 2A-receptor only. The values of most bioactive compounds correlated with each other. Acute physical and persistent mental stress influences platelets through distinct pathways, leading to differential changes in aggregability and content of bioactive compounds.

Recruitment of CD8(+) T cells into bone marrow might explain the suppression of megakaryocyte apoptosis through high expression of CX3CR1(+) in prolonged isolated thrombocytopenia after allogeneic hematopoietic stem cell transplantation

Prolonged isolated thrombocytopenia is a common complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT), which is associated with a poor prognosis. This study aimed to investigate the pathogenesis of prolonged isolated thrombocytopenia (PT). We analysed the expression of CX3CR1 on CD4 and CD8 T cells in bone marrow (BM) and peripheral blood (PB) at +90 days from allo-HSCT recipients with or without PT by flow cytometry analyses. We then determined the megakaryocytes ploidy distributions, apoptosis rate and Fas expression of recipients with or without PT in vitro directly or after depleting CD8(+) T cells or adding purified autologous CD8(+) T cells to CD8(+) T-dep MNCs. We found that the percentage of CD8(+) T cells in BM was higher in the patients with PT than in the controls. The elevated expression of the CX3CR1 was associated with PT. There was a marked increase in the percentage of low ploidy megakaryocytes in the recipients with PT. The depletion of CD8(+) T cells increased the apoptosis of megakaryocytes and decreased the expression of Fas, which could be corrected by re-adding purified autologous CD8(+) T cells. The increase of CD8(+) T cells and CD8(+)/CX3CR1(+) T cells in BM at +90 days were independent risk factors for PT according to multivariate analysis. Our data implied that the recruitment of CD8(+) T cells into BM might explain the suppression of megakaryocyte apoptosis through the elevated expression of CX3CR1(+) in PT after allo-HSCT. CX3CR1 might be a novel treatment target in recipients with PT.

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.

The role of P2Y₁₂ receptor and activated platelets during inflammation

Platelets play an important role not only during thrombosis, but also in modulating immune responses through their interaction with immune cells and by releasing inflammatory mediators upon activation. The P2Y12 receptor is a Gi-coupled receptor that not only regulates ADP-induced aggregation but can also dramatically potentiate secretion, when platelets are activated by other stimuli. Considering the importance of P2Y12 receptor in platelet function, a class of antiplatelet drugs, thienopyridines, have been designed and successfully used to prevent thrombosis. This review will focus on the role of activated platelets in inflammation and the effects that P2Y12 antagonism exerts on the inflammatory process. A change in platelet functions was noted in patients treated with thienopyridines during inflammatory conditions, suggesting that platelets may modulate the inflammatory response. Further experiments in a variety of animal models of diseases, such as sepsis, rheumatoid arthritis, myocardial infarction, pancreatitis and pulmonary inflammation have also demonstrated that activated platelets influence the inflammatory state. Platelets can secrete inflammatory modulators in a P2Y12-dependent manner, and, as a result, directly alter the inflammatory response. P2Y12 receptor may also be expressed in other cells of the immune system, indicating that thienopyridines could directly influence the immune system rather than only through platelets. Overall the results obtained to date strongly support the notion that activated platelets significantly contribute to the inflammatory process and that antagonizing P2Y12 receptor can influence the immune response.

Impact of platelet phenotype on myocardial infarction

In acute myocardial infarction patients the injured vascular wall triggers thrombus formation in the damage site. Fibrin fibers and blood cellular elements are the major components of thrombus formed in acute occlusion of coronary arteries. It has been established that the initial thrombus is primarily composed of activated platelets rapidly stabilized by fibrin fibers. This review highlights the role of platelet membrane phenotype in pathophysiology of myocardial infarction. Here, we regard platelet phenotype as quantitative and qualitative parameters of the plasma membrane outer surface, which are crucial for platelet participation in blood coagulation, development of local inflammation and tissue repair.

Biology of platelet-rich plasma and its clinical application in cartilage repair

Platelet-rich plasma (PRP) is an autologous concentrated cocktail of growth factors and inflammatory mediators, and has been considered to be potentially effective for cartilage repair. In addition, the fibrinogen in PRP may be activated to form a fibrin matrix to fill cartilage lesions, fulfilling the initial requirements of physiological wound healing. The anabolic, anti-inflammatory and scaffolding effects of PRP based on laboratory investigations, animal studies, and clinical trials are reviewed here. In vitro, PRP is found to stimulate cell proliferation and cartilaginous matrix production by chondrocytes and adult mesenchymal stem cells (MSCs), enhance matrix secretion by synoviocytes, mitigate IL-1β-induced inflammation, and provide a favorable substrate for MSCs. In preclinical studies, PRP has been used either as a gel to fill cartilage defects with variable results, or to slow the progression of arthritis in animal models with positive outcomes. Findings from current clinical trials suggest that PRP may have the potential to fill cartilage defects to enhance cartilage repair, attenuate symptoms of osteoarthritis and improve joint function, with an acceptable safety profile. Although current evidence appears to favor PRP over hyaluronan for the treatment of osteoarthritis, the efficacy of PRP therapy remains unpredictable owing to the highly heterogeneous nature of reported studies and the variable composition of the PRP preparations. Future studies are critical to elucidate the functional activity of individual PRP components in modulating specific pathogenic mechanisms.

Platelet turnover in stable coronary artery disease - influence of thrombopoietin and low-grade inflammation

Background

Newly formed platelets are associated with increased aggregation and adverse outcomes in patients with coronary artery disease (CAD). The mechanisms involved in the regulation of platelet turnover in patients with CAD are largely unknown.

Aim

To investigate associations between platelet turnover parameters, thrombopoietin and markers of low-grade inflammation in patients with stable CAD. Furthermore, to explore the relationship between platelet turnover parameters and type 2 diabetes, prior myocardial infarction, smoking, age, gender and renal insufficiency.

Methods

We studied 581 stable CAD patients. Platelet turnover parameters (immature platelet fraction, immature platelet count, mean platelet volume, platelet distribution width and platelet large cell-ratio) were determined using automated flow cytometry (Sysmex XE-2100). Furthermore, we measured thrombopoietin and evaluated low-grade inflammation by measurement of high-sensitive CRP and interleukin-6.

Results

We found strong associations between the immature platelet fraction, immature platelet count, mean platelet volume, platelet distribution width and platelet large cell ratio (r = 0.61–0.99, p<0.0001). Thrombopoietin levels were inversely related to all of the platelet turnover parameters (r = −0.17–−0.25, p<0.0001). Moreover, thrombopoietin levels were significantly increased in patients with diabetes (p = 0.03) and in smokers (p = 0.003). Low-grade inflammation evaluated by high-sensitive CRP correlated significantly, yet weakly, with immature platelet count (r = 0.10, p = 0.03) and thrombopoietin (r = 0.16, p<0.001). Also interleukin-6 correlated with thrombopoietin (r = 0.10, p = 0.02).

Conclusion

In stable CAD patients, thrombopoietin was inversely associated with platelet turnover parameters. Furthermore, thrombopoietin levels were increased in patients with diabetes and in smokers. However, low-grade inflammation did not seem to have a substantial impact on platelet turnover parameters.