Megakaryopoiesis

Management of a twin pregnancy patient with Glanzmann thrombasthenia might be caused by a novel ITGA2B gene mutation (c.2822G>A): a case report and family investigation

Mutations in the ITGA2B or ITGB3 gene that encodes for the αIIbβ3 platelet integrin usually cause Glanzmann thrombasthenia (GT). This study aims to investigate the clinical characteristics of a pedigree exhibiting an inherited hemorrhagic disorder resembling GT, elucidate its molecular pathogenesis and evaluate the efficacy of blood management strategies for a proband who is pregnant with twins. The clinical data of the pedigree with inherited hemorrhagic disorder were collected, including the assessment of clinical, laboratory and thromboelastography (TEG) profiles. DNA samples were obtained for next-generation sequencing, encompassing the exons and flanking sequences of the ITGA2B and ITGB3 genes, as well as other genes associated with blood and immune deficiency. Bioinformatics software tools, such as PolyPhen-2, SIFT and MutationTaster, were employed to analyze the functional impact of mutations. Protein structural models for the new mutation type were generated using PyMOL. The phenotype of the proband in this pedigree with inherited platelet dysfunction and bleeding disorder was in accordance with GT. The proband shows persistent blood accumulation in the uterine cavity. Laboratory findings indicate normal PLT morphology, PLT count, MPV, and PDW. However, there is a decreased PLT aggregation induced by agonists ADP, collagen, and AA while maintaining a normal response to ristocetin. The initial TEG examination results indicated that the patient presented with a hypocoagulable state, characterize d by a reduction in α angle (46.9), an extended K value (4.6) and a decreased maximum amplitude (35.1). The younger sister demonstrated comparable TEG performance to that of the proband and has a documented history of abnormal bleeding. A novel heterozygous mutation of ITGA2B at position c.2822 G>A (p.Trp941*) was identified in the proband and her familial counterparts-father, brother and sister. MutationTaster software predicted the new mutation to be pathogenic; however, PolyPhen-2 and SIFT software did not provide correlated predictions. The p.Trp941* mutation resulted in the premature termination of translation at residue 940Trp, leading to impaired protein function. Successful management was achieved during the perioperative period by administration of human immunoglobulin, platelets and antifibrinolytic drugs, followed by recombinant factor VIIa (rFVIIa), according to the thromboelastography tracings. The laboratory findings of the proband are consistent with GT, and a novel mutation in the ITGA2B gene at position c.2822 G>A (p.Trp941*) has been identified as a potential cause of GT. However, since GT is a recessive disorder and both the proband and her family members are heterozygous, it cannot be excluded that they may possess additional bleeding risk factors, including the presence of other undetected variants. This study also illustrates the significance of multidisciplinary planning, TEG analysis and judicious utilization of rFVIIa to minimize operative bleeding risk.

RBM22-depletion delays progression through all steps of cell cycle and increases ploidy in myeloid cells

RNA-Binding Motif 22 (RBM22) is a splicing factor and a transcription regulator that plays important roles in cancer. Our goal was to document further the implication of RBM22 in cell cycle progression. Using normal human haematopoietic stem and progenitor cells and myeloid cell lines (MDS-L, HL-60), we demonstrated that RBM22 depletion reduces proliferation by delaying the progression of the G1-phase, S-phase and G2/M phase. RBM22 depletion alters mitosis, generating endomitosis and altered megakaryocyte differentiation. Altogether, we propose, for the first time, RBM22 as an essential actor of the cell cycle regulation in human haematopoietic stem and progenitor cells and myeloid cells. We demonstrated that RBM22 alteration is partially responsible for the phenotype of cytopenia of myeloid cell lineages observed in myelodysplastic syndromes (MDS) with a partial deletion of chromosome 5 (MDS with del(5q)) where one allele of RBM22 is lost. We hypothesise that the impact of RBM22 on cell cycle progression could explain some phenotypic features of other cancers.

Sparse matrix factorization robust to sample sharing across GWAS reveals interpretable genetic components

Complex trait-associated genetic variation is highly pleiotropic. This extensive pleiotropy implies that multi-phenotype analyses are informative for characterizing genetic associations, as they facilitate the discovery of trait-shared and trait-specific variants and pathways ("genetic factors"). Previous efforts have estimated genetic factors using matrix factorization (MF) applied to numerous GWAS. However, existing methods are susceptible to spurious factors arising from residual confounding due to sample-sharing in biobank GWAS. Furthermore, MF approaches have historically estimated dense factors, loaded on most traits and variants, that are challenging to map onto interpretable biological pathways. To address these shortcomings, we introduce "GWAS latent embeddings accounting for noise and regularization" (GLEANR), a MF method for detection of sparse genetic factors from summary statistics. GLEANR accounts for sample sharing between studies and uses regularization to estimate a data-driven number of interpretable factors. GLEANR is robust to confounding induced by shared samples and improves the replication of genetic factors derived from distinct biobanks. We used GLEANR to evaluate 137 diverse GWAS from the UK Biobank, identifying 58 factors that decompose the genetic architecture of input traits and have distinct signatures of negative selection and degrees of polygenicity. These sparse factors can be interpreted with respect to disease, cell-type, and pathway enrichment. We highlight three such factors capturing platelet measure phenotypes and enriched for disease-relevant markers corresponding to distinct stages of platelet differentiation. Overall, GLEANR is a powerful tool for discovering both trait-specific and trait-shared pathways underlying complex traits from GWAS summary statistics.

Phosphoinositides take a central stage in regulating blood platelet production and function

Blood platelets are produced by megakaryocytes through a complex program of differentiation and play a critical role in hemostasis and thrombosis. These anucleate cells are the target of antithrombotic drugs that prevent them from clumping in cardiovascular disease conditions. Platelets also significantly contribute to various aspects of physiopathology, including interorgan communications, healing, inflammation, and thromboinflammation. Their production and activation are strictly regulated by highly elaborated mechanisms. Among them, those involving inositol lipids have drawn the attention of researchers. Phosphoinositides represent the seven combinatorially phosphorylated forms of the inositol head group of inositol lipids. They play a crucial role in regulating intracellular mechanisms, such as signal transduction, actin cytoskeleton rearrangements, and membrane trafficking, either by generating second messengers or by directly binding to specific domains of effector proteins. In this review, we will explore how phosphoinositides are implicated in controlling platelet production by megakaryocytes and in platelet activation processes. We will also discuss the diversity of phosphoinositides in platelets, their role in granule biogenesis and maintenance, as well as in integrin signaling. Finally, we will address the discovery of a novel pool of phosphatidylinositol 3-monophosphate in the outerleaflet of the plasma membrane of human and mouse platelets.

MXRA7 is involved in megakaryocyte differentiation and platelet production

Matrix remodeling is a critical process in hematopoiesis. The biology of MXRA7, as a matrix remodeling associated gene, has still not been reported in hematopoietic process. Public databases showed that MXRA7 expressed in hematopoietic stem cells, suggesting that it may be involved in hematopoiesis. We found that the amounts of megakaryocytes were lower in bone marrow and spleen from Mxra7-/- mice compared with that from wild-type mice. Knock-out of MXRA7 also reduced the amount of platelet in peripheral blood and affected the function of platelets. Knock-out of MXRA7 inhibited hematopoietic stem/progenitor cells differentiate to megakaryocytes possibly through down-regulating the expression of GATA-1 and FOG-1. Moreover, knockdown of MXRA7 in MEG-01 cells could inhibit the cell proliferation and cell apoptosis. Knockdown of MXRA7 inhibited the differentiation of MEG-01 cells and proplatelet formation through suppressing the ERK/MAPK signaling pathway and the expression of β-tubulin. In conclusion, the current study demonstrated the potential significance of MXRA7 in megakaryocyte differentiation and platelet production. The novel findings proposed a new target for the treatment of platelet-related diseases, and much more investigations are guaranteed to dissect the mechanisms of MXRA7 in megakaryocyte differentiation and platelet production.

The polymorphic landscape analysis of GATA1 exons uncovered the genetic variants associated with higher thrombocytopenia in dengue patients

The current study elucidated an association between gene variants and thrombocytopenia through the investigation of the exonic polymorphic landscape of hematopoietic transcription factor—GATA1 gene in dengue patients. A total of 115 unrelated dengue patients with dengue fever (DF) (N = 91) and dengue hemorrhagic fever (DHF) (N = 24) were included in the study. All dengue patients were confirmed through detection of NS1 antigen, IgM, and IgG antibodies against the dengue virus. Polymerase chain reaction using specific primers amplified the exonic regions of GATA1 while Sanger sequencing and chromatogram analyses facilitated the identification of variants. Variants G>A (at chX: 48792009) and C>A (at chX: 4879118) had higher frequency out of 13 variants identified (3 annotated and 10 newly recognized). Patients carrying either nonsynonymous or synonymous variants had significantly lower mean values of platelets compared to those harboring the reference nucleotides (NC_000023.11). Further analyses revealed that the change in amino acid residue leads to the altered three-dimensional structure followed by interaction with neighboring residues. Increased stability of the protein due to substitution of serine by asparagine (S129N at chX: 48792009) may cause increased rigidity followed by reduced structural flexibility which may ultimately disturb the dimerization (an important prerequisite for GATA1 to perform its biological activity) process of the GATA1 protein. This, in turn, may affect the function of GATA1 followed by impaired production of mature platelets which may be reflected by the lower platelet counts in individuals with such variation. In summary, we have identified new variants within the GATA1 gene which were found to be clinically relevant to the outcome of dengue patients and thus, have the potential as candidate biomarkers for the determination of severity and prognosis of thrombocytopenia caused by dengue virus. However, further validation of this study in a large number of dengue patients is warranted.

Trial Registration: number SLCTR/2019/037.

Dengue, a mosquito-borne viral disease, caused by dengue virus (DENV) of the Flaviviridae family has been the source of a global epidemic for decades, and it has recently spread to all parts of the globe. Hemorrhagic manifestations in dengue are caused by increased vascular permeability and thrombocytopenia (150K cells/L). GATA1 is considered to be the master transcription factor for regulating the differentiation and maturation of megakaryocytes (MKs). The exonic polymorphism landscape of hematopoietic transcription factor—GATA1 in dengue patients was investigated in our study to see if there is an association between genetic variations and thrombocytopenia. Out of the 13 variations identified, variants G>A (chX:48792009) and C>A (chX:4879118) had the highest frequencies. Patients harboring either nonsynonymous or synonymous variants had significantly lower mean platelet counts compared to those having wild-type nucleotides. We have identified new GATA1 gene variations that were proven to be clinically related to the outcome of dengue patients and hence holds the potential to be candidate biomarkers for determining the severity and prognosis of dengue thrombocytopenia. However, this study needs to be replicated in a large number of dengue patients.

MicroRNA-26b Attenuates Platelet Adhesion and Aggregation in Mice

Platelets are key regulators of haemostasis, making platelet dysfunction a major driver of thrombosis. Numerous processes that determine platelet function are influenced by microRNAs (miRs). MiR-26b is one of the highest-expressed miRs in healthy platelets, and its expression in platelets is changed in a diseased state. However, the exact effect of this miR on platelet function has not been studied yet. In this study, we made use of a whole-body knockout of miR-26b in ApoE-deficient mice in order to determine its impact on platelet function, thrombus formation and platelet signalling both ex vivo and in vivo. We show that a whole-body deficiency of miR-26b exacerbated platelet adhesion and aggregation ex vivo. Additionally, in vivo, platelets adhered faster, and larger thrombi were formed in mice lacking miR-26b. Moreover, isolated platelets from miR-26b-deficient mice showed a hyperactivated Src and EGFR signalling. Taken together, we show here for the first time that miR-26b attenuates platelet adhesion and aggregation, possibly through Src and EGFR signalling.

C-type lectin-like receptor 2 specifies a functionally distinct subpopulation within phenotypically defined hematopoietic stem cell population that contribute to emergent megakaryopoiesis

C-type lectin-like receptor 2 (CLEC-2) expressed on megakaryocytes plays important roles in megakaryopoiesis. We found that CLEC-2 was expressed in about 20% of phenotypical long-term hematopoietic stem cells (LT-HSCs), which expressed lower levels of HSC-specific genes and produced larger amounts of megakaryocyte-related molecules than CLEC-2low LT-HSCs. Although CLEC-2high LT-HSCs had immature clonogenic activity, cultured CLEC-2high LT-HSCs preferentially differentiated into megakaryocytes. CLEC-2high HSCs yielded 6.8 times more megakaryocyte progenitors (MkPs) and 6.0 times more platelets 2 weeks and 1 week after transplantation compared with CLEC-2low LT-HSCs. However, platelet yield from CLEC-2high HSCs gradually declined with the loss of MkPs, while CLEC-2low HSCs self-renewed long-term, indicating that CLEC-2high LT-HSCs mainly contribute to early megakaryopoiesis. Treatment with pI:C and LPS increased the proportion of CLEC-2high LT-HSCs within LT-HSCs. Almost all CLEC-2low LT-HSCs were in the G0 phase and barely responded to pI:C. In contrast, 54% of CLEC-2high LT-HSCs were in G0, and pI:C treatment obliged CLEC-2high LT-HSCs to enter the cell cycle and differentiate into megakaryocytes, indicating that CLEC-2high LT-HSCs are primed for cell cycle entry and rapidly yield platelets in response to inflammatory stress. In conclusion, CLEC-2high LT-HSCs appear to act as a reserve for emergent platelet production under stress conditions.

Vaccination Accelerates Liver-Intrinsic Expression of Megakaryocyte-Related Genes in Response to Blood-Stage Malaria

Erythropoiesis and megakaryo-/thrombopoiesis occur in the bone marrow proceeding from common, even bipotent, progenitor cells. Recently, we have shown that protective vaccination accelerates extramedullary hepatic erythroblastosis in response to blood-stage malaria of Plasmodium chabaudi. Here, we investigated whether protective vaccination also accelerates extramedullary hepatic megakaryo-/thrombopoiesis. Female Balb/c mice were twice vaccinated with a non-infectious vaccine before infecting with 106 P. chabaudi-parasitized erythrocytes. Using gene expression microarrays and quantitative real-time PCR, transcripts of genes known to be expressed in the bone marrow by cells of the megakaryo-/thrombocytic lineage were compared in livers of vaccination-protected and unprotected mice on days 0, 1, 4, 8, and 11 p.i. Livers of vaccination-protected mice responded with expression of megakaryo-/thrombocytic genes faster to P. chabaudi than those of unvaccinated mice, evidenced at early patency on day 4 p.i., when livers exhibited significantly higher levels of malaria-induced transcripts of the genes Selp and Pdgfb (p-values < 0.0001), Gp5 (p-value < 0.001), and Fli1, Runx1, Myb, Mpl, Gp1ba, Gp1bb, Gp6, Gp9, Pf4, and Clec1b (p-values < 0.01). Together with additionally analyzed genes known to be related to megakaryopoiesis, our data suggest that protective vaccination accelerates liver-intrinsic megakaryo-/thrombopoiesis in response to blood-stage malaria that presumably contributes to vaccination-induced survival of otherwise lethal blood-stage malaria.

Dengue Virus Dysregulates Master Transcription Factors and PI3K/AKT/mTOR Signaling Pathway in Megakaryocytes

Dengue virus (DENV) infection can cause either self-limited dengue fever or hemorrhagic complications. Low platelet count is one of the manifestations of dengue fever. Megakaryocytes are the sole producers of platelets. However, the role of both host and viral factors in megakaryocyte development, maturation, and platelet production is largely unknown in DENV infection. PI3K/AKT/mTOR pathway plays a significant role in cell survival, maturation, and megakaryocyte development. We were interested to check whether pathogenic insult can impact this pathway. We observed decreased expression of most of the major key molecules associated with the PI3K/AKT/mTOR pathway in DENV infected MEG-01 cells. In this study, the involvement of PI3K/AKT/mTOR pathway in megakaryocyte development and maturation was confirmed with the use of specific inhibitors in infected MEG-01 cells. Our results showed that direct pharmacologic inhibition of this pathway greatly impacted megakaryopoiesis associated molecule CD61 and some essential transcription factors (GATA-1, GATA-2, and NF-E2). Additionally, we observed apoptosis in megakaryocytes due to DENV infection. Our results may suggest that DENV impairs PI3K/AKT/mTOR axis and molecules involved in the development and maturation of megakaryocytes. It is imperative to investigate the role of these molecules in the context of megakaryopoiesis during DENV infection to better understand the pathways and mechanisms, which in turn might provide insights into the development of antiviral strategies.

RhoA/Cdc42 signaling drives cytoplasmic maturation but not endomitosis in megakaryocytes

Megakaryocytes (MKs), the precursors of blood platelets, are large, polyploid cells residing mainly in the bone marrow. We have previously shown that balanced signaling of the Rho GTPases RhoA and Cdc42 is critical for correct MK localization at bone marrow sinusoids in vivo. Using conditional RhoA/Cdc42 double-knockout (DKO) mice, we reveal here that RhoA/Cdc42 signaling is dispensable for the process of polyploidization in MKs but essential for cytoplasmic MK maturation. Proplatelet formation is virtually abrogated in the absence of RhoA/Cdc42 and leads to severe macrothrombocytopenia in DKO animals. The MK maturation defect is associated with downregulation of myosin light chain 2 (MLC2) and β1-tubulin, as well as an upregulation of LIM kinase 1 and cofilin-1 at both the mRNA and protein level and can be linked to impaired MKL1/SRF signaling. Our findings demonstrate that MK endomitosis and cytoplasmic maturation are separately regulated processes, and the latter is critically controlled by RhoA/Cdc42.

Adipocyte Fatty Acid Transfer Supports Megakaryocyte Maturation

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

Thrombopoietic effects of CCAAT/enhancer-binding protein β on the early-stage differentiation of megakaryocytes

CCAAT/enhancer-binding protein β (C/EBPβ) is a transcription factor that is involved in adipocytic and monocytic differentiation. However, the physiological role of C/EBPβ in megakaryocytes (MKs) is not clear. In this study, we investigated the effects of C/EBPβ on the early-stage differentiation of MKs, and explored the potential mechanisms of action. We established a cytosine arabinoside-induced thrombocytopenia mouse model using C57BL/6 mice. In the thrombocytopenia mice, the platelet count was found to be decreased, and the mRNA and protein expression levels of C/EBPβ in MKs were also reduced. Furthermore, the maturation of Dami (MKs cell line) cells was induced by phorbol 12-myristate 13-acetate. When C/EBPβ was silenced in Dami cells by transfection using C/EBPβ-small interfering RNA, the expression of MKs-specific markers CD41 and CD62P, was dramatically decreased, resulting in morphological changes and differentiation retardation in low ploidy, which were evaluated using flow cytometry, real-time polymerase chain reaction, western blot, and confocal microscopy. The mitogen activated protein kinase-extracellular signal-regulated kinase signaling pathway was found to be required for the differentiation of MKs; knockdown of C/EBPβ in MEK/ERK1/2 pathway attenuated MKs differentiation. Overexpression of C/EBPβ in MEK/ERK1/2 pathway inhibited by U0126 did not promote MKs differentiation. To the best of our knowledge, C/EBPβ plays an important role in MKs differentiation and polyploidy cell cycle control. Taken together, C/EBPβ may have thrombopoietic effects in the differentiation of MKs, and may assist in the development of treatments for various disorders.

HIRA, a DiGeorge Syndrome Candidate Gene, Confers Proper Chromatin Accessibility on HSCs and Supports All Stages of Hematopoiesis

HIRA is a histone chaperone that deposits the histone variant H3.3 in transcriptionally active genes. In DiGeorge syndromes, a DNA stretch encompassing HIRA is deleted. The syndromes manifest varied abnormalities, including immunodeficiency and thrombocytopenia. HIRA is essential in mice, as total knockout (KO) results in early embryonic death. However, the role of HIRA in hematopoiesis is poorly understood. We investigate hematopoietic cell-specific Hira deletion in mice and show that it dramatically reduces bone marrow hematopoietic stem cells (HSCs), resulting in anemia, thrombocytopenia, and lymphocytopenia. In contrast, fetal hematopoiesis is normal in Hira-KO mice, although fetal HSCs lack the reconstitution capacity. Transcriptome analysis reveals that HIRA is required for expression of many transcription factors and signaling molecules critical for HSCs. ATAC-seq analysis demonstrates that HIRA establishes HSC-specific DNA accessibility, including the SPIB/PU.1 sites. Together, HIRA provides a chromatin environment essential for HSCs, thereby steering their development and survival.

Platelet production using adipose-derived mesenchymal stem cells: Mechanistic studies and clinical application

Megakaryocytes (MKs) are platelet progenitor stem cells found in the bone marrow. Platelets obtained from blood draws can be used for therapeutic applications, especially platelet transfusion. The needs for platelet transfusions for clinical situation is increasing, due in part to the growing number of patients undergoing chemotherapy. Platelets obtained from donors, however, have the disadvantages of a limited storage lifespan and the risk of donor-related infection. Extensive effort has therefore been directed at manufacturing platelets ex vivo. Here, we review ex vivo technologies for MK development, focusing on human adipose tissue-derived mesenchymal stem/stromal cell line (ASCL)-based strategies and their potential clinical application. Bone marrow and adipose tissues contain mesenchymal stem/stromal cells that have an ability to differentiate into MKs, which release platelets. Taking advantage of this mechanism, we developed a donor-independent system for manufacturing platelets for clinical application using ASCL established from adipose-derived mesenchymal stem/stromal cells (ASCs). Culture of ASCs with endogenous thrombopoietin and its receptor c-MPL, and endogenous genes such as p45NF-E2 leads to MK differentiation and subsequent platelet production. ASCs compose heterogeneous cells, however, and are not suitable for clinical application. Thus, we established ASCLs, which expand into a more homogeneous population, and fulfill the criteria for mesenchymal stem cells set by the International Society for Cellular Therapy. Using our ASCL culture system with MK lineage induction medium without recombinant thrombopoietin led to peak production of platelets within 12 days, which may be sufficient for clinical application.

Deciphering the Role of the Coagulation Cascade and Autophagy in Cancer-Related Thrombosis and Metastasis

Thrombotic complications are the second leading cause of death among oncology patients worldwide. Enhanced thrombogenesis has multiple origins and may result from a deregulation of megakaryocyte platelet production in the bone marrow, the synthesis of coagulation factors in the liver, and coagulation factor signaling upon cancer and the tumor microenvironment (TME). While a hypercoagulable state has been attributed to factors such as thrombocytosis, enhanced platelet aggregation and Tissue Factor (TF) expression on cancer cells, further reports have suggested that coagulation factors can enhance metastasis through increased endothelial-cancer cell adhesion and enhanced endothelial cell activation. Autophagy is highly associated with cancer survival as a double-edged sword, as can both inhibit and promote cancer progression. In this review, we shall dissect the crosstalk between the coagulation cascade and autophagic pathway and its possible role in metastasis and cancer-associated thrombosis formation. The signaling of the coagulation cascade through the autophagic pathway within the hematopoietic stem cells, the endothelial cell and the cancer cell are discussed. Relevant to the coagulation cascade, we also examine the role of autophagy-related pathways in cancer treatment. In this review, we aim to bring to light possible new areas of cancer investigation and elucidate strategies for future therapeutic intervention.

miRNA 146b mediates the regulation of nucleolar size and activity in polyploid megakaryocytes

BACKGROUND INFORMATION

Megakaryocytes (MKs) follow a unique cell cycle duplication process, called endomitosis, resulting in polyploidisation of cells. It is hypothesised that polyploidy, as well as an increment in cytoplasm volume, allow more efficient platelets generation from MKs. Although polyploidy leads to an increase in the DNA amount, which impacts gene expression, little is known about ribosomal biogenesis in these polylobulated polyploid cells.

RESULTS

The nucleolus acts as a hub for ribosomal biogenesis, which in turn governs the protein synthesis rate of the cells. We therefore estimated the size and activity of the nucleolus in polyploid cells during megakaryopoiesis in vitro. Polyploid megakaryocytic cell lines and in vitro cultured MKs, which were obtained from human cord blood-derived CD 34⁺ cells, revealed that miRNA 146b regulated the activity of nucleolar and coiled-body phosphoprotein 1, which plays an integral role in determining nucleolar size and activity. Additionally, miRNA-146b was up-regulated during endomitosis and was found to promote megakaryopoiesis.

CONCLUSION

We propose that miRNA 146b regulates not only nucleolar size and activity, but also megakaryopoiesis.

SIGNIFICANCE

This study highlights the importance of nucleolar activity and miRNA in the progression of megakaryopoiesis and thrombopoiesis.

αIIbβ3 variants in ten families with autosomal dominant macrothrombocytopenia: Expanding the mutational and clinical spectrum

Background

Rare pathogenic variants in either the ITGA2B or ITGB3 genes have been linked to autosomal dominant macrothrombocytopenia associated with abnormal platelet production and function, deserving the designation of Glanzmann Thrombasthenia-Like Syndrome (GTLS) or ITGA2B/ITGB3-related thrombocytopenia.

Objectives

To describe a series of patients with familial macrothrombocytopenia and decreased expression of αIIbβ3 integrin due to defects in the ITGA2B or ITGB3 genes.

Methods

We reviewed the clinical and laboratory records of 10 Portuguese families with GTLS (33 patients and 11 unaffected relatives), including the functional and genetic defects.

Results

Patients had absent to moderate bleeding, macrothrombocytopenia, low αIIbβ3 expression, impaired platelet aggregation/ATP release to physiological agonists and low expression of activation-induced binding sites on αIIbβ3 (PAC-1) and receptor-induced binding sites on its ligand (bound fibrinogen), upon stimulation with TRAP-6 and ADP. Evidence for constitutive αIIbβ3 activation, occurred in 2 out of 9 patients from 8 families studied, but also in 2 out of 12 healthy controls. We identified 7 missense variants: 3 in ITGA2B (5 families), and 4 in ITGB3 (5 families). Three variants (αIIb: p.Arg1026Trp and p.Arg1026Gln and β3: p.Asp749His) were previously reported. The remaining (αIIb: p.Gly1007Val and β3: p.Thr746Pro, p.His748Pro and p.Arg760Cys) are new, expanding the αIIbβ3 defects associated with GTLS. The integration of the clinical and laboratory data allowed the identification of two GTLS subgroups, with distinct disease severity.

Conclusions

Previously reported ITGA2B and ITGB3 variants related to thrombocytopenia were clustered in a confined region of the membrane-proximal cytoplasmic domains, the inner membrane clasp. For the first time, variants are reported at the outer membrane clasp, at the transmembrane domain of αIIb, and at the membrane distal cytoplasmic domains of β3. This is the largest single-center series of inherited macrothrombocytopenia associated with αIIbβ3 variants published to date.

Evidence for the loss of plasminogen receptor KT gene in chicken

The loss of conserved genes has the potential to alter phenotypes drastically. Screening of vertebrate genomes for lineage-specific gene loss events has identified numerous natural knockouts associated with specific phenotypes. We provide evidence for the loss of a multi-exonic plasminogen receptor KT (PLGRKT) protein-encoding gene located on the Z chromosome in chicken. Exons 1 and 2 are entirely missing; remnants of exon 3 and a mostly intact exon 4 are identified in an assembly gap-free region in chicken with conserved synteny across species and verified using transcriptome and genome sequencing. PLGRKT gene disrupting changes are present in representative species from all five galliform families. In contrast to this, the presence of an intact transcriptionally active PLGRKT gene in species such as mallard, swan goose, and Anolis lizard suggests that gene loss occurred in the galliform lineage sometime between 68 and 80 Mya. The presence of galliform specific chicken repeat 1 (CR1) insertion at the erstwhile exon 2 of PLGRKT gene suggests repeat insertion-mediated loss. However, at least nine other independent PLGRKT coding frame disrupting changes in other bird species are supported by genome sequencing and indicate a role for relaxed purifying selection before CR1 insertion. The recurrent loss of a conserved gene with a role in the regulation of macrophage migration, efferocytosis, and blood coagulation is intriguing. Hence, we propose potential candidate genes that might be compensating the function of PLGRKT based on the presence of a C-terminal lysine residue, transmembrane domains, and gene expression patterns.

Dengue virus infection impedes megakaryopoiesis in MEG-01 cells where the virus envelope protein interacts with the transcription factor TAL-1

Dengue virus (DENV) infection causes dengue fever in humans, which can lead to thrombocytopenia showing a marked reduction in platelet counts, and dengue hemorrhagic fever. The virus may cause thrombocytopenia either by destroying the platelets or by interfering with their generation via the process of megakaryopoiesis. MEG-01 is the human megakaryoblastic leukemia cell line that can be differentiated in vitro by phorbol-12-myristate-13-acetate (PMA) treatment to produce platelet-like-particles (PLPs). We have studied DENV infection of MEG-01 cells to understand its effect on megakaryopoiesis and the generation of PLPs. We observed that DENV could infect only naive MEG-01 cells, and differentiated cells were refractory to virus infection/replication. However, DENV-infected MEG-01 cells, when induced for differentiation with PMA, supported an enhanced viral replication. Following the virus infection, the MEG-01 cells showed a marked reduction in the surface expression of platelet markers (CD41, CD42a, and CD61), a decreased polyploidy, and significantly reduced PLP counts. DENV infection caused an enhanced Notch signaling in MEG-01 cells where the virus envelope protein was shown to interact with TAL-1, a host protein important for megakaryopoiesis. These observations provide new insight into the role of DENV in modulating the megakaryopoiesis and platelet production process.

De novo lipogenesis is essential for platelet production in humans

Acetyl-CoA carboxylase (ACC) catalyses the first step of de novo lipogenesis (DNL). Pharmacologic inhibition of ACC has been of interest for therapeutic intervention in a wide range of diseases. We demonstrate here that ACC and DNL are essential for platelet production in humans and monkeys, but in not rodents or dogs. During clinical evaluation of a systemically distributed ACC inhibitor, unexpected dose-dependent reductions in platelet count were observed. While platelet count reductions were not observed in rat and dog toxicology studies, subsequent studies in cynomolgus monkeys recapitulated these platelet count reductions with a similar concentration response to that in humans. These studies, along with ex vivo human megakaryocyte maturation studies, demonstrate that platelet lowering is a consequence of DNL inhibition likely to result in impaired megakaryocyte demarcation membrane formation. These observations demonstrate that while DNL is a minor quantitative contributor to global lipid balance in humans, DNL is essential to specific lipid pools of physiological importance.

Increased platelet distribution width and reduced IL-2 and IL-12 are associated with thrombocytopenia in Plasmodium vivax malaria

BACKGROUND

Thrombocytopenia in malaria involves platelet destruction and consumption; however, the cellular response underlying this phenomenon has still not been elucidated.

OBJECTIVE

To find associations between platelet indices and unbalanced Th1/Th2/Th17 cytokines as a response to thrombocytopenia in Plasmodium vivax infected (Pv-MAL) patients.

METHODS

Platelet counts and quantification of Th1/Th2/Th17 cytokine levels were compared in 77 patients with uncomplicated P. vivax malaria and 37 healthy donors from the same area (endemic control group - ENCG).

FINDINGS

Thrombocytopenia was the main manifestation in 55 patients, but was not associated with parasitaemia. The Pv-MAL patients showed increases in the mean platelet volume (MPV), which may be consistent with larger or megaplatelets. Contrary to the findings regarding the endemic control group, MPV and platelet distribution width (PDW) did not show an inverse correlation, due the increase in the heterogeneity of platelet width. In addition, the Pv-MAL patients presented increased IL-1β and reduced IL-12p70 and IL-2 serum concentrations. Furthermore, the reduction of these cytokines was associated with PDW values.

MAIN CONCLUSIONS

Our data demonstrate that an increase in MPV and the association between reductions of IL-2 and IL-12 and PDW values may be an immune response to thrombocytopenia in uncomplicated P. vivax malaria.

Zebrafish for thrombocytopoiesis- and hemostasis-related researches and disorders

Platelets play vital roles in hemostasis, inflammation, and vascular biology. Platelets are also active participants in the immune responses. As vertebrates, zebrafish have a highly conserved hematopoietic system in the developmental, cellular, functional, biochemical, and genetic levels with mammals. Thrombocytes in zebrafish are functional homologs of mammalian platelets. Here, we summarized thrombocyte development, function, and related research techniques in zebrafish, and reviewed available zebrafish models of platelet-associated disorders, including congenital amegakaryocytic thrombocytopenia, inherited thrombocytopenia, essential thrombocythemia, and blood coagulation disorders such as gray platelet syndrome. These elegant zebrafish models and methods are crucial for understanding the molecular and genetic mechanisms of thrombocyte development and function, and provide deep insights into related human disease pathophysiology and drug development.

Platelet Disorders

After vascular injury and exposure of subendothelial matrix proteins to the intravascular space, mediators of hemostasis are triggered and allow for clot formation and restoration of vascular integrity. Platelets are the mediators of primary hemostasis, creating a platelet plug and allowing for initial cessation of bleeding. Platelet disorders, qualitative and quantitative, may result in bleeding signs and symptoms, particularly mucocutaneous bleeding such as epistaxis, bruising, petechiae, and heavy menstrual bleeding. Increasing evidence suggests that platelets have functional capabilities beyond hemostasis, but this review focuses solely on platelet hemostatic properties. Herein, normal platelet function as well as the effects of abnormal function and thrombocytopenia are reviewed.

Nobiletin Promotes Megakaryocytic Differentiation through the MAPK/ERK-Dependent EGR1 Expression and Exerts Anti-Leukemic Effects in Human Chronic Myeloid Leukemia (CML) K562 Cells

Differentiation therapy is an alternative strategy used to induce the differentiation of blast cells toward mature cells and to inhibit tumor cell proliferation for cancer treatment. Nobiletin (NOB), a polymethoxyflavone phytochemical, is present abundantly in citrus peels and has been reported to possess anti-cancer activity. In this study, we investigated the anti-leukemic effects of NOB on cell differentiation and its underlying mechanisms in human chronic myeloid leukemia (CML) K562 cells. NOB (100 μM) treatment for 24 and 48 h significantly decreased viability of K562 cells to 54.4 ± 5.3% and 46.2 ± 9.9%, respectively. NOB (10–100 μM) significantly inhibited cell growth in K562 cells. Flow cytometry analysis and immunoblotting data showed that NOB (40 and 80 μM) could modulate the cell cycle regulators including p21, p27, and cyclin D2, and induce G1 phase arrest. NOB also increased the messenger RNA (mRNA) and protein expression of megakaryocytic differentiation markers, such as CD61, CD41, and CD42 as well as the formation of large cells with multi-lobulated nuclei in K562 cells. These results suggested that NOB facilitated K562 cells toward megakaryocytic differentiation. Furthermore, microarray analysis showed that expression of EGR1, a gene associated with promotion of megakaryocytic differentiation, was markedly elevated in NOB-treated K562 cells. The knockdown of EGR1 expression by small interference RNA (siRNA) could significantly attenuate NOB-mediated cell differentiation. We further elucidated that NOB induced EGR1 expression and CD61 expression through increases in MAPK/ERK phosphorylation in K562 cells. These results indicate that NOB promotes megakaryocytic differentiation through the MAPK/ERK pathway-dependent EGR1 expression in human CML cells. In addition, NOB when combined with imatinib could synergistically reduce the viability of K562 cells. Our findings suggest that NOB may serve as a beneficial anti-leukemic agent for differentiation therapy.

Regulation of bone metabolism by megakaryocytes in a paracrine manner

Megakaryocytes (MKs) play key roles in regulating bone metabolism. To test the roles of MK-secreted factors, we investigated whether MK and promegakaryocyte (pro-MK) conditioned media (CM) may affect bone formation and resorption. K562 cell lines were differentiated into mature MKs. Mouse bone marrow macrophages were differentiated into mature osteoclasts, and MC3T3-E1 cells were used for osteoblastic experiments. Bone formation was determined by a calvaria bone formation assay in vivo. Micro-CT analyses were performed in the femurs of ovariectomized female C57B/L6 and Balb/c nude mice after intravenous injections of MK or pro-MK CM. MK CM significantly reduced in vitro bone resorption, largely due to suppressed osteoclastic resorption activity. Compared with pro-MK CM, MK CM suppressed osteoblastic differentiation, but stimulated its proliferation, resulting in stimulation of calvaria bone formation. In ovariectomized mice, treatment with MK CM for 4 weeks significantly increased trabecular bone mass parameters, such as bone volume fraction and trabecular thickness, in nude mice, but not in C57B/L6 mice. In conclusion, MKs may secrete anti-resorptive and anabolic factors that affect bone tissue, providing a novel insight linking MKs and bone cells in a paracrine manner. New therapeutic agents against metabolic bone diseases may be developed from MK-secreted factors.

microRNA-22 promotes megakaryocyte differentiation through repression of its target, GFI1

Precise control of microRNA expression contributes to development and the establishment of tissue identity, including in proper hematopoietic commitment and differentiation, whereas aberrant expression of various microRNAs has been implicated in malignant transformation. A small number of microRNAs are upregulated in megakaryocytes, among them is microRNA-22 (miR-22). Dysregulation of miR-22 leads to various hematologic malignancies and disorders, but its role in hematopoiesis is not yet well established. Here we show that upregulation of miR-22 is a critical step in megakaryocyte differentiation. Megakaryocytic differentiation in cell lines is promoted upon overexpression of miR-22, whereas differentiation is disrupted in CRISPR/Cas9-generated miR-22 knockout cell lines, confirming that miR-22 is an essential mediator of this process. RNA-sequencing reveals that miR-22 loss results in downregulation of megakaryocyte-associated genes. Mechanistically, we identify the repressive transcription factor, GFI1, as the direct target of miR-22, and upregulation of GFI1 in the absence of miR-22 inhibits megakaryocyte differentiation. Knocking down aberrant GFI1 expression restores megakaryocytic differentiation in miR-22 knockout cells. Furthermore, we have characterized hematopoiesis in miR-22 knockout animals and confirmed that megakaryocyte differentiation is similarly impaired in vivo and upon ex vivo megakaryocyte differentiation. Consistently, repression of Gfi1 is incomplete in the megakaryocyte lineage in miR-22 knockout mice and Gfi1 is aberrantly expressed upon forced megakaryocyte differentiation in explanted bone marrow from miR-22 knockout animals. This study identifies a positive role for miR-22 in hematopoiesis, specifically in promoting megakaryocyte differentiation through repression of GFI1, a target antagonistic to this process.

Blood donor-derived buffy coat to produce platelets in vitro

BACKGROUND AND OBJECTIVES

Platelet transfusion is a standard medical therapy used to treat several bleeding disorders. However, a critical drawback is the dependency on donor-derived platelets, which leads to concerns like insufficient availability and immunological complications. In vitro platelet production from hematopoietic progenitor cells (CD34) may represent a reasonable solution.

MATERIALS AND METHODS

CD34+ cells were isolated from either buffy coat or peripheral blood and compared in terms of platelet production in vitro. The number and the quality of magnetically isolated CD34+ cells and their capability to differentiate into mature megakaryocytes were investigated using flow cytometry. Additionally, the functionality of megakaryocytes in term of in vitro platelet production was tested.

RESULTS

Similar purity and quantity of CD34+ cells was found after their isolation from both cell sources. In contrast, after 6 days of culture, enhanced number of CD34+ cells isolated from buffy coat compared with peripheral blood was observed (5·3 x 106 vs. 3·0 x 106, respectively). Interestingly, despite a comparable nuclear maturation phenotype, the yield of platelets released from buffy coat-derived megakaryocytes was significantly higher than from peripheral blood cells (platelet yield pro MK: 7·2 vs. 2·7, respectively). Importantly, platelets produced from buffy coat-derived cells could be activated by agonists.

CONCLUSION

Haematopoietic progenitor cells isolated from buffy coat have increased yield of platelets released from mature megakaryocytes and enhanced in vitro functionality, compared with peripheral blood-derived cells. Our study, suggests that buffy coat, obtained during blood donation processing, might be a promising source of megakaryocytes for in vitro platelet production.

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.

Microvesicles promote megakaryopoiesis by regulating DNA methyltransferase and methylation of Notch1 promoter

Megakaryopoiesis is the process of formation of mature megakaryocytes that takes place in the bone marrow niche resulting in the release of platelets into the peripheral blood. It has been suggested that cell to cell communication in this dense bone marrow niche may influence the fate of the cells. Numerous studies point to the role of exosomes and microvesicles not only as a messenger of the cellular crosstalk but also in growth and developmental process of various cell types. In the current study, we explored the effects of megakaryocyte-derived microvesicles in hematopoietic cell lines in the context of differentiation. Our study demonstrated that microvesicles isolated from the induced megakaryocytic cell lines have the ability to stimulate noninduced cells specifically into that particular lineage. We showed that this lineage commencement comes from the change in the methylation status of Notch1 promoter, which is regulated by DNA methyltransferases.

Inherited Thrombocytopenia: Update on Genes and Genetic Variants Which may be Associated With Bleeding

Inherited thrombocytopenia (IT) is comprised of a group of hereditary disorders characterized by a reduced platelet count as the main feature, and often with abnormal platelet function, which can subsequently lead to impaired haemostasis. Inherited thrombocytopenia results from genetic mutations in genes implicated in megakaryocyte differentiation and/or platelet formation and clearance. The identification of the underlying causative gene of IT is challenging given the high degree of heterogeneity, but important due to the presence of various clinical presentations and prognosis, where some defects can lead to hematological malignancies. Traditional platelet function tests, clinical manifestations, and hematological parameters allow for an initial diagnosis. However, employing Next-Generation Sequencing (NGS), such as Whole Genome and Whole Exome Sequencing (WES) can be an efficient method for discovering causal genetic variants in both known and novel genes not previously implicated in IT. To date, 40 genes and their mutations have been implicated to cause many different forms of inherited thrombocytopenia. Nevertheless, despite this advancement in the diagnosis of IT, the molecular mechanism underlying IT in some patients remains unexplained. In this review, we will discuss the genetics of thrombocytopenia summarizing the recent advancement in investigation and diagnosis of IT using phenotypic approaches, high-throughput sequencing, targeted gene panels, and bioinformatics tools.

Megakaryocytes and platelets from a novel human adipose tissue-derived mesenchymal stem cell line

The clinical need for platelet transfusions is increasing; however, donor-dependent platelet transfusions are associated with practical problems, such as the limited supply and the risk of infection. Thus, we developed a manufacturing system for platelets from a donor-independent cell source: a human adipose-derived mesenchymal stromal/stem cell line (ASCL). The ASCL was obtained using an upside-down culture flask method and satisfied the minimal criteria for defining mesenchymal stem cells (MSCs) by The International Society for Cellular Therapy. The ASCL showed its proliferation capacity for ≥2 months without any abnormal karyotypes. The ASCL was cultured in megakaryocyte induction media. ASCL-derived megakaryocytes were obtained, with a peak at day 8 of culture, and ASCL-derived platelets (ASCL-PLTs) were obtained, with a peak at day 12 of culture. We observed that CD42b⁺ cells expressed an MSC marker (CD90) which is related to cell adhesion. Compared with peripheral platelets, ASCL-PLTs exhibit higher levels of PAC1 binding, P-selectin surface exposure, ristocetin-induced platelet aggregation, and ADP-induced platelet aggregation, as well as similar levels of fibrinogen binding and collagen-induced platelet aggregation. ASCL-PLTs have lower epinephrine-induced platelet aggregation. The pattern of in vivo kinetics after infusion into irradiated immunodeficient NOD.Cg-Prkdc^scidIl2rg^tm1Wjl/SzJ mice was similar to that of platelet concentrates. ASCL-PLTs have similar characteristics to those of peripheral platelets and might have an additional function as MSCs. The establishment of the ASCL and its differentiation into ASCL-PLTs do not require gene transfer, and endogenous thrombopoietin is used for differentiation. The present protocol is a simple method that does not require feeder cells, further enhancing the clinical application of our approach.

Neuro-Immune Hemostasis: Homeostasis and Diseases in the Central Nervous System

Coagulation and the immune system interact in several physiological and pathological conditions, including tissue repair, host defense, and homeostatic maintenance. This network plays a key role in diseases of the central nervous system (CNS) by involving several cells (CNS resident cells, platelets, endothelium, and leukocytes) and molecular pathways (protease activity, complement factors, platelet granule content). Endothelial damage prompts platelet activation and the coagulation cascade as the first physiological step to support the rescue of damaged tissues, a flawed rescuing system ultimately producing neuroinflammation. Leukocytes, platelets, and endothelial cells are sensitive to the damage and indeed can release or respond to chemokines and cytokines (platelet factor 4, CXCL4, TNF, interleukins), and growth factors (including platelet-derived growth factor, vascular endothelial growth factor, and brain-derived neurotrophic factor) with platelet activation, change in capillary permeability, migration or differentiation of leukocytes. Thrombin, plasmin, activated complement factors and matrix metalloproteinase-1 (MMP-1), furthermore, activate intracellular transduction through complement or protease-activated receptors. Impairment of the neuro-immune hemostasis network induces acute or chronic CNS pathologies related to the neurovascular unit, either directly or by the systemic activation of its main steps. Neurons, glial cells (astrocytes and microglia) and the extracellular matrix play a crucial function in a “tetrapartite” synaptic model. Taking into account the neurovascular unit, in this review we thoroughly analyzed the influence of neuro-immune hemostasis on these five elements acting as a functional unit (“pentapartite” synapse) in the adaptive and maladaptive plasticity and discuss the relevance of these events in inflammatory, cerebrovascular, Alzheimer, neoplastic and psychiatric diseases. Finally, based on the solid reviewed data, we hypothesize a model of neuro-immune hemostatic network based on protein–protein interactions. In addition, we propose that, to better understand and favor the maintenance of adaptive plasticity, it would be useful to construct predictive molecular models, able to enlighten the regulating logic of the complex molecular network, which belongs to different cellular domains. A modeling approach would help to define how nodes of the network interact with basic cellular functions, such as mitochondrial metabolism, autophagy or apoptosis. It is expected that dynamic systems biology models might help to elucidate the fine structure of molecular events generated by blood coagulation and neuro-immune responses in several CNS diseases, thereby opening the way to more effective treatments.

IGF-1 facilitates thrombopoiesis primarily through Akt activation

IGF-1 has the ability to promote megakaryocyte differentiation, PPF, and platelet release. The effect of IGF-1 on thrombopoiesis is mediated primarily by AKT activation with the assistance of SRC-3.

Efficient production of erythroid, megakaryocytic and myeloid cells, using single cell-derived iPSC colony differentiation

Hematopoietic differentiation of human induced pluripotent stem cells (iPSCs) provide opportunities not only for fundamental research and disease modelling/drug testing but also for large-scale production of blood effector cells for future clinical application. Although there are multiple ways to differentiate human iPSCs towards hematopoietic lineages, there is a need to develop reproducible and robust protocols. Here we introduce an efficient way to produce three major blood cell types using a standardized differentiation protocol that starts with a single hematopoietic initiation step. This system is feeder-free, avoids EB-formation, starts with a hematopoietic initiation step based on a novel single cell-derived iPSC colony differentiation and produces multi-potential progenitors within 8-10 days. Followed by lineage-specific growth factor supplementation these cells can be matured into well characterized erythroid, megakaryocytic and myeloid cells with high-purity, without transcription factor overexpression or any kind of pre-purification step. This standardized differentiation system provides a simple platform to produce specific blood cells in a reproducible manner for hematopoietic development studies, disease modelling, drug testing and the potential for future therapeutic applications.

Autophagy as a pharmacological target in hematopoiesis and hematological disorders

Autophagy is involved in many cellular processes, including cell homeostasis, cell death/survival balance and differentiation. Autophagy is essential for hematopoietic stem cell survival, quiescence, activation and differentiation. The deregulation of this process is associated with numerous hematological disorders and pathologies, including cancers. Thus, the use of autophagy modulators to induce or inhibit autophagy emerges as a potential therapeutic approach for treating these diseases and could be particularly interesting for differentiation therapy of leukemia cells. This review presents therapeutic strategies and pharmacological agents in the context of hematological disorders. The pros and cons of autophagy modulators in therapy will also be discussed.

Label free monitoring of megakaryocytic development and proplatelet formation in vitro

Megakaryopoiesis and platelet production are complex biological processes that require tight regulation of successive lineage commitment steps and are ultimately responsible for maintaining and renewing the pool of circulating platelets in the blood. Despite major advancements in the understanding of megakaryocytic biology, the detailed mechanisms driving megakaryocytic differentiation have yet to be elucidated. Here we show that automated image analysis algorithms applied to two-photon excited fluorescence (TPEF) images can non-invasively monitor structural and metabolic megakaryocyte behavior changes occurring during differentiation and platelet formation in vitro. Our results demonstrate that high-contrast, label-free two photon imaging holds great potential in studying the underlying physiological processes controlling the intricate process of platelet production.

Towards the Manufacture of Megakaryocytes and Platelets for Clinical Application

Platelet transfusions are used in standard clinical practice to prevent hemorrhage in patients suffering from thrombocytopenia or platelet dysfunctions. Recently, a constant rise on the demand of platelets for transfusion has been registered. This may be associated with several factors including demographic changes, population aging as well as incidence and prevalence of hematological diseases. In addition, platelet-regenerative properties have been started to be exploited in different areas such as tissue remodeling and anti-cancer therapies. These new applications are also expected to increase the future demand on platelets. Thus, in vitro generated platelets may constitute a highly desirable alternative to meet the rising demand on platelets. Several factors have been considered in the road trip of producing in vitro megakaryocytes and platelets for clinical application. From selection of the cell source, differentiation protocols and culture conditions to the design of optimal bioreactors, several strategies have been proposed to maximize production yields while preserving functionality. This review summarizes new advances in megakaryocyte and platelet differentiation and their production upscaling.

Transcription factor defects causing platelet disorders

Recent years have seen increasing recognition of a subgroup of inherited platelet function disorders which are due to defects in transcription factors that are required to regulate megakaryopoiesis and platelet production. Thus, germline mutations in the genes encoding the haematopoietic transcription factors RUNX1, GATA-1, FLI1, GFI1b and ETV6 have been associated with both quantitative and qualitative platelet abnormalities, and variable bleeding symptoms in the affected patients. Some of the transcription factor defects are also associated with an increased predisposition to haematologic malignancies (RUNX1, ETV6), abnormal erythropoiesis (GATA-1, GFI1b, ETV6) and immune dysfunction (FLI1). The persistence of MYH10 expression in platelets is a surrogate marker for FLI1 and RUNX1 defects. Characterisation of the transcription factor defects that give rise to platelet function disorders, and of the genes that are differentially regulated as a result, are yielding insights into the roles of these genes in platelet formation and function.

Human adipose tissue-derived stromal cells can differentiate into megakaryocytes and platelets by secreting endogenous thrombopoietin

Essentials Manufacturing platelets from a donor-independent source is highlighted in transfusion medicine. We examined the differentiation of adipose tissue-derived stromal cells (ASCs) into platelets. Endogenous thrombopoietin (TPO) induced ASCs differentiation into megakaryocytes and platelets. TPO secretion from ASCs was due to an interaction of transferrin with its receptor CD71.

SUMMARY

Background Ex vivo production of megakaryocytes (MKs) and platelets from a donor-independent source is currently of intense interest in transfusion medicine. Adipose tissue-derived stromal cells (ASCs) constitute an attractive candidate cell source, because inducing these cells into MK lineages requires no gene transfer and only endogenous transcription factors containing p45NF-E2/Maf, an MK-inducing factor. Objectives To examine whether ASCs differentiate into MK lineages by using endogenous thrombopoietin (TPO), a primary cytokine that drives MK lineages. Methods TPO levels were measured by quantitative real-time PCR and ELISA. To investigate the effects of endogenous TPO on MK and platelet production, surface marker expression and functions for platelets were analyzed in ASC-derived cells cultured in the presence or absence of recombinant TPO. Based on a screening test, the role of transferrin receptor CD71 in TPO production and MK differentiation was examined with anti-CD71 antibody, small interfering RNA (siRNA) against CD71 (siRNA-CD71), and CD71-positive/negative cells. Results ASCs secreted TPO during MK differentiation, and the endogenous TPO facilitated MK and platelet production from ASCs. TPO secretion from ASCs occurred in a transferrin-dependent manner. ASCs treated with anti-CD71 antibody or transfected with siRNA-CD71 produced markedly less TPO. The TPO levels and MK yield were significantly higher when CD71-positive ASCs were used than when CD71-negative ASCs were used. Conclusions CD71 might be an appropriate marker for MK progenitor cells among human ASCs, because of the higher capacity of CD71-positive cells to produce TPO and their ability to differentiate into MKs. These findings could help to establish an efficient method for platelet production.

Abnormal megakaryopoiesis and platelet function in cyclooxygenase-2-deficient mice

Previous studies suggest that cyclooxygenase-2 (COX-2) might influence megakaryocyte (MK) maturation and platelet production in vitro. Using a gene deletion model, we analysed the effect of COX-2 deficiency on megakaryopoiesis and platelet function. COX-2-/- mice (10-12 weeks old) have hyper-responsive platelets as suggested by their enhanced aggregation, TXA2 biosynthesis, CD62P and CD41/CD61 expression, platelet-fibrinogen binding, and increased thromboembolic death after collagen/epinephrine injection compared to wild-type (WT). Moreover, increased platelet COX-1 expression and reticulated platelet fraction were observed in COX-2-/- mice while platelet count was similar to WT. MKs were significantly reduced in COX-2-/- bone marrows (BMs), with high nuclear/cytoplasmic ratios, low ploidy and poor expression of lineage markers of maturation (CD42d, CD49b). However, MKs were significantly increased in COX-2-/- spleens, with features of MK maturation markers which were not observed in MKs of WT spleens. Interestingly, the expression of COX-1, prostacyclin and PGE2 synthases and prostanoid pattern were modified in BMs and spleens of COX-2-/- mice. Moreover, COX-2 ablation reduced the percentage of CD49b+ cells, the platelet formation and the haematopoietic stem cells in bone marrow and increased their accumulation in the spleen. Splenectomy decreased peripheral platelet number, reverted their hyper-responsive phenotype and protected COX-2-/- mice from thromboembolism. Interestingly, fibrosis was observed in spleens of old COX-2-/- mice (28 weeks old). In conclusion, COX-2 deletion delays BM megakaryopoiesis promoting a compensatory splenic MK hyperplasia, with a release of hyper-responsive platelets and increased thrombogenicity in vivo. COX-2 seems to contribute to physiological MK maturation and pro-platelet formation.

Growth factor use in medication-induced hematologic toxicity

Myelosuppression is a dose-limiting adverse effect with antineoplastic therapy and nonchemotherapy medications. Clinicians have data and guidelines to provide direction for the management of neutropenia and thrombocytopenia in patients with malignancies. Clinical situations outside oncology extrapolate these data along with limited data sets for those patients who demonstrate myelosuppressive effects from medications that are not traditionally considered cytotoxic. Pharmacological treatments can be used to help ameliorate the myelosuppressive toxicities. Recombinant technology has provided growth factors to counteract or lessen the degree of toxicity from myelosuppressive medications including chemotherapy. Clinical strategies and future trends on how to mitigate medication-related myelosuppression are discussed.

The biology of pediatric acute megakaryoblastic leukemia

Acute megakaryoblastic leukemia (AMKL) comprises between 4% and 15% of newly diagnosed pediatric acute myeloid leukemia patients. AMKL in children with Down syndrome (DS) is characterized by a founding GATA1 mutation that cooperates with trisomy 21, followed by the acquisition of additional somatic mutations. In contrast, non-DS-AMKL is characterized by chimeric oncogenes consisting of genes known to play a role in normal hematopoiesis. CBFA2T3-GLIS2 is the most frequent chimeric oncogene identified to date in this subset of patients and confers a poor prognosis.

Loss of autophagy leads to failure in megakaryopoiesis, megakaryocyte differentiation, and thrombopoiesis in mice

During hematopoiesis, megakaryopoiesis, megakaryocyte differentiation, and thrombopoiesis are regulated at multiple stages, which involve successive lineage commitment steps and proceed with polyploidization, maturation, and organized fragmentation of the cytoplasm, leading to the release of platelets in circulation. However, the cellular mechanisms by which megakaryocytes derive from their progenitors and differentiate into platelets have not fully been understood. Using an Atg7 hematopoietic conditional knockout mouse model, we found that loss of autophagy, a metabolic process essential in homeostasis and cellular remodeling, caused mitochondrial and cell cycle dysfunction, impeding megakaryopoiesis and megakaryocyte differentiation, as well as thrombopoiesis and subsequently produced abnormal platelets, larger in size and fewer in number, ultimately leading to severely impaired platelet production and failed hemostasis.