Megakaryocyte ontogeny: Clinical and molecular significance

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.

Nupr1 Negatively Regulates Endothelial to Hematopoietic Transition in the Aorta-Gonad-Mesonephros Region

In the aorta of mid-gestational mouse embryos, a specialized endothelial subpopulation termed hemogenic endothelial cells (HECs) develops into hematopoietic stem and progenitor cells (HSPCs), through a conserved process of endothelial-to-hematopoietic transition (EHT). EHT is tightly controlled by multiple intrinsic and extrinsic mechanisms. Nevertheless, the molecular regulators restraining this process remain poorly understood. Here, it is uncovered that, one of the previously identified HEC signature genes, Nupr1, negatively regulates the EHT process. Nupr1 deletion in endothelial cells results in increased HSPC generation in the aorta-gonad-mesonephros region. Furthermore, single-cell transcriptomics combined with serial functional assays reveals that loss of Nupr1 promotes the EHT process by promoting the specification of hematopoiesis-primed functional HECs and strengthening their subsequent hematopoietic differentiation potential toward HSPCs. This study further finds that the proinflammatory cytokine, tumor necrosis factor α (TNF-α), is significantly upregulated in Nupr1-deficient HECs, and the use of a specific TNF-α neutralizing antibody partially reduces excessive HSPC generation in the explant cultures from Nupr1-deficient embryos. This study identifies a novel negative regulator of EHT and the findings indicate that Nupr1 is a new potential target for future hematopoietic stem cell regeneration research.

Augmented Production of Platelets From Cord Blood With Euchromatic Histone Lysine Methyltransferase Inhibition

Cord blood hematopoietic stem/progenitor cells (CB-HSPCs) have emerged as a promising supply for functional platelets to potentially alleviate the increasing demand for platelet transfusions, but the clinical application has been limited by the undefined molecular mechanism and insufficient platelet production. Here, we performed single-cell profiling of more than 16 160 cells to construct a dynamic molecular landscape of human megakaryopoiesis from CB-HSPCs, enabling us to uncover, for the first time, cellular heterogeneity and unique features of neonatal megakaryocytes (MKs) and to also offer unique resources for the scientific community. By using this model, we defined the genetic programs underlying the differentiation process from megakaryocyte-erythroid progenitors (MEPs) to MKs via megakaryocyte progenitors (MKPs) and identified inhibitors of euchromatic histone lysine methyltransferase (EHMT), which, when applied at the early stage of differentiation, significantly increase the final platelet production. At the mechanistic level, we found that EHMT inhibitors act to selectively induce the expansion of MEPs and MKPs. Together, we uncover new mechanistic insights into human megakaryopoiesis and provide a novel chemical strategy for future large-scale generation and clinical applications of platelets.

A Rapid and Sensitive UHPLC-MS/MS Method for Determination of 2, 3, 8-Trimethylellagic, a Potent Active Compound from Sanguisorba officinalis L., and Its Application in the Pharmacokinetic Study within Thrombocytopenia Rats

To investigate the pharmacokinetics of 2, 3, 8-trimethylellagic (TMEA) in rats in vivo and determine the possible effects of the pathological conditions and compatibility, a rapid and sensitive ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method for quantitative determination was developed. TMEA and Artemetin (internal standard, IS) were separated on an Acquity Shim-pack GIST column with a total running time of 7 min using gradient elution at a flow rate of 0.3 mL/min. The intraday and interday relative standard deviations were <9.50%, and the relative error of accuracy was between −5.70% and 2.96%. The calibration curve of TMEA demonstrated good linearity with r2 = 0.9996, with the average recovery changing from 94.77% to 102.47% and the matrix effect from 93.16% to 100.15%. Compared with the normal group, the area under the plasma concentration-time curve from time 0 to the last time of quantifiable concentration (AUC(0 − t)), area under the plasma concentration-time curve from time 0 extrapolated to infinite time (AUC(0 − ∞)), and the maximum concentration (Cmax) of TMEA increased, whereas the time of maximum concentration (Tmax) and apparent clearance (CL/F) remarkably decreased in the TMEA group. With significantly reduced CL/F, AUC(0 − t), AUC(0 − ∞), and Cmax for TMEA were increased approximately one time after combining with 3, 7-Di-O-methylducheside A (DOMA). AUC(0 − t) and Cmax for TMEA in the 2, 3, 8-trimethylellagic-3, 8-dimethoxyellagic acid-2-oxyglucoside (TMEA-DMAG) group were significantly lower than that in the TMEA group with clearly prolonged Tmax and increased CL/F. These findings indicate that the changes in the pharmacokinetic parameters of TMEA may be caused by pathological and combination conditions.

Developmental differences of in vitro cultured murine bone marrow- and fetal liver-derived megakaryocytes

Multiple lines of evidence support differences in the megakaryopoiesis during development. Murine in vitro models to study megakaryopoiesis employ cultured megakaryocytes MKs derived from adult bone marrow (BM) or fetal livers (FL) of mouse embryos. Mouse models allow to study the molecular basis for cellular changes utilizing conditional or knock-out models and permit further in vitro genetic or pharmacological manipulations. Despite being extensively used, MKs cultured from these two sources have not been systematically compared. In the present study, we compared BM- and FL-derived MKs, assessing their size, proplatelet production capacity, expression of common MK markers (αIIb, β3, GPIb α, β) and cytoskeletal proteins (filamin A, β1-tubulin, actin), the subcellular appearance of α-granules (VWF), membranes (GPIbβ) and cytoskeleton (F-actin) throughout in vitro development. We demonstrate that FL MKs although smaller in size, spontaneously produce more proplatelets than BM MKs and at earlier stages express more β1-tubulin. In addition, early FL MKs show increased internal GPIbβ staining and present higher GPIbβ (early and late) and VWF (late stages) total fluorescence intensity (TFI)/cell size than BM MKs. BM MKs have up-regulated TPO signaling corresponding to their bigger size and ploidy, without changes in c-Mpl. Expressing endogenous β1-tubulin or the presence of heparin improves BM MKs ability to produce proplatelets. These data suggest that FL MKs undergo cytoplasmic maturation earlier than BM MKs and that this, in addition to higher β1-tubulin levels and GPIb, supported with an extensive F-actin network, could contribute to more efficient proplatelet formation in vitro.

Craniofacial Bone Tissue Engineering: Current Approaches and Potential Therapy

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

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.

Modeling genetic platelet disorders with human pluripotent stem cells: mega-progress but wanting more on our plate(let)

PURPOSE OF REVIEW

Megakaryocytes are rare hematopoietic cells that play an instrumental role in hemostasis, and other important biological processes such as immunity and wound healing. With the advent of cell reprogramming technologies and advances in differentiation protocols, it is now possible to obtain megakaryocytes from any pluripotent stem cell (PSC) via hematopoietic induction. Here, we review recent advances in PSC-derived megakaryocyte (iMK) technology, focusing on platform validation, disease modeling and current limitations.

RECENT FINDINGS

A comprehensive study confirmed that iMK can recapitulate many transcriptional and functional aspects of megakaryocyte and platelet biology, including variables associated with complex genetic traits such as sex and race. These findings were corroborated by several pathological models in which iMKs revealed molecular mechanisms behind inherited platelet disorders and assessed the efficacy of novel pharmacological interventions. However, current differentiation protocols generate primarily embryonic iMK, limiting the clinical and translational potential of this system.

SUMMARY

iMK are strong candidates to model pathologic mutations involved in platelet defects and develop innovative therapeutic strategies. Future efforts on generating definitive hematopoietic progenitors would improve current platelet generation protocols and expand our capacity to model neonatal and adult megakaryocyte disorders.

Characterization of Cellular Heterogeneity and an Immune Subpopulation of Human Megakaryocytes

Megakaryocytes (MKs) and their progeny platelets function in a variety of biological processes including coagulation, hemostasis, inflammation, angiogenesis, and innate immunity. However, the divergent developmental and cellular landscape of adult MKs remains mysterious. Here, by deriving the single-cell transcriptomic profiling of MKs from human adult bone marrow (BM), cellular heterogeneity within MKs is unveiled and an MK subpopulation with high enrichment of immune-associated genes is identified. By performing the dynamic single-cell transcriptomic landscape of human megakaryopoiesis in vitro, it is found that the immune signatures of MKs can be traced back to the progenitor stage. Furthermore, two surface markers, CD148 and CD48, are identified for mature MKs with immune characteristics. At the functional level, these CD148+ CD48+ MKs can respond rapidly to immune stimuli both in vitro and in vivo, exhibit high-level expression of immune receptors and mediators, and may function as immune-surveillance cells. The findings uncover the cellular heterogeneity and a novel immune subset of human adult MKs and should greatly facilitate the understanding of the divergent functions of MKs under physiological and pathological conditions.

Generation and manipulation of human iPSC-derived platelets

The discovery of iPSCs has led to the ex vivo production of differentiated cells for regenerative medicine. In the case of transfusion products, the derivation of platelets from iPSCs is expected to complement our current blood-donor supplied transfusion system through donor-independent production with complete pathogen-free assurance. This derivation can also overcome alloimmune platelet transfusion refractoriness by resulting in autologous, HLA-homologous or HLA-deficient products. Several developments were necessary to produce a massive number of platelets required for a single transfusion. First, expandable megakaryocytes were established from iPSCs through transgene expression. Second, a turbulent-type bioreactor with improved platelet yield and quality was developed. Third, novel drugs that enabled efficient feeder cell-free conditions were developed. Fourth, the platelet-containing suspension was purified and resuspended in an appropriate buffer. Finally, the platelet product needed to be assured for competency and safety including non-tumorigenicity through in vitro and in vivo preclinical tests. Based on these advancements, a clinical trial has started. The generation of human iPSC-derived platelets could evolve transfusion medicine to the next stage and assure a ubiquitous, safe supply of platelet products. Further, considering the feasibility of gene manipulations in iPSCs, other platelet products may bring forth novel therapeutic measures.

Decoding Human Megakaryocyte Development

Despite our growing understanding of embryonic immune development, rare early megakaryocytes (MKs) remain relatively understudied. Here we used single-cell RNA sequencing of human MKs from embryonic yolk sac (YS) and fetal liver (FL) to characterize the transcriptome, cellular heterogeneity, and developmental trajectories of early megakaryopoiesis. In the YS and FL, we found heterogeneous MK subpopulations with distinct developmental routes and patterns of gene expression that could reflect early functional specialization. Intriguingly, we identified a subpopulation of CD42b⁺CD14⁺ MKs in vivo that exhibit high expression of genes associated with immune responses and can also be derived from human embryonic stem cells (hESCs) in vitro. Furthermore, we identified THBS1 as an early marker for MK-biased embryonic endothelial cells. Overall, we provide important insights and invaluable resources for dissection of the molecular and cellular programs underlying early human megakaryopoiesis.

Ontogeny of platelet function

Although the hemostatic potential of adult platelets has been investigated extensively, regulation of platelet function during fetal life is less clear. Recent studies have provided increasing evidence for a developmental control of platelet function during fetal ontogeny. Fetal platelets feature distinct differences in reactive properties compared with adults. These differences very likely reflect a modified hemostatic and homeostatic environment in which platelet hyporeactivity contributes to prevent pathological clot formation on the one hand but still ensures sufficient hemostasis on the other hand. In this review, recent findings on the ontogeny of platelet function and reactivity are summarized, and implications for clinical practice are critically discussed. This includes current platelet-transfusion practice and its potential risk in premature infants and neonates.

Ontogenic Changes in Hematopoietic Hierarchy Determine Pediatric Specificity and Disease Phenotype in Fusion Oncogene-Driven Myeloid Leukemia

Fusion oncogenes are prevalent in several pediatric cancers, yet little is known about the specific associations between age and phenotype. We observed that fusion oncogenes, such as ETO2-GLIS2, are associated with acute megakaryoblastic or other myeloid leukemia subtypes in an age-dependent manner. Analysis of a novel inducible transgenic mouse model showed that ETO2-GLIS2 expression in fetal hematopoietic stem cells induced rapid megakaryoblastic leukemia whereas expression in adult bone marrow hematopoietic stem cells resulted in a shift toward myeloid transformation with a strikingly delayed in vivo leukemogenic potential. Chromatin accessibility and single-cell transcriptome analyses indicate ontogeny-dependent intrinsic and ETO2-GLIS2-induced differences in the activities of key transcription factors, including ERG, SPI1, GATA1, and CEBPA. Importantly, switching off the fusion oncogene restored terminal differentiation of the leukemic blasts. Together, these data show that aggressiveness and phenotypes in pediatric acute myeloid leukemia result from an ontogeny-related differential susceptibility to transformation by fusion oncogenes. SIGNIFICANCE: This work demonstrates that the clinical phenotype of pediatric acute myeloid leukemia is determined by ontogeny-dependent susceptibility for transformation by oncogenic fusion genes. The phenotype is maintained by potentially reversible alteration of key transcription factors, indicating that targeting of the fusions may overcome the differentiation blockage and revert the leukemic state.See related commentary by Cruz Hernandez and Vyas, p. 1653.This article is highlighted in the In This Issue feature, p. 1631.