Unipotent Megakaryopoietic Pathway Bridging Hematopoietic Stem Cells and Mature Megakaryocytes

Discovery of seven hox genes in zebrafish thrombopoiesis

Thrombopoiesis is the production of platelets from megakaryocytes in the bone marrow of mammals. In fish, thrombopoiesis involves the formation of thrombocytes without megakaryocyte-like precursors but derived from erythrocyte thrombocyte bi-functional precursor cells. One unique feature of thrombocyte differentiation involves the maturation of young thrombocytes in circulation. In this study, we investigated the role of hox genes in zebrafish thrombopoiesis to model platelet production. We selected hoxa10b, hoxb2a, hoxc5a, hoxd3a, and hoxc11b from thrombocyte RNA expression data, and checked whether they are expressed in young or mature thrombocytes. We found hoxa10b, hoxb2a, hoxc5a, and hoxd3a were expressed in both young and mature thrombocytes and hoxc11b was expressed in only young thrombocytes. We then performed knockdowns of these 5 hox genes and found hoxc11b knockdown resulted in thrombocytosis and the rest showed thrombocytopenia. To identify hox genes that could have been missed by the above datasets, we performed knockdowns 47 hox genes in the zebrafish genome and found hoxa9a, and hoxb1a knockdowns resulted in thrombocytopenia and they were expressed in both young and mature thrombocytes. In conclusion, our comprehensive knockdown study identified Hoxa10b, Hoxb2a, Hoxc5a, Hoxd3a, Hoxa9a, and Hoxb1a, as positive regulators and Hoxc11b, as a negative regulator for thrombocyte development.

A mouse model with high clonal barcode diversity for joint lineage, transcriptomic, and epigenomic profiling in single cells

Cellular lineage histories and their molecular states encode fundamental principles of tissue development and homeostasis. Current lineage-recording mouse models have insufficient barcode diversity and single-cell lineage coverage for profiling tissues composed of millions of cells. Here, we developed DARLIN, an inducible Cas9 barcoding mouse line that utilizes terminal deoxynucleotidyl transferase (TdT) and 30 CRISPR target sites. DARLIN is inducible, generates massive lineage barcodes across tissues, and enables the detection of edited barcodes in ∼70% of profiled single cells. Using DARLIN, we examined fate bias within developing hematopoietic stem cells (HSCs) and revealed unique features of HSC migration. Additionally, we established a protocol for joint transcriptomic and epigenomic single-cell measurements with DARLIN and found that cellular clonal memory is associated with genome-wide DNA methylation rather than gene expression or chromatin accessibility. DARLIN will enable the high-resolution study of lineage relationships and their molecular signatures in diverse tissues and physiological contexts.

Safety of romiplostim administered immediately after cord-blood transplantation: a phase 1 trial

Graft failure and delayed hematopoietic recovery are the major limitations of cord-blood transplantation (CBT). Romiplostim, a thrombopoietin-receptor agonist, promotes megakaryopoiesis and multilineage hematopoiesis in aplastic anemia. The decreased number of hematopoietic stem cells in the early phase after CBT and aplastic anemia share certain characteristics. Therefore, we hypothesized that romiplostim administration immediately after CBT may promote multilineage hematopoietic recovery. We investigated the safety and preliminary efficacy of administering romiplostim a day after CBT. This phase 1 dose-escalation study included six adults with hematologic malignancies in remission. Romiplostim was administered subcutaneously within 7 days after single-unit CBT, initially at doses of 5 µg/kg or 10 µg/kg in three patients, then once a week for 14 weeks or until platelet recovery. The maximum dose was 20 µg/kg. The median number of romiplostim administrations was 6 (range, 3-15). Romiplostim-related adverse events included bone pain (3/6) and injection site reaction (1/6). Non-hematological grade ≥ 3 toxicities were observed in four patients; febrile neutropenia was the most common (4/6). All patients achieved neutrophil engraftment and the median time was 14 days (range, 12-32). Platelet counts ≥ 50 × 109 /L were recorded in all patients except for one who died on day 48; the median time was 34 days (range, 29-98). No relapse, thrombosis, or bone marrow fibrosis was observed during a median follow-up of 34 months. Romiplostim may be safely administered in the early phase of CBT. Further phase 2 trial is warranted for its efficacy evaluation. Trial registration number: UMIN000033799, August 18, 2018.

Genetics of inherited thrombocytopenias

The inherited thrombocytopenia syndromes are a group of disorders characterized primarily by quantitative defects in platelet number, though with a variety demonstrating qualitative defects and/or extrahematopoietic findings. Through collaborative international efforts applying next-generation sequencing approaches, the list of genetic syndromes that cause thrombocytopenia has expanded significantly in recent years, now with over 40 genes implicated. In this review, we focus on what is known about the genetic etiology of inherited thrombocytopenia syndromes and how the field has worked to validate new genetic discoveries. We highlight the important role for the clinician in identifying a germline genetic diagnosis and strategies for identifying novel causes through research-based endeavors.

Developmental cues license megakaryocyte priming in murine hematopoietic stem cells

The fetal-to-adult switch in hematopoietic stem cell (HSC) behavior is characterized by alterations in lineage output and entry into deep quiescence. Here we identify the emergence of megakaryocyte (Mk)-biased HSCs as an event coinciding with this developmental switch. Single-cell chromatin accessibility analysis reveals a ubiquitous acquisition of Mk lineage priming signatures in HSCs during the fetal-to-adult transition. These molecular changes functionally coincide with increased amplitude of early Mk differentiation events after acute inflammatory insult. Importantly, we identify LIN28B, known for its role in promoting fetal-like self-renewal, as an insulator against the establishment of an Mk-biased HSC pool. LIN28B protein is developmentally silenced in the third week of life, and its prolonged expression delays emergency platelet output in young adult mice. We propose that developmental regulation of Mk priming may represent a switch for HSCs to toggle between prioritizing self-renewal in the fetus and increased host protection in postnatal life.

Differentiation Epitopes Define Hematopoietic Stem Cells and Change with Cell Cycle Passage

Hematopoietic stem cells express differentiation markers B220 and Gr1 and are proliferative. We have shown that the expression of these entities changes with cell cycle passage. Overall, we conclude that primitive hematopoietic stem cells alter their differentiation potential with cell cycle progression. Murine derived long-term hematopoietic stem cells (LT-HSC) are cycling and thus always changing phenotype. Here we show that over one half of marrow LT-HSC are in the population expressing differentiation epitopes and that B220 and Gr-1 positive populations are replete with LT-HSC after a single FACS separation but if subjected to a second separation these cells no longer contain LT-HSC. However, with second separated cells there is a population appearing that is B220 negative and replete with cycling c-Kit, Sca-1 CD150 positive LT-HSC. There is a 3-4 h interval between the first and second B220 or GR-1 FACS separation during which the stem cells continue to cycle. Thus, the LT-HSC have lost B220 or GR-1 expression as the cells progress through cell cycle, although they have maintained the c-kit, Sca-1 and CD150 stem cells markers over this time interval. These data indicate that cycling stem cells express differentiation epitopes and alter their differentiation potential with cell cycle passage.

CD62L expression level determines the cell fate of myeloid progenitors

Hematopoietic cells differentiate through several progenitors in a hierarchical manner, and recent single-cell analyses have revealed substantial heterogeneity within each progenitor. Although common myeloid progenitors (CMPs) are defined as a multipotent cell population that can differentiate into granulocyte-monocyte progenitors (GMPs) and megakaryocyte-erythrocyte progenitors (MEPs), and GMPs generate neutrophils and monocytes, these myeloid progenitors must contain some lineage-committed progenitors. Through gene expression analysis at single-cell levels, we identified CD62L as a marker to reveal the heterogeneity. We confirmed that CD62L-negative CMPs represent "bona fide" CMPs, whereas CD62L-high CMPs are mostly restricted to GMP potentials both in mice and humans. In addition, we identified CD62L-negative GMPs as the most immature subsets in GMPs and Ly6C+CD62L-intermediate and Ly6C+CD62L-high GMPs are skewed to neutrophil and monocyte differentiation in mice, respectively. Our findings contribute to more profound understanding about the mechanism of myeloid differentiation.

DMAG, a novel countermeasure for the treatment of thrombocytopenia

Background

Thrombocytopenia is one of the most common hematological disease that can be life-threatening caused by bleeding complications. However, the treatment options for thrombocytopenia remain limited.

Methods

In this study, giemsa staining, phalloidin staining, immunofluorescence and flow cytometry were used to identify the effects of 3,3ʹ-di-O-methylellagic acid 4ʹ-glucoside (DMAG), a natural ellagic acid derived from Sanguisorba officinalis L. (SOL) on megakaryocyte differentiation in HEL cells. Then, thrombocytopenia mice model was constructed by X-ray irradiation to evaluate the therapeutic action of DMAG on thrombocytopenia. Furthermore, the effects of DMAG on platelet function were evaluated by tail bleeding time, platelet aggregation and platelet adhesion assays. Next, network pharmacology approaches were carried out to identify the targets of DMAG. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to elucidate the underling mechanism of DMAG against thrombocytopenia. Finally, molecular docking simulation, molecular dynamics simulation and western blot analysis were used to explore the relationship between DAMG with its targets.

Results

DMAG significantly promoted megakaryocyte differentiation of HEL cells. DMAG administration accelerated platelet recovery and megakaryopoiesis, shortened tail bleeding time, strengthened platelet aggregation and adhesion in thrombocytopenia mice. Network pharmacology revealed that ITGA2B, ITGB3, VWF, PLEK, TLR2, BCL2, BCL2L1 and TNF were the core targets of DMAG. GO and KEGG pathway enrichment analyses suggested that the core targets of DMAG were enriched in PI3K–Akt signaling pathway, hematopoietic cell lineage, ECM-receptor interaction and platelet activation. Molecular docking simulation and molecular dynamics simulation further indicated that ITGA2B, ITGB3, PLEK and TLR2 displayed strong binding ability with DMAG. Finally, western blot analysis evidenced that DMAG up-regulated the expression of ITGA2B, ITGB3, VWF, p-Akt and PLEK.

Conclusion

DMAG plays a critical role in promoting megakaryocytes differentiation and platelets production and might be a promising medicine for the treatment of thrombocytopenia.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-021-00404-1.

Thrombopoietin maintains cell numbers of hematopoietic stem and progenitor cells with megakaryopoietic potential

Thrombopoietin has long been known to influence megakaryopoiesis and hematopoietic stem and progenitor cells, although the exact mechanisms through which it acts are unknown. Here we show that MPL expression correlates with megakaryopoietic potential of hematopoietic stem and progenitor cells and identify a population of quiescent hematopoietic stem and progenitor cells that show limited dependence on thrombopoietin signaling. We show that thrombopoietin is primarily responsible for maintenance of hematopoietic cells with megakaryocytic differentiation potential and their subsequent megakaryocyte differentiation and maturation. The loss of megakaryocytes in thrombopoietin knockout mouse models results in a reduction of megakaryocyte-derived chemokine platelet factor 4 (CXCL4/PF4) in the bone marrow and administration of recombinant CXCL4/PF4 rescues the loss of quiescence observed in these mice. CXCL4/PF4 treatment does not rescue reduced hematopoietic stem and progenitor cell numbers, suggesting that thrombopoietin maintains hematopoietic stem and progenitor cell numbers directly.

Study of inherited thrombocytopenia resulting from mutations in ETV6 or RUNX1 using a human pluripotent stem cell model

Inherited thrombocytopenia results in low platelet counts and increased bleeding. Subsets of these patients have monoallelic germline mutations in ETV6 or RUNX1 and a heightened risk of developing hematologic malignancies. Utilizing CRISPR-Cas9, we compared the in vitro phenotype of hematopoietic progenitor cells and megakaryocytes derived from induced pluripotent stem cell (iPSC) lines harboring mutations in either ETV6 or RUNX1. Both mutant lines display phenotypes consistent with a platelet-bleeding disorder. Surprisingly, these cellular phenotypes were largely distinct. The ETV6-mutant iPSCs yield more hematopoietic progenitor cells and megakaryocytes, but the megakaryocytes are immature and less responsive to agonist stimulation. On the contrary, RUNX1-mutant iPSCs yield fewer hematopoietic progenitor cells and megakaryocytes, but the megakaryocytes are more responsive to agonist stimulation. However, both mutant iPSC lines display defects in proplatelet formation. Our work highlights that, while patients harboring germline ETV6 or RUNX1 mutations have similar clinical phenotypes, the molecular mechanisms may be distinct.

JAK2-V617F and interferon-α induce megakaryocyte-biased stem cells characterized by decreased long-term functionality

We studied a subset of hematopoietic stem cells (HSCs) that are defined by elevated expression of CD41 (CD41hi) and showed bias for differentiation toward megakaryocytes (Mks). Mouse models of myeloproliferative neoplasms (MPNs) expressing JAK2-V617F (VF) displayed increased frequencies and percentages of the CD41hi vs CD41lo HSCs compared with wild-type controls. An increase in CD41hi HSCs that correlated with JAK2-V617F mutant allele burden was also found in bone marrow from patients with MPN. CD41hi HSCs produced a higher number of Mk-colonies of HSCs in single-cell cultures in vitro, but showed reduced long-term reconstitution potential compared with CD41lo HSCs in competitive transplantations in vivo. RNA expression profiling showed an upregulated cell cycle, Myc, and oxidative phosphorylation gene signatures in CD41hi HSCs, whereas CD41lo HSCs showed higher gene expression of interferon and the JAK/STAT and TNFα/NFκB signaling pathways. Higher cell cycle activity and elevated levels of reactive oxygen species were confirmed in CD41hi HSCs by flow cytometry. Expression of Epcr, a marker for quiescent HSCs inversely correlated with expression of CD41 in mice, but did not show such reciprocal expression pattern in patients with MPN. Treatment with interferon-α further increased the frequency and percentage of CD41hi HSCs and reduced the number of JAK2-V617F+ HSCs in mice and patients with MPN. The shift toward the CD41hi subset of HSCs by interferon-α provides a possible mechanism of how interferon-α preferentially targets the JAK2 mutant clone.

Biological Characteristics and Regulation of Early Megakaryocytopoiesis

For decades, megakaryocytopoiesis is believed to occur following a classical binary hierarchical developmental model. This model is based on an analysis of predefined flow-sorted cell populations by using cell surface markers. However, this classical model has been challenged by increasing evidences obtained with new techniques which integrating flow cytometric, transcriptomic and functional data at single-cell level and with lineage tracing technique. These recent advances in megakaryocytopoiesis proposed that commitment of haematopoietic stem cells (HSCs) towards megakaryocytic lineage occurs in much earlier stage than that postulated in the classical model. There may exist multipotent but megakaryocyte (MK)/platelet-biased HSCs within HSC compartment and even HSCs can directly differentiate into MKs in steady state or in response to stress. In this review, we focus on recent findings about differentiation from commitment of HSCs to MK and its regulation, and discuss future directions in this research field.

Unbiased phenotypic identification of functionally distinct hematopoietic progenitors

Background

Hematopoiesis is a model-system for studying cellular development and differentiation. Phenotypic and functional characterization of hematopoietic progenitors has significantly aided our understanding of the mechanisms that govern fate choice, lineage specification and maturity. Methods for progenitor isolation have historically relied on complex flow-cytometric strategies based on nested, arbitrary gates within defined panels of immunophenotypic markers. The resulted populations are then functionally assessed, although functional homogeneity or absolute linkage between function and phenotype is not always achieved, thus distorting our view on progenitor biology.

Method

In this study, we present a protocol for unbiased phenotypic identification and functional characterization which combines index sorting and clonogenic assessment of individual progenitor cells. Single-cells are plated into custom media allowing multiple hematopoietic fates to emerge and are allowed to give rise to unilineage colonies or mixed. After colony identification, lineage potential is assigned to each progenitor and finally the indexed phenotype of the initial cell is recalled and a phenotype is assigned to each functional output.

Conclusions

Our approach overcomes the limitations of the current protocols expanding beyond the established cell-surface marker panels and abolishing the need for nested gating. Using this method we were able to resolve the relationships of myeloid progenitors according to the revised model of hematopoiesis, as well as identify a novel marker for erythroid progenitors. Finally, this protocol can be applied to the characterization of any progenitor cell with measurable function.

Electronic supplementary material

The online version of this article (10.1186/s40709-019-0097-7) contains supplementary material, which is available to authorized users.

Diverse functions of the thrombopoietin receptor agonist romiplostim rescue individuals exposed to lethal radiation

In cases of radiological accidents, especially victims exposed to high-dose ionizing radiation, the administration of appropriate approved pharmaceutical drugs is the most rapid medical treatment. However, currently, there are no suitable candidates. The thrombopoietin receptor (TPOR) agonist romiplostim (RP) is a therapeutic agent for immune thrombocytopenia and has potential to respond to such victims. Here, we show that RP administration in mice exposed to lethal-dose radiation leads not only to the promotion of haematopoiesis in multiple organs, including the lungs but also a reduction in damage to organs and cells. RP also causes a rapid increase in the number of mesenchymal stem cells in the spleen. In addition, RP suppresses the expression of several miRNAs involved in radiation-induced leukemogenesis, suggesting the presence of targets other than TPOR. Among the currently approved pharmaceutical drugs, RP is the most suitable candidate for victims exposed to high-dose ionizing radiation.

Modulation of megakaryopoiesis and platelet production during inflammation

Megakaryocytes (MKs) are widely known as the progenitor cells of platelets. These large, polyploid cells are a derivative of the hematopoietic stem cell (HSC), and reside in the bone marrow, lining blood vessel walls where they release their platelet progeny into circulation. Although little is known about how MKs differ under various environmental stressors, both chronic and acute inflammation alter the differentiation and molecular content of MKs. Furthermore, evidence suggests that the release of inflammatory cytokines may induce MK rupture and rapid release of platelets as a mechanism to quickly replenish diminished platelet counts in response to inflammation. Similarities between MKs and their close relatives, white blood cells, have introduced the notion that MKs may play a role in combating infection by engulfing and presenting antigens, and passing this information to circulating platelets. In addition, MKs exposed to varying bone marrow environments produce different platelets which enter circulation primed to respond to and combat inflammation, infection, or injury. This review focuses on how inflammation alters MK production, maturation, and platelet production. In addition, it introduces the idea that inflammation reprograms MKs to create different, more pathogenic platelets and leads them to take on different roles as responders to deleterious conditions. In the future, studies determining how platelets are altered in disease states may lead to novel MK- and platelet-based therapeutic targets to mitigate inflammation-related morbidity and mortality.

The Gp1ba-Cre transgenic mouse: a new model to delineate platelet and leukocyte functions

Conditional knockout (KO) mouse models are invaluable for elucidating the physiological roles of platelets. The Platelet factor 4-Cre recombinase (Pf4-Cre) transgenic mouse is the current model of choice for generating megakaryocyte/platelet-specific KO mice. Platelets and leukocytes work closely together in a wide range of disease settings, yet the specific contribution of platelets to these processes remains unclear. This is partially a result of the Pf4-Cre transgene being expressed in a variety of leukocyte populations. To overcome this issue, we developed a Gp1ba-Cre transgenic mouse strain in which Cre expression is driven by the endogenous Gp1ba locus. By crossing Gp1ba-Cre and Pf4-Cre mice to the mT/mG dual-fluorescence reporter mouse and performing a head-to-head comparison, we demonstrate more stringent megakaryocyte lineage-specific expression of the Gp1ba-Cre transgene. Broader tissue expression was observed with the Pf4-Cre transgene, leading to recombination in many hematopoietic lineages, including monocytes, macrophages, granulocytes, and dendritic and B and T cells. Direct comparison of phenotypes of Csk, Shp1, or CD148 conditional KO mice generated using either the Gp1ba-Cre or Pf4-Cre strains revealed similar platelet phenotypes. However, additional inflammatory and immunological anomalies were observed in Pf4-Cre-generated KO mice as a result of nonspecific deletion in other hematopoietic lineages. By excluding leukocyte contributions to phenotypes, the Gp1ba-Cre mouse will advance our understanding of the role of platelets in inflammation and other pathophysiological processes in which platelet-leukocyte interactions are involved.

CD45 expression discriminates waves of embryonic megakaryocytes in the mouse

Embryonic megakaryopoiesis starts in the yolk sac on gestational day 7.5 as part of the primitive wave of hematopoiesis, and it continues in the fetal liver when this organ is colonized by hematopoietic progenitors between day 9.5 and 10.5, as the definitive hematopoiesis wave. We characterized the precise phenotype of embryo megakaryocytes in the liver at gestational day 11.5, identifying them as CD41⁺⁺CD45-CD9⁺⁺CD61⁺MPL⁺CD42c⁺ tetraploid cells that express megakaryocyte-specific transcripts and display differential traits when compared to those present in the yolk sac at the same age. In contrast to megakaryocytes from adult bone marrow, embryo megakaryocytes are CD45⁻ until day 13.5 of gestation, as are both the megakaryocyte progenitors and megakaryocyte/erythroid-committed progenitors. At gestational day 11.5, liver and yolk sac also contain CD41⁺CD45⁺ and CD41⁺CD45⁻ cells. These populations, and that of CD41⁺⁺CD45⁻CD42c⁺ cells, isolated from liver, differentiate in culture into CD41⁺⁺CD45⁻CD42c⁺ proplatelet-bearing megakaryocytes. Also present at this time are CD41⁻CD45⁺⁺CD11b⁺ cells, which produce low numbers of CD41⁺⁺CD45⁻CD42c⁺ megakaryocytes in vitro, as do fetal liver cells expressing the macrophage-specific Csf receptor-1 (Csf1r/CD115) from MaFIA transgenic mice, which give rise poorly to CD41⁺⁺CD45⁻CD42c⁺ embryo megakaryocytes both in vivo and in vitro. In contrast, around 30% of adult megakaryocytes (CD41⁺⁺CD45⁺⁺CD9⁺⁺CD42c⁺) from C57BL/6 and MaFIA mice express CD115. We propose that differential pathways operating in the mouse embryo liver at gestational day 11.5 beget CD41⁺⁺CD45⁻CD42c⁺ embryo megakaryocytes that can be produced from CD41⁺CD45⁻ or from CD41⁺CD45⁺ cells, at difference from those from bone marrow.

Kinetics of adult hematopoietic stem cell differentiation in vivo

The process whereby hematopoietic stem cells (HSCs) generate different blood cell types in the steady-state is poorly understood. Upadhaya et al. used inducible lineage tracing to characterize the earliest steps of adult HSC differentiation in vivo.

Adult hematopoiesis has been studied in terms of progenitor differentiation potentials, whereas its kinetics in vivo is poorly understood. We combined inducible lineage tracing of endogenous adult hematopoietic stem cells (HSCs) with flow cytometry and single-cell RNA sequencing to characterize early steps of hematopoietic differentiation in the steady-state. Labeled cells, comprising primarily long-term HSCs and some short-term HSCs, produced megakaryocytic lineage progeny within 1 wk in a process that required only two to three cell divisions. Erythroid and myeloid progeny emerged simultaneously by 2 wk and included a progenitor population with expression features of both lineages. Myeloid progenitors at this stage showed diversification into granulocytic, monocytic, and dendritic cell types, and rare intermediate cell states could be detected. In contrast, lymphoid differentiation was virtually absent within the first 3 wk of tracing. These results show that continuous differentiation of HSCs rapidly produces major hematopoietic lineages and cell types and reveal fundamental kinetic differences between megakaryocytic, erythroid, myeloid, and lymphoid differentiation.

Tyrosyl-tRNA synthetase stimulates thrombopoietin-independent hematopoiesis accelerating recovery from thrombocytopenia

Aminoacyl-tRNA synthetases (aaRSs) catalyze aminoacylation of tRNAs in the first step of protein synthesis in the cytoplasm. However, in higher eukaryotes, they acquired additional functions beyond translation. In the present study, we show that an activated form of tyrosyl-tRNA synthetase (YRS^ACT) functions to enhance megakaryopoiesis and platelet production in vitro and in vivo. These findings were confirmed with human megakaryocytes differentiated from peripheral blood CD34⁺ hematopoietic stem cells and with human induced pluripotent stem (iPS) cells. The activity of YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from iPS cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. These findings demonstrate a previously unrecognized function of an aaRS which may have implications for therapeutic interventions.

New mechanisms behind blood cell formation continue to be uncovered, with therapeutic approaches for hematological diseases being of great interest. Here we report an enzyme in protein synthesis, known for cell-based activities beyond translation, is a factor inducing megakaryocyte-biased hematopoiesis, most likely under stress conditions. We show an activated form of tyrosyl-tRNA synthetase (YRS^ACT), prepared either by rationally designed mutagenesis or alternative splicing, induces expansion of a previously unrecognized high-ploidy Sca-1⁺ megakaryocyte population capable of accelerating platelet replenishment after depletion. Moreover, YRS^ACT targets monocytic cells to induce secretion of transacting cytokines that enhance megakaryocyte expansion stimulating the Toll-like receptor/MyD88 pathway. Platelet replenishment by YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from induced pluripotent stem cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. We suggest megakaryocyte-biased hematopoiesis induced by YRS^ACT offers new approaches for treating thrombocytopenia, boosting yields from cell-culture production of platelet concentrates for transfusion, and bridging therapy for hematopoietic stem cell transplantation.

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.

Establishment of regulatory elements during erythro-megakaryopoiesis identifies hematopoietic lineage-commitment points

BACKGROUND

Enhancers and promoters are cis-acting regulatory elements associated with lineage-specific gene expression. Previous studies showed that different categories of active regulatory elements are in regions of open chromatin, and each category is associated with a specific subset of post-translationally marked histones. These regulatory elements are systematically activated and repressed to promote commitment of hematopoietic stem cells along separate differentiation paths, including the closely related erythrocyte (ERY) and megakaryocyte (MK) lineages. However, the order in which these decisions are made remains unclear.

RESULTS

To characterize the order of cell fate decisions during hematopoiesis, we collected primary cells from mouse bone marrow and isolated 10 hematopoietic populations to generate transcriptomes and genome-wide maps of chromatin accessibility and histone H3 acetylated at lysine 27 binding (H3K27ac). Principle component analysis of transcriptional and open chromatin profiles demonstrated that cells of the megakaryocyte lineage group closely with multipotent progenitor populations, whereas erythroid cells form a separate group distinct from other populations. Using H3K27ac and open chromatin profiles, we showed that 89% of immature MK (iMK)-specific active regulatory regions are present in the most primitive hematopoietic cells, 46% of which contain active enhancer marks. These candidate active enhancers are enriched for transcription factor binding site motifs for megakaryopoiesis-essential proteins, including ERG and ETS1. In comparison, only 64% of ERY-specific active regulatory regions are present in the most primitive hematopoietic cells, 20% of which containing active enhancer marks. These regions were not enriched for any transcription factor consensus sequences. Incorporation of genome-wide DNA methylation identified significant levels of de novo methylation in iMK, but not ERY.

CONCLUSIONS

Our results demonstrate that megakaryopoietic profiles are established early in hematopoiesis and are present in the majority of the hematopoietic progenitor population. However, megakaryopoiesis does not constitute a "default" differentiation pathway, as extensive de novo DNA methylation accompanies megakaryopoietic commitment. In contrast, erythropoietic profiles are not established until a later stage of hematopoiesis, and require more dramatic changes to the transcriptional and epigenetic programs. These data provide important insights into lineage commitment and can contribute to ongoing studies related to diseases associated with differentiation defects.

Thrombopoietin signaling to chromatin elicits rapid and pervasive epigenome remodeling within poised chromatin architectures

Thrombopoietin (TPO) is a critical cytokine regulating hematopoietic stem cell maintenance and differentiation into the megakaryocytic lineage. However, the transcriptional and chromatin dynamics elicited by TPO signaling are poorly understood. Here, we study the immediate early transcriptional and cis-regulatory responses to TPO in hematopoietic stem/progenitor cells (HSPCs) and use this paradigm of cytokine signaling to chromatin to dissect the relation between cis- regulatory activity and chromatin architecture. We show that TPO profoundly alters the transcriptome of HSPCs, with key hematopoietic regulators being transcriptionally repressed within 30 minutes of TPO. By examining cis-regulatory dynamics and chromatin architectures, we demonstrate that these changes are accompanied by rapid and extensive epigenome remodeling of cis-regulatory landscapes that is spatially coordinated within topologically associating domains (TADs). Moreover, TPO-responsive enhancers are spatially clustered and engage in preferential homotypic intra- and inter-TAD interactions that are largely refractory to TPO signaling. By further examining the link between cis-regulatory dynamics and chromatin looping, we show that rapid modulation of cis-regulatory activity is largely independent of chromatin looping dynamics. Finally, we show that, although activated and repressed cis-regulatory elements share remarkably similar DNA sequence compositions, transcription factor binding patterns accurately predict rapid cis-regulatory responses to TPO.

Clonal analysis of lineage fate in native haematopoiesis

Hematopoiesis, the process of mature blood and immune cell production, is functionally organized as a hierarchy, with self-renewing hematopoietic stem cells (HSCs) and multipotent progenitor (MPP) cells sitting at the very top^1,2. Multiple models have been proposed as to what the earliest lineage choices are in these primitive hematopoietic compartments, the cellular intermediates, and the resulting lineage trees that emerge from them^3–10. Given that the bulk of studies addressing lineage outcomes have been performed in the context of hematopoietic transplantation, current lineage branching models are more likely to represent roadmaps of lineage potential rather than native fate. Here, we utilize transposon (Tn) tagging to clonally trace the fates of progenitors and stem cells in unperturbed hematopoiesis. Our results describe a distinct clonal roadmap in which the megakaryocyte (Mk) lineage arises largely independently of other hematopoietic fates. Our data, combined with single cell RNAseq, identify a functional hierarchy of uni- and oligolineage producing clones within the MPP population. Finally, our results demonstrate that traditionally defined long-term HSCs (LT-HSCs) are a significant source of Mk-restricted progenitors, suggesting that the Mk-lineage is the predominant native fate of LT-HSCs. Our study provides evidence for a substantially revised roadmap for unperturbed hematopoiesis, and highlights unique properties of MPPs and HSCs in situ.

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

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

The Road Map for Megakaryopoietic Lineage from Hematopoietic Stem/Progenitor Cells

Megakaryocytes (Mgks) are terminally differentiated blood cells specified to produce platelets, whereas hematopoietic stem cells (HSCs) are the most undifferentiated blood cells that retain multipotency to produce all kinds of blood cells. As such, these two cell types reside at the bottom and the top of the hematopoietic hierarchy, respectively. In spite of this distance, they share several important cell surface molecules as well as transcription factors.

In the conventional step‐wise differentiation model, HSCs gradually lose their self‐renewal capacity and differentiate into multipotent progenitors (MPPs), which is the first branch point of myeloid and lymphoid lineage. In this model, common myeloid progenitors can differentiate into bipotent Mgk/erythroid progenitors (MEPs), and MEPs eventually differentiate into unipotent mature Mgks. However, it has been recently reported that a subpopulation within the HSC and MPP compartments demonstrates an Mgk‐biased differentiation potential. These reports imply that revisions to the HSC‐to‐Mgk differentiation pathway should be discussed. In this review, we summarize recent findings about Mgk differentiation from HSCs and discuss future directions in this research field. Stem Cells Translational Medicine2017;6:1661–1665

Foxp3+ regulatory T cells maintain the bone marrow microenvironment for B cell lymphopoiesis

Foxp3⁺ regulatory T cells (Treg cells) modulate the immune system and maintain self-tolerance, but whether they affect haematopoiesis or haematopoietic stem cell (HSC)-mediated reconstitution after transplantation is unclear. Here we show that B-cell lymphopoiesis is impaired in Treg-depleted mice, yet this reduced B-cell lymphopoiesis is rescued by adoptive transfer of affected HSCs or bone marrow cells into Treg-competent recipients. B-cell reconstitution is abrogated in both syngeneic and allogeneic transplantation using Treg-depleted mice as recipients. Treg cells can control physiological IL-7 production that is indispensable for normal B-cell lymphopoiesis and is mainly sustained by a subpopulation of ICAM1⁺ perivascular stromal cells. Our study demonstrates that Treg cells are important for B-cell differentiation from HSCs by maintaining immunological homoeostasis in the bone marrow microenvironment, both in physiological conditions and after bone marrow transplantation.

Treg cells suppress peripheral immune responses, but their function in haematopoiesis is unclear. Here the authors show they modulate the bone marrow microenvironment to sustain haematopoietic stem cell-driven generation of mature B cells.

Identification of unipotent megakaryocyte progenitors in human hematopoiesis

The developmental pathway for human megakaryocytes remains unclear, and the definition of pure unipotent megakaryocyte progenitor is still controversial. Using single-cell transcriptome analysis, we have identified a cluster of cells within immature hematopoietic stem- and progenitor-cell populations that specifically expresses genes related to the megakaryocyte lineage. We used CD41 as a positive marker to identify these cells within the CD34⁺CD38⁺IL-3Rα^dimCD45RA^- common myeloid progenitor (CMP) population. These cells lacked erythroid and granulocyte-macrophage potential but exhibited robust differentiation into the megakaryocyte lineage at a high frequency, both in vivo and in vitro. The efficiency and expansion potential of these cells exceeded those of conventional bipotent megakaryocyte/erythrocyte progenitors. Accordingly, the CD41⁺ CMP was defined as a unipotent megakaryocyte progenitor (MegP) that is likely to represent the major pathway for human megakaryopoiesis, independent of canonical megakaryocyte-erythroid lineage bifurcation. In the bone marrow of patients with essential thrombocythemia, the MegP population was significantly expanded in the context of a high burden of Janus kinase 2 mutations. Thus, the prospectively isolatable and functionally homogeneous human MegP will be useful for the elucidation of the mechanisms underlying normal and malignant human hematopoiesis.

Granulocyte colony-stimulating factor mobilizes dormant hematopoietic stem cells without proliferation in mice

Granulocyte colony-stimulating factor (G-CSF) is used clinically to treat leukopenia and to enforce hematopoietic stem cell (HSC) mobilization to the peripheral blood (PB). However, G-CSF is also produced in response to infection, and excessive exposure reduces HSC repopulation capacity. Previous work has shown that dormant HSCs contain all the long-term repopulation potential in the bone marrow (BM), and that as HSCs accumulate a divisional history, they progressively lose regenerative potential. As G-CSF treatment also induces HSC proliferation, we sought to examine whether G-CSF-mediated repopulation defects are a result of increased proliferative history. To do so, we used an established H2BGFP label retaining system to track HSC divisions in response to G-CSF. Our results show that dormant HSCs are preferentially mobilized to the PB on G-CSF treatment. We find that this mobilization does not result in H2BGFP label dilution of dormant HSCs, suggesting that G-CSF does not stimulate dormant HSC proliferation. Instead, we find that proliferation within the HSC compartment is restricted to CD41-expressing cells that function with short-term, and primarily myeloid, regenerative potential. Finally, we show CD41 expression is up-regulated within the BM HSC compartment in response to G-CSF treatment. This emergent CD41^Hi HSC fraction demonstrates no observable engraftment potential, but directly matures into megakaryocytes when placed in culture. Together, our results demonstrate that dormant HSCs mobilize in response to G-CSF treatment without dividing, and that G-CSF-mediated proliferation is restricted to cells with limited regenerative potential found within the HSC compartment.

Targeting expression to megakaryocytes and platelets by lineage-specific lentiviral vectors

Essentials Platelet phenotypes can be modified by lentiviral transduction of hematopoietic stem cells. Megakaryocyte-specific lentiviral vectors were tested in vitro and in vivo for restricted expression. The glycoprotein 6 vector expressed almost exclusively in megakaryocytes. The platelet factor 4 vector was the strongest but with activity in hematopoietic stem cells.

SUMMARY

Background Lentiviral transduction and transplantation of hematopoietic stem cells (HSCs) can be utilized to modify the phenotype of megakaryocytes and platelets. As the genetic modification in HSCs is transmitted onto all hematopoietic progenies, transgene expression from the vector should be restricted to megakaryocytes to avoid un-physiologic effects by ectopic transgene expression. This can be achieved by lentiviral vectors that control expression by lineage-specific promoters. Methods In this study, we introduced promoters of megakaryocyte/platelet-specific genes, namely human glycoprotein 6 (hGP6) and hGP9, into third generation lentiviral vectors and analyzed their functionality in vitro and in vivo in bone marrow transplantation assays. Their specificity and efficiency of expression was compared with lentiviral vectors utilizing the promoters of murine platelet factor 4 (mPf4) and hGP1BA, both with strong activity in megakaryocytes (MKs) used in earlier studies, and the ubiquitously expressing phosphoglycerate kinase (hPGK) and spleen focus forming virus (SFFV) enhancer/promoters. Results Expression from the mPf4 vector in MKs and platelets was the strongest similar to expression from the viral SFFV promoter, however, the mPf4 vector, also exhibited considerable off-target expression in hematopoietic stem and progenitor cells. In contrast, the newly generated hGP6 vector was highly specific to megakaryocytes and platelets. The specificity was also retained when reducing the promoter size to 350 bp, making it a valuable new tool for lentiviral expression in MKs/platelets. Conclusion MK-specific vectors express preferentially in the megakaryocyte lineage. These vectors can be applied to develop murine models to study megakaryocyte and platelet function, or for gene therapy targeting proteins to platelets.

Single-cell profiling of human megakaryocyte-erythroid progenitors identifies distinct megakaryocyte and erythroid differentiation pathways

Background

Recent advances in single-cell techniques have provided the opportunity to finely dissect cellular heterogeneity within populations previously defined by “bulk” assays and to uncover rare cell types. In human hematopoiesis, megakaryocytes and erythroid cells differentiate from a shared precursor, the megakaryocyte-erythroid progenitor (MEP), which remains poorly defined.

Results

To clarify the cellular pathway in erythro-megakaryocyte differentiation, we correlate the surface immunophenotype, transcriptional profile, and differentiation potential of individual MEP cells. Highly purified, single MEP cells were analyzed using index fluorescence-activated cell sorting and parallel targeted transcriptional profiling of the same cells was performed using a specifically designed panel of genes. Differentiation potential was tested in novel, single-cell differentiation assays. Our results demonstrate that immunophenotypic MEP comprise three distinct subpopulations: “Pre-MEP,” enriched for erythroid/megakaryocyte progenitors but with residual myeloid differentiation capacity; “E-MEP,” strongly biased towards erythroid differentiation; and “MK-MEP,” a previously undescribed, rare population of cells that are bipotent but primarily generate megakaryocytic progeny. Therefore, conventionally defined MEP are a mixed population, as a minority give rise to mixed-lineage colonies while the majority of cells are transcriptionally primed to generate exclusively single-lineage output.

Conclusions

Our study clarifies the cellular hierarchy in human megakaryocyte/erythroid lineage commitment and highlights the importance of using a combination of single-cell approaches to dissect cellular heterogeneity and identify rare cell types within a population. We present a novel immunophenotyping strategy that enables the prospective identification of specific intermediate progenitor populations in erythro-megakaryopoiesis, allowing for in-depth study of disorders including inherited cytopenias, myeloproliferative disorders, and erythromegakaryocytic leukemias.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-016-0939-7) contains supplementary material, which is available to authorized users.

Notch Stimulates Both Self-Renewal and Lineage Plasticity in a Subset of Murine CD9High Committed Megakaryocytic Progenitors

This study aimed at reinvestigating the controversial contribution of Notch signaling to megakaryocytic lineage development. For that purpose, we combined colony assays and single cells progeny analyses of purified megakaryocyte-erythroid progenitors (MEP) after short-term cultures on recombinant Notch ligand rDLL1. We showed that Notch activation stimulated the SCF-dependent and preferential amplification of Kit⁺ erythroid and bipotent progenitors while favoring commitment towards the erythroid at the expense of megakaryocytic lineage. Interestingly, we also identified a CD9^High MEP subset that spontaneously generated almost exclusively megakaryocytic progeny mainly composed of single megakaryocytes. We showed that Notch activation decreased the extent of polyploidization and maturation of megakaryocytes, increased the size of megakaryocytic colonies and surprisingly restored the generation of erythroid and mixed colonies by this CD9^High MEP subset. Importantly, the size increase of megakaryocytic colonies occurred at the expense of the production of single megakaryocytes and the restoration of colonies of alternative lineages occurred at the expense of the whole megakaryocytic progeny. Altogether, these results indicate that Notch activation is able to extend the number of divisions of MK-committed CD9^High MEPs before terminal maturation while allowing a fraction of them to generate alternative lineages. This unexpected plasticity of MK-committed progenitors revealed upon Notch activation helps to better understand the functional promiscuity between megakaryocytic lineage and hematopoietic stem cells.

Aryl hydrocarbon receptor-dependent enrichment of a megakaryocytic precursor with a high potential to produce proplatelets

The mechanisms regulating megakaryopoiesis and platelet production (thrombopoiesis) are still incompletely understood. Identification of a progenitor with enhanced thrombopoietic capacity would be useful to decipher these mechanisms and to improve our capacity to produce platelets in vitro. Differentiation of peripheral blood CD34(+) cells in the presence of bone marrow-human mesenchymal stromal cells (MSCs) enhanced the production of proplatelet-bearing megakaryocytes (MKs) and platelet-like elements. This was accompanied by enrichment in a MK precursor population exhibiting an intermediate level of CD41 positivity while maintaining its expression of CD34. Following sorting and subculture with MSCs, this CD34(+)CD41(low) population was able to efficiently generate proplatelet-bearing MKs and platelet-like particles. Similarly, StemRegenin 1 (SR1), an antagonist of the aryl hydrocarbon receptor (AhR) transcription factor known to maintain CD34 expression of progenitor cells, led to an enriched CD34(+)CD41(low) fraction and to an increased capacity to generate proplatelet-producing MKs and platelet-like elements ultrastructurally and functionally similar to circulating platelets. The effect of MSCs, like that of SR1, appeared to be mediated by an AhR-dependent mechanism because both culture conditions resulted in repression of its downstream effector CYP1B1. This newly described isolation of a precursor exhibiting strong MK potential could be exploited to study normal and abnormal thrombopoiesis and for in vitro platelet production.

Linkage between the mechanisms of thrombocytopenia and thrombopoiesis

Thrombocytopenia is defined as a status in which platelet numbers are reduced. Imbalance between the homeostatic regulation of platelet generation and destruction is 1 potential cause of thrombocytopenia. In adults, platelet generation is a 2-stage process entailing the differentiation of hematopoietic stem cells into mature megakaryocytes (MKs; known as megakaryopoiesis) and release of platelets from MKs (known as thrombopoiesis or platelet biogenesis). Until recently, information about the genetic defects responsible for congenital thrombocytopenia was only available for a few forms of the disease. However, investigations over the past 15 years have identified mutations in genes encoding >20 different proteins that are responsible for these disorders, which has advanced our understanding of megakaryopoiesis and thrombopoiesis. The underlying pathogenic mechanisms can be categorized as (1) defects in MK lineage commitment and differentiation, (2) defects in MK maturation, and (3) defect in platelet release. Using these developmental stage categories, we here update recently described mechanisms underlying megakaryopoiesis and thrombopoiesis and discuss the association between platelet generation systems and thrombocytopenia.

Hematopoietic stem/progenitor cell commitment to the megakaryocyte lineage

The classical model of hematopoiesis has long held that hematopoietic stem cells (HSCs) sit at the apex of a developmental hierarchy in which HSCs undergo long-term self-renewal while giving rise to cells of all the blood lineages. In this model, self-renewing HSCs progressively lose the capacity for self-renewal as they transit into short-term self-renewing and multipotent progenitor states, with the first major lineage commitment occurring in multipotent progenitors, thus giving rise to progenitors that initiate the myeloid and lymphoid branches of hematopoiesis. Subsequently, within the myeloid lineage, bipotent megakaryocyte-erythrocyte and granulocyte-macrophage progenitors give rise to unipotent progenitors that ultimately give rise to all mature progeny. However, over the past several years, this developmental scheme has been challenged, with the origin of megakaryocyte precursors being one of the most debated subjects. Recent studies have suggested that megakaryocytes can be generated from multiple pathways and that some differentiation pathways do not require transit through a requisite multipotent or bipotent megakaryocyte-erythrocyte progenitor stage. Indeed, some investigators have argued that HSCs contain a subset of cells with biased megakaryocyte potential, with megakaryocytes directly arising from HSCs under steady-state and stress conditions. In this review, we discuss the evidence supporting these nonclassical megakaryocytic differentiation pathways and consider their relative strengths and weaknesses as well as the technical limitations and potential pitfalls in interpreting these studies. Ultimately, such pitfalls will need to be overcome to provide a comprehensive and definitive understanding of megakaryopoiesis.

Origins of the Vertebrate Erythro/Megakaryocytic System

Vertebrate erythrocytes and thrombocytes arise from the common bipotent thrombocytic-erythroid progenitors (TEPs). Even though nonmammalian erythrocytes and thrombocytes are phenotypically very similar to each other, mammalian species have developed some key evolutionary improvements in the process of erythroid and thrombocytic differentiation, such as erythroid enucleation, megakaryocyte endoreduplication, and platelet formation. This brings up a few questions that we try to address in this review. Specifically, we describe the ontology of erythro-thrombopoiesis during adult hematopoiesis with focus on the phylogenetic origin of mammalian erythrocytes and thrombocytes (also termed platelets). Although the evolutionary relationship between mammalian and nonmammalian erythroid cells is clear, the appearance of mammalian megakaryocytes is less so. Here, we discuss recent data indicating that nonmammalian thrombocytes and megakaryocytes are homologs. Finally, we hypothesize that erythroid and thrombocytic differentiation evolved from a single ancestral lineage, which would explain the striking similarities between these cells.