Characterization of mouse clonogenic megakaryocyte progenitors

From Hematopoietic Stem Cells to Platelets: Unifying Differentiation Pathways Identified by Lineage Tracing Mouse Models

Platelets are the terminal progeny of megakaryocytes, primarily produced in the bone marrow, and play critical roles in blood homeostasis, clotting, and wound healing. Traditionally, megakaryocytes and platelets are thought to arise from multipotent hematopoietic stem cells (HSCs) via multiple discrete progenitor populations with successive, lineage-restricting differentiation steps. However, this view has recently been challenged by studies suggesting that (1) some HSC clones are biased and/or restricted to the platelet lineage, (2) not all platelet generation follows the "canonical" megakaryocytic differentiation path of hematopoiesis, and (3) platelet output is the default program of steady-state hematopoiesis. Here, we specifically investigate the evidence that in vivo lineage tracing studies provide for the route(s) of platelet generation and investigate the involvement of various intermediate progenitor cell populations. We further identify the challenges that need to be overcome that are required to determine the presence, role, and kinetics of these possible alternate pathways.

Differentiation route determines the functional outputs of adult megakaryopoiesis

Emerging evidence has revealed a direct differentiation route from hematopoietic stem cells to megakaryocytes (direct route), in addition to the classical differentiation route through a series of restricted hematopoietic progenitors (stepwise route). This raises the question of the importance of two alternative routes for megakaryopoiesis. Here, we developed fate-mapping systems to distinguish the two routes, comparing their quantitative and functional outputs. We found that megakaryocytes were produced through the two routes with comparable kinetics and quantity under homeostasis. Single-cell RNA sequencing of the fate-mapped megakaryocytes revealed that the direct and stepwise routes contributed to the niche-supporting and immune megakaryocytes, respectively, but contributed to the platelet-producing megakaryocytes together. Megakaryocytes derived from the two routes displayed different activities and were differentially regulated by chemotherapy and inflammation. Our work links differentiation route to the heterogeneity of megakaryocytes. Alternative differentiation routes result in variable combinations of functionally distinct megakaryocyte subpopulations poised for different physiological demands.

Synergistic roles of tristetraprolin family members in myeloid cells in the control of inflammation

Members of the tristetraprolin (TTP) family of RNA-binding proteins can bind to and promote the decay of specific transcripts containing AU-rich motifs. ZFP36 (TTP) is best known for regulating pro-inflammatory cytokine expression in myeloid cells; however, its mammalian paralogues ZFP36L1 and ZFP36L2 have not been viewed as important in controlling inflammation. We knocked out these genes in myeloid cells in mice, singly and together. Single-gene myeloid-specific knockouts resulted in almost no spontaneous phenotypes. In contrast, mice with myeloid cell deficiency of all three genes developed severe inflammation, with a median survival of 8 wk. Macrophages from these mice expressed many more stabilized transcripts than cells from myeloid-specific TTP knockout mice; many of these encoded pro-inflammatory cytokines and chemokines. The failure of weight gain, arthritis, and early death could be prevented completely by two normal alleles of any of the three paralogues, and even one normal allele of Zfp36 or Zfp36l2 was enough to prevent the inflammatory phenotype. Our findings emphasize the importance of all three family members, acting in concert, in myeloid cell function.

The hematopoietic microenvironment: a network of niches for the development of all blood cell lineages

Blood cell formation (hematopoiesis) takes place mainly in the bone marrow, within the hematopoietic microenvironment, composed of a number of different cell types and their molecular products that together shape spatially organized and highly specialized microstructures called hematopoietic niches. From the earliest developmental stages and throughout the myeloid and lymphoid lineage differentiation pathways, hematopoietic niches play a crucial role in the preservation of cellular integrity and the regulation of proliferation and differentiation rates. Current evidence suggests that each blood cell lineage develops under specific, discrete niches that support committed progenitor and precursor cells and potentially cooperate with transcriptional programs determining the gradual lineage commitment and specification. This review aims to discuss recent advances on the cellular identity and structural organization of lymphoid, granulocytic, monocytic, megakaryocytic, and erythroid niches throughout the hematopoietic microenvironment and the mechanisms by which they interconnect and regulate viability, maintenance, maturation, and function of the developing blood cells.

The inhibitor of MyoD Family A (I-MFA) regulates megakaryocyte lineage commitment and terminal differentiation

Hematopoiesis and lineage commitment are regulated by several conserved cell-intrinsic signaling pathways, including MAPKs and β-catenin/TCF/LEF. The Inhibitor of MyoD Family A (I-MFA), a transcriptional repressor and tumor suppressor gene, interacts with these pathways and is dysregulated in chronic and acute myeloid leukemias, suggesting it may play a role in development and differentiation during hematopoiesis. To study this, immune cell populations in the bone marrow (BM) and periphery were analyzed in mice lacking Mdfi, encoding I-MFA (I-MFA-/-), and wild type (WT) controls. I-MFA-/- mice had reduced spleen and BM cellularity, with significant hyposplenism, compared to WT mice. In blood, total red blood cells and platelet counts were significantly reduced in I-MFA-/- mice, accompanied by a reduction in megakaryocyte (MK)/erythrocyte progenitor cells and an increase in myeloid progenitors in BM compared to WT mice. The K562 cell line exhibits PMA-induced MK differentiation, and shRNA knockdown of I-MFA resulted in reduced differentiation compared to control, with an increase and prolongation in phospho-JNK and phospho-ERK signaling. Overexpression of I-MFA promoted MK differentiation. These results suggest I-MFA plays a cell-intrinsic role in the response to differentiation signals, an effect that can be explored in the context of hematological cancers or other blood proliferative disorders.

Role of YAP as a Mechanosensing Molecule in Stem Cells and Stem Cell-Derived Hematopoietic Cells

Yes-associated protein (YAP) and WW domain-containing transcription regulator protein 1 (WWTR1, also known as TAZ) are transcriptional coactivators in the Hippo signaling pathway. Both are well-known regulators of cell proliferation and organ size control, and they have significant roles in promoting cell proliferation and differentiation. The roles of YAP and TAZ in stem cell pluripotency and differentiation have been extensively studied. However, the upstream mediators of YAP and TAZ are not well understood. Recently, a novel role of YAP in mechanosensing and mechanotransduction has been reported. The present review updates information on the regulation of YAP by mechanical cues such as extracellular matrix stiffness, fluid shear stress, and actin cytoskeleton tension in stem cell behaviors and differentiation. The review explores mesenchymal stem cell fate decisions, pluripotent stem cells (PSCs), self-renewal, pluripotency, and differentiation to blood products. Understanding how cells sense their microenvironment or niche and mimic those microenvironments in vitro could improve the efficiency of producing stem cell products and the efficacy of the products.

Fate mapping of hematopoietic stem cells reveals two pathways of native thrombopoiesis

Hematopoietic stem cells (HSCs) produce highly diverse cell lineages. Here, we chart native lineage pathways emanating from HSCs and define their physiological regulation by computationally integrating experimental approaches for fate mapping, mitotic tracking, and single-cell RNA sequencing. We find that lineages begin to split when cells leave the tip HSC population, marked by high Sca-1 and CD201 expression. Downstream, HSCs either retain high Sca-1 expression and the ability to generate lymphocytes, or irreversibly reduce Sca-1 level and enter into erythro-myelopoiesis or thrombopoiesis. Thrombopoiesis is the sum of two pathways that make comparable contributions in steady state, a long route via multipotent progenitors and CD48^hi megakaryocyte progenitors (MkPs), and a short route from HSCs to developmentally distinct CD48^−/lo MkPs. Enhanced thrombopoietin signaling differentially accelerates the short pathway, enabling a rapid response to increasing demand. In sum, we provide a blueprint for mapping physiological differentiation fluxes from HSCs and decipher two functionally distinct pathways of native thrombopoiesis.

Hematopoietic stem cells produce diverse cell lineages. Here, the authors apply single-cell RNA-seq, computational integration of non-perturbative approaches for fate-mapping, and mitotic tracking to chart lineage decisions in native hematopoiesis and identify megakaryocyte progenitors that directly link HSCs to megakaryocytes.

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.

Concurrent stem- and lineage-affiliated chromatin programs precede hematopoietic lineage restriction

The emerging notion of hematopoietic stem and progenitor cells (HSPCs) as a low-primed cloud without sharply demarcated gene expression programs raises the question on how cellular-fate options emerge and at which stem-like stage lineage priming is initiated. Here, we investigate single-cell chromatin accessibility of Lineage-, cKit+, and Sca1+ (LSK) HSPCs spanning the early differentiation landscape. Application of a signal-processing algorithm to detect transition points corresponding to massive alterations in accessibility of 571 transcription factor motifs reveals a population of LSK FMS-like tyrosine kinase 3 (Flt3)intCD9high cells that concurrently display stem-like and lineage-affiliated chromatin signatures, pointing to a simultaneous gain of both lympho-myeloid and megakaryocyte-erythroid programs. Molecularly and functionally, these cells position between stem cells and committed progenitors and display multi-lineage capacity in vitro and in vivo but lack self-renewal activity. This integrative molecular analysis resolves the heterogeneity of cells along hematopoietic differentiation and permits investigation of chromatin-mediated transition between multipotency and lineage restriction.

Oral Trypanosoma cruzi Acute Infection in Mice Targets Primary Lymphoid Organs and Triggers Extramedullary Hematopoiesis

During the acute phase of Chagas disease, Trypanosoma cruzi circulation through the bloodstream leads to high tissue parasitism in the host. In primary lymphoid organs, progenitor cell reduction paralleled transient immunosuppression. Herein we showed that acute oral infection in mice promotes diffuse parasitism in bone marrow cells at 14 and 21 days post-infection (dpi), with perivascular regions, intravascular regions, and regions near the bone being target sites of parasite replication. Phenotypic analysis of hematopoietic differentiation in the bone marrow of infected mice showed that the cell number in the tissue is decreased (lineage-negative and lineage-positive cells). Interestingly, analysis of hematopoietic branching points showed that hematopoietic stem and progenitor cells (HSPCs) were significantly increased at 14 dpi. In addition, the pool of progenitors with stem plasticity (HSC-MPP3), as well as multipotent progenitors (MPPs) such as MPP4, also showed this pattern of increase. In contrast, subsequent progenitors that arise from MPPs, such as common lymphoid progenitors (CLPs), lymphoid-primed MPPs (LMPPs), and myeloid progenitors, were not enhanced; conversely, all presented numeric decline. Annexin V staining revealed that cell death increase in the initial hematopoietic branching point probably is not linked to CLPs and that myeloid progenitors decreased at 14 and 21 dpi. In parallel, our investigation provided clues that myeloid progenitor decrease could be associated with an atypical expression of Sca-1 in this population leading to a remarkable increase on LSK-like cells at 14 dpi within the HSPC compartment. Finally, these results led us to investigate HSPC presence in the spleen as a phenomenon triggered during emergency hematopoiesis due to mobilization or expansion of these cells in extramedullary sites. Splenocyte analysis showed a progressive increase in HSPCs between 14 and 21 dpi. Altogether, our study shows that the bone marrow is a target tissue in T. cruzi orally infected mice, leading to a hematopoietic disturbance with LSK-like cell bias accounting on HSPCs possibly affecting myeloid progenitor numbers. The LMPP and CLP reduction converges with defective thymocyte development. Lastly, it is tempting to speculate that the extramedullary hematopoiesis seen in the spleen is a mechanism involved in the hematological maintenance reported during the acute phase of oral T. cruzi infection.

Hematopoiesis in numbers

Hematopoiesis is a dynamic process in which stem and progenitor cells give rise to the ~10¹³ blood and immune cells distributed throughout the human body. We argue that a quantitative description of hematopoiesis can help consolidate existing data, identify knowledge gaps, and generate new hypotheses. Here, we review known numbers in murine and, where possible, human hematopoiesis, and consolidate murine numbers into a set of reference values. We present estimates of cell numbers, division and differentiation rates, cell size, and macromolecular composition for each hematopoietic cell type. We also propose guidelines to improve the reporting of measurements and highlight areas in which quantitative data are lacking. Overall, we show how quantitative approaches can be used to understand key properties of hematopoiesis.

CFU-S assay: a historical single-cell assay that offers modern insight into clonal hematopoiesis

Hematopoietic stem cells (HSCs) have been studied extensively since their initial functional description in 1961 when Dr. James Till and Dr. Ernest McCulloch developed the first in vivo clonal strategy, termed the spleen colony-forming unit (CFU-S) assay, to assess the functional capacity of bone marrow-derived hematopoietic progenitors at the single-cell level. Through transplantation of bone marrow cells and analysis of the resulting cellular nodules in the spleen, the CFU-S assay revealed both the self-renewal and clonal differentiation capacity of hematopoietic progenitors. Further development and use of this assay have identified highly proliferative, self-renewing, and differentiating HSCs that possess clonal, multilineage differentiation. The CFU-S strategy has also been adapted to interrogating single purified hematopoietic stem and progenitor cell populations, advancing our knowledge of the hematopoietic hierarchy. In this review, we explore the major discoveries made with the CFU-S assay, consider its modern use and recent improvements, and compare it with commonly used long-term transplantation assays to determine the continued value of the CFU-S assay for understanding HSC biology and hematopoiesis.

Megakaryocyte progenitor cell function is enhanced upon aging despite the functional decline of aged hematopoietic stem cells

Age-related morbidity is associated with a decline in hematopoietic stem cell (HSC) function, but the mechanisms of HSC aging remain unclear. We performed heterochronic HSC transplants followed by quantitative analysis of cell reconstitution. Although young HSCs outperformed old HSCs in young recipients, young HSCs unexpectedly failed to outcompete the old HSCs of aged recipients. Interestingly, despite substantial enrichment of megakaryocyte progenitors (MkPs) in old mice in situ and reported platelet (Plt) priming with age, transplanted old HSCs were deficient in reconstitution of all lineages, including MkPs and Plts. We therefore performed functional analysis of young and old MkPs. Surprisingly, old MkPs displayed unmistakably greater regenerative capacity compared with young MkPs. Transcriptome analysis revealed putative molecular regulators of old MkP expansion. Collectively, these data demonstrated that aging affects HSCs and megakaryopoiesis in fundamentally different ways: whereas old HSCs functionally decline, MkPs gain expansion capacity upon aging.

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Reconstitution deficit by old HSCs was observed by chimerism and absolute cell numbers

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Young HSCs did not outcompete resident HSCs in aged recipient mice

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Old MkPs display remarkable capacity to engraft, expand, and reconstitute platelets

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Aging is associated with changes in MkP genome-wide expression signatures

High-throughput enrichment and isolation of megakaryocyte progenitor cells from the mouse bone marrow

Megakaryocytes are a rare population of cells that develop in the bone marrow and function to produce platelets that circulate throughout the body and form clots to stop or prevent bleeding. A major challenge in studying megakaryocyte development, and the diseases that arise from their dysfunction, is the identification, classification, and enrichment of megakaryocyte progenitor cells that are produced during hematopoiesis. Here, we present a high throughput strategy for identifying and isolating megakaryocytes and their progenitor cells from a heterogeneous population of bone marrow samples. Specifically, we couple thrombopoietin (TPO) induction, image flow cytometry, and principal component analysis (PCA) to identify and enrich for megakaryocyte progenitor cells that are capable of self-renewal and directly differentiating into mature megakaryocytes. This enrichment strategy distinguishes megakaryocyte progenitors from other lineage-committed cells in a high throughput manner. Furthermore, by using image flow cytometry with PCA, we have identified a combination of markers and characteristics that can be used to isolate megakaryocyte progenitor cells using standard flow cytometry methods. Altogether, these techniques enable the high throughput enrichment and isolation of cells in the megakaryocyte lineage and have the potential to enable rapid disease identification and diagnoses ahead of severe disease progression.

Non-viral ex vivo genome-editing in mouse bona fide hematopoietic stem cells with CRISPR/Cas9

We conducted two lines of genome-editing experiments of mouse hematopoietic stem cells (HSCs) with the clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated protein 9 (Cas9). First, to evaluate the genome-editing efficiency in mouse bona fide HSCs, we knocked out integrin alpha 2b (Itga2b) with Cas9 ribonucleoprotein (Cas9/RNP) and performed serial transplantation in mice. The knockout efficiency was estimated at approximately 15%. Second, giving an example of X-linked severe combined immunodeficiency (X-SCID) as a target genetic disease, we showed a proof-of-concept of universal gene correction, allowing rescue of most of X-SCID mutations, in a completely non-viral setting. We inserted partial cDNA of interleukin-2 receptor gamma chain (Il2rg) into intron 1 of Il2rg via non-homologous end-joining (NHEJ) with Cas9/RNP and a homology-independent targeted integration (HITI)-based construct. Repaired HSCs reconstituted T lymphocytes and thymuses in SCID mice. Our results show that a non-viral genome-editing of HSCs with CRISPR/Cas9 will help cure genetic diseases.

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.

Platelet-derived alpha-granules are associated with inflammation in patients with NK/T-cell lymphoma-associated hemophagocytic syndrome

Due to the variable overlap of multiple symptoms, accurate early diagnosis of NK/T-cell lymphoma-associated hemophagocytic syndrome (NK/T-LAHS) is difficult, making the prognosis extremely poor. Hemophagocytic syndrome (HPS) is now diagnosed primarily based on the hemophagocytic lymphohistiocytosis (HLH)-2004 diagnostic criteria, and platelet count is one of the baseline evaluations. However, in our study, the data showed that decreased platelets were not only a clinical feature of HPS but also the key cells that regulate inflammation by releasing α-granules containing upregulated platelet factor 4 (PF4) and downregulated platelet-derived growth factors (PDGFs). Furthermore, we found that angiopoietin-4 (ANG-4), which has significant differential expression, has been less reported, that may affect hematopoiesis and proinflammatory responses and can be used as diagnostic biomarkers together with PF4 and PDGFs.

The GABA receptor GABRR1 is expressed on and functional in hematopoietic stem cells and megakaryocyte progenitors

GABRR1 is a rho subunit receptor of GABA, the major inhibitory neurotransmitter in the mammalian brain. While most investigations of its function focused on the nervous system, its regulatory role in hematopoiesis has not been reported. In this study, we found GABRR1 is mainly expressed on subsets of human and mouse hematopoietic stem cells (HSCs) and megakaryocyte progenitors (MkPs). GABRR1-negative (GR-) HSCs led to higher donor-derived hematopoietic chimerism than GABRR1-positive (GR+) HSCs. GR+ but not GR- HSCs and MkPs respond to GABA in patch clamp studies. Inhibition of GABRR1 via genetic knockout or antagonists inhibited MkP differentiation and reduced platelet numbers in blood. Overexpression of GABRR1 or treatment with agonists significantly promoted MkP generation and megakaryocyte colonies. Thus, this study identifies a link between the neural and hematopoietic systems and opens up the possibility of manipulating GABA signaling for platelet-required clinical applications.

New paradigms on hematopoietic stem cell differentiation

Ever since hematopoietic stem cells (HSCs) were first identified half a century ago, their differentiation roadmap has been extensively studied. The classical model of hematopoiesis has long held as a dogma that HSCs reside at the top of a hierarchy in which HSCs possess self-renewal capacity and can progressively give rise to all blood lineage cells. However, over the past several years, with advances in single cell technologies, this developmental scheme has been challenged. In this review, we discuss the evidence supporting heterogeneity within HSC and progenitor populations as well as the hierarchical models revised by novel approaches mainly in mouse system. These evolving views provide further understanding of hematopoiesis and highlight the complexity of hematopoietic differentiation.

BCG Educates Hematopoietic Stem Cells to Generate Protective Innate Immunity against Tuberculosis

The dogma that adaptive immunity is the only arm of the immune response with memory capacity has been recently challenged by several studies demonstrating evidence for memory-like innate immune training. However, the underlying mechanisms and location for generating such innate memory responses in vivo remain unknown. Here, we show that access of Bacillus Calmette-Guérin (BCG) to the bone marrow (BM) changes the transcriptional landscape of hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs), leading to local cell expansion and enhanced myelopoiesis at the expense of lymphopoiesis. Importantly, BCG-educated HSCs generate epigenetically modified macrophages that provide significantly better protection against virulent M. tuberculosis infection than naïve macrophages. By using parabiotic and chimeric mice, as well as adoptive transfer approaches, we demonstrate that training of the monocyte/macrophage lineage via BCG-induced HSC reprogramming is sustainable in vivo. Our results indicate that targeting the HSC compartment provides a novel approach for vaccine development.

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.

Screening for genes that regulate the differentiation of human megakaryocytic lineage cells

Different combinations of transcription factors (TFs) function at each stage of hematopoiesis, leading to distinct expression patterns of lineage-specific genes. The identification of such regulators and their functions in hematopoiesis remain largely unresolved. In this study, we utilized screening approaches to study the transcriptional regulators of megakaryocyte progenitor (MkP) generation, a key step before platelet production. Promising candidate genes were generated from a microarray platform gene expression commons and individually manipulated in human hematopoietic stem and progenitor cells (HSPCs). Deletion of some of the candidate genes (the hit genes) by CRISPR/Cas9 led to decreased MkP generation during HSPC differentiation, while more MkPs were produced when some hit genes were overexpressed in HSPCs. We then demonstrated that overexpression of these genes can increase the frequency of mature megakaryocytic colonies by functional colony forming unit-megakaryocyte (CFU-Mk) assay and the release of platelets after in vitro maturation. Finally, we showed that the histone deacetylase inhibitors could also increase MkP differentiation, possibly by regulating some of the newly identified TFs. Therefore, identification of such regulators will advance the understanding of basic mechanisms of HSPC differentiation and conceivably enable the generation and maturation of megakaryocytes and platelets in vitro.

Nfix Promotes Survival of Immature Hematopoietic Cells via Regulation of c-Mpl

Hematopoietic stem and progenitor cells (HSPCs) are necessary for life-long blood production and replenishment of the hematopoietic system during stress. We recently reported that nuclear factor I/X (Nfix) promotes HSPC survival post-transplant. Here, we report that ectopic expression of Nfix in primary mouse HSPCs extends their ex vivo culture from about 20 to 40 days. HSPCs overexpressing Nfix display hypersensitivity to supportive cytokines and reduced apoptosis when subjected to cytokine deprivation relative to controls. Ectopic Nfix resulted in elevated levels of c-Mpl transcripts and cell surface protein on primary murine HSPCs as well as increased phosphorylation of STAT5, which is known to be activated down-stream of c-MPL. Blocking c-MPL signaling by removal of thrombopoietin or addition of a c-MPL neutralizing antibody negated the antiapoptotic effect of Nfix overexpression on cultured HSPCs. Furthermore, NFIX was capable of binding to and transcriptionally activating a proximal c-Mpl promoter fragment. In sum, these data suggest that NFIX-mediated upregulation of c-Mpl transcription can protect primitive hematopoietic cells from stress ex vivo. Stem Cells 2018;36:943-950.

A novel prospective isolation of murine fetal liver progenitors to study in utero hematopoietic defects

In recent years, highly detailed characterization of adult bone marrow (BM) myeloid progenitors has been achieved and, as a result, the impact of somatic defects on different hematopoietic lineage fate decisions can be precisely determined. Fetal liver (FL) hematopoietic progenitor cells (HPCs) are poorly characterized in comparison, potentially hindering the study of the impact of genetic alterations on midgestation hematopoiesis. Numerous disorders, for example infant acute leukemias, have in utero origins and their study would therefore benefit from the ability to isolate highly purified progenitor subsets. We previously demonstrated that a Runx1 distal promoter (P1)-GFP::proximal promoter (P2)-hCD4 dual-reporter mouse (Mus musculus) model can be used to identify adult BM progenitor subsets with distinct lineage preferences. In this study, we undertook the characterization of the expression of Runx1-P1-GFP and P2-hCD4 in FL. Expression of P2-hCD4 in the FL immunophenotypic Megakaryocyte-Erythroid Progenitor (MEP) and Common Myeloid Progenitor (CMP) compartments corresponded to increased granulocytic/monocytic/megakaryocytic and decreased erythroid specification. Moreover, Runx1-P2-hCD4 expression correlated with several endogenous cell surface markers' expression, including CD31 and CD45, providing a new strategy for prospective identification of highly purified fetal myeloid progenitors in transgenic mouse models. We utilized this methodology to compare the impact of the deletion of either total RUNX1 or RUNX1C alone and to determine the fetal HPCs lineages most substantially affected. This new prospective identification of FL progenitors therefore raises the prospect of identifying the underlying gene networks responsible with greater precision than previously possible.

Lack of functional selectin-ligand interactions enhances innate immune resistance to systemic Listeria monocytogenes infection

Selectin-ligand interactions are important for leukocyte homing and functionality. The roles of selectin-ligand interactions in modulating immunity to intracellular infections are not completely understood. Mice lacking the expression of fucosyltransferase-IV and -VII (Fucosyltransferase-IV and -VII double knockout, FtDKO) exhibit deficient functionality of selectin-ligand interactions. We addressed the kinetics of infection and immunity to Listeria monocytogenes (LM), an intracellular pathogen, in FtDKO mice. These mice exhibited enhanced ability to clear infection and increased survival to a lethal dose of LM infection relative to wild-type (WT) C57BL/6J controls. This was associated with increased levels of neutrophils, monocytes, and dendritic cells (DCs) in the blood and/or infected organs. Adoptive transfer of bone marrow (BM) cells from FtDKO mice to WT mice resulted in enhanced neutrophil numbers and improved clearance of LM bacteria in recipients. In vivo depletion of myeloid innate immune cells, particularly neutrophils, monocytes, macrophages, and DCs, using anti-Ly-6G (RB6-8C5) monoclonal antibody, reduced the ability of FtDKO mice to curtail LM infection. Nevertheless, depletion using anti-Ly-6G (1A8) known to exclusively deplete neutrophils did not abrogate increased resistance of FtDKO mice to LM infection, suggesting a role for other myeloid innate immune cells in this model. Examination of BM hematopoietic progenitors through flow cytometry and cell culture colony-forming unit assay showed increased frequencies of granulocyte-macrophage progenitors in FtDKO relative to WT mice, Overall, our results indicate that functional selectin ligand deficiency enhances innate immune-mediated resistance to systemic LM infection despite defective leukocyte migration and lymphocyte homing.

Identification of a Multipotent Progenitor Population in the Spleen That Is Regulated by NR4A1

The developmental fate of hematopoietic stem and progenitor cells is influenced by their physiological context. Although most hematopoietic stem and progenitor cells are found in the bone marrow of the adult, some are found in other tissues, including the spleen. The extent to which the fate of stem cells is determined by the tissue in which they reside is not clear. In this study, we identify a new progenitor population, which is enriched in the mouse spleen, defined by cKit⁺CD71^lowCD24^high expression. This previously uncharacterized population generates exclusively myeloid lineage cells, including erythrocytes, platelets, monocytes, and neutrophils. These multipotent progenitors of the spleen (MPPS) develop from MPP2, a myeloid-biased subset of hematopoietic progenitors. We find that NR4A1, a transcription factor expressed by myeloid-biased long term-hematopoietic stem cells, guides the lineage specification of MPPS. In vitro, NR4A1 expression regulates the potential of MPPS to differentiate into erythroid cells. MPPS that express NR4A1 differentiate into a variety of myeloid lineages, whereas those that do not express NR4A1 primarily develop into erythroid cells. Similarly, in vivo, after adoptive transfer, Nr4a1-deficient MPPS contribute more to erythrocyte and platelet populations than do wild-type MPPS. Finally, unmanipulated Nr4a1^-/- mice harbor significantly higher numbers of erythroid progenitors in the spleen compared with wild-type mice. Together, our data show that NR4A1 expression by MPPS limits erythropoiesis and megakaryopoeisis, permitting development to other myeloid lineages. This effect is specific to the spleen, revealing a unique molecular pathway that regulates myeloid bias in an extramedullary niche.

A unique microenvironment in the developing liver supports the expansion of megakaryocyte progenitors

The fetal liver is the site of a major expansion of the hematopoietic stem cell (HSC) pool and is also a privileged organ to study megakaryocyte progenitor differentiation. We identified in the mouse fetal liver at day 13.5 a discrete stromal cell population harboring a CD45^-TER119^-CD31^-CD51⁺VCAM-1⁺PDGFRα^- (V⁺P^-) phenotype that lacked colony-forming unit fibroblast activity and harbored an hepatocyte progenitor signature. This previously undescribed V⁺P^- population efficiently supported megakaryocyte production from mouse bone marrow HSC and human peripheral blood HSC-myeloid progenitors cultured in the presence of limited cytokine concentrations. Megakaryocytes obtained in V⁺P^- cocultures were polyploid, positive for CD41/CD42c, and efficiently produced proplatelets. Megakaryocyte production appeared to be mediated by an expansion of the progenitor compartment through HSC-stromal cell contact. In conclusion, the fetal liver contains a unique cellular microenvironment that could represent a platform for the discovery of regulators of megakaryopoiesis.

Myeloid Cell Origins, Differentiation, and Clinical Implications

The hematopoietic stem cell (HSC) is a multipotent stem cell that resides in the bone marrow and has the ability to form all of the cells of the blood and immune system. Since its first purification in 1988, additional studies have refined the phenotype and functionality of HSCs and characterized all of their downstream progeny. The hematopoietic lineage is divided into two main branches: the myeloid and lymphoid arms. The myeloid arm is characterized by the common myeloid progenitor and all of its resulting cell types. The stages of hematopoiesis have been defined in both mice and humans. During embryological development, the earliest hematopoiesis takes place in yolk sac blood islands and then migrates to the fetal liver and hematopoietic organs. Some adult myeloid populations develop directly from yolk sac progenitors without apparent bone marrow intermediates, such as tissue-resident macrophages. Hematopoiesis also changes over time, with a bias of the dominating HSCs toward myeloid development as animals age. Defects in myelopoiesis contribute to many hematologic disorders, and some of these can be overcome with therapies that target the aberrant stage of development. Furthermore, insights into myeloid development have informed us of mechanisms of programmed cell removal. The CD47/SIRPα axis, a myeloid-specific immune checkpoint, limits macrophage removal of HSCs but can be exploited by hematologic and solid malignancies. Therapeutics targeting CD47 represent a new strategy for treating cancer. Overall, an understanding of hematopoiesis and myeloid cell development has implications for regenerative medicine, hematopoietic cell transplantation, malignancy, and many other diseases.

Mouse RUNX1C regulates premegakaryocytic/erythroid output and maintains survival of megakaryocyte progenitors

RUNX1 is crucial for the regulation of megakaryocyte specification, maturation, and thrombopoiesis. Runx1 possesses 2 promoters: the distal P1 and proximal P2 promoters. The major protein isoforms generated by P1 and P2 are RUNX1C and RUNX1B, respectively, which differ solely in their N-terminal amino acid sequences. RUNX1C is the most abundantly expressed isoform in adult hematopoiesis, present in all RUNX1-expressing populations, including the cKit+ hematopoietic stem and progenitor cells. RUNX1B expression is more restricted, being highly expressed in the megakaryocyte lineage but downregulated during erythropoiesis. We generated a Runx1 P1 knock-in of RUNX1B, termed P1-MRIPV This mouse line lacks RUNX1C expression but has normal total RUNX1 levels, solely comprising RUNX1B. Using this mouse line, we establish a specific requirement for the P1-RUNX1C isoform in megakaryopoiesis, which cannot be entirely compensated for by RUNX1B overexpression. P1 knock-in megakaryocyte progenitors have reduced proliferative capacity and undergo increased cell death, resulting in thrombocytopenia. P1 knock-in premegakaryocyte/erythroid progenitors demonstrate an erythroid-specification bias, evident from increased erythroid colony-forming ability and decreased megakaryocyte output. At a transcriptional level, multiple erythroid-specific genes are upregulated and megakaryocyte-specific transcripts are downregulated. In addition, proapoptotic pathways are activated in P1 knock-in premegakaryocyte/erythroid progenitors, presumably accounting for the increased cell death in the megakaryocyte progenitor compartment. Unlike in the conditional adult Runx1 null models, megakaryocytic maturation is not affected in the P1 knock-in mice, suggesting that RUNX1B can regulate endomitosis and thrombopoiesis. Therefore, despite the high degree of structural similarity, RUNX1B and RUNX1C isoforms have distinct and specific roles in adult megakaryopoiesis.

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.

Zebrafish Caudal Haematopoietic Embryonic Stromal Tissue (CHEST) Cells Support Haematopoiesis

Haematopoiesis is an essential process in early vertebrate development that occurs in different distinct spatial locations in the embryo that shift over time. These different sites have distinct functions: in some anatomical locations specific hematopoietic stem and progenitor cells (HSPCs) are generated de novo. In others, HSPCs expand. HSPCs differentiate and renew in other locations, ensuring homeostatic maintenance. These niches primarily control haematopoiesis through a combination of cell-to-cell signalling and cytokine secretion that elicit unique biological effects in progenitors. To understand the molecular signals generated by these niches, we report the generation of caudal hematopoietic embryonic stromal tissue (CHEST) cells from 72-hours post fertilization (hpf) caudal hematopoietic tissue (CHT), the site of embryonic HSPC expansion in fish. CHEST cells are a primary cell line with perivascular endothelial properties that expand hematopoietic cells in vitro. Morphological and transcript analysis of these cultures indicates lymphoid, myeloid, and erythroid differentiation, indicating that CHEST cells are a useful tool for identifying molecular signals critical for HSPC proliferation and differentiation in the zebrafish. These findings permit comparison with other temporally and spatially distinct haematopoietic-supportive zebrafish niches, as well as with mammalian haematopoietic-supportive cells to further the understanding of the evolution of the vertebrate hematopoietic system.

Alpha-Synuclein Expression in Acute Erythroleukaemia, Acute Megakaryoblastic Leukemia, and Normal Counterparts in Bone Marrow

BACKGROUND

Alpha-synuclein is a member of synuclein family of proteins with unidentified function localized in the cytoplasm, mitochondria of neurons, and presynaptic nerve endings. Although it is found in the Lewy bodies in synucleinopathies and in Alzheimer's disease, the protein could also be considered as a novel marker in diagnosis of diseases related to the hematopoietic system.

METHODS

The current study evaluated alpha-synuclein expression in bone marrow sections obtained from 9 patients with acute myeloblastic leukemia (AML)-M6, 2 patients with AML-M7, and 56 patients with other forms of AML by immunohistochemical (IHC) analysis.

RESULTS

Seven out of 9 cases with erythroleukemia (66.7%) and 1 of the 2 cases with M7 (50%) were positive. In contrast; the blasts in 2 out of 56 AML cases with non-M6/M7 (3.6%) showed positive staining. Accordingly, alpha-synuclein was positive in normal erythroid precursors and megakaryocytes (if existing) in these cases; while, it was negative in lymphoid and myeloid precursors.

CONCLUSION

Alpha-synuclein expression in non-neoplastic and neoplastic erythroid cells and megakaryocytes could be used as a complementary and useful marker for distinction between AML-M6/M7 and other types of AML.

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.

The Branching Point in Erythro-Myeloid Differentiation

Development of mature blood cell progenies from hematopoietic stem cells involves the transition through lineage-restricted progenitors. The first branching point along this developmental process is thought to separate the erythro-myeloid and lymphoid lineage fate by yielding two intermediate progenitors, the common myeloid and the common lymphoid progenitors (CMPs and CLPs). Here, we use single-cell lineage tracing to demonstrate that so-called CMPs are highly heterogeneous with respect to cellular output, with most individual CMPs yielding either only erythrocytes or only myeloid cells after transplantation. Furthermore, based on the labeling of earlier progenitors, we show that the divergence between the myeloid and erythroid lineage develops within multipotent progenitors (MPP). These data provide evidence for a model of hematopoietic branching in which multiple distinct lineage commitments occur in parallel within the MPP pool.

Cellular dissection of zebrafish hematopoiesis

Zebrafish as a model system have been instrumental in understanding early vertebrate development, especially of the hematopoietic system. The external development of zebrafish and their genetic amenability have allowed in-depth studies of multiple blood cell types and their respective genetic regulation. This chapter highlights some new data in zebrafish hematopoiesis regarding primitive and definitive hematopoiesis in the embryonic and adult fish, allowing the isolation of prospective progenitor subsets. It also highlights assays developed to examine the function of these progenitors in vivo and in vitro, allowing an evolutionary understanding of the hematopoietic system and how zebrafish can be better utilized as a model system for a multitude of hematopoietic disorders.

Aberrant Notch Signaling in the Bone Marrow Microenvironment of Acute Lymphoid Leukemia Suppresses Osteoblast-Mediated Support of Hematopoietic Niche Function

More than half of T-cell acute lymphoblastic leukemia (T-ALL) patients harbor gain-of-function mutations in the intracellular domain of Notch1. Diffuse infiltration of the bone marrow commonly occurs in T-ALL and relapsed B-cell acute lymphoblastic leukemia patients, and is associated with worse prognosis. However, the mechanism of leukemia outgrowth in the marrow and the resulting biologic impact on hematopoiesis are poorly understood. Here, we investigated targetable cellular and molecular abnormalities in leukemia marrow stroma responsible for the suppression of normal hematopoiesis using a T-ALL mouse model and human T-ALL xenografts. We found that actively proliferating leukemia cells inhibited normal hematopoietic stem and progenitor cell (HSPC) proliferation and homing to the perivascular region. In addition, leukemia development was accompanied by the suppression of the endosteum-lining osteoblast population. We further demonstrated that aberrant Notch activation in the stroma plays an important role in negatively regulating the expression of CXLC12 on osteoblasts and their differentiation. Notch blockade reversed attenuated HSPC cycling, leukemia-associated abnormal blood lineage distribution, and thrombocytopenia as well as recovered osteoblast and HSPC abundance and improved the hematopoietic-supportive functions of osteoblasts. Finally, we confirmed that reduced osteoblast frequency and enhanced Notch signaling were also features of the marrow stroma of human ALL tissues. Collectively, our findings suggest that therapeutically targeting the leukemia-infiltrated hematopoietic niche may restore HSPC homeostasis and improve the outcome of ALL patients.

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.

RUNX1B Expression Is Highly Heterogeneous and Distinguishes Megakaryocytic and Erythroid Lineage Fate in Adult Mouse Hematopoiesis

The Core Binding Factor (CBF) protein RUNX1 is a master regulator of definitive hematopoiesis, crucial for hematopoietic stem cell (HSC) emergence during ontogeny. RUNX1 also plays vital roles in adult mice, in regulating the correct specification of numerous blood lineages. Akin to the other mammalian Runx genes, Runx1 has two promoters P1 (distal) and P2 (proximal) which generate distinct protein isoforms. The activities and specific relevance of these two promoters in adult hematopoiesis remain to be fully elucidated. Utilizing a dual reporter mouse model we demonstrate that the distal P1 promoter is broadly active in adult hematopoietic stem and progenitor cell (HSPC) populations. By contrast the activity of the proximal P2 promoter is more restricted and its upregulation, in both the immature Lineage- Sca1high cKithigh (LSK) and bipotential Pre-Megakaryocytic/Erythroid Progenitor (PreMegE) populations, coincides with a loss of erythroid (Ery) specification. Accordingly the PreMegE population can be prospectively separated into "pro-erythroid" and "pro-megakaryocyte" populations based on Runx1 P2 activity. Comparative gene expression analyses between Runx1 P2+ and P2- populations indicated that levels of CD34 expression could substitute for P2 activity to distinguish these two cell populations in wild type (WT) bone marrow (BM). Prospective isolation of these two populations will enable the further investigation of molecular mechanisms involved in megakaryocytic/erythroid (Mk/Ery) cell fate decisions. Having characterized the extensive activity of P1, we utilized a P1-GFP homozygous mouse model to analyze the impact of the complete absence of Runx1 P1 expression in adult mice and observed strong defects in the T cell lineage. Finally, we investigated how the leukemic fusion protein AML1-ETO9a might influence Runx1 promoter usage. Short-term AML1-ETO9a induction in BM resulted in preferential P2 upregulation, suggesting its expression may be important to establish a pre-leukemic environment.

Zebrafish embryonic stromal trunk (ZEST) cells support hematopoietic stem and progenitor cell (HSPC) proliferation, survival, and differentiation

Forward genetic screens in zebrafish have been used to identify genes essential for the generation of primitive blood and the emergence of hematopoietic stem cells (HSCs), but have not elucidated the genes essential for hematopoietic stem and progenitor cell (HSPC) proliferation and differentiation because of the lack of methodologies to functionally assess these processes. We previously described techniques used to test the developmental potential of HSPCs by culturing them on zebrafish kidney stromal (ZKS) cells, derived from the main site of hematopoiesis in the adult teleost. Here we describe an additional primary stromal cell line we refer to as zebrafish embryonic stromal trunk (ZEST) cells, derived from tissue surrounding the embryonic dorsal aorta, the site of HSC emergence in developing fish. ZEST cells encouraged HSPC differentiation toward the myeloid, lymphoid, and erythroid pathways when assessed by morphologic and quantitative reverse transcription polymerase chain reaction analyses. Additionally, ZEST cells significantly expanded the number of cultured HSPCs in vitro, indicating that these stromal cells are supportive of both HSPC proliferation and multilineage differentiation. Examination of ZEST cells indicates that they express numerous cytokines and Notch ligands and possess endothelial characteristics. Further characterization of ZEST cells should prove to be invaluable in understanding the complex signaling cascades instigated by the embryonic hematopoietic niche required to expand and differentiate HSPCs. Elucidating these processes and identifying possibilities for the modulation of these molecular pathways should allow the in vitro expansion of HSPCs for a multitude of therapeutic uses.

Prospective isolation of human erythroid lineage-committed progenitors

Determining the developmental pathway leading to erythrocytes and being able to isolate their progenitors are crucial to understanding and treating disorders of red cell imbalance such as anemia, myelodysplastic syndrome, and polycythemia vera. Here we show that the human erythrocyte progenitor (hEP) can be prospectively isolated from adult bone marrow. We found three subfractions that possessed different expression patterns of CD105 and CD71 within the previously defined human megakaryocyte/erythrocyte progenitor (hMEP; Lineage(-) CD34(+) CD38(+) IL-3Rα(-) CD45RA(-)) population. Both CD71(-) CD105(-) and CD71(+) CD105(-) MEPs, at least in vitro, still retained bipotency for the megakaryocyte (MegK) and erythrocyte (E) lineages, although the latter subpopulation is skewed in differentiation toward the erythroid lineage. Notably, the proliferative and differentiation output of the CD71(intermediate(int)/+) CD105(+) subset of cells within the MEP population was completely restricted to the erythroid lineage with the loss of MegK potential. CD71(+) CD105(-) MEPs are erythrocyte-biased MEPs (E-MEPs) and CD71(int/+) CD105(+) cells are EPs. These previously unclassified populations may facilitate further understanding of the molecular mechanisms governing human erythroid development and serve as potential therapeutic targets in disorders of the erythroid lineage.

Angptl4 is upregulated under inflammatory conditions in the bone marrow of mice, expands myeloid progenitors, and accelerates reconstitution of platelets after myelosuppressive therapy

BACKGROUND

Upon inflammation, myeloid cell generation in the bone marrow (BM) is broadly enhanced by the action of induced cytokines which are produced locally and at multiple sites throughout the body.

METHODS

Using microarray studies, we found that Angptl4 is upregulated in the BM during systemic inflammation.

RESULTS

Recombinant murine Angptl4 (rmAngptl4) stimulated the proliferation of myeloid colony-forming units (CFUs) in vitro. Upon repeated in vivo injections, rmAngptl4 increased BM progenitor cell frequency and this was paralleled by a relative increase in phenotypically defined granulocyte-macrophage progenitors (GMPs). Furthermore, in vivo treatment with rmAngptl4 resulted in elevated platelet counts in steady-state mice while allowing a significant acceleration of reconstitution of platelets after myelosuppressive therapy. The administration of rmAngptl4 increased the number of CD61(+)CD41(low)-expressing megakaryocytes (MK) in the BM of steady-state and in the spleen of transplanted mice. Furthermore, rmAngptl4 improved the in vitro differentiation of immature MKs from hematopoietic stem and progenitor cells. Mechanistically, using a signal transducer and activator of transcription 3 (STAT3) reporter knockin model, we show that rmAngptl4 induces de novo STAT3 expression in immature MK which could be important for the effective expansion of MKs after myelosuppressive therapy.

CONCLUSION

Whereas the definitive role of Angptl4 in mediating the effects of lipopolysaccharide (LPS) on the BM has to be demonstrated by further studies involving multiple cytokine knockouts, our data suggest that Angptl4 plays a critical role during hematopoietic, especially megakaryopoietic, reconstitution following stem cell transplantation.

Unipotent Megakaryopoietic Pathway Bridging Hematopoietic Stem Cells and Mature Megakaryocytes

Recent identification of platelet/megakaryocyte-biased hematopoietic stem/repopulating cells requires revision of the intermediate pathway for megakaryopoiesis. Here, we show a unipotent megakaryopoietic pathway bypassing the bipotent megakaryocyte/erythroid progenitors (biEMPs). Cells purified from mouse bone marrow by CD42b (GPIbα) marking were demonstrated to be unipotent megakaryocytic progenitors (MKPs) by culture and transplantation. A subpopulation of freshly isolated CD41(+) cells in the lineage Sca1(+) cKit(+) (LSK) fraction (subCD41(+) LSK) differentiated only into MKP and mature megakaryocytes in culture. Although CD41(+) LSK cells as a whole were capable of differentiating into all myeloid and lymphoid cells in vivo, they produced unipotent MKP, mature megakaryocytes, and platelets in vitro and in vivo much more efficiently than Flt3(+) CD41(-) LSK cells, especially at the early phase after transplantation. In single cell polymerase chain reaction and thrombopoietin (TPO) signaling analyses, the MKP and a fraction of CD41(+) LSK, but not the biEMP, showed the similarities in mRNA expression profile and visible TPO-mediated phosphorylation. On increased demand of platelet production after 5-FU treatment, a part of CD41(+) LSK population expressed CD42b on the surface, and 90% of them showed unipotent megakaryopoietic capacity in single cell culture and predominantly produced platelets in vivo at the early phase after transplantation. These results suggest that the CD41(+) CD42b(+) LSK are straightforward progenies of megakaryocytes/platelet-biased stem/repopulating cells, but not progenies of biEMP. Consequently, we show a unipotent/highly biased megakaryopoietic pathway interconnecting stem/repopulating cells and mature megakaryocytes, the one that may play physiologic roles especially in emergency megakaryopoiesis.

Aurora kinase A is required for hematopoiesis but is dispensable for murine megakaryocyte endomitosis and differentiation

Aurora kinase A (AURKA) is a therapeutic target in acute megakaryocytic leukemia. However, its requirement in normal hematopoiesis and megakaryocyte development has not been extensively characterized. Based on its role as a cell cycle regulator, we predicted that an Aurka deficiency would lead to severe abnormalities in all hematopoietic lineages. Here we reveal that loss of Aurka in hematopoietic cells causes profound cell autonomous defects in the peripheral blood and bone marrow. Surprisingly, in contrast to the survival defects of nearly all hematopoietic lineages, deletion of Aurka was associated with increased differentiation and polyploidization of megakaryocytes both in vivo and in vitro. Furthermore, in contrast to other cell types examined, megakaryocytes continued DNA synthesis after loss of Aurka. Thus, like other cell cycle regulators such as Aurkb and survivin, Aurka is required for hematopoiesis, but is dispensable for megakaryocyte endomitosis. Our work supports a growing body of evidence that the megakaryocyte endomitotic cell cycle differs significantly from the proliferative cell cycle.

Cytosolic carboxypeptidase CCP6 is required for megakaryopoiesis by modulating Mad2 polyglutamylation

Ye et al. identify cytosolic carboxypeptidase CCP6 as a protein required for the regulation of bone marrow megakaryopoiesis in mice. The authors find that Mad2 (a core component of spindle checkpoint in mitosis) is a substrate of CCP6 in megakaryocytes and is polyglutamylated by proteins TTLL6 and TTLL4, subsequently affecting the activity of Aurora B kinase. Mad2 is thus additionally implicated in megakaryopoiesis regulation.

Bone marrow progenitor cells develop into mature megakaryocytes (MKs) to produce platelets for hemostasis and other physiological functions. However, the molecular mechanisms underlying megakaryopoiesis are not completely defined. We show that cytosolic carboxypeptidase (CCP) 6 deficiency in mice causes enlarged spleens and increased platelet counts with underdeveloped MKs and dysfunctional platelets. The prominent phenotypes of CCP6 deficiency are different from those of CCP1-deficient mice. We found that CCP6 and tubulin tyrosine ligase-like family (TTLL) members TTLL4 and TTLL6 are highly expressed in MKs. We identify Mad2 (mitotic arrest deficient 2) as a novel substrate for CCP6 and not CCP1. Mad2 can be polyglutamylated by TTLL4 and TTLL6 to modulate the maturation of MKs. CCP6 deficiency causes hyperglutamylation of Mad2 to promote activation of Aurora B, leading to suppression of MK maturation. We reveal that Mad2 polyglutamylation plays a critical role in the regulation of megakaryopoiesis.

Extrinsic and intrinsic control by EKLF (KLF1) within a specialized erythroid niche

The erythroblastic island provides an important nutritional and survival support niche for efficient erythropoietic differentiation. Island integrity is reliant on adhesive interactions between erythroid and macrophage cells. We show that erythroblastic islands can be formed from single progenitor cells present in differentiating embryoid bodies, and that these correspond to erythro-myeloid progenitors (EMPs) that first appear in the yolk sac of the early developing embryo. Erythroid Krüppel-like factor (EKLF; KLF1), a crucial zinc finger transcription factor, is expressed in the EMPs, and plays an extrinsic role in erythroid maturation by being expressed in the supportive macrophage of the erythroblastic island and regulating relevant genes important for island integrity within these cells. Together with its well-established intrinsic contributions to erythropoiesis, EKLF thus plays a coordinating role between two different cell types whose interaction provides the optimal environment to generate a mature red blood cell.

Improved quantitative analysis of primary bone marrow megakaryocytes utilizing imaging flow cytometry

Life-threatening thrombocytopenia can develop following bone marrow injury due to decreased platelet production from megakaryocytes (MKs). However, the study of primary MKs has been complicated by their low frequency in the bone marrow and by technical challenges presented by their unique maturation properties. More accurate and efficient methods for the analysis of in vivo MKs are needed to enhance our understanding of megakaryopoiesis and ultimately develop new therapeutic strategies for thrombocytopenia. Imaging flow cytometry (IFC) combines the morphometric capabilities of microscopy with the high-throughput analyses of flow cytometry (FC). Here, we investigate the application of IFC on the ImageStream(X) platform to the analysis of primary MKs isolated from murine bone marrow. Our data highlight and address technical challenges for conventional FC posed by the wide range of cellular size within the MK lineage as well as the shared surface phenotype with abundant platelet progeny. We further demonstrate that IFC can be used to reproducibly and efficiently quantify the frequency of primary murine MKs in the marrow, both at steady-state and in the setting of radiation-induced bone marrow injury, as well as assess their ploidy distribution. The ability to accurately analyze the full spectrum of maturing MKs in the bone marrow now allows for many possible applications of IFC to enhance our understanding of megakaryopoiesis and platelet production.

Deficiency of ATP-binding cassette transporter B6 in megakaryocyte progenitors accelerates atherosclerosis in mice

OBJECTIVE

The ATP-binding cassette (ABC) transporter B6 (ABCB6) is highly expressed in megakaryocyte progenitors, but its role in platelet production and disease has not been elucidated.

APPROACH AND RESULTS

Among various ABC transporters, ABCB6 was highly expressed in megakaryocyte progenitors, exhibiting the same pattern of expression of genes involved in heme synthesis pathway. Transplantation of Abcb6 deficient (Abcb6(-/-)) bone marrow into low density lipoprotein receptor deficient recipient mice resulted in expansion and proliferation of megakaryocyte progenitors, attributable to increased reactive oxygen species production in response to porphyrin loading. The enhanced megakaryopoiesis in Abcb6(-/-) bone marrow-transplanted mice was further illustrated by increased platelet counts, mean platelet volume, and platelet activity. Platelets from Abcb6(-/-) bone marrow-transplanted mice had higher levels of chemokine (C-C motif) ligand 5, which was associated with increased plasma chemokine (C-C motif) ligand 5 levels. There were also increased platelet-leukocyte aggregates, which resulted in leukocyte activation. Abcb6(-/-) bone marrow-transplanted mice had accelerated atherosclerosis which was associated with deposition of the chemotactic agent, chemokine (C-C motif) ligand 5 in atherosclerotic plaques, resulting in increased macrophage accumulation.

CONCLUSIONS

Our findings identify a new role of ABCB6 in preventing atherosclerosis development by dampening platelet production, reactivity, and chemokine (C-C motif) ligand 5 deposition in atherosclerotic lesions.