An intricate regulatory circuit between FLI1 and GATA1/GATA2/LDB1/ERG dictates erythroid vs. megakaryocytic differentiation.

During hematopoiesis, megakaryocytic erythroid progenitors (MEPs) differentiate into megakaryocytic or erythroid lineages in response to specific transcriptional factors, yet the regulatory mechanism remains to be elucidated. Using the MEP­like cell line HEL western blotting, RT­qPCR, lentivirus­mediated downregulation, flow cytometry as well as chromatin immunoprecipitation (ChIp) assay demonstrated that the E26 transformation­specific (ETS) transcription factor friend leukemia integration factor 1 (Fli­1) inhibits erythroid differentiation. The present study using these methods showed that while FLI1­mediated downregulation of GATA binding protein 1 (GATA1) suppresses erythropoiesis, its direct transcriptional induction of GATA2 promotes megakaryocytic differentiation. GATA1 is also involved in megakaryocytic differentiation through regulation of GATA2. By contrast to FLI1, the ETS member erythroblast transformation­specific­related gene (ERG) negatively controls GATA2 and its overexpression through exogenous transfection blocks megakaryocytic differentiation. In addition, FLI1 regulates expression of LIM Domain Binding 1 (LDB1) during erythroid and megakaryocytic commitment, whereas shRNA­mediated depletion of LDB1 downregulates FLI1 and GATA2 but increases GATA1 expression. In agreement, LDB1 ablation using shRNA lentivirus expression blocks megakaryocytic differentiation and modestly suppresses erythroid maturation. These results suggested that a certain threshold level of LDB1 expression enables FLI1 to block erythroid differentiation. Overall, FLI1 controlled the commitment of MEP to either erythroid or megakaryocytic lineage through an intricate regulation of GATA1/GATA2, LDB1 and ERG, exposing multiple targets for cell fate commitment and therapeutic intervention.

SDF-1 promotes metastasis of NSCLC by enhancing chemoattraction of megakaryocytes through the PI3K/Akt signaling pathway.

Lung cancer (LC) is the leading cause of cancer-associated deaths worldwide, among which non-small-cell lung cancer (NSCLC) accounts for 80%. Stromal cell-derived factor-1 (SDF-1) inhibition results in a significant depletion of NSCLC metastasis. Additionally, SDF-1 is the only natural chemokine known to bind and activate the receptor CXCR4. Thus, we attempted to clarify the molecular mechanism of SDF-1 underlying NSCLC progression. Transwell migration, adhesion, and G-LISA assays were used to assess megakaryocytic chemotaxis in vitro and in vivo in terms of megakaryocytic migration, adherence, and RhoA activation, respectively. Western blotting was used to assess PI3K/Akt-associated protein abundances in MEG-01 cells and primary megakaryocytes under the indicated treatment. A hematology analyzer and flow cytometry were used to assess platelet counts in peripheral blood and newly formed platelet counts in Lewis LC mice under different treatments. Immunochemistry and flow cytometry were used to measure CD41+ megakaryocyte numbers in Lewis LC mouse tissue under different treatments. ELISA was used to measure serum TPO levels, and H&E staining was used to detect NSCLC metastasis.SDF-1 receptor knockdown suppressed megakaryocytic chemotaxis in Lewis LC mice. SDF-1 receptor inhibition suppressed megakaryocytic chemotaxis via the PI3K/Akt pathway. SDF-1 receptor knockdown suppressed CD41+ megakaryocyte numbers in vivo through PI3K/Akt signaling. SDF-1 receptor inhibition suppressed CD41+ megakaryocytes to hinder NSCLC metastasis. SDF-1 facilitates NSCLC metastasis by enhancing the chemoattraction of megakaryocytes via the PI3K/Akt signaling pathway, which may provide a potential new direction for seeking therapeutic plans for NSCLC.

Activating SRC/MAPK signaling via 5-HT1A receptor contributes to the effect of vilazodone on improving thrombocytopenia.

Thrombocytopenia caused by long-term radiotherapy and chemotherapy exists in cancer treatment. Previous research demonstrates that 5-Hydroxtrayptamine (5-HT) and its receptors induce the formation of megakaryocytes (MKs) and platelets. However, the relationships between 5-HT1A receptor (5-HTR1A) and MKs is unclear so far. We screened and investigated the mechanism of vilazodone as a 5-HTR1A partial agonist in promoting MK differentiation and evaluated its therapeutic effect in thrombocytopenia. We employed a drug screening model based on machine learning (ML) to screen the megakaryocytopoiesis activity of Vilazodone (VLZ). The effects of VLZ on megakaryocytopoiesis were verified in HEL and Meg-01 cells. Tg (itga2b: eGFP) zebrafish was performed to analyze the alterations in thrombopoiesis. Moreover, we established a thrombocytopenia mice model to investigate how VLZ administration accelerates platelet recovery and function. We carried out network pharmacology, Western blot, and immunofluorescence to demonstrate the potential targets and pathway of VLZ. VLZ has been predicted to have a potential biological action. Meanwhile, VLZ administration promotes MK differentiation and thrombopoiesis in cells and zebrafish models. Progressive experiments showed that VLZ has a potential therapeutic effect on radiation-induced thrombocytopenia in vivo. The network pharmacology and associated mechanism study indicated that SRC and MAPK signaling are both involved in the processes of megakaryopoiesis facilitated by VLZ. Furthermore, the expression of 5-HTR1A during megakaryocyte differentiation is closely related to the activation of SRC and MAPK. Our findings demonstrated that the expression of 5-HTR1A on MK, VLZ could bind to the 5-HTR1A receptor and further regulate the SRC/MAPK signaling pathway to facilitate megakaryocyte differentiation and platelet production, which provides new insights into the alternative therapeutic options for thrombocytopenia.

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.

Thrombopoietin, the Primary Regulator of Platelet Production: From Mythos to Logos, a Thirty-Year Journey.

Thrombopoietin, the primary regulator of blood platelet production, was postulated to exist in 1958, but was only proven to exist when the cDNA for the hormone was cloned in 1994. Since its initial cloning and characterization, the hormone has revealed many surprises. For example, instead of acting as the postulated differentiation factor for platelet precursors, megakaryocytes, it is the most potent stimulator of megakaryocyte progenitor expansion known. Moreover, it also stimulates the survival, and in combination with stem cell factor leads to the expansion of hematopoietic stem cells. All of these growth-promoting activities have resulted in its clinical use in patients with thrombocytopenia and aplastic anemia, although the clinical development of the native molecule illustrated that "it's not wise to mess with mother nature", as a highly engineered version of the native hormone led to autoantibody formation and severe thrombocytopenia. Finally, another unexpected finding was the role of the thrombopoietin receptor in stem cell biology, including the development of myeloproliferative neoplasms, an important disorder of hematopoietic stem cells. Overall, the past 30 years of clinical and basic research has yielded many important insights, which are reviewed in this paper.

A subset of megakaryocytes regulates development of hematopoietic stem cell precursors.

Understanding the regulatory mechanisms facilitating hematopoietic stem cell (HSC) specification during embryogenesis is important for the generation of HSCs in vitro. Megakaryocyte emerged from the yolk sac and produce platelets, which are involved in multiple biological processes, such as preventing hemorrhage. However, whether megakaryocytes regulate HSC development in the embryonic aorta-gonad-mesonephros (AGM) region is unclear. Here, we use platelet factor 4 (PF4)-Cre;Rosa-tdTomato+ cells to report presence of megakaryocytes in the HSC developmental niche. Further, we use the PF4-Cre;Rosa-DTA (DTA) depletion model to reveal that megakaryocytes control HSC specification in the mouse embryos. Megakaryocyte deficiency blocks the generation and maturation of pre-HSCs and alters HSC activity at the AGM. Furthermore, megakaryocytes promote endothelial-to-hematopoietic transition in a OP9-DL1 coculture system. Single-cell RNA-sequencing identifies megakaryocytes positive for the cell surface marker CD226 as the subpopulation with highest potential in promoting the hemogenic fate of endothelial cells by secreting TNFSF14. In line, TNFSF14 treatment rescues hematopoietic cell function in megakaryocyte-depleted cocultures. Taken together, megakaryocytes promote production and maturation of pre-HSCs, acting as a critical microenvironmental control factor during embryonic hematopoiesis.

Long non-coding RNA NORAD regulates megakaryocyte differentiation and proplatelet formation via the DUSP6/ERK signaling pathway.

Megakaryopoiesis and platelet production is a complex process that is underpotential regulation at multiple stages. Many long non-coding RNAs (lncRNAs) are distributed in hematopoietic stem cells and platelets. lncRNAs may play important roles as key epigenetic regulators in megakaryocyte differentiation and proplatelet formation. lncRNA NORAD can affect cell ploidy by sequestering PUMILIO proteins, although its direct effect on megakaryocyte differentiation and thrombopoiesis is still unknown. In this study, we demonstrate NORAD RNA is highly expressed in the cytoplasm during megakaryocyte differentiation. Interestingly, we identified for the first time that NORAD has a strong inhibitory effect on megakaryocyte differentiation and proplatelet formation from cultured megakaryocytes. DUSP6/ERK1/2 pathway is activated in response to NORAD knockdown during megakaryocytopoiesis, which is achieved by sequestering PUM2 proteins. Finally, compared with the wild-type control mice, NORAD knockout mice show a faster platelet recovery after severe thrombocytopenia induced by 6 Gy total body irradiation. These findings demonstrate lncRNA NORAD has a key role in regulating megakaryocyte differentiation and thrombopoiesis, which provides a promising molecular target for the treatment of platelet-related diseases such as severe thrombocytopenia.

GFI1B and LSD1 repress myeloid traits during megakaryocyte differentiation.

The transcription factor Growth Factor Independence 1B (GFI1B) recruits Lysine Specific Demethylase 1 A (LSD1/KDM1A) to stimulate gene programs relevant for megakaryocyte and platelet biology. Inherited pathogenic GFI1B variants result in thrombocytopenia and bleeding propensities with varying intensity. Whether these affect similar gene programs is unknow. Here we studied transcriptomic effects of four patient-derived GFI1B variants (GFI1BT174N,H181Y,R184P,Q287*) in MEG01 megakaryoblasts. Compared to normal GFI1B, each variant affected different gene programs with GFI1BQ287* uniquely failing to repress myeloid traits. In line with this, single cell RNA-sequencing of induced pluripotent stem cell (iPSC)-derived megakaryocytes revealed a 4.5-fold decrease in the megakaryocyte/myeloid cell ratio in GFI1BQ287* versus normal conditions. Inhibiting the GFI1B-LSD1 interaction with small molecule GSK-LSD1 resulted in activation of myeloid genes in normal iPSC-derived megakaryocytes similar to what was observed for GFI1BQ287* iPSC-derived megakaryocytes. Thus, GFI1B and LSD1 facilitate gene programs relevant for megakaryopoiesis while simultaneously repressing programs that induce myeloid differentiation.

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.

In Vitro Generation of Megakaryocytes from Engineered Mouse Embryonic Stem Cells.

The in vitro differentiation of pluripotent stem cells into desired lineages enables mechanistic studies of cell transitions into more mature states that can provide insights into the design principles governing cell fate control. We are interested in reprogramming pluripotent stem cells with synthetic gene circuits to drive mouse embryonic stem cells (mESCs) down the hematopoietic lineage for the production of megakaryocytes, the progenitor cells for platelets. Here, we describe the methodology for growing and differentiating mESCs, in addition to inserting a transgene to observe its expression throughout differentiation. This entails four key methods: (1) growing and preparing mouse embryonic fibroblasts for supporting mESC growth and expansion, (2) growing and preparing OP9 feeder cells to support the differentiation of mESCs, (3) the differentiation of mESCs into megakaryocytes, and (4) utilizing an integrase-mediated docking site to insert transgenes for their stable integration and expression throughout differentiation. Altogether, this approach demonstrates a streamline differentiation protocol that emphasizes the reprogramming potential of mESCs that can be used for future mechanistic and therapeutic studies of controlling cell fate outcomes.

A let-7 microRNA-RALB axis links the immune properties of iPSC-derived megakaryocytes with platelet producibility.

We recently achieved the first-in-human transfusion of induced pluripotent stem cell-derived platelets (iPSC-PLTs) as an alternative to standard transfusions, which are dependent on donors and therefore variable in supply. However, heterogeneity characterized by thrombopoiesis-biased or immune-biased megakaryocytes (MKs) continues to pose a bottleneck against the standardization of iPSC-PLT manufacturing. To address this problem, here we employ microRNA (miRNA) switch biotechnology to distinguish subpopulations of imMKCLs, the MK cell lines producing iPSC-PLTs. Upon miRNA switch-based screening, we find imMKCLs with lower let-7 activity exhibit an immune-skewed transcriptional signature. Notably, the low activity of let-7a-5p results in the upregulation of RAS like proto-oncogene B (RALB) expression, which is crucial for the lineage determination of immune-biased imMKCL subpopulations and leads to the activation of interferon-dependent signaling. The dysregulation of immune properties/subpopulations, along with the secretion of inflammatory cytokines, contributes to a decline in the quality of the whole imMKCL population.

Osteomacs promote maintenance of murine hematopoiesis through megakaryocyte-induced upregulation of Embigin and CD166.

Maintenance of hematopoietic stem cell (HSC) function in the niche is an orchestrated event. Osteomacs (OM) are key cellular components of the niche. Previously, we documented that osteoblasts, OM, and megakaryocytes interact to promote hematopoiesis. Here, we further characterize OM and identify megakaryocyte-induced mediators that augment the role of OM in the niche. Single-cell mRNA-seq, mass spectrometry, and CyTOF examination of megakaryocyte-stimulated OM suggested that upregulation of CD166 and Embigin on OM augment their hematopoiesis maintenance function. CD166 knockout OM or shRNA-Embigin knockdown OM confirmed that the loss of these molecules significantly reduced the ability of OM to augment the osteoblast-mediated hematopoietic-enhancing activity. Recombinant CD166 and Embigin partially substituted for OM function, characterizing both proteins as critical mediators of OM hematopoietic function. Our data identify Embigin and CD166 as OM-regulated critical components of HSC function in the niche and potential participants in various in vitro manipulations of stem cells.

Resilient anatomy and local plasticity of naive and stress haematopoiesis.

The bone marrow adjusts blood cell production to meet physiological demands in response to insults. The spatial organization of normal and stress responses are unknown owing to the lack of methods to visualize most steps of blood production. Here we develop strategies to image multipotent haematopoiesis, erythropoiesis and lymphopoiesis in mice. We combine these with imaging of myelopoiesis1 to define the anatomy of normal and stress haematopoiesis. In the steady state, across the skeleton, single stem cells and multipotent progenitors distribute through the marrow enriched near megakaryocytes. Lineage-committed progenitors are recruited to blood vessels, where they contribute to lineage-specific microanatomical structures composed of progenitors and immature cells, which function as the production sites for each major blood lineage. This overall anatomy is resilient to insults, as it was maintained after haemorrhage, systemic bacterial infection and granulocyte colony-stimulating factor (G-CSF) treatment, and during ageing. Production sites enable haematopoietic plasticity as they differentially and selectively modulate their numbers and output in response to insults. We found that stress responses are variable across the skeleton: the tibia and the sternum respond in opposite ways to G-CSF, and the skull does not increase erythropoiesis after haemorrhage. Our studies enable in situ analyses of haematopoiesis, define the anatomy of normal and stress responses, identify discrete microanatomical production sites that confer plasticity to haematopoiesis, and uncover unprecedented heterogeneity of stress responses across the skeleton.

Altered platelet-megakaryocyte endocytosis and trafficking of albumin and fibrinogen in RUNX1 haplodeficiency.

ABSTRACT: Platelet α-granules have numerous proteins, some synthesized by megakaryocytes (MK) and others not synthesized but incorporated by endocytosis, an incompletely understood process in platelets/MK. Germ line RUNX1 haplodeficiency, referred to as familial platelet defect with predisposition to myeloid malignancies (FPDMMs), is associated with thrombocytopenia, platelet dysfunction, and granule deficiencies. In previous studies, we found that platelet albumin, fibrinogen, and immunoglobulin G (IgG) were decreased in a patient with FPDMM. We now show that platelet endocytosis of fluorescent-labeled albumin, fibrinogen, and IgG is decreased in the patient and his daughter with FPDMM. In megakaryocytic human erythroleukemia (HEL) cells, small interfering RNA RUNX1 knockdown (KD) increased uptake of these proteins over 24 hours compared with control cells, with increases in caveolin-1 and flotillin-1 (2 independent regulators of clathrin-independent endocytosis), LAMP2 (a lysosomal marker), RAB11 (a marker of recycling endosomes), and IFITM3. Caveolin-1 downregulation in RUNX1-deficient HEL cells abrogated the increased uptake of albumin, but not fibrinogen. Albumin, but not fibrinogen, partially colocalized with caveolin-1. RUNX1 KD resulted in increased colocalization of albumin with flotillin and fibrinogen with RAB11, suggesting altered trafficking of both proteins. The increased uptake of albumin and fibrinogen, as well as levels of caveolin-1, flotillin-1, LAMP2, and IFITM3, were recapitulated by short hairpin RNA RUNX1 KD in CD34+-derived MK. To our knowledge, these studies provide first evidence that platelet endocytosis of albumin and fibrinogen is impaired in some patients with RUNX1-haplodeficiency and suggest that megakaryocytes have enhanced endocytosis with defective trafficking, leading to loss of these proteins by distinct mechanisms. This study provides new insights into mechanisms governing endocytosis and α-granule deficiencies in RUNX1-haplodeficiency.

Transcription factor 3 is dysregulated in megakaryocytes in myelofibrosis.

Transcription factor 3 (TCF3) is a DNA transcription factor that modulates megakaryocyte development. Although abnormal TCF3 expression has been identified in a range of hematological malignancies, to date, it has not been investigated in myelofibrosis (MF). MF is a Philadelphia-negative myeloproliferative neoplasm (MPN) that can arise de novo or progress from essential thrombocythemia [ET] and polycythemia vera [PV] and where dysfunctional megakaryocytes have a role in driving the fibrotic progression. We aimed to examine whether TCF3 is dysregulated in megakaryocytes in MPN, and specifically in MF. We first assessed TCF3 protein expression in megakaryocytes using an immunohistochemical approach analyses and showed that TCF3 was reduced in MF compared with ET and PV. Further, the TCF3-negative megakaryocytes were primarily located near trabecular bone and had the typical "MF-like" morphology as described by the WHO. Genomic analysis of isolated megakaryocytes showed three mutations, all predicted to result in a loss of function, in patients with MF; none were seen in megakaryocytes isolated from ET or PV marrow samples. We then progressed to transcriptomic sequencing of platelets which showed loss of TCF3 in MF. These proteomic, genomic and transcriptomic analyses appear to indicate that TCF3 is downregulated in megakaryocytes in MF. This infers aberrations in megakaryopoiesis occur in this progressive phase of MPN. Further exploration of this pathway could provide insights into TCF3 and the evolution of fibrosis and potentially lead to new preventative therapeutic targets.

What is the context? We investigated TCF3 (transcription factor 3), a gene that regulates megakaryocyte development, for genomic and proteomic changes in myelofibrosis.Myelofibrosis is the aggressive phase of a group of blood cancers called myeloproliferative neoplasms, and abnormalities in development and maturation of megakaryocytes is thought to drive the development of myelofibrosis.What is new? We report detection of three novel TCF3 mutations in megakaryocytes and decreases in TCF3 protein and gene expression in primary megakaryocytes and platelets from patients with myelofibrosis.This is the first association between loss of TCF3 in megakaryocytes from patients and myelofibrosis.What is the impact? TCF3 dysregulation may be a novel mechanism that is responsible for the development of myelofibrosis and better understanding of this pathway could identify new drug targets.

Targeting Erbin-mitochondria axis in platelets/megakaryocytes promotes B cell-mediated antitumor immunity.

The roles of platelets/megakaryocytes (MKs), the key components in the blood system, in the tumor microenvironment and antitumor immunity are unclear. In patients with colorectal cancer, the number of platelets was significantly increased in patients with metastasis, and Erbin expression was highly expressed in platelets from patients with metastases. Moreover, Erbin knockout in platelets/MKs suppressed lung metastasis in mice and promoted aggregations of platelets. Mechanistically, Erbin-deficient platelets have increasing mitochondrial oxidative phosphorylation and secrete lipid metabolites like acyl-carnitine (Acar) by abolishing interaction with prothrombotic protein ESAM. Notably, Acar enhanced the activity of mitochondrial electron transport chain complex and mitochondrial oxidative phosphorylation in B cells by acetylation of H3K27 epigenetically. Targeting Erbin in platelets/MKs by a nanovesicle system dramatically attenuated lung metastasis in mice in vivo. Our study identifies an Erbin-mitochondria axis in platelets/MKs, which suppresses B cell-mediated antitumor immunity, suggesting a new way for the treatment of metastasis.

Decreased Expression of IL-35 and Its Receptor Contributes to Impaired Megakaryopoiesis in the Pathogenesis of Immune Thrombocytopenia.

Recent findings have shown that the level of interleukin-35 (IL-35) is abnormal in several autoimmune diseases. Nonetheless, whether IL-35 participates in the pathogenesis of immune thrombocytopenia (ITP) remains unclear. The current study investigates whether IL-35 modulates megakaryopoiesis. The results show that IL-35 receptors are progressively expressed on bone marrow megakaryocytes during the in vitro differentiation of CD34+ progenitors. IL-35 increases the number of megakaryocyte colony-forming units through the Akt pathway. The level of bone marrow IL-35 is reduced in ITP patients, and the decreased level of IL-35 may inhibit megakaryopoiesis. Then, the potential causes of decreased IL-35 in ITP patients are explored. The primary type of cell that secretes IL-35, known as IL-35-producing regulatory T cells (iTr35), is reduced in ITP patients. Bone marrow mesenchymal stem cells (MSCs) from ITP patients exhibit an impaired capability of inducing iTr35 due to enhanced apoptosis, which may contribute to the reduced level of bone marrow IL-35 in ITP patients. Iguratimod promotes megakaryocyte development and differentiation by elevating the expression of IL-35 receptors on megakaryocytes. Iguratimod improves response rates and reduces bleeding symptoms in corticosteroid-resistant ITP patients.