Transcriptomic and phospho-proteomic analyzes of erythroblasts expanded in vitro from normal donors and from patients with polycythemia vera

Deepening Our Understanding of the Factors Affecting Landscape of Myeloproliferative Neoplasms: What Do We Know about Them?

Myeloproliferative neoplasms (MPNs) arise from the uncontrolled proliferation of hematopoietic stem and progenitor cells in bone marrow. As with all tumors, the development of MPNs is a consequence of alterations in malignant cells and their interaction with other extrinsic factors that support and promote tumor progression. Since the discovery of driver mutations, much work has focused on studying and reviewing the genomic features of the disease but has neglected to delve into the important role that many other mechanisms may play. This review discusses the genetic component of MPNs but focuses mainly on some of the most relevant work investigating other non-genetic factors that may be crucial for the disease. The studies summarized here address MPN cell-intrinsic or -extrinsic factors and the interaction between them through transcriptomic, proteomic and microbiota studies, among others.

Functions of G protein-coupled receptor 56 in health and disease

Human G protein-coupled receptor 56 (GPR56) is encoded by gene ADGRG1 from chromosome 16q21 and is homologously encoded in mice, at chromosome 8. Both 687 and 693 splice forms are present in humans and mice. GPR56 has a 381 amino acid-long N-terminal extracellular segment and a GPCR proteolysis site upstream from the first transmembrane domain. GPR56 is mainly expressed in the heart, brain, thyroid, platelets, and peripheral blood mononuclear cells. Accumulating evidence indicates that GPR56 promotes the formation of myelin sheaths and the development of oligodendrocytes in the cerebral cortex of the central nervous system. Moreover, GPR56 contributes to the development and differentiation of hematopoietic stem cells, induces adipogenesis, and regulates the function of immune cells. The lack of GPR56 leads to nervous system dysfunction, platelet disorders, and infertility. Abnormal expression of GPR56 is related to the malignant transformation and tumor metastasis of several cancers including melanoma, neuroglioma, and gastrointestinal cancer. Metabolic disorders and cardiovascular diseases are also associated with dysregulation of GPR56 expression, and GPR56 is involved in the pharmacological resistance to some antidepressant and cancer drug treatments. In this review, the molecular structure, expression profile, and signal transduction of GPR56 are introduced, and physiological and pathological functions of GRP56 are comprehensively summarized. Attributing to its significant biological functions and its long N-terminal extracellular region that interacts with multiple ligands, GPR56 is becoming an attractive therapeutic target in treating neurological and hematopoietic diseases.

SLN124, a GalNac-siRNA targeting transmembrane serine protease 6, in combination with deferiprone therapy reduces ineffective erythropoiesis and hepatic iron-overload in a mouse model of β-thalassaemia

Beta‐thalassaemia is an inherited blood disorder characterised by ineffective erythropoiesis and anaemia. Consequently, hepcidin expression is reduced resulting in increased iron absorption and primary iron overload. Hepcidin is under the negative control of transmembrane serine protease 6 (TMPRSS6) via cleavage of haemojuvelin (HJV), a co‐receptor for the bone morphogenetic protein (BMP)‐mothers against decapentaplegic homologue (SMAD) signalling pathway. Considering the central role of the TMPRSS6/HJV/hepcidin axis in iron homeostasis, the inhibition of TMPRSS6 expression represents a promising therapeutic strategy to increase hepcidin production and ameliorate anaemia and iron overload in β‐thalassaemia. In the present study, we investigated a small interfering RNA (siRNA) conjugate optimised for hepatic targeting of Tmprss6 (SLN124) in β‐thalassaemia mice (Hbb^th3/+). Two subcutaneous injections of SLN124 (3 mg/kg) were sufficient to normalise hepcidin expression and reduce anaemia. We also observed a significant improvement in erythroid maturation, which was associated with a significant reduction in splenomegaly. Treatment with the iron chelator, deferiprone (DFP), did not impact any of the erythroid parameters. However, the combination of SLN124 with DFP was more effective in reducing hepatic iron overload than either treatment alone. Collectively, we show that the combination therapy can ameliorate several disease symptoms associated with chronic anaemia and iron overload, and therefore represents a promising pharmacological modality for the treatment of β‐thalassaemia and related disorders.

Phospho-proteomic discovery of novel signal transducers including thioredoxin-interacting protein as mediators of erythropoietin-dependent human erythropoiesis

Erythroid cell formation critically depends on signals transduced via erythropoietin (EPO)/EPO receptor (EPOR)/JAK2 complexes. This includes not only core response modules (e.g., JAK2/STAT5, RAS/MEK/ERK), but also specialized effectors (e.g., erythroferrone, ASCT2 glutamine transport, Spi2A). By using phospho-proteomics and a human erythroblastic cell model, we identify 121 new EPO target proteins, together with their EPO-modulated domains and phosphosites. Gene ontology (GO) enrichment for "Molecular Function" identified adaptor proteins as one top EPO target category. This includes a novel EPOR/JAK2-coupled network of actin assemblage modifiers, with adaptors DLG-1, DLG-3, WAS, WASL, and CD2AP as prime components. "Cellular Component" GO analysis further identified 19 new EPO-modulated cytoskeletal targets including the erythroid cytoskeletal targets spectrin A, spectrin B, adducin 2, and glycophorin C. In each, EPO-induced phosphorylation occurred at pY sites and subdomains, which suggests coordinated regulation by EPO of the erythroid cytoskeleton. GO analysis of "Biological Processes" further revealed metabolic regulators as a likewise unexpected EPO target set. Targets included aldolase A, pyruvate dehydrogenase α1, and thioredoxin-interacting protein (TXNIP), with EPO-modulated p-Y sites in each occurring within functional subdomains. In TXNIP, EPO-induced phosphorylation occurred at novel p-T349 and p-S358 sites, and was paralleled by rapid increases in TXNIP levels. In UT7epo-E and primary human stem cell (HSC)-derived erythroid progenitor cells, lentivirus-mediated short hairpin RNA knockdown studies revealed novel pro-erythropoietic roles for TXNIP. Specifically, TXNIP's knockdown sharply inhibited c-KIT expression; compromised EPO dose-dependent erythroblast proliferation and survival; and delayed late-stage erythroblast formation. Overall, new insight is provided into EPO's diverse action mechanisms and TXNIP's contributions to EPO-dependent human erythropoiesis.

Phosphoproteomic Landscaping Identifies Non-canonical cKIT Signaling in Polycythemia Vera Erythroid Progenitors

Although stem cell factor (SCF)/cKIT interaction plays key functions in erythropoiesis, cKIT signaling in human erythroid cells is still poorly defined. To provide new insights into cKIT-mediated erythroid expansion in development and disease, we performed phosphoproteomic profiling of primary erythroid progenitors from adult blood (AB), cord blood (CB), and Polycythemia Vera (PV) at steady-state and upon SCF stimulation. While AB and CB, respectively, activated transient or sustained canonical cKIT-signaling, PV showed a non-canonical signaling including increased mTOR and ERK1 and decreased DEPTOR. Accordingly, screening of FDA-approved compounds showed increased PV sensitivity to JAK, cKIT, and MEK inhibitors. Moreover, differently from AB and CB, in PV the mature 145kDa-cKIT constitutively associated with the tetraspanin CD63 and was not endocytosed upon SCF stimulation, contributing to unrestrained cKIT signaling. These results identify a clinically exploitable variegation of cKIT signaling/metabolism that may contribute to the great erythroid output occurring during development and in PV.

Dexamethasone Predisposes Human Erythroblasts Toward Impaired Lipid Metabolism and Renders Their ex vivo Expansion Highly Dependent on Plasma Lipoproteins

Cultures of stem cells from discarded sources supplemented with dexamethasone, a synthetic glucocorticoid receptor agonist, generate cultured red blood cells (cRBCs) in numbers sufficient for transfusion. According to the literature, however, erythroblasts generated with dexamethasone exhibit low enucleation rates giving rise to cRBCs that survive poorly in vivo. The knowledge that the glucocorticoid receptor regulates lipid metabolism and that lipid composition dictates the fragility of the plasma membrane suggests that insufficient lipid bioavailability restrains generation of cRBCs. To test this hypothesis, we first compared the expression profiling of erythroblasts generated with or without dexamethasone. This analysis revealed differences in expression of 55 genes, 6 of which encoding proteins involved in lipid metabolism. These were represented by genes encoding the mitochondrial proteins 3-Hydroxymethyl-3-Methylglutaryl-CoA lyase, upregulated, and 3-Oxoacid CoA-Transferase1 and glycerol-3-phosphate acyltransferase1, both downregulated, and the proteins ATP-binding cassette transporter1 and Hydroxysteroid-17-Beta-Dehydrogenase7, upregulated, and cAMP-dependent protein kinase catalytic subunit beta, downregulated. This profiling predicts that dexamethasone, possibly by interfering with mitochondrial functions, impairs the intrinsic lipid metabolism making the synthesis of the plasma membrane of erythroid cells depend on lipid-uptake from external sources. Optical and electron microscopy analyses confirmed that the mitochondria of erythroblasts generated with dexamethasone are abnormal and that their plasma membranes present pebbles associated with membrane ruptures releasing exosomes and micro-vesicles. These results indicate that the lipid supplements of media currently available are not adequate for cRBCs. To identify better lipid supplements, we determined the number of erythroblasts generated in synthetic media supplemented with either currently used liposomes or with lipoproteins purified from human plasma [the total lipoprotein fraction (TL) or its high (HDL), low (LDL) and very low (VLDL) density lipoprotein components]. Both LDL and VLDL generated numbers of erythroid cells 3-2-fold greater than that observed in controls. These greater numbers were associated with 2-3-fold greater amplification of erythroid cells due both to increased proliferation and to resistance to stress-induced death. In conclusion, dexamethasone impairs lipid metabolism making ex vivo expansion of erythroid cells highly dependent on lipid absorbed from external sources and the use of LDL and VLDL as lipid supplements improves the generation of cRBCs.

Developmental differences between neonatal and adult human erythropoiesis

Studies of human erythropoiesis have relied, for the most part, on the in vitro differentiation of hematopoietic stem and progenitor cells (HSPC) from different sources. Here, we report that despite the common core erythroid program that exists between cord blood (CB)- and peripheral blood (PB)-HSPC induced toward erythroid differentiation in vitro, significant functional differences exist. We undertook a comparative analysis of human erythropoiesis using these two different sources of HSPC. Upon in vitro erythroid differentiation, CB-derived cells proliferated 4-fold more than PB-derived cells. However, CB-derived cells exhibited a delayed kinetics of differentiation, resulting in an increased number of progenitors, notably colony-forming unit (CFU-E). The phenotypes of early erythroid differentiation stages also differed between the two sources with a significantly higher percentage of IL3R^- GPA^- CD34⁺ CD36⁺ cells generated from PB- than CB-HSPCs. This subset was found to generate both burst-forming unit (BFU-E) and CFU-E colonies in colony-forming assays. To further understand the differences between CB- and PB-HSPC, cells at eight stages of erythroid differentiation were sorted from each of the two sources and their transcriptional profiles were compared. We document differences at the CD34, BFU-E, poly- and orthochromatic stages. Genes exhibiting the most significant differences in expression between HSPC sources clustered into cell cycle- and autophagy-related pathways. Altogether, our studies provide a qualitative and quantitative comparative analysis of human erythropoiesis, highlighting the impact of the developmental origin of HSPCs on erythroid differentiation.

The Calreticulin control of human stress erythropoiesis is impaired by JAK2V617F in polycythemia vera

Calreticulin (CALR) is a Ca²⁺-binding protein that shuttles among cellular compartments with proteins bound to its N/P domains. The knowledge that activation of the human erythropoietin receptor induces Ca²⁺ fluxes prompted us to investigate the role of CALR in human erythropoiesis. As shown by Western blot analysis, erythroblasts generated in vitro from normal sources and JAK2V617F polycythemia vera (PV) patients expressed robust levels of CALR. However, Ca²⁺ regulated CALR conformation only in normal cells. Normal erythroblasts expressed mostly the N-terminal domain of CALR (N-CALR) on their cell surface (as shown by flow cytometry) and C-terminal domain (C-CALR) in their cytoplasm (as shown by confocal microscopy) and expression of both epitopes decreased with maturation. In the proerythroblast (proEry) cytoplasm, C-CALR was associated with the glucocorticoid receptor (GR), which initiated the stress response. In these cells, Ca²⁺ deprivation and inhibition of nuclear export increased GR nuclear localization while decreasing cytoplasmic detection of C-CALR and C-CALR/GR association and proliferation in response to the GR agonist dexamethasone (Dex). C-CALR/GR association and Dex responsiveness were instead increased by Ca²⁺ and erythropoietin. In contrast, JAK2V617F proErys expressed normal cell-surface levels of N-CALR but barely detectable cytoplasmic levels of C-CALR. These cells contained GR mainly in the nucleus and were Dex unresponsive. Ruxolitinib rescued cytoplasmic detection of C-CALR, C-CALR/GR association, and Dex responsiveness in JAK2V617F proErys and its effects were antagonized by nuclear export and Ca²⁺ flux inhibitors. These results indicates that Ca²⁺-induced conformational changes of CALR regulate nuclear export of GR in normal erythroblasts and that JAK2V617F deregulates this function in PV.

The spleen microenvironment influences disease transformation in a mouse model of KITD816V-dependent myeloproliferative neoplasm

Activating mutations leading to ligand-independent signaling of the stem cell factor receptor KIT are associated with several hematopoietic malignancies. One of the most common alterations is the D816V mutation. In this study, we characterized mice, which conditionally express the humanized KIT^D816V receptor in the adult hematopoietic system to determine the pathological consequences of unrestrained KIT signaling during blood cell development. We found that KIT^D816V mutant animals acquired a myeloproliferative neoplasm similar to polycythemia vera, marked by a massive increase in red blood cells and severe splenomegaly caused by excessive extramedullary erythropoiesis. Moreover, we found mobilization of stem cells from bone marrow to the spleen. Splenectomy prior to KIT^D816V induction prevented expansion of red blood cells, but rapidly lead to a state of aplastic anemia and bone marrow fibrosis, reminiscent of post polycythemic myeloid metaplasia, the spent phase of polycythemia vera. Our results show that the extramedullary hematopoietic niche microenvironment significantly influences disease outcome in KIT^D816V mutant mice, turning this model a valuable tool for studying the interplay between functionally abnormal hematopoietic cells and their microenvironment during development of polycythemia vera-like disease and myelofibrosis.

Preclinical rationale for TGF-β inhibition as a therapeutic target for the treatment of myelofibrosis

To assess the role of abnormal transforming growth factor-beta (TGF-β) signaling in the pathogenesis of primary myelofibrosis (PMF), the effects of the TGF-β receptor-1 kinase inhibitor SB431542 on ex vivo expansion of hematopoietic cells in cultures from patients with JAK2V617+-polycythemia vera (PV) or PMF (JAK2V617F+, CALRpQ365f+, or unknown) and from normal sources (adult blood, AB, or cord blood, CB) were compared. In cultures of normal sources, SB431542 significantly increased by 2.5-fold the number of progenitor cells generated by days 1-2 (CD34+) and 6 (colony-forming cells) (CB) and that of precursor cells, mostly immature erythroblasts, by days 14-17 (AB and CB). In cultures of JAK2V617F+-PV, SB431542 increased by twofold the numbers of progenitor cells by day 10 and had no effect on that of precursors cells by days 12-17 (∼fourfold increase in all cases). In contrast, SB431542 had no effect on the number of either progenitor or precursor cells in cultures of JAK2V617F+ and CALR pQ365fs+ PMF. These ontogenetic- and disease-specific effects were associated with variegation in the ability of SB431542 to induce CD34+ cells from AB (increased), CB (decreased), or PV and PMF (unaffected) into cycle and erythroblasts in proliferation (increased for AB and PV and unaffected for CB and PMF). Differences in expansion of erythroblasts from AB, CB, and PV were associated with differences in activation of TGF-β signaling (SHCY317, SMAD2S245/250/255, and SMAD1S/S/SMAD5S/S/SMAD8S/S) detectable in these cells by phosphoproteomic profiling. In conclusion, treatment with TGF-β receptor-1 kinase inhibitors may reactivate normal hematopoiesis in PMF patients, providing a proliferative advantage over the unresponsive malignant clone.

Adhesion GPCRs in Tumorigenesis

Alterations in the homeostasis of several adhesion GPCRs (aGPCRs) have been observed in cancer. The main cellular functions regulated by aGPCRs are cell adhesion, migration, polarity, and guidance, which are all highly relevant to tumor cell biology. Expression of aGPCRs can be induced, increased, decreased, or silenced in the tumor or in stromal cells of the tumor microenvironment, including fibroblasts and endothelial and/or immune cells. For example, ADGRE5 (CD97) and ADGRG1 (GPR56) show increased expression in many cancers, and initial functional studies suggest that both are relevant for tumor cell migration and invasion. aGPCRs can also impact the regulation of angiogenesis by releasing soluble fragments following the cleavage of their extracellular domain (ECD) at the conserved GPCR-proteolytic site (GPS) or other more distal cleavage sites as typical for the ADGRB (BAI) family. Interrogation of in silico cancer databases suggests alterations in other aGPCR members and provides the impetus for further exploration of their potential role in cancer. Integration of knowledge on the expression, regulation, and function of aGPCRs in tumorigenesis is currently spurring the first preclinical studies to examine the potential of aGPCR or the related pathways as therapeutic targets.

Kit and Scl regulation of hematopoietic stem cells

PURPOSE OF REVIEW

KIT tyrosine kinase receptor is essential for several tissue stem cells, especially for hematopoietic stem cells (HSCs). Moderately decreased KIT signaling is well known to cause anemia and defective HSC self-renewal, whereas gain-of-function mutations are infrequently found in leukemias. Thus, maintaining KIT signal strength is critically important for homeostasis. KIT signaling in HSCs involves effectors such as SHP2 and PTPN11. This review summarizes the recent developments on the novel mechanisms regulating or reinforcing KIT signal strength in HSCs and its perturbation in polycythemia vera.

RECENT FINDINGS

Stem cell leukemia (SCL) is a transcription factor that is essential for HSC development. Genetic experiments indicate that Kit, protein tyrosine phosphatase, nonreceptor type 11 (Ptpn11), or Scl control long-term HSC self-renewal, survival, and quiescence in adults. Kit is now shown to be centrally involved in two feedforward loops in HSCs, one with Ptpn11 and the other with Scl.

SUMMARY

Knowledge of the regulatory mechanisms that favor self-renewal divisions or a lineage determination process is central to the design of strategies to expand HSCs for the purpose of cell therapy. In addition, transcriptome and phosphoproteome analyses of erythroblasts in polycythemia vera identified lower SCL expression and hypophosphorylated KIT, suggesting that the KIT-SCL loop is relevant to the pathophysiology of human blood disorders as well.

Dexamethasone targeted directly to macrophages induces macrophage niches that promote erythroid expansion

Cultures of human CD34(pos) cells stimulated with erythroid growth factors plus dexamethasone, a model for stress erythropoiesis, generate numerous erythroid cells plus a few macrophages (approx. 3%; 3:1 positive and negative for CD169). Interactions occurring between erythroblasts and macrophages in these cultures and the biological effects associated with these interactions were documented by live phase-contrast videomicroscopy. Macrophages expressed high motility interacting with hundreds/thousands of erythroblasts per hour. CD169(pos) macrophages established multiple rapid 'loose' interactions with proerythroblasts leading to formation of transient erythroblastic island-like structures. By contrast, CD169(neg) macrophages established 'tight' interactions with mature erythroblasts and phagocytosed these cells. 'Loose' interactions of CD169(pos) macrophages were associated with proerythroblast cytokinesis (the M phase of the cell cycle) suggesting that these interactions may promote proerythroblast duplication. This hypothesis was tested by experiments that showed that as few as 103 macrophages significantly increased levels of 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide incorporation frequency in S/G2/M and cytokinesis expressed by proerythroblasts over 24 h of culture. These effects were observed also when macrophages were co-cultured with dexamethasone directly conjugated to a macrophage-specific CD163 antibody. In conclusion, in addition to promoting proerythroblast proliferation directly, dexamethasone stimulates expansion of these cells indirectly by stimulating maturation and cytokinesis supporting activity of macrophages.