Stem cell factor synergistically enhances thrombopoietin-induced STAT5 signaling in megakaryocyte progenitors through JAK2 and Src kinase

Janus kinase inhibitors ruxolitinib and baricitinib impair glycoprotein-VI mediated platelet function

Several Janus kinase (JAK) inhibitors (jakinibs) have recently been approved to treat inflammatory, autoimmune and hematological conditions. Despite emerging roles for JAKs and downstream signal transducer and activator of transcription (STAT) proteins in platelets, it remains unknown whether jakinibs affect platelet function. Here, we profile platelet biochemical and physiological responses in vitro in the presence of five different clinically relevant jakinibs, including ruxolitinib, upadacitinib, oclacitinib, baricitinib and tofacitinib. Flow cytometry, microscopy and other assays found that potent JAK1/2 inhibitors baricitinib and ruxolitinib reduced platelet adhesion to collagen, as well as platelet aggregation, secretion and integrin α_IIbβ₃ activation in response to the glycoprotein VI (GPVI) agonist collagen-related peptide (CRP-XL). Western blot analysis demonstrated that jakinibs reduced Akt phosphorylation and activation following GPVI activation, where ruxolitinib and baricitinib prevented DAPP1 phosphorylation. In contrast, jakinibs had no effects on platelet responses to thrombin. Inhibitors of GPVI and JAK signaling also abrogated platelet STAT5 phosphorylation following CRP-XL stimulation. Additional pharmacologic experiments supported roles for STAT5 in platelet secretion, integrin activation and cytoskeletal responses. Together, our results demonstrate that ruxolitinib and baricitinib have inhibitory effects on platelet function in vitro and support roles for JAK/STAT5 pathways in GPVI/ITAM mediated platelet function.

Cyba-deficient mice display an increase in hematopoietic stem cells and an overproduction of immunoglobulins

The regulation of protein function by reversible oxidation is increasingly recognized as a key mechanism for the control of cellular signaling, modulating crucial biological processes such as cell differentiation. In this scenario, NADPH oxidases must occupy a prominent position. Our results show that hematopoietic stem and progenitor cells express three p22^phox -dependent NADPH oxidase members (NOX1, NOX2 and NOX4). By deleting the p22^phox coding gene (Cyba), here we have analyzed the importance of this family of enzymes during in vivo hematopoiesis. Cyba^-/- mice show a myeloid bias, and an enrichment of hematopoietic stem cell populations. By means of hematopoietic transplant experiments we have also tried to dissect the specific role of the NADPH oxidases. While the absence of NOX1 or NOX2 provides a higher level of reconstitution, a lack of NOX4 rendered the opposite result, suggesting a functional specificity among the different NADPH oxidases. Cyba^-/- cells showed a hampered activation of AKT1 and a sharp decrease in STAT5 protein. This is in line with the diminished response to IL-7 shown by our results, which could explain the overproduction of immunoglobulins observed in Cyba^-/- mice.

Enhancing functional platelet release in vivo from in vitro-grown megakaryocytes using small molecule inhibitors

In vitro-grown megakaryocytes for generating platelets may have value in meeting the increasing demand for platelet transfusions. Remaining challenges have included the poor yield and quality of in vitro-generated platelets. We have shown that infusing megakaryocytes leads to intrapulmonary release of functional platelets. A Src kinase inhibitor (SU6656), a Rho-associated kinase inhibitor (Y27632), and an aurora B kinase inhibitor (AZD1152) have been shown to increase megakaryocyte ploidy and in vitro proplatelet release. We now tested whether megakaryocytes generated from CD34⁺ hematopoietic cells in the presence of these inhibitors could enhance functional platelet yield following megakaryocyte infusion. As expected, all inhibitors increased megakaryocyte ploidy, size, and granularity, but these inhibitors differed in whether they injured terminal megakaryocytes: SU6656 was protective, whereas Y27632 and AZD1152 increased injury. Upon infusion, inhibitor-treated megakaryocytes released threefold to ninefold more platelets per initial noninjured megakaryocyte relative to control, but only SU6656-treated megakaryocytes had a significant increase in platelet yield when calculated based on the number of initial CD34⁺ cells; this was fourfold over nontreated megakaryocytes. The released platelets from drug-treated, but healthy, megakaryocytes contained similar percentages of young, uninjured platelets that robustly responded to agonists and were well incorporated into a growing thrombus in vivo as controls. These studies suggest that drug screens that select megakaryocytes with enhanced ploidy, cell size, and granularity may include a subset of drugs that can enhance the yield and function of platelets, and may have clinical application for ex vivo-generated megakaryocytes and platelet transfusion.

Cytokine control of megakaryopoiesis

Platelets are anuclear blood cells required for haemostasis and are implicated in other processes including inflammation and metastasis. Platelets are produced by megakaryocytes, specialized cells that are themselves generated by a process of controlled differentiation and maturation of bone-marrow stem and progenitor cells. This process of megakaryopoiesis involves the coordinated interplay of transcription factor-controlled cellular programming with extra-cellular cues produced locally in supporting niches or as circulating factors. This review focuses on these external cues, the cytokines and chemokines, that drive production of megakaryocytes and support the terminal process of platelet release. Emphasis is given to thrombopoietin (Tpo), the major cytokine regulator of steady-state megakaryopoiesis, and its specific cell surface receptor, the Mpl protein, including normal and pathological roles as well as clinical application. The potential for alternative or supplementary regulatory mechanisms for platelet production, particularly in times of acute need, or in states of infection or inflammation are also discussed.

PTPN13 and β-Catenin Regulate the Quiescence of Hematopoietic Stem Cells and Their Interaction with the Bone Marrow Niche

The regulation of hematopoietic stem cells (HSCs) depends on the integration of the multiple signals received from the bone marrow niche. We show the relevance of the protein tyrosine phosphatase PTPN13 and β-catenin as intracellular signaling molecules to control HSCs adhesiveness, cell cycling, and quiescence. Lethally irradiated mice transplanted with Lin^– bone marrow cells in which PTPN13 or β-catenin had been silenced showed a significant increase of long-term (LT) and short-term (ST) HSCs. A decrease in cycling cells was also found, together with an increase in quiescence. The decreased expression of PTPN13 or β-catenin was linked to the upregulation of several genes coding for integrins and several cadherins, explaining the higher cell adhesiveness. Our data are consistent with the notion that the levels of PTPN13 and β-catenin must be strictly regulated by extracellular signaling to regulate HSC attachment to the niche and the balance between proliferation and quiescence.

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PTPN13 or β-catenin silencing increases LT-HSCs and ST-HSCs frequency in vivo

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The cell cycling of HSPCs was decreased by PTPN13 or β-catenin downregulation

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LT-HSCs and ST-HSCs quiescence was increased by PTPN13 or β-catenin downregulation

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PTPN13 and β-catenin levels modulate the interaction of HSPCs with the BM niche

Mammalian gonocyte and spermatogonia differentiation: recent advances and remaining challenges

The production of spermatozoa relies on a pool of spermatogonial stem cells (SSCs), formed in infancy from the differentiation of their precursor cells, the gonocytes. Throughout adult life, SSCs will either self-renew or differentiate, in order to maintain a stem cell reserve while providing cells to the spermatogenic cycle. By contrast, gonocytes represent a transient and finite phase of development leading to the formation of SSCs or spermatogonia of the first spermatogenic wave. Gonocyte development involves phases of quiescence, cell proliferation, migration, and differentiation. Spermatogonia, on the other hand, remain located at the basement membrane of the seminiferous tubules throughout their successive phases of proliferation and differentiation. Apoptosis is an integral part of both developmental phases, allowing for the removal of defective cells and the maintenance of proper germ–Sertoli cell ratios. While gonocytes and spermatogonia mitosis are regulated by distinct factors, they both undergo differentiation in response to retinoic acid. In contrast to postpubertal spermatogenesis, the early steps of germ cell development have only recently attracted attention, unveiling genes and pathways regulating SSC self-renewal and proliferation. Yet, less is known on the mechanisms regulating differentiation. The processes leading from gonocytes to spermatogonia have been seldom investigated. While the formation of abnormal gonocytes or SSCs could lead to infertility, defective gonocyte differentiation might be at the origin of testicular germ cell tumors. Thus, it is important to better understand the molecular mechanisms regulating these processes. This review summarizes and compares the present knowledge on the mechanisms regulating mammalian gonocyte and spermatogonial differentiation.

Role of retinoic acid and platelet-derived growth factor receptor cross talk in the regulation of neonatal gonocyte and embryonal carcinoma cell differentiation

Neonatal gonocytes are direct precursors of spermatogonial stem cells, the cell pool that supports spermatogenesis. Although unipotent in vivo, gonocytes express pluripotency genes common with embryonic stem cells. Previously, we found that all-trans retinoic acid (RA) induced the expression of differentiation markers and a truncated form of platelet-derived growth factor receptor (PDGFR)β in rat gonocytes, as well as in F9 mouse embryonal carcinoma cells, an embryonic stem cell-surrogate that expresses somatic lineage markers in response to RA. The present study is focused on identifying the signaling pathways involved in RA-induced gonocyte and F9 cell differentiation. Mitogen-activated protein kinase kinase (MEK) 1/2 activation was required during F9 cell differentiation towards somatic lineage, whereas its inhibition potentiated RA-induced Stra8 expression, suggesting that MEK1/2 acts as a lineage specification switch in F9 cells. In both cell types, RA increased the expression of the spermatogonial/premeiotic marker Stra8, which is in line with F9 cells being at a stage before somatic-germline lineage specification. Inhibiting PDGFR kinase activity reduced RA-induced Stra8 expression. Interestingly, RA increased the expression of PDGFRα variant forms in both cell types. Together, these results suggest a potential cross talk between RA and PDGFR signaling pathways in cell differentiation. RA receptor-α inhibition partially reduced RA effects on Stra8 in gonocytes, indicating that RA acts in part via RA receptor-α. RA-induced gonocyte differentiation was significantly reduced by inhibiting SRC (v-src avian sarcoma [Schmidt-Ruppin A-2] viral oncogene) and JAK2/STAT5 (Janus kinase 2/signal transducer and activator of transcription 5) activities, implying that these signaling molecules play a role in gonocyte differentiation. These results suggest that gonocyte and F9 cell differentiation is regulated via cross talk between RA and PDGFRs using different downstream pathways.

Pathological interactions between hematopoietic stem cells and their niche revealed by mouse models of primary myelofibrosis

Primary myelofibrosis (PMF) belongs to the Philadelphia-negative myeloproliferative neoplasms and is a hematological disorder caused by abnormal function of the hematopoietic stem cells. The disease manifests itself with a plethora of alterations, including anemia, splenomegaly and extramedullary hematopoiesis. Its hallmarks are progressive marrow fibrosis and atypical megakaryocytic hyperplasia, two distinctive features used to clinically monitor disease progression. In an attempt to investigate the role of abnormal megakaryocytopoiesis in the pathogenesis of PMF, several transgenic mouse models have been generated. These models are based either on mutations that interfere with the extrinsic (thrombopoietin and its receptor, MPL) and intrinsic (the GATA1 transcription factor) control of normal megakaryocytopoiesis, or on known genetic lesions associated with the human disease. Here we provide an up-to-date review on the insights into the pathobiology of human PMF achieved by studying these animal models, with particular emphasis on results obtained with Gata1(low) mice.

Challenges and promises for the development of donor-independent platelet transfusions

Platelet transfusions are often a life-saving intervention, and the use of platelet transfusions has been increasing. Donor-derived platelet availability can be challenging. Compounding this concern are additional limitations of donor-derived platelets, including variability in product unit quality and quantity, limited shelf life and the risks of product bacterial contamination, other transfusion-transmitted infections, and immunologic reactions. Because of these issues, there has been an effort to develop strategies to generate platelets from exogenously generated precursor cells. If successful, such platelets have the potential to be a safer, more consistent platelet product, while reducing the necessity for human donations. Moreover, ex vivo-generated autologous platelets or precursors may be beneficial for patients who are refractory to allogeneic platelets. For patients with inherited platelet disorders, ex vivo-generated platelets offer the promise of a treatment via the generation of autologous gene-corrected platelets. Theoretically, ex vivo-generated platelets also offer targeted delivery of ectopic proteins to sites of vascular injury. This review summarizes the current, state-of-the-art methodologies in delivering a clinically relevant ex vivo-derived platelet product, and it discusses significant challenges that must be overcome for this approach to become a clinical reality.

Megakaryopoiesis and thrombopoiesis: an update on cytokines and lineage surface markers

Megakaryopoiesis is the process by which mature megakaryocytes (MKs) develop from hematopoietic stem cells (HSCs). The biological function of MKs is to produce platelets, which play critical roles in hemostasis and contribute to angiogenesis and wound healing. The generation of platelets from MKs is termed thrombopoiesis. The cytokine thrombopoietin (TPO) is the major regulator of megakaryopoiesis and thrombopoiesis. It binds to its surface receptor, c-Mpl, and acts through multiple downstream signaling pathways, including the PI-3 kinase-Akt, MAPK, and ERK1/ERK2 pathways. However, non-TPO pathways, such as the SDF1/CXCR4 axis, Notch signaling, src family kinases, integrin signaling, and Platelet Factor 4/low-density lipoprotein receptor-related protein 1, have more recently been recognized to influence megakaryopoiesis and thrombopoiesis in vitro and in vivo. In this chapter, we review megakaryopoiesis and thrombopoiesis with emphasis on cell surface marker changes during their differentiation from HSCs, and the classical cytokines that affect these developmental stages. We also discuss non-TPO regulators and their effects on in vitro culture systems.

Adventures in time and space: Nonlinearity and complexity of cytokine effects on stem cell fate decisions

Cytokines are central factors in the control of stem cell fate decisions and, as such, they are invaluable to those interested in the manipulation of stem and progenitor cells for clinical or research purposes. In their in vivo niches or in optimized cultures, stem cells are exposed to multiple cytokines, matrix proteins and other cell types that provide individual and combinatorial signals that influence their self-renewal, proliferation and differentiation. Although the individual effects of cytokines are well-characterized in terms of increases or decreases in stem cell expansion or in the production of specific cell lineages, their interactions are often overlooked. Factorial design experiments in association with multiple linear regression is a powerful multivariate approach to derive response-surface models and to obtain a quantitative understanding of cytokine dose and interactions effects. On the other hand, cytokine interactions detected in stem cell processes can be difficult to interpret due to the fact that the cell populations examined are often heterogeneous, that cytokines can exhibit pleiotropy and redundancy and that they can also be endogenously produced. This perspective piece presents a list of possible biological mechanisms that can give rise to positive and negative two-way factor interactions in the context of in vivo and in vitro stem cell-based processes. These interpretations are based on insights provided by recent studies examining intra- and extra-cellular signaling pathways in adult and embryonic stem cells. Cytokine interactions have been classified according to four main types of molecular and cellular mechanisms: (i) interactions due to co-signaling; (ii) interactions due to sequential actions; (iii) interactions due to high-dose saturation and inhibition; and (iv) interactions due to intercellular signaling networks. For each mechanism, possible patterns of regression coefficients corresponding to the cytokine main effects, quadratic effects and two-way interactions effects are provided. Finally, directions for future mechanistic studies are presented.

Single-cell STAT5 signal transduction profiling in normal and leukemic stem and progenitor cell populations reveals highly distinct cytokine responses

Background

Signal Transducer and Activator of Transcription 5 (STAT5) plays critical roles in normal and leukemic hematopoiesis. However, the manner in which STAT5 responds to early-acting and lineage-restricted cytokines, particularly in leukemic stem/progenitor cells, is largely unknown.

Methodology/Principal Findings

We optimized a multiparametric flow cytometry protocol to analyze STAT5 phosphorylation upon cytokine stimulation in stem and progenitor cell compartments at a single-cell level. In normal cord blood (CB) cells, STAT5 phosphorylation was efficiently induced by TPO, IL-3 and GM-CSF within CD34⁺CD38⁻ hematopoietic stem cells (HSCs). EPO- and SCF-induced STAT5 phosphorylation was largely restricted to the megakaryocyte-erythroid progenitor (MEP) compartment, while G-CSF as well IL-3 and GM-CSF were most efficient in inducing STAT5 phosphorylation in the myeloid progenitor compartments. Strikingly, mobilized adult peripheral blood (PB) CD34⁺ cells responded much less efficiently to cytokine-induced STAT5 activation, with the exception of TPO. In leukemic stem and progenitor cells, highly distinct cytokine responses were observed, differing significantly from their normal counterparts. These responses could not be predicted by the expression level of cytokine receptors. Also, heterogeneity existed in cytokine requirements for long-term expansion of AML CD34⁺ cells on stroma.

Conclusions/Significance

In conclusion, our optimized multiparametric flow cytometry protocols allow the analysis of signal transduction at the single cell level in normal and leukemic stem and progenitor cells. Our study demonstrates highly distinctive cytokine responses in STAT5 phosphorylation in both normal and leukemic stem/progenitor cells.

Distinct roles of the mTOR components Rictor and Raptor in MO7e megakaryocytic cells

OBJECTIVE

During megakaryopoiesis, hematopoietic progenitor cells in the bone marrow proliferate and ultimately differentiate in mature megakaryocytes (MK). We and others have recently described a role for the mammalian target of Rapamycin (mTOR) in proliferation and differentiation of MK cells. Two non-redundant complexes of mTOR have been described; mTORC1 containing rapamycin-associated TOR protein (Raptor) and mTORC2 containing Rapamycin-insensitive companion of mTOR (Rictor). The individual roles of these complexes in MK development have so far not been elucidated, and were investigated in this study.

METHODS

We have used an siRNA approach to selectively knock down either Rictor or Raptor expression in MO7e megakaryoblastic cells. Using flow cytometry, nuclear ploidity, and cell cycling as assessed by BrdU incorporation were investigated. Electron microscopy and cotransductions with GFP-LC3 were used to quantify autophagy. Activation of intracellular signal transduction pathways was studied by Western blot analysis.

RESULTS

We observed a reduced cell cycling upon Rictor siRNA transduction, resulting in decreased numbers of polypoid cells. Knocking down Raptor expression resulted in a reduced expansion and a reduced cell size. In addition, increased autophagy was observed in Raptor siRNA-transduced cells, in correspondence with an attenuation of activation of the p70S6K/S6, and 4E-BP pathways.

CONCLUSIONS

The current study shows that the mTORC1 and mTORC2 complexes have distinct, non-redundant functions in MO7e MK cell proliferation, and development. The mTOR/Rictor complex affects megakaryopoiesis by regulating nuclear division and subsequent cell cycle progression, whereas Raptor signaling protects MK cells from autophagic cell death, enabling normal megakaryopoiesis to take place.

Expression, refolding, and characterization of recombinant thrombopoietin/stem cell factor fusion protein in Escherichia coli

Thrombopoietin/stem cell factor (TPO/SCF) is a novel fusion protein that combines the complementary biological effects of TPO and SCF into a single molecule. In this study, TPO/SCF gene was cloned into pET32a and expressed as a thioredoxin (Trx) fusion protein with a C-terminal 6His-tag in Escherichia coli BL21(DE3) under the control of T7 promoter. Trx-TPO/SCF protein approximately accounted for 20% of the total bacterial proteins and was found to accumulate in inclusion bodies. Inclusion bodies were separated from cellular debris, washed with buffer containing 2 M urea, and solubilized with 8 M urea. The refolding of Trx-TPO/SCF was then carried out by an on-column method. Soluble Trx-TPO/SCF was characterized for its dose-dependent effects on promoting cells proliferation in both TF1 and Mo7e cell lines. rhTPO/SCF was released by thrombin digestion and further purified by Ni(2+) affinity chromatography. Western blot analysis confirmed the identities of Trx-TPO/SCF and rhTPO/SCF.

Co expression of SCF and KIT in gastrointestinal stromal tumours (GISTs) suggests an autocrine/paracrine mechanism

KIT is a tyrosine kinase receptor expressed by several tumours, which has for specific ligand the stem cell factor (SCF). KIT is the main oncogene in gastrointestinal stromal tumours (GISTs), and gain-of-function KIT mutations are present in 70% of these tumours. The aim of the study was to measure and investigate the mechanisms of KIT activation in 80 KIT-positive GIST patients. KIT activation was quantified by detecting phosphotyrosine residues in Western blotting. SCF production was determined by reverse transcriptase-PCR, ELISA and/or immunohistochemistry. Primary cultures established from three GISTs were also analysed. The results show that KIT activation was detected in all cases, even in absence of KIT mutations. The fraction of activated KIT was not correlated with the mutational status of GISTs. Membrane and soluble isoforms of SCF mRNA were present in all GISTs analysed. Additionally, SCF was also detected in up to 93% of GISTs, and seen to be present within GIST cells. Likewise, the two SCF mRNA isoforms were found to be expressed in GIST-derived primary cultures. Thus, KIT activation in GISTs may in part result from the presence of SCF within the tumours.

Lyn kinase promotes erythroblast expansion and late-stage development

Lyn kinase is known to modulate the formation and function of B cells, monocytes, and mast cells. However, Lyn-/- mice also develop erythrosplenomegaly, and cases for both negative and positive erythropoietic actions of Lyn recently have been outlined. In phenylhydrazine-treated Lyn-/- mice, extramedullary splenic erythropoiesis was hyperactivated, but this did not lead to accelerated recovery from anemia. Furthermore, ex vivo analyses of the development of bone marrow-derived Lyn-/- erythroblasts in unique primary culture systems indicated positive roles for Lyn at 2 stages. Late-stage Lyn-/- erythroblasts exhibited deficit Ter119(pos) cell formation, and this was paralleled by increased apoptosis (and decreased Bcl-xL expression). During early development, Lyn-/- erythroblasts accumulated at a Kit(pos)CD71(high) stage, possessed decreased proliferative capacity, and were attenuated in entering an apparent G1/S cell-cycle phase. In proposed compensatory responses, Lyn-/- erythroblasts expressed increased levels of activated Akt and p60-Src and decreased levels of death-associated protein kinase-2. Stat5 activation and Bcl-xL expression, in contrast, were significantly decreased in keeping with decreased survival and developmental potentials. Lyn, therefore, is proposed to function via erythroid cell-intrinsic mechanisms to promote progenitor cell expansion beyond a Kit(pos)CD71(high) stage and to support subsequent late-stage development.

Expression, purification, and characterization of a novel recombinant fusion protein, rhTPO/SCF, in Escherichia coli

Thrombopoietin (TPO) is the principal regulatory cytokine of megakaryopoiesis and thrombopoiesis and promotes all aspects of megakaryocyte development. Stem cell factor (SCF) is mainly a pleiotropic cytokine acting on hematopoiesis by promoting the survival and proliferation of hematopoietic stem cells and has a potent synergistic effect on megakaryopoiesis in the presence of TPO. Here, we report the construction, expression, and purification of a novel recombinant human thrombopoietin/stem cell factor (rhTPO/SCF) fusion protein, which consists of a truncated human thrombopoietin (1-157 a.a.) plus a truncated human stem cell factor (1-145 a.a.), linked by a peptide (GGGGSPGGSGGGGSGG). The TPO/SCF gene was cloned into the Escherichia coli expression vector pET28a and expressed in BL21(DE3) strain. The rhTPO/SCF constituted up to 6% of the total bacterial protein. Co-expression with E. coli chaperones, Trigger Factor (TF) and GroES/GroEL, and lowering cultivation temperature cooperatively improved the solubility of expressed rhTPO/SCF, resulting in about fourfold increase in the yield soluble rhTPO/SCF. The rhTPO/SCF was purified to homogeneity using anion exchange followed by metal affinity chromatography. Western blot analysis confirmed the identity of the purified protein. rhTPO/SCF stimulated a dose-dependent cell proliferation in both TF1 and Mo7e cell lines.