Ubiquitination and degradation of the thrombopoietin receptor c-Mpl

Lysosomal degradation targets mutant calreticulin and the thrombopoietin receptor in myeloproliferative neoplasms

ABSTRACT

Somatic mutants of calreticulin (CRT) drive myeloproliferative neoplasms (MPNs) via binding to the thrombopoietin receptor (MPL) and aberrant activation of the JAK/STAT pathway. Compared with healthy donors, platelets from mutant CRT-expressing patients with MPN display low cell surface MPL. Additionally, coexpression of MPL with an MPN-linked CRT mutant (CRTDel52) reduces cell surface MPL, suggesting that CRTDel52 may induce MPL degradation. We show that lysosomal degradation is relevant to the turnover of CRTDel52 and MPL. Furthermore, CRTDel52 increases the lysosomal localization and degradation of MPL. Mammalian target of rapamycin (mTOR) inhibitors reduce cellular CRTDel52 and MPL, secreted CRTDel52 levels, and impair CRTDel52-mediated cell proliferation. mTOR inhibition also reduces colony formation and differentiation of CD34+ cells from patients with MPN but not from healthy donors. Together, these findings indicate that low-surface MPL is a biomarker of mutant CRT-mediated MPN and that induced degradation of CRTDel52 and MPL is an avenue for therapeutic intervention.

Differential in vivo roles of Mpl cytoplasmic tyrosine residues in murine hematopoiesis and myeloproliferative disease

Thrombopoietin (Tpo), which binds to its specific receptor, the Mpl protein, is the major cytokine regulator of megakaryopoiesis and circulating platelet number. Tpo binding to Mpl triggers activation of Janus kinase 2 (Jak2) and phosphorylation of the receptor, as well as activation of several intracellular signalling cascades that mediate cellular responses. Three tyrosine (Y) residues in the C-terminal region of the Mpl intracellular domain have been implicated as sites of phosphorylation required for regulation of major Tpo-stimulated signalling pathways: Mpl-Y565, Mpl-Y599 and Mpl-Y604. Here, we have introduced mutations in the mouse germline and report a consistent physiological requirement for Mpl-Y599, mutation of which resulted in thrombocytopenia, deficient megakaryopoiesis, low hematopoietic stem cell (HSC) number and function, and attenuated responses to myelosuppression. We further show that in models of myeloproliferative neoplasms (MPN), where Mpl is required for pathogenesis, thrombocytosis was dependent on intact Mpl-Y599. In contrast, Mpl-Y565 was required for negative regulation of Tpo responses; mutation of this residue resulted in excess megakaryopoiesis at steady-state and in response to myelosuppression, and exacerbated thrombocytosis associated with MPN.

Sensitivity to targeted UBA1 inhibition in a myeloid cell line model of VEXAS syndrome

Somatic UBA1 mutations in hematopoietic cells are a hallmark of Vacuoles, E1 enzyme, X-linked, Autoinflammatory, Somatic (VEXAS) syndrome, which is a late-onset inflammatory disease associated with bone marrow failure and high mortality. The majority of UBA1 mutations in VEXAS syndrome comprise hemizygous mutations affecting methionine-41 (M41), leading to the expression of UBA1M41T, UBA1M41V, or UBA1M41L and globally reduced protein polyubiquitination. Here, we used CRISPR-Cas9 to engineer isogenic 32D mouse myeloid cell lines expressing hemizygous Uba1WT or Uba1M41L from the endogenous locus. Consistent with prior analyses of patients with VEXAS syndrome samples, hemizygous Uba1M41L expression was associated with loss of the UBA1b protein isoform, gain of the UBA1c protein isoform, reduced polyubiquitination, abnormal cytoplasmic vacuoles, and increased production of interleukin-1β and inflammatory chemokines. Vacuoles in Uba1M41L cells contained a variety of endolysosomal membranes, including small vesicles, multivesicular bodies, and multilamellar lysosomes. Uba1M41L cells were more sensitive to the UBA1 inhibitor TAK243. TAK243 treatment promoted apoptosis in Uba1M41L cells and led to preferential loss of Uba1M41L cells in competition assays with Uba1WT cells. Knock-in of a TAK243-binding mutation, Uba1A580S, conferred TAK243 resistance. In addition, overexpression of catalytically active UBA1b in Uba1M41L cells restored polyubiquitination and increased TAK243 resistance. Altogether, these data indicate that loss of UBA1b underlies a key biochemical phenotype associated with VEXAS syndrome and renders cells with reduced UBA1 activity vulnerable to targeted UBA1 inhibition. Our Uba1M41L knock-in cell line is a useful model of VEXAS syndrome that will aid in the study of disease pathogenesis and the development of effective therapies.

Uncovering complex trait heritability hidden in the repeatome

Short tandem repeats (STRs) account for a substantial fraction of human genetic variation, but their contribution to complex human phenotypes is largely unknown. Margoliash et al. perform detailed genome-wide association analysis and fine-mapping of STRs in UK Biobank, identifying many STRs likely to influence variation in blood and serum traits.

Polymorphic short tandem repeats make widespread contributions to blood and serum traits

Short tandem repeats (STRs) are genomic regions consisting of repeated sequences of 1-6 bp in succession. Single-nucleotide polymorphism (SNP)-based genome-wide association studies (GWASs) do not fully capture STR effects. To study these effects, we imputed 445,720 STRs into genotype arrays from 408,153 White British UK Biobank participants and tested for association with 44 blood phenotypes. Using two fine-mapping methods, we identify 119 candidate causal STR-trait associations and estimate that STRs account for 5.2%-7.6% of causal variants identifiable from GWASs for these traits. These are among the strongest associations for multiple phenotypes, including a coding CTG repeat associated with apolipoprotein B levels, a promoter CGG repeat with platelet traits, and an intronic poly(A) repeat with mean platelet volume. Our study suggests that STRs make widespread contributions to complex traits, provides stringently selected candidate causal STRs, and demonstrates the need to consider a more complete view of genetic variation in GWASs.

Effects of calreticulin mutations on cell transformation and immunity

Myeloproliferative neoplasms (MPNs) are cancers involving dysregulated production and function of myeloid lineage hematopoietic cells. Among MPNs, Essential thrombocythemia (ET), Polycythemia Vera (PV) and Myelofibrosis (MF), are driven by mutations that activate the JAK-STAT signalling pathway. Somatic mutations of calreticulin (CRT), an endoplasmic reticulum (ER)-localized lectin chaperone, are driver mutations in approximately 25% of ET and 35% of MF patients. The MPN-linked mutant CRT proteins have novel frameshifted carboxy-domain sequences and lack an ER retention motif, resulting in their secretion. Wild type CRT is a regulator of ER calcium homeostasis and plays a key role in the assembly of major histocompatibility complex (MHC) class I molecules, which are the ligands for antigen receptors of CD8+ T cells. Mutant CRT-linked oncogenesis results from the dysregulation of calcium signalling in cells and the formation of stable complexes of mutant CRT with myeloproliferative leukemia (MPL) protein, followed by downstream activation of the JAK-STAT signalling pathway. The intricate participation of CRT in ER protein folding, calcium homeostasis and immunity suggests the involvement of multiple mechanisms of mutant CRT-linked oncogenesis. In this review, we highlight recent findings related to the role of MPN-linked CRT mutations in the dysregulation of calcium homeostasis, MPL activation and immunity.

New functions of C3G in platelet biology: Contribution to ischemia-induced angiogenesis, tumor metastasis and TPO clearance

C3G is a Rap1 guanine nucleotide exchange factor that controls platelet activation, aggregation, and the release of α-granule content. Transgenic expression of C3G in platelets produces a net proangiogenic secretome through the retention of thrombospondin-1. In a physiological context, C3G also promotes megakaryocyte maturation and proplatelet formation, but without affecting mature platelet production. The aim of this work is to investigate whether C3G is involved in pathological megakaryopoiesis, as well as its specific role in platelet mediated angiogenesis and tumor metastasis. Using megakaryocyte-specific C3G knockout and transgenic mouse models, we found that both C3G overexpression and deletion promoted platelet-mediated angiogenesis, induced by tumor cell implantation or hindlimb ischemia, through differential release of proangiogenic and antiangiogenic factors. However, only C3G deletion resulted in a higher recruitment of hemangiocytes from the bone marrow. In addition, C3G null expression enhanced thrombopoietin (TPO)-induced platelet production, associated with reduced TPO plasma levels. Moreover, after 5-fluorouracil-induced platelet depletion and rebound, C3G knockout mice showed a defective return to homeostatic platelet levels, indicating impaired platelet turnover. Mechanistically, C3G promotes c-Mpl ubiquitination by inducing Src-mediated c-Cbl phosphorylation and participates in c-Mpl degradation via the proteasome and lysosome systems, affecting TPO internalization. We also unveiled a positive role of platelet C3G in tumor cell-induced platelet aggregation, which facilitated metastatic cell homing and adhesion. Overall, these findings revealed that C3G plays a crucial role in platelet-mediated angiogenesis and metastasis, as well as in platelet level modulation in response to pathogenic stimuli.

Endocytosis of the thrombopoietin receptor Mpl regulates megakaryocyte and erythroid maturation in mice

Dnm2fl/fl Pf4-Cre (Dnm2Plt-/- ) mice lacking the endocytic GTPase dynamin 2 (DNM2) in platelets and megakaryocytes (MKs) develop hallmarks of myelofibrosis. At the cellular level, the tyrosine kinase JAK2 is constitutively active but decreased in expression in Dnm2Plt-/- platelets. Additionally, Dnm2Plt-/- platelets cannot endocytose the thrombopoietin (TPO) receptor Mpl, leading to elevated circulating TPO levels. Here, we assessed whether the hyperproliferative phenotype of Dnm2Plt-/- mice was due to JAK2 constitutive activation or to elevated circulating TPO levels. In unstimulated Dnm2Plt-/- platelets, STAT3 and, to a lower extent, STAT5 were phosphorylated, but their phosphorylation was slowed and diminished upon TPO stimulation. We further crossed Dnm2Plt-/- mice in the Mpl-/- background to generate Mpl-/-Dnm2Plt-/- mice lacking Mpl ubiquitously and DNM2 in platelets and MKs. Mpl-/- Dnm2Plt-/- platelets had severely reduced JAK2 and STAT3 but normal STAT5 expression. Mpl-/- Dnm2Plt-/- mice had severely reduced bone marrow MK and hematopoietic stem and progenitor cell numbers. Additionally, Mpl-/- Dnm2Plt-/- mice had severe erythroblast (EB) maturation defects, decreased expression of hemoglobin and heme homeostasis genes and increased expression of ribosome biogenesis and protein translation genes in spleen EBs, and developed anemia with grossly elevated plasma erythropoietin (EPO) levels, leading to early fatality by postnatal day 25. Mpl-/- Dnm2Plt+/+ mice had impaired EB development at three weeks of age, which normalized with adulthood. Together, the data shows that DNM2-dependent Mpl-mediated endocytosis in platelets and MKs is required for steady-state hematopoiesis and provides novel insights into a developmentally controlled role for Mpl in normal erythropoiesis, regulating hemoglobin and heme production.

Thrombocytopenia in chronic liver disease: Physiopathology and new therapeutic strategies before invasive procedures

Chronic liver disease is characterized by several hematological derangements resulting in a complex and barely rebalanced haemostatic environment. Thrombocytopenia is the most common abnormality observed in these patients and recent advances have led to researchers focus the attention on the multifactorial origin of thrombocytopenia and on the key role of thrombopoietin (TPO) in its physiopathology. Severe thrombocytopenia (platelet count < 50000/μL) complicates the management of patients with chronic liver disease by increasing the potential risk of bleeding for invasive procedures, which may be therefore delayed or canceled even if lifesaving. In the very last years, the development of new drugs which exceed the limits of the current standard of care (platelet transfusions, either immediately before or during the procedure) paves the way to a new scenario in the management of this population of patients. Novel agents, such as the TPO-receptor agonists avatrombopag and lusutrombopag, have been developed in order to increase platelet production as an alternative to platelet transfusions. These agents have demonstrated a good profile in terms of efficacy and safety and will hopefully allow reducing limitations and risks associated with platelet transfusion, without any delay in scheduled interventions. Altogether, it is expected that patients with chronic liver disease will be able to face invasive procedures with one more string in their bow.

The JAK2 mutation

Myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell (HSC) disorders with overproduction of mature myeloid blood cells, including essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF). In 2005, several groups identified a single gain-of-function point mutation JAK2V617F in the majority of MPN patients. The JAK2V617F mutation confers cytokine independent proliferation to hematopoietic progenitor cells by constitutively activating canonical and non-canonical downstream pathways. In this chapter, we focus on (1) the regulation of JAK2, (2) the molecular mechanisms used by JAK2V617F to induce MPNs, (3) the factors that are involved in the phenotypic diversity in MPNs, and (4) the effects of JAK2V617F on hematopoietic stem cells (HSCs). The discovery of the JAK2V617F mutation led to a comprehensive understanding of MPN; however, the question still remains about how one mutation can give rise to three distinct disease entities. Various mechanisms have been proposed, including JAK2V617F allele burden, differential STAT signaling, and host genetic modifiers. In vivo modeling of JAK2V617F has dramatically enhanced the understanding of the pathophysiology of the disease and provided the pre-clinical platform. Interestingly, most of these models do not show an increased hematopoietic stem cell self-renewal and function compared to wildtype controls, raising the question of whether JAK2V617F alone is sufficient to give a clonal advantage in MPN patients. In addition, the advent of modern sequencing technologies has led to a broader understanding of the mutational landscape and detailed JAK2V617F clonal architecture in MPN patients.

In and out: Traffic and dynamics of thrombopoietin receptor

Thrombopoiesis had long been a challenging area of study due to the rarity of megakaryocyte precursors in the bone marrow and the incomplete understanding of its regulatory cytokines. A breakthrough was achieved in the early 1990s with the discovery of the thrombopoietin receptor (TpoR) and its ligand thrombopoietin (TPO). This accelerated research in thrombopoiesis, including the uncovering of the molecular basis of myeloproliferative neoplasms (MPN) and the advent of drugs to treat thrombocytopenic purpura. TpoR mutations affecting its membrane dynamics or transport were increasingly associated with pathologies such as MPN and thrombocytosis. It also became apparent that TpoR affected hematopoietic stem cell (HSC) quiescence while priming hematopoietic stem cells (HSCs) towards the megakaryocyte lineage. Thorough knowledge of TpoR surface localization, dimerization, dynamics and stability is therefore crucial to understanding thrombopoiesis and related pathologies. In this review, we will discuss the mechanisms of TpoR traffic. We will focus on the recent progress in TpoR membrane dynamics and highlight the areas that remain unexplored.

MPL overexpression induces a high level of mutant-CALR/MPL complex: a novel mechanism of ruxolitinib resistance in myeloproliferative neoplasms with CALR mutations

Ruxolitinib (RUX), a JAK1/2-inhibitor, is effective for myeloproliferative neoplasm (MPN) with both JAK2V617 F and calreticulin (CALR) mutations. However, many MPN patients develop resistance to RUX. Although mechanisms of RUX-resistance in cells with JAK2V617 F have already been characterized, those in cells with CALR mutations remain to be elucidated. In this study, we established RUX-resistant human cell lines with CALR mutations and characterized mechanisms of RUX-resistance. Here, we found that RUX-resistant cells had high levels of MPL transcripts, overexpression of both MPL and JAK2, and increased phosphorylation of JAK2 and STAT5. We also found that mature MPL proteins were more stable in RUX-resistant cells. Knockdown of MPL in RUX-resistant cells by shRNAs decreased JAK/STAT signaling. Immunoprecipitation assays showed that binding of mutant CALR to MPL was increased in RUX-resistant cells. Reduction of mutated CALR decreased proliferation of the resistant cells. When resistant cells were cultured in the absence of RUX, the RUX-resistance was reversed, with reduction of the mutant-CALR/MPL complex. In conclusion, MPL overexpression induces higher levels of a mutant-CALR/MPL complex, which may cause RUX-resistance in cells with CALR mutations. This mechanism may be a new therapeutic target to overcome RUX-resistance.

GPIbα is the driving force of hepatic thrombopoietin generation

Thrombopoietin (TPO), a glycoprotein hormone produced predominantly in the liver, plays important roles in the hematopoietic stem cell (HSC) niche, and is essential for megakaryopoiesis and platelet generation. Long-standing understanding proposes that TPO is constitutively produced by hepatocytes, and levels are fine-tuned through platelet and megakaryocyte internalization/degradation via the c-Mpl receptor. However, in immune thrombocytopenia (ITP) and several other diseases, TPO levels are inconsistent with this theory. Recent studies showed that platelets, besides their TPO clearance, can induce TPO production in the liver. Our group also accidentally discovered that platelet glycoprotein (GP) Ibα is required for platelet-mediated TPO generation, which is underscored in both GPIbα-/- mice and patients with Bernard-Soulier syndrome. This review will introduce platelet versatilities and several new findings in hemostasis and platelet consumption but focus on its roles in TPO regulation. The implications of these new discoveries in hematopoiesis and the HSC niche, particularly in ITP, will be discussed.

The Role of the Proteasome in Platelet Function

Platelets are megakaryocyte-derived acellular fragments prepped to maintain primary hemostasis and thrombosis by preserving vascular integrity. Although they lack nuclei, platelets harbor functional genomic mediators that bolster platelet activity in a signal-specific manner by performing limited de novo protein synthesis. Furthermore, despite their limited protein synthesis, platelets are equipped with multiple protein degradation mechanisms, such as the proteasome. In nucleated cells, the functions of the proteasome are well established and primarily include proteostasis among a myriad of other signaling processes. However, the role of proteasome-mediated protein degradation in platelets remains elusive. In this review article, we recapitulate the developing literature on the functions of the proteasome in platelets, discussing its emerging regulatory role in platelet viability and function and highlighting how its functional coupling with the transcription factor NF-κB constitutes a novel potential therapeutic target in atherothrombotic diseases.

HCMV modulates c-Mpl/IEX-1 pathway-mediated megakaryo/thrombopoiesis via PDGFRα and αvβ3 receptors after allo-HSCT

Thrombocytopenia is a common complication of human cytomegalovirus (HCMV) infection in immunocompromised hosts, which contributes to poor prognosis even in patients receiving antiviral treatment. Here, we investigated the megakaryo/thrombopoiesis process, including the involvement of the c-Mpl/IEX-1 pathway, after HCMV infection, identified receptors mediating the interaction between megakaryocytes (MKs) and HCMV, and explored novel therapeutic targets. Our data shows that HCMV directly infects megakaryocytes in patients with HCMV DNAemia and influences megakaryopoiesis via the c-Mpl/IEX-1 pathway throughout megakaryocyte maturation, apoptosis, and platelet generation in vivo and in vitro. After treatment with inhibitors of PDGFRα and αvβ3, the HCMV infection rate in MKs was significantly reduced, suggesting that IMC-3G3 and anti-αvβ3 are potential therapeutic alternatives for viral infection. In summary, our study proposes a possible mechanism and potential treatments for thrombocytopenia caused by HCMV infection and other viral diseases associated with abnormal hemostasis.

The Role of Ubiquitin E3 Ligase in Atherosclerosis

Atherosclerosis is a chronic inflammatory vascular disease. Atherosclerotic cardiovascular disease is the main cause of death in both developed and developing countries. Many pathophysiological factors, including abnormal cholesterol metabolism, vascular inflammatory response, endothelial dysfunction and vascular smooth muscle cell proliferation and apoptosis, contribute to the development of atherosclerosis and the molecular mechanisms underlying the development of atherosclerosis are not fully understood. Ubiquitination is a multistep post-translational protein modification that participates in many important cellular processes. Emerging evidence suggests that ubiquitination plays important roles in the pathogenesis of atherosclerosis in many ways, including regulation of vascular inflammation, endothelial cell and vascular smooth muscle cell function, lipid metabolism and atherosclerotic plaque stability. This review summarizes important contributions of various E3 ligases to the development of atherosclerosis. Targeting ubiquitin E3 ligases may provide a novel strategy for the prevention of the progression of atherosclerosis.

C-Cbl regulates c-MPL receptor trafficking and its internalization

Thrombocyte formation from megakaryocyte and their progenitor cells is tightly regulated by thrombopoietin (TPO) and its receptor c‐MPL, thereby maintaining physiological functionality and numbers of circulating platelets. In patients, dysfunction of this regulation could cause thrombocytopenia or myeloproliferative syndromes. Since regulation of this pathway is still not completely understood, we investigated the role of the ubiquitin ligase c‐Cbl which was previously shown to negatively regulated c‐MPL signalling. We developed a new conditional mouse model using c‐Cbl^fl/flPf4^Cre mice and demonstrated that platelet‐specific knockout of c‐Cbl led to severe microthrombocytosis and impaired uptake of TPO and c‐MPL receptor internalization. Furthermore, we characterized a constitutive STAT5 activation c‐Cbl KO platelets. This study identified c‐Cbl as a potential player in causing megakaryocytic and thrombocytic disorders.

Signaling properties of murine MPL and MPL mutants after stimulation with thrombopoietin and romiplostim

Thrombopoietin (THPO) and its receptor myeloproliferative leukemia virus oncogene (MPL) regulate hematopoietic stem cell (HSC) quiescence and maintenance, but also megakaryopoiesis. Thrombocytopenias or aplastic anemias can be treated today with THPO peptide mimetics (romiplostim) or small-molecule THPO receptor agonists (e.g., eltrombopag). These THPO mimetics were designed for human application; however, many preclinical studies are performed in murine models. We investigated the activation of wild-type murine MPL (mMPL) by romiplostim. Romiplostim stimulated AKT, ERK1/2, and STAT5 phosphorylation without preference for one of these pathways, however, with a four- to fivefold lower phosphorylation intensity at high concentration. Faster internalization of mMPL after romiplostim binding could be one explanation of reduced signaling. In vitro megakaryocyte differentiation, proliferation, and maturation by romiplostim was less efficient compared with stimulation with mTHPO. We further dissected mMPL signaling by lentiviral overexpression of mMPL mutants with tyrosine (Y)-to-phenylalanine (F) substitutions in the distal cytoplasmic tyrosines 582 (Y582F), 616 (Y616F), and 621 (Y621F) individually and in combination (Y616F_Y621F) and in truncated receptors lacking 53 (Δ53) or 69 (Δ69) C-terminal amino acids. Mutation at tyrosine residue Y582F caused a gain-of-function with baseline activation and increased ERK1/2 phosphorylation upon stimulation. In agreement with this, proliferation in Y582F-32D cells was increased, yet did not rescue in vitro megakaryopoiesis from Mpl-deficient cells. Y616F and Y621F mutated receptors exhibited strongly impaired ERK1/2 and decreased AKT signaling and conferred reduced proliferation to 32D cells upon mTHPO stimulation but a partial correction of immature megakaryopoiesis in vitro.

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.

It's reticulated: the liver at the heart of atherosclerosis

Platelets play a critical role in both the initiation and progression of atherosclerosis, and even more so in the ensuing atherothrombotic complications. Low-dose aspirin remains the mainstay of antiplatelet therapy in high-risk patients by reducing the risk of myocardial ischemia, stroke or death due to cardiovascular disease. However, antiplatelet therapies lose their efficacy in people with diabetes mellitus, increasing the risk of future atherothrombotic events. The molecular mechanisms that promote platelet hyperactivity remain unclear but could be due to glycation-induced conformational changes of platelet membranes resulting in impaired aspirin entry or less-efficient acetylation/compensatory increase in COX-2 expression in newborn platelets. Emerging evidence from our laboratory and elsewhere suggest that enhanced platelet turnover (thrombopoiesis), particularly the production of immature reticulated platelets from the bone marrow, could contribute to atherosclerotic complications. We have identified a major role for neutrophil-derived S100A8/A9, a damage-associated molecular pattern, in driving reticulated thrombopoiesis by directly interacting with its receptors on Kupffer cells in the liver. In this review, we discuss the role of hepatic inflammation in driving reticulated platelet production and suggest potential targets to control their production, improve efficacy of current antiplatelet therapies and reduce the risk of atherothrombotic complications.

Beyond COX-1: the effects of aspirin on platelet biology and potential mechanisms of chemoprevention

After more than a century, aspirin remains one of the most commonly used drugs in western medicine. Although mainly used for its anti-thrombotic, anti-pyretic, and analgesic properties, a multitude of clinical studies have provided convincing evidence that regular, low-dose aspirin use dramatically lowers the risk of cancer. These observations coincide with recent studies showing a functional relationship between platelets and tumors, suggesting that aspirin's chemopreventive properties may result, in part, from direct modulation of platelet biology and biochemistry. Here, we present a review of the biochemistry and pharmacology of aspirin with particular emphasis on its cyclooxygenase-dependent and cyclooxygenase-independent effects in platelets. We also correlate the results of proteomic-based studies of aspirin acetylation in eukaryotic cells with recent developments in platelet proteomics to identify non-cyclooxygenase targets of aspirin-mediated acetylation in platelets that may play a role in its chemopreventive mechanism.

CBL family E3 ubiquitin ligases control JAK2 ubiquitination and stability in hematopoietic stem cells and myeloid malignancies

Here, Lv et al. report that the CBL family E3 ubiquitin ligases down-regulate JAK2 stability and signaling via the adaptor protein LNK/SH2B3. Their results reveal a novel signaling axis that regulates JAK2 in normal and malignant HSPCs and suggest new therapeutic strategies for treating CBL^mut myeloid malignancies.

Janus kinase 2 (JAK2) is a central kinase in hematopoietic stem/progenitor cells (HSPCs), and its uncontrolled activation is a prominent oncogenic driver of hematopoietic neoplasms. However, molecular mechanisms underlying the regulation of JAK2 have remained elusive. Here we report that the Casitas B-cell lymphoma (CBL) family E3 ubiquitin ligases down-regulate JAK2 stability and signaling via the adaptor protein LNK/SH2B3. We demonstrated that depletion of CBL/CBL-B or LNK abrogated JAK2 ubiquitination, extended JAK2 half-life, and enhanced JAK2 signaling and cell growth in human cell lines as well as primary murine HSPCs. Built on these findings, we showed that JAK inhibitor (JAKi) significantly reduced aberrant HSPCs and mitigated leukemia development in a mouse model of aggressive myeloid leukemia driven by loss of Cbl and Cbl-b. Importantly, primary human CBL mutated (CBL^mut) leukemias exhibited increased JAK2 protein levels and signaling and were hypersensitive to JAKi. Loss-of-function mutations in CBL E3 ubiquitin ligases are found in a wide range of myeloid malignancies, which are diseases without effective treatment options. Hence, our studies reveal a novel signaling axis that regulates JAK2 in normal and malignant HSPCs and suggest new therapeutic strategies for treating CBL^mut myeloid malignancies.

The genetics of myelodysplastic syndrome: from clonal haematopoiesis to secondary leukaemia

Myelodysplastic syndrome (MDS) is a clonal disease that arises from the expansion of mutated haematopoietic stem cells. In a spectrum of myeloid disorders ranging from clonal haematopoiesis of indeterminate potential (CHIP) to secondary acute myeloid leukaemia (sAML), MDS is distinguished by the presence of peripheral blood cytopenias, dysplastic haematopoietic differentiation and the absence of features that define acute leukaemia. More than 50 recurrently mutated genes are involved in the pathogenesis of MDS, including genes that encode proteins involved in pre-mRNA splicing, epigenetic regulation and transcription. In this Review we discuss the molecular processes that lead to CHIP and further clonal evolution to MDS and sAML. We also highlight the ways in which these insights are shaping the clinical management of MDS, including classification schemata, prognostic scoring systems and therapeutic approaches.

Cytokine Receptor Endocytosis: New Kinase Activity-Dependent and -Independent Roles of PI3K

Type I and II cytokine receptors are cell surface sensors that bind cytokines in the extracellular environment and initiate intracellular signaling to control processes such as hematopoiesis, immune function, and cellular growth and development. One key mechanism that regulates signaling from cytokine receptors is through receptor endocytosis. In this mini-review, we describe recent advances in endocytic regulations of cytokine receptors, focusing on new paradigms by which PI3K controls receptor endocytosis through both kinase activity-dependent and -independent mechanisms. These advances underscore the notion that the p85 regulatory subunit of PI3K has functions beyond regulating PI3K kinase activity, and that PI3K plays both positive and negative roles in receptor signaling. On the one hand, the PI3K/Akt pathway controls various aspects downstream of cytokine receptors. On the other hand, it stimulates receptor endocytosis and downregulation, thus contributing to signaling attenuation.

The Thrombopoietin Receptor: Structural Basis of Traffic and Activation by Ligand, Mutations, Agonists, and Mutated Calreticulin

A well-functioning hematopoietic system requires a certain robustness and flexibility to maintain appropriate quantities of functional mature blood cells, such as red blood cells and platelets. This review focuses on the cytokine receptor that plays a significant role in thrombopoiesis: the receptor for thrombopoietin (TPO-R; also known as MPL). Here, we survey the work to date to understand how this receptor functions at a molecular level throughout its lifecycle, from traffic to the cell surface, dimerization and binding cognate cytokine via its extracellular domain, through to its subsequent activation of associated Janus kinases and initiation of downstream signaling pathways, as well as the regulation of these processes. Atomic level resolution structures of TPO-R have remained elusive. The identification of disease-causing mutations in the receptor has, however, offered some insight into structure and function relationships, as has artificial means of receptor activation, through TPO mimetics, transmembrane-targeting receptor agonists, and engineering in dimerization domains. More recently, a novel activation mechanism was identified whereby mutated forms of calreticulin form complexes with TPO-R via its extracellular N-glycosylated domain. Such complexes traffic pathologically in the cell and persistently activate JAK2, downstream signal transducers and activators of transcription (STATs), and other pathways. This pathologic TPO-R activation is associated with a large fraction of human myeloproliferative neoplasms.

Genetic Alterations of the Thrombopoietin/MPL/JAK2 Axis Impacting Megakaryopoiesis

Megakaryopoiesis is an original and complex cell process which leads to the formation of platelets. The homeostatic production of platelets is mainly regulated and controlled by thrombopoietin (TPO) and the TPO receptor (MPL)/JAK2 axis. Therefore, any hereditary or acquired abnormality affecting this signaling axis can result in thrombocytosis or thrombocytopenia. Thrombocytosis can be due to genetic alterations that affect either the intrinsic MPL signaling through gain-of-function (GOF) activity (MPL, JAK2, CALR) and loss-of-function (LOF) activity of negative regulators (CBL, LNK) or the extrinsic MPL signaling by THPO GOF mutations leading to increased TPO synthesis. Alternatively, thrombocytosis may paradoxically result from mutations of MPL leading to an abnormal MPL trafficking, inducing increased TPO levels by alteration of its clearance. In contrast, thrombocytopenia can also result from LOF THPO or MPL mutations, which cause a complete defect in MPL trafficking to the cell membrane, impaired MPL signaling or stability, defects in the TPO/MPL interaction, or an absence of TPO production.

Hyaluronan Depolymerization by Megakaryocyte Hyaluronidase-2 Is Required for Thrombopoiesis

Hyaluronan is the predominant glycosaminoglycan component of the extracellular matrix with an emerging role in hematopoiesis. Modulation of hyaluronan polymer size is responsible for its control over cellular functions, and the balance of hyaluronan synthesis and degradation determines its molecular size. Although two active somatic hyaluronidases are expressed in mammals, only deficiency in hyaluronidase-2 (Hyal-2) results in thrombocytopenia of unknown mechanism. Our results reveal that Hyal-2 knockout mice accumulate hyaluronan within their bone marrow and within megakaryocytes, the cells responsible for platelet generation. Proplatelet formation by Hyal-2 knockout megakaryocytes was disrupted because of abnormal formation of the demarcation membrane system, which was dilated and poorly developed. Importantly, peptide-mediated delivery of exogenous hyaluronidase rescued deficient proplatelet formation in murine and human megakaryocytes lacking Hyal-2. Together, our data uncover a previously unsuspected mechanism of how hyaluronan and Hyal-2 control platelet generation.

Cholesterol in platelet biogenesis and activation

Hypercholesterolemia is a risk factor for atherothrombotic disease, largely attributed to its impact on atherosclerotic lesional cells such as macrophages. Platelets are involved in immunity and inflammation and impact atherogenesis, primarily by modulating immune and inflammatory effector cells. There is evidence that hypercholesterolemia increases the risk of atherosclerosis and thrombosis by modulating platelet biogenesis and activity. This review highlights recent findings on the impact of aberrant cholesterol metabolism on platelet biogenesis and activity and their relevance in atherosclerosis and thrombosis.

The pathophysiology of thrombocytopenia in chronic liver disease

Thrombocytopenia is the most common hematological abnormality encountered in patients with chronic liver disease (CLD). In addition to being an indicator of advanced disease and poor prognosis, it frequently prevents crucial interventions. Historically, thrombocytopenia has been attributed to hypersplenism, which is the increased pooling of platelets in a spleen enlarged by congestive splenomegaly secondary to portal hypertension. Over the past decade, however, there have been significant advances in the understanding of thrombopoiesis, which, in turn, has led to an improved understanding of thrombocytopenia in cirrhosis. Multiple factors contribute to the development of thrombocytopenia and these can broadly be divided into those that cause decreased production, splenic sequestration, and increased destruction. Depressed thrombopoietin levels in CLD, together with direct bone marrow suppression, result in a reduced rate of platelet production. Thrombopoietin regulates both platelet production and maturation and is impaired in CLD. Bone marrow suppression can be caused by viruses, alcohol, iron overload, and medications. Splenic sequestration results from hypersplenism. The increased rate of platelet destruction in cirrhosis also occurs through a number of pathways: increased shear stress, increased fibrinolysis, bacterial translocation, and infection result in an increased rate of platelet aggregation, while autoimmune disease and raised titers of antiplatelet immunoglobulin result in the immunologic destruction of platelets. An in-depth understanding of the complex pathophysiology of the thrombocytopenia of CLD is crucial when considering treatment strategies. This review outlines the recent advances in our understanding of thrombocytopenia in cirrhosis and CLD.

UBASH3B/Sts-1-CBL axis regulates myeloid proliferation in human preleukemia induced by AML1-ETO

The t(8;21) rearrangement, which creates the AML1-ETO fusion protein, represents the most common chromosomal translocation in acute myeloid leukemia (AML). Clinical data suggest that CBL mutations are a frequent event in t(8;21) AML, but the role of CBL in AML1-ETO-induced leukemia has not been investigated. In this study, we demonstrate that CBL mutations collaborate with AML1-ETO to expand human CD34+ cells both in vitro and in a xenograft model. CBL depletion by shRNA also promotes the growth of AML1-ETO cells, demonstrating the inhibitory function of endogenous CBL in t(8;21) AML. Mechanistically, loss of CBL function confers hyper-responsiveness to thrombopoietin and enhances STAT5/AKT/ERK/Src signaling in AML1-ETO cells. Interestingly, we found the protein tyrosine phosphatase UBASH3B/Sts-1, which is known to inhibit CBL function, is upregulated by AML1-ETO through transcriptional and miR-9-mediated regulation. UBASH3B/Sts-1 depletion induces an aberrant pattern of CBL phosphorylation and impairs proliferation in AML1-ETO cells. The growth inhibition caused by UBASH3B/Sts-1 depletion can be rescued by ectopic expression of CBL mutants, suggesting that UBASH3B/Sts-1 supports the growth of AML1-ETO cells partly through modulation of CBL function. Our study reveals a role of CBL in restricting myeloid proliferation of human AML1-ETO-induced leukemia, and identifies UBASH3B/Sts-1 as a potential target for pharmaceutical intervention.

Targeting glutamine metabolism in myeloproliferative neoplasms

JAK2(V617F) mutation can be detected in the majority of myeloproliferative neoplasm (MPN) patients. The JAK2 inhibitor Ruxolitinib is the first FDA-approved treatment for MPNs. However, its use is limited by various dose related toxicities. Here, we studied the metabolic state and glutamine metabolism of BaF3-hEPOR-JAK2V617F and BaF3-hEPOR-JAK2WT cells. We found that the JAK2(V617F)-mutant cells were associated with increased oxygen consumption rate and extracellular acidification rate than the JAK2(WT) cells and there was an increased glutamine metabolism in JAK2(V617F)-mutant cells compared to wild-type cells. Glutaminase (GLS), the key enzyme in glutamine metabolism, was upregulated in the JAK2(V617F)-mutant BaF3 cells compared to the JAK2(WT) BaF3 cells. In MPN patient peripheral blood CD34+ cells, GLS expression was increased in JAK2(V617F)-mutant progenitor cells compared to JAK2 wild-type progenitor cells from the same patients and GLS levels were increased at the time of disease progression compared to at earlier time points. Moreover, GLS inhibitor increased the growth inhibitory effect of Ruxolitinib in both JAK2(V617F)-mutant cell lines and peripheral blood CD34+ cells from MPN patients. Therefore, GLS inhibitor should be further explored to enhance the therapeutic effectiveness of JAK2 inhibitor and allow the administration of lower doses of the drug to avoid its toxicity.

Inhibition of Thrombopoietin/Mpl Signaling in Adult Hematopoiesis Identifies New Candidates for Hematopoietic Stem Cell Maintenance

Thrombopoietin (Thpo) signals via its receptor Mpl and regulates megakaryopoiesis, hematopoietic stem cell (HSC) maintenance and post-transplant expansion. Mpl expression is tightly controlled and deregulation of Thpo/Mpl-signaling is linked to hematological disorders. Here, we constructed an intracellular-truncated, signaling-deficient Mpl protein which is presented on the cell surface (dnMpl). The transplantation of bone marrow cells retrovirally transduced to express dnMpl into wildtype mice induced thrombocytopenia, and a progressive loss of HSC. The aplastic BM allowed the engraftment of a second BM transplant without further conditioning. Functional analysis of the truncated Mpl in vitro and in vivo demonstrated no internalization after Thpo binding and the inhibition of Thpo/Mpl-signaling in wildtype cells due to dominant-negative (dn) effects by receptor competition with wildtype Mpl for Thpo binding. Intracellular inhibition of Mpl could be excluded as the major mechanism by the use of a constitutive-dimerized dnMpl. To further elucidate the molecular changes induced by Thpo/Mpl-inhibition on the HSC-enriched cell population in the BM, we performed gene expression analysis of Lin-Sca1+cKit+ (LSK) cells isolated from mice transplanted with dnMpl transduced BM cells. The gene expression profile supported the exhaustion of HSC due to increased cell cycle progression and identified new and known downstream effectors of Thpo/Mpl-signaling in HSC (namely TIE2, ESAM1 and EPCR detected on the HSC-enriched LSK cell population). We further compared gene expression profiles in LSK cells of dnMpl mice with human CD34+ cells of aplastic anemia patients and identified similar deregulations of important stemness genes in both cell populations. In summary, we established a novel way of Thpo/Mpl inhibition in the adult mouse and performed in depth analysis of the phenotype including gene expression profiling.

Platelet-Rich Plasma Promotes Axon Regeneration, Wound Healing, and Pain Reduction: Fact or Fiction

Platelet-rich plasma (PRP) has been tested in vitro, in animal models, and clinically for its efficacy in enhancing the rate of wound healing, reducing pain associated with injuries, and promoting axon regeneration. Although extensive data indicate that PRP-released factors induce these effects, the claims are often weakened because many studies were not rigorous or controlled, the data were limited, and other studies yielded contrary results. Critical to assessing whether PRP is effective are the large number of variables in these studies, including the method of PRP preparation, which influences the composition of PRP; type of application; type of wounds; target tissues; and diverse animal models and clinical studies. All these variables raise the question of whether one can anticipate consistent influences and raise the possibility that most of the results are correct under the circumstances where PRP was tested. This review examines evidence on the potential influences of PRP and whether PRP-released factors could induce the reported influences and concludes that the preponderance of evidence suggests that PRP has the capacity to induce all the claimed influences, although this position cannot be definitively argued. Well-defined and rigorously controlled studies of the potential influences of PRP are required in which PRP is isolated and applied using consistent techniques, protocols, and models. Finally, it is concluded that, because of the purported benefits of PRP administration and the lack of adverse events, further animal and clinical studies should be performed to explore the potential influences of PRP.

Hematopoietic stem cell quiescence and function are controlled by the CYLD-TRAF2-p38MAPK pathway

Tesio at al. identify a novel pathway controlled by the tumor suppressor and deubiquitinase cylindromatosis (CYLD), which is involved in the regulation of hematopoietic stem cell quiescence and repopulation potential.

The status of long-term quiescence and dormancy guarantees the integrity of hematopoietic stem cells (HSCs) during adult homeostasis. However the molecular mechanisms regulating HSC dormancy remain poorly understood. Here we show that cylindromatosis (CYLD), a tumor suppressor gene and negative regulator of NF-κB signaling with deubiquitinase activity, is highly expressed in label-retaining dormant HSCs (dHSCs). Moreover, Cre-mediated conditional elimination of the catalytic domain of CYLD induced dHSCs to exit quiescence and abrogated their repopulation and self-renewal potential. This phenotype is dependent on the interactions between CYLD and its substrate TRAF2 (tumor necrosis factor–associated factor 2). HSCs expressing a mutant CYLD with an intact catalytic domain, but unable to bind TRAF2, showed the same HSC phenotype. Unexpectedly, the robust cycling of HSCs lacking functional CYLD–TRAF2 interactions was not elicited by increased NF-κB signaling, but instead by increased activation of the p38MAPK pathway. Pharmacological inhibition of p38MAPK rescued the phenotype of CYLD loss, identifying the CYLD–TRAF2–p38MAPK pathway as a novel important regulator of HSC function restricting HSC cycling and promoting dormancy.

Dynamin 2-dependent endocytosis is required for normal megakaryocyte development in mice

Dynamins are highly conserved large GTPases (enzymes that hydrolyze guanosine triphosphate) involved in endocytosis and vesicle transport, and mutations in the ubiquitous and housekeeping dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in thrombopoiesis, we generated Dnm2(fl/fl) Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2(fl/fl) Pf4-Cre mice had severe macrothrombocytopenia with moderately accelerated platelet clearance. Dnm2-null bone marrow MKs had altered demarcation membrane system formation in vivo due to defective endocytic pathway, and fetal liver-derived Dnm2-null MKs formed proplatelets poorly in vitro, showing that DNM2-dependent endocytosis plays a major role in MK membrane formation and thrombopoiesis. Endocytosis of the thrombopoietin receptor Mpl was impaired in Dnm2-null platelets, causing constitutive phosphorylation of the tyrosine kinase JAK2 and elevated circulating thrombopoietin levels. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as reflected by marked expansion of hematopoietic stem and progenitor cells, MK hyperplasia, myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. Taken together, our data demonstrate that unrestrained MK growth and proliferation results in rapid myelofibrosis and establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis, and bone marrow homeostasis.

Thrombopoiesis

The production of platelets is a complex process that involves hematopoietic stem cells (HSCs), their differentiated progeny, the marrow microenvironment and hematopoietic cytokines. Much has been learned in the 110 years since James Homer Wright postulated that marrow megakaryocytes were responsible for blood platelet production, at a time when platelets were termed the "dust of the blood". In the 1980s a number of in vitro culture systems were developed that could produce megakaryocytes, followed by the identification of several cytokines that could stimulate the process in vitro. However, none of these cytokines produced a substantial thrombocytosis when injected into animals or people, nor were blood levels inversely related to platelet count, the sine qua non of a physiological regulator. A major milestone in our understanding of thrombopoiesis occurred in 1994 when thrombopoietin, the primary regulator of platelet production was cloned and initially characterized. Since that time many of the molecular mechanisms of thrombopoiesis have been identified, including the effects of thrombopoietin on the survival, proliferation, and differentiation of hematopoietic stem and progenitor cells, the development of polyploidy and proplatelet formation, the final fragmentation of megakaryocyte cytoplasm to yield blood platelets, and the regulation of this process. While much progress has been made, several outstanding questions remain, such as the nature of the signals for final platelet formation, the molecular nature of the regulation of marrow stromal thrombopoietin production, and the role of these physiological processes in malignant hematopoiesis.

A hyperactive Mpl-based cell growth switch drives macrophage-associated erythropoiesis through an erythroid-megakaryocytic precursor

Several approaches for controlling hematopoietic stem and progenitor cell expansion, lineage commitment, and maturation have been investigated for improving clinical interventions. We report here that amino acid substitutions in a thrombopoietin receptor (Mpl)--containing cell growth switch (CGS) extending receptor stability improve the expansion capacity of human cord blood CD34(+) cells in the absence of exogenous cytokines. Activation of this CGS with a chemical inducer of dimerization (CID) expands total cells 99-fold, erythrocytes 70-fold, megakaryocytes 0.5-fold, and CD34(+) stem/progenitor cells 4.4-fold by 21 days of culture. Analysis of cells in these expanded populations identified a CID-dependent bipotent erythrocyte-megakaryocyte precursor (PEM) population, and a CID-independent macrophage population. The CD235a(+)/CD41a(+) PEM population constitutes up to 13% of the expansion cultures, can differentiate into erythrocytes or megakaryocytes, exhibits very little expansion capacity, and exists at very low levels in unexpanded cord blood. The CD206(+) macrophage population constitutes up to 15% of the expansion cultures, exhibits high-expansion capacity, and is physically associated with differentiating erythroblasts. Taken together, these studies describe a fundamental enhancement of the CGS expansion platform, identify a novel precursor population in the erythroid/megakaryocytic differentiation pathway of humans, and implicate an erythropoietin-independent, macrophage-associated pathway supporting terminal erythropoiesis in this expansion system.

Molecular genetic biomarkers in myeloid malignancies

CONTEXT

Recent studies using massively parallel sequencing technologies, so-called next-generation sequencing, have uncovered numerous recurrent, single-gene variants or mutations across the spectrum of myeloid malignancies.

OBJECTIVES

To review the recent advances in the understanding of the molecular basis of myeloid neoplasms, including their significance for diagnostic and prognostic purposes and the possible implications for the development of novel therapeutic strategies.

DATA SOURCES

Literature review.

CONCLUSIONS

The recurrent mutations found in myeloid malignancies fall into distinct functional categories. These include (1) cell signaling factors, (2) transcription factors, (3) regulators of the cell cycle, (4) regulators of DNA methylation, (5) regulators of histone modification, (6) RNA-splicing factors, and (7) components of the cohesin complex. As the clinical significance of these mutations and mutation combinations is established, testing for their presence is likely to become a routine part of the diagnostic workup. This review will attempt to establish a framework for understanding these mutations in the context of myeloproliferative neoplasms, myelodysplastic syndromes, and acute myeloid leukemia.

Cbl negatively regulates erythropoietin-induced growth and survival signaling through the proteasomal degradation of Src kinase

We examined the biological functions of the gene Cbl in erythropoietin (EPO) signaling using Cbl-deficient F-36P human erythroleukemia cells by the introduction of the Cbl siRNA expression vector. Knockdown of Cbl promoted EPO-dependent proliferation and survival of F-36P cells, especially at a low concentration of EPO (0.01U/mL), similar to serum concentrations of EPO in healthy volunteers (0.005-0.04U/mL). We found that Src was degraded mainly by the proteasomal pathway because the proteasome inhibitor MG-132 but not the lysosome inhibitor NH4Cl suppressed the EPO-induced degradation of Src in F-36P cells and that knockdown of Cbl inhibited EPO-induced ubiquitination and degradation of Src in F-36P cells. The experiments using the Src inhibitor PP1 and co-expression experiments further confirmed that Cbl and the kinase activity of Src are required for the EPO-induced ubiquitination of Src. In addition, the co-expression experiments and in vitro kinase assay demonstrated that the EPO-induced tyrosine phosphorylation and ubiquitination of Cbl were dependent on the kinase activity of Src but not Jak2. Thus, Cbl negatively regulates EPO signaling mainly through the proteasome-dependent degradation of Src, and the E3 ligase activity of Cbl and its tyrosine phosphorylation are regulated by Src but not Jak2.

PMA induces SnoN proteolysis and CD61 expression through an autocrine mechanism

Phorbol-12-myristate-13-acetate, also called PMA, is a small molecule that activates protein kinase C and functions to differentiate hematologic lineage cells. However, the mechanism of PMA-induced cellular differentiation is not fully understood. We found that PMA triggers global enhancement of protein ubiquitination in K562, a myelogenous leukemia cell line and one of the enhanced-ubiquitination targets is SnoN, an inhibitor of the Smad signaling pathway. Our data indicated that PMA stimulated the production of Activin A, a cytokine of the TGF-β family. Activin A then activated the phosphorylation of both Smad2 and Smad3. In consequence, SnoN is ubiquitinated by the APC(Cdh1) ubiquitin ligase with the help of phosphorylated Smad2. Furthermore, we found that SnoN proteolysis is important for the expression of CD61, a marker of megakaryocyte. These results indicate that protein ubiquitination promotes megakaryopoiesis via degrading SnoN, an inhibitor of CD61 expression, strengths the roles of ubiquitination in cellular differentiation.

Phosphorylated c-Mpl tyrosine 591 regulates thrombopoietin-induced signaling

Thrombopoietin (TPO) is the primary regulator of platelet production, affecting cell survival, proliferation, and differentiation through binding to and stimulation of the cell surface receptor the cellular myeloproliferative leukemia virus oncogene (c-Mpl). Activating mutations in c-Mpl constitutively stimulate downstream signaling pathways, leading to aberrant hematopoiesis, and contribute to development of myeloproliferative neoplasms. Several studies have mapped the tyrosine residues within the cytoplasmic domain of c-Mpl that mediate these cellular signals; however, secondary signaling pathways are incompletely understood. In this study, we focused on c-Mpl tyrosine 591 (Y591). We found Y591 of wild-type c-Mpl to be phosphorylated in the presence of TPO. Additionally, eliminating Y591 phosphorylation by mutation to Phe resulted in decreased total receptor phosphorylation. Using a Src homology 2/phosphotyrosine-binding (SH2/PTB) domain binding microarray, we identified novel c-Mpl binding partners for phosphorylated Y591, including Src homology region 2 domain-containing phosphatase-1 (SHP-1), spleen tyrosine kinase (SYK) and Bruton's tyrosine kinase (BTK). The functional significance of binding partners was determined through small interfering RNA treatment of Ba/F3-Mpl cells, confirming that the increase in pERK1/2 resulting from removal of Y591 may be mediated by spleen tyrosine kinase. These findings identify a novel negative regulatory pathway that controls TPO-mediated signaling, advancing our understanding of the mechanisms required for successful maintenance of hematopoietic stem cells and megakaryocyte development.

Thrombopoietin from beginning to end

In the two decades since its cloning, thrombopoietin (TPO) has emerged not only as a critical haematopoietic cytokine, but also serves as a great example of bench-to-bedside research. Thrombopoietin, produced by the liver, is the primary regulator of megakaryocyte progenitor expansion and differentiation. Additionally, as TPO is vital for the maintenance of haematopoietic stem cells, it can truly be described as a pan-haematopoietic cytokine. Since recombinant TPO became available, the molecular mechanisms of TPO function have been the subject of extensive research. Via its receptor, c-Mpl (also termed MPL), TPO activates a wide array of downstream signalling pathways, promoting cellular survival and proliferation. Due to its central, non-redundant role in haematopoiesis, alterations of both the hormone and its receptor contribute to human disease; congenital and acquired states of thrombocytosis and thrombocytopenia and aplastic anaemia as a result from dysregulated TPO expression or functional alterations of c-Mpl. With TPO mimetics now in clinical use, the story of this haematopoietic cytokine represents a great success for biomedical research.

Activation of Janus kinase/signal transducers and activators of transcription pathway involved in megakaryocyte proliferation induced by vanadium resembles some aspects of essential thrombocythemia

Vanadium (V) is an air pollutant released into the atmosphere by burning fossil fuels. Also, it has been recently evaluated for their carcinogenic potential to establish permissible limits of exposure at workplaces. We previously reported an increase in the number and size of platelets and their precursor cells and megakaryocytes in bone marrow and spleen. The aim of this study was to identify the involvement of Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway and thrombopoietin (TPO) receptor, and myeloproliferative leukemia virus oncogene (Mpl), in megakaryocyte proliferation induced by this compound. Mice were exposed twice a week to vanadium pentoxide inhalation (0.02 M) and were killed at 4th, 6th, and 8th week of exposure. Phosphorylated JAK2 (JAK2 ph), STAT3 (STAT3 ph), STAT5, and Mpl were identified in mice spleen megakaryocytes by cytofluorometry and immunohistochemistry. An increase in JAK2 ph and STAT3 ph, but a decrease in Mpl at 8-week exposure was identified in our findings. Taking together, we propose that the morphological findings, JAK/STAT activation, and decreased Mpl receptor induced by V leads to a condition comparable to essential thrombocythemia, so the effect on megakaryocytes caused by different mechanisms is similar. We also suggest that the decrease in Mpl is a negative feedback mechanism after the JAK/STAT activation. Since megakaryocytes are platelet precursors, their alteration affects platelet morphology and function, which might have implications in hemostasis as demonstrated previously, so it is important to continue evaluating the effects of toxics and pollutants on megakaryocytes and platelets.

Protein degradation systems in platelets

Protein synthesis and degradation are essential processes that allow cells to survive and adapt to their surrounding milieu. In nucleated cells, the degradation and/or cleavage of proteins is required to eliminate aberrant proteins. Cells also degrade proteins as a mechanism for cell signalling and complex cellular functions. Although the last decade has convincingly shown that platelets synthesise proteins, the roles of protein degradation in these anucleate cytoplasts are less clear. Here we review what is known about protein degradation in platelets placing particular emphasis on the proteasome and the cysteine protease calpain.

CBL linker region and RING finger mutations lead to enhanced granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling via elevated levels of JAK2 and LYN

Juvenile myelomonocytic leukemia (JMML) is characterized by hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF). SHP2, NF-1, KRAS, and NRAS are mutated in JMML patients, leading to aberrant regulation of RAS signaling. A subset of JMML patients harbor CBL mutations associated with 11q acquired uniparental disomy. Many of these mutations are in the linker region and the RING finger of CBL, leading to a loss of E3 ligase activity. We investigated the mechanism by which CBL-Y371H, a linker region mutant, and CBL-C384R, a RING finger mutant, lead to enhanced GM-CSF signaling. Expression of CBL mutants in the TF-1 cell line resulted in enhanced survival in the absence of GM-CSF. Cells expressing CBL mutations displayed increased phosphorylation of GM-CSF receptor βc subunit in response to stimulation, although expression of total GM-CSFR βc was lower. This suggested enhanced kinase activity downstream of GM-CSFR. JAK2 and LYN kinase expression is elevated in CBL-Y371H and CBL-C384R mutant cells, resulting in enhanced phosphorylation of CBL and S6 in response to GM-CSF stimulation. Incubation with the JAK2 inhibitor, TG101348, abolished the increased phosphorylation of GM-CSFR βc in cells expressing CBL mutants, whereas treatment with the SRC kinase inhibitor dasatinib resulted in equalization of GM-CSFR βc phosphorylation signal between wild type CBL and CBL mutant samples. Dasatinib treatment inhibited the elevated phosphorylation of CBL-Y371H and CBL-C384R mutants. Our study indicates that CBL linker and RING finger mutants lead to enhanced GM-CSF signaling due to elevated kinase expression, which can be blocked using small molecule inhibitors targeting specific downstream pathways.