Pronounced thrombocytosis in transgenic mice expressing reduced levels of Mpl in platelets and terminally differentiated megakaryocytes

Molecular Pathogenesis of Myeloproliferative Neoplasms: From Molecular Landscape to Therapeutic Implications

Despite distinct clinical entities, the myeloproliferative neoplasms (MPN) share morphological similarities, propensity to thrombotic events and leukemic evolution, and a complex molecular pathogenesis. Well-known driver mutations, JAK2, MPL and CALR, determining constitutive activation of JAK-STAT signaling pathway are the hallmark of MPN pathogenesis. Recent data in MPN patients identified the presence of co-occurrence somatic mutations associated with epigenetic regulation, messenger RNA splicing, transcriptional mechanism, signal transduction, and DNA repair mechanism. The integration of genetic information within clinical setting is already improving patient management in terms of disease monitoring and prognostic information on disease progression. Even the current therapeutic approaches are limited in disease-modifying activity, the expanding insight into the genetic basis of MPN poses novel candidates for targeted therapeutic approaches. This review aims to explore the molecular landscape of MPN, providing a comprehensive overview of the role of drive mutations and additional mutations, their impact on pathogenesis as well as their prognostic value, and how they may have future implications in therapeutic management.

Carica papaya ameliorates thrombocytopenia through upregulation of Interleukin-11 and modulation of thrombopoietin in mouse model of carboplatin-induced myelosuppression

INTRODUCTION

Carica papaya L. (C. papaya) is used as a folk medicine for the treatment of various diseases throughout the world. Recently, papaya leaves decoction has been effectively used for the prevention and treatment of thrombocytopenia. The current study was undertaken to evaluate the thrombopoietic and immunomodulatory activities of C. papaya leaves in the mouse model of carboplatin induced myelosuppression.

METHODS

Myelosuppression was induced by a single intraperitoneal injection of carboplatin (125 mg/kg b. w.). Aqueous extract of C. papaya leaves (15 mg/kg b. w.) was given orally by feeding tube from day 0-18 to preventive group to see the preventive effect and from day 6-18 to treatment group for treatment effect.

RESULTS

The results showed that the C. papaya leaves extract significantly decreased the fall in platelet count in preventive and treatment groups. Extract significantly prevented the fall in total WBCs count on day 12 and 18 in the preventive group, whereas it significantly elevated the WBCs count in treatment group on day 18. Significantly increased RBCs count in both groups was observed on day 18 after treatment with C. papaya leaves extract. Treatment with C. papaya leaves extract significantly upregulated the mRNA expression levels of thrombopoietic cytokine IL-11 in both preventive and treatment groups. It is also observed that restoration of normal platelet count might have been resulted owing to the synergistic effect of upregulated IL-11 which ultimately led to a significantly diminished TPO expression.

CONCLUSION

Our data suggest that aqueous extract of C. papaya leaves possesses significant preventive and curative properties against thrombocytopenia.

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.

The Thrombopoietin Receptor, MPL, Is a Therapeutic Target of Opportunity in the MPN

The myeloproliferative neoplasms, polycythemia vera, essential thrombocytosis and primary myelofibrosis share driver mutations that either activate the thrombopoietin receptor, MPL, or indirectly activate it through mutations in the gene for JAK2, its cognate tyrosine kinase. Paradoxically, although the myeloproliferative neoplasms are classified as neoplasms because they are clonal hematopoietic stem cell disorders, the mutations affecting MPL or JAK2 are gain-of-function, resulting in increased production of normal erythrocytes, myeloid cells and platelets. Constitutive JAK2 activation provides the basis for the shared clinical features of the myeloproliferative neoplasms. A second molecular abnormality, impaired posttranslational processing of MPL is also shared by these disorders but has not received the recognition it deserves. This abnormality is important because MPL is the only hematopoietic growth factor receptor expressed in hematopoietic stem cells; because MPL is a proto-oncogene; because impaired MPL processing results in chronic elevation of plasma thrombopoietin, and since these diseases involve normal hematopoietic stem cells, they have proven resistant to therapies used in other myeloid neoplasms. We hypothesize that MPL offers a selective therapeutic target in the myeloproliferative neoplasms since impaired MPL processing is unique to the involved stem cells, while MPL is required for hematopoietic stem cell survival and quiescent in their bone marrow niches. In this review, we will discuss myeloproliferative neoplasm hematopoietic stem cell pathophysiology in the context of the behavior of MPL and its ligand thrombopoietin and the ability of thrombopoietin gene deletion to abrogate the disease phenotype in vivo in a JAK2 V617 transgenic mouse model of PV.

The Role of Megakaryocytes in Myelofibrosis

Megakaryocytes give rise to platelets, which have a wide variety of functions in coagulation, immune response, inflammation, and tissue repair. Dysregulation of megakaryocytes is a key feature of in the myeloproliferative neoplasms, especially myelofibrosis. Megakaryocytes are among the main drivers of myelofibrosis by promoting myeloproliferation and bone marrow fibrosis. In vivo targeting of megakaryocytes by genetic and pharmacologic approaches ameliorates the disease, underscoring the important role of megakaryocytes in myeloproliferative neoplasms. Here we review the current knowledge of the function of megakaryocytes in the JAK2, CALR, and MPL-mutant myeloproliferative neoplasms.

Megakaryocytes use in vivo podosome-like structures working collectively to penetrate the endothelial barrier of bone marrow sinusoids

BACKGROUND

Blood platelets are anucleate cell fragments that prevent bleeding and minimize blood vessel injury. They are formed from the cytoplasm of megakaryocytes located in the bone marrow. For successful platelet production, megakaryocyte fragments must pass through the sinusoid endothelial barrier by a cell biology process unique to these giant cells as compared with erythrocytes and leukocytes. Currently, the mechanisms by which megakaryocytes interact and progress through the endothelial cells are not understood, resulting in a significant gap in our knowledge of platelet production.

OBJECTIVE

The aim of this study was to investigate how megakaryocytes interact and progress through the endothelial cells of mouse bone marrow sinusoids.

METHODS

We used a combination of fluorescence, electron, and three-dimensional microscopy to characterize the cellular events between megakaryocytes and endothelial cells.

RESULTS

We identified protrusive, F-actin-based podosome-like structures, called in vivo-MK podosomes, which initiate the formation of pores through endothelial cells. These structures present a collective and spatial organization through their interconnection via a contractile network of actomyosin, essential to regulate the endothelial openings. This ensures proper passage of megakaryocyte-derived processes into the blood circulation to promote thrombopoiesis.

CONCLUSION

This study provides novel insight into the in vivo function of podosomes of megakaryocytes with critical importance to platelet production.

MPN: The Molecular Drivers of Disease Initiation, Progression and Transformation and their Effect on Treatment

Myeloproliferative neoplasms (MPNs) constitute a group of disorders identified by an overproduction of cells derived from myeloid lineage. The majority of MPNs have an identifiable driver mutation responsible for cytokine-independent proliferative signalling. The acquisition of coexisting mutations in chromatin modifiers, spliceosome complex components, DNA methylation modifiers, tumour suppressors and transcriptional regulators have been identified as major pathways for disease progression and leukemic transformation. They also confer different sensitivities to therapeutic options. This review will explore the molecular basis of MPN pathogenesis and specifically examine the impact of coexisting mutations on disease biology and therapeutic options.

Thrombopoietin is required for full phenotype expression in a JAK2V617F transgenic mouse model of polycythemia vera

The myeloproliferative neoplasms, polycythemia vera, essential thrombocytosis and primary myelofibrosis are hematopoietic stem cell disorders and share driver mutations that either directly activate the thrombopoietin receptor, MPL, or activate it indirectly through gain-of-function mutations in the gene for JAK2, its cognate tyrosine kinase. Paradoxically, MPL surface expression in hematopoietic stem cells is also reduced in the myeloproliferative neoplasms due to abnormal post-translational glycosylation and premature destruction of JAK2, suggesting that the myeloproliferative neoplasms are disorders of MPL processing since MPL is the only hematopoietic growth factor receptor in hematopoietic stem cells. To examine this possibility, we genetically manipulated MPL expression and maturation in a JAK2^V617F transgenic mouse model of polycythemia vera. Elimination of MPL expression completely abrogated the polycythemia vera phenotype in this JAK2^V617F transgenic mouse model, which could only be partially restored by expression of one MPL allele. Most importantly, elimination of thrombopoietin gene expression abrogated the polycythemia vera phenotype in this JAK2^V617F transgenic mouse model, which could be completely restored by expression of a single thrombopoietin allele. These data indicate that polycythemia vera is in part a thrombopoietin-dependent disorder and that targeting the MPL-thrombopoietin axis could be an effective, nonmyelotoxic therapeutic strategy in this disorder.

Current Advances in Red Blood Cell Generation Using Stem Cells from Diverse Sources

Blood transfusions hold an indispensable part in the modern healthcare system. Up to date, the blood supply is largely dependent on donations. Unfortunately, collecting the clinical-grade blood products has become a challenging mission due to accelerated population aging, which not only increases the need for blood transfusions but also decreases the number of healthy donors. Moreover, individuals with severe hematological abnormalities or rare blood phenotypes need alternative therapeutic approaches instead of conventional blood transfusion. In these aspects, the concept of in vitro/ex vivo production of blood cells has been emerging and many attempts have been focused on manufacturing mature erythrocytes, so-called red blood cells (RBCs), the most common and important component among the blood derivatives. In this review, we provide a general overview regarding the current strategies for generating RBCs from various stem cell sources including pluripotent stem cells (PSCs) as well as circulating blood stem cells and the remaining challenges that must be overcome prior to their practical application.

Evaluating the Role of Janus Kinase Pathways in Platelet Homeostasis Using a Systems Modeling Approach

Maintaining platelet homeostasis is important to avoid spontaneous bleeding and organ damage. Thrombopoietin, the primary regulator of platelet production, is affected by and acts in part via Janus kinase (JAK)‐signal transducer and activator of transcription (STAT)–mediated mechanisms. Interleukin‐6 is also partly responsible for inducing thrombopoietin production via the JAK‐STAT pathway. Although current understanding suggests that JAK2 is a primary mediator of platelet regulation, the emerging data show that a JAK1‐specific inhibitor resulted in the modulation of platelet numbers following dosing. To gain a mechanistic understanding, a model describing platelet regulation based on known physiology and JAK‐STAT pathways was built. The model provides a tool to coalesce biological understanding of platelet physiology and an in silico experimental platform to explore drug effects on platelet homeostasis. In this article, we explain the model construction and demonstrate the use of JAK‐inhibitor programs as informing probes of the physiology, gaining insights on dosing paradigms that avoid platelet‐related safety concerns.

The role of the thrombopoietin receptor MPL in myeloproliferative neoplasms: recent findings and potential therapeutic applications

Introduction: Classical Myeloproliferative Neoplasms (MPNs) include three disorders: Polycythemia Vera (PV), Essential Thrombocythemia (ET) and Primary Myelofibrosis (PMF). MPNs are associated with constitutive activation of JAK2 leading to persistent cell signaling downstream of the dimeric myeloid cytokine receptors due to mutations in three genes encoding JAK2, calreticulin (CALR) and the thrombopoietin (TPO) receptor (MPL or TPOR). CALR and MPL mutants induce JAK2 activation that depends on MPL expression, thus explaining why they induce megakaryocyte pathologies including ET and PMF, but not PV. In contrast, JAK2 V617F drives all three diseases as it induces persistent signaling via EPOR, G-CSFR (CSF3R) and MPL. Areas Covered: Here, we review how different pathogenic mutations of MPL are translated into active receptors by inducing stable dimerization. We focus on the unique role of MPL on the hematopoietic stem cell (HSC), explaining why MPL is indispensable for the development of all MPNs. Last but not least, we describe how CALR mutants are pathogenic via binding and activation of MPL. Expert Opinion: Altogether, we believe that MPL is an important, but challenging, therapeutic target in MPNs that requires novel strategies to interrupt the specific conformational changes induced by each mutation or pathologic interaction without compromising the key functions of wild type MPL.

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.

Identification of MPL R102P Mutation in Hereditary Thrombocytosis

The molecular basis of hereditary thrombocytosis is germline mutations affecting the thrombopoietin (TPO)/TPO receptor (MPL)/JAK2 signaling axis. Here, we report one family presenting two cases with a mild thrombocytosis. By sequencing JAK2 and MPL coding exons, we identified a germline MPL R102P heterozygous mutation in the proband and his daughter. Concomitantly, we detected high TPO levels in the serum of these two patients. The mutation was not found in three other unaffected cases from the family except in another proband's daughter who did not present thrombocytosis but had a high TPO level. The MPL R102P mutation was first described in congenital amegakaryocytic thrombocytopenia in a homozygous state with a loss-of-function activity. It was previously shown that MPL R102P was blocked in the endoplasmic reticulum without being able to translocate to the plasma membrane. Thus, this case report identifies for the first time that MPL R102P mutation can differently impact megakaryopoiesis: thrombocytosis or thrombocytopenia depending on the presence of the heterozygous or homozygous state, respectively. The paradoxical effect associated with heterozygous MPL R102P may be due to subnormal cell-surface expression of wild-type MPL in platelets inducing a defective TPO clearance. As a consequence, increased TPO levels may activate megakaryocyte progenitors that express a lower, but still sufficient level of MPL for the induction of proliferation.

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.

An incomplete trafficking defect to the cell-surface leads to paradoxical thrombocytosis for human and murine MPL P106L

The mechanisms behind the hereditary thrombocytosis induced by the thrombopoietin (THPO) receptor MPL P106L mutant remain unknown. A complete trafficking defect to the cell surface has been reported, suggesting either weak constitutive activity or nonconventional THPO-dependent mechanisms. Here, we report that the thrombocytosis phenotype induced by MPL P106L belongs to the paradoxical group, where low MPL levels on platelets and mature megakaryocytes (MKs) lead to high serum THPO levels, whereas weak but not absent MPL cell-surface localization in earlier MK progenitors allows response to THPO by signaling and amplification of the platelet lineage. MK progenitors from patients showed no spontaneous growth and responded to THPO, and MKs expressed MPL on their cell surface at low levels, whereas their platelets did not respond to THPO. Transduction of MPL P106L in CD34⁺ cells showed that this receptor was more efficiently localized at the cell surface on immature than on mature MKs, explaining a proliferative response to THPO of immature cells and a defect in THPO clearance in mature cells. In a retroviral mouse model performed in Mpl^-/- mice, MPL P106L could induce a thrombocytosis phenotype with high circulating THPO levels. Furthermore, we could select THPO-dependent cell lines with more cell-surface MPL P106L localization that was detected by flow cytometry and [¹²⁵I]-THPO binding. Altogether, these results demonstrate that MPL P106L is a receptor with an incomplete defect in trafficking, which induces a low but not absent localization of the receptor on cell surface and a response to THPO in immature MK cells.

Recent advances in understanding myelofibrosis and essential thrombocythemia

The classic BCR-ABL-negative myeloproliferative neoplasms (MPNs), a form of chronic malignant hemopathies, have been classified into polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). ET and PMF are two similar disorders in their pathogenesis, which is marked by a key role of the megakaryocyte (MK) lineage. Whereas ET is characterized by MK proliferation, PMF is also associated with aberrant MK differentiation (myelodysplasia), leading to the release of cytokines in the marrow environment, which causes the development of myelofibrosis. Thus, PMF is associated with both myeloproliferation and different levels of myelodysplastic features. MPNs are mostly driven by mutated genes called MPN drivers, which abnormally activate the cytokine receptor/JAK2 pathway and their downstream effectors. The recent discovery of CALR mutations has closed a gap in our knowledge and has shown that this mutated endoplasmic reticulum chaperone activates the thrombopoietin receptor MPL and JAK2. These genetic studies have shown that there are two main types of MPNs: JAK2V617F-MPNs, including ET, PV, and PMF, and the MPL-/CALR-MPNs, which include only ET and PMF. These MPN driver mutations are associated with additional mutations in genes involved in epigenetics, splicing, and signaling, which can precede or follow the acquisition of MPN driver mutations. They are involved in clonal expansion or phenotypic changes or both, leading to myelofibrosis or leukemic transformation or both. Only a few patients with ET exhibit mutations in non-MPN drivers, whereas the great majority of patients with PMF harbor one or several mutations in these genes. However, the entire pathogenesis of ET and PMF may also depend on other factors, such as the patient’s constitutional genetics, the bone marrow microenvironment, the inflammatory response, and age. Recent advances allowed a better stratification of these diseases and new therapeutic approaches with the development of JAK2 inhibitors.

Cross-talk between PRMT1-mediated methylation and ubiquitylation on RBM15 controls RNA splicing

RBM15, an RNA binding protein, determines cell-fate specification of many tissues including blood. We demonstrate that RBM15 is methylated by protein arginine methyltransferase 1 (PRMT1) at residue R578, leading to its degradation via ubiquitylation by an E3 ligase (CNOT4). Overexpression of PRMT1 in acute megakaryocytic leukemia cell lines blocks megakaryocyte terminal differentiation by downregulation of RBM15 protein level. Restoring RBM15 protein level rescues megakaryocyte terminal differentiation blocked by PRMT1 overexpression. At the molecular level, RBM15 binds to pre-messenger RNA intronic regions of genes important for megakaryopoiesis such as GATA1, RUNX1, TAL1 and c-MPL. Furthermore, preferential binding of RBM15 to specific intronic regions recruits the splicing factor SF3B1 to the same sites for alternative splicing. Therefore, PRMT1 regulates alternative RNA splicing via reducing RBM15 protein concentration. Targeting PRMT1 may be a curative therapy to restore megakaryocyte differentiation for acute megakaryocytic leukemia.

DOI:http://dx.doi.org/10.7554/eLife.07938.001

The many different cell types in an adult animal all develop from a single fertilized egg. The development of cells into more specialized cell types is called ‘differentiation’. Proteins and other molecules from both inside and outside of the cells regulate the differentiation process.

RNA is a molecule that is similar to DNA, and performs several important roles inside cells. Perhaps most importantly, RNA molecules act as messengers and carry genetic instructions during gene expression. RBM15 is an RNA-binding protein that is found throughout nature, and is involved in a number of developmental processes. Previous research has linked the incorrect control of RBM15 with an increased risk of certain cancers, including megakaryocytic leukemia. However, it is not clear what role RNA-binding proteins such as RBM15 play during differentiation.

Now, Zhang, Tran, Su et al. have investigated the role of RBM15 during the development of large cells found in human bone marrow (called megakaryocytes). First, the experiments demonstrated that an enzyme called PRMT1 modifies RBM15. This enzyme adds a chemical mark called a methyl group at a specific site (an arginine amino acid) on the RNA-binding protein. Next, Zhang, Tran, Su et al. showed that the addition of this methyl group earmarks RBM15 for destruction. This means that an increase in PRMT1 levels reduces the amount of RBM15 in cells, while decreases in PRMT1 have the opposite effect.

Further experiments showed that RBM15 normally processes the RNA messengers that carry the genetic instructions needed for the differentiation of bone marrow cells. An excess of PRMT1 enzyme leads to a lack of this RNA-binding protein. This in turn interferes with the differentiation process, and can contribute to the development of cancers such as megakaryocytic leukemia. Future work will therefore explore whether targeting PRMT1 with drugs could represent an effective treatment for these kinds of cancers.

DOI:http://dx.doi.org/10.7554/eLife.07938.002

Platelet studies in autism spectrum disorder patients and first-degree relatives

BACKGROUND

Platelets have been proven to be a useful cellular model to study some neuropathologies, due to the overlapping biological features between neurons and platelets as granule secreting cells. Altered platelet dense granule morphology was previously reported in three autism spectrum disorder (ASD) patients with chromosomal translocations that disrupted ASD candidate genes NBEA, SCAMP5, and AMYSIN, but a systematic analysis of platelet function in ASD is lacking in contrast to numerous reports of elevated serotonin levels in platelets and blood as potential biomarker for ASD.

METHODS

We explored platelet count, size, epinephrine-induced activation, and dense granule ATP secretion in a cohort of 159 ASD patients, their 289 first-degree relatives (103 unaffected siblings, 99 mothers, and 87 fathers), 45 adult controls, and 65 pediatric controls. For each of the responses separately, a linear mixed model with gender as a covariate was used to compare the level between groups. We next investigated the correlation between platelet function outcomes and severity of impairments in social behavior (social responsiveness score (SRS)).

RESULTS

The average platelet count was increased in ASD patients and siblings vs. controls (ASD 320.3 × 10(9)/L, p = 0.003; siblings 332.0 × 10(9)/L, p < 0.001; controls 283.0 × 10(9)/L). The maximal platelet secretion-dependent aggregation response to epinephrine was not significantly lower for ASD patients. However, secondary wave responses following stimulation with epinephrine were more frequently delayed or absent compared to controls (ASD 52 %, siblings 45 %, parents 53 %, controls 22 %, p = 0.002). In addition, stimulated release of ATP from dense granules was reduced in ASD patients, siblings, and parents vs. controls following activation of platelets with either collagen (ASD 1.54 μM, p = 0.001; siblings 1.51 μM, p < 0.001; parents 1.67 μM, p = 0.021; controls 2.03 μM) or ADP (ASD 0.96 μM, p = 0.003; siblings 1.00 μM, p = 0.012; parents 1.17 μM, p = 0.21; controls 1.40 μM). Plasma serotonin levels were increased for ASD patients (n = 20, p = 0.005) and siblings (n = 20, p = 0.0001) vs. controls (n = 16). No significant correlations were found in the different groups between SRS scores and count, size, epinephrine aggregation, or ATP release.

CONCLUSIONS

We report increased platelet counts, decreased platelet ATP dense granule secretion, and increased serotonin plasma levels not only in ASD patients but also in their first-degree relatives. This suggests that potential genetic factors associated with platelet counts and granule secretion can be associated with, but are not fully penetrant for ASD.

The thrombopoietin receptor P106L mutation functionally separates receptor signaling activity from thrombopoietin homeostasis

The interaction between thrombopoietin (THPO) and its receptor c-Mpl regulates downstream cytokine signaling and platelet homeostasis. Hereditary mutations of c-Mpl can either result in loss-of-function and thrombocytopenia or in gain-of-function and thrombocythemia (HT), and are important models to analyze the mechanism of c-Mpl activity. We have analyzed the effect of the c-Mpl P106L gain-of-function and the nearby loss-of-function R102P and F104S mutations, which cause HT or thrombocytopenia, respectively, on posttranslational processing, intracellular trafficking, cell surface expression, and cell proliferation. In contrast to R102P and F104S, the P106L mutant confers cytokine-independent growth and stimulates downstream signaling after THPO treatment in Ba/F3 cells. Despite their opposite function, R102P and P106L, both lead to abnormal subcellular receptor distribution, lack of membrane localization, impaired glycosylation, and elevated THPO serum levels in effected patients. These findings indicate that the activation of downstream signaling by c-Mpl P106L does not require correct processing, trafficking, and cell surface expression of c-Mpl, whereas the negative feedback loop controlling THPO serum levels requires cell surface expression of the receptor. Thus, we propose that the P106L mutation functionally separates the activity of c-Mpl in downstream signaling from that in maintaining platelet homeostasis.

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.

The thrombopoietin receptor, MPL, is critical for development of a JAK2V617F-induced myeloproliferative neoplasm

The most frequent contributing factor in Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) is the acquisition of a V617F mutation in Janus kinase 2 (JAK2) in hematopoietic stem cells (HSCs). Recent evidence has demonstrated that to drive MPN transformation, JAK2V617F needs to directly associate with a functional homodimeric type I cytokine receptor, suggesting that, although acquiring JAK2V617F may promote disease, there are additional cellular components necessary for MPN development. Here we show that loss of the thrombopoietin (TPO) receptor (MPL) significantly ameliorates MPN development in JAK2V617F(+) transgenic mice, whereas loss of TPO only mildly affects the disease phenotype. Specifically, compared with JAK2V617F(+) mice, JAK2V617F(+)Mpl(-/-) mice exhibited reduced thrombocythemia, neutrophilia, splenomegaly, and neoplastic stem cell pool. The importance of MPL is highlighted as JAK2V617FMpl(+/-) mice displayed a significantly reduced MPN phenotype, indicating that Mpl level may have a substantial effect on MPN development and severity. Splenomegaly and the increased neoplastic stem cell pool were retained in JAK2V617F(+)Tpo(-/-) mice, although thrombocytosis was reduced compared with JAK2V617F(+) mice. These results demonstrate that Mpl expression, but not Tpo, is fundamental in the development of JAK2V617F(+) MPNs, highlighting an entirely novel target for therapeutic intervention.

Haemorrhagic and thrombotic diatheses in mouse models with thrombocytosis

We studied haemostasis in two mouse models with thrombocytosis caused by different pathogenic mechanisms. In one strain (Yall;Mpl-/-) thrombocytosis is driven by a misbalance between thrombopoietin and its receptor, whereas in the other strain, thrombocytosis is caused by expressing a human JAK2-V617F transgene (FF1) that depends on activation by Cre-recombinase (VavCre;FF1, MxCre;FF1). Thrombotic responses were increased following some, but not all types of challenges. In a vaso-occlusive thrombotic model following collagen-adrenaline injection we found increased mortality in both strains. Arterial thrombosis, examined after ferric chloride-induced carotid injury, was accelerated but with little impact on maximal thrombus size. In a vena cava stasis model, clots were of similar size as in wild-type controls, but exhibited a different composition with a higher platelet to fibrin ratio. Both thrombocytosis strains displayed increased haemorrhagic tendency in a tail bleeding assay. Yall;Mpl and VavCre;FF1 displayed a lower proportion of the more reactive high-molecular-weight forms of von Willebrand factor in their plasma, mimicking essential thrombocythaemia with very high platelet counts. Bleeding could not be explained by clear defects in platelet activation, which were normal or only weakly decreased. In conclusion, these models of thrombocytosis recapitulate several features of the haemorrhagic and thrombotic diatheses in ET and PV demonstrating potentials but also some limitations to study these major complications.

Mpl traffics to the cell surface through conventional and unconventional routes

Myeloproliferative neoplasms (MPNs) are often characterized by JAK2 or calreticulin (CALR) mutations, indicating aberrant trafficking in pathogenesis. This study focuses on Mpl trafficking and Jak2 association using two model systems: human erythroleukemia cells (HEL; JAK2V617F) and K562 myeloid leukemia cells (JAK2WT). Consistent with a putative chaperone role for Jak2, Mpl and Jak2 associate on both intracellular and plasma membranes (shown by proximity ligation assay) and siRNA-mediated knockdown of Jak2 led to Mpl trapping in the endoplasmic reticulum (ER). Even in Jak2 sufficient cells, Mpl accumulates in punctate structures that partially colocalize with ER-tracker, the ER exit site marker (ERES) Sec31a, the autophagy marker LC3 and LAMP1. Mpl was fused to miniSOG, a genetically encoded tag for correlated light and electron microscopy. Results suggest that a fraction of Mpl is taken up into autophagic structures from the ER and routed to autolyososomes. Surface biotinylation shows that both immature and mature Mpl reach the cell surface; in K562 cells Mpl is also released in exosomes. Both forms rapidly internalize upon ligand addition, while recovery is primarily attributed to immature Mpl. Mpl appears to reach the plasma membrane via both conventional ER-Golgi and autolysosome secretory pathways, as well as recycling.

JAK2 and MPL protein levels determine TPO-induced megakaryocyte proliferation vs differentiation

Megakaryopoiesis is a 2-step differentiation process, regulated by thrombopoietin (TPO), on binding to its cognate receptor myeloproliferative leukemia (MPL). This receptor associates with intracytoplasmic tyrosine kinases, essentially janus kinase 2 (JAK2), which regulates MPL stability and cell-surface expression, and mediates TPO-induced signal transduction. We demonstrate that JAK2 and MPL mediate TPO-induced proliferation arrest and megakaryocytic differentiation of the human megakaryoblastic leukemia cell line UT7-MPL. A decrease in JAK2 or MPL protein expression, and JAK2 chemical inhibition, suppress this antiproliferative action of TPO. The expression of JAK2 and MPL, which progressively increases along normal human megakaryopoiesis, is decreased in platelets of patients diagnosed with JAK2- or MPL-mutated essential thrombocytemia and primary myelofibrosis, 2 myeloproliferative neoplasms in which megakaryocytes (MKs) proliferate excessively. Finally, low doses of JAK2 chemical inhibitors are shown to induce a paradoxical increase in MK production, both in vitro and in vivo. We propose that JAK2 and MPL expression levels regulate megakaryocytic proliferation vs differentiation in both normal and pathological conditions, and that JAK2 chemical inhibitors could promote a paradoxical thrombocytosis when used at suboptimal doses.

Genetic studies reveal an unexpected negative regulatory role for Jak2 in thrombopoiesis

JAK inhibitor treatment is limited by the variable development of anemia and thrombocytopenia thought to be due to on-target JAK2 inhibition. We evaluated the impact of Jak2 deletion in platelets (PLTs) and megakaryocytes (MKs) on blood counts, stem/progenitor cells, and Jak-Stat signaling. Pf4-Cre-mediated Jak2 deletion in PLTs and MKs did not compromise PLT formation but caused thrombocytosis, and resulted in expansion of MK progenitors and Lin(-)Sca1(+)Kit+ cells. Serum thrombopoietin (TPO) was maintained at normal levels in Pf4-Cre-positive Jak2(f/f) mice, consistent with reduced internalization/turnover by Jak2-deficient PLTs. These data demonstrate that Jak2 in terminal megakaryopoiesis is not required for PLT production, and that Jak2 loss in PLTs and MKs results in non-autonomous expansion of stem/progenitors and of MKs and PLTs via dysregulated TPO turnover. This suggests that the thrombocytopenia frequently seen with JAK inhibitor treatment is not due to JAK2 inhibition in PLTs and MKs, but rather due to JAK2 inhibition in stem/progenitor cells.

Mpl expression on megakaryocytes and platelets is dispensable for thrombopoiesis but essential to prevent myeloproliferation

Thrombopoietin (TPO) acting via its receptor, the cellular homologue of the myeloproliferative leukemia virus oncogene (Mpl), is the major cytokine regulator of platelet number. To precisely define the role of specific hematopoietic cells in TPO-dependent hematopoiesis, we generated mice that express the Mpl receptor normally on stem/progenitor cells but lack expression on megakaryocytes and platelets (Mpl(PF4cre/PF4cre)). Mpl(PF4cre/PF4cre) mice displayed profound megakaryocytosis and thrombocytosis with a remarkable expansion of megakaryocyte-committed and multipotential progenitor cells, the latter displaying biological responses and a gene expression signature indicative of chronic TPO overstimulation as the underlying causative mechanism, despite a normal circulating TPO level. Thus, TPO signaling in megakaryocytes is dispensable for platelet production; its key role in control of platelet number is via generation and stimulation of the bipotential megakaryocyte precursors. Nevertheless, Mpl expression on megakaryocytes and platelets is essential to prevent megakaryocytosis and myeloproliferation by restricting the amount of TPO available to stimulate the production of megakaryocytes from the progenitor cell pool.

Toward a universal treatment for cancer: cell inflation assisted chemotherapy

Cancers show considerable genetic and functional heterogeneity, preventing the development of a universal anticancer drug. Here, I argue that it is nevertheless possible to elaborate a therapeutic strategy that can be used in almost every cancer, exploiting the negative feedback effect of normal cells on the proliferation of their precursors. This method, termed cell inflation assisted chemotherapy, is aimed at blocking normal cell division prior to high-dose antimitotic chemotherapy. Evidence for a negative feedback effect on granulocyte production suggests that it is possible to prevent neutropenia by transfusion of autologous granulocytes. In a first step, this protocol will be devised to protect neutrophils and to prevent granulopenia in patients treated with intensive chemotherapy. In its simplest form, it will consist of a leukapheresis–storage–reinjection sequence just prior to drug administration. Then, if the proof of concept is established, a more systematic use of intensive cell cycle-specific chemotherapy, together with protection of other lineages through temporary mitotic blockade might be a treatment applicable for most cancers.

Negative feedback effect of normal cells on the proliferation of their precursors may be used to protect them from high-dose antimitotic chemotherapy, preventing myelosuppression. In its simplest form, cell inflation assisted chemotherapy will consist of a leukapheresis-storage-reinjection sequence just prior to drug administration.

Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis

Platelets play a key role in atherogenesis and its complications. Both hypercholesterolemia and increased platelet production promote athero-thrombosis; however, a potential link between altered cholesterol homeostasis and platelet production has not been explored. Transplantation of bone marrow (BM) deficient in ABCG4, a transporter of unknown function, into Ldlr^−/− mice resulted in thrombocytosis, accelerated thrombosis and atherosclerosis. While not detected in lesions, Abcg4 was highly expressed in BM megakaryocyte progenitors (MkP). Abcg4^−/− MkPs displayed defective cholesterol efflux to HDL, increased cell surface levels of thrombopoietin (TPO) receptor (c-MPL) and enhanced proliferation. This appeared to reflect disruption of the negative feedback regulation of c-MPL levels and signaling by E3 ligase c-CBL and cholesterol-sensing LYN kinase. HDL infusions reduced platelet counts in Ldlr^−/− mice and in a mouse model of myeloproliferative neoplasm, in a completely ABCG4-dependent fashion. HDL infusions may offer a novel approach to reducing athero-thrombotic events associated with increased platelet production.

Imputation of exome sequence variants into population- based samples and blood-cell-trait-associated loci in African Americans: NHLBI GO Exome Sequencing Project

Researchers have successfully applied exome sequencing to discover causal variants in selected individuals with familial, highly penetrant disorders. We demonstrate the utility of exome sequencing followed by imputation for discovering low-frequency variants associated with complex quantitative traits. We performed exome sequencing in a reference panel of 761 African Americans and then imputed newly discovered variants into a larger sample of more than 13,000 African Americans for association testing with the blood cell traits hemoglobin, hematocrit, white blood count, and platelet count. First, we illustrate the feasibility of our approach by demonstrating genome-wide-significant associations for variants that are not covered by conventional genotyping arrays; for example, one such association is that between higher platelet count and an MPL c.117G>T (p.Lys39Asn) variant encoding a p.Lys39Asn amino acid substitution of the thrombopoietin receptor gene (p = 1.5 × 10(-11)). Second, we identified an association between missense variants of LCT and higher white blood count (p = 4 × 10(-13)). Third, we identified low-frequency coding variants that might account for allelic heterogeneity at several known blood cell-associated loci: MPL c.754T>C (p.Tyr252His) was associated with higher platelet count; CD36 c.975T>G (p.Tyr325(∗)) was associated with lower platelet count; and several missense variants at the α-globin gene locus were associated with lower hemoglobin. By identifying low-frequency missense variants associated with blood cell traits not previously reported by genome-wide association studies, we establish that exome sequencing followed by imputation is a powerful approach to dissecting complex, genetically heterogeneous traits in large population-based studies.

Dysmegakaryopoiesis of FPD/AML pedigrees with constitutional RUNX1 mutations is linked to myosin II deregulated expression

FPD/AML is a familial platelet disorder characterized by platelet defects, predisposition to acute myelogenous leukemia (AML) and germ-line heterozygous RUNX1 alterations. Here we studied the in vitro megakaryopoiesis of 3 FPD/AML pedigrees. A 60% to 80% decrease in the output of megakaryocytes (MKs) from CD34+ was observed. MK ploidy level was low and mature MKs displayed a major defect in proplatelet formation. To explain these defects, we focused on myosin II expression as RUNX1 has been shown to regulate MYL9 and MYH10 in an inverse way. In FPD/AML MKs, expression of MYL9 and MYH9 was decreased, whereas MYH10 expression was increased and the MYH10 protein was still present in the cytoplasm of mature MKs. Myosin II activity inhibition by blebbistatin rescued the ploidy defect of FPD/AML MKs. Finally, we demonstrate that MYH9 is a direct target of RUNX1 by chromatin immunoprecipitation and luciferase assays and we identified new RUNX1 binding sites in the MYL9 promoter region. Together, these results demonstrate that the defects in megakaryopoiesis observed in FPD/AML are, in part, related to a deregulation of myosin IIA and IIB expression leading to both a defect in ploidization and proplatelet formation.

JAK2 V617F down-modulates MPL

Decreased expression of the thrombopoietin receptor (TPOR or MPL) on the cell surface of platelets and megakaryocytes is an established feature of and myelofibrosis; however, the exact mechanism responsible for this phenomenon has gone largely unexplained. In this issue of Blood, Pecquet and colleagues publish an excellent study revealing that MPL expression is downregulated in the context of the mutant protein, JAK2V617F.

Thrombopoietin receptor down-modulation by JAK2 V617F: restoration of receptor levels by inhibitors of pathologic JAK2 signaling and of proteasomes

The constitutively active JAK2 V617F mutant is the major determinant of human myeloproliferative neoplasms (MPNs). We show that coexpression of murine JAK2 V617F and the murine thrombopoietin (Tpo) receptor (TpoR, c-MPL) in hematopoietic cell lines or heterozygous knock-in of JAK2 V617F in mice leads to down-modulation of TpoR levels. Enhanced TpoR ubiquitinylation, proteasomal degradation, reduced recycling, and maturation are induced by the constitutive JAK2 V617F activity. These effects can be prevented in cell lines by JAK2 and proteasome inhibitors. Restoration of TpoR levels by inhibitors could be detected in platelets from JAK2 inhibitor-treated myelofibrosis patients that express the JAK2 V617F mutant, and in platelets from JAK2 V617F knock-in mice that were treated in vivo with JAK2 or proteasome inhibitors. In addition, we show that Tpo can induce both proliferative and antiproliferative effects via TpoR at low and high JAK2 activation levels, respectively, or on expression of JAK2 V617F. The antiproliferative signaling and receptor down-modulation by JAK2 V617F were dependent on signaling via TpoR cytosolic tyrosine 626. We propose that selection against TpoR antiproliferative signaling occurs by TpoR down-modulation and that restoration of down-modulated TpoR levels could become a biomarker for the treatment of MPNs.

Megakaryocyte development is normal in mice with targeted disruption of Tescalcin

BACKGROUND

Tescalcin is an EF-hand calcium-binding protein that interacts with the Na+/H+ exchanger 1 (NHE1). Levay and Slepak recently proposed a role for tescalcin in megakaryopoiesis that was independent of NHE1 activity. Their studies using K562 and HEL cell lines, and human CD34+ hematopoietic stem cells suggested an essential role for tescalcin in megakaryocyte differentiation.

OBJECTIVE

To study the role of tescalcin in megakaryocyte development using a murine model of megakaryopoiesis.

METHODS

We generated a mouse with targeted disruption of tescalcin and investigated megakaryocyte development.

RESULTS

Tescalcin-deficient mice had a normal number of megakaryocytes and platelets. The morphology, polyploidization profile, and expression of Fli-1 in bone marrow-derived megakaryocytes were also normal.

CONCLUSION

Tescalcin does not appear to be necessary for normal megakaryocyte development.

Role of tumor suppressor p53 in megakaryopoiesis and platelet function

The pathobiological role of p53 has been widely studied, however, its role in normophysiology is relatively unexplored. We previously showed that p53 knock-down increased ploidy in megakaryocytic cultures. This study aims to examine the effect of p53 loss on in vivo megakaryopoiesis, platelet production, and function, and to investigate the basis for greater ploidy in p53(-/-) megakaryocytic cultures. Here, we used flow cytometry to analyze ploidy, DNA synthesis, and apoptosis in murine cultured and bone marrow megakaryocytes following thrombopoietin administration and to analyze fibrinogen binding to platelets in vitro. Culture of p53(-/-) marrow cells for 6 days with thrombopoietin gave rise to 1.7-fold more megakaryocytes, 26.1% ± 3.6% of which reached ploidy classes ≥64 N compared to 8.2% ± 0.9% of p53(+/+) megakaryocytes. This was due to 30% greater DNA synthesis in p53(-/-) megakaryocytes and 31% greater apoptosis in p53(+/+) megakaryocytes by day 4 of culture. Although the bone marrow and spleen steady-state megakaryocytic content and ploidy were similar in p53(+/+) and p53(-/-) mice, thrombopoietin administration resulted in increased megakaryocytic polyploidization in p53(-/-) mice. Although their platelet counts were normal, p53(-/-) mice exhibited significantly longer bleeding times and p53(-/-) platelets were less sensitive than p53(+/+) platelets to agonist-induced fibrinogen binding and P-selectin secretion. In summary, our in vivo and ex vivo studies indicate that p53 loss leads to increased polyploidization during megakaryopoiesis. Our findings also suggest for the first time a direct link between p53 loss and the development of fully functional platelets resulting in hemostatic deficiencies.

Hematological and acute-phase responses to diet-induced obesity in IL-6 KO mice

Obesity is associated with chronic inflammation and elevated levels of IL-6. The role of IL-6 in induction of acute-phase proteins and modulation of hematological responses has been demonstrated in models of inflammation and aging, but not in obesity. We hypothesized that IL-6 is necessary to regulate the acute-phase response and hematological changes associated with diet-induced obesity (DIO) in mice. Feeding a 60%kcal/fat diet for 13 weeks to C57BL6 WT male mice induced a significant increase in IL-6 expression in visceral adipose tissue (VAT), but not liver, compared to mice fed chow diet. Significantly elevated IL-6 levels were present in the peritoneal lavage fluid, but not plasma, of DIO compared to lean mice. A comparable degree of obesity, hepatomegaly, hyperleptinemia, VAT inflammation and insulin resistance was observed in DIO WT and IL-6 KO mice compared to WT and KO mice fed chow diet. Significant leukocytosis was observed in DIO WT but not DIO KO mice compared to lean groups. A significant reduction in platelet counts, without alterations in platelet size, percentage of circulating reticulated platelets and number of bone marrow megakaryocytes, was present in DIO KO mice compared to each other group. Hepatic expression of thrombopoietin was comparable in each group, with DIO WT and KO mice having reduced VAT expression compared to lean mice. Lean KO mice had significantly elevated plasma levels of thrombopoietin compared to each other group, whereas liver-associated thrombopoietin levels were comparable in each group. Deficiency of IL-6 resulted in blunted hepatic induction of the acute-phase protein serum amyloid A-1, whereas expression of hepcidin-1 and -2, LPS-binding protein, ceruloplasmin, plasminogen activator inhibitor-1 and thrombospondin-1 was IL-6-independent. In conclusion, in the absence of overt metabolic alterations, IL-6 modulates leukocytosis, thrombopoiesis and induction of SAA-1, but not other acute-phase proteins in obese mice.

Mouse models of diseases of megakaryocyte and platelet homeostasis

Platelets are the small anuclear blood cells that are the product of megakaryocytopoiesis, the process of hematopoietic stem cell commitment to megakaryocyte production and the differentiation and maturation of these cells for platelet release. Deregulation or disruption of megakaryocytopoiesis can result in platelet deficiencies, the thrombocytopenias, with attendant risk of hemorrhage or thrombocytosis, a pathological excess of platelet numbers. Mouse models, particularly those engineered to carry genetic alterations modeling mutations associated with human disease, have provided important insights into megakaryocytopoiesis and deregulation of this process in disease. This review focuses on mouse models of diseases of altered megakaryocyte and platelet number, illustrating the profound contribution of these models in validating suspected roles of disease-associated genetic alterations, promoting discovery of new links between genetic mutations and specific diseases, and providing unique tools for better understanding of disease pathophysiology and progression, as well as resources to define drug action or develop new therapeutic strategies.

Lentiviral gene transfer regenerates hematopoietic stem cells in a mouse model for Mpl-deficient aplastic anemia

Thpo/Mpl signaling plays an important role in the maintenance of hematopoietic stem cells (HSCs) in addition to its role in megakaryopoiesis. Patients with inactivating mutations in Mpl develop thrombocytopenia and aplastic anemia because of progressive loss of HSCs. Yet, it is unknown whether this loss of HSCs is an irreversible process. In this study, we used the Mpl knockout (Mpl(-/-)) mouse model and expressed Mpl from newly developed lentiviral vectors specifically in the physiologic Mpl target populations, namely, HSCs and megakaryocytes. After validating lineage-specific expression in vivo using lentiviral eGFP reporter vectors, we performed bone marrow transplantation of transduced Mpl(-/-) bone marrow cells into Mpl(-/-) mice. We show that restoration of Mpl expression from transcriptionally targeted vectors prevents lethal adverse reactions of ectopic Mpl expression, replenishes the HSC pool, restores stem cell properties, and corrects platelet production. In some mice, megakaryocyte counts were atypically high, accompanied by bone neo-formation and marrow fibrosis. Gene-corrected Mpl(-/-) cells had increased long-term repopulating potential, with a marked increase in lineage(-)Sca1(+)cKit(+) cells and early progenitor populations in reconstituted mice. Transcriptome analysis of lineage(-)Sca1(+)cKit(+) cells in Mpl-corrected mice showed functional adjustment of genes involved in HSC self-renewal.

Extracellular domain N-glycosylation controls human thrombopoietin receptor cell surface levels

The thrombopoietin receptor (TpoR) is a type I transmembrane protein that mediates the signaling functions of thrombopoietin (Tpo) in regulating megakaryocyte differentiation, platelet formation, and hematopoietic stem cell renewal. We probed the role of each of the four extracellular domain putative N-glycosylation sites for cell surface localization and function of the receptor. Single N-glycosylation mutants at any of the four sites were able to acquire the mature N-glycosylated pattern, but exhibited a decreased Tpo-dependent JAK2-STAT response in stably transduced Ba/F3 or Ba/F3-JAK2 cell lines. The ability of JAK2 to promote cell surface localization and stability of TpoR required the first N-glycosylation site (Asn117). In contrast, the third N-glycosylation site (Asn298) decreased receptor maturation and stability. TpoR mutants lacking three N-glycosylation sites were defective in maturation, but N-glycosylation on the single remaining site could be detected by sensitivity to PNGaseF. The TpoR mutant defective in all four N-glycosylation sites was severely impaired in plasma membrane localization and was degraded by the proteasome. N-glycosylation receptor mutants are not misfolded as, once localized on the cell surface in overexpression conditions, they can bind and respond to Tpo. Our data indicate that extracellular domain N-glycosylation sites regulate in a combinatorial manner cell surface localization of TpoR. We discuss how mutations around TpoR N-glycosylation sites might contribute to inefficient receptor traffic and disease.

Screening for MPL mutations in essential thrombocythemia and primary myelofibrosis: normal Mpl expression and absence of constitutive STAT3 and STAT5 activation in MPLW515L-positive platelets

OBJECTIVES

To evaluate the frequency of MPL W515L, W515K and S505N mutations in essential thrombocythemia (ET) and primary myelofibrosis (PMF) and to determine whether MPLW515L leads to impaired Mpl expression, constitutive STAT3 and STAT5 activation and enhanced response to thrombopoietin (TPO).

METHODS

Mutation detection was performed by allele-specific PCR and sequencing. Platelet Mpl expression was evaluated by flow cytometry, immunoblotting and real-time RT-PCR. Activation of STAT3 and STAT5 before and after stimulation with increasing concentrations of TPO was studied by immunoblotting. Plasma TPO was measured by ELISA.

RESULTS

MPLW515L was detected in 1 of 100 patients with ET and 1 of 11 with PMF. Platelets from the PMF patient showed 100% mutant allele, which was <50% in platelets from the ET patient, who also showed the mutation in granulocytes, monocytes and B cells. Mpl surface and total protein expression were normal, and TPO levels were mildly increased in the MPLW515L-positive ET patient, while MPL transcripts did not differ from controls in both MPLW515L-positive patients. Constitutive STAT3 and STAT5 phosphorylation was absent and dose response to TPO-induced phosphorylation was not enhanced.

CONCLUSIONS

The low frequency of MPL mutations in this cohort is in agreement with previous studies. The finding of normal Mpl levels in MPLW515L-positive platelets indicates this mutation does not lead to dysregulated Mpl expression, as frequently shown for myeloproliferative neoplasms. The lack of spontaneous STAT3 and STAT5 activation and the normal response to TPO is unexpected as MPLW515L leads to constitutive receptor activation and hypersensitivity to TPO in experimental models.

Gene therapy of MPL deficiency: challenging balance between leukemia and pancytopenia

Signaling of the thrombopoietin (THPO) receptor MPL is critical for the maintenance of hematopoietic stem cells (HSCs) and megakaryocytic differentiation. Inherited loss-of-function mutations of MPL cause severe thrombocytopenia and aplastic anemia, a syndrome called congenital amegakaryocytic thrombocytopenia (CAMT). With the aim to assess the toxicity of retroviral expression of Mpl as a basis for further development of a gene therapy for this disorder, we expressed Mpl in a murine bone marrow transplantation (BMT) model. Treated mice developed a profound yet transient elevation of multilineage hematopoiesis, which showed morphologic features of a chronic myeloproliferative disorder (CMPD) with progressive pancytopenia. Ten percent of mice (3/27) developed erythroleukemia, associated with insertional activation of Sfpi1 and Fli1. The majority of transplanted mice developed a progressive pancytopenia with histopathological features of a myelodysplastic syndrome (MDS)-like disorder. To avoid these adverse reactions, improved retroviral vectors were designed that mediate reduced and more physiological Mpl expression. Self-inactivating gamma-retroviral vectors were constructed that expressed Mpl from the phosphoglycerate kinase (PGK) or the murine Mpl promoter. Mice that received BM cells expressing Mpl from the Mpl promoter were free of any previously observed adverse reactions.

In essential thrombocythemia, multiple JAK2-V617F clones are present in most mutant-positive patients: a new disease paradigm

In essential thrombocythemia (ET), the JAK2-V617F mutation is usually restricted to a subpopulation of neutrophils and platelets, and production of JAK2 wild-type (WT) platelets is not suppressed. Nonmutated precursor cells may, therefore, be susceptible to the acquisition of further JAK2 mutations. We used a common single nucleotide polymorphism (SNP) in the JAK2 coding sequence to genotype V617F alleles obtained either by allele-specific restriction enzyme digestion (RED) or by cloning. Both SNP alleles were detected in JAK2 mutant-positive alleles from neutrophils of 10 of 11 ET patients studied using RED compared with 0 of 5 with polycythemia vera. These results were confirmed in cloned products from 5 ET patients and indicate the occurrence of at least 2 separate JAK2 mutation events in the majority of ET patients investigated. In a further ET patient, JAK2 mutant-positive erythroid colonies with either X-allele inactivated were detected, demonstrating they could not have arisen from a common clonal precursor. These results indicate that at least 2 independent JAK2-V617F events occur commonly in ET patients, and they may arise on a polyclonal background. The presence of a JAK2 mutation in ET patients should not, therefore, be equated with a malignant disease.

Other Transgenic Animal Models Used in Cardiovascular Studies

Previous chapters have described a large number of transgenic animal models used to study specific cardiovascular syndromes. This chapter will fill in some gaps. Many of these transgenic animals were developed to study normal and/or abnormal physiological responses in other organ systems, or to study basic biochemical and molecular reactions or pathways. These models were then discovered to also have effects on the cardiovascular system, some of them unanticipated.

A word of caution, particularly when highly inbred mouse strains are used to develop transgenic models - not all strains of a particular species are created equal. When cardiovascular parameters of age- and sex-matched A/J and C57BL/6J inbred mice were compared the C57BL/6J mice demonstrated eccentric physiologic ventricular hypertrophy, increased ventricular function, lower heart rates, and increased exercise endurance.¹