Cloning and functional characterization of a novel c-mpl variant expressed in human CD34 cells and platelets

Review on the Biogenesis of Platelets in Lungs and Its Alterations in SARS-CoV-2 Infection Patients

Thrombocytes (platelets) are the type of blood cells that are involved in hemostasis, thrombosis, etc. For the conversion of megakaryocytes into thrombocytes, the thrombopoietin (TPO) protein is essential which is encoded by the TPO gene. TPO gene is present in the long arm of chromosome number 3 (3q26). This TPO protein interacts with the c-Mpl receptor, which is present on the outer surface of megakaryocytes. As a result, megakaryocyte breaks into the production of functional thrombocytes. Some of the evidence shows that the megakaryocytes, the precursor of thrombocytes, are seen in the lung's interstitium. This review focuses on the involvement of the lungs in the production of thrombocytes and their mechanism. A lot of findings show that viral diseases, which affect the lungs, cause thrombocytopenia in human beings. One of the notable viral diseases is COVID-19 or severe acute respiratory syndrome caused by SARS-associated coronavirus 2 (SARS-CoV-2). SARS-CoV-2 caused a worldwide alarm in 2019 and a lot of people suffered because of this disease. It mainly targets the lung cells for its replication. To enter the cells, these virus targets the angiotensin-converting enzyme-2 (ACE-2) receptors that are abundantly seen on the surface of the lung cells. Recent reports of COVID-19-affected patients reveal the important fact that these peoples develop thrombocytopenia as a post-COVID condition. This review elaborates on the biogenesis of platelets in the lungs and the alterations of thrombocytes during the COVID-19 infection.

c-Mpl-del, a c-Mpl alternative splicing isoform, promotes AMKL progression and chemoresistance

Acute megakaryocytic leukemia (AMKL) is a clinically heterogeneous subtype of acute myeloid leukemia characterized by unrestricted megakaryoblast proliferation and poor prognosis. Thrombopoietin receptor c-Mpl is a primary regulator of megakaryopoeisis and a potent mitogenic receptor. Aberrant c-Mpl signaling has been implicated in a myriad of myeloid proliferative disorders, some of which can lead to AMKL, however, the role of c-Mpl in AMKL progression remains largely unexplored. Here, we identified increased expression of a c-Mpl alternative splicing isoform, c-Mpl-del, in AMKL patients. We found that c-Mpl-del expression was associated with enhanced AMKL cell proliferation and chemoresistance, and decreased survival in xenografted mice, while c-Mpl-del knockdown attenuated proliferation and restored apoptosis. Interestingly, we observed that c-Mpl-del exhibits preferential utilization of phosphorylated c-Mpl-del C-terminus Y607 and biased activation of PI3K/AKT pathway, which culminated in upregulation of GATA1 and downregulation of DDIT3-related apoptotic responses conducive to AMKL chemoresistance and proliferation. Thus, this study elucidates the critical roles of c-Mpl alternative splicing in AMKL progression and drug resistance, which may have important diagnostic and therapeutic implications for leukemia accelerated by c-Mpl-del overexpression.

Splicing Anomalies in Myeloproliferative Neoplasms: Paving the Way for New Therapeutic Venues

Since the discovery of spliceosome mutations in myeloid malignancies, abnormal pre-mRNA splicing, which has been well studied in various cancers, has attracted novel interest in hematology. However, despite the common occurrence of spliceosome mutations in myelo-proliferative neoplasms (MPN), not much is known regarding the characterization and mechanisms of splicing anomalies in MPN. In this article, we review the current scientific literature regarding "splicing and myeloproliferative neoplasms". We first analyse the clinical series reporting spliceosome mutations in MPN and their clinical correlates. We then present the current knowledge about molecular mechanisms by which these mutations participate in the pathogenesis of MPN or other myeloid malignancies. Beside spliceosome mutations, splicing anomalies have been described in myeloproliferative neoplasms, as well as in acute myeloid leukemias, a dreadful complication of these chronic diseases. Based on splicing anomalies reported in chronic myelogenous leukemia as well as in acute leukemia, and the mechanisms presiding splicing deregulation, we propose that abnormal splicing plays a major role in the evolution of myeloproliferative neoplasms and may be the target of specific therapeutic strategies.

RNA-Binding Proteins in Acute Leukemias

Acute leukemias are genetic diseases caused by translocations or mutations, which dysregulate hematopoiesis towards malignant transformation. However, the molecular mode of action is highly versatile and ranges from direct transcriptional to post-transcriptional control, which includes RNA-binding proteins (RBPs) as crucial regulators of cell fate. RBPs coordinate RNA dynamics, including subcellular localization, translational efficiency and metabolism, by binding to their target messenger RNAs (mRNAs), thereby controlling the expression of the encoded proteins. In view of the growing interest in these regulators, this review summarizes recent research regarding the most influential RBPs relevant in acute leukemias in particular. The reported RBPs, either dysregulated or as components of fusion proteins, are described with respect to their functional domains, the pathways they affect, and clinical aspects associated with their dysregulation or altered functions.

Cytokine receptor splice variants in hematologic diseases

Cytokine and cytokine receptors are important regulators of hematopoiesis. Hematopoietic stem cells (HSCs) and progenitors differentiate into the myeloid or lymphoid lineage in response to specific cytokines. Cell-type specific receptors are expressed on committed progenitors that bind to other late-acting cytokines that are involved in terminal differentiation of hematopoietic cells. In normal hematopoiesis, these receptors undergo alternative splicing and are developmentally regulated. Splicing changes can significantly affect the structure and function of the receptors resulting in alterations of either the extracellular ligand binding domain or the cytoplasmic signaling domain responsible for cellular growth and differentiation. Most alternatively spliced isoforms generally lose the ability to promote differentiation. Evidently, overexpression of naturally occurring cytokine receptor alternate isoforms are observed in multiple myeloid diseases such as myelodysplastic syndromes (MDS), acute myeloid leukemia (AML), and polycythemia vera (PV). The purpose of this review is to introduce the various isoforms of key cytokine receptors that play a crucial role in myeloid development and their potential role in myeloid diseases.

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

Ott1 (Rbm15) regulates thrombopoietin response in hematopoietic stem cells through alternative splicing of c-Mpl

Thrombopoietin (Thpo) signaling through the c-Mpl receptor promotes either quiescence or proliferation of hematopoietic stem cells (HSCs) in a concentration-dependent manner; however, in vivo Thpo serum levels are responsive to platelet mass rather than HSC demands, suggesting additional regulation exists. Ott1 (Rbm15), a spliceosomal component originally identified as a fusion partner in t(1;22)-associated acute megakaryocytic leukemia, is also essential for maintaining HSC quiescence under stress. Ott1 controls the alternative splicing of a dominant negative isoform, Mpl-TR, capable of inhibiting HSC engraftment and attenuating Thpo signaling. Ott1, which associates with Hdac3 and the histone methyltransferase, Setd1b, binds to both c-Mpl RNA and chromatin and regulates H4 acetylation and H3K4me3 marks. Histone deacetylase or histone methyltransferase inhibition also increases Mpl-TR levels, suggesting that Ott1 uses an underlying epigenetic mechanism to control alternative splicing of c-Mpl. Manipulation of Ott1-dependent alternative splicing may therefore provide a novel pharmacologic avenue for regulating HSC quiescence and proliferation in response to Thpo.

Identification and characterization of an alternative splice variant of Mpl with a high affinity for TPO and its activation of ERK1/2 signaling

The thrombopoietin receptor is a crucial element in thrombopoietin-initiated signaling pathways, which stimulates the differentiation of normal hematopoietic progenitor cells, the maturation of megakaryocytes, and the generation of platelets. In this study, we identified a novel activating variant of thrombopoietin receptor, termed Mpl-D, in human megakaryoblastic leukemia Dami cells and demonstrated that the binding affinity of the Mpl-D receptor for thrombopoietin is enhanced. Cell cycle analysis revealed that in the presence of thrombopoietin, most Mpl-D expressing NIH3T3 (NIH3T3/Mpl-D) cells were prevalent in G1 phase while the S and G2/M populations were less frequently observed. Unexpectedly, thrombopoietin induced strong and prolonged ERK1/2 signaling in NIH3T3/Mpl-D cells compared with its receptor wild-type expressing NIH3T3 (NIH3T3/Mpl-F) cells. Further analysis of the mRNA levels of cyclin D1/D2 in NIH3T3/Mpl-D cells demonstrated markedly down-regulated expression compared to NIH3T3/Mpl-F cells in the presence of thrombopoietin. Thus, the prolonged activation of ERK1/2 by Mpl-D might lead to G1 cell cycle arrest through a profound reduction of cyclin D1/D2 in order to support cell survival without proliferation. We also provided tertiary structural basis for the Mpl-D and thrombopoietin interaction, which might provide insights into how Mpl-D effectively increases binding to thrombopoietin and significantly contributes to its specific signaling pathway. These results suggest a new paradigm for the regulation of cytokine receptor expression and function through the alternative splicing variant of Mpl in Dami cells, which may play a role in the pathogenesis of megakaryoblastic leukemia.

Thrombopoietin factors

Megakaryopoiesis and thrombopoiesis are the central biological processes of platelet generation. Severe thrombocytopenia is a major morbidity and mortality factor in several diseases and represents a significant unmet medical need. Since the discovery of thrombopoietin (TPO) as the primary physiological regulator of megakaryopoiesis, a number of therapeutics have been developed for thrombocytopenia and been tested in preclinical models and human clinical trials. The TPO mimetics romiplostim (Nplate(®) or AMG531) and eltrombopag (Promacta(®)) have recently been approved for the treatment of adult chronic idiopathic (immune) thrombocytopenic purpura (ITP) and are successful examples of these endeavors. This chapter will review scientific progress over the last 20 years on various thrombopoietic factors with an emphasis on the biology, physiology, and pharmacology of TPO, its cognate receptor, c-Mpl, and various TPO mimetics.

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

We generated mice expressing a full-length Mpl transgene under the control of a 2-kb Mpl promoter in an Mpl−/− background, effectively obtaining mice that express full-length Mpl in the absence of other Mpl isoforms. These mice developed thrombocytosis with platelet levels approximately 5-fold higher than wild-type controls and markedly increased megakaryocyte numbers. The reintroduction of one wild-type Mpl allele restored normal platelet counts. We excluded the deletion of Mpl-tr, a dominant-negative isoform, as the underlying molecular cause for thrombocytosis. Instead, we found that transgene expression driven by the 2-kb Mpl promoter fragment was decreased during late megakaryocyte maturation, resulting in strongly diminished Mpl protein expression in platelets. Because platelets exert a negative feedback on thrombopoiesis by binding and consuming Tpo in the circulation through Mpl, we propose that the severe reduction of Mpl protein in platelets in Mpl-transgenic Mpl−/− mice shifts the equilibrium of this feedback loop, resulting in markedly elevated levels of megakaryocytes and platelets at steady state. Although the mechanism causing decreased expression of Mpl protein in platelets from patients with myeloproliferative disorders differs from this transgenic model, our results suggest that lowering Mpl protein in platelets could contribute to raising the platelet count.

The molecular mechanisms that control thrombopoiesis

Our understanding of thrombopoiesis--the formation of blood platelets--has improved greatly in the last decade, with the cloning and characterization of thrombopoietin, the primary regulator of this process. Thrombopoietin affects nearly all aspects of platelet production, from self-renewal and expansion of HSCs, through stimulation of the proliferation of megakaryocyte progenitor cells, to support of the maturation of these cells into platelet-producing cells. The molecular and cellular mechanisms through which thrombopoietin affects platelet production provide new insights into the interplay between intrinsic and extrinsic influences on hematopoiesis and highlight new opportunities to translate basic biology into clinical advances.

A truncated isoform of c-Mpl with an essential C-terminal peptide targets the full-length receptor for degradation

Thrombopoietin and its cognate receptor c-Mpl are the primary regulators of megakaryopoiesis and platelet production. They also play an important role in the maintenance of hematopoietic stem cells. Here, we have analyzed the function of a truncated Mpl receptor isoform (Mpl-tr), which results from alternative splicing. The mpl-tr variant is the only alternate mpl isoform conserved between mouse and humans, suggesting a relevant function in regulating Mpl signaling. Despite the presence of a signal peptide and the lack of a transmembrane domain, Mpl-tr is retained intracellularly. Our results provide evidence that Mpl-tr exerts a dominant-negative effect on thrombopoietin-dependent cell proliferation and survival. We demonstrate that this inhibitory effect is due to down-regulation of the full-length Mpl protein. The C terminus of Mpl-tr, consisting of 30 amino acids of unique sequence, is essential for the suppression of thrombopoietin-dependent proliferation and Mpl protein down-regulation. Cathepsin inhibitor-1 (CATI-1), an inhibitor of cathepsin-like cysteine proteases, counteracts the effect of Mpl-tr on Mpl protein expression, suggesting that Mpl-tr targets Mpl for lysosomal degradation. Together, these data suggest a new paradigm for the regulation of cytokine receptor expression and function through a proteolytic process directed by a truncated isoform of the same receptor.

Identification and characterization of an isoform of murine Mpl

A new isoform of the full-length murine thrombopoietin (Tpo) receptor was isolated from a murine spleen cDNA library. This isoform, c-mpl-II, differs from full-length c-mpl (c-mpl-I) by virtue of deletion of 180 nucleotides that encode 60 amino acids located in the extracellular domain of Mpl. Normal murine megakaryocytes were found to express both c-mpl-I and c-mpl-II transcripts. BaF3 cells transfected with c-mpl-I expressed a 95 kDa protein that was displayed on the cell surface and bound 125I-Tpo. BaF3 cells transfected with c-mpl-II expressed a 70 kDa protein. However, these cells were not able to bind 125I-Tpo and surface display of Mpl-II could not be detected. In summary, c-mpl-II is an isoform of murine Mpl expressed by megakaryocytes that lacks a 60 amino acid region required for surface expression of the protein.

MplK, a natural variant of the thrombopoietin receptor with a truncated cytoplasmic domain, binds thrombopoietin but does not interfere with thrombopoietin-mediated cell growth

OBJECTIVE

Interaction of thrombopoietin (TPO) with its receptor c-Mpl is responsible for the formation of megakaryocytes and platelets. In humans, there are two major c-mpl molecules, MplP and MplK, which are generated by alternative splicing. In contrast to MplP, MplK has none of the intracellular sequences required for typical signal transduction but instead has a unique 27 amino acid sequence that is coded by intron 10. We tested to determine if MplK exerts a negative effect on TPO Mpl signal transduction by interfering with the normal homodimerization of MplP.

MATERIALS AND METHODS

A cassette coding for MplK cDNA was introduced into parental and MplP-expressing BaF3 cells and TPO-mediated cell growth studied.

RESULTS

Cells expressing MplK alone did not respond to TPO compared to cells that expressed MplP. When MplK was coexpressed with MplP on the cell surface of BaF3, no modification in cell growth was observed when compared to those expressing MplP alone. To determine if the normal homodimerization process was negatively influenced, two genetically engineered variants of c-Mpl, one lacking the box1 sequence and the other containing only the first nine amino acids of the intracellular domain, were introduced into MplP-expressing cells. In contrast to MplK, these mutants had a dominant negative effect on TPO-mediated cell growth.

CONCLUSIONS

MplK does not influence TPO-mediated growth of Mpl-expressing cells. Our data suggest that the absence of a dominant negative effect of MplK most probably is due to the inability of MplK to dimerize with the MplP receptor.

The end is just the beginning: megakaryocyte apoptosis and platelet release

Under influence of hematopoietic growth factors, particularly thrombopoietin (TPO), hematopoietic stem cells in the bone marrow go through a process of commitment, proliferation, differentiation, and maturation and become mature megakaryocytes. At this critical point, terminally differentiated megakaryocytes face a new fate: ending the old life as mature megakaryocytes by induction of apoptosis and beginning a new life as platelets by fragmentation of the large megakaryocyte cytoplasm. These events are as important as megakaryocyte commitment, proliferation, differentiation, and maturation, but the molecular mechanisms regulating these events are not well established. Although TPO drives megakaryocyte proliferation and differentiation and protects hematopoietic progenitor cells from death, it does not appear to promote platelet release from terminally differentiated megakaryocytes. Although mature megakaryocyte apoptosis is temporally associated with platelet formation, premature megakaryocyte death directly causes thrombocytopenia in cancer therapy and in diseases such as mvelodysplastic syndromes and human immunodeficiency virus infection. Also, genetic studies have shown that accumulation of megakaryocytes in bone marrow is not necessarily sufficient to produce platelets. All of these findings suggest that platelet release from megakaryocytes is an important and regulated aspect of platelet production, in which megakaryocyte apoptosis may also play a role. This review summarizes recent research progress on megakaryocyte apoptosis and platelet release.

Thrombocytopenia caused by the development of antibodies to thrombopoietin

Thrombocytopenia developed in some individuals treated with a recombinant thrombopoietin (TPO), pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF). Three of the subjects who developed severe thrombocytopenia were analyzed in detail to determine the cause of their thrombocytopenia. Except for easy bruising and heavy menses, none of these subjects had major bleeding episodes; none responded to intravenous immunoglobulin or prednisone. Bone marrow examination revealed a marked reduction in megakaryocytes. All 3 thrombocytopenic subjects had antibody to PEG-rHuMGDF that cross-reacted with endogenous TPO and neutralized its biological activity. All anti-TPO antibodies were immunoglobulin G (IgG), with increased amounts of IgG4; no IgM antibodies to TPO were detected at any time. A quantitative assay for IgG antibody to TPO was developed and showed that the antibody concentration varied inversely with the platelet count. Anti-TPO antibody recognized epitopes located in the first 163 amino acids of TPO and prevented TPO from binding to its receptor. In 2 subjects, endogenous TPO levels were elevated, but the TPO circulated as a biologically inactive immune complex with anti-TPO IgG; the endogenous TPO in these complexes had an apparent molecular weight of 95 000, slightly larger than the full-length recombinant TPO. None of the subjects had atypical HLA or platelet antigens, and the TPO cDNA was normal in both that were sequenced. Treatment of one subject with cyclosporine eliminated the antibody and normalized the platelet count. These data demonstrate a new mechanism for thrombocytopenia in which antibody develops to TPO; because endogenous TPO is produced constitutively, thrombocytopenia ensues.