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

Single-cell gene and isoform expression analysis reveals signatures of ageing in haematopoietic stem and progenitor cells

Single-cell approaches have revealed that the haematopoietic hierarchy is a continuum of differentiation, from stem cell to committed progenitor, marked by changes in gene expression. However, many of these approaches neglect isoform-level information and thus do not capture the extent of alternative splicing within the system. Here, we present an integrated short- and long-read single-cell RNA-seq analysis of haematopoietic stem and progenitor cells. We demonstrate that over half of genes detected in standard short-read single-cell analyses are expressed as multiple, often functionally distinct, isoforms, including many transcription factors and key cytokine receptors. We observe global and HSC-specific changes in gene expression with ageing but limited impact of ageing on isoform usage. Integrating single-cell and cell-type-specific isoform landscape in haematopoiesis thus provides a new reference for comprehensive molecular profiling of heterogeneous tissues, as well as novel insights into transcriptional complexity, cell-type-specific splicing events and consequences of ageing.

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.

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.

Aβ42-mediated proteasome inhibition and associated tau pathology in hippocampus are governed by a lysosomal response involving cathepsin B: Evidence for protective crosstalk between protein clearance pathways

Impaired protein clearance likely increases the risk of protein accumulation disorders including Alzheimer’s disease (AD). Protein degradation through the proteasome pathway decreases with age and in AD brains, and the Aβ₄₂ peptide has been shown to impair proteasome function in cultured cells and in a cell-free model. Here, Aβ₄₂ was studied in brain tissue to measure changes in protein clearance pathways and related secondary pathology. Oligomerized Aβ₄₂ (0.5–1.5 μM) reduced proteasome activity by 62% in hippocampal slice cultures over a 4-6-day period, corresponding with increased tau phosphorylation and reduced synaptophysin levels. Interestingly, the decrease in proteasome activity was associated with a delayed inverse effect, >2-fold increase, regarding lysosomal cathepsin B (CatB) activity. The CatB enhancement did not correspond with the Aβ₄₂-mediated phospho-tau alterations since the latter occurred prior to the CatB response. Hippocampal slices treated with the proteasome inhibitor lactacystin also exhibited an inverse effect on CatB activity with respect to diminished proteasome function. Lactacystin caused earlier CatB enhancement than Aβ₄₂, and no correspondence was evident between up-regulated CatB levels and the delayed synaptic pathology indicated by the loss of pre- and postsynaptic markers. Contrasting the inverse effects on the proteasomal and lysosomal pathways by Aβ₄₂ and lactacystin, such were not found when CatB activity was up-regulated two-fold with Z-Phe-Ala-diazomethylketone (PADK). Instead of an inverse decline, proteasome function was increased marginally in PADK-treated hippocampal slices. Unexpectedly, the proteasomal augmentation was significantly pronounced in Aβ₄₂-compromised slices, while absent in lactacystin-treated tissue, resulting in >2-fold improvement for nearly complete recovery of proteasome function by the CatB-enhancing compound. The PADK treatment also reduced Aβ₄₂-mediated tau phosphorylation and synaptic marker declines, corresponding with the positive modulation of both proteasome activity and the lysosomal CatB enzyme. These findings indicate that proteasomal stress contributes to AD-type pathogenesis and that governing such pathology occurs through crosstalk between the two protein clearance pathways.

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.

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.

Novel anti-c-Mpl monoclonal antibodies identified multiple differentially glycosylated human c-Mpl proteins in megakaryocytic cells but not in human solid tumors

Thrombopoietin and its cognate receptor, c-Mpl, are the primary molecular regulators of megakaryocytopoiesis and platelet production. To date the pattern of c-Mpl expression in human solid tumors and the distribution and biochemical properties of c-Mpl proteins in hematopoietic tissues are largely unknown. We have recently developed highly specific mouse monoclonal antibodies (MAb) against human c-Mpl. In this study we used these antibodies to demonstrate the presence of full-length and truncated human c-Mpl proteins in various megakaryocytic cell types, and their absence in over 100 solid tumor cell lines and in the 12 most common primary human tumor types. Quantitative assays showed a cell context-dependent distribution of full-length and truncated c-Mpl proteins. All forms of human c-Mpl protein were found to be modified with extensive N-linked glycosylation but different degrees of sialylation and O-linked glycosylation. Of note, different variants of full-length c-Mpl protein exhibiting differential glycosylation were expressed in erythromegakaryocytic leukemic cell lines and in platelets from healthy human donors. This work provides a comprehensive analysis of human c-Mpl mRNA and protein expression on normal and malignant hematopoietic and non-hematopoietic cells and demonstrates the multiple applications of several novel anti-c-Mpl antibodies.

Thrombopoietin modulates the proliferation, migration and cytokine profile of decidual cell subsets during early gestation

During early gestation, a considerable increase in different leukocyte subsets can be observed in the decidualized endometrium concomitantly to the invasion of cytotrophoblast cells (CTB). To date, it is still in question which factors induce this accumulation of immune cells and whether it is evoked by an in situ proliferation or by a migratory process. Studies on hepatoblastoma cells identified thrombopoietin (TPO) as a novel factor, which elicits dose-dependent chemotactic and chemokinetic effects. However, the impact and function of TPO on decidual cells has not been clarified yet. This study analyses the expression and function of TPO and its receptor c-Mpl in decidua during early gestation. Applying western blot analysis, we detected that TPO is expressed by decidual immune cells (uNK cells and CD14+ monocytes) as well as CTB and decidual stromal cells (DSCs). Expression of the different isoforms of c-Mpl was found in uNK cells, CD14+ monocytes and DSC. Studying the signalling pathway proteins in the uNK cells, an activation of STAT3/Tyr by TPO, was detected. The investigation of the proliferative effects of TPO on the decidual cell subsets revealed that TPO enhances the proliferation of uNK cells and CTB. No change of the proliferative activity after TPO incubation was found in DSC and even a decrease in CD14+ monocytes. In addition, TPO was observed to induce significantly the migratory activity of uNK cells, CD14+ monocytes and CTB. Investigating the effects of TPO on the cytokine profile of the isolated decidual cells, we observed a decrease in the secretion of IL-8, IL-10 and IL-1β of isolated uNK cells, CD14+ monocytes and CTB, although these changes did not reach statistical significance. Thus, we here identified TPO as a novel factor modulating the proliferation, migration and possibly cytokine secretion of decidual cell subsets.

Inflammation stimulates thrombopoietin (Tpo) expression in rat brain-derived microvascular endothelial cells, but suppresses Tpo in astrocytes and microglia

Thrombopoietin (Tpo) and its receptor (c-Mpl; TpoR), which primary regulate megakaryopoiesis and platelet production, are also expressed in the central nervous system (CNS). Increased Tpo concentrations are present in the cerebrospinal fluid (CSF) of some patients with bacterial or viral meningitis. Since previous data implicated a proapoptotic role of Tpo on newly generated neuronal cells, we herein elucidated the regulation of Tpo in primary rat neurons (e17), astrocytes, and microglia (p0-p3), as well as in brain-derived vascular endothelial cells of 3-week-old rats after exposure to bacterial lipopolysaccharide (LPS). LPS inhibited Tpo gene expression in astrocytes and microglia, but not in neurons, most likely due to absence of Toll-like receptor 4 in neurons. While Tpo mRNA expression recovered in astrocytes after 24 h, it remained suppressed in microglia. Furthermore, we detected Tpo mRNA expression in primary brain-derived vascular endothelial cells, which also express the TpoR. In these cells, LPS significantly up-regulated Tpo mRNA expression. TpoR mRNA and protein expression remained constitutive in all cell types. Thus, our data provide evidence for a cell-type-specific modulation of Tpo mRNA expression by inflammation in brain-derived cells. Transient down-regulation of Tpo expression in astrocytes and microglia may limit Tpo-induced neuronal cell death in inflammatory brain disorders.

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.