Structure of the thrombopoietin-MPL receptor complex is a blueprint for biasing hematopoiesis

Thrombopoietin (THPO or TPO) is an essential cytokine for hematopoietic stem cell (HSC) maintenance and megakaryocyte differentiation. Here, we report the 3.4 Å resolution cryoelectron microscopy structure of the extracellular TPO-TPO receptor (TpoR or MPL) signaling complex, revealing the basis for homodimeric MPL activation and providing a structural rationalization for genetic loss-of-function thrombocytopenia mutations. The structure guided the engineering of TPO variants (TPOmod) with a spectrum of signaling activities, from neutral antagonists to partial- and super-agonists. Partial agonist TPOmod decoupled JAK/STAT from ERK/AKT/CREB activation, driving a bias for megakaryopoiesis and platelet production without causing significant HSC expansion in mice and showing superior maintenance of human HSCs in vitro. These data demonstrate the functional uncoupling of the two primary roles of TPO, highlighting the potential utility of TPOmod in hematology research and clinical HSC transplantation.

The thrombopoietin receptor: revisiting the master regulator of platelet production

Thrombopoietin (TPO) and its receptor, MPL, are the primary regulators of platelet production and critical for hematopoietic stem cell (HSC) maintenance. Since TPO was first cloned in 1994, the physiological and pathological roles of TPO and MPL have been well characterized, culminating in the first MPL agonists being approved for the treatment of chronic immune thrombocytopenia in 2008. Dysregulation of the TPO-MPL signaling axis contributes to the pathogenesis of hematological disorders: decreased expression or function results in severe thrombocytopenia progressing to bone marrow failure, while hyperactivation of MPL signaling, either by mutations in the receptor or associated Janus kinase 2 (JAK2), results in pathological myeloproliferation. Despite its importance, it was only recently that the long-running debate over the mechanism by which TPO binding activates MPL has been resolved. This review will cover key aspects of TPO and MPL structure and function and their importance in receptor activation, discuss how these are altered in hematological disorders and consider how a greater understanding could lead to the development of better-targeted and more efficacious therapies.

Hematological consequences of malaria in mice previously treated for visceral leishmaniasis

Background: Polyparasitism is commonplace in countries where endemicity for multiple parasites exists, and studies in animal models of coinfection have made significant inroads into understanding the impact of often competing demands on the immune system. However, few studies have addressed how previous exposure to and treatment for one infection impacts a subsequent heterologous infection.   Methods: We used a C57BL/6 mouse model of drug-treated Leishmania donovani infection followed by experimental Plasmodium chabaudi AS malaria, focusing on hematological dysfunction as a common attribute of both infections. We measured parasite burden, blood parameters associated with anemia and thrombocytopenia, and serum thrombopoietin. In addition, we quantified macrophage iNOS expression through immunohistological analysis of the liver and spleen.   Results: We found that the thrombocytopenia and anemia that accompanies primary L. donovani infection was rapidly reversed following single dose AmBisome® treatment, along with multiple other markers associated with immune activation (including restoration of tissue microarchitecture and reduced macrophage iNOS expression). Compared to naive mice, mice cured of previous L. donovani infection showed comparable albeit delayed clinical responses (including peak parasitemia and anemia) to P. chabaudi AS infection. Thrombocytopenia was also evident in these sequentially infected mice, consistent with a decrease in circulating levels of thrombopoietin. Architectural changes to the spleen were also comparable in sequentially infected mice compared to those with Plasmodium infection alone. Conclusions: Our data suggest that in this sequential infection model, previously-treated L. donovani infection has limited impact on the subsequent development of Plasmodium infection, but this issue deserves further attention in models of more severe disease or through longitudinal population studies in humans.

Hematological consequences of malaria infection in mice previously treated for visceral leishmaniasis

Background: Polyparasitism is commonplace in countries where endemicity for multiple parasites exists, and studies in animal models of coinfection have made significant inroads into understanding the impact of often competing demands on the immune system. However, few studies have addressed how previous exposure to and treatment for one infection impacts a subsequent heterologous infection.   Methods: We used a C57BL/6 mouse model of drug-treated Leishmania donovani infection followed by experimental Plasmodium chabaudi AS malaria, focusing on hematological dysfunction as a common attribute of both infections. We measured parasite burden, blood parameters associated with anemia and thrombocytopenia, and serum thrombopoietin. In addition, we quantified macrophage iNOS expression through immunohistological analysis of the liver and spleen.   Results: We found that the thrombocytopenia and anemia that accompanies primary L. donovani infection was rapidly reversed following single dose AmBisome® treatment, along with multiple other markers associated with immune activation (including restoration of tissue microarchitecture and reduced macrophage iNOS expression). Compared to naive mice, mice cured of previous VL showed comparable albeit delayed clinical responses (including peak parasitemia and anemia) to P. chabaudi AS infection. Thrombocytopenia was also evident in these sequentially infected mice, consistent with a decrease in circulating levels of thrombopoietin. Architectural changes to the spleen were also comparable in sequentially infected mice compared to those with malaria alone. Conclusions: Our data suggest that in this sequential infection model, previously-treated VL has limited impact on the subsequent development of malaria, but this issue deserves further attention in models of more severe disease or through longitudinal population studies in humans.

Dissecting pathways to thrombocytopenia in a mouse model of visceral leishmaniasis

Visceral leishmaniasis is an important yet neglected parasitic disease caused by infection with Leishmania donovani or L infantum. Disease manifestations include fever, weight loss, hepatosplenomegaly, immune dysregulation, and extensive hematological complications. Thrombocytopenia is a dominant hematological feature seen in both humans and experimental models, but the mechanisms behind this infection-driven thrombocytopenia remain poorly understood. Using a murine model of experimental visceral leishmaniasis (EVL), we demonstrated a progressive decrease in platelets from day 14 after infection, culminating in severe thrombocytopenia by day 28. Plasma thrombopoietin (TPO) levels were reduced in infected mice, at least in part because of the alterations in the liver microenvironment associated with granulomatous inflammation. Bone marrow (BM) megakaryocyte cytoplasmic maturation was significantly reduced. In addition to a production deficit, we identified significant increases in platelet clearance. L donovani-infected splenectomized mice were protected from thrombocytopenia compared with sham operated infected mice and had a greater response to exogenous TPO. Furthermore, infection led to higher levels of platelet opsonization and desialylation, both associated with platelet clearance in spleen and liver, respectively. Critically, these changes could be reversed rapidly by drug treatment to reduce parasite load or by administration of TPO agonists. In summary, our findings demonstrate that the mechanisms underpinning thrombocytopenia in EVL are multifactorial and reversible, with no obvious residual damage to the BM microenvironment.

Tuning MPL signaling to influence hematopoietic stem cell differentiation and inhibit essential thrombocythemia progenitors

Thrombopoietin (TPO) and the TPO-receptor (TPO-R, or c-MPL) are essential for hematopoietic stem cell (HSC) maintenance and megakaryocyte differentiation. Agents that can modulate TPO-R signaling are highly desirable for both basic research and clinical utility. We developed a series of surrogate protein ligands for TPO-R, in the form of diabodies (DBs), that homodimerize TPO-R on the cell surface in geometries that are dictated by the DB receptor binding epitope, in effect "tuning" downstream signaling responses. These surrogate ligands exhibit diverse pharmacological properties, inducing graded signaling outputs, from full to partial TPO agonism, thus decoupling the dual functions of TPO/TPO-R. Using single-cell RNA sequencing and HSC self-renewal assays we find that partial agonistic diabodies preserved the stem-like properties of cultured HSCs, but also blocked oncogenic colony formation in essential thrombocythemia (ET) through inverse agonism. Our data suggest that dampening downstream TPO signaling is a powerful approach not only for HSC preservation in culture, but also for inhibiting oncogenic signaling through the TPO-R.

Quantitative Optical Diffraction Tomography Imaging of Mouse Platelets

Platelets are specialized anucleate cells that play a major role in hemostasis following vessel injury. More recently, platelets have also been implicated in innate immunity and inflammation by directly interacting with immune cells and releasing proinflammatory signals. It is likely therefore that in certain pathologies, such as chronic parasitic infections and myeloid malignancies, platelets can act as mediators for hemostatic and proinflammatory responses. Fortunately, murine platelet function ex vivo is highly analogous to human, providing a robust model for functional comparison. However, traditional methods of studying platelet phenotype, function and activation status often rely on using large numbers of whole isolated platelet populations, which severely limits the number and type of assays that can be performed with mouse blood. Here, using cutting edge 3D quantitative phase imaging, holotomography, that uses optical diffraction tomography (ODT), we were able to identify and quantify differences in single unlabeled, live platelets with minimal experimental interference. We analyzed platelets directly isolated from whole blood of mice with either a JAK2V617F-positive myeloproliferative neoplasm (MPN) or Leishmania donovani infection. Image analysis of the platelets indicates previously uncharacterized differences in platelet morphology, including altered cell volume and sphericity, as well as changes in biophysical parameters such as refractive index (RI) and dry mass. Together, these data indicate that, by using holotomography, we were able to identify clear disparities in activation status and potential functional ability in disease states compared to control at the level of single platelets.

Single-Cell Analyses Reveal Megakaryocyte-Biased Hematopoiesis in Myelofibrosis and Identify Mutant Clone-Specific Targets

Myelofibrosis is a severe myeloproliferative neoplasm characterized by increased numbers of abnormal bone marrow megakaryocytes that induce fibrosis, destroying the hematopoietic microenvironment. To determine the cellular and molecular basis for aberrant megakaryopoiesis in myelofibrosis, we performed single-cell transcriptome profiling of 135,929 CD34+ lineage- hematopoietic stem and progenitor cells (HSPCs), single-cell proteomics, genomics, and functional assays. We identified a bias toward megakaryocyte differentiation apparent from early multipotent stem cells in myelofibrosis and associated aberrant molecular signatures. A sub-fraction of myelofibrosis megakaryocyte progenitors (MkPs) are transcriptionally similar to healthy-donor MkPs, but the majority are disease specific, with distinct populations expressing fibrosis- and proliferation-associated genes. Mutant-clone HSPCs have increased expression of megakaryocyte-associated genes compared to wild-type HSPCs, and we provide early validation of G6B as a potential immunotherapy target. Our study paves the way for selective targeting of the myelofibrosis clone and illustrates the power of single-cell multi-omics to discover tumor-specific therapeutic targets and mediators of tissue fibrosis.

Mechanism of homodimeric cytokine receptor activation and dysregulation by oncogenic mutations

Homodimeric class I cytokine receptors are assumed to exist as preformed dimers that are activated by ligand-induced conformational changes. We quantified the dimerization of three prototypic class I cytokine receptors in the plasma membrane of living cells by single-molecule fluorescence microscopy. Spatial and spatiotemporal correlation of individual receptor subunits showed ligand-induced dimerization and revealed that the associated Janus kinase 2 (JAK2) dimerizes through its pseudokinase domain. Oncogenic receptor and hyperactive JAK2 mutants promoted ligand-independent dimerization, highlighting the formation of receptor dimers as the switch responsible for signal activation. Atomistic modeling and molecular dynamics simulations based on a detailed energetic analysis of the interactions involved in dimerization yielded a mechanistic blueprint for homodimeric class I cytokine receptor activation and its dysregulation by individual mutations.

CD4+ T Cells Alter the Stromal Microenvironment and Repress Medullary Erythropoiesis in Murine Visceral Leishmaniasis

Human visceral leishmaniasis, a parasitic disease of major public health importance in developing countries, is characterized by variable degrees of severity of anemia, but the mechanisms underlying this change in peripheral blood have not been thoroughly explored. Here, we used an experimental model of visceral leishmaniasis in C57BL/6 mice to explore the basis of anemia following infection with Leishmania donovani. 28 days post-infection, mice showed bone marrow dyserythropoiesis by myelogram, with a reduction of TER119+ CD71-/+ erythroblasts. Reduction of medullary erythropoiesis coincided with loss of CD169high bone marrow stromal macrophages and a reduction of CXCL12-expressing stromal cells. Although the spleen is a site of extramedullary erythropoiesis and erythrophagocytosis, splenectomy did not impact the extent of anemia or affect the repression of medullary hematopoiesis that was observed in infected mice. In contrast, these changes in bone marrow erythropoiesis were not evident in B6.Rag2-/- mice, but could be fully reconstituted by adoptive transfer of IFNγ-producing but not IFNγ-deficient CD4+ T cells, mimicking the expansion of IFNγ-producing CD4+ T cells that occurs during infection in wild type mice. Collectively, these data indicate that anemia during experimental murine visceral leishmaniasis can be driven by defects associated with the bone marrow erythropoietic niche, and that this represents a further example of CD4+ T cell-mediated immunopathology affecting hematopoietic competence.

Thrombopoietin from beginning to end

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

Thrombopoietin promotes β1-integrin–mediated adhesion in hematopoietic cells via the small GTPase Rap1

OBJECTIVE

The interactions between cells and extracellular matrices in the bone marrow microenvironment are critical for normal hematopoiesis, controlling cell survival, proliferation, differentiation, and motility. A number of hematopoietic growth factors and cytokines can mediate these interactions by changing expression and/or activity of specific integrins, or by changing cell shape. Thrombopoietin (TPO) has previously been shown to stimulate adhesion. in certain hematopoietic cell types, although the exact mechanisms by which adhesion is promoted remain elusive.

MATERIALS AND METHODS

The role of TPO in hematopoietic cell adhesion was determined with fibronectin adhesion and binding assays, flow cytometry, and immunocytochemistry using the hematopoietic cell line UT-7/TPO and bone marrow-derived primary mouse megakaryocytes. The role of Rapl in TPO-mediated adhesion was determined using a RaplGAP overexpressing UT-7/TPO cell line, in which Rapl could not be activated.

RESULTS

We found that TPO promoted hematopoietic cell adhesion by causing cytoskeletal reorganization and not by increasing integrin expression, localization, or affinity, as previously hypothesized. Through studies using the UT-7/TPO-RaplGAP cell line, we found that TPO-mediated cell shape change occurred via activation of Rapl.

CONCLUSIONS

These data demonstrate an important role for TPO in mediating interactions in the bone marrow microenvironment and make a significant contribution to our understanding of how TPO may affect hematopoiesis.

Glutamate receptors on human melanocytes regulate the expression of MiTF

Glutamate is the major excitatory neurotransmitter in the central nervous system but has also important functions in the epidermis. It is involved in keratinocyte barrier function and in re-epithelialization processes after wounding. Recently, glutamate signalling has been suggested to be implicated in the development of melanoma. The present study examined the expression and functionality of metabotropic and ionotropic glutamate receptors on normal human melanocytes. We found that cultured melanocytes expressed the ionotropic glutamate receptors GluR2 and 4 [alpha-amino-3-hydroxy-5-methyl-4-isoxsazolepropionic acid (AMPA) receptors] and N-methyl-d-aspartate (NMDA) receptors 2A and 2C and possibly the metabotropic glutamate receptor 1. Melanocytes were also found to express specific glutamate transporters and decarboxylases, but appeared neither to produce nor to release l-glutamate. Stimulation with 10 or 100 microM AMPA or NMDA elevated intracellular calcium concentrations in melanocytes, and thus demonstrated the functionality of the glutamate receptors. Millimolar concentrations of l-glutamate did not induce melanocyte toxicity and had no stimulating effect on melanin production. However, blockage of AMPA and NMDA receptors with CFM-2, memantine or MK801 caused a rapid and reversible change in melanocyte morphology, which was associated with disorganisation of actin and tubulin microfilaments. After 24 h of treatment with the AMPA receptor inhibitor CFM-2, there was a sharp reduction in the expression of the crucial melanocyte differentiation and proliferation factor MiTF. The results of this study demonstrate a role for glutamate in melanocyte regulation that may have implications in melanocyte associated disorders.

Are unconventional NMDA receptors involved in slowly adapting type I mechanoreceptor responses?

Specific immunohistochemical staining for NMDA receptor NR2A/B subunits was found in the outer root sheath layer of rat sinus hair (whisker) follicle. Co-localization with CK 20 confirmed that Merkel cells were stained. The NR2A/B staining seen on Merkel cells was pericellular. In addition it appeared that NF70-positive staining was in close proximity to, but did not colocalise with NR2A/B immunoreactivity, indicating that NR2A/B was only expressed by Merkel cells and not their adjacent nerve terminals. Merkel cells and the nerve terminals have previously been associated with electrophysiological recordings from slowly adapting type I (St I) mechanoreceptor unit activity. Pharmacological experiments with isolated sinus hairs using a wide range of ionotropic glutamate receptor antagonists found that only certain NMDA receptor blockers depressed St I unit responses to mechanical stimuli. AMPA/kainate receptor antagonists (CNQX and NBQX, 100 microM) had no effect, nor did classical competitive NMDA receptor antagonists, D-AP5 (600 microM) and R-CPP (100 microM), nor the NMDA glycine site antagonist 5,7-dichlorokynurenic acid (100 microM). The only effective NMDA receptor blockers were those selective for the polyamine site: ifenprodil (IC50 20 microM) and Ro 25-6981 (IC50 approximately 50 microM), and the associated ion channel: MK 801, ketamine and (+/-)-1-(1,2-diphenylethyl)piperidine (IC50 < 100 microM). The two enantiomers of MK 801 were equipotent. All effects were long lasting, consistent with their non-/uncompetitive actions. The most potent drug tested, ifenprodil, at an effective dose of 30 microM, had a mean recovery time of 74 min. A three-fold increase in drug concentration was required to depress St II units (associated with non-synaptic lanceolate endings). Changes in Zn2+ did not affect St I unit responses. These data suggest that unconventional NMDA receptors are involved in St I unit responses, but question the notion of a glutamatergic synapse between the Merkel cell and nerve terminal.

Essential components for a glutamatergic synapse between Merkel cell and nerve terminal in rats

The exact role of Merkel cells and their possible involvement in mechanosensation is unclear. The aim of this study was to determine, in the adult rat sinus hair follicle, the expression pattern of a number of vesicular proteins involved in neurotransmitter release to provide a clearer understanding of Merkel cell signalling mechanisms. We identified prominent expression and co-localization of the glutamatergic vesicle loading proteins VGLUT1 and VGLUT2 at the site of the sinus hair follicle known to be densely populated with Merkel cells. We also found expression of the vesicle recycling proteins synaptogyrin and syntaxin-6 in the same region of the hair follicle. Our data suggest that glutamate signalling is involved in Merkel cell mechanosensation and that vesicular trafficking is commonplace in the Merkel cell-neurite complex.

Emerging neuroskeletal signalling pathways: a review

Recent work has demonstrated that neurotransmitters, signalling molecules primarily associated with the nervous system, can have profound effects on the skeleton. Bone cells express a broad range of neurotransmitter receptors and transporters, and respond to receptor activation by initiating diverse intracellular signalling pathways, which modulate cellular function. Evidence of neuronal innervation in skeletal tissues, neurotransmitter release directly from bone cells and functional effects of pharmacological manipulation support the existence of a complex and functionally significant neurotransmitter-mediated signalling network in bone. This review aims to concisely summarise our current understanding of how neurotransmitters affect the skeletal system, focusing on their origin, cellular targets and functional effects in bone.

NMDA receptor-mediated regulation of human megakaryocytopoiesis

Identification of the regulatory inputs that direct megakaryocytopoiesis and platelet production is essential for the development of novel therapeutic strategies for the treatment of thrombosis and related hematologic disorders. We have previously shown that primary human megakaryocytes express the N-methyl-d-aspartate acid (NMDA) receptor 1 (NR1) subunit of NMDA-type glutamate receptors, which appear to be pharmacologically similar to those identified at neuronal synapses, responsible for mediating excitatory neurotransmission in the central nervous system. However, the functional role of NMDA receptor signaling in megakaryocytopoiesis remains unclear. Here we provide evidence that demonstrates the fundamental importance of this signaling pathway during human megakaryocyte maturation in vitro. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of RNA extracted from CD34+-derived megakaryocytes identified expression of NR2A and NR2D receptor subunits in these cells, as well as the NMDA receptor accessory proteins, Yotiao and postsynaptic density protein 95 (PSD-95). In functional studies, addition of a selective NMDA receptor antagonist, MK-801 inhibited proplatelet formation, without affecting proliferation or apoptosis. Exposure of CD34+ cells to MK-801 cultured for 14 days in the presence of thrombopoietin induced a decrease in expression of the megakaryocyte cell surface markers CD61, CD41a, and CD42a compared with controls. At an ultrastructural level, MK-801-treated cells lacked alpha-granules, demarcated membranes, and multilobed nuclei, which were prominent in untreated mature megakaryocyte controls. Using immunohistochemistry on sections of whole tibiae from c-Mpl knockout mice we demonstrated that megakaryocytic NMDA receptor expression was maintained following c-Mpl ablation. These data support a fundamental role for glutamate signaling in megakaryocytopoiesis and platelet production, which is likely to be independent of thrombopoietin-mediated effects.

Enhanced expression of vimentin in motile prostate cell lines and in poorly differentiated and metastatic prostate carcinoma

BACKGROUND

The metastatic potential of a series of prostate cell lines was analysed by measuring motility and invasiveness, and further correlated to the expression of epithelial differentiation markers.

METHODS

Invasion and motility were measured using in vitro assays. Immunohistochemistry of cell lines and tissues was used to identify expression of cytokeratins 8 and 1, 5, 10, 14, vimentin, prostate specific antigen, prostate specific membrane antigen, androgen receptor, desmoglein, E-cadherin, beta1 integrin, CD44, hmet, vinculin and actin.

RESULTS

Expression of vimentin was the only marker to correlate with motility, no markers correlated to invasion. Lower vimentin expression was observed in cells with low motility (PNT2-C2) and high expression in cells with high motility (P4E6, PNT1a, PC-3). Vimentin expression was not detected in well differentiated tumours, moderately differentiated tumours contained vimentin positive cells (1/9 bone scan negative, 2/5 bone scan positive), but the majority of poorly differentiated cancers (4/11 bone scan negative, 9/14 bone scan positive) and bone metastases (7/8) had high vimentin expression in tumour cells.

CONCLUSIONS

Motile prostate cancer cell lines express vimentin. In tissue sections, the presence of vimentin positive tumour cells correlated positively to poorly differentiated cancers and the presence of bone metastases.