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Identification of early biomarkers of heart injury and drug-induced cardiotoxicity is important to eliminate harmful drug candidates early in preclinical development and to prevent severe drug effects. The main objective of this study was to investigate the expression of microRNAs (miRNAs) in human-induced pluripotent stem cell cardiomyocytes (hiPSC-CM) in response to a broad range of cardiotoxic drugs. Next generation sequencing was applied to hiPSC-CM treated for 72 h with 40 drugs falling into the categories of functional (i.e., ion channel blockers), structural (changes in cardiomyocytes structure), and general (causing both functional and structural) cardiotoxicants as well as non-cardiotoxic drugs. The largest changes in miRNAs expression were observed after treatments with structural or general cardiotoxicants. The number of deregulated miRNAs was the highest for idarubicin, mitoxantrone, and bortezomib treatments. RT-qPCR validation confirmed upregulation of several miRNAs across multiple treatments at therapeutically relevant concentrations: hsa-miR-187-3p, hsa-miR-146b-5p, hsa-miR-182-5p (anthracyclines); hsa-miR-365a-5p, hsa-miR-185-3p, hsa-miR-184, hsa-miR-182-5p (kinase inhibitors); hsa-miR-182-5p, hsa-miR-126-3p and hsa-miR-96-5p (common some anthracyclines, kinase inhibitors and bortezomib). Further investigations showed that an upregulation of hsa-miR-187-3p and hsa-miR-182-5p could serve as a potential biomarker of structural cardiotoxicity and/or an additional endpoint to characterize cardiac injury in vitro.

We applied a set of in silico and in vitro assays, compliant with the CiPA (Comprehensive In Vitro Proarrhythmia Assay) paradigm, to assess the risk of chloroquine or hydroxychloroquine‐mediated QT prolongation and Torsades de Pointes (TdP), alone and combined with erythromycin and azithromycin, drugs repurposed during the first wave of COVID‐19. Each drug or drug combination was tested in patch clamp assays on 7 cardiac ion channels, in in silico models of human ventricular electrophysiology (Virtual Assay®) using control (healthy) or high‐risk cell populations, and in human induced pluripotent stem cell (hiPSC)‐derived cardiomyocytes. In each assay, concentration‐response curves encompassing and exceeding therapeutic free plasma levels were generated. Both chloroquine and hydroxychloroquine showed blocking activity against some potassium, sodium and calcium currents. Chloroquine and hydroxychloroquine inhibited IKr (IC50: 1µM and 3‐7µM, respectively) and IK1 currents (IC50: 5 and 44µM, respectively). When combining hydroxychloroquine with azithromycin, no synergistic effects were observed. The two macrolides had no or very weak effects on the ion currents (IC50>300‐1000µM). Using Virtual Assay®, both antimalarials affected several TdP indicators, chloroquine being more potent than hydroxychloroquine. Effects were more pronounced in the high‐risk cell population. In hiPSC‐derived cardiomyocytes, all drugs showed early‐after‐depolarizations, except azithromycin. Combining chloroquine or hydroxychloroquine with a macrolide did not aggravate their effects. In conclusion, our integrated nonclinical CiPA dataset confirmed that, at therapeutic plasma concentrations relevant for malaria or off‐label use in COVID‐19, chloroquine and hydroxychloroquine use is associated with a proarrhythmia risk, which is higher in populations carrying predisposing factors but not worsened with macrolide combination.

Safety pharmacology is an essential part of drug development aiming to identify, evaluate and investigate undesirable pharmacodynamic properties of a drug primarily prior to clinical trials. In particular, cardiovascular adverse drug reactions (ADR) have halted many drug development programs. Safety pharmacology has successfully implemented a screening strategy to detect cardiovascular liabilities, but there is room for further refinement. In this setting, we present the INSPIRE project, a European Training Network in safety pharmacology for Early Stage Researchers (ESRs), funded by the European Commission's H2020-MSCA-ITN programme. INSPIRE has recruited 15 ESR fellows that will conduct an individual PhD-research project for a period of 36 months. INSPIRE aims to be complementary to ongoing research initiatives. With this as a goal, an inventory of collaborative research initiatives in safety pharmacology was created and the ESR projects have been designed to be complementary to this roadmap. Overall, INSPIRE aims to improve cardiovascular safety evaluation, either by investigating technological innovations or by adding mechanistic insight in emerging safety concerns, as observed in the field of cardio-oncology. Finally, in addition to its hands-on research pillar, INSPIRE will organize a number of summer schools and workshops that will be open to the wider community as well. In summary, INSPIRE aims to foster both research and training in safety pharmacology and hopes to inspire the future generation of safety scientists.

The development of improved, innovative models for the detection of toxicity of drugs, chemicals, or chemicals in cosmetics is crucial to efficiently bring new products safely to market in a cost-effective and timely manner. In addition, improvement in models to detect toxicity may reduce the incidence of unexpected post-marketing toxicity and reduce or eliminate the need for animal testing. The safety of novel products of the pharmaceutical, chemical, or cosmetics industry must be assured; therefore, toxicological properties need to be assessed. Accepted methods for gathering the information required by law for approval of substances are often animal methods. To reduce, refine, and replace animal testing, innovative organotypic in vitro models have emerged. Such models appear at different levels of complexity ranging from simpler, self-organized three-dimensional (3D) cell cultures up to more advanced scaffold-based co-cultures consisting of multiple cell types. This review provides an overview of recent developments in the field of toxicity testing with in vitro models for three major organ types: heart, skin, and liver. This review also examines regulatory aspects of such models in Europe and the UK, and summarizes best practices to facilitate the acceptance and appropriate use of advanced in vitro models.

Identification of improved translatable biomarkers of nephrotoxicity is an unmet safety biomarker need. Fatty-acid-binding protein 4 (FABP4) was previously found to be associated with clinical renal dysfunction and was proposed as a biomarker of glomerular damage. The aim of this study was to evaluate FABP4 as a potential preclinical biomarker of drug-induced kidney injury (DIKI). Han-Wistar rats were dosed with cisplatin [2.5 mg/kg, single, intraperitoneally (i.p.)], puromycin (10 mg/kg, daily, i.p.) or N-phenylanthranylic acid [NPAA, 500 mg/kg, daily, per os (p.o.)] over a 28-day period to induce proximal tubule, glomerular or collecting duct injury, respectively. An increase in urinary FABP4 levels was observed on days 1 and 3 after NPAA treatment and on days 14, 21, and 28 after puromycin treatment, whereas cisplatin treatment had no effect. No significant changes were reported for plasma levels of FABP4 after any treatment. Interestingly, immunohistochemical analysis showed a marked decrease in FABP4 expression in the loop of Henle on day 7 after NPAA treatment and a complete loss of FABP4 expression on day 14 after puromycin treatment. The magnitude of increase in FABP4 urinary levels in response to NPAA and puromycin was higher than for established preclinical biomarkers serum creatinine, clusterin, or cystatin C. Our results suggest that FABP4 has the potential for preclinical application as a biomarker of DIKI.

Most cell-surface receptors for cytokines and growth factors signal as dimers, but it is unclear whether remodeling receptor dimer topology is a viable strategy to “tune” signaling output. We utilized diabodies (DA) as surrogate ligands in a prototypical dimeric receptor-ligand system, the cytokine Erythropoietin (EPO) and its receptor (EpoR), to dimerize EpoR ectodomains in non-native architectures. Diabody-induced signaling amplitudes varied from full to minimal agonism, and structures of these DA/EpoR complexes differed in EpoR dimer orientation and proximity. Diabodies also elicited biased or differential activation of signaling pathways and gene expression profiles compared to EPO. Non-signaling diabodies inhibited proliferation of erythroid precursors from patients with a myeloproliferative neoplasm due to a constitutively active JAK2V617F mutation. Thus, intracellular oncogenic mutations causing ligand-independent receptor activation can be counteracted by extracellular ligands that re-orient receptors into inactive dimer topologies. This approach has broad applications for tuning signaling output for many dimeric receptor systems.

Primary familial and congenital polycythemia is characterized by erythropoietin hypersensitivity of erythroid progenitors due to germline nonsense or frameshift mutations in the erythropoietin receptor gene. All mutations so far described lead to the truncation of the C-terminal receptor sequence that contains negative regulatory domains. Their removal is presented as sufficient to cause the erythropoietin hypersensitivity phenotype. Here we provide evidence for a new mechanism whereby the presence of novel sequences generated by frameshift mutations is required for the phenotype rather than just extensive truncation resulting from nonsense mutations. We show that the erythropoietin hypersensitivity induced by a new erythropoietin receptor mutant, p.Gln434Profs*11, could not be explained by the loss of negative signaling and of the internalization domains, but rather by the appearance of a new C-terminal tail. The latter, by increasing erythropoietin receptor dimerization, stability and cell-surface localization, causes pre-activation of erythropoietin receptor and JAK2, constitutive signaling and hypersensitivity to erythropoietin. Similar results were obtained with another mutant, p.Pro438Metfs*6, which shares the same last five amino acid residues (MDTVP) with erythropoietin receptor p.Gln434Profs*11, confirming the involvement of the new peptide sequence in the erythropoietin hypersensitivity phenotype. These results suggest a new mechanism that might be common to erythropoietin receptor frameshift mutations. In summary, we show that primary familial and congenital polycythemia is more complex than expected since distinct mechanisms are involved in the erythropoietin hypersensitivity phenotype, according to the type of erythropoietin receptor mutation.

Investigation of genomic changes in cardiotoxicity can provide novel biomarkers and insights into molecular mechanisms of drug-induced cardiac injury (DICI). The main objective of this study was to identify and characterize dysregulated microRNAs (miRNAs) in the heart associated with cardiotoxicity. Wistar rats were dosed once with either isoproterenol (1.5 mg/kg, i.p), allylamine (100 mg/kg, p.o.) or the respective vehicle controls. Heart tissue was collected at 24 h, 48 h and 72 h post-drug administration and used for histopathological assessment, miRNA profiling, immunohistochemical analysis and in situ hybridization. Multiplex analysis of 68 miRNAs in the heart revealed a significant upregulation of several miRNAs (miR-19a-3p, miR-142-3p, miR-155-5p, miR-208b-3p, miR-21-5p) after isoproterenol and one miRNA (miR-21-5p) after allylamine administration. Localization of miR-21-5p was specific to inflammatory cell infiltrates in the heart after both treatments. Immunohistochemical analysis of Stat3, a known miR-21-5p regulator, also confirmed its upregulation in cardiomyocytes and inflammatory cell infiltrates. The toxicity signatures based on miRNA networks, identified in vivo, can potentially be used as mechanistic biomarkers as well as to study cardiotoxicity in vitro in order to develop sensitive tools for early hazard identification and risk assessment.

Impulsive-compulsive disorders in Parkinson’s disease patients have been described as behavioural or substance addictions including pathological gambling or compulsive medication use of dopamine replacement therapy. A substantial gap remains in the understanding of these disorders. We previously demonstrated that the rewarding effect of the D2/D3 agonist pramipexole was enhanced after repeated exposure to L-dopa and alpha-synuclein mediated dopaminergic nigral loss with specific transcriptional signatures suggesting a key involvement of the glutamatergic pathway. Here, we further investigate the therapeutic potential of metabotropic glutamate receptor 5 antagonism in Parkinson’s disease/dopamine replacement therapy related bias of reward-mediated associative learning. We identified protein changes underlying the striatal remodelling associated with the pramipexole-induced conditioned place preference. Acquisition and expression of the pramipexole-induced conditioned place preference were abolished by the metabotropic glutamate receptor 5 antagonist 2-methyl-6-phenylethynyl (pyridine) (conditioned place preference scores obtained with pramipexole conditioning were reduced by 12.5% and 125.8% when 2-methyl-6-phenylethynyl (pyridine) was co-administrated with pramipexole or after the pramipexole conditioning, respectively). Up-regulation of the metabotropic glutamate receptor 5 was found in the dorsomedial-striatum and nucleus accumbens core. Activation of these two brain sub-regions was also highlighted through FosB immunohistochemistry. Convergent molecular and pharmacological data further suggests metabotropic glutamate receptor 5 as a promising therapeutic target for the management of Parkinson’s disease/dopamine replacement therapy related reward bias.

Gain-of-function somatic mutations introducing cysteines to either the extracellular or to the transmembrane domain (TMD) in interleukin-7 receptor α (IL7R) or cytokine receptor-like factor 2 (CRLF2) have been described in acute lymphoblastic leukemias. Here we report noncysteine in-frame mutations in IL7R and CRLF2 located in a region of the TMD closer to the cytosolic domain. Biochemical and functional assays showed that these are activating mutations conferring cytokine-independent growth of progenitor lymphoid cells in vitro and are transforming in vivo. Protein fragment complementation assays suggest that despite the absence of cysteines, the mechanism of activation is through ligand-independent dimerization. Mutagenesis experiments and ConSurf calculations suggest that the mutations stabilize the homodimeric conformation, positioning the cytosolic kinases in predefined orientation to each other, thereby inducing spontaneous receptor activation independently of external signals. Hence, type I cytokine receptors may be activated in leukemia through 2 types of transmembrane somatic dimerizing mutations.

Mutations in the calreticulin gene (CALR) represented by deletions and insertions in exon 9 inducing a -1/+2 frameshift are associated with a significant fraction of myeloproliferative neoplasms (MPNs). The mechanisms by which CALR mutants induce MPN are unknown. Here, we show by transcriptional, proliferation, biochemical, and primary cell assays that the pathogenic CALR mutants specifically activate the thrombopoietin receptor (TpoR/MPL). No activation is detected with a battery of type I and II cytokine receptors, except granulocyte colony-stimulating factor receptor, which supported only transient and weak activation. CALR mutants induce ligand-independent activation of JAK2/STAT/phosphatydylinositol-3'-kinase (PI3-K) and mitogen-activated protein (MAP) kinase pathways via TpoR, and autonomous growth in Ba/F3 cells. In these transformed cells, no synergy is observed between JAK2 and PI3-K inhibitors in inhibiting cytokine-independent proliferation, thus showing a major difference from JAK2V617F cells where such synergy is strong. TpoR activation was dependent on its extracellular domain and its N-glycosylation, especially at N117. The glycan binding site and the novel C-terminal tail of the mutant CALR proteins were required for TpoR activation. A soluble form of TpoR was able to prevent activation of full-length TpoR provided that it was N-glycosylated. By confocal microscopy and subcellular fractionation, CALR mutants exhibit different intracellular localization from that of wild-type CALR. Finally, knocking down either MPL/TpoR or JAK2 in megakaryocytic progenitors from patients carrying CALR mutations inhibited cytokine-independent megakaryocytic colony formation. Taken together, our study provides a novel signaling paradigm, whereby a mutated chaperone constitutively activates cytokine receptor signaling.

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 [(125)I]-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. Stefan N. Constantinescu and William Vainchenker are co-last Author

Background: Autoantibodies, which are produced against tumor-associated antigens, are potential tumor markers and attract a growing interest for cancer detection, differential diagnostics and prognosis.Objective: To evaluate the diagnostic significance of 40 antigens identified by immunoscreening of cDNA libraries from thyroid and colon cancers by allogenic screening with different tumor types patients’ sera.Method: Plaque-spot serological assay.Results: Increased frequency of antibody response in sera of cancer patients compared with that of healthy donors was shown toward 14 antigens, 8 of which (CG016, BTN3A3, FKBP4, XRCC4, TSGA2, ACTR1A, FXYD3 and CTSH) have revealed exclusively cancer-related serological profile.Conclusion: Allogenic screening of 40 SEREX-antigens with sera from cancer patients and healthy donors allowed us to reveal 14 antigens with potential diagnostic significance. These antigens and their cognate autoantibodies could be considered as valuable targets for further analysis as poten...

Tumor-associated antigen MX35, which is overexpressed in 70-90% of epithelial ovarian cancers, has been recently identified as phosphate transporter NaPi2b. This finding has raised significant interest in understanding NaPi2b function under physiological conditions and its deregulation in human pathologies, such as cancer. As a member of the sodium-dependent phosphate transporter family, NaPi2b is primarily involved in the maintenance of phosphate homeostasis in the human body. The role of NaPi2b in oncogenic transformation and malignant growth is not well understood. To date, several monoclonal antibodies specific to NaPi2b have been reported. However, available monoclonal antibodies are not very efficient in recognizing endogenous NaPi2b under reducing conditions. In addition, these antibodies could not recognize the mutant form of transporter (NaPi2b-T330V). In this study we describe the production of monoclonal antibodies raised against the N-terminal region of NaPi2b. One of them, designated N-NaPi2b(15/1), possesses very useful immunological characteristics. We found that N-NaPi2b(15/1) specifically recognizes NaPi2b protein in immunohistochemical analysis and immunoprecipitation assay. Importantly, N-NaPi2b(15/1) antibody detects very efficiently endogenous and expressed wild-type and mutant forms of NaPi2b under both reducing and non-reducing conditions in Western blot analysis. These features make N-NaPi2b(15/1) antibody a very useful tool for studying the pattern of NaPi2b expression in health and pathologies.

The aim of present study was to develop a model for a functional analysis of a recently identified marker of the ovarian cancer - sodium-dependent phosphate transporter NaPi2b. For this purpose, we have created HEK293 stable cell lines expressing wild type or mutant forms of NaPi2b (T330V substitution in a large extracellular loop and a 6 amino acid residues deletion in the C-terminal cytoplasmic tail), revealed in the ovarian cancer cell lines. The expression of wild type and mutant forms NaPi2b in the stable cell lines created was confirmed by Western-blot analysis with monoclonal antibodies against NaPi2b. The cellular models described here will be useful for studying the function of sodium-dependent phosphate transporter NaPi2b in health and disease.

Homeostasis of inorganic phosphate in the human body is maintained by regulated absorption, metabolism, and excretion. Sodium-dependent phosphate transporters (NaPi) mediate the transport of inorganic phosphate (P(i)) in cells in response to dietary phosphate consumption, hormones, and growth factors. NaPi2b is a member of the sodium-dependent phosphate transporter family, with a distinct pattern of expression and regulation. Signaling pathways activated by mitogens, glucocorticoids, and metabolic factors have been implicated in regulating P(i) transport via NaPi2b. Inactivation of NaPi2b function by mutations has been linked to human pathologies, such as pulmonary alveolar microlithiasis. In this study, we describe the generation and characterization of monoclonal antibodies against human NaPi2b. The monoclonal antibodies were shown to recognize specifically transiently overexpressed and endogenous NaPi2b in commonly used immunoassays, including Western blotting, immunoprecipitation, and immunohistochemistry. These properties make them particularly valuable reagents for elucidating NaPi2b function in health and disease.

Ligand binding to the extracellular domain of the thrombopoietin receptor (TpoR) imparts a specific orientation on the receptors' transmembrane (TM) and intracellular domains that is required for physiologic activation via receptor dimerization. To map the inactive and active dimeric orientations of the TM helices, we performed asparagine (Asn) scanning mutagenesis of the TM domains of the murine and human TpoR. Substitution of Asn at only one position (S505N) activated the human receptor, while Asn substitutions at several positions activated the murine receptor. Second-site mutational studies indicate that His499 near the N-terminus of the TM domain is responsible for protecting the human receptor from activation by asparagine mutations. Structural studies reveal that the sequence preceding His499 is helical in the murine receptor, but non-helical in peptides corresponding to the TM domain of the inactive human receptor. The activating S505N mutation and the small molecule agonist eltrombopag both induce helix in this region of the TM domain, and are associated with dimerization and activation of the human receptor. Thus, His499 provides additional protection against activating mutations, such as oncogenic asparagine mutations in the TM domain, and regulates the activation of human TpoR. Stefan N. Constantinescu and Steven O. Smith are co-last Author

Despite intensive studies, the mechanisms of cytokine receptor activation are still not completely understood. Historically two models have been proposed: early studies suggested a mechanism involving ligand-induced di- merization of the receptors which results in the activation of downstream signaling pathways, whereas, in the absence of ligands, the receptors are thought to be in a monomeric inactive state; later studies uncovered evidence for the existence of many receptors as inactive preformed dimers, which upon ligand binding undergo conformational changes and/or relative rotation of the receptor molecules. Here we focus on the dimerization status of the single chain cytokine receptors, which are involved in the regulation of hematopoiesis. Незважаючи на інтенсивні дослідження, механізми активації цитокінових рецепторів наразі є не зовсім зрозумілими. Історично було запропоновано дві моделі: у ранніх роботах припускали механізм ліганд-індукованої димеризації рецепторів, що призводить до активації більш пізніх сигнальних шляхів, у той час як за відсутності лігандів рецептори перебувають у мономерому неактивованому стані; подальшими дослідженнями виявлено докази існування багатьох рецепторів у формі неактивованих сформованих димерів, які при зв’язуванні ліганда зазнають конформаційних змін та/або мономери яких починають ротаційно переміщуватися відносно один одного. Даний огляд сфокусовано на проблемі димеризації окремих гомодимерних рецепторів цитокінів, які причетні до регуляції кровотворення. Несмотря на интенсивные исследования, механизмы активации цитокиновых рецепторов до сих пор не полностью изучены. Исторически было предложено две модели: ранние работы предполагали механизм лиганд-индуцированной димеризации рецепторов, что приводит к активации более поздних сигнальных путей, в то время как при отсутствии лигандов рецепторы находятся в мономерном неактивном состоянии; последующие исследования выявили доказательства существования многих рецепторов в форме неактивированных сформированных димеров, которые при связывании лиганда претерпевают конформационные изменения и/или мономеры которых начинают ротационно перемещаться относительно друг друга. Данный обзор сфокусирован на проблеме димеризации отдельных гомодимерних рецепторов цитокинов, вовлеченнх в регуляцию кроветворения.

The discovery of the highly prevalent activating JAK (Janus kinase) 2 V617F mutation in myeloproliferative neoplasms, and of other pseudokinase domain-activating mutations in JAK2, JAK1 and JAK3 in blood cancers, prompted great interest in understanding how pseudokinase domains regulate kinase domains in JAKs. Recent functional and mutagenesis studies identified residues required for the V617F mutation to induce activation. Several X-ray crystal structures of either kinase or pseudokinase domains including the V617F mutant of JAK2 pseudokinase domains are now available, and a picture has emerged whereby the V617F mutation induces a defined conformational change around helix C of JH (JAK homology) 2. Effects of mutations on JAK2 can be extrapolated to JAK1 and TYK2 (tyrosine kinase 2), whereas JAK3 appears to be different. More structural information of the full-length JAK coupled to cytokine receptors might be required in order to define the structural basis of JH1 activation by JH2 mutants and eventually obtain mutant-specific inhibitors.

Dimerization of single-pass membrane receptors is essential for activation. In the human thrombopoietin receptor (TpoR), a unique amphipathic RWQFP motif separates the transmembrane (TM) and intracellular domains. Using a combination of mutagenesis, spectroscopy, and biochemical assays, we show that W515 of this motif impairs dimerization of the upstream TpoR TM helix. TpoR is unusual in that a specific residue is required for this inhibitory function, which prevents receptor self-activation. Mutations as diverse as W515K and W515L cause oncogenic activation of TpoR and lead to human myeloproliferative neoplasms. Two lines of evidence support a general mechanism in which W515 at the intracellular juxtamembrane boundary inhibits dimerization of the TpoR TM helix by increasing the helix tilt angle relative to the membrane bilayer normal, which prevents the formation of stabilizing TM dimer contacts. First, measurements using polarized infrared spectroscopy show that the isolated TM domain of the active W515K mutant has a helix tilt angle closer to the bilayer normal than that of the wild-type receptor. Second, we identify second-site R514W and Q516W mutations that reverse dimerization and tilt angle changes induced by the W515K and W515L mutations. The second-site mutations prevent constitutive activation of TpoR W515K/L, while preserving ligand-induced signaling. The ability of tryptophan to influence the angle and dimerization of the TM helix in wild-type TpoR and in the second-site revertants is likely associated with its strong preference to be buried in the headgroup region of membrane bilayers.

In this study we describe the identification of sodium-dependent phosphate transporter NaPi2b as MX35 cancer-associated antigen. To achieve this goal we have screened extensively a cDNA expressing library from ovarian cancer cell line OVCAR3 with monoclonal antibody MX35. To further confirm the authenticity of this finding, we showed that bacterially and baculovirally expressed NaPi2b is specifically recognized by MX35 antibody. Moreover, the validity of these results was verified in a parallel study involving affinity purification and mass spectrometry. The epitope for MX35 monoclonal antibody was mapped to the largest extracellular loop of NaPi2b. Taken together, this study uncovers the identity of MX35 antigen and provides molecular tools for studying its function in normal and cancer tissues. Натрій-залежний транспортер NaPi2b ідентифіковано як MX35 пухлинний антиген. Для цього здійснено скринування кДНК-експресуючої біблеотеки, отриманої з клітин OVCAR3, моноклональними антитілами MX35. Щоб підтвердити ці дослідження, встановлено, що NaPi2b, експресований в бактеріях та клітинах комах, специфічно розпізнається MX35 антитілами. Крім того, виявлено ділянку, яка містить епітоп для моноклональних антитіл на великому позаклітинному домені NaPi2b. Таким чином, нами визначено антиген MX35, що забезпечує молекулярну основу для подальшого вивчення його функції в нормальних тканинах та при раку. Натрий-зависимый транспортер NaPi2b идентифицирован как MX35 опухлевый антиген. Для достижения этого осуществлено скринирование кДНК-экспрессирующей библиотеки, полученной из OVCAR3 клеток, моноклональными антителами. Чтобы подтвердить эти результаты, установлено, что NaPi2b, экспрессированный в бактериях и в клетках насекомых, специфично узнается антителами MX35. Определен участок на внеклеточном домене NaPi2b, содержащий эпитоп для антител. Таким образом, нами идентифицирован антиген MX35, что создает молекулярную основу для дальнейшего изучения его функции в нормальных и раковых тканях.

Classical BCR-ABL-negative myeloproliferative neoplasms (MPN) include Polycythemia Vera (PV), Essential Thrombocytemia (ET) and Primary Myelofibrosis (PMF). They are malignant homeopathies resulting from the transformation of a multipotent hematopoietic stem cell (HSC). The common mechanism of transformation is the constitutive activation of the cytokine receptor/JAK2 pathway that leads to the myeloproliferation. The acquired point mutation JAK2V617F is the most prevalent (95% of PV and 60% of ET or PMF). In addition, other mutations affecting the same signaling pathway have been described such as JAK2 exon 12 mutations, mutations of MPL affecting W515, and loss-of-function mutations of LNK and also mutations of c-Cbl in 3% of PMF. Recently, whole exome sequencing allowed identifying a new recurrent genetic abnormalities in the exon 9 of the calreticulin gene (CALR) in about 30% of ET and PMF patients. All CALR mutants induce a frameshift of the same alternative reading frame and generate a novel C-terminus tail. To address the role of these new mutants in the pathophysiology of MPN, the goal of this study was to investigate the effect of the CALR mutant (del52 and ins5) expression by a retroviral mouse modeling. For that purpose, we transduced bone marrow cells with retrovirus expressing either CALRdel52, CALRins5, CALRWT or CALRDexon9 and performed a transplantation in lethally irradiated recipient mice (10 mice / group), which were then followed over one year. CALRdel52 expressing mice showed a rapid and strong increased in platelet counts (over 5 x106/mL) without any other changes in blood parameters during 6 months. In contrast, CALRins5 expressing mice presented platelet counts much lower than CALRdel52 but significantly higher than CALRWT or CALRDexon9 expressing mice. After 6 months, CALRdel52 expressing mice showed a decreased in platelets count associated with anemia and development of splenomegaly suggesting the progression to a myelofibrosis. Importantly, the disease was transplantable to secondary recipient for both CALRdel52 and CALRins5 mutants. The bone marrow and spleen were also analyzed over time. We observed a progressive increased in immature progenitors (SLAM cells) as well as a hypersensitivity of the megakaryocytic progenitors (CFU-MK) to thrombopoietin. Altogether, these results demonstrate that CALR mutants are able and sufficient to induce a thrombocytosis progressing to myelofibrosis in retroviral mouse model, thus mimicking the natural history of MPN patients. It will offer a good in vivo model to investigate therapeutic approaches for CALR-positive MPN. Disclosures No relevant conflicts of interest to declare.

Abstract 3449 Background and Aims. The thrombopoietin receptor (TpoR, c-Mpl) plays a dual role in hematopoiesis. Beside the fact that it is one of the main regulators of megacaryopoiesis and platelet formation, TpoR is also expressed in HSCs and maintains them quiescent. TpoR signals as a homodimer after ligand binding in a 1:2 (ligand to receptor) ratio. Similar to other single pass membrane receptor, it contains an extracellular ligand binding domain, one helical transmembrane domain (TMD) and a cytoplasmic part important for JAKs binding and to mediate signaling. Unlike other cytokine receptors, TpoR contains a unique amphipathic motif (RW515QFP) at the junction between the TMD and the cytoplasmic domain. Mutations within this motif, namely W515K or W515L, lead to activation of the receptor and are associated with JAK2 V617F negative myeloproliferative neoplasms, specifically essential thrombocytopenia and primary myelofibrosis. We asked i) how a tryptophan at position 515 prevents autonomous activation of TpoR and ii) exactly how mutants at W515 impact on TpoR function and lead to oncogenesis. Methods. A combination of site-directed mutagenesis, biochemical and signaling assays, and spectroscopy analysis were employed. We made several specific mutations at position 515 to see if Trp515 is regulatory because of it size and/or aromatic character. We then tested the effect of mutations at adjacent positions in the wild type or mutated W515 constructs. Data were recorded from transiently transfected g2A cells and the results were confirmed in stably transduced BaF3 cells using dual luciferase and growth assays in the presence or absence of ligand. Adoptive transfer in lethally irradiated mice was used for assessing in vivo effects of TpoR mutants. The oligomerization status of the full length TpoR and some of our mutants was tested by employing the Gaussia princeps luciferase complementation assay, where Gaussia princeps luciferase fragments were fused in frame to the carboxyl terminus of TpoR or TpoR mutants. Recombination of the luciferase fragments results in a positive signal and reveals dimerization. Deuterium magic angle spinning (MAS) NMR spectroscopy and analytical ultracentrifugation were used to assess dimerization of the region encompassing the TMD. The tilt angle of this segment was assessed by FTIR spectroscopy. Results. Tryptophan seems to be absolutely required at position 515 to maintain the receptor inactive in absence of ligand. We identified secondary mutations (to tryptophan) that can prevent W515L/K/A activation of TpoR. The doubly mutated receptors behave like wild type receptors exhibiting no activation in absence of TPO and a normal response to TPO. These data suggest that W515 mutants may be specifically targeted for therapy. Mutations at W515 induced an increase in full length TpoR dimerization, and promoted dimerization of the segment comprising the TMD. However, our secondary site mutations showed differences between the mechanisms by which K or L versus A activate at W515. Overall, W515 impairs TpoR dimerization in a conformation productive for JAK2 activation. Measurements of the dichroic ratio by FTIR indicated that W515 increases the tilt angle of the TMD, thus reducing the interface required for TMD dimerization. We further explored the role of W515 in receptor activation by using a model where coiled coils are used to induce distinct dimeric conformations when fused to the TMD-cytosolic domains. W515 was essential for imparting differential signaling to the different dimers, both in cell lines and in vivo, where phenotypes in reconstituted mice no longer differ between receptors that have different dimeric orientations. Conclusions. Tryptophan 515 prevents TpoR dimerization in a productive orientation for JAK2 activation. This effect is reversible when secondary site mutations to tryptophan are introduced at certain positions around W515. Dimerization of TMD induced by ligand rotates W515 so that it is displaced from the headgroup region of the membrane to the dimer interface. This is crucial for imparting specific signaling as a function of dimeric orientation. Our results indicate that the region around W515 is a major switch for receptor function and could be exploited therapeutically. Disclosures: Constantinescu: Novartis: Membership on an entity's Board of Directors or advisory committees; Shire: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Amgen: Membership on an entity's Board of Directors or advisory committees.

Sodium-dependent phosphate transporter NaPi2b is involved in transport of inorganic phosphate and the maintenance of phosphate homeostasisin human body. NaPi2b wasrecently identified as a marker of ovarian cancer, termed MX35. Monoclonal antibody (mAb) against transporter NaPi2b, called MX35, demonstrated therapeutic efficacy in radioimmunotherapy of patients with ovarian cancer. Aim. The present experiments explored whether MX35 antibody can affect function of NaPi2b to transport inorganic phosphate ions in cellular models. Methods. HEK293 cells stably expressing wild type NaPi2b and mutant NaPi2b_T330V, which could not be recognized by MX35 antibody, were incubated with MX35 antibody and analyzed by phosphate uptake assay. Results. Cells expressing wild type NaPi2b showed reduced phosphate uptake after incubation with MX35 antibody at concentration 50 µg/ml. No significant changes in phosphate transport were detected for cells expressing NaPi2b_T330V at the same experimental conditions. Conclusions. Our results demonstrate 1,8-fold decrease of NaPi2b-mediated phosphate transport in HEK293 cells stably expressing wild type NaPi2b after MX35 antibody application, which can be considered as a specific inhibitor of NaPi2b function. Keywords: NaPi2b, MX35 antibody. Натрій-залежний транспортер фосфату NaPi2b бере участь у перенесенні неорганічного фосфату та підтриманні фосфатного гомеостазу в організмі людини. Нещодавно NaPi2b описано як маркер раку яєчника МХ35. Моноклональні антитіла (МКАТ) проти транспортера NaPi2b, які отримали назву МХ35, виявляють терапевтичну ефективність при лікуванні хворих на рак яєчника. Мета даної роботи полягала в тому, щоб з’ясувати, чи впливає зв’язування МКАТ МХ35 з NaPi2b на його функцію транспортувати іони неорганічного фосфату в клітинних моделях. Методи. Клітини лінії НЕК293, що стабільно експресують дикий тип NaPi2b та мутантну форму NaPi2b_T330V, яка не розпізнається антитілами МХ35, інкубували з антитілами МХ35, після чого аналізували поглинання клітинами радіоактивно міченого фосфату. Результати. Після інкубації з антитілами МХ35 у концентрації 50 мкг/мл відмічено значне зменшення поглинання фосфату клітинами, які експресують дикий тип NaPi2b. Для клітин, що експресують мутантну форму NaPi2b_T330V, за аналогічних експериментальних умов значних змін у поглинанні фосфату не виявлено. Висновки. Наші дані свідчать про 1,8-разове зниження NaPi2b-залежного транспорту фосфату в результаті інкубації з МКАТ МХ35, які можуть слугувати специфічним інгібітором функції NaPi2b. Ключові слова: NaPi2b, моноклональні антитіла МХ35. Натрий-зависимый транспортер фосфата NaPi2b участвует в переносе неорганического фосфата и поддержании фосфатного гомеостаза в организме человека. Недавно NaPi2b описан как маркер рака яичника МХ35. Моноклональные антитела (МКАТ) против транспортера NaPi2b, названные МХ35, проявляют терапевтическую эффективность при лечении больных раком яичника. Цель данной работы состояла в выяснении того, влияет ли связывание МКАТ МХ35 с NaPi2b на его функцию транспортировать ионы неорганического фосфата в клеточных моделях. Методы. Клетки линии НЕК293, стабильно экспрессирующие дикий тип NaPi2b и мутантную форму NaPi2b_T330V, не распознаваемую антителами МХ35, инкубировали с антителами МХ35, после чего анализировали поглощение клетками радиоактивно меченного фосфата. Результаты. После инкубации с антителами МХ35 в концентрации 50 мкг/мл отмечено значительное уменьшение поглощения фосфата клетками, экспрессирующими дикий тип NaPi2b. Для клеток, экспрессирующих мутантную форму транспортера NaPi2b_T330V, не выявлено существенных изменений в поглощении фосфата при аналогичных экспериментальных условиях. Выводы. Наши результаты свидетельствуют о 1,8-кратном снижении NaPi2b-зависимого транспорта фосфата в результате инкубации с МКАТ МХ35, которые можно рассматривать как специфический ингибитор функции NaPi2b. Ключевые слова: NaPi2b, моноклональные антитела МХ35.