Dysregulation of galectins and global protein glycosylation have been reported in various cancers, but their role in myeloproliferative neoplasms (MPNs) have remained incompletely understood. We performed single-cell RNA sequencing (scRNA-seq) which revealed significant enrichment of galectin genes in MPN monocytes. Cell-cell communication analysis predicted monocytes as a pivotal mediator of cell interactions and galectin signaling as a robust input/output pathway for monocytes. We identified elevated expression of galectin-1 (Gal-1, LGALS1) in monocytes from both human MPN samples and mouse models. Mass cytometry (CyTOF) profiling of MPN blood samples demonstrated that recombinant galectin-1 (rGal-1) significantly increased levels of multiple inflammatory cytokines in monocytes without affecting other cell types. Incubation of CD14+ monocytes from MPN patients with rGal-1 led to markedly increased transcription and secretion of inflammatory cytokines. Mechanistically, we uncovered crosstalk between the TLR4 and Gal-1 signaling pathways, as evidenced by protein 3D modeling and co-immunoprecipitation. Notably, TLR4 inhibition abrogated Gal-1 mediated proinflammatory effects in monocytes. We further identified NF-κB-dependent signaling as a key downstream effector of Gal-1, as reporter assays demonstrated rGal-1 mediated activation of NF-κB signaling in a TLR4-dependent manner. We corroborated these findings in vivo in a murine model driven by MPLW515L in which genetic abrogation of Lgals1 ameliorated key MPN disease features, including leukocytosis and splenomegaly. Additionally, Gal-1 inhibition suppressed carrageenan-induced thrombosis and inflammation in vivo. In summary, we identify Gal-1 enrichment in MPN monocytes as a driver of monocyte-mediated inflammation through TLR4 and NF-κB activation and uncover a novel therapeutic avenue for MPNs.

ABSTRACT:Platelet shape and volume changes are early mechanical events contributing to platelet activation and thrombosis. Here, we identify single-nucleotide polymorphisms in leucine-rich repeat-containing 8 (LRRC8) protein subunits that form the volume-regulated anion channel (VRAC), which are independently associated with altered mean platelet volume. LRRC8A is required for functional VRAC in megakaryocytes (MKs) and regulates platelet volume; adhesion; and agonist-stimulated activation, aggregation, adenosine triphosphate (ATP) secretion, and calcium mobilization. MK-specific LRRC8A conditional knockout mice have reduced laser injury-induced cremaster arteriolar thrombus formation and prolonged FeCl3 induced carotid arterial thrombosis without prolonged bleeding times. Mechanistically, platelet LRRC8A mediates swell-induced cytosolic ATP release to amplify agonist-stimulated calcium-phosphoinositide 3-kinase-protein kinase B signaling. Small-molecule LRRC8 channel inhibitors recapitulate defects observed in LRRC8A-null platelets in vitro and in vivo. These studies identify the mechanoresponsive LRRC8 channel complex as an ATP release channel in platelets, which positively regulates platelet function and thrombosis, providing a proof of concept for a novel antithrombotic drug target.

During activation, platelets undergo both shape and volumetric changes which are mechanical events essential for thrombosis. Volume-Regulated Anion Channels (VRACs) are ubiquitously expressed mechanoresponsive, heterohexameric anion channels comprised of leucine-rich repeat containing protein 8 (LRRC8) A (essential for channel activity) in combination with B, C, D and/or E subunits. In this study, we identify single-nucleotide polymorphisms (SNPs) in three LRRC8 subunits (LRRC8A, C, and D) which are expressed in human platelets and are independently associated with altered mean platelet volume in human genetic studies – implicating the volume-sensing LRRC8 channel complex as a regulator of platelet function in humans. We show that LRRC8A is required for functional LRRC8/VRAC in megakaryocytes (MKs). MK-specific LRRC8A conditional knockout (cKO) mice give rise to LRRC8A-null platelets with larger mean volumes. LRRC8A-null platelets and CRISPR-edited human LRRC8A KO MKs (hiPSC MKs) have reduced agonist-stimulated P-selectin exposure and aIIbb3 integrin activation. LRRC8A-null platelets exhibit impaired adhesion to collagen-coated surfaces and reduced agonist-stimulated aggregation and ATP secretion. Additionally, VRAC in MKs mediate ATP currents which are abolished in LRRC8A-null MKs. In vivo, MK-specific LRRC8A cKO mice have reduced platelet thrombus formation in laser-induced cremaster arteriolar thrombosis, and prolonged occlusion times in FeCl 3 -induced carotid arterial thrombosis, compared to control mice, without affecting tail bleeding times. Mechanistically, platelet LRRC8A mediates swell-induced ATP release to amplify agonist-stimulated calcium influx and platelet aggregation. Small-molecule LRRC8 channel inhibitors largely recapitulate the defects observed in LRRC8A-null platelets. These studies identify the mechanoresponsive LRRC8 channel complex as an ATP release channel in platelets which regulates platelet function and arterial thrombosis, providing a proof-of-concept for a novel anti-thrombotic drug target.

Activating mutations in the calcium sensor STIM1 underlie Stormorken Syndrome (SS), a rare congenital disorder that causes thrombocytopenia and tubular aggregate myopathy. We recently identified two patients with SS and confirmed mutations in STIM1 (R304W and S88G), who were found to have features consistent with myelofibrosis (MF), as indicated by bone marrow biopsies demonstrating megakaryocyte hyperplasia and atypia in conjunction with marked reticulin fibrosis. The unusual MF findings in these two patients raised the hypothesis that altered calcium signaling represents a shared hallmark of congenital platelet disorders such as Stormorken Syndrome and myeloproliferative neoplasms (MPNs) including MF. Analyzing gene expression data from public datasets, we found increased expression of STIM1 in megakaryocyte progenitors and platelets, as well as CD34+ hematopoietic stem/progenitor cells (HSPCs), from MPN patients (including those with CALR or JAK2 mutations) vs healthy controls. These findings led us to hypothesize that dysregulation of STIM1 and store operated calcium entry (SOCE) signaling may contribute to altered megakaryopoiesis and development of fibrosis in both SS and MPNs. To further delineate these processes, we analyzed samples from our two SS patients, demonstrating constitutive activation of SOCE causing increased calcium flux, as well as decreased aggregation in mature platelets. We also performed transmission electron microscopy (TEM) and identified platelets with abnormal granularity in dense and alpha granules. We also performed single cell RNA-seq on SS patient peripheral blood mononuclear cells, which revealed enhanced NFkB inflammatory signaling, suggesting that altered signaling driven by STIM1 mutations may drive aberrant inflammation to contribute to MF development. To corroborate these initial observations, we identified a separate family cohort of 9 individuals in Italy with SS and confirmed STIM1 mutation (L92V). In ex vivo megakaryocytic differentiation assays, cells derived from these patients demonstrated a defect in thrombopoiesis with significantly decreased pro-platelet formation and adhesion to extracellular matrix. To further characterize the effects of the gain of function STIM1 mutations seen in our patients, we developed a novel, conditional knock-in mouse model of the heterozygous R304W mutation in the coiled coil (CC) domain of Stim1. Induction of hematopoietic-specific expression of mutant Stim1 via Vav-Cre resulted in recapitulation of hematological features of SS, including thrombocytopenia and a mild bleeding tendency, with confirmation of increased SOCE activity and calcium flux. Notably, we confirmed the development of bone marrow fibrosis, as well as findings of severe osteosclerosis, in these mice. TEM imaging confirmed that platelets from Stim1R304W/+Vav-Cre mice exhibit abnormal granularity, similar to what we observed from our SS patients. Noting that we identified increased STIM1 expression in samples from MPN patients harboring either JAK2 or CALR mutations, we further characterized the role of STIM1 in MPN disease development. NSGS mice were engrafted with CALR-mutant CD34+ cells subjected to CRISPR ablation of STIM1. Recipient mice exhibited decreased human CD45+ cell engraftment in conjunction with prolonged survival. In contrast, targeting of STIM1 in JAK2-mutant CD34+ cells led to exacerbated disease phenotypes, as manifested by enhanced human CD45+ cell engraftment, worsened splenomegaly, and early lethality. Taken together, these findings suggest a striking, differential relationship between mutant JAK2 and CALR and STIM1 activity and their relationship to MF development. Altogether, this study represents the first demonstration of MF development in patients with Stormorken Syndrome, and reveals a previously unrecognized hallmark of altered calcium signaling via aberrant STIM1 activation underlying SS and MPNs. We further demonstrate recapitulation of these hematologic features in a novel knock-in Stim1R304W/+Vav-Cre mouse model. These findings implicate an important role for STIM1and SOCE activity in MF development, and further uncover important distinctions between JAK2 vs CALR mutations and their interaction with altered STIM1 activity. These studies may contribute to the development of novel therapeutic approaches for these disorders.

Platelet shape and volume changes are early mechanical events contributing to platelet activation and thrombosis. Here, we identify single-nucleotide polymorphisms in Leucine-Rich Repeat Containing 8 (LRRC8) protein subunits that form the Volume-Regulated Anion Channel (VRAC) which are independently associated with altered mean platelet volume. LRRC8A is required for functional VRAC in megakaryocytes (MKs) and regulates platelet volume, adhesion, and agonist-stimulated activation, aggregation, ATP secretion and calcium mobilization. MK-specific LRRC8A cKO mice have reduced arteriolar thrombus formation and prolonged arterial thrombosis without affecting bleeding times. Mechanistically, platelet LRRC8A mediates swell-induced ATP/ADP release to amplify agonist-stimulated calcium and PI3K-AKT signaling via P2X1, P2Y 1 and P2Y 12 receptors. Small-molecule LRRC8 channel inhibitors recapitulate defects observed in LRRC8A-null platelets in vitro and in vivo . These studies identify the mechanoresponsive LRRC8 channel complex as an ATP/ADP release channel in platelets which regulates platelet function and thrombosis, providing a proof-of-concept for a novel anti-thrombotic drug target.

JDP and STO co-corresponding authors Previous studies by our group and others have elucidated a key role for monocytes in driving hyperinflammation in myeloproliferative neoplasms (MPNs). To further interrogate the cellular landscape of aberrant inflammation in MPNs, we subjected MPN patient samples to single cell RNA-seq (scRNA-seq) analysis and found significant enrichment of inflammation-related genes, including galectins, in MPN patient monocytes. Cell-cell communication networks inferred from expression of ligands and receptors predicted monocytes as a pivotal mediator of cell interactions and galectin signaling as one of the most robust input/output pathways for monocytes. Galectins are a class of proteins that bind to glycosylated proteins and mediate broad biological functions, including cell proliferation, apoptosis, adhesion, and inflammation. Dysregulation of galectins and global protein glycosylation has been reported in various cancers, but its role in MPN has remained incompletely understood. To validate our scRNA-seq findings, we performed flow cytometry analysis which demonstrated elevated expression of galectin-1 (Gal-1) in MPN patient CD14+ monocytes. Increased plasma levels of Gal-1 were also identified in MPN patients compared to healthy individuals. We also observed enrichment of Gal-1 in mouse CD11b+ myeloid cells, with further elevation in JAK2V617F knock-in mice compared to wild-type controls. Additionally, Gal-1 expression was induced by MPLW515L and JAK2V617F and inhibited by ruxolitinib, a JAK inhibitor, in Ba/F3 cells. Thus, our results confirm enrichment of Gal-1 in MPN monocytes, with evidence of direct contribution from specific MPN driver mutations. To comprehensively characterize functional effects of Gal-1, we performed mass cytometry (CyTOF) analysis of MPN samples which demonstrated that Gal-1 stimulation induced multiple inflammatory cytokines, including TNF and IL-6, in monocytes specifically without affecting other types of cells. We further incubated CD14+ monocytes from MPN patients with recombinant Gal-1 (rGal-1) and observed markedly stimulated transcription and secretion of inflammatory cytokines, such as IL-1α, IL-1β, IL-6, IL-8 and TNF. Pharmacologic inhibition of Gal-1 by OTX008 suppressed the expression and secretion of inflammatory cytokines in MPN monocytes and monocytic cell lines. Furthermore, we observed crosstalk between LPS-TLR4 and Gal-1 signaling pathways. Using protein 3D structure prediction and co-immunoprecipitation, we confirmed interaction of Gal-1 with TLR4. Notably, targeting TLR4 via both neutralizing antibody and pharmacologic inhibition (TAK-242) abrogated the proinflammatory effects of Gal-1 on monocytes. Taken together, these findings uncover a monocyte-specific pro-inflammatory effect of Gal-1 mediated by TLR4 in MPN. To explore molecular mechanisms underlying its proinflammatory functions, we performed gene co-expression analysis for Gal-1 in monocytes and identified OXPHOS and PI3K-AKT-mTOR signaling pathways as top candidates. Both genetic and pharmacologic inhibition of Gal-1 reduced cellular ATP levels and oxygen consumption rate, suggesting a metabolic reprogramming by Gal-1 in MPN. Incubation of monocytic cell lines and MPN monocytes with rGal-1 stimulated PI3K-AKT-mTOR signaling, represented by increased levels of phosphorylated mTOR, AKT and S6. Consistently, genetic and pharmacologic inhibition of Gal-1 inhibited activation of the PI3K-AKT-mTOR pathway. Across in vivo models, both Gal-1 knockout and OTX008 ameliorated key MPN disease features, including leukocytosis and splenomegaly, driven by MPLW515L and JAK2V617F. Gal-1 inhibition also suppressed carrageenan-induced thrombosis and inflammation in mice. We further evaluated therapeutic effects of targeting global glycosylation in MPN via 2-Deoxy-D-glucose (2-DG), a global glycosylation inhibitor, which decreased splenomegaly and reduced elevated platelets and hematocrit in JAK2V617F knock-in mice. In summary, we identify enrichment of Gal-1 in MPN monocytes, potentially due to activation of JAK2-driven signaling. We further demonstrate that Gal-1 fuels monocyte inflammation by interacting with TLR4 and activation of OXPHOS and PI3K-AKT-mTOR signaling pathways. Our results uncover a novel therapeutic avenue for targeting aberrant Gal-1 and global glycosylation in MPNs.

BACKGROUND:Platelet-rich thrombi occlude arteries causing fatal infarcts like heart attacks and strokes. Prevention of thrombi by current antiplatelet agents can cause major bleeding. Instead, we propose using N-acetyl cysteine (NAC) to act against the protein VWF (von Willebrand factor), and not platelets, to prevent arterial thrombi from forming.METHODS:NAC was assessed for its ability to prevent arterial thrombosis by measuring platelet accumulation rate and occlusion time using a microfluidic model of arterial thrombosis with human blood. Acute clot formation, clot stability, and tail bleeding were measured in vivo with the murine modified Folts model. The effect of NAC in the murine model after 6 hours was also measured to determine any persistent effects of NAC after it has been cleared from the blood.RESULTS:We demonstrate reduction of thrombi formation following treatment with NAC in vitro and in vivo. Human whole blood treated with 3 or 5 mmol/L NAC showed delayed thrombus formation 2.0× and 3.7× longer than control, respectively (P<0.001). Blood treated with 10 mmol/L NAC did not form an occlusive clot, and no macroscopic platelet aggregation was visible (P<0.001). In vivo, a 400-mg/kg dose of NAC prevented occlusive clots from forming in mice without significantly affecting tail bleeding times. A lower dose of NAC significantly reduced clot stability. Mice given multiple injections showed that NAC has a lasting and cumulative effect on clot stability, even after being cleared from the blood (P<0.001).CONCLUSIONS:Both preclinical models demonstrate that NAC prevents thrombus formation in a dose-dependent manner without significantly affecting bleeding time. This work highlights a new pathway for preventing arterial thrombosis, different from antiplatelet agents, using an amino acid derivative as an antithrombotic therapeutic.

BACKGROUND:Tissue factor pathway inhibitor (TFPI) regulates tissue factor-triggered coagulation. Humans and mice express transcripts encoding for multidistributed (endothelial, platelet, and plasma) 3-Kunitz domain TFPIα and endothelial membrane-anchored 2-Kunitz TFPIβ. Mice express a third transcript, γ, that encodes plasma lipoprotein-associated 2-Kunitz TFPI. In humans, proteolysis of α and/or β produces plasma lipoprotein-associated 2-Kunitz TFPI at lower levels. In clinical trials, monoclonal antibodies that target all TFPI isoforms extend coagulation and correct bleeding in hemophilic patients but with some thrombosis risks. OBJECTIVES:To determine the impact of TFPI isoform-specific deletions on promoting clotting in hemophilic mice. METHODS:Engineered TFPI isoform-specific, hemophilic (factor VIII-null) mice were evaluated for clotting. RESULTS:Mice expressing any single TFPI isoform were healthy. Thrombin generation assays identified TFPIγ as the dominant anticoagulation isoform in mouse plasma. Hemostasis was assessed by serial bleeding times from a tail vein laceration. Repeatedly, after a clot forms, it was manually disrupted; the number of clots/disruptions occurring over a 15-minute period were reported. C57BL/6 and hemophilic mice clot on average 25.6 vs 5.4 times, respectively. On a hemophilia background, TFPIβ or TFPIγ-specific deletion improved clotting to 14.6 and 15.2 times, respectively (P < .0001). TFPIα-specific deletion was without impact, clotting 5.1 times. Heterozygous deletion of TFPIβ was effective, clotting 11.8 times (P < .0001). Heterozygous deletion of TFPIα or TFPIγ alone was ineffective, clotting 3.0 and 6.1 times, respectively, but heterozygous TFPIαγ deletion improved clotting to 11.2 times (P < .001). CONCLUSION:In hemophilic mice, endothelial TFPIβ and plasma γ-derived 2-Kunitz TFPI individually contribute more to bleeding than total TFPIα.

Platelet shape and volume changes are early mechanical events in platelet activation proposed to contribute to arterial thrombosis. Here, we identify single-nucleotide polymorphisms (SNPs) in four leucine-rich repeat containing protein subunits (LRRC8A, B, C, and D) expressed in human platelets that are independently associated with increased mean platelet volume in human genetic studies - implicating the volume-sensing LRRC8 channel complex as regulating platelet function in humans. We show LRRC8A is required for functional LRRC8/VRAC in megakaryocytes (MKs). MK-specific LRRC8A conditional knockout mice give rise to LRRC8A-null platelets with larger mean volumes. LRRC8A-null platelets and CRISPR-edited human LRRC8A KO MKs (hiPSC MKs) have reduced agonist -stimulated P-selectin exposure, and αIIbβ3 integrin activation. LRRC8A-null platelets exhibit impaired adhesion to collagen-coated surfaces, agonist-induced platelet aggregation and ATP secretion. MK-specific LRRC8A conditional knockout mice have reduced platelet thrombus formation in laser-induced cremaster arteriolar thrombosis and prolonged occlusion times in FeCl 3 -induced carotid arterial thrombosis, compared to control mice, without affecting tail bleeding times. Mechanistically, LRRC8A mediates swelling-induced ATP release to amplify agonist-stimulated calcium influx and aggregation in platelets. Treatment of platelets with small-molecule LRRC8 channel inhibitors recapitulate the defects in LRRC8A-null platelets. These studies identify the mechanoresponsive LRRC8 channel complex as a regulator of platelet function and thrombosis - providing a proof-of-concept for a novel anti-thrombotic drug target.

Platelets from patients with myeloproliferative neoplasms (MPNs) exhibit a hyperreactive phenotype. Here, we found elevated P-selectin exposure and platelet-leukocyte aggregates indicating activation of platelets from essential thrombocythemia (ET) patients. Single cell RNA-seq analysis of primary samples revealed significant enrichment of transcripts related to platelet activation, mTOR and oxidative phosphorylation (OXPHOS) in ET patient platelets. These observations were validated via proteomic profiling. Platelet metabolomics revealed distinct metabolic phenotypes consisting of elevated ATP generation, accompanied by increases in the levels of multiple intermediates of the tricarboxylic acid (TCA) cycle, but lower alpha-ketoglutarate (α-KG) in MPN patients. Inhibition of PI3K/AKT/mTOR signaling significantly reduced metabolic responses and hyperreactivity in MPN patient platelets, while α-KG supplementation markedly reduced oxygen consumption and ATP generation. Ex vivo incubation of platelets from both MPN patients and Jak2 V617F mice with α-KG significantly reduced platelet activation responses. Oral α-KG supplementation of Jak2 V617F mice decreased splenomegaly and reduced hematocrit, monocyte and platelet counts. Finally, α-KG incubation significantly decreased proinflammatory cytokine secretion from MPN CD14+ monocytes. Our results reveal a previously unrecognized metabolic disorder in conjunction with aberrant PI3K/AKT/mTOR signaling, contributing to platelet hyperreactivity in MPN patients.

Key Points: Expression of any single TFPI isoform (α, β, or γ) is sufficient for mouse survival.Two-Kunitz domain TFPI isoforms, anchored β and circulating γ, each contribute to hemophilia bleeding.In vivo, TFPIβ is a more potent anticoagulant than TFPIα. Upon injury, tissue factor (TF) triggers coagulation by complexing with factor (F)VIIa to activate FIX and FX, leading to thrombin generation and a fibrin clot. Tissue factor pathway inhibitor limits TF-triggered coagulation. Humans and mice express transcripts encoding for 3-Kunitz domain TFPIα and membrane-anchored 2-Kunitz TFPIβ. Mice also express transcripts encoding untethered, 2-Kunitz TFPIγ. In humans, proteolysis of TFPIα and/or β produces “γ-like” 2-Kunitz TFPI. Hemophilia A and B, caused by deficiency in factors VIII and IX, respectively, are congenital X-linked recessive bleeding disorders that affect an estimated 1.1 million males worldwide. Hemophilia can cause excessive bleeding spontaneously or in response to trauma that can be life-threatening or significantly decrease the quality of life. Approved treatments for hemophilia include factor replacement, bypassing agents, the bispecific antibody Emicizumab, and recently approved gene therapies. Also in development are agents that reduce endogenous anticoagulant activity to counter the procoagulant deficiency of hemophiliacs, including monoclonal antibodies (Mab) to TFPI. These Mab target all TFPI isoforms to improve coagulation and correct bleeding in hemophilia patients, but with some demonstrated thrombosis risk. We investigated if TFPI-isoform-specific inhibition could provide equivalent efficacy which might reduce thrombotic risk. We generated TFPI-isoform-specific exon deletions in mice bred into hemophilia (FVIII-null) background. We find mice expressing any single TFPI isoform (α, β, or γ) appear healthy and reproduce. Our in vivo tail-vein re-bleeding assay is based on serial clot disruptions over 15 minutes, which we have shown is sensitive to anticoagulant and antiplatelet treatment, as well as factor replacement therapy. In this assay, we find C57Bl/6 WT mice clot 25.6 + 0.8 times (mean + SEM), whereas FVIII-null hemophilia mice clot 5.3 +0.4 times. In hemophilia mice, TFPIα-specific deletion is without impact, forming clots 5.1 + 0.8 times, whereas TFPIβ-specific or TFPIγ-specific deletion improves clot formation to 16.8 + 2.0 and 15.2 + 1.5 times, respectively (p<0001). Even hemophilia mice with heterozygotic deletion of the β exon form clots 11.8 + 0.8 times (p<0001). Thus, in vivo 2-Kunitz TFPI isoforms, anchored β and untethered γ each provides greater anticoagulant activity and impact on bleeding than 3-Kunitz TFPIα. Ex vivo plasma-based thrombin generation assays show that TFPIγ provides more anticoagulant activity than TFPIα. As shown in the figure, the deletion of TFPIγ increases thrombin generation to a greater extent than the deletion of TFPIα. The addition of an anti-TFPI antibody that effectively blocks TFPI activity shows a significant increase in endogenous thrombin potential in the plasma of TFPIαβ-deleted (γ-only) mice (370 nM.min), versus TFPIβγ-deleted (α-only) plasma (70 nM.min) and TFPIαγ-deleted (β-only) mice plasma (22 nM.min), (p<0.001). As expected, membrane-anchored TFPIβ is not detected in plasma. It is well established that 3-Kunitz TFPIα is a potent anticoagulant in plasma-based clotting time assays, while truncated 2-Kunitz TFPI, like mouse TFPIγ, appears inactive. Based on such assays, it was unexpected that TFPIγ-only expressing mice thrive, and in vivo, TFPIα is not the dominant endogenous anticoagulant. If mouse models are to be useful in providing insights into human hemostasis and TF-associated diseases, then it is critical to understand the similarities and differences between mice and humans. As humans do not express TFPIγ, the observed impact of depleting circulating 2-Kunitz TFPI on bleeding and thrombin generation in hemophilic mice is unlikely to translate to man. By contrast, our results demonstrating that, in vivo, TFPIβ is a more potent anticoagulant than TFPIα in mice has potential therapeutic implications in humans.

MB and VA equal contributors AB and STO co-corresponding authors This study was initiated following evaluation of a 32 year-old woman who presented with a history of thrombocytopenia identified in childhood who subsequently developed features of myelofibrosis (MF). A bone marrow biopsy demonstrated hypercellularity in conjunction with megakaryocyte hyperplasia and marked reticulin fibrosis. Molecular testing for JAK2, CALR, and MPL mutations was negative. Given the unusual association between congenital thrombocytopenia and MF in this patient, exome sequencing was performed, revealing a heterozygous R304W mutation in the coiled coil (CC) domain of STIM1. Activating mutations in the CC and EF hand domains of STIM1 have been associated with Stormorken syndrome, a rare congenital platelet disorder associated with abnormal store operated calcium entry (SOCE). We subsequently identified a second patient with MF associated with a STIM1 activating mutation. This individual was found to have severe thrombocytopenia at birth, and exome sequencing revealed a heterozygous S88G mutation in the EF hand of STIM1. A bone marrow biopsy obtained at 6 months of age revealed atypical megakaryocytes and grade 1-2 MF. A repeat biopsy at age 2 showed persistence of stable MF. The unusual finding of MF in these two patients suggested the possibility of altered calcium signaling as a shared mechanism driving congenital platelet disorders such as Stormorken syndrome and myeloproliferative neoplasms (MPNs) including MF. In support of this notion, we identified elevated STIM1 expression in MF vs normal megakaryocyte progenitors, as well as in platelets from patients with essential thrombocythemia (ET) vs healthy controls. Additionally, we found that STIM1 expression was significantly elevated in CD34+ hematopoietic stem/progenitor cells (HSPCs) from both ET and MF patients vs healthy controls. Notably, STIM1 expression was increased in both JAK2 and CALR-mutant MF patients. Collectively, these findings provide evidence of aberrant STIM1 expression in MPN patient cells. To determine the functional role of STIM1 in MPN disease development, colony assays and patient-derived xenograft (PDX) experiments were performed with MF patient CD34+ cells subjected to CRISPR ablation of STIM1. Strikingly discordant results were observed with JAK2 vs CALR-mutant patient samples. Abrogation of STIM1 in CALR-mutant CD34+ cells led to decreased colony formation, and NSGS mice engrafted in parallel with STIM1-targeted cells exhibited decreased human CD45+ cell engraftment in conjunction with prolonged survival. These findings suggest an important role for STIM1 in CALR-mutant MPN disease phenotypes. In contrast, targeting of STIM1 in JAK2-mutant CD34+ cells led to increased colony formation and exacerbated disease phenotypes in vivo as manifested by enhanced human CD45+ cell engraftment, worsened splenomegaly, and early lethality. Similar results were obtained in experiments utilizing pharmacologic inhibitors of SOCE activity. Taken together, these findings indicate that the consequences of aberrant STIM1 activity may be context-dependent relating to specific MPN driver mutations. To expand these observations, we identified a separate cohort of 9 family members in Italy with Stormorken syndrome and confirmed STIM1 EF hand mutations. In ex vivo megakaryocytic differentiation assays, cells from affected individuals exhibited a defect in proplatelet formation. These observations were corroborated by initial analyses of a newly generated Stim1 R304W conditional knock-in mouse which recapitulated the characteristic thrombocytopenia found in patients with Stormorken syndrome. In summary, this study represents the first demonstration of MF development in patients with Stormorken syndrome, thereby uncovering a previously unrecognized hallmark of altered calcium signaling via aberrant STIM1 activation underlying Stormorken syndrome and MPNs. Our findings suggest distinct mechanisms relating to the interaction between JAK2 vs CALR mutation and altered STIM1 activity. Further studies of these relationships may have important ramifications for potential therapeutic approaches targeting these pathways.

This report identifies a novel variant form of the inherited bleeding disorder Glanzmann thrombasthenia, exhibiting only mild bleeding in a physically active individual. The platelets cannot aggregate ex vivo with physiologic agonists of activation, although microfluidic analysis with whole blood displays moderate ex vivo platelet adhesion and aggregation consistent with mild bleeding. Immunocytometry shows reduced expression of & alpha;IIb & beta;3 on quiescent platelets that spontaneously bind/store fibrinogen, and activationdependent antibodies (ligand-induced binding site-319.4 and PAC-1) report & beta;3 extension suggesting an intrinsic activation phenotype. Genetic analysis reveals a single F153S & beta;3 substitution within the & beta;I-domain from a heterozygous T556C nucleotide substitution of ITGB3 exon 4 in conjunction with a previously reported IVS5(+1)G>A splice site mutation with undetectable platelet messenger RNA accounting for hemizygous expression of S153 & beta;3. F153 is completely conserved among & beta;3 of several species and all human & beta;-integrin subunits suggesting that it may play a vital role in integrin structure/function. Mutagenesis of & alpha;IIbF153S & beta;3 also displays reduced levels of a constitutively activated & alpha;IIb-S153 & beta;3 on HEK293T cells. The overall structural analysis suggests that a bulky aromatic, nonpolar amino acid (F,W)153 & beta;3 is critical for maintaining the resting conformation of & alpha;2- and & alpha;1-helices of the & beta;I-domain because small amino acid substitutions (S,A) facilitate an unhindered inward movement of the & alpha;2- and & alpha;1-helices of the & beta;I-domain toward the constitutively active & alpha;IIb & beta;3 conformation, while a bulky aromatic, polar amino acid (Y) hinders such movements and restrains & alpha;IIb & beta;3 activation. The data collectively demonstrate that disruption of F153 & beta;3 can significantly alter normal integrin/platelet function, although reduced expression of & alpha;IIbS153 & beta;3 may be compensated by a hyperactive conformation that promotes viable hemostasis.

JDP and STO co-corresponding authors Previous studies by our group and others have demonstrated that monocytes play a key role in driving hyperinflammation in myeloproliferative neoplasms (MPNs), and that aberrant inflammatory cytokine signaling contributes to disease progression and poor prognosis in MPNs. To interrogate relationships between cellular sources and targets of inflammatory cytokines, we performed single cell RNA-seq (scRNA-seq) in PBMCs and progenitor cells from MPN patients and healthy controls. Monocytes from MPN patients displayed significant enrichment of inflammation-relevant genes as expected. Cell-cell communication networks inferred from expression of ligands and receptors predicted monocytes as a pivotal mediator of cell interactions. Further analysis revealed galectin signaling as one of the most robust input/output pathways for monocytes and significant enrichment of LGALS1 (galectin-1) expression in MPN monocytes. Galectins are a class of proteins that bind specifically to β-galactoside carbohydrates, such as N-linked or O-linked glycosylated proteins. Abnormal expression of galectins in various cancers has been found to mediate broad biological functions, including cell proliferation, apoptosis, adhesion, and inflammation. However, the mechanisms by which galectins contribute to MPN pathogenesis remain incompletely understood. To corroborate our sRNA-seq findings, we performed flow cytometry analysis which demonstrated elevated expression of galectin-1 in MPN patient CD14+ monocytes. Increased plasma levels of galectin-1 were also identified in MPN patients compared to healthy individuals. Higher galectin-1 protein levels in both Kit+ progenitor cells and CD11b+ myeloid cells from Jak2 V617F knock-in mice were also observed when compared to wild type mice. Additionally, galectin-1 expression was induced by MPL W515L and JAK2 V617F and inhibited by ruxolitinib in Ba/F3 cells. Thus, our results show enrichment of galectin-1 in both MPN patients and mouse models, with evidence of direct contribution from specific MPN driver mutations. Our scRNA-seq dataset analysis showed significant correlations of galectin-1 expression with inflammatory and oxidative phosphorylation (OXPHOS) genes in MPN patient monocytes. To investigate the effects of galectin-1 in monocyte activation, we incubated CD14+ monocytes from MPN patients with recombinant galectin-1 (rGal-1) and observed markedly stimulated secretion of inflammatory cytokines, such as IL-1α, IL-1β, IL-6, IL-8 and TNF-α. Furthermore, OTX008, a galectin-1 inhibitor, inhibited the transcription and secretion of inflammatory cytokines in MPN patient CD14+ monocytes and monocytic cell lines. Notably, both genetic and pharmacologic inhibition of galectin-1 reduced cellular ATP level and oxygen consumption rate, suggesting a metabolic reprogramming in MPNs. Although galectin-1 mediates broad biological functions, the molecular mechanisms by which galectin-1 regulates signaling pathways and transcription in MPN cells had not been delineated. Our scRNA-seq heterogeneity analysis suggested that genes in the PI3K-AKT-mTOR pathway were differentially expressed between galectin-1 high- and low-expressing monocytes. We therefore incubated both monocytic cell lines and monocytes from MPN patients with rGal-1 and observed stimulation of the PI3K-AKT-mTOR pathway, represented by increased levels of phosphorylated mTOR, AKT and S6. Consistently, genetic and pharmacologic inhibition of galectin-1 inhibited activation of the PI3K-AKT-mTOR pathway. In summary, our results demonstrate upregulation of galectin-1 in MPN monocytes, potentially due to activation of JAK2-driven signaling. We further demonstrate that galectin-1 fuels inflammation potentially via metabolic reprogramming of monocytes and activation of PI3K-AKT-mTOR signaling. Our data also suggest galectin-1 as a putative therapeutic target in MPNs.

Sushi, von Willebrand factor type A, EGF and pentraxin domain containing 1 (SVEP1) is an extracellular matrix protein that causally promotes vascular disease and associates with platelet reactivity in humans. Here, using a human genomic and proteomic approach, we identify a high affinity, disease-relevant, and potentially targetable interaction between SVEP1 and the orphan receptor Platelet and Endothelial Aggregation Receptor 1 (PEAR1). This interaction promotes PEAR1 phosphorylation and disease associated AKT/mTOR signaling in vascular cells and platelets. Mice lacking SVEP1 have reduced platelet activation, and exogenous SVEP1 induces PEAR1-dependent activation of platelets. SVEP1 and PEAR1 causally and concordantly relate to platelet phenotypes and cardiovascular disease in humans, as determined by Mendelian Randomization. Targeting this receptor-ligand interaction may be a viable therapeutic strategy to treat or prevent cardiovascular and thrombotic disease.

von Willebrand factor (VWF) plays a key role in normal hemostasis, and deficiencies of VWF lead to clinically significant bleeding. We sought to identify novel modifiers of VWF levels in endothelial colony-forming cells (ECFCs) using single-cell RNA sequencing (scRNA-seq). ECFCs were isolated from patients with low VWF levels (plasma VWF antigen levels between 30 and 50 IU/dL) and from healthy controls. Human umbilical vein endothelial cells were used as an additional control cell line. Cells were characterized for their Weibel Palade body (WPB) content and VWF release. scRNA-seq of all cell lines was performed to evaluate for gene expression heterogeneity and for candidate modifiers of VWF regulation. Candidate modifiers identified by scRNA-seq were further characterized with small-interfering RNA (siRNA) experiments to evaluate for effects on VWF. We observed that ECFCs derived from patients with low VWF demonstrated alterations in baseline WPB metrics and exhibit impaired VWF release. scRNA-seq analyses of these endothelial cells revealed overall decreased VWF transcription, mosaicism of VWF expression, and genes that are differentially expressed in low VWF ECFCs and control endothelial cells (control ECs). An siRNA screen of potential VWF modifiers provided further evidence of regulatory candidates, and 1 such candidate, FLI1, alters the transcriptional activity of VWF. In conclusion, ECFCs from individuals with low VWF demonstrate alterations in their baseline VWF packaging and release compared with control ECs. scRNA-seq revealed alterations in VWF transcription, and siRNA screening identified multiple candidate regulators of VWF.

Background: Sushi, von Willebrand factor type A, EGF and pentraxin domain containing 1 (SVEP1) is an extracellular matrix protein that circulates in plasma and is causally related to cardiovascular disease, hypertension, and type 2 diabetes. A recent genome wide association study (GWAS) also implicates SVEP1 in platelet reactivity. The gene most strongly associated with platelet reactivity in the GWAS is Platelet and Endothelial Cell Receptor 1 ( PEAR1 ), a gene that encodes an orphan receptor tyrosine kinase-like protein that also associates with cardiovascular disease. Little is known about the molecular interactions and disease mechanisms of these proteins, despite their associations with cardiovascular disease and platelet reactivity. Methods: We used Mendelian Randomization (MR) and phenome-wide association studies to identify candidate SVEP1 protein interactions. We tested candidate interactions using recombinant proteins with molecular, cellular, and ex vivo assays. Results: A coding variant within the ectodomain of PEAR1 alters plasma levels of SVEP1 in humans (p=1.7x10^-17), suggesting these proteins may interact. MR demonstrates that genetically determined, increased plasma PEAR1 levels causally associate with decreased plasma SVEP1 (p=1.3x10^-11), further supporting an interaction. PEAR1 co-immunoprecipitates with SVEP1. Exposure to SVEP1 induces PEAR1 phosphorylation and activation of AKT/mTOR signaling in endothelial cells, vascular smooth muscle cells, and platelets. siRNA knockdown of PEAR1 abrogates this signaling. Endothelial cells adhere and proliferate in response to immobilized SVEP1. Platelets isolated from mice lacking SVEP1 have lower activation relative to controls and phenocopy platelets from mice lacking PEAR1. Conclusions: Despite robust evidence for the role of SVEP1 and PEAR1 in cardiovascular disease, critical gaps remain in our understanding of their disease mechanisms. Here, we identify SVEP1 as the first biological ligand of PEAR1 and a potent activator of AKT/mTOR signaling. The interaction between SVEP1 and PEAR1 is particularly intriguing, given the prominent role of AKT/mTOR in cardiovascular disease, and disrupting the interaction may be therapeutically beneficial.

This review describes von Willebrand factor (VWF)-mediated platelet function in inherited and acquired bleeding disorders, and current and novel approaches from bench to clinical practice used to perform research, diagnose, and treat bleeding disorders. Patients can present with either qualitative or quantitative defects in VWF and/or platelets, which can be inherited or acquired. Understanding the structure–function relationships of VWF and platelets has led to clinically useful classification and treatment of coagulopathy. However, current bench and clinical assays can be time-consuming and have limitations that preclude assessment of important mechanistic drivers, including physiologically relevant flow conditions pertinent to VWF-mediated platelet function. The purpose of this review is to discuss current and traditional laboratory-based assessment of VWF-mediated platelet function and the translation to clinical practice with specific focus on patient bleeding risks and the tools available for their management.

Platelet accumulation by VWF under high shear rates at the site of atherosclerotic plaque rupture leads to myocardial infarction and stroke. Current anti-platelet therapies remain ineffective for a large percentage of the population, while presenting significant risks for bleeding. We explore a novel way to inhibit arterial thrombus formation. Theoretically, a negative charge may influence the tertiary structure of VWF to favor the globular configuration by biophysical means without the use of platelet inactivating drugs. We tested this hypothesis experimentally for charged nanoparticles (CNPs) to inhibit thrombus formation in a microfluidic thrombosis assay (MTA). Several different CNPs demonstrated the ability to retard thrombotic occlusion in the MTA. A preliminary study in mice shows that thrombus stability is weaker with CNP administration and bleeding times are not markedly prolonged. The CNPs tested here show promise as a new class of antithrombotic therapies that act by biophysical means rather than biochemical pathways.