Constitutive hypercoagulability in pediatric sickle cell disease patients with hemoglobin SS genotype

Background

Constitutive inflammation and hemostatic activation have been identified as key contributors to the pathophysiology of sickle cell disease (SCD), leading to clinical consequences such as vaso-occlusive crises and stroke. Patients with hemoglobin SS (HbSS) and hemoglobin SC (HbSC) genotypes are reported to have different symptoms, as do patients in steady-state and crisis situations. Differences among these groups remain unclear in pediatric patients.

Objectives

To compare hemostatic activity in HbSS and HbSC pediatric patients during steady state, in crisis, and in clinical follow-up and compare HbSS and HbSC patients with normal healthy children.

Methods

Whole-blood coagulation assay thromboelastography (TEG) was used to assess hemostatic activity. In parallel, flow cytometry was used to assess procoagulant surface expression of platelets and red blood cells.

Results

TEG results indicated no significant differences in clotting onset (R time), clot maximum amplitude, or maximum rate of thrombus generation among steady-state, crisis, and follow-up subgroups of HbSS and HbSC patients. TEG parameters did not differ significantly between HbSC patients and healthy children, while HbSS patients showed significantly shorter R time and greater maximum amplitude and maximum rate of thrombus generation, all indicative of a constitutive hypercoagulable state. Flow cytometry results did not detect increased platelet integrin αIIbβ3 activation or red blood cell procoagulant surface expression in SCD patients compared with unaffected children.

Conclusion

Our results indicate that pediatric SCD patients with the HbSS genotype have constitutively activated hemostasis relative to HbSC patients and healthy children. It remains to be determined how treatments that improve clinical outcomes in SCD patients affect this constitutively hypercoagulable state.

Evaluation of human platelet granules by structured illumination laser fluorescence microscopy

Many roles of human platelets in health and disease are linked to their ability to transport and secrete a variety of small molecules and proteins carried in dense (δ-) and α-granules. Determination of granule number and content is important for diagnosis of platelet disorders and for studies of platelet structure, function, and development. We have optimized methods for detection and localization of platelet proteins via antibody and lectin staining, imaging via structured illumination laser fluorescence microscopy (SIM), and three-dimension (3D) image analysis. The methods were validated via comparison with published studies based on electron microscopy and high-resolution fluorescence microscopy. The α-granule cargo proteins thrombospondin-1 (TSP1), osteonectin (SPARC), fibrinogen (FGN), and Von Willebrand factor (VWF) were localized within the granule lumen, as was the proteoglycan serglycin (SRGN). Colocalization analysis indicates that staining with fluorescently labeled wheat germ agglutinin (WGA) allows detection of α-granules as effectively as immunostaining for cargo proteins, with the advantage of not requiring antibodies. RAB27B was observed to be concentrated at dense granules, allowing them to be counted via visual scoring and object analysis. We present a workflow for counting dense and α-granules via object analysis of 3D SIM images of platelets stained for RAB27B and with WGA.Abbreviation: SIM: structured illumination microscopy; WGA: wheat germ agglutinin; FGN: fibrinogen; TSP1: thrombospondin 1; ER: endoplasmic reticulum.

The Sec1/Munc18 protein VPS33B forms a uniquely bidirectional complex with VPS16B

Loss of function variants of VPS33B and VIPAS39 (encoding VPS16B) are causative for arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome, where early lethality of patients indicates that VPS33B and VPS16B play essential cellular roles. VPS33B is a member of the Sec1/Munc18 (SM) protein family, and thus thought to facilitate vesicular fusion via interaction with SNARE complexes, as does its paralog VPS33A in the homotypic fusion and vacuole sorting (HOPS) complex. VPS33B and VPS16B have been shown to associate, but little is known about the composition, structure or function of the VPS33B/VPS16B complex. We show here that human VPS33B/VPS16B is a high molecular weight complex, which we expressed in yeast to obtain material for structural, composition and stability analysis. Circular dichroism data indicate VPS33B/VPS16B has a well-folded α-helical secondary structure, for which size exclusion chromatography-multi angle light scattering revealed a MW of ∼315 kDa. Quantitative immunoblotting indicated the complex has a VPS33B:VPS16B ratio of 2:3. Expression of ARC syndrome-causing VPS33B missense variants showed that L30P disrupts complex formation, but not S243F or H344D. Truncated VPS16B containing amino acids 143-316 was sufficient to form a complex with VPS33B. Small angle X-ray scattering and negative staining electron microscopy revealed a two-lobed shape for VPS33B/VPS16B. Avidin tagging indicated that each lobe contains a VPS33B molecule, and they are oriented in opposite directions. From this we propose a structure for VPS33B/VPS16B that allows the copies of VPS33B at each end to interact with separate SNARE bundles and/or SNAREpins, plus their associated membrane components. Thus our observations reveal the only known potentially bidirectional SM protein complex.

Thromboelastography and thrombin generation assessments for pediatric severe hemophilia A patients are highly variable and not predictive of clinical phenotypes

Background

Severe hemophilia A (SHA) patients vary in severity of bleeding, arthropathy, and requirements for replacement factor VIII (FVIII). Baseline hemostatic activity assays using calibrated automated thrombography (CAT) and thromboelastography (TEG) may offer insights into the physiological basis of clinical heterogeneity.

Objectives

Use CAT and TEG to measure baseline hemostatic activity in a cohort of 30 pediatric SHA patients with available clinical data. Determine effect of contact activation inhibition with corn trypsin inhibitor (CTI). Assess heterogeneity among patients for baseline hemostatic activity and examine correlations between assay results and clinical parameters including FVIII dosing regimen, von Willebrand factor level, and Pettersson arthropathy score.

Methods

SHA blood after FVIII washout was subjected to TEG, and platelet‐rich (PRP) and platelet‐poor plasma was used for CAT assays. Varying concentrations of tissue factor (TF) were used. Statistical analysis examined relationships between assay results, and clinical parameters.

Results

CTI treatment was required to obtain TEG and CAT results representative of baseline hemostatic activity. Weak activity was observed in assays with low TF concentrations (0.5–2 pM), and most but not all samples approached normal activity levels at high TF concentrations (10–20 pM). A significant positive correlation was observed between results of TEG and CAT‐PRP assays. Correlations were not detected between hemostatic assay results and clinical parameters.

Conclusions

In vitro hemostatic assay results of samples containing platelets showed concordance. Assay results were not predictive of FVIII requirements or correlated with other clinical parameters. SHA patient heterogeneity is influenced by factors other than baseline hemostatic activity.

Platelet VPS16B is dependent on VPS33B expression, as determined in two siblings with arthrogryposis, renal dysfunction, and cholestasis syndrome

BACKGROUND

Platelet α-granule biogenesis in precursor megakaryocytes is critically dependent on VPS33B and VPS16B, as demonstrated by the platelet α-granule deficiency seen in the rare multisystem disorder arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome associated with biallelic pathogenic variants in VPS33B and VIPAS39 (encoding VPS16B). VPS33B and VPS16B are ubiquitously expressed proteins that are known to interact and play key roles in protein sorting and trafficking between subcellular locations. However, there remain significant gaps in our knowledge of the nature of these interactions in primary cells from patients with ARC syndrome.

OBJECTIVES

To use primary cells from patients with ARC syndrome to better understand the interactions and roles of VPS33B and VPS16B in platelets and precursor megakaryocytes.

PATIENTS/METHODS

The proband and his male sibling were clinically suspected to have ARC syndrome. Confirmatory genetic testing and platelet phenotyping, including electron microscopy and protein expression analysis, was performed with consent in a research setting.

RESULTS

We describe the first case of ARC syndrome identified in Costa Rica, associated with a novel homozygous nonsense VPS33B variant that is linked with loss of expression of both VPS33B and VPS16B in platelets.

CONCLUSION

These results indicate that stable expression of VPS16B in platelets, their precursor megakaryocytes, and other cells is dependent on VPS33B. We suggest that systematic evaluation of primary cells from patients with a range of VPS33B and VIPAS39 variants would help to elucidate the interactions and functions of these proteins.

Vascular endothelial cells evade complement-mediated membrane injury via Weibel-Palade body mobilization

BACKGROUND

Defective complement inhibition can lead to the formation of membrane attack complexes (MAC; C5b-9) on the plasma membranes of vascular endothelial cells, resulting in injury that drives the progression of thrombotic microangiopathy (TMA), a key pathology in kidney disease.

OBJECTIVE/METHODS

We examined the response of human endothelial cells to complement-mediated damage using blood outgrowth endothelial cells (BOECs) derived from healthy donors. BOECs were sensitized to complement factors present in normal human serum to induce the formation of C5b-9 on their plasma membranes.

RESULTS

This triggered an expected abrupt rise in intracellular Ca2+ reflecting membrane leakage. Remarkably, while intracellular Ca2+ remained elevated, membrane leakage ceased within 30 minutes, and cells did not show significant death. Extensive mobilization of Weibel-Palade bodies (WPBs) was observed along with secretion of von Willebrand factor (VWF). The potential role of WPBs and VWF in mitigating complement-mediated damage was examined by comparing the effects of C5b-9 on BOECs derived from von Willebrand disease (VWD) patients expressing reduced amounts of VWF, lacking expression of functional VWF, or lacking both VWF and WPBs. BOECs lacking WPBs were not resistant to complement-mediated damage, but became resistant when transfected to express VWF (and thus WPBs).

CONCLUSION

We conclude that BOECs exposed to C5b-9 attack respond by mobilizing WPBs, which mitigate and repair damage by fusing with the plasma membrane. We propose that a similar cell-specific response may protect the vascular endothelium from complement-mediated damage in vivo.

A Hypoxia-Inducible HIF1-GAL3ST1-Sulfatide Axis Enhances ccRCC Immune Evasion via Increased Tumor Cell-Platelet Binding

Clear cell renal cell carcinoma (ccRCC) is the most common form of kidney cancer and the major cause of mortality for individuals with von Hippel-Lindau (VHL) disease. ccRCC is characterized most frequently by inactivation of VHL tumor suppressor protein that mediates degradation of the alpha subunit of the hypoxia-inducible factor (HIF) transcription factor family. HIF has been implicated in disease progression and the aim of this study was to identify novel HIF target genes that may contribute to ccRCC. We show that GAL3ST1, an enzyme that catalyzes the sulfonation of the plasma membrane sulfolipid sulfatide, is among the top 50 upregulated genes in ccRCC tissue relative to matched normal tissue. Increased expression of GAL3ST1 in primary ccRCC correlates with decreased survival. We show that GAL3ST1 is a HIF target gene whose expression is induced upon VHL loss leading to the accumulation of its enzymatic product sulfatide. Notably, platelets bind more efficiently to renal cancer cells with high GAL3ST1-sulfatide expression than to GAL3ST1-sulfatide-negative counterparts, which protects ccRCC cells against natural killer cell-mediated cytotoxicity. These results suggest that GAL3ST1 is a HIF-responsive gene that may contribute to ccRCC development via promoting cancer cell evasion of immune surveillance. IMPLICATIONS: Cancer development is in part dependent on evasion of immune response. We identify a HIF target gene product GAL3ST1 that may play a role in this critical process.

NBEAL2 (Neurobeachin-Like 2) Is Required for Retention of Cargo Proteins by α-Granules During Their Production by Megakaryocytes

Objective- Human and mouse megakaryocytes lacking NBEAL2 (neurobeachin-like 2) produce platelets where α-granules lack protein cargo. This cargo is mostly megakaryocyte-synthesized, but some proteins, including FGN (fibrinogen), are endocytosed. In this study, we examined the trafficking of both types of cargo within primary megakaryocytes cultured from normal and NBEAL2-null mice, to determine the role of NBEAL2 in α-granule maturation. We also examined the interaction of NBEAL2 with the granule-associated protein P-selectin in human megakaryocytes and platelets. Approach and Results- Fluorescence microscopy was used to compare uptake of labeled FGN by normal and NBEAL2-null mouse megakaryocytes, which was similar in both. NBEAL2-null cells, however, showed decreased FGN retention, and studies with biotinylated protein showed rapid loss rather than increased degradation. Intracellular tracking via fluorescence microscopy revealed that in normal megakaryocytes, endocytosed FGN sequentially associated with compartments expressing RAB5 (Ras-related protein in brain 5), RAB7 (Ras-related protein in brain 7), and P-selectin, where it was retained. A similar initial pattern was observed in NBEAL2-null megakaryocytes, but then FGN passed from the P-selectin compartment to RAB11 (Ras-related protein in brain 11)-associated endosomes before release. Megakaryocyte-synthesized VWF (Von Willebrand factor) was observed to follow the same route out of NBEAL2-null cells. Immunofluorescence microscopy revealed intracellular colocalization of NBEAL2 with P-selectin in human megakaryocytes, proplatelets, and platelets. Native NBEAL2 and P-selectin were coimmunoprecipitated from platelets and megakaryocytes. Conclusions- NBEAL2 is not required for FGN uptake by megakaryocytes. NBEAL2 is required for the retention of both endocytosed and megakaryocyte-synthesized proteins by maturing α-granules, and possibly by platelet-borne granules. This function may involve interaction of NBEAL2 with P-selectin.

The Birth and Death of Platelets in Health and Disease

Blood platelets are involved in a wide range of physiological responses and pathological processes. Recent studies have considerably advanced our understanding of the mechanisms of platelet production and clearance, revealing new connections between the birth and death of these tiny, abundant cells. Key insights have also been gained into how physiological challenges such as inflammation, infection, and chemotherapy can affect megakaryocytes, the cells that produce platelets.

NBEAL2 mutations and bleeding in patients with gray platelet syndrome

Homozygosity/compound heterozygosity for loss of function mutations in neurobeachin-like 2 (NBEAL2) is causative for Gray platelet syndrome (GPS; MIM #139090), characterized by thrombocytopenia and large platelets lacking α-granules and cargo. Most GPS-associated NBEAL2 mutations generate nonsense codons; frameshifts causing premature translation termination and/or changes in mRNA splicing have also been observed. Data regarding NBEAL2 protein expression in GPS patients is limited. We observed absence of NBEAL2 in platelets from GPS patients with 3 different genotypes, and reduced/truncated platelet NBEAL2 has been reported for others. GPS is commonly associated with mild bleeding, but lifethreatening bleeding has been reported in some cases. A common long-term complication in GPS patients is myelofibrosis; splenomegaly is less common but sometimes of sufficient severity to merit splenectomy. Like GPS patients, mice lacking NBEAL2 expression exhibit macrothrombocytopenia, deficiency of platelet α-granules, splenomegaly, myelofibrosis, impaired platelet function and abnormalities in megakaryocyte development. Animal studies have also reported impaired platelet function in vivo using laser injury and thrombo-inflammation models. NBEAL2 is a large gene with 54 exons, and several putative functional domains have been identified in NBEAL2, including PH (pleckstrin homology) and BEACH (beige and Chediak-Higashi) domains shared with other members of a protein family that includes LYST and LRBA, also expressed by hematopoietic cells. Potential NBEAL2-interacting proteins have recently been identified, and it is expected that current and future efforts will reveal the cellular mechanisms by which NBEAL2 facilitates platelet development and supports hemostatic function.

Relative antibacterial functions of complement and NETs: NETs trap and complement effectively kills bacteria

Neutrophil extracellular traps (NETs) are web-like DNA structures released by activated neutrophils. These structures are decorated with antimicrobial proteins, and considered to trap and kill bacteria extracellularly. However, the exact functions of NETs remain elusive, and contradictory observations have been made with NETs functioning as an antimicrobial or a pathogentrapping mechanism. There is a disconnect in the interpretation of the involvement of other major immune mechanisms, such as the complement system, as effectors of the function of NETs. We have recently shown that NETs activate complement. In this study, we aimed to elucidate the relative antimicrobial roles of NETs in the absence and presence of complement. Using primary human neutrophils, human serum (normal, heat inactivated, and C5-depleted), P. aeruginosa (at multiplicity of infection, MOI, of 1 or 10), S. aureus (MOI of 1), colony-counting assays and confocal microscopy, we demonstrate that most bacteria trapped by NETs remain viable, indicating that NETs have limited bactericidal properties. By contrast, complement effectively killed bacteria, but NETs decreased the bactericidal ability of complement and degrading NETs by DNases restored complement-mediated killing. Experiments with conditions allowing for specific pathway activation showed that the complement classical and lectin, but not the alternative, pathway lead to bacterial killing. NETs under static conditions showed limited killing of bacteria while NETs under dynamic conditions showed enhanced bacteria trapping and reduced killing. Furthermore, NETs incubated with normal human serum depleted complement and reduced the hemolytic capacity of the serum. This report, for the first time, clarifies the relative bactericidal contributions of NETs and complement. We propose that - while NETs can ensnare bacteria such as P. aeruginosa - complement is necessary for efficient bacterial killing.

Imaging Platelets and Megakaryocytes by High-Resolution Laser Fluorescence Microscopy

Microscopy is central to studies of platelets and their precursor megakaryocytes. Here we describe methods to rapidly obtain high resolution images of fixed platelets, megakaryocytes and megakaryocytic cells via immunofluorescence microscopy. Protocols covered include: (1) isolation and preparation of cells suitable for fluorescence staining; (2) staining with antibodies and other molecules; (3) imaging via spinning-disc confocal and structured illumination laser fluorescence microscopy; (4) processing and presentation of images. Also included is a list of primary antibodies we have validated for use in staining specific proteins and subcellular structures in platelets and megakaryocytes.

Loss of the Arp2/3 complex component ARPC1B causes platelet abnormalities and predisposes to inflammatory disease

Human actin-related protein 2/3 complex (Arp2/3), required for actin filament branching, has two ARPC1 component isoforms, with ARPC1B prominently expressed in blood cells. Here we show in a child with microthrombocytopenia, eosinophilia and inflammatory disease, a homozygous frameshift mutation in ARPC1B (p.Val91Trpfs*30). Platelet lysates reveal no ARPC1B protein and greatly reduced Arp2/3 complex. Missense ARPC1B mutations are identified in an unrelated patient with similar symptoms and ARPC1B deficiency. ARPC1B-deficient platelets are microthrombocytes similar to those seen in Wiskott–Aldrich syndrome that show aberrant spreading consistent with loss of Arp2/3 function. Knockout of ARPC1B in megakaryocytic cells results in decreased proplatelet formation, and as observed in platelets from patients, increased ARPC1A expression. Thus loss of ARPC1B produces a unique set of platelet abnormalities, and is associated with haematopoietic/immune symptoms affecting cell lineages where this isoform predominates. In agreement with recent experimental studies, our findings suggest that ARPC1 isoforms are not functionally interchangeable.

ARPC1B is a component of the actin-related protein 2/3 complex (Arp2/3), which is required for actin filament branching. Kahr et al. show that ARPC1B deficiency in humans is associated with severe multisystem disease that includes platelet abnormalities, eosinophilia, eczema and other indicators of immune disease.

Non-genomic activities of retinoic acid receptor alpha control actin cytoskeletal events in human platelets

Essentials Platelets employ proteins/signaling pathways traditionally thought reserved for nuclear niche. We determined retinoic-acid-receptor alpha (RARα) expression and function in human platelets. RARα/actin-related protein-2/3 complex (Arp2/3) interact via non-genomic signaling in platelets. RARα regulates Arp2/3-mediated actin cytoskeletal dynamics and platelet spreading.

SUMMARY

Background Platelets utilize proteins and pathways classically reserved for the nuclear niche. Methods We determined whether human platelets express retinoic-acid-receptor family members, traditionally thought of as nuclear transcription factors, and deciphered the function of RARα. Results We found that RARα is robustly expressed in human platelets and megakaryocytes and interacts directly with actin-related protein-2/3 complex (Arp2/3) subunit 5 (Arp2/3s5). Arp2/3s5 co-localized with RARα in situ and regulated platelet cytoskeletal processes. The RARα ligand all-trans retinoic acid (atRA) disrupted RARα-Arp2/3 interactions. When isolated human platelets were treated with atRA, rapid cytoskeletal events (e.g. platelet spreading) were inhibited. In addition, when platelets were cultured for 18 h in the presence of atRA, actin-dependent morphological changes (e.g. extended cell body formation) were similarly inhibited. Using in vitro actin branching assays, RARα and Arp2/3-regulated complex actin branch formation was demonstrated. Consistent with inhibition of cytoskeletal processes in platelets, atRA, when added to this branching assay, resulted in dysregulated actin branching. Conclusion Our findings identify a previously unknown mechanism by which RARα regulates Arp2/3-mediated actin cytoskeletal dynamics through a non-genomic signaling pathway. These findings have broad implications in both nucleated and anucleate cells, where actin cytoskeletal events regulate cell morphology, movement and division.

Intracellular Trafficking, Localization, and Mobilization of Platelet-Borne Thiol Isomerases

OBJECTIVE

Thiol isomerases facilitate protein folding in the endoplasmic reticulum, and several of these enzymes, including protein disulfide isomerase and ERp57, are mobilized to the surface of activated platelets, where they influence platelet aggregation, blood coagulation, and thrombus formation. In this study, we examined the synthesis and trafficking of thiol isomerases in megakaryocytes, determined their subcellular localization in platelets, and identified the cellular events responsible for their movement to the platelet surface on activation.

APPROACH AND RESULTS

Immunofluorescence microscopy imaging was used to localize protein disulfide isomerase and ERp57 in murine and human megakaryocytes at various developmental stages. Immunofluorescence microscopy and subcellular fractionation analysis were used to localize these proteins in platelets to a compartment distinct from known secretory vesicles that overlaps with an inner cell-surface membrane region defined by the endoplasmic/sarcoplasmic reticulum proteins calnexin and sarco/endoplasmic reticulum calcium ATPase 3. Immunofluorescence microscopy and flow cytometry were used to monitor thiol isomerase mobilization in activated platelets in the presence and absence of actin polymerization (inhibited by latrunculin) and in the presence or absence of membrane fusion mediated by Munc13-4 (absent in platelets from Unc13d(Jinx) mice).

CONCLUSIONS

Platelet-borne thiol isomerases are trafficked independently of secretory granule contents in megakaryocytes and become concentrated in a subcellular compartment near the inner surface of the platelet outer membrane corresponding to the sarco/endoplasmic reticulum of these cells. Thiol isomerases are mobilized to the surface of activated platelets via a process that requires actin polymerization but not soluble N-ethylmaleimide-sensitive fusion protein attachment receptor/Munc13-4-dependent vesicular-plasma membrane fusion.

NETosing Neutrophils Activate Complement Both on Their Own NETs and Bacteria via Alternative and Non-alternative Pathways

Neutrophils deposit antimicrobial proteins, such as myeloperoxidase and proteases on chromatin, which they release as neutrophil extracellular traps (NETs). Neutrophils also carry key components of the complement alternative pathway (AP) such as properdin or complement factor P (CFP), complement factor B (CFB), and C3. However, the contribution of these complement components and complement activation during NET formation in the presence and absence of bacteria is poorly understood. We studied complement activation on NETs and a Gram-negative opportunistic bacterial pathogen Pseudomonas aeruginosa (PA01, PAKwt, and PAKgfp). Here, we show that anaphylatoxin C5a, formyl-methionyl-leucyl-phenylalanine (fMLP) and phorbol myristate acetate (PMA), which activates NADPH oxidase, induce the release of CFP, CFB, and C3 from neutrophils. In response to PMA or P. aeruginosa, neutrophils secrete CFP, deposit it on NETs and bacteria, and induce the formation of terminal complement complexes (C5b-9). A blocking anti-CFP antibody inhibited AP-mediated but not non-AP-mediated complement activation on NETs and P. aeruginosa. Therefore, NET-mediated complement activation occurs via both AP- and non AP-based mechanisms, and AP-mediated complement activation during NETosis is dependent on CFP. These findings suggest that neutrophils could use their "AP tool kit" to readily activate complement on NETs and Gram-negative bacteria, such as P. aeruginosa, whereas additional components present in the serum help to fix non-AP-mediated complement both on NETs and bacteria. This unique mechanism may play important roles in host defense and help to explain specific roles of complement activation in NET-related diseases.

Germline mutations in ETV6 are associated with thrombocytopenia, red cell macrocytosis and predisposition to lymphoblastic leukemia

Some familial platelet disorders are associated with predisposition to leukemia, myelodysplastic syndrome (MDS) or dyserythropoietic anemia. We identified a family with autosomal dominant thrombocytopenia, high erythrocyte mean corpuscular volume (MCV) and two occurrences of B cell-precursor acute lymphoblastic leukemia (ALL). Whole-exome sequencing identified a heterozygous single-nucleotide change in ETV6 (ets variant 6), c.641C>T, encoding a p.Pro214Leu substitution in the central domain, segregating with thrombocytopenia and elevated MCV. A screen of 23 families with similar phenotypes identified 2 with ETV6 mutations. One family also had a mutation encoding p.Pro214Leu and one individual with ALL. The other family had a c.1252A>G transition producing a p.Arg418Gly substitution in the DNA-binding domain, with alternative splicing and exon skipping. Functional characterization of these mutations showed aberrant cellular localization of mutant and endogenous ETV6, decreased transcriptional repression and altered megakaryocyte maturation. Our findings underscore a key role for ETV6 in platelet formation and leukemia predisposition.

Abstract 380: PDI and ERp57 Localization in Platelets Is Regulated by Actin Polymerization

Introduction: PDI and ERp57 are members of the thiol isomerase family of enzymes that are released to the platelet surface where they contribute to a range of platelet responses. PDI has been reported to be present in subcellular structures in resting platelets, although it is excluded from α - and δ -granules, and lysosomes. It was, however, recently shown to co-localise with TLR9 in T-granules, intracellular bodies underlying the plasma membrane. The subcellular localisation of other platelet thiol isomerases or their mechanisms of translocation to the cell surface have not been established.

Hypothesis and Methods: Using spinning disk confocal microscopy we sought to explore whether platelet thiol isomerases ERp57 and PDI are co-localised in resting and activated platelets and to test the hypothesis that translocation of thiol isomerases to the platelet surface is dependent on actin polymerization.

Results and Conclusions: In resting platelets, PDI and ERp57 were organized in punctate structures both on the platelet surface and in the cytoplasm. They did not colocalise with P-selectin consistent with them residing outside α -granules. In contrast to expectations, TLR9 did not colocalize with PDI and ERp57, suggesting their exclusion from T-granules. Partial colocalization was observed in resting platelets between PDI and ERp57 in granule-like structures. Upon platelet activation, this degree of co-distribution persisted with PDI and ERp57 migrating to the platelet surface. Latrunculin A, an inhibitor of actin polymerization, decreased P-selectin exposure on the platelet surface, as measured by flow cytometry, and agonist-stimulated platelets treated with this agent retained a rounded shape, as shown by tubulin staining. Importantly, latrunculin A also blocked the translocation of PDI and ERp57 from internal granule-like structures to the platelet surface. We conclude that in resting platelets PDI and ERp57 are organized, and partially co-localised, in punctuate structures unlikely to represent α- or T-granules. The polymerization of actin during platelet activation exerts a fundamental role in the relocalization of PDI and ERp57 from these cytoplasmic structures to the platelet surface.

Eculizumab therapy in children with severe hematopoietic stem cell transplantation-associated thrombotic microangiopathy

We recently observed that dysregulation of the complement system may be involved in the pathogenesis of hematopoietic stem cell transplantation-associated thrombotic microangiopathy (HSCT-TMA). These findings suggest that the complement inhibitor eculizumab could be a therapeutic option for this severe HSCT complication with high mortality. However, the efficacy of eculizumab in children with HSCT-TMA and its dosing requirements are not known. We treated 6 children with severe HSCT-TMA using eculizumab and adjusted the dose to achieve a therapeutic level >99 μg/mL. HSCT-TMA resolved over time in 4 of 6 children after achieving therapeutic eculizumab levels and complete complement blockade, as measured by low total hemolytic complement activity (CH50). To achieve therapeutic drug levels and a clinical response, children with HSCT-TMA required higher doses or more frequent eculizumab infusions than currently recommended for children with atypical hemolytic uremic syndrome. Two critically ill patients failed to reach therapeutic eculizumab levels, even after dose escalation, and subsequently died. Our data indicate that eculizumab may be a therapeutic option for HSCT-TMA, but HSCT patients appear to require higher medication dosing than recommended for other conditions. We also observed that a CH50 level ≤ 4 complement activity enzyme units correlated with therapeutic eculizumab levels and clinical response, and therefore CH50 may be useful to guide eculizumab dosing in HSCT patients as drug level monitoring is not readily available.

Monitoring and modeling treatment of atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS), is mainly present in children, who have high risks of end-stage kidney disease (ESKD), post-transplant recurrence and death. aHUS is linked to defective regulation of the complement alternative pathway (AP), with a prominent cause being mutation/inhibition of the negative regulator complement factor H (CFH). CFH function can be restored via infusion of fresh frozen plasma (FFP), a treatment that was effective for several years in a patient heterozygous for a cfh mutation, before the patient progressed to ESKD. While on dialysis, FFP was replaced with eculizumab, which blocks C5 cleavage and thus halts progression of the terminal complement pathway. Patient plasma samples collected during FFP and eculizumab treatment phases were assessed for AP activity (via erythrocyte lysis assays) and for overall complement activity (via ELISA-based screen). Assay results indicated that FFP partially restored AP regulation, an observation supported by in vitro modeling of FFP treatment using purified CFH, while eculizumab completely blocked complement activity. The same approach was used to model in vitro a potential aHUS treatment approach based on blocking the AP effector properdin (complement factor P; CFP) with an anti-properdin antibody. These results provide insights into the efficacy of aHUS treatment and highlight the usefulness of in vitro assays in monitoring and predicting therapeutic responses and testing new treatment possibilities.

The cell motility modulator Slit2 is a potent inhibitor of platelet function

BACKGROUND

Vascular injury and atherothrombosis involve vessel infiltration by inflammatory leukocytes, migration of medial vascular smooth muscle cells to the intimal layer, and ultimately acute thrombosis. A strategy to simultaneously target these pathological processes has yet to be identified. The secreted protein, Slit2, and its transmembrane receptor, Robo-1, repel neuronal migration in the developing central nervous system. More recently, it has been appreciated that Slit2 impairs chemotaxis of leukocytes and vascular smooth muscle cells toward diverse inflammatory attractants. The effects of Slit2 on platelet function and thrombus formation have never been explored.

METHODS AND RESULTS

We detected Robo-1 expression in human and murine platelets and megakaryocytes and confirmed its presence via immunofluorescence microscopy and flow cytometry. In both static and shear microfluidic assays, Slit2 impaired platelet adhesion and spreading on diverse extracellular matrix substrates by suppressing activation of Akt. Slit2 also prevented platelet activation on exposure to ADP. In in vivo studies, Slit2 prolonged bleeding times in murine tail bleeding assays. Using intravital microscopy, we found that after mesenteric arteriolar and carotid artery injury, Slit2 delayed vessel occlusion time and prevented the stable formation of occlusive arteriolar thrombi.

CONCLUSIONS

These data demonstrate that Slit2 is a powerful negative regulator of platelet function and thrombus formation. The ability to simultaneously block multiple events in vascular injury may allow Slit2 to effectively prevent and treat thrombotic disorders such as myocardial infarction and stroke.

Antibody mediated rejection associated with complement factor h-related protein 3/1 deficiency successfully treated with eculizumab

Antibody mediated rejection (AMR) activates the classical complement pathway and can be detrimental to graft survival. AMR can be accompanied by thrombotic microangiopathy (TMA). Eculizumab, a monoclonal C5 antibody prevents induction of the terminal complement cascade (TCC) and has recently emerged as a therapeutic option for AMR. We present a highly sensitized 13-year-old female with end-stage kidney disease secondary to spina bifida-associated reflux nephropathy, who developed severe steroid-, ATG- and plasmapheresis-resistant AMR with TMA 1 week post second kidney transplant despite previous desensitization therapy with immunoglobulin infusions. Eculizumab rescue therapy resulted in a dramatic improvement in biochemical (C3; creatinine) and hematological (platelets) parameters within 6 days. The patient was proven to be deficient in complement Factor H-related protein 3/1 (CFHR3/1), a plasma protein that regulates the complement cascade at the level of C5 conversion and has been involved in the pathogenesis of atypical hemolytic uremic syndrome caused by CFH autoantibodies (DEAP-HUS). CFHR1 deficiency may have worsened the severe clinical progression of AMR and possibly contributed to the development of donor-specific antibodies. Thus, screening for CFHR3/1 deficiency should be considered in patients with severe AMR associated with TMA.

Diannexin, an annexin A5 homodimer, binds phosphatidylserine with high affinity and is a potent inhibitor of platelet-mediated events during thrombus formation

BACKGROUND

Shielding of procoagulant phosphatidylserine (PS) with annexin A5 attenuates thrombosis, but annexin A5 (35.7 kDa) is rapidly cleared from the circulation. In contrast, Diannexin, a 73.1 kDa homodimer of annexin A5, has an extended half-life.

OBJECTIVES

To quantify the affinity of Diannexin for PS, examine its interaction with activated platelets and determine its effects on platelet-mediated events during thrombus formation.

METHODS

The affinities of Diannexin and annexin A5 for PS-containing lipid bilayers were compared using surface plasmon resonance, and binding to activated platelets was assessed by flow cytometry. Calibrated automated thrombography and thromboelastography were employed to study the effects of Diannexin on thrombin generation and platelet-fibrin clot formation, respectively, whereas intravital videomicroscopy was used to examine its effect on platelet accumulation and activation after laser-induced injury to murine cremaster arterioles, and a tail tip bleeding model was used to explore its effects on hemostasis.

RESULTS

Diannexin and annexin A5 bind PS with K(D) values of 0.6 and 5 nm, respectively, and both bind to the same subpopulation of PS-exposing platelets. Diannexin inhibited thrombin generation and platelet-fibrin clot formation in vitro at 10 nm (P<0.05-0.001 compared with control), and reduced platelet accumulation at 1 μg g(-1) (P<0.05) and activation at 0.25 μg g(-1) (P<0.001) in experimentally induced arterial thrombi in mice while increasing blood loss at 1 μg g(-1) (P<0.01).

CONCLUSIONS

Diannexin binds to PS with high affinity and is a potent inhibitor of platelet-mediated events during thrombus formation.

Mutations in NBEAL2, encoding a BEACH protein, cause gray platelet syndrome

Next-generation RNA sequence analysis of platelets from an individual with autosomal recessive gray platelet syndrome (GPS, MIM139090) detected abnormal transcript reads, including intron retention, mapping to NBEAL2 (encoding neurobeachin-like 2). Genomic DNA sequencing confirmed mutations in NBEAL2 as the genetic cause of GPS. NBEAL2 encodes a protein containing a BEACH domain that is predicted to be involved in vesicular trafficking and may be critical for the development of platelet α-granules.

Abnormalities in villin gene expression and canalicular microvillus structure in progressive cholestatic liver disease of childhood

BACKGROUND

The molecular basis of clinical cholestasis is a subject of intense investigation. Villin is an actin binding, bundling, and severing protein needed for maintenance of structural integrity of canalicular microvilli, in which membrane transporters required for bile secretion are located. We aimed to investigate the role of canalicular cytoskeletal proteins in three genetically unrelated children with a biliary atresia-like clinical disorder, each of whom developed liver failure requiring liver transplantation.

METHODS

Explanted livers from the three patients were examined by standard pathological methods followed by transmission and cryoimmunoelectron microscopy. With archival tissue samples, a panel of cytoskeletal proteins was investigated by immunohistochemistry and western blotting, with purified canalicular membrane preparations. Villin mRNA analyses were undertaken on liver homogenates, with primers from coding regions of the human villin gene. Classic biliary atresia, other types of cholestasis, and normal livers served as controls.

FINDINGS

In patients, pronounced ultrastructural deformities of canaliculi and especially of their microvilli were noted, which correlated with absence of villin protein by immunostaining of liver tissue sections and by western blot analysis. Additionally, villin mRNA was strikingly reduced or absent. These results differed greatly from those in controls.

INTERPRETATION

These results suggest that the disorder described mimics biliary atresia, but structural and molecular pathological findings differ. We propose that a functional abnormality in villin gene expression is key to the mechanism of cholestasis in patients with progressive cholestasis and hepatic failure.

Rapid purification of high-activity Taq DNA polymerase

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Superoxide dismutase halts cycling of murine erythroid progenitor cells prior to S phase in vitro and possibly in vivo

Mice of the C57BL/6 (B6) strain show a much lower proportion of marrow erythroid progenitor cells (BFU-E) in DNA synthesis in vivo than mice of the congeneic B6S strain. However, when assayed in vitro marrow cells from both strains show high proportions of BFU-E in S-phase. BFU-E from normal B6 mice have been previously shown to be specifically inhibited from entering S-phase in vitro by the antioxidant enzyme superoxide dismutase (SOD), however, in this study we have found that BFU-E taken from the marrow of B6S mice or B6 mice which have been subjected to bleeding are insensitive to SOD inhibition in vitro. Comparisons of results from in vivo and in vitro cycling assays done with cells from both strains indicate that a large proportion of marrow BFU-E in normal B6 mice are halted in the pre-S portion of the cell cycle in vivo, and these halted cells are prevented from going into S-phase in vitro by SOD. The insensitivity to SOD inhibition shown by BFU-E from B6S and bled B6 mice can be attributed to the absence of accumulation of SOD-susceptible cells in pre-S phase in these mice in vivo, and there is evidence to suggest that the difference in BFU-E cycling seen in vivo may be due to interactions between SOD and factors which stimulate cycling of BFU-E.

Superoxide dismutase as an inhibitor of erythroid progenitor cell cycling

C57Bl/6 (B6) mice and mice of a congeneic strain, B6S, differ in the proportions of erythroid progenitor cells (BFU-E) typically seen in DNA synthesis in in vivo cell suicide assays, and bone marrow supernatants (MS) prepared from B6 mice can inhibit BFU-E cycling in vitro. Using in vitro BFU-E DNA synthesis assays and a model system of BFU-E in culture (DA-1 cells) as screening methods for the detection of inhibitors of BFU-E cycling, we have purified the protein that is apparently responsible for the inhibitory effects of MS on progenitor cells and that is also an antagonist of the stimulatory effects of interleukin-3 (IL-3) on DA-1 cell proliferation in culture. We have identified this protein as the Cu,Zn-containing form of the antioxidant enzyme superoxide dismutase (SOD), which is normally present in large amounts in erythrocytes. MS from B6S mice does not inhibit BFU-E DNA synthesis. However, measurements of SOD activity showed no differences between B6 and B6S mice; thus the difference between the effects of B6S-MS and B6-MS is not due to differences in the levels of SOD present. The inhibitory effects of SOD on BFU-E in vitro are opposed by the stimulatory effects of IL-3 in a dose-dependent manner, and similar interactions between stimulatory and inhibitory factors also appear to determine the effects of mouse-derived preparations on erythroid cells. If the interactions seen in vitro are applicable to the state in vivo, SOD may be a constitutive inhibitor of erythroid progenitor cell cycling in mice, acting in opposition to stimulatory factors whose expression varies in response to genetic and physiological influences.

Superoxide dismutase specifically inhibits erythroid cell DNA synthesis and proliferation

The antioxidant enzyme superoxide dismutase (SOD) was previously shown to inhibit both the proliferation of murine erythroid DA-1 cells growing in the presence of Interleukin-3 (IL-3) and the DNA synthesis of marrow erythroid progenitor cells (BFU-E) in vitro. We show here that the inhibition of marrow cell DNA synthesis by SOD is specific for BFU-E and erythroid precursors (CFU-E), with other myeloid progenitors (CFU-GM) and stem cells (CFU-S) being unaffected, and IL-3 blocks the inhibitory effects of SOD on BFU-E in a dose-dependent manner. Extending earlier observations on the effects of SOD on cell proliferation, it was found that SOD was capable of inhibiting DA-1 cell proliferation supported by either IL-3 or erythropoietin (epo), but had no effect on IL-3 dependent FDCP-1 cells, nor on epo-dependent HCD-57 cells. Of several murine erythroleukemia cell lines tested, only those transformed with Friend SFFVa virus were inhibited by SOD, while those transformed with Friend SFFVp or MuLV virus were not affected. These results show that the effects of SOD are not antagonistic to particular growth factors but rather the inhibition is specific for erythroid cells, and cells of the proper stage can be inhibited even if they have been transformed to factor independence.

Purification of an inhibitor of erythroid progenitor cell cycling and antagonist to interleukin 3 from mouse marrow cell supernatants and its identification as cytosolic superoxide dismutase

We have isolated a protein from media conditioned by a murine marrow-derived cell line (PB6) and from mouse marrow supernatants that antagonizes interleukin 3-dependent proliferation of cells in culture and reversibly inhibits DNA synthesis of erythroid progenitor cells (BFU-E) in vitro. This protein, p16 (monomer Mr = 16 kD on SDS-PAGE), was purified to homogeneity and amino acid sequencing of a polypeptide fragment yielded a sequence identical to that of murine cytosolic Cu,Zn-containing superoxide dismutase (SOD). The identification of p16 as SOD was confirmed by the detection of SOD enzymatic activity in pure p16 fractions, and when a commercial human erythrocytic SOD preparation was tested it showed the same cell inhibitory activities as p16. These observations show that superoxide dismutase is able to affect the cycling and growth factor responses of hematopoietic cells, activities that have not previously been associated with this enzyme.

The accumulation of three yeast ribosomal proteins under conditions of excess mRNA is determined primarily by fast protein decay

The suggestion that compensation for overabundant mRNA of the genes for Saccharomyces cerevisiae ribosomal protein (r-protein) L3, L29, or rp59 occurs by translation repression has been reinvestigated. First, analysis of the distribution of these three mRNAs in polysome profiles revealed no differences between normal and mRNA-overproducing strains, indicating that initiation of r-protein translation is not repressed under conditions of mRNA overaccumulation. Second, experiments involving radioactive pulse-labeling of proteins were done by using a modified method of data collection and analysis that allows quantitation and correction for fast decay during the pulse. These measurements revealed that the synthesis rate of the three r-proteins is increased when their mRNA levels are elevated and that their decay rate is also high, with half-lives ranging from a fraction of a minute to more than 10 min. We conclude that accumulation of excess r-protein mRNA has no effect on translation rate; rapid decay of protein during the course of the labeling period can account for the apparent discrepancy between mRNA levels and protein synthesis rates. Yeast r-proteins, when produced in excess, are among the most rapidly degraded proteins so far described.