An acquired inhibitor to the GPVI platelet collagen receptor in a patient with lupus nephritis

BACKGROUND

GPVI is a major platelet collagen signaling receptor. In rare cases of immune thrombocytopenic purpura (ITP), autoantibodies to GPVI result in receptor shedding.

OBJECTIVES

To investigate a possible pathogenic role of plasma anti-GPVI antibody located in a woman with lupus nephritis.

METHODS

Measured were (i) platelet aggregation to collagen and convulxin, (ii) platelet GPVI expression (flow cytometry and western blotting), (iii) plasma soluble GPVI (sGPVI, dual antibody ELISA), and (iv) plasma anti-GPVI antibody (ELISA using recombinant sGPVI).

RESULTS

In 2006 and early 2007, the patient had a normal platelet count but a virtual absence of platelet aggregation to collagen and convulxin. Her platelets responded normally to other agonists including cross-linking ITAM-dependent FcgammaRIIA by monoclonal antibody, IV.3. Flow cytometry and western blotting showed a platelet deficiency of GPVI. Plasma sGPVI levels were undetectable whereas ELISA confirmed the presence of anti-GPVI antibody. Sequencing revealed a normal GPVI cDNA structure. The patient's plasma and the isolated IgG3 fraction activated and induced GPVI shedding from normal platelets. A deteriorating clinical condition led to increasingly strict immunosuppressive therapy. This was globally associated with a fall in plasma anti-GPVI titres, the restoration of platelet GPVI and the convulxin response, and the loss of her nephrotic syndrome.

CONCLUSIONS

Our results show that this patient acquired a potent anti-GPVI IgG3 antibody with loss of GPVI and collagen-related platelet function. Further studies are required to determine whether anti-GPVI antibodies occur in other lupus patients with nephritis.

Mutations in the β3 gene giving rise to type I Glanzmann thrombasthenia in two families in Portugal

Glazzmann thrombasthenia is an inherited bleeding syndrome in which an absence of platelet aggregation is associated with quantitative or qualitative deficiencies of the alphaIIbbeta3 integrin. We now describe biochemical and molecular studies on two Portuguese families where platelets lack both surface and intracellular pools of alphaIIbbeta3. DNA extraction was followed by PCR-SSCP analysis of all exons and intronic boundaries in the alphaIIb and beta3 genes. Migration abnormalities were found for PCR fragments encompassing exon 12 (family 1) and exon 10 (family 2). For patient 1, there was a homozygous G to T transition at position 1846 which resulted in a stop codon at codon 616 in the beta3 gene. For patient 2, direct sequencing revealed a homozygous 1347C insert which led to a stop codon at codon 444 in the beta3 gene. For both patients a single mutated allele was inherited from each parent. Evidence is accumulating that nonsense mutations leading to a truncated beta3 may be a frequent cause of type I Glanzmann thrombasthenia in the Iberian peninsula.

Altered megakaryocytopoiesis in von Willebrand type 2B disease

Type 2B von Willebrand disease (VWD2B) is caused by gain-of-function amino acid substitutions in the von Willebrand factor (VWF) A1 domain. These allow facilitated binding of mutated VWF to platelet GPIbalpha with prolonged lifetimes of VWF bonds and enhanced ADAMTS-13 cleavage of large VWF multimers. A bleeding rather than prothrombotic syndrome is due to: (i) decreased large VWF multimers in plasma; (ii) limited thrombus formation; and (iii) thrombocytopenia affecting some but not all patients. Accumulating evidence points to an altered megakaryocytopoiesis in VWD2B with the production of enlarged or giant platelets showing an abnormal ultrastructure and, in a cohort of patients, the presence of circulating platelet agglutinates. In fact, evidence from in vitro cultures and marrow aspirates suggests that the upregulated VWF function can lead to abnormal VWF trafficking in megakaryocytes, a modified platelet production with interacting proplatelets, and the presence or even release of platelet agglutinates in the bone marrow.

The transport of high amounts of vascular endothelial growth factor by blood platelets underlines their potential contribution in systemic sclerosis angiogenesis

OBJECTIVES

Altered angiogenesis is a characteristic feature in SSc and remains ill-understood. VEGF is believed to play a central role. Serum VEGF is elevated in SSc patients but questions remain concerning the source of circulating VEGF. Here we investigated platelet activation and the role of platelets as a source of VEGF and other angiogenic mediators in this disease.

METHODS

A cohort of 40 patients with SSc was included. Age- and sex-matched healthy subjects and subjects presenting a primary RP were included as controls. Platelets were isolated, activated with thrombin and the secretion of VEGF, platelet derived growth factor, homodimeric form BB (PDGF-BB), TGF-beta1 and angiopoietins-1 and -2 measured. Plasma concentrations of these mediators and the functionality of platelet-derived VEGF were also studied. Platelet activation was assayed by measuring plasma beta-thromboglobulin and expression of P-selectin on platelets. The effect of iloprost on VEGF secretion by platelets was studied.

RESULTS

Platelets from SSc patients, in contrast to controls, secreted large amounts of VEGF when activated, but not PDGF-BB, TGF-beta1 or angiopoietins. Increased expression of membrane P-selectin confirmed platelet activation in the patients. Iloprost inhibited VEGF secretion by platelets both in vivo and in vitro, through inhibition of platelet activation.

CONCLUSIONS

Platelets transport high levels of VEGF in SSc. They may contribute to circulating VEGF because of ongoing activation in the course of the disease. If activated at the contact of injured endothelium, platelets may be important in the altered angiogenesis associated with the disease through the secretion of high levels of VEGF.

Tissue inhibitors of matrix metalloproteinases in platelets and megakaryocytes: a novel organization for these secreted proteins

OBJECTIVE

Expression of tissue inhibitors of matrix metalloproteinases (TIMPs) is one way that activated platelets intervene in tissue remodeling and angiogenesis. Our study was designed to investigate their synthesis in megakaryocytes (MKs) and their storage in platelets.

MATERIALS AND METHODS

TIMP expression in MKs derived from blood CD34(+) progenitor cells of normal donors and a megakaryocytic cell line (CHRF-288-11) grown in serum-free conditions and platelets from normal donors or two patients with gray platelet syndrome was studied by immunofluorescence labeling, reverse transcription-polymerase chain reaction, and western blotting.

RESULTS

Biosynthesis of TIMPs 1-4 in MKs was indicated by presence of their messenger RNAs as shown by polymerase chain reaction and of their proteins. Immunofluorescence labeling suggested a primarily granular localization of TIMPs in MKs and platelets. But when colocalization with von Willebrand factor, fibrinogen, P-selectin, and other alpha-granule proteins was assessed in platelets by confocal microscopy, TIMP-1, -2, and -4 were localized as distinct fluorescent patches apart from the established alpha-granule markers and largely independent of platelet metalloproteinases. TIMP-3 differed for it also had an alpha-granule location. Western blotting confirmed the presence of TIMPs 1-4 in platelets and thrombin activation resulted in their extensive release to the medium. Platelets from two patients with gray platelet syndrome, congenitally deficient in alpha-granules, showed sparse labeling of von Willebrand factor and fibrinogen confined to vestigial alpha-granules; however, localization of the TIMPs was unchanged.

CONCLUSIONS

TIMPs are synthesized and organized in MKs and platelets independently of other secreted proteins present in alpha-granule pools.

New-generation drugs that stimulate platelet production in chronic immune thrombocytopenic purpura

Idiopathic thrombocytopenic purpura is an acquired disease characterised by a low platelet count. Development of autoantibodies is a main cause of the disease. Although many patients have few symptoms, life-threatening bleeding can arise and hence, when platelet counts fall to unacceptable values treatment should be initiated. However, conventional immunosuppressive approaches can fail, perhaps because of the heterogeneous nature of the disease. Newly developed agents that increase platelet production by stimulating megakaryocytes-such as drugs that bind to the thrombopoietin receptor c-MPL-offer an alternative treatment strategy. Although initial thrombopoietin analogues caused adverse immune reactions, second-generation thrombopoietin-receptor agonists that are in late-stage clinical development seem promising. In particular, eltrombopag and romiplostim safely increase and maintain platelet production in patients with refractory disease. However, long-term side-effects are being assessed and the exact role of these agents in the overall treatment strategy of chronic idiopathic thrombocytopenic purpura remains to be established.

Phenotypic heterogeneity in the Gray platelet syndrome extends to the expression of TREM family member, TLT-1

The Gray platelet syndrome (GPS) is a rare inherited disorder linked to undefined molecular abnormalities that prevent the formation and maturation of alpha-granules. Here, we report studies on two patients from unrelated families that confirm phenotypic heterogeneity in the disease. First we used immunoelectron microscopy (I-EM) to confirm that TREM-like transcript-1 (TLT-1) is mostly localized to alpha-granule membranes of normal platelets. Then we performed Western blotting (WB) and flow cytometry with permeabilized platelets to show that TLT-1 is selectively reduced in the platelets of patient 1, previously noted to be deficient in glycoprotein (GP)VI (Nurden et al., Blood 2004; 104: 107-114). Yet both TLT-1 and GPVI were normally expressed in platelets of patient 2. Usual levels of JAM-C and claudin-5, also members of the immunoglobulin receptor family, were detected in platelets of both patients. In contrast, P-selectin was markedly decreased for patient 1 but not patient 2. Two metalloproteases, MMP-2 and MMP-9 were normally present. As predicted, platelets of patient 1 showed little labelling for TLT-1 in I-EM, whereas residual Fg was seen in small vesicular structures and P-selectin lining vacuoles or channels of what may be elements of the surface-connected canalicular system. Our results identify TLT-1 as a glycoprotein potentially targeted in platelets of GPS patients, while decreases in at least three membrane glycoproteins suggest that an unidentified proteolytic activity may contribute to the phenotype in some patients with this rare disease.

Platelets and wound healing

Platelets help prevent blood loss at sites of vascular injury. To do this, they adhere, aggregate and form a procoagulant surface favoring thrombin generation and fibrin formation. In addition, platelets express and release substances that promote tissue repair and influence processes such as angiogenesis, inflammation and the immune response. They contain large secretable pools of biologically active proteins, while newly synthesized active metabolites are also released. Although anucleate, activated platelets possess a spliceosome and can synthesize tissue factor and interleukin-1beta. The binding of secreted proteins within a developing fibrin mesh or to the extracellular matrix can create chemotactic gradients favoring the recruitment of stem cells, stimulating cell migration and differentiation, and promoting repair. The therapeutic use of platelets in a fibrin clot has a positive influence in clinical situations requiring rapid healing. Dental implant surgery, orthopaedic surgery, muscle and tendon repair, skin ulcers, hole repair in eye surgery and cardiac surgery are situations where the use of autologous platelets accelerates healing. We now review the ways in which platelets participate in these processes.

Platelet-released growth factors enhance the secretion of hyaluronic acid and induce hepatocyte growth factor production by synovial fibroblasts from arthritic patients

OBJECTIVES

Autologous platelet-secreted growth factors (GFs) may have therapeutic effects in osteoarthritis (OA) capsular joints via multiple mechanisms. Our aim was to examine the effect of a platelet-derived preparation rich in growth factors (PRGFs) in OA synovial cell biology.

METHODS

Synovial cells were isolated from 10 osteoarthritic patients and cultured in serum-free media (basal conditions) and exposed to either a platelet-poor preparation or PRGF for 72 h. Cells activated with interleukin-1beta (IL-1beta) for 48 h were also exposed to PRGF. Changes in several events relevant to joint homeostasis including (i) hyaluronic acid (HA) secretion, (ii) the balance between metalloproteinase-1, -3 and -13 (MMP-1, MMP-3 and MMP-13) and tissue inhibitor-1 (TIMP-1) and (iii) the secretion of transforming growth factor-beta1(TGF-beta1), vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF), were all assessed.

RESULTS

PRGF significantly enhanced HA secretion compared with platelet-poor preparations, P < 0.05; at the same time release of TIMP-1, MMP-1, MMP-3 and MMP-13 were not affected. An increased HGF production was observed (P < 0.05) but VEGF and TGF-beta1 levels remained unchanged. PRGF significantly enhanced the secretion of HA induced by IL-1beta activation, P < 0.05, but it did not modify the IL-1beta-induced rise in MMP-1, MMP-3 and VEGF. In contrast, PRGF-induced HGF production was abolished by the presence of IL-1beta during PRGF treatment, P < 0.05.

CONCLUSIONS

Intra-articular administration of PRGF might be beneficial in restoring HA concentration and switching angiogenesis to a more balanced status but does not halt the effects of IL-1beta on synovial cells.

Simultaneous detection of changes in cytoplasmic Ca(2+), aminophospholipid exposure and micro-vesiculation in activated platelets

We have used flow cytometry to compare the temporal relationship between cytoplasmic Ca(2+)-fluxes and micro-vesiculation during platelet activation. Changes in fluorescence of the Ca(2+)-dye, fluo-3, and in forward light scatter as a measure of the decrease in platelet size that accompanies micro-vesiculation, were assessed simultaneously. In other experiments, changes in Ca(2+) levels and aminophospholipid exposure were assessed using fura-red, which is a long wavelength range indicator, and FITC-annexin V. Results obtained using the ionophore A23 187 and the ATPase inhibitor, thapsigargin, showed that micro-vesiculation is a relatively late event compared with intracellular Ca(2+) elevation. The relatively slow binding kinetics of annexin V prevented the establishment of a temporal relationship between increases in intracellular Ca(2+) and aminophospholipid exposure. Nevertheless, the combined use of fura-red and annexin V highlighted the heterogeneous response seen on some occasions with thapsigargin and always with a thrombin plus collagen mixture, and confirmed that individual platelets that bound annexin V were also those with elevated intracellular Ca(2+) levels.

Family screening for a Glanzmann's thrombasthenia mutation using PCR-SSCP

Genetic counselling is often requested in Glanzmann's thrombasthenia, but measurements of GPIIb-IIIa density on platelets are often too inconclusive to allow a precise assessment of whether prospective parents are obligate heterozygotes for this disease by this measure alone. The recent application of PCR technology to Glanzmann's thrombasthenia has resulted in the identification of a large number of mutations, i.e. insertions/ deletions, splicing defects, in the genes for both GPIIb and GPIIIa. Among the reported abnormalities is an intronic G-->A substitution at the splice donor site of intron 15 in the GPIIb gene of a European gypsy tribe. This gives rise to an abnormal splicing, of an 8-bp deletion located at the 3' end of exon 15, a reading-frame shift and a premature stop codon in the mRNA for GPIIb. In applying PCR-SSCP to the elucidation of the genetic defects of a series of Glanzmann's patients, we have found the above-cited abnormality in three more gypsy families in France. The presence of the mutation was initially established by sequencing the amplified fragment, and its presence in family members was confirmed by both PCR-SSCP and HphI restriction analysis. Evaluation of the intronic G-->A mutation enabled genetic counselling to prospective parents within these families.

Clopidogrel: a review of its mechanism of action

The search for active antiplatelet drugs within the original chemical class of the thienopyridines, led to the discovery of clopidogrel, a novel ADP-selective agent whose antiaggregating properties are several times higher than those of ticlopidine. The antiaggregating properties of this compound are well known and, very recently, new results have clarified its mechanism of action. Clopidogrel is active only after intravenous or oral administration, and no circulating activity has been found in the plasma of treated animals or human volunteers. Experiments in rats have demonstrated that the antiaggregating activity was caused by a shortlasting metabolite generated in the liver by a cytochrome P450-dependent pathway. The antiaggregating property of clopidogrel is caused by an inhibition of the binding of ADP to its platelet receptors, and more specifically to the low affinity receptors, the high affinity binding sites being unaffected by clopidogrel. Several events in the ADP activation process, including adenylyl cyclase down-regulation, protein tyrosine phosphorylation, activation of the GPIIb-IIIa complex, fibrinogen binding, aggregation and release, were inhibited by clopidogrel and indicate their close relationship with the activation of a low affinity receptor by ADP. In contrast, binding of ADP to its high affinity binding sites (clopidogrel-resistant receptors) induced shape change, cytosolic calcium increase and phosphorylations of several other proteins, some events which were clopidogrel-sensitive. Thus, clopidogrel not only constitutes a potent antithrombotic drug in humans but also a good tool to study the effect of ADP on platelets.

Reciprocal actions of platelet-secreted TGF-beta1 on the production of VEGF and HGF by human tendon cells

BACKGROUND

Autologous platelet-rich matrices can be an aid in surgery by promoting and accelerating tissue healing because of the release of growth factors including transforming growth factor (TGF)-beta1 and platelet-derived growth factor (PDGF) from platelet alpha-granules.

METHODS

PDGF and TGF-beta1 were quantified in supernatants collected from platelet-rich matrices prepared in vitro (n = 45 donors) and they correlated with the number of platelets and showed a constant ratio (p < 0.05). Tendon cells in culture were exposed to the supernatants (n = 4 donors) from either platelet-rich or platelet-poor matrices, differing in their content of platelet-secreted molecules. These treatments were modified by either neutralizing or adding PDGF or TGF-beta1. Effects were compared in terms of proliferation, procollagen I, vascular endothelial growth factor (VEGF), and hepatocyte growth factor (HGF) production.

RESULTS

PDGF was a partial contributor to cell proliferation, whereas exogenous TGF-beta1 acted as a negative modulator (p < 0.05). The production of type I collagen was similar regardless of differences in the concentration of TGF-beta1. Moreover, addition of exogenous TGF-beta1 promoted a significant increase in collagen synthesis only in the absence of other platelet-released substances (p < 0.05). Exogenous TGF-beta1 increased the synthesis of VEGF and simultaneously abolished the production of HGF. Furthermore, antibody-mediated neutralization of TGF-beta1 induced a decrease in VEGF synthesis and concomitantly a substantial production of HGF (p < 0.05).

CONCLUSION

The balance between TGF-beta1 and the pools of platelet-secreted molecules may have important therapeutic implications in the control of angiogenesis and fibrosis.

The gray platelet syndrome: Clinical spectrum of the disease

The gray platelet syndrome (GPS) is a rare inherited disorder of the megakaryocyte (MK) lineage. Thrombocytopenia and enlarged platelets are associated with a specific absence of alpha-granules and their contents. GPS patients exhibit much heterogeneity both in bleeding severity and in their response to platelet function testing. A unique feature is that proteins endogenously synthesised by megakaryocytes (MK) or endocytosed by MK or platelets fail to enter into the secretable storage pools that characterise alpha-granules of normal platelets. Although the molecular basis of the disease is unknown, evidence suggests that alpha-granules simply fail to mature during MK differentiation. One result is a continued leakage of growth factors and cytokines into the marrow causing myelofibrosis. While for some patients platelet function may be only moderately affected, for others thrombin and/or collagen-induced platelet aggregation is markedly modified and an acquired lack of the GPVI collagen receptor has been reported. In this review, we document the clinical and molecular heterogeneity in GPS, a unique disease of the biogenesis of platelet alpha-granules and of the storage of growth factors and secretable proteins.

Identification of human scFvs targeting atherosclerotic lesions: selection by single round in vivo phage display

Our aim was to investigate by in vivo biopanning the lesions developed early in atherosclerosis and identify human antibodies that home to diseased regions. We have designed a two-step approach for a rapid isolation of human Monoclonal phage-display single-chain antibodies (MoPhabs) reactive with proteins found in lesions developed in an animal model of atherosclerosis. After a single round of in vivo biopanning, the MoPhabs were eluted from diseased sections of rabbit aorta identified by histology and NMR microscopy. MoPhabs expressed in situ were selected by subtractive colony filter screening for their capacity to recognize atherosclerotic but not normal aorta. MoPhabs selected by our method predominantly bind atherosclerotic lesions. Two of them, B3.3G and B3.GER, produced as scFv fragments, recognized an epitope present on the surface in early atherosclerotic lesions and within the intimal thickness in more complex plaques. These human MoPhabs homed to atherosclerotic lesions in ApoE(-/-) mice after in vivo injection. A protein of approximately 56 kDa recognized by B3.3G was affinity-purified and identified by mass spectrometry analysis as vitronectin. This is the first time that single round in vivo biopanning has been used to select human antibodies as candidates for diagnostic imaging and for obtaining insight into targets displayed in atherosclerotic plaques.