ALBERT

Description: Platelet glycoprotein (GP) V is a major surface protein cleaved during thrombin-induced platelet activation. GPV associates noncovalently with the GPIb-IX complex to form GPIb-V–IX, a receptor for von Willebrand factor and thrombin. We describe the cloning of the genes coding for rat and mouse GPV and compare them with the human gene. The two rodent genes have a similar structure and resemble the human GPV gene with a coding sequence (≈1,700 nucleotides) entirely contained in one exon and a single intron (≈900 nucleotides) in the 5′ untranslated region. Both genes have megakaryocyte-type promoters with conserved tandem Ets and GATA recognition motifs and lack a TATA box. The mature rat and mouse proteins comprise 551 amino acids, have 70% sequence identity, and contain an additional 8–amino acid intracellular segment as compared with the human protein. As in human GPV, there is an NH2 -terminal leucine-rich region of 15 repeats and a thrombin cleavage recognition sequence. Whereas the rat and human thrombin cleavage sites are similar, the mouse cleavage site resembles that of the human thrombin receptor. Functionality of these sites was demonstrated by thrombin cleavage of synthetic peptides and analysis by high-performance liquid chromatography (HPLC) or mass spectrometry. Cleavage of native rat GPV was confirmed by means of a polyclonal antibody directed against the new NH2 -terminal peptide exposed after thrombin cleavage. This antibody specifically recognized thrombin-activated rat platelets by fluorescence-activated cell sorting (FACS) analysis. In addition, we raised monoclonal antibodies specific for rat GPV (88 kD), which recognized the NH2 -terminal soluble fragment (70 kD) liberated after thrombin cleavage. Knowledge of these rodent GPV genes and availability of species-specific peptides and antibodies will be essential to further studies aiming to define the exact in vivo function of platelet GPV using animal models of thrombosis and gene inactivation experiments.

Description: The aim of this study was to investigate the inhibitory effects of human leukocyte elastase (HLE), cathepsin G (Cat G), and proteinase 3 (PR3) on the activation of endothelial cells (ECs) and platelets by thrombin and to elucidate the underlying mechanisms. Although preincubation of ECs with HLE or Cat G prevented cytosolic calcium mobilization and prostacyclin synthesis induced by thrombin, these cell responses were not affected when triggered by TRAP42-55, a synthetic peptide corresponding to the sequence of the tethered ligand (Ser42-Phe55) unmasked by thrombin on cleavage of its receptor. Using IIaR-A, a monoclonal antibody directed against the sequence encompassing this cleavage site, flow cytometry analysis showed that the surface expression of this epitope was abolished after incubation of ECs with HLE or Cat G. Further experiments conducted with platelets indicated that not only HLE and Cat G but also PR3 inhibited cell activation induced by thrombin, although they were again ineffective when TRAP42-55 was the agonist. Similar to that for ECs, the epitope for IIaR-A disappeared on treatment of platelets with either proteinase. These results suggested that the neutrophil enzymes proteolyzed the thrombin receptor dowstream of the thrombin cleavage site (Arg41-Ser42) but left intact the TRAP42-55 binding site (Gln83-Ser93) within the extracellular aminoterminal domain. The capacity of these proteinases to cleave five overlapping synthetic peptides mapping the portion of the receptor from Asn35 to Pro85 was then investigated. Mass spectrometry studies showed several distinct cleavage sites, ie, two for HLE (Val72-Ser73 and Ile74-Asn75), three for Cat G (Arg41-Ser42, Phe55-Trp56 and Tyr69-Arg70), and one for PR3 (Val72-Ser73). We conclude that this singular susceptibility of the thrombin receptor to proteolysis accounts for the ability of neutrophil proteinases to inhibit cell responses to thrombin.

Description: Abstract 1923 Poster Board I-946 We recently characterized CD148 as a potential marker for mantle cell lymphoma using mass spectrometry analysis of cell-derived microvesicles in a restricted set of patients (ASH Annual Meeting Abstracts, Nov 2008; 112: 1766). CD148 is a plasma membrane receptor with phosphatase activity related to CD45, composed of an extracellular domain containing 8 fibronectin type II-like domains, a transmembrane region and a single intracellular phosphatase domain. Interestingly, it was recently shown that deletion of the phosphatase domain of CD148 in mice blocks B cell development at the transitional stage, with a dramatic increase of marginal zone B cells. BCR signalling events are also substantially altered in CD148/CD45 doubly deficient mice. In the present study, we analyzed the expression of CD148 using flow cytometry in a larger group of controls and patients with circulating pathologic B-cells, including 93 chronic lymphocytic leukemia (CLL), 46 small lymphocytic lymphomas with Matutes score inferior or equal to 3 (SLL), 35 MCL, all harboring (11;14) translocation and/or cyclin D1 overexpression, 5 marginal zone lymphomas (MZL), 5 splenic lymphoma with villous lymphocytes (SLVL), as well as 30 controls. Mean fluorescence intensity of direct CD148 staining with phycoerythrin conjugated 143−41 clone was used for expression comparison. CD148 MFI of the 30 control cells was weak and very homogeneous (mean = 168, SD = 31), as well as in the 93 CLL (mean = 199, SD = 84). SLL cases (n=46) were stained slightly higher (mean = 297, SD = 138) revealing a slight but significant difference with CLL (p

Description: The diagnosis of mature B-cell neoplasms remains difficult in a number of cases, especially leukemic phases of non Hodgkin lymphomas, for which discriminating criteria or marker are often lacking. In order to identify new surface markers, we developed an original proteomic approach based on mass spectrometry analysis of plasma membrane microparticles derived from chronic B-cell lymphoproliferations: chronic lymphocytic leukemia/ small cell lymphoma (CLL/SLL) and mantle cell lymphoma (MCL). The approach consisted of protein extraction from plasma membrane microparticles (MPs), generated following actinomycin D stimulation and recovered using differential centrifugation. The complex protein mixture was further separated using 1D-gel electrophoresis separation and gel bands were systematically cutted at 2 mm intervals, trypsin digested and the resulting peptide mixtures was subsequently analysed by tandem mass spectrometry prior to protein identification. The lists of the proteins obtained for each pathology were then examined with respect to the membrane localization of the proteins in order to fulfill biological requirements needed for its further clinical use. Comparison of the lists of the proteins obtained for each pathology identified CD148, a membrane receptor with phosphatase activity, as a candidate for a discriminating marker detected in MCL but not in CLL in this approach. Flow cytometry studies, performed on 55 patients and 10 controls, showed that an anti-CD148 antibody stained significantly higher MCL (n=22) than CLL/SLL (n=33) circulating cells (p

Description: Plasma membrane antigens are critical for the classification and diagnosis of chronic mature B-cell lymphocytic malignancies, illustrated by the extensive use of the Matutes score for CLL diagnosis. Nevertheless, new surface markers that could be detected using flow cytometry are still needed for the diagnostic and classification of a number of chronic lymphoid malignancies. In this aim, proteomic tools could be of great interest in order to detect such antigens, but analysis of plasma membrane proteins is limited by the difficulty of their purification and separation of membrane proteins with usual 2-dimensional electrophoresis have often been poor. For all these reasons, we proposed to take advantage of the physiological production of plasma membrane microvesicles in order to study plasma membrane proteins of malignant lymphoid cells. Indeed, when eukaryotic cells are submitted to stress conditions (including mitogenic activation or apoptosis), the constitutive asymmetry between the inner and outer leaflet of the plasma membrane is disrupted and fragments are released from the plasma membrane, constituting the so-called microparticles (MPs). Although MPs reflect at least in vitro the cell state, little is known on their protein composition. We describe here the first set of experiments aiming to characterize the MPs proteome. First, two ways of triggering MPs formation from a T-lymphocytic cell line were analyzed using a 1D-gel approach coupled with Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS). 390 proteins were identified in MPs among which 34% were localized to the plasma membrane. Replicate experiments from independent samples identified only very few nucleus or organelles proteins, but revealed a broad representation of plasma membrane proteins including seventeen hematopoietic clusters of differentiation and numerous signal transduction proteins. This approach was then successfully applied to circulating cells of a small-lymphocytic B-cell lymphoma. 414 proteins were identified in MPs, including 118 membrane proteins. In addition to various CD antigens, annexins, immunoglobulin chains and HLA antigens, proteins involved in the B-cell receptor signal transduction were recovered, including tyrosine kinases LYN, SYK, BTK, CSK, but also phospho-inositide 3-kinase (PI3K) phospholipase Cγ (PLCγ 2), hemopoietic cell kinase (HCK) and JAK1. Proteomic analysis of MPs may represent a new tool to study plasma membrane proteins, displaying the advantages of reproducibility, minimal organelle contamination and potentially applicable to most cell types. Extensive proteomic analysis of MPs may thus be a way to investigate differential expression of plasma membrane components and to identify new pathology markers that could help for diagnosis and classification.