A comprehensive proteomics and genomics analysis reveals novel transmembrane proteins in human platelets and mouse megakaryocytes including G6b-B, a novel immunoreceptor tyrosine-based inhibitory motif protein

Proteomic analysis of integrin alphaIIbbeta3 outside-in signaling reveals Src-kinase-independent phosphorylation of Dok-1 and Dok-3 leading to SHIP-1 interactions

BACKGROUND AND OBJECTIVES

Outside-in integrin alphaIIbbeta3 signaling involves a series of tyrosine kinase reactions that culminate in platelet spreading on fibrinogen. The aim of this study was to identify novel tyrosine phosphorylated signaling proteins downstream of alphaIIbbeta3, and explore their role in platelet signaling.

METHODS AND RESULTS

Utilizing proteomics to search for novel platelet proteins that contribute to outside-in signaling by the integrin alphaIIbbeta3, we identified 27 proteins, 17 of which were not previously shown to be part of a tyrosine phosphorylation-based signaling complex downstream of alphaIIbbeta3. The proteins identified include the novel immunoreceptors G6f and G6b-B, and two members of the Dok family of adapters, Dok-1 and Dok-3, which underwent increased tyrosine phosphorylation following platelet spreading on fibrinogen. Dok-3 was also inducibly phosphorylated in response to the GPVI-specific agonist collagen-related peptide (CRP) and the PAR-1 and -4 agonist thrombin, independently of the integrin alphaIIbbeta3. Tyrosine phosphorylation of Dok-1 and Dok-3 was primarily Src kinase-independent downstream of the integrin, whereas it was Src kinase-dependent downstream of GPVI. Moreover, both proteins inducibly interacted with Grb-2 and SHIP-1 in fibrinogen-spread platelets.

CONCLUSIONS

This study provides new insights into the molecular mechanism regulating alphaIIbbeta3-mediated platelet spreading on fibrinogen. The novel platelet adapter Dok-3 and the structurally related Dok-1 are tyrosine phosphorylated in an Src kinase-independent manner downstream of alphaIIbbeta3 in human platelets, leading to an interaction with Grb2 and SHIP-1.

The Y's that bind: negative regulators of Src family kinase activity in platelets

Members of the Src family of protein tyrosine kinases play important roles in platelet adhesion, activation, and aggregation. The purpose of this review is to summarize current knowledge regarding how Src family kinase activity is regulated in general, to describe what is known about mechanisms underlying SFK activation in platelets, and to discuss platelet proteins that contribute to SFK inactivation, particularly those that use phosphotyrosine-containing sequences to recruit phosphatases and kinases to sites of SFK activity.

Platelet membrane proteomics: a novel repository for functional research

Being central players in thrombosis and hemostasis, platelets react in manifold and complex ways to extracellular stimuli. Cell-matrix and cell-cell interactions are mandatory for initial adhesion as well as for final development of stable plugs. Primary interfaces for interactions are plasma membrane proteins, of which many have been identified over the past decades in individual studies. However, due to their enucleate structure, platelets are not accessible to large-scale genomic screens and thus a comprehensive inventory of membrane proteins is still missing. For this reason, we here present an advanced proteomic setup for the detailed analysis of enriched platelet plasma membranes and the so far most complete collection of platelet membrane proteins. In summary, 1282 proteins were identified, of which more than half are termed to be of membrane origin. This study provides a brief overview of gene ontology subcellular and functional classification, as well as interaction network analysis. In addition, the mass spectrometric data were used to assemble a first tentative relative quantification of large-scale data on the protein level. We therefore estimate the presented data to be of major interest to the platelet research field and to support rational design of functional studies.

The tyrosine phosphatase CD148 is an essential positive regulator of platelet activation and thrombosis

Platelets play a fundamental role in hemostasis and thrombosis. They are also involved in pathologic conditions resulting from blocked blood vessels, including myocardial infarction and ischemic stroke. Platelet adhesion, activation, and aggregation at sites of vascular injury are regulated by a diverse repertoire of tyrosine kinase–linked and G protein–coupled receptors. Src family kinases (SFKs) play a central role in initiating and propagating signaling from several platelet surface receptors; however, the underlying mechanism of how SFK activity is regulated in platelets remains unclear. CD148 is the only receptor-like protein tyrosine phosphatase identified in platelets to date. In the present study, we show that mutant mice lacking CD148 exhibited a bleeding tendency and defective arterial thrombosis. Basal SFK activity was found to be markedly reduced in CD148-deficient platelets, resulting in a global hyporesponsiveness to agonists that signal through SFKs, including collagen and fibrinogen. G protein–coupled receptor responses to thrombin and other agonists were also marginally reduced. These results highlight CD148 as a global regulator of platelet activation and a novel antithrombotic drug target.

Identification of Tspan9 as a novel platelet tetraspanin and the collagen receptor GPVI as a component of tetraspanin microdomains

Platelets are essential for wound healing and inflammatory processes, but can also play a deleterious role by causing heart attack and stroke. Normal platelet activation is dependent on tetraspanins, a superfamily of glycoproteins that function as ‘organisers’ of cell membranes by recruiting other receptors and signalling proteins into tetraspanin-enriched microdomains. However, our understanding of how tetraspanin microdomains regulate platelets is hindered by the fact that only four of the 33 mammalian tetraspanins have been identified in platelets. This is because of a lack of antibodies to most tetraspanins and difficulties in measuring mRNA, due to low levels in this anucleate cell. To identify potentially platelet-expressed tetraspanins, mRNA was measured in their nucleated progenitor cell, the megakaryocyte, using serial analysis of gene expression and DNA microarrays. Amongst 19 tetraspanins identified in megakaryocytes, Tspan9, a previously uncharacterized tetraspanin, was relatively specific to these cells. Through generating the first Tspan9 antibodies, Tspan9 expression was found to be tightly regulated in platelets. The relative levels of CD9, CD151, Tspan9 and CD63 were 100, 14, 6 and 2 respectively. Since CD9 was expressed at 49000 cell surface copies per platelet, this suggested a copy number of 2800 Tspan9 molecules. Finally, Tspan9 was shown to be a component of tetraspanin microdomains that included the collagen receptor GPVI (glycoprotein VI) and integrin α6β1, but not the von Willebrand receptor GPIbα or the integrins αIIbβ3 or α2β1. These findings suggest a role for Tspan9 in regulating platelet function in concert with other platelet tetraspanins and their associated proteins.

Analysis of human plasma proteins: a focus on sample collection and separation using free-flow electrophoresis

Due to ease of accessibility, plasma has become the sample of choice for proteomics studies directed towards biomarker discovery intended for use in diagnostics, prognostics and even in theranostics. The result of these extensive efforts is a long list of potential biomarkers, very few of which have led to clinical utility. Why have so many potential biomarkers failed validation? Herein, we address certain issues encountered, which complicate biomarker discovery efforts originating from plasma. The advantages of stabilizing the sample at collection by the addition of protease inhibitors are discussed. The principles of free-flow electrophoresis (FFE) separation are provided together with examples applying to various studies. Finally, particular attention is given to plasma or serum analysis using multidimensional separation strategies into which the FFE is incorporated. The advantages of using FFE separation in these workflows are discussed.

G6b-B inhibits constitutive and agonist-induced signaling by glycoprotein VI and CLEC-2

Platelets play an essential role in wound healing by forming thrombi that plug holes in the walls of damaged blood vessels. To achieve this, platelets express a diverse array of cell surface receptors and signaling proteins that induce rapid platelet activation. In this study we show that two platelet glycoprotein receptors that signal via an immunoreceptor tyrosine-based activation motif (ITAM) or an ITAM-like domain, namely the collagen receptor complex glycoprotein VI (GPVI)-FcR γ-chain and the C-type lectin-like receptor 2 (CLEC-2), respectively, support constitutive (i.e. agonist-independent) signaling in a cell line model using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay that can detect low level activation of phospholipase Cγ (PLCγ). Constitutive and agonist signaling by both receptors is dependent on Src and Syk family kinases, and is inhibited by G6b-B, a platelet immunoglobulin receptor that has two immunoreceptor tyrosine-based inhibitory motifs in its cytosolic tail. Mutation of the conserved tyrosines in the two immunoreceptor tyrosine-based inhibitory motifs prevents the inhibitory action of G6b-B. Interestingly, the inhibitory activity of G6b-B is independent of the Src homology 2 (SH2)-domain containing tyrosine phosphatases, SHP1 and SHP2, and the inositol 5′-phosphatase, SHIP. Constitutive signaling via Src and Syk tyrosine kinases is observed in platelets and is associated with tyrosine phosphorylation of GPVI-FcR γ-chain and CLEC-2. We speculate that inhibition of constitutive signaling through Src and Syk tyrosine kinases by G6b-B may help to prevent unwanted platelet activation.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

Phosphoproteome of resting human platelets

Beside their main physiological function in hemostasis, platelets are also highly involved in pathological processes, such as atherothrombosis and inflammation. During hemostasis, binding of adhesive substrates to tyrosine-kinase-linked adhesion receptors and/or soluble agonists to G-protein coupled receptors leads to a cascade of intracellular signaling processes based on substrate (de)phosphorylation. The same mechanisms are involved in platelet activation at sites of atherosclerotic plaque rupture, contributing to vessel occlusion and consequently to pathologic states, such as myocardial infarction, stroke, or peripheral artery disease. To gain a deeper insight into platelet function, we analyzed the phosphoproteome of resting platelets and identified 564 phosphorylation sites from more than 270 proteins, of which many have not been described in platelets before. Among those were several unknown potential protein kinase A (PKA) and protein kinase G (PKG) substrates. Because platelet inhibition is tightly regulated especially by PKA and PKG activity, these proteins may represent important new targets for cardiovascular research. Thus, our finding that GPIbalpha is phosphorylated at Ser603 in resting platelets may represent a novel mechanism for the regulation of one of the most important platelet receptor (GPIb-IX-V) mediated signaling pathways by PKA/PKG.

Mammalian plasma membrane proteomics

Plasma membrane proteins serve essential functions for cells, interacting with both cellular and extracellular components, structures and signaling molecules. Additionally, plasma membrane proteins comprise more than two-thirds of the known protein targets for existing drugs. Consequently, defining membrane proteomes is crucial to understanding the role of plasma membranes in fundamental biological processes and for finding new targets for action in drug development. MS-based identification methods combined with chromatographic and traditional cell-biology techniques are powerful tools for proteomic mapping of proteins from organelles. However, the separation and identification of plasma membrane proteins remains a challenge for proteomic technology because of their hydrophobicity and microheterogeneity. Creative approaches to solve these problems and potential pitfalls will be discussed. Finally, a representative overview of the impressive achievements in this field will also be given.

Exploring the platelet proteome via combinatorial, hexapeptide ligand libraries

A combinatorial ligand library, composed of millions of diverse hexapeptide baits, able to capture and concentrate the "low-abundance" proteome while drastically cutting the concentration of the most abundant species, has been applied to the exploration of the soluble platelet proteome. Mass spectrometry analysis of untreated and library-treated platelets has resulted in the identification of 435 unique gene products. Of those, 147 entries (35% of the total) have not been described among the list of >1100 proteins in proteomic platelet investigations reported before. In addition, the analysis of excised spots from two-dimensional electrophoresis analysis allowed 57 other proteins to be added that were not found in LC-MS analysis, 33 of them not described before in proteomics studies, bringing the total number of new gene products to 180. Thus, the present data add a non-negligible number of species for continuing the "cartography" of the proteomic asset of platelets, in view of completing the mapping procedure for a deeper understanding of the physiology and pathology of this blood cell. Because the capturing process is performed under physiological conditions, by exploiting, for binding to the combinatorial library, the native protein configuration, the described technique is not adapted to capture highly hydrophobic proteins, which need strong denaturing and solubilizing conditions that are incompatible with our working procedure. Thus, our list reports essentially hydrophilic proteins, with negative GRAVY indexes.

Proteins at membrane surfaces-a review of approaches

Membrane proteins are critical for normal cellular differentiation and function, and alterations in these proteins often leads to cell dysfunction and disease. Membrane proteomics aims to identify the membrane protein constituents, their posttranslational modifications, protein-protein interactions, and dynamics. Efforts to identify membrane proteins and elucidate their dynamics have been plagued by the challenges presented by studying water insoluble proteins that are distributed among a range of membranes in a cell and often occur at a relatively low abundance. This brief review presents a summary of the literature related to membrane proteomics with an emphasis on efforts to develop effective protocols for the enrichment of membrane proteins, particularly those located in the plasma membrane.

Two-dimensional electrophoresis of membrane proteins

One third of all genes of various organisms encode membrane proteins, emphasizing their crucial cellular role. However, due to their high hydrophobicity, membrane proteins demonstrate low solubility and a high tendency for aggregation. Indeed, conventional two-dimensional gel electrophoresis (2-DE), a powerful electrophoretic method for the separation of complex protein samples that applies isoelectric focusing (IEF) in the first dimension and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) in the second dimension, has a strong bias against membrane proteins. This review describes two-dimensional electrophoretic techniques that can be used to separate membrane proteins. Alternative methods for performing conventional 2-DE are highlighted; these involve replacing the IEF with electrophoresis using cationic detergents, namely 16-benzyldimethyl-n-hexadecylammonium chloride (16-BAC) and cetyl trimethyl ammonium bromide (CTAB), or the anionic detergent SDS. Finally, the separation of native membrane protein complexes through the application of blue and clear native gel electrophoresis (BN/CN-PAGE) is reviewed, as well as the free-flow electrophoresis (FFE) of membranes.