Purification and characterization of endogenous protein activator of human platelet proteasome

Platelet EVs contain an active proteasome involved in protein processing for antigen presentation via MHC-I molecules

In addition to their hemostatic role, platelets play a significant role in immunity. Once activated, platelets release extracellular vesicles (EVs) formed by budding of their cytoplasmic membranes. Because of their heterogeneity, platelet EVs (PEVs) are thought to perform diverse functions. It is unknown, however, whether the proteasome is transferred from platelets to PEVs or whether its function is retained. We hypothesized that functional protein processing and antigen presentation machinery is transferred to PEVs by activated platelets. Using molecular and functional assays, we show that the active 20S proteasome is enriched in PEVs along with MHC-I and lymphocyte costimulatory molecules (CD40L and OX40L). Proteasome-containing PEVs were identified in healthy donor blood, but did not increase in platelet concentrates that caused adverse transfusion reactions. They were, however, augmented after immune complex injections in mice. The complete biodistribution of murine PEVs following injection into mice revealed that they could principally reach lymphoid organs such as spleen and lymph nodes, in addition to the bone marrow, and to a lesser extent liver and lungs. The PEV proteasome processed exogenous ovalbumin (OVA) and loaded its antigenic peptide onto MHC-I molecules which promoted OVA-specific CD8+ T lymphocyte proliferation. These results suggest that PEVs contribute to adaptive immunity through cross-presentation of antigens and have privileged access to immune cells through the lymphatic system, a tissue location that is inaccessible to platelets.

The Role of the Proteasome in Platelet Function

Platelets are megakaryocyte-derived acellular fragments prepped to maintain primary hemostasis and thrombosis by preserving vascular integrity. Although they lack nuclei, platelets harbor functional genomic mediators that bolster platelet activity in a signal-specific manner by performing limited de novo protein synthesis. Furthermore, despite their limited protein synthesis, platelets are equipped with multiple protein degradation mechanisms, such as the proteasome. In nucleated cells, the functions of the proteasome are well established and primarily include proteostasis among a myriad of other signaling processes. However, the role of proteasome-mediated protein degradation in platelets remains elusive. In this review article, we recapitulate the developing literature on the functions of the proteasome in platelets, discussing its emerging regulatory role in platelet viability and function and highlighting how its functional coupling with the transcription factor NF-κB constitutes a novel potential therapeutic target in atherothrombotic diseases.

Role of platelets and megakaryocytes in adaptive immunity

The immune system is comprised of two principal interconnected components called innate and adaptive immunity. While the innate immune system mounts a nonspecific response that provides protection against the spread of foreign pathogens, the adaptive immune system has developed to specifically recognize a given pathogen and lead to immunological memory. Platelets are small fragments produced from megakaryocytes in bone marrow and lungs. They circulate throughout the blood to monitor the integrity of the vasculature and to prevent bleeding. Given their large repertoire of immune receptors and inflammatory molecules, platelets and megakaryocytes can contribute to both innate and adaptive immunity. In adaptive immunity, platelets and megakaryocytes can process and present antigens to lymphocytes. Moreover, platelets, via FcγRIIA, rapidly respond to pathogens in an immune host when antibodies are present. This manuscript reviews the reported contributions of platelets and megakaryocytes with emphasis on antigen presentation and antibody response in adaptive immunity.

Structure and function of the ubiquitin-proteasome system in platelets

Platelets are small anucleate blood cells with a life span of 7 to 10 days. They are main regulators of hemostasis. Balanced platelet activity is crucial to prevent bleeding or occlusive thrombus formation. Growing evidence supports that platelets also participate in immune reactions, and interaction between platelets and leukocytes contributes to both thrombosis and inflammation. The ubiquitin-proteasome system (UPS) plays a key role in maintaining cellular protein homeostasis by its ability to degrade non-functional self-, foreign, or short-lived regulatory proteins. Platelets express standard and immunoproteasomes. Inhibition of the proteasome impairs platelet production and platelet function. Platelets also express major histocompatibility complex (MHC) class I molecules. Peptide fragments released by proteasomes can bind to MHC class I, which makes it also likely that platelets can activate epitope specific cytotoxic T lymphocytes (CTLs). In this review, we focus on current knowledge on the significance of the proteasome for the functions of platelets as critical regulators of hemostasis as well as modulators of the immune response.

The proteasome regulates collagen-induced platelet aggregation via nuclear-factor-kappa-B (NFĸB) activation

INTRODUCTION

Platelets possess critical hemostatic functions in the system of thrombosis and hemostasis, which can be affected by a multitude of external factors. Previous research has shown that platelets have the capacity to synthesize proteins de novo and more recently a multicatalytic protein complex, the proteasome, has been discovered in platelets. Due to its vital function for cellular integrity, the proteasome has become a therapeutic target for anti-proliferative drug therapies in cancer. Clinically thrombocytopenia is a frequent side-effect, but the aggregatory function of platelets also appears to be affected. Little is known however about underlying regulatory mechanisms and functional aspects of proteasome inhibition on platelets. Our study aims to investigate the role of the proteasome in regulating collagen-induced platelet aggregation and its interaction with NFkB in this context.

MATERIAL AND METHODS

Using fluorescence activity assays, platelet aggregometry and immunoblotting, we investigate regulatory interactions of the proteasome and Nuclear-factor-kappa-B (NFkB) in collagen-induced platelet aggregation.

RESULTS

We show that collagen induces proteasome activation in platelets and collagen-induced platelet aggregation can be reduced with proteasome inhibition by the specific inhibitor epoxomicin. This effect does not depend on Rho-kinase/ROCK activation or thromboxane release, but rather depends on NFkB activation. Inhibition of the proteasome prevented cleavage of NFκB-inhibitor protein IκBα and decreased NFκB activity after collagen stimulation. Inhibition of the NFκB-pathway in return reduced collagen-induced platelet proteasome activity and cleavage of proteasome substrates.

CONCLUSIONS

This work offers novel explanations how the proteasome influences collagen-dependent platelet aggregation by involving non-genomic functions of NFkB.

Autophagy is induced upon platelet activation and is essential for hemostasis and thrombosis

Autophagy is important for maintaining cellular homeostasis, and thus its deficiency is implicated in a broad spectrum of human diseases. Its role in platelet function has only recently been examined. Our biochemical and imaging studies demonstrate that the core autophagy machinery exists in platelets, and that autophagy is constitutively active in resting platelets. Moreover, autophagy is induced upon platelet activation, as indicated by agonist-induced loss of the autophagy marker LC3II. Additional experiments, using inhibitors of platelet activation, proteases, and lysosomal acidification, as well as platelets from knockout mouse strains, show that agonist-induced LC3II loss is a consequence of platelet signaling cascades and requires proteases, acidic compartments, and membrane fusion. To assess the physiological role of platelet autophagy, we generated a mouse strain with a megakaryocyte- and platelet-specific deletion of Atg7, an enzyme required for LC3II production. Ex vivo analysis of platelets from these mice shows modest defects in aggregation and granule cargo packaging. Although these mice have normal platelet numbers and size distributions, they exhibit a robust bleeding diathesis in the tail-bleeding assay and a prolonged occlusion time in the FeCl3-induced carotid injury model. Our results demonstrate that autophagy occurs in platelets and is important for hemostasis and thrombosis.

Global proteome analysis identifies active immunoproteasome subunits in human platelets

The discovery of new functions for platelets, particularly in inflammation and immunity, has expanded the role of these anucleate cell fragments beyond their primary hemostatic function. Here, four in-depth human platelet proteomic data sets were generated to explore potential new functions for platelets based on their protein content and this led to the identification of 2559 high confidence proteins. During a more detailed analysis, consistently high expression of the proteasome was discovered, and the composition and function of this complex, whose role in platelets has not been thoroughly investigated, was examined. Data set mining resulted in identification of nearly all members of the 26S proteasome in one or more data sets, except the β5 subunit. However, β5i, a component of the immunoproteasome, was identified. Biochemical analyses confirmed the presence of all catalytically active subunits of the standard 20S proteasome and immunoproteasome in human platelets, including β5, which was predominantly found in its precursor form. It was demonstrated that these components were assembled into the proteasome complex and that standard proteasome as well as immunoproteasome subunits were constitutively active in platelets. These findings suggest potential new roles for platelets in the immune system. For example, the immunoproteasome may be involved in major histocompatibility complex I (MHC I) peptide generation, as the MHC I machinery was also identified in our data sets.

Dissection of autophagy in human platelets

Continuous turnover of intracellular components by autophagy is necessary to preserve cellular homeostasis in all tissues. Despite recent advances in identifying autophagy-related genes and understanding the functions of autophagy in developmental and pathological conditions, so far, the role of autophagy in platelet, a specific anucleate cell type, is poorly understood. In this study, we showed that human platelets express the autophagy-related proteins ATG5, ATG7, and LC3. The same as in nucleated mammalian cells, autophagy was stimulated by cell starvation or the MTOR inhibitor rapamycin in a phosphatidylinositol 3-kinase (PtdIns3K)-dependent manner. Disruption of autophagic flux led to impairment of platelet aggregation and adhesion. Furthermore, Becn1 heterozygous knockout mice displayed a prolonged bleeding time and reduced platelet aggregation. These results suggest a potential role of autophagy in the regulation of platelet function, and imply that gene transcription may not be an essential prerequisite for adaptive autophagy.

Proteasome proteolysis supports stimulated platelet function and thrombosis

OBJECTIVE

Proteasome inhibitors used in the treatment of hematologic cancers also reduce thrombosis. Whether the proteasome participates in platelet activation or function is unclear because little is known of the proteasome in these terminally differentiated cells.

APPROACH AND RESULTS

Platelets displayed all 3 primary proteasome protease activities, which MG132 and bortezomib (Velcade) inhibited. Proteasome substrates are marked by ubiquitin, and platelets contained a functional ubiquitination system that modified the proteome by monoubiquitination and polyubiquitination. Systemic MG132 strongly suppressed the formation of occlusive, platelet-rich thrombi in FeCl3-damaged carotid arteries. Transfusion of platelets treated ex vivo with MG132 and washed before transfusion into thrombocytopenic mice also reduced carotid artery thrombosis. Proteasome inhibition reduced platelet aggregation by low thrombin concentrations and ristocetin-stimulated agglutination through the glycoprotein Ib-IX-V complex. This receptor was not appropriately internalized after proteasome inhibition in stimulated platelets, and spreading and clot retraction after MG132 exposure also were decreased. The effects of proteasome inhibitors were not confined to a single receptor as MG132 suppressed thrombin-stimulated, ADP-stimulated, and lipopolysaccharide-stimulated microparticle shedding. Proteasome inhibition increased ubiquitin decoration of cytoplasmic proteins, including the cytoskeletal proteins Filamin A and Talin-1. Mass spectrometry revealed a single MG132-sensitive tryptic cleavage after R1745 in an extended Filamin A loop, which would separate its actin-binding domain from its carboxy terminal glycoprotein Ibα-binding domain.

CONCLUSIONS

Platelets contain a ubiquitin/proteasome system that marks cytoskeletal proteins for proteolytic modification to promote productive platelet-platelet and platelet-wall interactions.

Protein degradation systems in platelets

Protein synthesis and degradation are essential processes that allow cells to survive and adapt to their surrounding milieu. In nucleated cells, the degradation and/or cleavage of proteins is required to eliminate aberrant proteins. Cells also degrade proteins as a mechanism for cell signalling and complex cellular functions. Although the last decade has convincingly shown that platelets synthesise proteins, the roles of protein degradation in these anucleate cytoplasts are less clear. Here we review what is known about protein degradation in platelets placing particular emphasis on the proteasome and the cysteine protease calpain.

Regulatory role of proteasome in determination of platelet life span

Limit of platelet life span (8-10 days) is determined by the activity of a putative "internal clock" composed of Bcl-2 family proteins, whereas the role of other molecular players in this process remains obscure. Here, we sought to establish a central role of proteasome in platelet life span regulation. Administration of mice with inhibitors of proteasome peptidase activity induced significant thrombocytopenia. This was associated with enhanced clearance of biotin-labeled platelets from circulation and reduction in average platelet half-life from 66 to 37 h. Cells pretreated in vitro with proteasome inhibitors exhibited augmented annexin V binding and a drop in mitochondrial transmembrane potential indicative of apoptotic cell death and decreased platelet life span. These cells were preferentially phagocytosed by monocyte-derived macrophages, thus linking proteasome activity with platelet survival. The decisive role of proteasome in this process was underscored from enhanced expression of conformationally active Bax in platelets with attenuated proteasome activity, which was consistent with pro-apoptotic phenotype of these cells. The present study establishes a critical role of proteasome in delimiting platelet life span ostensibly through constitutive elimination of the conformationally active Bax. These findings bear potential implications in clinical settings where proteasome peptidase activities are therapeutically targeted.

Regulation of proteasome activity in activated human platelets

Ubiquitin-proteasome system has emerged a central player in regulation of diverse cellular processes. However, relevance of proteasome activity in platelets, which are terminally differentiated enucleate cells, is not clear. In this report we show that activation of platelets with physiological agonists was associated with 7-10 -fold rise in proteasomal activity. Elevation of cytosolic calcium with A23187 or thapsigargin resulted in significant increase in enzymatic activity, while treatment with intracellular calcium chelator or inhibitor of inositol trisphosphate receptor attenuated proteasomal enzymes in collagen-stimulated platelets. Specific inhibitors of protein kinase C as well as calpain, too, downregulated proteasome function. To conclude, proteasomal enzymatic activity in platelets is regulated by cytosolic calcium through Ca(2+)-dependent downstream effectors like calpain and protein kinase C.

Mevastatin induces degeneration and decreases viability of cAMP-induced differentiated neuroblastoma cells in culture by inhibiting proteasome activity, and mevalonic acid lactone prevents these effects

Statins with a closed-ring structure (mevastatin, lovastatin, and simvastatin) and with an open-ring structure (pravastatin and fluvastatin) are widely used in the human population to manage hypercholesterolemia. These statins may have neuroprotective or neurotoxic effects, but these effects remain controversial. We have utilized adenosine 3',5'-cyclic monophosphate-induced terminally differentiated murine neuroblastoma (NB) cells in culture as an experimental model to study the effect of statins. Results showed that mevastatin induced degenerative changes and reduced the viability of differentiated NB cells by inhibiting proteasome activity. Lactacystin, an established inhibitor of proteasome, also produced similar degenerative changes in these cells. In contrast, pravastatin neither affected the degeneration and viability of differentiated NB cells nor the proteasome activity. High-performance liquid chromatography (HPLC) analysis of the extract obtained from mevastatin-treated growth medium and differentiated cells revealed that about 50% of mevastatin is converted to an open-ring structure in the growth medium; however, differentiated cells did not convert any portion of mevastatin into an open-ring structure and accumulated only mevastatin with a closed-ring structure. Mevalonic acid lactone by itself did not affect the viability of differentiated NB cells or the proteasome activity, but it completely prevented mevastatin-induced degeneration and decreased viability by reducing the uptake of mevastatin and by blocking its action on proteasome activity. Mevalonic acid failed to prevent lactacystin-induced degeneration and inhibition of proteasome activity. Our results suggest that mevastatin could act as a neurotoxic agent or neuroprotective agent, depending upon the extent of its hydrolysis to an open-ring structure and the level of mevalonic acid.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Structural and functional properties of proteasome activator PA28

The proteasome activator PA28 or 11S regulator is a protein complex composed of two different but homologous polypeptides, termed PA28alpha and PA28beta. The purified activator protein (approximately 200 kDa) is a ring-shaped heteromultimer containing the two polypeptides, possibly with an (alpha3beta3 stoichiometry. The activator, which by itself shows no hydrolytic activity elicits activation of the proteasome's multiple peptidase activities by binding to the terminal rings of the proteinase. In vitro, active PA28 can be reconstituted from isolated alpha and beta subunits, yielding two different oligomers: with the single alpha subunit, PA28alpha homomultimers with moderate stimulatory activity toward 20S proteasomes are obtained whereas isolated beta-subunits are unable to form oligomers and are devoid of stimulatory activity. However, in the presence of both subunits, alphabeta heteromultimers form, concomitant with restoration of full stimulatory activity. The recent finding that PA28 modulates the proteasome-catalyzed production of antigenic peptides presented to the immune system on MHC class I molecules indicates a cellular function of the activator in antigen processing.