Alterations of the platelet proteome in type I Glanzmann thrombasthenia caused by different homozygous delG frameshift mutations in ITGA2B

Proteomic landscapes of inherited platelet disorders with different etiologies

BACKGROUND

Inherited platelet disorders (IPDs) are a heterogeneous group of rare diseases that are caused by the defects in early megakaryopoiesis, proplatelet formation, and/or mature platelet function. Although genomic sequencing is increasingly used to identify genetic variants underlying IPD, this technique does not disclose resulting molecular changes that impact platelet function. Proteins are the functional units that shape platelet function; however, insights into how variants that cause IPDs impact platelet proteomes are limited.

OBJECTIVES

The objective of this study was to profile the platelet proteomics signatures of IPDs.

METHODS

We performed unbiased label-free quantitative mass spectrometry (MS)-based proteome profiling on platelets of 34 patients with IPDs with variants in 13 ISTH TIER1 genes that affect different stages of platelet development.

RESULTS

In line with the phenotypical heterogeneity between IPDs, proteomes were diverse between IPDs. We observed extensive proteomic alterations in patients with a GFI1B variant and for genetic variants in genes encoding proteins that impact cytoskeletal processes (MYH9, TUBB1, and WAS). Using the diversity between IPDs, we clustered protein dynamics, revealing disrupted protein-protein complexes. This analysis furthermore grouped proteins with similar cellular function and location, classifying mitochondrial protein constituents and identifying both known and putative novel alpha granule associated proteins.

CONCLUSIONS

With this study, we demonstrate a MS-based proteomics perspective to IPDs. By integrating the effects of IPDs that impact different aspects of platelet function, we dissected the biological contexts of protein alterations to gain further insights into the biology of platelet (dys)function.

Differential Inhibition of Platelet Reactivity by Dual Therapy With Aspirin and Low-Dose Rivaroxaban in Peripheral Arterial Disease: A Pilot Study

Patients with peripheral arterial disease (PAD) benefit from combination therapy with acetylsalicylic acid (ASA, 100 mg, one time per day) plus low-dose rivaroxaban (2.5 mg, two times per day) compared to ASA monotherapy. In particular, major adverse cardiac and limb events were significantly reduced after peripheral endovascular revascularization (EVR). In this pilot study, the platelet activation status in vivo and platelet reactivity in vitro were longitudinally analyzed by flow cytometric assays and calibrated automated thrombography in platelet-rich plasma (PRP) from 10 patients with PAD receiving ASA (100 mg, one time per day) before EVR, ASA plus clopidogrel (75 mg, one time per day) after EVR, and ASA plus rivaroxaban (2.5 mg, two times per day) during a long-term follow-up. Platelet responsiveness to clopidogrel was compared to additional 10 patients with stable PAD and clopidogrel (75 mg, one time per day) monotherapy. ASA plus rivaroxaban treatment resulted in a significantly decreased thrombin peak in PRP for two triggers, namely, low concentration of tissue factor (TF) and thrombin, compared to ASA monotherapy. TF-controlled thrombin generation was additionally characterized by a significantly prolonged lag time in PRP and platelet-free plasma during ASA plus rivaroxaban combination therapy. In comparison, ASA plus clopidogrel treatment presented a significant reduction of the thrombin peak in PRP, which was less pronounced than during subsequent ASA plus rivaroxaban therapy. Platelet responsiveness to clopidogrel was observed for 60% of patients receiving ASA plus clopidogrel and clopidogrel monotherapy, respectively. Blocking of CD36 on the platelet surface further reduced the thrombin peak in PRP induced by TF for all three therapy regimes. Platelet activation in vivo and in response to the GPVI-agonist convulxin or thrombin in vitro was similar, whereas integrin αIIbβ3 activation and α-granule release induced by the PAR-1 activating peptide TRAP-6 were significantly diminished during ASA plus rivaroxaban treatment compared to ASA monotherapy. In conclusion, the data of this pilot study indicate an inhibitory effect of rivaroxaban on the thrombin propagation phase of CD36-sensitive platelet thrombin formation in patients with PAD treated with ASA plus rivaroxaban combination therapy, which is associated with decreased PAR-1 but not thrombin-mediated platelet activation.

Molecular Proteomics and Signalling of Human Platelets in Health and Disease

Platelets are small anucleate blood cells that play vital roles in haemostasis and thrombosis, besides other physiological and pathophysiological processes. These roles are tightly regulated by a complex network of signalling pathways. Mass spectrometry-based proteomic techniques are contributing not only to the identification and quantification of new platelet proteins, but also reveal post-translational modifications of these molecules, such as acetylation, glycosylation and phosphorylation. Moreover, target proteomic analysis of platelets can provide molecular biomarkers for genetic aberrations with established or non-established links to platelet dysfunctions. In this report, we review 67 reports regarding platelet proteomic analysis and signalling on a molecular base. Collectively, these provide detailed insight into the: (i) technical developments and limitations of the assessment of platelet (sub)proteomes; (ii) molecular protein changes upon ageing of platelets; (iii) complexity of platelet signalling pathways and functions in response to collagen, rhodocytin, thrombin, thromboxane A₂ and ADP; (iv) proteomic effects of endothelial-derived mediators such as prostacyclin and the anti-platelet drug aspirin; and (v) molecular protein changes in platelets from patients with congenital disorders or cardiovascular disease. However, sample sizes are still low and the roles of differentially expressed proteins are often unknown. Based on the practical and technical possibilities and limitations, we provide a perspective for further improvements of the platelet proteomic field.

Profiling the Genetic and Molecular Characteristics of Glanzmann Thrombasthenia: Can It Guide Current and Future Therapies?

Glanzmann thrombasthenia (GT) is the most widely studied inherited disease of platelet function. Platelets fail to aggregate due to a defect in platelet-to-platelet attachment. The hemostatic plug fails to form and a moderate to severe bleeding diathesis results. Classically of autosomal recessive inheritance, GT is caused by defects within the ITGA2B and ITGB3 genes that encode the αIIbβ3 integrin expressed at high density on the platelet surface and also in intracellular pools. Activated αIIbβ3 acts as a receptor for fibrinogen and other adhesive proteins that hold platelets together in a thrombus. Over 50 years of careful clinical and biological investigation have provided important advances that have improved not only the quality of life of the patients but which have also contributed to an understanding of how αIIbβ3 functions. Despite major improvements in our knowledge of GT and its genetic causes, extensive biological and clinical variability with respect to the severity and intensity of bleeding remains poorly understood. I now scan the repertoire of ITGA2B and ITGB3 gene defects and highlight the wide genetic and biological heterogeneity within the type II and variant subgroups especially with regard to bleeding, clot retraction, the internal platelet Fg storage pool and the nature of the mutations causing the disease. I underline the continued importance of gene profiling and biological studies and emphasize the multifactorial etiology of the clinical expression of the disease. This is done in a manner to provide guidelines for future studies and future treatments of a disease that has not only aided research on rare diseases but also contributed to advances in antithrombotic therapy.

Assessment of a complete and classified platelet proteome from genome-wide transcripts of human platelets and megakaryocytes covering platelet functions

Novel platelet and megakaryocyte transcriptome analysis allows prediction of the full or theoretical proteome of a representative human platelet. Here, we integrated the established platelet proteomes from six cohorts of healthy subjects, encompassing 5.2 k proteins, with two novel genome-wide transcriptomes (57.8 k mRNAs). For 14.8 k protein-coding transcripts, we assigned the proteins to 21 UniProt-based classes, based on their preferential intracellular localization and presumed function. This classified transcriptome-proteome profile of platelets revealed: (i) Absence of 37.2 k genome-wide transcripts. (ii) High quantitative similarity of platelet and megakaryocyte transcriptomes (R = 0.75) for 14.8 k protein-coding genes, but not for 3.8 k RNA genes or 1.9 k pseudogenes (R = 0.43–0.54), suggesting redistribution of mRNAs upon platelet shedding from megakaryocytes. (iii) Copy numbers of 3.5 k proteins that were restricted in size by the corresponding transcript levels (iv) Near complete coverage of identified proteins in the relevant transcriptome (log2fpkm > 0.20) except for plasma-derived secretory proteins, pointing to adhesion and uptake of such proteins. (v) Underrepresentation in the identified proteome of nuclear-related, membrane and signaling proteins, as well proteins with low-level transcripts. We then constructed a prediction model, based on protein function, transcript level and (peri)nuclear localization, and calculated the achievable proteome at ~ 10 k proteins. Model validation identified 1.0 k additional proteins in the predicted classes. Network and database analysis revealed the presence of 2.4 k proteins with a possible role in thrombosis and hemostasis, and 138 proteins linked to platelet-related disorders. This genome-wide platelet transcriptome and (non)identified proteome database thus provides a scaffold for discovering the roles of unknown platelet proteins in health and disease.

Age-Dependent Control of Collagen-Dependent Platelet Responses by Thrombospondin-1-Comparative Analysis of Platelets from Neonates, Children, Adolescents, and Adults

Platelet function is developmentally regulated. Healthy neonates do not spontaneously bleed, but their platelets are hypo-reactive to several agonists. The mechanisms underlying immature platelet function in neonates are incompletely understood. This critical issue remains challenging for the establishment of age-specific reference ranges. In this study, we evaluated platelet reactivity of five pediatric age categories, ranging from healthy full-term neonates up to adolescents (11–18 years) in comparison to healthy adults (>18 years) by flow cytometry. We confirmed that platelet hypo-reactivity detected by fibrinogen binding, P-selectin, and CD63 surface expression was most pronounced in neonates compared to other pediatric age groups. However, maturation of platelet responsiveness varied with age, agonist, and activation marker. In contrast to TRAP and ADP, collagen-induced platelet activation was nearly absent in neonates. Granule secretion markedly remained impaired at least up to 10 years of age compared to adults. We show for the first time that neonatal platelets are deficient in thrombospondin-1, and exogenous platelet-derived thrombospondin-1 allows platelet responsiveness to collagen. Platelets from all pediatric age groups normally responded to the C-terminal thrombospondin-1 peptide RFYVVMWK. Thus, thrombospondin-1 deficiency of neonatal platelets might contribute to the relatively impaired response to collagen, and platelet-derived thrombospondin-1 may control distinct collagen-induced platelet responses.

Proteomics: A Tool to Study Platelet Function

Platelets are components of the blood that are highly reactive, and they quickly respond to multiple physiological and pathophysiological processes. In the last decade, it became clear that platelets are the key components of circulation, linking hemostasis, innate, and acquired immunity. Protein composition, localization, and activity are crucial for platelet function and regulation. The current state of mass spectrometry-based proteomics has tremendous potential to identify and quantify thousands of proteins from a minimal amount of material, unravel multiple post-translational modifications, and monitor platelet activity during drug treatments. This review focuses on the role of proteomics in understanding the molecular basics of the classical and newly emerging functions of platelets. including the recently described role of platelets in immunology and the development of COVID-19.The state-of-the-art proteomic technologies and their application in studying platelet biogenesis, signaling, and storage are described, and the potential of newly appeared trapped ion mobility spectrometry (TIMS) is highlighted. Additionally, implementing proteomic methods in platelet transfusion medicine, and as a diagnostic and prognostic tool, is discussed.

Platelet Phenotyping and Function Testing in Thrombocytopenia

Patients who suffer from inherited or acquired thrombocytopenia can be also affected by platelet function defects, which potentially increase the risk of severe and life-threatening bleeding complications. A plethora of tests and assays for platelet phenotyping and function analysis are available, which are, in part, feasible in clinical practice due to adequate point-of-care qualities. However, most of them are time-consuming, require experienced and skilled personnel for platelet handling and processing, and are therefore well-established only in specialized laboratories. This review summarizes major indications, methods/assays for platelet phenotyping, and in vitro function testing in blood samples with reduced platelet count in relation to their clinical practicability. In addition, the diagnostic significance, difficulties, and challenges of selected tests to evaluate the hemostatic capacity and specific defects of platelets with reduced number are addressed.

Cyclin Y is expressed in Platelets and Modulates Integrin Outside-in Signaling

Diabetes is associated with platelet hyper-reactivity and enhanced risk of thrombosis development. Here we compared protein expression in platelets from healthy donors and diabetic patients to identify differentially expressed proteins and their possible function in platelet activation. Mass spectrometry analyses identified cyclin Y (CCNY) in platelets and its reduced expression in platelets from diabetic patients, a phenomenon that could be attributed to the increased activity of calpains. To determine the role of CCNY in platelets, mice globally lacking the protein were studied. CCNY^-/- mice demonstrated lower numbers of circulating platelets but platelet responsiveness to thrombin and a thromboxane A₂ analogue were comparable with that of wild-type mice, as was agonist-induced α and dense granule secretion. CCNY-deficient platelets demonstrated enhanced adhesion to fibronectin and collagen as well as an attenuated spreading and clot retraction, indicating an alteration in "outside in" integrin signalling. This phenotype was accompanied by a significant reduction in the agonist-induced tyrosine phosphorylation of β3 integrin. Taken together we have shown that CCNY is present in anucleated platelets where it is involved in the regulation of integrin-mediated outside in signalling associated with thrombin stimulation.

CD36-fibrin interaction propagates FXI-dependent thrombin generation of human platelets

Thrombin converts fibrinogen to fibrin and activates blood and vascular cells in thrombo-inflammatory diseases. Platelets are amplifiers of thrombin formation when activated by leukocyte- and vascular cell-derived thrombin. CD36 on platelets acts as sensitizer for molecules with damage-associated molecular patterns, thereby increasing platelet reactivity. Here, we investigated the role of CD36 in thrombin-generation on human platelets, including selected patients with advanced chronic kidney disease (CKD). Platelets deficient in CD36 or blocked by anti-CD36 antibody FA6.152 showed impaired thrombin generation triggered by thrombin in calibrated automated thrombography. Using platelets with congenital function defects, blocking antibodies, pharmacological inhibitors, and factor-depleted plasma, CD36-sensitive thrombin generation was dependent on FXI, fibrin, and platelet signaling via GPIbα and SFKs. CD36-deficiency or blocking suppressed thrombin-induced platelet αIIbβ3 activation, granule exocytosis, binding of adhesion proteins and FV, FVIII, FIX, FX, but not anionic phospholipid exposure determined by flow cytometry. CD36 ligated specifically soluble fibrin, which recruited distinct coagulation factors via thiols. Selected patients with CKD showed elevated soluble fibrin plasma levels and enhanced thrombin-induced thrombin generation, which was normalized by CD36 blocking. Thus, CD36 is an important amplifier of platelet-dependent thrombin generation when exposure of anionic phospholipids is limited. This pathway might contribute to hypercoagulability in CKD.

cAMP- and cGMP-elevating agents inhibit GPIbα-mediated aggregation but not GPIbα-stimulated Syk activation in human platelets

Background

The glycoprotein (GP) Ib-IX-V complex is a unique platelet plasma membrane receptor, which is essential for platelet adhesion and thrombus formation. GPIbα, part of the GPIb-IX-V complex, has several physiological ligands such as von Willebrand factor (vWF), thrombospondin and distinct coagulation factors, which trigger platelet activation. Despite having an important role, intracellular GPIb-IX-V signaling and its regulation by other pathways are not well defined. Our aim was to establish the intracellular signaling response of selective GPIbα activation in human platelets, in particular the role of the tyrosine kinase Syk and its regulation by cAMP/PKA and cGMP/PKG pathways, respectively. We addressed this using echicetin beads (EB), which selectively bind to GPIbα and induce platelet aggregation.

Methods

Purified echicetin from snake Echis carinatus venom was validated by mass spectrometry. Washed human platelets were incubated with EB, in the presence or absence of echicetin monomers (EM), Src family kinase (SFK) inhibitors, Syk inhibitors and the cAMP- and cGMP-elevating agents iloprost and riociguat, respectively. Platelet aggregation was analyzed by light transmission aggregometry, protein phosphorylation by immunoblotting. Intracellular messengers inositolmonophosphate (InsP1) and Ca²⁺_i were measured by ELISA and Fluo-3 AM/FACS, respectively.

Results

EB-induced platelet aggregation was dependent on integrin α_IIbβ₃ and secondary mediators ADP and TxA₂, and was antagonized by EM. EB stimulated Syk tyrosine phosphorylation at Y352, which was SFK-dependent and Syk-independent, whereas Y525/526 phosphorylation was SFK-dependent and partially Syk-dependent. Furthermore, phosphorylation of both Syk Y352 and Y525/526 was completely integrin α_IIbβ₃-independent but, in the case of Y525/526, was partially ADP/TxA₂-dependent. Syk activation, observed as Y352/ Y525/Y526 phosphorylation, led to the phosphorylation of direct substrates (LAT Y191, PLCγ2 Y759) and additional targets (Akt S473). PKA/PKG pathways inhibited EB-induced platelet aggregation and Akt phosphorylation but, surprisingly, enhanced Syk and LAT/PLCγ2 tyrosine phosphorylation. A similar PKA/PKG effect was confirmed with convulxin−/GPVI-stimulated platelets. EB-induced InsP1 accumulation/InsP3 production and Ca²⁺-release were Syk-dependent, but only partially inhibited by PKA/PKG pathways.

Conclusion

EB and EM are specific agonists and antagonists, respectively, of GPIbα-mediated Syk activation leading to platelet aggregation. The cAMP/PKA and cGMP/PKG pathways do not inhibit but enhance GPIbα−/GPVI-initiated, SFK-dependent Syk activation, but strongly inhibit further downstream responses including aggregation. These data establish an important intracellular regulatory network induced by GPIbα.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0428-1) contains supplementary material, which is available to authorized users.

Detecting post-translational modification signatures as potential biomarkers in clinical mass spectrometry

INTRODUCTION

Numerous diseases are caused by changes in post-translational modifications (PTMs). Therefore, the number of clinical proteomics studies that include the analysis of PTMs is increasing. Combining complementary information-for example changes in protein abundance, PTM levels, with the genome and transcriptome (proteogenomics)-holds great promise for discovering important drivers and markers of disease, as variations in copy number, expression levels, or mutations without spatial/functional/isoform information is often insufficient or even misleading. Areas covered: We discuss general considerations, requirements, pitfalls, and future perspectives in applying PTM-centric proteomics to clinical samples. This includes samples obtained from a human subject, for instance (i) bodily fluids such as plasma, urine, or cerebrospinal fluid, (ii) primary cells such as reproductive cells, blood cells, and (iii) tissue samples/biopsies. Expert commentary: PTM-centric discovery proteomics can substantially contribute to the understanding of disease mechanisms by identifying signatures with potential diagnostic or even therapeutic relevance but may require coordinated efforts of interdisciplinary and eventually multi-national consortia, such as initiated in the cancer moonshot program. Additionally, robust and standardized mass spectrometry (MS) assays-particularly targeted MS, MALDI imaging, and immuno-MALDI-may be transferred to the clinic to improve patient stratification for precision medicine, and guide therapies.

Mass Cytometry Reveals Distinct Platelet Subtypes in Healthy Subjects and Novel Alterations in Surface Glycoproteins in Glanzmann Thrombasthenia

Mass cytometry (MC) uses mass spectrometry to simultaneously detect multiple metal-conjugated antibodies on single cells, thereby enabling the detailed study of cellular function. Here, for the first time, we applied MC to the analysis of platelets. We developed a panel of 14 platelet-specific metal-tagged antibodies (targeting cluster of differentiation [CD] 9, CD29, CD31, CD36, CD41, CD42a, CD42b, CD61, CD62P, CD63, CD107a, CD154, glycoprotein [GP] VI and activated integrin αIIbβ3) and compared this panel with two fluorescence flow cytometry (FFC) panels (CD41, CD42b, and CD61; or CD42b, CD62P, and activated integrin αIIbβ3) in the evaluation of activation-dependent changes in glycoprotein expression on healthy subject and Glanzmann thrombasthenia (GT) platelets. High-dimensional analysis of surface markers detected by MC identified previously unappreciated subpopulations of platelets in healthy donors. As expected, MC and FFC revealed that GT platelets had significantly reduced CD41, CD61, and activated integrin αIIbβ3 surface expression. MC also revealed that surface expression of CD9, CD42a and CD63 were elevated, CD31, CD154 and GPVI were reduced and CD29, CD36, CD42b, CD62P and CD107a were similar on GT platelets compared to healthy donor platelets. In summary, MC revealed distinct platelet subtypes in healthy subjects and novel alterations in surface glycoproteins on GT platelets.

Platelet proteomics: from discovery to diagnosis

INTRODUCTION

Platelets are the smallest cells within the circulating blood with key roles in physiological hemostasis and pathological thrombosis regulated by the onset of activating/inhibiting processes via receptor responses and signaling cascades. Areas covered: Proteomics as well as genomic approaches have been fundamental in identifying and quantifying potential targets for future diagnostic strategies in the prevention of bleeding and thrombosis, and uncovering the complexity of platelet functions in health and disease. In this article, we provide a critical overview on current functional tests used in diagnostics and the future perspectives for platelet proteomics in clinical applications. Expert commentary: Proteomics represents a valuable tool for the identification of patients with diverse platelet associated defects. In-depth validation of identified biomarkers, e.g. receptors, signaling proteins, post-translational modifications, in large cohorts is decisive for translation into routine clinical diagnostics.

Effects of the NO/soluble guanylate cyclase/cGMP system on the functions of human platelets

Platelets are circulating sentinels of vascular integrity and are activated, inhibited, or modulated by multiple hormones, vasoactive substances or drugs. Endothelium- or drug-derived NO strongly inhibits platelet activation via activation of the soluble guanylate cyclase (sGC) and cGMP elevation, often in synergy with cAMP-elevation by prostacyclin. However, the molecular mechanisms and diversity of cGMP effects in platelets are poorly understood and sometimes controversial. Recently, we established the quantitative human platelet proteome, the iloprost/prostacyclin/cAMP/protein kinase A (PKA)-regulated phosphoproteome, and the interactions of the ADP- and iloprost/prostacyclin-affected phosphoproteome. We also showed that the sGC stimulator riociguat is in vitro a highly specific inhibitor, via cGMP, of various functions of human platelets. Here, we review the regulatory role of the cGMP/protein kinase G (PKG) system in human platelet function, and our current approaches to establish and analyze the phosphoproteome after selective stimulation of the sGC/cGMP pathway by NO donors and riociguat. Present data indicate an extensive and diverse NO/riociguat/cGMP phosphoproteome, which has to be compared with the cAMP phosphoproteome. In particular, sGC/cGMP-regulated phosphorylation of many membrane proteins, G-proteins and their regulators, signaling molecules, protein kinases, and proteins involved in Ca²⁺ regulation, suggests that the sGC/cGMP system targets multiple signaling networks rather than a limited number of PKG substrate proteins.