Phosphoproteomic Analysis of Platelets in Severe Obesity Uncovers Platelet Reactivity and Signaling Pathways Alterations

Platelet Lipidome Fingerprint: New Assistance to Characterize Platelet Dysfunction in Obesity

Obesity is associated with a pro-inflammatory and pro-thrombotic state that supports atherosclerosis progression and platelet hyper-reactivity. During the last decade, the platelet lipidome has been considered a treasure trove, as it is a source of biomarkers for preventing and treating different pathologies. The goal of the present study was to determine the lipid profile of platelets from non-diabetic, severely obese patients compared with their age- and sex-matched lean controls. Lipids from washed platelets were isolated and major phospholipids, sphingolipids and neutral lipids were analyzed either by gas chromatography or by liquid chromatography coupled to mass spectrometry. Despite a significant increase in obese patient’s plasma triglycerides, there were no significant differences in the levels of triglycerides in platelets among the two groups. In contrast, total platelet cholesterol was significantly decreased in the obese group. The profiling of phospholipids showed that phosphatidylcholine and phosphatidylethanolamine contents were significantly reduced in platelets from obese patients. On the other hand, no significant differences were found in the sphingomyelin and ceramide levels, although there was also a tendency for reduced levels in the obese group. The outline of the glycerophospholipid and sphingolipid molecular species (fatty-acyl profiles) was similar in the two groups. In summary, these lipidomics data indicate that platelets from obese patients have a unique lipid fingerprint that may guide further studies and provide mechanistic-driven perspectives related to the hyperactivate state of platelets in obesity.

Toward platelet transcriptomics in cancer diagnosis, prognosis and therapy

Widespread adoption of next-generation techniques such as RNA-sequencing (RNA-seq) has enabled research examining the transcriptome of anucleate blood platelets in health and disease, thus revealing a rich platelet transcriptomic signature that is reprogrammed in response to disease. Platelet signatures not only capture information from parent megakaryocytes and progenitor hematopoietic stem cells but also the bone marrow microenvironment, and underlying disease states. In cancer, the substantive body of research in patients with solid tumours has identified distinct signatures in 'tumour-educated platelets', reflecting influences of the tumour, stroma and vasculature on splicing, sequestration of tumour-derived RNAs, and potentially cytokine and microvesicle influences on megakaryocytes. More recently, platelet RNA expression has emerged as a highly sensitive approach to profiling chronic progressive haematologic malignancies, where the combination of large data cohorts and machine-learning algorithms enables precise feature selection and potential prognostication. Despite these advances, however, our ability to translate platelet transcriptomics toward clinical diagnostic and prognostic efforts remains limited. In this Perspective, we present a few actionable steps for our basic, translational and clinical research communities in advancing the utility of the platelet transcriptome as a highly sensitive biomarker in cancer and collectively enable efforts toward clinical translation and patient benefit.

Spontaneous Platelet Aggregation in Blood Is Mediated by FcγRIIA Stimulation of Bruton’s Tyrosine Kinase

High platelet reactivity leading to spontaneous platelet aggregation (SPA) is a hallmark of cardiovascular diseases; however, the mechanism underlying SPA remains obscure. Platelet aggregation in stirred hirudin-anticoagulated blood was measured by multiple electrode aggregometry (MEA) for 10 min. SPA started after a delay of 2–3 min. In our cohort of healthy blood donors (n = 118), nine donors (8%) with high SPA (>250 AU*min) were detected. Pre-incubation of blood with two different antibodies against the platelet Fc-receptor (anti-FcγRIIA, CD32a) significantly reduced high SPA by 86%. High but not normal SPA was dose-dependently and significantly reduced by blocking Fc of human IgG with a specific antibody. SPA was completely abrogated by blood pre-incubation with the reversible Btk-inhibitor (BTKi) fenebrutinib (50 nM), and 3 h after intake of the irreversible BTKi ibrutinib (280 mg) by healthy volunteers. Increased SPA was associated with higher platelet GPVI reactivity. Anti-platelet factor 4 (PF4)/polyanion IgG complexes were excluded as activators of the platelet Fc-receptor. Our results indicate that high SPA in blood is due to platelet FcγRIIA stimulation by unidentified IgG complexes and mediated by Btk activation. The relevance of our findings for SPA as possible risk factor of cardiovascular diseases and pathogenic factor contributing to certain autoimmune diseases is discussed.

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.

Platelet Proteomes, Pathways, and Phenotypes as Informants of Vascular Wellness and Disease

Platelets rapidly undergo responsive transitions in form and function to repair vascular endothelium and mediate hemostasis. In contrast, heterogeneous platelet subpopulations with a range of primed or refractory phenotypes gradually arise in chronic inflammatory and other conditions in a manner that may indicate or support disease. Qualitatively distinguishable platelet phenotypes are increasingly associated with a variety of physiological and pathological circumstances; however, the origins and significance of platelet phenotypic variation remain unclear and conceptually vague. As changes in platelet function in disease exhibit many similarities to platelets following the activation of platelet agonist receptors, the intracellular responses of platelets common to hemostasis and inflammation may provide insights to the molecular basis of platelet phenotype. Here, we review concepts around how protein-level relations-from platelet receptors through intracellular signaling events-may help to define platelet phenotypes in inflammation, immune responses, aging, and other conditions. We further discuss how representing systems-wide platelet proteomics data profiles as circuit-like networks of causally related intracellular events, or, pathway maps, may inform molecular definitions of platelet phenotype. In addition to offering insights into platelets as druggable targets, maps of causally arranged intracellular relations underlying platelet function can also advance precision and interceptive medicine efforts by leveraging platelets as accessible, dynamic, endogenous, circulating biomarkers of vascular wellness and disease. Graphic Abstract: A graphic abstract is available for this article.