Extracellular tyrosyl-tRNA synthetase cleaved by plasma proteinases and stored in platelet α-granules: Potential role in monocyte activation

Characterisation of Platelet Releasate Proteome in Relapsing-Remitting Multiple Sclerosis Reveals Dysregulation of Inflammatory Signalling and Extracellular Vesicle Dynamics

PURPOSE

Multiple Sclerosis is an inflammatory neurodegenerative disease characterised by blood-brain barrier dysfunction and leukocyte infiltration into the CNS. Platelets are best known for their contributions to haemostasis, however, upon activation, platelets release an abundance of soluble and vesicular-associated proteins, termed the platelet releasate (PR). This milieu contains numerous inflammatory and vasoactive proteins, that can attract leukocytes and alter endothelial permeability.

EXPERIMENTAL DESIGN

We aimed to characterise the PR of Relapsing-Remitting multiple sclerosis (RRMS) patients, previously characterized regarding thrombin generation dynamics compared to healthy controls. We carried out LFQ proteomic profiling of the PR from 15 RRMS and 19 aged-matched healthy controls.

RESULTS

We identified 9 proteins increased and 16 proteins decreased in the PR of RRMS patients. Platelet/endothelial cell-adhesion molecule-1 (PECAM-1) was uniquely found in healthy control PR and circulating levels of PECAM-1 were significantly lower in RRMS patient samples. GO analysis revealed a strong link between altered proteins and extracellular vesicles (EVs). Small EV levels were significantly reduced in RRMS PR compared to healthy PR and showed a negative correlation with PECAM-1 levels in RRMS plasma.

CONCLUSIONS AND CLINICAL RELEVANCE

Our findings suggest that platelet reactivity may be linked to disease activity, even in periods of disease remission.

Mesenchymal Stromal Cell Secretome Is Affected by Tissue Source and Donor Age

Variation in mesenchymal stromal cell (MSC) function depending on their origin is problematic, as it may confound clinical outcomes of MSC therapy. Current evidence suggests that the therapeutic benefits of MSCs are attributed to secretion of biologically active factors (secretome). However, the effect of donor characteristics on the MSC secretome remains largely unknown. Here, we examined the influence of donor age, sex, and tissue source, on the protein profile of the equine MSC secretome. We used dynamic metabolic labeling with stable isotopes combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS) to identify secreted proteins in MSC conditioned media (CM). Seventy proteins were classified as classically secreted based on the rate of label incorporation into newly synthesized proteins released into the extracellular space. Next, we analyzed CM of bone marrow- (n = 14) and adipose-derived MSCs (n = 16) with label-free LC-MS/MS. Clustering analysis of 314 proteins detected across all samples identified tissue source as the main factor driving variability in MSC CM proteomes. Linear modelling applied to the subset of 70 secreted proteins identified tissue-related difference in the abundance of 23 proteins. There was an age-related decrease in the abundance of CTHRC1 and LOX, further validated with orthogonal techniques. Due to the lack of flow cytometry characterization of MSC surface markers, the analysis could not account for the potential effect of cell population heterogeneity. This study provides evidence that tissue source and donor age contribute to differences in the protein composition of MSC secretomes which may influence the effects of MSC therapy.

Is autologous platelet activation the key step in ovarian therapy for fertility recovery and menopause reversal?

Platelets are a uniquely mammalian physiologic feature. As the only non-marine vertebrates to experience menopause, humans have a substantial post-reproductive lifespan and are believed to have a limited, non-renewable oocyte supply. Ovarian reserve typically declines after about age 35yrs, marking losses which cannot be recovered by available fertility medications. When in vitro fertilization fails due to low or absent ovarian response, gonadotropin adjustments are often ineffectual and if additional oocytes are occasionally harvested, egg quality is usually poor. This problem was confronted by Greek researchers who developed a new surgical method to insert autologous platelet-rich plasma (PRP) into ovaries; the first ovarian PRP success to improve reproductive outcomes was published from Athens in 2016. This innovation influenced later research with condensed platelet-derived growth factors, leading to correction of oocyte ploidy error, normal blastocyst development, and additional term livebirths. Yet women's health was among the last clinical domains to explore PRP, and its role in 'ovarian rejuvenation' remains unsettled. One critical aspect in this procedure is platelet activation, a commonly overlooked step in the cytokine release cascade considered essential for successful transition of undifferentiated ovarian stem cells to an oocyte lineage. Poor activation of platelets thus becomes an unforced error, potentially diminishing or even negating post-treatment ovarian follicular response. To answer this query, relevant theory, current disagreements, and new data on platelet activation are presented, along with clinical challenges for regenerative fertility practice.

Inflammatory platelet production stimulated by tyrosyl-tRNA synthetase mimicking viral infection

Platelets play a role not only in hemostasis and thrombosis, but also in inflammation and innate immunity. We previously reported that an activated form of tyrosyl-tRNA synthetase (YRSACT) has an extratranslational activity that enhances megakaryopoiesis and platelet production in mice. Here, we report that YRSACT mimics inflammatory stress inducing a unique megakaryocyte (MK) population with stem cell (Sca1) and myeloid (F4/80) markers through a mechanism dependent on Toll-like receptor (TLR) activation and type I interferon (IFN-I) signaling. This mimicry of inflammatory stress by YRSACT was studied in mice infected by lymphocytic choriomeningitis virus (LCMV). Using Sca1/EGFP transgenic mice, we demonstrated that IFN-I induced by YRSACT or LCMV infection suppressed normal hematopoiesis while activating an alternative pathway of thrombopoiesis. Platelets of inflammatory origin (Sca1/EGFP+) were a relevant proportion of those circulating during recovery from thrombocytopenia. Analysis of these "inflammatory" MKs and platelets suggested their origin in myeloid/MK-biased hematopoietic stem cells (HSCs) that bypassed the classical MK-erythroid progenitor (MEP) pathway to replenish platelets and promote recovery from thrombocytopenia. Notably, inflammatory platelets displayed enhanced agonist-induced activation and procoagulant activities. Moreover, myeloid/MK-biased progenitors and MKs were mobilized from the bone marrow, as evidenced by their presence in the lung microvasculature within fibrin-containing microthrombi. Our results define the function of YRSACT in platelet generation and contribute to elucidate platelet alterations in number and function during viral infection.

Aminoacyl-tRNA Synthetases: On Anti-Synthetase Syndrome and Beyond

Anti-synthetase syndrome (ASSD) is an autoimmune disease characterized by the presence of autoantibodies targeting one of several aminoacyl t-RNA synthetases (aaRSs) along with clinical features including interstitial lung disease, myositis, Raynaud’s phenomenon, arthritis, mechanic’s hands, and fever. The family of aaRSs consists of highly conserved cytoplasmic and mitochondrial enzymes, one for each amino acid, which are essential for the RNA translation machinery and protein synthesis. Along with their main functions, aaRSs are involved in the development of immune responses, regulation of transcription, and gene-specific silencing of translation. During the last decade, these proteins have been associated with cancer, neurological disorders, infectious responses, and autoimmune diseases including ASSD. To date, several aaRSs have been described to be possible autoantigens in different diseases. The most commonly described are histidyl (HisRS), threonyl (ThrRS), alanyl (AlaRS), glycyl (GlyRS), isoleucyl (IleRS), asparaginyl (AsnRS), phenylalanyl (PheRS), tyrosyl (TyrRS), lysyl (LysRS), glutaminyl (GlnRS), tryptophanyl (TrpRS), and seryl (SerRS) tRNA synthetases. Autoantibodies against the first eight autoantigens listed above have been associated with ASSD while the rest have been associated with other diseases. This review will address what is known about the function of the aaRSs with a focus on their autoantigenic properties. We will also describe the anti-aaRSs autoantibodies and their association to specific clinical manifestations, and discuss their potential contribution to the pathogenesis of ASSD.