The clinical significance of platelet microparticle-associated microRNAs

Circulating blood platelets play a central role in the maintenance of hemostasis. They adhere to subendothelial extracellular matrix proteins that become exposed upon vessel wall damage, which is followed by platelet activation, further platelet recruitment, platelet aggregation and formation of an occlusive, or non-occlusive, platelet thrombus. Platelets host a surprisingly diverse transcriptome, which is comprised of ~9500 messenger RNAs (mRNAs) and different classes of non-coding RNAs, including microRNAs, as well as a significant repertoire of proteins that contribute to their primary (adhesion, aggregation, granule secretion) and alternative (RNA transfer, mRNA translation, immune regulation) functions. Platelets have the propensity to release microparticles (MPs; 0.1–1 μm in diameter) upon activation, which may mediate inflammatory responses and contribute to exacerbate inflammatory diseases and conditions. Carrying components of the platelets’ cytoplasm, platelet MPs may exert their effects on recipient cells by transferring their content in platelet-derived bioactive lipid mediators, cytokines, mRNAs and microRNAs. Platelet MP-associated microRNAs may thus function also outside of platelets and play an important role in intercellular signaling and gene expression programming across the entire circulatory system. The role and importance of platelet MP-associated microRNAs in various aspects of biology and pathophysiology are increasingly recognized, and now provide the scientific basis and rationale to support further translational research and clinical studies. The clinical significance, pathophysiological role as well as the diagnostic and therapeutic potential of platelet MP-associated microRNAs in cardiovascular diseases, platelet transfusion and cancer will be discussed

Hypothesis: A Role for Fragile X Mental Retardation Protein in Mediating and Relieving MicroRNA-Guided Translational Repression?

MicroRNA (miRNA)-guided messenger RNA (mRNA) translational repression is believed to be mediated by effector miRNA-containing ribonucleoprotein (miRNP) complexes harboring fragile X mental retardation protein (FMRP). Recent studies documented the nucleic acid chaperone properties of FMRP and characterized its role and importance in RNA silencing in mammalian cells. We propose a model in which FMRP could facilitate miRNA assembly on target mRNAs in a process involving recognition of G quartet structures. Functioning within a duplex miRNP, FMRP may also mediate mRNA targeting through a strand exchange mechanism, in which the miRNA* of the duplex is swapped for the mRNA. Furthermore, FMRP may contribute to the relief of miRNA-guided mRNA repression through a reverse strand exchange reaction, possibly initiated by a specific cellular signal, that would liberate the mRNA for translation. Suboptimal utilization of miRNAs may thus account for some of the molecular defects in patients with the fragile X syndrome

MicroRNAs as modulators of the platelet proteome

MicroRNAs are short 21- to 24-nucleotide (nt) RNA species that act as key regulators of gene expression. Known primarily to modulate mRNA translation through recognition of specific binding sites located in the 3untranslated region (UTR) of messenger RNA (mRNA) targets, microRNAs may regulate between 30% to 92% of the genes in human, thereby controlling a plethora of biological processes. Although devoid of a nucleus and lacking genomic DNA, platelets may be no exception, as recent experimental evidences indicate that they contain all the protein and RNA components and features required for microRNA-regulated mRNA translation: (i) the platelet transcriptome is astonishingly diverse, representing between 15 and 32% of all human genes, (ii) platelet mRNAs can be translated into proteins, (iii) platelets contain an abundant and diverse array of microRNAs, and (iv) the host Dicer and Argonaute 2 (Ago2) complexes. The latter ones are functional in microRNA biogenesis and function, respectively. In this review article, we will summarize and discuss the experimental evidences as well as the most recent advances supporting a role for microRNAs as modulators of the platelet proteome. Expected to play a central role in health and disease, a dysfunctional microRNA-based regulation of gene expression in platelets may represent an important etiologic factor underlying platelet-related and cardiovascular diseases

Protein interactions and complexes in human microRNA biogenesis and function

Encoded in the genome of most eukaryotes, microRNAs (miRNAs) have been proposed to regulate specifically up to 90% of human genes through a process known as miRNA-guided RNA silencing. The aim of this review is to present this process as the integration of a succession of specialized molecular machines exerting well defined functions. The nuclear microprocessor complex initially recognizes and processes its primary miRNA substrate into a miRNA precursor (pre-miRNA). This structure is then exported to the cytoplasm by the Exportin-5 complex where it is presented to the pre-miRNA processing complex. Following pre-miRNA conversion into a miRNA:miRNA* duplex, this complex is assembled into a miRNA-containing ribonucleoprotein (miRNP) complex, after which the miRNA strand is selected. The degree of complementarity of the miRNA for its messenger RNA (mRNA) target guides the recruitment of the miRNP complex. Initially repressing its translation, the miRNP-silenced mRNA is directed to the P-bodies, where the mRNA is either released from its inhibition upon a cellular signal and/or actively degraded. The potency and specificity of miRNA biogenesis and function rely on the distinct protein x protein, protein x RNA and RNA:RNA interactions found in different complexes, each of which fulfill a specific function in a well orchestrated process

MicroRNA-298 and MicroRNA-328 regulate expression of mouse ß-Amyloid precursor protein-converting enzyme 1

MicroRNAs (miRNAs) are key regulatory RNAs known to repress mRNA translation through recognition of specific binding sites located mainly in their 3′-untranslated region (UTR). Loss of specific miRNA control of gene expression is thus expected to underlie serious genetic diseases. Intriguingly, previous post-mortem analyses showed higher β-amyloid precursor protein-converting enzyme (BACE) protein, but not mRNA, levels in the brain of patients that suffered from Alzheimer disease (AD). Here we also observed a loss of correlation between BACE1 mRNA and protein levels in the hippocampus of a mouse model of AD. Consistent with an impairment of miRNA-mediated regulation of BACE1 expression, these findings prompted us to investigate the regulatory role of the BACE1 3′-UTR element and the possible involvement of specific miRNAs in cultured neuronal (N2a) and fibroblastic (NIH 3T3) cells. Through various experimental approaches, we validated computational predictions and demonstrated that miR-298 and miR-328 recognize specific binding sites in the 3′-UTR of BACE1 mRNA and exert regulatory effects on BACE1 protein expression in cultured neuronal cells. Our results may provide the molecular basis underlying BACE1 deregulation in AD and offer new perspectives on the etiology of this neurological disorder

Identification of protein markers for extracellular vesicle (EV) subsets in cow’s milk

Extracellular vesicles (EVs), like exosomes, are small membrane vesicles involved in cell-to-cell communications that modulate numerous biological processes. We previously discovered a new EV subset in milk (sedimenting at 35,000 g; 35 K) that protected its cargo (RNAs and proteins) during simulated digestion and was more enriched in microRNAs than exosomes (sedimenting at 100 K). Here, we used LC-MS/MS to push further the comparison between these two pellets. Commonly used EV markers were not differentially enriched between the pellets, questioning their use with cow's milk EVs. Similarly, the majority of the quantified proteins were equally enriched between the two pellets. Nevertheless, 20 proteins were specific to 35 K, while 41 were specifically enriched in 100 K (p < 0.05), suggesting their potential use as specific markers. Loaded with these proteins, the EVs in these pellets might regulate translation, proliferation and cell survival for 35 K, and metabolism, extracellular matrix turnover and immunity for 100 K. This approach also brought new insights into milk EV-associated integrins and their possible role in specifically targeting recipient cell types. These findings may help better discriminate between milk EVs, improve our understanding of milk EV-associated protein function and their possible use as therapeutic tools for the management of immunity- and metabolism-associated disorders

The repertoire and features of human platelet microRNAs

Playing a central role in the maintenance of hemostasis as well as in thrombotic disorders, platelets contain a relatively diverse messenger RNA (mRNA) transcriptome as well as functional mRNA-regulatory microRNAs, suggesting that platelet mRNAs may be regulated by microRNAs. Here, we elucidated the complete repertoire and features of human platelet microRNAs by high-throughput sequencing. More than 492 different mature microRNAs were detected in human platelets, whereas the list of known human microRNAs was expanded further by the discovery of 40 novel microRNA sequences. As in nucleated cells, platelet microRNAs bear signs of post-transcriptional modifications, mainly terminal adenylation and uridylation. In vitro enzymatic assays demonstrated the ability of human platelets to uridylate microRNAs, which correlated with the presence of the uridyltransferase enzyme TUT4. We also detected numerous microRNA isoforms (isomiRs) resulting from imprecise Drosha and/or Dicer processing, in some cases more frequently than the reference microRNA sequence, including 5′ shifted isomiRs with redirected mRNA targeting abilities. This study unveils the existence of a relatively diverse and complex microRNA repertoire in human platelets, and represents a mandatory step towards elucidating the intraplatelet and extraplatelet role, function and importance of platelet microRNAs

Recent exposure to ultrafine particles in school children alters miR-222 expression in the extracellular fraction of saliva

Background: Ultrafine particles (< 100 nm) are ubiquitous present in the air and may contribute to adverse cardiovascular effects. Exposure to air pollutants can alter miRNA expression, which can affect downstream signaling pathways. miRNAs are present both in the intracellular and extracellular environment. In adults, miR-222 and miR-146a were identified as associated with particulate matter exposure. However, there is little evidence of molecular effects of ambient air pollution in children. This study examined whether exposure to fine and ultrafine particulate matter (PM) is associated with changes in the extracellular content of miR-222 and miR-146a of children. Methods: Saliva was collected from 80 children at two different time points, circa 11 weeks apart and stabilized for RNA preservation. The extracellular fraction of saliva was obtained by means of differential centrifugation and ultracentrifugation. Expression levels of miR-222 and miR-146a were profiled by qPCR. We regressed the extracellular miRNA expression against recent exposure to ultrafine and fine particles measured at the school site using mixed models, while accounting for sex, age, BMI, passive smoking, maternal education, hours of television use, time of the day and day of the week. Results: Exposure to ultrafine particles (UFP) at the school site was positively associated with miR-222 expression in the extracellular fraction in saliva. For each IQR increase in particles in the class room (+8504 particles/cm(3)) or playground (+ 28776 particles/cm(3)), miR-222 was, respectively 23.5 % (95 % CI: 3.5 %-41.1 %; p = 0.021) or 29.9 % (95 % CI: 10.6 %-49.1 %; p = 0.0027) higher. No associations were found between miR-146a and recent exposure to fine and ultrafine particles. Conclusions: Our results suggest a possible epigenetic mechanism via which cells respond rapidly to small particles, as exemplified by miR-222 changes in the extracellular fraction of saliva