miR-125b modulates megakaryocyte maturation by targeting the cell-cycle inhibitor p19 INK4D. Cell Death Dis. 2016;7:e2430. [PMID: 27763644 PMCID: PMC5133966 DOI: 10.1038/cddis.2016.288]

Roles of non-coding RNA in megakaryocytopoiesis and thrombopoiesis: new target therapies in ITP

Noncoding RNAs (ncRNAs) are a group of RNA molecules that cannot encode proteins, and a better understanding of the complex interaction networks coordinated by ncRNAs will provide a theoretical basis for the development of therapeutics targeting the regulatory effects of ncRNAs. Platelets are produced upon the differentiation of hematopoietic stem cells into megakaryocytes, 10¹¹ per day, and are renewed every 8-9 days. The process of thrombopoiesis is affected by multiple factors, in which ncRNAs also exert a significant regulatory role. This article reviewed the regulatory roles of ncRNAs, mainly microRNAs (miRNAs), circRNAs (circular RNAs), and long non-coding RNAs (lncRNAs), in thrombopoiesis in recent years as well as their roles in primary immune thrombocytopenia (ITP).

The accumulation of miR-125b-5p is indispensable for efficient erythroblast enucleation

Erythroblast enucleation is a precisely regulated but not clearly understood process. Polycythemia shows pathological erythroblast enucleation, and we discovered a low miR-125b-5p level in terminal erythroblasts of patients with polycythemia vera (PV) compared to those of healthy controls. Exogenous upregulation of miR-125b-5p levels restored the enucleation rate to normal levels. Direct downregulation of miR-125b-5p in mouse erythroblasts simulated the enucleation issue found in patients with PV, and miR-125b-5p accumulation was found in enucleating erythroblasts, collectively suggesting the importance of miR-125b-5p accumulation for erythroblast enucleation. To elucidate the role of miR-125b-5p in enucleation, gain- and loss-of-function studies were performed. Overexpression of miR-125b-5p improved the enucleation of erythroleukemia cells and primary erythroblasts. Infused erythroblasts with higher levels of miR-125b-5p also exhibited accelerated enucleation. In contrast, miR-125b-5p inhibitors significantly suppressed erythrocyte enucleation. Intracellular imaging revealed that in addition to cytoskeletal assembly and nuclear condensation, miR-125b-5p overexpression resulted in mitochondrial reduction and depolarization. Real-time PCR, western blot analysis, luciferase reporter assays, small molecule inhibitor supplementation and gene rescue assays revealed that Bcl-2, as a direct target of miR-125b-5p, was one of the key mediators of miR-125b-5p during enucleation. Following suppression of Bcl-2, the activation of caspase-3 and subsequent activation of ROCK-1 resulted in cytoskeletal rearrangement and enucleation. In conclusion, this study is the first to reveal the pivotal role of miR-125b-5p in erythroblast enucleation.

Expression of miR-223 to predict outcomes after transcatheter aortic valve implantation

BACKGROUND

Transcatheter aortic valve implantation (TAVI) is an established treatment for aortic stenosis (AS) in patients at increased surgical risk. Up to 29% of patients annually experience major adverse cardiac and cerebrovascular events (MACCE) after TAVI. MicroRNAs (miRNA) are currently widely investigated as novel cardiovascular biomarkers. The aim of this study was to determine the influence of TAVI on the expressions of selected miRNAs associated with platelet function (miR-125a-5p, miR-125b and miR-223), and evaluate the predictive value of these miRNAs for MACCE in 65 patients undergoing TAVI.

METHODS

Venous blood samples for miRNA expression analysis were collected 1 day before TAVI and at hospital discharge. The expression of miR-223, miR-125a-5p, miR-125b was evaluated in platelet-depleted plasma.

RESULTS

The expression of miR-223 and miR-125b increased after TAVI, compared to the measurement before (p = 0.020, p = 0.003, respectively). Among 63 patients discharged from the hospital, 18 patients experienced MACCE (29%) during the median 15 months of observation. Baseline low miR-223 expression was a predictor of MACCE in univariate Cox regression analysis (hazard ratio [HR]: 2.71, 95% confidence interval [CI]: 1.04-7.01; p = 0.041). After inclusion of covariates, age, gender (male), New York Heart Association class and diabetes into the multivariate Cox regression model, miR-223 did not reach statistical significance (HR: 2.56, 95% CI: 0.79-8.33; p = 0.118).

CONCLUSIONS

To conclude, miR-223 might improve risk stratification after TAVI. Further studies are required to confirm the clinical applicability of this promising biomarker.

The Landscape of Coding and Noncoding RNAs in Platelets

Significance: Levels of platelet noncoding RNAs (ncRNAs) are altered by disease, and ncRNAs may exert functions inside and outside of platelets. Their role in physiologic hemostasis and pathologic thrombosis remains to be explored. Recent Advances: The number of RNA classes identified in platelets has been growing since the past decade. Apart from coding messenger RNAs, the RNA landscape in platelets comprises ncRNAs such as microRNAs, circular RNAs, long ncRNAs, YRNAs, and potentially environmentally derived exogenous ncRNAs. Recent research has focused on the function of platelet RNAs beyond platelets, mediated through protective RNA shuttles or even cellular uptake of entire platelets. Multiple studies have also explored the potential of platelet RNAs as novel biomarkers. Critical Issues: Platelet preparations can contain contaminating leukocytes. Even few leukocytes may contribute a substantial amount of RNA. As biomarkers, platelet RNAs have shown associations with platelet activation, but it remains to be seen whether their measurements could improve diagnostics. It also needs to be clarified whether platelet RNAs influence processes beyond platelets. Future Directions: Technological advances such as single-cell RNA-sequencing might help to identify hyperreactive platelet subpopulations on a single-platelet level, avoid the common problem of leukocyte contamination in platelet preparations, and allow simultaneous profiling of native megakaryocytes and their platelet progeny to clarify to what extent the platelet RNA content reflects their megakaryocyte precursors or changes in the circulation. Antioxid. Redox Signal. 34, 1200-1216.

p19INK4d: More than Just a Cyclin-Dependent Kinase Inhibitor

Cyclin-dependent kinase inhibitors (CDKIs) are important cell cycle regulators. The CDKI family is composed of the INK4 family and the CIP/KIP family. p19INK4d belongs to the INK4 gene family and is involved in a series of normal physiological activities and the pathogenesis of diseases. Many factors play regulatory roles in the p19INK4d gene expression at the transcriptional and posttranscriptional levels. p19INK4d not only regulates the cell cycle but also plays regulatory roles in apoptosis, DNA damage repair, cell differentiation of hematopoietic cells, and cellular senescence. In this review, the regulatory network of the p19INK4d gene expression and its biological functions are summarized, which provides a basis for further study of p19INK4d as a drug target for disease treatment.

Platelet-derived microparticles enhance megakaryocyte differentiation and platelet generation via miR-1915-3p

Thrombosis leads to platelet activation and subsequent degradation; therefore, replenishment of platelets from hematopoietic stem/progenitor cells (HSPCs) is needed to maintain the physiological level of circulating platelets. Platelet-derived microparticles (PMPs) are protein- and RNA-containing vesicles released from activated platelets. We hypothesized that factors carried by PMPs might influence the production of platelets from HSPCs, in a positive feedback fashion. Here we show that, during mouse acute liver injury, the density of megakaryocyte in the bone marrow increases following an increase in circulating PMPs, but without thrombopoietin (TPO) upregulation. In vitro, PMPs are internalized by HSPCs and drive them toward a megakaryocytic fate. Mechanistically, miR-1915-3p, a miRNA highly enriched in PMPs, is transported to target cells and suppresses the expression levels of Rho GTPase family member B, thereby inducing megakaryopoiesis. In addition, direct injection of PMPs into irradiated mice increases the number of megakaryocytes and platelets without affecting TPO levels. In conclusion, our data reveal that PMPs have a role in promoting megakaryocytic differentiation and platelet production.

Platelets derive from megakaryocytes, which differentiate from hematopoietic stem/progenitor cells (HSPCs). Here, Qu et al show that platelet-derived microparticles carrying miR-1915-3p target HSPCs and promote megakaryopoiesis by suppressing RHOB expression levels.

Microvesicles promote megakaryopoiesis by regulating DNA methyltransferase and methylation of Notch1 promoter

Megakaryopoiesis is the process of formation of mature megakaryocytes that takes place in the bone marrow niche resulting in the release of platelets into the peripheral blood. It has been suggested that cell to cell communication in this dense bone marrow niche may influence the fate of the cells. Numerous studies point to the role of exosomes and microvesicles not only as a messenger of the cellular crosstalk but also in growth and developmental process of various cell types. In the current study, we explored the effects of megakaryocyte-derived microvesicles in hematopoietic cell lines in the context of differentiation. Our study demonstrated that microvesicles isolated from the induced megakaryocytic cell lines have the ability to stimulate noninduced cells specifically into that particular lineage. We showed that this lineage commencement comes from the change in the methylation status of Notch1 promoter, which is regulated by DNA methyltransferases.

miRNA and platelet genetic machinery

Platelets are responsible for blood haemostasis. Although anucleate, a complete translational machinery has been found in platelets, which is responsible for new protein generation. Recently, the role of miRNAs in platelets has started to become apparent. In this editorial I highlight this topic in the hope that other scientists may be attracted to work in this area to aid a more complete understanding of protein regulation in platelets and its impact on platelet function.

Systems biology approach to study the role of miRNA in promoter targeting during megakaryopoiesis

The distinct process of megakaryopoiesis requires occurrence of endomitosis for polyploidization of the megakaryocytes. Although, Cyclins, CDKs and have been described to regulate endomitosis, the exact mechanism still remains an enigma. miRNA which were otherwise known as post transcriptional gene silencers are now emerging with various non-canonical functions including gene regulation at pre-transcriptional level by miRNA binding at promoter region. Out of the many processes they regulate, miRNA have been manifested to play a role in megakaryocyte differentiation. In this study an attempt has been made to identify miRNA that could regulate cell cycle genes (Cyclins and CDKs) by targeting their promoters, during megakaryopoiesis. A new computational algorithm was implemented using Perl programming to identify putative targets of miRNA in CDK and Cyclin promoters. Perl script was also used to check nuclear localizing miRNA based on the presence of a consensus sequence. Real-time PCR was performed to analyze the expression of miRNA and their predicted targets in Dami vs. PMA treated Dami cells. Putative targets of miRNAs with longest, high complementarity matches in CDK/Cyclin promoters were obtained. We identified two significant miRNA, miR-1273g-3p and miR-619-5p with longest seed sequence matches. We further identified three main targets (CDK10, CDK11, Cyclin F) through which these two miRNA could regulate cell cycle during megakaryopoiesis. Our results reinforce the role of promoting targeting miRNA in regulation of cell cycle through certain CDK/Cyclins to support the process of endomitosis during megakaryopoiesis.

Megakaryocyte ontogeny: Clinical and molecular significance

Fetal megakaryocytes (Mks) differ from adult Mks in key parameters that affect their capacity for platelet production. However, despite being smaller, more proliferative, and less polyploid, fetal Mks generally mature in the same manner as adult Mks. The phenotypic features unique to fetal Mks predispose patients to several disease conditions, including infantile thrombocytopenia, infantile megakaryoblastic leukemias, and poor platelet recovery after umbilical cord blood stem cell transplantations. Ontogenic Mk differences also affect new strategies being developed to address global shortages of platelet transfusion units. These donor-independent, ex vivo production platforms are hampered by the limited proliferative capacity of adult-type Mks and the inferior platelet production by fetal-type Mks. Understanding the molecular programs that distinguish fetal versus adult megakaryopoiesis will help in improving approaches to these clinical problems. This review summarizes the phenotypic differences between fetal and adult Mks, the disease states associated with fetal megakaryopoiesis, and recent advances in the understanding of mechanisms that determine ontogenic Mk transitions.