Platelet mRNA: the meaning behind the message

Abstract

Purpose of review: It is now well appreciated that megakaryocytes invest platelets with a diverse repertoire of messenger RNAs (mRNAs), which are competent for translation. Herein we describe what is currently known regarding the expression, function, and clinical significance of mRNAs in platelets.

Recent findings: Although mRNA was detected in platelets nearly 30 years ago, we are only beginning to understand the roles of mRNA in platelet biology and human disease. Recent studies have shown that megakaryocytes specifically sort, rather than randomly transfer, mRNA to platelets during thrombopoiesis. As a result, platelets are released into the circulation with thousands of mRNAs. The emergence of next-generation RNA sequencing has demonstrated that platelet mRNAs possess classic structural features, which include untranslated regions and open reading frames. There is also growing evidence that platelet mRNA expression patterns are altered in human disease.

Summary: Intense investigation of platelet mRNA has shed considerable light on predicted functions of platelets and identified previously unrecognized attributes of platelets. Lessons learned from platelet mRNA is presented in this review.

FIGURE 1

Distribution of next-generation RNA-sequencing reads in human CD34-derived megakaryocytes and platelets. Pie charts represent the percentage of sequencing reads from CD34-derived megakaryocytes (left) and freshlyisolated human platelets (right) that map to the specified genomic regions. Only high-quality alignments following de-novo alignment are represented. As shown, the majority of sequence reads map to known exonic regions (combined Refseq, UCSC, and ENSEMBL annotations) of genes (light blue, annotated exons). The remaining reads map to annotated introns (dark green) or intergenic regions (light green). Reads that map to intergenic regions probably mark novel genic regions expressed in platelets.

FIGURE 2

Interleukin (IL)-1β messenger RNA (mRNA) expression in platelets. mRNA was collected from freshly isolated, unactivated platelets and processed as recently described. This figure is a visualization using the Integrated Genome Browser (IGB) of paired-end next-generation RNA-sequencing (next-gen RNA-seq) reads that are aligned to genes for secreted protein acidic and rich in cysteine (SPARC, top panel) or IL-1β (bottom panel) (genes are in a 3′ to 5′ orientation). Gene maps (bottom portion of each figure) are represented by thick (exons) and thin (introns) horizontal lines. The bars immediately above the gene represent sequencing reads from platelet transcripts that were fragmented, sequenced, and aligned to IL-1β. The lines at the top represent paired-end sequence reads that align within exons only, introns only, or span exon/exon or exon/intron junctions. As shown, exon/exon junction spanning sequences are common for SPARC and rare for IL-1β, whereas intronic and intron/exon junction spanning reads are common for IL-1β and rare for SPARC.

FIGURE 3

Abnormal sequence reads in transcripts from platelet RNA facilitate the discovery of neurobeachin-like 2 (NBEAL2) as the causative gene for gray platelet syndrome (GPS). The top panel displays a snapshot of NBEAL2 transcripts expressed in platelets from a healthy (control) and GPS patient. The red box outlines a region in which introns are abnormally retained in NBEAL2 transcripts isolated from the GPS patient. The bottom panel shows exon/intron spanning RNA sequencing (RNA-seq) reads from the GPS patient, which contain the c.1029+ 1G>A mutation [47^▪▪]. The red box on the right shows the zoomed region where the G>A mutation is located. Reproduced with permission from [47^▪▪].