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Abundant Circular RNA expression in terminal erythropoiesis and megakaryopoiesis shows limited evidence for translation or circRNA-mediated translation regulation [RNA-seq]

ABSTRACT: Erythrocytes and platelets have high production rates, with 1012 cells released daily into the blood stream. This output from bone-marrow residing hematopoietic stem cells is tightly regulated by transcription factors and epigenetic modifications. Whether and how non-coding RNAs such as circular RNAs (circRNAs) contribute to the differentiation and/or identity of hematopoietic cells is to date not well understood. We recently published a circRNA expression map of hematopoietic cells, which showed that erythrocytes and platelets contain the highest levels and numbers of circRNAs. Whether and how circRNA expression alters during differentiation of erythrocytes and platelet precursors is however not known. Therefore, we here provide the first detailed and comprehensive analysis of circRNA expression during red blood cell and megakaryocyte differentiation. CircRNA expression significantly increased during erythroid precursor differentiation into red blood cells, and in differentiating megakaryocytes, in particular upon enucleation. Many functions have been attributed to circRNAs. To dissect their possible function in hematopoietic differentiation, we first focused on translation regulation. We compared circRNA and mRNA expression to ribosomal foot printing data, and found that only 20 (2.6%) circRNAs associated with translation regulation of their mRNA counterparts. We also identified thousands of putative open reading frames in circRNAs, suggesting that circRNAs may also encode proteins. However, deep ribosome-footprinting sequencing data and in-depth mass spectrometry data analysis provided little evidence for translation of endogenously expressed circRNAs in erythroblasts, megakaryocytes and platelets. In conclusion, circRNAs in platelets and red blood cells are highly abundant and alter their expression profile during differentiation, yet their contribution to regulate cellular processes remains enigmatic.

ORGANISM(S): Homo sapiens

PROVIDER: GSE159577 | GEO | 2021/03/04

REPOSITORIES: GEO

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Project description:In platelets, splicing and translation occur in the absence of a nucleus. However, the integrity and stability of mRNAs derived from megakaryocyte progenitor cells remain poorly quantified on a transcriptome-wide level. As circular RNAs (circRNAs) are resistant to degradation by exonucleases, their abundance relative to linear RNAs can be used as a surrogate marker for mRNA stability in the absence of transcription. Here we show that circRNAs are enriched in human platelets 17-188 fold relative to nucleated tissues, and 14-26 fold relative to samples digested with RNAseR to selectively remove linear RNA. We compare RNAseq read depths inside and outside circRNAs to provide in silico evidence of transcript circularity, show that exons within circRNAs are enriched ~13X in platelets relative to nucleated tissues, and identify 3162 genes significantly enriched for circRNAs including some where all RNAs appear to be derived from circular molecules. We also confirm that this is a feature of other anucleate cells through transcriptome sequencing of mature erythrocytes, demonstrate that circRNAs are not enriched in megakaryocytes, and that linear RNAs decay more rapidly than circRNAs in platelet preparations. Collectively, these results suggest that circulating platelets have lost on aveage over 90% of their progenitor mRNAs, and that translation in platelets occurrs against the backdrop of a highly degraded transcriptome. Finally, we find that transcripts classified as products of reverse transcriptase template switching are both enriched in platelets and resistant to decay, countering the recent suggestion that up to 50% of rearranged RNAs are artefacts. A single rRNA depleted total RNA sample was sequenced. This together with 25 publicly available rRNA depleted total RNA samples (including 3 from platelets) were analysed using PTESFinder v 1 (http://sourceforge.net/projects/ptesfinder-v1/) to identify back-splice junctions, characteristic of circRNA transcripts. The contribution of circRNA producing exons was analysed on a gene by gene basis as follows: All circRNA transcripts identified in any sample were first pooled to define exons which can contribute to circRNA generation using custom scripts (available on request). For each sample, expression estimates (RPKMI) across all circRNA producing exons were computed for each locus using the total size of exons (in bp) and the read counts mapping to them. Similarly, total size and exonic read counts for exons for which no circRNA were detected in any sample were used to compute expression estimates (RPKME) for non-circRNA producing exons for each locus. Abundance ratios (RPKMI/RPKME and RPKMI/RPKMI+RPKME) were calculated and compared between Platelets and human tissues using Wilcoxon signed-rank test. Please note that the '25sample_info_accn_no.txt' contains the accession numbers and tissue/cell type information for 25 samples analyzed together.

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Abundant Circular RNA expression in terminal erythropoiesis and megakaryopoiesis shows limited evidence for translation or circRNA-mediated translation regulation [RNA-seq]

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