Abstract
Circular RNAs (circRNAs) have re-emerged as an interesting RNA species. Using deep RNA profiling in different mouse tissues, we observed that circRNAs were substantially enriched in brain and a disproportionate fraction of them were derived from host genes that encode synaptic proteins. Moreover, on the basis of separate profiling of the RNAs localized in neuronal cell bodies and neuropil, circRNAs were, on average, more enriched in the neuropil than their host gene mRNA isoforms. Using high-resolution in situ hybridization, we visualized circRNA punctae in the dendrites of neurons. Consistent with the idea that circRNAs might regulate synaptic function during development, many circRNAs changed their abundance abruptly at a time corresponding to synaptogenesis. In addition, following a homeostatic downscaling of neuronal activity many circRNAs exhibited substantial up- or downregulation. Together, our data indicate that brain circRNAs are positioned to respond to and regulate synaptic function.
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Acknowledgements
We thank I. Bartnik, N. Fuerst and A. Staab for the preparation of cultured hippocampal neurons and M. Feldkamp and C. Langnick for their excellent technical assistance on sequencing. We thank C. Schanzenbaecher and J. Langer for generating mass spectrometry data and for the bioinformatic evaluation of circRNA translation. E.M.S. is funded by the Max Planck Society, an Advanced Investigator award from the European Research Council, DFG CRC 902: Molecular Principles of RNA-based Regulation, DFG CRC 1080: Molecular and Cellular Mechanisms of Neural Homeostasis, and the DFG Cluster of Excellence for Macromolecular Complexes, Goethe University. As part of the Berlin Institute for Medical Systems Biology at the MDC, the research group of W.C. is funded by the Federal Ministry for Education and Research (BMBF) and the Senate of Berlin, Germany (BIMSB 0315362A, 0315362C). H.L., T.C. and W.S. are supported by the Chinese Scholarship Council (CSC).
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X.Y. and G.T. designed and performed all of the bioinformatic analysis and edited the manuscript. I.V., A.B., T.W., I.E. and G.A. designed, conducted and analyzed the experiments and edited the manuscript, with the help of M.W., C.G., C.Q., T.C., X.W., J.H., H.L. and W.S. S.S. performed and analyzed the electrophysiology experiments. W.C. and E.M.S. conceived and supervised the project, and wrote the manuscript.
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Integrated supplementary information
Supplementary Figure 1 circRNAs are poly(A) tail–depleted, RNAse R–resistant and enriched in brain
A. circRNAs are depleted in the polyA-enriched RNA-seq library (n = 1 from 1 mouse). Each circle represents one circRNA. Values on X/Y axis denote the abundance of circRNAs in poly(A)-enriched library (Y) and in rRNA-depleted library (X). Inset shows that circRNAs are strongly depleted in the poly(A)-enriched library. Whiskers show extreme data points no more than 1.5 times the interquartile range from the box. B. circRNAs from mouse and rat are at least 5-fold more RNase R resistant than their linear transcripts. C. circRNAs are enriched in mouse brain. rRNA depleted RNA-seq were performed with five tissues from one wild type C57B6 male mice at the age of 14 weeks. Left. The percentage of circular junction reads from all the reads mapped on the genome is shown for different tissues, with the highest value in brain, followed by testis. Middle. The number of circRNA host genes that are exclusively expressed in one tissue is shown across different tissues, with the highest value in brain, Right. The relative contribution of circRNA to the total transcription output of the same gene locus i.e. the ratio between the abundance of each circRNA and the total transcriptional output (TTO) of the hosting gene loci (measured in TPM, transcripts per million) is significantly higher in brain compared to all the other tissues, The ratios of the relative contribution between brain and other four tissues are significantly larger than one (two-sided one-sample t-test, brain/heart: n=1176, p< 2.2e-16; brain/liver: n=579, p< 2.2e-16; brain/lung: n=919, p< 2.2e-16; brain/testis: n=1860, p< 2.2e-16). D CircRNAs are enriched in rat brain. RNA-Seq data from the five rat tissues, each with four biological replicates from rat with matched age (6 weeks), two from males and two from females (with the exception of testis), was downloaded from GEO (Accession No. GSE53960, Ying Yu et al. Nat Commun. 2014;5:3230). We applied our computational pipeline to this dataset. Twenty to 60 million 50nt-long reads were obtained from each sample. Due to the short read length, the number of circRNAs identified with this dataset is much smaller. Nevertheless, compared to other tissues, the enrichment of circRNA in brain is still obvious,. Left. The percentage of circular junction reads from all the reads mapped on the genome is shown for different tissues, with the highest value in brain, followed by testis. Middle. The number of circRNA host genes that are exclusively expressed in one tissue is shown across different tissues, with the highest value in brain. Right. The relative contribution of circRNA to the total transcription output of the same gene locus i.e. the ratio between the abundance of each circRNA and the total transcriptional output (TTO) of the hosting gene loci (measured in TPM, transcripts per million) is significantly higher in brain compared to all the other tissues. The ratios of the relative contribution between brain and other four tissues are significantly larger than one (two-sided one-sample t-test, brain/heart: n=293, p=1.412e-09; brain/liver: n=130, p=0.0001614; brain/lung: n=330, p=3.153e-16; brain/testis: n=327, p=1.806e-07).
Supplementary Figure 2 circRNAs are enriched in synaptic tissue
A. Gene ontology enrichment analysis of the genes producing brain expressed circRNAs. Functional groups related to synaptic function were overrepresented regardless the background gene set used for comparison. We chose a background gene set consisting of the 1000, 2000, 3000, 5000, or 10000 most highly expressed genes (top1k, top2k, top3k, top5k, top10k), and then tested for the enrichment of GO terms for the circRNA-hosting genes within the same gene set, respectively. B. The abundance of circRNA and total transcriptional output of protein-coding gene loci (measured in TPM) were compared between mouse hippocampal synaptosomes (X-axis) and mouse hippocampal homogenates (Y-axis). Each red dot represents one circRNA, and each dark dot represents one protein-coding gene (n=1 for homogenate prepared from 10 mice and n=1 for synaptosomes prepared from 10 mice). C. The abundance of circRNA and total transcriptional output of protein-coding gene loci (measured in TPM) were compared between neuropil (X-axis) and the somatic layer of the hippocampus (Y axis) in rat. Each red dot represents one circRNA, and each dark dot represents one protein-coding gene locus (n=1 for the somata sample prepared from 7 rats and n=1 for the neuropil sample prepared from 7 rats). Insets in B and C show that the abundance of circRNAs, but not total transcriptional output, is significantly higher in the synaptosome and neuropil fractions (two-side unpaired Student's T-test with Welch's correction, ***p < 2.2E-16). Whiskers show the extreme data points no more than 1.5 times the interquartile range from the box. D. Overlap between circRNAs detected in mouse synaptosomes and neuropil. E. Overlap between circRNAs identified in mouse and rat neuropil. F. Cumulative distribution of the circRNA abundance shown in E. The common circRNAs are of higher abundance (blue circles for mouse, orange x for rat) than circRNAs only detected in either mouse (green circles) or rat (pink x).
Supplementary Figure 3 circRNA localization in neuronal compartments
A. Schematics of circRNA probe for in situ hybridization. Probe is shown as connected rectangles targeted at the circRNA junction (black line). Probe originates from first 10nt of one exon (red) and last 10nt of the other exon (blue) that form the head-to-tail junction. B. Exon control probe consists of start and end of exons that do not form circular junction. C-J. High resolution in situ hybridization experiments in cultured hippocampal neurons using probe sets designed to detect the indicated circRNA (E-J), no probe (C) or control probe (D). In each case, many circRNA-positive particles (green) are apparent in the cell bodies, but also in the dendritic processes, identified using an anti-MAP2 antibody (red). The absence of a probe (no probe) or a control (scrambled) probe designed to lack detection of any known exonic sequence (see Methods) yielded just a few background particles. Scale bar = 20 microns.
Supplementary Figure 4 circRNA in situ hybridization is specific
A. Relative expression (measured as TPM) of circRmst, circKlhl2 and circGigyf2 in brain, liver and lung based on RNA-seq experiments. Error bars represent standard deviation. B. Validation of expression level of the circRNAs in liver and lung using in situ hybridization. No probe control indicates background levels of fluorescence. circRmst and circKlhl2 are not expressed while circGigyf2, by comparison, is moderately expressed, as expected based on RNA-sequencing data. Nuclei were stained by DAPI (blue); circRNA in situ hybridization puncta are shown in green and anti- antibody staining (albumin for liver and Heme-Oxygenase 1 for lung) is shown in red. Scale bar = 10 microns in large images and 5 microns in zoomed images.
Supplementary Figure 5 circRims1 and its host gene mRNA do not colocalize
In situ hybridization of circRims1 (purple) and linear Rims1 (green) transcript in the same neuron shows lack of co-localization. Whole image and blow up of soma and dendrites are shown. Scale bar = 20μm for whole image and 10μm for zoom and dendrite.
Supplementary Figure 6 circRNA: no enrichment of miRNA/RBP binding and translation
A. Based on nucleotide sequences, circRNAs (red) as a population do not possess a higher density of miRNA binding sites than either the 3′ UTR (black) or CDS (coding sequence, yellow) of the mRNAs. This trend persists when circRNAs of different abundances are examined. B. CircRNAs (red) have a lower density of RBP binding sites than either 3′ UTR (black) and CDS (yellow) of the mRNAs. This trend also persists when circRNAs of different abundances are examined. C. CircRNAs are enriched in the non-ribosome fraction and are depleted drastically from the mono-/poly-ribosome fraction. Values on Y-axis denote the percentage of circRNAs in the RNA fractions marked on X-axis. n=2 whole brains. D. circRNAs (red dots) are enriched in the non-ribosome fraction, whilst mRNAs (black dots) are enriched in the poly-ribosome fraction. Classical noncoding RNAs such as snRNAs and snoRNAs are shown in blue. The inset shows that circRNAs show significantly less association with ribosomes than those classical noncoding RNAs (two-sided unpaired Student's t-test with Welch's correction, ****p < 2.2E-16 and ***p = 3.999e-10). Whiskers show extreme data points no more than 1.5 times the interquartile range from the box; n = 1 from 10 mice).
Supplementary Figure 7 RNA-sequencing of the circRNAs is reproducible
A-E. Sequencing replicates for mouse tissues: brain (A), heart (B), liver (C), lung (D) and testis (E). F-I: Sequencing replicates for mouse developmental stages: E18 (F), P1 (G), P10 (H), P30 (I). J-K Sequencing replicates for control (J) and bicuculline treatment (K). Black dots represent mRNAs and lincRNAs, and red dots represent circRNAs. Values on X/Y axis denote the abundance of the genes (in TPM) in biological replicates (n = 6 − 7, 3, 3 and 3 − 4 in each of two replicates for developmental stages E18, P1, P10 and P30, respectively).
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You, X., Vlatkovic, I., Babic, A. et al. Neural circular RNAs are derived from synaptic genes and regulated by development and plasticity. Nat Neurosci 18, 603–610 (2015). https://doi.org/10.1038/nn.3975
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DOI: https://doi.org/10.1038/nn.3975
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