Discovery and characterization of miRNA genes in Atlantic salmon (Salmo salar) by use of a deep sequencing approach

Identification of General and Heart-Specific miRNAs in Sheep (Ovis aries)

MicroRNAs (miRNAs or miRs) are small regulatory RNAs crucial for modulation of signaling pathways in multiple organs. While the link between miRNAs and heart disease has grown more readily apparent over the past three years, these data are primarily limited to small animal models or cell-based systems. Here, we performed a high-throughput RNA sequencing (RNAseq) analysis of left ventricle and other tissue from a pre-clinical ovine model. We identified 172 novel miRNA precursors encoding a total of 264 mature miRNAs. Notably, 84 precursors were detected in both the left ventricle and other tissues. However, 10 precursors, encoding 11 mature sequences, were specific to the left ventricle. Moreover, the total 168 novel miRNA precursors included 22 non-conserved ovine-specific sequences. Our data identify and characterize novel miRNAs in the left ventricle of sheep, providing fundamental new information for our understanding of protein regulation in heart and other tissues.

Micromanaging metabolism-a role for miRNAs in teleost energy metabolism

MicroRNAs (miRNAs) are small, non-protein coding RNA sequences, which are found in most eukaryotes. Since their initial discovery, miRNAs have emerged as important regulators of many biological processes. One of the most important processes profoundly regulated by miRNAs is energy metabolism. Traditionally, metabolic functions of miRNAs have been studied in genome-sequenced mammalian organisms, especially the mouse model. However, partially driven by commercial interest in aquaculture, increasingly feasible large-scale molecular techniques have resulted in the characterization of miRNA repertoires, and importantly, several genome sequences of several (commercially important) teleost species, which also hold important roles as research models in the comparative physiology of energy metabolism. This review aims to introduce the recent advances in miRNA research in teleost fish and to describe the current knowledge of miRNA function in teleost energy metabolism. The most pressing research needs and questions to determine metabolic roles of miRNAs in teleost models are presented, as well as applicable technical approaches and current bottlenecks. Rainbow trout, which possess the advantages of newly available molecular tools and a long history as comparative research model in teleost energy metabolism, are discussed as a promising research model to address these questions.

Two Virus-Induced MicroRNAs Known Only from Teleost Fishes Are Orthologues of MicroRNAs Involved in Cell Cycle Control in Humans

MicroRNAs (miRNAs) are ~22 base pair-long non-coding RNAs which regulate gene expression in the cytoplasm of eukaryotic cells by binding to specific target regions in mRNAs to mediate transcriptional blocking or mRNA cleavage. Through their fundamental roles in cellular pathways, gene regulation mediated by miRNAs has been shown to be involved in almost all biological phenomena, including development, metabolism, cell cycle, tumor formation, and host-pathogen interactions. To address the latter in a primitive vertebrate host, we here used an array platform to analyze the miRNA response in rainbow trout (Oncorhynchus mykiss) following inoculation with the virulent fish rhabdovirus Viral hemorrhagic septicaemia virus. Two clustered miRNAs, miR-462 and miR-731 (herein referred to as miR-462 cluster), described only in teleost fishes, were found to be strongly upregulated, indicating their involvement in fish-virus interactions. We searched for homologues of the two teleost miRNAs in other vertebrate species and investigated whether findings related to ours have been reported for these homologues. Gene synteny analysis along with gene sequence conservation suggested that the teleost fish miR-462 and miR-731 had evolved from the ancestral miR-191 and miR-425 (herein called miR-191 cluster), respectively. Whereas the miR-462 cluster locus is found between two protein-coding genes (intergenic) in teleost fish genomes, the miR-191 cluster locus is found within an intron of a protein-coding gene (intragenic) in the human genome. Interferon (IFN)-inducible and immune-related promoter elements found upstream of the teleost miR-462 cluster locus suggested roles in immune responses to viral pathogens in fish, while in humans, the miR-191 cluster functionally associated with cell cycle regulation. Stimulation of fish cell cultures with the IFN inducer poly I:C accordingly upregulated the expression of miR-462 and miR-731, while no stimulatory effect on miR-191 and miR-425 expression was observed in human cell lines. Despite high sequence conservation, evolution has thus resulted in different regulation and presumably also different functional roles of these orthologous miRNA clusters in different vertebrate lineages.

Identification of MicroRNA for Intermuscular Bone Development in Blunt Snout Bream (Megalobrama amblycephala)

Intermuscular bone (IB), which occurs only in the myosepta of the lower teleosts, is attracting more attention of researchers due to its particular development and lack of genetic information. MicroRNAs (miRNAs) are emerging as important regulators for biological processes. In the present study, miRNAs from IBs and connective tissue (CT; encircled IBs) from six-month-old Megalobrama amblycephala were characterized and compared. The results revealed the sequences and expression levels of 218 known miRNA genes (belonging to 97 families). Of these miRNAs, 44 known microRNA sequences exhibited significant expression differences between the two libraries, with 24 and 20 differentially-expressed miRNAs exhibiting higher expression in the CT and IBs libraries, respectively. The expressions of 11 miRNAs were selected to validate in nine tissues. Among the high-ranked predicted gene targets, differentiation, cell cycle, metabolism, signal transduction and transcriptional regulation were implicated. The pathway analysis of differentially-expressed miRNAs indicated that they were abundantly involved in regulating the development and differentiation of IBs and CT. This study characterized the miRNA for IBs of teleosts for the first time, which provides an opportunity for further understanding of miRNA function in the regulation of IB development.

Validation of miRNA genes suitable as reference genes in qPCR analyses of miRNA gene expression in Atlantic salmon (Salmo salar)

Background

MicroRNAs (miRNAs) are an abundant class of endogenous small RNA molecules that downregulate gene expression at the post-transcriptional level. They play important roles by regulating genes that control multiple biological processes, and recent years there has been an increased interest in studying miRNA genes and miRNA gene expression. The most common method applied to study gene expression of single genes is quantitative PCR (qPCR). However, before expression of mature miRNAs can be studied robust qPCR methods (miRNA-qPCR) must be developed. This includes identification and validation of suitable reference genes. We are particularly interested in Atlantic salmon (Salmo salar). This is an economically important aquaculture species, but no reference genes dedicated for use in miRNA-qPCR methods has been validated for this species. Our aim was, therefore, to identify suitable reference genes for miRNA-qPCR methods in Salmo salar.

Results

We used a systematic approach where we utilized similar studies in other species, some biological criteria, results from deep sequencing of small RNAs and, finally, experimental validation of candidate reference genes by qPCR to identify the most suitable reference genes. Ssa-miR-25-3p was identified as most suitable single reference gene. The best combinations of two reference genes were ssa-miR-25-3p and ssa-miR-455-5p. These two genes were constitutively and stably expressed across many different tissues. Furthermore, infectious salmon anaemia did not seem to affect their expression levels. These genes were amplified with high specificity, good efficiency and the qPCR assays showed a good linearity when applying a simple cybergreen miRNA-PCR method using miRNA gene specific forward primers.

Conclusions

We have identified suitable reference genes for miRNA-qPCR in Atlantic salmon. These results will greatly facilitate further studies on miRNA genes in this species. The reference genes identified are conserved genes that are identical in their mature sequence in many aquaculture species. Therefore, they may also be suitable as reference genes in other teleosts. Finally, the systematic approach used in our study successfully identified suitable reference genes, suggesting that this may be a useful strategy to apply in similar validation studies in other aquaculture species.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-0500-7-945) contains supplementary material, which is available to authorized users.

A new model army: Emerging fish models to study the genomics of vertebrate Evo-Devo

Many fields of biology--including vertebrate Evo-Devo research--are facing an explosion of genomic and transcriptomic sequence information and a multitude of fish species are now swimming in this "genomic tsunami." Here, we first give an overview of recent developments in sequencing fish genomes and transcriptomes that identify properties of fish genomes requiring particular attention and propose strategies to overcome common challenges in fish genomics. We suggest that the generation of chromosome-level genome assemblies--for which we introduce the term "chromonome"--should be a key component of genomic investigations in fish because they enable large-scale conserved synteny analyses that inform orthology detection, a process critical for connectivity of genomes. Orthology calls in vertebrates, especially in teleost fish, are complicated by divergent evolution of gene repertoires and functions following two rounds of genome duplication in the ancestor of vertebrates and a third round at the base of teleost fish. Second, using examples of spotted gar, basal teleosts, zebrafish-related cyprinids, cavefish, livebearers, icefish, and lobefin fish, we illustrate how next generation sequencing technologies liberate emerging fish systems from genomic ignorance and transform them into a new model army to answer longstanding questions on the genomic and developmental basis of their biodiversity. Finally, we discuss recent progress in the genetic toolbox for the major fish models for functional analysis, zebrafish, and medaka, that can be transferred to many other fish species to study in vivo the functional effect of evolutionary genomic change as Evo-Devo research enters the postgenomic era.

MicroRNA in teleost fish

MicroRNAs (miRNAs) are transcriptional and posttranscriptional regulators involved in nearly all known biological processes in distant eukaryotic clades. Their discovery and functional characterization have broadened our understanding of biological regulatory mechanisms in animals and plants. They show both evolutionary conserved and unique features across Metazoa. Here, we present the current status of the knowledge about the role of miRNA in development, growth, and physiology of teleost fishes, in comparison to other vertebrates. Infraclass Teleostei is the most abundant group among vertebrate lineage. Fish are an important component of aquatic ecosystems and human life, being the prolific source of animal proteins worldwide and a vertebrate model for biomedical research. We review miRNA biogenesis, regulation, modifications, and mechanisms of action. Specific sections are devoted to the role of miRNA in teleost development, organogenesis, tissue differentiation, growth, regeneration, reproduction, endocrine system, and responses to environmental stimuli. Each section discusses gaps in the current knowledge and pinpoints the future directions of research on miRNA in teleosts.

Conserved microRNA editing in mammalian evolution, development and disease

Background

Mammalian microRNAs (miRNAs) are sometimes subject to adenosine-to-inosine RNA editing, which can lead to dramatic changes in miRNA target specificity or expression levels. However, although a few miRNAs are known to be edited at identical positions in human and mouse, the evolution of miRNA editing has not been investigated in detail. In this study, we identify conserved miRNA editing events in a range of mammalian and non-mammalian species.

Results

We demonstrate deep conservation of several site-specific miRNA editing events, including two that date back to the common ancestor of mammals and bony fishes some 450 million years ago. We also find evidence of a recent expansion of an edited miRNA family in placental mammals and show that editing of these miRNAs is associated with changes in target mRNA expression during primate development and aging. While global patterns of miRNA editing tend to be conserved across species, we observe substantial variation in editing frequencies depending on tissue, age and disease state: editing is more frequent in neural tissues compared to heart, kidney and testis; in older compared to younger individuals; and in samples from healthy tissues compared to tumors, which together suggests that miRNA editing might be associated with a reduced rate of cell proliferation.

Conclusions

Our results show that site-specific miRNA editing is an evolutionarily conserved mechanism, which increases the functional diversity of mammalian miRNA transcriptomes. Furthermore, we find that although miRNA editing is rare compared to editing of long RNAs, miRNAs are greatly overrepresented among conserved editing targets.

Identification and characterization of Cynoglossus semilaevis microRNA response to Vibrio anguillarum infection through high-throughput sequencing

MicroRNAs (miRNA) play key regulatory roles in diverse biological processes. Cynoglossus semilaevis is an important commercial mariculture fish species in China. To identify miRNAs and investigate immune-related miRNAs of C. semilaevis, we performed high-throughput sequencing on three small RNA libraries prepared from C. semilaevis immune tissues (liver, head kidney, spleen, and intestine). One library was prepared under normal conditions (control, CG); two were prepared during Vibrio anguillarum infection, where vibriosis symptoms were obvious and non-obvious (HOSG and NOSG, respectively). We obtained 11,216,875, 12,313,404, and 11,398,695 clean reads per library, respectively. Bioinformatic analysis identified 452 miRNAs, including 24 putative novel miRNAs. We analyzed differentially expressed miRNAs between two libraries using pairwise comparison. For NOSG-CG, there was significant differential expression of 175 (38.72%) miRNAs. There was significant differential expression of 215 (47.57%) miRNAs between HOSG and CG. Compared with CG, The HOSG-NOSG comparison revealed significantly different expression of 122 (26.99%) miRNAs respectively. Real-time quantitative PCR (RT-qPCR) experiments were performed for 10 miRNAs of the three samples, and agreement was found between the sequencing and RT-qPCR data. For miRNAs that were significantly differentially expressed, functional annotation of target genes by Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analysis indicated that a set of miRNAs that were expressed highly abundantly and significantly differentially were might involved in immune system development and immune response. To our understanding, this is the first report of comprehensive identification of C. semilaevis miRNAs being differentially regulated in immune tissues (liver, head kidney, spleen, and intestine) in normal conditions relating to V. anguillarum infection. Many miRNAs were differentially regulated upon pathogen exposure. This work provides an opportunity for further understanding of the molecular mechanisms of miRNA regulation in C. semilaevis host-pathogen interactions.