Developmental functions of piRNAs and transposable elements: a Drosophila point-of-view

The Novel Role of Noncoding RNAs in Modulating Platelet Function: Implications in Activation and Aggregation

It is currently believed that plaque complication, with the consequent superimposed thrombosis, is a key factor in the clinical occurrence of acute coronary syndromes (ACSs). Platelets are major players in this process. Despite the considerable progress made by the new antithrombotic strategies (P2Y12 receptor inhibitors, new oral anticoagulants, thrombin direct inhibitors, etc.) in terms of a reduction in major cardiovascular events, a significant number of patients with previous ACSs treated with these drugs continue to experience events, indicating that the mechanisms of platelet remain largely unknown. In the last decade, our knowledge of platelet pathophysiology has improved. It has been reported that, in response to physiological and pathological stimuli, platelet activation is accompanied by de novo protein synthesis, through a rapid and particularly well-regulated translation of resident mRNAs of megakaryocytic derivation. Although the platelets are anucleate, they indeed contain an important fraction of mRNAs that can be quickly used for protein synthesis following their activation. A better understanding of the pathophysiology of platelet activation and the interaction with the main cellular components of the vascular wall will open up new perspectives in the treatment of the majority of thrombotic disorders, such as ACSs, stroke, and peripheral artery diseases before and after the acute event. In the present review, we will discuss the novel role of noncoding RNAs in modulating platelet function, highlighting the possible implications in activation and aggregation.

Genome-wide profiling of piRNAs in the whitefly Bemisia tabaci reveals cluster distribution and association with begomovirus transmission

The whitefly Bemisia tabaci MEAM1 is a notorious vector capable of transmitting many plant viruses, resulting in serious crop loss and food shortage around the world. To investigate potential sRNA-mediated regulatory mechanisms in whiteflies that are affected by virus acquisition and transmission, we conducted small RNA (sRNA) deep sequencing and performed genome-wide profiling of piwi-interacting RNAs (piRNAs) in whiteflies that were fed on tomato yellow leaf curl virus (TYLCV)-infected or non-infected tomato plants for 24, 48, and 72 h. In the present study, piRNA reads ranging from 564,395 to 1,715,652 per library were identified and shown to distribute unevenly in clusters (57 to 96 per library) on the whitefly (B. tabaci MEAM1) genome. Among them, 53 piRNA clusters were common for all treatments. Comparative analysis between libraries generated from viruliferous and non-viruliferous whiteflies identified five TYLCV-induced and 24 TYLCV-suppressed piRNA clusters. Approximately 62% of piRNAs were derived from non-coding sequences including intergenic regions, introns, and untranslated regions (UTRs). The remaining 38% were derived from coding sequences (CDS) or repeat elements. Interestingly, six protein coding genes were targeted by the TYLCV-induced piRNAs. We identified a large number of piRNAs that were distributed in clusters across the whitefly genome, with 60% being derived from non-coding regions. Comparative analysis revealed that feeding on a virus-infected host caused induction and suppression of only a small number of piRNA clusters in whiteflies. Although piRNAs primarily regulate the activity of transposable elements, our results suggest that they may have additional functions in regulating protein coding genes and in insect-virus interactions.

Transposable Elements in Human Cancer: Causes and Consequences of Deregulation

Transposable elements (TEs) comprise nearly half of the human genome and play an essential role in the maintenance of genomic stability, chromosomal architecture, and transcriptional regulation. TEs are repetitive sequences consisting of RNA transposons, DNA transposons, and endogenous retroviruses that can invade the human genome with a substantial contribution in human evolution and genomic diversity. TEs are therefore firmly regulated from early embryonic development and during the entire course of human life by epigenetic mechanisms, in particular DNA methylation and histone modifications. The deregulation of TEs has been reported in some developmental diseases, as well as for different types of human cancers. To date, the role of TEs, the mechanisms underlying TE reactivation, and the interplay with DNA methylation in human cancers remain largely unexplained. We reviewed the loss of epigenetic regulation and subsequent genomic instability, chromosomal aberrations, transcriptional deregulation, oncogenic activation, and aberrations of non-coding RNAs as the potential mechanisms underlying TE deregulation in human cancers.

Developmental piRNA profiles of the invasive vector mosquito Aedes albopictus

Background

In eukaryotic organisms, Piwi-interacting RNAs (piRNAs) control the activities of mobile genetic elements and ensure genome maintenance. Recent evidence indicates that piRNAs are involved in multiple biological pathways, including transcriptional regulation of protein-coding genes, sex determination and even interactions between host and pathogens. Aedes albopictus is a major invasive species that transmits a number of viral diseases in humans. Ae. albopictus has the largest genome and the highest abundance of repetitive sequences when compared with members that belong to Culicidae with a published genome. Analysis of piRNA profiles will provide a developmental and evolutionary perspective on piRNAs in Ae. albopictus.

Methods

piRNAs were identified and characterized during the development of Ae. albopictus, and piRNA expression patterns in adult males and females as well as sugar-fed females and blood-fed females were compared.

Results

Our results reveal that, despite the large genome size of Ae. albopictus, the piRNA pool of Ae. albopictus (1.2 × 10⁷) is smaller than those of Aedes aegypti (1.7 × 10⁷) and Drosophila melanogaster (1.6 × 10⁷). In Ae. albopictus, piRNAs displayed the highest abundance at the embryo stage and the lowest abundance at the pupal stage. Approximately 50 % of the piRNAs mapped to intergenic regions with no known functions. Approximately 30 % of the piRNAs mapped to repetitive elements, and 77.69 % of these repeat-derived piRNAs mapped to Class I TEs; 45.42 % of the observed piRNA reads originated from piRNA clusters, and most of the top 10 highest expressed piRNA clusters and 100 highest expressed piRNAs from each stage displayed biased expression patterns across the developmental stages. All anti-sense-derived piRNAs displayed a preference for uridine at the 5′ end; however, the sense-derived piRNAs showed adenine bias at the tenth nucleotide position and a typical ping-pong signature, suggesting that the biogenesis of piRNAs was conserved throughout development. Our results also show that 962 piRNAs displayed sex-biased expression, and 522 piRNAs showed higher expression in the blood-fed females than in the sugar-fed females.

Conclusions

Our results suggest that piRNAs, aside from silencing transposable elements in Ae. albopictus, may have a role in other biological pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1815-8) contains supplementary material, which is available to authorized users.

Do human transposable element small RNAs serve primarily as genome defenders or genome regulators?

It is currently thought that small RNA (sRNA) based repression mechanisms are primarily employed to mitigate the mutagenic threat posed by the activity of transposable elements (TEs). This can be achieved by the sRNA guided processing of TE transcripts via Dicer-dependent (e.g., siRNA) or Dicer-independent (e.g., piRNA) mechanisms. For example, potentially active human L1 elements are silenced by mRNA cleavage induced by element encoded siRNAs, leading to a negative correlation between element mRNA and siRNA levels. On the other hand, there is emerging evidence that TE derived sRNAs can also be used to regulate the host genome. Here, we evaluated these two hypotheses for human TEs by comparing the levels of TE derived mRNA and TE sRNA across six tissues. The genome defense hypothesis predicts a negative correlation between TE mRNA and TE sRNA levels, whereas the genome regulatory hypothesis predicts a positive correlation. On average, TE mRNA and TE sRNA levels are positively correlated across human tissues. These correlations are higher than seen for human genes or for randomly permuted control data sets. Overall, Alu subfamilies show the highest positive correlations of element mRNA and sRNA levels across tissues, although a few of the youngest, and potentially most active, Alu subfamilies do show negative correlations. Thus, Alu derived sRNAs may be related to both genome regulation and genome defense. These results are inconsistent with a simple model whereby TE derived sRNAs reduce levels of standing TE mRNA via transcript cleavage, and suggest that human cells efficiently process TE transcripts into sRNA based on the available message levels. This may point to a widespread role for processed TE transcripts in genome regulation or to alternative roles of TE-to-sRNA processing including the mitigation of TE transcript cytotoxicity.

Protein interactions with piALU RNA indicates putative participation of retroRNA in the cell cycle, DNA repair and chromatin assembly

Recent analyses suggest that transposable element-derived transcripts are processed to yield a variety of small RNA species that play critical functional roles in gene regulation and chromatin organization as well as genome stability and maintenance. Here we report a mass spectrometry analysis of an RNA-affinity complex isolation using a piRNA homologous sequence derived from Alu retrotransposal RNA. Our data point to potential roles for piALU RNAs in DNA repair, cell cycle and chromatin regulations.

Argonaute-dependent small RNAs derived from single-stranded, non-structured precursors

A general feature of Argonaute-dependent small RNAs is their base-paired precursor structures, and precursor duplex structures are often required for confident annotation of miRNA genes. However, this rule has been broken by discoveries of functional small RNA species whose precursors lack a predictable double-stranded (ds-) RNA structure, arguing that duplex structures are not prerequisite for small RNA loading to Argonautes. The biological significance of single-stranded (ss-) RNA loading has been recognized particularly in systems where active small RNA amplification mechanisms are involved, because even a small amount of RNA molecules can trigger the production of abundant RNA species leading to profound biological effects. However, even in the absence of small RNA amplification mechanisms, recent studies have demonstrated that potent gene silencing can be achieved using chemically modified synthetic ssRNAs that are resistant to RNases in mice. Therefore, such ssRNA-mediated gene regulation may have broader roles than previously recognized, and the findings have opened the door for further research to optimize the design of ss-siRNAs toward future pharmaceutical and biomedical applications of gene silencing technologies. In this review, we will summarize studies about endogenous ssRNA species that are bound by Argonaute proteins and how ssRNA precursors are recognized by various small RNA pathways.

The intertwining of transposable elements and non-coding RNAs

Growing evidence shows a close association of transposable elements (TE) with non-coding RNAs (ncRNA), and a significant number of small ncRNAs originate from TEs. Further, ncRNAs linked with TE sequences participate in a wide-range of regulatory functions. Alu elements in particular are critical players in gene regulation and molecular pathways. Alu sequences embedded in both long non-coding RNAs (lncRNA) and mRNAs form the basis of targeted mRNA decay via short imperfect base-pairing. Imperfect pairing is prominent in most ncRNA/target RNA interactions and found throughout all biological kingdoms. The piRNA-Piwi complex is multifunctional, but plays a major role in protection against invasion by transposons. This is an RNA-based genetic immune system similar to the one found in prokaryotes, the CRISPR system. Thousands of long intergenic non-coding RNAs (lincRNAs) are associated with endogenous retrovirus LTR transposable elements in human cells. These TEs can provide regulatory signals for lincRNA genes. A surprisingly large number of long circular ncRNAs have been discovered in human fibroblasts. These serve as "sponges" for miRNAs. Alu sequences, encoded in introns that flank exons are proposed to participate in RNA circularization via Alu/Alu base-pairing. Diseases are increasingly found to have a TE/ncRNA etiology. A single point mutation in a SINE/Alu sequence in a human long non-coding RNA leads to brainstem atrophy and death. On the other hand, genomic clusters of repeat sequences as well as lncRNAs function in epigenetic regulation. Some clusters are unstable, which can lead to formation of diseases such as facioscapulohumeral muscular dystrophy. The future may hold more surprises regarding diseases associated with ncRNAs andTEs.

Control of maternal mRNA stability in germ cells and early embryos

mRNA regulation is essential in germ cells and early embryos. In particular, late oogenesis and early embryogenesis occur in the absence of transcription and rely on maternal mRNAs stored in oocytes. These maternal mRNAs subsequently undergo a general decay in embryos during the maternal-to-zygotic transition in which the control of development switches from the maternal to the zygotic genome. Regulation of mRNA stability thus plays a key role during these early stages of development and is tightly interconnected with translational regulation and mRNA localization. A common mechanism in these three types of regulation implicates variations in mRNA poly(A) tail length. Recent advances in the control of mRNA stability include the widespread and essential role of regulated deadenylation in early developmental processes, as well as the mechanisms regulating mRNA stability which involve RNA binding proteins, microRNAs and interplay between the two. Also emerging are the roles that other classes of small non-coding RNAs, endo-siRNAs and piRNAs play in the control of mRNA decay, including connections between the regulation of transposable elements and cellular mRNA regulation through the piRNA pathway. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Novel features of a PIWI-like protein homolog in the parasitic protozoan Leishmania

In contrast to nearly all eukaryotes, the Old World Leishmania species L. infantum and L. major lack the bona fide RNAi machinery genes. Interestingly, both Leishmania genomes code for an atypical Argonaute-like protein that possesses a PIWI domain but lacks the PAZ domain found in Argonautes from RNAi proficient organisms. Using sub-cellular fractionation and confocal fluorescence microscopy, we show that unlike other eukaryotes, the PIWI-like protein is mainly localized in the single mitochondrion in Leishmania. To predict PIWI function, we generated a knockout mutant for the PIWI gene in both L. infantum (Lin) and L. major species by double-targeted gene replacement. Depletion of PIWI has no effect on the viability of insect promastigote forms but leads to an important growth defect of the mammalian amastigote lifestage in vitro and significantly delays disease pathology in mice, consistent with a higher expression of the PIWI transcript in amastigotes. Moreover, amastigotes lacking PIWI display a higher sensitivity to apoptosis inducing agents than wild type parasites, suggesting that PIWI may be a sensor for apoptotic stimuli. Furthermore, a whole-genome DNA microarray analysis revealed that loss of LinPIWI in Leishmania amastigotes affects mostly the expression of specific subsets of developmentally regulated genes. Several transcripts encoding surface and membrane-bound proteins were found downregulated in the LinPIWI^(−/−) mutant whereas all histone transcripts were upregulated in the null mutant, supporting the possibility that PIWI plays a direct or indirect role in the stability of these transcripts. Although our data suggest that PIWI is not involved in the biogenesis or the stability of small noncoding RNAs, additional studies are required to gain further insights into the role of this protein on RNA regulation and amastigote development in Leishmania.

Male germline control of transposable elements

Repetitive sequences, especially transposon-derived interspersed repetitive elements, account for a large fraction of the genome in most eukaryotes. Despite the repetitive nature, these transposable elements display quantitative and qualitative differences even among species of the same lineage. Although transposable elements contribute greatly as a driving force to the biological diversity during evolution, they can induce embryonic lethality and genetic disorders as a result of insertional mutagenesis and genomic rearrangement. Temporary relaxation of the epigenetic control of retrotransposons during early germline development opens a risky window that can allow retrotransposons to escape from host constraints and to propagate abundantly in the host genome. Because germline mutations caused by retrotransposon activation are heritable and thus can be deleterious to the offspring, an adaptive strategy has evolved in host cells, especially in the germline. In this review, we will attempt to summarize general defense mechanisms deployed by the eukaryotic genome, with an emphasis on pathways utilized by the male germline to confer retrotransposon silencing.