Requirements for gene silencing mediated by U1 snRNA binding to a target sequence

N-6-methyladenosine (m6A) promotes the nuclear retention of mRNAs with intact 5' splice site motifs

In humans, misprocessed mRNAs containing intact 5' Splice Site (5'SS) motifs are nuclear retained and targeted for decay by ZFC3H1, a component of the Poly(A) Exosome Targeting complex, and U1-70K, a component of the U1 snRNP. In S. pombe, the ZFC3H1 homolog, Red1, binds to the YTH domain-containing protein Mmi1 and targets certain RNA transcripts to nuclear foci for nuclear retention and decay. Here we show that YTHDC1 and YTHDC2, two YTH domain-containing proteins that bind to N-6-methyladenosine (m6A) modified RNAs, interact with ZFC3H1 and U1-70K, and are required for the nuclear retention of mRNAs with intact 5'SS motifs. Disruption of m6A deposition inhibits both the nuclear retention of these transcripts and their accumulation in YTHDC1-enriched foci that are adjacent to nuclear speckles. Endogenous RNAs with intact 5'SS motifs, such as intronic poly-adenylated transcripts, tend to be m6A-modified at low levels. Thus, the m6A modification acts on a conserved quality control mechanism that targets misprocessed mRNAs for nuclear retention and decay.

Engineering U1-Based Tetracycline-Inducible Riboswitches to Control Gene Expression in Mammals

Synthetic riboswitches are promising regulatory devices due to their small size, lack of immunogenicity, and ability to fine-tune gene expression in the absence of exogenous trans-acting factors. Based on a gene inhibitory system developed at our lab, termed U1snRNP interference (U1i), we developed tetracycline (TC)-inducible riboswitches that modulate mRNA polyadenylation through selective U1 snRNP recruitment. First, we engineered different TC-U1i riboswitches, which repress gene expression unless TC is added, leading to inductions of gene expression of 3-to-4-fold. Second, we developed a technique called Systematic Evolution of Riboswitches by Exponential Enrichment (SEREX), to isolate riboswitches with enhanced U1 snRNP binding capacity and activity, achieving inducibilities of up to 8-fold. Interestingly, by multiplexing riboswitches we increased inductions up to 37-fold. Finally, we demonstrated that U1i-based riboswitches are dose-dependent and reversible and can regulate the expression of reporter and endogenous genes in culture cells and mouse models, resulting in attractive systems for gene therapy applications. Our work probes SEREX as a much-needed technology for the in vitro identification of riboswitches capable of regulating gene expression in vivo.

Regulation of alternative splicing and polyadenylation in neurons

Cell-type-specific gene expression is a fundamental feature of multicellular organisms and is achieved by combinations of regulatory strategies. Although cell-restricted transcription is perhaps the most widely studied mechanism, co-transcriptional and post-transcriptional processes are also central to the spatiotemporal control of gene functions. One general category of expression control involves the generation of multiple transcript isoforms from an individual gene, whose balance and cell specificity are frequently tightly regulated via diverse strategies. The nervous system makes particularly extensive use of cell-specific isoforms, specializing the neural function of genes that are expressed more broadly. Here, we review regulatory strategies and RNA-binding proteins that direct neural-specific isoform processing. These include various classes of alternative splicing and alternative polyadenylation events, both of which broadly diversify the neural transcriptome. Importantly, global alterations of splicing and alternative polyadenylation are characteristic of many neural pathologies, and recent genetic studies demonstrate how misregulation of individual neural isoforms can directly cause mutant phenotypes.

The discovery and development of RNA-based therapies for treatment of HIV-1 infection

INTRODUCTION

Long-term control of HIV-1 infection can potentially be achieved using autologous stem cell transplants with gene-modified cells. Non-coding RNAs represent a diverse class of therapeutic agents including ribozymes, RNA aptamers and decoys, small interfering RNAs, short hairpin RNAs, and U1 interference RNAs that can be designed to inhibit HIV-1 replication. They have been engineered for delivery as drugs to complement current HIV-1 therapies and as gene therapies for a potential HIV-1 functional cure.

AREAS COVERED

This review surveys the past three decades of development of these RNA technologies with a focus on their efficacy and safety for treating HIV-1 infections. We describe the mechanisms of each RNA-based agent, targets they have been developed against, efforts to enhance their stability and efficacy, and we evaluate their performance in past and ongoing preclinical and clinical trials.

EXPERT OPINION

RNA-based technologies are among the top candidates for gene therapies where they can be stably expressed for long-term suppression of HIV-1. Advances in both gene and drug delivery strategies and improvements to non-coding RNA stability and antiviral properties will cooperatively drive forward progress in improving drug therapy and engineering HIV-1 resistant cells.

U1A is a positive regulator of the expression of heterologous and cellular genes involved in cell proliferation and migration

Here, we show that direct recruitment of U1A to target transcripts can increase gene expression. This is a new regulatory role, in addition to previous knowledge showing that U1A decreases the levels of U1A mRNA and other specific targets. In fact, genome-wide, U1A more often increases rather than represses gene expression and many U1A-upregulated transcripts are directly bound by U1A according to individual nucleotide resolution crosslinking and immunoprecipitation (iCLIP) studies. Interestingly, U1A-mediated positive regulation can be transferred to a heterologous system for biotechnological purposes. Finally, U1A-bound genes are enriched for those involved in cell cycle and adhesion. In agreement with this, higher U1A mRNA expression associates with lower disease-free survival and overall survival in many cancer types, and U1A mRNA levels positively correlate with those of some oncogenes involved in cell proliferation. Accordingly, U1A depletion leads to decreased expression of these genes and the migration-related gene CCN2/CTGF, which shows the strongest regulation by U1A. A decrease in U1A causes a strong drop in CCN2 expression and CTGF secretion and defects in the expression of CTGF EMT targets, cell migration, and proliferation. These results support U1A as a putative therapeutic target for cancer treatment. In addition, U1A-binding sequences should be considered in biotechnological applications.

ZFC3H1 and U1-70K promote the nuclear retention of mRNAs with 5' splice site motifs within nuclear speckles

Quality control of mRNA represents an important regulatory mechanism for gene expression in eukaryotes. One component of this quality control is the nuclear retention and decay of misprocessed RNAs. Previously, we demonstrated that mature mRNAs containing a 5' splice site (5'SS) motif, which is typically found in misprocessed RNAs such as intronic polyadenylated (IPA) transcripts, are nuclear retained and degraded. Using high-throughput sequencing of cellular fractions, we now demonstrate that IPA transcripts require the zinc finger protein ZFC3H1 for their nuclear retention and degradation. Using reporter mRNAs, we demonstrate that ZFC3H1 promotes the nuclear retention of mRNAs with intact 5'SS motifs by sequestering them into nuclear speckles. Furthermore, we find that U1-70K, a component of the spliceosomal U1 snRNP, is also required for the nuclear retention of these reporter mRNAs and likely functions in the same pathway as ZFC3H1. Finally, we show that the disassembly of nuclear speckles impairs the nuclear retention of reporter mRNAs with 5'SS motifs. Our results highlight a splicing independent role of U1 snRNP and indicate that it works in conjunction with ZFC3H1 in preventing the nuclear export of misprocessed mRNAs by sequestering them into nuclear speckles.

U1 snRNP telescripting: molecular mechanisms and beyond

U1 snRNP is one of the most abundant ribonucleoprotein (RNP) complexes in eukaryotic cells and is estimated to be approximately 1 million copies per cell. Apart from its canonical role in mRNA splicing, this complex has emerged as a key regulator of eukaryotic mRNA length via inhibition of mRNA 3'-end processing at numerous intronic polyadenylation sites, in a process that is also termed 'U1 snRNP telescripting'. Several reviews have extensively described the concept of U1 telescripting and subsequently highlighted its potential impacts in mRNA metabolism. Here, we review what is currently known regarding the underlying mechanisms of this important phenomenon and discuss open questions and future challenges.

Functional Long Non-coding RNAs Evolve from Junk Transcripts

Transcriptome studies reveal pervasive transcription of complex genomes, such as those of mammals. Despite popular arguments for functionality of most, if not all, of these transcripts, genome-wide analysis of selective constraints indicates that most of the produced RNA are junk. However, junk is not garbage. On the contrary, junk transcripts provide the raw material for the evolution of diverse long non-coding (lnc) RNAs by non-adaptive mechanisms, such as constructive neutral evolution. The generation of many novel functional entities, such as lncRNAs, that fuels organismal complexity does not seem to be driven by strong positive selection. Rather, the weak selection regime that dominates the evolution of most multicellular eukaryotes provides ample material for functional innovation with relatively little adaptation involved.

Phylogenetic comparison of 5' splice site determination in central spliceosomal proteins of the U1-70K gene family, in response to developmental cues and stress conditions

Intron-containing genes have the ability to generate multiple transcript isoforms by splicing, thereby greatly expanding the eukaryotic transcriptome and proteome. In eukaryotic cells, precursor mRNA (pre-mRNA) splicing is performed by a mega-macromolecular complex defined as a spliceosome. Among its splicing components, U1 small nuclear ribonucleoprotein (U1 snRNP) is the smallest subcomplex involved in early spliceosome assembly and 5'-splice site recognition. Its central component, named U1-70K, has been extensively characterized in animals and yeast. Very few investigations on U1-70K genes have been conducted in plants, however. To this end, we performed a comprehensive study to systematically identify 115 U1-70K genes from 67 plant species, ranging from algae to angiosperms. Phylogenetic analysis suggested that the expansion of the plant U1-70K gene family was likely to have been driven by whole-genome duplications. Subsequent comparisons of gene structures, protein domains, promoter regions and conserved splicing patterns indicated that plant U1-70Ks are likely to preserve their conserved molecular function across plant lineages and play an important functional role in response to environmental stresses. Furthermore, genetic analysis using T-DNA insertion mutants suggested that Arabidopsis U1-70K may be involved in response to osmotic stress. Our results provide a general overview of this gene family in Viridiplantae and will act as a reference source for future mechanistic studies on this U1 snRNP-specific splicing factor.

A U1i RNA that Enhances HIV-1 RNA Splicing with an Elongated Recognition Domain Is an Optimal Candidate for Combination HIV-1 Gene Therapy

U1 interference (U1i) RNAs can be designed to correct splicing defects and target pathogenic RNA, such as HIV-1 RNA. In this study, we show that U1i RNAs that enhance HIV-1 RNA splicing are more effective at inhibiting HIV-1 production compared to top U1i RNAs that inhibit polyadenylation of HIV-1 RNA. A U1i RNA was also identified targeting a site upstream of the first splice acceptor site in the Gag coding region that was effective at inhibiting HIV-1 production. U1-T6, which enhanced HIV-1 RNA splicing, was superior to an antiviral short hairpin RNA (shRNA) currently in clinical trials. To increase specificity, the recognition domain of U1-T6 was elongated by 3–6 nt. The elongated molecules inhibited HIV-1 production from different HIV-1 strains, including one with a mismatch in the target site. These results suggest that lengthening the recognition domain can enhance the specificity of U1i RNAs for their intended target sites while at the same time allowing them to tolerate single mismatch mutations. Overall, our results demonstrate that U1-T6 with an elongated recognition domain inhibits HIV-1 production and has both the efficacy and specificity to be a promising candidate for HIV-1 gene therapy.

Molecular Therapies for Inherited Retinal Diseases-Current Standing, Opportunities and Challenges

Inherited retinal diseases (IRDs) are both genetically and clinically highly heterogeneous and have long been considered incurable. Following the successful development of a gene augmentation therapy for biallelic RPE65-associated IRD, this view has changed. As a result, many different therapeutic approaches are currently being developed, in particular a large variety of molecular therapies. These are depending on the severity of the retinal degeneration, knowledge of the pathophysiological mechanism underlying each subtype of IRD, and the therapeutic target molecule. DNA therapies include approaches such as gene augmentation therapy, genome editing and optogenetics. For some genetic subtypes of IRD, RNA therapies and compound therapies have also shown considerable therapeutic potential. In this review, we summarize the current state-of-the-art of various therapeutic approaches, including the pros and cons of each strategy, and outline the future challenges that lie ahead in the combat against IRDs.

Suboptimal RNA-RNA interaction limits U1 snRNP inhibition of canonical mRNA 3' processing

It is increasingly appreciated that U1 snRNP transcriptomically suppresses the usage of intronic polyadenylation site (PAS) of mRNAs, an outstanding question is why frequently used PASs are not suppressed. Here we found that U1 snRNP could be transiently associated with sequences upstream of actionable PASs in human cells, and RNA-RNA interaction might contribute to the association. By focusing on individual PAS, we showed that the stable assembly of U1 snRNP near PAS might be generally required for U1 inhibition of mRNA 3' processing. Therefore, actionable PASs that often lack optimal U1 snRNP docking site nearby is free from U1 inhibitory effect. Consistently, natural 5' splicing site (5'-SS) is moderately enriched ~250 nt upstream of intronic PASs whose usage is sensitive to functional knockdown of U1 snRNA. Collectively, our results provided an insight into how U1 snRNP selectively inhibits the usage of PASs in a cellular context, and supported a prevailing model that U1 snRNP scans pre-mRNA through RNA-RNA interaction to find a stable interaction site to exercise its function in pre-mRNA processing, including repressing the usage of cryptic PASs.

Sequence Determinants for Nuclear Retention and Cytoplasmic Export of mRNAs and lncRNAs

Eukaryotes are divided into two major compartments: the nucleus where RNA is synthesized and processed, and the cytoplasm, where mRNA is translated into proteins. Although many different RNAs are made, only a subset is allowed access to the cytoplasm, primarily RNAs involved in protein synthesis (mRNA, tRNA, and rRNA). In contrast, nuclear retained transcripts are mostly long non-coding RNAs (lncRNAs) whose role in cell physiology has been a source of much investigation in the past few years. In addition, it is likely that many non-functional RNAs, which arise by spurious transcription and misprocessing of functional RNAs, are also retained in the nucleus and degraded. In this review, the main sequence features that dictate whether any particular mRNA or lncRNA is a substrate for retention in the nucleus, or export to the cytoplasm, are discussed. Although nuclear export is promoted by RNA-splicing due to the fact that the spliceosome can help recruit export factors to the mature RNA, nuclear export does not require splicing. Indeed, most stable unspliced transcripts are well exported and associate with these same export factors in a splicing-independent manner. In contrast, nuclear retention is promoted by specialized cis-elements found in certain RNAs. This new understanding of the determinants of nuclear retention and cytoplasmic export provides a deeper understanding of how information flow is regulated in eukaryotic cells. Ultimately these processes promote the evolution of complexity in eukaryotes by shaping the genomic content through constructive neutral evolution.

Therapeutic activity of modified U1 core spliceosomal particles

Modified U1 snRNAs bound to intronic sequences downstream of the 5′ splice site correct exon skipping caused by different types of mutations. Here we evaluate the therapeutic activity and structural requirements of these exon-specific U1 snRNA (ExSpeU1) particles. In a severe spinal muscular atrophy, mouse model, ExSpeU1, introduced by germline transgenesis, increases SMN2 exon 7 inclusion, SMN protein production and extends life span. In vitro, RNA mutant analysis and silencing experiments show that while U1A protein is dispensable, the 70K and stem loop IV elements mediate most of the splicing rescue activity through improvement of exon and intron definition. Our findings indicate that precise engineering of the U1 core spliceosomal RNA particle has therapeutic potential in pathologies associated with exon-skipping mutations.

Modification of the spliceosome is being tested as a potential therapy for exon-skipping diseases, such as spinal muscular atrophy (SMA). Here the authors show that 70K and stem loop IV structural elements of a modified U1 particle are essential for splicing enhancement and effective treatment of SMA mice.

The reciprocal regulation between splicing and 3'-end processing

Most eukaryotic precursor mRNAs are subjected to RNA processing events, including 5′‐end capping, splicing and 3′‐end processing. These processing events were historically studied independently; however, since the early 1990s tremendous efforts by many research groups have revealed that these processing factors interact with each other to control each other's functions. U1 snRNP and its components negatively regulate polyadenylation of precursor mRNAs. Importantly, this function is necessary for protecting the integrity of the transcriptome and for regulating gene length and the direction of transcription. In addition, physical and functional interactions occur between splicing factors and 3′‐end processing factors across the last exon. These interactions activate or inhibit splicing and 3′‐end processing depending on the context. Therefore, splicing and 3′‐end processing are reciprocally regulated in many ways through the complex protein–protein interaction network. Although interesting questions remain, future studies will illuminate the molecular mechanisms underlying the reciprocal regulation. WIREs RNA 2016, 7:499–511. doi: 10.1002/wrna.1348

For further resources related to this article, please visit the WIREs website.

Conditional U1 Gene Silencing in Toxoplasma gondii

The functional characterisation of essential genes in apicomplexan parasites, such as Toxoplasma gondii or Plasmodium falciparum, relies on conditional mutagenesis systems. Here we present a novel strategy based on U1 snRNP-mediated gene silencing. U1 snRNP is critical in pre-mRNA splicing by defining the exon-intron boundaries. When a U1 recognition site is placed into the 3'-terminal exon or adjacent to the termination codon, pre-mRNA is cleaved at the 3'-end and degraded, leading to an efficient knockdown of the gene of interest (GOI). Here we describe a simple method that combines endogenous tagging with DiCre-mediated positioning of U1 recognition sites adjacent to the termination codon of the GOI which leads to a conditional knockdown of the GOI upon rapamycin-induction. Specific knockdown mutants of the reporter gene GFP and several endogenous genes of T. gondii including the clathrin heavy chain gene 1 (chc1), the vacuolar protein sorting gene 26 (vps26), and the dynamin-related protein C gene (drpC) were silenced using this approach and demonstrate the potential of this technology. We also discuss advantages and disadvantages of this method in comparison to other technologies in more detail.

The consensus 5' splice site motif inhibits mRNA nuclear export

In eukaryotes, mRNAs are synthesized in the nucleus and then exported to the cytoplasm where they are translated into proteins. We have mapped an element, which when present in the 3’terminal exon or in an unspliced mRNA, inhibits mRNA nuclear export. This element has the same sequence as the consensus 5’splice site motif that is used to define the start of introns. Previously it was shown that when this motif is retained in the mRNA, it causes defects in 3’cleavage and polyadenylation and promotes mRNA decay. Our new data indicates that this motif also inhibits nuclear export and promotes the targeting of transcripts to nuclear speckles, foci within the nucleus which have been linked to splicing. The motif, however, does not disrupt splicing or the recruitment of UAP56 or TAP/Nxf1 to the RNA, which are normally required for nuclear export. Genome wide analysis of human mRNAs, lncRNA and eRNAs indicates that this motif is depleted from naturally intronless mRNAs and eRNAs, but less so in lncRNAs. This motif is also depleted from the beginning and ends of the 3’terminal exons of spliced mRNAs, but less so for lncRNAs. Our data suggests that the presence of the 5’splice site motif in mature RNAs promotes their nuclear retention and may help to distinguish mRNAs from misprocessed transcripts and transcriptional noise.

RNA-binding proteins in regulation of alternative cleavage and polyadenylation

Almost all eukaryotic pre-mRNAs are processed at the 3' end by the cleavage and polyadenylation (C/P) reaction, which preludes termination of transcription and gives rise to the poly(A) tail of mature mRNA. Genomic studies in recent years have indicated that most eukaryotic mRNA genes have multiple cleavage and polyadenylation sites (pAs), leading to alternative cleavage and polyadenylation (APA) products. APA isoforms generally differ in their 3' untranslated regions (3' UTRs), but can also have different coding sequences (CDSs). APA expands the repertoire of transcripts expressed from the genome, and is highly regulated under various physiological and pathological conditions. Growing lines of evidence have shown that RNA-binding proteins (RBPs) play important roles in regulation of APA. Some RBPs are part of the machinery for C/P; others influence pA choice through binding to adjacent regions. In this chapter, we review cis elements and trans factors involved in C/P, the significance of APA, and increasingly elucidated roles of RBPs in APA regulation. We also discuss analysis of APA using transcriptome-wide techniques as well as molecular biology approaches.

U1 snRNP-Dependent Suppression of Polyadenylation: Physiological Role and Therapeutic Opportunities in Cancer

Pre-mRNA splicing and polyadenylation are critical steps in the maturation of eukaryotic mRNA. U1 snRNP is an essential component of the splicing machinery and participates in splice-site selection and spliceosome assembly by base-pairing to the 5' splice site. U1 snRNP also plays an additional, nonsplicing global function in 3' end mRNA processing; it actively suppresses the polyadenylation machinery from using early, mostly intronic polyadenylation signals which would lead to aberrant, truncated mRNAs. Thus, U1 snRNP safeguards pre-mRNA transcripts against premature polyadenylation and contributes to the regulation of alternative polyadenylation. Here, we review the role of U1 snRNP in 3' end mRNA processing, outline the evidence that led to the recognition of its physiological, general role in inhibiting polyadenylation, and finally highlight the possibility of manipulating this U1 snRNP function for therapeutic purposes in cancer.

Competition within Introns: Splicing Wins over Polyadenylation via a General Mechanism

Most eukaryotic messenger RNAs are capped, spliced, and polyadenylated via co-transcriptional processes that are coupled to each other and to the transcription machinery. Coordination of these processes ensures correct RNA maturation and provides for the diversity of the transcribed isoforms. Thus, RNA processing is a chain of events in which the completion of one event is coupled to the initiation of the next one. In this context, the relationship between splicing and polyadenylation is an important aspect of gene regulation. We have found that cryptic polyadenylation signals are widely distributed over the intron sequences of Drosophila melanogaster. As shown by analyzing the distribution of genes arranged in a nested pattern, where one gene is fully located within an intron of another gene, overlapping of putative polyadenylation signals is a fairly common event affecting about 17% of all genes. Here we show that polyadenylation signals are silenced within introns: the poly(A) signal is utilized in the exonic but not in the intronic regions of the transcript. The transcription does not end within the introns, either in a transient reporter system or in the genomic context, while deletion of the 5'-splice site restores their functionality. According to a full Drosophila transcriptome analysis, utilization of intronic polyadenylation signals occurs very rarely and such events are likely to be inducible. These results confirm that the transcription apparatus ignores premature polyadenylation signals for as long as they are intronic.

Chemical synthesis of U1 snRNA derivatives

U1 snRNA is an interesting biological tool for splicing correction and regulation of gene expression. However, U1 snRNA has never been chemically synthesized. In this study, the first chemical synthesis of U1snRNA and its analogues was carried out. Moreover, it was found that the binding affinity of the modified U1 snRNA with an ethylene glycol linkage to snurportin 1 (nuclear import adaptor) was as high as that of the unmodified RNA.

A simple model to explain evolutionary trends of eukaryotic gene architecture and expression: how competition between splicing and cleavage/polyadenylation factors may affect gene expression and splice-site recognition in eukaryotes

Enormous phylogenetic variation exists in the number and sizes of introns in protein-coding genes. Although some consideration has been given to the underlying role of the population-genetic environment in defining such patterns, the influence of the intracellular environment remains virtually unexplored. Drawing from observations on interactions between co-transcriptional processes involved in splicing and mRNA 3'-end formation, a mechanistic model is proposed for splice-site recognition that challenges the commonly accepted intron- and exon-definition models. Under the suggested model, splicing factors that outcompete 3'-end processing factors for access to intronic binding sites concurrently favor the recruitment of 3'-end processing factors at the pre-mRNA tail. This hypothesis sheds new light on observations such as the intron-mediated enhancement of gene expression and the negative correlation between intron length and levels of gene expression.

hnRNP L and hnRNP A1 induce extended U1 snRNA interactions with an exon to repress spliceosome assembly

Pre-mRNA splicing is catalyzed through the activity of the spliceosome, a dynamic enzymatic complex. Forcing aberrant interactions within the spliceosome can reduce splicing efficiency and alter splice site choice; however, it is unknown whether such alterations are naturally exploited mechanisms of splicing regulation. Here, we demonstrate that hnRNP L represses CD45 exon 4 by recruiting hnRNP A1 to a sequence upstream of the 5' splice site. Together, hnRNP L and A1 induce extended contacts between the 5' splice site-bound U1 snRNA and neighboring exonic sequences that, in turn, inhibit stable association of U6 snRNA and subsequent catalysis. Importantly, analysis of several exons regulated by hnRNP L shows a clear relationship between the potential for binding of hnRNP A1 and U1 snRNA and the effect of hnRNP L on splicing. Together, our results demonstrate that conformational perturbations within the spliceosome are a naturally occurring and generalizable mechanism for controlling alternative splicing decisions.

Dual suppression of hemangiogenesis and lymphangiogenesis by splice-shifting morpholinos targeting vascular endothelial growth factor receptor 2 (KDR)

The KDR gene, which participates in angiogenesis and lymphangiogenesis, produces two functionally distinct protein products, membrane-bound KDR (mbKDR) and its isoform, soluble KDR (sKDR). Since sKDR does not have a tyrosine kinase domain and does not dimerize, it is principally an antagonist of lymphangiogenesis by sequestering VEGF-C. Alternative polyadenylation of exon 30 or intron 13 leads to the production of mbKDR or sKDR, respectively, yet the regulatory mechanisms are unknown. Here we show that an antisense morpholino oligomer directed against the exon 13-intron 13 junction increases sKDR (suppressing lymphangiogenesis) and decreases mbKDR (inhibiting hemangiogenesis). The latent polyadenylation site in intron 13 of KDR is activated by blocking the upstream 5' splicing site with an antisense morpholino oligomer. Intravitreal morpholino injection suppressed laser choroidal neovascularization while increasing sKDR. In the mouse cornea, subconjunctival injection of the morpholino-inhibited corneal angiogenesis and lymphangiogenesis, and suppressed graft rejection after transplantation. Thus, this morpholino can be used for concurrent suppression of hemangiogenesis and lymphangiogenesis. This study offers new insight into the mechanisms and potential therapeutic modulation of alternative polyadenylation.

The increasing functional repertoire of U1 snRNA

Splicing is a key process for mRNA maturation, particularly in higher eukaryotes where most protein-coding transcripts contain multiple introns. It is achieved by the concerted action of five snRNAs (small nuclear RNAs) and hundreds of accessory proteins that form the spliceosome. Although snRNAs are present in equal amounts in the spliceosome, there is an overall excess of U1 in human cells. This finding led to the opinion that U1 might be involved in processes other than splicing. Research has shown that this is indeed the case and some examples found from studies in human cell systems are described briefly in the present review.

Increased in vivo inhibition of gene expression by combining RNA interference and U1 inhibition

Inhibition of gene expression can be achieved with RNA interference (RNAi) or U1 small nuclear RNA—snRNA—interference (U1i). U1i is based on U1 inhibitors (U1in), U1 snRNA molecules modified to inhibit polyadenylation of a target pre-mRNA. In culture, we have shown that the combination of RNAi and U1i results in stronger inhibition of reporter or endogenous genes than that obtained using either of the techniques alone. We have now used these techniques to inhibit gene expression in mice. We show that U1ins can induce strong inhibition of the expression of target genes in vivo. Furthermore, combining U1i and RNAi results in synergistic inhibitions also in mice. This is shown for the inhibition of hepatitis B virus (HBV) sequences or endogenous Notch1. Surprisingly, inhibition obtained by combining a U1in and a RNAi mediator is higher than that obtained by combining two U1ins or two RNAi mediators. Our results suggest that RNAi and U1i cooperate by unknown mechanisms to result in synergistic inhibitions. Analysis of toxicity and specificity indicates that expression of U1i inhibitors is safe. Therefore, we believe that the combination of RNAi and U1i will be a good option to block damaging endogenous genes, HBV and other infectious agents in vivo.

Nucleophosmin deposition during mRNA 3' end processing influences poly(A) tail length

During polyadenylation, the multi-functional protein nucleophosmin (NPM1) is deposited onto all cellular mRNAs analysed to date. Premature termination of poly(A) tail synthesis in the presence of cordycepin abrogates deposition of the protein onto the mRNA, indicating natural termination of poly(A) addition is required for NPM1 binding. NPM1 appears to be a bona fide member of the complex involved in 3' end processing as it is associated with the AAUAAA-binding CPSF factor and can be co-immunoprecipitated with other polyadenylation factors. Furthermore, reduction in the levels of NPM1 results in hyperadenylation of mRNAs, consistent with alterations in poly(A) tail chain termination. Finally, knockdown of NPM1 results in retention of poly(A)(+) RNAs in the cell nucleus, indicating that NPM1 influences mRNA export. Collectively, these data suggest that NPM1 has an important role in poly(A) tail length determination and may help network 3' end processing with other aspects of nuclear mRNA maturation.

AAV-mediated in vivo knockdown of luciferase using combinatorial RNAi and U1i

RNA interference (RNAi) has been successfully employed for specific inhibition of gene expression; however, safety and delivery of RNAi remain critical issues. We investigated the combinatorial use of RNAi and U1 interference (U1i). U1i is a gene-silencing technique that acts on the pre-mRNA by preventing polyadenylation. RNAi and U1i have distinct mechanisms of action in different cellular compartments and their combined effect allows usage of minimal doses, thereby avoiding toxicity while retaining high target inhibition. As a proof of concept, we investigated knockdown of the firefly luciferase reporter gene by combinatorial use of RNAi and U1i, and evaluated their inhibitory potential both in vitro and in vivo. Co-transfection of RNAi and U1i constructs showed additive reduction of luciferase expression up to 95% in vitro. We attained similar knockdown when RNAi and U1i constructs were hydrodynamically transfected into murine liver, demonstrating for the first time successful in vivo application of U1i. Moreover, we demonstrated long-term gene silencing by AAV-mediated transduction of murine muscle with RNAi/U1i constructs targeting firefly luciferase. In conclusion, these results provide a proof of principle for the combinatorial use of RNAi and U1i to enhance target gene knockdown in vivo.

U1 adaptors result in reduction of multiple pre-mRNA species principally by sequestering U1snRNP

U1 Adaptors are a recently reported novel approach for targeted reduction of mRNA transcripts. A U1 adaptor oligonucleotide comprising of a target-complimentary hybridization domain and a U1 recruitment domain, directs the U1 snRNP complex to the terminal exon of a targeted gene, subsequently inhibiting poly(A) tail addition and leading to degradation of that RNA species within the nucleus. Here, we present data demonstrating U1 adapter-mediated gene silencing can result in significant ‘off-target’ silencing effects as demonstrated by the reduction of multiple mRNA species that were not intended to be targeted. Our data suggest that a substantial portion of this U1 adaptor-mediated off-target mRNA reduction is the result of sequestration U1 snRNP at levels sufficient to affect splicing and processing of non-target transcripts.

Toward a durable treatment of HIV-1 infection using RNA interference

RNA interference (RNAi) is a cellular mechanism that mediates sequence-specific gene silencing at the posttranscriptional level. RNAi can be used as an antiviral approach against human pathogens. An attractive target for RNAi therapeutics is the human immunodeficiency virus type 1 (HIV-1), and the first clinical trial using a lentiviral gene therapy was initiated in early 2008. In this chapter, we focus on some basic principles of such an RNAi-based gene therapy against HIV-1. This includes the subjects of target site selection within the viral RNA genome, the phenomenon of viral escape, and therapeutic strategies to prevent viral escape. The latter antiescape strategies include diverse combinatorial RNAi approaches that are all directed against the HIV-1 RNA genome. As an alternative strategy, we also discuss the possibilities and restrictions of targeting cellular cofactors that are essential for virus replication, but less important for cell physiology.

Combination of RNA interference and U1 inhibition leads to increased inhibition of gene expression

RNA interference (RNAi) has been revolutionary for the specific inhibition of gene expression. However, the application of RNAi has been hampered by the fact that many siRNAs induce dose-dependent unwanted secondary effects. Therefore, new methods to increase inhibition of gene expression with low doses of inhibitors are required. We have tested the combination of RNAi and U1i (U1 small nuclear RNA—snRNA—interference). U1i is based on U1 inhibitors (U1in), U1 snRNA molecules modified to target a pre-mRNA and inhibit its gene expression by blocking nuclear polyadenylation. The combination of RNAi and U1i resulted in stronger inhibition of reporter or endogenous genes than that obtained using either of the techniques alone. The increased inhibition observed is stable over time and allows higher inhibition than the best obtained with either of the inhibitors alone even with decreased doses of the inhibitors. We believe that the combination of RNAi and U1i will be of interest when higher inhibition is required or when potent inhibitors are not available. Also, the combination of these techniques would allow functional inhibition with a decreased dose of inhibitors, avoiding toxicity due to dose-dependent unwanted effects.

The intronic splicing code: multiple factors involved in ATM pseudoexon definition

Abundance of pseudo splice sites in introns can potentially give rise to innumerable pseudoexons, outnumbering the real ones. Nonetheless, these are efficiently ignored by the splicing machinery, a process yet to be understood completely. Although numerous 5' splice site-like sequences functioning as splicing silencers have been found to be enriched in predicted human pseudoexons, the lack of active pseudoexons pose a fundamental challenge to how these U1snRNP-binding sites function in splicing inhibition. Here, we address this issue by focusing on a previously described pathological ATM pseudoexon whose inhibition is mediated by U1snRNP binding at intronic splicing processing element (ISPE), composed of a consensus donor splice site. Spliceosomal complex assembly demonstrates inefficient A complex formation when ISPE is intact, implying U1snRNP-mediated unproductive U2snRNP recruitment. Furthermore, interaction of SF2/ASF with its motif seems to be dependent on RNA structure and U1snRNP interaction. Our results suggest a complex combinatorial interplay of RNA structure and trans-acting factors in determining the splicing outcome and contribute to understanding the intronic splicing code for the ATM pseudoexon.

Next-generation SELEX identifies sequence and structural determinants of splicing factor binding in human pre-mRNA sequence

Many splicing factors interact with both mRNA and pre-mRNA. The identification of these interactions has been greatly improved by the development of in vivo cross-linking immunoprecipitation. However, the output carries a strong sampling bias in favor of RNPs that form on more abundant RNA species like mRNA. We have developed a novel in vitro approach for surveying binding on pre-mRNA, without cross-linking or sampling bias. Briefly, this approach entails specifically designed oligonucleotide pools that tile through a pre-mRNA sequence. The pool is then partitioned into bound and unbound fractions, which are quantified by a two-color microarray. We applied this approach to locating splicing factor binding sites in and around approximately 4000 exons. We also quantified the effect of secondary structure on binding. The method is validated by the finding that U1snRNP binds at the 5' splice site (5'ss) with a specificity that is nearly identical to the splice donor motif. In agreement with prior reports, we also show that U1snRNP appears to have some affinity for intronic G triplets that are proximal to the 5'ss. Both U1snRNP and the polypyrimidine tract binding protein (PTB) avoid exonic binding, and the PTB binding map shows increased enrichment at the polypyrimidine tract. For PTB, we confirm polypyrimidine specificity and are also able to identify structural determinants of PTB binding. We detect multiple binding motifs enriched in the PTB bound fraction of oligonucleotides. These motif combinations augment binding in vitro and are also enriched in the vicinity of exons that have been determined to be in vivo targets of PTB.

Adenovirus VA RNA-derived miRNAs target cellular genes involved in cell growth, gene expression and DNA repair

Adenovirus virus-associated (VA) RNAs are processed to functional viral miRNAs or mivaRNAs. mivaRNAs are important for virus production, suggesting that they may target cellular or viral genes that affect the virus cell cycle. To look for cellular targets of mivaRNAs, we first identified genes downregulated in the presence of VA RNAs by microarray analysis. These genes were then screened for mivaRNA target sites using several bioinformatic tools. The combination of microarray analysis and bioinformatics allowed us to select the splicing and translation regulator TIA-1 as a putative mivaRNA target. We show that TIA-1 is downregulated at mRNA and protein levels in infected cells expressing functional mivaRNAs and in transfected cells that express mivaRNAI-138, one of the most abundant adenoviral miRNAs. Also, reporter assays show that TIA-1 is downregulated directly by mivaRNAI-138. To determine whether mivaRNAs could target other cellular genes we analyzed 50 additional putative targets. Thirty of them were downregulated in infected or transfected cells expressing mivaRNAs. Some of these genes are important for cell growth, transcription, RNA metabolism and DNA repair. We believe that a mivaRNA-mediated fine tune of the expression of some of these genes could be important in adenovirus cell cycle.

RNA-interference-based Gene Therapy Approaches to HIV Type-1 Treatment: Tackling the Hurdles from Bench to Bedside

RNA interference (RNAi) is a cellular mechanism that can be induced by small interfering RNAs (siRNAs) to mediate sequence-specific gene silencing by cleavage of the targeted messenger RNA. RNAi can be used as an antiviral approach to silence HIV type-1 (HIV-1) through stable expression of precursors, such as short hairpin RNAs (shRNAs), which are processed into siRNAs that can elicit degradation of HIV-1 RNAs. At the beginning of 2008, the first clinical trial using a lentivirus with an RNA-based gene therapy against HIV-1 was initiated. The antiviral molecules in this gene therapy consist of three RNA effectors, one of which triggers the RNAi pathway. This review article focuses on the basic principles of an RNAi-based gene therapy against HIV-1, including delivery methods, target selection, viral escape possibilities, systems for multiplexing siRNAs to achieve a durable therapy and the in vitro and in vivo test systems to evaluate the efficacy and safety of such a therapy.

Gene silencing by synthetic U1 adaptors

We describe a gene silencing method that employs a mechanism of action distinct from those of antisense and RNA interference. U1 Adaptors are bifunctional oligonucleotides with a 'target domain' complementary to a site in the target gene's terminal exon and a 'U1 domain' that binds to the U1 small nuclear RNA component of the U1 small nuclear ribonucleoprotein (U1 snRNP) splicing factor. Tethering of U1 snRNP to the target pre-mRNA inhibits poly(A)-tail addition, causing degradation of that RNA species in the nucleus. U1 Adaptors can inhibit both endogenous and reporter genes in a sequence-specific manner. Comparison of U1 Adaptors with small interfering RNA (siRNA) using a genome-wide microarray analysis indicates that U1 Adaptors have limited off-target effects and no detectable adverse effects on splicing. Further, targeting the same gene either with multiple U1 Adaptors or with a U1 Adaptor and siRNA strongly enhances gene silencing.

Where do introns come from?

The systems for mRNA surveillance, capping, and cleavage/polyadenylation are proposed to play pivotal roles in the physical establishment and distribution of spliceosomal introns along a transcript.