Native spliceosomes assemble with pre-mRNA to form supraspliceosomes

A novel role for nucleolin in splice site selection

Latent 5' splice sites, not normally used, are highly abundant in human introns, but are activated under stress and in cancer, generating thousands of nonsense mRNAs. A previously proposed mechanism to suppress latent splicing was shown to be independent of NMD, with a pivotal role for initiator-tRNA independent of protein translation. To further elucidate this mechanism, we searched for nuclear proteins directly bound to initiator-tRNA. Starting with UV-crosslinking, we identified nucleolin (NCL) interacting directly and specifically with initiator-tRNA in the nucleus, but not in the cytoplasm. Next, we show the association of ini-tRNA and NCL with pre-mRNA. We further show that recovery of suppression of latent splicing by initiator-tRNA complementation is NCL dependent. Finally, upon nucleolin knockdown we show activation of latent splicing in hundreds of coding transcripts having important cellular functions. We thus propose nucleolin, a component of the endogenous spliceosome, through its direct binding to initiator-tRNA and its effect on latent splicing, as the first protein of a nuclear quality control mechanism regulating splice site selection to protect cells from latent splicing that can generate defective mRNAs.

Biology of the mRNA Splicing Machinery and Its Dysregulation in Cancer Providing Therapeutic Opportunities

Dysregulation of messenger RNA (mRNA) processing—in particular mRNA splicing—is a hallmark of cancer. Compared to normal cells, cancer cells frequently present aberrant mRNA splicing, which promotes cancer progression and treatment resistance. This hallmark provides opportunities for developing new targeted cancer treatments. Splicing of precursor mRNA into mature mRNA is executed by a dynamic complex of proteins and small RNAs called the spliceosome. Spliceosomes are part of the supraspliceosome, a macromolecular structure where all co-transcriptional mRNA processing activities in the cell nucleus are coordinated. Here we review the biology of the mRNA splicing machinery in the context of other mRNA processing activities in the supraspliceosome and present current knowledge of its dysregulation in lung cancer. In addition, we review investigations to discover therapeutic targets in the spliceosome and give an overview of inhibitors and modulators of the mRNA splicing process identified so far. Together, this provides insight into the value of targeting the spliceosome as a possible new treatment for lung cancer.

Spliceosome-Associated microRNAs Signify Breast Cancer Cells and Portray Potential Novel Nuclear Targets

MicroRNAs (miRNAs) act as negative regulators of gene expression in the cytoplasm. Previous studies have identified the presence of miRNAs in the nucleus. Here we study human breast cancer-derived cell-lines (MCF-7 and MDA-MB-231) and a non-tumorigenic cell-line (MCF-10A) and compare their miRNA sequences at the spliceosome fraction (SF). We report that the levels of miRNAs found in the spliceosome, their identity, and pre-miRNA segmental composition are cell-line specific. One such miRNA is miR-7704 whose genomic position overlaps HAGLR, a cancer-related lncRNA. We detected an inverse expression of miR-7704 and HAGLR in the tested cell lines. Specifically, inhibition of miR-7704 caused an increase in HAGLR expression. Furthermore, elevated levels of miR-7704 slightly altered the cell-cycle in MDA-MB-231. Altogether, we show that SF-miR-7704 acts as a tumor-suppressor gene with HAGLR being its nuclear target. The relative levels of miRNAs found in the spliceosome fractions (e.g., miR-100, miR-30a, and let-7 family) in non-tumorigenic relative to cancer-derived cell-lines was monitored. We found that the expression trend of the abundant miRNAs in SF was different from that reported in the literature and from the observation of large cohorts of breast cancer patients, suggesting that many SF-miRNAs act on targets that are different from the cytoplasmic ones. Altogether, we report on the potential of SF-miRNAs as an unexplored route for cancerous cell state.

Dynamic Supraspliceosomes Are Assembled on Different Transcripts Regardless of Their Intron Number and Splicing State

Splicing and alternative splicing of pre-mRNA are key sources in the formation of diversity in the human proteome. These processes have a central role in the regulation of the gene expression pathway. Yet, how spliceosomes are assembled on a multi-intronic pre-mRNA is at present not well understood. To study the spliceosomes assembled in vivo on transcripts with variable number of introns, we examined a series of three related transcripts derived from the β-globin gene, where two transcript types contained increasing number of introns, while one had only an exon. Each transcript had multiple MS2 sequence repeats that can be bound by the MS2 coat protein. Using our protocol for isolation of endogenous spliceosomes under native conditions from cell nuclei, we show that all three transcripts are found in supraspliceosomes – 21 MDa dynamic complexes, sedimenting at 200S in glycerol gradients, and composed of four native spliceosomes connected by the transcript. Affinity purification of complexes assembled on the transcript with most introns (termed E6), using the MS2 tag, confirmed the assembly of E6 in supraspliceosomes with components such as Sm proteins and PSF. Furthermore, splicing inhibition by spliceostatin A did not inhibit the assembly of supraspliceosomes on the E6 transcript, yet increased the percentage of E6 pre-mRNA supraspliceosomes. These findings were corroborated in intact cells, using RNA FISH to detect the MS2-tagged E6 mRNA, together with GFP-tagged splicing factors, showing the assembly of splicing factors SRSF2, U1-70K, and PRP8 onto the E6 transcripts under normal conditions and also when splicing was inhibited. This study shows that different transcripts with different number of introns, or lacking an intron, are assembled in supraspliceosomes even when splicing is inhibited. This assembly starts at the site of transcription and can continue during the life of the transcript in the nucleoplasm. This study further confirms the dynamic and universal nature of supraspliceosomes that package RNA polymerase II transcribed pre-mRNAs into complexes composed of four native spliceosomes connected by the transcript, independent of their length, number of introns, or splicing state.

Splicing Players Are Differently Expressed in Sporadic Amyotrophic Lateral Sclerosis Molecular Clusters and Brain Regions

Splicing is a tightly orchestrated process by which the brain produces protein diversity over time and space. While this process specializes and diversifies neurons, its deregulation may be responsible for their selective degeneration. In amyotrophic lateral sclerosis (ALS), splicing defects have been investigated at the singular gene level without considering the higher-order level, involving the entire splicing machinery. In this study, we analyzed the complete spectrum (396) of genes encoding splicing factors in the motor cortex (41) and spinal cord (40) samples from control and sporadic ALS (SALS) patients. A substantial number of genes (184) displayed significant expression changes in tissue types or disease states, were implicated in distinct splicing complexes and showed different topological hierarchical roles based on protein–protein interactions. The deregulation of one of these splicing factors has a central topological role, i.e., the transcription factor YBX1, which might also have an impact on stress granule formation, a pathological marker associated with ALS.

Small non-coding RNA within the endogenous spliceosome and alternative splicing regulation

Splicing and alternative splicing (AS), which occur in the endogenous spliceosome, play major roles in regulating gene expression, and defects in them are involved in numerous human diseases including cancer. Although the mechanism of the splicing reaction is well understood, the regulation of AS remains to be elucidated. A group of essential regulatory factors in gene expression are small non-coding RNAs (sncRNA): e.g. microRNA, mainly known for their inhibitory role in translation in the cytoplasm; and small nucleolar RNA, known for their role in methylating non-coding RNA in the nucleolus. Here I highlight a new aspect of sncRNAs found within the endogenous spliceosome. Assembled in non-canonical complexes and through different base pairing than their canonical ones, spliceosomal sncRNAs can potentially target different RNAs. Examples of spliceosomal sncRNAs regulating AS, regulating gene expression, and acting in a quality control of AS are reviewed, suggesting novel functions for spliceosomal sncRNAs. This article is part of a Special Issue entitled: RNA structure and splicing regulation edited by Francisco Baralle, Ravindra Singh and Stefan Stamm.

Small RNA sequences derived from pre-microRNAs in the supraspliceosome

MicroRNAs (miRNAs) are short non-coding RNAs that negatively regulate the expression and translation of genes in healthy and diseased tissues. Herein, we characterize short RNAs from human HeLa cells found in the supraspliceosome, a nuclear dynamic machine in which pre-mRNA processing occurs. We sequenced small RNAs (<200 nt) extracted from the supraspliceosome, and identified sequences that are derived from 200 miRNAs genes. About three quarters of them are mature miRNAs, whereas the rest account for various defined regions of the pre-miRNA, and its hairpin-loop precursor. Out of these aligned sequences, 53 were undetected in cellular extract, and the abundance of additional 48 strongly differed from that in cellular extract. Notably, we describe seven abundant miRNA-derived sequences that overlap non-coding exons of their host gene. The rich collection of sequences identical to pre-miRNAs at the supraspliceosome suggests overlooked nuclear functions. Specifically, the abundant hsa-mir-99b may affect splicing of LINC01129 primary transcript through base-pairing with its exon-intron junction. Using suppression and overexpression experiments, we show that hsa-mir-7704 negatively regulates the level of the lncRNA HAGLR. We claim that in cases of extended base-pairing complementarity, such supraspliceosomal pre-miRNA sequences might have a role in transcription attenuation, maturation and processing.

Structural studies of the endogenous spliceosome - The supraspliceosome

Pre-mRNA splicing is executed in mammalian cell nuclei within a huge (21MDa) and highly dynamic molecular machine - the supraspliceosome - that individually package pre-mRNA transcripts of different sizes and number of introns into complexes of a unique structure, indicating their universal nature. Detailed structural analysis of this huge and complex structure requires a stepwise approach using hybrid methods. Structural studies of the supraspliceosome by room temperature electron tomography, cryo-electron tomography, and scanning transmission electron microscope mass measurements revealed that it is composed of four native spliceosomes, each resembling an in vitro assembled spliceosome, which are connected by the pre-mRNA. It also elucidated the arrangement of the native spliceosomes within the intact supraspliceosome. Native spliceosomes and supraspliceosomes contain all five spliceosomal U snRNPs together with other splicing factors, and are active in splicing. The structure of the native spliceosome, at a resolution of 20Å, was determined by cryo-electron microscopy, and a unique spatial arrangement of the spliceosomal U snRNPs within the native spliceosome emerged from in silico studies. The supraspliceosome also harbor components for all pre-mRNA processing activities. Thus the supraspliceosome - the endogenous spliceosome - is a stand-alone complete macromolecular machine capable of performing splicing, alternative splicing, and encompass all nuclear pre-mRNA processing activities that the pre-mRNA has to undergo before it can exit from the nucleus to the cytoplasm to encode for protein. Further high-resolution cryo-electron microscopy studies of the endogenous spliceosome are required to decipher the regulation of alternative splicing, and elucidate the network of processing activities within it.

Mechanistic Insights Into Catalytic RNA-Protein Complexes Involved in Translation of the Genetic Code

The contemporary world is an "RNA-protein world" rather than a "protein world" and tracing its evolutionary origins is of great interest and importance. The different RNAs that function in close collaboration with proteins are involved in several key physiological processes, including catalysis. Ribosome-the complex megadalton cellular machinery that translates genetic information encoded in nucleotide sequence to amino acid sequence-epitomizes such an association between RNA and protein. RNAs that can catalyze biochemical reactions are known as ribozymes. They usually employ general acid-base catalytic mechanism, often involving the 2'-OH of RNA that activates and/or stabilizes a nucleophile during the reaction pathway. The protein component of such RNA-protein complexes (RNPCs) mostly serves as a scaffold which provides an environment conducive for the RNA to function, or as a mediator for other interacting partners. In this review, we describe those RNPCs that are involved at different stages of protein biosynthesis and in which RNA performs the catalytic function; the focus of the account is on highlighting mechanistic aspects of these complexes. We also provide a perspective on such associations in the context of proofreading during translation of the genetic code. The latter aspect is not much appreciated and recent works suggest that this is an avenue worth exploring, since an understanding of the subject can provide useful insights into how RNAs collaborate with proteins to ensure fidelity during these essential cellular processes. It may also aid in comprehending evolutionary aspects of such associations.

The nuts and bolts of the endogenous spliceosome

The complex life of pre-mRNA from transcription to the production of mRNA that can be exported from the nucleus to the cytoplasm to encode for proteins entails intricate coordination and regulation of a network of processing events. Coordination is required between transcription and splicing and between several processing events including 5' and 3' end processing, splicing, alternative splicing and editing that are major contributors to the diversity of the human proteome, and occur within a huge and dynamic macromolecular machine-the endogenous spliceosome. Detailed mechanistic insight of the splicing reaction was gained from studies of the in vitro spliceosome assembled on a single intron. Because most pre-mRNAs are multiintronic that undergo alternative splicing, the in vivo splicing machine requires additional elements to those of the in vitro machine, to account for all these diverse functions. Information about the endogenous spliceosome is emerging from imaging studies in intact and live cells that support the cotranscriptional commitment to splicing model and provide information about splicing kinetics in vivo. Another source comes from studies of the in vivo assembled spliceosome, isolated from cell nuclei under native conditions-the supraspliceosome-that individually package pre-mRNA transcripts of different sizes and number of introns into complexes of a unique structure, indicating their universal nature. Recent years have portrayed new players affecting alternative splicing and novel connections between splicing, transcription and chromatin. The challenge ahead is to elucidate the structure and function of the endogenous spliceosome and decipher the regulation and coordination of its network of processing activities. WIREs RNA 2017, 8:e1377. doi: 10.1002/wrna.1377 For further resources related to this article, please visit the WIREs website.

Dual function of C/D box small nucleolar RNAs in rRNA modification and alternative pre-mRNA splicing

C/D box small nucleolar RNAs (SNORDs) are small noncoding RNAs, and their best-understood function is to target the methyltransferase fibrillarin to rRNA (for example, SNORD27 performs 2'-O-methylation of A27 in 18S rRNA). Unexpectedly, we found a subset of SNORDs, including SNORD27, in soluble nuclear extract made under native conditions, where fibrillarin was not detected, indicating that a fraction of the SNORD27 RNA likely forms a protein complex different from canonical snoRNAs found in the insoluble nuclear fraction. As part of this previously unidentified complex,SNORD27 regulates the alternative splicing of the transcription factor E2F7p re-mRNA through direct RNA-RNA interaction without methylating the RNA, likely by competing with U1 small nuclear ribonucleoprotein (snRNP). Furthermore, knockdown of SNORD27 activates previously "silent" exons in several other genes through base complementarity across the entire SNORD27 sequence, not just the antisense boxes. Thus, some SNORDs likely function in both rRNA and pre-mRNA processing, which increases the repertoire of splicing regulators and links both processes.

Connecting the dots: chromatin and alternative splicing in EMT

Nature has devised sophisticated cellular machinery to process mRNA transcripts produced by RNA Polymerase II, removing intronic regions and connecting exons together, to produce mature RNAs. This process, known as splicing, is very closely linked to transcription. Alternative splicing, or the ability to produce different combinations of exons that are spliced together from the same genomic template, is a fundamental means of regulating protein complexity. Similar to transcription, both constitutive and alternative splicing can be regulated by chromatin and its associated factors in response to various signal transduction pathways activated by external stimuli. This regulation can vary between different cell types, and interference with these pathways can lead to changes in splicing, often resulting in aberrant cellular states and disease. The epithelial to mesenchymal transition (EMT), which leads to cancer metastasis, is influenced by alternative splicing events of chromatin remodelers and epigenetic factors such as DNA methylation and non-coding RNAs. In this review, we will discuss the role of epigenetic factors including chromatin, chromatin remodelers, DNA methyltransferases, and microRNAs in the context of alternative splicing, and discuss their potential involvement in alternative splicing during the EMT process.

Quantification of pre-mRNA escape rate and synergy in splicing

Splicing reactions generally combine high speed with accuracy. However, some of the pre-mRNAs escape the nucleus with a retained intron. Intron retention can control gene expression and increase proteome diversity. We calculated the escape rate for the yeast PTC7 intron and pre-mRNA. This prediction was facilitated by the observation that splicing is a linear process and by deriving simple algebraic expressions from a model of co- and post-transcriptional splicing and RNA surveillance that determines the rate of the nonsense-mediated decay (NMD) of the pre-mRNAs with the retained intron. The escape rate was consistent with the observed threshold of splicing rate below which the mature mRNA level declined. When an mRNA contains multiple introns, the outcome of splicing becomes more difficult to predict since not only the escape rate of the pre-mRNA has to be considered, but also the possibility that the splicing of each intron is influenced by the others. We showed that the two adjacent introns in the SUS1 mRNA are spliced cooperatively, but this does not counteract the escape of the partially spliced mRNA. These findings will help to infer promoter activity and to predict the behavior of and to control splicing regulatory networks.

The Supraspliceosome - A Multi-Task Machine for Regulated Pre-mRNA Processing in the Cell Nucleus

Pre-mRNA splicing of Pol II transcripts is executed in the mammalian cell nucleus within a huge (21 MDa) and highly dynamic RNP machine — the supraspliceosome. It is composed of four splicing active native spliceosomes, each resembling an in vitro assembled spliceosome, which are connected by the pre-mRNA. Supraspliceosomes harbor protein splicing factors and all the five-spliceosomal U snRNPs. Recent analysis of specific supraspliceosomes at defined splicing stages revealed that they harbor all five spliceosomal U snRNAs at all splicing stages. Supraspliceosomes harbor additional pre-mRNA processing components, such as the 5′-end and 3′-end processing components, and the RNA editing enzymes ADAR1 and ADAR2. The structure of the native spliceosome, at a resolution of 20 Å, was determined by cryo-EM. A unique spatial arrangement of the spliceosomal U snRNPs within the native spliceosome emerged from in-silico studies, localizing the five U snRNPs mostly within its large subunit, and sheltering the active core components deep within the spliceosomal cavity. The supraspliceosome provides a platform for coordinating the numerous processing steps that the pre-mRNA undergoes: 5′ and 3′-end processing activities, RNA editing, constitutive and alternative splicing, and processing of intronic microRNAs. It also harbors a quality control mechanism termed suppression of splicing (SOS) that, under normal growth conditions, suppresses splicing at abundant intronic latent 5′ splice sites in a reading frame-dependent fashion. Notably, changes in these regulatory processing activities are associated with human disease and cancer. These findings emphasize the supraspliceosome as a multi-task master regulator of pre-mRNA processing in the cell nucleus.

Supraspliceosomes at defined functional states portray the pre-assembled nature of the pre-mRNA processing machine in the cell nucleus

When isolated from mammalian cell nuclei, all nuclear pre-mRNAs are packaged in multi-subunit large ribonucleoprotein complexes-supraspliceosomes-composed of four native spliceosomes interconnected by the pre-mRNA. Supraspliceosomes contain all five spliceosomal U snRNPs, together with other splicing factors, and are functional in splicing. Supraspliceosomes studied thus far represent the steady-state population of nuclear pre-mRNAs that were isolated at different stages of the splicing reaction. To analyze specific splicing complexes, here, we affinity purified Pseudomonas aeruginosa phage 7 (PP7)-tagged splicing complexes assembled in vivo on Adenovirus Major Late (AdML) transcripts at specific functional stages, and characterized them using molecular techniques including mass spectrometry. First, we show that these affinity purified splicing complexes assembled on PP7-tagged AdML mRNA or on PP7-tagged AdML pre-mRNA are assembled in supraspliceosomes. Second, similar to the general population of supraspliceosomes, these defined supraspliceosomes populations are assembled with all five U snRNPs at all splicing stages. This study shows that dynamic changes in base-pairing interactions of U snRNA:U snRNA and U snRNA:pre-mRNA that occur in vivo during the splicing reaction do not require changes in U snRNP composition of the supraspliceosome. Furthermore, there is no need to reassemble a native spliceosome for the splicing of each intron, and rearrangements of the interactions will suffice.

Interplay between pre-mRNA splicing and microRNA biogenesis within the supraspliceosome

MicroRNAs (miRNAs) are central regulators of gene expression, and a large fraction of them are encoded in introns of RNA polymerase II transcripts. Thus, the biogenesis of intronic miRNAs by the microprocessor and the splicing of their host introns by the spliceosome require coordination between these processing events. This cross-talk is addressed here. We show that key microprocessor proteins Drosha and DGCR8 as well as pre-miRNAs cosediment with supraspliceosomes, where nuclear posttranscriptional processing is executed. We further show that inhibition of splicing increases miRNAs expression, whereas knock-down of Drosha increases splicing. We identified a novel splicing event in intron 13 of MCM7, where the miR-106b-25 cluster is located. The unique splice isoform includes a hosted pre-miRNA in the extended exon and excludes its processing. This indicates a possible mechanism of altering the levels of different miRNAs originating from the same transcript. Altogether, our study indicates interplay between the splicing and microprocessor machineries within a supraspliceosome context.

Quantifying the ratio of spliceosome components assembled on pre-mRNA

RNA processing by the splicing machinery removes intronic sequences from pre-mRNA to generate mature mRNA transcripts. Many splicing events occur co-transcriptionally when the pre-mRNA is still associated with the transcription machinery. This mechanism raises questions regarding the number of spliceosomes associated with the pre-mRNA at a given time. In this protocol, we present a quantitative FISH approach that measures the ratio of intensities between two different spliceosomal components associated on a nascent mRNA, and compares to the number of introns in the mRNA, thereby calculating the number of spliceosome complexes assembled with each transcript.

Pyrvinium pamoate changes alternative splicing of the serotonin receptor 2C by influencing its RNA structure

The serotonin receptor 2C plays a central role in mood and appetite control. It undergoes pre-mRNA editing as well as alternative splicing. The RNA editing suggests that the pre-mRNA forms a stable secondary structure in vivo. To identify substances that promote alternative exons inclusion, we set up a high-throughput screen and identified pyrvinium pamoate as a drug-promoting exon inclusion without editing. Circular dichroism spectroscopy indicates that pyrvinium pamoate binds directly to the pre-mRNA and changes its structure. SHAPE (selective 2'-hydroxyl acylation analysed by primer extension) assays show that part of the regulated 5'-splice site forms intramolecular base pairs that are removed by this structural change, which likely allows splice site recognition and exon inclusion. Genome-wide analyses show that pyrvinium pamoate regulates >300 alternative exons that form secondary structures enriched in A-U base pairs. Our data demonstrate that alternative splicing of structured pre-mRNAs can be regulated by small molecules that directly bind to the RNA, which is reminiscent to an RNA riboswitch.

The 5' untranslated region of the serotonin receptor 2C pre-mRNA generates miRNAs and is expressed in non-neuronal cells

The serotonin receptor 2C (HTR2C) gene encodes a G protein-coupled receptor that is exclusively expressed in neurons. Here, we report that the 5′ untranslated region of the receptor pre-mRNA as well as its hosted miRNAs is widely expressed in non-neuronal cell lines. Alternative splicing of HTR2C is regulated by MBII-52. MBII-52 and the neighboring MBII-85 cluster are absent in people with Prader–Willi syndrome, which likely causes the disease. We show that MBII-52 and MBII-85 increase expression of the HTR2C 5′ UTR and influence expression of the hosted miRNAs. The data indicate that the transcriptional unit expressing HTR2C is more complex than previously recognized and likely deregulated in Prader–Willi syndrome.

A unique spatial arrangement of the snRNPs within the native spliceosome emerges from in silico studies

The spliceosome is a mega-Dalton ribonucleoprotein (RNP) assembly that processes primary RNA transcripts, producing functional mRNA. The electron microscopy structures of the native spliceosome and of several spliceosomal subcomplexes are available; however, the spatial arrangement of the latter within the native spliceosome is not known. We designed a computational procedure to efficiently fit thousands of conformers into the spliceosome envelope. Despite the low resolution limitations, we obtained only one model that complies with the available biochemical data. Our model localizes the five small nuclear RNPs (snRNPs) mostly within the large subunit of the native spliceosome, requiring only minor conformation changes. The remaining free volume presumably accommodates additional spliceosomal components. The constituents of the active core of the spliceosome are juxtaposed, forming a continuous surface deep within the large spliceosomal cavity, which provides a sheltered environment for the splicing reaction.

Regulation of alternative splicing within the supraspliceosome

Alternative splicing is a fundamental feature in regulating the eukaryotic transcriptome, as ~95% of multi-exon human Pol II transcripts are subject to this process. Regulated splicing operates through the combinatorial interplay of positive and negative regulatory signals present in the pre-mRNA, which are recognized by trans-acting factors. All these RNA and protein components are assembled in a gigantic, 21 MDa, ribonucleoprotein splicing machine - the supraspliceosome. Because most alternatively spliced mRNA isoforms vary between different cell and tissue types, the ability to perform alternative splicing is expected to be an integral part of the supraspliceosome, which constitutes the splicing machine in vivo. Here we show that both the constitutively and alternatively spliced mRNAs of the endogenous human pol II transcripts: hnRNP A/B, survival of motor neuron (SMN) and ADAR2 are predominantly found in supraspliceosomes. This finding is consistent with our observations that the splicing regulators hnRNP G as well as all phosphorylated SR proteins are predominantly associated with supraspliceosomes. We further show that changes in alternative splicing of hnRNP A/B, affected by up regulation of SRSF5 (SRp40) or by treatment with C6-ceramide, occur within supraspliceosomes. These observations support the proposed role of the supraspliceosome in splicing regulation and alternative splicing.

The differential interaction of snRNPs with pre-mRNA reveals splicing kinetics in living cells

GFP-tagged snRNP components reveal the dynamics and rate for spliceosome assembly in vivo.

Precursor messenger RNA (pre-mRNA) splicing is catalyzed by the spliceosome, a large ribonucleoprotein (RNP) complex composed of five small nuclear RNP particles (snRNPs) and additional proteins. Using live cell imaging of GFP-tagged snRNP components expressed at endogenous levels, we examined how the spliceosome assembles in vivo. A comprehensive analysis of snRNP dynamics in the cell nucleus enabled us to determine snRNP diffusion throughout the nucleoplasm as well as the interaction rates of individual snRNPs with pre-mRNA. Core components of the spliceosome, U2 and U5 snRNPs, associated with pre-mRNA for 15–30 s, indicating that splicing is accomplished within this time period. Additionally, binding of U1 and U4/U6 snRNPs with pre-mRNA occurred within seconds, indicating that the interaction of individual snRNPs with pre-mRNA is distinct. These results are consistent with the predictions of the step-wise model of spliceosome assembly and provide an estimate on the rate of splicing in human cells.

A potential role for initiator-tRNA in pre-mRNA splicing regulation

The translation initiator-tRNA plays a crucial role in the initiation of protein synthesis in both prokaryotic and eukaryotic cells, by employing specific base pairing between its anticodon triplet CAU and the general initiation codon AUG in the mRNA. Here we show that the initiator-tRNA may also act, in a manner that is independent of its role in protein translation, as a pre-mRNA splicing regulator. Specifically, we show that alternative splicing events that are induced by mutations in the translation initiation AUG codon can be suppressed by expressing initiator-tRNA constructs carrying anticodon mutations that compensate for the AUG mutations. These mutated initiator-tRNAs appeared to be uncharged with an amino acid. Our results imply that recognition of the initiation AUG sequence by the anticodon triplet of initiator-tRNA in its unloaded state plays a role in quality control of splicing in the cell nucleus by a yet unresolved mechanism. Identifying the initiator-tRNA as a transacting splicing regulator suggests a novel involvement of this molecule in splicing regulation and provides a critical step toward deciphering this intriguing mechanism.

A novel tissue-specific alternatively spliced form of the A-to-I RNA editing enzyme ADAR2

ADAR2, a member of the adenosine deaminase family of proteins, is the enzyme that edits the Q/R site in the GluR-B transcript, an important physiological A-to-I editing event. ADAR2 pre-mRNA undergoes a number of known alternative splicing events, affecting its function. Here we describe a novel alternatively spliced exon, located within intron 7 of the human gene, which we term "exon 7a". This alternatively spliced exon is highly conserved in the mammalian ADAR2 gene. It has stop codons in all three frames and is down regulated by NMD. We show that the level of exon 7a inclusion differs between different human tissues, with the highest levels of inclusion in skeletal muscle, heart and testis. In the brain, where the level of editing is known to be high, the level of exon 7a inclusion is low. The new alternative form was also found in supraspliceosomes, which constitute the nuclear pre-mRNA processing machine. The high conservation of the novel ADAR2 alternative exon in mammals indicates a physiological importance for this exon.

Heterogeneous nuclear ribonucleoprotein G regulates splice site selection by binding to CC(A/C)-rich regions in pre-mRNA

Almost every protein-coding gene undergoes pre-mRNA splicing, and the majority of these pre-mRNAs are alternatively spliced. Alternative exon usage is regulated by the transient formation of protein complexes on the pre-mRNA that typically contain heterogeneous nuclear ribonucleoproteins (hnRNPs). Here we characterize hnRNP G, a member of the hnRNP class of proteins. We show that hnRNP G is a nuclear protein that is expressed in different concentrations in various tissues and that interacts with other splicing regulatory proteins. hnRNP G is part of the supraspliceosome, where it regulates alternative splice site selection in a concentration-dependent manner. Its action on alternative exons can occur without a functional RNA-recognition motif by binding to other splicing regulatory proteins. The RNA-recognition motif of hnRNP G binds to a loose consensus sequence containing a CC(A/C) motif, and hnRNP G preferentially regulates alternative exons where this motif is clustered in close proximity. The X-chromosomally encoded hnRNP G regulates different RNAs than its Y-chromosomal paralogue RNA-binding motif protein, Y-linked (RBMY), suggesting that differences in alternative splicing, evoked by the sex-specific expression of hnRNP G and RBMY, could contribute to molecular sex differences in mammals.

Structure and function of the Pre-mRNA splicing machine

Most eukaryotic pre-mRNAs contain non-coding sequences (introns) that must be removed in order to accurately place the coding sequences (exons) in the correct reading frame. This critical regulatory pre-mRNA splicing event is fundamental in development and cancer. It occurs within a mega-Dalton multicomponent machine composed of RNA and proteins, which undergoes dynamic changes in RNA-RNA, RNA-protein, and protein-protein interactions during the splicing reaction. Recent years have seen progress in functional and structural analyses of the splicing machine and its subcomponents, and this review is focused on structural aspects of the pre-mRNA splicing machine and their mechanistic implications on the splicing of multi-intronic pre-mRNAs. It brings together, in a comparative manner, structural information on spliceosomes and their intermediates in the stepwise assembly process in vitro, and on the preformed supraspliceosomes, which are isolated from living cell nuclei, with a view of portraying a consistent picture.

The editing enzyme ADAR1 and the mRNA surveillance protein hUpf1 interact in the cell nucleus

Posttranscriptional regulation is an important step in the regulation of gene expression. In this article, we show an unexpected connection between two proteins that participate in different processes of posttranscriptional regulation that ensures the production of functional mRNA molecules. Specifically, we show that the A-to-I RNA editing protein adenosine deaminase that acts on RNA 1 (ADAR1) and the human Upf1 (hUpf1) protein involved in RNA surveillance are found associated within nuclear RNA-splicing complexes. A potential functional role for this association was revealed by RNAi-mediated down-regulation of ADAR1, which was accompanied by up-regulation of a number of genes previously shown to undergo A-to-I editing in Alu repeats and to be down-regulated by hUpf1. This study suggests a regulatory pathway by a combination of ADAR1 A-to-I editing enzyme and RNA degradation presumably with the aid of hUpf1.

Regulation of splicing by protein phosphorylation

Most eukaryotic messenger RNAs are transcribed as precursors that necessitate specific and exact processing of intron boundaries. Furthermore, the choice of these boundaries appears to be fluid and adaptive to the rate of transcription and the developmental and physiological state of the cell. A central regulator of splicing reactions and choice are kinases that work through phosphorylation of specific factors like RNA polymerase II, which influences the pace of transcription and of SR splicing factors. While very different in their mechanisms both regulatory pathways will impact on splicing site choice. This chapter summarizes the biology of splicing-related phosphorylation activity, emphasizing plant-specific aspects in relation to the metazoan counterpart.

Alternative RNA splicing regulation in the testis

Alternative splicing regulation has been shown to be critically important for several developmental pathways. It is particularly prevalent in the testis, which is the site of an extensive adult developmental programme. Alternative splicing is controlled by a splicing code, in which transcripts respond to subtle cell type-specific variations in positive and negative trans-acting RNA-binding proteins according to their unique set of binding sites for these proteins. Because of their unique combinations of cis-acting sequence elements, specific transcripts are able to respond individually to this code. In this review, we discuss how this code may be deciphered in germ cells to mediate a splicing response.

Proteomic analysis of in vivo-assembled pre-mRNA splicing complexes expands the catalog of participating factors

Previous compositional studies of pre-mRNA processing complexes have been performed in vitro on synthetic pre-mRNAs containing a single intron. To provide a more comprehensive list of polypeptides associated with the pre-mRNA splicing apparatus, we have determined the composition of the bulk pre-mRNA processing machinery in living cells. We purified endogenous nuclear pre-mRNA processing complexes from human and chicken cells comprising the massive (>200S) supraspliceosomes (a.k.a. polyspliceosomes). As expected, RNA components include a heterogeneous mixture of pre-mRNAs and the five spliceosomal snRNAs. In addition to known pre-mRNA splicing factors, 5′ end binding factors, 3′ end processing factors, mRNA export factors, hnRNPs and other RNA binding proteins, the protein components identified by mass spectrometry include RNA adenosine deaminases and several novel factors. Intriguingly, our purified supraspliceosomes also contain a number of structural proteins, nucleoporins, chromatin remodeling factors and several novel proteins that were absent from splicing complexes assembled in vitro. These in vivo analyses bring the total number of factors associated with pre-mRNA to well over 300, and represent the most comprehensive analysis of the pre-mRNA processing machinery to date.

Exploring the architecture of the intact supraspliceosome using electron microscopy

Splicing of pre-mRNA takes place on a massive macromolecular machine in the nucleus of eukaryotic cells, the supraspliceosome. This particle is a multicomponent biological complex of RNA and proteins. It is composed of four sub-structures termed native spliceosomes that splice pre-mRNA. The structure of the native spliceosome, determined by cryo-EM at 20 A resolution, showed that it is composed of two distinct subunits. Previously, medium resolution structural analysis of supraspliceosomes by electron tomography was performed, yet little is known of how the native spliceosomes are arranged within the intact particle. To address this question the native spliceosomes were analyzed and reconstructed in the context of the intact particle, using electron microscopy combined with image processing. Good correlation was obtained between the structure of the isolated native spliceosome, solved by cryo-EM, and the native spliceosome within the intact supraspliceosome. An ordered assembly was revealed with different potential roles assigned to the small and large subunits of the native spliceosome. The edges of the small subunits, which are in the center of the supraspliceosome, form a right angle and thus facilitate close contacts between the small subunits generating a 4-fold pattern. The analysis of sub-complex orientation within the particle suggests a possible route within the supraspliceosome for the passage of pre-mRNA, which is known to hold the particle together.

Alternative RNA splicing complexes containing the scaffold attachment factor SAFB2

The scaffold attachment factor SAFB1 and its recently discovered homologue SAFB2 might provide an important link between pre-mRNA splicing, intracellular signalling and transcription. Using novel mono-specific antisera, we found endogenous SAFB2 protein has a different spatial distribution from SAFB1 within the nucleus where it is found in much larger nuclear complexes (up to 670 kDa in size), and a distinct pattern of expression in adult human testis. By contrast, SAFB1 protein predominantly exists either as smaller complexes or as a monomeric protein. Our results suggest stable core complexes containing components comprised of SAFB1, SAFB2 and the RNA binding proteins Sam68 and hnRNPG exist in parallel with free SAFB1 protein. We found that SAFB2 protein, like SAFB1, acts as a negative regulator of a tra2β variable exon. Despite showing an involvement in splicing, we detected no stable interaction between SAFB proteins and SR or SR-related splicing regulators, although these were also found in stable higher molecular mass complexes. Each of the detected alternative splicing regulator complexes exists independently of intact nucleic acids, suggesting they might be pre-assembled and recruited to nascent transcripts as modules to facilitate alternative splicing, and/or they represent nuclear storage compartments from which active proteins are recruited.

Intranuclear binding kinetics and mobility of single native U1 snRNP particles in living cells

Uridine-rich small nuclear ribonucleoproteins (U snRNPs) are splicing factors, which are diffusely distributed in the nucleoplasm and also concentrated in nuclear speckles. Fluorescently labeled, native U1 snRNPs were microinjected into the cytoplasm of living HeLa cells. After nuclear import single U1 snRNPs could be visualized and tracked at a spatial precision of 30 nm at a frame rate of 200 Hz employing a custom-built microscope with single-molecule sensitivity. The single-particle tracks revealed that most U1 snRNPs were bound to specific intranuclear sites, many of those presumably representing pre-mRNA splicing sites. The dissociation kinetics from these sites showed a multiexponential decay behavior on time scales ranging from milliseconds to seconds, reflecting the involvement of U1 snRNPs in numerous distinct interactions. The average dwell times for U1 snRNPs bound at sites within the nucleoplasm did not differ significantly from those in speckles, indicating that similar processes occur in both compartments. Mobile U1 snRNPs moved with diffusion constants in the range from 0.5 to 8 microm2/s. These values were consistent with uncomplexed U1 snRNPs diffusing at a viscosity of 5 cPoise and U1 snRNPs moving in a largely restricted manner, and U1 snRNPs contained in large supramolecular assemblies such as spliceosomes or supraspliceosomes.