An Independently folding RNA G-quadruplex domain directly recruits the 40S ribosomal subunit

5'UTR G-quadruplex structure enhances translation in size dependent manner

Translation initiation in bacteria is frequently regulated by various structures in the 5' untranslated region (5'UTR). Previously, we demonstrated that G-quadruplex (G4) formation in non-template DNA enhances transcription. In this study, we aim to explore how G4 formation in mRNA (RG4) at 5'UTR impacts translation using a T7-based in vitro translation system and in E. coli. We show that RG4 strongly promotes translation efficiency in a size-dependent manner. Additionally, inserting a hairpin upstream of the RG4 further enhances translation efficiency, reaching up to a 12-fold increase. We find that the RG4-dependent effect is not due to increased ribosome affinity, ribosome binding site accessibility, or mRNA stability. We propose a physical barrier model in which bulky structures in 5'UTR biases ribosome movement toward the downstream start codon, thereby increasing the translation output. This study provides biophysical insights into the regulatory role of 5'UTR structures in in vitro and bacterial translation, highlighting their potential applications in tuning gene expression.

Recruitment of the 40S ribosomal subunit by the West Nile virus 3' UTR promotes the cross-talk between the viral genomic ends for translation regulation

Flaviviral RNA genomes are composed of discrete RNA structural units arranged in an ordered fashion and grouped into complex folded domains that regulate essential viral functions, e.g. replication and translation. This is achieved by adjusting the overall structure of the RNA genome via the establishment of inter- and intramolecular interactions. Translation regulation is likely the main process controlling flaviviral gene expression. Although the genomic 3' UTR is a key player in this regulation, little is known about the molecular mechanisms underlying this role. The present work provides evidence for the specific recruitment of the 40S ribosomal subunit by the 3' UTR of the West Nile virus RNA genome, showing that the joint action of both genomic ends contributes the positioning of the 40S subunit at the 5' end. The combination of structural mapping techniques revealed specific conformational requirements at the 3' UTR for 40S binding, involving the highly conserved SL-III, 5'DB, 3'DB and 3'SL elements, all involved in the translation regulation. These results point to the 40S subunit as a bridge to ensure cross-talk between both genomic ends during viral translation and support a link between 40S recruitment by the 3' UTR and translation control.

5'UTR G-quadruplex structure enhances translation in size dependent manner

Translation initiation in bacteria is frequently regulated by various structures in the 5' untranslated region (5'UTR). Previously, we demonstrated that G-quadruplex (G4) formation in non-template DNA enhances transcription. In this study, we aimed to explore how G4 formation in mRNA (RG4) at 5'UTR impacts translation using a T7-based in vitro translation system and in E. coli. We showed that RG4 strongly promotes translation efficiency in a size-dependent manner. Additionally, inserting a hairpin upstream of the RG4 further enhances translation efficiency, reaching up to a 12-fold increase. We found that the RG4-dependent effect is not due to increased ribosome affinity, ribosome binding site accessibility, or mRNA stability. We proposed a physical barrier model in which bulky structures in 5'UTR prevent ribosome dislodging and thereby increase the translation output. This study provides biophysical insights into the regulatory role of 5'UTR structures in bacterial translation, highlighting their potential applications in tuning gene expression.

G-quadruplexes and associated proteins in aging and Alzheimer's disease

Aging is a prominent risk factor for many neurodegenerative disorders, such as Alzheimer's disease (AD). Alzheimer's disease is characterized by progressive cognitive decline, memory loss, and neuropsychiatric and behavioral symptoms, accounting for most of the reported dementia cases. This disease is now becoming a major challenge and burden on modern society, especially with the aging population. Over the last few decades, a significant understanding of the pathophysiology of AD has been gained by studying amyloid deposition, hyperphosphorylated tau, synaptic dysfunction, oxidative stress, calcium dysregulation, and neuroinflammation. This review focuses on the role of non-canonical secondary structures of DNA/RNA G-quadruplexes (G4s, G4-DNA, and G4-RNA), G4-binding proteins (G4BPs), and helicases, and their roles in aging and AD. Being critically important for cellular function, G4s are involved in the regulation of DNA and RNA processes, such as replication, transcription, translation, RNA localization, and degradation. Recent studies have also highlighted G4-DNA's roles in inducing DNA double-strand breaks that cause genomic instability and G4-RNA's participation in regulating stress granule formation. This review emphasizes the significance of G4s in aging processes and how their homeostatic imbalance may contribute to the pathophysiology of AD.

Genome sequence analysis suggests coevolution of the DIS, SD, and Psi hairpins in HIV-1 genomes

HIV-1 RNA dimerization is a critical step in viral life cycle. It is a prerequisite for genome packaging and plays an important role in reverse transcription and recombination. Dimerization is promoted by the DIS (dimerization initiation site) hairpin located in the 5' leader of HIV-1 genome. Despite the high genetic diversity in HIV-1 group M, only five apical loops (AAGCGCGCA, AAGUGCGCA, AAGUGCACA, AGGUGCACA and AGUGCAC) are commonly found in DIS hairpins. We refer to the parent DISes with these apical loops as DIS_Lai, DIS_Trans, DIS_F, DIS_Mal, and DIS_C, respectively. Based on identity or similarity of DIS hairpins to parent DISes, we distributed HIV-1 M genomes into five dimerization groups. Comparison of the primary and secondary structures of DIS, SD and Psi hairpins in about 3000 HIV-1 M genomes showed that the mutation frequencies at particular nucleotide positions of these hairpins differ among the dimerization groups, and DIS_F may be an origin of other parent DISes. We found that DIS, SD and Psi hairpins have hundreds of variants, only some of them occurring rather frequently. The lower part of DIS hairpin with G x AGG internal loop is highly conserved in both HIV-1 and SIV genomes. We supposed that the G-quadruplex, located 56 nts downstream of the Gag start codon, may participate in switching of HIV-1 leader RNA from BMH (branched multiple hairpins) to LDI (long distance interaction) conformation.

Secondary structures in RNA synthesis, splicing and translation

Even though the functional role of mRNA molecules is primarily decided by the nucleotide sequence, several properties are determined by secondary structure conformations. Examples of secondary structures include long range interactions, hairpins, R-loops and G-quadruplexes and they are formed through interactions of non-adjacent nucleotides. Here, we discuss advances in our understanding of how secondary structures can impact RNA synthesis, splicing, translation and mRNA half-life. During RNA synthesis, secondary structures determine RNA polymerase II (RNAPII) speed, thereby influencing splicing. Splicing is also determined by RNA binding proteins and their binding rates are modulated by secondary structures. For the initiation of translation, secondary structures can control the choice of translation start site. Here, we highlight the mechanisms by which secondary structures modulate these processes, discuss advances in technologies to detect and study them systematically, and consider the roles of RNA secondary structures in disease.

Reversal of G-Quadruplexes’ Role in Translation Control When Present in the Context of an IRES

G-quadruplexes (GQs) are secondary nucleic acid structures that play regulatory roles in various cellular processes. G-quadruplex-forming sequences present within the 5′ UTR of mRNAs can function not only as repressors of translation but also as elements required for optimum function. Based upon previous reports, the majority of the 5′ UTR GQ structures inhibit translation, presumably by blocking the ribosome scanning process that is essential for detection of the initiation codon. However, there are certain mRNAs containing GQs that have been identified as positive regulators of translation, as they are needed for translation initiation. While most cellular mRNAs utilize the 5′ cap structure to undergo cap-dependent translation initiation, many rely on cap-independent translation under certain conditions in which the cap-dependent initiation mechanism is not viable or slowed down, for example, during development, under stress and in many diseases. Cap-independent translation mainly occurs via Internal Ribosomal Entry Sites (IRESs) that are located in the 5′ UTR of mRNAs and are equipped with structural features that can recruit the ribosome or other factors to initiate translation without the need for a 5′ cap. In this review, we will focus only on the role of RNA GQs present in the 5′ UTR of mRNAs, where they play a critical role in translation initiation, and discuss the potential mechanism of this phenomenon, which is yet to be fully delineated.

G-Quadruplex Regulation of VEGFA mRNA Translation by RBM4

In eukaryotes, mRNAs translation is mainly mediated in a cap-dependent or cap-independent manner. The latter is primarily initiated at the internal ribosome entry site (IRES) in the 5'-UTR of mRNAs. It has been reported that the G-quadruplex structure (G4) in the IRES elements could regulate the IRES activity. We previously confirmed RBM4 (also known as LARK) as a G4-binding protein in human. In this study, to investigate whether RBM4 is involved in the regulation of the IRES activity by binding with the G4 structure within the IRES element, the IRES-A element in the 5'-UTR of vascular endothelial growth factor A (VEGFA) was constructed into a dicistronic reporter vector, psiCHECK2, and the effect of RBM4 on the IRES activity was tested in 293T cells. The results showed that the IRES insertion significantly increased the FLuc expression activity, indicating that this G4-containing IRES was active in 293T cells. When the G4 structure in the IRES was disrupted by base mutation, the IRES activity was significantly decreased. The IRES activity was notably increased when the cells were treated with G4 stabilizer PDS. EMSA results showed that RBM4 specifically bound the G4 structure in the IRES element. The knockdown of RBM4 substantially reduced the IRES activity, whereas over-expressing RBM4 increased the IRES activity. Taking all results together, we demonstrated that RBM4 promoted the mRNA translation of VEGFA gene by binding to the G4 structure in the IRES.

A Small Ligand That Selectively Binds to the G-quadruplex at the Human Vascular Endothelial Growth Factor Internal Ribosomal Entry Site and Represses the Translation

G-quadruplexes are believed to have important biological functions, so many small molecules have been screened or developed for targeting G-quadruplexes. However, it is still a major challenge to find molecules that recognize specific G-quadruplexes. Here, by using a combination of surface plasmon resonance, electrospray ionization mass spectrometry, circular dichroism, Western blot, luciferase assay, and reverse transcriptase stop assay, we observed a small molecule, namely, oxymatrine (OMT) that could selectively bind to the RNA G-quadruplex in 5′-untranslated regions (UTRs) of human vascular endothelial growth factor (hVEGF), but could not bind to other G-quadruplexes. OMT could selectively repress the translation of VEGF in cervical cancer cells. Furthermore, it could recognize VEGF RNA G-quadruplexes in special conformations. The results indicate that OMT may serve as a potentially special tool for studying the VEGF RNA G-quadruplex in cells and as a valuable scaffold for the design of ligands that recognize different G-quadruplexes.

RNA G-quadruplexes (rG4s): genomics and biological functions

G-quadruplexes (G4s) are non-classical DNA or RNA secondary structures that have been first observed decades ago. Over the years, these four-stranded structural motifs have been demonstrated to have significant regulatory roles in diverse biological processes, but challenges remain in detecting them globally and reliably. Compared to DNA G4s (dG4s), the study of RNA G4s (rG4s) has received less attention until recently. In this review, we will summarize the innovative high-throughput methods recently developed to detect rG4s on a transcriptome-wide scale, highlight the many novel and important functions of rG4 being discovered in vivo across the tree of life, and discuss the key biological questions to be addressed in the near future.

G-quadruplex regulation of neural gene expression

G-quadruplexes are four-stranded helical nucleic acid structures characterized by stacked tetrads of guanosine bases. These structures are widespread throughout mammalian genomic DNA and RNA transcriptomes, and prevalent across all tissues. The role of G-quadruplexes in cancer is well-established, but there has been a growing exploration of these structures in the development and homeostasis of normal tissue. In this review, we focus on the roles of G-quadruplexes in directing gene expression in the nervous system, including the regulation of gene transcription, mRNA processing, and trafficking, as well as protein translation. The role of G-quadruplexes and their molecular interactions in the pathology of neurological diseases is also examined. Outside of cancer, there has been only limited exploration of G-quadruplexes as potential intervention targets to treat disease or injury. We discuss studies that have used small-molecule ligands to manipulate G-quadruplex stability in order to treat disease or direct neural stem/progenitor cell proliferation and differentiation into therapeutically relevant cell types. Understanding the many roles that G-quadruplexes have in the nervous system not only provides critical insight into fundamental molecular mechanisms that control neurological function, but also provides opportunities to identify novel therapeutic targets to treat injury and disease.

Small molecule targeting of biologically relevant RNA tertiary and quaternary structures

Initial successes in developing small molecule ligands for non-coding RNAs have underscored their potential as therapeutic targets. More recently, these successes have been aided by advances in biophysical and structural techniques for identification and characterization of more complex RNA structures; these higher-level folds present protein-like binding pockets that offer opportunities to design small molecules that could achieve a degree of selectivity often hard to obtain at the primary and secondary structure level. More specifically, identification and small molecule targeting of RNA tertiary and quaternary structures have allowed researchers to probe several human diseases and have resulted in promising clinical candidates. In this review we highlight a selection of diverse and exciting successes and the experimental approaches that led to their discovery. These studies include examples of recent developments in RNA-centric assays and ligands that provide insight into the features responsible for the affinity and biological outcome of RNA-targeted chemical probes. This report highlights the potential and emerging opportunities to selectively target RNA tertiary and quaternary structures as a route to better understand and, ultimately, treat many diseases.

The regulation and functions of DNA and RNA G-quadruplexes

DNA and RNA can adopt various secondary structures. Four-stranded G-quadruplex (G4) structures form through self-recognition of guanines into stacked tetrads, and considerable biophysical and structural evidence exists for G4 formation in vitro. Computational studies and sequencing methods have revealed the prevalence of G4 sequence motifs at gene regulatory regions in various genomes, including in humans. Experiments using chemical, molecular and cell biology methods have demonstrated that G4s exist in chromatin DNA and in RNA, and have linked G4 formation with key biological processes ranging from transcription and translation to genome instability and cancer. In this Review, we first discuss the identification of G4s and evidence for their formation in cells using chemical biology, imaging and genomic technologies. We then discuss possible functions of DNA G4s and their interacting proteins, particularly in transcription, telomere biology and genome instability. Roles of RNA G4s in RNA biology, especially in translation, are also discussed. Furthermore, we consider the emerging relationships of G4s with chromatin and with RNA modifications. Finally, we discuss the connection between G4 formation and synthetic lethality in cancer cells, and recent progress towards considering G4s as therapeutic targets in human diseases.

Epigenetic Modulation of Chromatin States and Gene Expression by G-Quadruplex Structures

G-quadruplexes are four-stranded helical nucleic acid structures formed by guanine-rich sequences. A considerable number of studies have revealed that these noncanonical structural motifs are widespread throughout the genome and transcriptome of numerous organisms, including humans. In particular, G-quadruplexes occupy strategic locations in genomic DNA and both coding and noncoding RNA molecules, being involved in many essential cellular and organismal functions. In this review, we first outline the fundamental structural features of G-quadruplexes and then focus on the concept that these DNA and RNA structures convey a distinctive layer of epigenetic information that is critical for the complex regulation, either positive or negative, of biological activities in different contexts. In this framework, we summarize and discuss the proposed mechanisms underlying the functions of G-quadruplexes and their interacting factors. Furthermore, we give special emphasis to the interplay between G-quadruplex formation/disruption and other epigenetic marks, including biochemical modifications of DNA bases and histones, nucleosome positioning, and three-dimensional organization of chromatin. Finally, epigenetic roles of RNA G-quadruplexes in post-transcriptional regulation of gene expression are also discussed. Undoubtedly, the issues addressed in this review take on particular importance in the field of comparative epigenetics, as well as in translational research.

Hidden Structural Modules in a Cooperative RNA Folding Transition

Large-scale, cooperative rearrangements underlie the functions of RNA in RNA-protein machines and gene regulation. To understand how such rearrangements are orchestrated, we used high-throughput chemical footprinting to dissect a seemingly concerted rearrangement in P5abc RNA, a paradigm of RNA folding studies. With mutations that systematically disrupt or restore putative structural elements, we found that this transition reflects local folding of structural modules, with modest and incremental cooperativity that results in concerted behavior. First, two distant secondary structure changes are coupled through a bridging three-way junction and Mg²⁺-dependent tertiary structure. Second, long-range contacts are formed between modules, resulting in additional cooperativity. Given the sparseness of RNA tertiary contacts after secondary structure formation, we expect that modular folding and incremental cooperativity are generally important for specifying functional structures while also providing productive kinetic paths to these structures. Additionally, we expect our approach to be useful for uncovering modularity in other complex RNAs.

The role of RNA G-quadruplexes in human diseases and therapeutic strategies

G-quadruplexes (GQs) are four-stranded secondary structures formed by G-rich nucleic acid sequence(s). DNA GQs are present abundantly in the genome and affect a wide range of processes associated with DNA. Recent studies show that RNA GQs are present in different transcripts, including coding and noncoding areas of mRNA, telomeric RNA as well as in other premature and mature noncoding RNAs. When present at specific locations within the RNAs, GQs play important roles in key biological functions, including the regulation of gene expression and telomere homeostasis. RNA GQs regulate pre-mRNA processing, such as splicing and polyadenylation. Evidently, among other processes, RNA GQs also control mRNA translation, miRNA and piRNA biogenesis, and RNA localization. The regulatory mechanisms controlled by RNA GQs mainly involve binding to RNA binding protein that modulate GQ conformation or serve as an entity in recruiting additional protein regulators to act as a block element to the processing machinery. Here we provide an overview of the ever-increasing number of discoveries revealing the role of RNA GQs in biology and their relevance in human diseases and therapeutics. This article is categorized under: RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA in Disease and Development > RNA in Disease.

Toward predictive R-loop computational biology: genome-scale prediction of R-loops reveals their association with complex promoter structures, G-quadruplexes and transcriptionally active enhancers

R-loops are three-stranded RNA:DNA hybrid structures essential for many normal and pathobiological processes. Previously, we generated a quantitative R-loop forming sequence (RLFS) model, quantitative model of R-loop-forming sequences (QmRLFS) and predicted ∼660 000 RLFSs; most of them located in genes and gene-flanking regions, G-rich regions and disease-associated genomic loci in the human genome. Here, we conducted a comprehensive comparative analysis of these RLFSs using experimental data and demonstrated the high performance of QmRLFS predictions on the nucleotide and genome scales. The preferential co-localization of RLFS with promoters, U1 splice sites, gene ends, enhancers and non-B DNA structures, such as G-quadruplexes, provides evidence for the mechanical linkage between DNA tertiary structures, transcription initiation and R-loops in critical regulatory genome regions. We introduced and characterized an abundant class of reverse-forward RLFS clusters highly enriched in non-B DNA structures, which localized to promoters, gene ends and enhancers. The RLFS co-localization with promoters and transcriptionally active enhancers suggested new models for in cis and in trans regulation by RNA:DNA hybrids of transcription initiation and formation of 3D-chromatin loops. Overall, this study provides a rationale for the discovery and characterization of the non-B DNA regulatory structures involved in the formation of the RNA:DNA interactome as the basis for an emerging quantitative R-loop biology and pathobiology.

Combinatorial role of two G-quadruplexes in 5' UTR of transforming growth factor β2 (TGFβ2)

Albeit most studies demonstrate the inhibitory role of G-quadruplex in the 5' Untranslated Region (5' UTR), our previous report depicted its completely contrasting activating role in the 5' UTR of transforming growth factor β2 (TGFβ2) mRNA. Therefore, we screened the 5' UTR of TGFβ2 manually and identified a second putative G-quadruplex sequence. Our in vitro experiments encompassing CD and UV spectroscopy confirmed the ability of this sequence to form a G-quadruplex and in cellulo studies further indicated its activating role in modulation of TGFβ2 gene expression. Our study suggests that these two 5' UTR G-quadruplexes most probably operate additively to substantially increase gene expression of TGFβ2. Neither of the two G-quadruplex alone is sufficient enough to drastically augment protein production. Both G-quadruplexes are essential for increasing protein output. To the best of our knowledge, our study is the first report showcasing the combinatorial role of two G-quadruplexes in the 5' UTR of an mRNA.

Alternatively spliced variants of the 5'-UTR of the ARPC2 mRNA regulate translation by an internal ribosome entry site (IRES) harboring a guanine-quadruplex motif

The 5'-UTR of the actin-related protein 2/3 complex subunit 2 (ARPC2) mRNA exists in two variants. Using a bicistronic reporter construct, the present study demonstrates that the longer variant of the 5'-UTR harbours an internal ribosome entry site (IRES) which is lacking in the shorter one. Multiple control assays confirmed that only this variant promotes cap-independent translation. Furthermore, it includes a guanine-rich region that is capable of forming a guanine-quadruplex (G-quadruplex) structure which was found to contribute to the IRES activity. To investigate the cellular function of the IRES element, we determined the expression level of ARPC2 at various cell densities. At high cell density, the relative ARPC2 protein level increases, supporting the presumed function of IRES elements in driving the expression of certain genes under stressful conditions that compromise cap-dependent translation. Based on chemical probing experiments and computer-based predictions, we propose a structural model of the IRES element, which includes the G-quadruplex motif exposed from the central stem-loop element. Taken together, our study describes the functional relevance of two alternative 5'-UTR splice variants of the ARPC2 mRNA, one of which contains an IRES element with a G-quadruplex as a central motif, promoting translation under stressful cellular conditions.

Translation initiation by cap-dependent ribosome recruitment: Recent insights and open questions

Gene expression universally relies on protein synthesis, where ribosomes recognize and decode the messenger RNA template by cycling through translation initiation, elongation, and termination phases. All aspects of translation have been studied for decades using the tools of biochemistry and molecular biology available at the time. Here, we focus on the mechanism of translation initiation in eukaryotes, which is remarkably more complex than prokaryotic initiation and is the target of multiple types of regulatory intervention. The "consensus" model, featuring cap-dependent ribosome entry and scanning of mRNA leader sequences, represents the predominantly utilized initiation pathway across eukaryotes, although several variations of the model and alternative initiation mechanisms are also known. Recent advances in structural biology techniques have enabled remarkable molecular-level insights into the functional states of eukaryotic ribosomes, including a range of ribosomal complexes with different combinations of translation initiation factors that are thought to represent bona fide intermediates of the initiation process. Similarly, high-throughput sequencing-based ribosome profiling or "footprinting" approaches have allowed much progress in understanding the elongation phase of translation, and variants of them are beginning to reveal the remaining mysteries of initiation, as well as aspects of translation termination and ribosomal recycling. A current view on the eukaryotic initiation mechanism is presented here with an emphasis on how recent structural and footprinting results underpin axioms of the consensus model. Along the way, we further outline some contested mechanistic issues and major open questions still to be addressed. This article is categorized under: Translation > Translation Mechanisms Translation > Translation Regulation RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Functional 5' UTR mRNA structures in eukaryotic translation regulation and how to find them

RNA molecules can fold into intricate shapes that can provide an additional layer of control of gene expression beyond that of their sequence. In this Review, we discuss the current mechanistic understanding of structures in 5' untranslated regions (UTRs) of eukaryotic mRNAs and the emerging methodologies used to explore them. These structures may regulate cap-dependent translation initiation through helicase-mediated remodelling of RNA structures and higher-order RNA interactions, as well as cap-independent translation initiation through internal ribosome entry sites (IRESs), mRNA modifications and other specialized translation pathways. We discuss known 5' UTR RNA structures and how new structure probing technologies coupled with prospective validation, particularly compensatory mutagenesis, are likely to identify classes of structured RNA elements that shape post-transcriptional control of gene expression and the development of multicellular organisms.

Intermolecular G-Quadruplex Induces Hyaluronic Acid-DNA Superpolymers Causing Cancer Cell Swelling, Blebbing, and Death

Over the past decade, nanomedicine has gained considerable attraction through its relevance, for example, in "smart" delivery, thus creating platforms for novel treatments. Here, we report a natural polymer-DNA conjugate that undergoes self-assembly in a K⁺-dependent fashion to form a G-quadruplex (GQ) and generate superpolymeric structures. We derivatized a thiolated conjugate of the naturally occurring glycosaminoglycan polymer hyaluronic acid (HASH) with short G-rich DNA (HASH-DNA) that can form an intermolecular noncanonical GQ structure. Gel mobility shift assay and circular dichroism measurements confirmed HASH conjugation to DNA and K⁺-dependent GQ formation, respectively. Transmission electron microscopy and scanning electron microscopy results indicated that the addition of K⁺ to the HASH-DNA conjugate led to the formation of micron-range structures, whereas control samples remained unordered and as a nebulous globular form. Confocal microscopy of a fluorescently labeled form of the superpolymer verified increased cellular uptake. The HASH-DNA conjugates showed toxicity in HeLa cells, whereas a scrambled DNA (Mut) conjugate HASH-Mut showed no cytotoxicity, presumably because of nonformation of the superpolymeric structure. To understand the mechanism of cell death and if the superpolymeric structure is responsible for it, we monitored the cell size and observed an average of 23% increase in size compared to 4.5% in control cells at 4.5 h. We believe that cellular stress is generated presumably by the intracellular assembly of this large superpolymeric nanostructure causing cell blebbing with no exit option. This approach provides a new strategy of cellular delivery of a targeted naturally occurring polymer and a novel way to induce superpolymeric structure formation that acts as a therapeutic.

RNA G-Quadruplexes in Biology: Principles and Molecular Mechanisms

G-quadruplexes (G4s) are extremely stable DNA or RNA secondary structures formed by sequences rich in guanine. These structures are implicated in many essential cellular processes, and the number of biological functions attributed to them continues to grow. While DNA G4s are well understood on structural and, to some extent, functional levels, RNA G4s and their functions have received less attention. The presence of bona fide RNA G4s in cells has long been a matter of debate. The development of G4-specific antibodies and ligands hinted on their presence in vivo, but recent advances in RNA sequencing coupled with chemical footprinting suggested the opposite. In this review, we will critically discuss the biology of RNA G4s focusing on the molecular mechanisms underlying their proposed functions.

Discovery of Small Molecules for Repressing Cap-Independent Translation of Human Vascular Endothelial Growth Factor (hVEGF) as Novel Antitumor Agents

Angiogenesis is important in tumorigenesis and tumor progression. Human vascular endothelial growth factor (hVEGF) is an angiogenic growth factor that plays a crucial role in tumor progression. The G-rich region within the 5'-untranslated regions (5'-UTR) of hVEGF-A mRNA can form a "switchable" RNA G-quadruplex structure that is essential for a cap-independent translation initiation. We screened our small-molecule library for binders of this G-tract. One novel quinazoline derivative, compound 1, showed a significant specific interaction with the G-tract and destabilized the G-quadruplex structure. The results of cellular experiments revealed that compound 1 down-regulated hVEGF-A translation and significantly impeded tumor cells migration. We also found that compound 1 exhibited tumor-inhibiting activity in MCF-7 xenograft tumors, which might be related to its ability to reduce hVEGF expression. These findings present a new strategy of hVEGF-A translational control in which small molecules interact with G-quadruplex structure in the 5'UTR.

Engineered domain swapping indicates context dependent functional role of RNA G-quadruplexes

RNA domain swapping typically demonstrates conservation of the native function of the domain in a non-native context. In contrast, we employed RNA engineering to demonstrate deviation of G-quadruplex (GQ) function that is contingent upon its context dependent location, which is opposite to their native functional role. Known translation repressing RNA GQs were engineered into human VEGF IRES A replacing the endogenous GQ domain essential for translation. Alternatively, the translation inhibitory GQ motif within the 5'-UTR of MT3-MMP mRNA was replaced with two known GQ motifs that are essential for translation. The results indicate that the engineered GQ domains can adopt GQ structures in a foreign environment with a functional role reversal to accommodate the need of the endogenous swapped motifs. The observations establish the functionality and context dependent modularity of RNA GQ structures.

Structure, Properties, and Biological Relevance of the DNA and RNA G-Quadruplexes: Overview 50 Years after Their Discovery

G-quadruplexes (G4s), which are known to have important roles in regulation of key biological processes in both normal and pathological cells, are the most actively studied non-canonical structures of nucleic acids. In this review, we summarize the results of studies published in recent years that change significantly scientific views on various aspects of our understanding of quadruplexes. Modern notions on the polymorphism of DNA quadruplexes, on factors affecting thermodynamics and kinetics of G4 folding–unfolding, on structural organization of multiquadruplex systems, and on conformational features of RNA G4s and hybrid DNA–RNA G4s are discussed. Here we report the data on location of G4 sequence motifs in the genomes of eukaryotes, bacteria, and viruses, characterize G4-specific small-molecule ligands and proteins, as well as the mechanisms of their interactions with quadruplexes. New information on the structure and stability of G4s in telomeric DNA and oncogene promoters is discussed as well as proof being provided on the occurrence of G-quadruplexes in cells. Prominence is given to novel experimental techniques (single molecule manipulations, optical and magnetic tweezers, original chemical approaches, G4 detection in situ, in-cell NMR spectroscopy) that facilitate breakthroughs in the investigation of the structure and functions of G-quadruplexes.

G-Quadruplex in the NRF2 mRNA 5' Untranslated Region Regulates De Novo NRF2 Protein Translation under Oxidative Stress

Inhibition of protein synthesis serves as a general measure of cellular consequences of chemical stress. A few proteins are translated selectively and influence cell fate. How these proteins can bypass the general control of translation remains unknown. We found that low to mild doses of oxidants induce de novo translation of the NRF2 protein. Here we demonstrate the presence of a G-quadruplex structure in the 5' untranslated region (UTR) of NRF2 mRNA, as measured by circular dichroism, nuclear magnetic resonance, and dimethylsulfate footprinting analyses. Such a structure is important for 5'-UTR activity, since its removal by sequence mutation eliminated H₂O₂-induced activation of the NRF2 5' UTR. Liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based proteomics revealed elongation factor 1 alpha (EF1a) as a protein binding to the G-quadruplex sequence. Cells responded to H₂O₂ treatment by increasing the EF1a protein association with NRF2 mRNA, as measured by RNA-protein interaction assays. The EF1a interaction with small and large subunits of ribosomes did not appear to change due to H₂O₂ treatment, nor did posttranslational modifications, as measured by two-dimensional (2-D) Western blot analysis. Since NRF2 encodes a transcription factor essential for protection against tissue injury, our data have revealed a novel mechanism of cellular defense involving de novo NRF2 protein translation governed by the EF1a interaction with the G-quadruplex in the NRF2 5' UTR during oxidative stress.

Metal Cations in G-Quadruplex Folding and Stability

This review is focused on the structural and physicochemical aspects of metal cation coordination to G-Quadruplexes (GQ) and their effects on GQ stability and conformation. G-quadruplex structures are non-canonical secondary structures formed by both DNA and RNA. G-quadruplexes regulate a wide range of important biochemical processes. Besides the sequence requirements, the coordination of monovalent cations in the GQ is essential for its formation and determines the stability and polymorphism of GQ structures. The nature, location, and dynamics of the cation coordination and their impact on the overall GQ stability are dependent on several factors such as the ionic radii, hydration energy, and the bonding strength to the O6 of guanines. The intracellular monovalent cation concentration and the localized ion concentrations determine the formation of GQs and can potentially dictate their regulatory roles. A wide range of biochemical and biophysical studies on an array of GQ enabling sequences have generated at a minimum the knowledge base that allows us to often predict the stability of GQs in the presence of the physiologically relevant metal ions, however, prediction of conformation of such GQs is still out of the realm.