Identification and Structural Aspects of G-Quadruplex-Forming Sequences from the Influenza A Virus Genome

An insight into G-quadruplexes: Identification and potential therapeutic targets in livestock viruses

G-quadruplexes (G4s) are non-canonical nucleic acids secondary structures that involve in the regulation of some key biological processes, such as replication, transcription, and translation. G4s have been extensively described in the genomes of human and related diseases. In recent years, G4s were identified in several livestock viruses, including those of the emerging epidemics, like Nipah virus (NiV). Since their discovery, G4s have been developed as the potential antiviral targets, and the employment of G4 ligands or interacting proteins has helped to expound the viral infectivity and pathogenesis through G4-mediated mechanisms, and highlight the potential as therapeutic approaches. However, the comprehensively studies of G4s in livestock viruses have not been summarized. This review delves into the reported literatures of G4s in livestock viruses, particular focus on the presence, biophysical identification, and possible function of G4s in viral genome, summarizing the G4 ligands, interacted proteins and aptamers on antiviral applications. The strengths and the challenges of G4 targeting in this field are also discussed. Therefore, this review will shed new light on the future development of highly potent and targeting antiviral therapy.

Opportunities and challenges with G-quadruplexes as promising targets for drug design

INTRODUCTION

G-quadruplexes (G4s) are secondary structures formed in guanine-rich regions of nucleic acids (both DNA and RNA). G4s are significantly enriched at regulatory genomic regions and are associated with important biological processes ranging from telomere homeostasis and genome instability to transcription and translation. Importantly, G4s are related to health and diseases such as cancer, neurological diseases, as well as infections with viruses and microbial pathogens. Increasing evidence suggests the potential of G4s for designing new diagnostic and therapeutic strategies although in vivo studies are still at early stages.

AREAS COVERED

This review provides an updated summary of the literature describing the impact of G4s in human diseases and different approaches based on G4 targeting in therapy.

EXPERT OPINION

Within the G4 field, most of the studies have been performed in vitro and in a descriptive manner. Therefore, detailed mechanistic understanding of G4s in the biological context remains to be deciphered. In clinics, the use of G4s as therapeutic targets has been hindered due to the low selectivity profile and poor drug-like properties of G4 ligands. Future research on G4s may overcome current methodological and interventional limitations and shed light on these unique structural elements in the pathogenesis and treatment of diseases.

Stabilization of G-Quadruplex Structures of the SARS-CoV-2 Genome by TMPyP4, BRACO19, and PhenDC3

The G-quadruplex is one of the non-canonical structures formed by nucleic acids, which can be formed by guanine-rich sequences. They became the focus of much research when they were found in several oncogene promoter regions and also in the telomeres. Later on, they were discovered in viruses as well. Various ligands have been developed in order to stabilize DNA G-quadruplexes, which were believed to have an anti-cancer or antiviral effect. We investigated three of these ligands, and whether they can also affect the stability of the G-quadruplex-forming sequences of the RNA genome of SARS-CoV-2. All three investigated oligonucleotides showed the G-quadruplex form. We characterized their stability and measured their thermodynamic parameters using the Förster resonance energy transfer method. The addition of the ligands caused an increase in the unfolding temperature, but this effect was smaller compared to that found earlier in the case of G-quadruplexes of the hepatitis B virus, which has a DNA genome.

G-Quadruplexes in the Regulation of Viral Gene Expressions and Their Impacts on Controlling Infection

G-quadruplexes (G4s) are noncanonical nucleic acid structures that play significant roles in regulating various biological processes, including replication, transcription, translation, and recombination. Recent studies have identified G4s in the genomes of several viruses, such as herpes viruses, hepatitis viruses, and human coronaviruses. These structures are implicated in regulating viral transcription, replication, and virion production, influencing viral infectivity and pathogenesis. G4-stabilizing ligands, like TMPyP4, PhenDC3, and BRACO19, show potential antiviral properties by targeting and stabilizing G4 structures, inhibiting essential viral life-cycle processes. This review delves into the existing literature on G4's involvement in viral regulation, emphasizing specific G4-stabilizing ligands. While progress has been made in understanding how these ligands regulate viruses, further research is needed to elucidate the mechanisms through which G4s impact viral processes. More research is necessary to develop G4-stabilizing ligands as novel antiviral agents. The increasing body of literature underscores the importance of G4s in viral biology and the development of innovative therapeutic strategies against viral infections. Despite some ligands' known regulatory effects on viruses, a deeper comprehension of the multifaceted impact of G4s on viral processes is essential. This review advocates for intensified research to unravel the intricate relationship between G4s and viral processes, paving the way for novel antiviral treatments.

G-Quadruplexes in the Viral Genome: Unlocking Targets for Therapeutic Interventions and Antiviral Strategies

G-quadruplexes (G4s) are unique non-canonical four-stranded nucleic acid secondary structures formed by guanine-rich DNA or RNA sequences. Sequences with the potential to form quadruplex motifs (pG4s) are prevalent throughout the genomes of all organisms, spanning from prokaryotes to eukaryotes, and are enriched within regions of biological significance. In the past few years, the identification of pG4s within most of the Baltimore group viruses has attracted increasing attention due to their occurrence in regulatory regions of the genome and the subsequent implications for regulating critical stages of viral life cycles. In this context, the employment of specific G4 ligands has aided in comprehending the intricate G4-mediated regulatory mechanisms in the viral life cycle, showcasing the potential of targeting viral G4s as a novel antiviral strategy. This review offers a thorough update on the literature concerning G4s in viruses, including their identification and functional significance across most of the human-infecting viruses. Furthermore, it delves into potential therapeutic avenues targeting G4s, encompassing various G4-binding ligands, G4-interacting proteins, and oligonucleotide-based strategies. Finally, the article highlights both progress and challenges in the field, providing valuable insights into leveraging this unusual nucleic acid structure for therapeutic purposes.

Bispecific G-quadruplexes as inhibitors of cancer cells growth

A therapeutic system with the ability to target more than one protein is an important aim of cancer therapy since tumor growth is accompanied by dysregulation of many biological pathways. G-quadruplexes (G4s) are non-canonical structures formed by guanine-rich DNA or RNA oligonucleotides, with the ability to bind to different targets. In this study, we constructed ten novel bispecific G-quadruplex conjugates based on AT11, TBA, T40214 and T40231 aptamer structures, with the ability to bind two different targets at once in cancer cells. We analyzed the physicochemical aspects and the anticancer properties of novel molecules relating them with the single G-quadruplex unit and attempted to comprehend the correlation between the structures of bispecific G-quadruplexes with their biological activity. Our studies uncovered conjugates with considerable antiproliferative potential in HeLa and MCF-7 cancer cell lines, however with relatively low thermal stability or low nuclease resistance. Three conjugates among all studied oligonucleotides possess improved antiproliferative activity in MCF-7 cell line in comparison to their single G-quadruplex units leading to up to 90% inhibition of cancer cells growth, but their inhibitory potential is rather comparable to the effect observed for mix of two separate G-quadruplex units. Importantly, the conjugation enhances oligonucleotides enzymatic stability leading to the improvement of their therapeutic profile. The comprehensive studies presented herein indicate new approach for possibly effective cancer therapy and for the design of G4-based drugs.

Targeting G-quadruplexes to achieve antiviral activity

With the emergence of new viruses in the human population and the fast mutation rates of existing viruses, new antiviral targets and compounds are needed. Most existing antiviral drugs are active against proteins of a handful of viruses. Most of these proteins in the end affect viral nucleic acid processing, but direct nucleic acid targeting is less represented due to the difficulty of selectively acting at the nucleic acid of interest. Recently, nucleic acids have been shown to fold in structures alternative to the classic double helix and Watson and Crick base-pairing. Among these non-canonical structures, G-quadruplexes (G4s) have attracted interest because of their key biological roles that are being discovered. Molecules able to selectively target G4s have been developed and since G4s have been investigated as targets in several human pathologies, including viral infections. Here, after briefly introducing viruses, G4s and the G4-binding molecules with antiviral properties, we comment on the mechanisms at the base of the antiviral activity reported for G4-binding molecules. Understanding how G4-ligands act in infected cells will possibly help designing and developing next-generation antiviral drugs.

G-quadruplexes in the monkeypox virus are potential antiviral targets

Monkeypox virus (MPXV) is a member of Orthopoxvirus in the Poxviridae family, causing a Public Health Emergency of International Concern. The number of cases and geographic range has increased significantly in 2022. Identification of MPXV-specific therapeutic targets is urgent. G-quadruplex (GQ) secondary structures attract great attention as potential targets for antiviral strategy. Whether GQs are present in the MPXV genome remains inconclusive. In this study, we aim to characterize the GQs encoded by MPXV. Through a series of biophysical experiments, we characterized the formation potential of MPXV-encoded GQs and evaluated the binding and stabilization abilities of GQ ligands including BRACO-19, pyridostatin, and TMPyP4 to GQs encoded by MPXV. Moreover, GQ ligands suppressed the gene transcription of MPXV sequences containing GQ. BRACO-19 and TMPyP4 were able to inhibit vaccinia virus replication. We demonstrated the existence of MPXV GQ and reinforced the idea that GQs could be novel antiviral targets. Targeting these GQ sequences with GQ-binding molecules may represent a new approach for MPXV therapy.

Examples of Structural Motifs in Viral Genomes and Approaches for RNA Structure Characterization

The relationship between conserved structural motifs and their biological function in the virus replication cycle is the interest of many researchers around the world. RNA structure is closely related to RNA function. Therefore, technological progress in high-throughput approaches for RNA structure analysis and the development of new ones are very important. In this mini review, we discuss a few perspectives on the structural elements of viral genomes and some methods used for RNA structure prediction and characterization. Based on the recent literature, we describe several examples of studies concerning the viral genomes, especially severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza A virus (IAV). Herein, we emphasize that a better understanding of viral genome architecture allows for the discovery of the structure-function relationship, and as a result, the discovery of new potential antiviral therapeutics.

Alternative RNA Conformations: Companion or Combatant

RNA molecules, in one form or another, are involved in almost all aspects of cell physiology, as well as in disease development. The diversity of the functional roles of RNA comes from its intrinsic ability to adopt complex secondary and tertiary structures, rivaling the diversity of proteins. The RNA molecules form dynamic ensembles of many interconverting conformations at a timescale of seconds, which is a key for understanding how they execute their cellular functions. Given the crucial role of RNAs in various cellular processes, we need to understand the RNA molecules from a structural perspective. Central to this review are studies aimed at revealing the regulatory role of conformational equilibria in RNA in humans to understand genetic diseases such as cancer and neurodegenerative diseases, as well as in pathogens such as bacteria and viruses so as to understand the progression of infectious diseases. Furthermore, we also summarize the prior studies on the use of RNA structures as platforms for the rational design of small molecules for therapeutic applications.

Discovering functional motifs in long noncoding RNAs

Long noncoding RNAs (lncRNAs) are products of pervasive transcription that closely resemble messenger RNAs on the molecular level, yet function through largely unknown modes of action. The current model is that the function of lncRNAs often relies on specific, typically short, conserved elements, connected by linkers in which specific sequences and/or structures are less important. This notion has fueled the development of both computational and experimental methods focused on the discovery of functional elements within lncRNA genes, based on diverse signals such as evolutionary conservation, predicted structural elements, or the ability to rescue loss-of-function phenotypes. In this review, we outline the main challenges that the different methods need to overcome, describe the recently developed approaches, and discuss their respective limitations. This article is categorized under: RNA Evolution and Genomics > Computational Analyses of RNA RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs.

Characterization of G-Quadruplexes in Enterovirus A71 Genome and Their Interaction with G-Quadruplex Ligands

Human enteroviruses cause many diseases; however, there is no specific therapeutic drug. G-quadruplex is an atypical secondary structure formed in the guanine rich region of DNA or RNA, which can exist in the viral genome. The different positions of G-quadruplex play an important role in the regulation of virus replication and infection. Whether G-quadruplexes are present in human enteroviruses is unknown. In current study, we analyzed the potential quadruplex forming sequences of human enteroviruses, especially EV-A71 virus, which causes hand, foot, and mouth disease. The results showed that there were a certain number of potential quadruplex-forming sequences in human enteroviruses. Through a variety of experimental methods, we evaluated the formation potential of EV-A71 encoded G-quadruplex and analyzed the binding ability of G-quadruplex ligands, including BRACO-19, pyridostatin and TMPyP4 to virus encoded G-quadruplexes. G-quadruplex ligands BRACO-19, PDS and TMPyP4 could inhibit the transcription of constructs containing EV-A71 G-quadruplex sequences. Moreover, we found that BRACO-19 was able to inhibit the replication of EV-A71, suggesting that targeting G-quadruplexes in EV-A71 genome by G-quadruplex ligands could be a novel antiviral way against EV-A71. Our finding not only uncovered the G-quadruplexes in human enteroviruses, but also would provide a new strategy for human enteroviruses therapy. IMPORTANCE G-quadruplex is a stable nucleic acid secondary structure formed by the folding of guanine rich nucleic acid. The important regulatory function of G-quadruplex makes it an attractive target of antiviral effect. Human enteroviruses cause a variety of human diseases, including common cold, nervous system diseases, cardiovascular damage, and diabetes. Enterovirus A71 (EV-A71) is one of pathogens causing hand, foot, and mouth disease; however, whether G-quadruplexes are present in the genomes of human enteroviruses is unknown. The function of G-quadruplexes in the EV-A71 genomes is not clear. We predicted and characterized G-quadruplex sequences in EV-A71. G-quadruplex ligands were identified to stabilize EV-A71 G-quadruplexes with high affinities. We also demonstrated G-quadruplex ligand BRACO-19 inhibited EV-A71 replication. Our studies provide a framework for targeting G-quadruplexes in the enteroviruses genome, which will be a new way to develop antiviral agents against human enteroviruses.

Identification of G-quadruplex DNA sequences in SARS-CoV2

G-quadruplex structure or Putative Quadruplex Sequences (PQSs) are abundant in human, microbial, DNA, or RNA viral genomes. These sequences in RNA viral genome play critical roles in integration into human genome as LTR (Long Terminal Repeat), genome replication, chromatin rearrangements, gene regulation, antigen variation (Av), and virulence. Here, we investigated whether the genome of SARS-CoV2, an RNA virus, contained such potential G-quadruplex structures. Using bioinformatic tools, we searched for such sequences and found thirty-seven (forward strand (twenty-five) + reverse strand (Twelve)) QGRSs (Quadruplex forming G-Rich Sequences)/PQSs in SARS-CoV2 genome. These sequences are dispersed mainly in the upstream of SARS-CoV2 genes. We discuss whether existing PQS/QGRS ligands could inhibit the SARS-CoV2 replication and gene transcription as has been observed in other RNA viruses. Further experimental validation would determine the role of these G-quadruplex sequences in SARS-CoV2 genome function to survive in the host cells and identify therapeutic agents to destabilize these PQSs/QGRSs.

G-Quadruplexes in Neurobiology and Virology: Functional Roles and Potential Therapeutic Approaches

A G-quadruplex (G4) is a four-stranded nucleic acid secondary structure maintained by Hoogsteen hydrogen bonds established between four guanines. Experimental studies and bioinformatics predictions support the hypothesis that these structures are involved in different cellular functions associated with both DNA and RNA processes. An increasing number of diseases have been shown to be associated with abnormal G4 regulation. Here, we describe the existence of G4 and then discuss G4-related pathogenic mechanisms in neurodegenerative diseases and the viral life cycle. Furthermore, we focus on the role of G4s in the design of antiviral therapy and neuropharmacology, including G4 ligands, G4-based aptamers, G4-related proteins, and CRISPR-based sequence editing, along with a discussion of limitations and insights into the prospects of this unusual nucleic acid secondary structure in therapeutics. Finally, we highlight progress and challenges in this field and the potential G4-related research fields.

G-Quadruplex Targeting in the Fight against Viruses: An Update

G-quadruplexes (G4s) are noncanonical nucleic acid structures involved in the regulation of key cellular processes, such as transcription and replication. Since their discovery, G4s have been mainly investigated for their role in cancer and as targets in anticancer therapy. More recently, exploration of the presence and role of G4s in viral genomes has led to the discovery of G4-regulated key viral pathways. In this context, employment of selective G4 ligands has helped to understand the complexity of G4-mediated mechanisms in the viral life cycle, and highlighted the possibility to target viral G4s as an emerging antiviral approach. Research in this field is growing at a fast pace, providing increasing evidence of the antiviral activity of old and new G4 ligands. This review aims to provide a punctual update on the literature on G4 ligands exploited in virology. Different classes of G4 binders are described, with emphasis on possible antiviral applications in emerging diseases, such as the current COVID-19 pandemic. Strengths and weaknesses of G4 targeting in viruses are discussed.