RNA synthesis is modulated by G-quadruplex formation in Hepatitis C virus negative RNA strand

Role of RNA G-Quadruplexes in the Japanese Encephalitis Virus Genome and Their Recognition as Prospective Antiviral Targets

G-quadruplexes (GQs) have been primarily studied in the context of cancer and neurodegenerative pathologies. However, recent research has shifted focus to their existence and functional roles in viral genomes, revealing GQ-regulated key pathways in various human pathogenic viruses. While GQ structures have been reported in the genomes of emerging and re-emerging viruses, RNA viruses have been understudied compared to DNA viruses, including notable examples such as human immunodeficiency virus-1, hepatitis C virus, Ebola virus, Nipah virus, Zika virus, and SARS-CoV-2. The flavivirus family, comprising the Japanese encephalitis virus (JEV), poses a significant global threat due to recurring outbreaks yet lacks approved antivirals. In this study, we identified and characterized eight putative G-quadruplex-forming motifs within essential genes involved in genome replication, assembly, and internalization in the host cell, conserved across different JEV isolates. The formation and stability of these motifs were validated through a multitude of biophysical and cell-based assays. The interaction and binding affinity of these motifs with the known GQ-binding ligand BRACO-19 were supported by biophysical assays, confirming the capability of these motifs to form GQ structures. Notably, BRACO-19 also exerted antiviral properties through reduction of viral replication and infectious virus titers as well as inhibition of viral protein expression, as evaluated by the cell-based assays. This comprehensive molecular characterization of G-quadruplex structures within the JEV genome highlights their potential as promising antiviral targets for intervention strategies against JEV infection through GQ-specific ligands.

The UP1 domain is essential for the facilitation effect of HnRNP A1 on PRRSV-2 replication

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes reproductive failure and respiratory distress and is a serious threat to the swine industry, given its continuous and rapid emergence. The knowledge of viral-host interaction could facilitate anti-PRRSV drug development. HnRNP A1 is an abundantly expressed protein which associates with RNA metabolic processes and plays multifarious roles during the reproduction cycle of multiple viruses. However, the function of porcine HnRNP A1 in PRRSV-2 replication is still unknown. Herein, HnRNP A1 was identified as a nucleocapsid (N)-binding protein for PRRSV-2. Overexpression of porcine HnRNP A1 promoted the expression of viral RNA, and viral proteins, corresponding to enhanced virus titers. While deletion of the UP1 domain abolished the HnRNP A1-mediated enhancement of PRRSV-2 replication. In addition, HnRNP A1-silencing confirmed its pro-viral effect on PRRSV-2 infectivity in porcine alveolar macrophages (PAMs). RNA pull-down and RNA immunoprecipitation verification confirmed that the UP1 domain is important for the recognition of the guanine-rich sequence (GRS) in PRRSV-2 negative RNA. Eventually, supplementation with TMPyP4, a G4 ligand, efficiently provokes the release of HnRNP A1 from GRS, thereby limiting PRRSV-2 replication. Together, these findings help to inform the mechanism by which HnRNP A1 accelerates PRRSV-2 replication, and facilitate antiviral drug design.

Asymmetric distribution of G-quadruplex forming sequences in genomes of retroviruses

Retroviruses are among the most extensively studied viral families, both historically and in contemporary research. They are primarily investigated in the fields of viral oncogenesis, reverse transcription mechanisms, and other infection-specific aspects. These include the integration of endogenous retroviruses (ERVs) into host genomes, a process widely utilized in genetic engineering, and the ongoing search for HIV/AIDS treatment. G-quadruplexes (G4) have emerged as potential therapeutic targets in antiviral therapy and have been identified in important regulatory regions of viral genomes. In this study, we examine the presence of potential G-quadruplex-forming sequences (PQS) across all currently available unique retroviral genomes. Given that these retroviral genomes typically consist of single-stranded RNA (ssRNA) molecules, we also investigated whether the localization of PQSs is strand-dependent. This is particularly relevant since antisense transcripts have been detected in HIV, and ERV integration into the host genome involves reverse transcription from genomic positive strand ssRNA to double-stranded DNA (dsDNA), implicating both strands in this process. We show that in most mammalian retroviruses, including human retroviruses, PQSs are significantly more prevalent on the negative (antisense) strand, with some notable exceptions such as HIV-1. In sharp contrast, avian retroviruses exhibit a higher prevalence of PQSs on the positive (sense) strand.

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.

Viral hijacking of hnRNPH1 unveils a G-quadruplex-driven mechanism of stress control

Viral genomes are enriched with G-quadruplexes (G4s), non-canonical structures formed in DNA or RNA upon assembly of four guanine stretches into stacked quartets. Because of their critical roles, G4s are potential antiviral targets, yet their function remains largely unknown. Here, we characterize the formation and functions of a conserved G4 within the polymerase coding region of orthoflaviviruses of the Flaviviridae family. Using yellow fever virus, we determine that this G4 promotes viral replication and suppresses host stress responses via interactions with hnRNPH1, a host nuclear protein involved in RNA processing. G4 binding to hnRNPH1 causes its cytoplasmic retention with subsequent impacts on G4-containing tRNA fragments (tiRNAs) involved in stress-mediated reductions in translation. As a result, these host stress responses and associated antiviral effects are impaired. These data reveal that the interplay between hnRNPH1 and both host and viral G4 targets controls the integrated stress response and viral replication.

Stress less: Viral mastery of the RNA G-quadruplex

RNA G-quadruplexes are dynamically regulated during stress and infection. In this issue of Cell Host & Microbe, Schult et al.1 demonstrate that an RNA G-quadruplex conserved across orthoflaviviruses binds hnRNPH1 to mitigate the host stress response, highlighting the potential of this dynamic proviral RNA structure as a pan-flaviviral target.

Structure of a DNA G-quadruplex that Modulates SP1 Binding Sites Architecture in HIV-1 Promoter

Nucleic acid sequences containing guanine tracts are able to form non-canonical DNA or RNA structures known as G-quadruplexes (or G4s). These structures, based on the stacking of G-tetrads, are involved in various biological processes such as gene expression regulation. Here, we investigated a G4 forming sequence, HIVpro2, derived from the HIV-1 promoter. This motif is located 60 nucleotides upstream of the proviral Transcription Starting Site (TSS) and overlaps with two SP1 transcription factor binding sites. Using NMR spectroscopy, we determined that HIVpro2 forms a hybrid type G4 structure with a core that is interrupted by a single nucleotide bulge. An additional reverse-Hoogsteen AT base pair is stacked on top of the tetrad. SP1 transcription factor is known to regulate transcription activity of many genes through the recognition of Guanine-rich duplex motifs. Here, the formation of HIVpro2 G4 may modulate SP1 binding sites architecture by competing with the formation of the canonical duplex structure. Such DNA structural switch potentially participates to the regulation of viral transcription and may also interfere with HIV-1 reactivation or viral latency.

A sodium/potassium switch for G4-prone G/C-rich sequences

Metal ions are essential components for the survival of living organisms. For most species, intracellular and extracellular ionic conditions differ significantly. As G-quadruplexes (G4s) are ion-dependent structures, changes in the [Na+]/[K+] ratio may affect the folding of genomic G4s. More than 11000 putative G4 sequences in the human genome (hg19) contain at least two runs of three continuous cytosines, and these mixed G/C-rich sequences may form a quadruplex or a competing hairpin structure based on G-C base pairing. In this study, we examine how the [Na+]/[K+] ratio influences the structures of G/C-rich sequences. The natural G4 structure with a 9-nt long central loop, CEBwt, was chosen as a model sequence, and the loop bases were gradually replaced by cytosines. The series of CEB mutations revealed that the presence of cytosines in G4 loops does not prevent G4 folding or decrease G4 stability but increases the probability of forming a competing structure, either a hairpin or an intermolecular duplex. Slow conversion to the quadruplex in vitro (in a potassium-rich buffer) and cells was demonstrated by NMR. 'Shape-shifting' sequences may respond to [Na+]/[K+] changes with delayed kinetics.

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.

G-quadruplex ligands as therapeutic agents against cancer, neurological disorders and viral infections

G-quadruplexes (G4s) within the human genome have undergone extensive molecular investigation, with a strong focus on telomeres, gene promoters and repetitive regulatory sequences. G4s play central roles in regulating essential biological processes, including telomere maintenance, replication, transcription and translation. Targeting these molecular processes with G4-binding ligands holds substantial therapeutic potential in anticancer treatments and has also shown promise in treating neurological, skeletal and muscular disorders. The presence of G4s in bacterial and viral genomes also suggests that G4-binding ligands could be a critical tool in fighting infections. This review provides an overview of the progress and applications of G4-binding ligands, their proposed mechanisms of action, challenges faced and prospects for their utilization in anticancer treatments, neurological disorders and antiviral activities.

Understanding the Interactions of Guanine Quadruplexes with Peptides as Novel Strategies for Diagnosis or Tuning Biological Functions

Guanine quadruplexes (G4s) are nucleic acid structures exhibiting a complex structural behavior and exerting crucial biological functions in both cells and viruses. The specific interactions of peptides with G4s, as well as an understanding of the factors driving the specific recognition are important for the rational design of both therapeutic and diagnostic agents. In this review, we examine the most important studies dealing with the interactions between G4s and peptides, highlighting the strengths and limitations of current analytic approaches. We also show how the combined use of high-level molecular simulation techniques and experimental spectroscopy is the best avenue to design specifically tuned and selective peptides, thus leading to the control of important biological functions.

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.

Can G-quadruplex become a promising target in HBV therapy?

The chronic infection with hepatitis B virus (HBV) is an important health problem that affects millions of people worldwide. Current therapies for HBV always suffer from a poor response rate, common side effects, and the need for lifelong treatment. Novel therapeutic targets are expected. Interestingly, non-canonical structures of nucleic acids play crucial roles in the regulation of gene expression. Especially the formation of G-quadruplexes (G4s) in G-rich strands has been demonstrated to affect many bioprocesses including replication, transcription, and translation, showing great potential as targets in anticancer and antiviral therapies. In this review, we summarize recent antiviral studies about G4s and discuss the potential roles of G4 structures in antiviral therapy for HBV.

Deciphering RNA G-quadruplex function during the early steps of HIV-1 infection

G-quadruplexes (G4s) are four-stranded nucleic acid structures formed by the stacking of G-tetrads. Here we investigated their formation and function during HIV-1 infection. Using bioinformatics and biophysics analyses we first searched for evolutionary conserved G4-forming sequences in HIV-1 genome. We identified 10 G4s with conservation rates higher than those of HIV-1 regulatory sequences such as RRE and TAR. We then used porphyrin-based G4-binders to probe the formation of the G4s during infection of human cells by native HIV-1. The G4-binders efficiently inhibited HIV-1 infectivity, which is attributed to the formation of G4 structures during HIV-1 replication. Using a qRT-PCR approach, we showed that the formation of viral G4s occurs during the first 2 h post-infection and their stabilization by the G4-binders prevents initiation of reverse transcription. We also used a G4-RNA pull-down approach, based on a G4-specific biotinylated probe, to allow the direct detection and identification of viral G4-RNA in infected cells. Most of the detected G4-RNAs contain crucial regulatory elements such as the PPT and cPPT sequences as well as the U3 region. Hence, these G4s would function in the early stages of infection when the viral RNA genome is being processed for the reverse transcription step.

RNA G-quadruplex forming regions from SARS-2, SARS-1 and MERS coronoviruses

COVID-19 (Corona Virus Disease 2019), SARS (Severe Acute Respiratory Syndrome) and MERS (Middle East Respiratory Syndrome) are infectious diseases each caused by coronavirus outbreaks. Small molecules and other therapeutics are rapidly being developed to treat these diseases, but the threat of new variants and outbreaks argue for the identification of additional viral targets. Here we identify regions in each of the three coronavirus genomes that are able to form G-quadruplex (G4) structures. G4s are structures formed by DNA or RNA with a core of two or more stacked planes of guanosine tetrads. In recent years, numerous DNA and RNA G4s have emerged as promising pharmacological targets for the treatment of cancer and viral infection. We use a combination of bioinformatics and biophysical approaches to identify conserved RNA G4 regions from the ORF1A and S sequences of SARS-CoV, SARS-CoV-2 and MERS-CoV. Although a general depletion of G4-forming regions is observed in coronaviridae, the preservation of these selected G4 sequences support a significance in viral replication. Targeting these RNA structures may represent a new antiviral strategy against these viruses distinct from current approaches that target viral proteins.

Hepatitis C virus nonstructural protein NS3 unfolds viral G-quadruplex RNA structures

Hepatitis C virus (HCV) is a major cause of liver-related diseases and hepatocellular carcinoma. The helicase domain of one of the nonstructural proteins of HCV, NS3 (nonstructural protein 3), is essential for viral replication; however, its specific biological role is still under investigation. Here, we set out to determine the interaction between a purified recombinant full length NS3 and synthetic guanine-rich substrates that represent the conserved G-quadruplex (G4)-forming sequences in the HCV-positive and HCV-negative strands. We performed fluorescence anisotropy binding, G4 reporter duplex unwinding, and G4RNA trapping assays to determine the binding and G4 unfolding activity of NS3. Our data suggest that NS3 can unfold the conserved G4 structures present within the genome and the negative strand of HCV. Additionally, we found the activity of NS3 on a G4RNA was reduced significantly in the presence of a G4 ligand. The ability of NS3 to unfold HCV G4RNA could imply a novel biological role of the viral helicase in replication.

Targeting the RNA G-Quadruplex and Protein Interactome for Antiviral Therapy

In recent years, G-quadruplexes (G4s), types of noncanonical four-stranded nucleic acid structures, have been identified in many viruses that threaten human health, such as HIV and Epstein-Barr virus. In this context, G4 ligands were designed to target the G4 structures, among which some have shown promising antiviral effects. In this Perspective, we first summarize the diversified roles of RNA G4s in different viruses. Next, we introduce small-molecule ligands developed as G4 modulators and highlight their applications in antiviral studies. In addition to G4s, we comprehensively review the medical intervention of G4-interacting proteins from both the virus (N protein, viral-encoded helicases, severe acute respiratory syndrome-unique domain, and Epstein-Barr nuclear antigen 1) and the host (heterogeneous nuclear ribonucleoproteins, RNA helicases, zinc-finger cellular nucelic acid-binding protein, and nucleolin) by inhibitors as an alternative way to disturb the normal functions of G4s. Finally, we discuss the challenges and opportunities in G4-based antiviral therapy.

Innate immune suppression by SARS-CoV-2 mRNA vaccinations: The role of G-quadruplexes, exosomes, and MicroRNAs

The mRNA SARS-CoV-2 vaccines were brought to market in response to the public health crises of Covid-19. The utilization of mRNA vaccines in the context of infectious disease has no precedent. The many alterations in the vaccine mRNA hide the mRNA from cellular defenses and promote a longer biological half-life and high production of spike protein. However, the immune response to the vaccine is very different from that to a SARS-CoV-2 infection. In this paper, we present evidence that vaccination induces a profound impairment in type I interferon signaling, which has diverse adverse consequences to human health. Immune cells that have taken up the vaccine nanoparticles release into circulation large numbers of exosomes containing spike protein along with critical microRNAs that induce a signaling response in recipient cells at distant sites. We also identify potential profound disturbances in regulatory control of protein synthesis and cancer surveillance. These disturbances potentially have a causal link to neurodegenerative disease, myocarditis, immune thrombocytopenia, Bell's palsy, liver disease, impaired adaptive immunity, impaired DNA damage response and tumorigenesis. We show evidence from the VAERS database supporting our hypothesis. We believe a comprehensive risk/benefit assessment of the mRNA vaccines questions them as positive contributors to public health.

Interaction of HnRNP F with the guanine-rich segments in viral antigenomic RNA enhances porcine reproductive and respiratory syndrome virus-2 replication

Background

Heterogeneous nuclear ribonucleoprotein (HnRNP) F is a member of HnRNP family proteins that participate in splicing of cellular newly synthesized mRNAs by specifically recognizing tandem guanine-tracts (G-tracts) RNA sequences. Whether HnRNP F could recognize viral-derived tandem G-tracts and affect virus replication remain poorly defined.

Methods

The effect of HnRNP F on porcine reproductive and respiratory syndrome virus (PRRSV) propagation was evaluated by real-time PCR, western blotting, and plaque-forming unit assay. The association between HnRNP F and PRRSV guanine-rich segments (GRS) were analyzed by RNA pulldown and RNA immunoprecipitation. The expression pattern of HnRNP F was investigated by western blotting and nuclear and cytoplasmic fractionation.

Results

Knockdown of endogenous HnRNP F effectively blocks the synthesis of viral RNA and nucleocapsid (N) protein. Conversely, overexpression of porcine HnRNP F has the opposite effect. Moreover, RNA pulldown and RNA immunoprecipitation assays reveal that the qRMM1 and qRRM2 domains of HnRNP F recognize the GRS in PRRSV antigenomic RNA. Finally, HnRNP F is redistributed into the cytoplasm and forms a complex with guanine-quadruplex (G4) helicase DHX36 during PRRSV infection.

Conclusions

These findings elucidate the potential functions of HnRNP F in regulating the proliferation of PRRSV and contribute to a better molecular understanding of host-PRRSV interactions.

Guanine quadruplexes in the RNA genome of the tick-borne encephalitis virus: their role as a new antiviral target and in virus biology

We have identified seven putative guanine quadruplexes (G4) in the RNA genome of tick-borne encephalitis virus (TBEV), a flavivirus causing thousands of human infections and numerous deaths every year. The formation of G4s was confirmed by biophysical methods on synthetic oligonucleotides derived from the predicted TBEV sequences. TBEV-5, located at the NS4b/NS5 boundary and conserved among all known flaviviruses, was tested along with its mutated variants for interactions with a panel of known G4 ligands, for the ability to affect RNA synthesis by the flaviviral RNA-dependent RNA polymerase (RdRp) and for effects on TBEV replication fitness in cells. G4-stabilizing TBEV-5 mutations strongly inhibited RdRp RNA synthesis and exhibited substantially reduced replication fitness, different plaque morphology and increased sensitivity to G4-binding ligands in cell-based systems. In contrast, strongly destabilizing TBEV-5 G4 mutations caused rapid reversion to the wild-type genotype. Our results suggest that there is a threshold of stability for G4 sequences in the TBEV genome, with any deviation resulting in either dramatic changes in viral phenotype or a rapid return to this optimal level of G4 stability. The data indicate that G4s are critical elements for efficient TBEV replication and are suitable targets to tackle TBEV infection.

Major Achievements in the Design of Quadruplex-Interactive Small Molecules

Organic small molecules that can recognize and bind to G-quadruplex and i-Motif nucleic acids have great potential as selective drugs or as tools in drug target discovery programs, or even in the development of nanodevices for medical diagnosis. Hundreds of quadruplex-interactive small molecules have been reported, and the challenges in their design vary with the intended application. Herein, we survey the major achievements on the therapeutic potential of such quadruplex ligands, their mode of binding, effects upon interaction with quadruplexes, and consider the opportunities and challenges for their exploitation in drug discovery.

Comprehensive Analysis of G-Quadruplexes in African Swine Fever Virus Genome Reveals Potential Antiviral Targets by G-Quadruplex Stabilizers

African Swine Fever Virus (ASFV), a lethal hemorrhagic fever of the swine, poses a major threat to the world's swine population and has so far resulted in devastating socio-economic consequences. The situation is further compounded by the lack of an approved vaccine or antiviral drug. Herein, we investigated a novel anti-ASFV approach by targeting G-Quadruplexes (G4s) in the viral genome. Bioinformatics analysis of putative G-quadruplex-forming sequences (PQSs) in the genome of ASFV BA71V strain revealed 317 PQSs on the forward strand and 322 PQSs on the reverse strand of the viral genome, translating to a density of 3.82 PQSs/kb covering 9.52% of the entire genome, which means that 85% of genes in the ASFV genome have at least 1 PQS on either strand. Biochemical characterization showed that 8 out of 13 conserved PQSs could form stable G4s in the presence of K⁺, and 4 of them could be stabilized by G4 ligands, N-Methyl Mesoporphyrin (NMM), and pyridostatin (PDS) in vitro. An enhanced green fluorescent protein (EGFP)-based reporter system revealed that the expression of two G4-containing genes, i.e., P1192R and D117L, could be significantly suppressed by NMM and PDS in 293T cells. In addition, a virus infection model showed that NMM could inhibit the replication of ASFV in Porcine Alveolar Macrophages (PAM) cells with an EC₅₀ value of 1.16 μM. Altogether, the present study showed that functional PQSs existent in the promoters, CDS, 3' and 5' UTRs of the ASFV genome could be stabilized by G4 ligands, such as NMM and PDS, and could serve as potential targets for antivirals.

RNA helicases required for viral propagation in humans

RNA viruses cause many routine illnesses, such as the common cold and the flu. Recently, more deadly diseases have emerged from this family of viruses. The hepatitis C virus has had a devastating impact worldwide. Despite the cures developed in the U.S. and Europe, economically disadvantaged countries remain afflicted by HCV infection due to the high cost of these medications. More recently, COVID-19 has swept across the world, killing millions and disrupting economies and lifestyles; the virus responsible for this pandemic is a coronavirus. Our understanding of HCV and SARS CoV-2 replication is still in its infancy. Helicases play a critical role in the replication, transcription and translation of viruses. These key enzymes need extensive study not only as an essential player in the viral lifecycle, but also as targets for antiviral therapeutics. In this review, we highlight the current knowledge for RNA helicases of high importance to human health.

Characterization of a G-quadruplex from hepatitis B virus and its stabilization by binding TMPyP4, BRACO19 and PhenDC3

Specific guanine rich nucleic acid sequences can form non-canonical structures, like the four stranded G-quadruplex (GQ). We studied the GQ-forming sequence (named HepB) found in the genome of the hepatitis B virus. Fluorescence-, infrared- and CD-spectroscopy were used. HepB shows a hybrid form in presence of K⁺, but Na⁺, Li⁺, and Rb⁺ induce parallel structure. Higher concentrations of metal ions increase the unfolding temperature, which was explained by a short thermodynamic calculation. Temperature stability of the GQ structure was determined for all these ions. Na⁺ has stronger stabilizing effect on HepB than K⁺, which is highly unusual. The transition temperatures were 56.6, 53.8, 58.5 and 54.4 °C for Na⁺, K⁺, Li⁺, and Rb⁺ respectively. Binding constants for Na⁺ and K⁺ were 10.2 mM and 7.1 mM respectively. Study of three ligands designed in cancer research for GQ targeting (TMPyP4, BRACO19 and PhenDC3) showed unequivocally their binding to HepB. Binding was proven by the increased stability of the bound form. The stabilization was higher than 20 °C for TMPyP4 and PhenDC3, while it was considerably lower for BRACO19. These results might have medical importance in the fight against the hepatitis B virus.

The different activities of RNA G-quadruplex structures are controlled by flanking sequences

The role of G-quadruplex (G4) RNA structures is multifaceted and controversial. Here, we have used as a model the EBV-encoded EBNA1 and the Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded LANA1 mRNAs. We have compared the G4s in these two messages in terms of nucleolin binding, nuclear mRNA retention, and mRNA translation inhibition and their effects on immune evasion. The G4s in the EBNA1 message are clustered in one repeat sequence and the G4 ligand PhenDH2 prevents all G4-associated activities. The RNA G4s in the LANA1 message take part in similar multiple mRNA functions but are spread throughout the message. The different G4 activities depend on flanking coding and non-coding sequences and, interestingly, can be separated individually. Together, the results illustrate the multifunctional, dynamic and context-dependent nature of G4 RNAs and highlight the possibility to develop ligands targeting specific RNA G4 functions. The data also suggest a common multifunctional repertoire of viral G4 RNA activities for immune evasion.

Characterization of the SARS-CoV-2 genomes in Egypt in first and second waves of infection

At Wuhan, in December 2019, the SRAS-CoV-2 outbreak was detected and it has been the pandemic worldwide. This study aims to investigate the mutations in sequence of the SARS-CoV-2 genome and characterize the mutation patterns in Egyptian COVID-19 patients during different waves of infection. The samples were collected from 250 COVID-19 patients and the whole genome sequencing was conducted using Next Generation Sequencing. The viral sequence analysis showed 1115 different genome from all Egyptian samples in the second wave mutations including 613 missense mutations, 431 synonymous mutations, 25 upstream gene mutations, 24 downstream gene mutations, 10 frame-shift deletions, and 6 stop gained mutation. The Egyptian genomic strains sequenced in second wave of infection are different to that of the first wave. We observe a shift of lineage prevalence from the strain B.1 to B.1.1.1. Only one case was of the new English B.1.1.7. Few samples have one or two mutations of interest from the Brazil and South Africa isolates. New clade 20B appear by March 2020 and 20D appear by May 2020 till January 2021.

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.

Gold(III) porphyrins: Synthesis and interaction with G-quadruplex DNA

G-quadruplex nucleic acids (G4s) are RNA and DNA secondary structures involved in the regulation of multiple key biological processes. They can be found in telomeres, oncogene promoters, RNAs, but also in viral genomes. Due to their unique structural features, very distinct from the canonical duplexes or single-strands, G4s represent promising pharmacological targets for small molecules, namely G4-ligands. Gold(III) penta-cationic porphyrins, as specific G4 ligands, are able to inhibit HIV-1 infectivity and their antiviral activity correlates with their affinity for G4s. Up to now, one of the best antiviral compounds is meso-5,10,15,20-tetrakis[4-(N-methyl-pyridinium-2-yl)phenyl]porphyrinato gold(III) (1). Starting from this compound, we report a structure/affinity relationship study of gold(III) cationic porphyrins to find out the best porphyrin candidate for functionalization, in order to study the antiviral mechanism of action of these gold(III) porphyrins.

Guanine quadruplexes and their roles in molecular processes

The role of guanine quadruplexes (G4) in fundamental biological processes like DNA replication, transcription, translation and telomere maintenance is recognized. G4 structure dynamics is regulated by G4 structure binding proteins and is thought to be crucial for the maintenance of genome integrity in both prokaryotic and eukaryotic cells. Growing research over the last decade has expanded the existing knowledge of the functional diversity of G4 (DNA and RNA) structures across the working models. The control of G4 structure dynamics using G4 binding drugs has been suggested as the putative targets in the control of cancer and bacterial pathogenesis. This review has brought forth the collections of recent information that indicate G4 (mostly G4 DNA) roles in microbial pathogenesis, DNA damaging stress response in bacteria and mammalian cells. Studies in mitochondrial gene function regulation by G4s have also been underscored. Finally, the interdependence of G4s and epigenetic modifications and their speculated medical implications through G4 interacting proteins has been discussed.

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

Influenza A virus (IAV) causes seasonal epidemics and sporadic pandemics, therefore is an important research subject for scientists around the world. Despite the high variability of its genome, the structure of viral RNA (vRNA) possesses features that remain constant between strains and are biologically important for virus replication. Therefore, conserved structural motifs of vRNA can represent a novel therapeutic target. Here, we focused on the presence of G-rich sequences within the influenza A/California/07/2009(H1N1) genome and their ability to form RNA G-quadruplex structures (G4s). We identified 12 potential quadruplex-forming sequences (PQS) and determined their conservation among the IAV strains using bioinformatics tools. Then we examined the propensity of PQS to fold into G4s by various biophysical methods. Our results revealed that six PQS oligomers could form RNA G-quadruplexes. However, three of them were confirmed to adopt G4 structures by all utilized methods. Moreover, we showed that these PQS motifs are present within segments encoding polymerase complex proteins indicating their possible role in the virus biology.

SARS-CoV-2 hot-spot mutations are significantly enriched within inverted repeats and CpG island loci

SARS-CoV-2 is an intensively investigated virus from the order Nidovirales (Coronaviridae family) that causes COVID-19 disease in humans. Through enormous scientific effort, thousands of viral strains have been sequenced to date, thereby creating a strong background for deep bioinformatics studies of the SARS-CoV-2 genome. In this study, we inspected high-frequency mutations of SARS-CoV-2 and carried out systematic analyses of their overlay with inverted repeat (IR) loci and CpG islands. The main conclusion of our study is that SARS-CoV-2 hot-spot mutations are significantly enriched within both IRs and CpG island loci. This points to their role in genomic instability and may predict further mutational drive of the SARS-CoV-2 genome. Moreover, CpG islands are strongly enriched upstream from viral ORFs and thus could play important roles in transcription and the viral life cycle. We hypothesize that hypermethylation of these loci will decrease the transcription of viral ORFs and could therefore limit the progression of the disease.

Analyses of viral genomes for G-quadruplex forming sequences reveal their correlation with the type of infection

G-quadruplexes contribute to the regulation of key molecular processes. Their utilization for antiviral therapy is an emerging field of contemporary research. Here we present comprehensive analyses of the presence and localization of putative G-quadruplex forming sequences (PQS) in all viral genomes currently available in the NCBI database (including subviral agents). The G4Hunter algorithm was applied to a pool of 11,000 accessible viral genomes representing 350 Mbp in total. PQS frequencies differ across evolutionary groups of viruses, and are enriched in repeats, replication origins, 5'UTRs and 3'UTRs. Importantly, PQS presence and localization is connected to viral lifecycles and corresponds to the type of viral infection rather than to nucleic acid type; while viruses routinely causing persistent infections in Metazoa hosts are enriched for PQS, viruses causing acute infections are significantly depleted for PQS. The unique localization of PQS identifies the importance of G-quadruplex-based regulation of viral replication and life cycle, providing a tool for potential therapeutic targeting.

Tracing dsDNA Virus-Host Coevolution through Correlation of Their G-Quadruplex-Forming Sequences

The importance of gene expression regulation in viruses based upon G-quadruplex may point to its potential utilization in therapeutic targeting. Here, we present analyses as to the occurrence of putative G-quadruplex-forming sequences (PQS) in all reference viral dsDNA genomes and evaluate their dependence on PQS occurrence in host organisms using the G4Hunter tool. PQS frequencies differ across host taxa without regard to GC content. The overlay of PQS with annotated regions reveals the localization of PQS in specific regions. While abundance in some, such as repeat regions, is shared by all groups, others are unique. There is abundance within introns of Eukaryota-infecting viruses, but depletion of PQS in introns of bacteria-infecting viruses. We reveal a significant positive correlation between PQS frequencies in dsDNA viruses and corresponding hosts from archaea, bacteria, and eukaryotes. A strong relationship between PQS in a virus and its host indicates their close coevolution and evolutionarily reciprocal mimicking of genome organization.

CNBP Binds and Unfolds In Vitro G-Quadruplexes Formed in the SARS-CoV-2 Positive and Negative Genome Strands

The Coronavirus Disease 2019 (COVID-19) pandemic has become a global health emergency with no effective medical treatment and with incipient vaccines. It is caused by a new positive-sense RNA virus called severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2). G-quadruplexes (G4s) are nucleic acid secondary structures involved in the control of a variety of biological processes including viral replication. Using several G4 prediction tools, we identified highly putative G4 sequences (PQSs) within the positive-sense (+gRNA) and negative-sense (−gRNA) RNA strands of SARS-CoV-2 conserved in related betacoronaviruses. By using multiple biophysical techniques, we confirmed the formation of two G4s in the +gRNA and provide the first evidence of G4 formation by two PQSs in the −gRNA of SARS-CoV-2. Finally, biophysical and molecular approaches were used to demonstrate for the first time that CNBP, the main human cellular protein bound to SARS-CoV-2 RNA genome, binds and promotes the unfolding of G4s formed by both strands of SARS-CoV-2 RNA genome. Our results suggest that G4s found in SARS-CoV-2 RNA genome and its negative-sense replicative intermediates, as well as the cellular proteins that interact with them, are relevant factors for viral genes expression and replication cycle, and may constitute interesting targets for antiviral drugs development.

Assessing G4-Binding Ligands In Vitro and in Cellulo Using Dimeric Carbocyanine Dye Displacement Assay

G-quadruplexes (G4) are the most actively studied non-canonical secondary structures formed by contiguous repeats of guanines in DNA or RNA strands. Small molecule mediated targeting of G-quadruplexes has emerged as an attractive tool for visualization and stabilization of these structures inside the cell. Limited number of DNA and RNA G4-selective assays have been reported for primary ligand screening. A combination of fluorescence spectroscopy, AFM, CD, PAGE, and confocal microscopy have been used to assess a dimeric carbocyanine dye B6,5 for screening G4-binding ligands in vitro and in cellulo. The dye B6,5 interacts with physiologically relevant DNA and RNA G4 structures, resulting in fluorescence enhancement of the molecule as an in vitro readout for G4 selectivity. Interaction of the dye with G4 is accompanied by quadruplex stabilization that extends its use in primary screening of G4 specific ligands. The molecule is cell permeable and enables visualization of quadruplex dominated cellular regions of nucleoli using confocal microscopy. The dye is displaced by quarfloxin in live cells. The dye B6,5 shows remarkable duplex to quadruplex selectivity in vitro along with ligand-like stabilization of DNA G4 structures. Cell permeability and response to RNA G4 structures project the dye with interesting theranostic potential. Our results validate that B6,5 can serve the dual purpose of visualization of DNA and RNA G4 structures and screening of G4 specific ligands, and adds to the limited number of probes with such potential.

Whole Genome Identification of Potential G-Quadruplexes and Analysis of the G-Quadruplex Binding Domain for SARS-CoV-2

The coronavirus disease 2019 (COVID-19) pandemic caused by SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) has become a global public health emergency. G-quadruplex, one of the non-canonical secondary structures, has shown potential antiviral values. However, little is known about the G-quadruplexes of the emerging SARS-CoV-2. Herein, we characterized the potential G-quadruplexes in both positive and negative-sense viral strands. The identified potential G-quadruplexes exhibited similar features to the G-quadruplexes detected in the human transcriptome. Within some bat- and pangolin-related betacoronaviruses, the G-tracts rather than the loops were under heightened selective constraints. We also found that the amino acid sequence similar to SUD (SARS-unique domain) was retained in SARS-CoV-2 but depleted in some other coronaviruses that can infect humans. Further analysis revealed that the amino acid residues related to the binding affinity of G-quadruplexes were conserved among 16,466 SARS-CoV-2 samples. Moreover, the dimer of the SUD-homology structure in SARS-CoV-2 displayed similar electrostatic potential patterns to the SUD dimer from SARS. Considering the potential value of G-quadruplexes to serve as targets in antiviral strategy, our fundamental research could provide new insights for the SARS-CoV-2 drug discovery.

Stabilization of human telomeric RNA G-quadruplex by the water-compatible optically pure and biologically-active metallohelices

RNA G-quadruplexes have been suggested to play key roles in fundamental biological processes and are linked to human diseases. Thus, they also represent good potential therapeutic targets. Here, we describe, using the methods of molecular biophysics, interactions of a series of biologically-active supramolecular cationic metallohelices with human telomeric RNA G-quadruplex. We demonstrate that the investigated metallohelices bind with a high affinity to human telomeric RNA G-quadruplex and that their binding selectivity considerably differs depending on the dimensions and overall shape of the metallohelices. Additionally, the investigated metallohelices inhibit DNA synthesis on the RNA template containing four repeats of the human telomeric sequence by stabilizing the RNA G-quadruplex structure. Collectively, the results of this study suggest that stabilization of RNA sequences capable of G-quadruplex formation by metallohelices investigated in this work might contribute to the mechanism of their biological activity.

Mechanisms of RNA N6-Methyladenosine in Hepatocellular Carcinoma: From the Perspectives of Etiology

N6-Methyladenosine (m⁶A) is the most common RNA internal modification in eukaryotic cells. Its regulatory effects at the post-transcriptional level on both messenger RNAs (mRNAs) and noncoding RNAs have been widely studied; these include alternative splicing, stability, translation efficiency, nucleus export, and degradation. m⁶A modification is implicated in a series of physiological and pathological activities, such as embryonic stem cell differentiation, immunoregulation, adipogenesis, and cancer development. Recently, the significance of m⁶A methylation has been identified in both viral hepatitis and non-alcohol fatty liver disease (NAFLD), which are major risk factors in the development of hepatocellular carcinoma (HCC). Given the high incidence and mortality rate of HCC worldwide, it is of great importance to elucidate the mechanisms underlying HCC initiation and progression. m⁶A as an emerging research focus has great potential to facilitate the understanding of HCC, particularly from an etiological perspective. Thus, in this review, we summarize recent progress in understanding m⁶A modification related to viral hepatitis, NAFLD, and HCC, including their mechanisms and clinical applications.

In-Depth Bioinformatic Analyses of Nidovirales Including Human SARS-CoV-2, SARS-CoV, MERS-CoV Viruses Suggest Important Roles of Non-canonical Nucleic Acid Structures in Their Lifecycles

Non-canonical nucleic acid structures play important roles in the regulation of molecular processes. Considering the importance of the ongoing coronavirus crisis, we decided to evaluate genomes of all coronaviruses sequenced to date (stated more broadly, the order Nidovirales) to determine if they contain non-canonical nucleic acid structures. We discovered much evidence of putative G-quadruplex sites and even much more of inverted repeats (IRs) loci, which in fact are ubiquitous along the whole genomic sequence and indicate a possible mechanism for genomic RNA packaging. The most notable enrichment of IRs was found inside 5'UTR for IRs of size 12+ nucleotides, and the most notable enrichment of putative quadruplex sites (PQSs) was located before 3'UTR, inside 5'UTR, and before mRNA. This indicates crucial regulatory roles for both IRs and PQSs. Moreover, we found multiple G-quadruplex binding motifs in human proteins having potential for binding of SARS-CoV-2 RNA. Non-canonical nucleic acids structures in Nidovirales and in novel SARS-CoV-2 are therefore promising druggable structures that can be targeted and utilized in the future.

Viral G-quadruplexes: New frontiers in virus pathogenesis and antiviral therapy

Viruses are the most abundant organisms on our planet, affecting all living beings: some of them are responsible for massive epidemics that concern health, national economies and the overall welfare of societies. Although advances in antiviral research have led to successful therapies against several human viruses, still some of them cannot be eradicated from the host and most of them do not have any treatment available. Consequently, innovative antiviral therapies are urgently needed. In the past few years, research on G-quadruplexes (G4s) in viruses has boomed, providing powerful evidence for the regulatory role of G4s in key viral steps. Comprehensive bioinformatics analyses have traced putative G4-forming sequences in the genome of almost all human viruses, showing that their distribution is statistically significant and their presence highly conserved. Since the genomes of viruses are remarkably variable, high conservation rates strongly suggest a crucial role of G4s in the viral replication cycle and evolution, emphasizing the possibility of targeting viral G4s as a new pharmacological approach in antiviral therapy. Recent studies have demonstrated the formation and function of G4s in pathogens responsible for serious diseases, such as HIV-1, Hepatitis B and C, Ebola viruses, to cite a few. In this chapter, we present the state of the art on the structural and functional characterization of viral G4s in RNA viruses, DNA viruses and retroviruses. We also present the G4 ligands that provide further details on the viral G4 role and which, showing promising antiviral activity, which could be exploited for the development of innovative antiviral agents.

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.

Relationship Between G-Quadruplex Sequence Composition in Viruses and Their Hosts

A subset of guanine-rich nucleic acid sequences has the potential to fold into G-quadruplex (G4) secondary structures, which are functionally important for several biological processes, including genome stability and regulation of gene expression. Putative quadruplex sequences (PQSs) G₃₊N_1-7G₃₊N_1-7G₃₊N_1-7G₃₊ are widely found in eukaryotic and prokaryotic genomes, but the base composition of the N_1-7 loops is biased across species. Since the viruses partially hijack their hosts' cellular machinery for proliferation, we examined the PQS motif size, loop length, and nucleotide compositions of 7370 viral genome assemblies and compared viral and host PQS motifs. We studied seven viral taxa infecting five distant eukaryotic hosts and created a resource providing a comprehensive view of the viral quadruplex motifs. Overall, short-looped PQSs are predominant and with a similar composition across viral taxonomic groups, albeit subtle trends emerge upon classification by hosts. Specifically, there is a higher frequency of pyrimidine loops in viruses infecting animals irrespective of the viruses' genome type. This observation is confirmed by an in-depth analysis of the Herpesviridae family of viruses, which showed a distinctive accumulation of thermally stable C-looped quadruplexes in viruses infecting high-order vertebrates. The occurrence of viral C-looped G4s, which carry binding sites for host transcription factors, as well as the high prevalence of viral TTA-looped G4s, which are identical to vertebrate telomeric motifs, provide concrete examples of how PQSs may help viruses impinge upon, and benefit from, host functions. More generally, these observations suggest a co-evolution of virus and host PQSs, thus underscoring the potential functional significance of G4s.

Colocalization of m6A and G-Quadruplex-Forming Sequences in Viral RNA (HIV, Zika, Hepatitis B, and SV40) Suggests Topological Control of Adenosine N 6-Methylation

This Outlook calls attention to two seemingly disparate and emerging fields regarding viral genomics that may be correlated in a way previously overlooked. First, we describe identification of conserved potential G-quadruplex-forming sequences (PQSs) in viral genomes relevant to human health. Studies have demonstrated that PQSs are highly conserved and can fold to G-quadruplexes (G4s) to regulate viral processes. Key examples include G4s as a countermeasure to the host's immune system or G4-guided regulation of replication or transcription. Second, emerging data are discussed concerning the epitranscriptomic modification N 6-methyladenosine (m6A) in viral RNA installed by host proteins in a consensus sequence favoring 5'-GG(m6A)C-3'. The proposed pathways by which m6A is written, read, and erased in viral RNA genomes and the impact this has on viral replication are described. The structural reason why certain sites are selected for modification while others are not is still mysterious. Finally, we discuss our new observations regarding these previous sequencing data that identify m6A installation within the loops of two-tetrad PQSs in the RNA genomes of the Zika, HIV, hepatitis B, and SV40 viruses. We hypothesize that conserved viral PQSs can provide a framework (sequence and/or structural) for m6A installation. We also discuss literature sources suggesting that PQSs as sites of RNA modification could be a general phenomenon. We anticipate our observations will provide ample opportunities for exciting discoveries regarding the interplay between G4 structures and epitranscriptomic modifications of RNA.

Deciphering the Enigmatic Biological Functions of RNA Guanine-Quadruplex Motifs in Human Cells

Guanine-rich sequences in nucleic acids can form noncanonical structures known as guanine quadruplexes (G-quadruplexes), which constitute a not yet fully elucidated layer of regulatory function for central cellular processes. RNA G-quadruplexes have been shown to be involved in the modulation of translation, the regulation of (alternative) splicing, and the subcellular transport of mRNAs, among other processes. However, in living cells, an equilibrium between the formation of G-quadruplex structures and their unwinding by RNA helicases is likely. The extent to which G-rich sequences adopt G-quadruplex structures in living eukaryotic cells is currently a matter of debate. Multiple lines of evidence confirm the intracellular formation of G-quadruplex structures, such as their detection by immunochemical approaches, fluorogenic probes, and in vivo nuclear magnetic resonance. However, intracellular chemical probing suggests most if not all are in an unfolded state. It is therefore tempting to speculate that some G-quadruplex structures are only temporarily formed when they are required to contribute to the fine-tuning of the processes mentioned above. Future research should focus on the analysis of G-quadruplex formation under physiological conditions, which will allow the re-evaluation of the biological function of G-quadruplex motifs in regulatory processes in their natural environment and at physiological expression levels. This will help in the elucidation of their significance in the regulation of central processes in molecular biology and the exploitation of their potential as therapeutic targets.