Binding of oligopyrimidines to the RNA hairpin responsible for the ribosome gag-pol frameshift in HIV-1

Triplex-forming peptide nucleic acids as emerging ligands to modulate structure and function of complex RNAs

Over the last three decades, our view of RNA has changed from a simple intermediate supporting protein synthesis to a major regulator of biological processes. In the expanding area of RNA research, peptide nucleic acid (PNA) is emerging as a promising ligand for triple-helical recognition of complex RNAs. As discussed in this feature article, the key advantages of PNAs are high sequence specificity and affinity for RNA (>10 fold higher than for DNA) that are difficult to achieve with small molecule ligands. Emerging studies demonstrate that triple-helical binding of PNAs can modulate biological function and control dynamic conformational equilibria of complex folded RNAs. These results suggest that PNA has a unique potential as a research tool and therapeutic compound targeting RNA. The remaining problems hampering advances in these directions are limitations of sequences that can be recognized by Hoogsteen triplexes (typically purine rich tracts), poor cellular uptake and bioavailability of PNA, and potential off-target effects in biological systems. Recent exciting studies are discussed that illustrate how synthetic nucleic acid chemistry provides innovative solutions for these problems.

Triplex-Forming Peptide Nucleic Acid Controls Dynamic Conformations of RNA Bulges

RNA folding is driven by the formation of double-helical segments interspaced by loops of unpaired nucleotides. Among the latter, bulges formed by one or several unpaired nucleotides are one of the most common structural motifs that play an important role in stabilizing RNA-RNA, RNA-protein, and RNA-small molecule interactions. Single-nucleotide bulges can fold in alternative structures where the unpaired nucleobase is either looped-out (flexible) in a solvent or stacked-in (intercalated) between the base pairs. In the present study, we discovered that triplex-forming peptide nucleic acids (PNAs) had unusually high affinity for single-purine-nucleotide bulges in double-helical RNA. Depending on the PNA's sequence, the triplex formation shifted the equilibrium between looped-out and stacked-in conformations. The ability to control the dynamic equilibria of RNA's structure will be an important tool for studying structure-function relationships in RNA biology and may have potential in novel therapeutic approaches targeting disease-related RNAs.

Fluorescence Sensing of the Panhandle Structure of the Influenza A Virus RNA Promoter by Thiazole Orange Base Surrogate-Carrying Peptide Nucleic Acid Conjugated with Small Molecule

We have developed a new class of triplex-forming peptide nucleic acid (PNA)-based fluorogenic probes for sensing of the panhandle structure of the influenza A virus (IAV) RNA promoter region. Here, a small molecule (DPQ) capable of selectively binding to the internal loop structure was conjugated with triplex-forming forced intercalation of the thiazole orange (tFIT) probe with natural PNA nucleobases. The resulting conjugate, tFIT-DPQ, showed a significant light-up response (83-fold) upon strong (K_d = 107 nM) and structure-selective binding to the IAV RNA promoter region under physiological conditions (pH 7.0, 100 mM NaCl). We demonstrated the conjugation of these two units through the suitable spacer was key to show useful binding and fluorogenic signaling functions. tFIT-DPQ facilitated the sensitive and selective detection of IAV RNA based on its binding to the promoter region. Furthermore, we found that tFIT-DPQ could work as a sensitive indicator for screening of test compounds targeting the IAV RNA promoter region in the fluorescence indicator displacement assay.

Evolution of peptide nucleic acid with modifications of its backbone and application in biotechnology

Peptide nucleic acids (PNAs) are getting prodigious interest currently in the biomedical and diagnostic field as an extremely powerful tool because of their potentiality to hybridize with natural nucleic acids. Although PNA has strong affinity and sequence specificity to DNA/RNA, there is a considerable ongoing effort to further enhance their special chemical and biological properties for potential application in numerous fields, notably in the field of therapeutics. The toolbox for backbone modified PNAs synthesis has been extended substantially in recent decades, providing a more efficient synthesis of peptides with numerous scaffolds and modifications. This paper reviews the various strategies that have been developed so far for the modification of the PNA backbone, challenging the search for new PNA systems with improved chemical and physical properties lacking in the original aegPNA backbone. The various practical issues and limitations of different PNA systems are also summarized. The focus of this review is on the evolution of PNA by its backbone modification to improve the cellular uptake, sequence specificity, and compatibility of PNA to bind to DNA/RNA. Finally, an insight was also gained into major applications of backbone modified PNAs for the development of biosensors.

Triplex-Forming Peptide Nucleic Acids with Extended Backbones

Peptide nucleic acid (PNA) forms a triple helix with double-stranded RNA (dsRNA) stabilized by a hydrogen-bonding zipper formed by PNA's backbone amides (N-H) interacting with RNA phosphate oxygens. This hydrogen-bonding pattern is enabled by the matching ∼5.7 Å spacing (typical for A-form dsRNA) between PNA's backbone amides and RNA phosphate oxygens. We hypothesized that extending the PNA's backbone by one -CH2 - group might bring the distance between PNA amide groups closer to 7 Å, which is favourable for hydrogen bonding to the B-form dsDNA phosphate oxygens. Extension of the PNA backbone was expected to selectively stabilize PNA-DNA triplexes compared to PNA-RNA. To test this hypothesis, we synthesized triplex-forming PNAs that had the pseudopeptide backbones extended by an additional -CH2 - group in three different positions. Isothermal titration calorimetry measurements of the binding affinity of these extended PNA analogues for the matched dsDNA and dsRNA showed that, contrary to our structural reasoning, extending the PNA backbone at any position had a strong negative effect on triplex stability. Our results suggest that PNAs might have an inherent preference for A-form-like conformations when binding double-stranded nucleic acids. It appears that the original six-atom-long PNA backbone is an almost perfect fit for binding to A-form nucleic acids.

Synthetic, Structural, and RNA Binding Studies on 2-Aminopyridine-Modified Triplex-Forming Peptide Nucleic Acids

The development of new RNA-binding ligands is attracting increasing interest in fundamental science and the pharmaceutical industry. The goal of this study was to improve the RNA binding properties of triplex-forming peptide nucleic acids (PNAs) by further increasing the pK_a of 2-aminopyridine (M). Protonation of M was the key for enabling triplex formation at physiological pH in earlier studies. Substitution on M by an electron-donating 4-methoxy substituent resulted in slight destabilization of the PNA-dsRNA triplex, contrary to the expected stabilization due to more favorable protonation. To explain this unexpected result, the first NMR structural studies were performed on an M-modified PNA-dsRNA triplex which, combined with computational modeling identified unfavorable steric and electrostatic repulsion between the 4-methoxy group of M and the oxygen of the carbonyl group connecting the adjacent nucleobase to PNA backbone. The structural studies also provided insights into hydrogen-bonding interactions that might be responsible for the high affinity and unusual RNA-binding preference of PNAs.

Incorporation of thio-pseudoisocytosine into triplex-forming peptide nucleic acids for enhanced recognition of RNA duplexes

Peptide nucleic acids (PNAs) have been developed for applications in biotechnology and therapeutics. There is great potential in the development of chemically modified PNAs or other triplex-forming ligands that selectively bind to RNA duplexes, but not single-stranded regions, at near-physiological conditions. Here, we report on a convenient synthesis route to a modified PNA monomer, thio-pseudoisocytosine (L), and binding studies of PNAs incorporating the monomer L. Thermal melting and gel electrophoresis studies reveal that L-incorporated 8-mer PNAs have superior affinity and specificity in recognizing the duplex region of a model RNA hairpin to form a pyrimidine motif major-groove RNA₂–PNA triplex, without appreciable binding to single-stranded regions to form an RNA–PNA duplex or, via strand invasion, forming an RNA–PNA₂ triplex at near-physiological buffer condition. In addition, an L-incorporated 8-mer PNA shows essentially no binding to single-stranded or double-stranded DNA. Furthermore, an L-modified 6-mer PNA, but not pseudoisocytosine (J) modified or unmodified PNA, binds to the HIV-1 programmed −1 ribosomal frameshift stimulatory RNA hairpin at near-physiological buffer conditions. The stabilization of an RNA₂–PNA triplex by L modification is facilitated by enhanced van der Waals contacts, base stacking, hydrogen bonding and reduced dehydration energy. The destabilization of RNA–PNA and DNA–PNA duplexes by L modification is due to the steric clash and loss of two hydrogen bonds in a Watson–Crick-like G–L pair. An RNA₂–PNA triplex is significantly more stable than a DNA₂–PNA triplex, probably because the RNA duplex major groove provides geometry compatibility and favorable backbone–backbone interactions with PNA. Thus, L-modified triplex-forming PNAs may be utilized for sequence-specifically targeting duplex regions in RNAs for biological and therapeutic applications.

Recent advances in chemical modification of Peptide nucleic acids

Peptide nucleic acid (PNA) has become an extremely powerful tool in chemistry and biology. Although PNA recognizes single-stranded nucleic acids with exceptionally high affinity and sequence selectivity, there is considerable ongoing effort to further improve properties of PNA for both fundamental science and practical applications. The present paper discusses selected recent studies that improve on cellular uptake and binding of PNA to double-stranded DNA and RNA. The focus is on chemical modifications of PNA's backbone and heterocyclic nucleobases. The paper selects representative recent studies and does not attempt to provide comprehensive coverage of the broad and vibrant field of PNA modification.

Short peptide nucleic acids bind strongly to homopurine tract of double helical RNA at pH 5.5

The important role that noncoding RNA plays in cell biology makes it an attractive target for molecular recognition. However, the discovery of small molecules that bind double helical RNA selectively and may serve as biochemical probes and potential drug leads has been relatively slow. Herein, we show that peptide nucleic acids, as short as six nucleobases, bind very strongly (K(a) > 10(7)) and sequence selectively to a homopurine tract of double helical RNA at pH 5.5. The isothermal titration calorimetry and circular dichroism experiments suggest that the binding mode may be a sequence selective triple helix formation. Our results have implications for development of biochemical probes to study function of noncoding RNAs and design of compounds with potential antibacterial and antiviral activity.

Programmed ribosomal frameshifting in HIV-1 and the SARS-CoV

Ribosomal frameshifting is a mechanism of gene expression used by several RNA viruses to express replicase enzymes. This article focuses on frameshifting in two human pathogens, the retrovirus human immunodeficiency virus type 1 (HIV-1) and the coronavirus responsible for severe acute respiratory syndrome (SARS). The nature of the frameshift signals of HIV-1 and the SARS–CoV will be described and the impact of this knowledge on models of frameshifting will be considered. The role of frameshifting in the replication cycle of the two pathogens and potential antiviral therapies targeting frameshifting will also be discussed.

Characterization of the frameshift stimulatory signal controlling a programmed -1 ribosomal frameshift in the human immunodeficiency virus type 1

Synthesis of the Gag-Pol protein of the human immunodeficiency virus type 1 (HIV-1) requires a programmed -1 ribosomal frameshifting when ribosomes translate the unspliced viral messenger RNA. This frameshift occurs at a slippery sequence followed by an RNA structure motif that stimulates frameshifting. This motif is commonly assumed to be a simple stem-loop for HIV-1. In this study, we show that the frameshift stimulatory signal is more complex than believed and consists of a two-stem helix. The upper stem-loop corresponds to the classic stem-loop, and the lower stem is formed by pairing the spacer region following the slippery sequence and preceding this classic stem-loop with a segment downstream of this stem-loop. A three-purine bulge interrupts the two stems. This structure was suggested by enzymatic probing with nuclease V1 of an RNA fragment corresponding to the gag/pol frameshift region of HIV-1. The involvement of the novel lower stem in frameshifting was supported by site-directed mutagenesis. A fragment encompassing the gag/pol frameshift region of HIV-1 was inserted in the beginning of the coding sequence of a reporter gene coding for the firefly luciferase, such that expression of luciferase requires a -1 frameshift. When the reporter was expressed in COS cells, mutations that disrupt the capacity to form the lower stem reduced frameshifting, whereas compensatory changes that allow re-formation of this stem restored the frameshift efficiency near wild-type level. The two-stem structure that we propose for the frameshift stimulatory signal of HIV-1 differs from the RNA triple helix structure recently proposed.

Modulation of RNA function by oligonucleotides recognizing RNA structure

Numerous RNA structures are responsible for regulatory processes either because they constitute a signal, like the hairpins or pseudoknots involved in ribosomal frameshifting, or because they are binding sites for proteins such as the trans-activating responsive RNA element of the human immunodeficiency virus whose binding to the viral protein Tat and cellular proteins allows full-length transcription of the retroviral genome. Selective ligands able to bind with high affinity to such RNA motifs may serve as tools for dissecting the molecular mechanisms in which they are involved. Such ligands might also constitute prototypes of therapeutic agents when RNA structures play a role in the expression of dysfunctional genes or in the multiplication of pathogens. Different classes of ligands (aminoglycosides, interacalating agents, peptides) are of interest to this aim. However, oligonucleotides deserve particular consideration. They have been extensively used in the frame of the antisense strategy. The apparent simplicity of this rational approach is, at first sight, very attractive. Indeed, numerous successful studies have been published describing the efficient inhibition of translation, splicing, or reverse transcription in cell-free systems, in cultured cells, or in vivo by oligomers complementary to an RNA region. However, RNA structures restrict the access of the target site to the antisense sequence: The competition between the intramolecular association of RNA regions weakens or even abolishes the antisense effect. Various possibilities have been developed to circumvent this limitation. This includes both rational and combinatorial strategies. High-affinity oligomers were designed to invade the RNA structure. Alternatively, triplex-forming oligonucleotides (TFO) and aptamers may recognize the folded RNA motif. Whereas the use of TFOs is rather limited owing to the strong sequence constraints for triple-helix formation, in vitro selection offers a way to explore vast oligoribo or oligodeoxyribo libraries to identify strong, selective oligonucleotide binders. The candidates (aptamers) selected against the TAR RNA element of HIV-1, which form stable loop-loop (kissing) complexes with the target, provide interesting examples of oligonucleotides recognizing a functional RNA structure through an important contribution of tertiary interactions.

RNA and N3'-->P5' kissing aptamers targeted to the trans-activation responsive (TAR) RNA of the human immunodeficiency virus-1

We used in vitro selection to identify RNA aptamers able to selectively bind to the TAR RNA motif of HIV-1, an unperfect RNA hairpin involved in the transcription of the retroviral genome. We selected aptameric RNA hairpins giving rise to kissing complexes with TAR. The N3'-->P5' phosphoramidate variant of the aptamer bind to TAR with a Kd in the low nanomolar range. However, only the RNA-RNA loop-loop complex is recognized by the Rop protein of E. coli which is specific for kissing complexes.

Inhibition of in vitro and ex vivo translation by a transplatin-modified oligo(2'-O-methylribonucleotide) directed against the HIV-1 gag-pol frameshift signal

A 2'-O-methylribooligonucleotide containing a G1.U.G3 triad modified by trans-diamminedichloro-platinum(II) was targeted to the RNA region responsible for the gag-pol frameshifting during translation of the HIV-1 mRNA. The binding of the platinated oligonucleotide to its target RNA induced a rearrangement of the (G1, G3)-intrastrand crosslink, leading to the formation of an intermolecular oligonucleotide-RNA G-A crosslink. This resulted in the selective arrest of translation of a luciferase gene placed downstream of the HIV-1 frameshift signal both in a cell-free extract (rabbit reticulocyte lysate) and in RNA-transfected cells. A specific inhibition of luciferase activity was still observed when the oligonucleotide-RNA complex was not pre-formed prior to either translation or transfection. Moreover, a selective inhibition was also observed when the oligonucleotide and the plasmid DNA encoding the luciferase and bearing the RNA gag- pol frameshifting signal were co-transfected in NIH 3T3 cultured cells. Therefore the intra-strand-->interstrand conversion of the platinum crosslink kinetically competes with the translation machinery and blocks the polypeptide elongation. These transplatin-modified oligonucleotides which operate within a live cell on a 'real-time' basis and do not need an external triggering signal constitute a promising new class of selective reactive probes.