Anti-HIV activity of new higher order G-quadruplex aptamers obtained from tetra-end-linked oligonucleotides

Cytosine-rich oligonucleotides incorporating a non-nucleotide loop: A further step towards the obtainment of physiologically stable i-motif DNA

i-Motifs, also known as i-tetraplexes, are secondary structures of DNA occurring in cytosine-rich oligonucleotides (CROs) that recall increasing interest in the scientific community for their relevance in various biological processes and DNA nanotechnology. This study reports the design of new structurally modified CROs, named Double-Ended-Linker-CROs (DEL-CROs), capable of forming stable i-motif structures. Here, two C-rich strands having sequences d(AC₄A) and d(C₆) have been attached, in a parallel fashion, to the two linker's edges by their 3' or 5' ends. The resulting DEL-CROs have been investigated for their capability to form i-motif structures by circular dichroism, poly-acrylamide gel electrophoresis, HPLC-size-exclusion chromatography, and NMR studies. This investigation established that DEL-CROs could form more stable i-motif structures than the corresponding unmodified CROs. In particular, the i-motif formed by DEL-5'-d(C₆)₂ resulted stable enough to be detected even at near physiological conditions (37 °C, pH 7.0). The results open the way to developing pH-switchable nanocarriers and aptamers based on suitably functionalized DEL-CROs.

Nucleic Acids as Biotools at the Interface between Chemistry and Nanomedicine in the COVID-19 Era

The recent development of mRNA vaccines against the SARS-CoV-2 infection has turned the spotlight on the potential of nucleic acids as innovative prophylactic agents and as diagnostic and therapeutic tools. Until now, their use has been severely limited by their reduced half-life in the biological environment and the difficulties related to their transport to target cells. These limiting aspects can now be overcome by resorting to chemical modifications in the drug and using appropriate nanocarriers, respectively. Oligonucleotides can interact with complementary sequences of nucleic acid targets, forming stable complexes and determining their loss of function. An alternative strategy uses nucleic acid aptamers that, like the antibodies, bind to specific proteins to modulate their activity. In this review, the authors will examine the recent literature on nucleic acids-based strategies in the COVID-19 era, focusing the attention on their applications for the prophylaxis of COVID-19, but also on antisense- and aptamer-based strategies directed to the diagnosis and therapy of the coronavirus pandemic.

Structure- and Interaction-Based Design of Anti-SARS-CoV-2 Aptamers

Aptamer selection against novel infections is a complicated and time-consuming approach. Synergy can be achieved by using computational methods together with experimental procedures. This study aims to develop a reliable methodology for a rational aptamer in silico et vitro design. The new approach combines multiple steps: (1) Molecular design, based on screening in a DNA aptamer library and directed mutagenesis to fit the protein tertiary structure; (2) 3D molecular modeling of the target; (3) Molecular docking of an aptamer with the protein; (4) Molecular dynamics (MD) simulations of the complexes; (5) Quantum-mechanical (QM) evaluation of the interactions between aptamer and target with further analysis; (6) Experimental verification at each cycle for structure and binding affinity by using small-angle X-ray scattering, cytometry, and fluorescence polarization. By using a new iterative design procedure, structure- and interaction-based drug design (SIBDD), a highly specific aptamer to the receptor-binding domain of the SARS-CoV-2 spike protein, was developed and validated. The SIBDD approach enhances speed of the high-affinity aptamers development from scratch, using a target protein structure. The method could be used to improve existing aptamers for stronger binding. This approach brings to an advanced level the development of novel affinity probes, functional nucleic acids. It offers a blueprint for the straightforward design of targeting molecules for new pathogen agents and emerging variants.

Aptamers in Virology-A Consolidated Review of the Most Recent Advancements in Diagnosis and Therapy

The use of short oligonucleotide or peptide molecules as target-specific aptamers has recently garnered substantial attention in the field of the detection and treatment of viral infections. Based on their high affinity and high specificity to desired targets, their use is on the rise to replace antibodies for the detection of viruses and viral antigens. Furthermore, aptamers inhibit intracellular viral transcription and translation, in addition to restricting viral entry into host cells. This has opened up a plethora of new targets for the research and development of novel vaccines against viruses. Here, we discuss the advances made in aptamer technology for viral diagnosis and therapy in the past decade.

Aptamers against the β-Conglutin Allergen: Insights into the Behavior of the Shortest Multimeric (Intra)Molecular DNA G-Quadruplex

In previous work, a 93-mer aptamer was selected against the anaphylactic allergen, β-conglutin and truncated to an 11-mer, improving the affinity by two orders of magnitude, whilst maintaining the specificity. This 11-mer was observed to fold in a G-quadruplex, and preliminary results indicated the existence of a combination of monomeric and higher-order structures. Building on this previous work, in the current study, we aimed to elucidate a deeper understanding of the structural forms of this 11-mer and the effect of the structure on its binding ability. A battery of techniques including polyacrylamide gel electrophoresis, high-performance liquid chromatography in combination with electrospray ionization time-of-flight mass spectrometry, matrix-assisted laser desorption/ionization time-of-flight, thermal binding analysis, circular dichroism and nuclear magnetic resonance were used to probe the structure of both the 11-mer and the 11-mer flanked with TT- at either the 5′ or 3′ end or at both ends. The TT-tail at the 5′ end hinders stacking effects and effectively enforces the 11-mer to maintain a monomeric form. The 11-mer and the TT- derivatives of the 11-mer were also evaluated for their ability to bind its cognate target using microscale thermophoresis and surface plasmon resonance, and biolayer interferometry confirmed the nanomolar affinity of the 11-mer. All the techniques utilized confirmed that the 11-mer was found to exist in a combination of monomeric and higher-order structures, and that independent of the structural form present, nanomolar affinity was observed.

Dimeric and Multimeric DNA Aptamers for Highly Effective Protein Recognition

Multivalent interactions frequently occur in biological systems and typically provide higher binding affinity and selectivity in target recognition than when only monovalent interactions are operative. Thus, taking inspiration by nature, bivalent or multivalent nucleic acid aptamers recognizing a specific biological target have been extensively studied in the last decades. Indeed, oligonucleotide-based aptamers are suitable building blocks for the development of highly efficient multivalent systems since they can be easily modified and assembled exploiting proper connecting linkers of different nature. Thus, substantial research efforts have been put in the construction of dimeric/multimeric versions of effective aptamers with various degrees of success in target binding affinity or therapeutic activity enhancement. The present review summarizes recent advances in the design and development of dimeric and multimeric DNA-based aptamers, including those forming G-quadruplex (G4) structures, recognizing different key proteins in relevant pathological processes. Most of the designed constructs have shown improved performance in terms of binding affinity or therapeutic activity as anti-inflammatory, antiviral, anticoagulant, and anticancer agents and their number is certainly bound to grow in the next future.

Biological macromolecule binding and anticancer activity of synthetic alkyne-containing L-phenylalanine derivatives

Herein, we described the synthesis of two L-phenylalanines α-derivatized with a terminal alkyne moiety whose structures differed by phenyl ring halogen substitution (two o-Cl in 1 vs. one p-Br in 2) and investigated their effect on biological macromolecules and living cells. We explored their interaction with quadruplex DNA (G4 DNA), using tel₂₆ and c-myc as models, and bovine serum albumin (BSA). By CD spectroscopy, we found that 1 caused minor tel26 secondary structure changes, leading also to a slight thermal stabilization of this hybrid antiparallel/parallel G4 structure, while the c-myc parallel topology remained essentially unchanged upon 1 binding. Other CD evidences showed the ability of 1 to bind BSA, while molecular docking studies suggested that the same molecule could be housed into the hydrophobic cavity between sub-domains IIA, IIB, and IIIA of the protein. Furthermore, preliminary aggregation studies, based on concentration-dependent spectroscopic experiments, suggested the ability of 1 to aggregate forming noncovalent polymeric systems in aqueous solution. Differently from 1, the bromine-modified compound was able to bind Cu(II) ion, likely with the formation of a CuL₂ complex, as found by UV spectroscopy. Finally, cell tests excluded any cytotoxic effect of both compounds toward normal cells, but showed slight antiproliferative effects of 2 on PC3 cancerous cells at 24 h, and of 1 on both T98G and MDA-MB-231 cancer cells at 48 h.

Evaluation of an Analogue of the Marine ε-PLL Peptide as a Ligand of G-quadruplex DNA Structures

ε-poly-l-Lysine (ε-PLL) peptide is a product of the marine bacterium Bacillus subtilis with antibacterial and anticancer activity largely used worldwide as a food preservative. ε-PLL and its synthetic analogue α,ε-poly-l-lysine (α,ε-PLL) are also employed in the biomedical field as enhancers of anticancer drugs and for drug and gene delivery applications. Recently, several studies reported the interaction between these non-canonical peptides and DNA targets. Among the most important DNA targets are the DNA secondary structures known as G-quadruplexes (G4s) which play relevant roles in many biological processes and disease-related mechanisms. The search for novel ligands capable of interfering with G4-driven biological processes elicits growing attention in the screening of new classes of G4 binders. In this context, we have here investigated the potential of α,ε-PLL as a G4 ligand. In particular, the effects of the incubation of two different models of G4 DNA, i.e., the parallel G4 formed by the Pu22 (d[TGAGGGTGGGTAGGGTGGGTAA]) sequence, a mutated and shorter analogue of the G4-forming sequence known as Pu27 located in the promoter of the c-myc oncogene, and the hybrid parallel/antiparallel G4 formed by the human Tel22 (d[AGGGTTAGGGTTAGGGTTAGGG]) telomeric sequence, with α,ε-PLL are discussed in the light of circular dichroism (CD), UV, fluorescence, size exclusion chromatography (SEC), and surface plasmon resonance (SPR) evidence. Even though the SPR results indicated that α,ε-PLL is capable of binding with µM affinity to both the G4 models, spectroscopic and SEC investigations disclosed significant differences in the structural properties of the resulting α,ε-PLL/G4 complexes which support the use of α,ε-PLL as a G4 ligand capable of discriminating among different G4 topologies.

Scaffold Simplification Strategy Leads to a Novel Generation of Dual Human Immunodeficiency Virus and Enterovirus-A71 Entry Inhibitors

Currently, there are only three FDA-approved drugs that inhibit human immunodeficiency virus (HIV) entry-fusion into host cells. The situation is even worse for enterovirus EV71 infection for which no antiviral therapies are available. We describe here the discovery of potent entry dual inhibitors of HIV and EV71. These compounds contain in their structure three or four tryptophan (Trp) residues linked to a central scaffold. Critical for anti-HIV/EV71 activity is the presence of extra phenyl rings, bearing one or two carboxylates, at the C2 position of the indole ring of each Trp residue. The most potent derivatives, 22 and 30, inhibit early steps of the replicative cycles of HIV-1 and EV-A71 by interacting with their respective viral surfaces (glycoprotein gp120 of HIV and the fivefold axis of the EV-A71 capsid). The high potency, low toxicity, facile chemical synthesis, and great opportunities for chemical optimization make them useful prototypes for future medicinal chemistry studies.

Insights into the G-rich VEGF-binding aptamer V7t1: when two G-quadruplexes are better than one!

The G-quadruplex-forming VEGF-binding aptamer V7t1 was previously found to be highly polymorphic in a K⁺-containing solution and, to restrict its conformational preferences to a unique, well-defined form, modified nucleotides (LNA and/or UNA) were inserted in its sequence. We here report an in-depth biophysical characterization of V7t1 in a Na⁺-rich medium, mimicking the extracellular environment in which VEGF targeting should occur, carried out combining several techniques to analyse the conformational behaviour of the aptamer and its binding to the protein. Our results demonstrate that, in the presence of high Na⁺ concentrations, V7t1 behaves in a very different way if subjected or not to annealing procedures, as evidenced by native gel electrophoresis, size exclusion chromatography and dynamic light scattering analysis. Indeed, not-annealed V7t1 forms both monomeric and dimeric G-quadruplexes, while the annealed oligonucleotide is a monomeric species. Remarkably, only the dimeric aptamer efficiently binds VEGF, showing higher affinity for the protein compared to the monomeric species. These findings provide new precious information for the development of improved V7t1 analogues, allowing more efficient binding to the cancer-related protein and the design of effective biosensors or theranostic devices based on VEGF targeting.

DNA Quadruplex-Based Inhibitor With Flexible Fragments at the 3' Terminal Shows Enhanced Anti-HIV-1 Fusion Activity

Chemically optimizing the molecular structure of aptamers may enhance properties such as biological activity or metabolic stability. DNA quadruplex-based HIV-1 fusion inhibitors were found to interact with HIV-1 surface glycoprotein in aptamer mode. In this work, a series of quadruplex-based HIV-1 fusion inhibitors with flexible oligodeoxynucleotide fragments at the 3' terminal was discovered. The flexible extension did not greatly influence quadruplex formation at the 5'-end. Increasing the length of the flexible fragment may increase antifusion activity. Compared with a traditional inhibitor, d(^5'TGGGAG^3')₄, these novel inhibitors showed enhanced interaction with HIV-1 glycoproteins gp120 and gp41, which increased inhibition of 6-helical bundle formation during the course of virus fusion. These inhibitors also showed improved stability, compared with natural oligodeoxynucleotide. This work may inform the design of anti-HIV-1 DNA helix-based inhibitors with new structures or mechanisms.

New G-Quadruplex-Forming Oligodeoxynucleotides Incorporating a Bifunctional Double-Ended Linker (DEL): Effects of DEL Size and ODNs Orientation on the Topology, Stability, and Molecularity of DEL-G-Quadruplexes

G-quadruplexes (G4s) are unusual secondary structures of DNA occurring in guanosine-rich oligodeoxynucleotide (ODN) strands that are extensively studied for their relevance to the biological processes in which they are involved. In this study, we report the synthesis of a new kind of G4-forming molecule named double-ended-linker ODN (DEL-ODN), in which two TG₄T strands are attached to the two ends of symmetric, non-nucleotide linkers. Four DEL-ODNs differing for the incorporation of either a short or long linker and the directionality of the TG₄T strands were synthesized, and their ability to form G4 structures and/or multimeric species was investigated by PAGE, HPLC⁻size-exclusion chromatography (HPLC⁻SEC), circular dichroism (CD), and NMR studies in comparison with the previously reported monomeric tetra-ended-linker (TEL) analogues and with the corresponding tetramolecular species (TG₄T)₄. The structural characterization of DEL-ODNs confirmed the formation of stable, bimolecular DEL-G4s for all DEL-ODNs, as well as of additional DEL-G4 multimers with higher molecular weights, thus suggesting a way towards the obtainment of thermally stable DNA nanostructures based on reticulated DEL-G4s.

Peptide Nucleic Acid-Functionalized Adenoviral Vectors Targeting G-Quadruplexes in the P1 Promoter of Bcl-2 Proto-Oncogene: A New Tool for Gene Modulation in Anticancer Therapy

The B-cell lymphoma 2 (Bcl-2) gene encodes for an antiapoptotic protein associated with the onset of many human tumors. Several oligonucleotides (ONs) and ON analogues are under study as potential tools to counteract the Bcl-2 expression. Among these are Peptide Nucleic Acids (PNAs). The absence of charges on PNA backbones allows the formation of PNA/DNA complexes provided with higher stability than the corresponding natural DNA/DNA counterparts. To date, the use of PNAs in antigene or antisense strategies is strongly limited by their inability to efficiently cross the cellular membranes. With the aim of downregulating the expression of Bcl-2, we propose here a novel antigene approach which uses oncolytic adenoviral vectors (OAds) as a new cancer cell-targeted PNA delivery system. The ability of oncolytic Ad5D24 vectors to selectively infect and kill cancer cells was exploited to transfect with high efficiency and selectivity a short cytosine-rich PNA complementary to the longest loop of the main G-quadruplex formed by the 23-base-long bcl2midG4 sequence located 52-30 bp upstream of the P1 promoter of Bcl-2 gene. Physico-chemical and biological investigations confirmed the ability of the PNA-conjugated Ad5D24 vectors to load and transfect their PNA cargo into human A549 and MDA-MB-436 cancer cell lines, as well as the synergistic (OAd+PNA) cytotoxic effect against the same cell lines. This approach holds promise for safer chemotherapy because of reduced toxicity to healthy tissues and organs.