Hydrogen Bonding (Base Pairing) in Antiviral Activity.

Base pairing based on hydrogen bonding has, since its inception, been crucial in the antiviral activity of arabinosyladenine, 2'-deoxyuridines (i.e., IDU, TFT, BVDU), acyclic nucleoside analogues (i.e., acyclovir) and nucleoside reverse transcriptase inhibitors (NRTIs). Base pairing based on hydrogen bonding also plays a key role in the mechanism of action of various acyclic nucleoside phosphonates (ANPs) such as adefovir, tenofovir, cidofovir and O-DAPYs, thus explaining their activity against a wide array of DNA viruses (human hepatitis B virus (HBV), human immunodeficiency (HIV) and human herpes viruses (i.e., human cytomegalovirus)). Hydrogen bonding (base pairing) also seems to be involved in the inhibitory activity of Cf1743 (and its prodrug FV-100) against varicella-zoster virus (VZV) and in the activity of sofosbuvir against hepatitis C virus and that of remdesivir against SARS-CoV-2 (COVID-19). Hydrogen bonding (base pairing) may also explain the broad-spectrum antiviral effects of ribavirin and favipiravir. This may lead to lethal mutagenesis (error catastrophe), as has been demonstrated with molnutegravir in its activity against SARS-CoV-2.

Thermodynamic Analysis of 1 x 2 Internal Loop Regions of SL1 of SARS-CoV-2 and SL of U6 snRNA

The genomic material of SARS-CoV-2 is a positive-sense single-stranded RNA. SARS-CoV-2 produces non-structural protein 1 (NSP1), which inhibits host cell translation by binding its' N-terminal to the host's 40S ribosomal subunit. Once NSP1 is bound its C-terminal domain folds and binds to the mRNA entry channel. Stem loop 1 (SL1) in the 5'-UTR of the viral mRNA binds to NSP1 to abrogate translation inhibition leading to the expression of viral proteins. SL1 contains a 1 x 2 internal loop that is not seen in other coronaviruses and may be involved in conformational changes that influence SL1-NSP1 interactions. The 1 x 2 internal loop of SL1 contains a putative A*C non-canonical base pair. The U6 snRNA also contains a 1 x 2 internal loop known to undergo conformation changes in response to pH and magnesium ion binding. Here we examine the thermodynamic properties and magnesium binding of the 1 x 2 internal loop of SL1 in varying helical contexts. Thermal denaturation experiments were performed on various DNA and RNA constructs in the presence of 1 M KCl or 10 mM magnesium chloride at a pH of 5.5 and 7. We show that formation of the A+*C base pair and the construct stability in the presence of magnesium ions is dependent on the helical context.Copyright © 2023 The American Society for Biochemistry and Molecular Biology, Inc.

Computational tools for aptamer identification and optimization

Aptamers are single-stranded DNA or RNA oligonucleotides that can selectively bind to a specific target. They are generally obtained by SELEX, but the procedure is challenging and time-consuming. Moreover, the identified aptamers tend to be insufficient in stability, specificity, and affinity. Thus, only a handful of aptamers have entered the practical use stage. Recently, computational approaches have demonstrated a significant capacity to assist in the discovery of high-performance aptamers. This review discusses the advances achieved in several aspects of computational tools in this field, as well as the new progress in machine learning and deep learning, which are used in aptamer identification and optimization. To illustrate these computationally aided processes, aptamer selections against SARS-CoV-2 are discussed in detail as a case study. We hope that this review will aid and motivate researchers to develop and utilize more computational techniques to discover ideal aptamers effectively. Copyright © 2022 Elsevier B.V.

Self-assembling short immunostimulatory duplex RNAs with broad-spectrum antiviral activity.

The current coronavirus disease 2019 (COVID-19) pandemic highlights the need for broad-spectrum antiviral therapeutics. Here we describe a new class of self-assembling immunostimulatory short duplex RNAs that potently induce production of type I and type III interferon (IFN-I and IFN-III). These RNAs require a minimum of 20 base pairs, lack any sequence or structural characteristics of known immunostimulatory RNAs, and instead require a unique sequence motif (sense strand, 5'-C; antisense strand, 3'-GGG) that mediates end-to-end dimer self-assembly. The presence of terminal hydroxyl or monophosphate groups, blunt or overhanging ends, or terminal RNA or DNA bases did not affect their ability to induce IFN. Unlike previously described immunostimulatory small interfering RNAs (siRNAs), their activity is independent of Toll-like receptor (TLR) 7/8, but requires the RIG-I/IRF3 pathway that induces a more restricted antiviral response with a lower proinflammatory signature compared with immunostimulant poly(I:C). Immune stimulation mediated by these duplex RNAs results in broad-spectrum inhibition of infections by many respiratory viruses with pandemic potential, including severe acute respiratory syndrome coronavirus (SARS-CoV)-2, SARS-CoV, Middle East respiratory syndrome coronavirus (MERS-CoV), human coronavirus (HCoV)-NL63, and influenza A virus in cell lines, human lung chips that mimic organ-level lung pathophysiology, and a mouse SARS-CoV-2 infection model. These short double-stranded RNAs (dsRNAs) can be manufactured easily, and thus potentially could be harnessed to produce broad-spectrum antiviral therapeutics.