Inhibitory Effects on RNA Binding and RNase H Induction Activity of Prodrug-Type Oligodeoxynucleotides Modified with a Galactosylated Self-Immolative Linker Cleavable by β-Galactosidase

Prodrug-type oligonucleotides (prodrug-ONs) are a class of oligonucleotide designed for activation under specific intracellular conditions or external stimuli. Prodrug-ONs can be activated in the target tissues or cells, thereby reducing the risk of adverse effects. In this study, we synthesized prodrug-type oligodeoxynucleotides activated by β-galactosidase, an enzyme that is overexpressed in cancer and senescent cells. These oligodeoxynucleotides (ODNs) contain a modified thymidine conjugated with galactose via a self-immolative linker at the O4-position. UV-melting analysis revealed that the modifications decreased the melting temperature (Tm) compared with that of the unmodified ODN when hybridized with complementary RNA. Furthermore, cleavage of the glycosidic bond by β-galactosidase resulted in the spontaneous removal of the linker from the nucleobase moiety, generating unmodified ODNs. Additionally, the introduction of multiple modified thymidines into ODNs completely inhibited the RNase H-mediated cleavage of complementary RNA. These findings suggest the possibility of developing prodrug-ONs, which are specifically activated in cancer cells or senescent cells with high β-galactosidase expression.

Chemical Synthesis of Modified RNA

Ribonucleic acids (RNAs) play a vital role in living organisms. Many of their cellular functions depend critically on chemical modification. Methods to modify RNA in a controlled manner-both in vitro and in vivo-are thus essential to evaluate and understand RNA biology at the molecular and mechanistic levels. The diversity of modifications, combined with the size and uniformity of RNA (made up of only 4 nucleotides) makes its site-specific modification a challenging task that needs to be addressed by complementary approaches. One such approach is solid-phase RNA synthesis. We discuss recent developments in this field, starting with new protection concepts in the ongoing effort to overcome current size limitations. We continue with selected modifications that have posed significant challenges for their incorporation into RNA. These include deazapurine bases required for atomic mutagenesis to elucidate mechanistic aspects of catalytic RNAs, and RNA containing xanthosine, N4-acetylcytidine, 5-hydroxymethylcytidine, 3-methylcytidine, 2'-OCF3, and 2'-N3 ribose modifications. We also discuss the all-chemical synthesis of 5'-capped mRNAs and the enzymatic ligation of chemically synthesized oligoribonucleotides to obtain long RNA with multiple distinct modifications, such as those needed for single-molecule FRET studies. Finally, we highlight promising developments in RNA-catalyzed RNA modification using cofactors that transfer bioorthogonal functionalities.

Microwave-Dependent Thermo-Release Approach for Oligonucleotides 5'-Phosphorylation

A method for phosphorylating oligonucleotides using a thermosensitive "trigger" is hereby presented. The recovery of the phosphate specifically takes place under neutral conditions when subjected to an elevated temperature. Two identical thermolabile protecting groups are differentially removed with the initial release occurring swiftly and the second at a more gradual pace. The delayed deprotection of the second group led to the development of a method for the purification of 5'-phosphorylated oligonucleotides. Microwave irradiation enables the rapid attainment of complete deprotection, in contrast to conventional heating methods.

Oligonucleotide Synthesis on Porous Glass Resins Containing Activators

For the advancement of nucleic acid-related research, high-efficiency, low-cost synthesis of high-purity oligonucleotides is necessary. Herein, we introduced hydroxybenzotriazole (HOBt) activators on controlled pore glass resins to improve the efficiency of chain elongation (the synthesis efficiency increased from 48% without an activator to 92% with an activator). In particular, the use of the resin containing 6-trifluoromethyl HOBt with a linker of lauric acid and succinic acid significantly improved the synthesis efficiency for both DNA and RNA syntheses.

A prebiotically plausible scenario of an RNA-peptide world

The RNA world concept¹ is one of the most fundamental pillars of the origin of life theory^2–4. It predicts that life evolved from increasingly complex self-replicating RNA molecules^1,2,4. The question of how this RNA world then advanced to the next stage, in which proteins became the catalysts of life and RNA reduced its function predominantly to information storage, is one of the most mysterious chicken-and-egg conundrums in evolution^3–5. Here we show that non-canonical RNA bases, which are found today in transfer and ribosomal RNAs^6,7, and which are considered to be relics of the RNA world^8–12, are able to establish peptide synthesis directly on RNA. The discovered chemistry creates complex peptide-decorated RNA chimeric molecules, which suggests the early existence of an RNA–peptide world¹³ from which ribosomal peptide synthesis¹⁴ may have emerged^15,16. The ability to grow peptides on RNA with the help of non-canonical vestige nucleosides offers the possibility of an early co-evolution of covalently connected RNAs and peptides^13,17,18, which then could have dissociated at a higher level of sophistication to create the dualistic nucleic acid–protein world that is the hallmark of all life on Earth.

Peptide synthesis can take place directly on RNA, which suggests how a nucleic acid–protein world might have originated on early Earth.

Chemical synthesis and biochemical characterization of cyclic oligonucleotides containing acyl groups at both 5'- and 3'-terminal positions

Modified oligonucleotides, whose ON-OFF switch of hybridization can be controlled by an external stimulus, are important to understanding life phenomena and efficient treatment of diseases. The ON-OFF switch can be completely controlled by chemical modification of the oligonucleotide such as cyclization. However, their chemical modifications of the previous cyclic oligonucleotides remain after the addition of an external stimulus. To overcome this problem, we carried out the first synthesis of cyclic oligonucleotides containing acyl groups at both 5'- and 3'-terminal positions, which can be hydrolyzed by intracellular esterase. The cyclic oligonucleotides were successfully synthesized via disulfide bond formation and the phosphoramidite method without base protection on polymer supports containing a silyl linker. Subsequently, we were able to introduce a functional group into the cyclic oligonucleotide using the corresponding isothiocyanate reagent. Additionally, a cyclic oligonucleotide with acyl groups was found to have a much lower binding ability than the corresponding linear oligonucleotide. Moreover, we demonstrated its structural conversion to the corresponding linear oligonucleotide with two thiol groups under reducing conditions using dithiothreitol. It was also confirmed that the two terminal acyl groups of the linear oligonucleotide were hydrolyzed by pig liver esterase. These results indicate that hybridization of cyclic acylated nucleic acid drugs with high nuclease resistance is regulated by intracellular esterase under the reducing conditions in the cell cytoplasm.

Fluorescent 2-(Pyridin-2-yl)vinyl Pyridine Dyes and Their Thermocontrolled Release

The generation of unique thermosensitive fluorescent dyes via heteroaromatic Heck cross-coupling and N-pyridin-2-yl nucleophilic substitution was described. To demonstrate thermosensitive properties, the precursor was converted into carbonates or phosphates and heated at various temperatures and for various time periods. Significant changes in the fluorescence intensity and emission wavelengths, between carbonates and the cyclic product, were observed, and it was proved that the dyes may serve as removable fluorescent labels with large Stokes shifts (>80 nm). The application of thermosensitive fluorescent dyes in oligonucleotide labeling has been demonstrated.

Stimuli-responsive oligonucleotides in prodrug-based approaches for gene silencing

Oligonucleotides (ONs) have been envisaged for therapeutic applications for more than thirty years. However, their broad use requires overcoming several hurdles such as instability in biological fluids, low cell penetration, limited tissue distribution, and off-target effects. With this aim, many chemical modifications have been introduced into ONs definitively as a means of modifying and better improving their properties as gene silencing agents and some of them have been successful. Moreover, in the search for an alternative way to make efficient ON-based drugs, the general concept of prodrugs was applied to the oligonucleotide field. A prodrug is defined as a compound that undergoes transformations in vivo to yield the parent active drug under different stimuli. The interest in stimuli-responsive ONs for gene silencing functions has been notable in recent years. The ON prodrug strategies usually help to overcome limitations of natural ONs due to their low metabolic stability and poor delivery. Nevertheless, compared to permanent ON modifications, transient modifications in prodrugs offer the opportunity to regulate ON activity as a function of stimuli acting as switches. Generally, the ON prodrug is not active until it is triggered to release an unmodified ON. However, as it will be described in some examples, the opposite effect can be sought. This review examines ON modifications in response to various stimuli. These stimuli may be internal or external to the cell, chemical (glutathione), biochemical (enzymes), or physical (heat, light). For each stimulus, the discussion has been separated into sections corresponding to the site of the modification in the nucleotide: the internucleosidic phosphate, the nucleobase, the sugar or the extremities of ONs. Moreover, the review provides a current and detailed account of stimuli-responsive ONs with the main goal of gene silencing. However, for some stimuli-responsive ONs reported in this review, no application for controlling gene expression has been shown, but a certain potential in this field could be demonstrated. Additionally, other applications in different domains have been mentioned to extend the interest in such molecules.

pH-Dependent Switching of Base Pairs Using Artificial Nucleobases with Carboxyl Groups

In this study, we report the synthesis of modified oligonucleotides consisting of benzoic acid or isophthalic acid residues as new nucleobases. As evaluated by UV thermal denaturation analysis at different pH conditions (5.0, 6.0, 7.0, and 8.0), these modified oligonucleotides exhibited pH-dependent recognition of natural nucleobases and one is first found to be capable of base pair switching in response to a pH change. The isophthalic acid residue incorporated into the oligonucleotide on a d-threoninol backbone could preferentially bind with adenine but with guanine in response to a change in the pH conditions from pH 5 to pH 7 (or 8) without significant difference in duplex stability. These findings would be valuable for further developing pH-responsive DNA-based molecular devices.

Nucleobase Protection of Deoxyribo- and Ribonucleosides

Oligonucleotides carrying a variety of chemical modifications including conjugates are finding increasing applications in therapeutics, diagnostics, functional genomics, proteomics, and as research tools in chemical and molecular biology. The successful synthesis of oligonucleotides primarily depends on the use of appropriately protected nucleoside building blocks including the exocyclic amino groups of the nucleobases, the hydroxyl groups at the 2'-, 3'-, and 5'-positions of the sugar moieties, and the internucleotide phospho-linkage. This unit is a thoroughly revised update of the previously published version and describes the recent development of various protecting groups that facilitate reliable oligonucleotide synthesis. In addition, various protecting groups for the imide/lactam function of thymine/uracil and guanine, respectively, are described to prevent irreversible nucleobase modifications that may occur in the presence of reagents used in oligonucleotide synthesis. © 2017 by John Wiley & Sons, Inc.

2-Pyridinyl Thermolabile Groups as General Protectants for Hydroxyl, Phosphate, and Carboxyl Functions

Application of 2-pyridinyl thermolabile protecting groups (2-PyTPGs) for protection of hydroxyl, phosphate, and carboxyl functions is presented in this unit. Their characteristic feature is a unique removal process following the intramolecular cyclization mechanism and induced only by temperature rise. Deprotection rate of 2-PyTPGs is dependent on certain parameters, such as solvent (aqueous or non-aqueous medium), pH values, and electron distribution in a pyridine ring. The presented approach pertains not only to protecting groups but also to an advanced system of controlling certain properties of 2-pyridinyl derivatives. We improved the "chemical switch" method, allowing us to regulate the protecting group stability by inversing the electron distribution in 2-PyTPG. Together with pH values manipulation, this allows us to regulate the protecting group stability. Moreover, phosphite cyclization to oxazaphospholidine provides a very stable but easily reversible tool for phosphate protection/modifications. For all TPGs we confirmed their utility in a system of protecting groups. This concept can contribute to designing the general protecting group that could be useful in bioorganic chemistry. © 2017 by John Wiley & Sons, Inc.

2-Pyridinyl-N-(2,4-difluorobenzyl)aminoethyl Group As Thermocontrolled Implement for Protection of Carboxylic Acids

A thermolabile protecting group strategy for carboxylic acids is expanded. Thermosensitive esters are readily prepared using a known procedure, and their stability under neutral condition is investigated. Effective thermolytic deprotection initiated only by temperature for different carboxylic acids is demonstrated, and the compatibility of a thermolytic protecting group with acidic and basic protecting groups in an orthogonal protection strategy is also presented. This study showed interesting correlations between the pKa of acids and the deprotection rate of their well-protected moieties.

Modulating the Stability of 2-Pyridinyl Thermolabile Hydroxyl Protecting Groups via the "Chemical Switch" Approach

A novel and effective method is presented for modulating the stability of 2-Pyridinyl Thermolabile Protecting Groups (2-Py TPGs) in the "chemical switch" approach. The main advantage of the discussed approach is the possibility of changing the nucleophilic character of pyridine nitrogen using different switchable factors, which results in an increase or decrease in the thermal deprotection rate. One of the factors is transformation of a nitro into an amine group via reduction with a low-valent titanium in mild conditions. The usefulness of our approach is corroborated using 3'-O-acetyl nucleosides as model compounds. Their stability in various solvents and temperatures before and after reduction is also examined. Pyridine N-oxide and pH are other factors responsible for the nucleophilicity and stability of 2-Pyridinyl Thermolabile Protecting Groups in thermal deprotection. Protonation of 4-amino 2-Pyridinyl Thermolabile Protecting Groups is demonstrated by (1)H-(15)N HMBC and HSQC NMR analysis.

H+-Assisted fluorescent differentiation of Cu+ and Cu2+: effect of Al3+-induced acidity on chemical sensing and generation of two novel and independent logic gating pathways

A novel logic gate containing protecting groups interacts with various species (acetonitrile) with fluorescence responses relating to ligand non innocence.