Nucleotides Part XL. Synthesis and Characterization of Modified 2?-5? Adenylate Trimers-Potential Antiviral Agents

[Protection of 2'-hydroxyls in the chemical synthesis of oligoribonucleotides]

The review is devoted to the chemical synthesis of oligoribonucleotides and the protecting groups used. In particular the existent methods of blocking 2'-OH function in nucleotide monomers for the RNA synthesis are discussed in detail.

Solventless protocol for efficient bis-N-boc protection of adenosine, cytidine, and guanosine derivatives

A solvent-free reaction employing a simple low-energy ball mill apparatus converts the amino groups of adenosine, 2-deoxyadenosine, cytidine, 2-deoxycytidine, guanosine, and 2-deoxyguanosine as well as some of their ribosyl O-protected derivatives to the corresponding bis-N-Boc carbamates. In the case of guanosine compounds, the carbonyl group of the base moiety was also blocked as its O-Boc enol carbonate. A variation of this approach using transient in situ O-silylation permitted the preparation of bis-N-Boc nucleosides in which the sugar hydroxyls were unprotected. The ball mill reactions were rapid, convenient, and very high-yielding except in the case of the guanosine compounds. This highly efficient method protects the amino groups of these nucleosides with a base stable and acid labile group suitable for further synthetic manipulation.

Endowing RNase H-inactive antisense with catalytic activity: 2-5A-morphants

A convergent synthetic approach was used to conjugate 2',5'-oligoadenylate (2-5A, p5'A2' [p5'A2'](n)()p5'A) to phosphorodiamidate morpholino oligomers (morphants). To provide requisite quantities of 2-5A starting material, commercially and readily available synthons for solid-phase synthesis were adapted for larger scale solution synthesis. Thus, the tetranucleotide 5'-phosphoryladenylyl(2'-->5')adenylyl(2'-->5')adenylyl(2'-->5')adenosine (p5'A2'p5'A2'](2)p5'A2', tetramer 2-5A, 9) was synthesized starting with 2',3'-O-dibenzoyl-N(6),N(6)-dibenzoyl adenosine prepared from commercially available 5'-O-(4-monomethoxytrityl) adenosine. Coupling with N(6)-benzoyl-5'-O-(4,4'-dimethoxytrityl)-3'-O-(tert-butyldimethylsilyl) adenosine-2'-(N,N-diisopropyl-2-cyanoethyl)phosphoramidite, followed by oxidization and deprotection, generated 5'-deprotected dimer 2-5A. Similar procedures lengthened the chain to form protected tetramer 2-5 A. The title product 9 p5'A(2'p5'A)(3) (tetramer 2-5A) was obtained through phosphorylation of the terminal 5'-hydroxy of the protected tetramer and removal of remaining protecting groups using concentrated ammonium hydroxide-ethanol (3:1, v/v) at 55 degrees C and tetrabutylammonium fluoride (TBAF) in THF at room temperature, respectively. The 2-5A-phosphorodiamidate morpholino antisense chimera 11 (2-5A-morphant) was synthesized by covalently linking an aminolinker-functionalized phosphorodiamidate morpholino oligomer with periodate oxidized 2-5A tetramer (p5'A2'[p5'A2'](2)p5'A). The resulting Schiff base was reduced with cyanoborohydride thereby transforming the ribose of the 2'-terminal nucleotide of 2-5A N-substituted morpholine. RNase L assays demonstrated that this novel 2-5A-antisense chimera had significant biological activity, thereby providing another potential tool for RNA ablation.

The 2-5A system: modulation of viral and cellular processes through acceleration of RNA degradation

The 2-5A system is an RNA degradation pathway that can be induced by the interferons (IFNs). Treatment of cells with IFN activates genes encoding several double-stranded RNA (dsRNA)-dependent synthetases. These enzymes generate 5'-triphosphorylated, 2',5'-phosphodiester-linked oligoadenylates (2-5A) from ATP. The effects of 2-5A in cells are transient since 2-5A is unstable in cells due to the activities of phosphodiesterase and phosphatase. 2-5A activates the endoribonuclease 2-5A-dependent RNase L, causing degradation of single-stranded RNA with moderate specificity. The human 2-5A-dependent RNase is an 83.5 kDa polypeptide that has little, if any, RNase activity, unless 2-5A is present. 2-5A binding to RNase L switches the enzyme from its off-state to its on-state. At least three 2',5'-linked oligoadenylates and a single 5'-phosphoryl group are required for maximal activation of the RNase. Even though the constitutive presence of 2-5A-dependent RNase is observed in nearly all mammalian cell types, cellular amounts of 2-5A-dependent mRNA and activity can increase after IFN treatment. One well-established role of the 2-5A system is as a host defense against some types of viruses. Since virus infection of cells results in the production and secretion of IFNs, and since dsRNA is both a frequent product of virus infection and an activator of 2-5A synthesis, the replication of encephalomyocarditis virus, which produces dsRNA during its life cycle, is greatly suppressed in IFN-treated cells as a direct result of RNA decay by the activated 2-5A-dependent RNase. This review covers the organic chemistry, enzymology, and molecular biology of 2-5A and its associated enzymes. Additional possible biological roles of the 2-5A system, such as in cell growth and differentiation, human immunodeficiency virus replication, heat shock, atherosclerotic plaque, pathogenesis of Type I diabetes, and apoptosis, are presented.