Modulation of the helicase activity of eIF4A by eIF4B, eIF4H, and eIF4F.

Eukaryotic initiation factor (eIF) 4A is a DEAD box RNA helicase that works in conjunction with eIF4B, eIF4H, or as a subunit of eIF4F to unwind secondary structure in the 5'-untranslated region of mRNA, which facilitates binding of the mRNA to the 40 S ribosomal subunit. This study demonstrates how the helicase activity of eIF4A is modulated by eIF4B, eIF4H, or as a subunit of eIF4F. Results indicate that a linear relationship exists between the initial rate or amplitude of unwinding and duplex stability for all factor combinations tested. eIF4F, like eIF4A, behaves as a non-processive helicase. Either eIF4B or eIF4H stimulated the initial rate and amplitude of eIF4A-dependent duplex unwinding, and the magnitude of stimulation is dependent on duplex stability. Furthermore, eIF4A (or eIF4F) becomes a slightly processive helicase in the presence of eIF4B or eIF4H. All combinations of factors tested indicate that the rate of duplex unwinding is equivalent in the 5' --> 3' and 3' --> 5' directions. However, the optimal rate of unwinding was dependent on the length of the single-stranded region of the substrate when different combinations of factors were used. The combinations of eIF4A, eIF4A + eIF4B, eIF4A + eIF4H, and eIF4F showed differences in their ability to unwind chemically modified duplexes. A simple model of how eIF4B or eIF4H affects the duplex unwinding mechanism of eIF4A is proposed.

Investigating the structure of human RNase H1 by site-directed mutagenesis.

In this study we examine for the first time the roles of the various domains of human RNase H1 by site-directed mutagenesis. The carboxyl terminus of human RNase H1 is highly conserved with Escherichia coli RNase H1 and contains the amino acid residues of the putative catalytic site and basic substrate-binding domain of the E. coli RNase enzyme. The amino terminus of human RNase H1 contains a structure consistent with a double-strand RNA (dsRNA) binding motif that is separated from the conserved E. coli RNase H1 region by a 62-amino acid sequence. These studies showed that although the conserved amino acid residues of the putative catalytic site and basic substrate-binding domain are required for RNase H activity, deletion of either the catalytic site or the basic substrate-binding domain did not ablate binding to the heteroduplex substrate. Deletion of the region between the dsRNA-binding domain and the conserved E. coli RNase H1 domain resulted in a significant loss in the RNase H activity. Furthermore, the binding affinity of this deletion mutant for the heteroduplex substrate was approximately 2-fold tighter than the wild-type enzyme suggesting that this central 62-amino acid region does not contribute to the binding affinity of the enzyme for the substrate. The dsRNA-binding domain was not required for RNase H activity, as the dsRNA-deletion mutants exhibited catalytic rates approximately 2-fold faster than the rate observed for wild-type enzyme. Comparison of the dissociation constant of human RNase H1 and the dsRNA-deletion mutant for the heteroduplex substrate indicates that the deletion of this region resulted in a 5-fold loss in binding affinity. Finally, comparison of the cleavage patterns exhibited by the mutant proteins with the cleavage pattern for the wild-type enzyme indicates that the dsRNA-binding domain is responsible for the observed strong positional preference for cleavage exhibited by human RNase H1.

Further characterization of the helicase activity of eIF4A. Substrate specificity.

Eukaryotic initiation factor (eIF) 4A is the archetypal member of the DEAD box family of RNA helicases and is proposed to unwind structures in the 5'-untranslated region of mRNA to facilitate binding of the 40 S ribosomal subunit. The helicase activity of eIF4A has been further characterized with respect to substrate specificity and directionality. Results confirm that the initial rate and amplitude of duplex unwinding by eIF4A is dependent on the overall stability, rather than the length or sequence, of the duplex substrate. eIF4A helicase activity is minimally dependent on the length of the single-stranded region adjacent to the double-stranded region of the substrate. Interestingly, eIF4A is able to unwind blunt-ended duplexes. eIF4A helicase activity is also affected by substitution of 2'-OH (RNA) groups with 2'-H (DNA) or 2'-methoxyethyl groups. These observations, taken together with results from competitive inhibition experiments, suggest that eIF4A may interact directly with double-stranded RNA, and recognition of helicase substrates occurs via chemical and/or structural features of the duplex. These results allow for refinement of a previously proposed model for the mechanism of action of eIF4A helicase activity.

Highly efficient endonucleolytic cleavage of RNA by a Cys(2)His(2) zinc-finger peptide.

We have identified a 30-aa peptide that efficiently cleaves single-stranded RNA. The peptide sequence corresponds to a single zinc finger of the human male-associated ZFY protein; a transcription factor belonging to the Cys(2)His(2) family of zinc-finger proteins. RNA cleavage was observed only in the absence of zinc. Coordination with zinc resulted in complete loss of ribonuclease activity. The ribonuclease active structure was determined to be a homodimeric form of the peptide. Dimerization of the peptide occurred through a single intermolecular disulfide between two of the four cystines. The observed hydrolytic activity was single-stranded RNA-specific. Single-stranded DNA, double-stranded RNA and DNA, and 2'-methoxy-modified sequences were not degraded by the peptide. The peptide specifically cleaved pyrimidines within single-stranded RNA and the dinucleotide sequence 5'-pyr-A-3' was preferred. The RNA cleavage products consisted of a 3' phosphate and 5' hydroxyl. The initial rates of cleavage (V(0)) observed for the finger peptide were comparable to rates observed for human ribonucleases, and the catalytic rate (K(cat)) was comparable to rates observed for the group II intron rybozymes. The pH profile exhibited by the peptide is characteristic of general acid-base catalytic mechanisms observed with other ribonucleases. These observations raise interesting questions about the potential biological roles of zinc-finger proteins.

Properties of cloned and expressed human RNase H1.

We have characterized cloned His-tag human RNase H1. The activity of the enzyme exhibited a bell-shaped response to divalent cations and pH. The optimum conditions for catalysis consisted of 1 mM Mg(2+) and pH 7-8. In the presence of Mg(2+), Mn(2+) was inhibitory. Human RNase H1 shares many enzymatic properties with Escherichia coli RNase H1. The human enzyme cleaves RNA in a DNA-RNA duplex resulting in products with 5'-phosphate and 3'-hydroxy termini, can cleave overhanging single strand RNA adjacent to a DNA-RNA duplex, and is unable to cleave substrates in which either the RNA or DNA strand has 2' modifications at the cleavage site. Human RNase H1 binds selectively to "A-form"-type duplexes with approximately 10-20-fold greater affinity than that observed for E. coli RNase H1. The human enzyme displays a greater initial rate of cleavage of a heteroduplex-containing RNA-phosphorothioate DNA than an RNA-DNA duplex. Unlike the E. coli enzyme, human RNase H1 displays a strong positional preference for cleavage, i.e. it cleaves between 8 and 12 nucleotides from the 5'-RNA-3'-DNA terminus of the duplex. Within the preferred cleavage site, the enzyme displays modest sequence preference with GU being a preferred dinucleotide. The enzyme is inhibited by single-strand phosphorothioate oligonucleotides and displays no evidence of processivity. The minimum RNA-DNA duplex length that supports cleavage is 6 base pairs, and the minimum RNA-DNA "gap size" that supports cleavage is 5 base pairs.

Molecular cloning and expression of cDNA for human RNase H.

We have cloned, expressed, and purified to electrophoretic homogeneity a human RNase H. The enzyme has a molecular weight of 32 kDa, is Mg2+ dependent, and is inhibited by Mn2+ and N-ethylmaleimide. Its molecular weight and cleavage characteristics are consistent with type 2 human RNase H. The human RNase H we have cloned is highly homologous to Escherichia coli RNase HI (33.6% amino acid identity) and to other RNase H enzymes homologous to E. coli RNase HI. The enzyme is encoded by a single gene that is at least 10 kb in length and is expressed ubiquitously in human cells and tissues.

Identification and partial purification of human double strand RNase activity. A novel terminating mechanism for oligoribonucleotide antisense drugs.

We have identified a double strand RNase (dsRNase) activity that can serve as a novel mechanism for chimeric antisense oligonucleotides comprised of 2'-methoxy 5' and 3' "wings" on either side of an oligoribonucleotide gap. Antisense molecules targeted to the point mutation in codon 12 of Harvey Ras (Ha-Ras) mRNA resulted in a dose-dependent reduction in Ha-Ras RNA. Reduction in Ha-Ras RNA was dependent on the oligoribonucleotide gap size with the minimum gap size being four nucleotides. An antisense oligonucleotide of the same composition, but containing four mismatches, was inactive. When chimeric antisense oligonucleotides were prehybridized with 17-mer oligoribonucleotides, extracts prepared from T24 cells, cytosol, and nuclei resulted in cleavage in the oligoribonucleotide gap. Both strands were cleaved. Neither mammalian nor Escherichia coli RNase HI cleaved the duplex, nor did single strand nucleases. The dsRNase activity resulted in cleavage products with 5'-phosphate and 3'-hydroxyl termini. Partial purification of dsRNase from rat liver cytosolic and nuclear fractions was effected. The cytosolic enzyme was purified approximately 165-fold. It has an approximate molecular weight of 50,000-65,000, a pH optimum of approximately 7.0, requires divalent cations, and is inactivated by approximately 300 mM NaCl. It is inactivated by heat, proteinase K, and also by a number of detergents and several organic solvents.

Cleavage of single strand RNA adjacent to RNA-DNA duplex regions by Escherichia coli RNase H1.

RNase H1 from Escherichia coli cleaves single strand RNA extending 3' from an RNA-DNA duplex. Substrates consisting of a 25-mer RNA annealed to complementary DNA ranging in length from 9-17 nucleotides were designed to create overhanging single strand RNA regions extending 5' and 3' from the RNA-DNA duplex. Digestion of single strand RNA was observed exclusively within the 3' overhang region and not the 5' overhang region. RNase H digestion of the 3' overhang region resulted in digestion products with 5'-phosphate and 3'-hydroxyl termini. The number of single strand RNA residues cleaved by RNase H is influenced by the sequence of the single strand RNA immediately adjacent to the RNA-DNA duplex and appears to be a function of the stacking properties of the RNA residues adjacent to the RNA-DNA duplex. RNase H digestion of the 3' overhang region was not observed for a substrate that contained a 2'-methoxy antisense strand. The introduction of 3 deoxynucleotides at the 5' terminus of the 2'-methoxy antisense oligonucleotide resulted in cleavage. These results offer additional insights into the binding directionality of RNase H with respect to the heteroduplex substrate.

The influence of antisense oligonucleotide-induced RNA structure on Escherichia coli RNase H1 activity.

The ability of Escherichia coli RNase H1 to hydrolyze structured substrates containing antisense oligonucleotides preannealed to a 47-mer RNA was compared with its ability to hydrolyze unstructured substrates containing antisense oligonucleotides duplexed with 13-mer RNA. These results demonstrate that when antisense oligonucleotides were bound to structured RNA, the resultant duplexes were cleaved at rates significantly slower than when the same oligonucleotides were bound to unstructured oligoribonucleotides. Structured substrates exhibited fewer cleavage sites, and each cleavage site was cleaved less rapidly than in unstructured substrates. Furthermore, the enzymatic activity of E. coli RNase H1 for the structured substrates was most affected when the cleavage sites corresponding to the enzymatically most active sites on the unstructured substrates were blocked in the structured substrates. Molecular modeling suggests that the observed ablation of RNase H activity was due to the steric hindrance of the enzyme by the structured RNA, i.e. steric interference of the phosphate groups on the substrate and/or the binding site of the enzyme. When chimeric oligonucleotides composed of a five-base deoxynucleotide sequence flanked by chemically modified nucleotides were bound to structured RNA, the resultant duplexes were even worse substrates for RNase H. These results offer further insights into the role of antisense-induced RNA structure on RNase H activity and may facilitate the design of effective antisense oligonucleotides.

Control of complexity constraints on combinatorial screening for preferred oligonucleotide hybridization sites on structured RNA.

We have explored the use of short (10-mer), fully sequence-randomized oligonucleotide libraries for affinity-based screening in solution for energetically preferred sites of hybridization of a model 47-nucleotide (nt) mutant Ha-ras mRNA stem-loop fragment. In characterizing the model, binding studies using either a gel mobility-shift assay or an RNase ONE footprinting assay indicated the presence of a greatly preferred hybridization site for individual antisense RNA oligonucleotides on the 5'-most side of the ras RNA 19-nt loop. However, initial attempts to affinity-titrate combinatorial uniform 2'-O-methyl-substituted oligonucleotide libraries for selective binding to this 5'-loop site using an RNase ONE footprinting assay that can discriminate between binding to different sites on ras RNA were unsuccessful. By reducing the complexity of the library to a mix of seven RNA oligonucleotides complementary to a range of sites on ras RNA and with no self-complements, footprinting evidence for binding was obtained but was characterized by ras RNA site-specific binding constants differing dramatically from binding constants for individual oligonucleotides. The library complexity was reduced further to three different cases of two RNA oligonucleotides, one of which for all cases was the highest affinity 5'-loop complement. Detailed kinetic and thermodynamic binding analyses revealed a good fit of the data to independent (5'-loop and ascending stem sites), competitive (overlapping 5'-loop sites), or mutually allosteric (5'-loop and 3'-loop sites) formalisms and an energetics description showed that ras 5'-loop site-specific binding could be achieved by affinity titration only for the independent case. Reconstruction of events with the full complexity library suggested that there was the emergence of multiple, linked binding interactions and implied that successful hybridization affinity screening would be achieved only if all possible bimolecular binding interactions of individual library oligonucleotides with target RNA could be made mutually independent. Accordingly, by holding the calculated concentration of unique oligonucleotide sequences of a full complexity DNA library well below the value for the dissociation constant for binding of individual complement to the 5'-loop site and then titrating the concentration of ras RNA through this value, hybridization specific to the 5'-side of the ras loop was demonstrated as assayed either by sequential gel mobility-shift resolution of bimolecular complexes and RNase ONE footprinting in situ in gel slices or by RNase H cleavage of complexes in solution. Because this strategy uses an unbiased oligonucleotide library it should combinatorially identify energetically preferred hybridization sites on folded RNA targets of any sequence and of undetermined structure. This should enable a focused in vitro optimization of antisense oligonucleotide length, sequence, and chemical composition for preferred site binding affinity and specificity which, in turn, may be expected to provide for enhanced biological potency and specificity (Lima et al., 1996). Finally, the complexity constraints encountered and the fundamental requirement to control them presented here also should be applicable to interactions with any biomolecule target of any chemical class of combinatorial library when screened in solution in pooled mixes.

Combinatorial screening and rational optimization for hybridization to folded hepatitis C virus RNA of oligonucleotides with biological antisense activity.

We describe our initial application of a biochemical strategy, comprising combinatorial screening and rational optimization, which directly identifies oligonucleotides with maximum affinity (per unit length), specificity, and rates of hybridization to structurally preferred sites on folded RNA, to the problem of design of antisense oligonucleotides active against the hepatitis C virus (HCV). A fully randomized sequence DNA oligonucleotide (10-mer) library was equilibrated with each of two folded RNA fragments (200 and 370 nucleotides (nt)), together spanning the 5' 440 nt of an HCV transcript (by overlapping 130 nt), which were varied over a range of concentrations. The equilibrations were performed in solution under conditions determined to preserve RNA structure and to limit all RNA-DNA library oligonucleotide interactions to 1:1 stoichiometry. Subsequent Escherichia coli RNase H (endoribonuclease H: EC 3.1.26.4) cleavage analysis identified two preferred sites of highest affinity heteroduplex hybridization. The lengths and sequences of different substitute chemistry oligonucleotides complementary to these sites were rationally optimized using an iterative and quantitative analysis of binding affinity and specificity. Thus, DNA oligonucleotides that hybridized with the same affinity to the preferred sites in the folded RNA fragments found by screening as to short (< or = 25 nt) RNA complements were identified but were found to vary in length (10-18 nt) from site to site. Phosphorothioate (P=S) and 2'-fluoro (2'-F) uniformly substituted oligonucleotides also were found, which hybridized optimally to these sites, supporting the design of short (10-15-nt) and maximally specific oligonucleotides that are more nuclease-resistant (via P=S) and have higher affinity (via 2'-F) than DNA. Finally, the affinities of DNA and uniform 2'-F-, P=S-substituted 10-20-mer oligonucleotide complements for the best hybridization site, from HCV nt 355 to nt 364-374, closely corresponded to antisense mechanism inhibition activities in an in vitro translation assay and in a human cell-based HCV core protein expression assay, respectively. These results validate our strategy for the selection of hybridization-optimized and biologically active antisense oligonucleotides targeting HCV RNA and support the potential for utility in further applications.

Binding affinity and specificity of Escherichia coli RNase H1: impact on the kinetics of catalysis of antisense oligonucleotide-RNA hybrids.

In this study we report for the first time the binding affinity of RNase H1 for oligonucleotide duplexes. We used a previously described 17-mer antisense sequence [Monia, B. P., Johnston, J. F., Ecker, D. J., Zounes, M. A., Lima, W. F., & Freier, S. M. (1992) J. Biol. Chem. 267, 19954-19962] hybridized to a complementary oligoribonucleotide to evaluate both the binding affinity and the catalytic rate of RNase H1. The dissociation constants (Kd) of RNase H1 for the various substrates tested were determined by inhibition analysis using chemically modified noncleavable oligonucleotide heteroduplexes. Catalytic rates were determined using heteroduplex substrates containing chimeric antisense oligonucleotides composed of a five-base deoxynucleotide sequence flanked on either side by chemically modified nucleotides. We find that the enzyme preferentially binds A-form duplexes: RNase H bound A-form duplexes (RNA:RNA and DNA:RNA) approximately 60-fold tighter than B-form duplexes (DNA:DNA) and approximately 300-fold tighter than single-strand oligonucleotides. The enzyme exhibited equal affinity for both the wild type (RNA:DNA) oligonucleotide substrate and heteroduplexes containing various 2'-sugar modifications, while the cleavage rates for these chemically modified substrates were without exception slower than for the wild type substrate. The introduction of a single positively charged 2'-propoxyamine modification into the chimeric antisense oligonucleotide portion of the heteroduplex substrate resulted in both decreased binding affinity and a slower rate of catalysis by RNase H. The cleavage rates for heteroduplexes containing single-base mismatch sequences within the chimeric oligonucleotide portion varied depending on the position of the mismatch but had no effect on the binding affinity of the enzyme. These results offer further insights into the physical binding properties of the RNase H-substrate interaction as well as the design of effective antisense oligonucleotides.

Evaluation of 2'-modified oligonucleotides containing 2'-deoxy gaps as antisense inhibitors of gene expression.

We have used a previously described 17-mer phosphorothioate (Monia, B.P., Johnston, J.F., Ecker, D. J., Zounes, M.A., Lima, W.F., and Freier, S.M. (1992) J. Biol. Chem. 267, 19954-19962) for structure-function analysis of 2'-sugar modifications including 2'-O-methyl, 2'-O-propyl, 2'-O-pentyl, and 2'-fluoro. These modifications were analyzed for hybridization affinity to complementary RNA and for antisense activity against the Ha-ras oncogene in cells using a highly sensitive transactivation reporter gene system. Hybridization analysis demonstrated that all of the 2'-modified oligonucleotides hybridized with greater affinity to RNA than an unmodified 2'-deoxy oligonucleotide with the rank order of affinity being 2'-fluoro > 2'-O-methyl > 2'-O-propyl > 2'-O-pentyl > 2'-deoxy. Evaluation of antisense activities of uniformly 2'-modified oligonucleotides revealed that these compounds were completely ineffective in inhibiting Ha-ras gene expression. Activity was restored if the compound contained a stretch of at least five 2'-deoxy residues. This minimum deoxy length correlated perfectly with the minimum length required for efficient RNase H activation in vitro using partially purified mammalian RNase H enzyme. These chimeric 2'-modified/deoxy phosphorothioates displayed greater antisense potencies in inhibiting Ha-ras gene expression, compared with the unmodified uniform deoxy phosphorothioate. Furthermore, antisense potency correlated directly with affinity of a given 2' modification for it's complementary RNA. These results demonstrate the importance of target affinity in the action of antisense oligonucleotides and of RNase H as a mechanism by which these compounds exert their effects.

Implication of RNA structure on antisense oligonucleotide hybridization kinetics.

A 47-nucleotide transcript of the activated Ha-ras gene was prepared and determined, by enzymatic structure mapping, to form a stable hairpin structure. Six antisense decaribonucleotides were designed, and association constants (Ka) for the hairpin- and length-matched complements were measured. Two of the antisense oligonucleotides targeted to the loop had nearly equal affinity for the transcript compared to the complement. The others, including one oligonucleotide complementary to the 3' side of the single-stranded loop, bound 10(5)-10(6)-fold less tightly to the transcript than to the short complement. We propose the difference in affinity is due to the target structure, both the secondary structure of the stem and the structure in the loop. Measurement of the bimolecular association rate constant, k1, and the dissociation rate constant, k-1, for these oligonucleotides indicates the observed relationship between affinity and structure is primarily due to k1.

Selective inhibition of mutant Ha-ras mRNA expression by antisense oligonucleotides.

A biological reporter gene assay was employed to determine the crucial parameters for maximizing selective targeting of a Ha-ras codon 12 point mutation (G----T) using phosphorothioate antisense oligonucleotides. We have tested a series of oligonucleotides ranging in length between 5 and 25 bases, each centered around the codon 12 point mutation. Our results indicate that selective targeting of this point mutation can be achieved with phosphorothioate antisense oligonucleotides, but this selectivity is critically dependent upon oligonucleotide length and concentration. The maximum selectivity observed in antisense experiments, 5-fold for a 17-base oligonucleotide, was closely predicted by a simple thermodynamic model that relates the fraction of mutant to wild type target bound as a function of oligonucleotide concentration and affinity. These results suggest thermodynamic analysis of oligonucleotide/target interactions is useful in predicting the specificity that can be achieved by an antisense oligonucleotide targeted to a single base point mutation.

Antisense oligonucleotides inhibit intercellular adhesion molecule 1 expression by two distinct mechanisms.

Intercellular adhesion molecule 1 (ICAM-1) is a glycoprotein expressed on the surface of both hemopoietic and nonhemopoietic cells that mediates, in part, the emigration of leukocytes out of the vasculature. Expression of ICAM-1 on the surface of human umbilical vein endothelial cells and a human lung carcinoma cell line (A549) was increased by interleukin-1 beta, tumor necrosis factor alpha, and interferon gamma in a concentration-dependent manner. Phosphorothioate antisense oligonucleotides designed to hybridize to 10 target sites on the human ICAM-1 mRNA were tested for inhibition of ICAM-1 expression in both cell lines by an ICAM-1 enzyme-linked immunosorbent assay. Based upon potency and unique mRNA target sites, two oligonucleotides were studied in greater detail: ISIS 1570, which targeted the AUG translation initiation codon, and ISIS 1939, which targeted specific sequences in the 3'-untranslated region of the mRNA. Both oligonucleotides specifically inhibit expression of ICAM-1 as analyzed by immunoprecipitation of 35S-labeled proteins. Treatment of cells with ISIS 1939 promoted a reduction in ICAM-1 mRNA, whereas ISIS 1570 did not change the level of ICAM-1 mRNA, suggesting that the two oligonucleotides may be inhibiting ICAM-1 expression by two different mechanisms. The activity of both oligonucleotides was blocked by hybridization of the oligonucleotide to its complementary sense strand prior to addition to the cells. Neither ISIS 1570 nor ISIS 1939 changed the transcriptional rate of the ICAM-1 gene, demonstrating that both oligonucleotides were working through a post-transcriptional mechanism. 2'-O-Methyl phosphorothioate analogs, which do not support RNase H-mediated cleavage of target mRNA, were used to determine if the active antisense oligonucleotides inhibited ICAM-1 expression by an RNase H-dependent mechanism. The 2'-O-methyl phosphorothioate analog of ISIS 1939 did not significantly reduce interleukin-1 beta-induced ICAM-1 expression, whereas the 2'-O-methyl phosphorothioate analog of ISIS 1570 did inhibit ICAM-1 expression, suggesting that the reduction of ICAM-1 mRNA following treatment with ISIS 1939 was due, in part, to RNase H-mediated hydrolysis. Adherence of HL-60 cells to human umbilical vein cell monolayers was inhibited by ISIS 1570 and ISIS 1939, demonstrating that the reduced levels of ICAM-1 impact on ICAM-1-associated function.