Cryo-EM structure of the human α1β3γ2 GABAA receptor in a lipid bilayer

Type A γ-aminobutyric acid (GABAA) receptors are pentameric ligand-gated ion channels and the main drivers of fast inhibitory neurotransmission in the vertebrate nervous system1,2. Their dysfunction is implicated in a range of neurological disorders, including depression, epilepsy and schizophrenia3,4. Among the numerous assemblies that are theoretically possible, the most prevalent in the brain are the α1β2/3γ2 GABAA receptors5. The β3 subunit has an important role in maintaining inhibitory tone, and the expression of this subunit alone is sufficient to rescue inhibitory synaptic transmission in β1-β3 triple knockout neurons6. So far, efforts to generate accurate structural models for heteromeric GABAA receptors have been hampered by the use of engineered receptors and the presence of detergents7-9. Notably, some recent cryo-electron microscopy reconstructions have reported 'collapsed' conformations8,9; however, these disagree with the structure of the prototypical pentameric ligand-gated ion channel the Torpedo nicotinic acetylcholine receptor10,11, the large body of structural work on homologous homopentameric receptor variants12 and the logic of an ion-channel architecture. Here we present a high-resolution cryo-electron microscopy structure of the full-length human α1β3γ2L-a major synaptic GABAA receptor isoform-that is functionally reconstituted in lipid nanodiscs. The receptor is bound to a positive allosteric modulator 'megabody' and is in a desensitized conformation. Each GABAA receptor pentamer contains two phosphatidylinositol-4,5-bisphosphate molecules, the head groups of which occupy positively charged pockets in the intracellular juxtamembrane regions of α1 subunits. Beyond this level, the intracellular M3-M4 loops are largely disordered, possibly because interacting post-synaptic proteins are not present. This structure illustrates the molecular principles of heteromeric GABAA receptor organization and provides a reference framework for future mechanistic investigations of GABAergic signalling and pharmacology.

Design, Synthesis, and Antiviral Activity of 2‘-Deoxy-2‘-fluoro-2‘-C-methylcytidine, a Potent Inhibitor of Hepatitis C Virus Replication

The pyrimidine nucleoside beta-d-2'-deoxy-2'-fluoro-2'-C-methylcytidine (1) was designed as a hepatitis C virus RNA-dependent RNA polymerase (HCV RdRp) inhibitor. The title compound was obtained by a DAST fluorination of N(4)-benzoyl-1-(2-methyl-3,5-di-O-benzoyl-beta-d-arabinofuranosyl]cytosine to provide N(4)-benzoyl-1-[2-fluoro-2-methyl-3,5-di-O-benzoyl-beta-d-ribofuranosyl]cytosine. The protected 2'-C-methylcytidine was obtained as a byproduct from the DAST fluorination and allowed for the preparation of two biologically active compounds from a common precursor. Compound 1 and 2'-C-methylcytidine were assayed in a subgenomic HCV replicon assay system and found to be potent and selective inhibitors of HCV replication. Compound 1 shows increased inhibitory activity in the HCV replicon assay compared to 2'-C-methylcytidine and low cellular toxicity.

Novel route to oligo(deoxyribonucleoside phosphorothioates). Stereocontrolled synthesis of P-chiral oligo(deoxyribonucleoside phosphorothioates)

The synthesis and separation of diastereoisomerically pure 5'-O-DMT-nucleoside 3'-O-(2-thio-1,3,2-oxathiaphospholane) allows their use as synthons in DBU-catalyzed reaction with the 5'-hydroxyl function of solid-support-bound nucleoside moiety. Since this reaction is stereospecific (greater than 99%), this novel method allows preparation of oligo(nucleoside phosphorothioates) with predetermined chirality at each P-chiral internucleotide phosphorothioate centre.

Inhibitory action of certain cyclophosphate derivatives of cAMP on cAMP-dependent protein kinases

A series cAMP derivatives with modifications in the adenine, ribose and cyclophosphate moiety were screened for their binding affinity for the two types of cAMP-binding sites in mammalian protein kinase type 1. In addition, the activation of the kinase by these analogs was monitored. The binding data indicate that cAMP is bound to both sites in a comparable manner: the adenine appears to have no hydrogen-bond interactions with the binding sites, whereas the ribose may be bound by three hydrogen bonds involving the 2', 3' and 5' positions of cAMP. The binding data are not conclusive about the nature of the interaction with the exocyclic oxygen atoms on phosphorus, though a charge interaction seems to be absent. The cAMP molecule seems to be bound in the syn conformation. The results of activation experiments show that modifications in the adenine and ribose moiety do not affect the maximal activation level, while alteration of the two exocyclic oxygen atoms may result in a reduced maximal activation level and in one case, (Rp)-adenosine 3', 5'-monophosphorothioate [Rp-cAMPS], in total absence of activation even at concentrations at which the analog saturates both binding sites. Since occupancy of the cAMP-binding sites by this derivative apparently did not lead to activation of the enzyme, we examined whether this compound could antagonize the activation by cAMP. Indeed (Rp)-cAMPS was found to inhibit cAMP stimulated kinase activity at concentrations compatible to its binding affinity. Also with mammalian protein kinase type II (Rp)-cAMPS showed antagonistic activity, while with a cAMP-dependent protein kinase from Dictyostelium discoideum partial agonistic activity was observed. Previously a mechanism for activation of protein kinase type I was proposed involving a charge interaction between the equatorial exocyclic oxygen atom and the binding site [De Wit et. al. (1982) Eur. J. Biochem 122, 95-99]. This was based on measurements with impure preparations of (Rp)-cAMPS and the Rp and Sp isomers adenosine 3', 5'-monophosphodimethylamidate. cAMPN(CH3)2. The present work using highly purified compounds suggests the absence of a charge interaction, since the uncharged analog (Sp)-cAMPN(CH3)2 activates the kinase effectively. The data seem compatible with an activation model involving the formation of a covalent bond with phosphorus in both cAMP binding sites.

Recent advances in the elucidation of the mechanisms of action of ribozymes

The cleavage of RNA can be accelerated by a number of factors. These factors include an acidic group (Lewis acid) or a basic group that aids in the deprotonation of the attacking nucleophile, in effect enhancing the nucleophilicity of the nucleophile; an acidic group that can neutralize and stabilize the leaving group; and any environment that can stabilize the pentavalent species that is either a transition state or a short-lived intermediate. The catalytic properties of ribozymes are due to factors that are derived from the complicated and specific structure of the ribozyme-substrate complex. It was postulated initially that nature had adopted a rather narrowly defined mechanism for the cleavage of RNA. However, recent findings have clearly demonstrated the diversity of the mechanisms of ribozyme-catalyzed reactions. Such mechanisms include the metal-independent cleavage that occurs in reactions catalyzed by hairpin ribozymes and the general double-metal-ion mechanism of catalysis in reactions catalyzed by the Tetrahymena group I ribozyme. Furthermore, the architecture of the complex between the substrate and the hepatitis delta virus ribozyme allows perturbation of the pK(a) of ring nitrogens of cytosine and adenine. The resultant perturbed ring nitrogens appear to be directly involved in acid/base catalysis. Moreover, while high concentrations of monovalent metal ions or polyamines can facilitate cleavage by hammerhead ribozymes, divalent metal ions are the most effective acid/base catalysts under physiological conditions.

Interaction of cAMP derivatives with the 'stable' cAMP-binding site in the cAMP-dependent protein kinase type I

cAMP binding to the 'stable' cAMP-binding sites in the regulatory subunit of the cAMP-dependent protein kinase type I was investigated using a set of 18 selected derivatives. All the tested analogues were competitive with [3H]cAMP and inhibitor constants from 12 nM to 20 microM with the free regulatory subunit were determined. The cAMP molecule seemed to be bound by these specific hydrogen bonds to the 5' and 3' oxygen, the 2' hydroxyl, and an ion pair interaction between the negative charge in equatorial position and a positively charged amino acid side chain. The adenine base is rather unspecifically bound with no hydrogen bonds involved. This binding specificity of the 'stable' site is similar to the requirement for dissociation as determined by the activation of the kinase by a respective analogue. This indicates that occupation of the 'stable' sites leads to activation of the protein kinase. The presence of the catalytic subunit reduced the affinity of most analogues. The binding of one derivative with the negative charge fixed in the axial position is not influenced by the addition of the catalytic subunit and ATP. A plausible model for a conformational change during the activation process in the 'stable' site is discussed.

Reversed-phase high-performance liquid chromatographic separation of diastereomeric phosphorothioate analogues of oligodeoxyribonucleotides and other backbone-modified congeners of DNA

Diastereomeric phosphorothioate analogues of oligodeoxyribonucleotides, which were synthesized by an automated, solid-phase, phosphoramidite-coupling method, were conveniently separated by reversed-phase high-performance liquid chromatography on a muBondapak C18 column with a gradient of acetonitrile in triethylammonium acetate buffer. These synthetic and chromatographic methods were also used to obtain diastereomerically pure bis-phosphorothioate, alkanephosphonate, and O-alkyl phosphotriester analogues of DNA for exploratory studies of stereochemistry and phosphorolytic enzymes.

Structure and dynamics of a DNA.RNA hybrid duplex with a chiral phosphorothioate moiety: NMR and molecular dynamics with conventional and time-averaged restraints

The three-dimensional structure of two thiophosphate-modified DNA.RNA hybrid duplexes d(GCTATAApsTGG).r(CCAUUAUAGC), one with R-thiophosphate chirality and one with S-thiophosphate chirality, have been determined by restrained molecular dynamics simulations (rMD). As the two yielded almost identical results, a description of results can be presented in the singular. The conformational flexibility of this hybrid has been investigated by employing time-averaged constraints during the molecular dynamics simulations (MD-tar). A set of structural restraints, comprising 322 precise interproton distance constraints obtained by a complete relaxation matrix analysis of the 2D NOE intensities as well as J coupling constants obtained from quantitative simulations of DQF-COSY cross-peaks in deoxyriboses, was reported in our previous paper [González, C., Stec, W., Kobylanska, A., Hogrefe, R. I., Reynolds, M., & James, T. L. (1994) Biochemistry 33, 11062-11072]. Multiple conformations of the deoxyribose moieties were evident from the scalar coupling constant analysis. Accurate distance constraints, obtained from complete relaxation matrix analysis, yielded a time-averaged solution structure via conventional restrained molecular dynamics which is not compatible with the experimental J coupling constants (root-mean-square deviation in J value approximately 2 Hz). However, vicinal coupling constant information can be reproduced when time-averaged constraints are used during the molecular dynamics calculations instead of the conventional restraints (Jrms approximately 0.6 Hz). MD-tar simulations also improve the NMR R factors. This improvement is more evident in the DNA than in the RNA strand, where no indication of conformational flexibility had been obtained. Analysis of the MD-tar trajectories confirms that deoxyriboses undergo pucker transitions between the S and N domain, with the major conformer in the S domain. The ribose moieties in the RNA strand, however, remain in the N domain during the entire simulation. Conformations of deoxyriboses in the intermediate domain near O4'-endo are obtained when the average structure is calculated with conventional NMR restraints. Since these conformations cannot account for the experimental J coupling information, and they only appear in a very low population in the MD-tar ensemble, we conclude that intermediate E sugar puckers are artifacts produced by the attempt to fit all the structural constraints simultaneously when in reality more than one conformer is present. Most structural features of the duplex remain the same in the average structure and in the MD-tar ensemble, e.g., the minor groove width, exhibiting an intermediate value compared with those of canonical A- and B-like structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereodifferentiation--the effect of P chirality of oligo(nucleoside phosphorothioates) on the activity of bacterial RNase H

P stereoregular phosphorothioate analogs of pentadecamer 5'-d(AGATGTTTGAGCTCT)-3' were synthesized by the oxathiaphospholane method. Their diastereomeric purity was assigned by means of enzymatic degradation with nuclease P1 and, independently, with snake venom phosphodiesterase. DNA-RNA hybrids formed by phosphorothioate oligonucleotides (PS-oligos) with the corresponding complementary pentadecaribonucleotide were treated with bacterial RNase H. The DNA-RNA complex containing the PS-oligo of [all-RP] configuration was found to be more susceptible to RNase H-dependent degradation of the pentadecaribonucleotide compared with hybrids containing either the [all-SP] counterpart or the so called 'random mixture of diastereomers' of the pentadeca(nucleoside phosphorothioate). This stereodependence of RNase H action was also observed for a polyribonucleotide (475 nt) hybridized with these phosphorothioate oligonucleotides. The results of melting studies of PS-oligo-RNA hybrids allowed a rationalization of the observed stereodifferentiation in terms of the higher stability of heterodimers formed between oligoribonucleotides and [all-RP]-oligo(nucleoside phosphorothioates), compared with the less stable heterodimers formed with [all-SP]-oligo(nucleoside phosphorothioates) or the random mixture of diastereomers.

Specificity of cyclic GMP activation of a multi-substrate cyclic nucleotide phosphodiesterase from rat liver

Cyclic nucleotide derivatives have been used as a tool to investigate the existence of distinctive activating and hydrolytic sites on the phosphodiesterase from rat liver activated by cGMP (guanosine 3',5'-monophosphate). This positively cooperative enzyme was stimulated up to 30-fold by 3 microM cGMP when 3 microM cAMP (adenosine 3',5'-monophosphate) was used as substrate. All analogues were less potent activators than cGMP. Most cAMP derivatives were inactive, with two exceptions: 7-deazaadenosine 3',5'-monophosphate and 3'-amino-3'-deoxy-adenosine 3',5'-monophosphate. Benzimidazole ribonucleoside 3',5'-monophosphate, where the two atoms of nitrogen of the pyrimidine ring are missing was a better stimulator than the intact purine-related cyclic derivative. When cAMP and cGMP with identical chemical ligands substituted at the same position were compared, the cGMP analogue was always the more potent activator suggesting that the activating site is sensitive to a guanine-type cyclic nucleotide structure. Degradation of the derivatives by the enzyme was measured by high-performance liquid chromatography: no relation could be established between hydrolysis and effectiveness of activation. In addition, there was no parallelism between inhibitory and activating potency for ten cyclic nucleotide derivatives. Since the chemical interactions between the analogues at the activating site on the one hand and at the catalytic site on the other, are different, it is proposed that the sites are distinct. Consequently, it is suggested that the enzyme operates in steps. In the first activating step, cGMP is fixed by at least two hydrogen bonds at a specific binding site of the enzyme. This is followed by a conformational change of the protein and subsequently a change of the kinetic parameters. In a rather unspecific process and in a second hydrolytic step, several purine-related cyclic nucleotides are converted to the corresponding 5' nucleotides.

Stability of stereoregular oligo(nucleoside phosphorothioate)s in human plasma: diastereoselectivity of plasma 3'-exonuclease

The stability of stereoregular oligo(nucleoside phosphorothioate)s (PS-oligos) in human plasma has been studied. 3'-Exonuclease present in human plasma appeared to be RP specific, that is, it cleaves internucleotide phosphorothioate linkages of [RP]-configuration and not those of [SP]-configuration. Therefore, PS-oligos containing all phosphorothioate internucleotide linkages of [RP]-configuration [RP-PS-oligos]) are more effectively degraded by the enzyme than PS-oligos prepared via nonstereo-controlled methods (so-called random mixture of diastereomers [Mix-PS-oligos]), whereas oligo(nucleoside phosphorothioate)s of [S(P)]-configuration remain intact. The enzyme activity depends on the sequence of nucleobases. The presence of deoxycytidine units (three or more residues) at the 3'-end of PS-oligo substrate significantly inhibits the enzyme activity.

Phosphorothioate-modified oligodeoxyribonucleotides. III. NMR and UV spectroscopic studies of the Rp-Rp, Sp-Sp, and Rp-Sp duplexes, [d(GGSAATTCC)]2, derived from diastereomeric O-ethyl phosphorothioates

2D-NOE and 1H NMR chemical shift data obtained for the title oligonucleotides were compared with similar data previously reported [Broido et al. (1985) Eur. J. Biochem. 150, 117-128] for the unmodified "parent" structure, [d(GGAATTCC)]2. The spectroscopically detectable structural perturbations caused by replacement of phosphate oxygen with sulfur were mostly localized within the GsA moiety, and were greater for the Rp configuration wherein sulfur is oriented into the major groove of the B-helix. UV-derived Tm measurements gave the following order of stability for the duplexes in 0.4 M NaCl: unmodified (33.9 +/- 0.1 degrees C) approximately Sp-Sp (34.1 degrees C) greater than Rp-Rp (31.7 degrees C). The title compounds were prepared by a new and convenient synthetic route which utilized HPLC to separate the diastereomeric O-ethyl phosphorothioate precursors, (Rp)- and (Sp)-d[GG(S,Et)AATTCC], for subsequent de-ethylation by ammonia in water.

Binding of phosphorothioate oligodeoxynucleotides to basic fibroblast growth factor, recombinant soluble CD4, laminin and fibronectin is P-chirality independent

Antisense oligodeoxynucleotides can selectively inhibit the expression of individual genes and thus have potential applications in anticancer and antiviral therapy. A critical prerequisite to their use as therapeutic agents is the understanding of their non-specific interactions with biological structures, e.g. proteins. In this study we examined the interactions of P-chiral phosphorothioate oligodeoxynucleotides with several proteins. The Rp- and Sp- diastereomers, and racemic machine-made mixtures, or M-oligodeoxynucleotides were used independently as competitors of the binding of a probe, phosphodiester oligodeoxynucleotide bearing a 5' alkylating moiety, to rsCD4, bFGF and laminin. These oligodeoxynucleotides were also used as competitors of the binding of a non-alkylating probe M-phosphorothioate oligodeoxynucleotide, 5'-32P-SdT18 to fibronectin. The average values of and quantitative estimates for the IC50 of competition and the constant of competition (Kc) of Rp-, Sp- and M-stereoisomers of several homo- and heteropolymer oligodeoxynucleotides were determined and compared. Surprisingly, in the proteins we studied, the values of IC50 and Kc for the Rp-, Sp- and M-oligodeoxynucleotides were essentially identical. Thus, the ability of the phosphorothioate oligodeoxynucleotides we employed, to bind to the proteins studied in this work, is virtually independent of P-chirality. Our results also imply that the role of the purine and pyrimidine bases in oligodeoxynucleotide-protein interactions, as well as the nature of the contact points (sulfur versus oxygen) between the oligomer and the protein, may be relatively unimportant.

Octa(thymidine methanephosphonates) of partially defined stereochemistry: synthesis and effect of chirality at phosphorus on binding to pentadecadeoxyriboadenylic acid

Block condensation of MePOCI2 or MeP(NEt2)2 with appropriately protected tetra(thymidine methanephosphonates) of predetermined sense of chirality at asymmetric phosphonate centres gave two pairs of diastereomeric mixtures, namely (SpSpSpSpSpSpSp + SpSpSpRpSpSpSp) 5a and (RpRpRpRpRpRpRp + RpRpRpSpRpRpRp) 5b. A comparison of the CD spectra of 5a and 5b with those of octathymidylic acid (7) and a random mixture of diastereomers of octa(thymidine methanephosphonate) (6), and also a comparison of the Tm of complexes formed between 5a, 5b, 6 or 7, and pentadecadeoxyriboadenylic acid (8), indicates that octamer 5b and its complex with its complementary oligonucleotide has a well-ordered structure due to the 'outward' or 'pseudoequatorial' orientation of the methyl group of each internucleotide methanephosphonate function of Rp configuration. Results presented in this report clearly indicate that the stability of hybrids formed between octa(thymidine methanephosphonate) and pentadecadeoxyriboadenylic acid depends on the stereochemistry of each internucleotide methanephosphonate function and strongly suggests that stereoselective synthesis of P-chiral oligonucleotide analogues is an important goal.