Extracellular Tau Oligomers Produce An Immediate Impairment of LTP and Memory

Non-fibrillar soluble oligomeric forms of amyloid-β peptide (oAβ) and tau proteins are likely to play a major role in Alzheimer's disease (AD). The prevailing hypothesis on the disease etiopathogenesis is that oAβ initiates tau pathology that slowly spreads throughout the medial temporal cortex and neocortices independently of Aβ, eventually leading to memory loss. Here we show that a brief exposure to extracellular recombinant human tau oligomers (oTau), but not monomers, produces an impairment of long-term potentiation (LTP) and memory, independent of the presence of high oAβ levels. The impairment is immediate as it raises as soon as 20 min after exposure to the oligomers. These effects are reproduced either by oTau extracted from AD human specimens, or naturally produced in mice overexpressing human tau. Finally, we found that oTau could also act in combination with oAβ to produce these effects, as sub-toxic doses of the two peptides combined lead to LTP and memory impairment. These findings provide a novel view of the effects of tau and Aβ on memory loss, offering new therapeutic opportunities in the therapy of AD and other neurodegenerative diseases associated with Aβ and tau pathology.

Detection of rRNA from four respiratory pathogens using an automated Q beta replicase assay

Ribosomal RNA targets from Mycobacterium avium complex (23S), Mycoplasma pneumoniae (16S), Pneumocystis carinii (18S) and Legionella pneumophila (16S) were detected in four separate assays on a model automated Q-beta amplification instrument. Sandwich hybridization, reversible target capture, detector probe amplification and fluorescent signal detection occurred in closed, disposable packs at 38 degrees C. Packs were injected with 0.5 ml samples in 3.06 M guanidine thiocyanate. Ten samples per run were read after 7 h, requiring only 4 min loading time. Synthetic RNA transcripts and purified, natural RNAs from up to four different strains per assay were diluted to 10(6) or fewer molecules per sample (approximately 100 cells for prokaryotes, 10 cells for Pneumocystis). All analytes were detected at 10(6) targets. The limits of detection were found at 10(5) to 10(4). Discrimination against competitor RNA was tested using up to 10(9) molecules (1000 X excess) of appropriate test strains. Samples containing either zero targets or 10(7) competitors produced negative results in 95 to 100% of the samples, depending on the assay. Closely related Legionella and Mycoplasma species cross-reacted at high challenge levels of 10(9) molecules as a result of sequence similarities in the target regions. These results demonstrate the utility and versatility of an automated, high sensitivity, closed system for amplified analysis of direct-from-sample testing of respiratory pathogens.

Energetics of base pair opening in a DNA dodecamer containing an A3T3 tract

Nuclear magnetic resonance spectroscopy has been used to characterize the kinetics and energetics of opening of base pairs in the DNA dodecamer [d(CGCAAATTTGCG)]2. The dodecamer contains an A3T3 tract that induces intrinsic curvature of the helix axis. Previous studies from this and other laboratories have shown that the kinetics of base pair opening in AnTn tracts is unique: the opening rates of the A.T base pairs in the interior of the tract are much lower than that of the A.T base pair at the 5'-end of the tract. In the present work, we have investigated the energetics of the pathways for opening of the A.T base pairs in the A3T3 tract. The energetic parameters of the activated state(s) are obtained from the temperature dependence of the opening rate constants. The lower opening rates for the A.T base pairs situated in the interior of the tract are shown to originate from higher activation enthalpies which are compensated, in part, by increases in the activation entropies. We have also obtained an energetic characterization of the open state(s) of the A.T base pairs in the dodecamer by measuring the equilibrium constants for base pair opening and their temperature dependence. The results suggest that the transitions from closed to open state(s) in the A.T base pairs of the A3T3 tract are energetically similar.

Structure of a duplex oligodeoxynucleotide containing propanodeoxyguanosine opposite a two-base deletion in the (CpG)3 frame shift hotspot of Salmonella typhimurium hisD3052 determined by 1H NMR and restrained molecular dynamics

Structural refinement from solution 1H NMR data was performed on the 5'-d[ATCGC(PdG)-CGGCATG]-3'.5'-d[CATGCCGCGAT]-3' duplex, in which the adducted oligodeoxynucleotide containing the exocyclic lesion 1,N2-propano-2'-deoxyguanosine (PdG) was annealed with the complementary strand which contained a CpG deletion. The resulting duplex required PdG and one adjacent cytosine to be unpaired. A total of 352 distances were utilized to restrain molecular dynamics calculations, of which 264 were NOE-derived. These distances were calculated using complete relaxation matrix methods from hybrid matrices, which were comprised of the experimentally determined distances and additional distances derived from either A-form or B-form DNA. A simulated annealing protocol combined with the distance restraints was able to refine a single structure with an average rms deviation of < 1.35 A. The accuracy of the refined structure was assessed using full relaxation matrix calculations, which gave good agreement with measured NOE intensities. PdG was found to be stacked into the helix below base pair C3.G18, whereas C5 was found to be unpaired and extruded toward the major groove and parallel to base pair G6.C17. This created a localized bend in the DNA helix of approximately 20-35 degrees at the junction between PdG and C5. The bending corroborated previous assays performed on this modified sequence [Moe, J. G., Reddy, G. R., Marnett, L. J., & Stone, M. P. (1994) Chem. Res. Toxicol. 7, 319-328].

1H NMR characterization of a duplex oligodeoxynucleotide containing propanodeoxyguanosine opposite a two-base deletion in the (CpG)3 frameshift hotspot of Salmonella typhimurium hisD3052

The exocyclic lesion 1,N2-propano-2'-deoxyguanosine (PdG) was incorporated into 5'-d[ATCGC(PdG)CGGCATG]-3', derived from the hisD3052 gene of Salmonella typhimurium. The modified oligodeoxynucleotide was annealed with the complementary strand 5'-d[CATGCCGCGAT]-3' which contained a CpG deletion. The resulting duplex 5'-d[ATCGC(PdG)CGGCATG]-3'.5'-d[CATGCCGCGAT]-3' required PdG and one adjacent cytosine to be unpaired. Four thymine imino 1H NMR resonances were observed at temperatures below 25 degrees C, which demonstrated formation of a stable duplex with a two-base bulge. PdG was accommodated within the DNA helix, whereas the 3'-neighbor cytosine was poorly stacked and appeared to be extrahelical. The sequential nuclear Overhauser enhancement connectivities between aromatic and H1' protons along the modified strand were interrupted between PdG and the 3'-neighboring unpaired cytosine. On the complementary strand no interruptions were observed. An NOE was observed between the PdG methylene protons H8a,b and the imino proton of the 5'-neighbor base pair. Weaker NOEs were observed between the PdG H8a,b protons and the imino proton from guanine two nucleotides removed in the 3'-direction, and to the amino proton of cytosine located in the complementary strand two nucleotides removed in the 3'-direction. Chemical shift perturbations were also observed for the latter cytosine as compared to the corresponding cytosine in the unmodified fully complementary duplex. These observations provided evidence for a poorly stacked or an extrahelical conformation of this unpaired cytosine. The amino proton resonances of the 3'-neighbor cytosine were not observed, presumably due to increased exchange with solvent. The methylene protons from PdG were shifted upfield relative to the monomer PdG, probably as a result of aromatic ring current shielding, consistent with an intrahelical location. Multimeric derivative oligonucleotides containing the PdG bulge migrated anomalously on nondenaturing polyacrylamide gels, consistent with a structure in which the unpaired nucleotides induced a bend in the DNA.

1H NMR of an oligodeoxynucleotide containing a propanodeoxyguanosine adduct positioned in a (CG)3 frameshift hotspot of Salmonella typhimurium hisD3052: Hoogsteen base-pairing at pH 5.8

The exocyclic DNA adduct 1,N2-propano-2'-deoxyguanosine (PdG) was inserted into the oligodeoxynucleotide 5'-CGC(PdG)CGGCATG-3' and annealed to the complementary oligodeoxynucleotide 5'-CATGCCGCGCG-3'. This sequence is derived from a spontaneous revertant of the hisD3052 gene in a frameshift-sensitive tester strain of Salmonella typhimurium and is a hotspot for two-base pair deletions. The solution structure of the modified duplex was examined by 1H NMR spectroscopy. The exocyclic lesions resulted in loss of Watson-Crick base-pairing capability. Modification resulted in an approximately 24 degrees C decrease in Tm of the duplex. NMR experiments revealed pH-dependent conformational equilibria, which involved the modified base pair and its 3'-neighbor base pair. At pH 5.8, the lesion resulted in a localized perturbation of the B-form helix. PdG was rotated about the glycosyl bond from the anti to the syn conformation, thus placing the propano moiety into the major groove. This resulted in the observation of a strong NOE between the imidazole proton of PdG and the anomeric proton of the attached deoxyribose. Additional NOEs were observed between the methylene protons of the propano moiety and H5 and H6 of the 5'-neighbor cytosine. An imino proton resonance from the cytosine complementary to PdG and protonated at N3, characteristic of a Hoogsteen base pair, was observed at 15 ppm, but was broadened due to exchange with water. The amino protons of the complementary cytosine were shifted downfield from the other cytosine amino protons, characteristic of a Hoogsteen-like conformation at the site of modification. A second equilibrium involved the 3'-neighbor base pair, which alternated between Watson-Crick and Hoogsteen pairing, also via rotation of the guanosine glycosyl bond from the anti to the syn conformer. The conformational exchange of the 3'-neighbor base pair was sufficiently slow on the NMR time scale to allow simultaneous observation of resonances from the Watson-Crick and the Hoogsteen conformers.

Kinetics and energetics of base-pair opening in 5'-d(CGCGAATTCGCG)-3' and a substituted dodecamer containing G.T mismatches

Proton nuclear magnetic resonance (NMR) spectroscopy is used to characterize the kinetics and energetics of base-pair opening in the dodecamers 5'-d(CGCGAATTCGCG)-3' and 5'-d(CGCGAATTTGCG)-3'. The latter dodecamer contains two symmetrical G.T mismatched base pairs. The exchange kinetics of imino protons is measured from resonance line widths and selective longitudinal relaxation times. For the G.T pair, the two imino protons (G-N1H and T-N3H) provide probes for the opening of each base in the mismatched pair. The lifetimes of individual base pairs in the closed state and the equilibrium constants for formation of the open state are obtained from the dependence of the exchange rates on the concentration of ammonia catalyst. The activation energies and standard enthalpy changes for base-pair opening are obtained from the temperature dependence of the lifetimes and equilibrium constants, respectively. The results indicate that the G.T mismatched pairs are kinetically and energetically destabilized relative to normal, Watson-Crick base pairs. The lifetimes of the G.T pairs are of the order of 1 ms or less, over the temperature range from 0 to 20 degrees C. The equilibrium constants for base-pair opening, at 20 degrees C, are increased up to 4000-fold, relative to those of normal base pairs. The energetic destabilization of the G.T base pairs is, at least in part, enthalpic in origin. The presence of the G.T mismatched base pairs destabilizes also neighboring base pairs.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton exchange and base-pair opening kinetics in 5'-d(CGCGAATTCGCG)-3' and related dodecamers

We have used nuclear magnetic resonance (NMR) spectroscopy to measure the lifetimes of individual base-pairs in the palindromic DNA oligonucleotide 5'-d(CGCGAATTCGCG)-3' and in three other dodecamers with symmetrical base substitutions in the sites underlined. The resonances of the hydrogen-bonded imino protons in each of the substituted oligomers in the duplex form have been assigned using one dimensional nuclear Overhauser effect (1-D NOE) experiments. The lifetimes have been obtained from the dependence of selective longitudinal relaxation times and linewidths of the imino proton resonances on the concentration of base catalyst (Tris) at 25 degrees C and in the presence of 50 mM NaCl. The lifetimes of the central A.T base-pairs have been found to depend on base sequence. They are greatly increased in the dodecamer 5'-d(CGCAAATTTGCG)-3' which contains an A3T3 tract. The lifetimes of the central A.T base-pairs in 5'-d(CGCGAATTCGCG)-3', 5'-d(CGCTAATTAGCG)-3' and 5'-d(CGCCAATTGGCG)-3' are comparable. In all dodecamers, the lifetime of the A.T base-pair at the 5'-end of the AnTn tract is the shortest. The anomalous opening kinetics of the A.T base-pairs can be correlated to the bending properties of the corresponding sequences.

Isoleucyl transfer ribonucleic acid synthetase. Steady-state kinetic analysis

A steady-state kinetic analysis was conducted of the overall aminoacylation reaction catalyzed by isoleucyl-tRNA synthetase. The patterns of Lineweaver-Burk plots obtained indicated that tRNA adds to the enzyme only after isoleucyl adenylate formation and pyrophosphate release. These kinetic patterns were consistent with the bi-uni-uni-bi Ping Pong mechanism generally accepted for this aminoacyl-tRNA synthetase, but they could also be accommodated by a mechanism in which a second molecule of L-isoleucine added to the enzyme between isoleucyl adenylate formation and aminoacylation of tRNA [Fersht, A.R., & Kaethner, M.M. (1976) Biochemistry 15, 818]. The values of the kinetic parameters favor the latter mechanism. The results of this kinetic analysis indicated that the affinity of isoleucyl-tRNA synthetase for Mg.ATP was enhanced upon binding of L-isoleucine and vice versa. It also indicated that the affinity of the enzyme for L-isoleucine is decreased upon binding tRNA and vice versa. The values of dissociation constants calculated for each of the substrates by this study generally compared well with those determined by other authors using a variety of kinetic and equilibrium methods.

Isoleucyl transfer ribonucleic acid synthetase. Competitive inhibition with respect to transfer ribonucleic acid by blue dextran

The inhibitory effects of blue dextran and a small dye molecule derived from it (F3GA-OH) on the steady-state reaction catalyzed by Escherichia coli isoleucy-tRNA synthetase have been studied. Blue dextran gave uncompetitive inhibition with respect to Mg.ATP, mixed inhibition with respect to L-isoleucine, and competitive inhibition with respect to tRNA. The small dye molecule (F3GA-OH) was also competitive with respect to tRNA. These inhibition patterns were not consistent with the bi-uni-uni-bi Ping Pong mechanism generally accepted for aminoacyl-tRNA synthetases. They were consistent with a mechanism in which a second L-isoleucine is bound after isoleucyl-AMP synthesis and before transfer of the isoleucyl moiety to tRNA. Enzyme-bound L-isoleucine lowered the affinity of the enzyme for blue dextran approximately fivefold, a value comparable to the ninefold lowering of the enzyme's affinity for tRNA upon binding L-isoleucine. The affinity of the synthetase for F3GA-OH (K1 = 1.0 X 10(-7) M) is approximately fivefold higher than its affinity for blue dextran (K1 = 5.3 X 10(-7) M). These results indicate that blue dextran and its derivatives may be useful for kinetic and physical studies of polynucleotide binding sites on proteins as well as NAD and ATP sites.