Membrane fusogenic activity of the Alzheimer's peptide A beta(1-42) demonstrated by small-angle neutron scattering

Amyloid-β peptide (Aβ) is considered a triggering agent of Alzheimer's disease. In relation to a therapeutic treatment of the disease, the interaction of Aβ with the cell membrane has to be elucidated at the molecular level to understand its mechanism of action. In previous works, we had ascertained by neutron diffraction on stacked lipid multilayers that a toxic fragment of Aβ is able to penetrate and perturb the lipid bilayer. Here, the influence of Aβ(1-42), the most abundant Aβ form in senile plaques, on unilamellar lipid vesicles of phospholipids is investigated by small-angle neutron scattering. We have used the recently proposed separated form factor method to fit the data and to obtain information about the vesicle diameter and structure of the lipid bilayer and its change upon peptide administration. The lipid membrane parameters were obtained with different models of the bilayer profile. As a result, we obtained an increase in the vesicle radii, indicating vesicle fusion. This effect was particularly enhanced at pH 7.0 and at a high peptide/lipid ratio. At the same time, a thinning of the lipid bilayer occurred. A fusogenic activity of the peptide may have very important consequences and may contribute to cytotoxicity by destabilizing the cell membrane. The perturbation of the bilayer structure suggests a strong interaction and/or insertion of the peptide into the membrane, although its localization remains beyond the limit of the experimental resolution.     

Crystal structure of glutamyl-queuosine tRNAAsp synthetase complexed with L-glutamate: structural elements mediating tRNA-independent activation of glutamate and glutamylation of tRNAAsp anticodon

Glutamyl-queuosine tRNA^Asp synthetase (Glu-Q-RS) from Escherichia coli is a paralog of the catalytic core of glutamyl-tRNA synthetase (GluRS) that catalyzes glutamylation of queuosine in the wobble position of tRNA^Asp. Despite important structural similarities, Glu-Q-RS and GluRS diverge strongly by their functional properties. The only feature common to both enzymes consists in the activation of Glu to form Glu-AMP, the intermediate of transfer RNA (tRNA) aminoacylation. However, both enzymes differ by the mechanism of selection of the cognate amino acid and by the mechanism of its activation. Whereas GluRS selects l-Glu and activates it only in the presence of the cognate tRNA^Glu, Glu-Q-RS forms Glu-AMP in the absence of tRNA. Moreover, while GluRS transfers the activated Glu to the 3′ accepting end of the cognate tRNA^Glu, Glu-Q-RS transfers the activated Glu to Q34 located in the anticodon loop of the noncognate tRNA^Asp. In order to gain insight into the structural elements leading to distinct mechanisms of amino acid activation, we solved the three-dimensional structure of Glu-Q-RS complexed to Glu and compared it to the structure of the GluRS·Glu complex. Comparison of the catalytic site of Glu-Q-RS with that of GluRS, combined with binding experiments of amino acids, shows that a restricted number of residues determine distinct catalytic properties of amino acid recognition and activation by the two enzymes. Furthermore, to explore the structural basis of the distinct aminoacylation properties of the two enzymes and to understand why Glu-Q-RS glutamylates only tRNA^Asp among the tRNAs possessing queuosine in position 34, we performed a tRNA mutational analysis to search for the elements of tRNA^Asp that determine recognition by Glu-Q-RS. The analyses made on tRNA^Asp and tRNA^Asn show that the presence of a C in position 38 is crucial for glutamylation of Q34. The results are discussed in the context of the evolution and adaptation of the tRNA glutamylation system.     

Anti-oligomeric Abeta single-chain variable domain antibody blocks Abeta-induced toxicity against human neuroblastoma cells

The Amyloid-β (Aβ) peptide is a major component of the amyloid plaques associated with Alzheimer's disease (AD). Recent studies suggest that the most toxic forms of Aβ are small, soluble oligomeric aggregates. Here, we report the isolation and characterization of a single-chain variable domain (scFv) antibody isolated against oligomeric Aβ using a protocol developed in our laboratory that combines phage display technology and atomic force microscopy (AFM). Starting with a randomized, single framework phage display library, after three rounds of selection against oligomeric Aβ, we identified an scFv that bound oligomeric Aβ specifically, but not monomeric or fibrillar forms. The anti-oligomeric scFv inhibits Aβ aggregation and toxicity, and reduces the toxicity of preformed oligomeric Aβ towards human neuroblastoma cells. When used to probe samples of human brain tissue, the scFv reacted with AD tissue but not a healthy control or Parkinson's disease brain samples. The anti-oligomeric Aβ scFv therefore has potential therapeutic and diagnostic applications in specifically targeting or identifying the toxic morphologies of Aβ in AD brains.     

ATP-induced allostery in the eukaryotic chaperonin CCT is abolished by the mutation G345D in CCT4 that renders yeast temperature-sensitive for growth

Saccharomyces cerevisiae yeast cells containing the chaperonin CCT (chaperonin-containing t-complex polypeptide 1 (TCP-1)) with the G345D mutation in subunit CCT4 (anc2-1) are temperature-sensitive for growth and display defects in organization of actin structure, budding and cell shape. In this first structure-function analysis of CCT, we show that this mutation abolishes both intra- and inter-ring cooperativity in ATP binding by CCT. The finding that a single mutation in only one subunit in each CCT ring has such drastic effects highlights the importance of allostery for its in vivo function. These results, together with other kinetic data for wild-type CCT reported in this study, provide support for the sequential model for ATP-dependent allosteric transitions in CCT.     

Antibody-based approaches in Alzheimer's research: safety, pharmacokinetics, metabolism, and analytical tools

High morbidity, enormous socioeconomic costs, and lack of specific treatments emphasize the importance of research on protective therapies against Alzheimer's disease. The efficacy of anti-amyloid immunization strategies has been demonstrated preclinically, prompting the design of clinical studies. However, the detailed mechanisms of action of therapeutic antibodies, especially their influence on the complex amyloid β peptide (Aβ) metabolism and various Aβ-equilibria present both within and outside the CNS, are far from being clear. Furthermore, physiological Aβ metabolism is poorly understood and the analytical tools to characterize and quantify treatment effects on Aβ metabolism are suboptimal. Thus, the design of immunization strategies with optimized benefit-to-risk ratios for patients is subjected to significant obstacles. Indeed, an active immunization trial with Aβ was discontinued because of severe adverse effects. Anti-Aβ immunization protocols designed to attain high blood levels of antibodies bear the potential to induce brain inflammation and/or hemorrhage, thus directing the biomedical research towards development of more predictable therapies for minimizing the risk of adverse effects. The focus of this review is to summarize current knowledge of Aβ metabolism under physiological and antibody-based therapeutic conditions and to introduce a promising approach, namely the passive immunization using antibody fragments, which are characterized by entirely different pharmacokinetic and pharmacodynamic properties compared with conventional monoclonal antibodies.     

K+ACh channel activation with carbachol increases atrial ANP release

Although it has been known that atrial natriuretic peptide (ANP) release is regulated through muscarinic acetylcholine receptors (mAChR), the mechanism by which this neurotransmitter regulates atrial ANP release is largely unknown. This study tested the hypothesis that K(+)(ACh) channels mediate the action of mAChR on atrial myocyte ANP release. Experiments were performed in perfused beating rabbit atria. Carbachol (CCh), an agonist of cardiac mAChR, increased atrial myocyte ANP release concomitantly with a decrease in stroke volume and intra-atrial pulse pressure in a concentration-dependent manner. Isoproterenol, a beta-adrenoceptor agonist, decreased ANP release concomitantly with an increase in cAMP and mechanical dynamics. In the presence of isoproterenol, the CCh-induced increase in ANP release and decrease in cAMP efflux levels and mechanical dynamics were able to be repeated. The CCh-induced changes were blocked by selective M(2) mAChR antagonists. Tertiapin, a selective G-protein-gated K(+)(ACh) channel blocker, attenuated the CCh-induced increase in ANP release and decrease in mechanical dynamics in a concentration-dependent manner, but without a significant effect on the CCh-induced decrease in cAMP efflux levels. The CCh-induced changes in ANP release and atrial dynamics were inhibited in the atria from pertussis toxin-pretreated rabbits. These findings demonstrate that G-protein-gated K(+)(ACh) channels regulate atrial myocyte ANP release. The present study also shows that mAChR and adrenoceptors have opposing roles in the regulation of ANP release.     

Huperzine A Reverses Cholinergic and Monoaminergic Dysfunction Induced by Bilateral Nucleus Basalis Magnocellularis Injection of β-Amyloid Peptide (1–40) in Rats

(1) Huperzine A, a promising therapeutic agent for Alzheimer’s disease (AD), was tested for its effects on cholinergic and monoaminergic dysfunction induced by injecting β-amyloid peptide-(1–40) into nucleus basalis magnocellularis of the rat. (2) Bilateral injection of 10 μg β-amyloid peptide-(1–40) into nucleus basalis magnocellularis produced local deposits of amyloid plaque and functional abnormalities detected by microdialysis. In medial prefrontal cortex, reductions in the basal levels and stimulated release of acetylcholine, dopamine, norepinephrine, and 5-hydroxytryptamine were observed. However, oral huperzine A (0.18 mg/kg, once daily for 21 consecutive days) markedly reduced morphologic abnormalities at the injection site in rats infused with β-amyloid peptide-(1–40). Likewise, this treatment ameliorated the β-amyloid peptide-(1–40)-induced deficits in extracellular acetylcholine, dopamine, and norepinephrine (though not 5-hydroxytryptamine) in medial prefrontal cortex, and lessened the reduction in nicotine or methoctramine-stimulated release of acetylcholine and K⁺-evoked releases of acetylcholine and dopamine. (3) The present results provide the first direct evidence that huperzine A acts to oppose neurotoxic effects of β-amyloid peptide on cholinergic, dopaminergic, and noradrenergic systems of the rat forebrain.     

甲壳动物精氨酸激酶的结构与功能

精氨酸激酶（Arginine kinase）是调节无脊椎动物能量代谢的重要酶,在调节无脊椎动物体内磷酸精氨酸与ATP之间的能量平衡过程中具有重要作用。甲壳动物是节肢动物门内最重要的类群之一,并具有重要的经济价值。本文综述了甲壳动物体内精氨酸激酶的分子构象、表达变化及生理功能等方面的研究进展,为深入研究甲壳动物的能量代谢调控机制提供必要的参考。另外,文中对甲壳动物精氨酸激酶的重要性和研究中存在的问题进行了讨论。