The amyloid-␤ peptide (A␤) can generate cytotoxic oligomers, and their accumulation is thought to underlie the neuropathologic changes found in Alzheimer's disease. Known inhibitors of A␤ polymerization bind to undefined structures and can work as nonspecific aggregators, and inhibitors that target conformations that also occur in larger A␤ assemblies may even increase oligomerderived toxicity. Here we report on an alternative approach whereby ligands are designed to bind and stabilize the 13-26 region of A␤ in an ␣-helical conformation, inspired by the postulated A␤ native structure. This is achieved with 2 different classes of compounds that also reduce A␤ toxicity to cells in culture and to hippocampal slice preparations, and that do not show any nonspecific aggregatory properties. In addition, when these inhibitors are administered to Drosophila melanogaster expressing human A␤ 1-42 in the central nervous system, a prolonged lifespan, increased locomotor activity, and reduced neurodegeneration is observed. We conclude that stabilization of the central A␤ ␣-helix counteracts polymerization into toxic assemblies and provides a strategy for development of specific inhibitors of A␤ polymerization.amyloid fibrils ͉ neurodegenerative disease ͉ protein misfolding ͉ Alzheimer's disease A lzheimer's disease is a progressive neurodegenerative disorder that is characterized by cerebral extracellular amyloid plaques and intracellular neurofibrillary tangles (1). Classically, the amyloid cascade hypothesis (2) states that Alzheimer's disease is caused by fibril and plaque formation of amyloid-␤ peptide (A␤) in the central nervous system. More recently, the hypothesis has been modified to include A␤ assemblies of sizes intermediate to monomeric and fibrillar forms, which today are considered to be the main source of cytotoxicity (3). Such A␤ assemblies include low-number oligomers and larger assemblies known as protofibrils, globulomers, Alzheimer's disease diffusible ligands, or A␤*56 (4-7). A␤ is cleaved from an integral membrane protein, the amyloid ␤ precursor protein (A␤PP), predominantly as a 40-residue peptide (A␤ 1-40 ). In addition, a C-terminally elongated 42-residue version can be excised (A␤ 1-42 ); it is this longer variant that is the main constituent of parenchymal amyloid deposits (8).The link between A␤ aggregation and Alzheimer's disease implies that inhibitors of this process should be able to slow down disease progression. A number of low-molecular-mass A␤ aggregation inhibitors have been identified by use of screens of compound libraries as well as rational design strategies. The resulting inhibitors include such chemically diverse compounds as curcumin, inositol, and nicotine (9, 10). The screens have identified inhibitors of fibril formation that similarly to the rationally designed inhibitors are predicted to bind to A␤ in an elongated, ␤-strand-like conformation and prevent its polymerization. A potential problem with this strategy is that blocking the later stages of fibril formation will favor t...

The amyloid-␤ hypothesis of Alzheimer's Disease (AD) focuses on accumulation of amyloid-␤ peptide (A␤) as the main culprit for the myriad physiological changes seen during development and progression of AD including desynchronization of neuronal action potentials, consequent development of aberrant brain rhythms relevant for cognition, and final emergence of cognitive deficits.The aim of this study was to elucidate the cellular and synaptic mechanisms underlying the A␤-induced degradation of gamma oscillations in AD, to identify aggregation state(s) of A␤ that mediate the peptides neurotoxicity, and to test ways to prevent the neurotoxic A␤ effect.We show that A␤ 1-42 in physiological concentrations acutely degrades mouse hippocampal gamma oscillations in a concentration-and time-dependent manner. The underlying cause is an A␤-induced desynchronization of action potential generation in pyramidal cells and a shift of the excitatory/inhibitory equilibrium in the hippocampal network. Using purified preparations containing different aggregation states of A␤, as well as a designed ligand and a BRICHOS chaperone domain, we provide evidence that the severity of A␤ neurotoxicity increases with increasing concentration of fibrillar over monomeric A␤ forms, and that A␤-induced degradation of gamma oscillations and excitatory/inhibitory equilibrium is prevented by compounds that interfere with A␤ aggregation.Our study provides correlative evidence for a link between A␤-induced effects on synaptic currents and AD-relevant neuronal network oscillations, identifies the responsible aggregation state of A␤ and proofs that strategies preventing peptide aggregation are able to prevent the deleterious action of A␤ on the excitatory/inhibitory equilibrium and on the gamma rhythm.

Hydrolytic reactions of the R? and Sp diastereomers of the phosphoromonothioate analog of uridylyl-(3',5')uridine (3',5'-UpU), having a nonbridging oxygen replaced with sulfur, have been followed by HPLC over a wide pH range at 363.2 K. Under neutral and acidic conditions three reactions compete: (i) desulfurization to an equilibrium mixture of 3',5'-and 2',5'-UpU, (ii) hydrolysis to uridine 2'-and 3'-monophosphates with release of uridine (either via a 2',3'-cyclic phosphoromonothioate, or a desulfurized cyclic triester), and (iii) isomerization to the 2',5'-dinucleoside phosphoromonothioate. With both diastereomers, desulfurization predominates over hydrolysis and migration at pH 1-8. Migration proceeds by retention of configuration at phosphorus and is most pronounced in very acidic solutions (Hq < 0.2, i.e., [HC1] > 0.5 mol Lr1), representing 20-30% of the total disappearance of the starting material. At pH 3-6, the proportion of this reaction is less than 10%. In the latter pH range, all the reactions are pH-independent. At lower pH, first-order dependence on acidity is observed, but at Ho < 0.2 desulfurization becomes slower than the competing reactions. The R? diastereomer is at pH < 7 up to three times as reactive as the Sp isomer. Under alkaline conditions (pH > 9), only base-catalyzed hydrolysis to uridine 2'-and 3'thiophosphates with release of uridine takes place. At pH < 1, the thioate analogs are more than 1 order of magnitude more stable than UpU, while at higher pH the reactivities are comparable.

A Bronsted graph with a convex break at pK(a)(Lg)= 12.58 provides compelling evidence for an intermediate in the alkaline cyclisation of uridine 3'-phosphate esters. The transient pentacoordinated oxyphosphorane dianion intermediate collapses to reactant and cyclic uridine 2',3'-monophosphate faster than it can pseudo-rotate and isomerise to the 2'-isomer.

Native pulmonary-surfactant-associated lipopolypeptide SP-C, its chemically depalmitoylated form and several synthetic analogues lacking the palmitoylcysteine residues were analysed for secondary structure in phospholipid micelles and for biophysical activity in 1,2-dipalmitoyl-sn-glycero-3- phosphocholine/phosphatidylglycerol/palmitic acid (68:22:9, by wt.). Compared with the native molecule, with the entire poly-valyl part in a known alpha-helical conformation, depalmitoylated SP-C was found to be still mainly alpha-helical, but with an approx. 20% decrease in the helical content. A synthetic hybrid polypeptide where the entire poly-valyl alpha-helical part of native SP-C had been replaced with the amino acid sequence of a transmembrane helix of bacteriorhodopsin is also predominantly alpha-helical. In contrast, synthetic SP-C analogues lacking only the palmitoyl groups, by replacement of the palmitoylcysteine residues with cysteine, phenylalanine or serine, or lacking the positively charged amino acids by replacement with alanine, are considerably less alpha-helical than both native and depalmitoylated SP-C. The data indicate that the SP-C palmitoyl groups are important for maintenance of the alpha-helical conformation in parts of the polypeptide, and that the poly-valyl alpha-helical conformation is not fully formed in synthetic SP-C polypeptides. Furthermore, the helical structure of both native and depalmitoylated SP-C in dodecylphosphocholine micelles is very resistant to thermal denaturation, exhibiting ordered structure at 90 degrees C. The alpha-helical content grossly parallels the peptide-induced acceleration of the spreading of phospholipids at an air/water interface and the increase of surface pressure. The data suggest that the alpha-helical conformation itself, rather than just the covalent structure, is of prime importance for the biological function of synthetic pulmonary-surfactant peptides.