High resolution scanning tunnelling microscopy of the beta-amyloid protein (Abeta1-40) of Alzheimer's disease suggests a novel mechanism of oligomer assembly

The aggregation of the beta-amyloid protein (Abeta) is an important step in the pathogenesis of Alzheimer's disease. There is increasing evidence that lower molecular weight oligomeric forms of Abeta may be the most toxic species in vivo. However, little is known about the structure of Abeta oligomers. In this study, scanning tunnelling microscopy (STM) was used to examine the structure of Abeta monomers, dimers and oligomers. Abeta1-40 was visualised by STM on a surface of atomically flat gold. At low concentrations (0.5 microM) small globular structures were observed. High resolution STM of these structures revealed them to be monomers of Abeta. The monomers measured approximately 3-4 nm in diameter. Internal structure was seen in many of the monomers consistent with a conformation in which the polypeptide chain is folded into 3 or 4 domains. Oligomers were seen after ageing the Abeta solution for 24 h. The oligomers were also 3-4 nm in width and appeared to be formed by the end-to-end association of monomers with the polypeptide chain oriented at 90 degrees to the axis of the oligomer. The results suggest that the oligomer formation can proceed through a mechanism involving the linear association of monomers.     

PYY3-36 injection in mice produces an acute anorexigenic effect followed by a delayed orexigenic effect not observed with other anorexigenic gut hormones

Peptide YY (PYY) is secreted postprandially from the endocrine L cells of the gastrointestinal tract. PYY(3-36), the major circulating form of the peptide, is thought to reduce food intake in humans and rodents via high-affinity binding to the autoinhibitory neuropeptide Y (NPY) receptor within the arcuate nucleus. We studied the effect of early light-phase injection of PYY(3-36) on food intake in mice fasted for 0, 6, 12, 18, 24, and 30 h and show that PYY(3-36) produces an acute anorexigenic effect regardless of the duration of fasting. We also show evidence of a delayed orexigenic effect in ad libitum-fed mice injected with PYY(3-36) in the early light phase. This delayed orexigenic effect also occurs in mice administered a potent analog of PYY(3-36), d-Allo Ile(3) PYY(3-36), but not following injection of other anorectic agents (glucagon-like-peptide 1, oxyntomodulin, and lithium chloride). Early light-phase injection of PYY(3-36) to ad libitum-fed mice resulted in a trend toward increased levels of hypothalamic NPY and agouti-related peptide mRNA and a decrease in proopiomelanocortin mRNA at the beginning of the dark phase. Furthermore, plasma levels of ghrelin were increased significantly, and there was a trend toward decreased plasma PYY(3-36) levels at the beginning of the dark phase. These data indicate that PYY(3-36) injection results in an acute anorexigenic effect followed by a delayed orexigenic effect.     

Interactions of cholesterol esters with phospholipids: cholesteryl myristate and dimyristoyl lecithin.

The ternary phase diagram of cholesteryl myristate--dimyristoyl lecithin--water has been determined by polarizing light microscopy, scanning calorimetry, and x-ray diffraction. Hydrated dimyristoyl lecithin forms a lamellar liquid--crystalline phase (L alpha) at temperatures greater than 23 degrees C into which limited amounts of cholesteryl myristate (less than 5 wt. %) can be incorporated. The amount of cholesterol ester incorporated is dependent upon the degree of hydration of the L alpha phase. Below 23 degrees C dimyristoyl lecithin forms ordered hydrocarbon chain structures (L beta' and P beta') which do not incorporate cholesterol ester. Comparison with other phospholipid--cholesterol ester--water phase diagrams suggests the following general principles: i) the incorporation of cholesterol ester occurs only into liquid crystalling phospholipid bilayers, ii) the extent of incorporation is temperature-dependent, with increasing amounts of cholesterol ester being incorporated at higher temperatures, and iii) unsaturated cholesterol esters induce increased disordering of the phospholipid bilayers.     

Calcium induces cyclic GMP formation in Dictyostelium

Cyclic GMP is rapidly formed a few seconds after binding of chemotactic signalling molecules to specific receptors on the cell surface of Dictyostelium amoebae. This phenomenon could be mimicked by addition of a pulse of Ca2+ to permeabilised amoebae. The concentration of Ca2+ for half-maximal response was 60 microM. Other ions (K+, Na+, Mg+ or Mn+) had no effect. A pulse of 5 microM IP3 produced a cyclic GMP response of similar magnitude but IP2 elicited no response. The data provide strong support for the hypothesis that cell surface receptor binding induces cyclic GMP formation by liberating Ca2+ from internal stores.     

New temperature-sensitive alleles of ftsZ in Escherichia coli

We isolated five new temperature-sensitive alleles of the essential cell division gene ftsZ in Escherichia coli, using P1-mediated, localized mutagenesis. The five resulting single amino acid changes (Gly109-->Ser109 for ftsZ6460, Ala129-->Thr129 for ftsZ972, Val157-->Met157 for ftsZ2066, Pro203-->Leu203 for ftsZ9124, and Ala239-->Val239 for ftsZ2863) are distributed throughout the FtsZ core region, and all confer a lethal cell division block at the nonpermissive temperature of 42 degrees C. In each case the division block is associated with loss of Z-ring formation such that fewer than 2% of cells show Z rings at 42 degrees C. The ftsZ9124 and ftsZ6460 mutations are of particular interest since both result in abnormal Z-ring formation at 30 degrees C and therefore cause significant defects in FtsZ polymerization, even at the permissive temperature. Neither purified FtsZ9124 nor purified FtsZ6460 exhibited polymerization when it was assayed by light scattering or electron microscopy, even in the presence of calcium or DEAE-dextran. Hence, both mutations also cause defects in FtsZ polymerization in vitro. Interestingly, FtsZ9124 has detectable GTPase activity, although the activity is significantly reduced compared to that of the wild-type FtsZ protein. We demonstrate here that unlike expression of ftsZ84, multicopy expression of the ftsZ6460, ftsZ972, and ftsZ9124 alleles does not complement the respective lethalities at the nonpermissive temperature. In addition, all five new mutant FtsZ proteins are stable at 42 degrees C. Therefore, the novel isolates carrying single ftsZ(Ts) point mutations, which are the only such strains obtained since isolation of the classical ftsZ84 mutation, offer significant opportunities for further genetic characterization of FtsZ and its role in cell division.     

High-pressure and stark hole-burning studies of chlorosome antennas from Chlorobium tepidum

Results from high-pressure and Stark hole-burning experiments on isolated chlorosomes from the green sulfur bacterium Chlorobium tepidum are presented, as well as Stark hole-burning data for bacteriochlorophyll c (BChl c) monomers in a poly(vinyl butyral) copolymer film. Large linear pressure shift rates of -0.44 and -0.54 cm(-1)/MPa were observed for the chlorosome BChl c Q(y)-band at 100 K and the lowest Q(y)-exciton level at 12 K, respectively. It is argued that approximately half of the latter shift rate is due to electron exchange coupling between BChl c molecules. The similarity between the above shift rates and those observed for the B875 and B850 BChl a rings of the light-harvesting complexes of purple bacteria is emphasized. For BChl c monomer, fDeltamu++ = 0.35 D, where Deltamu+ is the dipole moment change for the Q(y) transition and f is the local field correction factor. The data establish that Deltamu+ is dominated by the matrix-induced contribution. The change in polarizability (Deltaalpha) for the Q(y) transition of the BChl c monomer is estimated at 19 A(3), which is essentially identical to that of the Chl a monomer. Interestingly, no Stark effects were observed for the lowest exciton level of the chlorosomes (maximum Stark field of 10(5) V/cm). Possible explanations for this are given, and these include consideration of structural models for the chlorosome BChl c aggregates.     

Single Administration of p2TA (AB103), a CD28 Antagonist Peptide,Prevents Inflammatory and Thrombotic Reactions and Protects against Gastrointestinal Injury in Total-Body Irradiated Mice

The goal of this study was to elucidate the action of the CD28 mimetic peptide p2TA (AB103) that attenuates an excessive inflammatory response in mitigating radiation-induced inflammatory injuries. BALB/c and A/J mice were divided into four groups: Control (C), Peptide (P; 5 mg/kg of p2TA peptide), Radiation (R; total body irradiation with 8 Gy γ-rays), and Radiation + Peptide (RP; irradiation followed by p2TA peptide 24 h later). Gastrointestinal tissue damage was evaluated by analysis of jejunum histopathology and immunohistochemistry for cell proliferation (Cyclin D1) and inflammation (COX-2) markers, as well as the presence of macrophages (F4/80). Pro-inflammatory cytokines IL-6 and KC as well as fibrinogen were quantified in plasma samples obtained from the same mice. Our results demonstrated that administration of p2TA peptide significantly reduced the irradiation-induced increase of IL-6 and fibrinogen in plasma 7 days after exposure. Seven days after total body irradiation with 8 Gy of gamma rays numbers of intestinal crypt cells were reduced and villi were shorter in irradiated animals compared to the controls. The p2TA peptide delivery 24 h after irradiation led to improved morphology of villi and crypts, increased Cyclin D1 expression, decreased COX-2 staining and decreased numbers of macrophages in small intestine of irradiated mice. Our study suggests that attenuation of CD28 signaling is a promising therapeutic approach for mitigation of radiation-induced tissue injury.     

Calorimetry of apolipoprotein-A1 binding to phosphatidylcholine-triolein-cholesterol emulsions.

The thermotropic properties of triolein-rich, low-cholesterol dipalmitoyl phosphatidylcholine (DPPC) emulsion particles with well-defined chemical compositions (approximately 88% triolein, 1% cholesterol, 11% diacyl phosphatidylcholine) and particle size distributions (mean diameter, approximately 1000-1100 A) were studied in the absence and presence of apolipoprotein-A1 by a combination of differential scanning and titration calorimetry. The results are compared to egg yolk PC emulsions of similar composition and size. Isothermal titration calorimetry at 30 degrees C was used to saturate the emulsion surface with apo-A1 and rapidly quantitate the binding constants (affinity Ka = 11.1 +/- 3.5 x 10(6) M-1 and capacity N = 1.0 +/- 0.09 apo-A1 per 1000 DPPC) and heats of binding (enthalpy H = -940 +/- 35 kcal mol-1 apo-A1 or -0.92 +/- 0.12 kcal mol-1 DPPC). The entropy of association is -3070 cal deg-1 mol-1 protein or -3 cal deg-1 mol-1 DPPC. Without protein on the surface, the differential scanning calorimetry heating curve of the emulsion showed three endothermic transitions at 24.3 degrees C, 33.0 degrees C, and 40.0 degrees C with a combined enthalpy of 1.53 +/- 0.2 kcal mol-1 DPPC. With apo-A1 on the surface, the heating curve showed the three transitions more clearly, in particular, the second transition became more prominent by significant increases in both the calorimetric and Van't Hoff enthalpies. The combined enthalpy was 2.70 +/- 0.12 kcal mol-1 DPPC and remained constant upon repeated heating and cooling. Indicating that the newly formed DPPC emulsion-Apo-A1 complex is thermally reversible during calorimetry. Thus there is an increase in delta H of 1.17 kcal mol-1 DPPC after apo-A1 is bound, which is roughly balanced by the heat released during binding (-0.92 kcal) of apo-A1. The melting entropy increase, +3.8 cal deg-1 mol-1 DPPC of the three transitions after apo-A1 binds, also roughly balances the entropy (-3 cal deg-1 mol-1 DPPC) of association of apo-A1. These changes indicate that apo-A1 increases the amount of ordered gel-like phase on the surface of DPPC emulsions when added at 30 degrees C. From the stoichiometry of the emulsions we calculate that the mean area of DPPC at the triolein/DPPC interface is 54.5 A2 at 41 degrees C and 54.2 A2 at 30 degrees C. The binding of apo-A1 at 30 degrees C to the emulsion reduces the surface area per DPPC molecule from 54.2 A2 to 50.8 A2. At 30 degrees apo-A1 binds with high affinity and low capacity to the surface of DPPC emulsions and increases the packing density of the lipid domain to which it binds. Apo-A1 was also titrated onto DPPC emulsions at 45 degrees C. This temperature is above the gel liquid crystal transition. No heat was released or adsorbed. Furthermore, egg yolk phosphatidylcholine emulsions of nearly identical composition were also titrated at 30 degrees C with apo-A1 and were euthermic. Association constants were previously measured using a classical centrifugation assay and were used to calculate the entropy of apo-A1 binding (+28 cal deg-1 mol-1 apo-A1). This value indicates that apo-A1 binding to a fluid surface like egg yolk phosphatidylcholine or probably DPPC at 45 degrees C is hydrophobic and is consistent with hydrocarbon lipid or protein moities coming together and excluding water. Thus the binding of apo-A1 to partly crystalline surfaces is entropically negative and increases the order of the already partly ordered phases, whereas binding to liquid surfaces is mainly an entropically driven hydrophobic process.     

Cholesterol is necessary both for the toxic effect of Abeta peptides on vascular smooth muscle cells and for Abeta binding to vascular smooth muscle cell membranes

Accumulation of beta amyloid (Abeta) in the brain is central to the pathogenesis of Alzheimer's disease. Abeta can bind to membrane lipids and this binding may have detrimental effects on cell function. In this study, surface plasmon resonance technology was used to study Abeta binding to membranes. Abeta peptides bound to synthetic lipid mixtures and to an intact plasma membrane preparation isolated from vascular smooth muscle cells. Abeta peptides were also toxic to vascular smooth muscle cells. There was a good correlation between the toxic effect of Abeta peptides and their membrane binding. 'Ageing' the Abeta peptides by incubation for 5 days increased the proportion of oligomeric species, and also increased toxicity and the amount of binding to lipids. The toxicities of various Abeta analogs correlated with their lipid binding. Significantly, binding was influenced by the concentration of cholesterol in the lipid mixture. Reduction of cholesterol in vascular smooth muscle cells not only reduced the binding of Abeta to purified plasma membrane preparations but also reduced Abeta toxicity. The results support the view that Abeta toxicity is a direct consequence of binding to lipids in the membrane. Reduction of membrane cholesterol using cholesterol-lowering drugs may be of therapeutic benefit because it reduces Abeta-membrane binding.