Search

We have previously reported that protein lipidation in the form of palmitoylation and farnesylation is critical for the production of Aβ (amyloid β-peptide), the dimerization of β-secretase and its trafficking into cholesterol-rich microdomains. As statins influence these lipid modifications in addition to their effects on cholesterol biosynthesis, we have investigated the effects of lovastatin and SIMVA (simvastatin) at a range of concentrations chosen to distinguish different cellular effects on Aβ production and β-secretase structure and its localization in bHEK cells [HEK-293 cells (human embryonic kidney cells) transfected with the Asp-2 gene plus a polyhistidine coding tag] cells. We have compared the changes brought about by statins with those brought about by the palmitoylation inhibitor cerulenin and the farnesyltransferase inhibitor CVFM (Cys-Val-Phe-Met). The statin-mediated reduction in Aβ production correlated with an inhibition of β-secretase dimerization into its more active form at all concentrations of statin investigated. These effects were reversed by the administration of mevalonate, showing that these effects were mediated via 3-hydroxy-3-methylglutaryl-CoA-dependent pathways. At low (1 μM) statin concentrations, reduction in Aβ production and inhibition of β-secretase dimerization were mediated by inhibition of isoprenoid synthesis. At high (>10 μM) concentrations of statins, inhibition of β-secretase palmitoylation occurred, which we demonstrated to be regulated by intracellular cholesterol levels. There was also a concomitant concentration-dependent change in β-secretase subcellular trafficking. Significantly, Aβ release from cells was markedly higher at 50 μM SIMVA than at 1 μM, whereas these concentrations resulted in similar reductions in total Aβ production, suggesting that low-dose statins may be more beneficial than high doses for the therapeutic treatment of Alzheimer's disease.

β-amyloid, the 39-43 amino acid peptide fragment originating from amyloid precursor protein, is today, generally accepted as the biological entity responsible for causing the debilitating human disorder Alzheimer's disease. Understanding the exact biological effects of β-amyloid in vitro and in vivo is clearly important to provide therapeutic strategies for the disease. Recent in vitro studies have focused on the production of reactive oxygen species by aggregating β- amyloid, but the cellular effects of β-amyloid induced reactive oxygen species production have not been fully elucidated.

This article represents the proceedings of a symposium at the 2002 RSA‐ISBRA Meeting in San Francisco. The chairs were Vinood B. Patel and Victor R. Preedy. The presentations were (1) Macromolecular structural analysis, by Vinood B. Patel; (2) Profiling and imaging of proteins in tissue sections using mass spectrometry as a discovery tool in biological research, by Pierre Chaurand and Richard M. Caprioli; (3) The use of SELDI ProteinChip™ arrays, by Brian M. Austen, Emma R. Frears, Francesca Manca, and Huw Davies; (4) DNA hybridization array technologies, by Kent E. Vrana; and (5) Adeno‐ and adeno‐associated viral mediated gene transfer approaches for alcoholic liver disease, by Michael Wheeler. Concluding remarks were by Victor R. Preedy.

Familial British dementia (FBD) is an autosomal dominant neurodegenerative disorder, with biochemical and pathological similarities to Alzheimer’s disease. FBD is associated with a point mutation in the stop codon of the BRI gene. The mutation extends the length of the wild-type protein by 11 amino acids, and following proteolytic cleavage, results in the production of a cyclic peptide (ABri) 11 amino acids longer than the wild-type (WT) peptide produced from the normal gene BRI. ABri was found to be the main component of amyloid deposits in FBD brains. However, pathological examination of FBD brains has shown the presence of ABri as non-fibrillar deposits as well as amyloid fibrils. Taken together, the genetic, pathological and biochemical data support the hypothesis that ABri deposits play a central role in the pathogenesis of FBD. Here we report that ABri, but not WT peptide, can oligomerise and form amyloid-like fibrils. We show for the first time that ABri induces apoptotic cell death, whereas WT is not toxic to cells. Moreover, we report the novel findings that non-fibrillar oligomeric species of ABri are more toxic than protofibrils and mature fibrils. These findings provide evidence that non-fibrillar oligomeric species are likely to play a critical role in the pathogenesis of FBD and suggest that a similar process may also operate in other neurodegenerative diseases.

β‐Amyloid (Aβ), a 39‐43 residue peptide, is the principal component of senile plaques found in the brains of patients with Alzheimer's disease (AD). There are two main lines of evidence that its deposition is the cause of neurodegeneration. First, mutations found in three genes in familial Alzheimer's cases give rise to increased production of the longest, most toxic, form, Aβ 1‐42. Second, aggregated Aβ is toxic to neuronal cells in culture. Inhibitors of the proteases involved in its release from the amyloid precursor protein are, therefore, of major therapeutic interest. The best candidates for the releasing proteases are both aspartyl proteases, which are integrated into the membranes of the endoplasmic reticulum and Golgi network. A sensitive assay using Ciphergen's Seldi™ system has been developed to measure all the variants of Aβ in culture supernatants, which will be of great value in screening inhibitors of these proteases. With this assay, it has been shown that increasing intracellular cholesterol increases the activities of both β‐secretase, and γ‐secretase‐42. Moreover, changing the intracellular targeting of amyloid precursor glycoprotein (APP) yields increased α‐secretase cleavage, and increases in the amounts of oxidized/nitrated forms of Aβ. Copyright © 2000 European Peptide Society and John Wiley & Sons, Ltd.

β-Amyloid protein (Aβ) is the major component of senile plaques found in the brains of Alzheimer's patients. A novel ELISA has been developed which probes the early stages of oligomerization of Aβ. Incubation of Aβ solutions at 37°C and pH 7.4 produces soluble oligomers in a concentration-dependent manner. Fresh Aβ42 solutions rapidly form soluble oligomers, whereas Aβ40 solutions require prolonged incubation to produce oligomers. Fresh Aβ42 solutions are more toxic to human neuroblastoma SH-SY5Y cells than Aβ40 solutions, possibly mediated by soluble oligomers. The differences between Aβ42 and Aβ40 could explain the association of the longer form with familial early-onset Alzheimer's disease. We also report a new strategy for solid-phase synthesis of Aβ peptides which gives high yield and purity of the initial crude preparation.

A novel ELISA has been developed which detects oligomerization of β-amyloid (Aβ). Oligomerization, fibrillization and neurotoxicity of native Aβ associated with Alzheimer's disease (AD) type has been compared with E22Q Aβ (amyloid β-protein containing residues 1-40 with the native Glu at residue 22 changed to Gln) implicated in Dutch cerebral haemorrhage disease. Solutions of Aβ rapidly yield soluble oligomers in a concentration-dependent manner, which are detected by the ELISA, and by size-exclusion gel chromatography. Conformational changes from disordered to β-sheet occur more slowly than oligomerization, and fibrils are produced after prolonged incubation. The E22Q Aβ oligomerizes, changes conformation and fibrillizes more rapidly than the native form and produces shorter stubbier fibrils. Aged fibrillar preparations of E22Q Aβ are more potent than aged fibrils of native Aβ in inducing apoptotic changes and toxic responses in human neuroblastoma cell lines, whereas low-molecular-mass oligomers in briefly incubated solutions are much less potent. The differences in the rates of oligomerization of the two Aβ forms, their conformational behaviour over a range of pH values, and NMR data reported elsewhere, are consistent with a molecular model of oligomerization in which strands of Aβ monomers initially overcome charge repulsion to form dimers in parallel β-sheet arrangement, stabilized by intramolecular hydrophobic interactions, with amino acids of adjacent chains in register.

ADDITION of the β-hydroxy-β-methylglutaryl-CoA (HmG-CoA) reductase inhibitor lovastatin to human HEK cells transfected with the amyloid precursor protein (APP) reduces intracellular cholesterol/protein ratios by 50, and markedly inhibits β-secretase cleavage of newly-synthesized APP. Exogenous water-solubilized cholesterol at 200 μg/ml concentration increases newly synthesized β-amyloidogenic products four-fold. These intracellular changes are detectable by immunoprecipitation and immunofluorescent labelling. Analyses of the fragments captured from culture medium by an N-terminal anti-β-amyloid antibody on ProteinChip arrays and detected using surface-enhanced laser desorption/ionization (SELDI) mass spectrometry revealed that culture with cholesterol (200 μg/ml) increased secretion of β-amyloid 1–40 by 1.8-fold, and increased secretion of β-amyloid 1–42. Changes in APP processing by cholesterol may mediate the way in which the ApoE4 allele increases risk of developing Alzheimer's disease (AD) in western populations.

A solid phase approach has been used to synthesize a large branched disulphide peptide from IgG Fc, Ac‐F‐C*‐A‐K‐V‐N‐N‐K‐D‐L‐P‐A‐P‐I‐E‐K(Ac‐E‐L‐L‐G‐G‐P‐S‐V‐F)‐C*‐I‐NH2. This peptide combines the lower hinge region of IgG and a proximal β‐hairpin loop, both implicated in binding to FcγRI. Solid phase Tl(tfa)3cyclization of the linear branched peptide resulted in a poor yield of cyclic hinge–loop peptide (11%) most likely due to steric hindrance caused by the branch. However, if addition of the branch was preceded by solid phase Tl(tfa)3cyclization of the loop, the yield was excellent at 75%. Cyclic hinge–loop peptide was active in displacing IgG2a from FcγRI expressed on monocyte cell lines with an IC50of 40 μM, whereas the linear form of this peptide was inactive. The Fc hinge–loop peptide demonstrates the potential for a non‐mAb high affinity, immunomodulatory ligand for FcγRI. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.

Surface properties of ovalbumin and of its putative signal sequence, and their interactions with phospholipids at an air-water interface, have been studied. The mature protein can form an interfacial film spontaneously from its bulk solution, whereas the signal sequence cannot. Mature ovalbumin also penetrates phospholipid monolayers from the subphase (independently of the type of phospholipid present), whereas its signal sequence does not. The surface stability of a spread film of the signal sequence is, however, higher than that of a film of mature ovalbumin. Above specific threshold concentrations of signal peptide and of mature ovalbumin in mixed films with phospholipids, two separate phases are formed. In such immiscible films, the signal sequence peptide is also able to support a higher lateral surface pressure than mature ovalbumin, at corresponding areas of peptide and mature protein in the mixed monolayers. It is suggested that the differing lateral stabilities of ovalbumin and of its putative signal sequence may be relevant to the translocation of ovalbumin across the membrane of the endoplasmic reticulum, and a scheme for its translocation is proposed that is based on these properties.

An azidophenacyl derivative of a chemically synthesized consensus signal peptide has been prepared. The peptide, when photoactivated in the presence of rough or high-salt-stripped microsomes from pancreas, leads to inhibition of their activity in cotranslational processing of secretory pre-proteins translated from their mRNA in vitro. The peptide binds specifically with high affinity to components in the microsomal membranes from pancreas and liver, and photoreaction of a radioactive form of the azidophenacyl derivative leads to covalent linkage to yield two closely related radiolabelled proteins of Mr about 45,000. These proteins are integrated into the membrane, with large 30,000-Mr domains embedded into the phospholipid bilayer to which the signal peptide binds. A smaller, endopeptidase-sensitive, domain is exposed on the cytoplasmic surface of the microsomal vesicles. The specificity and selectivity of the binding of azidophenacyl-derivatized consensus signal peptide was demonstrated by concentration-dependent inhibition of photolabelling by the ‘cold’ synthetic consensus signal peptide and by a natural internal signal sequence cleaved and isolated from ovalbumin. The properties of the labelled 45,000-Mr protein-signal peptide complexes, i.e. mass, pI, ease of dissociation from the membrane by detergent or salts and immunological properties, distinguish them from other proteins, e.g. subunits of signal recognition particle, docking protein and signal peptidase, already known to be involved in targetting and processing of nascent secretory proteins at the rough endoplasmic reticulum membrane. Although the 45,000-Mr signal peptide binding protein displays properties similar to those of the signal peptidase, a component of the endoplasmic reticulum, the azido-derivatized consensus signal peptide does not interact with it. It is proposed that the endoplasmic reticulum proteins with which the azidophenacyl-derivatized consensus signal peptide interacts to yield the 45,000-Mr adducts may act as receptors for signals in nascent secretory pre-proteins in transduction of changes in the endoplasmic reticulum which bring about translocation of secretory protein across the membrane.

The 54-kDa subunit of the signal recognition particle (SRP) binds nascent secretory polypeptides, binds the 7SL RNA (SRP RNA) component of SRP, and hydrolyzes GTP. Limited proteolysis of SRP 54-kDa suggests the protein has two domains, termed the G (GTP-binding) and M (methionine-rich) domains. The M domain is predicted to contain a number of amphiphilic helices, which provide a binding cleft for signal sequences. In order to obtain sufficient material for studies of relationships between structure and function, we have expressed the canine cDNA encoding the 54-kDa subunit inEscherichia coliusing a T7 expression system. To aid purification, the protein was expressed with an amino-terminal extension encoding an initiating methionine and 10 histidine residues followed by an enterokinase cleavage site; 0.3mg of HIS:SRP 54-kDa was purified to give a single band on SDS–PAGE in 20% yield from 500 ml of culturedE. coli.Purified HIS:SRP 54-kDa was shown to be folded into the G and M domains, to inhibit the translocation of pre-prolactin into canine microsomes, and to bind mammalian SRP RNA only in the presence of the 19-kDa subunit of SRP.

The surface behaviour of three signal-sequence polypeptides (the pretrypsinogen 2 signal sequence, a synthetic consensus signal sequence and the putative signal sequence of ovalbumin) were studied at an air-water interface. It was found that the surface stabilities of the spread polypeptide films were higher than those of polypeptides and proteins previously investigated (including melittin and membrane proteins), and that the signal peptides had a much lower affinity for the interface than had other peptides and proteins. The observed molecular areas of the signal-sequence peptides indicated that the molecules have a considerable degree of secondary structure at the surface interface.

Most signal sequences are found to vary considerably in length and primary sequence, but possess some common structural features. Analysis of known signal sequences has led to the design of a 19-residue sequence that, although not a naturally occurring signal, possesses the structural features that commonly occur in pre-proteins. This peptide has been synthesized by solid-phase methods, and has been shown to inhibit, in a concentration-dependent manner, the processing in vitro of nascent pre-prolactin, pre-forms of pancreatic digestive enzymes, and pre-placental lactogen. The peptide acts at the cytoplasmic surface of microsomal vesicles added to the protein translation system, preventing translocation of the nascent chains to the lumenal space of vesicles where signal peptidase normally cleaves to remove the signal from nascent pre-proteins.

A procedure is described for the preparation of rough membrane vesicles of endoplasmic-reticular origin from the pancreas of sheep. These isolated membranes translocate, process and glycosylate in vitro-translated heterologous proteins in a manner comparable with that exhibited by dog pancreatic microsomes.

The Vpr protein of human immunodeficiency virus type 1 (HIV-1) is incorporated into the virion by the Gag polyprotein precursor Pr55gag. The importance of the p6gagsequence at the C-terminal end of Pr55gaghas a crucial role in Vpr incorporation. To identify the Gag sequences directly involved in Vpr binding, we compared the Vpr binding affinities of the 71 amino acid nucleocapsid protein p7, the C-terminal peptide (35–71) p7C and p6gagby affinity chromatography. p7 and p7C have the strongest Vpr binding activities compared to p6gag. These results suggest that the nucleocapsid protein and its C-terminal domain may be important for the incorporation of Vpr into the mature HIV-1 virion and the subsequent localisation of viral nucleic acid to the cell nucleus by Vpr.

A 24-residue transit peptide based on the sequence of a precursor of the small subunit of wheat ribulose-1,5-bisphosphate carboxylase (Rubisco) was synthesized. The transit peptide was converted into a radioactive azido derivatised analogue. Photoactivation of the radiolabelled transit peptide analogue with isolated inner and outer membranes of the chloroplast envelope intensely labelled two proteins of 30 kDa and 52 kDa. In the outer membrane only the 52 kDa polypeptide was labelled. These findings are consistent with a recent report on the identification of the 30 kDa receptor protein for protein import in the chloroplast envelope contact zones [(1988) Nature 331, 232–237].

A monoclonal antibody to porcine β-lipotropin has been produced which binds to the N-terminal (λ-lipotropin) portion of the molecule. The antibody can be used to detect β-lipotropin as well as other β-endorphin precursors (predominantly a Mr38000 polypeptide) using radiobinding assay or the immunoblotting technique. Purification of the peptides can be readily achieved by affinity chromatography using the monoclonal antibody covalently bound to Sepharose 4B. As the antibody recognises the N-terminal part of β-lipotropin, it can be used to detect and purify β-lipotropin and other β-endorphin precursors in the presence of β-endorphin.

A peptide corresponding to the N-terminal sequence of the rat malate dehydrogenase, comprising the transit sequence and two residues of the mature protein (MLSALARPVGAALR-RSFSTSAQNNAK) has been chemically synthesized, and its structural characteristics investigated by Fourier-transform i.r. (FT-IR), c.d. and 1H-n.m.r. spectroscopy. FT-IR and c.d. spectra of the peptide were recorded in a variety of environments (aqueous solution, trifluoroethanol) and after incorporation into phospholipid bilayers. The peptide was found to be mainly in aperiodic or undefined conformation in aqueous solution. However, in trifluoroethanol a marked increase in alpha-helical content was observed. An increase in alpha-helical content was also observed in negatively charged lipids (dimyristoylphosphatidylglycerol and cardiolipin). However, when reconstituted in a zwitterionic phospholipid (dimyristoylphosphatidylcholine), no alpha-helical structure was observed. N.m.r. spectroscopy was used to characterize the helical structure in greater detail in trifluoroethanol. The 1H-n.m.r. spectrum of the peptide in this solvent was assigned using standard homonuclear two-dimensional methods. The observed patterns of nuclear Overhauser enhancements confirmed the deductions obtained from c.d. and FT-1R spectroscopy concerning the solution conformation, suggesting a region of flexible nascent helix between Ala-4 and Ser-18. This structure is discussed in terms of the possible function of the peptide.

The mechanisms whereby isolated or synthetic signal peptides inhibit processing of newly synthesized prolactin in microsome-supplemented lysates from reticulocytes and wheat-germ were investigated. At a concentration of 5 microM, a consensus signal peptide reverses the elongation arrest imposed by the signal-recognition particle (SRP), and at higher concentrations in addition inhibits elongation of both secretory and non-secretory proteins. A photoreactive form of a synthetic signal peptide cross-links under u.v. illumination to the 54 kDa and 68 kDa subunits of SRP, whereas the major cross-linked protein produced after photoreaction of rough microsomes is of 45 kDa. As SRP-mediated elongation arrest is unlikely to be essential for translocation, it is suggested that signal peptides may interact with components other than SRP in the translation system in vitro.

The two-dimensional polyacrylamide-gel electrophoresis technique of O'Farrell [(1975) J. Biol. Chem 250, 4007-4021] was applied to resolve and analyse the polypeptide composition of dog pancreatic rough microsomal membranes, which were shown to be active in co-translational processing of preprolactin synthesized from pituitary mRNA in a translation system in vitro. About 100 polypeptides are resolved. Treatment of rough microsomal membranes with EDTA and high KCl concentration yielded membranes stripped of their ribosomes with retention of activity for translocation and processing. Stripped microsomal membranes showed a selective concentration of approximately 25 polypeptides in the membranes when analysed by two-dimensional polyacrylamide-gel electrophoresis. The two-dimensional electrophoretic profile was catalogued into polypeptides that are glycoproteins, those that contain free thiol groups disposed at the cytosolic surface of microsomal vesicles and those that are of secretory origin but have been entrapped in the microsomal preparation. Several secretory components, including amylase, procarboxypeptidases, lipase and anionic trypsinogen, were tentatively identified among the microsomal polypeptides. The rough and stripped microsomal membranes from dog pancreas show a characteristic set of seven major acidic polypeptides, which are also identifiable in microsomal-membrane preparations isolated from dog liver and rat liver. One of these polypeptides was identified as protein disulphide-isomerase (EC 5.3.4.1).

Parts of the primary structure of the NAD-specific glutamate dehydrogenase [L-glutamate:NAD oxidoreductase (deaminating), EC 1.4.1.2] from Neurospora crassa are presented. Segments of the sequence representing 886 unique amino-acid residues have been determined; the largest contains 267 residues. There are only short regions of possible homology between this enzyme and the glutamate dehydrogenases of bovine liver or the NADP-specific enzyme of Neurospora. The large size of the subunit (116,000 molecular weight) of the NAD-specific glutamate dehydrogenase is unusual when compared to other known dehydrogenases.

Fatigue tests on 835M30 steel in hydrogen gas at atmospheric pressure have confirmed that two distinct forms of corrosion fatigue crack growth can occur. The dominant mechanism of corrosion fatigue is governed by whether the cyclic stress intensity level is above or below the threshold stress intensity value for hydrogen-induced crack growth under static load, and by the stress ratio and cyclic frequency employed during testing. It has been shown that existing mechanistic models are unable to account quantitatively for the observed corrosion fatigue behaviour at stress intensity levels below the static load threshold. However, a model designated the ‘Process Competition Model’ can be used for quantitative predictions of corrosion fatigue behaviour at stress intensity levels above the static load threshold. An extended version of the model is also described in which the effects of cyclic waveform are incorporated. The translation of crack propagation rates to cyclic lifetimes is discussed and the effects of corrosion fatigue on S/Ncurves is emphasized. The direction of future research on quantitative models of corrosion fatigue crack growth is discussed.

Two 5%Ni steels have been subjected to corrosion fatigue at 70 Hz in a variety of environments. Paristype behaviour was observed, with an additional ‘tail’ region at low ∆K, attributed to fatigue crack initiation. A significant increase in propagation rate resulted from the presence of water as vapour or liquid, especially when oxygen was simultaneously present, an effect consistent with hydrogen embrittlement.

Glycopeptides containing mainly four amino acid residues in the sequence Asn-Leu-Thr-Ser, with small amounts of additional amino acid residues, were isolated from enzymic hydrolysates of hen's-egg albumin. Heterogeneity of the carbohydrate moiety was confirmed. Acid-base titrations showed that the alpha-amino group has a pKa value of 6.43 at 25 degrees C. The standard free engery and entropy changes associated with the ionization at 25 degrees C were 37.2kJ-mol-1 and -0.014kJ-mol-1- K-1 respectively. The complications arising in the interpretation of titration curves of the glycopeptides, which are heterogeneous with respect to the peptide chain, were considered and discussed in the light of the earlier suggestion that the titration curve of the glycopeptide might be interpreted as being due in part to a structure in which the hydroxyl group of the threonine residue is hydrogen-bonded to the beta-aspartamido oxygen atom [Neuberger & Marshall (1968) in Symposium on Foods - Carbohydrates and their Roles (Schultz, H.W., Cain, R.F. & Wrolstad, R.W., eds.), pp. 115-132, Avi Publishing Co., Westport, CT]. It is concluded that either the glycopeptides do not contain a hydrogen bond of that type, or, if they do, that it cannot be recognized by acid-base-titration studies.

Acute pancreatic oedema with hyperamylasaemia was induced in mice by subcutaneous administration of cholecystokinin octapeptide (CCK8). Comparison with effect of caerulein showed that cholecystokinin is less potent in vivo. To investigate the observed difference in response, threonine3CCKS and methionine5caerulein were synthesised and evaluated. Comparison of these peptides suggests that difference in bioactivity is related to possession of extra N-terminal residues rather than substitution of threonine for methionine.