Progressive deposition of amyloid 3-protein In brain, whether A,B has any physiological role is unknown. Studies aimed at elucidating the biological effects of A/3 on the nervous system have yielded equivocal results. Addition of synthetic peptides corresponding to the first 28 or 42 amino acids of the Al3 sequence to culture medium resulted in a dose-dependent effect on survival and neurite outgrowth of rat hippocampal neurons (3, 4). With a similar paradigm, AP-(1-40) peptide was found to be both trophic and toxic to hippocampal neurons, depending on the peptide dose and maturity ofthe neurons in vitro (5). In such systems, increased evidence favors a role of excitatory amino acids in A,B-induced neurotoxicity (6), possibly because A,B destabilizes calcium homeostasis (7). A recent study further suggested that peptide aggregation may enhance A,B toxicity in cultured hippocampal neurons (8). Finally, two reports suggested that transfection of either full-length APP or its C-terminal 100 residues, both ofwhich contain the AP region, into neural cells may result in cell degeneration upon neuronal differentiation (9, 10).Central to many of these studies is the addition of soluble AP3 in relatively high concentrations to culture medium. In one study, acetonitrile and trifluoroacetic acid were used to solubilize A,B before addition to culture medium (5 show that immobilized A,B has no detrimental effect on neurite outgrowth. On the contrary, in the presence of low amounts of extracellular matrix (ECM) components, A,B promotes neurite outgrowth in a dose-dependent manner.
MATERIALS AND METHODSPeptides. Six synthetic peptides were used: AP-(1-40),A,B-(1-28), and A,B-(21-37) (all from the human APP sequence) and, as controls, a 40-residue peptide having all A,B amino acids but randomly scrambled (NH2-VIEVLVGE-LDAFDGHVAGFSHDQKGNHVKGGARYMVSIFE), the 37-residue islet amyloid polypeptide (amylin; Peninsula Laboratories), and a 10-residue substance P peptide (Peninsula Laboratories). All peptides were HPLC purified, and the first four peptides were additionally analyzed by mass spectroscopy. All peptides were dissolved in deionized water at a concentration of 1 mg/ml. A sample of AP3 peptide was aggregated before use by dissolving in phosphate-buffered saline at 10 mg/ml, incubating at 37°C for 48 hr, and storing at 4°C for 1 week. The peptide was resuspended in water at 1 mg/ml just before use. On SDS/PAGE, the sample consisted primarily of mono-and dimeric forms, with smaller amounts of tetrameric aggregates. Substrate Preparation. Tissue culture substrates were prepared under sterile conditions by coating 35-mm Petri dishes with a premixed solution consisting of peptide, ECM, or both. All dishes were coated with peptide concentrations ranging from 0.5 to 5.0 pug/cm2. The ECM component con- §To whom reprint requests should be addressed.
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