Dual role of glutamatergic neurotransmission on amyloid beta(1-42) aggregation and neurotoxicity in embryonic avian retina.

The effects of glutamate receptor antagonists on the toxicity of the beta-amyloid peptide (Abeta(1-42)) in embryonic chick retina were investigated. When used alone or in combination, the N-methyl-D-asparate antagonist, MK-801, the (+/-)-alphaamino-3-hydroxyl-5-methylisoxazole-4-propionic acid/kainate antagonist, DNQX, and the metabotropic receptor antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid, blocked the neurotoxicity of Abeta(1-42). Aggregation of Abeta(1-42) was significantly increased in the presence of acidic glutamate solutions, but not in the presence of other neurotransmitters. These results point to a dual role of glutamatergic transmission in Alzheimer's disease (AD): (i) Abeta neurotoxicity requires activation of glutamate receptors and its blockade prevents cell death; (ii) high concentrations of glutamate in the synaptic cleft indirectly enhance Abeta aggregation through acidification of the medium, resulting in increased amounts of neurotoxic amyloid fibrils. These results suggest that glutamatergic neurotransmission may represent a novel target for therapeutic approaches in AD.

Effect of hydrostatic pressure on the fluorescence of indole derivatives.

Effects of hydrostatic pressure on the fluorescence emission of L-tryptophan, N-acetyl-L-trytophanamide and indole were investigated. An increase in pressure ranging from 1 bar to 2.4 kbar results in reversible red-shifts of the emission of the three fluorophores. The pressure-induced redshift amounts to about 170 cm(-1) at 2.4 kbar, and appears related to changes in Stokes shift of the fluorophores caused by pressure effects on the dielectric constant and/or refractive index of the medium. As the pressure range investigated here is the range commonly used in studies of protein subunit association and/or folding, these observations raise the need for caution in interpreting pressure-induced spectral shifts. The significance of these observations to pressure studies of proteins is illustrated by investigation of pressure effects on human Cu,Zn Superoxide dismutase (SOD) and azurin fromPseudomonas aeruginosa. A reversible 170 cm(-1) red-shift of the emission of SOD was observed upon pressurization to 2.4 kbar. This might be interpreted as pressure-induced conformational changes of the protein. However, further studies using SOD that had been fully unfolded by guanidine hydrochloride, and fluorescence anisotropy measurements indicated that the observed red-shift was likely due to a direct effect of pressure on the fluorescence of the single tryptophan residue of SOD. Similar pressure-induced red-shifts were also observed for the buried tryptophan residue of azurin or for azurin that had been previously denatured by guanidine hydrochloride. These observations further suggest that the effective dielectric constant of the protein matrix is affected by pressure similarly to water.