Amyloid-beta (Abeta) appears critical to Alzheimer's disease. To clarify possible mechanisms of Abeta action, we have quantified Abeta-induced gene expression in vitro by using Abeta-treated primary cortical neuronal cultures and in vivo by using mice transgenic for the Abeta precursor (AbetaP). Here, we report that aggregated, but not nonaggregated, Abeta increases the level of the mRNAs encoding tissue plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). Moreover, tPA and uPA were also upregulated in aged AbetaP overexpressing mice. Because others have reported that Abeta aggregates can substitute for fibrin aggregates in activating tPA post-translationally, the result of tPA induction by Abeta would be cleavage of plasminogen to the active protease plasmin. To gain insights into the possible actions of plasmin, we evaluated the hypotheses that tPA and plasmin may mediate Abeta in vitro toxicity or, alternatively, that plasmin activation may lead to Abeta degradation. In evaluating these conflicting hypotheses, we found that purified plasmin degrades Abeta with physiologically relevant efficiency, i.e., approximately 1/10th the rate of plasmin on fibrin. Mass spectral analyses show that plasmin cleaves Abeta at multiple sites. Electron microscopy confirms indirect assays suggesting that plasmin degrades Abeta fibrils. Moreover, exogenously added plasmin blocks Abeta neurotoxicity. In summation, we interpret these results as consistent with the possibility that the plasmin pathway is induced by aggregated Abeta, which can lead to Abeta degradation and inhibition of Abeta actions.

The excitatory neurotransmitter glutamate is believed to play important roles in development, synaptic plasticity, and neurodegenerative conditions. Recent studies have shown that neurotrophic factors can modulate neuronal excitability and survival and neurite outgrowth responses to glutamate, but the mechanisms are unknown . The present study tested the hypothesis that neurotrophic factors modulate responses to glutamate by affecting the expression of specific glutamate-receptor proteins . Exposure of cultured embryonic rat hippocampal cells to basic fibroblast growth factor (bFGF) resulted in a concentration-dependent increase in levels of a-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor subunit GIuRl protein as determined by western blot, dot-blot, and immunocytochemical analyses . In contrast, bFGF did not alter levels of GIuR2/3, GIuR4, or the NMDA-receptor subunit NR1 . Nerve growth factor did not affect GIuRl levels . Calcium-imaging studies revealed that elevation of [Ca 2+1;, resulting from selective AMPA-receptor activation, was enhanced in bFGFpretreated neurons . On the other hand, [Ca 2 +] ; responses to NMDA-receptor activation were suppressed in bFGFtreated neurons, consistent with previous studies showing that bFGF can protect neurons against NMDA toxicity . Moreover, neurons pretreated with bFGF were relatively resistant to the toxicities of glutamate and AMPA, both of which were shown to be mediated by NMDA receptors . These data suggest that differential regulation of the expression of specific glutamate-receptor subunits may be an important mechanism whereby neurotrophic factors modulate activity-dependent neuronal plasticity and vulnerability to excitotoxicity . Key Words : Excitotoxicity-Fura-2 calcium imaging-Glutamate-Nerve growth factor-N-Methyl-D-aspartate-Neurotrophic factor .Abbreviations used: AMPA, a-amino-3-hydroxy-5-methylisoxazole-4-propionate ; APV, DL-2-amino-5-phosphonovaleric acid ;bFGF, basic fibroblast growth factor ; I Ca 21 ],, intracellular free calcium concentration; NGF, nerve growth factor ; PBS, phosphatebuffered saline ; RT-PCR, reverse transcriptase-polymerase chain reaction ; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis ; TNF, tumor necrosis factor.