Aggregated beta amyloid peptide 1-40 decreases Ca2+- and cholinergic receptor-mediated phosphoinositide degradation by alteration of membrane and cytosolic phospholipase C in brain cortex

The effects of full-length amyloid beta protein, A beta (1-40), on phosphoi nositide-specific phospholipase C (PLC) were investigated in synaptic plasm a membranes (SPM) and cytosol prepared from the cerebral cortex of adult ra ts. Moreover, the role of A beta (1-40) on the activation of lipid peroxida tion was evaluated. The activity of phospholipase C (PLC) acting on phospha tidylinositol (PI) and phosphatidylinositol-4,5-bisphosphate (PIP2) was det ermined using exogenous labeled substrates, The subcellular fractions were the source of enzyme(s). The radioactivity of lipid messengers derived from degradation of [C-14- arachidonoyl] PI was also determined. The stable agg regated form of beta-amyloid peptide (1-40) at 25 mu M concentration exerte d reproducible effects. The aggregated form of A beta (1-40) inhibited Ca2-regulated PI and PIP2 degradation by SPM and cytosolic enzymes. Aggregated A beta also decreased significantly the level of diacylglycerol, the produ ct of PLC. This additionally supports the inhibitory effect of A beta on me mbrane-bound and cytosolic PLC. Moreover, A beta (1-40) significantly decre ased the basal activity of the PIP2-PLC in SPM and the enzyme activity regu lated through cholinergic receptors. However, in spite of the lower enzyme activity, the percentage distribution of inositol (1,4,5) P-3 radioactivity (IP3) in the total pool of inositol metabolites was not significantly chan ged. The aggregated neurotoxic fragment, A beta (25-35), mimicked the effec t of full-length A beta (1-40). A beta (1-40) enhanced the level of malondi aldehyde indicating an activation of free radical stimulated membrane lipid peroxidation that may be involved in alteration of phospholipase(s) activi ty. Our results indicated that aggregated A beta (1-40) alters Ca2+-depende nt phosphoinositide degradation affecting synaptic plasma membrane and cyto solic phospholipase(s) activity. Moreover, this peptide significantly decre ased the phosphoinositide-dependent signal transduction mediated by choline rgic receptors. The effect of aggregated A beta (1-40) is more pronounced t han that of the neurotoxic fragment A beta (25-35). Our study suggests that the deposition of aggregated A beta may alter phosphoinositide signaling i n brain.