Amyloid beta peptide 25-35 modulates hydrolysis of phosphoinositides by membrane phospholipase(s) C of adult brain cortex

Phosphoinositide-specific phospholipase C (PLC) is a key enzyme in signal t ransduction. A subset of muscarinic cholinergic receptors are linked to G-p roteins that activate phospholipase C. Cholinergic pathways are important i n learning and memory, and deficits in cholinergic transmission have been i mplicated in Alzheimer's disease (AD). AD is also associated with increased beta-amyloid plaques. In the present study, we have investigated the effect of the amyloid beta ( A beta)synthetic peptide homologous to residue 25-35 of A beta in nonaggreg ated and aggregated forms on the degradation of inositol phospholipids. Syn aptic plasma membranes (SPM) and the cytosolic fraction from rat brain cort ex served as a source of enzymes. The studies were carried out with radioac tive inositol phospholipids in the presence of endogenous and 2 mM CaCl2. T he enzyme(s) activity was evaluated by determination of the product formati on of [H-3]inositol-1-phosphate (IP1) or [H-3]inositol-1,4,5-trisphosphate (IP3). Results show that the PI-PLC activity was significantly higher in cy tosol compared to SPM, and this enzyme was stimulated by 2 mM CaCl2, but no t by GTP gamma S or carbachol, a cholinergic receptor agonist. Activity of the SPM-bound PIP2-PLC was similar to that in cytosol and was not activated by 2 mM CaCl2. The SPM PIP-PLC was significantly stimulated by GTP gamma S together with the cholinergic agonist, carbachol. Freshwater-soluble A bet a 25-35 activated PI-PLC in SPM. markedly by two- to threefold, but this ef fect was absent in the presence of 2 mM CaCl2. Moreover, A beta 25-35 had n o effect on basal PIP2-PLC activity and cytosolic PI-PLC and PIP2-PLC. The aggregated form of A beta 25-35 significantly inhibited PIP2-PLC only in th e presence of endogenous CaCl2. It also inhibited the carbachol and GTP(gam ma)S-stimulated PIP2-PLC. Our findings show that depending on the aggregati on state and Ca2+ concentration, A beta modulates phosphoinositide degradat ion differently and exclusively in brain synaptic plasma membranes. Our dat a suggested that aggregated A beta peptide may be responsible for the signi ficant impairment of phosphoinositide signaling found in brain membranes du ring AD.