Cholesterol decreases secretion of the secreted form of amyloid precursor protein by interfering with glycosylation in the protein secretory pathway

Cerebral deposits of beta-amyloid (beta A) are a major feature in Alzheimer 's disease. beta A is derived from amyloid precursor protein (APP). APP is subject to N- and O-glycosylation and undergoes a series of proteolytic cle avages that lead to the release of beta A or of a non-amyloidogenic secrete d form of APP (APPs). We used primary neuronal and glial cultures to invest igate how cholesterol affects the production and secretion of APPs. Exposur e to cholesterol for 2 h did not change the neuronal release of APPs; after 6 h APPs release was slightly lower, whereas 24 h of exposure decreased AP Ps in the medium by approx. 60%. The time courses were similar in astrocyte s and microglia preparations. To verify whether the effect of cholesterol w as a consequence of membrane rigidification we tested the activity of gangl ioside GM1 and prion protein fragment PrP 106-126, which affect membrane fl uidity similarly to cholesterol, on APPs secretion. Neither altered the pro duction of APPs. APP mRNA and the total amount of APP in the cells were sli ghtly decreased by cholesterol after 2 and 24 h respectively. Immunoblot an alysis of APP associated with neuronal cells and astrocytes indicated that cholesterol progressively decreased the glycosylated forms of the protein; a similar tendency was noted in cells treated with brefeldin A and monensin , two substances that interfere with protein glycosylation. The cell-surfac e biotinylation method showed that in cholesterol-treated cells APP reached the plasma membrane. Our results indicate that cholesterol decreases the s ecretion of APPs by interfering with APP maturation and inhibiting glycosyl ation of the protein; although APP is inserted in the membrane it is not cl eaved by alpha-secretase.