TURNOVER OF AMYLOID BETA-PROTEIN IN MOUSE-BRAIN AND ACUTE REDUCTION OF ITS LEVEL BY PHORBOL ESTER

Fibrillar amyloid deposits are defining pathological lesions in Alzhei mer's disease brain and are thought to mediate neuronal death. Amyloid is composed primarily of a 39-42 amino acid protein fragment of the a myloid precursor protein (APP), called amyloid beta-protein (A beta). Because deposition of fibrillar amyloid in vitro has been shown to be highly dependent on A beta concentration, reducing the proteolytic rel ease of A beta is an attractive, potentially therapeutic target. Here, the turnover rate of brain A beta has been determined to define treat ment intervals over which a change in steady-state concentration of A beta could be measured. Mice producing elevated levels of human A beta were used to determine approximate turnover rates for A beta and two of its precursors, C99 and APP. The t 1/2 for brain A beta was between 1.0 and 2.5 hr, whereas for C99, immature, and fully glycosylated for ms of APP695 the approximate t 1/2 values were 3, 3, and 7 hr, respect ively. Given the rapid A beta turnover rate, acute studies were design ed using phorbol 12-myristate 13-acetate (PMA), which had been demonst rated previously to reduce A beta secretion from cells in vitro via in duction of protein kinase C (PKC) activity. Six hours after intracorti cal injection of PMA, A beta levels were significantly reduced, as mea sured by both A beta 40- and A beta 42-selective ELISAs, returning to normal by 12 hr. An inactive structural analog of PMA, 4 alpha-PMA, ha d no effect on brain A beta levels. Among the secreted N-terminal APP fragments, APP beta levels were significantly reduced by PMA treatment , whereas APP alpha levels were unchanged, in contrast to most cell cu lture studies. These results indicate that A beta is rapidly turned ov er under normal conditions and support the therapeutic potential of el evating PKC activity for reduction of brain A beta.