The amyloid beta-protein precursor of Alzheimer's disease is degraded extracellularly by a Kunitz protease inhibitor domain-sensitive trypsin-like serine protease in cultures of chick sympathetic neurons

The amyloid P-protein precursor (APP) of Alzheimer's disease (AD) is cleave d either by alpha-secretase to generate an N-terminally secreted fragment, or by beta- and gamma-secretases to generate the beta-amyloid protein (A be ta). The accumulation of A beta in the brain is an important step in the pa thogenesis of AD. Alternative mRNA splicing can generate isoforms of APP wh ich contain a Kunitz protease inhibitor (KPI) domain. However, little is kn own about the physiological function of this domain. In the present study, the metabolic turnover of APP was examined in cultured chick sympathetic ne urons. APP was labelled by incubating neurons for 5 h with [S-35]methionine and [35S]cysteine. Intracellular labelled APP decayed in a biphasic patter n suggesting that trafficking occurs through two metabolic compartments. Th e half-lives for APP in each compartment were 1.5 and 5.7 h, respectively. A small fraction (10%) of the total APP was secreted into the culture mediu m where it was degraded with a half-life of 9h. Studies using specific prot ease inhibitors demonstrated that this extracellular breakdown was due to c leavage by a trypsin-like serine protease that was secreted into the cultur e medium. Significantly, this protease was inhibited by a recombinant isofo rm of APP (sAPP(751)), which contains a region homologous to the Kunitz pro tease,inhibitor (KPI) domain. These results suggest that KPI forms of APP r egulate extracellular cleavage of secreted APP by inhibiting the activity o f a secreted APP-degrading protease.