The amyloid protein, A␤, which accumulates in the brains of Alzheimer patients, is derived by proteolysis of the amyloid protein precursor (APP). APP can undergo endoproteolytic processing at three sites, one at the amino terminus of the A␤ domain (␤-cleavage), one within the A␤ domain (␣-cleavage), and one at the carboxyl terminus of the A␤ domain (␥-cleavage). The enzymes responsible for these activities have not been unambiguously identified. By the use of gene disruption (knockout), we now demonstrate that TACE (tumor necrosis factor ␣ converting enzyme), a member of the ADAM family (a disintegrin and metalloprotease-family) of proteases, plays a central role in regulated ␣-cleavage of APP. Our data suggest that TACE may be the ␣-secretase responsible for the majority of regulated ␣-cleavage in cultured cells. Furthermore, we show that inhibiting this enzyme affects both APP secretion and A␤ formation in cultured cells.The amyloid protein, A␤, which accumulates in the brains of Alzheimer patients, is derived by proteolysis of the amyloid protein precursor (APP) 1 (1-3). APP can undergo endoproteolytic processing at three sites, one at the amino terminus of the A␤ domain (␤-cleavage), one within the A␤ domain (␣-cleavage), and one at the carboxyl terminus of the A␤ domain (␥-cleavage). The enzymes responsible for these activities have not been unambiguously identified.In most cells in culture, a fraction (10 -30%) of all APP undergoes ␣-cleavage (4 -7). This results in the secretion of the large extracellular domain of APP into the medium. This secreted APP (APP s ) is a major APP-related species found in cerebrospinal fluid and brain homogenates (8,9) and is thought to interact with components of the extracellular matrix and with receptors on cells. In cultured cells it has been shown that the fraction of APP that is converted to APP s can be increased by activating second messenger cascades including those involving protein kinase C, protein kinase A, mitogen-activated protein kinase, protein phosphatase 1, protein phosphatase 2B (calcineurin), and calcium (4 -6, 10 -14). In most cells in culture, activating protein kinase C causes the majority (80 -95%) of the APP to undergo ␣-cleavage ("regulated" ␣-cleavage). Stimulation of APP s formation and secretion by activating second messenger cascades is not due to the phosphorylation of APP (15, 16) but may be due to protein phosphorylation leading to alterations in the trafficking of APP (17) or in the activity of an ␣-secretase. Importantly, stimulating ␣-cleavage of APP leads to a significant decrease in A␤ formation (18 -20).The potential importance of regulated cleavage of APP is indicated by the ability of acetylcholine, a critical neurotransmitter altered in Alzheimer's disease, working through muscarinic receptors, to stimulate regulated cleavage (4,10,21,22). Activation of other metabotropic receptors also leads to activation of regulated secretion of APP (4,21,23,24). Regulated cleavage of APP appears to occur in vivo under conditions in which protein ...

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