Brain Pyroglutamate Amyloid-β is Produced by Cathepsin B and is Reduced by the Cysteine Protease Inhibitor E64d, Representing a Potential Alzheimer's Disease Therapeutic

Pyroglutamate amyloid-beta peptides (pGlu-A beta) are particularly pernicious forms of amyloid-beta peptides (A beta) present in Alzheimer's disease (AD) brains. pGlu-A beta peptides are N-terminally truncated forms of full-length A beta peptides (flA beta((1-40/42))) in which the N-terminal glutamate is cyclized to pyroglutamate to generate pGlu-A beta((3-40/42)). beta-secretase cleavage of amyloid-beta precursor protein (A beta PP) produces flA beta((1-40/42)), but it is not yet known whether the beta-secretase BACE1 or the alternative beta-secretase cathepsin B (CatB) participate in the production of pGlu-A beta. Therefore, this study examined the effects of gene knockout of these proteases on brain pGlu-A beta levels in transgenic A beta PPLon mice, which express A beta PP isoform 695 and have the wild-type (wt) beta-secretase activity found in most AD patients. Knockout or overexpression of the CatB gene reduced or increased, respectively, pGlu-A beta((3-40/42)), flA beta((1-40/42)), and pGlu-A beta plaque load, but knockout of the BACE1 gene had no effect on those parameters in the transgenic mice. Treatment of A beta PPLon mice with E64d, a cysteine protease inhibitor of CatB, also reduced brain pGlu-A beta((3-42)), flA beta((1-40/42)), and pGlu-A beta plaque load. Treatment of neuronal-like chromaffin cells with CA074Me, an inhibitor of CatB, resulted in reduced levels of pGlu-A beta((3-40)) released from the activity-dependent, regulated secretory pathway. Moreover, CatB knockout and E64d treatment has been previously shown to improve memory deficits in the A beta PPLon mice. These data illustrate the role of CatB in producing pGlu-A beta and flA beta that participate as key factors in the development of AD. The advantages of CatB inhibitors, especially E64d and its derivatives, as alternatives to BACE1 inhibitors in treating AD patients are discussed.