Calpain inhibitor MDL28170 modulates Abeta formation by inhibiting the formation of intermediate Abeta46 and protecting Abeta from degradation.

The observations that three major cleavages within the transmembrane domain of APP, namely, the gamma-cleavage, -cleavage, and the newly identified zeta-cleavage, are involved in the generation of secreted Abeta40 and Abeta42 prompted us to determine how the calpain inhibitor III MDL 28170 influences these three cleavages and Abeta formation. With the use of a cell culture system, our data demonstrate that 1) at either high concentrations, or at a low range of concentrations, at early time points, MDL 28170 inhibits the formation of secreted Abeta40 and Abeta42. However, this effect is due to inhibition of the intermediate Abeta46 generation by zeta-cleavage and not due to direct inhibition of the gamma-cleavage that produces Abeta40/42 from Abeta46; 2) at low range of concentrations and at late time points, MDL 28170 causes an increase in secreted Abeta40/42 that likely results from inhibition of degradation of both the initial substrate, CTFbeta, and the final product, Abeta40/42, of gamma-secretase. These data strongly suggest that formation of Abeta46 is a key step in the gamma-secretase mediated generation of Abeta40/42 and provide a new target for the development of Abeta inhibitors. These data also suggest that calpain and related proteases, which are sensitive to MDL 28170, play an important role in the accumulation of secreted Abeta.