Here we show that prolyl isomerase Pin I is involved in the A beta production central to the pathogenesis of Alzheimer's disease. Enzyme immumoassay of brains of the Pin1-deficient mice revealed that production of A beta 40 and A beta 42 was lower than that of the wild-type mice, indicating that Pin1 promotes A beta production in the brain. GST-Pin1 pull-down and immunoprecipitation assay revealed that Pin1 binds phosphorylated Thr668-Pro of C99. In the Pin1(-/-) MEF transfected with C99, Pin1 co-transfection enhanced the levels of A beta 40 and A beta 42 compared to that without Pin1 co-transfection. In COS7 cells transfected with C99, Pin1 co-transfection enhanced the generation of A beta 40 and A beta 42, and reduced the expression level of C99, facilitating the C99 turnover. Thus, Pin1 interacts with C99 and promotes its gamma-cleavage, generating A beta 40 and A beta 42. Further, GSK3 inhibitor lithium blocked Pin1 binding to C99 by decreasing Thr668 phosphorylation and attenuated A beta generation, explaining the inhibitory effect of lithium on A beta generation. (c) 2005 Elsevier Inc. All rights reserved.