ApoER2 expression enhances γ-secretase activity. N2a and CHO LRP1-null cells expressing full length ApoER2 or transfected with pcDNA3 alone were lysed in CHAPSO containing buffer. (A), Measurement of γ-secretase activity using a fluorogenic substrate assay, which is based on the secretase-dependent cleavage of a γ-secretase-specific substrate conjugated with a fluorescent molecule. Data represent mean ± S.E.M of three experiments in duplicated, presented as fold values of the basal activity. * P < 0.05, ** P < 0.01. (B), 50 μg of protein/lane were subjected to 12% SDS-PAGE and endogenous PS1 levels were determined by Western blot. In N2a cells almost all PS1 is detected as PS1-NTF but in CHO LRP1-null cells a significant amount of Holo-PS1 is also detected. ApoER2 expression increases PS1-NTF and decreases Holo-PS1 compared to control CHO LRP1-null cells. The expression of actin was determined in the same blot as a loading control. (C), densitometric analysis of Western blots as in (B) confirms that total PS1 levels were slightly increased in CHO-ApoER2 cells compared to the pcDNA3 cells. However, a further increase in the PS1-NTF fraction is clearly observed in the Western blot.