γ-Secretase-mediated regulation of neprilysin: influence of cell density and aging and modulation by imatinib.

Proteolytic degradation has emerged as a key pathway involved in controlling levels of the Alzheimer's disease (AD)-associated amyloid-β peptides (Aβ) in the brain. The ectopeptidase, neprilysin (NEP), has been reported as the major Aβ-degrading enzyme in mice and human brains. We have previously shown that NEP expression and activity are regulated by AICD, the intracellular domain of the amyloid-β protein precursor (AβPP) generated by γ-secretase. Thus, NEP transcription, expression, and enzymatic activity are dramatically reduced in fibroblasts devoid of AβPP (the precursor of AICD) or lacking both presenilin (PS) 1 and 2 (two parent proteins contributing to AICD formation). We demonstrate here that NEP expression and activity are influenced by a number of cell passages and density, and we confirm a drastic reduction of NEP expression and activity in AβPP and PS null fibroblasts examined at similar passages and cell densities. Furthermore, Imatinib (Gleevec), a known tyrosine kinase inhibitor was recently shown to elevate AICD in H4 human neuroglioma cells, and this was accompanied by concomitant increases of NEP protein, mRNA levels, and activity. However, the demonstration of a causal link between Imatinib and AICD levels was still lacking. We show here an Imatinib-dependent effect on NEP expression and activity in murine fibroblasts and establish that Imatinib-induced modulation of NEP was abolished by the depletion of AβPP or its homologues APLP1 and APLP2, thereby confirming that Imatinib-mediated control of NEP could indeed be accounted for its effect on AICD.