Mercury Reduces the Enzymatic Activity of Neprilysin in Differentiated SH-SY5Y Cells

Levels of amyloid beta (A beta) in the central nervous system are regulated by the balance between its synthesis and degradation. Neprilysin (NEP) is associated with Alzheimer's disease (AD) by its ability to degrade A beta. Some studies have involved the exposure to mercury (Hg) in AD pathogenesis; therefore, our aim was to investigate the effects on the anabolism and catabolism of A beta in differentiated SH-SY5Y cells incubated with 1-20 mu M of Hg. Exposure to 20 A mu M of Hg induced an increase in A beta-42 secretion, but did not increase the expression of the amyloid precursor protein (APP). Hg incubation (10 and 20 A mu M) increased NEP protein levels; however, it did not change NEP mRNA levels nor the levels of the amyloid intracellular domain peptide, a protein fragment with transcriptional activity. Interestingly, Hg reduced NEP activity at 10 and 20 A mu M, and circular dichroism analysis using human recombinant NEP showed conformational changes after incubation with molar equivalents of Hg. This suggests that the Hg-induced inhibition of NEP activity may be mediated by a conformational change resulting in reduced A beta-42 degradation. Finally, the comparative effects of lead (Pb, 50 mu M) were evaluated. We found a significant increase in A beta-42 levels and a dramatic increase in APP protein levels; however, no alteration in NEP levels was observed nor in the enzymatic activity of this metalloprotease, despite the fact that Pb slightly modified the rhNEP conformation. Overall, our data suggest that Hg and Pb increase A beta levels by different mechanisms.