APOLIPOPROTEIN-E (APOE) PEPTIDE REGULATES TAU-PHOSPHORYLATION VIA 2 DIFFERENT SIGNALING PATHWAYS

Previous studies have shown that treating rat cortical neurons in prim ary culture with apolipoprotein E (apoE) peptide increased cytoplasmic Ca2+ by 2 mechanisms: 1) an influx of extracellular Ca2+ resulting ii om the activation of a cell surface Ca2+ channel; and 2) release of C a2+ from infernal Ca2+ stores via a G-protein-coupled pathway (Wang an d Gruenstein, 1997). These studies employed a biologically active apoE synthetic peptide (apoE(dp)) derived from the receptor binding domain of apoE. III the present study we examined whether activation of thes e 2 signal transduction pathways affects phosphorylation, of microtubu le-associated protein tan, The levels of tau phosphorylation at thr231 , ser235, and ser396 were quantified bg ELISA employing monoclonal ant ibodies PHF-6, SMI33, and PHF-1, ApoE(dp) treatment resulted in a conc entration-and time-dependent dephosphorylation of tan al: all 3 phosph orylation sites, The apoE(dp)-induced dephosphorylation of tan at thr2 31, and ser235 was dependent on the influx of extracellular Ca2+, whil e dephosphorylation at ser396 was mediated by a pertusis toxin-sensiti ve G-protein pathway, The involvement of protein phosphatases in media ting the apoE(dp)-induced dephosphorylation of tan was examined, Pretr eatment with the protein phosphatase 2B inhibitor cyclosporin A blocke d the apoE(dp)-induced dephosphorylation of tau at thr231 and ser235 b ut not at ser396. Pretreatment with the protein phosophatase 2A/1 inhi bitor okadaic acid blocked the apoE(dp)-induced dephosphorylation of t au at all 3 sites, while pretreatment with the protein phosphates 1 in hibitor tautomycin was without effect, The present study suggests that apnE may affect several Ca2+-associated signal transduction pathways that increase the activity of protein phosphatases 2A and 2B, which in turn dephosphorylate tau. (C) 1998 Wiley-Liss, Inc.