Secreted beta-amyloid precursor protein counteracts the proapoptotic action of mutant presenilin-1 by activation of NF-kappaB and stabilization of calcium homeostasis.

Mutations in the presenilin-1 (PS-1) gene account for approximately 50% of the cases of autosomal dominant, early onset, inherited forms of Alzheimer's disease (AD). PS-1 is an integral membrane protein expressed in neurons and is localized primarily in the endoplasmic reticulum (ER). PS-1 mutations may promote neuronal degeneration by altering the processing of the beta-amyloid precursor protein (APP) and/or by engaging apoptotic pathways. Alternative processing of APP in AD may increase production of neurotoxic amyloid beta-peptide (Abeta) and reduce production of the neuroprotective alpha-secretase-derived form of APP (sAPPalpha). In differentiated PC12 cells expressing an AD-linked PS-1 mutation (L286V), sAPPalpha activated the transcription factor NF-kappaB and prevented apoptosis induced by Abeta. Treatment of cells with kappaB decoy DNA blocked the antiapoptotic action of sAPPalpha, demonstrating the requirement for NF-kappaB activation in the cytoprotective action of sAPPalpha. Cells expressing mutant PS-1 exhibited an aberrant pattern of NF-kappaB activity following exposure to Abeta, which was characterized by enhanced early activation of NF-kappaB followed by a prolonged depression of activity. Blockade of NF-kappaB activity in cells expressing mutant PS-1 by kappaB decoy DNA was associated with enhanced Abeta-induced increases of [Ca2+]i and mitochondrial dysfunction. Treatment of cells with sAPPalpha stabilized [Ca2+]i and mitochondrial function and suppressed oxidative stress by a mechanism involving activation of NF-kappaB. Blockade of ER calcium release prevented (and stimulation of ER calcium release by thapsigargin induced) apoptosis in cells expressing mutant PS-1, suggesting a pivotal role for ER calcium release in the proapoptotic action of mutant PS-1. Finally, a role for NF-kappaB in preventing apoptosis induced by ER calcium release was demonstrated by data showing that sAPPalpha prevents thapsigargin-induced apoptosis, an effect blocked by kappaB decoy DNA. We conclude that sAPPalpha stabilizes cellular calcium homeostasis and protects neural cells against the proapoptotic action of mutant PS-1 by a mechanism involving activation of NF-kappaB. The data further suggest that PS-1 mutations result in aberrant NF-kappaB regulation that may render neurons vulnerable to apoptosis.