Presenilin-dependent regulation of neuronal tau pathology via the autophagy and proteasome pathways

Mutations in the presenilin (PS/PSEN) genes cause early-onset familial Alzheimer's disease (AD) by enhancing cerebral accumulation of amyloid-β (Aβ) peptides and microtubule-associated protein tau (MAPT). How PS mutations affect Aβ generation is well characterized, but the precise cellular mechanisms by which PS dysfunction drives neuronal tau pathology are not fully understood. Here, we investigated the mechanisms linking PS/γ-secretase-dependent tau pathology and autophagy/proteasome by employing pathological, imaging and molecular approaches in human brains, fibroblasts and induced pluripotent stem cells (iPSC)-derived neurons from PSEN1-linked familial AD carriers, and in a novel neuronal PS-deficient tauopathy transgenic mouse. We found enhanced levels and colocalization of pathological phosphorylated tau (pTau) and ubiquitin factor p62 in the hippocampus of dementia patients with familial AD-linked PSEN1 mutations, corticobasal degeneration and Pick's disease, suggesting disrupted proteasomal degradation in tauopathies. Human primary fibroblasts from PSEN1 G206D and/or L286P carriers showed elevated LC3-I and autolysosomes indicating autophagy flux alterations. Human iPSC-derived neurons harboring the familial-AD linked PSEN1 G206D mutation showed increased aggregated tau and reduced secreted tau, whereas pharmacological proteasome inhibition reduced significantly total and pTau (Ser396/404) while increasing its release. Consistently, proteasomal inhibition decreased intracellular tau and pTau and promoted tau release in human tau-expressing neurons through a mechanism that partially depends on PS. In the hippocampus of neuronal PS-deficient mice, Akt activation and GSK3β inhibition were associated with elevated levels of phosphorylated and aggregated tau and the ubiquitin-binding protein p62. In conclusion, PS function is required for autophagy/proteasome-mediated tau elimination in neurons, whereas that FAD-linked PSEN1 mutations cause progressive tau pathology by disrupting the proteasome and autophagy/lysosomal pathways.