Alpha-synuclein expression influences the processing of the amyloid precursor protein

In certain neurodegenerative diseases such Dementia with Lewy Bodies (DLB), it is hypothesised that misfolded α-synuclein (α-syn) and β-amyloid both contribute to pathology. α-Syn and β-amyloid have been suggested to synergistically promote one another’s accumulation and aggregation, but the mechanisms are unknown. β-Amyloid is generated from β-/γ-secretase-mediated processing of the amyloid precursor protein (APP). This study investigated how α-syn overexpression in cells affects β-amyloid production from APP, using multiplex assays, luciferase reporter assays, and western blotting. Wildtype α-syn expression induces β-amyloid generation from APP in SH-SY5Y human neuroblastoma cells, and similar changes to APP processing occur in another neuronal cell model. Dominant-negative overexpression of α-syn mutants revealed that disrupting the N-terminal domain can increase APP amyloidogenic processing. Secretase enzymes that perform APP processing were next investigated. γ-Secretase activity, measured by a luciferase reporter, was not increased by α-syn overexpression. A higher ratio of β- to α-secretase processing was hypothesised, which led to expression and activity studies of the major β- and α-secretases, BACE1 and ADAM10 respectively. It was shown that the BACE1 protein expression is post-transcriptionally upregulated in α-syn cells, with increased APP cleavage in cells. ADAM10 protein expression is transcriptionally suppressed in wild-type α-syn cells, reducing total levels of catalytically active enzyme. However the change in ADAM10-mediated APP processing may be negligible since, critically, plasma membrane expression of ADAM10 appears to be maintained. To aid understanding of the mechanism that connects α-syn to APP processing, BACE1 expression was used in pharmacological studies of cell stress signalling. This approach revealed that in α-syn cells BACE1 lysosomal and/or proteasomal degradation may be disturbed. Additionally, BACE1 expression is induced by translational de-repression mediated by eIF2α ser-51 phosphorylation, which was increased in α-syn cells. Although preliminary, the data suggests a role for oxidative stress mediating the increased BACE1 expression in wild-type α-syn cells.