Amyloid Precursor Protein Dimerisation Reduces Neurite Outgrowth

The amyloid precursor protein (APP) undergoes extensive metabolism, and its transport and proteolytic processing can be modulated by its ability to form a homodimer. We have investigated the functional consequences of stabilised APP dimer expression in cells by studying the engineered dimerisation of the APP(L17C) (residue 17 in A sequence) construct, which is associated with a 30% increase in APP dimer expression, on APP's neurite outgrowth promoting activity. Overexpression of APP(L17C) in SH-SY5Y cells decreased neurite outgrowth upon retinoic acid differentiation as compared to overexpressing APP(WT) cells. The APP(L17C) phenotype was rescued by replacing the APP(L17C) media with conditioned media from APP(WT) cells, indicating that the APP(L17C) mutant is impairing the secretion of a neuritogenic promoting factor. APP(L17C) had altered transport and was localised in the endoplasmic reticulum. Defining the molecular basis of the APP(L17C) phenotype showed that RhoA GTPase activity, a negative regulator of neurite outgrowth, was increased in APP(L17C) cells. RhoA activity was decreased after APP(WT) conditioned media rescue. Moreover, treatment with the RhoA inhibitor, Y27632, restored a wild-type morphology to the APP(L17C) cells. Small RNAseq analysis of APP(L17C) and APP(WT) cells identified several differentially expressed miRNAs relating to neurite outgrowth. Of these, miR-34a showed the greatest decrease in expression. Lentiviral-mediated overexpression of miR-34a rescued neurite outgrowth in APP(L17C) cells to APP(WT) levels and changed RhoA activation. This study has identified a novel link between APP dimerisation and its neuritogenic activity which is mediated by miR-34a expression.