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 Aβ 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.