Down syndrome (DS), caused by trisomy 21, almost invariably develops early-onset Alzheimer’s disease (AD). The amyloid precursor protein (APP) gene, located on chromosome 21, is dosage-sensitive and a critical driver of AD, promoting β-amyloid (Aβ) overproduction and accumulation. However, the mechanisms by which APP overexpression triggers AD in DS remain unclear, as cognitive decline is not always correlated with Aβ levels. Aβ binding to full-length APP induces multimerization, neuronal dystrophy, and activation of the Go/Gβγ/p38 MAPK pathway, leading to dendritic atrophy. Elevated phosphorylated tau (p-tau), another AD hallmark, has been detected early in DS and is linked to dendritic and neuritic damage. p38-MAPK directly phosphorylates tau at Ser396/404, while Aβ oligomers activate CIP2A, inhibiting PP2A-mediated tau dephosphorylation, thereby increasing p-tau. Enhanced APP expression at the neuronal membrane in DS may amplify these cascades, contributing to cytoskeletal dysfunction. This project will investigate the role of APP dosage and signaling in regulating tau/p-tau and cytoskeletal structure in DS neurons, using induced pluripotent stem cells (DS-iPSCs) and isogenic APP-reduced variants. Results may reveal novel mechanisms by which APP dosage influences AD progression in DS.