Decoding the secretase puzzle in amyloid-β generation: A state-of-the-art overview of the protease-mediated APP processing cascade in Alzheimer's disease.

The accumulation of amyloid β (Aβ) protein in the brain is a central pathological hallmark of Alzheimer's disease (AD). This process has become a major focus of interdisciplinary research and a critical target in drug development. Aβ is produced through the proteolytic processing of amyloid precursor protein (APP) by a group of enzymes known as secretases. They belong to different protease classes and operate through proteolytic cleavage of the peptide bond through several catalytic hydrolysis. Dysregulation of the expression or/and activity of proteases involved in APP processing disrupts the balance between the amyloidogenic and non-amyloidogenic pathways-often shifting it toward the amyloidogenic route. This shift leads to excessive production and further aggregation of Aβ peptides, ultimately resulting in neuronal toxicity. In this review, we integrate current state-of-the-art knowledge on all proteases reported to cleave APP, encompassing both canonical and non-canonical pathways, and offer detailed examination of cleavage-site topology, and catalytic mechanisms. By integrating the spatial and sequential hierarchy of APP proteolysis across cellular compartments, we establish a unifying mechanistic framework that captures the complexity of the process. We further delineate how distinct proteases-through defined active-site architectures, conserved catalytic motifs, and nucleophile-driven peptide bond hydrolysis-precisely regulate APP processing. This mechanistic perspective advances our molecular understanding of A pathogenesis and delineates critical catalytic control nodes amenable to therapeutic intervention. By defining these targets at a mechanistic level, it establishes a rational framework for precision drug design and the development of next-generation therapeutics.