| β-Amyloid 1-42 is a peptide fragment released from the amyloid precursor protein through sequential proteolytic cleavage, representing the primary product of the amyloidogenic processing branch of APP metabolism. Full-length APP undergoes cleavage by β-secretase (BACE1) at the cell surface and within endocytic compartments, generating a soluble N-terminal fragment, sAPPβ, and a membrane-retained C-terminal fragment, C99. Sequential cleavage of C99 by the γ-secretase complex, composed of presenilin, nicastrin, Pen-2, and Aph-1 subunits, releases Aβ into the extracellular space along with the intracellular domain AICD, which itself acts as a transcriptional regulator influencing further APP expression. This amyloidogenic route operates in competition with the non-amyloidogenic pathway, where α-secretase cleavage within the Aβ domain precludes intact peptide formation and instead yields the neuroprotective fragment sAPPα, positioning secretase balance as the determinant of Aβ output. Once generated, Aβ1-42 self-associates into soluble oligomers and higher-order fibrils that accumulate as extracellular plaques, interacting with neuronal membrane lipids, ion channels, and surface receptors to disrupt synaptic signaling and calcium homeostasis. Aβ pathology intersects with tau biology through shared kinase signaling intermediates that promote tau hyperphosphorylation, driving microtubule destabilization and neurofibrillary tangle formation as a parallel pathological cascade. BACE1 cleavage constitutes the rate-limiting step of the amyloidogenic pathway, and its activity is influenced by cellular trafficking, membrane lipid raft composition, and stress-responsive kinase signaling, including JNK and p38 MAPK pathways that elevate BACE1 expression under oxidative conditions. Aβ1-42 also accumulates within cerebral blood vessel walls, contributing to cerebral amyloid angiopathy as a distinct but overlapping vascular pathology. |
