| β‑Amyloid (Aβ) 1‑42 is a C‑terminal peptide fragment of the amyloid precursor protein (APP) generated by sequential proteolytic cleavage of APP by β‑secretase at the extracellular/lumenal side and γ‑secretase within the transmembrane region, and belongs to the family of APP‑derived Aβ peptides that aggregate in brain parenchyma and vessel walls. The Aβ1‑42 sequence extends two residues beyond Aβ1‑40 at the C‑terminus, has higher hydrophobicity, and shows a greater intrinsic tendency to self‑associate into low‑n oligomers, protofibrils, and fibrils that adopt cross‑β sheet–rich conformations and form the fibrillar core of neuritic plaques in Alzheimer‑type pathology. APP processing that favors the amyloidogenic β/γ pathway increases Aβ1‑42 production, and many familial Alzheimer’s disease mutations in APP or presenilin shift the Aβ profile toward elevated absolute Aβ1‑42 levels and an increased Aβ1‑42:Aβ1‑40 ratio, which correlates with early amyloid deposition and plaque burden. Aβ1‑42 interacts with a range of neuronal and glial surface receptors and binding partners, including NMDA receptors, cellular prion protein, nicotinic acetylcholine receptors, and RAGE, and modulates intracellular signaling cascades that involve Ca²⁺ influx, kinase activation, and synaptic receptor trafficking. Soluble oligomeric forms of Aβ1‑42 inhibit hippocampal long‑term potentiation and alter synaptic plasticity through a defined signaling pathway that includes caspase‑3 activation, decreased Akt activity, and increased glycogen synthase kinase‑3β activity, linking Aβ1‑42 exposure to impaired synaptic strengthening and tau‑phosphorylation‑related changes. Aβ1‑42 also affects non‑neuronal cells; it activates microglia and astrocytes, induces production of pro‑inflammatory cytokines and reactive oxygen species, and alters microglial survival and phagocytic function through pattern‑recognition receptors and downstream inflammatory pathways, contributing to a persistent neuroinflammatory environment. At a physiological level, low concentrations of Aβ, including Aβ1‑42, participate in regulation of synaptic activity and plasticity, modulation of neurogenesis, and antimicrobial defense, indicating that Aβ peptides have defined normal functions that depend on concentration, aggregation state, and context. In Alzheimer’s disease, biochemical and biomarker analyses identify altered Aβ1‑42 production, aggregation, and clearance as core features; decreased soluble Aβ1‑42 in cerebrospinal fluid and accumulation in plaques serve as diagnostic markers, and Aβ pathway dyshomeostasis interacts with tau pathology, neuroimmune activation, and neurotransmitter imbalance along the disease course. |
