A Review of the Effect of Peripheral Amyloid β on the Central Nervous System

Alzheimer's disease (AD) is a progressive neurodegenerative disorder classically defined by cerebral amyloid β (Aβ) plaque deposition and tau pathology. In recent years, AD has increasingly been recognized as a multisystem disorder rather than a purely brain-restricted condition, as mounting evidence indicates that Aβ metabolism is a dynamic, bidirectional process involving both central and peripheral compartments. Peripheral tissues, particularly platelets, liver, kidneys, and the gastrointestinal tract, contribute substantially to circulating Aβ levels and influence cerebral amyloid burden. Platelets are now considered the predominant source of peripheral Aβ, accounting for the majority of plasma Aβ under physiological and pathological conditions, while the liver and kidneys play critical roles in Aβ clearance through receptor-mediated uptake, enzymatic degradation and excretion. Disruption of these peripheral clearance pathways elevates circulating Aβ, increasing its transport into the brain via blood-brain barrier (BBB) mechanisms by enhanced RAGE-mediated influx and impaired LRP1-dependent efflux in AD. Peripheral Aβ entry into the central nervous system exacerbates neuroinflammation, mitochondrial dysfunction, and oxidative stress, thereby accelerating neuronal damage and disease progression. This review synthesizes updated evidence on peripheral sources of Aβ, differences between central and peripheral Aβ pools, mechanisms of Aβ transport across the BBB, pathological consequences of peripheral Aβ on the brain and emerging therapeutic strategies targeting peripheral Aβ metabolism, highlighting the importance of a systemic perspective in AD pathogenesis and treatment.