Mechanisms of amyloid beta protein-induced modification in ion transport systems: implications for neurodegenerative diseases

1. Alzheimer's disease (AD) is a neurodegenerative disorder that affects the cognitive function of the brain. Pathological changes in AD are characterized by the formation of amyloid plaques and neurofibrillary tangles as well as extensive neuronal loss. Abnormal proteolytic processing of amyloid precursor protein (APP) is the central step that leads to formation of amyloid plaque, neurofibrillary tangles, and neuronal loss.2. The plaques, which accumulate extracellularly in the brain, are composed of aggregates and cause direct neurotoxic effects and/or increase neuronal vulnerability to excitotoxic insults. The aggregates consist of soluble pathologic amyloid beta peptides AP1–42] and AP1–43] and soluble nonpathologic AP1–40]. Both APP and AP interact with ion transport systems. AP induces a wide range of effects as the result of activating a cascade of mechanisms.3. The major mechanisms proposed for AP-induced cytotoxicity involve the loss of Ca²⁺ homeostasis and the generation of reactive oxygen species (ROS). The changes in Ca²⁺ homeostasis could be the result of (1) changes in endogenous ion transport systems, e.g. Ca²⁺ and K⁺ channels and Na⁺/K⁺-ATPase, and membrane receptor proteins, such as ligand-driven ion channels and G-protein-driven releases of second messengers, and (2) formation of heterogeneous ion channels.4. The consequences of changes in Ca²⁺-homeostasis-induced generation of ROS are (a) direct modification of intrinsic ion transport systems and their regulatory mechanisms, and (b) indirect effects on ion transport systems via peroxidation of phospholipids in the membrane, inhibition of phosphorylation, and reduction of ATP levels and cytoplasmic pH.5. We propose that in AD, AP with its different conformations alters cell regulation by modifying several ion transport systems and also by forming heterogeneous ion channels. The changes in membrane transport systems are proposed as early steps in impairing neuronal function preceding plaque formation. We conclude that these changes damage the membrane by compromising its integrity and increasing its ion permeability. This mechanism of membrane damage is not only central for AD but also may explain other malfunctioned protein-processing–related pathologies.