Ceramides in Alzheimer’s Disease: Key Mediators of Neuronal Apoptosis Induced by Oxidative Stress and AβAccumulation

Abstract

Alzheimer’s disease (AD), the most common chronic and progressive neurodegenerative disorder, is characterized by extracellular deposits of amyloidβ-peptides (Aβ) and intracellular deposits of hyperphosphorylated tau (phospho-tau) protein. Ceramides, the major molecules of sphingolipid metabolism and lipid second messengers, have been associated with AD progression and pathology via Aβgeneration. Enhanced levels of ceramides directly increase Aβthrough stabilization ofβ-secretase, the key enzyme in the amyloidogenic processing of Aβprecursor protein (APP). As a positive feedback loop, the generated oligomeric and fibrillar Aβinduces a further increase in ceramide levels by activating sphingomyelinases that catalyze the catabolic breakdown of sphingomyelin to ceramide. Evidence also supports important role of ceramides in neuronal apoptosis. Ceramides may initiate a cascade of biochemical alterations, which ultimately leads to neuronal death by diverse mechanisms, including depolarization and permeabilization of mitochondria, increased production of reactive oxygen species (ROS), cytochrome c release, Bcl-2 depletion, and caspase-3 activation, mainly by modulating intracellular signalling, particularly along the pathways related to Akt/PKB kinase and mitogen-activated protein kinases (MAPKs). This review summarizes recent findings related to the role of ceramides in oxidative stress-driven neuronal apoptosis and interplay with Aβin the cascade of events ending in neuronal degeneration.