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Review
, 2015, 346783

Ceramides in Alzheimer's Disease: Key Mediators of Neuronal Apoptosis Induced by Oxidative Stress and Aβ Accumulation

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Review

Ceramides in Alzheimer's Disease: Key Mediators of Neuronal Apoptosis Induced by Oxidative Stress and Aβ Accumulation

Maja Jazvinšćak Jembrek et al. Oxid Med Cell Longev.

Abstract

Alzheimer's disease (AD), the most common chronic and progressive neurodegenerative disorder, is characterized by extracellular deposits of amyloid β-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 Aβ generation. Enhanced levels of ceramides directly increase Aβ through stabilization of β-secretase, the key enzyme in the amyloidogenic processing of Aβ precursor protein (APP). As a positive feedback loop, the generated oligomeric and fibrillar Aβ induces 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 Aβ in the cascade of events ending in neuronal degeneration.

Figures

Figure 1
Figure 1
Ceramide formation via de novo synthesis, recycling, and degradation. In the catabolic pathway, ceramides are generated through sphingomyelin hydrolysis by sphingomyelinases (SMases). In de novo synthesis, serine palmitoyltransferase (SPT) is the rate-limiting enzyme. In the recycling pathway, sphingosine, the product of sphingolipid catabolism, is salvaged through reacylation, resulting in ceramide production.
Figure 2
Figure 2
Role of ceramides in neuronal apoptosis. Overexpressed ceramides may provoke apoptosis by directly participating in ROS generation, through modulation of mitogen-activated protein kinases (MAPKs) signalling pathways, and by inhibition of prosurvival phosphatidylinositol 3-kinase (PI3-K)/Akt pathway via protein phosphatase 2A (PP2A) dephosphorylation, leading to activation of Bad and glycogen synthase 3β (GSK-3β). These cascades of biochemical alterations ultimately lead to neuronal death by diverse mechanisms, including depolarization and permeabilization of mitochondria (Δψ m), Bcl-2 depletion and Bax increase, cytochrome c release, and caspase-3 activation.
Figure 3
Figure 3
Interplay between ceramides, Aβ and oxidative stress. Increased levels of ceramides directly increase levels of Aβ by increasing the half-life of BACE1 through posttranslational stabilization. As a positive feedback loop, the generated Aβ induces a further increase in ceramide levels by activating SMases activities in a ROS-dependent fashion. Aβ promotes accumulation of free radicals via NADPH oxidase activation and glutathione (GSH) depletion and induces membrane oxidative stress leading to generation of 4-hydroxynonenal (HNE), the neurotoxic product that exacerbates neuronal death through protein adduct formation, impairment of glucose metabolism, and ATP depletion.
Figure 4
Figure 4
Interplay between neurons and astroglia in sphingolipid metabolism. Conditioned medium from palmitic acid- (PA-) treated astrocytes upregulates neuronal β-secretase (BACE1) leading to Aβ accumulation. PA increases de novo synthesis of ceramide and downregulates glucose uptake in astroglia. PA-induced activation of serine palmitoyltransferase (SPT) leads to production of proinflammatory cytokines, which after release in surrounding media activate neuronal sphingomyelinases (SMases), increase the ceramide pool, and ultimately lead to increase in Aβ accumulation.

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