Presenilin Deficiency Results in Cellular Cholesterol Accumulation by Impairment of Protein Glycosylation and NPC1 Function.

Presenilin proteins (PS1 and PS2) represent the catalytic subunit of γ-secretase and play a critical role in the generation of the amyloid ß (Aß) peptide and the pathogenesis of Alzheimer disease (AD). However, PS proteins also exert multiple functions beyond Aß generation. In this study, we examine the individual roles of PS1 and PS2 in cellular cholesterol metabolism. Deletion of PS1 or PS2 in mouse models led to cholesterol accumulation in cerebral neurons. Cholesterol accumulation was also observed in the lysosomes of embryonic fibroblasts from Psen1-knockout (PS1-KO) and Psen2-KO (PS2-KO) mice and was associated with decreased expression of the Niemann-Pick type C1 (NPC1) protein involved in intracellular cholesterol transport in late endosomal/lysosomal compartments. Mass spectrometry and complementary biochemical analyses also revealed abnormal N-glycosylation of NPC1 and several other membrane proteins in PS1-KO and PS2-KO cells. Interestingly, pharmacological inhibition of N-glycosylation resulted in intracellular cholesterol accumulation prominently in lysosomes and decreased NPC1, thereby resembling the changes in PS1-KO and PS2-KO cells. In turn, treatment of PS1-KO and PS2-KO mouse embryonic fibroblasts (MEFs) with the chaperone inducer arimoclomol partially normalized NPC1 expression and rescued lysosomal cholesterol accumulation. Additionally, the intracellular cholesterol accumulation in PS1-KO and PS2-KO MEFs was prevented by overexpression of NPC1. Collectively, these data indicate that a loss of PS function results in impaired protein N-glycosylation, which eventually causes decreased expression of NPC1 and intracellular cholesterol accumulation. This mechanism could contribute to the neurodegeneration observed in PS KO mice and potentially to the pathogenesis of AD.

Carboxy-terminal fragment of amyloid precursor protein mediates lipid droplet accumulation upon γ-secretase inhibition.

γ-Secretase is a protease catalysing the proteolysis of type-I membrane proteins usually after precedent ectodomain shedding of the respective protein substrates. Since proteolysis of membrane proteins is involved in fundamental cellular signaling pathways, dysfunction of γ-secretase can have significant impact on cellular metabolism and differentiation. Here, we examined the role of γ-secretase in cellular lipid metabolism using neuronally differentiated human SH-SY5Y cells. The pharmacological inhibition of γ-secretase induced lipid droplet (LD) accumulation. The LD accumulation was significantly attenuated by preventing the accumulation of C-terminal fragment of the amyloid precursor protein (APP-CTF), which is a direct substrate of γ-secretase. Additionally, LD accumulation upon γ-secretase inhibition was not induced in APP-knock out (APP-KO) mouse embryonic fibroblasts (MEFs), suggesting significant involvement of APP-CTF accumulation in LD accumulation upon γ-secretase inhibition. On the other hand, γ-secretase inhibition-dependent cholesterol accumulation was not attenuated by inhibition of APP-CTF accumulation in the differentiated SH-SY5Y cells nor in APP-KO MEFs. These results suggest that γ-secretase inhibition can induce accumulation of LD and cholesterol differentially via APP-CTF accumulation.

Importance of γ-secretase in the regulation of liver X receptor and cellular lipid metabolism.

Presenilins (PS) are the catalytic components of γ-secretase complexes that mediate intramembrane proteolysis. Mutations in the PS genes are a major cause of familial early-onset Alzheimer disease and affect the cleavage of the amyloid precursor protein, thereby altering the production of the amyloid ß-peptide. However, multiple additional protein substrates have been identified, suggesting pleiotropic functions of γ-secretase. Here, we demonstrate that inhibition of γ-secretase causes dysregulation of cellular lipid homeostasis, including up-regulation of liver X receptors, and complex changes in the cellular lipid composition. Genetic and pharmacological inhibition of γsecretase leads to strong accumulation of cytoplasmic lipid droplets, associated with increased levels of acylglycerols, but lowered cholesteryl esters. Furthermore, accumulation of lipid droplets was augmented by increasing levels of amyloid precursor protein C-terminal fragments, indicating a critical involvement of this γ-secretase substrate. Together, these data provide a mechanism that functionally connects γ-secretase activity to cellular lipid metabolism. These effects were also observed in human astrocytic cells, indicating an important function of γ-secretase in cells critical for lipid homeostasis in the brain.

Presenilins and γ-Secretase in Membrane Proteostasis.

The presenilin (PS) proteins exert a crucial role in the pathogenesis of Alzheimer disease (AD) by mediating the intramembranous cleavage of amyloid precursor protein (APP) and the generation of amyloid ß-protein (Aß). The two homologous proteins PS1 and PS2 represent the catalytic subunits of distinct γ-secretase complexes that mediate a variety of cellular processes, including membrane protein metabolism, signal transduction, and cell differentiation. While the intramembrane cleavage of select proteins by γ-secretase is critical in the regulation of intracellular signaling pathways, the plethora of identified protein substrates could also indicate an important role of these enzyme complexes in membrane protein homeostasis. In line with this notion, PS proteins and/or γ-secretase has also been implicated in autophagy, a fundamental process for the maintenance of cellular functions and homeostasis. Dysfunction in the clearance of proteins in the lysosome and during autophagy has been shown to contribute to neurodegeneration. This review summarizes the recent knowledge about the role of PS proteins and γ-secretase in membrane protein metabolism and trafficking, and the functional relation to lysosomal activity and autophagy.

Imbalance in fatty-acid-chain length of gangliosides triggers Alzheimer amyloid deposition in the precuneus.

Amyloid deposition, a crucial event of Alzheimer's disease (AD), emerges in distinct brain regions. A key question is what triggers the assembly of the monomeric amyloid ß-protein (Aß) into fibrils in the regions. On the basis of our previous findings that gangliosides facilitate the initiation of Aß assembly at presynaptic neuritic terminals, we investigated how lipids, including gangliosides, cholesterol and sphingomyelin, extracted from synaptic plasma membranes (SPMs) isolated from autopsy brains were involved in the Aß assembly. We focused on two regions of the cerebral cortex; precuneus and calcarine cortex, one of the most vulnerable and one of the most resistant regions to amyloid deposition, respectively. Here, we show that lipids extracted from SPMs isolated from the amyloid-bearing precuneus, but neither the amyloid-free precuneus nor the calcarine cortex, markedly accelerate the Aß assembly in vitro. Through liquid chromatography-mass spectrometry of the lipids, we identified an increase in the ratio of the level of GD1b-ganglioside containing C20:0 fatty acid to that containing C18:0 as a cause of the enhanced Aß assembly in the precuneus. Our results suggest that the local glycolipid environment play a critical role in the initiation of Alzheimer amyloid deposition.

Influence of APOE genotype and the presence of Alzheimer's pathology on synaptic membrane lipids of human brains.

The APOE genotype is the major risk factor for Alzheimer's disease (AD); however, it remains unclarified how the ε4 allele accelerates whereas the ε2 allele suppresses AD development, compared with the more common ε3 allele. On the basis of the previous finding that the assembly of the amyloid-ß protein (Aß) into fibrils in the brain, an early and invariable pathological feature of AD, depends on the lipid environment, we determined the levels of synaptic membrane lipids in aged individuals of different APOE genotypes. In the comparison between amyloid-free ε2/ε3 and ε3/ε3 brains, the presence of the ε2 allele significantly decreased the level of cholesterol. Alternatively, in the comparison among ε3/ε3 brains, the presence of AD pathology substantially decreased the levels of cholesterol. This study suggests that the ε2 allele suppresses the initiation of AD development by lowering the cholesterol levels in synaptic membranes.

The γ-secretase inhibitor DAPT increases the levels of gangliosides at neuritic terminals of differentiating PC12 cells.

Mutations in presenilins are the major cause of early onset familial Alzheimer disease. It has recently been argued that clinical presenilin mutations work as loss-of-function but not toxic gain-of-function. To investigate whether presenilins are involved in the regulation of the distribution of neuronal membrane lipids, we treated neuronally differentiated PC12 cells with DAPT, an inhibitor of presenilin-dependent γ-secretase, and performed lipid analyses of neuritic terminals, which is an initial site of Aß deposition in brains, using liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) in combination with multiple reaction monitoring (MRM). With DAPT treatment, levels of sphingomyelin, phosphatidylcholine, and cholesterol remained unchanged. However, DAPT treatment increased the ganglioside levels in PC12 neuritic terminals. Together with a previous finding that accumulation of gangliosides at neuritic terminals facilitates Aß assembly and deposition, the present data suggest that the loss-of-function of presenilins, i.e., a decrease in γ-secretase activity, has an impact on neuronal membrane architecture in a way that eventually exacerbates Alzheimer pathology.

Alzheimer-type tau pathology in advanced aged nonhuman primate brains harboring substantial amyloid deposition.

We elucidated how Alzheimer-type pathologies of amyloid beta-protein (Abeta) and tau spatiotemporally emerge in brains of nontransgenic nonhuman primate, cynomolgus monkey, in the present study. To examine the accumulation of deposited Abeta, phosphorylated tau accumulation, intracellular tau accumulation, and neurofibrillary tangle formation, the brains, mainly temporal cortex and hippocampus, of 34 cynomolgus monkeys aged 6 to 36 years were studied by biochemical and histochemical analyses. Biochemically, first, the accumulation of insoluble Abeta was detected in the neocortical (temporal and frontal) and hippocampal regions of animals as young as mid-20s and their levels were extremely high in those of advanced age. The accumulation of phosphorylated tau in the same regions occurred before the age of 20 with poor correlation to the levels of insoluble Abeta. Histologically, intraneuronal and intraoligodendroglial tau accumulation was observed in temporal cortex and hippocampus of animals before the age of 20. In an advanced aged 36-year-old individual, argyrophilic tangles and tau-accumulated dystrophic neurites were markedly observed in the medial temporal area contiguous to limbic structures. Notably, these tau pathologies also emerged, to a lesser extent, in the temporal cortices of advanced aged animals harboring significant amounts of insoluble Abeta. These results suggest that the cynomolgus monkey can be used to elucidate the age-dependent sequence of Abeta and tau pathologies.

Gender effect on the accumulation of hyperphosphorylated tau in the brain of locus-ceruleus-injured APP-transgenic mouse.

Locus ceruleus (LC) neurons are preferentially and initially affected in Alzheimer disease (AD); however, the impact of the loss of LC neurons on the pathological sequence of AD, including amyloid beta-protein (Abeta) deposition and neurofibrillary tangle formation, has not been elucidated. In this study, we chemically injured LC neurons of the brains of familial AD-related amyloid precursor protein (APP)-transgenic mice using the LC-noradrenergic neuron-selective neurotoxin, N-(2-chloroethyl)-N-ethyl-bromo-benzylamine (DSP4). The levels of noradrenaline significantly decreased in the cerebral cortices of DSP4-treated mice. The deposition of amyloid fibrils was biochemically observed in the APP-transgenic mouse brains; however, those levels were not significantly altered following DSP4 treatment. In contrast, the levels of accumulated hyperphosphorylated tau markedly increased in the cerebral cortices of DSP4-treated female but not male APP-transgenic mice. Our results suggest that innervation from LC neurons and testosterone secretion are potent and mutually independent suppressors of amyloid-related accumulation of hyperphosphorylated tau in the brain.

Gangliosides determine the amyloid pathology of Alzheimer's disease.

Gangliosides, GM3 and GM1, are suggested to accelerate the deposition of the amyloid beta-protein as amyloid angiopathy and senile plaques, respectively, in the Alzheimer brain. We investigated the profile of amyloid deposition in the brains of transgenic mice expressing a mutant amyloid precursor protein with a disrupted GM2 synthase gene, in which GM3 accumulates whereas GM1 is lacking. These mice showed a significantly increased level of deposited amyloid beta-protein in the vascular tissues. Furthermore, formation of severe dyshoric-form amyloid angiopathy, in which amyloid extended from the blood vessel walls deeply into the surrounding parenchyma was observed. Our results indicate that the expression of gangliosides is a critical determinant for the amyloid pathology in the Alzheimer brain.