Plant sterols the better cholesterol in Alzheimer's disease? A mechanistical study.

Amyloid-ß (Aß), major constituent of senile plaques in Alzheimer's disease (AD), is generated by proteolytic processing of the amyloid precursor protein (APP) by ß- and γ-secretase. Several lipids, especially cholesterol, are associated with AD. Phytosterols are naturally occurring cholesterol plant equivalents, recently been shown to cross the blood-brain-barrier accumulating in brain. Here, we investigated the effect of the most nutritional prevalent phytosterols and cholesterol on APP processing. In general, phytosterols are less amyloidogenic than cholesterol. However, only one phytosterol, stigmasterol, reduced Aß generation by (1) directly decreasing ß-secretase activity, (2) reducing expression of all γ-secretase components, (3) reducing cholesterol and presenilin distribution in lipid rafts implicated in amyloidogenic APP cleavage, and by (4) decreasing BACE1 internalization to endosomal compartments, involved in APP ß-secretase cleavage. Mice fed with stigmasterol-enriched diets confirmed protective effects in vivo, suggesting that dietary intake of phytosterol blends mainly containing stigmasterol might be beneficial in preventing AD.

PS dependent APP cleavage regulates glucosylceramide synthase and is affected in Alzheimer's disease.

BACKGROUND: Gangliosides were found to be associated with Alzheimer's disease (AD). Here we addressed a potential function of γ-secretase (presenilin) dependent cleavage of the amyloid-precursor-protein (APP) in the regulation of ganglioside de novo synthesis. METHODS: To identify a potential role of γ-secretase and APP in ganglioside de novo synthesis we used presenilin (PS) deficient and APP deficient cells and mouse brains, mutated PS as well as transgenic mice and AD post mortem brains. Changes in glucosylceramide synthase (GCS) activity were identified by incorporation of radiolabeled UDP-glucose in glucosylceramide, changes in gene expression via real-time PCR and Western blot analysis. Alterations in ganglioside levels were determined by thin layer chromatography and mass spectrometry. RESULTS: We found that PS and APP deficiency, in vitro and in vivo, resulted in increased GCS gene expression, elevated enzyme activity and thus increased glucosylceramide and total ganglioside level. Using a specific γ-secretase inhibitor revealed that PS proteolytic activity alters ganglioside homeostasis. By the use of mutated PS causing early onset AD in cell culture and transgenic mice we found that GCS is increased in AD, further substantiated by the use of AD post mortem brains, suffering from sporadic AD. CONCLUSION: APP processing regulates ganglioside de novo synthesis and is affected in AD.

Impact of Vitamin D on amyloid precursor protein processing and amyloid-ß peptide degradation in Alzheimer's disease.

Ninety percent of the elderly population has a vitamin D hypovitaminosis, and several lines of evidence suggest that there might be a potential causal link between Alzheimer's disease (AD) and a non-sufficient supply with vitamin D. However, the mechanisms linking AD to vitamin D have not been completely understood. The aim of our study is to elucidate the impact of 25(OH) vitamin D3 on amyloid precursor protein processing in mice and N2A cells utilizing very moderate and physiological vitamin D hypovitaminosis in the range of 20-30% compared to wild-type mice. We found that already under such mild conditions, amyloid-ß peptide (Aß) is significantly increased, which is caused by an increased ß-secretase activity and BACE1 protein level. Additionally, neprilysin (NEP) expression is downregulated resulting in a decreased NEP activity further enhancing the effect of decreased vitamin D on the Aß level. In line with the in vivo findings, corresponding effects were found with N2A cells supplemented with 25(OH) vitamin D3. Our results further strengthen the link between AD and vitamin D3 and suggest that supplementation of vitamin D3 might have a beneficial effect in AD prevention.

Effect of Different Phospholipids on α-Secretase Activity in the Non-Amyloidogenic Pathway of Alzheimer's Disease.

Alzheimer's disease (AD) is characterized by extracellular accumulation of amyloid-ß peptide (Aß), generated by proteolytic processing of the amyloid precursor protein (APP) by ß- and γ-secretase. Aß generation is inhibited when the initial ectodomain shedding is caused by α-secretase, cleaving APP within the Aß domain. Therefore, an increase in α-secretase activity is an attractive therapeutic target for AD treatment. APP and the APP-cleaving secretases are all transmembrane proteins, thus local membrane lipid composition is proposed to influence APP processing. Although several studies have focused on γ-secretase, the effect of the membrane lipid microenvironment on α-secretase is poorly understood. In the present study, we systematically investigated the effect of fatty acid (FA) acyl chain length (10:0, 12:0, 14:0, 16:0, 18:0, 20:0, 22:0, 24:0), membrane polar lipid headgroup (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine), saturation grade and the FA double-bond position on α-secretase activity. We found that α-secretase activity is significantly elevated in the presence of FAs with short chain length and in the presence of polyunsaturated FAs, whereas variations in the phospholipid headgroups, as well as the double-bond position, have little or no effect on α-secretase activity. Overall, our study shows that local lipid membrane composition can influence α-secretase activity and might have beneficial effects for AD.

Docosahexaenoic acid reduces amyloid beta production via multiple pleiotropic mechanisms.

Alzheimer disease is characterized by accumulation of the ß-amyloid peptide (Aß) generated by ß- and γ-secretase processing of the amyloid precursor protein (APP). The intake of the polyunsaturated fatty acid docosahexaenoic acid (DHA) has been associated with decreased amyloid deposition and a reduced risk in Alzheimer disease in several epidemiological trials; however, the exact underlying molecular mechanism remains to be elucidated. Here, we systematically investigate the effect of DHA on amyloidogenic and nonamyloidogenic APP processing and the potential cross-links to cholesterol metabolism in vivo and in vitro. DHA reduces amyloidogenic processing by decreasing ß- and γ-secretase activity, whereas the expression and protein levels of BACE1 and presenilin1 remain unchanged. In addition, DHA increases protein stability of α-secretase resulting in increased nonamyloidogenic processing. Besides the known effect of DHA to decrease cholesterol de novo synthesis, we found cholesterol distribution in plasma membrane to be altered. In the presence of DHA, cholesterol shifts from raft to non-raft domains, and this is accompanied by a shift in γ-secretase activity and presenilin1 protein levels. Taken together, DHA directs amyloidogenic processing of APP toward nonamyloidogenic processing, effectively reducing Aß release. DHA has a typical pleiotropic effect; DHA-mediated Aß reduction is not the consequence of a single major mechanism but is the result of combined multiple effects.

The role of APP proteolytic processing in lipid metabolism.

Amyloid plaques in brains are one of the major pathological hallmarks of Alzheimer's disease (AD). These plaques are mainly formed by aggregated Aß, generated by proteolytic cleavage of the amyloid precursor protein (APP). Therefore, APP processing and Aß production have been one of the central scopes in AD research in the past. Now, accumulating evidence suggests that besides its pathological impact, APP and its cleavage products also contribute to physiological functions. Proteolytic cleavage of APP is tightly regulated, and several lipids such as cholesterol and sphingolipids have been shown to influence APP processing and Aß generation. In turn, Aß as well as other APP cleavage products plays an essential role in regulating lipid homeostasis arguing for complex regulatory cycles in which lipids control APP processing and vice versa. This balanced regulation is disrupted under pathological conditions such as in AD. This article will review the physiological function of APP and its proteolytic products, especially Aß and AICD, in regulating lipid homeostasis and which lipid species modulate APP processing. Furthermore, we summarize the alterations in lipid metabolism observed in AD patients and AD mouse models.

Plasmalogens inhibit APP processing by directly affecting γ-secretase activity in Alzheimer's disease.

Lipids play an important role as risk or protective factors in Alzheimer's disease (AD). Previously it has been shown that plasmalogens, the major brain phospholipids, are altered in AD. However, it remained unclear whether plasmalogens themselves are able to modulate amyloid precursor protein (APP) processing or if the reduced plasmalogen level is a consequence of AD. Here we identify the plasmalogens which are altered in human AD postmortem brains and investigate their impact on APP processing resulting in Aß production. All tested plasmalogen species showed a reduction in γ-secretase activity whereas ß- and α-secretase activity mainly remained unchanged. Plasmalogens directly affected γ-secretase activity, protein and RNA level of the secretases were unaffected, pointing towards a direct influence of plasmalogens on γ-secretase activity. Plasmalogens were also able to decrease γ-secretase activity in human postmortem AD brains emphasizing the impact of plasmalogens in AD. In summary our findings show that decreased plasmalogen levels are not only a consequence of AD but that plasmalogens also decrease APP processing by directly affecting γ-secretase activity, resulting in a vicious cycle: Aß reduces plasmalogen levels and reduced plasmalogen levels directly increase γ-secretase activity leading to an even stronger production of Aß peptides.

Plasmalogen synthesis is regulated via alkyl-dihydroxyacetonephosphate-synthase by amyloid precursor protein processing and is affected in Alzheimer's disease.

Lipids play an important role as risk or protective factors in Alzheimer's disease, which is characterized by amyloid plaques composed of aggregated amyloid-beta. Plasmalogens are major brain lipids and controversially discussed to be altered in Alzheimer's disease (AD) and whether changes in plasmalogens are cause or consequence of AD pathology. Here, we reveal a new physiological function of the amyloid precursor protein (APP) in plasmalogen metabolism. The APP intracellular domain was found in vivo and in vitro to increase the expression of the alkyl-dihydroxyacetonephosphate-synthase (AGPS), a rate limiting enzyme in plasmalogen synthesis. Alterations in APP dependent changes of AGPS expression result in reduced protein and plasmalogen levels. Under the pathological situation of AD, increased amyloid-beta level lead to increased reactive oxidative species production, reduced AGPS protein and plasmalogen level. Accordingly, phosphatidylethanol plasmalogen was decreased in the frontal cortex of AD compared to age matched controls. Our findings elucidate that plasmalogens are decreased as a consequence of AD and regulated by APP processing under physiological conditions.

Special lipid-based diets alleviate cognitive deficits in the APPswe/PS1dE9 transgenic mouse model of Alzheimer's disease independent of brain amyloid deposition.

Dietary fish oil, providing n3 polyunsaturated fatty acids like eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), associates with reduced dementia risk in epidemiological studies and reduced amyloid accumulation in Alzheimer mouse models. We now studied whether additional nutrients can improve the efficacy of fish oil in alleviating cognitive deficits and amyloid pathology in APPswe/PS1dE9 transgenic and wild-type mice. We compared four isocaloric (5% fat) diets. The fish oil diet differed from the control diet only by substituted fish oil. Besides fish oil, the plant sterol diet was supplemented with phytosterols, while the Fortasyn diet contained as supplements precursors and cofactors for membrane synthesis, viz. uridine-monophosphate; DHA and EPA; choline; folate; vitamins B6, B12, C and E; phospholipids and selenium. Mice began the special diets at 5 months and were sacrificed at 14 months after behavioral testing. Transgenic mice, fed with control chow, showed poor spatial learning, hyperactivity in exploring a novel cage and reduced preference to explore novel odors. All fish-oil-containing diets increased exploration of a novel odor over a familiar one. Only the Fortasyn diet alleviated the spatial learning deficit. None of the diets influenced hyperactivity in a new environment. Fish-oil-containing diets strongly inhibited ß- and γ-secretase activity, and the plant sterol diet additionally reduced amyloid-ß 1-42 levels. These data indicate that beneficial effects of fish oil on cognition in Alzheimer model mice can be enhanced by adding other specific nutrients, but this effect is not necessarily mediated via reduction of amyloid accumulation.

Trans fatty acids enhance amyloidogenic processing of the Alzheimer amyloid precursor protein (APP).

Hydrogenation of oils and diary products of ruminant animals leads to an increasing amount of trans fatty acids in the human diet. Trans fatty acids are incorporated in several lipids and accumulate in the membrane of cells. Here we systematically investigate whether the regulated intramembrane proteolysis of the amyloid precursor protein (APP) is affected by trans fatty acids compared to the cis conformation. Our experiments clearly show that trans fatty acids compared to cis fatty acids increase amyloidogenic and decrease nonamyloidogenic processing of APP, resulting in an increased production of amyloid beta (Aß) peptides, main components of senile plaques, which are a characteristic neuropathological hallmark for Alzheimer's disease (AD). Moreover, our results show that oligomerization and aggregation of Aß are increased by trans fatty acids. The mechanisms identified by this in vitro study suggest that the intake of trans fatty acids potentially increases the AD risk or causes an earlier onset of the disease.

Amyloid precursor protein (APP) mediated regulation of ganglioside homeostasis linking Alzheimer's disease pathology with ganglioside metabolism.

Gangliosides are important players for controlling neuronal function and are directly involved in AD pathology. They are among the most potent stimulators of Aß production, are enriched in amyloid plaques and bind amyloid beta (Aß). However, the molecular mechanisms linking gangliosides with AD are unknown. Here we identified the previously unknown function of the amyloid precursor protein (APP), specifically its cleavage products Aß and the APP intracellular domain (AICD), of regulating GD3-synthase (GD3S). Since GD3S is the key enzyme converting a- to b-series gangliosides, it therefore plays a major role in controlling the levels of major brain gangliosides. This regulation occurs by two separate and additive mechanisms. The first mechanism directly targets the enzymatic activity of GD3S: Upon binding of Aß to the ganglioside GM3, the immediate substrate of the GD3S, enzymatic turnover of GM3 by GD3S was strongly reduced. The second mechanism targets GD3S expression. APP cleavage results, in addition to Aß release, in the release of AICD, a known candidate for gene transcriptional regulation. AICD strongly down regulated GD3S transcription and knock-in of an AICD deletion mutant of APP in vivo, or knock-down of Fe65 in neuroblastoma cells, was sufficient to abrogate normal GD3S functionality. Equally, knock-out of the presenilin genes, presenilin 1 and presenilin 2, essential for Aß and AICD production, or of APP itself, increased GD3S activity and expression and consequently resulted in a major shift of a- to b-series gangliosides. In addition to GD3S regulation by APP processing, gangliosides in turn altered APP cleavage. GM3 decreased, whereas the ganglioside GD3, the GD3S product, increased Aß production, resulting in a regulatory feedback cycle, directly linking ganglioside metabolism with APP processing and Aß generation. A central aspect of this homeostatic control is the reduction of GD3S activity via an Aß-GM3 complex and AICD-mediated repression of GD3S transcription.

Intracellular APP Domain Regulates Serine-Palmitoyl-CoA Transferase Expression and Is Affected in Alzheimer's Disease.

Lipids play an important role as risk or protective factors in Alzheimer's disease (AD), a disease biochemically characterized by the accumulation of amyloid beta peptides (Aß), released by proteolytic processing of the amyloid precursor protein (APP). Changes in sphingolipid metabolism have been associated to the development of AD. The key enzyme in sphingolipid de novo synthesis is serine-palmitoyl-CoA transferase (SPT). In the present study we identified a new physiological function of APP in sphingolipid synthesis. The APP intracellular domain (AICD) was found to decrease the expression of the SPT subunit SPTLC2, the catalytic subunit of the SPT heterodimer, resulting in that decreased SPT activity. AICD function was dependent on Fe65 and SPTLC2 levels are increased in APP knock-in mice missing a functional AICD domain. SPTLC2 levels are also increased in familial and sporadic AD postmortem brains, suggesting that SPT is involved in AD pathology.