Cholesterol and the biology of Alzheimer's disease

Recent results implicating cholesterol metabolism in the pathophysiology of Alzheimer's disease (AD) bring cholesterol to the forefront of AD research. Research from genetics, epidemiology, and cell biology all converge, suggesting that cholesterol plays a central role in the biology of amyloid precursor protein and the toxic peptide generated by its cleavage, beta-amyloid (Abeta). The ability of cholesterol to modulate Abeta production suggests opportunities for therapeutic intervention, although the functional significance underlying the connection between cholesterol and Abeta remains to be investigated.     

Commonalities between genetics of cardiovascular disease and neurodegenerative disorders

PURPOSE OF REVIEW: Numerous epidemiological and clinical data suggest that neurodegenerative disorders, such as Alzheimer's disease, may be related directly or indirectly to cardiovascular risk. Genetic studies have demonstrated that they share at least one common susceptibility gene, encoding apolipoprotein E, a modulator of cardiac risk and of cognitive impairment. Several studies have suggested that other genes involved in the development of cardiovascular diseases may be involved. Previous studies indicated that additional genes contribute to Alzheimer's disease, in particular to the sporadic, more common late-onset form. In this review, the authors focus on recent findings concerning the modulation of the risk of Alzheimer's disease by genes also involved in the development of cardiovascular diseases. RECENT FINDINGS: The intensive search conducted in the past year gave rise to many publications, more than half of which were related to genes common to cardiovascular and neurodegenerative disorders. The majority of the genes studied are involved in cholesterol metabolism, hypertension, lipid oxidation and detoxication, or inflammatory processes. SUMMARY: In the past year, approximately 100 studies concerning the genetics of Alzheimer's disease were published around the world. Results suggest that the risk of Alzheimer's disease is modulated by various genes encoding proteins involved in cholesterol metabolism, in the detoxication of lipoprotein oxidation or encoding cytokines.     

Apolipoprotein E and cholesterol metabolism in the pathogenesis and treatment of Alzheimer's disease

There is much evidence suggesting that there is a strong relationship between the deterioration of brain lipid homeostasis, vascular changes and the pathogenesis of Alzheimer's disease (AD). These associations include: (1). recognition that a key cholesterol transporter, apolipoprotein E type 4, acts a major genetic risk factor for both familial and sporadic AD; (2). epidemiological studies linking cardiovascular risk factors, such as hypertension and high plasma cholesterol, to dementia; (3). the discovery that small strokes can precipitate clinical dementia in cognitively normal elderly subjects; (4). the modulation of degradation of the amyloid precursor protein by cholesterol administration in cell culture and in animal models of beta-amyloid overproduction; and (5). the beneficial effect of cholesterol-lowering drugs, such as Probucol and statins, in combating common AD. The recent finding that there is a genetic association between the HMGR gene locus and sporadic AD further suggests that brain cholesterol metabolism is central to AD pathophysiology, and a potential therapeutic target for disease stabilization and primary disease prevention.     

The cholesterol transporter ABCG1 modulates the subcellular distribution and proteolytic processing of beta-amyloid precursor protein

Although intracellular cholesterol levels are known to influence the proteolysis of beta-amyloid precursor protein (APP), the effect of specific genes that regulate cholesterol metabolism on APP processing remains poorly understood. The cholesterol transporter ABCG1 facilitates cholesterol efflux to HDL and is expressed in brain. Notably, the human ABCG1 gene maps to chromosome 21q22.3, and individuals with Down syndrome (DS) typically manifest with Alzheimer's disease (AD) neuropathology in their 30s. Here, we demonstrate that expression of ABCG1 enhances amyloid-beta protein (Abeta) production in transfected HEK cells in a manner that requires functional cholesterol transporter activity. ABCG1-expressing cells also exhibit increased secreted APP (sAPP)alpha and sAPPbeta secretion and display increased cell surface-associated APP. These results suggest that ABCG1 increases the availability of APP as a secretase substrate for both the amyloidogenic and nonamyloidogenic pathways. In vivo, ABCG1 mRNA levels are 2-fold more abundant in DS brain compared with age- and sex-matched normal controls. Finally, both Abeta and sAPPalpha levels are increased in DS cortex relative to normal controls. These findings suggest that altered cholesterol metabolism and APP trafficking mediated by ABCG1 may contribute to the accelerated onset of AD neuropathology in DS.     

Oxidation of cholesterol by amyloid precursor protein and beta-amyloid peptide

Alzheimer's disease (AD) is characterized by accumulation of the neurotoxic peptide beta-amyloid, which is produced by proteolysis of amyloid precursor protein (APP). APP is a large membrane-bound copper-binding protein that is essential in maintaining synaptic function and may play a role in synaptogenesis. beta-Amyloid has been shown to contribute to the oxidative stress that accompanies AD. Later stages of AD are characterized by neuronal apoptosis. However, the biochemical function of APP and the mechanism of the toxicity of beta-amyloid are still unclear. In this study, we show that both beta-amyloid and APP can oxidize cholesterol to form 7beta-hydroxycholesterol, a proapoptotic oxysterol that was neurotoxic at nanomolar concentrations. 7beta-Hydroxycholesterol inhibited secretion of soluble APP from cultured rat hippocampal H19-7/IGF-IR neuronal cells and inhibited tumor necrosis factor-alpha-converting enzyme alpha-secretase activity but had no effect on beta-site APP-cleaving enzyme 1 activity. 7beta-Hydroxycholesterol was also a potent inhibitor of alpha-protein kinase C, with a K(i) of approximately 0.2 nm. The rate of reaction between cholesterol and beta-amyloid was comparable to the rates of cholesterol-metabolizing enzymes (k(cat) = 0.211 min(-)1). The rate of production of 7beta-hydroxycholesterol by APP was approximately 200 times lower than by beta-amyloid. Oxidation of cholesterol was accompanied by stoichiometric production of hydrogen peroxide and required divalent copper. The results suggest that a function of APP may be to produce low levels of 7-hydroxycholesterol. Higher levels produced by beta-amyloid could contribute to the oxidative stress and cell loss observed in Alzheimer's disease.     

Compartmentalization of beta-secretase (Asp2) into low-buoyant density, noncaveolar lipid rafts

Recent epidemiological studies show a reduced prevalence of Alzheimer's disease (AD) in patients treated with inhibitors of cholesterol biosynthesis. Moreover, the cholesterol-transport protein, apolipoprotein E4, and elevated cholesterol are important risk factors for AD. Additionally, in vitro and in vivo studies show that intracellular cholesterol levels can modulate the processing of amyloid precursor protein (APP) to beta-amyloid, the major constituent of senile plaques. Cholesterol plays a crucial role in maintaining lipid rafts in a functional state. Lipid rafts are cholesterol-enriched membrane microdomains implicated in signal transduction, protein trafficking, and proteolytic processing. Since APP, beta-amyloid, and the putative gamma-secretase, presenilin-1 (PS-1), have all been found in lipid rafts, we hypothesized that the recently identified beta-secretase, Asp2 (BACE1), might also be present in rafts. Here, we report that recombinant Asp2 expressed in three distinct cell lines is raft associated. Using both detergent and nondetergent methods, Asp2 protein and activity were found in a light membrane raft fraction that also contained other components of the amyloidogenic pathway. Immunoisolation of caveolin-containing vesicles indicated that Asp2 was present in a unique raft population distinct from caveolae. Finally, depletion of raft cholesterol abrogated association of Asp2 with the light membrane fraction. These observations are consistent with the raft localization of APP processing and suggest that the partitioning of Asp2 into lipid rafts may underlie the cholesterol sensitivity of beta-amyloid production.     

The conflicting role of brain cholesterol in Alzheimer's disease: lessons from the brain plasminogen system

Retrospective clinical studies indicate that individuals chronically treated with cholesterol synthesis inhibitors, statins, are at lower risk of developing AD (Alzheimer's disease). Moreover, treatment of guinea pigs with high doses of simvastatin or drastic reduction of cholesterol in cultured cells decrease Abeta (beta-amyloid peptide) production. These data sustain the concept that high brain cholesterol is responsible for Abeta accumulation in AD, providing the scientific support for the proposed use of statins to prevent this disease. However, a number of unresolved issues raise doubts that high brain cholesterol is to blame. First, it has not been shown that higher neuronal cholesterol increases Abeta production. Secondly, it has not been demonstrated that neurons in AD have more cholesterol than control neurons. On the contrary, the brains of AD patients show a specific down-regulation of seladin-1, a protein involved in cholesterol synthesis, and low membrane cholesterol was observed in hippocampal membranes of ApoE4 (apolipoprotein E4) AD cases. This effect was also evidenced by altered cholesterol-rich membrane domains (rafts) and raft-mediated functions, such as diminished generation of the Abeta-degrading enzyme plasmin. Thirdly, numerous genetic defects that cause neurodegeneration are due to defective cholesterol metabolism. Fourthly, in female mice, the most brain-permeant statin induces neurodegeneration and high amyloid production. Altogether, this evidence makes it difficult to accept that statins are beneficial through acting as brain cholesterol-synthesis inhibitors. It appears more likely that their advantageous role arises from improved brain oxygenation.     

Cholesterol metabolism, apolipoprotein E, adenosine triphosphate-binding cassette transporters, and Alzheimer's disease

PURPOSE OF REVIEW: Recent evidence suggests that cholesterol metabolism participates in the pathogenesis of Alzheimer's disease. Apolipoprotein E is the main lipid carrier in the brain and the best-established risk factor for late-onset Alzheimer's disease. Intracellular cholesterol levels influence the generation of amyloid-beta peptides, the toxic species thought to be a primary cause of Alzheimer's disease. Finally, compounds that modulate cholesterol metabolism affect amyloid-beta generation. This review summarizes data linking apolipoprotein E and adenosine triphosphate-binding cassette transporters to aspects of cholesterol metabolism and Alzheimer's disease pathogenesis. RECENT FINDINGS: In vivo, the lipidation status of apolipoprotein E affects amyloid-beta burden in mice with Alzheimer's disease, which appears to caused by the modulation of amyloid-beta deposition or clearance rather than amyloid-beta production. State-of-the-art in-vivo assays reveal that amyloid-beta is cleared from the brain by multiple pathways. Members of the adenosine triphosphate-binding cassette superfamily of transporters regulate lipid homeostasis and apolipoprotein metabolism in the brain, and may affect Alzheimer's disease pathogenesis by modulating apolipoprotein E lipidation as well as intracellular sterol homeostasis. SUMMARY: Proteins involved in brain cholesterol metabolism may affect the pathogenesis of Alzheimer's disease. Compounds that modulate the expression of these proteins may be of therapeutic benefit in Alzheimer's disease.     

Beta-amyloid (Abeta40, Abeta42) binding to modified LDL accelerates macrophage foam cell formation

Apart from its role as a risk factor in arteriosclerosis, plasma cholesterol is increasingly recognized to play a major role in the pathogenesis of Alzheimer's disease (AD). Moreover, alterations of intracellular cholesterol metabolism in neuronal and vascular cells are of considerable importance for the understanding of AD. Cellular cholesterol accumulation enhances the deposition of insoluble beta-amyloid peptides, which is considered a hallmark in the pathogenesis of AD. In order to test the hypothesis, whether exogenous beta-amyloid peptides (Abeta42, Abeta40) might contribute to cellular cholesterol accumulation by opsonization of lipoproteins, we compared the binding and uptake of native LDL, enzymatically modified LDL (E-LDL), copper oxidized LDL (Ox-LDL) and HDL as control, preincubated either in the absence or presence of Abeta42 or Abeta40, by human monocytes or monocyte-derived macrophages. Incubation of monocytes and macrophages with Abeta-lipoprotein-complexes lead to increased cellular free and esterified cholesterol when compared to non-opsonized lipoproteins, except for HDL. Furthermore, the cellular uptake of these complexes regulated Abeta-receptors such as FPRL-1 or LRP/CD91. In summary, our results suggest that Abeta42 and Abeta40 act as potent opsonins for LDL, E-LDL and Ox-LDL and enhance cellular cholesterol accumulation as well as Abeta-deposition in vessel wall macrophages.     

Can statin therapy really reduce the risk of Alzheimer's disease and slow its progression?

PURPOSE OF REVIEW: Statins are the most used cholesterol-lowering agents worldwide. Earlier studies suggested that they may have preventive effects in Alzheimer's disease. However, prospective studies have questioned this hypothesis. RECENT FINDINGS: Statins regulate beta-amyloid metabolism and microglial activation. Pathologically, patients with Alzheimer's disease have more severe atherosclerosis in cerebral arteries than do controls. Such lesions may cause cerebral hypoperfusion, a risk factor for dementia and cognitive decline. Although most population-based studies have failed to show a beneficial effect of statins in Alzheimer's disease, two randomized controlled trials suggested that statins slow cognitive decline in mild to moderate Alzheimer's disease. SUMMARY: There is still some hope that statins reduce the incidence of Alzheimer's disease and slow its progression. Large-scale randomized controlled trials of simvastatin and atorvastatin for mild to moderate Alzheimer's disease are underway, which might provide more conclusive results than earlier studies.     

The choline-leakage hypothesis for the loss of acetylcholine in Alzheimer''s disease.

We present a hypothesis for the loss of acetylcholine in Alzheimer's disease that is based on two recent experimental results: that beta-amyloid causes leakage of choline across cell membranes and that decreased production of acetylcholine increases the production of beta-amyloid. According to the hypothesis, an increase in beta-amyloid concentration caused by proteolysis of the amyloid precursor protein results in an increase in the leakage of choline out of cells. This leads to a reduction in intracellular choline concentration and hence a reduction in acetylcholine production. The reduction in acetylcholine production, in turn, causes an increase in the concentration of beta-amyloid. The resultant positive feedback between decreased acetylcholine and increased beta-amyloid accelerates the loss of acetylcholine. We compare the predictions of the choline-leakage hypothesis with a number of experimental observations. We also approximate it with a pair of ordinary differential equations. The solutions of these equations indicate that the loss of acetylcholine is very sensitive to the initial rate of beta-amyloid production.     

Cholesterol changes in Alzheimer's disease: methods of analysis and impact on the formation of enlarged endosomes

An increasing number of results implicating cholesterol metabolism in the pathophysiology of Alzheimer's disease (AD) suggest cholesterol as a target for treatment. Research in genetics, pathology, epidemiology, biochemistry, and cell biology, as well as in animal models, suggests that cholesterol, its transporter in the brain, apolipoprotein E, amyloid precursor protein, and amyloid-β all interact in AD pathogenesis. Surprisingly, key questions remain unanswered due to the lack of sensitive and specific methods for assessing cholesterol levels in the brain at subcellular resolution. The aims of this review are not only to discuss the various methods for measuring cholesterol and its metabolites and to catalog results obtained from AD patients but also to discuss some new data linking high plasma membrane cholesterol with modifications of the endocytic compartments. These studies are particularly relevant to AD pathology, since enlarged endosomes are believed to be the first morphological change observed in AD brains, in both sporadic cases and Down syndrome.     

Amyloid beta as a regulator of lipid homeostasis

The beta-amyloid peptide (A beta) is widely considered to be the molecule that causes Alzheimer's disease (AD). Besides this pathological function of A beta, recently published data reveal that A beta also has an essential physiological role in lipid homeostasis. Cholesterol increases A beta production, and conversely A beta production causes a decrease in cholesterol synthesis. The latter appears to be mediated by the inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR), a key enzyme in cholesterol synthesis, in an action similar to that of statins. Moreover, A beta regulates sphingolipid metabolism by directly activating sphingomyelinases (SMases). This review summarizes the molecular basis for the known physiological functions of A beta and amyloid precursor protein (APP), the roles of A beta and APP in lipid homeostasis and the medical implications of addressing lipid homeostasis in respect to AD. This knowledge might provide new insights for current and future therapeutic approaches to AD.     

Statin pharmacogenomics: what have we learned, and what remains unanswered?

PURPOSE OF REVIEW: Statins are widely prescribed and are established as first-line therapy for the primary and secondary prevention of coronary heart disease. Response to treatment varies considerably from person to person; however, inherited traits (genetic variability) may play a central role in this inter-individual variation. The purpose of this review is to summarize recent progress in the research for exploring genetic determinants of clinical efficacy and safety of statin therapy. RECENT FINDINGS: In addition to 41 previous studies of 19 genes, the results of 17 pharmacogenomic studies investigating the relationship between common genetic variants and response to statin therapy in terms of lipid responses, clinical outcomes, and adverse events have been reported since January 2004 - 15 candidate genes related to pharmacodynamics and three to pharmacokinetics of statins. These reported data suggest that genetic variations influencing intestinal cholesterol absorption, cholesterol production, and lipoprotein catabolism may all play a role in modulating responsiveness, as well as genes involved in drug metabolism of statins. They also suggest that combined analysis of multiple variants in several genes, all of which have possible functional relations, is more likely to give significant results, especially when being performed with a larger number of participants. SUMMARY: Pharmacogenomic studies of statin therapy will provide a better picture as to who is most likely and least likely to benefit from treatment, which results in more individualized management of coronary artery disease.     

Inhibition of glycosphingolipid biosynthesis reduces secretion of the beta-amyloid precursor protein and amyloid beta-peptide

Alzheimer disease is associated with extracellular deposits of amyloid beta-peptides in the brain. Amyloid beta-peptides are generated by proteolytic processing of the beta-amyloid precursor protein by beta- and gamma-secretases. The cleavage by secretases occurs predominantly in post-Golgi secretory and endocytic compartments and is influenced by cholesterol, indicating a role of the membrane lipid composition in proteolytic processing of the beta-amyloid precursor protein. To analyze the role of glycosphingolipids in these processes we inhibited glycosyl ceramide synthase, which catalyzes the first step in glycosphingolipid biosynthesis. The depletion of glycosphingolipids markedly reduced the secretion of endogenous beta-amyloid precursor protein in different cell types, including human neuroblastoma SH-SY5Y cells. Importantly, secretion of amyloid beta-peptides was also strongly decreased by inhibition of glycosphingolipid biosynthesis. Conversely, the addition of exogenous brain gangliosides to cultured cells reversed these effects. Biochemical and cell biological experiments demonstrate that the pharmacological reduction of cellular glycosphingolipid levels inhibited maturation and cell surface transport of the beta-amyloid precursor protein. In the glycosphingolipid-deficient cell line GM95, cellular levels and maturation of beta-amyloid precursor protein were also significantly reduced as compared with normal B16 cells. Together, these data demonstrate that glycosphingolipids are implicated in the regulation of the subcellular transport of the beta-amyloid precursor protein in the secretory pathway and its proteolytic processing. Thus, enzymes involved in glycosphingolipid metabolism might represent targets to inhibit the production of amyloid beta-peptides.     

beta -Amyloid peptide-induced apoptosis regulated by a novel protein containing a g protein activation module

Degeneration of neurons in Alzheimer's disease is mediated by beta-amyloid peptide by diverse mechanisms, which include a putative apoptotic component stimulated by unidentified signaling events. This report describes a novel beta-amyloid peptide-binding protein (denoted BBP) containing a G protein-coupling module. BBP is one member of a family of three proteins containing this conserved structure. The BBP subtype bound human beta-amyloid peptide in vitro with high affinity and specificity. Expression of BBP in cell culture induced caspase-dependent vulnerability to beta-amyloid peptide toxicity. Expression of a signaling-deficient dominant negative BBP mutant suppressed sensitivity of human Ntera-2 neurons to beta-amyloid peptide mediated toxicity. These findings suggest that BBP is a target of neurotoxic beta-amyloid peptide and provide new insight into the molecular pathophysiology of Alzheimer's disease.     

Chronic exposure to U18666A is associated with oxidative stress in cultured murine cortical neurons

Findings that antioxidant treatment may be beneficial in Alzheimer's disease indicate that oxidative stress is an important factor in its pathogenesis. Studies have also suggested that cholesterol imbalance in the brain might be related to the development of neurological disorders. Previously, we have reported that U18666A, a cholesterol transport-inhibiting agent, leads to apoptosis and intracellular cholesterol accumulation in primary cortical neurons. In this study, we found that neuronal apoptosis mediated by U18666A is associated with oxidative stress in the treated cortical neurons. Cortical neurons treated with U18666A also showed decreased secretion and increased intraneuronal accumulation of beta-amyloid. The association of neuronal apoptosis with oxidative stress and Abeta accumulation may provide clues to the pathogenesis of Alzheimer's disease, as well as the role oxidative stress plays in other neurodegenerative diseases.     

ADF/cofilin and actin dynamics in disease

ADF/cofilins are key regulators of actin dynamics in normal cells. Recent findings suggest that, under cellular stress, the wild-type proteins might form complexes with actin that can alter cell function. Owing to their rapid formation, these complexes might initiate or aid in the progression of diseases as diverse as Alzheimer's disease and ischemic kidney disease. Although evidence for their involvement in diseases other than Alzheimer's and ischemic kidney disease is tenuous, recent studies suggest that altered production, regulation or localization of these proteins might lead to cognitive impairment, inflammation, infertility, immune deficiencies and other pathophysiological defects.