Cholesterol, a modulator of membrane-associated Abeta-fibrillogenesis

The intriguing dose-dependent effect of selected amphiphilic compounds on insulin amyloid aggregation: Focus on a cholesterol-based detergent, Chobimalt

The amyloidogenic self-assembly of many peptides and proteins largely depends on external conditions. Among amyloid-prone proteins, insulin attracts attention because of its physiological and therapeutic importance. In the present work, the amyloid aggregation of insulin is studied in the presence of cholesterol-based detergent, Chobimalt. The strategy to elucidate the Chobimalt-induced effect on insulin fibrillogenesis is based on performing the concentration- and time-dependent analysis using a combination of different experimental techniques, such as ThT fluorescence assay, CD, AFM, SANS, and SAXS. While at the lowest Chobimalt concentration (0.1 µM; insulin to Chobimalt molar ratio of 1:0.004) the formation of insulin fibrils was not affected, the gradual increase of Chobimalt concentration (up to 100 µM; molar ratio of 1:4) led to a significant increase in ThT fluorescence, and the maximal ThT fluorescence was 3-4-fold higher than the control insulin fibril’s ThT fluorescence intensity. Kinetic studies confirm the dose-dependent experimental results. Depending on the concentration of Chobimalt, either (i) no effect is observed, or (ii) significantly, ∼10-times prolonged lag-phases accompanied by the substantial, ∼ 3-fold higher relative ThT fluorescence intensities at the steady-state phase are recorded. In addition, at certain concentrations of Chobimalt, changes in the elongation-phase are noticed. An increase in the Chobimalt concentrations also triggers the formation of insulin fibrils with sharply altered morphological appearance. The fibrils appear to be more flexible and wavy-like with a tendency to form circles. SANS and SAXS data also revealed the morphology changes of amyloid fibrils in the presence of Chobimalt. Amyloid aggregation requires the formation of unfolded intermediates, which subsequently generate amyloidogenic nuclei. We hypothesize that the different morphology of the formed insulin fibrils is the result of the gradual binding of Chobimalt to different binding sites on unfolded insulin. A similar explanation and the existence of such binding sites with different binding energies was shown previously for the nonionic detergent. Thus, the data also emphasize the importance of a protein partially-unfolded state which undergoes the process of fibrils formation; i.e., certain experimental conditions or the presence of additives may dramatically change not only kinetics but also the morphology of fibrillar aggregates.

Hypothyroidism and Diabetes-Related Dementia: Focused on Neuronal Dysfunction, Insulin Resistance, and Dyslipidemia

The incidence of dementia is steadily increasing worldwide. The risk factors for dementia are diverse, and include genetic background, environmental factors, sex differences, and vascular abnormalities. Among the subtypes of dementia, diabetes-related dementia is emerging as a complex type of dementia related to metabolic imbalance, due to the increase in the number of patients with metabolic syndrome and dementia worldwide. Thyroid hormones are considered metabolic regulatory hormones and affect various diseases, such as liver failure, obesity, and dementia. Thyroid dysregulation affects various cellular mechanisms and is linked to multiple disease pathologies. In particular, hypothyroidism is considered a critical cause for various neurological problems-such as metabolic disease, depressive symptoms, and dementia-in the central nervous system. Recent studies have demonstrated the relationship between hypothyroidism and brain insulin resistance and dyslipidemia, leading to diabetes-related dementia. Therefore, we reviewed the relationship between hypothyroidism and diabetes-related dementia, with a focus on major features of diabetes-related dementia such as insulin resistance, neuronal dysfunction, and dyslipidemia.

Repositioning medication for cardiovascular and cerebrovascular disease to delay the onset and prevent progression of Alzheimer's disease

Alzheimer's disease (AD) is a complex, progressive, neurodegenerative disorder. As with other common chronic diseases, multiple risk factors contribute to the onset and progression of AD. Many researchers have evaluated the epidemiologic and pathophysiological association between AD, cardiovascular diseases (CVDs), and cerebrovascular diseases (CBVDs), including commonly reported risk factors such as diabetes, hypertension, and dyslipidemia. Relevant therapies of CVDs/CBVDs for the attenuation of AD have also been empirically investigated. Considering the challenges of new drug development, in terms of cost and time, multifactorial approaches such as therapeutic repositioning of CVD/CBVD medication should be explored to delay the onset and progression of AD. Thus, in this review, we discuss our current understanding of the association between cardiovascular risk factors and AD, as revealed by clinical and non-clinical studies, as well as the therapeutic implications of CVD/CBVD medication that may attenuate AD. Furthermore, we discuss future directions by evaluating ongoing trials in the field.

Dyslipidemia and the risk of Alzheimer's disease

Whether cholesterol is implicated in the pathogenesis of Alzheimer's disease (AD) is still controversial. Several studies that explored the association between lipids and/or lipid-lowering treatment and AD indicate a harmful effect of dyslipidemia on AD risk. The findings are supported by genetic linkage and association studies that have clearly identified several genes involved in cholesterol metabolism or transport as AD susceptibility genes, including apolipoprotein E (APOE), apolipoprotein J (APOJ, CLU), ATP-binding cassette subfamily A member 7(ABCA7), and sortilin-related receptor (SORL1). Functional cell biology studies further support a critical involvement of lipid raft cholesterol in the modulation of Aβ precursor protein processing by β-secretase and γ-secretase resulting in altered Aβ production. However, conflicting evidence comes from epidemiological studies showing no or controversial association between dyslipidemia and AD risk, randomized clinical trials observing no beneficial effect of statin therapy, and cell biology studies suggesting that there is little exchange between circulating and brain cholesterol, that increased membrane cholesterol level is protective by inhibiting loss of membrane integrity through amyloid cytotoxicity, and that cellular cholesterol inhibits colocalization of β-secretase 1 and Aβ precursor protein in nonraft membrane domains, thereby increasing generation of plasmin, an Aβ-degrading enzyme. The aim of this article is to provide a comprehensive review of the findings of epidemiological, genetic, and cell biology studies aiming to elucidate the role of cholesterol in the pathogenesis of AD.

Dyslipidemia and dementia: current epidemiology, genetic evidence, and mechanisms behind the associations

The role of cholesterol in the etiology of Alzheimer's disease (AD) is still controversial. Some studies exploring the association between lipids and/or lipid lowering treatment and AD indicate a harmful effect of dyslipidemia and a beneficial effect of statin therapy on AD risk. The findings are supported by genetic linkage and association studies that have clearly identified several genes involved in cholesterol metabolism or transport as AD susceptibility genes, including apolipoprotein E, apolipoprotein J, and the sortilin-related receptor. Functional cell biology studies support a critical involvement of lipid raft cholesterol in the modulation of amyloid-β protein precursor (AβPP) processing by β- and γ-secretase resulting in altered amyloid-β production. Contradictory evidence comes from epidemiological studies showing no or controversial association between dyslipidemia and AD risk. Additionally, cell biology studies suggest that there is little exchange between circulating and brain cholesterol, that increased membrane cholesterol is protective by inhibiting loss of membrane integrity through amyloid cytotoxicity, and that cellular cholesterol inhibits co-localization of BACE1 and AβPP in non-raft membrane domains, thereby increasing generation of plasmin, an amyloid-β-degrading enzyme. The aim of this review is to summarize the findings of epidemiological and cell biological studies to elucidate the role of cholesterol in AD etiology.

Interaction of hIAPP with model raft membranes and pancreatic beta-cells: cytotoxicity of hIAPP oligomers

Type II diabetes mellitus (T2DM) is associated with beta-cell failure, which correlates with the formation of pancreatic islet amyloid deposits. The human islet amyloid polypeptide (hIAPP) is the major component of islet amyloid and undergoes structural changes followed by self-association and pathological tissue deposition during aggregation in T2DM. There is clear evidence that the aggregation process is accelerated in the presence of particular lipid membranes. Whereas hIAPP aggregation has been extensively studied in homogeneous model membrane systems, especially negatively charged lipid bilayers, information on the interaction of hIAPP with heterogeneous model raft membranes has been missing until now. In the present study, we focus on the principles of aggregation and amyloid formation of hIAPP in the presence of model raft membranes. Time-lapse tapping mode AFM and confocal fluorescence microscopy experiments followed membrane permeabilization and localization of hIAPP in the raft membrane. Together with the ThT and WST-1 assay, the data revealed elevated cytotoxicity of hIAPP oligomers on INS-1E cells.

The effects of cholesterol on learning and memory

Cholesterol is vital to normal brain function including learning and memory but that involvement is as complex as the synthesis, metabolism and excretion of cholesterol itself. Dietary cholesterol influences learning tasks from water maze to fear conditioning even though cholesterol does not cross the blood brain barrier. Excess cholesterol has many consequences including peripheral pathology that can signal brain via cholesterol metabolites, pro-inflammatory mediators and antioxidant processes. Manipulations of cholesterol within the central nervous system through genetic, pharmacological, or metabolic means circumvent the blood brain barrier and affect learning and memory but often in animals already otherwise compromised. The human literature is no less complex. Cholesterol reduction using statins improves memory in some cases but not others. There is also controversy over statin use to alleviate memory problems in Alzheimer's disease. Correlations of cholesterol and cognitive function are mixed and association studies find some genetic polymorphisms are related to cognitive function but others are not. In sum, the field is in flux with a number of seemingly contradictory results and many complexities. Nevertheless, understanding cholesterol effects on learning and memory is too important to ignore.

Plasma lipoprotein beta-amyloid in subjects with Alzheimer's disease or mild cognitive impairment

BACKGROUND

Plasma amyloid beta-peptide (Abeta) can compromise the blood-brain barrier, contributing to cerebrovascular alterations and amyloid angiopathy in Alzheimer's disease (AD). The objectives of this study were to investigate the distribution of lipoprotein-bound plasma-Abeta isoforms.

METHODS

This involved a case-control study of subjects with AD or amnestic mild cognitive impairment (MCI) versus controls. Lipoprotein Abeta distribution was determined in fasted plasma. For assessment of chylomicron homeostasis in the postabsorptive state, subjects were bled 4 h after a low-fat meal. The main outcome measures were plasma lipoprotein Abeta isoform distribution and lipid homeostasis.

RESULTS

We found the majority of plasma Abeta to be associated with triglyceride-rich lipoproteins (TRLs) encompassing chylomicrons, VLDL and IDL. For all lipoprotein groups, Abeta1-40 was the predominant isoform, accounting for approximately 50% of the total. Thereafter, equivalent amounts of the isoforms 1-42, 2-40, 1-38, 1-37 and 1-39 were found. Abeta1-37, Abeta1-38 and Abeta2-40 isoforms were significantly enriched within the TRL fraction of AD/MCI subjects and similar trends were observed for isoforms Abeta1-39, Abeta1-40 and Abeta1-42. Lipoprotein-Abeta was inversely associated with plasma total- and LDL cholesterol. AD/MCI subjects were not dyslipidaemic, however, there was evidence of accumulation of chylomicrons in the postabsorptive state.

CONCLUSIONS

Our data show that Abeta was found to be associated with plasma lipoproteins, especially those enriched with triglyceride. We find that Abeta may be increased in normolipidaemic AD subjects, commensurate with possible disturbances in postprandial lipoprotein homeostasis.