Impaired astrocytic synaptic function by peripheral cholesterol metabolite 27-hydroxycholesterol

Astrocytes represent the most abundant cell type in the brain, where they play critical roles in synaptic transmission, cognition, and behavior. Recent discoveries show astrocytes are involved in synaptic dysfunction during Alzheimer's disease (AD). AD patients have imbalanced cholesterol metabolism, demonstrated by high levels of side-chain oxidized cholesterol known as 27-hydroxycholesterol (27-OH). Evidence from our laboratory has shown that elevated 27-OH can abolish synaptic connectivity during neuromaturation, but its effect on astrocyte function is currently unclear. Our results suggest that elevated 27-OH decreases the astrocyte function in vivo in Cyp27Tg, a mouse model of brain oxysterol imbalance. Here, we report a downregulation of glutamate transporters in the hippocampus of CYP27Tg mice together with increased GFAP. GLT-1 downregulation was also observed when WT mice were fed with high-cholesterol diets. To study the relationship between astrocytes and neurons, we have developed a 3D co-culture system that allows all the cell types from mice embryos to differentiate in vitro. We report that our 3D co-cultures reproduce the effects of 27-OH observed in 2D neurons and in vivo. Moreover, we found novel degenerative effects in astrocytes that do not appear in 2D cultures, together with the downregulation of glutamate transporters GLT-1 and GLAST. We propose that this transporter dysregulation leads to neuronal hyperexcitability and synaptic dysfunction based on the effects of 27-OH on astrocytes. Taken together, these results report a new mechanism linking oxysterol imbalance in the brain and synaptic dysfunction through effects on astrocyte function.

Cholesterol Metabolism in Neurodegenerative Diseases: Molecular Mechanisms and Therapeutic Targets

Cholesterol is an indispensable component of the cell membrane and plays vital roles in critical physiological processes. Brain cholesterol accounts for a large portion of total cholesterol in the human body, and its content must be tightly regulated to ensure normal brain function. Disorders of cholesterol metabolism in the brain are linked to neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and other atypical cognitive deficits that arise at old age. However, the specific role of cholesterol metabolism disorder in the pathogenesis of neurodegenerative diseases has not been fully elucidated. Statins that are a class of lipid-lowering drugs have been reported to have a positive effect on neurodegenerative diseases. Herein, we reviewed the physiological and pathological conditions of cholesterol metabolism and discussed the possible mechanisms of cholesterol metabolism and statin therapy in neurodegenerative diseases.

The relationship between blood lipids and plasma amyloid beta is depend on blood pressure: a population-based cross-sectional study

Background

It is believed that deposition of amyloid beta (Aβ) in the brain is the central pathological changes of Alzheimer’s disease (AD), which triggers a series of pathological processes. However, the relationship between dyslipidemia and AD is uncertain. Considering the peripheral Aβ levels are related to brain Aβ deposition, we explore the relationships between blood lipids and plasma Aβ.

Methods

Participants who lived in the selected village of Xi’an for more than 3 years were enrolled, aged 40–85 years (n = 1282, 37.9% male). Fasting blood lipid, plasma Aβ levels, basic information and living habits were measured. Multiple linear regressions were used.

Results

In total population, blood lipids were not associated with plasma Aβ. After stratified by blood pressure, serum total cholesterol (TC) and low-density lipoprotein (LDL-c) were positively associated with plasma Aβ₄₂ levels (β_TC = 0.666, P_TC = 0.024; β_LDL-c = 0.743, P_LDL-c = 0.011, respectively) in normal blood pressure. LDL-c was negatively associated with plasma Aβ₄₀ levels (β = − 0.986, P = 0.037) in high blood pressure.

Conclusion

Elevated plasma Aβ₄₂ levels are associated with higher TC and LDL-c in normal blood pressure. Elevated plasma Aβ₄₀ levels are associated with lower LDL-c in high blood pressure. This indicated that the relationships between blood lipids and plasma Aβ were confounded by blood pressure.

The etiological effect of a new low-frequency ESR1 variant on Mild Cognitive Impairment and Alzheimer's Disease: a population-based study

Latent genetic variations of cholesterol metabolism-related genes in late-onset Alzheimer’s disease, especially, as well as in mild cognitive impairment pathogenesis are still to be studied extensively. Thus, we performed the targeted-sequencing of 12 nuclear receptor genes plus APOE which were involved in cholesterol content modulation to screen susceptible genetic variants and focused on a new risk variant ESR1 rs9340803 at 6q25.1 for both late-onset Alzheimer’s disease (OR=3.30[1.84~4.22], p<0.001) and mild cognitive impairment (OR=3.08[1.75~3.89], p<0.001). This low-frequency variant was validated in three independent cohorts totaling 854 late-onset Alzheimer’s disease cases, 1059 mild cognitive impairment cases and 1254 controls from nine provinces of China mainland. Preliminary functional study on it revealed decreased ESR1 expression in vitro. Besides, we detected higher serum Aβ1-40 concentration in participants carrying this variant (p=0.038) and lower plasma total cholesterol level in this variant carriers with late-onset Alzheimer’s disease (p=0.009). In summary, we identified a susceptible variant which might contribute to developing mild cognitive impairment at earlier stage and Alzheimer’s Disease later. Our study would provide new insight into the disease causation of late-onset Alzheimer’s disease and could be exploited therapeutically.

Current and emerging avenues for Alzheimer's disease drug targets

Alzheimer's disease (AD), the most frequent cause of dementia, is escalating as a global epidemic, and so far, there is neither cure nor treatment to alter its progression. The most important feature of the disease is neuronal death and loss of cognitive functions, caused probably from several pathological processes in the brain. The main neuropathological features of AD are widely described as amyloid beta (Aβ) plaques and neurofibrillary tangles of the aggregated protein tau, which contribute to the disease. Nevertheless, AD brains suffer from a variety of alterations in function, such as energy metabolism, inflammation and synaptic activity. The latest decades have seen an explosion of genes and molecules that can be employed as targets aiming to improve brain physiology, which can result in preventive strategies for AD. Moreover, therapeutics using these targets can help AD brains to sustain function during the development of AD pathology. Here, we review broadly recent information for potential targets that can modify AD through diverse pharmacological and nonpharmacological approaches including gene therapy. We propose that AD could be tackled not only using combination therapies including Aβ and tau, but also considering insulin and cholesterol metabolism, vascular function, synaptic plasticity, epigenetics, neurovascular junction and blood-brain barrier targets that have been studied recently. We also make a case for the role of gut microbiota in AD. Our hope is to promote the continuing research of diverse targets affecting AD and promote diverse targeting as a near-future strategy.

Alterations in cholesterol metabolism as a risk factor for developing Alzheimer's disease: Potential novel targets for treatment

Alzheimer's disease (AD) is the most common form of dementia and it is characterized by the deposition of amyloid-β (Aβ) plaques and neurofibrillary tangles in the brain. However, the complete pathogenesis of the disease is still unknown. High level of serum cholesterol has been found to positively correlate with an increased risk of dementia and some studies have reported a decreased prevalence of AD in patients taking cholesterol-lowering drugs. Years of research have shown a strong correlation between blood hypercholesterolemia and AD, however cholesterol is not able to cross the Blood Brain Barrier (BBB) into the brain. Cholesterol lowering therapies have shown mixed results in cognitive performance in AD patients, raising questions of whether brain cholesterol metabolism in the brain should be studied separately from peripheral cholesterol metabolism and what their relationship is. Unlike cholesterol, oxidized cholesterol metabolites known as oxysterols are able to cross the BBB from the circulation into the brain and vice-versa. The main oxysterols present in the circulation are 24S-hydroxycholesterol and 27-hydroxycholesterol. These oxysterols and their catalysing enzymes have been found to be altered in AD brains and there is evidence indicating their influence in the progression of the disease. This review gives a broad perspective on the relationship between hypercholesterolemia and AD, cholesterol lowering therapies for AD patients and the role of oxysterols in pathological and non-pathological conditions. Also, we propose cholesterol metabolites as valuable targets for prevention and alternative AD treatments.

Toward common mechanisms for risk factors in Alzheimer's syndrome

The global strategic goal of reducing health care cost, especially the prospects for massive increases due to expanding markets for health care services demanded by aging populations and/or people with a wide range of chronic disorders-disabilities, is a complex and formidable challenge with many facets. Current projections predict marked increases in the demand for health driven by both the exponential climb in the prevalence of chronic disabilities and the increases in the absolute numbers of people in need of some form of health care. Thus, the looming predicament for the economics of health care systems worldwide mandates the formulation of a strategic goal to foster significant expansion of global R&D efforts to discover and develop wide-ranging interventions to delay and/or prevent the onset of chronic disabling conditions. The rationale for adopting such a tactical objective is based on the premise that the costs and prevalence of chronic disabling conditions will be reduced by half even if a modest delay of 5 years in the onset of disability is obtained by a highly focused multinational research initiative. Because of the recent history of many failures in drug trials, the central thesis of this paper is to argue for the exploration-adoption of novel mechanistic ideas, theories, and paradigms for developing wide range and/or types of interventions. Although the primary focus of our discussion has been on biological approaches to therapy, we recognize the importance of emerging knowledge on nonpharmacological interventions and their potential impact in reducing health care costs. Although we may not find a drug to cure or prevent dementia for a long time, research is starting to demonstrate the potential contributes of nonpharmacological interventions toward the economics of health care in terms of rehabilitation, promoting autonomy, and potential to delay institutionalization, thus promoting healthy aging and reductions in the cost of care.

Atorvastatin ameliorates cognitive impairment, Aβ1-42 production and Tau hyperphosphorylation in APP/PS1 transgenic mice

Amyloid-beta (Aβ) interacts with the serine/threonine protein kinase AKT (also known as protein kinase B)/glycogen synthase kinase 3β (GSK3β) pathway and deactivates GSK3β signaling, which result in microtubule protein tau phosphorylation. Atorvastatin, a HMG-CoA reductase inhibitor, has been proven to improve learning and memory performance, reduce Aβ and phosphorylated tau levels in mouse model of Alzheimer's disease (AD). However, it still remains unclear whether atorvastatin is responsible for regulation of AKT/GSK3β signaling and contributes to subsequent down-regulation of Aβ1-42 and phosphorylated tau in APP/PS1 transgenic (Tg APP/PS1) mice. Herein, we aimed to investigate the possible impacts of atorvastatin (10 mg/kg, p.o.) on the memory deficit by behavioral tests and changes of AKT/GSK3β signaling in hippocampus and prefrontal cortex by western blot test in Tg APP/PS1 mice. The results showed that treatment with atorvastatin significantly reversed the memory deficit in the Tg APP/PS1 mice in a novel object recognition and the Morris water maze tests. Moreover, atorvastatin significantly attenuated Aβ1-42 accumulation and phosphorylation of tau (Ser396) in the hippocampus and prefrontal cortex of Tg APP/PS1 mice. In addition, atorvastatin treatment also increased phosphorylation of AKT, inhibited GSK3β activity by increasing phosphorylation of GSK3β (Ser9) and decreasing the beta-site APP cleaving enzyme 1 (BACE1) expression. These results indicated that the memory ameliorating effect of atorvastatin may be, in part, by regulation the AKT/GSK3β signaling which may contribute to down-regulation of Aβ1-42 and tau hyperphosphorylation.

Endocrine Risk Factors for Cognitive Impairment

Cognitive impairment, including Alzheimer's disease and other kinds of dementia, is a major health problem in older adults worldwide. Although numerous investigators have attempted to develop effective treatment modalities or drugs, there is no reasonably efficacious strategy for preventing or recovering from cognitive impairment. Therefore, modifiable risk factors for cognitive impairment have received attention, and the growing literature of metabolic risk factors for cognitive impairment has expanded from epidemiology to molecular pathogenesis and therapeutic management. This review focuses on the epidemiological evidence for the association between cognitive impairment and several endocrine risk factors, including insulin resistance, dyslipidemia, thyroid dysfunction, vitamin D deficiency, and subclinical atherosclerosis. Researches suggesting possible mechanisms for this association are reviewed. The research investigating modifiable endocrine risk factors for cognitive impairment provides clues for understanding the pathogenesis of cognitive impairment and developing novel treatment modalities. However, so far, interventional studies investigating the beneficial effect of the "modification" of these "modifiable risk factors" on cognitive impairment have reported variable results. Therefore, well-designed, randomized prospective interventional studies are needed.

Metabolic Risk Factors of Sporadic Alzheimer's Disease: Implications in the Pathology, Pathogenesis and Treatment

Alzheimer's disease (AD), the major cause of dementia among the elderly world-wide, manifests in familial and sporadic forms, and the latter variety accounts for the majority of the patients affected by this disease. The etiopathogenesis of sporadic AD is complex and uncertain. The autopsy studies of AD brain have provided limited understanding of the antemortem pathogenesis of the disease. Experimental AD research with transgenic animal or various cell based models has so far failed to explain the complex and varied spectrum of AD dementia. The review, therefore, emphasizes the importance of AD related risk factors, especially those with metabolic implications, identified from various epidemiological studies, in providing clues to the pathogenesis of this complex disorder. Several metabolic risk factors of AD like hypercholesterolemia, hyperhomocysteinemia and type 2 diabetes have been studied extensively both in epidemiology and experimental research, while much less is known about the role of adipokines, pro-inflammatory cytokines and vitamin D in this context. Moreover, the results from many of these studies have shown a degree of variability which has hindered our understanding of the role of AD related risk factors in the disease progression. The review also encompasses the recent recommendations regarding clinical and neuropathological diagnosis of AD and brings out the inherent uncertainty and ambiguity in this area which may have a distinct impact on the outcome of various population-based studies on AD-related risk factors.

Insulin, cognition, and dementia

Cognitive disorders of aging represent a serious threat to the social and economic welfare of current society. It is now widely recognized that pathology related to such conditions, particularly Alzheimer's disease, likely begins years or decades prior to the onset of clinical dementia symptoms. This revelation has led researchers to consider candidate mechanisms precipitating the cascade of neuropathological events that eventually lead to clinical Alzheimer's disease. Insulin, a hormone with potent effects in the brain, has recently received a great deal of attention for its potential beneficial and protective role in cognitive function. Insulin resistance, which refers to the reduced sensitivity of target tissues to the favorable effects of insulin, is related to multiple chronic conditions known to impact cognition and increase dementia risk. With insulin resistance-associated conditions reaching epidemic proportions, the prevalence of Alzheimer's disease and other cognitive disorders will continue to rise exponentially. Fortunately, these chronic insulin-related conditions are amenable to pharmacological intervention. As a result, novel therapeutic strategies that focus on increasing insulin sensitivity in the brain may be an important target for protecting or treating cognitive decline. The following review will highlight our current understanding of the role of insulin in brain, potential mechanisms underlying the link between insulin resistance and dementia, and current experimental therapeutic strategies aimed at improving cognitive function via modifying the brain's insulin sensitivity.

Hypercholesterolemia accelerates amyloid β-induced cognitive deficits

Hypercholesterolemia is a known risk factor for Alzheimer's disease (AD). In the present study, we investigated whether diet-induced hypercholesterolemia affects AD-like pathologies such as amyloid β-peptide (Aβ) deposition, tau pathology, inflammation and cognitive impairment, using an Aβ25-35-injected AD-like pathological mouse model. Hypercholesterolemia was induced by providing apolipoprotein E knock out (Apo E KO) mice with a high-fat diet for 4 weeks prior to Aβ25-35 injection and for 4 weeks following Aβ25-35 injection, for a total of 8 weeks of treatment. Our data showed that intracerebroventricular injection of C57BL/6J mice with Aβ25-35 resulted in increased immunoreactivity of Aβ and phosphorylated-tau (p-tau), which was accompanied by enhanced microglial CD11b-like immunoreactivity in the brain. Moreover, hypercholesterolemia slightly increased Aβ and p-tau levels and microglial activation in the vehicle group, while further increasing the Aβ and p-tau levels and microglial activation in Aβ25-35-injected mice. Consistent with the neuropathological analysis, hypercholesterolemia resulted in significant spatial learning and memory deficits in Aβ25-35-injected mice as revealed by water maze testing. Collectively, these findings demonstrated that hypercholesterolemia accelerated Aβ accumulation and tau pathology, which was accompanied by microglial activation and subsequent aggravation of memory impairment induced by Aβ25-35. Thus, we suggest that the modulation of cholesterol can be used to reduce the risk of developing AD.

Insulin resistance and pathological brain ageing

Sir Harold Himsworth's prescient observations 75 years ago have recently been expanded to include a clear relationship between insulin resistance and central nervous system function. Insulin is a master regulator of corporeal ageing in all known species, determining the rate and expression of ageing in multiple body systems. Thus, it is not surprising that insulin also plays an important role in brain ageing and cognitive decline that is associated with pathological brain ageing. Brain ageing is accompanied by reduced insulin effectiveness, either by an inadequate cellular response to insulin or by insulin deficiency attributable to reduced insulin transport across the blood-brain barrier. Age-associated brain insulin abnormalities may contribute to cognitive decline in ageing, as have been documented in older adults with Type 2 diabetes mellitus and hypertension. With more extreme pathology, brain insulin resistance may be associated with neurogenerative diseases such as Alzheimer's disease, and the condition which precedes Alzheimer's disease, known as amnestic mild cognitive impairment. In the following review, we discuss the mechanisms through which insulin resistance may induce or potentiate pathological brain ageing and thereby create a neurobiological environment that promotes neurodegeneration and associated cognitive decline. This topic is timely, given that insulin resistance-associated conditions such as diabetes and obesity have reached epidemic proportions. The prevalence of such chronic conditions, in combination with a rapidly ageing population, may result in a corresponding increase in the prevalence of Alzheimer's disease and other cognitive disorders. Fortunately, insulin resistance-associated conditions are amenable to both pharmacologic and lifestyle interventions that may reduce the deleterious impact of insulin resistance on the ageing brain.