Cholesterol Metabolism in Neurodegenerative Diseases

SIGNIFICANCE

Cholesterol plays a crucial role in brain, where it is highly concentrated and tightly regulated to support normal brain functions. It serves as a vital component of cell membranes, ensuring their integrity, and acts as a key regulator of various brain processes. Dysregulation of cholesterol metabolism in the brain has been linked to impaired brain function and the onset of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD).

RECENT ADVANCES

A significant advancement has been the identification of astrocyte-derived ApoE as a key regulator of de novo cholesterol biosynthesis in neurons, providing insights into how extracellular signals influence neuronal cholesterol levels. Additionally, the development of antibody-based therapies, particularly for AD, presents promising opportunities for therapeutic interventions.

CRITICAL ISSUES

Despite significant research, the association between cholesterol and neurodegenerative diseases remains inconclusive. It is crucial to distinguish between plasma cholesterol and brain cholesterol, as these pools are relatively independent. This differentiation should be considered when evaluating statin-based treatment approaches. Assessing not only the total cholesterol content in the brain but also its distribution among different types of brain cells is essential.

FUTURE DIRECTION

Establishing a causal link between changes in brain/plasma cholesterol levels and the onset of brain dysfunction/neurodegenerative diseases remains a key objective. Additionally, conducting cell-specific analyses of cholesterol homeostasis in various types of brain cells under pathological conditions will enhance our understanding of cholesterol metabolism in neurodegenerative diseases. Manipulating cholesterol levels to restore homeostasis may represent a novel approach for alleviating neurological symptom.

Oxysterols as Biomarkers of Aging and Disease

Oxysterols derive from either enzymatic or non-enzymatic oxidation of cholesterol. Even though they are produced as intermediates of bile acid synthesis pathway, they are recognised as bioactive compounds in cellular processes. Therefore, their absence or accumulation have been shown to be associated with disease phenotypes. This chapter discusses the contribution of oxysterol to ageing, age-related diseases such as neurodegeneration and various disorders such as cancer, cardiovascular disease, diabetes, metabolic and ocular disorders. It is clear that oxysterols play a significant role in development and progression of these diseases. As a result, oxysterols are being investigated as suitable markers for disease diagnosis purposes and some drug targets are in development targeting oxysterol pathways. However, further research will be needed to confirm the suitability of these potentials.

24S-Hydroxycholesterol in Neuropsychiatric Diseases: Schizophrenia, Autism Spectrum Disorder, and Bipolar Disorder

Neuropsychiatric diseases (NPDs) are severe, debilitating psychiatric conditions that affect the nervous system. These are among the most challenging disorders in medicine. Some examples include Alzheimer's, anxiety disorders, autism spectrum disorder, bipolar disorder, and schizophrenia. NPDs represent an ever-increasing burden on public health and are prevalent throughout the world. For most of these diseases, the particular etiopathogeneses are still enigmatic. NPDs are also associated with structural and functional changes in the brain, along with altered neurotransmitter and neuroendocrine systems.Approximately 25% of the total human body cholesterol is located in the brain. Its involvement in neuronal functions starts in the early growth stages and remains important throughout adulthood. It is also an integral part of the neuronal membrane, ensuring membrane lipid organization and regulating membrane fluidity. The main mechanism for removing cholesterol from the brain is cholesterol 24-hydroxylation by cytochrome P450 46A1 (CYP46A1), an enzyme specifically found in the central nervous system. Although research on 24S-OHC and its role in neuropsychiatric diseases is still in its early stages, this oxidized cholesterol metabolite is thought to play a crucial role in the etiology of NPDs. 24S-OHC can affect neurons, astrocytes, oligodendrocytes, and vascular cells. In addition to regulating the homeostasis of cholesterol in the brain, this oxysterol is involved in neurotransmission, oxidative stress, and inflammation. The role of 24S-OHC in NPDs has been found to be controversial in terms of the findings so far. There are several intriguing discrepancies in the data gathered so far regarding 24S-OHC and NPDs. In fact, 24S-OHC levels were reported to have decreased in a number of NPDs and increased in others.Hence, in this chapter, we first summarize the available data regarding 24S-OHC as a biomarker in NPDs, including schizophrenia, autism spectrum disorder, and bipolar disorder. Then, we present a brief synopsis of the pharmacological targeting of 24S-OHC levels through the modulation of CYP46A1 activity.

Enzymatically Formed Oxysterols and Cell Death

The side-chain hydroxylation of cholesterol by specific enzymes produces 24(S)-hydroxycholesterol, 25-hydroxycholesterol, 27-hydroxycholesterol, and other products. These enzymatically formed side-chain oxysterols act as intermediates in the biosynthesis of bile acids and serve as signaling molecules that regulate cholesterol homeostasis. Besides these intracellular functions, an imbalance in oxysterol homeostasis is implicated in pathophysiology. Furthermore, growing evidence reveals that oxysterols affect cell proliferation and cause cell death. This chapter provides an overview of the pathophysiological role of side-chain oxysterols in developing human diseases. We also summarize our understanding of the molecular mechanisms underlying the induction of various forms of cell death by side-chain oxysterols.

The potential of CYP46A1 as a novel therapeutic target for neurological disorders: An updated review of mechanisms

Cholesterol is a key component of the cell membrane that impacts the permeability, fluidity, and functions of membrane-bound proteins. It also participates in synaptogenesis, synaptic function, axonal growth, dendrite outgrowth, and microtubule stability. Cholesterol biosynthesis and metabolism are in balance in the brain. Its metabolism in the brain is mediated mainly by CYP46A1 or cholesterol 24-hydroxylase. It is responsible for eliminating about 80% of the cholesterol excess from the human brain. CYP46A1 converts cholesterol to 24S-hydroxycholesterol (24HC) that readily crosses the blood-brain barrier and reaches the liver for the final elimination process. Studies show that cholesterol and 24HC levels change during neurological diseases and conditions. So, it was hypothesized that inhibition or activation of CYP46A1 would be an effective therapeutic strategy. Accordingly, preclinical studies, using genetic and pharmacological interventions, assessed the role of CYP46A1 in main neurodegenerative disorders such as Parkinson's disease, Huntington's disease, Alzheimer's disease, multiple sclerosis, spinocerebellar ataxias, and amyotrophic lateral sclerosis. In addition, its role in seizures and brain injury was evaluated. The recent development of soticlestat, as a selective and potent CYP46A1 inhibitor, with significant anti-seizure effects in preclinical and clinical studies, suggests the importance of this target for future drug developments. Previous studies have shown that both activation and inhibition of CYP46A1 are of therapeutic value. This article, using recent studies, highlights the role of CYP46A1 in various brain diseases and insults.

24-Hydroxycholesterol Induces Tau Proteasome-Dependent Degradation via the SIRT1/PGC1α/Nrf2 Pathway: A Potential Mechanism to Counteract Alzheimer’s Disease

Considerable evidence indicates that cholesterol oxidation products, named oxysterols, play a key role in several events involved in Alzheimer’s disease (AD) pathogenesis. Although the majority of oxysterols causes neuron dysfunction and degeneration, 24-hydroxycholesterol (24-OHC) has recently been thought to be neuroprotective also. The present study aimed at supporting this concept by exploring, in SK-N-BE neuroblastoma cells, whether 24-OHC affected the neuroprotective SIRT1/PGC1α/Nrf2 axis. We demonstrated that 24-OHC, through the up-regulation of the deacetylase SIRT1, was able to increase both PGC1α and Nrf2 expression and protein levels, as well as Nrf2 nuclear translocation. By acting on this neuroprotective pathway, 24-OHC favors tau protein clearance by triggering tau ubiquitination and subsequently its degradation through the ubiquitin–proteasome system. We also observed a modulation of SIRT1, PGC1α, and Nrf2 expression and synthesis in the brain of AD patients with the progression of the disease, suggesting their potential role in neuroprotection. These findings suggest that 24-OHC contributes to tau degradation through the up-regulation of the SIRT1/PGC1α/Nrf2 axis. Overall, the evidence points out the importance of avoiding 24-OHC loss, which can occur in the AD brain, and of limiting SIRT1, PGC1α, and Nrf2 deregulation in order to prevent the neurotoxic accumulation of hyperphosphorylated tau and counteract neurodegeneration.

α-Tocopherol suppresses 24(S)-hydroxycholesterol-induced cell death via inhibition of endoplasmic reticulum membrane disruption

The brain-specific cholesterol metabolite 24(S)-hydroxycholesterol (24S-OHC) has been shown to cause neuronal cell death when subjected to esterification by acyl-CoA:cholesterol acyltransferase 1 (ACAT1). Accumulating 24S-OHC esters in the endoplasmic reticulum (ER) provoked ER membrane disruption and an integrated stress response (ISR), a signaling pathway that regulates adaptation to various stresses. We have previously reported that α-tocopherol (α-Toc) but not α-tocotrienol (α-Toc3), among vitamin E homologs, suppressed 24S-OHC-induced cell death without affecting ACAT1 activity in human neuroblastoma SH-SY5Y cells. However, the precise mechanisms underlying the inhibitory activity of α-Toc have yet to be elucidated. In the present study, we aimed to investigate the effects of α-Toc on the 24S-OHC-induced cell death machinery. We showed that α-Toc, but not α Toc3, suppressed 24S-OHC-induced ISR and downstream eukaryotic translation initiator factor 2α (eIF2α) phosphorylation. We also found that α-Toc inhibited stress granule formation and robust downregulation of nascent protein synthesis, which were induced by 24S-OHC treatment. Furthermore, disruption of ER membrane integrity was suppressed by α-Toc, but not by α-Toc3. Our findings suggest that the inhibitory effects of α-Toc on 24S-OHC-induced cell death may be attributed to its protective function against ER membrane disruption.

Integrated stress response is involved in the 24(S)-hydroxycholesterol-induced unconventional cell death mechanism

Perturbation of proteostasis triggers the adaptive responses that contribute to the homeostatic pro-survival response, whereas disruption of proteostasis can ultimately lead to cell death. Brain-specific oxysterol-i.e., 24(S)-hydroxycholesterol (24S-OHC)-has been shown to cause cytotoxicity when esterified by acyl-CoA:cholesterol acyltransferase 1 (ACAT1) in the endoplasmic reticulum (ER). Here, we show that the accumulation of 24S-OHC esters caused phosphorylation of eukaryotic translation initiator factor 2α (eIF2α), dissociation of polysomes, and formation of stress granules (SG), resulting in robust downregulation of global protein de novo synthesis in human neuroblastoma SH-SY5Y cells. We also found that integrated stress response (ISR) activation through PERK and GCN2 activation induced by 24S-OHC treatment caused eIF2α phosphorylation. 24S-OHC-inducible SG formation and cell death were suppressed by inhibition of ISR. These results show that ACAT1-mediated 24S-OHC esterification induced ISR and formation of SG, which play crucial roles in 24S-OHC-inducible protein synthesis inhibition and unconventional cell death.

Oxysterols are potential physiological regulators of ageing

Delaying and even reversing ageing is a major public health challenge with a tremendous potential to postpone a plethora of diseases including cancer, metabolic syndromes and neurodegenerative disorders. A better understanding of ageing as well as the development of innovative anti-ageing strategies are therefore an increasingly important field of research. Several biological processes including inflammation, proteostasis, epigenetic, oxidative stress, stem cell exhaustion, senescence and stress adaptive response have been reported for their key role in ageing. In this review, we describe the relationships that have been established between cholesterol homeostasis, in particular at the level of oxysterols, and ageing. Initially considered as harmful pro-inflammatory and cytotoxic metabolites, oxysterols are currently emerging as an expanding family of fine regulators of various biological processes involved in ageing. Indeed, depending of their chemical structure and their concentration, oxysterols exhibit deleterious or beneficial effects on inflammation, oxidative stress and cell survival. In addition, stem cell differentiation, epigenetics, cellular senescence and proteostasis are also modulated by oxysterols. Altogether, these data support the fact that ageing is influenced by an oxysterol profile. Further studies are thus required to explore more deeply the impact of the "oxysterome" on ageing and therefore this cholesterol metabolic pathway constitutes a promising target for future anti-ageing interventions.

Different functions of vitamin E homologues in the various types of cell death induced by oxysterols

24(S)-Hydroxycholesterol (24S-OHC) and 25-hydroxycholesterol (25-OHC) are produced by cholesterol 24-hydroxylase and cholesterol 25-hydroxylase, respectively. The purpose of the present study was to determine the type of cell death induced by these oxysterols in neuronal cells, hepatic cells, and keratinocytes, and to elucidate the inhibitory effect of vitamin E homologues on various types of cell death. In human neuronal cells (SH-SY5Y cells), 24S-OHC and 25-OHC caused a cell death that was independent of caspase activation. We reported previously that the esterification of 24S-OHC by acyl-CoA:cholesterol acyltransferase 1 (ACAT1) and the resulting formation of a lipid droplet (LD)-like structure are responsible for the 24S-OHC-induced neuronal cell death. Here, we found that 25-OHC also induced ACAT1-mediated 25-OHC esterification and LD formation in neuronal cells. 25-OHC-induced cell death was inhibited by α-tocopherol (α-Toc) but not by α-tocotrienol (α-Toc3), as observed for 24S-OHC-induced cell death in SH-SY5Y cells. In human hepatic cells (HepG2 cells), these oxysterols caused a cell death that was caspase- and oxysterol-esterification-independent. This cell death was suppressed by both α-Toc and α-Toc3, suggesting the involvement of free-radical-mediated lipid peroxidation in the cell death induced by these oxysterols in hepatic cells. In human keratinocytes (HaCaT cells), these oxysterols caused a caspase-dependent but oxysterol-esterification-independent cell death that was inhibited by α-Toc but not by α-Toc3. These results suggest that α-Toc and α-Toc3 act as radical-scavenging antioxidants against oxysterol-induced cell death in the same way in hepatic cells, whereas their behavior is different in inhibition of cell death in neuronal cells and keratinocytes. Collectively, these results demonstrated that 24S-OHC and 25-OHC induced the same type of cell death in each of the cell types examined, and that α-Toc and α-Toc3 exerted different effects, depending on the type of cell death.

The Controversial Role of 24-S-Hydroxycholesterol in Alzheimer's Disease

The development of Alzheimer’s disease (AD) is influenced by several events, among which the dysregulation of cholesterol metabolism in the brain plays a major role. Maintenance of brain cholesterol homeostasis is essential for neuronal functioning and brain development. To maintain the steady-state level, excess brain cholesterol is converted into the more hydrophilic metabolite 24-S-hydroxycholesterol (24-OHC), also called cerebrosterol, by the neuron-specific enzyme CYP46A1. A growing bulk of evidence suggests that cholesterol oxidation products, named oxysterols, are the link connecting altered cholesterol metabolism to AD. It has been shown that the levels of some oxysterols, including 27-hydroxycholesterol, 7β-hydroxycholesterol and 7-ketocholesterol, significantly increase in AD brains contributing to disease progression. In contrast, 24-OHC levels decrease, likely due to neuronal loss. Among the different brain oxysterols, 24-OHC is certainly the one whose role is most controversial. It is the dominant oxysterol in the brain and evidence shows that it represents a signaling molecule of great importance for brain function. However, numerous studies highlighted the potential role of 24-OHC in favoring AD development, since it promotes neuroinflammation, amyloid β (Aβ) peptide production, oxidative stress and cell death. In parallel, 24-OHC has been shown to exert several beneficial effects against AD progression, such as preventing tau hyperphosphorylation and Aβ production. In this review we focus on the current knowledge of the controversial role of 24-OHC in AD pathogenesis, reporting a detailed overview of the findings about its levels in different AD biological samples and its noxious or neuroprotective effects in the brain. Given the relevant role of 24-OHC in AD pathophysiology, its targeting could be useful for disease prevention or slowing down its progression.

Phytosterols: Targeting Neuroinflammation in Neurodegeneration

Plant-derived sterols, phytosterols, are well known for their cholesterol-lowering activity in serum and their anti-inflammatory activities. Recently, phytosterols have received considerable attention due to their beneficial effects on various non-communicable diseases, and recommended use as daily dietary components. The signaling pathways mediated in the brain by phytosterols have been evaluated, but little is known about their effects on neuroinflammation, and no clinical studies have been undertaken on phytosterols of interest. In this review, we discuss the beneficial roles of phytosterols, including their attenuating effects on inflammation, blood cholesterol levels, and hallmarks of the disease, and their regulatory effects on neuroinflammatory disease pathways. Despite recent advancements made in phytosterol pharmacology, some critical questions remain unanswered. Therefore, we have tried to highlight the potential of phytosterols as viable therapeutics against neuroinflammation and to direct future research with respect to clinical applications.

Oxysterols and retinal degeneration

Retinal degeneration, characterised by the progressive death of retinal neurons, is the most common cause of visual impairment. Oxysterols are the cholesterol derivatives produced via enzymatic and/or free radical oxidation that regulate cholesterol homeostasis in the retina. Preclinical and clinical studies have suggested a connection between oxysterols and retinal degeneration. Here, we summarise early and recent work related to retina oxysterol-producing enzymes and the distribution of oxysterols in the retina. We examine the impact of loss of oxysterol-producing enzymes on retinal pathology and explore the molecular mechanisms associated with the toxic or protective roles of individual oxysterols in different types of retinal degeneration. We conclude that increased efforts to better understand the oxysterol-associated pathophysiology will help in the development of effective retinal degeneration therapies. LINKED ARTICLES: This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Preferred Endocytosis of Amyloid Precursor Protein from Cholesterol-Enriched Lipid Raft Microdomains

Amyloid precursor protein (APP) at the plasma membrane is internalized via endocytosis and delivered to endo/lysosomes, where neurotoxic amyloid-β (Aβ) is produced via β-, γ-secretases. Hence, endocytosis plays a key role in the processing of APP and subsequent Aβ generation. β-, γ-secretases as well as APP are localized in cholesterol-enriched lipid raft microdomains. However, it is still unclear whether lipid rafts are the site where APP undergoes endocytosis and whether cholesterol levels affect this process. In this study, we found that localization of APP in lipid rafts was increased by elevated cholesterol level. We also showed that increasing or decreasing cholesterol levels increased or decreased APP endocytosis, respectively. When we labeled cell surface APP, APP localized in lipid rafts preferentially underwent endocytosis compared to nonraft-localized APP. In addition, APP endocytosis from lipid rafts was regulated by cholesterol levels. Our results demonstrate for the first time that cholesterol levels regulate the localization of APP in lipid rafts affecting raft-dependent APP endocytosis. Thus, regulating the microdomain localization of APP could offer a new therapeutic strategy for Alzheimer’s disease.

24S-hydroxycholesterol: Cellular effects and variations in brain diseases

The adult brain exhibits a characteristic cholesterol homeostasis, with low synthesis rate and active catabolism. Brain cholesterol turnover is possible thanks to the action of the enzyme cytochrome P450 46A1 (CYP46A1) or 24-cholesterol hydroxylase, that transforms cholesterol into 24S-hydroxycholesterol (24S-HC). But before crossing the blood-brain barrier (BBB), this oxysterol, that is the most abundant in the brain, can act locally, affecting the functioning of neurons, astrocytes, oligodendrocytes, and vascular cells. The first part of this review addresses different aspects of 24S-HC production and elimination from the brain. The second part concentrates in the effects of 24S-HC at the cellular level, describing how this oxysterol affects cell viability, amyloid β production, neurotransmission, and transcriptional activity. Finally, the role of 24S-HC in Alzheimer, Huntington and Parkinson diseases, multiple sclerosis and amyotrophic lateral sclerosis, as well as the possibility of using this oxysterol as predictive and/or evolution biomarker in different brain disorders is discussed.

Novel therapeutic strategies for Alzheimer's disease targeting brain cholesterol homeostasis

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia. Aβ plaques and tauopathy are two major concerns associated with AD. Moreover, excessive Aβ accumulation can lead to other nonspecific metabolic brain abnormalities. There are various genetic, environmental, and other risk factors associated with AD. Identification of risk factors and its mechanisms by which these factors impart role in AD pathology would be helpful for the prevention of AD progression. Altered cholesterol homeostasis could be considered as a risk factor for AD progression. Brain cholesterol dysmetabolism is recognized as one of the crucial attributes for AD that affect major hallmarks of AD including neurodegeneration. To fill the gap between altered cholesterol levels in the brain and AD, the researchers started focusing on statins as re-purposing drugs for AD treatment. The various other hypothesis has been suggested due to a lack of beneficial results of statins in clinical trials, such as reduced brain cholesterol could underlie poor cognition. Unfortunately, it is still unclear, whether an increase or decrease in brain cholesterol levels responsible for Alzheimer's disease or not. Presently, scientists believed that managing the level of cholesterol in the brain may help as an alternative treatment strategy for AD. In this review, we focused on the therapeutic strategies for AD by targeting brain cholesterol levels.

Curcumin Derivative GT863 Inhibits Amyloid-Beta Production via Inhibition of Protein N-Glycosylation

Amyloid-β (Aβ) peptides play a crucial role in the pathogenesis of Alzheimer's disease (AD). Aβ production, aggregation, and clearance are thought to be important therapeutic targets for AD. Curcumin has been known to have an anti-amyloidogenic effect on AD. In the present study, we performed screening analysis using a curcumin derivative library with the aim of finding derivatives effective in suppressing Aβ production with improved bioavailability of curcumin using CHO cells that stably express human amyloid-β precursor protein and using human neuroblastoma SH-SY5Y cells. We found that the curcumin derivative GT863/PE859, which has been shown to have an inhibitory effect on Aβ and tau aggregation in vivo, was more effective than curcumin itself in reducing Aβ secretion. We further found that GT863 inhibited neither β- nor γ-secretase activity, but did suppress γ-secretase-mediated cleavage in a substrate-dependent manner. We further found that GT863 suppressed N-linked glycosylation, including that of the γ-secretase subunit nicastrin. We also found that mannosidase inhibitors that block the mannose trimming step of N-glycosylation suppressed Aβ production in a similar fashion, as was observed as a result of treatment with GT863. Collectively, these results suggest that GT863 downregulates N-glycosylation, resulting in suppression of Aβ production without affecting secretase activity.

Brain Targeting of Acyl-CoA:Cholesterol O-Acyltransferase-1 Inhibitor K-604 via the Intranasal Route Using a Hydroxycarboxylic Acid Solution

An acyl-CoA:cholesterol O-acyltransferase-1 (ACAT-1/SOAT-1) inhibitor, K-604 is a promising drug candidate for the treatment of Alzheimer's disease and glioblastoma; however, it exhibits poor solubility in neutral water and low permeability across the blood-brain barrier. In this study, we report the successful delivery of K-604 to the brain via the intranasal route in mice using a hydroxycarboxylic acid solution. In cerebral tissue, the AUC of K-604 after intranasal administration (10 μL; 108 μg of K-604/mouse) was 772 ng·min/g, whereas that after oral administration (166 μg of K-604/mouse) was 8.9 ng·min/g. Thus, the index of brain-targeting efficiency was 133-fold based on the dose conversion. Even with intranasal administration of K-604 once per day for 7 days, the level of cholesteryl esters markedly decreased from 0.70 to 0.04 μmol/g in the mouse brain. Thus, this application will be a crucial therapeutic solution for ACAT-1 overexpressing diseases in the brain.

24(S)-Hydroxycholesterol induces ER dysfunction-mediated unconventional cell death

Endoplasmic reticulum (ER) stress induced by disruption of protein folding activates the unfolded protein response (UPR), which while generally pro-survival in effect can also induce cell death under severe ER stress. 24(S)-hydroxycholesterol (24S-OHC), which is enzymatically produced in the ER of neurons, plays an important role in maintaining brain cholesterol homeostasis but also shows neurotoxicity when subjected to esterification by acyl-CoA:cholesterol acyltransferase 1 (ACAT1) in the ER. In this study, we demonstrated that the accumulation of 24S-OHC esters in human neuroblastoma SH-SY5Y cells evoked the UPR with substantially no pro-survival adaptive response but with significant activation of pro-death UPR signaling via regulated IRE1-dependent decay (RIDD). We further found that accumulation of 24S-OHC esters caused disruption of ER membrane integrity and release of ER luminal proteins into cytosol. We also found that de novo synthesis of global proteins was robustly suppressed in 24S-OHC-treated cells. Collectively, these results show that ER dysfunction and the accompanying RIDD-mediated pro-death UPR signaling and global protein synthesis inhibition are responsible for 24S-OHC ester-induced unconventional cell death.

Oxysterols and the NeuroVascular Unit (NVU): A far true love with bright and dark sides

The brain is isolated from the whole body by the blood-brain barrier (BBB) which is located in brain microvessel endothelial cells (ECs). Through physical and metabolic properties induced by brain pericytes, astrocytes and neurons (these cells and the ECs referred to as the neurovascular unit (NVU)), the BBB hardly restricts exchanges of molecules between the brain and the bloodstream. Among them, cholesterol exchanges between these two compartments are very limited and occur through the transport of LDLs across the BBB. Oxysterols (mainly 24S and 27-hydroxycholesterol) daily cross the BBB and regulate molecule/cholesterol exchanges via Liver X nuclear Receptors (LXRs). In addition, these oxysterols have been linked to pathological processes in neurodegenerative diseases such as Alzheimer's disease. Here we propose an overview of the actual knowledge concerning oxysterols and the NVU cells in physiological and in Alzheimer's disease.

Serum 24-hydroxycholesterol in probable Alzheimer's dementia: Reexploring the significance of a tentative Alzheimer's disease biomarker

OBJECTIVE

This study measured and analyzed the serum levels of 24-hydroxycholesterol in patients with probable Alzheimer's disease (AD) and age-/sex-matched controls.

METHODS

A case-control study involving 40 AD patients and 40 controls was performed at a tertiary neurological teaching hospital in eastern India. Blood and serum samples were collected for APOE genotyping and 24-hydroxycholesterol levels, respectively.

RESULTS

Serum 24-hydroxycholesterol was significantly lower in AD patients compared to controls (median concentration: controls, 47.14 ng/mL (interquartile range, 16.34); AD patients, 32.93 ng/mL (interquartile range, 9.45); P < 0.001) but showed no significant correlation with Mini Mental State Examination (MMSE) score in AD cases (r = -0.169, P = 0.298) or in controls (r = 0.18, P = 0.26). No statistically significant difference was observed between serum 24-hydroxycholesterol levels of the APOE4-positive and -negative subgroups in AD patients (P = 0.79). Findings were consistent and unchanged even when the ratio of serum 24-hydroxycholesterol to serum total cholesterol was considered.

CONCLUSION

The decreased 24-hydroxycholesterol level in peripheral circulation in AD cases observed in the present study may suggest its role in AD pathogenesis. The lack of a clear correlation between serum levels of 24-hydroxycholesterol and MMSE score-a surrogate marker of AD severity-raises the question as to whether 24-hydroxycholesterol level declines with decreasing neuronal mass or whether the steroid continues to play a protective role.

A Crosstalk Between Brain Cholesterol Oxidation and Glucose Metabolism in Alzheimer's Disease

In Alzheimer’s disease (AD), both cholesterol and glucose dysmetabolism precede the onset of memory deficit and contribute to the disease’s progression. It is indeed now believed that oxidized cholesterol in the form of oxysterols and altered glucose uptake are the main triggers in AD affecting production and clearance of Aβ, and tau phosphorylation. However, only a few studies highlight the relationship between them, suggesting the importance of further extensive studies on this topic. Recently, a molecular link was demonstrated between cholesterol oxidative metabolism and glucose uptake in the brain. In particular, 27-hydroxycholesterol, a key linker between hypercholesterolemia and the increased AD risk, is considered a biomarker for reduced glucose metabolism. In fact, its excess increases the activity of the renin-angiotensin system in the brain, thus reducing insulin-mediated glucose uptake, which has a major impact on brain functioning. Despite this important evidence regarding the role of 27-hydroxycholesterol in regulating glucose uptake by neurons, the involvement of other cholesterol oxidation products that have been clearly demonstrated to be key players in AD cannot be ruled out. This review highlights the current understanding of the potential role of cholesterol and glucose dysmetabolism in AD progression, and the bidirectional crosstalk between these two phenomena.

The key genes, phosphoproteins, processes, and pathways affected by efavirenz-activated CYP46A1 in the amyloid-decreasing paradigm of efavirenz treatment

Efavirenz (EFV) is an anti-HIV drug, and cytochrome P450 46A1 (CYP46A1) is the major brain cholesterol hydroxylase. Previously, we discovered that EFV activates CYP46A1 and improves behavioral performance in 5XFAD mice, an Alzheimer's disease model. Herein, the unbiased omics and other approaches were used to study 5XFAD mice in the amyloid-decreasing paradigm of CYP46A1 activation by EFV. These approaches revealed increases in the brain levels of postsynaptic density protein 95, gephyrin, synaptophysin, synapsin, glial fibrillary acidic protein, and CYP46A1 and documented altered expression and phosphorylation of 66 genes and 77 proteins, respectively. The data obtained pointed to EFV effects at the synaptic level, plasmin-depended amyloid clearance, inflammation and microglia phenotype, oxidative stress and cellular hypoxia, autophagy and ubiquitin-proteasome systems as well as apoptosis. These effects could be realized in part via changes in the Ca²⁺-, small GTPase, and catenin signaling. A model is proposed, in which CYP46A1-dependent lipid raft rearrangement and subsequent decrease of protein phosphorylation are central in EFV effects and explain behavioral improvements in EFV-treated 5XFAD mice.-Petrov, A. M., Mast, N., Li, Y., Pikuleva, I. A. The key genes, phosphoproteins, processes, and pathways affected by efavirenz-activated CYP46A1 in the amyloid-decreasing paradigm of efavirenz treatment.

6-Hydroxydopamine induces secretion of PARK7/DJ-1 via autophagy-based unconventional secretory pathway

PARK7/DJ-1 is a Parkinson disease- and cancer-associated protein that functions as a multifunctional protein involved in gene transcription regulation and anti-oxidative defense. Although PARK7 lacks the secretory signal sequence, it is secreted and plays important physiological and pathophysiological roles. Whereas secretory proteins that lack the endoplasmic reticulum-targeting signal sequence are secreted from cells by way of what is called the unconventional secretion mechanism, the specific processes responsible for causing PARK7 to be secreted across the plasma membrane have remained unclear. In the present study, we found that PARK7 secretion was increased by treatment with 6-OHDA via the unconventional secretory pathway in human neuroblastoma SH-SY5Y cells and MEF cells. We also found that 6-OHDA-induced PARK7 secretion was suppressed in Atg5-, Atg9-, or Atg16l1-deficient MEF cells or ATG16L1 knockdown SH-SY5Y cells, indicating that the autophagy-based unconventional secretory pathway is involved in PARK7 secretion. We moreover observed that 6-OHDA-derived electrophilic quinone induced oxidative stress as indicated by a decrease in glutathione levels, and that this was suppressed by pretreatment with antioxidant NAC. We further found that NAC treatment suppressed autophagy and PARK7 secretion. We also observed that 6-OHDA-induced autophagy was associated with activation of AMPK and ULK1 via a pathway which was independent of MTOR. Collectively these results suggest that electrophilic 6-OHDA quinone enhances oxidative stress, and that this is followed by AMPK-ULK1 pathway activation and induction of secretory autophagy to produce unconventional secretion of PARK7.

Abbreviations: 6-OHDA: 6-hydroxydopamine; AMPK: AMP-activated protein kinase; ATG: autophagy related; CAV1: caveolin 1; ER: endoplasmic reticulum; FN1: fibronectin 1; GSH: glutathione; IDE: insulin degrading enzyme; IL: interleukin; LDH: lactate dehydrogenase; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MEF: mouse embryonic fibroblast; MTOR: mechanistic target of rapamycin kinase; NAC: N-acetyl-L-cysteine; PARK7/DJ-1: Parkinsonism associated deglycase; PD: Parkinson disease; RPS6KB1/p70S6K: ribosomal protein S6 kinase B1; RPN1: ribophorin I; ROS: reactive oxygen species; ULK1: unc-51 like autophagy activating kinase 1; WT: wild-type

A silver lining for 24-hydroxycholesterol in Alzheimer's disease: The involvement of the neuroprotective enzyme sirtuin 1

It is now established that cholesterol oxidation products (oxysterols) are involved in several events underlying Alzheimer's disease (AD) pathogenesis. Of note, certain oxysterols cause neuron dysfunction and degeneration but, recently, some of them have been shown also to have neuroprotective effects. The present study, which aimed to understand the potential effects of 24-hydroxycholesterol (24-OH) against the intraneuronal accumulation of hyperphosphorylated tau protein, stressed these latter effects. A beneficial effect of 24-OH was demonstrated in SK-N-BE neuroblastoma cells, and is due to its ability to modulate the deacetylase sirtuin 1 (SIRT1), which contributes to preventing the neurotoxic accumulation of the hyperphosphorylated tau protein. Unlike 24-OH, 7-ketocholesterol (7-K) did not modulate the SIRT1-dependent neuroprotective pathway. To confirm the neuroprotective role of 24-OH, in vivo experiments were run on mice that express human tau without spontaneously developing tau pathology (hTau mice), by means of the intracerebroventricular injection of 24-OH. 24-OH, unlike 7-K, was found to completely prevent the hyperphosphorylation of tau induced by amyloid β monomers. These data highlight the importance of preventing the loss of 24-OH in the brain, and of maintaining high levels of the enzyme SIRT1, in order to counteract neurodegeneration.

Graphical abstract

A hypothetical scheme of the molecular mechanisms underlying the effects of 24-OH on hyperphosphorylated tau accumulation.fx1

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24-OH, unlike 7-K, upregulates the neuroprotective enzyme SIRT1.

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24-OH induces a cell redox imbalance leading to SIRT1 activation.

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24-OH prevents the accumulation of hyperphosphorylated tau.

Cholesterol-metabolizing enzyme cytochrome P450 46A1 as a pharmacologic target for Alzheimer's disease

Cytochrome P450 46A1 (CYP46A1 or cholesterol 24-hydroxylase) controls cholesterol elimination from the brain and plays a role in higher order brain functions. Genetically enhanced CYP46A1 expression in mouse models of Alzheimer's disease mitigates the manifestations of this disease. We enhanced CYP46A1 activity pharmacologically by treating 5XFAD mice, a model of rapid amyloidogenesis, with a low dose of the anti-HIV medication efavirenz. Efavirenz was administered from 1 to 9 months of age, and mice were evaluated at specific time points. At one month of age, cholesterol homeostasis was already disturbed in the brain of 5XFAD mice. Nevertheless, efavirenz activated CYP46A1 and mouse cerebral cholesterol turnover during the first four months of administration. This treatment time also reduced amyloid burden and microglia activation in the cortex and subiculum of 5XFAD mice as well as protein levels of amyloid precursor protein and the expression of several genes involved in inflammatory response. However, mouse short-term memory and long-term spatial memory were impaired, whereas learning in the context-dependent fear test was improved. Additional four months of drug administration (a total of eight months of treatment) improved long-term spatial memory in the treated as compared to the untreated mice, further decreased amyloid-β content in 5XFAD brain, and also decreased the mortality rate among male mice. We propose a mechanistic model unifying the observed efavirenz effects. We suggest that CYP46A1 activation by efavirenz could be a new anti-Alzheimer's disease treatment and a tool to study and identify normal and pathological brain processes affected by cholesterol maintenance.

Effect of vitamin E on 24(S)-hydroxycholesterol-induced necroptosis-like cell death and apoptosis

24(S)-Hydroxycholesterol (24S-OHC) has diverse physiological and pathological functions. In particular, cytotoxic effects of 24S-OHC in neuronal cells are important in development of neurodegenerative diseases. 24S-OHC induces necroptosis-like cell death in SH-SY5Y cells expressing little caspase-8. In the present study, 24S-OHC was found to induce apoptosis as determined by caspase-3 activation in all-trans-retinoic acid (atRA)-treated SH-SY5Y cells in which expression of caspase-8 was induced. 24S-OHC-induced cell death was inhibited by α-tocopherol (α-Toc) but not by α-tocotrienol (α-Toc3) in SH-SY5Y cells regardless of whether cells were treated with atRA. In contrast, cumene hydroperoxide (CumOOH)-induced cell death was significantly inhibited by α-Toc and α-Toc3. In atRA-treated SH-SY5Y cells, generation of reactive oxygen species (ROS) was induced by stimulation with CumOOH but was not induced by stimulation with 24S-OHC. These results suggest that inhibition of 24S-OHC-induced cell death by α-Toc cannot be explained by its radical scavenging antioxidant activity. Esterification of 24S-OHC followed by lipid droplet (LD) formation due to acyl-CoA:cholesterol acyltransferase 1 (ACAT1) are key events in 24S-OHC-induced cell death in atRA-treated SH-SY5Y cells as demonstrated by inhibition of cell death by ACAT1 inhibitor. LD number was not changed by treatment with either α-Toc or α-Toc3. The different physical properties of α-Toc and α-Toc3 may account for their different inhibitory effects on 24S-OHC-induced cell death.

Yi-Zhi-Fang-Dai Formula Protects against Aβ1-42 Oligomer Induced Cell Damage via Increasing Hsp70 and Grp78 Expression in SH-SY5Y Cells

Yi-Zhi-Fang-Dai formula (YZFDF) is an experiential prescription used to cure dementia cases like Alzheimer's disease (AD). In this study, the main effective compounds of YZFDF have been identified from this formula, and the neuroprotective effect against Aβ_1–42 oligomer of YZFDF has been tested in SH-SY5Y cells. Our results showed that YZFDF could increase cell viability and could attenuate endothelial reticula- (ER-) mediated apoptosis. Evidence indicated that protein folding and endothelial reticula stress (ERS) played an important role in the AD pathological mechanism. We further explored the expression of Hsp70, an important molecular chaperon facilitating the folding of other proteins, and Grp78, the marker protein of ERS in SH-SY5Y cells. Data told us that YZFDF pretreatment could influence the mRNA and protein expression of these two proteins. At last, we also found that YZFDF pretreatment could activate Akt in SH-SY5Y cells. All these above indicate that YZFDF could be a potent therapeutic candidate for AD treatment.

Cholesterol 24-hydroxylase: Brain cholesterol metabolism and beyond

Dysfunctions in brain cholesterol homeostasis have been extensively related to brain disorders. The major elimination pathway of brain cholesterol is its hydroxylation into 24 (S)-hydroxycholesterol by the cholesterol 24-hydroxylase (CYP46A1). Interestingly, there seems to be an association between CYP46A1 and high-order brain functions, in a sense that increased expression of this hydroxylase improves cognition, while a reduction leads to a poor cognitive performance. Moreover, increasing amount of epidemiological, biochemical and molecular evidence, suggests that CYP46A1 has a role in the pathogenesis or progression of neurodegenerative disorders, in which up-regulation of this enzyme is clearly beneficial. However, the mechanisms underlying these effects are poorly understood, which highlights the importance of studies that further explore the role of CYP46A1 in the central nervous system. In this review we summarize the major findings regarding CYP46A1, and highlight the several recently described pathways modulated by this enzyme from a physiological and pathological perspective, which might account for novel therapeutic strategies for neurodegenerative disorders.

Esterification of 24S-OHC induces formation of atypical lipid droplet-like structures, leading to neuronal cell death

The 24(S)-hydroxycholesterol (24S-OHC), which plays an important role in maintaining brain cholesterol homeostasis, has been shown to possess neurotoxicity. We have previously reported that 24S-OHC esterification by ACAT1 and the resulting lipid droplet (LD) formation are responsible for 24S-OHC-induced cell death. In the present study, we investigate the functional roles of 24S-OHC esters and LD formation in 24S-OHC-induced cell death, and we identify four long-chain unsaturated fatty acids (oleic acid, linoleic acid, arachidonic acid, and DHA) with which 24S-OHC is esterified in human neuroblastoma SH-SY5Y cells treated with 24S-OHC. Here, we find that cotreatment of cells with 24S-OHC and each of these four unsaturated fatty acids increases prevalence of the corresponding 24S-OHC ester and exacerbates induction of cell death as compared with cell death induced by treatment with 24S-OHC alone. Using electron microscopy, we find in the present study that 24S-OHC induces formation of LD-like structures coupled with enlarged endoplasmic reticulum (ER) lumina, and that these effects are suppressed by treatment with ACAT inhibitor. Collectively, these results illustrate that ACAT1-catalyzed esterification of 24S-OHC with long-chain unsaturated fatty acid followed by formation of atypical LD-like structures at the ER membrane is a critical requirement for 24S-OHC-induced cell death.

24(S)-Hydroxycholesterol as a Modulator of Neuronal Signaling and Survival

The major cholesterol metabolite in brain, 24(S)-hydroxycholesterol (24S-HC), serves as a vehicle for cholesterol removal. Its effects on neuronal function, however, have only recently begun to be investigated. Here, we review that nascent work. Our own studies have demonstrated that 24S-HC has potent positive modulatory effects on N-methyl-d-aspartate (NMDA) receptor (NMDAR) function. This could have implications not only for brain plasticity but also for pathological NMDAR overuse. Other work has demonstrated effects of 24S-HC on neuronal survival and as a possible biomarker of neurodegenerative disease. Depending on circumstances, both upregulation/mimicry of 24S-HC signaling and down-regulation/antagonism may have therapeutic potential. We are interested in the possibility that synthetic analogues of 24S-HC with positive effects at NMDARs may hold neurotherapeutic promise, given the role of NMDA receptor hypofunction in certain neuropsychiatric disorders.

Oxysterols and mechanisms of survival signaling

Oxysterols, a family of oxidation products of cholesterol, are increasingly drawing attention of scientists to their multifaceted biochemical properties, several of them of clear relevance to human pathophysiology. Taken up by cells through both vesicular and non-vesicular ways or often generated intracellularly, oxysterols contribute to modulate not only the inflammatory and immunological response but also cell viability, metabolism and function by modulating several signaling pathways. Moreover, they have been recognized as elective ligands for the most important nuclear receptors. The outcome of such a complex network of intracellular reactions promoted by these cholesterol oxidation products appears to be largely dependent not only on the type of cells, the dynamic conditions of the cellular and tissue environment but also on the concentration of the oxysterols. Here focus has been given to the cascade of molecular events exerted by relatively low concentrations of certain oxysterols that elicit survival and functional signals in the cells, with the aim to contribute to further expand the knowledge about the biological and physiological potential of the biochemical reactions triggered and modulated by oxysterols.

CYP46A1, the rate-limiting enzyme for cholesterol degradation, is neuroprotective in Huntington's disease

Huntington’s disease is an autosomal dominant neurodegenerative disease caused by abnormal polyglutamine expansion in huntingtin (Exp-HTT) leading to degeneration of striatal neurons. Altered brain cholesterol homeostasis has been implicated in Huntington’s disease, with increased accumulation of cholesterol in striatal neurons yet reduced levels of cholesterol metabolic precursors. To elucidate these two seemingly opposing dysregulations, we investigated the expression of cholesterol 24-hydroxylase (CYP46A1), the neuronal-specific and rate-limiting enzyme for cholesterol conversion to 24S-hydroxycholesterol (24S-OHC). CYP46A1 protein levels were decreased in the putamen, but not cerebral cortex samples, of post-mortem Huntington’s disease patients when compared to controls. Cyp46A1 mRNA and CYP46A1 protein levels were also decreased in the striatum of the R6/2 Huntington’s disease mouse model and in ST hdh Q111 cell lines. In vivo , in a wild-type context, knocking down CYP46A1 expression in the striatum, via an adeno-associated virus-mediated delivery of selective shCYP46A1, reproduced the Huntington’s disease phenotype, with spontaneous striatal neuron degeneration and motor deficits, as assessed by rotarod. In vitro , CYP46A1 restoration protected ST hdh Q111 and Exp-HTT-expressing striatal neurons in culture from cell death. In the R6/2 Huntington’s disease mouse model, adeno-associated virus-mediated delivery of CYP46A1 into the striatum decreased neuronal atrophy, decreased the number, intensity level and size of Exp-HTT aggregates and improved motor deficits, as assessed by rotarod and clasping behavioural tests. Adeno-associated virus-CYP46A1 infection in R6/2 mice also restored levels of cholesterol and lanosterol and increased levels of desmosterol. In vitro , lanosterol and desmosterol were found to protect striatal neurons expressing Exp-HTT from death. We conclude that restoring CYP46A1 activity in the striatum promises a new therapeutic approach in Huntington’s disease.

ACAT1/SOAT1 as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia with no cure at present. Cholesterol metabolism is closely associated with AD at several stages. ACAT1 converts free cholesterol to cholesteryl esters, and plays important roles in cellular cholesterol homeostasis. Recent studies show that in a mouse model, blocking ACAT1 provides multiple beneficial effects on AD. Here we review the current evidence that implicates ACAT1 as a therapeutic target for AD. We also discuss the potential usage of various ACAT inhibitors currently available to treat AD.

Understanding the cholesterol metabolism-perturbing effects of docosahexaenoic acid by gas chromatography-mass spectrometry targeted metabonomic profiling

PURPOSE

Over the past few decades, docosahexaenoic acid (DHA) has gained special attention for management of cholesterol-associated metabolic disorders and neurodegenerative diseases such as Alzheimer's disease (AD) owing to its neuroprotective, anti-inflammatory and hypolipidemic properties. Several epidemiological studies have reported the effect of DHA in reducing the risk of developing AD by lowering cholesterol. Hypercholesterolemia is a pro-amyloidogenic factor influencing the enzymatic processing of amyloid-β precursor protein (AβPP) to toxic β-amyloid. However, the mechanism by which DHA modulates the cholesterol pathway has not been established. Thus, the objective of this study was to investigate the mechanism of regulation of cholesterol metabolism by DHA in an AβPP₆₉₅ overexpressing AD cell model.

METHODS

A gas chromatography/mass spectrometry method was developed and validated for the targeted profiling of 11 cholesterol metabolites in DHA-treated Chinese hamster ovary wild-type (CHO-wt) and AβPP₆₉₅ overexpressing (CHO-AβPP₆₉₅) cells. The differential metabolite profiles between DHA- and vehicle-treated groups were further analyzed using fold change values of the ratio of concentration of metabolites in CHO-AβPP₆₉₅ to CHO-wt cells. Effect of DHA on key rate-limiting enzymatic activities within the cholesterol pathway was established using biochemical assays.

RESULTS

Our results showed that DHA reduced the levels of key cholesterol anabolites and catabolites in CHO-AβPP₆₉₅ cells as compared to CHO-wt cells. Further enzymatic studies revealed that the cholesterol-lowering effect of DHA was mediated by regulating HMG-CoA reductase and squalene epoxidase enzyme activities.

CONCLUSION

We demonstrate for the first time the dual effects of DHA in inhibiting HMG-CoA reductase and squalene epoxidase and modulating the sterol biosynthesis axis of the cholesterol pathway in AβPP₆₉₅ overexpressing AD. Our novel findings underscore the potential of DHA as a multi-target hypocholesterolemic agent for the prophylaxis of AD and other cholesterol-associated diseases.

New aspects of 24(S)-hydroxycholesterol in modulating neuronal cell death

24(S)-Hydroxycholesterol (24S-OHC), which is enzymatically produced in the brain, has been known to play an important role in maintaining cholesterol homeostasis in the brain and has been proposed as a possible biomarker of neurodegenerative disease. Recent studies have revealed diverse functions of 24S-OHC and gained increased attention. For example, 24S-OHC at sublethal concentrations has been found to induce an adaptive response via activation of the liver X receptor signaling pathway, thereby protecting neuronal cells against subsequent oxidative stress. It has also been found that physiological concentrations of 24S-OHC suppress amyloid-β production via downregulation of amyloid precursor protein trafficking in neuronal cells. On the other hand, high concentrations of 24S-OHC have been found to induce a type of nonapoptotic programmed cell death in neuronal cells expressing little caspase-8. Because neuronal cell death induced by 24S-OHC has been found to proceed by a unique mechanism, which is different from but in some ways similar to necroptosis-necroptosis being a type of programmed necrosis induced by tumor necrosis factor α-neuronal cell death induced by 24S-OHC has been called "necroptosis-like" cell death. 24S-OHC-induced cell death is dependent on the formation of 24S-OHC esters but not on oxidative stress. This review article discusses newly reported aspects of 24S-OHC in neuronal cell death and sheds light on the possible importance of controlling 24S-OHC levels in the brain for preventing neurodegenerative disease.

Cholestenoic acid, an endogenous cholesterol metabolite, is a potent γ-secretase modulator

BACKGROUND

Amyloid-β (Aβ) 42 has been implicated as the initiating molecule in the pathogenesis of Alzheimer's disease (AD); thus, therapeutic strategies that target Aβ42 are of great interest. γ-Secretase modulators (GSMs) are small molecules that selectively decrease Aβ42. We have previously reported that many acidic steroids are GSMs with potencies ranging in the low to mid micromolar concentration with 5β-cholanic acid being the most potent steroid identified GSM with half maximal effective concentration (EC50) of 5.7 μM.

RESULTS

We find that the endogenous cholesterol metabolite, 3β-hydroxy-5-cholestenoic acid (CA), is a steroid GSM with enhanced potency (EC50 of 250 nM) relative to 5β-cholanic acid. CA i) is found in human plasma at ~100-300 nM concentrations ii) has the typical acidic GSM signature of decreasing Aβ42 and increasing Aβ38 levels iii) is active in in vitro γ-secretase assay iv) is made in the brain. To test if CA acts as an endogenous GSM, we used Cyp27a1 knockout (Cyp27a1-/-) and Cyp7b1 knockout (Cyp7b1-/-) mice to investigate if manipulation of cholesterol metabolism pathways relevant to CA formation would affect brain Aβ42 levels. Our data show that Cyp27a1-/- had increased brain Aβ42, whereas Cyp7b1-/- mice had decreased brain Aβ42 levels; however, peripheral dosing of up to 100 mg/kg CA did not affect brain Aβ levels. Structure-activity relationship (SAR) studies with multiple known and novel CA analogs studies failed to reveal CA analogs with increased potency.

CONCLUSION

These data suggest that CA may act as an endogenous GSM within the brain. Although it is conceptually attractive to try and increase the levels of CA in the brain for prevention of AD, our data suggest that this will not be easily accomplished.

24(S)-Hydroxycholesterol induces RIPK1-dependent but MLKL-independent cell death in the absence of caspase-8

24(S)-Hydroxycholesterol (24S-OHC), which is enzymatically produced in the brain, is known to play an important role in maintaining brain cholesterol homeostasis. We have previously reported that 24S-OHC induces a type of non-apoptotic programmed necrosis in neuronal cells expressing little caspase-8. Necroptosis has been characterized as a type of programmed necrosis in which activation of receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like (MLKL) is involved in the signaling pathway. In the present study, we investigated the involvement of these three proteins in 24S-OHC-induced cell death. We found that RIPK1 but neither RIPK3 nor MLKL was expressed in human neuroblastoma SH-SY5Y cells, while all three proteins were expressed in human T lymphoma caspase-8-deficient Jurkat (Jurkat(Cas8-/-)) cells. In Jurkat(Cas8-/-) cells, tumor necrosis factor α (TNFα)-induced cell death was significantly suppressed by treatment with respective inhibitors of RIPK1, RIPK3, and MLKL. In contrast, only RIPK1 inhibitor showed significant suppression of 24S-OHC-induced cell death, and even this was less prominent than was observed in TNFα-induced cell death. In Jurkat(Cas8-/-) cells, knockdown of either RIPK1 or RIPK3 caused moderate but significant suppression of 24S-OHC-induced cell death, but no such effect was observed as a result of knockdown of MLKL. Collectively, these results suggest that, for both SH-SY5Y cells and Jurkat(Cas8-/-) cells, 24S-OHC-induced cell death is dependent on RIPK1 but not on MLKL. We therefore conclude that, in the absence of caspase-8 activity, 24S-OHC induces a necroptosis-like cell death which is RIPK1-dependent but MLKL-independent.

Oxidized cholesterol as the driving force behind the development of Alzheimer's disease

Alzheimer’s disease (AD), the most common neurodegenerative disorder associated with dementia, is typified by the pathological accumulation of amyloid Aβ peptides and neurofibrillary tangles (NFT) within the brain. Considerable evidence indicates that many events contribute to AD progression, including oxidative stress, inflammation, and altered cholesterol metabolism. The brain’s high lipid content makes it particularly vulnerable to oxidative species, with the consequent enhancement of lipid peroxidation and cholesterol oxidation, and the subsequent formation of end products, mainly 4-hydroxynonenal and oxysterols, respectively from the two processes. The chronic inflammatory events observed in the AD brain include activation of microglia and astrocytes, together with enhancement of inflammatory molecule and free radical release. Along with glial cells, neurons themselves have been found to contribute to neuroinflammation in the AD brain, by serving as sources of inflammatory mediators. Oxidative stress is intimately associated with neuroinflammation, and a vicious circle has been found to connect oxidative stress and inflammation in AD. Alongside oxidative stress and inflammation, altered cholesterol metabolism and hypercholesterolemia also significantly contribute to neuronal damage and to progression of AD. Increasing evidence is now consolidating the hypothesis that oxidized cholesterol is the driving force behind the development of AD, and that oxysterols are the link connecting the disease to altered cholesterol metabolism in the brain and hypercholesterolemia; this is because of the ability of oxysterols, unlike cholesterol, to cross the blood brain barrier (BBB). The key role of oxysterols in AD pathogenesis has been strongly supported by research pointing to their involvement in modulating neuroinflammation, Aβ accumulation, and cell death. This review highlights the key role played by cholesterol and oxysterols in the brain in AD pathogenesis.

The role of leptin in the sporadic form of Alzheimer's disease. Interactions with the adipokines amylin, ghrelin and the pituitary hormone prolactin

Leptin (Lep) is emerging as a pivotal molecule involved in both the early events and the terminal phases of Alzheimer's disease (AD). In the canonical pathway, Lep acts as an anorexigenic factor via its effects on hypothalamic nucleus. However, additional functions of Lep in the hippocampus and cortex have been unravelled in recent years. Early events in the sporadic form of AD likely involve cellular level alterations which can have an effect on food intake and metabolism. Thus, AD can be conceivably interpreted as a multiorgan pathology that not only results in a dramatic neuronal loss in brain areas such as the hippocampus and the cortex (ultimately leading to a significant cognitive impairment) but as a disease which also affects body-weight homeostasis. According to this view, body-weight control disruptions are to be expected in both the early- and late-stage AD, concomitant with changes in serum Lep content, alterations in Lep transport across the blood-brain barrier (BBB) and Lep receptor-related signalling abnormalities. Lep is a member of the adipokine family of molecules, while the Lep receptor belongs to the class I cytokine receptors. Since cellular response to adipokine signalling can be either potentiated or diminished as a result of specific ligand-receptor interactions, Lep interactions with other members of the adipokine family including amylin, ghrelin and hormones such as prolactin require further investigation. In this review, we provide a general perspective on the functions of Lep in the brain, with a particular focus on the sporadic AD.

Adipokine pathways are altered in hippocampus of an experimental mouse model of Alzheimer's disease

A growing body of evidence suggests that β-amyloid peptides (Aβ) are unlikely to be the only factor involved in Alzheimer's disease (AD) aetiology. In fact, a strong correlation has been established between AD patients and patients with type 2 diabetes and/or cholesterol metabolism alterations. In addition, a link between adipose tissue metabolism, leptin signalling in particular, and AD has also been demonstrated. In the present study we analyzed the expression of molecules related to metabolism, with the main focus on leptin and prolactin signalling pathways in an APPswe/PS1dE9 (APP/PS1) transgenic mice model, at 3 and 6 months of age, compared to wild-type controls. We have chosen to study 3 months-old APP/PS1 animals at an age when neither the cognitive deficits nor significant Aβ plaques in the brain are present, and to compare them to the 6 months-old mice, which exhibit elevated levels of Aβ in the hippocampus and memory loss. A significant reduction in both mRNA and protein levels of the prolactin receptor (PRL-R) was detected in the hippocampi of 3 months old APP/PS1 mice, with a decrease in the levels of the leptin receptor (OB-R) first becoming evident at 6 months of age. We proceeded to study the expression of the intracellular signalling molecules downstream of these receptors, including stat (1-5), sos1, kras and socs (1-3). Our data suggest a downregulation in some of these molecules such as stat-5b and socs (1-3), in 3 months-old APP/PS1 brains. Likewise, at the same age, we detected a significant reduction in mRNA levels of lrp1 and cyp46a1, both of which are involved in cholesterol homeostasis. Taken together, these results demonstrate a significative impairment in adipokine receptors signalling and cholesterol regulation pathways in the hippocampus of APP/PS1 mice at an early age, prior to the Aβ plaque formation.

Lipids in the nervous system: from biochemistry and molecular biology to patho-physiology

Lipids in the nervous system accomplish a great number of key functions, from synaptogenesis to impulse conduction, and more. Most of the lipids of the nervous system are localized in myelin sheaths. It has long been known that myelin structure and brain homeostasis rely on specific lipid-protein interactions and on specific cell-to-cell signaling. In more recent years, the growing advances in large-scale technologies and genetically modified animal models have provided valuable insights into the role of lipids in the nervous system. Key findings recently emerged in these areas are here summarized. In addition, we briefly discuss how this new knowledge can open novel approaches for the treatment of diseases associated with alteration of lipid metabolism/homeostasis in the nervous system. This article is part of a Special Issue entitled Linking transcription to physiology in lipidomics.

Role of cholesterol metabolism in the pathogenesis of Alzheimer's disease

PURPOSE OF REVIEW

Cholesterol has been shown to stimulate the cleavage of amyloid precursor protein (APP) into amyloid peptides involved in Alzheimer's disease. However, high level of peripheral cholesterol as a risk factor for Alzheimer's disease is still debated. This current review provides an update of the recent literature on cholesterol and APP metabolisms in the brain.

RECENT FINDINGS

First, a new relationship between neuronal APP and cholesterol has been shown in which this protein controls cholesterol turnover required for neuronal activity. Second, oxysterols are able to stimulate the synthesis of ATP-binding cassette transporters involved in the exchange of amyloid peptides between the blood and the brain. Third, changes in APP targeting to lipid rafts and/or their composition in cholesterol regulate amyloid peptide production.

SUMMARY

These recent findings open new areas of investigations to control the neuronal activity and to decrease the amyloid peptide levels in brain, opening on new preventive and therapeutic strategies for Alzheimer's disease.