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.

Mass Spectrometry Imaging of Cholesterol and Oxysterols

Mass spectrometry imaging (MSI) is a new technique in the toolbox of the analytical biochemist. It allows the generation of a compound-specific image from a tissue slice where a measure of compound abundance is given pixel by pixel, usually displayed on a color scale. As mass spectra are recorded at each pixel, the data can be interrogated to generate images of multiple different compounds all in the same experiment. Mass spectrometry (MS) requires the ionization of analytes, but cholesterol and other neutral sterols tend to be poorly ionized by the techniques employed in most MSI experiments, so despite their high abundance in mammalian tissues, cholesterol is poorly represented in the MSI literature. In this chapter, we discuss some of the MSI studies where cholesterol has been imaged and introduce newer methods for its analysis by MSI. Disturbed cholesterol metabolism is linked to many disorders, and the potential of MSI to study cholesterol, its precursors, and its metabolites in animal models and from human biopsies will be discussed.

Efficacy, safety, and tolerability of soticlestat as adjunctive therapy for the treatment of seizures in patients with Dup15q syndrome or CDKL5 deficiency disorder in an open-label signal-finding phase II study (ARCADE)

OBJECTIVE

Chromosome 15q duplication (Dup15q) syndrome and cyclin‑dependent kinase-like 5 deficiency disorder (CDD) are rare neurodevelopmental disorders associated with epileptic encephalopathies, with a lack of specifically approved treatment options. ARCADE assessed the efficacy and safety of adjunctive soticlestat (TAK-935) for the treatment of seizures in patients with Dup15q syndrome or CDD (NCT03694275).

METHODS

ARCADE was a phase II, open-label, pilot study of soticlestat (≤300 mg/day twice daily, weight-adjusted) in pediatric and adult patients 2-55 years of age with Dup15q syndrome or CDD who experienced ≥3 motor seizures per month in the 3 months before screening and at baseline. The 20-week treatment period consisted of a dose-optimization period and a 12-week maintenance period. Efficacy endpoints included the change from baseline in motor seizure frequency during the maintenance period and the proportion of treatment responders. Safety endpoints included the incidence of treatment-emergent adverse effects (TEAEs).

RESULTS

The modified-intent-to-treat population included 20 participants who received ≥1 dose of soticlestat and had ≥1 efficacy assessment (Dup15q syndrome, n = 8; CDD, n = 12). Soticlestat administration during the maintenance period was associated with a median change from baseline in motor seizure frequency of +11.7% in the Dup15q syndrome group and -23.6% in the CDD group. Reductions in all seizure frequency of -23.4% and -30.5% were also observed during the maintenance period in the Dup15q syndrome group and the CDD group, respectively. Most TEAEs were of mild or moderate severity. Serious TEAEs were reported by three patients (15.0%); none were considered drug related. The most common TEAEs were constipation, rash, and seizure. No deaths were reported.

CONCLUSIONS

Adjunctive soticlestat treatment was associated with a decrease in motor seizure frequency from baseline in patients with CDD and a decrease in all seizure frequency in both patient groups. Soticlestat treatment was associated with an increase in motor seizure frequency in patients with Dup15q syndrome.

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.

Characterization of three naturally occurring lignans, sesamol, sesamolin, and sesamin, as potent inhibitors of human cytochrome P450 46A1: Implications for treating excitatory neurotoxicity

CYP46A1 is a brain-specific enzyme responsible for cholesterol homeostasis. Inhibition of CYP46A1 activity serves as a therapeutic target for excitatory neurotoxicity. Sesame is a common medicine and food resource; its component lignans possess various pharmacological activities. In this study, the inhibitory effects of sesame lignans on CYP46A1 activity were investigated. Inhibition kinetics analyses revealed that sesamin and sesamolin produce mixed partial competitive inhibition of CYP46A1, while sesamol produces non-competitive inhibition. Notably, molecular simulations revealed that the sesame lignans have excellent orientations within the active cavity of CYP46A1. Importantly, the sesame lignans had high permeability coefficients and low efflux ratios. Furthermore, sesamin significantly reduced the levels of 24S-hydroxycholesterol in rat plasma and brain tissues, and down-regulated the protein expressions of CYP46A1, NMDAR2A, NMDAR2B, and HMGCR. Collectively, sesame lignans exhibit significant inhibitory effects on CYP46A1 activity, highlighting their potential therapeutic role in treating excitatory neurotoxicity.

Association of Cholesterol and Oxysterols in Adipose Tissue With Obesity and Metabolic Syndrome Traits

OBJECTIVE

Adipose tissue stores a substantial amount of body cholesterol in humans. Obesity is associated with decreased concentrations of serum cholesterol. During weight gain, adipose tissue dysfunction might be one of the causes of metabolic syndrome. The aim of this study is to evaluate cholesterol storage and oxidized metabolites in adipose tissue and their relationship with metabolic clinical characteristics.

METHODS

Concentrations of cholesterol and oxysterols (27-hydroxycholesterol and 24S-hydroxycholesterol) in subcutaneous and visceral adipose tissue were determined by high-performance liquid chromatography with tandem mass spectrometry in 19 adult women with body mass index between 23 and 40 kg/m2 from the FAT expandability (FATe) study. Tissue concentration values were correlated with biochemical and clinical characteristics using nonparametric statistics.

RESULTS

Insulin correlated directly with 24S-hydroxycholesterol in both adipose tissues and with 27-hydroxycholesterol in visceral tissue. Leptin correlated directly with 24S-hydroxycholesterol in subcutaneous adipose tissue. Tissue cholesterol correlated directly with 27-hydroxycholesterol in both adipose tissues and with 24S-hydroxycholesterol in visceral tissue, where cholesterol correlation with 24S-hydroxycholesterol was higher than with 27-hydroxycholesterol. In addition, some tendencies were observed: serum high-density lipoprotein cholesterol tended to be inversely correlated with visceral adipose tissue cholesterol; high-sensitivity C-reactive protein tended to be correlated directly with subcutaneous adipose 24S-hydroxycholesterol and inversely with visceral 27-hydroxycholesterol.

CONCLUSIONS

Adipose tissue oxysterols are associated with blood insulin and insulin resistance. Tissue cholesterol correlated more with 27-hydroxycholesterol in subcutaneous adipose tissue and with 24S-hydroxycholesterol in visceral adipose tissue. Levels of adipose 24S-hydroxycholesterol seem to be correlated with some metabolic syndrome symptoms and inflammation while adipose 27-hydroxycholesterol could represent some protection against them.

Liquid chromatography coupled to tandem mass spectrometry methods for the selective and sensitive determination of 24S-hydroxycholesterol, its sulfate, and/or glucuronide conjugates in plasma

24S-hydroxycholesterol (i.e., cerebrosterol, 24S-OH-Chol) is the main form of cholesterol elimination from the brain. Liquid chromatography-tandem mass spectrometry methods were developed for the quantification of the total and unesterified/unbound fractions of 24S-OH-Chol, its monosulfate, monoglucuronide, and diconjugate derivatives (24S-OH-Chol-3sulfate [3S], 24S-OH-Chol-24glucuronide [24G] and 24S-OH-Chol-3S, 24G, respectively) in human plasma. Linearity, precision, accuracy, and extraction recovery were validated within the typical physiological and pathological ranges of concentrations for each compound. The lower limit of quantifications was 2.00, 0.33, 0.26, and 0.74 ng/ml for 24S-OH-Chol, 24S-OH-Chol-24G, 24S-OH-Chol-3S, and 24-OH-Chol-3S, 24G, respectively. Extraction recovery values in total and unbound plasma fractions were also analyzed in murine and monkey plasma and varied from 73% in mouse to 113% in cynomolgus monkey. The methods could rapidly (less than 7 min) quantify individual compounds with high sensitivity, accuracy (bias ≤15%), and reproducibility (coefficient of variation [CV] ≤ 17%). Their clinical applications were validated by measuring levels of the 4 compounds in samples from 20 noncholestatic donors, 5 cholestatic patients suffering from primary biliary cirrhosis, and 10 patients suffering from biliary stenosis. Results highlight the abundance of 24S-OH-Chol in the total fraction and the abundance of 24S-OH-Chol-3S and 24G in the unbound ones. While the latter strongly accumulate in plasma fractions of cholestatic patients, levels of 24S-OH-Chol remained similar to those of healthy donors. Our results indicate that this approach is suitable for monitoring cerebrosterol and its conjugates in large-scale clinical studies.

Regulation of Th17/Treg Balance by 27-Hydroxycholesterol and 24S-Hydroxycholesterol Correlates with Learning and Memory Ability in Mice

Dysregulation of cholesterol metabolism and its oxidative products-oxysterols-in the brain is known to be associated with neurodegenerative diseases. It is well-known that 27-hydroxycholesterol (27-OHC) and 24S-hydroxycholesterol (24S-OHC) are the main oxysterols contributing to the pathogenesis of Alzheimer's disease (AD). However, the molecular mechanism of how 27-OHC and 24S-OHC cause cognitive decline remains unclear. To verify whether 27-OHC and 24S-OHC affect learning and memory by regulating immune responses, C57BL/6J mice were subcutaneously injected with saline, 27-OHC, 24S-OHC, 27-OHC+24S-OHC for 21 days. The oxysterols level and expression level of related metabolic enzymes, as well as the immunomodulatory factors were measured. Our results indicated that 27-OHC-treated mice showed worse learning and memory ability and higher immune responses, but lower expression level of interleukin-10 (IL-10) and interferon (IFN-λ2) compared with saline-treated mice, while 24S-OHC mice performed better in the Morris water maze test than control mice. No obvious morphological lesion was observed in these 24S-OHC-treated mice. Moreover, the expression level of interleukin-17A (IL-17A), granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage inflammatory protein 3α (MIP-3α) were significantly decreased after 24S-OHC treatment. Notably, compared with 27-OHC group, mice treated with 27-OHC+24S-OHC showed higher brain 24S-OHC level, accompanied by increased CYP46A1 expression level while decreased CYP7B1, retinoic acid-related orphan receptor gamma t (RORγt) and IL-17A expression level. In conclusion, our study indicated that 27-OHC is involved in regulating the expression of RORγt, disturbing Th17/Treg balance-related immune responses which may be associated with the learning and memory impairment in mice. In contrast, 24S-OHC is neuroprotective and attenuates the neurotoxicity of 27-OHC.

Pharmacokinetics, pharmacodynamics and safety assessment of multiple doses of soticlestat in healthy volunteers

Aims

Soticlestat, a first‐in‐class inhibitor of cholesterol 24‐hydroxylase (also known as cytochrome P450 46A1), is currently in development for the treatment of developmental and epileptic encephalopathies. Here, we report safety, tolerability, pharmacokinetic and pharmacodynamic outcomes from a phase I, randomized, double‐blind, placebo‐controlled, multiple‐rising‐dose study of soticlestat in healthy adults.

Methods

Five cohorts of healthy subjects (n = 8 each, randomized 6:2 soticlestat:placebo) received oral soticlestat 100‐600 mg once daily (QD) or 300 mg twice daily (BID) for 10‐14 days. Serial blood and urine samples were obtained on days 1, 7 (blood only) and 14.

Results

Soticlestat in the dose range 100‐400 mg/day for up to 14 days was generally well tolerated. In total, 45 treatment‐emergent adverse events (TEAEs) were reported; most (91%) were transient and mild in intensity. Two subjects experienced TEAEs leading to discontinuation: one receiving soticlestat 600 mg QD reported a severe event of acute psychosis; another receiving 300 mg BID reported a mild event of confusional state. Steady‐state exposure to soticlestat increased in a slightly greater than dose‐proportional manner across the dose range 100‐400 mg QD. Peak plasma concentrations were reached within 0.33‐0.5 hour, and soticlestat elimination half‐life was approximately 4 hours. Renal excretion of soticlestat was negligible. Soticlestat 100‐400 mg QD reduced 24S‐hydroxycholesterol levels by 46.8 (coefficient of variation [CV%] −9.2) to −62.7% (CV% −7.3) at steady state; values of enzymatic inhibition were compatible with antiepileptic effects observed in preclinical models.

Conclusion

The pharmacokinetic and pharmacodynamic profiles of soticlestat characterized here provided a data‐driven rationale for clinical trial dose selection.

Increased Plasma Level of 24S-Hydroxycholesterol and Polymorphism of CYP46A1 SNP (rs754203) Are Associated With Mild Cognitive Impairment in Patients With Type 2 Diabetes

Background

Abnormal cholesterol metabolism is common in type 2 diabetes mellitus (T2DM) and causes dementia. Cholesterol 24S-hydroxylase (CYP46A1) converts cholesterol into 24S-hydroxycholesterol (24-OHC) and maintains cholesterol homeostasis in the brain.

Objective

This study aimed to investigate the roles of 24-OHC and the CYP46A1 (rs754203) polymorphism in patients with T2DM and mild cognitive impairment (MCI).

Methods

A total of 193 Chinese patients with T2DM were recruited into two groups according to the Montreal Cognitive Assessment (MoCA). Demographic and clinical data were collected, and neuropsychological tests were conducted. Enzyme-linked immunosorbent assay (ELISA) and Seqnome method were used to detect the concentration of plasma 24-OHC and the CYP46A1 rs754203 genotype, respectively.

Results

Compared with 118 healthy cognition participants, patients with MCI (n = 75) displayed a higher plasma level of 24-OHC and total cholesterol concentration (all p = 0.031), while no correlation was found between them. In the overall diabetes population, the plasma level of 24-OHC was negatively correlated with MoCA (r = −0.150, p = 0.039), and it was further proved to be an independent risk factor of diabetic MCI (OR = 1.848, p = 0.001). Additionally, patients with MCI and the CC genotype of CYP46A1 rs754203 showed the highest plasma level of 24-OHC even though the difference was not statistically significant, and they obtained low scores in both the verbal fluency test and Stroop color and word test A (p = 0.008 and p = 0.029, respectively).

Conclusion

In patients with T2DM, high plasma level of 24-OHC and the CC genotype carrier of CYP46A1 rs754203 may portend a high risk of developing early cognitive impairment, including attention and executive deficits.

Safety, tolerability, pharmacokinetics, pharmacodynamics, bioavailability and food effect of single doses of soticlestat in healthy subjects

Aims

Soticlestat is a first‐in‐class selective inhibitor of cholesterol 24‐hydroxylase, the enzyme that converts brain cholesterol to 24S‐hydroxycholesterol (24HC), a positive allosteric modulator of N‐methyl‐D‐aspartate receptors. Soticlestat is under development as treatment for rare developmental and epileptic encephalopathies.

Methods

In this first‐in‐human study, 48 healthy men and women received single ascending doses of soticlestat oral solution or placebo. Subsequently, nine healthy subjects received soticlestat tablets under fed and fasting conditions to assess the relative oral bioavailability and effects of food. Serial blood and urine samples were collected for pharmacokinetic and pharmacodynamic assessments.

Results

Soticlestat appeared to be well tolerated up to a single dose of 1350 mg. Adverse events (AEs) were mild in intensity, and dose‐dependent increase in AE prevalence was not apparent. Soticlestat administered via oral solution was rapidly absorbed (median time to maximum plasma concentration [C_max] 0.250–0.520 h). Mean C_max and area under plasma concentration–time curve from zero to infinity increased by 183‐ and 581‐fold, respectively, over a 90‐fold dose increase. Mean terminal elimination half‐life was 0.820–7.16 hours across doses. Renal excretion was negligible. Administration of soticlestat tablets, and with food, lowered C_max but did not affect overall exposure. Plasma 24HC concentrations generally decreased with increasing dose.

Conclusions

Soticlestat appeared to be well tolerated after a single oral administration of up to 1350 mg and dose‐dependently reduced plasma 24HC concentrations. Systemic exposure increased in a greater than dose‐proportional manner over the dose range evaluated but was not affected by formulation or administration with food.

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.

Effects of CYP46A1 Inhibition on Long-Term-Depression in Hippocampal Slices ex vivo and 24S-Hydroxycholesterol Levels in Mice in vivo

The manipulation of cholesterol and its metabolites has been hypothesized to be therapeutically beneficial for mood disorders, neurodegenerative disorders, and epilepsies. A major regulator of cholesterol clearance and turnover in the central nervous system is CYP46A1, a brain enriched enzyme responsible for metabolism of cholesterol into 24S-hydroxycholesterol. Inhibition of this enzyme may negatively modulate NMDARs as 24S-hydroxycholesterol was shown to enhance NMDAR function. In addition, alterations of local cholesterol or other changes mediated by CYP46A1 activity could have important influences on central nervous system function. Here we demonstrate that humans and mice display brain region specific and similar CYP46A1 and 24S-hydroxycholesterol distribution. Treatment with distinct classes of CYP46A1 inhibitors led to central 24S-hydroxycholesterol reduction in vivo and ablation of long term depression in hippocampal slices. Our results suggest that rodents show similarity to humans for studying the impact of CYP46A1 inhibitors and that rapid, local modulation of oxysterols can be achieved through CYP46A1 inhibition.

Neuro-oxysterols and neuro-sterols as ligands to nuclear receptors, GPCRs, ligand-gated ion channels and other protein receptors

The brain is the most cholesterol rich organ in the body containing about 25% of the body's free cholesterol. Cholesterol cannot pass the blood-brain barrier and be imported or exported; instead, it is synthesised in situ and metabolised to oxysterols, oxidised forms of cholesterol, which can pass the blood-brain barrier. 24S-Hydroxycholesterol is the dominant oxysterol in the brain after parturition, but during development, a myriad of other oxysterols are produced, which persist as minor oxysterols after birth. During both development and in later life, sterols and oxysterols interact with a variety of different receptors, including nuclear receptors, membrane bound GPCRs, the oxysterol/sterol sensing proteins INSIG and SCAP, and the ligand-gated ion channel NMDA receptors found in nerve cells. In this review, we summarise the different oxysterols and sterols found in the CNS whose biological activity is transmitted via these different classes of protein receptors. 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.

Orphan Nuclear Receptor RORα Regulates Enzymatic Metabolism of Cerebral 24S-Hydroxycholesterol through CYP39A1 Intronic Response Element Activation

Oxysterols, important regulators of cholesterol homeostasis in the brain, are affected by neurodegenerative diseases. Early-onset Alzheimer’s disease is associated with higher levels of circulating brain-derived 24S-hydroxycholesterol (24S-OHC). Conversion of cholesterol to 24S-OHC is mediated by cholesterol 24S-hydroxylase in the brain, which is the major pathway for oxysterol elimination, followed by oxidation through hepatic first-pass metabolism by CYP39A1. Abnormal CYP39A1 expression results in accumulation of 24S-OHC, influencing neurodegenerative disease-related deterioration; thus, it is important to understand the normal elimination of 24S-OHC and the system regulating CYP39A1, a selective hepatic metabolic enzyme of 24S-OHC. We examined the role of transcriptional regulation by retinoic acid receptor-related orphan receptor α (RORα), a nuclear receptor that responds to oxysterol ligands. In humans, the promoter and first intronic regions of CYP39A1 contain two putative RORα response elements (ROREs). RORα binding and responses of these ROREs were assessed using electrophoretic mobility shift, chromatin immunoprecipitation, and luciferase reporter assays. CYP39A1 was upregulated by RORα overexpression in HEK293 cells, while RORα knockdown by siRNA significantly downregulated CYP39A1 expression in human hepatoma cells. Additionally, CYP39A1 was induced by RORα agonist treatment, suggesting that CYP39A1 expression is activated by RORα nuclear receptors. This may provide a way to increase CYP39A1 activity using RORα agonists, and help halt 24S-OHC accumulation in neurodegenerative illnesses.

Localization of sterols and oxysterols in mouse brain reveals distinct spatial cholesterol metabolism

The brain is a remarkably complex organ and cholesterol homeostasis underpins brain function. It is known that cholesterol is not evenly distributed across different brain regions; however, the precise map of cholesterol metabolism in the brain remains unclear. If cholesterol metabolism is to be correlated with brain function it is essential to generate such a map. Here we describe an advanced mass spectrometry platform to reveal spatial cholesterol metabolism in situ at 400-µm spot diameter on 10-µm tissue slices from mouse brain. We mapped, not only cholesterol, but also other biologically active sterols arising from cholesterol turnover in both wild type and mice lacking cholesterol 24S-hydroxylase (CYP46A1), the major cholesterol metabolizing enzyme.

Dysregulated cholesterol metabolism is implicated in a number of neurological disorders. Many sterols, including cholesterol and its precursors and metabolites, are biologically active and important for proper brain function. However, spatial cholesterol metabolism in brain and the resulting sterol distributions are poorly defined. To better understand cholesterol metabolism in situ across the complex functional regions of brain, we have developed on-tissue enzyme-assisted derivatization in combination with microliquid extraction for surface analysis and liquid chromatography-mass spectrometry to locate sterols in tissue slices (10 µm) of mouse brain. The method provides sterolomic analysis at 400-µm spot diameter with a limit of quantification of 0.01 ng/mm². It overcomes the limitations of previous mass spectrometry imaging techniques in analysis of low-abundance and difficult-to-ionize sterol molecules, allowing isomer differentiation and structure identification. Here we demonstrate the spatial distribution and quantification of multiple sterols involved in cholesterol metabolic pathways in wild-type and cholesterol 24S-hydroxylase knockout mouse brain. The technology described provides a powerful tool for future studies of spatial cholesterol metabolism in healthy and diseased tissues.

Formation and metabolism of oxysterols and cholestenoic acids found in the mouse circulation: Lessons learnt from deuterium-enrichment experiments and the CYP46A1 transgenic mouse

While the presence and abundance of the major oxysterols and cholestenoic acids in the circulation is well established, minor cholesterol metabolites may also have biological importance and be of value to investigate. In this study by observing the metabolism of deuterium-labelled cholesterol in the pdgfb^ret/ret mouse, a mouse model with increased vascular permeability in brain, and by studying the sterol content of plasma from the CYP46A1 transgenic mouse overexpressing the human cholesterol 24S-hydroxylase enzyme we have been able to identify a number of minor cholesterol metabolites found in the circulation, make approximate-quantitative measurements and postulate pathways for their formation. These "proof of principle" data may have relevance when using mouse models to mimic human disease and in respect of the increasing possibility of treating human neurodegenerative diseases with pharmaceuticals designed to enhance the activity of CYP46A1 or by adeno-associated virus delivery of CYP46A1.

Ligand-binding properties and catalytic activity of the purified human 24-hydroxycholesterol 7α-hydroxylase, CYP39A1

Oxysterols are derivatives of cholesterol and biologically active molecules that are involved in a number of functions, including cholesterol homeostasis, immune response, embryogenic development and pathophysiology of neurodegenerative diseases. Enzymes catalyzing their synthesis and metabolism are of particular interest as potential or evaluated drug targets. Here we report for the first time biochemical analysis of purified human oxysterol 7α-hydroxylase selective for 24-hydroxycholesterol. Binding analyses indicated a tight binding of the oxysterols and estrone. Ligand screening revealed that CYP39A1 binds with high affinity antifungal drugs and prostate cancer drug galeterone (TOK-001). Site-directed mutagenesis of conserved Asn residue in the active site revealed its crucial role for protein folding and heme incorporation. Developed protocol for expression and purification enables further investigation of this hepatic enzyme as off-target in development of specific drugs targeting cytochrome P450 enzymes.

On the fluxes of side-chain oxidized oxysterols across blood-brain and blood-CSF barriers and origin of these steroids in CSF (Review)

In contrast to cholesterol itself the side-chain oxidized metabolites 24S-hydroxycholesterol (24OH) and 27-hydroxycholesterol (27OH) are able to pass the blood-brain barrier and the blood-CSF barrier. Most 27OH in circulation is formed extracerebrally and according to catheterization experiments about 5 mg of it is taken up by the brain per 24 h. 24OH is almost exclusively produced in the brain and about 6 mg fluxes from the brain into the circulation per 24 h. In addition to these major fluxes a very minor fraction of these two oxysterols flux from the circulation into CSF. Isotope experiments have shown that almost all 27OH in CSF originates from the circulation and evidence has been presented that this is the case also with a substantial part of 24OH. The levels of both 24OH and 27OH in CSF are thus affected by the integrity of the blood-CSF barrier with higher levels when the barrier is defect. Both levels of 24OH and 27OH in CSF are increased in connection with neurodegeneration and in general the increase in 24OH levels is higher than the increase in 27OH levels. A number of observations in different type of patients including measurements of other biochemical markers support that the increase in levels of 24OH due to neurodegeneration is due to a release of this oxysterol or its precursor cholesterol from dying neuronal cells. In contrast the increase in levels of 27OH is likely to be a consequence of reduced metabolism due to loss of the neuronal enzyme CYP7B1. We discuss the driving forces behind the fluxes of oxysterols in the brain, the limitations in the flux across the barriers and the diagnostic potential for side-chain oxidized oxysterols in CSF.

Preferential enhancement of GluN2B-containing native NMDA receptors by the endogenous modulator 24S-hydroxycholesterol in hippocampal neurons

24S-hydroxycholesterol (24HC) is the major metabolic breakdown product of cholesterol in the brain. Among its other effects on neurons, 24HC modulates N-methyl-d-aspartate (NMDA or GluN) receptors, but our understanding of this mechanism is poor. We used whole-cell patch clamp recordings and various pharmacological approaches in mouse brain slices to record isolated NMDAR-mediated (I_NMDA) tonic and evoked synaptic currents. 24HC (1 μΜ) significantly enhanced tonic, but not evoked, I_NMDA of dentate gyrus granule cells. The I_NMDA had both GluN2A and GluN2B-mediated components. Preincubation of the slices with PEAQX (a GluN2A antagonist) or Ro25-6981 (a GluN2B antagonist) dramatically changed the I_NMDA modulatory potential of 24HC. Ro25-6981 blocked the enhancing effect of 24HC on tonic I_NMDA, while preincubation with PEAQX had no effect. In cholesterol 24-hydroxylase (CYP46A1) knockout mice, in sharp contrast to WT, 24HC slightly decreased the tonic I_NMDA of granule cells. Furthermore, 24HC had no effect on tonic I_NMDA of dentate gyrus parvalbumin interneurons (PV-INs), known to express different GluN subunits than granule cells. Taken together, our results revealed a specific enhancement of GluN2B-containing NMDARs by 24HC, indicating a novel endogenous pathway to influence a subclass of NMDARs critically involved in cortical plasticity and in numerous neurological and psychiatric disorders.

24S-Hydroxycholesterol Correlates With Tau and Is Increased in Cerebrospinal Fluid in Parkinson's Disease and Corticobasal Syndrome

24S-hydroxycholesterol (24OHC) and Tau are produced in neuronal cells and neurodegeneration leads to increased flux of both of them into cerebrospinal fluid (CSF). In the present study, CSF levels of 24OHC and 27S-hydroxycholesterol (27OHC) along with those of Tau, P-Thr¹⁸¹-Tau and Aβ₄₂ were measured in patients with early Parkinson's disease (PD), Corticobasal syndrome (CBS), Corticobasal Degeneration (CBD), and controls. Using mouse models with increased or no formation of Tau protein and increased production of 24OHC, we have also tested the hypothesis that there is a direct association between neuronal turnover of 24OHC and Tau. The levels of 24OHC are increased, at a group level, in patients with PD or CBS. We found significant correlations between levels of 24OHC and Tau or P-Thr¹⁸¹-Tau in CSF from patients with PD, CBS or CBD. There were no similar correlations between 24OHC and Aβ₄₂ in CSF from these patients. The neuronal levels of 24OHC were not altered in Tau knockout or Tau overexpressing mice. Vice versa, Tau species levels were not changed in Cyp46 overexpressing mice with increased neuronal levels of 24OHC. We conclude that the strongly correlative fluxes of 24OHC and Tau from neuronal cells to CSF are likely to be secondary to neurodegeneration and not due to direct interaction between the two factors. We suggest that this high correlation reflects a rapid neurodegeneration of specific neuronal subtypes with simultaneous release of 24OHC and Tau into the CSF.

Cytoprotective Activities of Milk Thistle Seed Oil Used in Traditional Tunisian Medicine on 7-Ketocholesterol and 24S-Hydroxycholesterol-Induced Toxicity on 158N Murine Oligodendrocytes

The Asteraceae family is economically very important, because many of these plants are grown mainly for their food value, such as lettuce (Lactuca), chicory (Cichorium), and sunflower (Heliantus aminus). One of the typical properties of this family, which includes milk thistle (Sylibum marianum), is the richness of the oil in various compounds (flavonoids, alkaloids, tocopherols, and unsaturated fatty acids). Currently, and for the coming decades, age-related diseases, including neurodegenerative diseases, are a major public health problem. Preventing their appearance or opposing their evolution is a major objective. In this context, the cytoprotective activities of milk thistle seed oil produced in Tunisia were studied on the 158N model using 7-ketocholesterol (7KC) and 24S-hydroxycholesterol (24S) as cytotoxic agents. 7KC and 24S were used because they can be increased in the brain and body fluids of patients with major age-related neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases. In order to evaluate the cytoprotective properties of milk thistle seed oil, complementary techniques of microscopy, flow cytometry, and biochemistry were used. The chemical composition of milk thistle seed oil has also been determined by various chromatography techniques. Milk thistle seed oils from different area of Tunisia are rich in tocopherols and are strongly antioxidant according to various biochemical tests (KRL (Kit Radicaux Libres), FRAP (Ferric Reducing Antioxidant Power), and DPPH (2,2-diphenyl-1-picrylhydrazyl)). The main fatty acids are linoleic acid (C18:2 n-6) and oleic acid (C18:1 n-9). The main polyphenols identified are homovanillic acid, p-coumaric acid, quercetin, and apigenin, with a predominance of vanillic acid. On 158N cells, milk thistle seed oil attenuates the cytotoxicity of 7KC and 24S including: loss of cell adhesion, increased plasma membrane permeability, mitochondrial dysfunction, overproduction of reactive oxygen species, induction of apoptosis, and autophagy. The attenuation of the cytotoxicity of 7KC and 24S observed with the milk thistle seed oil is in the order of that observed with α-tocopherol used as a positive control. In the presence of nigella seed oil, considered potentially cytotoxic, no cytoprotective effects were observed. Given the chemical characteristics, antioxidant properties, and cytoprotective activities of milk thistle seed oil, our results highlight the potential benefit of this oil for human health.

24S-hydroxycholesterol suppresses neuromuscular transmission in SOD1(G93A) mice: A possible role of NO and lipid rafts

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by the initial denervation of skeletal muscle and subsequent death of motor neurons. A dying-back pattern of ALS suggests a crucial role for neuromuscular junction dysfunction. In the present study, microelectrode recording of postsynaptic currents and optical detection of synaptic vesicle traffic (FM1-43 dye) and intracellular NO levels (DAF-FM DA) were used to examine the effect of the major brain-derived cholesterol metabolite 24S-hydroxycholesterol (24S-HC, 0.4 μM) on neuromuscular transmission in the diaphragm of transgenic mice carrying a mutant superoxide dismutase 1 (SOD^G93A). We found that 24S-HC suppressed spontaneous neurotransmitter release and neurotransmitter exocytosis during high-frequency stimulation. The latter was accompanied by a decrease in both the rate of synaptic vesicle recycling and activity-dependent enhancement of NO production. Inhibition of NO synthase with L-NAME also attenuated synaptic vesicle exocytosis during high-frequency stimulation and completely abolished the effect of 24S-HC itself. Of note, 24S-HC enhanced the labeling of synaptic membranes with B-subunit of cholera toxin, suggesting an increase in lipid ordering. Lipid raft-disrupting agents (methyl-β-cyclodextrin, sphingomyelinase) prevented the action of 24S-HC on both lipid raft marker labeling and NO synthesis. Together, these experiments indicate that 24S-HC is able to suppress the exocytotic release of neurotransmitter in response to intense activity via a NO/lipid raft-dependent pathway in the neuromuscular junctions of SOD^G93A mice.

Lovastatin fails to improve motor performance and survival in methyl-CpG-binding protein2-null mice

Previous studies provided evidence for the alteration of brain cholesterol homeostasis in 129.Mecp2-null mice, an experimental model of Rett syndrome. The efficacy of statins in improving motor symptoms and prolonging survival of mutant mice suggested a potential role of statins in the therapy of Rett syndrome. In the present study, we show that Mecp2 deletion had no effect on brain and reduced serum cholesterol levels and lovastatin (1.5 mg/kg, twice weekly as in the previous study) had no effects on motor deficits and survival when Mecp2 deletion was expressed on a background strain (C57BL/6J; B6) differing from that used in the earlier study. These findings indicate that the effects of statins may be background specific and raise important issues to consider when contemplating clinical trials. The reduction of the brain cholesterol metabolite 24S-hydroxycholesterol (24S-OHC) found in B6.Mecp2-null mice suggests the occurrence of changes in brain cholesterol metabolism and the potential utility of using plasma levels of 24S-OHC as a biomarker of brain cholesterol homeostasis in RTT.

DOI:http://dx.doi.org/10.7554/eLife.22409.001

Endogenous 24S-hydroxycholesterol modulates NMDAR-mediated function in hippocampal slices

N-methyl-D-aspartate receptors (NMDARs), a major subtype of glutamate receptors mediating excitatory transmission throughout the central nervous system (CNS), play critical roles in governing brain function and cognition. Because NMDAR dysfunction contributes to the etiology of neurological and psychiatric disorders including stroke and schizophrenia, NMDAR modulators are potential drug candidates. Our group recently demonstrated that the major brain cholesterol metabolite, 24S-hydroxycholesterol (24S-HC), positively modulates NMDARs when exogenously administered. Here, we studied whether endogenous 24S-HC regulates NMDAR activity in hippocampal slices. In CYP46A1(-/-) (knockout; KO) slices where endogenous 24S-HC is greatly reduced, NMDAR tone, measured as NMDAR-to-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) excitatory postsynaptic current (EPSC) ratio, was reduced. This difference translated into more NMDAR-driven spiking in wild-type (WT) slices compared with KO slices. Application of SGE-301, a 24S-HC analog, had comparable potentiating effects on NMDAR EPSCs in both WT and KO slices, suggesting that endogenous 24S-HC does not saturate its NMDAR modulatory site in ex vivo slices. KO slices did not differ from WT slices in either spontaneous neurotransmission or in neuronal intrinsic excitability, and exhibited LTP indistinguishable from WT slices. However, KO slices exhibited higher resistance to persistent NMDAR-dependent depression of synaptic transmission induced by oxygen-glucose deprivation (OGD), an effect restored by SGE-301. Together, our results suggest that loss of positive NMDAR tone does not elicit compensatory changes in excitability or transmission, but it protects transmission against NMDAR-mediated dysfunction. We expect that manipulating this endogenous NMDAR modulator may offer new treatment strategies for neuropsychiatric dysfunction.

Evidence for altered cholesterol metabolism in Huntington's disease post mortem brain tissue

AIMS

Cholesterol plays an essential role in membrane structure and function, being especially important in the brain. Alteration of brain cholesterol synthesis and metabolism has been demonstrated in several Huntington's disease (HD) mouse and cell models; however, less is known about these alterations in human tissue. This study aimed to identify alterations to cholesterol synthetic and metabolic pathways in human HD brain tissue.

METHODS

A broad range of cholesterol synthetic precursors, metabolites and oxidation products were measured by gas chromatography-tandem mass spectrometry in five regions of human post mortem HD brain and compared with age- and sex-matched control tissues. The level of enzymes that regulate cholesterol homeostasis, cholesterol 24-hydroxylase and delta(24)-sterol reductase were investigated by Western blotting and qPCR in putamen.

RESULTS

The most significant changes were localized to the putamen, where a 60% decrease in 24(S)-hydroxycholesterol, 30% increase in cholesterol and 100-200% increase in synthetic precursors (lathosterol, zymosterol and desmosterol) was detected. The enzymes cholesterol 24-hydroxylase and delta(24)-sterol reductase were also significantly decreased in HD putamen as compared with control tissues. Free radical-generated cholesterol oxidation products 7-keto cholesterol and 7β-hydroxycholesterol were also increased by 50-70% in HD putamen.

CONCLUSION

Human HD brain has significantly decreased cholesterol metabolism and disrupted cholesterol homeostasis. Our data also indicate that lipid oxidative stress accompanies HD pathology.

Reduced cerebrospinal fluid concentrations of oxysterols in response to natalizumab treatment of relapsing remitting multiple sclerosis

BACKGROUND

Natalizumab therapy reduces inflammation and degeneration of the CNS in relapsing-remitting multiple sclerosis (RRMS). In cerebrospinal fluid (CSF) the concentration of 24S-hydroxycholesterol (24OHC) reflect neurodegeneration, whereas 27-hydroxycholesterol (27OHC) is dependent on the integrity of the blood-brain barrier (BBB).

OBJECTIVE

To measure the impact from natalizumab treatment on 24OHC and 27OHC concentrations in serum and CSF of RRMS.

METHODS

In serum and CSF obtained from 31 patients before and following 12 months of natalizumab treatment, 24OHC and 27OHC were analyzed by isotope-dilution mass spectrometry.

RESULTS

Natalizumab treatment reduced CSF-24OHC concentrations (p=0.002), CSF-27OHC concentrations (p=0.01) and serum-24OHC concentrations (p=0.029). There was no significant effect of the treatment on serum-27OHC concentrations. Serum concentrations of 24OHC correlated with Symbol Digit Modalities Test scores before (r=0.5, p=0.007) and after natalizumab treatment (r=0.403, p=0.033).

CONCLUSIONS

We showed for the first time that natalizumab treatment of RRMS reduced the concentrations of 24- and 27OHC in CSF, indicating reduced neurodegeneration and improved integrity of the BBB, respectively. Our results imply a role for serum 24OHC as a biomarker of cognition (visuo-spatial ability and processing speed) in RRMS.

CYP46A1 inhibition, brain cholesterol accumulation and neurodegeneration pave the way for Alzheimer's disease

Abnormalities in neuronal cholesterol homeostasis have been suspected or observed in several neurodegenerative disorders including Alzheimer's disease, Parkinson's disease and Huntington's disease. However, it has not been demonstrated whether an increased abundance of cholesterol in neurons in vivo contributes to neurodegeneration. To address this issue, we used RNA interference methodology to inhibit the expression of cholesterol 24-hydroxylase, encoded by the Cyp46a1 gene, in the hippocampus of normal mice. Cholesterol 24-hydroxylase controls cholesterol efflux from the brain and thereby plays a major role in regulating brain cholesterol homeostasis. We used an adeno-associated virus vector encoding short hairpin RNA directed against the mouse Cyp46a1 mRNA to decrease the expression of the Cyp46a1 gene in hippocampal neurons of normal mice. This increased the cholesterol concentration in neurons, followed by cognitive deficits and hippocampal atrophy due to apoptotic neuronal death. Prior to neuronal death, the recruitment of the amyloid protein precursor to lipid rafts was enhanced leading to the production of β-C-terminal fragment and amyloid-β peptides. Abnormal phosphorylation of tau and endoplasmic reticulum stress were also observed. In the APP23 mouse model of Alzheimer's disease, the abundance of amyloid-β peptides increased following inhibition of Cyp46a1 expression, and neuronal death was more widespread than in normal mice. Altogether, these results suggest that increased amounts of neuronal cholesterol within the brain may contribute to inducing and/or aggravating Alzheimer's disease.

Cholesterol metabolites exported from human brain

The human brain contains approximately 25% of the body's cholesterol. The brain is separated from the circulation by the blood brain barrier. While cholesterol will not passes this barrier, oxygenated forms of cholesterol can cross the barrier. Here by measuring the difference in the oxysterol content of blood plasma in the jugular vein and in a forearm vein by mass spectrometry (MS) we were able to determine the flux of more than 20 cholesterol metabolites between brain and the circulation. We confirm that 24S-hydroxycholesterol is exported from brain at a rate of about 2-3mg/24h. Gas chromatography (GC)-MS data shows that the cholesterol metabolites 5α-hydroxy-6-oxocholesterol (3β,5α-dihydroxycholestan-6-one), 7β-hydroxycholesterol and 7-oxocholesterol, generally considered to be formed through reactive oxygen species, are similarly exported from brain at rates of about 0.1, 2 and 2mg/24h, respectively. Although not to statistical significance both GC-MS and liquid chromatography (LC)-MS methods indicate that (25R)26-hydroxycholesterol is imported to brain, while LC-MS indicates that 7α-hydroxy-3-oxocholest-4-enoic acid is exported from brain.

Arterial tissue and plasma concentration of enzymatic-driven oxysterols are associated with severe peripheral atherosclerotic disease and systemic inflammatory activity

INTRODUCTION

Cholesterol undergoes oxidation via both enzymatic stress- and free radical-mediated mechanisms, generating a wide range of oxysterols. In contrast to oxidative stress-driven metabolites, enzymatic stress-derived oxysterols are scarcely studied in their association with atherosclerotic disease in humans.

METHODS

24S-hydroxycholesterol (24S-HC), 25-hydroxycholesterol (25-HC), and 27-hydroxycholesterol (27-HC) were assessed in plasma and arteries with atherosclerotic plaques from 10 patients (54-84 years) with severe peripheral artery disease (PAD) as well as arteries free of atherosclerotic plaques from 13 individuals (45-78 years, controls).

RESULTS

Plasma 25-HC was higher in PAD individuals than in controls (6.3[2] vs. 3.9[1.9] ng/mgCol; p = 0.004). 24S-HC and 27-HC levels were, respectively, five- and 20-fold higher in the arterial tissue of PAD individuals than in those of the controls (p = 0.016 and p = 0.001). Plasma C-reactive protein correlated with plasma 24-HC (r = 0.51; p = 0.010), 25-HC (r = 0.75; p < 0.001), 27-HC (r = 0.48; p = 0.015), and with tissue 24S-HC (r = 0.4; p = 0.041) and 27-HC (r = 0.46; p = 0.023).

CONCLUSION

Arterial intima accumulation of 27-HC and 24S-HC is associated with advanced atherosclerotic disease and systemic inflammatory activity in individuals with severe PAD.

24S,25-Epoxycholesterol in mouse and rat brain

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24S,25-Epoxycholesterol identified and quantified in rodent brain.

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Knock out of Cyp27a1 leads to a decrease in 24S,25-epoxycholesterol.

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Knock out of Cyp7b1 leads to an increase in 24S,25-epoxycholesterol.

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24S,25-Epoxycholesterol is metabolised by Cyp7b1 but not Cyp27a1.

24S,25-Epoxycholesterol is formed in a shunt of the mevalonate pathway that produces cholesterol. It is one of the most potent known activators of the liver X receptors and can inhibit sterol regulatory element-binding protein processing. Until recently analysis of 24S,25-epoxycholesterol at high sensitivity has been precluded by its thermal lability and lack of a strong chromophore. Here we report on the analysis of 24S,25-epoxycholesterol in rodent brain where its level was determined to be of the order of 0.4–1.4 μg/g wet weight in both adult mouse and rat. For comparison the level of 24S-hydroxycholesterol in brain of both rodents was of the order of 20 μg/g, while that of cholesterol in mouse was 10–20 mg/g. By exploiting knockout mice for the enzyme oxysterol 7α-hydroxylase (Cyp7b1) we show that this enzymes is important for the subsequent metabolism of the 24S,25-epoxide.

Adaptive responses induced by 24S-hydroxycholesterol through liver X receptor pathway reduce 7-ketocholesterol-caused neuronal cell death

Lipid peroxidation products have been known to induce cellular adaptive responses and enhance tolerance against subsequent oxidative stress through up-regulation of antioxidant compounds and enzymes. 24S-hydroxycholesterol (24SOHC) which is endogenously produced oxysterol in the brain plays an important role in maintaining brain cholesterol homeostasis. In this study, we evaluated adaptive responses induced by brain-specific oxysterol 24SOHC in human neuroblastoma SH-SY5Y cells. Cells treated with 24SOHC at sub-lethal concentrations showed significant reduction in cell death induced by subsequent treatment with 7-ketocholesterol (7KC) in both undifferentiated and retinoic acid-differentiated SH-SY5Y cells. These adaptive responses were also induced by other oxysterols such as 25-hydroxycholesterol and 27-hydroxycholesterol which are known to be ligands of liver X receptor (LXR). Co-treatment of 24SOHC with 9-cis retinoic acid, a retinoid X receptor ligand, enhanced the adaptive responses. Knockdown of LXRβ by siRNA diminished the adaptive responses induced by 24SOHC almost completely. The treatment with 24SOHC induced the expression of LXR target genes, such as ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1). The 24SOHC-induced adaptive responses were significantly attenuated by siRNA for ABCG1 but not by siRNA for ABCA1. Taken together, these results strongly suggest that 24SOHC at sub-lethal concentrations induces adaptive responses via transcriptional activation of LXR signaling pathway, thereby protecting neuronal cells from subsequent 7KC-induced cytotoxicity.

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24SOHC induces adaptive responses against 7KC-induced cell death in neuronal cells.

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Co-treatment of 24SOHC with 9cRA, an RXR ligand enhances adaptive responses.

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Knockdown of LXRβ suppresses 24SOHC-induced adaptive responses.

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ABCG1 is involved in LXR-mediated adaptive responses by 24SOHC.