APOB100 transgenic mice exemplify how the systemic circulation content may affect the retina without altering retinal cholesterol input.

Apolipoprotein B (APOB) is a constituent of unique lipoprotein particles (LPPs) produced in the retinal pigment epithelium (RPE), which separates the neural retina from Bruch's membrane (BrM) and choroidal circulation. These LPPs accumulate with age in BrM and contribute to the development of age-related macular degeneration, a major blinding disease. The APOB100 transgenic expression in mice, which unlike humans lack the full-length APOB100, leads to lipid deposits in BrM. Herein, we further characterized APOB100 transgenic mice. We imaged mouse retina in vivo and assessed chorioretinal lipid distribution, retinal sterol levels, retinal cholesterol input, and serum content as well as tracked indocyanine green-bound LPPs in mouse plasma and retina after an intraperitoneal injection. Retinal function and differentially expressed proteins were also investigated. APOB100 transgenic mice had increased serum LDL content and an additional higher density HDL subpopulation; their retinal cholesterol levels (initially decreased) became normal with age. The LPP cycling between the RPE and choroidal circulation was increased. Yet, LPP trafficking from the RPE to the neural retina was limited, and total retinal cholesterol input did not change. There were lipid deposits in the RPE and BrM, and retinal function was impaired. Retinal proteomics provided mechanistic insights. Collectively, our data suggested that the serum LDL/HDL ratio may not affect retinal pathways of cholesterol input as serum LPP load is mainly handled by the RPE, which offloads LPP excess to the choroidal circulation rather than neural retina. Different HDL subpopulations should be considered in studies linking serum LPPs and age-related macular degeneration.

Unbiased insights into the multiplicity of the CYP46A1 brain effects in 5XFAD mice treated with low dose-efavirenz.

Cytochrome P450 46A1 (CYP46A1) is the CNS-specific cholesterol 24-hydroxylase that controls cholesterol elimination and turnover in the brain. In mouse models, pharmacologic CYP46A1 activation with low-dose efavirenz or by gene therapy mitigates the manifestations of various brain disorders, neurologic, and nonneurologic, by affecting numerous, apparently unlinked biological processes. Accordingly, CYP46A1 is emerging as a promising therapeutic target; however, the mechanisms underlying the multiplicity of the brain CYP46A1 activity effects are currently not understood. We proposed the chain reaction hypothesis, according to which CYP46A1 is important for the three primary (unifying) processes in the brain (sterol flux through the plasma membranes, acetyl-CoA, and isoprenoid production), which in turn affect a variety of secondary processes. We already identified several processes secondary to changes in sterol flux and herein undertook a multiomics approach to compare the brain proteome, acetylproteome, and metabolome of 5XFAD mice (an Alzheimer's disease model), control and treated with low-dose efavirenz. We found that the latter had increased production of phospholipids from the corresponding lysophospholipids and a globally increased protein acetylation (including histone acetylation). Apparently, these effects were secondary to increased acetyl-CoA production. Signaling of small GTPases due to their altered abundance or abundance of their regulators could be affected as well, potentially via isoprenoid biosynthesis. In addition, the omics data related differentially abundant molecules to other biological processes either reported previously or new. Thus, we obtained unbiased mechanistic insights and identified potential players mediating the multiplicity of the CYP46A1 brain effects and further detailed our chain reaction hypothesis.

The normalizing effects of the CYP46A1 activator efavirenz on retinal sterol levels and risk factors for glaucoma in Apoj-/- mice.

Apolipoprotein J (APOJ) is a multifunctional protein with genetic evidence suggesting an association between APOJ polymorphisms and Alzheimer's disease as well as exfoliation glaucoma. Herein we conducted ocular characterizations of Apoj-/- mice and found that their retinal cholesterol levels were decreased and that this genotype had several risk factors for glaucoma: increased intraocular pressure and cup-to-disk ratio and impaired retinal ganglion cell (RGC) function. The latter was not due to RGC degeneration or activation of retinal Muller cells and microglia/macrophages. There was also a decrease in retinal levels of 24-hydroxycholesterol, a suggested neuroprotectant under glaucomatous conditions and a positive allosteric modulator of N-methyl-D-aspartate receptors mediating the light-evoked response of the RGC. Therefore, Apoj-/- mice were treated with low-dose efavirenz, an allosteric activator of CYP46A1 which converts cholesterol into 24-hydroxycholesterol. Efavirenz treatment increased retinal cholesterol and 24-hydroxycholesterol levels, normalized intraocular pressure and cup-to-disk ratio, and rescued in part RGC function. Retinal expression of Abcg1 (a cholesterol efflux transporter), Apoa1 (a constituent of lipoprotein particles), and Scarb1 (a lipoprotein particle receptor) was increased in EVF-treated Apoj-/- mice, indicating increased retinal cholesterol transport on lipoprotein particles. Ocular characterizations of Cyp46a1-/- mice supported the beneficial efavirenz treatment effects via CYP46A1 activation. The data obtained demonstrate an important APOJ role in retinal cholesterol homeostasis and link this apolipoprotein to the glaucoma risk factors and retinal 24-hydroxycholesterol production by CYP46A1. As the CYP46A1 activator efavirenz is an FDA-approved anti-HIV drug, our studies suggest a new therapeutic approach for treatment of glaucomatous conditions.

7,8-Dihydroxy Efavirenz Is Not as Effective in CYP46A1 Activation In Vivo as Efavirenz or Its 8,14-Dihydroxy Metabolite.

High dose (S)-efavirenz (EFV) inhibits the HIV reverse transcriptase enzyme and is used to lower HIV load. Low-dose EFV allosterically activates CYP46A1, the key enzyme for cholesterol elimination from the brain, and is investigated as a potential treatment for Alzheimer's disease. Simultaneously, we evaluate EFV dihydroxymetabolites for in vivo brain effects to compare with those of (S)-EFV. We have already tested (rac)-8,14dihydroxy EFV on 5XFAD mice, a model of Alzheimer's disease. Herein, we treated 5XFAD mice with (rac)-7,8dihydroxy EFV. In both sexes, the treatment modestly activated CYP46A1 in the brain and increased brain content of acetyl-CoA and acetylcholine. Male mice also showed a decrease in the brain levels of insoluble amyloid ß40 peptides. However, the treatment had no effect on animal performance in different memory tasks. Thus, the overall brain effects of (rac)-7,8dihydroxy EFV were weaker than those of EFV and (rac)-8,14dihydroxy EFV and did not lead to cognitive improvements as were seen in treatments with EFV and (rac)-8,14dihydroxy EFV. An in vitro study assessing CYP46A1 activation in co-incubations with EFV and (rac)-7,8dihydroxy EFV or (rac)-8,14dihydroxy EFV was carried out and provided insight into the compound doses and ratios that could be used for in vivo co-treatments with EFV and its dihydroxymetabolite.

Quantitative characterizations of the cholesterol-related pathways in the retina and brain of hamsters.

The retina and brain are separated from the systemic circulation by the anatomical barriers, which are permeable (the outer blood-retinal barrier) and impermeable (the blood-brain and inner blood-retina barriers) to cholesterol. Herein we investigated whether whole-body cholesterol maintenance affects cholesterol homeostasis in the retina and brain. We used hamsters, whose whole-body cholesterol handling is more similar to those in humans than in mice, and conducted separate administrations of deuterated water and deuterated cholesterol. We assessed the quantitative significance of the retinal and brain pathways of cholesterol input and compared the results with those from our previous studies in mice. The utility of the measurements in the plasma of deuterated 24-hydroxycholesterol, the major cholesterol elimination product from the brain, was investigated as well. We established that despite a sevenfold higher serum LDL to HDL ratio and other cholesterol-related differences, in situ biosynthesis remained the major source of cholesterol for hamster retina, although its quantitative significance was reduced to 53% as compared to 72%-78% in the mouse retina. In the brain, the principal pathway of cholesterol input was also the same, in situ biosynthesis, accounting for 94% of the total brain cholesterol input (96% in mice); the interspecies differences pertained to the absolute rates of the total cholesterol input and turnover. We documented the correlations between deuterium enrichments of the brain 24-hydroxycholesterol, brain cholesterol, and plasma 24-hydroxycholesterol, which suggested that deuterium enrichment of plasma 24-hydroxycholesteol could be an in vivo marker of cholesterol elimination and turnover in the brain.

2-Hydroxypropyl-ß-cyclodextrin mitigates pathological changes in a mouse model of retinal cholesterol dyshomeostasis.

CYP46A1 is a CNS-specific enzyme, which eliminates cholesterol from the brain and retina by metabolism to 24-hydroxycholesterol, thus contributing to cholesterol homeostasis in both organs. 2-Hydroxypropyl-ß-cyclodextrin (HPCD), a Food and Drug Administration-approved formulation vehicle, is currently being investigated off-label for treatment of various diseases, including retinal diseases. HPCD was shown to lower retinal cholesterol content in mice but had not yet been evaluated for its therapeutic benefits. Herein, we put Cyp46a1-/- mice on high fat cholesterol-enriched diet from 1 to 14 months of age (control group) and at 12 months of age, started to treat a group of these animals with HPCD until the age of 14 months. We found that as compared with mature and regular chow-fed Cyp46a1-/- mice, control group had about 6-fold increase in the retinal total cholesterol content, focal cholesterol and lipid deposition in the photoreceptor-Bruch's membrane region, and retinal macrophage activation. In addition, aged animals had cholesterol crystals at the photoreceptor-retinal pigment epithelium interface and changes in the Bruch's membrane ultrastructure. HPCD treatment mitigated all these manifestations of retinal cholesterol dyshomeostasis and altered the abundance of six groups of proteins (genetic information transfer, vesicular transport, and cytoskeletal organization, endocytosis and lysosomal processing, unfolded protein removal, lipid homeostasis, and Wnt signaling). Thus, aged Cyp46a1-/- mice on high fat cholesterol-enriched diet revealed pathological changes secondary to retinal cholesterol overload and supported further studies of HPCD as a potential therapeutic for age-related macular degeneration and diabetic retinopathy associated with retinal cholesterol dyshomeostasis.

Cholesterol crystal formation is a unifying pathogenic mechanism in the development of diabetic retinopathy.

AIMS/HYPOTHESIS: Hyper-reflective crystalline deposits found in retinal lesions have been suggested to predict the progression of diabetic retinopathy, but the nature of these structures remains unknown. METHODS: Scanning electron microscopy and immunohistochemistry were used to identify cholesterol crystals (CCs) in human donor, pig and mouse tissue. The effects of CCs were analysed in bovine retinal endothelial cells in vitro and in db/db mice in vivo using quantitative RT-PCR, bulk RNA sequencing, and cell death and permeability assays. Cholesterol homeostasis was determined using 2H2O and 2H7-cholesterol. RESULTS: We identified hyper-reflective crystalline deposits in human diabetic retina as CCs. Similarly, CCs were found in the retina of a diabetic mouse model and a high-cholesterol diet-fed pig model. Cell culture studies demonstrated that treatment of retinal cells with CCs can recapitulate all major pathogenic mechanisms leading to diabetic retinopathy, including inflammation, cell death and breakdown of the blood-retinal barrier. Fibrates, statins and α-cyclodextrin effectively dissolved CCs present in in vitro models of diabetic retinopathy, and prevented CC-induced endothelial pathology. Treatment of a diabetic mouse model with α-cyclodextrin reduced cholesterol levels and CC formation in the retina, and prevented diabetic retinopathy. CONCLUSIONS/INTERPRETATION: We established that cholesterol accumulation and CC formation are a unifying pathogenic mechanism in the development of diabetic retinopathy.

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

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.

The Hydroxylation Position Rather than Chirality Determines How Efavirenz Metabolites Activate Cytochrome P450 46A1 In Vitro.

(S)-Efavirenz (EFV) is a reverse transcriptase inhibitor and an antiviral drug. In addition, (S)-EFV can interact off target with CYP46A1, the major cholesterol hydroxylating enzyme in the mammalian brain, and allosterically activate CYP46A1 at a small dose in mice and humans. Studies with purified CYP46A1 identified two allosteric sites on the enzyme surface, one for (S)-EFV and the second site for L-glutamate (Glu), a neurotransmitter that also activates CYP46A1 either alone or in the presence of (S)-EFV. Previously, we found that racemic (rac)-7-hydroxyefavirenz, (rac)-8-hydroxyefavirenz, (S)-8-hydroxyefavirenz, and (rac)-8,14-dihydroxyefavirenz, compounds with the hydroxylation positions corresponding to the metabolism of (S)-EFV in the liver, activated CYP46A1 in vitro. Yet, these compounds differed from (S)-EFV in how they allosterically interacted with CYP46A1. Herein, we further characterized (rac)-7-hydroxyefavirenz, (rac)-8-hydroxyefavirenz, (S)-8-hydroxyefavirenz, and (rac)-8,14-dihydroxyefavirenz, and, in addition, (R)-EFV, (S)-7-hydroxyefavirenz, (rac)-7,8-dihydroxyefavirenz, (S)-7,8-dihydroxyefavirenz, and (S)-8,14-dihydroxyefavirenz for activation and binding to CYP46A1 in vitro. We found that the spatial configuration of all tested compounds neither affected the CYP46A1 activation nor the sites of binding to CYP46A1. Yet, the hydroxylation position determined whether the hydroxylated metabolite interacted with the allosteric site for (S)-EFV [(R)-EFV, (rac)-7,8-dihydroxyefavirenz, and (S)-7,8-dihydroxyefavirenz], L-Glu [(rac)- and (S)-8,14-dihydroxyefavirenz], or both [(rac)-7-hydroxyefavirenz, (S)-7-hydroxyefavirenz, (rac)-8-hydroxyefavirenz, and (S)-8-hydroxyefavirenz]. This difference in binding to the allosteric sites determined, in turn, how CYP46A1 activity was changed in the coincubations with (S)-EFV and either its metabolite or L-Glu. The results suggest EFV metabolites that could be more potent for CYP46A1 activation in vivo than (S)-EFV. SIGNIFICANCE STATEMENT: This study found that not only efavirenz but also all its hydroxylated metabolites allosterically activate CYP46A1 in vitro. The enzyme activation depended on the hydroxylation position but not the metabolite spatial configuration and involved either one or two allosteric sites-for efavirenz, L-glutamate, or both. The results suggest that the hydroxylated efavirenz metabolites may differ from efavirenz in how they interact with the CYP46A1 allosteric and active sites.

Studies of ApoD-/- and ApoD-/-ApoE-/- mice uncover the APOD significance for retinal metabolism, function, and status of chorioretinal blood vessels.

Apolipoprotein D (APOD) is an atypical apolipoprotein with unknown significance for retinal structure and function. Conversely, apolipoprotein E (APOE) is a typical apolipoprotein with established roles in retinal cholesterol transport. Herein, we immunolocalized APOD to the photoreceptor inner segments and conducted ophthalmic characterizations of ApoD-/- and ApoD-/-ApoE-/- mice. ApoD-/- mice had normal levels of retinal sterols but changes in the chorioretinal blood vessels and impaired retinal function. The whole-body glucose disposal was impaired in this genotype but the retinal glucose metabolism was unchanged. ApoD-/-ApoE-/- mice had altered sterol profile in the retina but apparently normal chorioretinal vasculature and function. The whole-body glucose disposal and retinal glucose utilization were enhanced in this genotype. OB-Rb, both leptin and APOD receptor, was found to be expressed in the photoreceptor inner segments and was at increased abundance in the ApoD-/- and ApoD-/-ApoE-/- retinas. Retinal levels of Glut4 and Cd36, the glucose transporter and scavenger receptor, respectively, were increased as well, thus linking APOD to retinal glucose and fatty acid metabolism and suggesting the APOD-OB-Rb-GLUT4/CD36 axis. In vivo isotopic labeling, transmission electron microscopy, and retinal proteomics provided additional insights into the mechanism underlying the retinal phenotypes of ApoD-/- and ApoD-/-ApoE-/- mice. Collectively, our data suggest that the APOD roles in the retina are context specific and could determine retinal glucose fluxes into different pathways. APOD and APOE do not play redundant, complementary or opposing roles in the retina, rather their interplay is more complex and reflects retinal responses elicited by lack of these apolipoproteins.

Increased Acetylcholine Levels and Other Brain Effects in 5XFAD Mice after Treatment with 8,14-Dihydroxy Metabolite of Efavirenz.

Efavirenz (EFV), an FDA-approved anti-HIV drug, has off-target binding to CYP46A1, the CNS enzyme which converts cholesterol to 24-hydroxycholesterol. At small doses, EFV allosterically activates CYP46A1 in mice and humans and mitigates some of the Alzheimer's disease manifestations in 5XFAD mice, an animal model. Notably, in vitro, all phase 1 EFV hydroxymetabolites activate CYP46A1 as well and bind either to the allosteric site for EFV, neurotransmitters or both. Herein, we treated 5XFAD mice with 8,14-dihydroxyEFV, the binder to the neurotransmitter allosteric site, which elicits the highest CYP46A1 activation in vitro. We found that treated animals of both sexes had activation of CYP46A1 and cholesterol turnover in the brain, decreased content of the amyloid beta 42 peptide, increased levels of acetyl-CoA and acetylcholine, and altered expression of the brain marker proteins. In addition, male mice had improved performance in the Barnes Maze test and increased expression of the acetylcholine-related genes. This work expands our knowledge of the beneficial CYP46A1 activation effects and demonstrates that 8,14-dihydroxyEFV crosses the blood-brain barrier and has therapeutic potential as a CYP46A1 activator.

Retinal Vascular Abnormalities and Microglia Activation in Mice with Deficiency in Cytochrome P450 46A1-Mediated Cholesterol Removal.

CYP46A1 is the cytochrome P450 enzyme that converts cholesterol to 24-hydroxycholesterol, a cholesterol elimination product and a potent liver X receptor (LXR) ligand. We conducted retinal characterizations of Cyp46a1-/- mice that had normal fasting blood glucose levels but up to a 1.8-fold increase in retinal cholesterol. The retina of Cyp46a1-/- mice exhibited venous beading and tortuosity, microglia/macrophage activation, and increased vascular permeability, features commonly associated with diabetic retinopathy. The expression of Lxrα and Lxrß was increased in both the whole Cyp46a1-/- retina and retinal macroglia/macrophages. The LXR-target genes were affected as well, primarily in activated microglial cells and macrophages. In the latter, the LXR-transactivated genes (Abca1, Abcg1, Apod, Apoe, Mylip, and Arg2) were up-regulated; similarly, there was an up-regulation of the LXR-transrepressed genes (Ccl2, Ptgs2, Cxcl1, Il1b, Il6, Nos2, and Tnfa). For comparison, gene expression was investigated in bone marrow-derived macrophages from Cyp46a1-/- mice as well as retinal and bone marrow-derived macrophages from Cyp27a1-/- and Cyp27a1-/-Cyp46a1-/- mice. CYP46A1 expression was detected in retinal endothelial cells, and this expression was increased in the proinflammatory environment. Retinal Cyp46a1-/- phosphoproteome revealed altered phosphorylation of 30 different proteins, including tight junction protein zonula occludens 1 and aquaporin 4. Collectively, the data obtained establish metabolic and regulatory significance of CYP46A1 for the retina and suggest pharmacologic activation of CYP46A1 as a potential therapeutic approach to dyslipidemia-induced retinal damage.

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.

Misdiagnosis of CTX due to propofol: The interference of total intravenous propofol anaesthesia with bile acid profiling.

BACKGROUND: Cerebrotendinous xanthomatosis (CTX) is a rare genetic disorder, characterised by chronic diarrhoea, xanthomas, cataracts, and neurological deterioration. CTX is caused by CYP27A1 deficiency, which leads to abnormal cholesterol and bile acid metabolism. Urinary bile acid profiling (increased m/z 627: glucuronide-5ß-cholestane-pentol) serves as diagnostic screening for CTX. However, this led to a false positive CTX diagnosis in two patients, who had received total intravenous anaesthesia (TIVA) with propofol. METHODS: To determine the influence of propofol on bile acid profiling, 10 urinary samples and 2 blood samples were collected after TIVA with propofol Fresenius 7 to 10 mg/kg/h from 12 subjects undergoing scoliosis correction. Urinary bile acids were analysed using flow injection negative electrospray mass spectrometry. Propofol binding to recombinant CYP27A1, the effects of propofol on recombinant CYP27A1 activity, and CYP27A1 expression in liver organoids were investigated using spectral binding, enzyme activity assays, and qPCR, respectively. Accurate masses were determined with high-resolution mass spectrometry. RESULTS: Abnormal urinary profiles were identified in all subjects after TIVA, with a trend correlating propofol dose per kilogramme and m/z 627 peak intensity. Propofol only induced a weak CYP27A1 response in the spectral binding assay, minimally affected CYP27A1 activity and did not affect CYP27A1 expression. The accurate mass of m/z 627 induced by propofol differed >10 PPM from m/z 627 observed in CTX. CONCLUSIONS: TIVA with propofol invariably led to a urinary profile misleadingly suggestive of CTX, but not through CYP27A1 inhibition. To avoid further misdiagnoses, propofol administration should be considered when interpreting urinary bile acid profiles.

Synthesis and pharmacokinetic study of a 11C-labeled cholesterol 24-hydroxylase inhibitor using 'in-loop' [11C]CO2 fixation method.

Cholesterol 24-hydroxylase, also known as CYP46A1 (EC 1.14.13.98), is a monooxygenase and a member of the cytochrome P450 family. CYP46A1 is specifically expressed in the brain where it controls cholesterol elimination by producing 24S-hydroxylcholesterol (24-HC) as the major metabolite. Modulation of CYP46A1 activity may affect Aß deposition and p-tau accumulation by changing 24-HC formation, which thereafter serves as potential therapeutic pathway for Alzheimer's disease. In this work, we showcase the efficient synthesis and preliminary pharmacokinetic evaluation of a novel cholesterol 24-hydroxylase inhibitor 1 for use in positron emission tomography.

Mechanisms that minimize retinal impact of apolipoprotein E absence.

Apolipoprotein E (APOE) is a component of lipid-transporting particles and a recognition ligand for receptors, which bind these particles. The APOE isoform ε2 is a risk factor for age-related macular degeneration; nevertheless, APOE absence in humans and mice does not significantly affect the retina. We found that retinal cholesterol biosynthesis and the levels of retinal cholesterol were increased in Apoe-/- mice, whereas cholesterol elimination by metabolism was decreased. No focal cholesterol deposits were observed in the Apoe-/- retina. Retinal proteomics identified the most abundant cholesterol-related proteins in WT mice and revealed that, of these cholesterol-related proteins, only APOA4 had increased expression in the Apoe-/- retina. In addition, there were changes in retinal abundance of proteins involved in proinflammatory and antiinflammatory responses, cellular cytoskeleton maintenance, vesicular traffic, and retinal iron homeostasis. The data obtained indicate that when APOE is absent, particles containing APOA1, APOA4, and APOJ still transport cholesterol in the intraretinal space, but these particles are not taken up by retinal cells. Therefore, cholesterol biosynthesis inside retinal cells increase, whereas metabolism to oxysterols decreases to prevent cells from cholesterol depletion. These and other compensatory changes underlie only a minor retinal phenotype in Apoe-/- mice.

Drugs and Scaffold That Inhibit Cytochrome P450 27A1 In Vitro and In Vivo.

Cytochrome P450 27A1 (CYP27A1) is a ubiquitous enzyme that hydroxylates cholesterol and other sterols. Complete CYP27A1 deficiency owing to genetic mutations is detrimental to human health, whereas 50% of activity retention is not and does not affect the whole body cholesterol levels. CYP27A1 is considered a potential therapeutic target in breast cancer and age-related neurodegenerative diseases; however, CYP27A1 inhibition should be ≤50%. Herein, 131 pharmaceuticals were tested for their effect on CYP27A1-mediated cholesterol 27-hydroxylation by in vitro enzyme assay. Of them, 14 drugs inhibited CYP27A1 by ≥75% and were evaluated for in vitro binding to the enzyme active site and for inhibition constants. All drugs except one (dasatinib) elicited a spectral response in CYP27A1 and had Ki values for cholesterol 27-hydroxylation either in the submicromolar (clevidipine, delavirdine, etravirine, felodipine, nicardipine, nilotinib, and sorafenib) or low micromolar range (abiratone, candesartan, celecoxib, dasatinib, nilvadipine, nimodipine, and regorafenib). Clevidipine, felodipine, nicardipine, nilvadipine, and nimodipine have the same 1,4-dihydropyridine scaffold and are indicated for hypertension. We used two of these antihypertensives (felodipine and nilvadipine) for administration to mice at a 1-mg/kg of body weight dose, daily, for 7 days. Mouse 27-hydroxycholesterol levels in the plasma, brain, and liver were reduced, whereas tissue levels of total cholesterol were unchanged. Structure-activity relationships within the 1,4-dihydropyridine scaffold were investigated, and features important for CY27A1 inhibition were identified. We confirmed our previous finding that CYP27A1 is a druggable enzyme and found additional drugs as well as the scaffold with potential for partial CYP27A1 inhibition in humans.

Cytochrome P450 27A1 Deficiency and Regional Differences in Brain Sterol Metabolism Cause Preferential Cholestanol Accumulation in the Cerebellum.

Cytochrome P450 27A1 (CYP27A1 or sterol 27-hydroxylase) is a ubiquitous, multifunctional enzyme catalyzing regio- and stereospecific hydroxylation of different sterols. In humans, complete CYP27A1 deficiency leads to cerebrotendinous xanthomatosis or nodule formation in tendons and brain (preferentially in the cerebellum) rich in cholesterol and cholestanol, the 5α-saturated analog of cholesterol. In Cyp27a1-/- mice, xanthomas are not formed, despite a significant cholestanol increase in the brain and cerebellum. The mechanism behind cholestanol production has been clarified, yet little is known about its metabolism, except that CYP27A1 might metabolize cholestanol. It also is unclear why CYP27A1 deficiency results in preferential cholestanol accumulation in the cerebellum. We hypothesized that cholestanol might be metabolized by CYP46A1, the principal cholesterol 24-hydroxylase in the brain. We quantified sterols along with CYP27A1 and CYP46A1 in mouse models (Cyp27a1-/-, Cyp46a1-/-, Cyp27a1-/-Cyp46a1-/-, and two wild type strains) and human brain specimens. In vitro experiments with purified P450s were conducted as well. We demonstrate that CYP46A1 is involved in cholestanol removal from the brain and that several factors contribute to the preferential increase in cholestanol in the cerebellum arising from CYP27A1 deficiency. These factors include (i) low cerebellar abundance of CYP46A1 and high cerebellar abundance of CYP27A1, the lack of which probably selectively increases the cerebellar cholestanol production; (ii) spatial separation in the cerebellum of cholesterol/cholestanol-metabolizing P450s from a pool of metabolically available cholestanol; and (iii) weak cerebellar regulation of cholesterol biosynthesis. We identified a new physiological role of CYP46A1, an important brain enzyme and cytochrome P450 that could be activated pharmacologically.

In vitro cytochrome P450 46A1 (CYP46A1) activation by neuroactive compounds.

Cytochrome P450 46A1 (CYP46A1, cholesterol 24-hydroxylase) is the enzyme responsible for the majority of cholesterol elimination from the brain. Previously, we found that the anti-HIV drug efavirenz (EFV) can pharmacologically activate CYP46A1 in mice. Herein, we investigated whether CYP46A1 could also be activated by endogenous compounds, including major neurotransmitters. In vitro experiments with purified recombinant CYP46A1 indicated that CYP46A1 is activated by l-glutamate (l-Glu), l-aspartate, γ-aminobutyric acid, and acetylcholine, with l-Glu eliciting the highest increase (3-fold) in CYP46A1-mediated cholesterol 24-hydroxylation. We also found that l-Glu and other activating neurotransmitters bind to the same site on the CYP46A1 surface, which differs from the EFV-binding site. The other principal differences between EFV and l-Glu in CYP46A1 activation include an apparent lack of l-Glu binding to the P450 active site and different pathways of signal transduction from the allosteric site to the active site. EFV and l-Glu similarly increased the CYP46A1 kcat, the rate of the "fast" phase of the enzyme reduction by the redox partner NADPH-cytochrome P450 oxidoreductase, and the amount of P450 reduced. Spectral titrations with cholesterol, in the presence of EFV or l-Glu, suggest that water displacement from the heme iron can be affected in activator-bound CYP46A1. Moreover, EFV and l-Glu synergistically activated CYP46A1. Collectively, our in vitro data, along with those from previous cell culture and in vivo studies by others, suggest that l-Glu-induced CYP46A1 activation is of physiological relevance.

Retinal Cholesterol Content Is Reduced in Simvastatin-Treated Mice Due to Inhibited Local Biosynthesis Albeit Increased Uptake of Serum Cholesterol.

Statins, a class of cholesterol-lowering drugs, are currently being investigated for treatment of age-related macular degeneration, a retinal disease. Herein, retinal and serum concentrations of four statins (atorvastatin, simvastatin, pravastatin, and rosuvastatin) were evaluated after mice were given a single drug dose of 60 mg/kg body weight. All statins, except rosuvastatin, were detected in the retina: atorvastatin and pravastatin at 1.6 pmol and simvastatin at 4.1 pmol. Serum statin concentrations (pmol/ml) were 223 (simvastatin), 1401 (atorvastatin), 2792 (pravastatin), and 9050 (rosuvastatin). Simvastatin was then administered to mice daily for 6 weeks at 60 mg/kg body weight. Simvastatin treatment reduced serum cholesterol levels by 18% and retinal content of cholesterol and lathosterol (but not desmosterol) by 24% and 21%, respectively. The relative contributions of retinal cholesterol biosynthesis and retinal uptake of serum cholesterol to total retinal cholesterol input were changed as well. These contributions were 79% and 21%, respectively, in vehicle-treated mice and 69% and 31%, respectively, in simvastatin-treated mice. Thus, simvastatin treatment lowered retinal cholesterol because a compensatory upregulation of retinal uptake of serum cholesterol was not sufficient to overcome the effect of inhibited retinal biosynthesis. Simultaneously, simvastatin-treated mice had a 2.9-fold increase in retinal expression of Cd36, the major receptor clearing oxidized low-density lipoproteins from Bruch's membrane. Notably, simvastatin treatment essentially did not affect brain cholesterol homeostasis. Our results reveal the statin effect on the retinal and brain cholesterol input and are of value for future clinical investigations of statins as potential therapeutics for age-related macular degeneration.