Formation of 7-dehydrocholesterol-containing membrane rafts in vitro and in vivo, with relevance to the Smith-Lemli-Opitz syndrome

Dishevelled localization and function are differentially regulated by structurally distinct sterols

The Dishevelled (DVL) family of proteins form supramolecular protein and lipid complexes at the cytoplasmic interface of the plasma membrane to regulate tissue patterning, proliferation, cell polarity, and oncogenic processes through DVL-dependent signaling, such as Wnt/β-catenin. While DVL binding to cholesterol is required for its membrane association, the specific structural requirements and cellular impacts of DVL-sterol association are unclear. We report that intracellular sterols which accumulate within normal and pathological conditions cause aberrant DVL activity. In silico and molecular analyses suggested orientation of the β- and α-sterol face within the DVL-PDZ domain regulates DVL-sterol binding. Intracellular accumulation of naturally occurring sterols impaired DVL2 plasma membrane association, inducing DVL2 nuclear localization via Foxk2. Changes to intracellular sterols also selectively impaired DVL2 protein-protein interactions This work identifies sterol specificity as a regulator of DVL signaling, suggests intracellular sterols cause distinct impacts on DVL activity, and supports a role for intracellular sterol homeostasis in cell signaling.

Inhibition of 7-dehydrocholesterol reductase prevents hepatic ferroptosis under an active state of sterol synthesis

Recent evidence indicates ferroptosis is implicated in the pathophysiology of various liver diseases; however, the organ-specific regulation mechanism is poorly understood. Here, we demonstrate 7-dehydrocholesterol reductase (DHCR7), the terminal enzyme of cholesterol biosynthesis, as a regulator of ferroptosis in hepatocytes. Genetic and pharmacological inhibition (with AY9944) of DHCR7 suppress ferroptosis in human hepatocellular carcinoma Huh-7 cells. DHCR7 inhibition increases its substrate, 7-dehydrocholesterol (7-DHC). Furthermore, exogenous 7-DHC supplementation using hydroxypropyl β-cyclodextrin suppresses ferroptosis. A 7-DHC-derived oxysterol metabolite, 3β,5α-dihydroxycholest-7-en-6-one (DHCEO), is increased by the ferroptosis-inducer RSL-3 in DHCR7-deficient cells, suggesting that the ferroptosis-suppressive effect of DHCR7 inhibition is associated with the oxidation of 7-DHC. Electron spin resonance analysis reveals that 7-DHC functions as a radical trapping agent, thus protecting cells from ferroptosis. We further show that AY9944 inhibits hepatic ischemia-reperfusion injury, and genetic ablation of Dhcr7 prevents acetaminophen-induced acute liver failure in mice. These findings provide new insights into the regulatory mechanism of liver ferroptosis and suggest a potential therapeutic option for ferroptosis-related liver diseases.

DHCR7 promotes lymph node metastasis in cervical cancer through cholesterol reprogramming-mediated activation of the KANK4/PI3K/AKT axis and VEGF-C secretion

Cervical cancer (CC) patients with lymph node metastasis (LNM) have a poor prognosis. However, the molecular mechanism of LNM in CC is unclear, and there is no effective clinical treatment. Here, we found that 7-dehydrocholesterol reductase (DHCR7), an enzyme that catalyzes the last step of cholesterol synthesis, was upregulated in CC and closely related to LNM. Gain-of-function and loss-of-function experiments proved that DHCR7 promoted the invasion ability of CC cells and lymphangiogenesis in vitro and induced LNM in vivo. The LNM-promoting effect of DHCR7 was partly mediated by upregulating KN motif and ankyrin repeat domains 4 (KANK4) expression and subsequently activating the PI3K/AKT signaling pathway. Alternatively, DHCR7 promoted the secretion of vascular endothelial growth factor-C (VEGF-C), and thereby lymphangiogenesis. Interestingly, cholesterol reprogramming was needed for the DHCR7-mediated promotion of activation of the KANK4/PI3K/AKT axis, VEGF-C secretion, and subsequent LNM. Importantly, treatment with the DHCR7 inhibitors AY9944 and tamoxifen (TAM) significantly inhibited LNM of CC, suggesting the clinical application potential of DHCR7 inhibitors in CC. Collectively, our results uncover a novel molecular mechanism of LNM in CC and identify DHCR7 as a new potential therapeutic target.

Inhibition of pseudorabies virus replication via upregulated interferon response by targeting 7-dehydrocholesterol reductase

Pseudorabies virus (PRV) is an alpha-herpesvirus capable of infecting a range of animal species, particularly its natural host, pigs, resulting in substantial economic losses for the swine industry. Recent research has shed light on the significant role of cholesterol metabolism in the replication of various viruses. However, the specific role of cholesterol metabolism in PRV infection remains unknown. Here, we demonstrated that the expression of 7-dehydrocholesterol reductase (DHCR7) is upregulated following PRV infection, as evidenced by the proteomic analysis. Subsequently, we showed that DHCR7 plays a crucial role in promoting PRV replication by converting 7-dehydrocholesterol (7-DHC) into cholesterol, leading to increased cellular cholesterol levels. Importantly, DHCR7 inhibits the phosphorylation of interferon regulatory factor 3 (IRF3), resulting in reduced levels of interferon-beta (IFN-β) and interferon-stimulated genes (ISGs). Finally, we revealed that the DHCR7 inhibitor, trans-1,4-bis(2-chlorobenzylaminomethyl) cyclohexane dihydrochloride (AY9944), significantly suppresses PRV replication both in vitro and in vivo. Taken together, the study has established a connection between cholesterol metabolism and PRV replication, offering novel insights that may guide future approaches to the prevention and treatment of PRV infections.

Role of rafts in neurological disorders

INTRODUCTION

Rafts are protein-lipid structural nanodomains involved in efficient signal transduction and the modulation of physiological processes of the cell plasma membrane. Raft disruption in the nervous system has been associated with a wide range of disorders.

DEVELOPMENT

We review the concept of rafts, the nervous system processes in which they are involved, and their role in diseases such as Parkinson's disease, Alzheimer disease, and Huntington disease.

CONCLUSIONS

Based on the available evidence, preservation and/or reconstitution of rafts is a promising treatment strategy for a wide range of neurological disorders.

Cholesterol metabolism pathway in autism spectrum disorder: From animal models to clinical observations

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by a persistent impairment of social skills, including aspects of perception, interpretation, and response, combined with restricted and repetitive behavior. ASD is a complex and multifactorial condition, and its etiology could be attributed to genetic and environmental factors. Despite numerous clinical and experimental studies, no etiological factor, biomarker, and specific model of transmission have been consistently associated with ASD. However, an imbalance in cholesterol levels has been observed in many patients, more specifically, a condition of hypocholesterolemia, which seems to be shared between ASD and ASD-related genetic syndromes such as fragile X syndrome (FXS), Rett syndrome (RS), and Smith- Lemli-Opitz (SLO). Furthermore, it is known that alterations in cholesterol levels lead to neuroinflammation, oxidative stress, impaired myelination and synaptogenesis. Thus, the aim of this review is to discuss the cholesterol metabolic pathways in the ASD context, as well as in genetic syndromes related to ASD, through clinical observations and animal models. In fact, SLO, FXS, and RS patients display early behavioral markers of ASD followed by cholesterol disturbances. Several studies have demonstrated the role of cholesterol in psychiatric conditions and how its levels modulate brain neurodevelopment. This review suggests an important relationship between ASD pathology and cholesterol metabolism impairment; thus, some strategies could be raised - at clinical and pre-clinical levels - to explore whether cholesterol metabolism disturbance has a generally adverse effect in exacerbating the symptoms of ASD patients.

Smith-Lemli-Opitz syndrome: A pathophysiological manifestation of the Bloch hypothesis

The biosynthesis of cholesterol, an essential component of higher eukaryotic membranes, was worked out by Konrad Bloch (and Feodor Lynen) in the 1960s and they received the Nobel Prize around that time in recognition of their pioneering contributions. An elegant consequence of this was a hypothesis proposed by Konrad Bloch (the Bloch hypothesis) which suggests that each subsequent intermediate in the cholesterol biosynthesis pathway is superior in supporting membrane function in higher eukaryotes relative to its precursor. In this review, we discuss an autosomal recessive metabolic disorder, known as Smith-Lemli-Opitz syndrome (SLOS), associated with a defect in the Kandutsch-Russell pathway of cholesterol biosynthesis that results in accumulation of the immediate precursor of cholesterol in its biosynthetic pathway (7-dehydrocholesterol) and an altered cholesterol to total sterol ratio. Patients suffering from SLOS have several developmental, behavioral and cognitive abnormalities for which no drug is available yet. We characterize SLOS as a manifestation of the Bloch hypothesis and review its molecular etiology and current treatment. We further discuss defective Hedgehog signaling in SLOS and focus on the role of the serotonin_1A receptor, a representative neurotransmitter receptor belonging to the GPCR family, in SLOS. Notably, ligand binding activity and cellular signaling of serotonin_1A receptors are impaired in SLOS-like condition. Importantly, cellular localization and intracellular trafficking of the serotonin_1A receptor (which constitute an important determinant of a GPCR cellular function) are compromised in SLOS. We highlight some of the recent developments and emerging concepts in SLOS pathobiology and suggest that novel therapies based on trafficking defects of target receptors could provide new insight into treatment of SLOS.

Temporal changes in the brain lipidome during neurodevelopment of Smith-Lemli-Opitz syndrome mice

Neurodevelopment is an intricately orchestrated program of cellular events that occurs with tight temporal and spatial regulation. While it is known that the development and proper functioning of the brain, which is the second most lipid rich organ behind adipose tissue, greatly rely on lipid metabolism and signaling, the temporal lipidomic changes that occur throughout the course of neurodevelopment have not been investigated. Smith-Lemli-Opitz syndrome is a metabolic disorder caused by genetic mutations in the DHCR7 gene, leading to defective 3β-hydroxysterol-Δ⁷-reductase (DHCR7), the enzyme that catalyzes the last step of the Kandutsch-Russell pathway of cholesterol synthesis. Due to the close regulatory relationship between sterol and lipid homeostasis, we hypothesize that altered or dysregulated lipid metabolism beyond the primary defect of cholesterol biosynthesis is present in the pathophysiology of SLOS. Herein, we applied our HILIC-IM-MS method and LiPydomics Python package to streamline an untargeted lipidomics analysis of developing mouse brains in both wild-type and Dhcr7-KO mice, identifying lipids at Level 3 (lipid species level: lipid class/subclass and fatty acid sum composition). We compared relative lipid abundances throughout development, from embryonic day 12.5 to postnatal day 0 and determined differentially expressed brain lipids between wild-type and Dhcr7-KO mice at specific developmental time points, revealing lipid metabolic pathways that are affected in SLOS beyond the cholesterol biosynthesis pathway, such as glycerolipid, glycerophospholipid, and sphingolipid metabolism. Implications of the altered lipid metabolic pathways in SLOS pathophysiology are discussed.

Medication effects on developmental sterol biosynthesis

Cholesterol is essential for normal brain function and development. Genetic disruptions of sterol biosynthesis result in intellectual and developmental disabilities. Developing neurons synthesize their own cholesterol, and disruption of this process can occur by both genetic and chemical mechanisms. Many commonly prescribed medications interfere with sterol biosynthesis, including haloperidol, aripiprazole, cariprazine, fluoxetine, trazodone and amiodarone. When used during pregnancy, these compounds might have detrimental effects on the developing brain of the offspring. In particular, inhibition of dehydrocholesterol-reductase 7 (DHCR7), the last enzyme in the biosynthesis pathway, results in accumulation of the immediate cholesterol precursor, 7-dehydrocholesterol (7-DHC). 7-DHC is highly unstable, giving rise to toxic oxysterols; this is particularly pronounced in a mouse model when both the mother and the offspring carry the Dhcr7^+/- genotype. Studies of human dermal fibroblasts from individuals who carry DCHR7^+/- single allele mutations suggest that the same gene*medication interaction also occurs in humans. The public health relevance of these findings is high, as DHCR7-inhibitors can be considered teratogens, and are commonly used by pregnant women. In addition, sterol biosynthesis inhibiting medications should be used with caution in individuals with mutations in sterol biosynthesis genes. In an age of precision medicine, further research in this area could open opportunities to improve patient and fetal/infant safety by tailoring medication prescriptions according to patient genotype and life stage.

Sterols lower energetic barriers of membrane bending and fission necessary for efficient clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is critical for cellular signal transduction, receptor recycling, and membrane homeostasis in mammalian cells. Acute depletion of cholesterol disrupts CME, motivating analysis of CME dynamics in the context of human disorders of cholesterol metabolism. We report that inhibition of post-squalene cholesterol biosynthesis impairs CME. Imaging of membrane bending dynamics and the CME pit ultrastructure reveals prolonged clathrin pit lifetimes and shallow clathrin-coated structures, suggesting progressive impairment of curvature generation correlates with diminishing sterol abundance. Sterol structural requirements for efficient CME include 3′ polar head group and B-ring conformation, resembling the sterol structural prerequisites for tight lipid packing and polarity. Furthermore, Smith-Lemli-Opitz fibroblasts with low cholesterol abundance exhibit deficits in CME-mediated transferrin internalization. We conclude that sterols lower the energetic costs of membrane bending during pit formation and vesicular scission during CME and suggest that reduced CME activity may contribute to cellular phenotypes observed within disorders of cholesterol metabolism.

Anderson et al. demonstrate that sterol abundance and identity play a dominant role in facilitating clathrin-mediated endocytosis. Detailed analyses of clathrin-coated pits under sterol depletion support a requirement for sterol-mediated membrane bending during multiple stages of endocytosis, implicating endocytic dysfunction within the pathogenesis of disorders of cholesterol metabolism.

Effect of Cholesterol Versus Ergosterol on DPPC Bilayer Properties: Insights from Atomistic Simulations

Sterols have been ascribed a major role in the organization of biological membranes, in particular for the formation of liquid ordered domains in complex lipid mixtures. Here, we employed molecular dynamics simulations to compare the effects of cholesterol and ergosterol as the major sterol of mammalian and fungal cells, respectively, on binary mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as a proxy for saturated lipids. In agreement with previous work, we observe that the addition of sterol molecules modifies the order of DPPC both in the gel phase and in the liquid phase. When disentangling the overall tilt angle and the structure of the tail imposed by trans/gauche configurations of torsion angles in the tail, respectively, a more detailed picture of the impact of sterols can be formulated, revealing, for example, an approximate temperature-concentration superposition ranging from the liquid to the gel phase. Furthermore, a new quantitative measure to identify the presence of collective sterol effects is discussed. Moreover, when comparing both types of sterols, addition of cholesterol has a noticeably stronger impact on phospholipid properties than that of ergosterol. The observed differences can be attributed to higher planarity of the cholesterol ring system. This planarity combined with an inherent asymmetry in its molecular interactions leads to better alignment and hence stronger interaction with saturated acyl chains. Our results suggest that the high order demonstrated for ergosterol in fungal plasma membranes must therefore be generated via additional mechanisms.

Role of rafts in neurological disorders

INTRODUCTION

Rafts are function-structural cell membrane nano-domains. They contribute to explain the efficiency of signal transduction at the low physiological membrane concentrations of the signaling partners by their clustering inside specialized signaling domains.

DEVELOPMENT

In this article, we review the current model of the membrane rafts and their physio-pathological relevance in the nervous system, including their role in Parkinson, Alzheimer, and Huntington diseases.

CONCLUSIONS

Rafts disruption/dysfunction has been shown to relate diverse neurological diseases. Therefore, it has been suggested that preservation of membrane rafts may represent a strategy to prevent or delay neuronal dysfunctions in several diseases.

Ultrafast excited state dynamics of provitamin D3 and analogs in solution and in lipid bilayers

The photochemical ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D₃) is responsible for the light-initiated formation of vitamin D₃ in mammalian skin membranes. Visible transient absorption spectroscopy was used to explore the excited state dynamics of DHC and two analogs: ergosterol (provitamin D₂) and DHC acetate free in solution and confined to lipid bilayers chosen to model the biological cell membrane. In solution, the excited state dynamics of the three compounds are nearly identical. However, when confined to lipid bilayers, the heterogeneity of the lipid membrane and packing forces imposed on the molecule by the lipid alter the excited state dynamics of these compounds. When confined to lipid bilayers in liposomes formed using DPPC, two solvation environments are identified. The excited state dynamics for DHC and analogs in fluid-like regions of the liposome membrane undergo internal conversion and ring-opening on 1 ps-2 ps time scales, similar to those observed in isotropic solution. In contrast, the excited state lifetime of a subpopulation in regions of lower fluidity is 7 ps-12 ps. The long decay component is unique to these liposomes and results from the structural properties of the lipid bilayer. Additional measurements in liposomes prepared with lipids having slightly longer or shorter alkane tails support this conclusion. In the lipid environments studied, the longest lifetimes are observed for DHC. The unsaturated sterol tail of ergosterol and the acetate group of DHC acetate disrupt the packing around the molecule and permit faster internal conversion and relaxation back to the ground state.

Targeting 7-Dehydrocholesterol Reductase Integrates Cholesterol Metabolism and IRF3 Activation to Eliminate Infection

Recent work suggests that cholesterol metabolism impacts innate immune responses against infection. However, the key enzymes or the natural products and mechanisms involved are not well elucidated. Here, we have shown that upon DNA and RNA viral infection, macrophages reduced 7-dehydrocholesterol reductase (DHCR7) expression. DHCR7 deficiency or treatment with the natural product 7-dehydrocholesterol (7-DHC) could specifically promote phosphorylation of IRF3 (not TBK1) and enhance type I interferon (IFN-I) production in macrophages. We further elucidated that viral infection or 7-DHC treatment enhanced AKT3 expression and activation. AKT3 directly bound and phosphorylated IRF3 at Ser385, together with TBK1-induced phosphorylation of IRF3 Ser386, to achieve IRF3 dimerization. Deletion of DHCR7 and the DHCR7 inhibitors including AY9944 and the chemotherapy drug tamoxifen promoted clearance of Zika virus and multiple viruses in vitro or in vivo. Taken together, we propose that the DHCR7 inhibitors and 7-DHC are potential therapeutics against emerging or highly pathogenic viruses.

Mycobacterial Membrane Domain, or a Primordial Organelle?

Mycobacteria, like many other prokaryotic organisms, do not appear to have membrane-bound organelles to organize the subcellular space. Nevertheless, mycobacteria and related bacteria grow their cell envelope in a spatially controlled manner, restricting cell elongation to the polar regions of the rod-shaped cell. This spatial organization demands that de novo synthesized cell envelope components must be supplied to the polar ends of the cell. Because many cell envelope components are either lipids or built as lipid-anchored precursors, the plasma membrane is the major site of the biosynthesis. Thus, there are logistical questions of where in the plasma membrane these lipids and lipid precursors are made and how they are subsequently delivered to the growing poles of the cell. Our discovery of an intracellular membrane domain (IMD) fills in this gap. Currently available data suggest that the IMD is a membrane domain within the plasma membrane of mycobacteria, which mediates key biosynthetic reactions for cell envelope and other lipid biosynthetic reactions. Consistent with its role in polar growth, the IMD is enriched in the polar regions of actively growing cells and becomes less polarized when the cells experience non-growing conditions. We discuss how such membrane compartmentalization may be generated and maintained in a mycobacterial cell and why it has not evolved into a bona fide organelle. In a broader perspective, we suggest that segregation of biosynthetic pathways into different domains of a planar membrane could be more widespread than we currently think.

Multi-omics investigation reveals benzalkonium chloride disinfectants alter sterol and lipid homeostasis in the mouse neonatal brain

Lipids are critical for neurodevelopment; therefore, disruption of lipid homeostasis by environmental chemicals is expected to have detrimental effects on this process. Previously, we demonstrated that the benzalkonium chlorides (BACs), a class of commonly used disinfectants, alter cholesterol biosynthesis and lipid homeostasis in neuronal cell cultures in a manner dependent on their alkyl chain length. However, the ability of BACs to reach the neonatal brain and alter sterol and lipid homeostasis during neurodevelopment in vivo has not been characterized. Therefore, the goal of this study was to use targeted and untargeted mass spectrometry and transcriptomics to investigate the effect of BACs on sterol and lipid homeostasis, and to predict the mechanism of toxicity of BACs on neurodevelopmental processes. After maternal dietary exposure to 120 mg BAC/kg body weight/day, we quantified BAC levels in the mouse neonatal brain, demonstrating for the first time that BACs can cross the blood-placental barrier and enter the developing brain. Transcriptomic analysis of neonatal brains using RNA sequencing revealed alterations in canonical pathways related to cholesterol biosynthesis, liver X receptor-retinoid X receptor (LXR/RXR) signaling, and glutamate receptor signaling. Mass spectrometry analysis revealed decreases in total sterol levels and downregulation of triglycerides and diglycerides, which were consistent with the upregulation of genes involved in sterol biosynthesis and uptake as well as inhibition of LXR signaling. In conclusion, these findings demonstrate that BACs target sterol and lipid homeostasis and provide new insights for the possible mechanisms of action of BACs as developmental neurotoxicants.

Effect of sterol structure on ordered membrane domain (raft) stability in symmetric and asymmetric vesicles

Sterol structure influences liquid ordered domains in membranes, and the dependence of biological functions on sterol structure can help identify processes dependent on ordered domains. In this study we compared the effect of sterol structure on ordered domain formation in symmetric vesicles composed of mixtures of sphingomyelin, 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol, and in asymmetric vesicles in which sphingomyelin was introduced into the outer leaflet of vesicles composed of DOPC and cholesterol. In most cases, sterol behavior was similar in symmetric and asymmetric vesicles, with ordered domains most strongly stabilized by 7-dehydrocholesterol (7DHC) and cholesterol, stabilized to a moderate degree by lanosterol, epicholesterol and desmosterol, and very little if at all by 4-cholesten-3-one. However, in asymmetric vesicles desmosterol stabilized ordered domain almost as well as cholesterol, and to a much greater degree than epicholesterol, so that the ability to support ordered domains decreased in the order 7-DHC > cholesterol > desmosterol > lanosterol > epicholesterol > 4-cholesten-3-one. This contrasts with values for intermediate stabilizing sterols in symmetric vesicles in which the ranking was cholesterol > lanosterol ~ desmosterol ~ epicholesterol or prior studies in which the ranking was cholesterol ~ epicholesterol > lanosterol ~ desmosterol. The reasons for these differences are discussed. Based on these results, we re-evaluated our prior studies in cells and conclude that endocytosis levels and bacterial uptake are even more closely correlated with the ability of sterols to form ordered domains than previously thought, and do not necessarily require that a sterol have a 3β-OH group.

Determining the target of membrane sterols on voltage-gated potassium channels

Cholesterol, an essential lipid component of cellular plasma membranes, regulates fluidity, mechanical integrity, raft structure and may specifically interact with membrane proteins. Numerous effects on ion channels by cholesterol, including changes in current amplitude, voltage dependence and gating kinetics, have been reported. We have previously described such changes in the voltage-gated potassium channel K_v1.3 of lymphocytes by cholesterol and its analog 7-dehydrocholesterol (7DHC). In voltage-gated channels membrane depolarization induces movement of the voltage sensor domains (VSD), which is transmitted by a coupling mechanism to the pore domain (PD) to open the channel. Here, we investigated whether cholesterol effects were mediated by the VSD to the pore or the PD was the direct target. Specificity was tested by comparing K_v1.3 and K_v10.1 channels having different VSD-PD coupling mechanisms. Current recordings were performed with two-electrode voltage-clamp fluorometry, where movement of the VSDs was monitored by attaching fluorophores to external cysteine residues introduced in the channel sequence. Loading the membrane with cholesterol or 7DHC using methyl-β-cyclodextrin induced changes in the steady-state and kinetic parameters of the ionic currents while leaving fluorescence parameters mostly unaffected in both channels. Non-stationary noise analysis revealed that reduction of single channel conductance rather than that of open probability caused the observed current decrease. Furthermore, confocal laser scanning and stimulated emission depletion microscopy demonstrated significant changes in the distribution of these ion channels in response to sterol loading. Our results indicate that sterol-induced effects on ion channel gating directly target the pore and do not act via the VSD.

Oxysterols and Retinal Degeneration in a Rat Model of Smith-Lemli-Opitz Syndrome: Implications for an Improved Therapeutic Intervention

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive human disease caused by mutations in the gene encoding 7-dehydrocholesterol (7DHC) reductase (DHCR7), resulting in abnormal accumulation of 7DHC and reduced levels of cholesterol in bodily tissues and fluids. A rat model of the disease has been created by treating normal rats with the DHCR7 inhibitor, AY9944, which causes progressive, irreversible retinal degeneration. Herein, we review the features of this disease model and the evidence linking 7DHC-derived oxysterols to the pathobiology of the disease, with particular emphasis on the associated retinal degeneration. A recent study has shown that treating the rat model with cholesterol plus suitable antioxidants completely prevents the retinal degeneration. These findings are discussed with regard to their translational implications for developing an improved therapeutic intervention for SLOS over the current standard of care.

Prevention of Retinal Degeneration in a Rat Model of Smith-Lemli-Opitz Syndrome

Smith-Lemli-Opitz Syndrome (SLOS) is a recessive human disease caused by defective cholesterol (CHOL) synthesis at the level of DHCR7 (7-dehydrocholesterol reductase), which normally catalyzes the conversion of 7-dehydrocholesterol (7DHC) to CHOL. Formation and abnormal accumulation of 7DHC and 7DHC-derived oxysterols occur in SLOS patients and in rats treated with the DHCR7 inhibitor AY9944. The rat SLOS model exhibits progressive and irreversible retinal dysfunction and degeneration, which is only partially ameliorated by dietary CHOL supplementation. We hypothesized that 7DHC-derived oxysterols are causally involved in this retinal degeneration, and that blocking or reducing their formation should minimize the phenotype. Here, using the SLOS rat model, we demonstrate that combined dietary supplementation with CHOL plus antioxidants (vitamins E and C, plus sodium selenite) provides better outcomes than dietary CHOL supplementation alone with regard to preservation of retinal structure and function and lowering 7DHC-derived oxysterol formation. These proof-of-principle findings provide a translational, pre-clinical framework for designing clinical trials using CHOL-antioxidant combination therapy as an improved therapeutic intervention over the current standard of care for the treatment of SLOS.

7-Dehydrocholesterol (7-DHC), But Not Cholesterol, Causes Suppression of Canonical TGF-β Signaling and Is Likely Involved in the Development of Atherosclerotic Cardiovascular Disease (ASCVD)

For several decades, cholesterol has been thought to cause ASCVD. Limiting dietary cholesterol intake has been recommended to reduce the risk of the disease. However, several recent epidemiological studies do not support a relationship between dietary cholesterol and/or blood cholesterol and ASCVD. Consequently, the role of cholesterol in atherogenesis is now uncertain. Much evidence indicates that TGF-β, an anti-inflammatory cytokine, protects against ASCVD and that suppression of canonical TGF-β signaling (Smad2-dependent) is involved in atherogenesis. We had hypothesized that cholesterol causes ASCVD by suppressing canonical TGF-β signaling in vascular endothelium. To test this hypothesis, we determine the effects of cholesterol, 7-dehydrocholesterol (7-DHC; the biosynthetic precursor of cholesterol), and other sterols on canonical TGF-β signaling. We use Mv1Lu cells (a model cell system for studying TGF-β activity) stably expressing the Smad2-dependent luciferase reporter gene. We demonstrate that 7-DHC (but not cholesterol or other sterols) effectively suppresses the TGF-β-stimulated luciferase activity. We also demonstrate that 7-DHC suppresses TGF-β-stimulated luciferase activity by promoting lipid raft/caveolae formation and subsequently recruiting cell-surface TGF-β receptors from non-lipid raft microdomains to lipid rafts/caveolae where TGF-β receptors become inactive in transducing canonical signaling and undergo rapid degradation upon TGF-β binding. We determine this by cell-surface 125 I-TGF-β-cross-linking and sucrose density gradient ultracentrifugation. We further demonstrate that methyl-β-cyclodextrin (MβCD), a sterol-chelating agent, reverses 7-DHC-induced suppression of TGF-β-stimulated luciferase activity by extrusion of 7-DHC from resident lipid rafts/caveolae. These results suggest that 7-DHC, but not cholesterol, promotes lipid raft/caveolae formation, leading to suppression of canonical TGF-β signaling and atherogenesis. J. Cell. Biochem. 118: 1387-1400, 2017. © 2016 Wiley Periodicals, Inc.

7-dehydrocholesterol efficiently supports Ret signaling in a mouse model of Smith-Opitz-Lemli syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a rare disorder of cholesterol synthesis. Affected individuals exhibit growth failure, intellectual disability and a broad spectrum of developmental malformations. Among them, renal agenesis or hypoplasia, decreased innervation of the gut, and ptosis are consistent with impaired Ret signaling. Ret is a receptor tyrosine kinase that achieves full activity when recruited to lipid rafts. Mice mutant for Ret are born with no kidneys and enteric neurons, and display sympathetic nervous system defects causing ptosis. Since cholesterol is a critical component of lipid rafts, here we tested the hypothesis of whether the cause of the above malformations found in SLOS is defective Ret signaling owing to improper lipid raft composition or function. No defects consistent with decreased Ret signaling were found in newborn Dhcr7^−/− mice, or in Dhcr7^−/− mice lacking one copy of Ret. Although kidneys from Dhcr7^−/− mice showed a mild branching defect in vitro, GDNF was able to support survival and downstream signaling of sympathetic neurons. Consistently, GFRα1 correctly partitioned to lipid rafts in brain tissue. Finally, replacement experiments demonstrated that 7-DHC efficiently supports Ret signaling in vitro. Taken together, our findings do not support a role of Ret signaling in the pathogenesis of SLOS.

Differential cytotoxic effects of 7-dehydrocholesterol-derived oxysterols on cultured retina-derived cells: Dependence on sterol structure, cell type, and density

Tissue accumulation of 7-dehydrocholesterol (7DHC) is a hallmark of Smith-Lemli-Opitz Syndrome (SLOS), a human inborn error of the cholesterol (CHOL) synthesis pathway. Retinal 7DHC-derived oxysterol formation occurs in the AY9944-induced rat model of SLOS, which exhibits a retinal degeneration characterized by selective loss of photoreceptors and associated functional deficits, Müller cell hypertrophy, and engorgement of the retinal pigment epithelium (RPE) with phagocytic inclusions. We evaluated the relative effects of four 7DHC-derived oxysterols on three retina-derived cell types in culture, with respect to changes in cellular morphology and viability. 661W (photoreceptor-derived) cells, rMC-1 (Müller glia-derived) cells, and normal diploid monkey RPE (mRPE) cells were incubated for 24 h with dose ranges of either 7-ketocholesterol (7kCHOL), 5,9-endoperoxy-cholest-7-en-3β,6α-diol (EPCD), 3β,5α-dihydroxycholest-7-en-6-one (DHCEO), or 4β-hydroxy-7-dehydrocholesterol (4HDHC); CHOL served as a negative control (same dose range), along with appropriate vehicle controls, while staurosporine (Stsp) was used as a positive cytotoxic control. For 661W cells, the rank order of oxysterol potency was: EPCD > 7kCHOL >> DHCEO > 4HDHC ≈ CHOL. EC50 values were higher for confluent vs. subconfluent cultures. 661W cells exhibited much higher sensitivity to EPCD and 7kCHOL than either rMC-1 or mRPE cells, with the latter being the most robust when challenged, either at confluence or in sub-confluent cultures. When tested on rMC-1 and mRPE cells, EPCD was again an order of magnitude more potent than 7kCHOL in compromising cellular viability. Hence, 7DHC-derived oxysterols elicit differential cytotoxicity that is dose-, cell type-, and cell density-dependent. These results are consistent with the observed progressive, photoreceptor-specific retinal degeneration in the rat SLOS model, and support the hypothesis that 7DHC-derived oxysterols are causally linked to that retinal degeneration as well as to SLOS.

Metabolic Precursor of Cholesterol Causes Formation of Chained Aggregates of Liquid-Ordered Domains

7-Dehydrocholesterol, an immediate metabolic predecessor of cholesterol, can accumulate in tissues due to some metabolic abnormalities, causing an array of symptoms known as Smith-Lemli-Opitz syndrome. Enrichment of cellular membranes with 7-dehydrocholesterol interferes with normal cell-signaling processes, which involve interaction between rafts and formation of the so-called signaling platforms. In model membranes, cholesterol-based ordered domains usually merge upon contact. According to our experimental data, ordered domains in the model systems where cholesterol is substituted for 7-dehydrocholesterol never merge on the time scale of the experiment, but clusterize into necklace-like aggregates. We attribute such different dynamical behavior to altered properties of the domain boundary. In the framework of thickness mismatch model, we analyzed changes of interaction energy profiles of two approaching domains caused by substitution of cholesterol by 7-dehydrocholesterol. The energy barrier for domain merger is shown to increase notably, with simultaneous appearance of another distinct local energy minimum. Such energy profile is in perfect qualitative agreement with the experimental observations. The observed change of domain dynamics can impair proper interaction between cellular rafts underlying pathologies associated with deviations in cholesterol metabolism.

Pathogenesis, Epidemiology, Diagnosis and Clinical Aspects of Smith-Lemli-Opitz Syndrome

INTRODUCTION

Smith-Lemli-Opitz Syndrome (SLOS) is a malformation syndrome inherited in an autosomal recessive fashion. It is due to a metabolic defect in the conversion of 7-dehydrocholesterol to cholesterol, which leads to an accumulation of 7-dehydrocholesterol and frequently a deficiency of cholesterol. The syndrome is characterized by typical dysmorphic facial features, multiple malformations, and intellectual disability.

AREAS COVERED

In this paper we provide an overview of the clinical phenotype and discuss how the manifestations of the syndrome vary depending on the age of the patients. We then explore the underlying biochemical defect and pathophysiological alterations that may contribute to the many disease manifestations. Subsequently we explore the epidemiology and succinctly discuss population genetics as they relate to SLOS. The next section presents the diagnostic possibilities. Thereafter, the treatment and management as is standard of care are presented.

EXPERT OPINION

Even though the knowledge of the underlying molecular mutations and the biochemical alterations is being rapidly accumulated, there is currently no efficacious therapy addressing neurological dysfunction. We discuss the difficulty of treating this disorder, which manifests as a combination of a malformation syndrome and an inborn error of metabolism. A very important factor in developing new therapies is the need to rigorously establish efficacy in controlled trials.

Cholesterol in myelin biogenesis and hypomyelinating disorders

The largest pool of free cholesterol in mammals resides in myelin membranes. Myelin facilitates rapid saltatory impulse propagation by electrical insulation of axons. This function is achieved by ensheathing axons with a tightly compacted stack of membranes. Cholesterol influences myelination at many steps, from the differentiation of myelinating glial cells, over the process of myelin membrane biogenesis, to the functionality of mature myelin. Cholesterol emerged as the only integral myelin component that is essential and rate-limiting for the development of myelin in the central and peripheral nervous system. Moreover, disorders that interfere with sterol synthesis or intracellular trafficking of cholesterol and other lipids cause hypomyelination and neurodegeneration. This review summarizes recent results on the roles of cholesterol in CNS myelin biogenesis in normal development and under different pathological conditions. This article is part of a Special Issue entitled Brain Lipids.

HFE gene variants, iron, and lipids: a novel connection in Alzheimer's disease

Iron accumulation and associated oxidative stress in the brain have been consistently found in several neurodegenerative diseases. Multiple genetic studies have been undertaken to try to identify a cause of neurodegenerative diseases but direct connections have been rare. In the iron field, variants in the HFE gene that give rise to a protein involved in cellular iron regulation, are associated with iron accumulation in multiple organs including the brain. There is also substantial epidemiological, genetic, and molecular evidence of disruption of cholesterol homeostasis in several neurodegenerative diseases, in particular Alzheimer's disease (AD). Despite the efforts that have been made to identify factors that can trigger the pathological events associated with neurodegenerative diseases they remain mostly unknown. Because molecular phenotypes such as oxidative stress, synaptic failure, neuronal loss, and cognitive decline, characteristics associated with AD, have been shown to result from disruption of a number of pathways, one can easily argue that the phenotype seen may not arise from a linear sequence of events. Therefore, a multi-targeted approach is needed to understand a complex disorder like AD. This can be achieved only when knowledge about interactions between the different pathways and the potential influence of environmental factors on them becomes available. Toward this end, this review discusses what is known about the roles and interactions of iron and cholesterol in neurodegenerative diseases. It highlights the effects of gene variants of HFE (H63D- and C282Y-HFE) on iron and cholesterol metabolism and how they may contribute to understanding the etiology of complex neurodegenerative diseases.

Evaluation of cytotoxic effects of 7-dehydrocholesterol on melanoma cells

Ultraviolet radiation is the main cause of skin cancers, and melanoma is the most serious form of tumor. There is no therapy for advanced-stage melanoma and its metastasis because of their high resistance to various anticancer therapies. Human skin is an important metabolic organ in which occurs photoinduced synthesis of vitamin D3 from 7-dehydrocholesterol (7-DHC). 7-DHC, the precursor of cholesterol biosynthesis, is highly reactive and easily modifiable to produce 7-DHC-derived compounds. The intracellular levels of 7-DHC or its derivatives can have deleterious effects on cellular functionality and viability. In this study we evaluated the effects on melanoma cell lines of 7-DHC as such and for this aim we used much care to minimize 7-DHC modifications. We found that from 12 to 72 h of treatment 82-86% of 7-DHC entered the cells, and the levels of 7-DHC-derived compounds were not significant. Simultaneously, reactive oxygen species production was significantly increased already after 2h. After 24 h and up to 72 h, 7-DHC-treated melanoma cells showed a reduction in cell growth and viability. The cytotoxic effect of 7-DHC was associated with an increase in Bax levels, decrease in Bcl-2/Bax ratio, reduction of mitochondrial membrane potential, increase in apoptosis-inducing factor levels, unchanged caspase-3 activity, and absence of cleavage of PARP-1. These findings could explain the mechanism through which 7-DHC exerts its cytotoxic effects. This is the first report in which the biological effects found in melanoma cells are mainly attributable to 7-DHC as such.

Oxysterol-binding protein (OSBP) is required for the perinuclear localization of intra-Golgi v-SNAREs

OSBP regulates the Golgi cholesterol level. This study demonstrates that OSBP and cholesterol are essential for localization of Golgi v-SNAREs. Knockdown of ArfGAP1 restores v-SNARE localization in OSBP-depleted cells, suggesting that OSBP-regulated cholesterol ensures proper COP-I vesicle transport.

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) have been implicated in the distribution of sterols among intracellular organelles. OSBP regulates the Golgi cholesterol level, but how it relates to Golgi function is elusive. Here we report that OSBP is essential for the localization of intra-Golgi soluble vesicle N-ethylmaleimide-sensitive fusion attachment protein receptors (v-SNAREs). Depletion of OSBP by small interfering RNA causes mislocalization of intra-Golgi v-SNAREs GS28 and GS15 throughout the cytoplasm without affecting the perinuclear localization of Golgi target-SNARE syntaxin5 and reduces the abundance of a Golgi enzyme, mannosidase II (Man II). GS28 mislocalization and Man II reduction are also induced by cellular cholesterol depletion. Three domains of OSBP—an endoplasmic reticulum–targeting domain, a Golgi-targeting domain, and a sterol-binding domain—are all required for Golgi localization of GS28. Finally, GS28 mislocalization and Man II reduction in OSBP-depleted cells are largely restored by depletion of ArfGAP1, a regulator of the budding of coat protein complex (COP)-I vesicles. From these results, we postulate that Golgi cholesterol level, which is controlled by OSBP, is essential for Golgi localization of intra-Golgi v-SNAREs by ensuring proper COP-I vesicle transport.

No evidence for mevalonate shunting in moderately affected children with Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is caused by a genetic deficiency in 7-dehydrocholesterol (7-DHC) reductase (EC 1.3.1.21), the last enzyme of the cholesterol synthetic pathway. In SLOS, plasma cholesterol concentration is reduced and immediate precursor concentration (7-DHC) is elevated. Surprisingly, total sterol synthesis is reduced but HMG-CoA reductase activity, a rate-limiting enzyme in cholesterol synthesis is unaltered as judged by normal urinary excretion of mevalonic acid (MVA) (Pappu et al. J Lipid Res 43:1661-1669, 2002). These findings raise the possibility of increased diversion of MVA into the MVA shunt pathway away from sterol synthesis, by activation of the shunt pathway enzymes. To test this hypothesis, we measured the urinary excretion of 3-methylglutaconic acid (U-3MGC), a by-product of the shunt pathway, in 19 mildly to moderately severely affected SLOS subjects (ten males, nine females) receiving either a cholesterol-free or a high cholesterol diet, and in 20 age- and sex-matched controls. U-3MGC was similar in SLOS and controls, and was unaffected by dietary cholesterol intake. Further, no change in U-3MGC was observed in a subset of SLOS subjects (n = 9) receiving simvastatin. In contrast, U-MVA was reduced by cholesterol supplementation (~54%, p < 0.05) and by simvastatin (~50%, p < 0.04). There was no correlation between U-3MGC and either plasma sterol concentrations, urinary isoprenoids, or the subjects' clinical severity score. However U-3MGC was inversely correlated with age (p < 0.04) and body weight (p < 0.02), and higher in females than in males (~65%, p < 0.025). The data show that DHCR7 deficiency does not result in 3MGC accumulation in SLOS and suggest that the MVA shunt pathway is not activated in patients with the condition.

Impact of sterol tilt on membrane bending rigidity in cholesterol and 7DHC-containing DMPC membranes

Cholesterol is so essential to the proper function of mammalian cell membranes that even strikingly small inborn errors in cholesterol synthesis can be devastating. Here we combine molecular dynamics simulations with small angle x-ray diffraction experiments to compare mixed sterol/DMPC membranes over a wide range of sterol compositions for two types of sterols: cholesterol and its immediate metabolic precursor 7DHC, that differs from cholesterol by one double bond. We find that while most membrane properties are only slightly affected by the replacement of one sterol by the other, the tilt degree of freedom, as gauged by the tilt modulus, is significantly larger for cholesterol than for 7DHC over a large range of concentrations. In silico mutations of one sterol into the other further support these findings. Moreover, bending rigidities calculated from simulations and estimated in experiments show that cholesterol stiffens membranes to a larger extent than 7DHC. We discuss the possible mechanistic link between sterol tilt and the way it impacts the membrane mechanical properties, and comment on how this link may shed light on the way replacement of cholesterol by 7DHC leads to disease.

Adrenal function in Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a multiple malformation syndrome due to mutations of the 7-dehydrocholesterol reductase gene (DHCR7), which leads to a deficiency of cholesterol synthesis and an accumulation of 7-dehydrocholesterol. The SLOS clinical spectrum ranges from multiple major malformations to a mild phenotype with minor anomalies and intellectual disability. Several children with SLOS and adrenal insufficiency have been described. We performed ovine corticotropin (oCRH) testing in 35 SLOS patients and 16 age- and gender-matched controls. We reviewed prior adrenocorticotropin (ACTH) stimulation tests of our SLOS patients (19 of 35 available) and reviewed results of ACTH stimulation tests from 10 additional SLOS patients. Results from oCRH testing showed that patients with SLOS had significantly higher ACTH baseline values than healthy controls (24.8 ± 15.3 pg/ml vs. 17.8 ± 7.5 pg/ml, P = 0.034). However, no statistically significant differences were noted for peak ACTH values (74.4 ± 35.0 pg/ml vs. 64.0 ± 24.9 pg/ml, P = 0.303) and for baseline (14.2 ± 7.8 mcg/dl vs. 14.2 ± 6.3 mcg/dl, P = 0.992) and peak cortisol values (28.2 ± 7.9 mcg/dl vs. 24.8 ± 8.1 mcg/dl, P = 0.156). The area-under-the-curve (AUC) was not significantly different in SLOS patients compared to controls for both ACTH (250.1 ± 118.7 pg/ml vs. 195.3 ± 96.6 pg/ml, P = 0.121) as well as cortisol secretion (83.1 ± 26.1 mcg/dl vs. 77.8 ± 25.9 mcg/dl, P = 0.499). ACTH stimulation test results were normal in 28 of 29 tests. The individual with the abnormal test results had subsequent normal oCRH tests. The slightly increased baseline ACTH level seen during oCRH testing may be due to compensated adrenocortical insufficiency. However, we were able to show that our patients with SLOS had an adequate glucocorticoid response, and thus, in mild to moderate cases of SLOS stress steroid coverage may not be warranted.

Alterations in membrane caveolae and BKCa channel activity in skin fibroblasts in Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is an inherited disorder of cholesterol synthesis caused by mutations in DHCR7 which encodes the final enzyme in the cholesterol synthesis pathway. The immediate precursor to cholesterol synthesis, 7-dehydrocholesterol (7-DHC) accumulates in the plasma and cells of SLOS patients which has led to the idea that the accumulation of abnormal sterols and/or reduction in cholesterol underlies the phenotypic abnormalities of SLOS. We tested the hypothesis that 7-DHC accumulates in membrane caveolae where it disturbs caveolar bilayer structure-function. Membrane caveolae from skin fibroblasts obtained from SLOS patients were isolated and found to accumulate 7-DHC. In caveolar-like model membranes containing 7-DHC, subtle, but complex alterations in intermolecular packing, lipid order and membrane width were observed. In addition, the BK(Ca) K(+) channel, which co-migrates with caveolin-1 in a membrane fraction enriched with cholesterol, was impaired in SLOS cells as reflected by reduced single channel conductance and a 50 mV rightward shift in the channel activation voltage. In addition, a marked decrease in BK(Ca) protein but not mRNA expression levels was seen suggesting post-translational alterations. Accompanying these changes was a reduction in caveolin-1 protein and mRNA levels, but membrane caveolar structure was not altered. These results are consistent with the hypothesis that 7-DHC accumulation in the caveolar membrane results in defective caveolar signaling. However, additional cellular alterations beyond mere changes associated with abnormal sterols in the membrane likely contribute to the pathogenesis of SLOS.

Review: Transport of maternal cholesterol to the fetal circulation

Data obtained from recent studies in humans, rodents, and cell culture demonstrate that circulating maternal cholesterol can be transported to the fetus. The two major cell types responsible for the transport are trophoblasts and endothelial cells of the fetoplacental vasculature. Maternal lipoprotein-cholesterol is initially taken up by trophoblasts via receptor-mediated and receptor-independent processes, is transported by any number of the sterol transport proteins expressed by cells, and is effluxed or secreted out of the basal side via protein-mediated processes or by aqueous diffusion. This cholesterol is then taken up by the endothelium and effluxed to acceptors within the fetal circulation. The ability to manipulate the mass of maternal cholesterol that is taken up by the placenta and crosses to the fetus could positively impact development of fetuses affected with the Smith-Lemli-Opitz Syndrome (SLOS) that have reduced ability to synthesize cholesterol and possibly impact growth of fetuses unaffected by SLOS but with low birthweights.

Malformation syndromes caused by disorders of cholesterol synthesis

Cholesterol homeostasis is critical for normal growth and development. In addition to being a major membrane lipid, cholesterol has multiple biological functions. These roles include being a precursor molecule for the synthesis of steroid hormones, neuroactive steroids, oxysterols, and bile acids. Cholesterol is also essential for the proper maturation and signaling of hedgehog proteins, and thus cholesterol is critical for embryonic development. After birth, most tissues can obtain cholesterol from either endogenous synthesis or exogenous dietary sources, but prior to birth, the human fetal tissues are dependent on endogenous synthesis. Due to the blood-brain barrier, brain tissue cannot utilize dietary or peripherally produced cholesterol. Generally, inborn errors of cholesterol synthesis lead to both a deficiency of cholesterol and increased levels of potentially bioactive or toxic precursor sterols. Over the past couple of decades, a number of human malformation syndromes have been shown to be due to inborn errors of cholesterol synthesis. Herein, we will review clinical and basic science aspects of Smith-Lemli-Opitz syndrome, desmosterolosis, lathosterolosis, HEM dysplasia, X-linked dominant chondrodysplasia punctata, Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects Syndrome, sterol-C-4 methyloxidase-like deficiency, and Antley-Bixler syndrome.

Quantitative proteomics analysis of inborn errors of cholesterol synthesis: identification of altered metabolic pathways in DHCR7 and SC5D deficiency

Smith-Lemli-Opitz syndrome (SLOS) and lathosterolosis are malformation syndromes with cognitive deficits caused by mutations of 7-dehydrocholesterol reductase (DHCR7) and lathosterol 5-desaturase (SC5D), respectively. DHCR7 encodes the last enzyme in the Kandutsch-Russel cholesterol biosynthetic pathway, and impaired DHCR7 activity leads to a deficiency of cholesterol and an accumulation of 7-dehydrocholesterol. SC5D catalyzes the synthesis of 7-dehydrocholesterol from lathosterol. Impaired SC5D activity leads to a similar deficiency of cholesterol but an accumulation of lathosterol. Although the genetic and biochemical causes underlying both syndromes are known, the pathophysiological processes leading to the developmental defects remain unclear. To study the pathophysiological mechanisms underlying SLOS and lathosterolosis neurological symptoms, we performed quantitative proteomics analysis of SLOS and lathosterolosis mouse brain tissue and identified multiple biological pathways affected in Dhcr7(Delta3-5/Delta3-5) and Sc5d(-/-) E18.5 embryos. These include alterations in mevalonate metabolism, apoptosis, glycolysis, oxidative stress, protein biosynthesis, intracellular trafficking, and cytoskeleton. Comparison of proteome alterations in both Dhcr7(Delta3-5/Delta3-5) and Sc5d(-/-) brain tissues helps elucidate whether perturbed protein expression was due to decreased cholesterol or a toxic effect of sterol precursors. Validation of the proteomics results confirmed increased expression of isoprenoid and cholesterol synthetic enzymes. This alteration of isoprenoid synthesis may underlie the altered posttranslational modification of Rab7, a small GTPase that is functionally dependent on prenylation with geranylgeranyl, that we identified and validated in this study. These data suggested that although cholesterol synthesis is impaired in both Dhcr7(Delta3-5/Delta3-5) and Sc5d(-/-) embryonic brain tissues the synthesis of nonsterol isoprenoids may be increased and thus contribute to SLOS and lathosterolosis pathology. This proteomics study has provided insight into the pathophysiological mechanisms of SLOS and lathosterolosis, and understanding these pathophysiological changes will help guide clinical therapy for SLOS and lathosterolosis.

Comparison of the liquid-ordered bilayer phases containing cholesterol or 7-dehydrocholesterol in modeling Smith-Lemli-Opitz syndrome

The phase behavior of egg sphingomyelin (ESM) mixtures with cholesterol or 7-dehydrocholesterol (7-DHC) has been investigated by independent methods: fluorescence microscopy, X-ray diffraction, and electron spin resonance spectroscopy. In giant vesicles, cholesterol-enriched domains appeared as large and clearly delineated domains assigned to a liquid-ordered (Lo) phase. The domains containing 7-DHC were smaller and had more diffuse boundaries. Separation of a gel phase assigned by X-ray examination to pure sphingomyelin domains coexisting with sterol-enriched domains was observed at temperatures less than 38 degrees C in binary mixtures containing 10-mol% sterol. At higher sterol concentrations, the coexistence of liquid-ordered and liquid-disordered phases was evidenced in the temperature range 20 degrees -50 degrees C. Calculated electron density profiles indicated the location of 7-DHC was more loosely defined than cholesterol, which is localized precisely at a particular depth along the bilayer normal. ESR spectra of spin-labeled fatty acid partitioned in the liquid-ordered component showed a similar, high degree of order for both sterols in the center of the bilayer, but it was higher in the coexisting disordered phase for 7-DHC. The differences detected in the models of the lipid membrane matrix are said to initiate the deleterious consequences of the Smith-Lemli-Opitz syndrome.

Activation of Rho GTPases in Smith-Lemli-Opitz syndrome: pathophysiological and clinical implications

Smith-Lemli-Opitz syndrome (SLOS) is a malformation syndrome with neurocognitive deficits due to mutations of DHCR7 that impair the reduction of 7-dehydrocholesterol to cholesterol. To investigate the pathological processes underlying the neurocognitive deficits, we compared protein expression in Dhcr7(+/+) and Dhcr7(Delta3-5/Delta3-5) brain tissue. One of the proteins identified was cofilin-1, an actin depolymerizing factor which regulates neuronal dendrite and axon formation. Differential expression of cofilin-1 was due to increased phosphorylation. Phosphorylation of cofilin-1 is regulated by Rho GTPases through Rho-Rock-Limk-Cofilin-1 and Rac/Cdc42-Pak-Limk-Cofilin-1 pathways. Pull-down assays were used to demonstrate increased activation of RhoA, Rac1 and Cdc42 in Dhcr7(Delta3-5/Delta3-5) brains. Consistent with increased activation of these Rho GTPases, we observed increased phosphorylation of both Limk and Pak in mutant brain tissue. Altered Rho/Rac signaling impairs normal dendritic and axonal formation, and mutations in genes encoding regulators and effectors of the Rho GTPases underlie other human mental retardation syndromes. Thus, we hypothesized that aberrant activation of Rho/Rac could have functional consequences for dendrite and axonal growth. In vitro analysis of Dhcr7(Delta3-5/Delta3-5) hippocampal neurons demonstrated both axonal and dendritic abnormalities. Developmental abnormalities of neuronal process formation may contribute to the neurocognitive deficits found in SLOS and may represent a potential target for therapeutic intervention.

A comparison of the packing behavior of egg phosphatidylcholine with cholesterol and biogenically related sterols in Langmuir monolayer films

Cholesterol and selected derivatives were studied as mixed Langmuir monolayers with egg phosphatidylcholine (PC). As an extension of our earlier work, which employed binary sterol/PC mixtures, here we examined ternary mixed monolayers containing cholesterol along with an alternate sterol and PC in different molar ratios, using pressure-area isotherms. The ternary systems behaved similarly to the binary sterol/PC systems reported previously, with similar condensation noted for the sterol/PC films. To better understand how variations in sterol structure affect sterol packing in such membrane monolayers, binary mixtures containing cholestenone, cholestanol, and lanosterol with PC were also studied. Cholestanol behaved similarly to cholesterol when incorporated with PC, while cholestenone and lanosterol did not cause as much film condensation. The observed differences in molecular packing, and attributed sterol structural differences, are considered within the context of sterol/phospholipid mixtures in biological membranes.