Differential effects of cholesterol and 7-dehydrocholesterol on ligand binding of solubilized hippocampal serotonin1A receptors: implications in SLOS

Cholesterol Dependent Activity of the Adenosine A2A Receptor Is Modulated via the Cholesterol Consensus Motif

BACKGROUND

Membrane cholesterol dysregulation has been shown to alter the activity of the adenosine A_2A receptor (A_2AR), a G protein-coupled receptor, thereby implicating cholesterol levels in diseases such as Alzheimer's and Parkinson's. A limited number of A_2AR crystal structures show the receptor interacting with cholesterol, as such molecular simulations are often used to predict cholesterol interaction sites.

METHODS

Here, we use experimental methods to determine whether a specific interaction between amino acid side chains in the cholesterol consensus motif (CCM) of full length, wild-type human A_2AR, and cholesterol modulates activity of the receptor by testing the effects of mutational changes on functional consequences, including ligand binding, G protein coupling, and downstream activation of cyclic AMP.

RESULTS AND CONCLUSIONS

Our data, taken with previously published studies, support a model of receptor state-dependent binding between cholesterol and the CCM, whereby cholesterol facilitates both G protein coupling and downstream signaling of A_2AR.

Late endosomal/lysosomal accumulation of a neurotransmitter receptor in a cellular model of Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a congenital and developmental malformation syndrome associated with defective cholesterol biosynthesis. It is characterized by accumulation of 7-dehydrocholesterol (the immediate biosynthetic precursor of cholesterol in the Kandutsch-Russell pathway) and an altered cholesterol to total sterol ratio. Because SLOS is associated with neurological malfunction, exploring the function and trafficking of neuronal receptors and their interaction with membrane lipids under these conditions assume significance. In this work, we generated a cellular model of SLOS in HEK-293 cells stably expressing the human serotonin_1A receptor (an important neurotransmitter G-protein coupled receptor) using AY 9944, an inhibitor for the enzyme 3β-hydroxy-steroid-∆⁷ -reductase (7-DHCR). Using a quantitative flow cytometry based assay, we show that the plasma membrane population of serotonin_1A receptors was considerably reduced under these conditions without any change in total cellular expression of the receptor. Interestingly, the receptors were trafficked to sterol-enriched LysoTracker positive compartments, which accumulated under these conditions. To the best of our knowledge, our results constitute one of the first reports demonstrating intracellular accumulation and misregulated traffic of a neurotransmitter GPCR in SLOS-like conditions. We believe these results assume relevance in our overall understanding of the molecular basis underlying the functional relevance of neurotransmitter receptors in SLOS.

Endogenous ω-3 fatty acids in Fat-1 mice attenuated depression-like behaviors, spatial memory impairment and relevant changes induced by olfactory bulbectomy

OBJECTIVES

Olfactory bulbectomy (OB) induced behaviors, hypercortisolism, inflammation and neurotrophin dysfunctions are similar to those observed in depressed patients. Omega (n)-3 polyunsaturated fatty acids (PUFAs) can effectively treat depression via anti-inflammatory and neuroprotective effects. However, n-3 PUFA purities, caloric contents, and ratios in different diets often cause contradictive results. This study used Fat-1 mice, which can convert n-6 to n-3 PUFAs in the brain, to study the effect of n-3 PUFAs on OB-induced behaviors and related changes.

METHODS

Fat-1 and wild-type littermates were fed safflower oil for 3 months. Behaviors were tested on day 21 after surgery. Monoamine neurotransmitters were measured by HPLC. Macrophage activity was measured by MTT assay. Astrocyte phenotypes A1 S100β, A2 BDNF and cholesterol level were measured by ELISA and total cholesterol assay kits respectively. PUFA profile and membrane fluidity were detected by GC and DPH fluorescence probe respectively.

RESULTS

OB significantly induced animal hyperactivity and spatial memory impairment, while decreased sucrose consumption and social contact with decreased 5-HT turnover, increased the macrophage activity and S100β/BDNF ratio. Meanwhile, n-3/n-6 PUFAs ratio and total cholesterol level were reduced in OB mice. Whereas, OB-induced behavioral changes were attenuated, which were associated with increasing 5-HT turnover, decrease macrophage activity, restored S100β/BDNF and n-3/n-6 PUFAs ratios, and total cholesterol concentrations in Fat-1 mice.

CONCLUSION

The present study for the first time demonstrated that endogenous n-3 PUFAs attenuated OB-induced depression-like behaviors and spatial memory impairment through modulating serotonergic and immune function, balancing the astrocyte A1/A2 phenotypes, and normalizing PUFAs profile and membrane function.

An ω-3, but Not an ω-6 Polyunsaturated Fatty Acid Decreases Membrane Dipole Potential and Stimulates Endo-Lysosomal Escape of Penetratin

Although the largely positive intramembrane dipole potential (DP) may substantially influence the function of transmembrane proteins, its investigation is deeply hampered by the lack of measurement techniques suitable for high-throughput examination of living cells. Here, we describe a novel emission ratiometric flow cytometry method based on F66, a 3-hydroxiflavon derivative, and demonstrate that 6-ketocholestanol, cholesterol and 7-dehydrocholesterol, saturated stearic acid (SA) and ω-6 γ-linolenic acid (GLA) increase, while ω-3 α-linolenic acid (ALA) decreases the DP. These changes do not correlate with alterations in cell viability or membrane fluidity. Pretreatment with ALA counteracts, while SA or GLA enhances cholesterol-induced DP elevations. Furthermore, ALA (but not SA or GLA) increases endo-lysosomal escape of penetratin, a cell-penetrating peptide. In summary, we have developed a novel method to measure DP in large quantities of individual living cells and propose ALA as a physiological DP lowering agent facilitating cytoplasmic entry of penetratin.

Depressed prostaglandins and leukotrienes in veterans with Gulf War illness

Background: There is need to understand biological markers and mechanisms in Gulf War illness (GWI). Goal: To examine whether and how eicosanoids - prostaglandins and leukotrienes - are altered in veterans with GWI. Methods: Seventy participants including 37 GWI and 33 healthy controls, shared exposure information, and had plasma eicosanoids assessed - prostaglandin F2 alpha (pgf2α), prostaglandin D2 (pgd2), leukotriene B4 (lb4) among others. Values were compared for GWI versus controls. Eicosanoid intercorrelations were compared in cases vs. controls. For the most significantly altered eicosanoid in GWI, exposure and symptom relations were assessed. Results: Prostaglandins and leukotrienes were depressed in GWI, strongest for pgf2α, then lb4. Eicosanoid intercorrelations differed in GWI vs. controls. Fuel-solvent, pesticide, radioactive chemicals and metal exposures related negatively to pgf2α; as, in GWI, did chemical attack and vaccines. Multivariate predictors included fuels-solvents and radioactive chemicals (negative); tetanus vaccine and herbicides (positive). Fuels-solvents and radioactive chemicals predicted lower pgf2α in cases, controls, and all participants controlled for case status. Lower pgf2α related to GWI "Kansas criteria" domains of pain, respiratory, and (borderline significantly) skin symptoms. Conclusion: Multiple eicosanoids are depressed in GWI, particularly pgf2α and lb4. Prior fuel-solvent exposures, radioactive chemicals, and (in GWI cases) vaccines were linked to lower pgf2α.

A Critical Analysis of Molecular Mechanisms Underlying Membrane Cholesterol Sensitivity of GPCRs

G protein-coupled receptors (GPCRs) are the largest and a diverse family of proteins involved in signal transduction across biological membranes. GPCRs mediate a wide range of physiological processes and have emerged as major targets for the development of novel drug candidates in all clinical areas. Since GPCRs are integral membrane proteins, regulation of their organization, dynamics, and function by membrane lipids, in particular membrane cholesterol, has emerged as an exciting area of research. Cholesterol sensitivity of GPCRs could be due to direct interaction of cholesterol with the receptor (specific effect). Alternately, GPCR function could be influenced by the effect of cholesterol on membrane physical properties (general effect). In this review, we critically analyze the specific and general mechanisms of the modulation of GPCR function by membrane cholesterol, taking examples from representative GPCRs. While evidence for both the proposed mechanisms exists, there appears to be no clear-cut distinction between these two mechanisms, and a combination of these mechanisms cannot be ruled out in many cases. We conclude that classifying the mechanism underlying cholesterol sensitivity of GPCR function merely into these two mutually exclusive classes could be somewhat arbitrary. A more holistic approach could be suitable for analyzing GPCR-cholesterol interaction.

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.

Structural Stringency of Cholesterol for Membrane Protein Function Utilizing Stereoisomers as Novel Tools: A Review

Cholesterol is an important lipid in the context of membrane protein function. The function of a number of membrane proteins, including G protein-coupled receptors (GPCRs) and ion channels, has been shown to be dependent on membrane cholesterol. However, the molecular mechanism underlying such regulation is still being explored. In some cases, specific interaction between cholesterol and the protein has been implicated. In other cases, the effect of cholesterol on the membrane properties has been attributed for the regulation of protein function. In this article, we have provided an overview of experimental approaches that are useful for determining the degree of structural stringency of cholesterol for membrane protein function. In the process, we have highlighted the role of immediate precursors in cholesterol biosynthetic pathway in the function of membrane proteins. Special emphasis has been given to the application of stereoisomers of cholesterol in deciphering the structural stringency required for regulation of membrane protein function. A comprehensive examination of these processes would help in understanding the molecular basis of cholesterol regulation of membrane proteins in subtle details.

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.

Reduced cholesterol levels impair Smoothened activation in Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a common autosomal-recessive disorder that results from mutations in the gene encoding the cholesterol biosynthetic enzyme 7-dehydrocholesterol reductase (DHCR7). Impaired DHCR7 function is associated with a spectrum of congenital malformations, intellectual impairment, epileptiform activity and autism spectrum disorder. Biochemically, there is a deficit in cholesterol and an accumulation of its metabolic precursor 7-dehydrocholesterol (7DHC) in developing tissues. Morphological abnormalities in SLOS resemble those seen in congenital Sonic Hedgehog (SHH)-deficient conditions, leading to the proposal that the pathogenesis of SLOS is mediated by aberrant SHH signalling. SHH signalling is transduced through the transmembrane protein Smoothened (SMO), which localizes to the primary cilium of a cell on activation and is both positively and negatively regulated by sterol molecules derived from cholesterol biosynthesis. One proposed mechanism of SLOS involves SMO dysregulation by altered sterol levels, but the salient sterol species has not been identified. Here, we clarify the relationship between disrupted cholesterol metabolism and reduced SHH signalling in SLOS by modelling the disorder in vitro. Our results indicate that a deficit in cholesterol, as opposed to an accumulation of 7DHC, impairs SMO activation and its localization to the primary cilium.

Two classes of cholesterol binding sites for the β2AR revealed by thermostability and NMR

Cholesterol binding to G protein-coupled receptors (GPCRs) and modulation of their activities in membranes is a fundamental issue for understanding their function. Despite the identification of cholesterol binding sites in high-resolution x-ray structures of the ?2 adrenergic receptor (β2AR) and other GPCRs, the binding affinity of cholesterol for this receptor and exchange rates between the free and bound cholesterol remain unknown. In this study we report the existence of two classes of cholesterol binding sites in β2AR. By analyzing the β2AR unfolding temperature in lipidic cubic phase (LCP) as a function of cholesterol concentration we observed high-affinity cooperative binding of cholesterol with sub-nM affinity constant. In contrast, saturation transfer difference (STD) NMR experiments revealed the existence of a second class of cholesterol binding sites, in fast exchange on the STD NMR timescale. Titration of the STD signal as a function of cholesterol concentration provided a lower limit of 100 mM for their dissociation constant. However, these binding sites are specific for both cholesterol and β2AR, as shown with control experiments using ergosterol and a control membrane protein (KpOmpA). We postulate that this specificity is mediated by the high-affinity bound cholesterol molecules and propose the formation of transient cholesterol clusters around the high-affinity binding sites.

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.

GPCRs: Lipid-Dependent Membrane Receptors That Act as Drug Targets

G protein-coupled receptors (GPCRs) are the largest class of molecules involved in signal transduction across cell membranes and represent major targets in the development of novel drug candidates in all clinical areas. Although there have been some recent leads, structural information on GPCRs is relatively rare due to the difficulty associated with crystallization. A specific reason for this is the intrinsic flexibility displayed by GPCRs, which is necessary for their functional diversity. Since GPCRs are integral membrane proteins, interaction of membrane lipids with them constitutes an important area of research in GPCR biology. In particular, membrane cholesterol has been reported to have a modulatory role in the function of a number of GPCRs. The role of membrane cholesterol in GPCR function is discussed with specific example of the serotonin1A receptor. Recent results show that GPCRs are characterized with structural motifs that preferentially associate with cholesterol. An emerging and important concept is oligomerization of GPCRs and its role in GPCR function and signaling. The role of membrane cholesterol in GPCR oligomerization is highlighted. Future research in GPCR biology would offer novel insight in basic biology and provide new avenues for drug discovery.

Stereospecific requirement of cholesterol in the function of the serotonin1A receptor

The serotonin1A receptor is an important member of the G protein-coupled receptor (GPCR) family. It is involved in the generation and modulation of a variety of cognitive and behavioral functions and serves as a drug target. Previous work from our laboratory has established the sensitivity of the function of the serotonin1A receptor to membrane cholesterol. Solubilization of the hippocampal serotonin1A receptor utilizing the zwitterionic detergent CHAPS is accompanied by loss of cholesterol and results in reduction in specific ligand binding. Replenishment of cholesterol to solubilized membranes restores specific ligand binding to the receptor. We utilized this strategy of sterol replenishment of solubilized membranes to explore the stereospecific stringency of cholesterol for receptor function. We used two stereoisomers of cholesterol, ent-cholesterol (enantiomer of cholesterol) and epi-cholesterol (a diastereomer of cholesterol), for this purpose. Importantly, we show here that while ent-cholesterol could replace cholesterol in supporting receptor function, epi-cholesterol could not. These results imply that the requirement of membrane cholesterol for the serotonin1A receptor function is diastereospecific, yet not enantiospecific. Our results extend and help define specificity of the interaction of membrane cholesterol with the serotonin1A receptor, and represent the first report utilizing ent-cholesterol to examine stereospecificity of GPCR-cholesterol interaction.

Yeast and cancer cells - common principles in lipid metabolism

One of the paradigms in cancer pathogenesis is the requirement of a cell to undergo transformation from respiration to aerobic glycolysis - the Warburg effect - to become malignant. The demands of a rapidly proliferating cell for carbon metabolites for the synthesis of biomass, energy and redox equivalents, are fundamentally different from the requirements of a differentiated, quiescent cell, but it remains open whether this metabolic switch is a cause or a consequence of malignant transformation. One of the major requirements is the synthesis of lipids for membrane formation to allow for cell proliferation, cell cycle progression and cytokinesis. Enzymes involved in lipid metabolism were indeed found to play a major role in cancer cell proliferation, and most of these enzymes are conserved in the yeast, Saccharomyces cerevisiae. Most notably, cancer cell physiology and metabolic fluxes are very similar to those in the fermenting and rapidly proliferating yeast. Both types of cells display highly active pathways for the synthesis of fatty acids and their incorporation into complex lipids, and imbalances in synthesis or turnover of lipids affect growth and viability of both yeast and cancer cells. Thus, understanding lipid metabolism in S. cerevisiae during cell cycle progression and cell proliferation may complement recent efforts to understand the importance and fundamental regulatory mechanisms of these pathways in cancer.

7-Dehydrocholesterol-derived oxysterols and retinal degeneration in a rat model of Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a recessive disease characterized by markedly elevated levels of 7-dehydrocholesterol (7-DHC) and reduced levels of cholesterol in tissues and fluids of affected individuals, due to defective 3β-hydroxysterol-Δ(7)-reductase (Dhcr7). Treatment of Sprague Dawley rats with AY9944 (an inhibitor of Dhcr7) leads to similar biochemical features as observed in SLOS. Eighteen oxysterols previously have been identified as oxidation products of 7-DHC (most of them distinct from cholesterol (Chol)-derived oxysterols) in solution, in cells, and in brains obtained from Dhcr7-KO mice and AY9944-treated rats, formed either via free radical oxidation (peroxidation) or P450-catalyzed enzymatic oxidation. We report here the identification of five 7-DHC-derived oxysterols, including 3β,5α-dihydroxycholest-7-en-6-one (DHCEO), 4α- and 4β-hydroxy-7-DHC, 24-hydroxy-7-DHC and 7-ketocholesterol (7-kChol, an oxysterol that is normally derived from Chol), in the retinas of AY9944-treated rats by comparing the retention times and mass spectrometric characteristics with corresponding synthetic standards in HPLC-MS analysis. Levels of 4α- and 4β-hydroxy-7-DHC, DHCEO, and 7-kChol were quantified using d(7)-DHCEO as an internal standard. Among the five oxysterols identified, only 7-kChol was observed in retinas of control rats, but the levels of 7-kChol in retinas of AY9944-rats were 30-fold higher. Intravitreal injection of 7-kChol (0.25μmol) into a normal rat eye induced panretinal degeneration within one week; by comparison, contralateral (control) eyes injected with vehicle alone exhibited normal histology. These findings are discussed in the context of the potential involvement of 7-DHC-derived oxysterols in the retinal degeneration associated with the SLOS rat model and in SLOS patients.

Mass spectrometry in India

This review emphasizes the mass spectrometry research being performed at academic and established research institutions in India. It consists of three main parts covering the work done in organic, atomic and biological mass spectrometry. The review reveals that the use of mass spectrometry techniques started in the middle of the 20th century and was applied to research in the fields of organic, nuclear, geographical and atomic chemistry. Later, with the advent of soft and atmospheric ionization techniques it has been applied to pharmaceutical and biological research. In due course, several research centers with advanced mass spectrometry facilities have been established for specific areas of research such as gas-phase ion chemistry, ion-molecule reactions, proscribed chemicals, pesticide residues, pharmacokinetics, protein/peptide chemistry, nuclear chemistry, geochronological studies, archeology, petroleum industry, proteomics, lipidomics and metabolomics. Day-by-day the mass spectrometry centers/facilities in India have attracted young students for their doctoral research and other advanced research applications.

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.

GPCR production in a novel yeast strain that makes cholesterol-like sterols

The activities of many mammalian membrane proteins including G-protein coupled receptors are cholesterol-dependent. Unlike higher eukaryotes, yeast do not make cholesterol. Rather they make a related molecule called ergosterol. As cholesterol and ergosterol are biologically non-equivalent, the potential of yeast as hosts for overproducing mammalian membrane proteins has never been fully realised. To address this problem, we are trying to engineer a novel strain of Saccharomyces cerevisiae in which the cholesterol biosynthetic pathway of mammalian cells has been fully reconstituted. Thus far, we have created a modified strain that makes cholesterol-like sterols which has an increased capacity to make G-protein coupled receptors compared to control yeast.

Desmosterol replaces cholesterol for ligand binding function of the serotonin(1A) receptor in solubilized hippocampal membranes: support for nonannular binding sites for cholesterol?

The serotonin(1A) receptor is an important member of the G-protein coupled receptor family, and is involved in the generation and modulation of a variety of cognitive and behavioral functions. Solubilization of the hippocampal serotonin(1A) receptor by CHAPS is accompanied by loss of cholesterol that results in a reduction in specific agonist binding activity. Replenishment of cholesterol to solubilized membranes restores membrane cholesterol content and significantly recovers specific agonist binding. In order to test the stringency of cholesterol requirement, we solubilized native hippocampal membranes followed by replenishment with desmosterol. Desmosterol is the immediate biosynthetic precursor of cholesterol in the Bloch pathway differing only in a double bond at the 24th position. Our results show that replenishment with desmosterol restores ligand binding of serotonin(1A) receptors. This is consistent with earlier results showing that desmosterol can replace cholesterol in a large number of cases. However, these results appear to be contradictory to our earlier findings, performed by sterol manipulation utilizing methyl-β-cyclodextrin, in which we observed that replacing cholesterol with desmosterol is unable to restore specific ligand binding of the hippocampal serotonin(1A) receptor. We discuss the possible molecular mechanism, in terms of nonannular lipid binding sites around the receptor, giving rise to these differences.

Modeling cholesterol metabolism by gene expression profiling in the hippocampus

An important part of the challenge of building models of biochemical reactions is determining reaction rate constants that transform substrates into products. We present a method to derive enzymatic kinetic values from mRNA expression levels for modeling biological networks without requiring further tuning. The core metabolic reactions of cholesterol in the brain, particularly in the hippocampus, were simulated. To build the model the baseline mRNA expression levels of genes involved in cholesterol metabolism were obtained from the Allen Mouse Brain Atlas. The model is capable of replicating the trends of relative cholesterol levels in Alzheimer's and Huntington's diseases; and reliably simulated SLOS, desmosterolosis, and Dhcr14/Lbr knockout studies. A sensitivity analysis correctly uncovers the Hmgcr, Idi2 and Fdft1 sites that regulate cholesterol homeostasis. Overall, our model and methodology can be used to pinpoint key reactions, which, upon manipulation, may predict altered cholesterol levels and reveal insights into potential drug therapy targets under diseased conditions.

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.

Lipid trajectories as predictors of depressive symptoms: the Young Finns Study

OBJECTIVE

The aim of this study was to identify common trajectories of lipid levels across childhood and early adulthood life span.

DESIGN

The sample was a subpopulation of 824 young adults (3 to 9 years of age at baseline in 1980) of the ongoing population-based prospective Cardiovascular Risk in Young Finns Study. Lipid levels were determined in 1980, 1983, 1986, and 2001.

MAIN OUTCOME MEASURES

Depressive symptoms were assessed using a modified version of Beck's Depression Inventory (Beck, 1967) in 1992 and 2001.

RESULTS

The 2 triglycerides trajectories (steeply vs. moderately increasing) were differently related to depressive symptoms in adulthood. The trajectory showing steep increase over time was associated with higher level of depressive symptoms (M 2.18, 95% CI [2.08, 2.28] vs. 1.99, [1.95, 2.04]). This relationship persisted after adjustments for various risk factors. These triglycerides trajectories accounted for part of the association between high body mass index and depressive symptoms.

CONCLUSION

A pattern of steeply increasing triglyceride levels throughout childhood and adulthood may be associated with increased the risk of depressive symptoms in adulthood. This pattern may also be one link between obesity and depressive symptoms.

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.

Membrane cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of higher eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The G-protein coupled receptors (GPCRs) are the largest class of molecules involved in signal transduction across membranes, and represent major targets in the development of novel drug candidates in all clinical areas. Membrane cholesterol has been reported to have a modulatory role in the function of a number of GPCRs. Two possible mechanisms have been previously suggested by which membrane cholesterol could influence the structure and function of GPCRs (i) through a direct/specific interaction with GPCRs, or (ii) through an indirect way by altering membrane physical properties in which the receptor is embedded, or due to a combination of both. Recently reported crystal structures of GPCRs have shown structural evidence of cholesterol binding sites. Against this backdrop, we recently proposed a novel mechanism by which membrane cholesterol could affect structure and function of GPCRs. According to our hypothesis, cholesterol binding sites in GPCRs could represent 'nonannular' binding sites. Interestingly, previous work from our laboratory has demonstrated that membrane cholesterol is required for the function of the serotonin(1A) receptor (a representative GPCR), which could be due to specific interaction of the receptor with cholesterol. Based on these results, we envisage that there could be specific/nonannular cholesterol binding site(s) in the serotonin(1A) receptor. We have analyzed putative cholesterol binding sites from protein databases in the serotonin(1A) receptor. Our analysis shows that cholesterol binding sites are inherent characteristic features of serotonin(1A) receptors and are conserved through natural evolution. Progress in deciphering molecular details of the GPCR-cholesterol interaction in the membrane would lead to better insight into our overall understanding of GPCR function in health and disease, thereby enhancing our ability to design better therapeutic strategies to combat diseases related to malfunctioning of GPCRs.

Differential effects of cholesterol and desmosterol on the ligand binding function of the hippocampal serotonin(1A) receptor: implications in desmosterolosis

Cholesterol is a unique molecule in terms of high level of in-built stringency, fine tuned by natural evolution for its ability to optimize physical properties of higher eukaryotic cell membranes in relation to biological functions. We previously demonstrated the requirement of membrane cholesterol in maintaining the ligand binding activity of the hippocampal serotonin(1A) receptor. In order to test the molecular stringency of the requirement of cholesterol, we depleted cholesterol from native hippocampal membranes followed by replenishment with desmosterol. Desmosterol is an immediate biosynthetic precursor of cholesterol in the Bloch pathway differing only in a double bond at the 24th position in the alkyl side chain. Our results show that replenishment with desmosterol does not restore ligand binding activity of the serotonin(1A) receptor although replenishment with cholesterol led to significant recovery of ligand binding. This is in spite of similar membrane organization (order) in these membranes, as monitored by fluorescence anisotropy measurements. The requirement for restoration of ligand binding activity therefore appears to be more stringent than the requirement for the recovery of overall membrane order. These novel results have potential implications in understanding the interaction of membrane lipids with this important neuronal receptor in diseases such as desmosterolosis.