DISTRIBUTION OF STEROLS IN ORGANISMS AND IN TISSUES

Functional expression of opioid receptors and other human GPCRs in yeast engineered to produce human sterols

The yeast Saccharomyces cerevisiae is powerful for studying human G protein-coupled receptors as they can be coupled to its mating pathway. However, some receptors, including the mu opioid receptor, are non-functional, which may be due to the presence of the fungal sterol ergosterol instead of cholesterol. Here we engineer yeast to produce cholesterol and introduce diverse mu, delta, and kappa opioid receptors to create sensitive opioid biosensors that recapitulate agonist binding profiles and antagonist inhibition. Additionally, human mu opioid receptor variants, including those with clinical relevance, largely display expected phenotypes. By testing mu opioid receptor-based biosensors with systematically adjusted cholesterol biosynthetic intermediates, we relate sterol profiles to biosensor sensitivity. Finally, we apply sterol-modified backgrounds to other human receptors revealing sterol influence in SSTR5, 5-HTR4, FPR1, and NPY1R signaling. This work provides a platform for generating human G protein-coupled receptor-based biosensors, facilitating receptor deorphanization and high-throughput screening of receptors and effectors.

Molecular markers of brain cholesterol homeostasis are unchanged despite a smaller brain mass in a mouse model of cholesteryl ester storage disease

Lysosomal acid lipase (LAL), encoded by the gene LIPA, facilitates the intracellular processing of lipids by hydrolyzing cholesteryl esters and triacylglycerols present in newly internalized lipoproteins. Loss-of-function mutations in LIPA result in cholesteryl ester storage disease (CESD) or Wolman disease when mutations cause complete loss of LAL activity. Although the phenotype of a mouse CESD model has been extensively characterized, there has not been a focus on the brain at different stages of disease progression. In the current studies, whole-brain mass and the concentrations of cholesterol in both the esterified (EC) and unesterified (UC) fractions were measured in Lal-/- and matching Lal+/+ mice (FVB-N strain) at ages ranging from 14 up to 280 days after birth. Compared to Lal+/+ controls at 50, 68-76, 140-142, and 230-280 days of age, Lal-/- mice had brain weights that averaged approximately 6%, 7%, 18%, and 20% less, respectively. Brain EC levels were higher in the Lal-/- mice at every age, being elevated 27-fold at 230-280 days. Brain UC concentrations did not show a genotypic difference at any age. The elevated brain EC levels in the Lal-/- mice did not reflect EC in residual blood. An mRNA expression analysis for an array of genes involved in the synthesis, catabolism, storage, and transport of cholesterol in the brains of 141-day old mice did not detect any genotypic differences although the relative mRNA levels for several markers of inflammation were moderately elevated in the Lal-/- mice. The possible sites of EC accretion in the central nervous system are discussed.

Identification of Correlative Shifts in Indices of Brain Cholesterol Metabolism in the C57BL6/Mecp2tm1.1Bird Mouse, a Model for Rett Syndrome

Rett syndrome (RS) is a pervasive neurodevelopmental disorder resulting from loss-of-function mutations in the X-linked gene methyl-Cpg-binding protein 2 (MECP2). Using a well-defined model for RS, the C57BL6/Mecp2^tm1.1Bird mouse, we have previously found a moderate but persistently lower rate of cholesterol synthesis, measured in vivo, in the brains of Mecp2^-/y mice, starting from about the third week after birth. There was no genotypic difference in the total cholesterol concentration throughout the brain at any age. This raised the question of whether the lower rate of cholesterol synthesis in the mutants was balanced by a fall in the rate at which cholesterol was converted via cholesterol 24-hydroxylase (Cyp46A1) to 24S-hydroxycholesterol (24S-OHC), the principal route through which cholesterol is ordinarily removed from the brain. Here, we show that while there were no genotypic differences in the concentrations in plasma and liver of three cholesterol precursors (lanosterol, lathosterol, and desmosterol), two plant sterols (sitosterol and campesterol), and two oxysterols (27-hydroxycholesterol [27-OHC] and 24S-OHC), the brains of the Mecp2 ^-/y mice had significantly lower concentrations of all three cholesterol precursors, campesterol, and both oxysterols, with the level of 24S-OHC being ~20% less than in their Mecp2 ^+/y controls. Together, these data suggest that coordinated regulation of cholesterol synthesis and catabolism in the central nervous system is maintained in this model for RS. Furthermore, we speculate that the adaptive changes in these two pathways conceivably resulted from a shift in the permeability of the blood-brain barrier as implied by the significantly lower campesterol and 27-OHC concentrations in the brains of the Mecp2^-/y mice.

Measurement of Rates of Cholesterol and Fatty Acid Synthesis In Vivo Using Tritiated Water

Every organ in the body is capable of synthesizing cholesterol de novo but at rates that vary with a constellation of factors. A significant proportion of the hydrogen atoms present in cholesterol that is synthesized in the body are derived from water. Thus, although water ordinarily makes up the bulk of body mass, the acute enrichment of the body water pool with a sufficiently large amount of tritiated water over a short interval of time (usually 1 h) yields measurable rates of incorporation of the labeled water into newly generated cholesterol and also fatty acids. Such data can provide a quantitative measure of how specific genetic, dietary, and pharmacological manipulations impact not just the rate of cholesterol synthesis in particular organs but also rates of whole-body cholesterol production and turnover.

Preferential incorporation of docosahexaenoic acid into nonphosphorus lipids and phosphatidylethanolamine protects rats from dietary DHA-stimulated lipid peroxidation

In a previous study, we found that dietary docosahexaenoic acid (DHA)-stimulated tissue lipid peroxide formation was suppressed to a lesser extent than expected from the peroxidizability index of tissue total lipids. This suppression was presumed to be potentiated by mechanisms other than the lipid peroxide-scavenging system. In this study, we focused primarily on the incorporation of DHA into tissue nonphosphorus lipids and phospholipid species. DHA and different levels of dietary vitamin E (VE; 7.5, 54, 134 and 402 mg/kg of diet) were fed to rats for 32 d. In rats with poor VE status, liver chemiluminescence intensity and kidney and testis thiobarbituric acid (TBA) values correlated with the tissue's peroxidizability index. In rats with normal VE nutriture, liver lipid peroxide formation was suppressed to a level below that expected from the peroxidizability index, likely because DHA was present in nonphosphorus lipids and utilized preferentially for phosphatidylethanolamine synthesis. In the kidney, differences in the TBA values were associated with differences in the peroxidizability index of total lipids, even in the DHA groups fed VE at higher than normal levels. This may be because the levels of lipid peroxide scavengers were lower than those of liver and because DHA was utilized preferentially for phosphatidylcholine synthesis. In testis, the lipid peroxide levels were not as high as expected from the peroxidizability index, even in rats fed a high DHA diet containing the normal level of VE. This may be because the testis was composed of a high proportion of (n-6) polyunsaturated fatty acids (PUFA), which are low in unsaturation, and thus the proportion of DHA was low. In addition, in testis, VE and ascorbic acid, which act as antioxidants, were retained at higher levels in rats with particularly poor and normal VE nutriture than those of liver and kidney. These results suggest that antioxidant protection against dietary DHA-stimulated lipid peroxidation below the extent expected from the peroxidizability index of tissue total lipids differed from tissue to tissue. The suppression was likely due to not only the lipid peroxide scavenging system but also preferential incorporation of DHA into nonphosphorus lipids and phosphatidylethanolamine, particularly in liver.