Sterols and oxysterols in brain and the immune system.

This project investigates sterol and oxysterol content of murine brain and macrophages. Oxysterols are oxidised forms of cholesterol implicated in a wide array of biological functions. The compounds were analysed with LC-MS LTQ-Orbitrap high resolution system, which provides a highly sensitive and accurate tool for analysis of metabolites. We profiled a sterol content of newborn murine brain and identified a broad spectrum of oxysterols. Some of these compounds are implicated in neurogenesis, a number of other oxysterols derived from desmosterol were identified in brain tissue for the first time. We analysed murine model of human disease arising from reduced ability to produce cholesterol (SLOS). The sterol profile showed an altered level of cholesterol precursors and generally lowered concentration of oxysterols. Next, we moved to analysis of brain lipidome from animals with disrupted mechanisms of cholesterol metabolism. Cholesterol hydroxylase Cyp46al provides the mechanism of cholesterol removal from the brain. The study of the knock out mice did not reveal the existence of any compensatory mechanisms. The results showed mainly the reduction in cholesterol synthesis. We profiled a sterol content of Cyp27al-/- mouse brain. Cyp27al participates in bile acid synthesis. The steroid profile revealed changed pattern of mono- and polihydroxylated sterols suggesting an upregulation of an alternative pathway of bile acid synthesis. We also analysed the consequence of deficiency of another enzyme involved in bile acids synthesis Cyp7bl. In brain of Cyp7bl-/- mouse we found elevated levels of known Cyp7bl substrates, and increased concentration of other, putative substrates for this enzyme. The last experimental chapter concentrates on analysis of murine macrophages treated with interferon beta and gamma. The treatment induced an increased production of 25-hydroxycholesterol. This links sterol metabolism with mechanisms of immune defence

Mining for Oxysterols in Cyp7b1−/− Mouse Brain and Plasma: Relevance to Spastic Paraplegia Type 5

Deficiency in cytochrome P450 (CYP) 7B1, also known as oxysterol 7α-hydroxylase, in humans leads to hereditary spastic paraplegia type 5 (SPG5) and in some cases in infants to liver disease. SPG5 is medically characterized by loss of motor neurons in the corticospinal tract. In an effort to gain a better understanding of the fundamental biochemistry of this disorder, we have extended our previous profiling of the oxysterol content of brain and plasma of Cyp7b1 knockout (-/-) mice to include, amongst other sterols, 25-hydroxylated cholesterol metabolites. Although brain cholesterol levels do not differ between wild-type (wt) and knockout mice, we find, using a charge-tagging methodology in combination with liquid chromatography–mass spectrometry (LC–MS) and multistage fragmentation (MSn), that there is a build-up of the CYP7B1 substrate 25-hydroxycholesterol (25-HC) in Cyp7b1-/- mouse brain and plasma. As reported earlier, levels of (25R)26-hydroxycholesterol (26-HC), 3β-hydroxycholest-5-en-(25R)26-oic acid and 24S,25-epoxycholesterol (24S,25-EC) are similarly elevated in brain and plasma. Side-chain oxysterols including 25-HC, 26-HC and 24S,25-EC are known to bind to INSIG (insulin-induced gene) and inhibit the processing of SREBP-2 (sterol regulatory element-binding protein-2) to its active form as a master regulator of cholesterol biosynthesis. We suggest the concentration of cholesterol in brain of the Cyp7b1-/- mouse is maintained by balancing reduced metabolism, as a consequence of a loss in CYP7B1, with reduced biosynthesis. The Cyp7b1-/- mouse does not show a motor defect; whether the defect in humans is a consequence of less efficient homeostasis of cholesterol in brain has yet to be uncovered

Synthesis and biological activity of (24E)- and (24Z)-26-hydroxydesmosterol

Using 3b-hydroxychol-5-en-24-oic acid (4) as starting material, the diastereoisomeric allylic alcohols (24E)-26-hydroxydesmosterol (2) and (24Z)-26-hydroxydesmosterol (3) have been synthesised in six steps with 67% and 12% overall yield, respectively. Both of these isomers are found in newborn mouse brain where sterol synthesis is high. Unlike desmosterol (1), neither of these isomers is a ligand to the liver x receptors and thus represents a novel biological deactivation mechanism avoiding cholesterol synthesis

24S,25-Epoxycholesterol in mouse and rat brain

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

Oxysterols in the brain of the cholesterol 24-hydroxylase knockout mouse

Oxysterols are oxidised forms of cholesterol or its precursors. In this study we utilised the cholesterol 24-hydroxylase knockout mouse (Cyp46a1-/-) to study the sterol and oxysterol content of brain. Despite a great reduction in the abundance of 24S-hydroxycholesterol, the dominant metabolite of cholesterol in wild type brain, no other cholesterol metabolite was found to quantitatively replace this oxysterol in the Cyp46a1-/- mouse. Only minor amounts of other side-chain oxysterols including 22R-, 24R-, 25- and (25R),26-hydroxycholesterols were detected. In line with earlier studies, levels of cholesterol were similar in Cyp46a1-/- and wild type animals. However, the level of the cholesterol precursor, desomsterol, and its parallel metabolite formed via a shut of the mevalonate pathway, 24S,25-epoxycholesterol, were reduced in the Cyp46a1-/- mouse. The reduction in abundance of 24S,25-epoxycholesterol is interesting in light of a recent report indicating that this oxysterol promotes dopaminergic neurogenesi

Analysis by Liquid Chromatography - Mass Spectrometry of Sterols and Oxysterols in Brain of the Newborn Dhcr7Δ3-5/T93M Mouse: A Model of Smith-Lemli-Opitz Syndrome

In this study the sterol and oxysterol profile of newborn brain from the Dhcr7Δ3-5/T93M mouse model of Smith-Lemli-Opitz syndrome (SLOS) has been investigated. This is a viable mouse model which is compound heterozygous containing one null allele and one T93M mutation on Dhcr7. We find the SLOS mouse has reduced levels of cholesterol and desmosterol and increased levels of 7- and 8-dehydrocholesterol and of 7- and 8-dehydrodesmosterol in brain compared to the wild type. The profile of enzymatically formed oxysterols in the SLOS mouse resembles that in the wild type but the level of 24S-hydroxycholesterol, the dominating cholesterol metabolite, is reduced in a similar proportion to that of cholesterol. A number of oxysterols abundant in the SLOS mouse are probably derived from 7-dehydrocholesterol, however, the mechanism of their formation is unclear