Targeted lipidomic analysis of oxysterols in the embryonic central nervous system

Abstract

In this study two regions of embryonic (E11) mouse central nervous system (CNS) have been profiled for their unesterified sterol content. Using high-performance liquid chromatography (HPLC)–mass spectrometry (MS) and tandem mass spectrometry (MSⁿ) low levels of oxysterols (estimated 2–165 ng g⁻¹ wet weight) were identified in cortex (Ctx) and spinal cord (Sc). The identified oxysterols include 7α-, 7β-, 22R-, 24S-, 25- and 27-hydroxycholesterol; 24,25- and 24,27-dihydroxycholesterol; and 24S,25-epoxycholesterol. Of these, 24S-hydroxycholesterol is biosynthesised exclusively in brain. In comparison to adult mouse where the 24S-hydroxycholesterol level is about 40 μg g⁻¹ in brain the level of 24S-hydroxycholesterol reported here (estimated 26 ng g⁻¹ in Ctx and 13 ng g⁻¹ in Sc) is extremely low. Interestingly, the level of 24S,25-epoxycholesterol in both CNS regions (estimated 165 ng g⁻¹ in Ctx and 91 ng g⁻¹ in Sc) is somewhat higher than the levels of the hydroxycholesterols. This oxysterol is formed in parallel to cholesterolvia a shunt of the mevalonate pathway and its comparatively high abundance may be a reflection of a high rate of cholesterol synthesis at this stage of development. Levels of cholesterol (estimated 1.25 mg g⁻¹ in Ctx and 1.15 mg g⁻¹ in Sc) and its precursors were determined by gas chromatography–mass spectrometry (GC-MS). In both CNS regions cholesterol levels were found to be lower than those reported in the adult, but in relation to cholesterol the levels of cholesterol precursors were higher than found in adult indicating a high rate of cholesterol synthesis. In summary, our data provide evidence for the presence of endogenous oxysterols in two brain regions of the developing CNS. Moreover, while most of the enzymes involved in hydroxysterol synthesis are minimally active at E11, our results suggest that the mevalonate pathway is significantly active, opening up the possibility for a function of 24S,25-epoxycholesterol during brain development.