Cholesterol 24-hydroxylase: an enzyme of cholesterol turnover in the brain

Abstract

Cholesterol 24-hydroxylase is a highly conserved cytochrome P450 that is responsible for the majority of cholesterol turnover in the vertebrate central nervous system. The enzyme is expressed in neurons, including hippocampal and cortical neurons that are important for learning and memory formation. Disruption of the cholesterol 24-hydroxylase gene in the mouse reduces both cholesterol turnover and synthesis in the brain but does not alter steady-state levels of cholesterol in the tissue. The decline in synthesis reduces the flow of metabolites through the cholesterol biosynthetic pathway, of which one, geranylgeraniol diphosphate, is required for learning in the whole animal and for synaptic plasticity in vitro. This review focuses on how the link between cholesterol metabolism and higher-order brain function was experimentally established.

Figure 1

Cholesterol metabolism in the brain. (a) An abbreviated cholesterol biosynthetic pathway is shown in which key intermediates and enzymes are highlighted. Statins reduce the flow of intermediates through the biosynthetic pathway by inhibiting 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase. Farnesyl diphosphate (farnesyl-PP) and geranylgeranyl diphosphate are polyisoprenoid end products of the pathway. Knockout of the cholesterol 24-hydroxylase gene causes cholesterol accumulation and activation of a negative feedback pathway that suppresses the reductase enzyme as well as others in the pathway and subsequently decreases cholesterol synthesis. Redrawn from Reference 79. (b) Cholesterol synthesis and accumulation in mouse brain as a function of age of the animal.

Figure 2

Cholesterol turnover in the brain. Cholesterol is converted into 24S-hydroxycholesterol by cholesterol 24-hydroxylase and spontaneously diffuses into the circulation. The oxysterol associates with high-density and low-density lipoproteins, which are cleared by the liver. The 24S-hydroxycholesterol is metabolized into bile acids and excreted into the bile.

Figure 3

Cholesterol 24-hydroxylase. (a) The reaction catalyzed by cholesterol 24-hydroxylase. (b) The structure of the human enzyme as determined by X-ray crystallography. Twelve α-helices and four β-pleated sheets are shown in various colors together with the heme cofactor and a cholesterol sulfate substrate. Redrawn and reprinted with permission from Reference 53. (c) Surface representation of the human enzyme. Twenty amino acid sequence differences with the mouse enzyme, which are concentrated on one side of the protein, are shown as space-filling representations. Regions of the protein at the N and C termini that are more highly conserved are indicated by ribbon diagrams. Data extracted from Protein Data Bank entry 2q9f.

Figure 4

Expression of cholesterol 24-hydroxylase in the brain. (a) Wild-type mouse cortical section stained with monoclonal antibody 1A7 showing expression of the enzyme (green stain) in pyramidal neurons of layers II/III, V, and VI. (b) Section from cortex of cholesterol 24-hydroxylase knockout mouse stained with monoclonal antibody 1A7. (c) Three-dimensional reconstruction of cholesterol 24-hydroxylase staining and cytochrome P450 reductase staining showing colocalization of the two proteins in the dendritic endoplasmic reticulum of a cultured primary cortical neuron. Reprinted with permission from Reference 62.

Figure 5

Cholesterol synthesis and turnover in the wild-type and cholesterol 24-hydroxylase knockout mouse brains.

Figure 6

Morris water maze test. (a) Animals of the indicated cholesterol 24-hydroxylase genotypes were subjected to Morris water maze tests to assess spatial learning. Average swim time to find a submerged platform is plotted versus trial day. (b) The submerged platform was removed on the indicated day of the experiment, and the number of times animals swum across the former location of the platform were counted. Data redrawn and published with permission from Reference 80. Abbreviations: +/+, wild type for cholesterol 24-hydroxylase; +/−, heterozygous for cholesterol 24-hydroxylase mutation; −/−, homozygous for cholesterol 24-hydroxylase mutation.

Figure 7

Hippocampal long-term potentiation (LTP) in wild-type and cholesterol 24-hydroxylase knockout mice. (a) Baseline responses to monophasic stimulation were recorded for 20 min in slices from 5-week-old animals of the indicated genotype, and then the tissue was subjected to high-frequency stimulation, θ-burst (arrow). Subsequent responses to monophasic stimulation were strengthened in wild-type slices and less so in knockout slices. Insets show recorded potentials before (dashed lines) and 55 min after (solid lines) tetanization. (b) Effect of inhibition of cholesterol synthesis with a statin on hippocampal LTP. Redrawn and reprinted with permission from Reference 79. Abbreviations: fEPSP slope, slope of extracellularly recorded (field) excitatory postsynaptic potentials; +/+, wild type for cholesterol 24-hydroxylase; −/−, homozygous for cholesterol 24-hydroxylase mutation.

Figure 8

Restoration of hippocampal long-term potentiation by geranylgeraniol treatment of slices from cholesterol 24-hydroxylase knockout mice (a) and those from wild-type mice treated with a statin (b). Data in (a) redrawn and reprinted with permission from Reference 79. Abbreviations: fEPSP slope, slope of extracellularly recorded (field) excitatory postsynaptic potentials; +/+, wild type for cholesterol 24-hydroxylase; −/−, homozygous for cholesterol 24-hydroxylase mutation; arrow, time of θ-burst stimulation; NS, not significant.

Figure 9

Biphasic requirement for geranylgeraniol in hippocampal long-term potentiation (LTP). (a) A 5-min geranylgeraniol treatment (purple bar) of slices from cholesterol 24-hydroxylase knockout mice (−/−) induces but does not maintain LTP. (b) Two 5-min geranylgeraniol treatments induce and maintain LTP. Redrawn and reprinted with permission from Reference 80. Abbreviations: fEPSP slope, slope of extracellularly recorded (field) excitatory postsynaptic potentials; +/+, wild type for cholesterol 24-hydroxylase; −/−, homozygous for cholesterol 24-hydroxylase mutation; arrow, time of θ-burst stimulation.