Most cholesterol turnover takes place in the liver and involves the conversion of cholesterol into soluble and readily excreted bile acids. The synthesis of bile acids is limited to the liver, but several enzymes in the bile acid biosynthetic pathway are expressed in extra-hepatic tissues and there also may contribute to cholesterol turnover. An example of the latter type of enzyme is cholesterol 24-hydroxylase, a cytochrome P450 (CYP46A1) that is expressed at 100-fold higher levels in the brain than in the liver. Cholesterol 24-hydroxylase catalyzes the synthesis of the oxysterol 24(S)-hydroxycholesterol. To assess the relative contribution of the 24-hydroxylation pathway to cholesterol turnover, we performed balance studies in mice lacking the cholesterol 24-hydroxylase gene (Cyp46a1 ؊/؊ mice). Parameters of hepatic cholesterol and bile acid metabolism in the mutant mice remained unchanged relative to wild type controls. In contrast to the liver, the synthesis of new cholesterol was reduced by ϳ40% in the brain, despite steady-state levels of cholesterol being similar in the knockout mice. These data suggest that the synthesis of new cholesterol and the secretion of 24(S)-hydroxycholesterol are closely coupled and that at least 40% of cholesterol turnover in the brain is dependent on the action of cholesterol 24-hydroxylase. We conclude that cholesterol 24-hydroxylase constitutes a major tissue-specific pathway for cholesterol turnover in the brain.The major site for the turnover of cholesterol 1 in the body is the liver. This process involves the secretion of cholesterol and of bile acids derived from cholesterol. A pathway of 16 enzymes is responsible for the conversion of cholesterol to primary bile acids like cholic acid and chenodeoxycholic acid, which are then secreted into the bile and eventually excreted from the body (1). Cholesterol for bile acid synthesis is derived from three sources, including de novo synthesis in the liver, absorption from the diet, and delivery from peripheral tissues via lipoprotein-mediated transport. The transfer of cholesterol from the peripheral tissues to circulating lipoprotein particles occurs at the surfaces of cells and represents the major pathway by which extra-hepatic tissues turn over cholesterol (2); an exception is the brain, where the blood-brain barrier prevents transfer of cholesterol to circulating lipoproteins. The brain is thought to utilize an alternate mechanism of turnover in which cholesterol is converted into 24(S)-hydroxycholesterol, an oxysterol that may be able to diffuse across the blood-brain barrier (3-6). Once in the circulation, 24(S)-hydroxycholesterol is cleared by the liver and therein converted into 7␣-hydroxylated intermediates in the bile acid synthetic pathway by a dedicated enzyme (7-9). In humans, the mass of cholesterol converted into 24(S)-hydroxycholesterol by the brain is estimated to be 0.09 mg/day/kg of body weight (10). The role of 24-hydroxylation has not been defined in other species, but the total amount of sterol excreted fr...

Summary An in vivo screen was performed in search of chemicals capable of enhancing neuron formation in the hippocampus of adult mice. Eight of 1,000 small molecules tested enhanced neuron formation in the subgranular zone of the dentate gyrus. Among these was an aminopropyl carbazole, designated P7C3, endowed with favorable pharmacological properties. In vivo studies gave evidence that P7C3 exerts its pro-neurogenic activity by protecting newborn neurons from apoptosis. Mice missing the gene encoding neuronal PAS domain protein 3 (NPAS3) are devoid of hippocampal neurogenesis and display malformation and electrophysiological dysfunction of the dentate gyrus. Prolonged administration of P7C3 to npas3-/- mice corrected these deficits by normalizing levels of apoptosis of newborn hippocampal neurons. Prolonged administration of P7C3 to aged rats also enhanced neurogenesis in the dentate gyrus, impeded neuron death, and preserved cognitive capacity as a function of terminal aging.

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.

The mevalonate pathway produces cholesterol and nonsterol isoprenoids, such as geranylgeraniol. In the brain, a fraction of cholesterol is metabolized in neurons by the enzyme cholesterol 24-hydroxylase, and this depletion activates the mevalonate pathway. Brains from mice lacking 24-hydroxylase excrete cholesterol more slowly, and the tissue compensates by suppressing the mevalonate pathway. Here we report that this suppression causes a defect in learning. 24-Hydroxylase knockout mice exhibit severe deficiencies in spatial, associative, and motor learning, and in hippocampal long-term potentiation (LTP). Acute treatment of wild-type hippocampal slices with an inhibitor of the mevalonate pathway (a statin) also impairs LTP. The effects of statin treatment and genetic elimination of 24-hydroxylase on LTP are reversed by a 20-min treatment with geranylgeraniol but not by cholesterol. We conclude that cholesterol turnover in brain activates the mevalonate pathway and that a constant production of geranylgeraniol in a small subset of neurons is required for LTP and learning.cholesterol 24-hydroxylase ͉ isoprenoids ͉ neurons ͉ synaptic plasticity ͉ cytochrome P450

Alzheimer's disease (AD) is a currently incurable neurodegenerative disorder and the most common form of dementia in people over the age of 65. The predominant genetic risk factor for AD is the ε4 allele encoding apolipoprotein E (ApoE4). The secreted glycoprotein Reelin, which is a physiological ligand for the multifunctional ApoE receptors Apolipoprotein E receptor 2 (Apoer2) and very low-density lipoprotein receptor (Vldlr), enhances synaptic plasticity. We have previously shown that the presence of ApoE4 renders neurons unresponsive to Reelin by impairing the recycling of the receptors, thereby decreasing its protective effects against amyloid β (Aβ) oligomer-induced synaptic toxicity in vitro. Here, we show that when Reelin was knocked out in adult mice, these mice behaved normally without overt learning or memory deficits. However, they were strikingly sensitive to amyloid-induced synaptic suppression, and had profound memory and learning disabilities at very low amounts of amyloid deposition. Our findings highlight the physiological importance of Reelin in protecting the brain against Aβ-induced synaptic dysfunction and memory impairment.