B. Inventario de Recursos Turísticos
5.3. Discusión
The brain is the most cholesterol-rich organ in the body. It contains about 25%
of all free cholesterol present in the body, while it represents only 2% of the total body weight. Essentially all cholesterol in the brain is unesterifed.
Surprisingly relatively little is still known about the maintenance of cholesterol homeostasis and about its specific roles within the central nervous system (Snipes and Suter U 1997 ).
Almost all of the cholesterol present in the brain is synthesized locally. In spite of the high cholesterol content, cholesterol turnover in the brain was found to be much slower than that in the rest of the body (20- to 80-fold lower than in the liver; Spady and Dietschy 1983) and its concentration is kept remarkably stable.
This may explain why, until about 15 years ago, little attention had been paid to cholesterol metabolism in the brain. This changed when it turned out that cerebral cholesterol homeostasis is much more dynamic than initially thought. It is gener-ally assumed that, under non-diseased conditions, cholesterol from plasma lipo-proteins does not enter the brain, since it cannot cross the endothelial cells of the BBB. The cerebral capillary endothelium is the anatomical substrate of the BBB, isolating the brain neuropil from the systemic circulation. The cerebral endothe-lium lining the blood vessel lumen consists of a single layer of cells joined together by tight intercellular junctions. This layer of cells is supported by a base-ment membrane, which is the laminar structure formed by the fusion of the endothelial and glial vascular basement membrane. The end feet of astrocytes make up a discontinuous sheath at the abluminal surface of the basement mem-brane (Fig. 5.1 ). This has been confirmed by studies in mice, sheep and rabbits with use of either labeled lipoproteins or labeled cholesterol (for a review, see Dietschy and Turley 2001) . For example, apoE-deficient mice display dramati-cally increased plasma cholesterol levels, but no alterations in brain cholesterol levels (Lomnitski et al. 1999) . However, a recent study with guinea-pigs sug-gested that minor amounts (~1%) of cholesterol might cross the BBB (Lutjohann et al. 2004) .
Cholesterol, or better its steroid ring structure, cannot be degraded in the human body and high concentrations of free cholesterol can lead to the formation of crystals which are toxic to cells, in particular neurons (Lemaire-Ewing et al. 2005 ; Travert et al. 2006) . Therefore, excess cholesterol is secreted from the brain into the circulation (Brown and Goldstein 1997) and finally released from the body via the liver where it is converted into bile acids (Lutjohann et al. 1996) . Within the brain, cholesterol can be modified into its major brain metabolite 24( S )-hydroxycholesterol by the enzyme CYP46A1. Based on experiments with mice deficient for the enzyme CYP46A1, it was calculated that about 64% is being secreted in the form of this polar cholesterol metabolite (Fig. 5.2 ; Bjorkhem et al. 1997 ; Xie et al. 2003) . The remaining 36% of cholesterol is secreted from the brain via another, yet unknown pathway that may involve apoE (Lund et al. 2003) .
24( S )-Hydroxycholesterol is, in contrast with cholesterol, able to traverse the BBB (Bjorkhem et al. 1997, 1998 ; Lutjohann et al. 1996) . At first it looks quite controversially that a compound more polar than cholesterol is able to pass a lipophilic barrier. As with other plasma membranes, the membranes of the endo-thelial cells are freely permeable to water (Panzenboeck et al. 2002) . Introduction of an hydroxyl group in the side-chain of the cholesterol molecule leads to a local reordering of membrane phospholipids such that it is energetically more favourable to expel the oxysterol (Kessel et al. 2001) . When a sterol like cholesterol is hydro-xylated, there is an increase in the maximal aequous activity that can be achieved, but a reduction in the passive permeability coefficient. However, the increase in solubility is proportionately greater than the reduction in the permeability coeffi-cient, so that the net effect of hydroxylation is to greatly increase the maximal rate of passive diffusion of the molecule across the BBB.
Astrocyte endfeet
Tight junction
Basement membrane Mitochondria
Fig. 5.1 Schematic representation of the blood–brain barrier. The blood–brain barrier consists of endothelial cells lining the vessel wall, connected via tight junctions. These are surrounded by a thick basement membrane which is covered by astrocytic endfeet
The enzyme CYP46A1 has been characterized at the molecular level. Its gene contains 15 exons and is located on human chromosome 14q32.1. CYP46A1 is pre-dominantly found in the brain, mainly located in a subset of specific neurons.
Deficiency for CYP46A1 in mice results in suppression of cholesterol synthesis in brain by about 25% (Lund et al. 2003 ; Xie et al. 2003) , probably to compensate for the decreased efflux of cholesterol from the brain. Evidence has been obtained indi-cating a role for CYP46A1 in neurosteroid metabolism (Mast et al. 2003) . Besides converting cholesterol into 24( S )-hydroxycholesterol, CYP46A1 can also inactivate neurosteroids that may either be synthesized endogenously or derived from the circulation.
Cholesterol in the brain resides in three major compartments with different turnover rates. The largest pool (70–80%, or 260 mg kg –1 of the total 330 mg kg –1 ) has the slowest turnover (>1%) and is present in myelin membranes. Of the remaining 70 mg kg– 1, about 10% reside in neurons, which represent about 10% of the brain cells, and therefore contain about 7 mg kg --1 of cholesterol.
The remaining 63 mg kg --1 are present in glial cells (Davison 1965 ; Muse et al 2001 ; Xie et al. 2003) .
60%
40%
apoE?
LIVER CIRCULATION CHOLESTEROL
CHOLESTEROL SYNTHESIS
NEURON
24(S)-HYDROXYCHOLESTEROL
CYP46
Fig. 5.2 Schematic representation of the overall cholesterol turnover in the brain. Cholesterol is synthesized endogenously predominantly by astrocytes and is converted by the enzyme CYP46A1 that resides in a subset of neurons, whereafter it is released from the brain into the circulation. This pathway is thought to be responsible for about 60% of the secretion of cholesterol from the brain, while the remaining 40% is secreted via a yet unknown pathway that may involve apoE
As mentioned, the CYP46A1 enzyme is expressed in a subset of metabolically active neurons such as pyramidal cells of the cortex and Purkinje cells of the cerebellum (Lund et al. 1999) . The cholesterol turnover in these neurons must be very high, since they are estimated to contain about 4 mg kg–1 of cholesterol while their turnover of 24-hydroxycholesterol is about 0.9 mg kg–1 day –1 , which is more than 20% day–1 . The overall turnover of cholesterol in the body is about 0.8% day–1 , i.e.
a value similar to the CYP46A1-independent cholesterol turnover in brain, which is about 0.5 mg kg–-1 day –1 of a pool of about 63 mg kg–1 day–1 .