The document discusses cholesterol 24-hydroxylase, an enzyme that converts cholesterol to oxysterols in the brain. It is exclusively expressed in neurons of the hippocampus and cortex. Knockout mice studies show this enzyme is responsible for most cholesterol turnover in the brain and disruption reduces learning and memory. Oxysterols produced may play a role in neurological diseases like Alzheimer's by regulating amyloid beta protein levels and lipid raft domains in neurons.
2. Introduction
Cholesterol 24-Hydroxylase is a member of Cytochrome-
p450 family (~500 a.a)
Synonyms: Cytochrome P450 46A1, CP46, CYP46
Genes: Cyp46a1 (Mouse), CYP46A1 (Humans)
Catalysis: Conversion of Cholesterol into Oxysterols
Membranes of neurons in CNS (Brain & Spinal Cord)
Exclusively present in the Brain & small amounts in testis
and liver of the mouse
3. Cont….
Responsible for majority of cholesterol turnover in the
Vetebrate Central Nervous System
Expressed in neurons of hippocampal (learning), cortical
neurons (memory)
Disruption of the gene in mouse reduces both cholesterol
turnover and synthesis in the brain
No alteration in steady state levels of cholesterol in other
tissues
4. Brain is a metabolically active tissue
Cont….
Occupies 2% of body mass in humans and tissue
consumes 20% of resting oxygen
Burns an estimated 4x1021 molecules of ATP/min of
the day
Mainly fueled by Glucose metabolism and on prolonged
starvation, it depends on ketonebodies
Composed of lipids, including 20% of the body’s total
cholesterol content
5. Cholesterol Levels in Tissues
Mouse Brain having cholesterol ~15mg/g tissue
Adrenal gland ~19mg/g
Lung ~6mg/g
Cholesterol in the Brain exists in two pools
One pool is largely containing ~70% (white
matter), conc. ~40 mg/g tissue
Second pool is small having ~30% (gray matter),
conc. ~8mg/g tissue
6. Cont….
Origin of Brain Cholesterol
Two sources: Exogenous (plasma lipoproteins)
Endogenous (from Acetate)
High vascularized tissues, (adrenal gland and liver) have
access to circulating lipoproteins (exogenous pathway)
Brain exclusively depends on endogenous pathway
Cholesterol is transported within the CNS in the form of
apo-E from Glial cells
8. Cholesterol metabolism in the Brain
Brain cholesterol located in myelin membranes of
oligodendrocytes & small amount in PM of neurons
Cholesterol synthesis and accumulation as a function of Age
dependent
Synthesis are increased during active myelination after birth
Cholesterol synthesis in the adult brain is larger than the
accumulation rate
9. Cont….
Apo-E & membrane transporter of the ABC family are
expressed in CNS
Apo-E containing lipoproteins from glial cells are bind
to neuronal surface receptor & then uptake into it
Cholesterol 24-hydroxylase catalyzes conversion of
cholesterol into 24s-hydroxycholesterol
Interplay between glial & neuronal cells
Regulates cholesterol homeostasis in brain
10. Cholesterol Turnover:
Cont….
It also catalyzes 24,25 and 24,27-dihydroxycholesterols
in brain & cerebrospinal fluid of mice & other mammals
Oxysterols is initially formed within the brain and then gain
access to the circulation
11. Cont….
In plasma, it associates with lipoproteins, which are cleared
by the liver
Mevalonate pathway (MP) synthesis cholesterol & nonsterol
isoprenoids (Geranylgeraniol) responsible for learning
Lacking 24-hydroxylase excrete cholesterol more slowly &
tissue compensates by suppressing Mevalonate pathway
Causes a severe deficiencies in spatial, associative, motor
learning & hippocampal long-term potentiation (LTP)
12. Metabolism of 24s-hydroxycholesterol:
Cont….
Having short half life & rapidly converted into Bile acids
Infants have high levels in the circulation
Cholesterol 7α & 7β-hydroxycholesterols hydryoxlates 24s-
hydroxycholesterol and 24,25 and 24,27 hydroxy-cholesterol in the liver
In mouse CYP39 have a high 7αhydroxylase activity
24s-hydroxycholesterol is a less efficient precursor for bile acids than 7α-
hydroxycholesterol
Half of it is conjugated and eliminated in bile as such or as a conjugate of a
27-OH cholesterol
13. Cont….
Functions: Atherosclerosis, Apoptosis, Necrosis,
Inflammation, Immune suppression, Development of Gall-
stones
Efficient suppressor of HMG-CoA reductase
14. Cont….
24s-hydroxycholesterol as a marker:
Marker for neurological and neurodegenerative diseases
Brain-dead patients had a reduction in the conc. 50%
In Multiple sclerosis appear to have increased levels in the
circulation
Alzheimer’s disease (AD) had significantly reduced plasma levels
of the oxysterols
Patients with AD were found to have increased levels 24s-OH
cholesterol in cerebrospinalfluid and parallel with decreased
levels in circulation
15. Cont….
Cholesterol 27-hydroxylase (CYP27):
It is present in all most all cells in the body
Catalysis conversion of cholesterol into 27-OH cholesterol
27-OH cholesterol is further converted into 7α-hydroxy-3-
oxo-4-cholestenoic acid by Cholesterol7β1-hydroxylase
(CYP7β1) in glial cells
AD patients had increased levels of 27-OH cholesterol in
brain
Alternatively for the high levels may be a reduced
metabolism
16. Alzheimer’s Disease (AD):
It is neurological disease, characterized by death of neurons
& even greater loss of synapses in the brain
Neurons death seems to be deposition of β-amyloid proteins
Neurons with reduced arborization have less surface area &
corresponds less plasma membrane
Cholesterol catabolized through 2 main routes
Can be esterified & stored within neurons as a cholesterol
esters
Oxidised at 24 or 27 to form 24-OHC & 27-OHC
17. Possible role of Oxysterols in AD:
Cont….
CYP46A1 are expressed in neurons & some astrocytes in
the normal brain
CYP27A1 present in oligodendrocytes & absent in neurons
In AD CYP46A1 shows prominent expression in astrocytes
& around amyloid plaques
Invitro studies shows that 24s-OHC & 27-OHC inhibits β-
amyloid proteins (Aβ)
Oxysterols also involved in signaling transduction pathway
18. Cont….
Oxysterols have dual function:
Both are cholesterol catabolites
Ligand of the nuclear transcription factor LXR
24-OHC levels high in plasma of patients of AD
Due to high levels its affect APP metabolism
Processing of APP by β & r-secretases produce Aβ, takes
place in high cholesterol levels of lipid raft domains
(Amyloid Protien Precursor)
19. Cont….
Cholesterol esters increase Aβ & inhibiting the LCAT
reduces Aβ production
Synthetic Oxysterol 22-OHC inhibits Aβ production in
some neuroblastoma cells
Oxysterols inhibits APP processing in neurons through a
mechanism mediated by LXR
Activation of LXR system is known to increase ABC
(ATPase binding cassette) levels
20. Summary & Conclusion
Cholesterol 24-hydroxylase is a highly conserved P450 that is
expressed in some, but not all, neurons of the brain
Cholesterol
24-hydroxylase converts cholesterol, into 24S-
hydroxy-cholesterol, that diffuses from the brain and is
subsequently metabolized by the liver
Disruption of the mouse cholesterol 24-hydroxylase gene causes
an ∼50% decrease in cholesterol turnover, which is compensated
for by an equal decrease in the rate of de novo cholesterol
synthesis
21. Cont….
Cholesterol 24-hydroxylase knockout mice are deficient in
spatial, associative, and motor learning, and have abnormal
hippocampal LTP
The defect in LTP can be reversed by geranylgeraniol, a poly-
isoprenoid end product of the cholesterol biosynthetic
pathway
22. Future Issues
What role does cholesterol turnover play in the human brain?
Are there naturally occurring mutations in the human cholesterol
24-hydroxylase gene, and if so, what are their clinical consequences?
What other pathways exist for cholesterol catabolism in the central
nervous system?
Are these pathways present in the peripheral nervous system?
What transport processes bring cholesterol to the enzyme in the
endoplasmic reticulum membrane?