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.

1. “Cholesterol 24-Hydroxylase: An
Enzyme Turnover in the Brain"
SPEAKER
P.RAMESH
Ph.D (Animal Biochemistry)

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

7. Fig:1 Blood-Brain barrier prevents exchange with plasma Cholesterol

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?

24. Fig:

25. Fig: Synthesis of A)24-hydroxy-cholesterol &B) cholesterol

27. Fig: Interaction between Astrocytes (glial cells) and Neuronal cells in
Cholesterol Homeostatsis in the Brain

28. Fig: Flux of 24s-hydroxycholesterol from the Brain