Natalia V. Koudinova1, Anatol Kontush2,3, Temirbolat T. Berezov1, Alexei R. Koudinov1§ CA
1 Russian Academy Med Sci, Moscow, Russian Federation
2 INSERM Unit 551, Hospital Pitie, Paris, France
3 Inst Med Biochem, U Hospital Eppendorf, Hamburg, Germany
To date great number of articles were devoted to cholesterol (chol) but only few articles studied the role for chol in neuron function/degeneration. For decades this molecule had been known to be important for atherosclerosis and heart disease. First indication of the involvement of chol in Alzheimer disease (AD), however, come from the mid 1990s. At that time it was shown that heart disease patients develop brain deposits of amyloid beta (Ab), a major dogmatic molecule of AD; that apoE (a chol transport apolipoprotein) allele e4 is a major genetic risk factor for AD; and that lab animals fed a chol diet express brain amyloid.1 These days it turns out that Ab, long thought to be exclusively a pathologic protein, is a normal and functional apolipoprotein constituent of high density lipoproteins in plasma and CSF. Thus, we and others showed that Ab modulates chol and phospholipid synthesis, and affects chol esterification.1 Protection of lipoproteins and other biomolecules from oxidation may represent another important function of Ab.2 We also discovered that neuronal chol homeostasis failure and the lack of chol supply to neurons by means of lipoprotein transport causes AD features, such as the failure of the neurotransmission and synaptic plasticity, degeneration of neuronal cell processes, and tau protein pathology.3, 4
Neurobiol. Lipids Vol.1, 6 (2003).
Since 1966 more than one hundred thousand research papers were devoted to cholesterol and only few articles dealt with the role of cholesterol in neuronal function, synaptic plasticity and neurodegeneration (according to the PubMed and HighWire Press search engines). This mysterious molecule is accused in atherosclerosis and heart disease, but was largely understudied in relation to brain function and neural structural and functional (i.e. activity dependent) plasticity. Living cells (including neurons and glial cells) produce their own cholesterol and can receive or donate cholesterol via lipoprotein transit, an attested body lipid transportation system. This system operates a number of vehicle classes, including well-known (and considered to be “bad”) LDLs and “good“ HDLs [1, 2]
Apart from this whole-body-system stands distinct brain lipid transport authority that uses different subtypes of HDLs and normally does not maintain LDLs . This vital service is in charge of cholesterol redistribution inside the brain and cholesterol export out of the brain border to liver for excretion. There is no reported cholesterol import to the brain, an issue that makes brain cholesterol availability entirely dependent on local manufacturing. Brain lipid transportation must have good management and operating capacity, because the quantity of cholesterol in the brain is much higher then anywhere else in the rest of the body. Thus, having just two percent of the body weight, brain has a quarter of cholesterol present in the whole individual.
Conceivable, the break in any element of the harmonized system of brain/neuronal cholesterol transport (caused by genetic defects of one of the enzyme or receptor associated with cholesterol turnover; by pharmacological modulation or environmentally) may result in abnormal homeostasis of cholesterol in the brain and impair fine tuning of synaptic function (see online Refs. 2, 3, 4 for instant access to detailed bibliography).
Cholesterol (and phospholipids) is a building block of any cell membrane (the nervous system wrapping material, where neuronal information in the form of electric activity is generated and propagated) and specialized membrane structures, lipid rafts and synaptic vesicles. In the brain the information (coded as nerve impulse) is transmitted from presynaptic neuron-transmitter to a postsynaptic neuron-receiver at the tiny gap between cells called synapse. The first neuron output nerve ending (called axon) releases synaptic vesicles containing chemicals called neurotransmitters. The neurotransmitter molecules then bind to receptors of the neuron-receiver input processes, located in postsynaptic membrane functional domains, called lipid rafts. After the minute interaction with a receptor on the neuron-receiver, neurotransmitter molecules normally return back to the nerve ending (from where they were released) for recycling in order to be ready for the next act of neurotransmission. This transient neurotransmitter-receptor interaction represents the quantum of neurotransmission and synaptic function. It launches a number of chemical changes inside the postsynaptic neuron-receiver, essential for the nerve signal processing, synaptic amplification or modulation (also called synaptic plasticity), and for the formation of memory .
CHOLESTEROL AND ALZHEIMER’S DISEASE BACKGROUND
The turn of the century was marked by several reports that demonstrated Alzheimer’s diseases features in neuronal cells, brain slices and laboratory animals that model cholesterol pathology (see Refs. 2, 4, 5 for details). Our related research during past decade included the study of the interaction of lipid (particularly cholesterol) metabolism and amyloid b (Ab) protein (insoluble brain deposits of Ab are widely believed to be Alzheimer's characteristic feature and the cause of the disease, see our another presentation at the 32nd Society for Neuroscience Annual Meeting 2002) as a "missing link in Alzheimer's puzzle" (for brief review also see Ref. 1). This relation became especially important recently because more then dozen studies showed that cholesterol modulation affects generation of Ab and the processing of its precursor [4, 5, 6].
In the past, however, just few reports implicated cholesterol in basic synaptic function, particularly in trafficking and recycling of synaptic vesicles, in receptor function, activity of accessory synaptic proteins, and in modulation of membrane biophysical properties (see bibliography in Refs. 2, 4).
In a separate set of experiments by modifying rat cholesterol status with the diet containing 2 % cholesterol for several months we generated animal model, that expressed brain amyloid similar to amyloid deposition of Alzheimer's disease brain specimens [4, 6, fr3, fr4, fr5]. We then prepared slices from the hippocampus, maintained them live in vitro, and subjected to the study of cholesterol synthesis by labeling with radioactive acetate. After labeling the lipids were extracted from slices, separated by thin layer chromatography, and quantitated by radioactivity counting. This methodology allowed us to quantitate the higher rate of cholesterol and phospholipids synthesis in rats fed a cholesterol diet, and to conclude that brain cholesterol synthesis upregulation is a possible cause (but NOT a consequence) of Alzheimer's-like brain amyloid. Most important, we also performed electrophysiological analysis of slices. We found that cholesterol-fed rats lack hippocampal LTP and thus have impaired synaptic plasticity [4, 6] It is notable that impaired LTP could be reversed by the reversal of a 2% cholesterol diet to a regular diet for an extended period of time. It is in accord with the notion that the rate of cholesterol turnover in the brain is very low, and thus requires long time for a disturbance (or a correction in case of the opposite order of events) to yield appreciable change. Our functional analysis data significantly extended several earlier histochemical reports that demonstrated amyloid buildup in rabbits and in amyloid precursor protein transgenic mice fed a cholesterol diet (see Refs. 4, 5, 6 for details).
CHOLESTEROL, AMYLOID b AND ALZHEIMER’S CSF-HDL
Our other study  showed that Alzheimer’s patients have unique pattern of HDL distribution in the CSF, the lipoprotein fraction that is especially important in cholesterol transport in tissue environment and across the body, and that has soluble form of Ab as an apolipoprotein constituent . Particularly we observed an increase in content of soluble Ab and selected apolipoproteins in the HDL subfraction called HDL1. Remembering a special role for HDL1 in reverse cholesterol transport on one hand, and the role for Ab in cholesterol esterification (that causes HDL size change and the formation of HDL1, Ref.3) we interpreted our data as an additional piece in Alzheimer’s cholesterol puzzle [3, 4].
Scheme 1 SCHEMATIC REPRESENTATION OF THE CASCADE OF THE COMMON SPORADIC FORMS OF ALZHEIMER'S DISEASE
(see Ref.4 for the extended bibliography for the scheme)
Neuronal cholesterol dynamics misregulation causes the key Alzheimer's disease (AD) feature of learning and memory failure as a result of the impairment of neuronal function, neurotransmission and synaptic plasticity through the mechanisms precise molecular nature which remains to be identified.
Cholesterol-mediated change in neurochemistry of amyloid beta, tau phosphorylation, neuronal cytoskeleton rearrangements and the modulation of physiological equilibrium of oxidative stress reactions could provide physiological transitory mechanisms aiming to compensate impaired brain cholesterol dynamics and neurotransmission and synaptic plasticity.
The break in neuronal cholesterol homeostasis may require very long (i.e. chronic) onset time frame due to the physiologically slow turnover of the central nervous system (CNS) cholesterol. Such condition may be genetically set (right top) and be assisted environmentally by the long term dietary habits. While during the past 30 years the concept of healthy food has become synonymous with avoiding dietary cholesterol, the question of how this avoidance and its compensation affects brain cholesterol chemistry, learning and memory remained non-addressed for many years. Several basic reports, however, documented that brain cholesterol is a delicate substance very sensitive to many influences, ranging from lipid preparation diets and chemical delivery systems for drugs and food additives (cyclodextrins, for example) to learning process itself. It is thus possible that antifat lifestyle “soft science” doctrine contributed to the increase of dementia and Alzheimer's prevalence in industrialised countries during 1970s and 1980s.
The indicated physiological compensatory changes may slowly invert when neuronal cholesterol dynamics is recovering slow to the initial physiological level. Such reversibility was proved experimentally (see Ref. 4) and certified by nature as an important mechanism of the CNS plasticity, as exampled by high expression of PHF-phosphorylated tau during an ontogenic period of cholesterol-demanding intense neuritic outgrowth. General compensatory nature of amyloid and tau neurochemistry modulation was proposed previously and is illustrated by its change observed under related to cholesterol (but different from AD) cardiovascular and Niemann-Pick type C pathologies, as well as in normal cases and during aging.
When neuronal cholesterol dynamics is not recovering compensatory mechanisms fail yielding (yet possible reversible) the development of conventional Alzheimer's disease hallmarks (right). These hallmarks, however, are not causative for the sporadic AD, and thus unlikely represent the proper target for the efficient AD therapy, as supported by the cognitive decline and dementia in AD patients without detectable lesions. Of these disease markers demonized amyloid beta pathology is the key enemy for the amyloid cascade hypothesis.
Plaque amyloid may itself impair (dotted arrows) synaptic plasticity and learning, neural networks, protein phosphorylation and oxidative stress status. Therefore it may have separate pathogenetic significance for the familial forms of AD, caused by the mutations in amyloid precursor protein and presenilins genes.
Similarly, oxidative stress independently disrupts synaptic plasticity and thus may have separate pathogenic value for the Down syndrome (characterized by upregulation of the reactions of oxidative stress due to the possible overexpression of the enzyme Cu/Zn-superoxide dismutase (SOD1), a chromosome 21 gene product) and for the pre-plaque stages of AD. The hallmarks trigger third order events of microglia activation, astrocytosis, cytokine/acute-phase protein release and cell death (not shown). This may convert physiological compensation into the pathological final and lock the cascade and the disease irreversibility.
ALZHEIMER’S DISEASE, CHOLESTEROL & AMYLOID b: DOGMA PREVAILS
Up to now, cholesterols’ role in Alzheimer's disease was mainly explained in terms of the dogmatic view that a reduction of amyloid burden by lowering cholesterol is beneficial [5, fr8], fr9, fr10]. This viewpoint (that become questioned in October 2002 Neurology article by Fassbender et al. [fr9]). was based on the in vitro data on the importance of cholesterol in amyloid precursor protein processing and Ab generation [4, 5]. Two recent articles further showed that cellular generation of Ab is modulated by cholesterol compartmentation and intracellular cholesteryl-ester levels [fr8].
The biochemical relation of cholesterol and Ab, however, is bidirectional (See Ref.4 for detailed bibliography).
Moreover, the modulation of neuronal cholesterol dynamics by Ab may have important functional consequences.
Particularly, Ab modulates neuronal cholesterol esterification, influx, efflux, and thus may regulate neural cholesterol intracellular compartmentation and extracellular trafficking . Ab also modulates neuronal physical property of membrane fluidity important for receptor function, and it is well possible that this effect is mediated by the peptide antioxidant properties (see next section). Additionally, Ab increases neural lipid synthesis, in contrast to the peptide inhibitory effect, observed in human hepatic HepG2 and in HEK293 cells, in fetal rat liver and in neuronal tissue under the condition of potassium-evoked depolarization and under oxidative stress. The latter results highlight the importance of developmental, tissue and neuronal functional specificity of Ab-cholesterol biochemical relation, which may vary in different brain regions and be of special importance in determining Alzheimer's specific areas of neurodegeneration. The latter data also suggest that Ab may serve a molecular messanger function and manage the crosstalk of hepatic, systemic and brain cholesterol, and thus maintain the tissue-specific coordinate regulation of cholesterol biosynthesis. Taken together, the above functional consideration and recent data on the importance of cholesterol compartmentation for Ab generation indicate feedback mechanism between cholesterol and Ab homeostasis, additionally supported by a dependency of amyloid precursor protein processing and Ab production on the site 2 processing of SREBP and associated inability of cells to upregulate the expression of several enzymes and proteins involved in cholesterol synthesis and turnover .
CACorresponding author: Alexei Koudinov, M.D., Ph.D., P.O.Box 1665, Rehovot 76100, Israel ; e-mail: [email protected] ; URL: http://anzwers.org/free/neurology
#Footnote 1: Prior to the 32nd Society for Neuroscience Meeting (Orlando, Nov. 2-7, 2002) Neurobiology of Lipids published the editorial (2002, Vol.1, 5) that compiled the abstracts on the subject of the journal scope. Neurobiology of Lipids editors were invited to select the most noteworthy abstracts from the list of more then two hundered presentations. The short list yielded eighteen presentations. The authors of these presentations were invited to publish with us proceedings articles (matching the content of the SFN 2002 presentations) without additional peer review. This article represents the one of eight bronze choice presentations. The acceptance date represents the day of the article appearence at the 32nd SFN Annual Meeting in Orlando, Florida, Nov 2-7, 2002.
To review other neurobiology of lipids SFN Annual Meeting abstracts 2002 and the editors' choice shortlist please click here.
§Footnote 2: The presented proceedings article is based on a History of Neuroscience session of the 32nd Society for Neuroscience Annual Meeting 2002, and represents the research by two groups. Dr. Kontush and Dr. Arlt author the presentation section entitled "Amyloid b is a potential physiological antioxidant for lipoproteins in cerebrospinal fluid and plasma". Dr. Koudinova, Dr. Koudinov and Dr. Berezov contributed to the rest of the article. Dr. Arlt (Hamburg, Germany) decided that he did not contribute sufficiently to justify his authorship of this proceedings article.
1. Koudinov AR, Berezov TT, Koudinova NV. Alzheimer's amyloid beta and lipid metabolism: a missing link? FASEB J. 12, 1097-99 (1998)
2. Koudinov AR, Koudinova NV. Essential role for cholesterol in synaptic plasticity and neuronal degeneration. FASEB J. 15, 1858-60 (2001). Originally published online June 27, 2001, 10.1096/ fj.00-0815fje
3. Koudinov AR, Berezov TT, Koudinova NV. The levels of soluble Ab in different HDL subfractions distinguish Alzheimer's/ normal aging CSF: implication for brain cholesterol pathology? Neurosci Lett. 314, 115-118 (2001)
17. Kontush A, Donarski N, Beisiegel U. Resistance of human cerebrospinal fluid to in vitro oxidation is directly related to its amyloid-beta content. Free Radic Res. 35, 507-17 (2001)18. Zou K, Gong JS, Yanagisawa K, Michikawa M. A novel function of monomeric amyloid beta-protein serving as an antioxidant molecule against metal-induced oxidative damage. J Neurosci. 22, 4833-41 (2002)
24. Koudinov AR, Koudinova NV. Amyloid beta protein restores hippocampal long term potentiation: a central role for cholesterol? Soc Neurosci Abstr online. Program No.884.1 Published online 23 September 2002.