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§
From the Departments of Pharmacology,*
Pathology,
and
Medicine,
and the Centers for Molecular
Neuroscience and Molecular Toxicology,§
Vanderbilt University Medical Center, Nashville, Tennessee; and the
Sanders-Brown Center on Aging¶
and the
Departments of Pathology and Neurology, University of Kentucky,
Lexington, Kentucky
| Abstract |
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| Introduction |
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Isoprostanes (IsoPs) are exclusive products of free
radical damage to arachidonic acid (AA)
(C20:4
6) that are formed esterified to lipid (bound) and then are
hydrolyzed (free).11
Measurement of the major class of
IsoPs, F2-IsoPs, has been used widely to quantify
free radical damage in vivo.12
Compared to
controls, F2-IsoP levels are elevated in
cerebrospinal fluid from probable AD patients early in the course of
dementia, and from definite AD patients where
F2-IsoP levels correlate with pathological
measures of AD severity.13-15
Bound
F2-IsoP levels in the frontal cortex obtained
with short post mortem intervals from definite AD patients also are
significantly greater than age-matched controls.16
However, in another study, bound F2-IsoP
concentrations in occipital, temporal, and parietal cortex were not
different between definite AD patients and controls,17
raising the intriguing possibility that F2-IsoP
generation is limited to the frontal lobe in AD.
Unlike AA that is evenly distributed in gray and white matter,
docosahexaenoic acid (DHA) (C22:6
3) is enriched in gray matter of
the central nervous system, where it is synthesized in astrocytes and
then transported and concentrated in neurons.18,19
Previously, we described the formation of neuroprostanes (NPs) from
free radical catalyzed peroxidation of DHA via reactions analogous to
IsoP generation.20
We proposed that NPs may provide more
specific information on free radical damage in DHA-containing
compartments, ie, neurons, and that NPs may be more sensitive markers
of free radical damage because DHA is more labile to peroxidation than
is AA. Free F4-NPs are increased in cerebrospinal
fluid of definite AD patients compared to controls, and the levels of
cerebrospinal fluid F4-NPs are greater than
F2-IsoPs.20
Interestingly, others
demonstrated that bound F4-NPs (called
F4-IsoP by these authors) are increased in
occipital and temporal cortex, but not parietal cortex, of AD patients
compared to controls.17
This regional pattern does not
correspond to the distribution of pathological changes in AD, and
suggests that lipid peroxidation may be more widespread in AD brain
than are histopathological changes.
IsoP and NP formation proceeds through bicyclic endoperoxide
intermediates that are reduced to F-ring compounds or undergo
rearrangement to D/E-ring compounds (Figure 1)
. Ex vivo oxidation of rat
hepatic microsomes produces higher levels of D/E-ring than F-ring
IsoPs, the reverse of what is observed after oxidation of liver
in vivo.21
Ex vivo oxidation of rat
brain synaptosomes similarly yields increasing levels of D/E-ring and
F-ring NPs at a ratio of 8:1, respectively.22
In
vitro, increasing the concentration of cellular reductants, such
as glutathione, favors reduction of the endoperoxide intermediates
resulting in greater amounts of F-ring compounds and lower amounts of
D/E ring compounds.21
Therefore, calculation of the F-ring
to D/E-ring ratio supplies information on the reducing environment in
which lipid peroxidation occurred.
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| Materials and Methods |
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All tissue sections were dissected at the time of autopsy and kept
frozen at -80°C until used. Lipids from specimens of hippocampus at
the level of the lateral geniculate nucleus, superior and middle
temporal gyri, IPL, and cerebellar cortex were extracted by the method
of Folch.15
D2/E2-IsoPs and
D4/E4-NPs esterified in
tissue were converted to O-methyloxime derivatives in Folch
solution. IsoPs and NPs were hydrolyzed by chemical saponification,
extracted using C-18 and silica Sep-Pak cartridges, purified by thin
layer chromatography, converted to pentaflurobenzyl ester
trimethylsilyl ether derivatives, and quantified by stable isotope
dilution techniques using gas chromatography/negative ion chemical
ionization/mass spectrometry using
[2H4]-8-iso-PGF2
and
[2H4]-PGE2
as internal standards as previously described.22
The
derivatized D/E-ring IsoPs or NPs co-migrate on silica thin layer
chromatography plates and GC and had identical masses, therefore the
levels of these isomers are reported as combined values. AA and DHA
concentrations were determined as previously described.29
Briefly, a 1-ml aliquot of Folch extract from each tissue sample was
transmethylated and the total fatty acid composition quantified using
gas chromatography with flame ionization detection.
Statistical analyses were preformed using GraphPad Prism software (San Diego, CA). Students t-tests was used for paired comparisons. All two-way analyses of variance (ANOVA) were used for data stratified by AD versus control and by four brain regions (1 x 3 degrees of freedom). One-way ANOVA with Bonferronis repeated comparison correction was used for post hoc analysis. Spearmans ranked correlation was used for discontinuous data such as Braak stage and APOE genotype.
| Results |
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4 allele frequency that
is similar to the general population. AD patients had an
APOE
4 allele frequency that was increased compared to
controls, but within the expected range for AD patients.30
AD patients had characteristic average disease duration, as well as
significantly lower brain weights than controls
(P < 0.05).
|
F4-NPs were by far the most abundant of the compounds measured, having an overall average (all individuals, all regions) level of 13.7 ± 0.8 ng/g. The corresponding overall average level of F2-IsoPs was 4.9 ± 0.3 ng/g, 2.8-fold less than F4-NPs. Levels of D/E-ring IsoPs and NPs were the lowest, averaging 1.5 ± 0.1 ng/g and 1.4 ± 0.2 ng/g, respectively. Tissue levels of F2-IsoPs did not correlate with F4-NPs concentrations. In addition, tissue levels of F-ring compounds did not correlate with the concentration of the corresponding D/E-ring compounds.
Levels of F-ring plus D/E-ring compounds were determined to assess the
magnitude of free radical damage to AA and DHA. Two-way ANOVA for
tissue levels of NPs was performed after stratifying data by the
presence of AD and by brain region (Figure 2A)
. Tissue levels of F- plus D/E-NPs
were significantly higher in AD patients versus controls
(P < 0.0001), and were significantly associated
with brain region (P < 0.05), a consequence of
NPs being higher in cerebral cortical regions than in cerebellum of AD
patients. An analogous two-way ANOVA for tissue levels of F-ring plus
D/E-ring IsoPs was not significant for presence of AD or brain region
(P > 0.05). Two-way ANOVAs for only F-ring
compounds were significant for AD versus control for both
F4-NPs (P < 0001) and
F2-IsoPs (P < 0.05);
however, only the F4-NPs were significantly
associated with brain region (Figure 2B)
. Although
D4/E4-NP tended to be
greater in AD patients than in controls, two-way ANOVA for
D4/E4-NP was not
significant for AD or brain region. Similarly,
D2/E2-IsoPs were not
statistically significant different with respect to AD or brain region.
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| Discussion |
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The lack of difference in total IsoP levels between AD patients and age-matched controls indicated that gray matter did not experience significantly more lipid peroxidation in AD patients compared to age-matched controls. However, neurons are only one component of gray matter. Our results with total NP levels indicated that the subset of gray matter that contains DHA did experience increased levels of free radical damage in AD patients compared to controls. In combination, these results suggest that free radical damage in AD is focused in DHA-containing compartments, mostly neurons, and is not evenly distributed within gray matter.31 Moreover, our results showed that increased free radical damage to DHA occurred in cerebral cortex and hippocampus but not cerebellar cortex. It should be noted that this regional distribution of elevated NPs, although expected if one proposes that free radical damage is an element in AD pathogenesis, is different from what was observed by others.17
Corroborating the report of others, we observed significantly elevated F2-IsoPs in AD brain regions compared to controls.16 Furthermore, this result is consistent with our earlier observations of significantly elevated F2-IsoPs in the cerebrospinal fluid of definite and probable AD patients compared to controls.13-15 However, if the elevation in F2-IsoPs in AD derived exclusively from the same compartment as the more dramatically elevated NPs, one would predict a similar shift in the F-ring to D/E-ring ratio in IsoPs as was observed with NPs. Because there was no significant change in the F2-ring to D2/E2-ring ratio, it seems likely that a smaller amount of lipid peroxidation may occur in tissue elements other than neurons, eg, reactive astrocytes or activated microglia, in AD.
Reports on the concentrations of glutathione and other cellular reductants in AD have been conflicting.1 This is an important issue to resolve because cellular reductants play an important role in anti-oxidant defenses, and would offer an accessible therapeutic target in AD. The 40 to 70% decreases in F-ring to D/E-ring NP ratio in AD patients with unchanged IsoP ratio indicated that DHA-containing compartments had significantly diminished reducing capacity in AD. In contrast, recent in situ data has demonstrated increased reductants in neuronal cytoplasm in regions of brain involved by AD.32 One interpretation of these apparently conflicting results is that reducing capacity may vary among different microenvironments within tissue and even within neurons. Our data indicates that reducing capacity is diminished within DHA-containing microenvironments in AD brain, but cannot be extrapolated to include other subcellullar compartments, eg, neuronal cytoplasm.
It is important to note that the F-ring to D/E-ring NP ratio was lower in all AD brain regions including cerebellar cortex. However, the levels of NPs were elevated only in cerebral regions and not in cerebellar cortex. This comparison suggests that the lowered reducing capacity in DHA-containing compartments in AD brain is not necessarily a consequence of increased free radical damage. Moreover, because cerebellar cortex is not considered a site for AD pathological changes, our results raise the possibility that diminished reducing capacity in DHA-containing compartments may be a feature of patients who are vulnerable to developing AD and not an outcome of AD pathological changes.
Although our data showed that there was no significant difference in the concentration of DHA or AA in the brain regions studied between AD patients and controls, it is possible that the cellular or subcellular distribution of DHA or AA is somehow altered by AD or by reactions to injury, such as gliosis or microgliosis. If this were the case, then interpretation of our data would be complicated by differential distribution of substrate in controls and AD patients. Nevertheless, our data would still indicate that DHA, a fatty acid essential to proper neuronal function, is significantly oxidized in AD cerebrum and that this may derive, in part, from decreased reducing capacity in certain lipid microenvironments in AD. However, the possibility of significant DHA redistribution in AD seems unlikely to be a major confounding variable because our results are entirely consistent with numerous histochemical and immunohistochemical reports localizing increased accumulation of lipid peroxidation products in neurons. The advantage of our complimentary approach is that it allows an unbiased, robust quantification of these events. Such quantification will be critical to future studies that attempt to determine the efficacy of therapeutic interventions that limit lipid peroxidation to brain in AD.
| Footnotes |
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Supported by National Institutes of Health grants AG00774, AG16835, AG05144, AG05119, DK48831, GM15431, DK26657, and CA77839, as well as grants from the Alzheimers Association (to T. J. M.), the Abercrombie Foundation (to W. R. M.), and a Burroughs-Wellcome Clinical Scientist Award in Translational Research (to J. D. M.).
Accepted for publication September 20, 2000.
| References |
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3) and arachidonic acid (20:4
6). J Neurochem 1991, 56:518-524[Medline]
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