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§
From the Departments of Pathology,*
Medicine,
and
Pharmacology,§
Vanderbilt University Medical
Center, Nashville, Tennessee and Institute of
Pathology,
Case Western Reserve University,
Cleveland, Ohio
| Abstract |
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| Introduction |
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There are several deleterious outcomes from excess oxidative stress in nigral neurons. These include modification of macromolecules by ROS or oxidized catechols, depletion of intracellular thiols, and inhibition of mitochondrial function.9-15 Indeed, some of these processes already have been shown to readily induce cell death in experimental systems.11,16,17 Among these potential mechanisms of neurodegeneration, oxidation of nucleic acid may be especially damaging because it can result in permanent modifications that may contribute to neurodegeneration over years. This may be particularly true for mitochondrial DNA whose repair systems are much less efficient than those of nuclear DNA.18 Indeed, increasing evidence is now beginning to suggest that some neurodegenerative diseases may derive from defects in mitochondrial DNA.19
Mitochondrial point mutations and a 4977-bp "common deletion," both of which are likely related to oxygen radical attack, have been reported in some studies to occur either exclusively or with increased frequency in midbrain from patients with PD.19-22 However, subsequent studies could not confirm these results; rather, it was suggested that the mitochondrial point mutations and common deletion are associated with aging and not PD.23,24 One possibility for these apparently conflicting results is the inherent problem in trying to quantify mitochondrial DNA mutations in whole midbrain while only a subset of midbrain neurons are degenerating in PD. Using a different experimental approach, others have measured 8-hydroxyguanine, a product of free radical attack, in SN of PD patients and controls using gas chromatography with mass spectrometric detection.7 These investigators showed that 8-hydroxyguanine levels were increased in PD patients, although it is not known how much it is derived from oxidative damage to mitochondrial versus nuclear DNA. Importantly, all of these techniques are limited because they have not addressed either the cell types or the subcellular distribution of these DNA mutations or adducts within midbrain from PD patients.
In this study, we have tested the hypothesis that oxidative damage to nucleic acid in midbrain of PD patients is present largely in the nigral neurons, possibly within mitochondria, using an immunohistochemical assay for an antibody to one of the common products from nucleoside oxidation, 8-hydroxyguanosine (8OHG). We first determined whether there was any difference in midbrain 8OHG immunoreactivity between age-matched controls and patients with PD. Then we investigated whether 8OHG immunoreactivity was simply a reflection of neurodegeneration or specifically related to PD by quantifying 8OHG staining in patients with multiple system atrophy-Parkinsonian type (MSA-P) or dementia with Lewy bodies (DLB). Finally, since 8OHG antibody binds to both ·OH modified DNA and RNA, we assessed 8OHG immunoreactivity with or without pretreatment with DNase and RNase. We found that 8OHG was not only selectively increased in midbrain, especially in SN neurons of PD patients, but also that 8OHG immunoreactivity was limited to the cytoplasm of all cell populations. Moreover, 8OHG signals were present in cytoplasmic RNA in addition to the cytoplasmic DNA, ie, mitochondrial DNA.
| Materials and Methods |
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Brain tissue was obtained from autopsies performed at Vanderbilt University Medical Center or the University of Kentucky Medical Center. All control individuals were volunteers in a rapid autopsy program who had annual physical and neurological examinations that were within normal limits. Neuropathological examination of control individuals showed age-related changes only. All patients had been diagnosed during life with an extrapyramidal movement disorder or dementia. Final diagnoses were established by neuropathological examination according to established criteria.1
Antibodies
Mouse monoclonal anti-8OHG was kindly supplied by Dr. Regina Santella25 and the specificity of this antibody in paraffin-embedded tissue has been tested previously and the results show that it binds to both 8OHG and 8OHdG and co-localizes with nucleic acids.26
Immunohistochemistry
Eight-µm sections were cut from formalin-fixed,
paraffin-embedded blocks. The sections examined included midbrain from
patients with PD, MSA-P, or DLB at the level containing the red nucleus
and the proximal portion of cranial nerve III. In cases of PD,
additional sections were cut from blocks of the cerebellum and cerebral
cortex. In cases of DLB, additional sections were cut from blocks of
the hippocampus. Tissue sections from the corresponding brain regions
were obtained from controls. All sections were hydrated through graded
ethanol following deparaffinization with xylene, and then processed on
a Ventana ES automated immunohistochemistry system according to the
manufacturer's specifications including Ventana's protease I
pretreatment for 4 minutes and a standard alkaline phosphatase method
with fast red chromogen. Negative controls omitted the primary
antibody. Specificity of anti-8OHG was confirmed by absorption of the
antibody with purified 8OHG (see Figure 3A
).
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Quantification
The SN at the level of cranial nerve III exit was divided into six anatomical regions based on the method of Fearnley and Lees27 : ventromedial, ventral intermediate, ventrolateral, dorsomedial, dorsolateral, and pars lateralis regions. Cases containing at least 100 remaining neurons in the SN were used for quantification of total neurons and 8OHG-positive neurons to assure accuracy. Midbrain sections from eight control individuals, six PD patients, four MSA-P patients, and four DLB patients met this criterion. Neuron counting was performed by two independent observers, and the average for each region was used for statistical analysis.
| Results |
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A total of 22 individuals were included in this study. There was
no significant difference in age, gender ratio, or postmortem interval
among the four groups (Table 1)
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The number of SN neurons in midbrain tissue sections from control
individuals was 252 ± 4, 77 ± 2, 228 ± 7, 62 ±
2, 93 ± 6, and 59 ± 3 (mean ± SEM) for ventromedial,
ventral intermediate, ventrolateral, dorsomedial, dorsolateral, and
pars lateralis regions, respectively. There was significant reduction
in the number of SN neurons in PD, MSA-P, and DLB patients compared to
age-matched controls. Moreover, nigral neuron loss varied among regions
in PD, MSA-P, and DLB patients (Table 2)
.
In cases of PD, neuron loss was greatest in the ventrolateral region
and least in the dorsomedial region consistent with previously reported
results.27
In addition to loss of ventral tier neurons,
MSA-P and DLB patients displayed greater neurodegeneration in the pars
lateralis of the SN than did PD patients. These data also are
consistent with reports from other laboratories.27
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Prominent 8OHG cytoplasmic immunoreactivity was commonly present
in soma of SN neurons from patients with PD, MSA-P, or DLB, but was
rarely observed in age-matched controls (Figure 1, A and B)
. There was no correlation
between age or postmortem interval and 8OHG formation in any group of
patients. Neuronal cytoplasmic 8OHG was granular and did not extend
into either dendrites or axon. Nuclear 8OHG immunoreactivity was not
observed in any individual. 8OHG immunoreactivity was completely
ablated by preabsorbing anti-8OHG with purified 8OHG (Sigma) before
incubation with tissue sections (Figure 3A)
.
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Specificity of cytoplasmic 8OHG immunoreactivity was investigated by examining hippocampal and cerebellar tissue sections from PD and DLB patients and control subjects. Rare pyramidal neuron cytoplasmic 8OHG immunoreactivity was observed in the cerebellum and hippocampus from PD patients and age-matched control subjects (data not shown). In contrast, DLB patients showed substantially more pyramidal neuron cytoplasmic 8OHG immunoreactivity in the hippocampus, corroborating the results of others.26 Cerebellar 8OHG immunoreactivity for DLB patients was indistinguishable from PD patients and control subjects. Taken together, these findings suggest that neuron cytoplasmic 8OHG immunoreactivity is associated with affected brain regions in these neurodegenerative diseases.
Target of Hydroxyl Radical Adduction
Anti-8OHG recognizes ·OH adducts on both DNA and RNA. To analyze
the subcellular targets responsible for 8OHG immunoreactivity,
immunohistochemical experiments with anti-8OHG were performed with
midbrain tissue sections that were preincubated with DNase, RNase, a
combination of both enzymes, or buffer alone. DNase or RNase each
significantly reduced 8OHG cytoplasmic immunoreactivity, and both
enzymes completely eliminated 8OHG immunoreactivity (Figure 3B)
. These experiments were repeated with
DNase or RNase that had been boiled for 30 minutes before incubating
with tissue sections. As expected, boiled DNase did not alter 8OHG
immunoreactivity while boiled RNase remained active. Protease
pretreatment did not decrease 8OHG immunoreactivity as nucleases, a
result similar to the data published previously.26
These
data indicate that both mitochondrial DNA and cytoplasmic RNA are
targets of oxidative damage to nucleic acid.
| Discussion |
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Our results demonstrated that severe dopaminergic neurodegeneration was
present in the SN of patients with PD, MSA-P or DLB and that the loss
of neurons followed the regional distribution described by
others.27
8OHG immunoreactivity in the remaining SN
neurons was increased in PD, MSA-P, and DLB compared to controls, and
the proportion of 8OHG-positive neurons was significantly greater in PD
than in MSA-P or DLB. In addition, the ventral tier, the ventrolateral
SN in particular, contained the highest proportion of 8OHG-positive
neurons in PD patients. This subregion of the SN is the most
extensively damaged in PD.27
Of note, 8OHG staining also
was observed in areas outside of the SN including the nucleus raphe
dorsalis, oculomotor nucleus, and some glial cells (Figure 1C)
. The
extra-SN staining was moderate in the PD patients, weak in MSA-P and
DLB, and absent in age-matched controls. Importantly, there was no
significant staining in the cerebellum or cerebral cortex of PD
patients. This pattern of immunoreactivity is identical to observations
of others using a different immunohistochemical probe for oxidative
damage in the midbrain of PD patients.5
In combination,
these results suggest that oxidative damage to nucleic acid is
increased in midbrain structures of PD, MSA-P, and DLB patients
compared to age-matched control individuals, is greatest in SN neurons
compared to other midbrain structures, and mirrors the regional
distribution of neurodegeneration both within the substantia nigra and
in other brain regions.
One reason why SN neurons may experience more oxidative damage is that those neurons use dopamine as their major neurotransmitter.9,11-13 Dopamine is metabolized enzymatically to produce H2O2 and ultimately dihydroxyphenylacetate (dopac). Also, dopamine and related o-catechols are unstable molecules that can oxidize in the presence of transition metals to yield O2- and quinoid species.28 O2-, a product of catechol autoxidation, can readily oxidize catechols thereby propagating ROS generation.29,30 In addition, O2- can yield H2O2 either spontaneously or enzymatically and then ·OH in the presence of transition metals such as iron. Furthermore, catechol metabolism may contribute indirectly to ·OH production by depleting cellular reduced thiols, a major defense mechanism against H2O2, via catechol thioether formation.8,31 ·OH is the primary species that attacks nucleic acid to yield 8OHG. Indeed, intense oxidative DNA damage with 8OHG formation has been shown in vitro by catechols in the presence of transition metals.32
A major consequence of increased ROS production is inhibition of mitochondrial function, a phenomenon first observed in midbrain tissue from PD patients about 10 years ago.33 In line with this observation, our results suggest that mitochondrial DNA is one of the primary targets of oxidative damage in PD patients, since 8OHG immunoreactivity was limited to the cytoplasm of neurons and the immunoreactivity was significantly diminished by DNase pretreatment. It is perhaps not surprising that mitochondrial DNA would accumulate substantially more oxidative damage than nuclear DNA, because mitochondrial DNA has no protective histone coat and its repair mechanisms are much less efficient than those of nuclear DNA.18 Our study showed that 8OHG adducts were significantly more common in SN neurons of PD patients compared to age-matched controls as well as to MSA-P and DLB patients, even though there was a similar degree of neuronal loss in the SN of patients from all three groups. This important result indicates that 8OHG formation is not simply a consequence of the imminent cell death and suggests that mechanisms of oxidative damage to SN neurons in PD may be different from in MSA-P or DLB. It is possible that this greater 8OHG adduct accumulation in PD may contribute to the mitochondrial dysfunction characteristic of this disease because many subunits of mitochondrial complex I are encoded by mitochondrial DNA.34 Consistent with this hypothesis, others have been unable to demonstrate mitochondrial dysfunction in midbrain from MSA-P patients.35,36
In addition to mitochondrial DNA, our results suggest that the other major target of oxidative damage is cytoplasmic RNA. Like mitochondrial DNA, RNA is not covered by protective histones and does not have advanced repair mechanisms. Also, because RNA is single-stranded it is even more vulnerable to free radical-mediated damage. The consequences of free radical-mediated damage to RNA are not fully understood. It is conceivable that mRNA damage may result in abnormal protein translation. tRNA and rRNA damage could result in dysfunction of protein synthesis. In an experimental system, inactivation of ribosomes has been demonstrated when 28S rRNA is damaged by free radicals.37 Little is known about the role of damaged RNA in neurodegenerative diseases. However, there is recent evidence that posttranscriptional modifications of RNA and protein synthesis may be altered in Alzheimer's disease38,39 and that aberrant RNA is associated with decreased activity of a glutamate transporter in amyotrophic lateral sclerosis.40 Certainly, more work is required to understand the potential role of oxidatively damaged RNA in the pathogenesis of neurodegeneration.
In summary, we observed increased frequency of 8OHG immunoreactive SN neurons in PD, MSA-P, and DLB patients compared to age-matched control individuals, with significantly more 8OHG immunoreactivity in PD than in MSA-P or DLB patients. 8OHG immunoreactivity was cytoplasmic and was present on both DNA and RNA. These results point to larger increases in oxidative damage to cytoplasmic nucleic acids in SN neurons from PD patients compared to patients with other nigral degenerative diseases and raise the possibility that oxidative damage to mitochondrial DNA and RNA may contribute to neurodegeneration.
| Acknowledgements |
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| Footnotes |
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Supported by grants from the National Institutes of Health to J.Z. (ES05842), D.R. (HL56693, NS33460), D.G.G. (ES02611), and T.J.M. (AG00774), grants from the National Parkinson Foundation to D.R., and grants from the Alzheimer's Disease and Related Disorders Association and the American Foundation for Aging Research to T.J.M.
Accepted for publication February 9, 1999.
| References |
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