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Short Communications |





From the Center for Neurodegenerative Disease Research and
Department of Pathology and Laboratory Medicine,*
and the
Department of Neurology,
The University of
Pennsylvania, Philadelphia; the Stokes Research Institute and
Department of Biochemistry and Biophysics,
Childrens Hospital of Philadelphia and The University of
Pennsylvania, Philadelphia, Pennsylvania
| Abstract |
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-synuclein
when compared to other in vitro nitrated constituents of
these pathological lesions, such as neurofilament subunits and
microtubules. Collectively, these findings imply that
-synuclein is nitrated in pathological lesions. The widespread
presence of nitrated
-synuclein in diverse intracellular inclusions
suggests that oxidation/nitration is involved in the onset and/or
progression of neurodegenerative diseases.
| Introduction |
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-Synuclein (
-syn) is a 140-amino
acid long highly conserved protein that is abundant in neurons,
particularly in presynaptic terminals.16,17
Two
mutations in the
-syn gene have been shown to be pathogenic for
familial Parkinsons disease in rare kindreds,18-20
and
it has been demonstrated that
-syn is the major component of LBs and
Lewy neurites (LNs) in Parkinsons disease, DLB, and the LB variant of
Alzheimers disease (LBVAD).21-27
More recently,
-syn
has been recognized to be a major component of the glial (GCIs) and
neuronal cytoplasmic inclusions in multiple system atrophy (MSA)
brains28-34
as well as of the LB-like inclusions,
neuraxonal spheroids, and LNs in neurodegeneration with brain iron
accumulation type 1 (NBIA1; previously known as Hallervorden-Spatz
disease).33,35,36
Thus, neurodegenerative disorders
characterized neuropathologically by
-syn lesions now are referred
to as synucleinopathies.
Here, we report that the majority of
-syn inclusions in DLB, LBVAD,
MSA, and NBIA1 contain nitrated proteins. Further, we also demonstrate
that
-syn, nitrated in vitro, is recognized by the rabbit
anti-3-NT polyclonal antibody (3-NT pAb) in Western blots thereby
implicating
-syn as a plausible target for nitrative modification in
these inclusions. Thus, we infer from these data that
oxidative/nitrative modifications of
-syn are involved in mechanisms
underlying neurodegenerative synucleinopathies.
| Materials and Methods |
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To assess the relative specificity of the 3-NT pAb for proteins
previously detected in synucleinopathy lesions, we performed Western
blot analyses with this antibody on purified proteins after in
vitro nitration. Recombinant human
-syn was expressed and
purified from bacteria as previously described.37
Recombinant mouse low molecular weight neurofilament (NF) protein (NFL)
were expressed in Escherichia coli BL21 (DE3) using a mouse
NFL cDNA cloned into the pET-23d expression vector (Novagen, Inc.
Madison, WI) after which transformed bacteria were selected and
maintained in Luria-Bertani medium (10 g/ml bacto-tryptone, 5 g/ml
bacto-yeast extract, 10 g/ml NaCl) or Terrific Broth (12 g/ml
bacto-tryptone, 24 g/ml bacto-yeast extract, 0.4% gycerol, 17 mmol/L
KH2PO4, 72 mmol/L
K2PO4) containing 100
µg/ml ampicillin. Bacteria were grown to an
OD600 of 0.6 and the expression of the
recombinant protein was induced with 0.5 mmol/L of
isopropyl-ß-D-thiogalactopyranoside for 2
hours. To recover bacterially expressed NFL, cells were pelleted,
resuspended into lysis buffer (25% sucrose, 1 mmol/L
ethylenediaminetetraacetic acid, 50 mmol/L Tris, pH 8.0, 2 mg/ml
lysozyme, and a cocktail of protease inhibitors) and incubated on ice
for 30 minutes. Ten mmol/L of MgCl2, 1 mmol/L
MnCl2, 10 µg/ml DNase 1 and 10 µg/ml RNase A
were added to the homogenate, which was incubated on ice for another 30
minutes. Two ml of detergent buffer (0.2 mol/L NaCl, 1% deoxycholic
acid, 1% Nonidet P-40, 20 mmol/L Tris, pH 7.5, 2 mmol/L
ethylenediaminetetraacetic acid) per ml of lysis buffer were added and,
after vigorous mixing, the insoluble material was sedimented at
5,000 x g for 30 minutes. The supernatant was
discarded and the pellet was repeatedly washed with buffer containing
0.5% Triton and 1 mmol/L ethylenediaminetetraacetic acid to generate a
highly compact pellet which was resuspended in 8 mol/L urea, 1%
ß-mercaptoethanol, 10 mmol/L NaPO4, pH 7.0, for
subsequent purification of NFL using hydroxylapatite (Bio-Rad
Laboratories, Richmond, CA).38
Medium (NFM) and high (NFH)
molecular weight NF proteins were purified from bovine spinal cords as
previously described.39
Tubulin proteins recovered from
phosphocellulose purified bovine microtubules were purchased from
Cytoskeleton, Inc., Denver, CO. All of these proteins were
nitrated with a 10-fold molar excess of peroxynitrite as previously
described,40
and then the relative specificity of the 3-NT
pAb for each of these proteins was assessed by Western blot methods
reported earlier.33,37,40
Tissue Collection and Processing
The harvesting, fixation, and further processing of the brain
tissue specimens were conducted as previously
described.22,41
Briefly, tissue blocks were removed at
autopsy from the cingulate cortex and mesencephalon of six DLB and five
LBVAD brains, the cerebellar white matter from seven MSA brains, and
the insular cortex and globus pallidus from one NBIA1 brain (see Table 1
). The diagnostic assessment of all
cases was performed in concordance with published
guidelines.42-44
Samples from these brains were fixed by
immersion in 70% ethanol with 150 mmol/L NaCl for 24 to 36 hours, and
paraffin embedded according to a previously described
schedule.45
These blocks were then cut into multiple, near
serial, 6-µm sections for immunohistochemical staining.
|
The presence of 3-NT modified substrates in human tissue was detected by immunohistochemistry using the 3-NT pAb (generously provided by Joseph S. Beckman; previously characterized by Beckman et al46 and Ye et al47 ), and the avidin-biotin complex (ABC) system (Vectastain ABC Elite Kit, Vector Laboratories, Burlingame, CA) with the chromagen 3,3'-diaminobenzidine as previously described.48 Briefly, sections were heated at 60°C for 60 minutes, deparaffinized and hydrated through graded ethanols, rinsed in 0.1 mol/L phosphate-buffered saline (PBS), pH 7.1, and endogenous peroxidases were neutralized with 5% H2O2 in methanol for 20 minutes. Sections were then washed and blocked in PBS containing 10% goat serum and 1% bovine serum albumin for 30 minutes at 37°C. 3-NT pAb was diluted 1:400 in PBS containing 10% goat serum and 1% bovine serum albumin, and incubated at 37°C for 90 minutes. Slides were washed and incubated with biotinylated goat anti-rabbit antibody at 37°C for 40 minutes. Slides were washed and incubated with ABC solution at 37°C for 30 minutes. Bound antibody complexes were visualized with 3,3'-diaminobenzidine followed by a brief wash in distilled water. The sections were then lightly counterstained with hematoxylin, dehydrated, and coverslipped. Negative controls for the 3-NT pAb included preabsorption of this antibody with a 10-fold excess of purified 3-NT and pre-incubation of tissue sections with 100 mmol/L dithionite as described previously.48
Quantitation of GCIs and LBs
Consecutive 6-µm sections were taken from each of the MSA
(cerebellum) or DLB/LBVAD (cingulate cortex) brains and immunostained
with either the 3-NT pAb as described above, or a mouse
anti-
-syn-specific monoclonal antibody, Syn 202,49
to
count the total number of GCIs or LBs in a given field. Immunostaining
with the Syn 202 was performed as described
previously.45,50
Briefly, the sections were
deparaffinized, hydrated through graded ethanols, treated with 5%
H2O2 in methanol, and
blocked in 2% donor horse serum in 0.1 mol/L Tris (Tris/donor horse
serum) for 5 minutes. Primary incubation was performed with Syn
202 diluted 1:1,500 in Tris/DHS overnight at 4°C. After washing,
sections were sequentially incubated with biotinylated secondary
antibodies for 1 hour and avidin-horseradish peroxidase complex
(Vectastain Standard ABC kit; Vector Laboratories) for 1 hour. Bound
antibody complexes were visualized using 3,3'-diaminobenzidine.
For GCI quantification, five adjacent medium power (x200) photomicrographs were taken from the MSA cerebellar white-matter tissue sections stained with either the 3-NT pAb or Syn 202 and the total number of GCIs in all five photomicrographs was obtained. For LB quantification, the total number of LBs recognized by each antibody within entire tissue sections of cingulate cortex from DLB and LBVAD cases were counted for each antibody. The percentage of LBs or GCIs labeled with the 3-NT pAb was determined as a ratio of 3-NT pAb inclusion counts over Syn 202 inclusion counts.
| Results |
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In all of the DLB and LBVAD brain sections (see Table 1
for a
summary of cases studied), the 3-NT pAb robustly stained cortical and
nigral LBs, as well as many LNs (Figure 1, ac)
. In the substantia nigra pars
compacta, the core and peripheral halo of classical LBs were labeled by
this antibody (Figure 1c)
, whereas robust 3-NT immunoreactivity was
observed throughout many GCIs in the cerebellar white matter in all MSA
brains examined here (Figure 1, df)
, and these 3-NT-positive GCIs
were seen in satellite, interfascicular, and perivascular
oligodendrocytes.
|
Quantitation of 3-NT Immunoreactive LBs and GCIs
The percentage of
-syn-positive cortical LBs that were also
3-NT immunoreactive ranged from 61 to 100% (mean, 76.2%) in DLB
brains and from 57 to 80% (mean, 67.8%) in LBVAD brains (Table 2)
. Presumably because of
section-to-section variation in the number of LBS, one DLB brain had
slightly more 3-NT-immunolabeled LBs than those labeled by antibodies
to
-syn in adjacent sections, and this case (identified by an
asterisk in Table 2
) was assigned a grade of 100%. Further, the
percentage of
-syn-positive GCIs in the MSA brains that were 3-NT
immunoreactive ranged from 56 to 76% (mean, 65.4%). Finally, there
was no apparent relationship between the percentage of 3-NT-positive
LBs or GCIs and the severity of the neuropathology, as reflected by the
abundance of these
-syn inclusions.
|
-Syn is the major component of LBs and GCIs, however other
proteins such as NF subunits41,51,52
and
tubulins53,54
are prominent constituents of LBs and GCIs,
respectively. Because it is plausible that the 3-NT pAb could recognize
a variety of nitrated proteins, Western blot analysis was used to
compare the relative specificity of this antibody for these proteins
after in vitro nitration. Human
-syn, bovine
microtubule-derived tubulins, as well as mouse NFL and bovine NFM and
NFH were exposed under identical conditions to the same quantity of
peroxynitrite in the presence of CO2. As shown in
Figure 2
, the immunoreactivity for
in vitro-nitrated
-syn was significantly more intense
than that for in vitro-nitrated tubulins or NF proteins.
This dramatic difference may be because of more extensive nitration of
-syn, or greater affinity of this 3-NT pAb for nitrated
-syn
which is known to be nitrated in vitro at all four of its
tyrosine residues, primarily because of the random coil conformation
that this protein assumes in solution.40
Indeed, other
proteins may assume alternative secondary structures that confer a
selective susceptibility of their tyrosine residues to nitration, as in
NFL.55
Notably, the higher molecular mass species of
-syn seen in Figure 2
probably reflect
peroxynitrite-induced o-o'-dityrosine cross-link
formation because
-syn has been show to be modified by this reaction
more readily than other proteins.40
|
| Discussion |
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-syn rich neuropathological hallmarks
of MSA, NBIA1, DLB, or LBVAD (ie, LNs, GCIs, neuronal cytoplasmic
inclusions, and neuraxonal spheroids) and our quantitative analysis
showed that the majority of these inclusions contain 3-NT modified
protein.
Taken together with previous observations, our studies suggest that
-syn is nitrated in the
-syn lesions detected here by the 3-NT
pAb. The abundance of
-syn in LBs and GCIs and the recent
observation that
-syn is an excellent substrate for
nitration40
provide compelling support for this view.
Further, because we also showed that nitrated
-syn was far more
intensely labeled by the 3-NT pAb than any of the other nitrated
proteins examined here by Western blot, it is highly likely that
-syn is the major 3-NT-modified protein in the lesions we studied.
Nonetheless, additional studies are needed to confirm this using
complementary methods and additional synucleinopathy brains.
Because the majority (57 to 100%) of the inclusions counted here, (ie,
LBs and GCIs), contained 3-NT immunoreactivity, it is tempting to
speculate that nitrative or oxidative damage may cause aggregation of
proteins to form these inclusions. However, not all inclusions were
labeled by the 3-NT pAb and this may be because of the fact that the
generation of 3-NT by nitration is enzymatically reversible, and
denitration of 3-NT may occur throughout an extended period of time if
normal cellular reductive capacities are re-established. Thus, the
generation of additional antibodies that specifically recognize the
more stable o-o'-dityrosine modification induced by
nitration may demonstrate the presence of nitrative injury in all of
these
-syn inclusions.
The effects of
-syn nitration are undetermined, but it is possible
that nitration may render
-syn more resistant to proteolysis or
alter other properties of this synaptic protein thereby playing a
mechanistic role in the formation of
-syn lesions as well as in the
onset/progression of synucleinopathies. Nitrating species also may
contribute to the pathogenesis of
-syn lesions by oxidizing tyrosine
residues to form o-o'-dityrosine resulting in the covalent
cross-linking of
-syn and the formation of stable
-syn
polymers.40
Indeed, o-o'-dityrosine formation
may be more damaging than the 3-NT modification because this alteration
may be reversed enzymatically whereas o-o'-dityrosine
cross-linking is more stable and it may retard or prevent the removal
of abnormal protein aggregates.57,58
These uncertainties not withstanding, based on the data described here,
together with evidence that o-o'-dityrosine cross-linking of
-syn leads to the formation of stable
-syn
polymers,40
we suggest that loss of neuronal or glial
oxidative protective mechanisms may have deleterious effects on the
normal functions or metabolism of
-syn and thereby contribute to the
onset/progression of neurodegenerative synucleinopathies. Thus, further
studies of the role of oxidative and nitrative injury in mechanisms
underlying these and other neurodegenerative disorders may lead to the
identification of therapeutic targets for the prevention or reversal of
these diseases.
| Acknowledgements |
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| Footnotes |
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Supported by grants from the National Institutes of Health and by a Pioneer Award from the Alzheimers Association. B. I. G. is the recipient of a fellowship from the Human Frontier Science Program Organization.
Accepted for publication August 3, 2000.
| References |
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