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1 From the Departments of Ophthalmology and Visual Sciences, 2 Molecular Biology and Pharmacology, and 3 Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri.
| Abstract |
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METHODS. To identify retinal antigens that are recognized by the sera of patients with glaucoma, immunoreactive bands were separated, by using two-dimensional gel electrophoresis of the bovine retinal soluble fraction. A 29-kDa band was then selected for further analysis. Tryptic peptides of the 29-kDa band were analyzed using electrospray mass spectrometry to identify the protein. After protein identification, immunoreactivity against this newly identified protein was studied by Western blot analysis using sera from 65 patients with glaucoma (25 with primary open-angle glaucoma [POAG]; 40 with normal-pressure glaucoma [NPG]) and 25 age-matched healthy subjects. In addition, serum antibody titers were compared in these groups, by using a specific enzyme-linked immunosorbent assay (ELISA).
RESULTS. The 29-kDa band was identified as glutathione
S-transferase (GST). Western blot analysis revealed that
serum antibodies against GST antigen were recognized in 34 (52%) of 65
patients with glaucoma (22 of NPG and 12 of POAG) and 5 (20%) of 25
age-matched control subjects (
2 test,
P < 0.05). By ELISA, it was also found that
patients with glaucoma had higher titers of anti-GST antibody, compared
with the control group (MannWhitney test; NPG versus control,
P = 0.013; POAG versus control,
P = 0.0006).
CONCLUSIONS. These findings indicate that GST is one of the retinal antigens targeted by the serum antibodies detected in some patients with glaucoma.
| Introduction |
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B-crystallin,7
as well as rhodopsin,5
are
among the retinal antigens targeted by serum antibodies detected in
patients with glaucoma. In addition, our recent observations have shown
that the mechanism of hsp27 antibody-mediated cytotoxicity to retinal
ganglion cells is associated with the disruption of the integrity of
cytoskeleton.9 Because our previous findings indicate that there may be additional target antigens in the retina of patients with glaucoma,5 6 7 8 we sought to identify previously uncharacterized specific autoantibodies against retinal proteins in patients with glaucoma. We report here that glutathione S-transferase (GST) is one of the retinal antigens targeted by serum antibodies detected in some patients with glaucoma.
| Materials and Methods |
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Western Blot Analysis
Bovine retinas were dissected from eyes obtained at a local
abattoir within 3 hours of death and homogenized in ice-cold lysis
buffer containing 2 mM HEPES, 2 mM EDTA (pH 7.4), and protease
inhibitors (50 µM phenylmethylsulfonyl fluoride and 1 µg/ml each of
aprotinin, antipain, bacitracin, bestanin, chymostatin, leupeptin, and
pepstatin A) for 5 minutes at 4°C. After centrifugation at
1,000g for 10 minutes, the pellet (consisting of nuclei and
unbroken cells) was discarded, and the membrane fraction was pelleted
by centrifugation at 35,000g for 20 minutes. The supernatant
or soluble fraction was saved and the membrane pellet washed twice with
a solution containing 50 mM Tris, 154 mM NaCl (pH 7.4), and homogenized
in lysis buffer. Fractions were stored at -80°C until use. The
protein concentrations in the membrane and soluble fractions were
determined using the bicinchroninic acid (BCA) method (Sigma,
St. Louis, MO).
Samples, including bovine retinal proteins and purified GST (Sigma) were separated by electrophoresis in 12% sodium dodecyl sulfate (SDS)-polyacrylamide gels at 160 V for 1 hour and electrophoretically transferred to polyvinylidene fluoride membranes (Millipore, Marlboro, MA) using a semidry transfer system (Bio-Rad, Hercules, CA). After transfer, membranes were blocked in a buffer (50 mM Tris-HCl, 154 mM NaCl, and 0.1% Tween-20 [pH 7.5]) containing 5% nonfat dry milk for 1 hour and then overnight in the same buffer containing a dilution of primary antibody and sodium azide. Primary antibodies were either patient sera (dilution, 1:1000) or antibody against GST (dilution, 1:2000; Sigma). After several washes and a second blocking for 20 minutes, the membranes were incubated with secondary antibodies (IgG or IgM) conjugated with horseradish peroxidase (1:2000; Fisher Scientific, Pittsburgh, PA) for 1 hour. Immunoreactive bands were visualized by enhanced chemiluminescence using commercially available reagents (Amersham Life Science, Arlington Heights, IL).
Two-Dimensional Gel Electrophoresis
Two-dimensional gel electrophoresis of bovine retinal proteins
was performed according to the manufacturers instructions (Bio-Rad).
During the electrophoresis, the capillary tubes (Bio-Rad) were used in
first dimension for isoelectric focusing. The tubes were filled with
gel monomer solution, containing 9.2 M urea, 4.5% acrylamide/bis
stock, 1% Bio-Lyte pH 57 ampholyte, 4% Bio-Lyte pH 310
ampholyte, 5% detergent solution (0.5 M 3-([3-cholamidopropyl]
dimethylammonio)-2-hydroxy-1-propanesulfonate [CHAPS] and 10%
Nonidet P-40), 0.1% N,N,N,'N',-tetramethyl-ethylenediamine
(TEMED), and 0.2% of 10% ammonium persulfate (APS). The
samples were treated with a solution containing 0.15 mM dithiothreitol
(DTT) and 0.35 mM SDS. Upper running buffer was 20 mM NaOH, and lower
running buffer was 10 mM
H3PO4. Electrophoresis was
performed at 200 V for 2 hours, followed by 500 V for 2 hours, and 800
V overnight. The proteins were then further separated by
electrophoresis in 12% SDS-polyacrylamide gels, transferred to
membranes, and subjected to Western blot analysis as described.
Electrospray Mass Spectrometry
Protein identification by electrospray mass spectrometry was
performed at the Protein and Nucleic Acid Chemistry Laboratories of
Washington University. Protein containing acrylamide gel pieces was
first digested with sequencing-grade trypsin (Promega, Madison, WI).
Tryptic digests were then separated on a high-pressure liquid
chromatograph (model UMA; Michrom BioResources, Auburn, CA) using a
reversed-phase trifluoroacetic acid (TFA)-lite solvent system on a C-18
column. The flow stream from the high-pressure liquid chromatograph,
immediately after UV detection, was introduced into an ion-trap
electrospray mass spectrometer (ThermoQuest model LCQ Classic;
Finnigan, San Jose, CA) for mass spectrometry of the tryptic peptides.
The mass spectrometry data for each peptide was then sequentially
compared with the database of tryptic digests of all known proteins
using commercial software (Sequest; Sequest Technologies, Lisle, IL)
that matches the mass spectra against the electronically produced mass
spectra of all possible tryptic peptides proteins in the database.
Standard reagents and conditions were used.
Enzyme-Linked Immunosorbent Assay
For the enzyme-linked immunosorbent assay (ELISA) 96-well
microtiter plates (Packard Instruments, Meriden, CT) coated with GST
(50 ng/ml in sodium carbonate buffer [pH 8.8]) were incubated
overnight at 4°C. After the plates were washed, any remaining binding
sites were blocked using 1% normal goat serum and 0.1% sodium azide
at room temperature for 2 hours. Sera from patients or control
subjects, diluted 1:500 in phosphate-buffered saline (PBS) containing
Tween-20, plus 1% normal goat serum and sodium azide, were added to
duplicate wells of antigen-coated plates and incubated overnight at
4°C. The serum was removed by washing with PBS, and secondary
antibody (goat anti-human IgM conjugated with horseradish peroxidase;
1:2000; Fisher) was added. After 2 hours incubation at room
temperature, the secondary antibody was washed with PBS, and color was
developed using a kit (Biotrin, Dublin, Ireland). The plates were read
at 410 nM with a plate reader (Packard Instruments). Negative control
wells prepared without antigen or primary antibody and positive control
wells in which increased concentrations of specific antibody to GST
were used as primary antibody were simultaneously processed. Optical
density values were recorded at three independent measurements, and the
average value was calculated for each patient. Optical densities are
presented as the mean ± SD calculated within each study group.
The sensitivity ranges of this method were 0 to 2000 pg/ml, with
intra-assay precision of less than 10%.
| Results |
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2 test; NPG
versus control, P = 0.005; POAG versus control,
P = 0.04). No statistical difference was detected
between POAG and NPG (P = 0.58).
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| Discussion |
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The reason for the selection of an immunoreactive band at 29 kDa for further characterization was based on the strength of the signal detected on Western blot analysis. In addition, the serum autoantibody recognizing the 29-kDa protein in retinal extracts was IgM. IgM has been identified to play a pathogenic role in producing autoimmune polyneuropathy, and the antigenic targets of IgM are more readily identified than the targets of IgG or IgA.11
The GST supergene family, which encodes detoxification enzymes, is widely expressed in mammalian tissue cytosols and membranes. GSTs catalyze the conjugation of reduced glutathione with a wide variety of electrophiles that include known carcinogens and various compounds that are products of oxidative stress, including oxidized DNA and lipid. Indeed, several lines of evidence suggest that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals and oxidative stress.12 13
The cytosolic enzymes are encoded by at least five distantly related
gene families designated GST classes
, µ,
,
, and
.14
15
16
All the cytosolic subunits have a molecular
weight of 45,000 and are dissociable into subunits of approximately
26,000 Da. The molecular weight of the immunoreactive band that we
selected appeared higher than 26 kDa, because the prestained marker
that we used migrated more slowly than nonstained markers. This was
further confirmed by using 26-kDa purified GST, which also migrated as
a 29-kDa band under these conditions.
GST is present in glial and neuronal cells of the central nervous system and in the retina, as well as all throughout the human body,17 18 19 20 21 22 and its subclasses are similar in various mammalian tissues.23 In addition, regional and cellular distribution of immunoreactivity to different subunits of GST was studied in rat and bovine retinas, and class µ immunoreactivity was detected throughout the retina, including the retinal ganglion cell layer.21 22 High levels of GST in retinal glial cells suggest one native neuroprotective mechanism provided by the glial cells in which they conjugate toxicants with glutathione through the action of GST.24 This may be an important component of adaptive retinal responses in the glaucomatous retina, because oxidative stress and the generation of reactive oxygen species appear to be involved in neuronal cell death during glaucomatous neurodegeneration.25 26 Increased titers of autoantibodies to GST in some patients with glaucoma may therefore represent a generalized response to tissue stress and/or damage as a consequence of the glaucomatous neurodegeneration process and thereby secondary production of serum antibodies to GST in the glaucomatous retina. Similarly, the members of the GST gene family have been identified as antigens in autoimmune hepatitis,27 in which GST has been identified to be an important component of hepatic detoxification mechanisms.
Whether the circulating antibodies against GST are a consequence of tissue stress and damage in the retina or whether they have a pathogenic significance should be further evaluated. The data presented herein support previous observations3 4 5 6 7 8 that activated immunity may be involved in the etiology and/or sustaining of glaucomatous optic neuropathy in some patients. Further studies will facilitate better understanding of the role of activated immunity in glaucomatous neurodegeneration.
| Footnotes |
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Submitted for publication August 31, 2000; revised November 13, 2000; accepted January 8, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Martin B. Wax, Department of Ophthalmology & Visual Sciences, Washington University School of Medicine, Box 8096, 660 South Euclid Avenue, St. Louis, MO 63110. wax{at}vision.wustl.edu
| References |
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and
class glutathione transferase in normal human tissues Pharmacol Toxicol 72,321-331[Medline][Order article via Infotrieve]
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