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Treatment
1 From the Glaucoma Center and the Department of Ophthalmology, University of California San Diego, La Jolla; the 2 Department of Ophthalmology, University of California San Francisco; and the 3 Department of Ophthalmology and Visual Sciences, University of Wisconsin-Madison.
| Abstract |
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METHODS. Vehicle was topically applied to one eye, and 2 µg
PGF2
-isopropyl ester (PGF2
-IE) was
applied to the other eye of cynomolgus monkeys twice daily for 5 days.
Pressure reductions of 5 mm Hg in the PGF2
-IEtreated
eyes were confirmed. The eyes were then fixed and paraffin sections
were cut from each eye. The distribution of TIGR protein in the ciliary
muscle was determined by confocal scanning laser microscopy. Additional
sections were immunostained with a polyclonal antibody to recombinant
TIGR protein or with a polyclonal antibody to a synthetic peptide
corresponding to the leucine zipper region within the TIGR protein.
Staining intensity in the ciliary muscle was assessed by measuring
optical density (OD) along two line segments overlying the ciliary
muscle, by using a high-resolution imaging densitometer.
RESULTS. TIGR protein immunoreactivity was observed in ciliary muscle fibers
throughout the ciliary muscle. Extracellular TIGR immunoreactivity
colocalized with collagen type IV immunoreactivity. Intracellular
staining also was present. Immunoreactivity was less intense in the
sections from the PGF2
-IEtreated eyes compared with
the vehicle-treated eyes. This was reflected in the reduction of mean
OD scores in each monkey. Overall, the reduction of mean OD scores in
the treated eyes was 42.1% ± 9.9% (P < 0.005)
with the anti-recombinant TIGR antibody and 27.3% ± 10.4% with the
anti-TIGR peptide antibody (P < 0.005).
CONCLUSIONS. TIGR protein immunoreactivity was present both intracellularly and
extracellularly in the ciliary muscle of the cynomolgus monkey. This
suggests that extracellular TIGR protein is in contact with aqueous
humor in the uveoscleral outflow pathway. Moreover, IOP-lowering
topical PGF2
-IE treatment decreases the amount of TIGR
protein in the ciliary muscle.
| Introduction |
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Recent studies have shown that topical treatment with prostaglandin
(PG)F2
, which facilitates uveoscleral outflow,
is associated with reductions in the amount of collagen types I, III,
and IV in the ciliary muscle.18
Because these collagens
are contained in the extracellular space through which uveoscleral
outflow passes, it is possible that such reductions contribute to
increased uveoscleral outflow by reducing hydraulic resistance to
aqueous flow.19
20
In view of the presence of TIGR protein
within the ciliary muscle and evidence that, in other cell systems,
TIGR protein is secreted,1
2
3
it is possible that topical
PG treatment also influences TIGR protein expression in the ciliary
muscle. The present study was undertaken to investigate this
possibility.
| Materials and Methods |
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-IE applied unilaterally for 5 days before
quantitative immunohistochemical evaluation of collagen changes in the
ciliary muscle.18
All procedures were conducted in
compliance with the ARVO Statement for the Use of Animals in Ophthalmic
and Vision Research.
The monkeys initially were evaluated by biomicroscopy to confirm normal
appearance and by fluorophotometry to confirm bloodaqueous barrier
integrity. Briefly, intravenous fluorescein, 10 mg/kg of 10% solution,
was administered through the saphenous vein under intramuscular
ketamine hydrochloride anesthesia (10 mg/kg). After a wash with 2 ml
lactated Ringer solution, corneal and anterior chamber (AC)
fluorescence was measured (Fluorotron Master Scanning Ocular
Fluorophotometer equipped with an anterior segment adapter; Coherent
Medical, Palo Alto, CA) at 20, 40, 60, 90, 120, 180, and 240 minutes
after injection. For inclusion in the present study, AC fluorescence
levels and time course of appearance and decay in both eyes had to be
similar and within the range of values obtained for well-characterized
control eyes. The following week, each of the qualifying monkeys
received 2 µg PGF2
-IE (in 5 µl) in the
morning and in the evening for 5 days. On the fourth and fifth days,
IOP was measured at 3, 3.5, and 4 hours after the morning treatment to
confirm IOP reduction. On the fifth day, the eyes were evaluated by
biomicroscopy. A monkey was excluded from further study if it had less
than 5 mm Hg reduction in the treated eye (compared with pretreatment
ipsilateral baseline and simultaneous contralateral control) or if AC
cells or flare were observed. Eight eyes were obtained from four
monkeys that met the criteria.
Quantitative Analysis
Each animal was killed, its vascular bed cleared by perfusion,
and the two eyes collected, fixed, and processed into paraffin, as
previously described.18
Each anterior segment was embedded
in a single block. For histopathologic analysis, several sections from
each eye were stained with hematoxylin and eosin. For the present
study, five midsagittal sections from each eye were cut at 10 µm, a
thickness determined by previous studies to be optimal for our
quantitative immunohistochemistry protocol. These sections were
cleared, rehydrated, subjected to antigen retrieval solution and
melanin extraction, and blocked with 0.1% bovine serum albumin, as
previously described.18
The sections were then exposed
overnight to a rabbit polyclonal antibody raised against the
recombinant human 55-kDa form of the TIGR protein, diluted to 1:500.
The usefulness of this antibody has been demonstrated by Western blot
analysis and immunoprecipitation assays of dexamethasone-induced human
trabecular meshwork cells and in tissue studies of the human trabecular
meshwork.12
Parallel experiments were conducted with a
polyclonal antibody generated in rabbits that had been raised against a
synthetic polypeptide corresponding to the amino acid sequence
DKSVLEEEKKRLRQ and characterized by Western blot analysis (diluted
1:500).15
This sequence is found within the leucine zipper
portion of the TIGR protein.16
Subsequently, the sections
were rinsed, exposed to biotinylated goat anti-rabbit antibody for
exactly 20 minutes, rinsed, exposed to horseradish
peroxidaseconjugated streptavidin for exactly 20 minutes, rinsed, and
then developed with 3,3'-diaminobenzidine chromogen for 10 minutes, as
previously described. To facilitate comparability, sections from the
control vehicle-treated and PG-treated eye of each monkey were
immunostained at the same time.21
22
To serve as control
samples for nonspecific staining, sections from each eye were
simultaneously processed by the same protocol but without the primary
antibody.
Immunohistochemical staining intensity was directly measured using a high-resolution imaging densitometer, as previously described.18 23 Measurements from multiple sections stained at the same time facilitated assessment of measurement precision and permitted statistical comparison of differences among control and experimental eyes. Immunostained sections were scanned by placing the slides directly on the platen of an imaging densitometer at a resolution of 1200 dots per inch (50-µm-wide pixels), as previously described. The scanned digital data were displayed in a masked fashion and analyzed using an image analysis program (Molecular Analyst, ver. 2.1; Bio-Rad, Hercules, CA). The optical density (OD) along two line segments positioned perpendicular to the long axis of the ciliary body and near the widest region of the ciliary muscle was measured in each section, by two-dimensionalimaging densitometry. The positioning of these line segments over the anterior segment tissues avoided any remaining cluster of pigment granules.18 23
Confocal Scanning Laser Microscopy
To determine the distribution of the TIGR protein within ciliary
muscle tissue, sections were immunostained with the anti-peptide
antibody. Other sections were immunostained using mouse monoclonal
antibodies to human collagen type IV (1:200; Biogenix, San Rafael, CA)
or with both antibodies. After washing, the sections were incubated
with either goat antibody to rabbit IgG conjugated to a rhodamine
derivative that fluoresces at 510 nm when illuminated by 488 nm light
(Alexa488, diluted 1:200; Molecular Probes, Eugene, OR), goat antibody
to mouse IgG conjugated to a rhodamine derivative that fluoresces at
610 nm when illuminated with 568 nm light (Alexa568, 1:200; Molecular
Probes), or both antibodies. To minimize nonspecific staining, the
secondary antibodies were preabsorbed to a homogenate of human ciliary
muscle tissue before use. The sections were washed and then a coverslip
was mounted using an aqueous mounting medium. Examination was performed
using a two-channel confocal scanning laser microscope (Fluoview;
Olympus, Lake Success, NY) fitted with a krypton-argon laser
(Melles-Griot, Carlsbad, CA).
| Results |
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-IE
-IEtreated eyes were less intensely
stained than in the corresponding vehicle-treated eyes (Figs. 1B
1C)
.
The pattern of immunostaining was similar in the treated and control
eyesthat is, intense staining in the ciliary muscle cell bundles and
less intense staining in the interstitial extracellular matrix.
Quantitative analysis using imaging densitometry revealed that for all
four monkeys analyzed using the antibody to recombinant TIGR protein,
immunoreactivity was less in the
PGF2
-IEtreated eye than in the
vehicle-treated eye. Specifically, there was less intense
immunostaining in the ciliary muscle of the treated eye than in that of
the control eye (Fig. 3A
). Combining these data showed that the average mean OD score reduction
in the treated eyes was 42.1% ± 9.9% (SEM; Fig. 3B
,
P < 0.005, paired Students t-test).
|
| Discussion |
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-IE treatments were associated with a
reduction in the amount of TIGR-protein expression within the monkey
ciliary muscle. Control experiments indicate that this effect was due
to the drug treatment.
As in human ciliary muscle,13
TIGR protein
immunoreactivity was not present in the interstitial extracellular
matrix of monkey ciliary muscle. However, TIGR protein immunoreactivity
was present inside and adjacent to the outside surface of ciliary
muscle cells. This extracellular TIGR immunoreactivity exactly
coincided with immunoreactivity for collagen type IV, which typically
is found in the basement membrane of normal tissues.24
25
Moreover, it appeared to be a substantial fraction of the total TIGR
protein present. Supporting its physiological association with basement
membrane is the observation that a molecular binding site is present in
TIGR protein with affinity for an artificial extracellular matrix
containing normal basement membrane components such as collagen type IV
(Matrigel; Becton-Dickinson, Bedford, MA).16
This may
indicate that PGF2
-IE treatment leads to
reduction of TIGR protein associated with basement membrane
extracellular matrix in the ciliary muscle.
Two antibodies were used in the present investigation. The first was raised against recombinant human TIGR protein, which is unglycosylated. It recognizes an epitope, or epitopes, that appear to be masked for certain extracellular forms of the molecule.16 In contrast, the present confocal microscopy results and prior immunoelectron microscopy results indicate that the anti-peptide antibody recognizes both intracellular and extracellular TIGR protein.15 Therefore, the measurements obtained using this antibody provided an overview of both intracellular and extracellular changes in TIGR protein with treatment. The results obtained with both antibodies were consistent with each other within the sensitivity limits of the present study.
A reduction in TIGR protein associated with basement membrane could be
of significance in uveoscleral outflow. Tracer studies indicate that
uveoscleral flow passes through extracellular spaces among ciliary
muscle bundles.26
If TIGR protein represents a substantial
volume fraction of basement membraneassociated proteins in the
ciliary muscle, it would be plausible that specific reduction of
extracellular TIGR protein would reduce the thickness of the stationary
boundary to the ciliary muscle uveoscleral flow compartment. Such
thickness reduction may produce an enlargement of the uveoscleral flow
compartment and perhaps thereby reduce uveoscleral flow resistance.
Alternatively, if TIGR protein retention in ciliary muscle
extracellular matrix reflects only its binding to collagen type IV,
then the reduction of collagen type IV (that has been previously
documented in the same eyes used in the present study18
may explain the presently observed reduction of TIGR protein. In either
case, the result would be an expansion of the extracellular spaces
within the ciliary muscle that could facilitate increased uveoscleral
outflow. This link between enlargement of the extracellular space in
ciliary muscle and increased uveoscleral outflow facility was first
suggested by early tracer studies27
28
and was
subsequently supported by observation of increases in the size of these
spaces in monkey eyes that had received topical
PGF2
.29
30
Because uveoscleral
flow appears to be predominantly through the interstitial spaces in the
ciliary muscle,26
the significance of TIGR protein and
collagen type IV reduction may be minimal.
A possible explanation for reduction of extracellular TIGR protein with
PGF2
-IE is that it reflects matrix
metalloproteinase (MMP) induction. Previous experiments have shown that
topical PGF2
-IE treatment of monkey eyes
induces reduction of collagen types I, III, and IV in the ciliary
muscle.18
PG treatment of cultured ciliary muscle cells
also leads to reduced collagen types I, III, and IV in the cell
layer.20
Concomitantly, there is increased secretion of
MMPs,31
32
enzymes that can initiate the degradation of
extracellular matrix molecules.33
In view of the close
association of extracellular TIGR protein with collagen type IV in the
ciliary muscle, it is possible that either as a result of MMP action on
collagen type IV, or directly on TIGR protein, there is mobilization of
TIGR protein within the basement membranes facing the interstitial
compartment and that it is subsequently washed out of the ciliary
muscle. Therefore, PGF2
-IEmediated induction
of MMP secretion may explain the reduction of both collagens and
extracellular TIGR protein in the ciliary muscle. These reductions each
could contribute to increased uveoscleral outflow.
Both intracellular and extracellular TIGR protein changes also may
reflect reduced TIGR/MYOC gene expression.
PGF2
treatment of cultured ciliary muscle
cells can increase intracellular messenger signals such as
cAMP.34
Moreover, activation of the FP prostanoid receptor
may trigger other signals associated with G-protein
activation.35
36
TIGR/MYOC gene
expression in trabecular meshwork cells has been shown to be regulated
by several intracellular signals, including steroid receptor
activation, oxidative signals, phorbol esterrelated signals, and
various hormonal signals.2
In astrocytes, several
different DNA-binding elements as well as an upstream regulatory factor
have been shown to modulate TIGR/MYOC gene
expression.14
In view of these observations, it is
possible that PGF2
-IE treatments lead to
altered TIGR/MYOC gene expression in ciliary
muscle cells that lead to reduced TIGR protein biosynthesis. Further
work is needed to determine the regulatory mechanisms specific to
ciliary muscle cells.
It also is possible that PGF2
-IE treatment
influences TIGR protein destined for intracellular targets within
ciliary muscle cells. Ultrastructural immunohistochemical analysis of
TIGR protein distribution in trabecular meshwork in situ and in
cultured trabecular meshwork cells found immunoreactivity in
association with mitochondria and cytoplasmic filaments.15
In the present study, some TIGR protein immunoreactivity was present
intracellularly in ciliary muscle cells. Thus, the reduced TIGR protein
occurring with IOP-lowering PGF2
-IE treatment,
detected using the antibody to recombinant protein, may have
consequences for as yet undefined cytoplasmic functions in ciliary
muscle cells. These considerations support the appropriateness of
further investigations addressing the possible role of TIGR protein in
uveoscleral outflow as well as in other ciliary muscle functions.
| Acknowledgements |
|---|
| Footnotes |
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Submitted for publication November 2, 2000; revised February 26, 2001; accepted April 6, 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: Robert N. Weinreb, Glaucoma Center, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0946. weinreb{at}eyecenter.ucsd.edu
| References |
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treatment reduces collagen types I, III, and IV in the monkey uveoscleral outflow pathway Arch Ophthalmol 117,794-801
Exp Eye Res 47,761-769[Medline][Order article via Infotrieve]
. An ultrastructural study in rhesus and cynomolgus monkeys Mech Ageing Dev 51,101-120[Medline][Order article via Infotrieve]
receptor J Lipid Med Cell Sig 12,405-411[Medline][Order article via Infotrieve]This article has been cited by other articles:
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S. Bakalash, J. Kipnis, E. Yoles, and M. Schwartz Resistance of Retinal Ganglion Cells to an Increase in Intraocular Pressure Is Immune-Dependent Invest. Ophthalmol. Vis. Sci., August 1, 2002; 43(8): 2648 - 2653. [Abstract] [Full Text] [PDF] |
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M. S. Filla, X. Liu, T. D. Nguyen, J. R. Polansky, C. R. Brandt, P. L. Kaufman, and D. M. Peters In Vitro Localization of TIGR/MYOC in Trabecular Meshwork Extracellular Matrix and Binding to Fibronectin Invest. Ophthalmol. Vis. Sci., January 1, 2002; 43(1): 151 - 161. [Abstract] [Full Text] [PDF] |
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