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From the Glaucoma Center, University of California San Diego, La Jolla, California.
| Abstract |
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METHODS. Primary cultures of HSFs were established from donor eyes. Also, sclera from human donor eyes was snap frozen and sectioned. Immunocytochemistry was performed on HSFs and tissue sections with subtype-specific antibodies to the EP1, EP2, EP3, EP4, and FP receptors. The presence of mRNA for the receptor subtypes was examined from total RNA obtained from human sclera and confirmed with restriction digest analysis.
RESULTS. Positive EP1 and FP receptor immunoreactivity was observed in fibroblasts within the sections from human sclera. In primary cultures of HSFs, EP1 and FP labeling was observed over the entire cell surface. EP2 immunoreactivity within HSFs was mostly present in the juxtanuclear region. RT-PCR analysis of total RNA isolated from human sclera and HSFs confirmed the presence of EP1, EP2, and FP receptor subtypes. The identity of the polymerase chain reaction products was confirmed by restriction enzyme analysis. No mRNA or immunoreactivity above basal levels was detected for the EP3 and EP4 prostanoid receptor subtypes in tissue sections or primary cultures.
CONCLUSIONS. The EP1, EP2, and FP receptor subtypes are present in HSFs, suggesting that these cells may respond to endogenous PGs and their structural analogues through interaction with these receptor subtypes.
| Introduction |
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analogue
latanoprost is used widely to lower intraocular pressure (IOP) and
treat human glaucoma.3
Although the precise mechanism for the IOP-lowering effect of
latanoprost after topical application is not known, there is
considerable evidence that supports the idea that
PGF2
, and possibly latanoprost, lowers IOP by
enhancing uveoscleral outflow.4
5
6
7
It is possible that
this is mediated by activation of prostanoid receptors in ciliary
muscle and other tissues within the uveoscleral outflow pathway.
However, our knowledge of the distribution of the prostanoid receptor
subtypes in the eye is still limited. This paucity of knowledge
prevents a complete understanding of the ocular effects of the PGs and
their analogues.
The uveoscleral outflow pathway consists of the iris root, the ciliary muscle cells, the supraciliary and suprachoroidal spaces, and the sclera.8 The extent to which the sclera is involved in the uveoscleral outflow pathway is not known. Sclera is composed of collagen fibrils embedded within a proteoglycan matrix. The collagen and proteoglycans in the sclera are produced and maintained by fibroblasts. These fibroblasts may play an integral part in the regulation of the scleral extracellular matrix. To determine whether scleral fibroblasts might express receptors for endogenous or topically applied PGs, we examined the expression of the EP1, EP2, EP3, EP4, and FP receptor subtypes in human sclera and in cultured scleral fibroblasts.
| Materials and Methods |
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Connective tissue, blood vessels, muscle, and conjunctiva were dissected from the exterior surface of the globes. The anterior chamber was removed by equatorial incision approximately 4 to 5 mm behind the corneal limbus. After cutting the zonule, the lens was removed and the remaining anterior segment was embedded in optimal cutting temperature compound (OCT; Tissue-Tek, Miles, Inc., Elkhart, IN) and snap frozen by immersion in a dry-ice and ethanol bath. Sections 10 µm thick were cut using a cryostat and then placed on positive-charged microscope slides (OptiPlus; BioGenex, San Francisco, CA).
Primary human scleral fibroblast (HSF) cultures were established from whole-tissue explants. Briefly, an incision was made (35 mm) anterior to the optic nerve head. Scleral pieces approximately 1 to 2 mm thick were placed uveal side down under sterile coverslips. Tissue segments were incubated in DMEM-F-12 medium supplemented with 10% fetal calf serum (FCS; Gemini, Calabasas, CA), 100 U/ml penicillin, 100 µg/ml streptomycin, 2.5 mg/ml amphotericin B (Fungizone; all antibiotics from Gibco BRL, Grand Island, NY) and 1 ng/ml basic fibroblast growth factor (R&D Systems, Minneapolis, MN). After 3 to 4 weeks, primary cells were passaged once to 75-cm2 flasks to establish the cell lines and then to 35-mm dishes for analysis.
Isolation of RNA
Tissue fragments were homogenized in 10 ml of reagent (TRIzol;
Gibco BRL; Polytron homogenizer; Brinkman Instruments, Westbury, NY).
The homogenate was transferred to sterile 1.5-ml microcentrifuge tubes
in 1-ml aliquots and incubated for 5 minutes at 25°C. Chloroform (200
µl) was added to each tube. The tubes were mixed by brief agitation
and incubated for an additional 3 minutes at 25°C. The samples were
then centrifuged (12,000g) for 15 minutes at 4°C, and the
aqueous phase was carefully transferred to fresh sterile 1.5-ml
microcentrifuge tubes. Isopropanol (500 µl/tube) was added and
allowed to incubate for 10 minutes at 25°C to initiate RNA
precipitation. Samples were centrifuged (12,000g) for 10
minutes at 4°C and the supernatant was removed. The RNA pellet was
washed with 75% ethanol-diethylpyrocarbonate (DEPC)-treated water and
air dried. RNA was resuspended in a total volume of 40 µl of
DEPC-treated water, and quality was checked by gel electrophoresis.
Reverse TranscriptionPolymerase Chain Reaction
RT-PCR was performed essentially as previously
described,9
with total RNA isolated from human sclera.
Primers were chosen to amplify unique regions within the individual
human FP and EP receptor subtypes. All PCR primer pairs were 100%
homologous with the reported cloned sequences of the human PG receptors
(Table 1) . The PCR reaction mix (final volume, 50 µl) contained 5 µl
of the RT reaction, 5 µl of 10x PCR buffer (Gibco BRL), 1 µl of 10
mM dNTP mixture, 1.5 µl of 50 mM MgCl2, 2.5
µl of the sense and antisense primers (20 µM), and 0.5 µl
Taq polymerase (5 U/µl; Gibco BRL). The PCR program
consisted of an initial step at 95°C for 3 minutes, followed by 30
cycles at 95°C for 1 minute, 55°C for 1 minute, and 72°C for 1
minute, and a final step at 72°C for 7 minutes. Products were
analyzed by electrophoresis in 1% agarose gels. The human plasmids
encoding the EP1, EP2,
EP3, EP4, and FP receptors
used for the positive controls of the RT-PCR reactions were a gift from
John Regan (University of Arizona, Tucson, AZ).
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For immunolabeling, human sclera tissue (10-µm-thick anterior segments) were postfixed in 4% paraformaldehyde and phosphate-buffered saline (PBS) for 10 minutes at room temperature. Sections were washed once in PBS and then placed in 30 mM sodium chloride and 300 mM SSC buffer for 10 minutes at room temperature. Sections were placed in 100 mM glycine solution for 15 minutes, then washed in SSC buffer and permeabilized with SSC containing 0.1% Triton X-100 for 1 hour. After an overnight incubation at 4°C with the primary antibody (1:1001:300 dilution), the sections were washed with SSC, incubated for 1 hour at room temperature with secondary antibody at a dilution of 1:1000, washed again, and mounted under coverslips for viewing. Before immunostaining, HSFs were grown on glass coverslips. The cells were fixed with 4% formaldehyde and PBS solution (freshly prepared from paraformaldehyde) for 5 minutes at room temperature. Labeling with the antibodies was performed as previously described.
| Results |
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Human PG Receptor Subtypes in Primary Cultured Scleral Fibroblasts
To evaluate the expression of PG receptors in HSFs, primary HSF
cultures were established from tissue explants and immunostained using
each of the prostanoid receptor antibodies. Immunoreactivity for the
EP1, EP2, and FP receptors
was detected in second-passage cultures of HSFs (Fig. 2)
. Immunoreactivity for the EP1 and FP receptor
antibodies was observed within every cell in the culture and was evenly
distributed over the entire cell surface. In contrast, the
immunoreactivity for the EP2 receptor antibody
was much weaker and more diffuse and appeared to be primarily localized
to the juxtanuclear region of the HSFs. This difference may account for
the absence of EP2 immunoreactivity in the
scleral sections. There was no difference in this staining pattern
among second-, third-, or fourth-passage cultures. As with the tissue
labeling, we were unable to detect EP3 and
EP4 receptor immunoreactivity (Figs. 2D
2E)
.
These data are consistent with the labeling observed in tissue sections
and further suggest that the receptor expression can be maintained in
culture for several weeks.
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| Discussion |
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and PGE2 in the
human ciliary muscle, but did not observe scleral binding sites.
Ocklind et al.18
found immunohistochemical FP receptor
expression in the ciliary muscle and sclera of the monkey
eye.18
However, in situ hybridization experiments in the
same study did not detect FP receptor transcripts in sclera.
EP1, EP2, and
EP4 receptors also have been identified in human
ciliary muscle cells, using RT-PCR, by Mukhopadhyay et
al.13
The same investigators recently used in situ
hybridization to identify mRNA transcripts for
EP1 and FP receptors in the blood vessels of
iris, the choroid, and ciliary muscle fibers in a human
eye.11
However, in neither of these studies were PG
receptors observed in sclera. To the best of our knowledge, there has
not been any prior observation of FP, EP1, and
EP2 receptors with human sclera. Perhaps these
prior methods did not have sufficient sensitivity to detect weak
signals from the low-density scleral fibroblasts.
PGF2
and latanoprost, a
PGF2
analogue, reduce IOP by initiating a
cascade of cellular events that lead to increased uveoscleral
outflow.19
20
21
22
Although the effects of this cascade are
not fully understood, they may in part increase scleral permeability.
Kim et al.23
found that PG treatment of isolated sclera
result in a dose-dependent and time-dependent increase in dextran
permeability. This increase in transscleral permeability may be related
to the decreased collagen immunoreactivity and increased matrix
metalloproteinase (MMP) immunoreactivity that also have been observed
in the sclera of monkey eyes after topical
PGF2
-isopropyl ester
treatment.24
25
It is possible that one or more of the PG
receptors on scleral fibroblasts mediate these changes.
In conclusion, the data presented in this study demonstrate that EP1, EP2, and FP receptors are present in human scleral fibroblasts. It is not yet known whether activation of the EP1, EP2, or FP receptors influences MMP production by scleral fibroblasts or alter transscleral permeability.
| Acknowledgements |
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| Footnotes |
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Submitted for publication June 19, 2001; revised August 15, 2001; accepted September 5, 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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increases uveoscleral outflow in the cynomolgus monkey Exp Eye Res 49,389-402[Medline][Order article via Infotrieve]
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on trabecular outflow facility in cynomolgus monkeys Exp Eye Res 51,87-91[Medline][Order article via Infotrieve]
receptors in the human trabecular meshwork Invest Ophthalmol Vis Sci 39,315-321
analogue Adv Prostaglandin Thromboxane Leukot Res 23,513-518[Medline][Order article via Infotrieve]
ester prodrugs and receptor selective prostaglandin analogs J Ocul Pharmacol 10,177-193[Medline][Order article via Infotrieve]
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on calcium efflux Invest Ophthalmol Vis Sci 33,2679-2686
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treatment reduces collagen types I, III, and IV in the monkey uveoscleral outflow pathway Arch Ophthalmol 117,794-801
-isopropyl ester Arch Ophthalmol 119,1165-1170This article has been cited by other articles:
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