The extracellular matrix and its modulation in the trabecular meshwork

A novel gene (oculomedin) induced by mechanical stretching in human trabecular cells of the eye

To understand molecular mechanisms underlying the response to pressure in human trabecular cells of the eye, genes induced by cyclic mechanical stretching were isolated by subtractive hybridization assisted by polymerase chain reaction. A novel gene containing an Alu repetitive element in the 5' untranslated region was identified, and its expression was confirmed by Northern blot analysis to be stretch-specific in trabecular cells. The gene was also expressed in the retina, but not in the other tissues, including the brain. The gene encoded a putative small protein with 44 amino acids, which showed homology with neuromedin K. The putative novel protein was named as "oculomedin," and would be used as a candidate gene for glaucoma.

Alteration of cytoskeletal structure, integrin distribution, and migratory activity by phagocytic challenge in cells from an ocular tissue--the trabecular meshwork

The trabecular meshwork is a specialized tissue in the anterior chamber of the eye that regulates the aqueous humor outflow and controls the intraocular pressure. Cells in the trabecular meshwork are believed to be essential for maintenance of the outflow system, and their malfunctioning may lead to elevation of intraocular pressure and development of glaucoma. These cells are avid phagocytes. Using an in vitro tissue culture system, we have previously shown that bovine trabecular meshwork cells exhibited a short-term loss of cell-matrix adhesiveness after exposure to latex microspheres. The current study showed that 4 h after phagocytosis, the cytoskeletal structure in trabecular meshwork cells was disrupted, the formation of focal contact formation was limited, and the cellular migratory activity was increased. These in vitro responses paralleled those that occur in vivo. By 24 h, all the changes demonstrated returned to normal. Our data suggest that the short-term loss in cell-matrix cohesiveness observed after phagocytic challenge may be related to the reorganization of cytoskeletal structures and the decline of focal contact formation. The altered cell migration may also be interlinked.

Growth factor and cytokine modulation of trabecular meshwork matrix metalloproteinase and TIMP expression

PURPOSE

We hypothesize that regulated trabecular extracellular matrix (ECM) turnover, initiated by the matrix metalloproteinases, is critical for the maintenance of normal aqueous humor outflow rates. However, very little is known about the regulation of trabecular ECM turnover. To identify candidate trabecular regulators, we evaluated the effects of several growth factors and cytokines on trabecular matrix metalloproteinase and TIMP expression.

METHODS

Porcine trabecular meshwork cells were treated with several doses of a variety of growth factors and cytokines and culture media was analyzed after 24, 48, and 72 h. Zymograms were used to evaluate stromelysin, gelatinase A and B activity levels, while immunoblots of Western transfers were used to evaluate stromelysin, collagenase, TIMP-1 and TIMP-2 protein levels.

RESULTS

A phorbol mitogen (TPA), and TNF alpha and beta, interleukin-1 alpha and PDGF BB stimulate gelatinase B, stromelysin, interstitial collagenase and TIMP-1 expression, while having negligible effects on gelatinase A expression; TIMP-2 levels are reduced by TNF but not affected by the other treatments. Acidic and basic FGF, IL-1 beta, TGF beta and PDGF AB produce similar but smaller effects, while HGF, VEGF, EGF, KGF, and LIF produce small to moderate elevations in stromelysin with minimal other responses. PDGF AA, gamma INF, oncostatin-M and endothelin-1 produce negligible changes in these proteinases and inhibitors.

CONCLUSIONS

In addition to providing potential ways to modulate trabecular metalloproteinase and TIMP levels, the responsiveness of these cells to some of these growth factors and cytokines suggests possible roles in normal or pathogenic trabecular cell regulation and some may affect aqueous humor outflow.

In situ visualization of messenger RNA for basic fibroblast growth factor in living cells

We examined whether messenger RNA for basic fibroblast growth factor (bFGF) could be visualized specifically by a fluorescent probe in living cells. A 15-nucleotide-long antisense or sense sequence for human bFGF was sandwiched between two complementary 5-nucleotide-long arm sequences. A fluorophore, 5-(2'-aminoethyl)aminonaphthalene-1-sulfonic acid (EDANS), was joined to the 5'-terminal phosphate, while 4-(4'-dimethylaminophenylazo)benzoic acid, quencher for EDANS, was joined to the 3'-terminal hydroxyl group. The probe emitted blue fluorescence only upon hybridization with the complementary 18-nucleotide-long sequence under ultraviolet light. The antisense or sense probe carried with liposome was delivered to human cells, trabecular cells of the eye, in a glass-bottom culture dish placed on the stage of an inverted microscope. Cells with the antisense probe did, but not with the sense probe, show blue fluorescence under ultraviolet light. The present study opens a way to measure the changing levels of a specific messenger RNA in living cells.

Effects of ascorbic acid on levels of fibronectin, laminin and collagen type 1 in bovine trabecular meshwork in organ culture

PURPOSE

Fibronectin, laminin and collagen type I are important extracellular matrix products of trabecular meshwork cells. This study was performed to examine the effects of ascorbic acid, a significant component in the aqueous humor, on the levels of these proteins in trabecular meshwork cells maintained in organ culture.

METHODS

The anterior segment of freshly enucleated bovine eyes was perfused in a modified organ culture system. Three cultures were set up simultaneously. One received serum-free medium containing 100 micrograms/ml of ascorbic acid, one received 250 micrograms/ml of ascorbic acid and one served as a control. After 72 h, the tissues were processed for paraffin sections and immunostaining was conducted using an avidin-biotin-peroxidase complex method. Western blot and dot blot assays were performed on tissue extracts.

RESULTS

Compared with the controls, the staining for fibronectin and laminin was markedly enhanced in trabecular meshwork tissues treated with both concentrations of ascorbic acid. Increased collagen type I production by trabecular meshwork cells was also demonstrated in the presence of ascorbic acid. Western blot and dot blot results confirmed the immunostaining findings.

CONCLUSIONS

Ascorbic acid promotes production of fibronectin, laminin and collagen type I by trabecular meshwork cells. The organ culture results are consistent with those obtained previously from tissue culture studies.

A mechanism for trabecular meshwork cell retraction: ethacrynic acid initiates the dephosphorylation of focal adhesion proteins

Ethacrynic acid (ECA) increases aqueous humor outflow facility in human and animal model systems, and causes cellular retraction in cultured trabecular meshwork (TM) cells. ECA-induced retraction, a possible correlate to the opening of spaces in the outflow pathway in vivo, takes place coincident with disruption of cell-cell attachments and actin stress fibers. Tyrosine phosphorylated proteins are located predominantly where actin filaments terminate at sites of cell-to-cell and cell-to-substrate adhesion, and are understood to regulate cellular adhesions and filamentous (F) actin organization in many cell types. In the present study we investigated whether ECA might affect cell adhesions and F-actin in TM cells by altering levels of phosphotyrosine. We analysed levels of phosphotyrosine in cultured human TM and calf pulmonary artery endothelial cells after exposure to ECA. Using immunoflourescence microscopy and antibodies to phosphotyrosinated proteins we found a rapid decrease in phosphotyrosine levels at the focal contacts of cells treated with ECA. Immunoblots of whole cell extracts showed a decrease in phosphotyrosine predominantly in a band running at about 120 kD, with a more subtle decrease in a band about 65 kD. Reprobing the blot with antibodies to pp120 focal adhesion kinase (FAK) or paxillin indicated that the 120 kD band was FAK and the 65 kD band was likely paxillin. Immunoprecipitation of FAK or paxillin and probing the resulting blot with antibodies to phosphotyrosine confirmed that these proteins were rapidly dephosphorylated after ECA addition. Loss of FAK and paxillin proteins in cells was then confirmed using immunofluorescence microscopy. Dephosphorylation of these proteins was detected before the onset of retraction, stress fiber disruption, or complete disruption of focal adhesions. A pure microtubule inhibitor (colchicine), did not cause stress fiber disruption or decrease focal adhesion phosphorylation. We postulate that dephosphorylation of FAK and paxillin by ECA disrupts signaling pathways that normally maintain the stability of the actin cytoskeleton and cellular adhesions, and that this action leads both to cell shape change in culture, and to facility changes in vivo.

Proteoglycan expression by human trabecular meshworks

PURPOSE

Proteoglycans may serve important roles in trabecular meshwork structure or function. Detailed molecular characterization and identification of specific trabecular proteoglycan core proteins has been limited.

METHODS

Radiolabeled proteoglycans were extracted from cultured human trabecular meshworks and subjected to ion exchange and molecular sieve chromatography. Peaks were subjected to glycosaminoglycan content analysis. Reverse transcription with polymerase chain reaction was used to identify trabecular mRNAs of several common proteoglycan core proteins. Western immunoblots of trabecular extracts were also utilized to identify these proteoglycan core proteins.

RESULTS

The proteoglycans elute from ion exchange columns at high salt and molecular sieve column profiles, and they exhibit broad peaks typical of the proteoglycan microheterogeneity seen in other tissues. The four common glycosaminoglycan side-chains were identified on these proteoglycans. Trabecular cells in organ or cell culture contain mRNAs coding for decorin, biglycan, versican, perlecan and a basement membrane glycoprotein, SPARC. Syndecan-1 transcripts were present at very low levels, while aggrecan transcripts were not detectable. Decorin, biglycan, versican and perlecan core proteins were also identified by immunoblots of trabecular cell extracts.

CONCLUSIONS

Several common proteoglycans are expressed by trabecular cells in organ explant or cell culture. Their general characteristics are not unlike those found in other tissues. These proteoglycans may serve important functions in the trabecular outflow pathway.