97th DOG Annual Meeting 1999



U. Welge-Lüßen1,2 ,E. Lütjen-Drecoll1

Purpose: In most eyes with primary open angle glaucoma (POAG) aqueous outflow resistance is increased. The mechanism of diminished outflow is attributed to the accumulation of extracellular matrix (ECM). Ultrastructural investigations revealed that there is not only an increase in ECM, but that parts of the ECM appeared as crosslinked plaque-like aggregates. Tissue transglutaminase (tTgase) is a calcium dependent enzyme which catalyzes the cross-linking of e -(g -glutamyl) lysine bonds. This crosslinking is highly resistant to degradation. As in a number of glaucoma eyes an increase in transforming growth factor-beta (TGF-ß) is present in the auqueous humor we studied the expression of tTgase by human trabecular meshwork (HTM) as well as the influence of TGF-ß on the expression of tTgase in cultured HTM cells.

Material and methods: Anterior segments of 6 normal human eyes were stained with antibody to tTgase. Monolayer cultures of HTM cells from eyes of 5 human donors were treated with TGF-ß1, TGF-ß2 and dexamethasone (DEX) for 12-96 h. Induction of tTgase and the related mRNA was investigated by western- and northern-blot analysis. External tTgase activity was measured by the ability to form polymerized fibronectin and the incorporation of biotinylated cadaverine into fibronectin.

Results: Labeling for tTgase occurred in the entire HTM. Cultured HTM cells expressed tTgase intra- and extracellularly. Treatment of cultured HTM cells with TGF-ß and TGF-ß2 increased the expression of tTgase and the corresponding mRNA, whereas DEX had no effect. TGF-ß treated HTM cells showed a significant higher increase in polymerized and unpolymerized fibronectin. Incorporation of biotinylated cadaverine was markedly increased when HTM cells were treated 24 h with TGF-ß before seeding.

Conclusion: The enzyme tTgase is expressed in the HTM, and inducible by TGF-ß in cultured HTM cells. The extracellular tTgase is functionally active to polymerize fibronectin. It is therefore tempting to speculate, that increased levels of TGF-ß2 in the aqueous humor, leads to an increase of tTgase expression and activity in the HTM, causing an increase of irreversibly crosslinked ECM proteins. This mechanism might be essential for increased outflow resistance in glaucomatous eyes.

1Anatomie II, Universitätsstr. 19, D-91054 Erlangen
2 Univ.-Augenklinik, Mathildenstr. 8, D-80336 München