Transglutaminase activity in the eye: Cross-linking in epithelia and connective tissue structures

PURPOSE. To assess the distribution of transglutaminase (TGase) activity in ocular tissues and the target structures for cross-linking. METHODS. Cryosections from human and cynomolgus monkey eyes were incubated with the biotinylated amine donor substrate cadaverine (biot), which was su bsequently visualized with streptavidin-peroxidase. Confocal laser scanning was used to colocalize biotC and fibrillin, a major component of elastic m icrofibrils and the zonular fibers in particular. Cryosections and isolated bovine zonules were treated with purified TGase 2 and biotC. The distribut ion of different TGases (1, 2, 3, and factor; XIII) was confirmed immunohis tochemically. RESULTS. Virtually all ocular tissues showed TGase activity with a remarkab le preponderance for the ciliary body, zonular fibers, and blued vessel wal ls. Confocal laser scanning revealed fibrillin-containing microfibrils as a major target for TGase activity, in particular the ciliary zonules. Cornea l epithelium and basement membrane showed a TGase cross-linking pattern sim ilar to sl;in. Treatment of cryosections and isolated bovine zonular fibers with purified TGase 2 led to additional incorporation of biotC into extrac ellular matrix, particularly zonular fibers. The immunohistochemically pred ominant TGase 2 was associated with epithelia and particularly with connect ive tissue fibers. TGase I was restricted to the corneal epithelium, wherea s factor XIII was found to be associated only with blood vessels. TGase 3 w as absent. CONCLUSIONS. TGase 2 appears to be an important cross-linker and thus stabi lizer of ocular connective tissue. In particular, the zonular fibers are 3 major target for TGase 2. This is of relevance in hereditary microfibrillop athies such as Marfan syndrome, which exhibits distinct ocular manifestatio ns such as elongated bulbus, retinal detachment, and subluxation of the len s. Purified or recombinant TGase might be of therapeutic use in the future.