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Transglutaminase activity in the eye: cross-linking in epithelia and connective tissue structures
M Raghunath1, R Cankay, U Kubitscheck
1Department of Dermatology, University of Münster, Germany. raghuna@uni-muenster.de
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 (biotC), which was subsequently visualized with streptavidin-peroxidase. Confocal laser scanning was used to colocalize biotC and fibrillin, a major component of elastic microfibrils and the zonular fibers in particular. Cryosections and isolated bovine zonules were treated with purified TGase 2 and biotC. The distribution of different TGases (1, 2, 3, and factor XIII) was confirmed immunohistochemically.
Results:
Virtually all ocular tissues showed TGase activity with a remarkable preponderance for the ciliary body, zonular fibers, and blood vessel walls. Confocal laser scanning revealed fibrillin-containing microfibrils as a major target for TGase activity, in particular the ciliary zonules. Corneal epithelium and basement membrane showed a TGase cross-linking pattern similar to skin. Treatment of cryosections and isolated bovine zonular fibers with purified TGase 2 led to additional incorporation of biotC into extracellular matrix, particularly zonular fibers. The immunohistochemically predominant TGase 2 was associated with epithelia and particularly with connective tissue fibers. TGase 1 was restricted to the corneal epithelium, whereas factor XIII was found to be associated only with blood vessels. TGase 3 was absent.
Conclusions:
TGase 2 appears to be an important cross-linker and thus stabilizer of ocular connective tissue. In particular, the zonular fibers are a major target for TGase 2. This is of relevance in hereditary microfibrillopathies such as Marfan syndrome, which exhibits distinct ocular manifestations such as elongated bulbus, retinal detachment, and subluxation of the lens. Purified or recombinant TGase might be of therapeutic use in the future.
Insights
Transglutaminase (TGase) activity stabilizes ocular connective tissues, particularly zonular fibers. This finding is relevant for hereditary conditions like Marfan syndrome and suggests potential therapeutic uses for TGase.
Area of Science:
- Ophthalmology
- Biochemistry
- Molecular Biology
Background:
- Transglutaminases (TGases) are enzymes involved in protein cross-linking.
- Ocular tissues possess unique structural components requiring stabilization.
- Understanding TGase distribution is crucial for ocular health and disease.
Purpose of the Study:
- To map the distribution of transglutaminase (TGase) activity within ocular tissues.
- To identify specific ocular structures targeted by TGase for cross-linking.
Main Methods:
- Human and monkey ocular cryosections were incubated with biotinylated cadaverine (biotC).
- Confocal laser scanning visualized biotC colocalization with fibrillin in ocular tissues.
- Immunohistochemistry identified the distribution of TGase 1, 2, 3, and factor XIII.
Main Results:
- Significant TGase activity was observed in the ciliary body, zonular fibers, and blood vessel walls.
- Fibrillin-containing microfibrils, especially ciliary zonules, were identified as major TGase targets.
- TGase 2 was predominant in epithelia and connective tissue fibers, while TGase 1 was in corneal epithelium, and Factor XIII in blood vessels.
Conclusions:
- Transglutaminase 2 (TGase 2) is a key cross-linker stabilizing ocular connective tissue, notably zonular fibers.
- This TGase activity is relevant to ocular manifestations of microfibrillopathies like Marfan syndrome.
- Future therapeutic strategies may involve purified or recombinant TGase for ocular conditions.