Tissue transglutaminase in apoptosis of photoreceptor cells in rat retina

Purpose, The possible involvement of tissue transglutaminase (tTG) in apoptosis during photoreceptor degeneration was examined in retinal photic injury in rats and in retinal dystrophy of Royal College of Surgeons (RCS) rats. Methods, Retinal photic injury was induced in 48 male Lewis albino rats by exposure to green fluorescent light of 300 to 320 foot-candles. The retinal tTG was examined by enzyme assay, immunohistochemistry, and Western blot analysis after 9, 12, or 24 hours of exposure or at 6 or 24 hours of dark adaptation after 24 hours of light exposure. Retinas from RCS rats at various stages of degeneration also were examined with similar methods. Results, There was a progressive increase in retinal tTG activity after 300 to 320 ft-c of light exposure, reaching a peak after 24 hours of light exposure. In the RCS rats, tTG activity increased with age. Western blot analysis revealed an immunoreactive band at 80 kDa, which increased in accordance with the transglutaminase activity in both models. In normal, rat retinas, tTG immunolabeling was present only in the outer segments. There was an increased number of immunolabeled photoreceptor nuclei from 12 hours of light exposure to 24 hours of light exposure. In the RCS rat, increasing numbers of immunopositive photoreceptor nuclei from 20 to 50 days of age were noted. Conclusions. The data associated increased retinal tTG activity and enzyme levels with photoreceptor cells undergoing apoptosis. The tTG-dependent irreversible cross-linking of intracellular protein may play an important role in causing the structural changes in cells undergoing apoptosis in the retina.