Abstract
Purpose: :
Glucocorticoids (GCs) block photoreceptor cell death in inherited and light-induced retinal degenerations. Steroids can exert potent antiapoptotic effects through different pathways. However, GC-dependent survival pathways in the retina are still controversial. Therefore we evaluated the effects of GC receptor (GR) inhibition on light-induced photoreceptor degeneration and sought a correlation between these effects and levels of the antiapoptotic protein Bcl-XL.
Methods: :
Experimental work was done according to the ARVO Statement for the use of animals. Male Balb-c mice (35-45 days-old), bred under standard illumination conditions (12:12 h light: dark; < 60 lux), remained in complete darkness for 24 h. They were divided in two groups: those returning to standard illumination, and those exposed to 1,500 lux during 2-4 days. Animals from both groups received mifepristone (MFP, a GR inhibitor) or dexamethasone (DEX) or saline. Photoreceptor degeneration was evaluated by histology, cleaved caspase-3 (CC3) immunostaining and opsin protein levels. GR and Bcl-XL was immunochemically detected in cryosections and Western blots.
Results: :
In normal mice, GR-ir appeared in cell nuclei from all retinal layers. Light-exposed retinas showed stronger immunostaining. After 2 days under 1,500 lux, retinas displayed shortening of photoreceptor outer segments and decreased opsin. Damage after 4 days included loss of photoreceptor cell nuclei and decrease of opsin below detection levels. Opsin decrease at 2 days was prevented by DEX and increased by MFP administration. GR inhibition also increased the number of CC3+ cell nuclei. Opsin levels were unchanged in animals remaining under standard illumination, with or without DEX or MPF administration.Animals exposed to 1,500 lux during 4 days and returned to standard illumination for another 6 days showed partial recovery of opsin levels. Opsin recovery did not occur in mice receiving MFP during light exposure.Bcl-XL levels decreased after 2 days under 1,500 lux. This effect was prevented by DEX. Administration of MFP determined a large reduction of Bcl-XL levels, even in animals remaining under standard illumination.
Conclusions: :
Our observations indicate that MFP increased photoreceptor damage and death after exposure to toxic light levels. GR-mediated activation of the antiapoptotic molecule Bcl-XL would be an important component of photoreceptor response to light injury. Retinas deficient in Bcl-XL might be abnormally sensitive to apoptotic stimuli.
Keywords: photoreceptors • retinal degenerations: cell biology • apoptosis/cell death