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M. Donovan; Differential Expression of Bcl–2 Family Members During Retinal Development and Age–dependent Susceptibility of Photoreceptors to the Mitochondrial Cell Death Pathway . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1645.
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Purpose: We have previously demonstrated that photoreceptor cell death in several mouse models of retinal degeneration occurs independently of caspase activation and that preclusion of cytochrome–c release is an apical event in these systems. In contrast, retinal cell death during development appears to involve caspase activation. To determine why these cells employ different pathways of cell death depending on their stage of development, we examined the expression pattern of several Bcl–2 family members. Methods: Cell death was analyzed by TUNEL during postnatal development (postnatal day 7 and 8) and following light exposure (5000 lux for 2hrs). Immunohistochemistry (IHC) was employed to detect caspase–9 and caspase–3 cleavage. Expression of pro–apoptotic Bcl–2 family members during light–induced photoreceptor apoptosis and retinal development was determined by RT–PCR and western blot. Various death stimuli known to target the mitochondria including oxidative stress, calcium overload and ultraviolet light were employed to induce death in retinal explants at various stages of development. Results: We show that pro–apoptotic members such as Bax, Bak, Bid, and the three isoforms of Bim are down regulated during postnatal retinal development and are not re–expressed in the adult retina during light–induced retinal degeneration. Furthermore, we demonstrate that early postnatal retinal explants exposed to various stimuli display typical features of apoptosis including TUNEL labelling and caspase–9 and –3 activation. However, adult retinal explants exposed to the same insults are completely resistant to apoptosis even under conditions of increasing stress. Conclusions: Our results indicate that transcriptional regulation of ‘death genes’ such as pro–apoptotic Bcl–2 family members affords protection in adult post–mitotic neurons by preventing execution of the archetypal caspase–dependent pathway. These findings have important implications for the pathophysiology of photoreceptor cell death in adult retinal disorders.
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