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Imaging Ca2+ Dynamics in Cone Photoreceptor Axon Terminals of the Mouse Retina
Published on: May 6, 2015
Caspase-independent photoreceptor apoptosis in mouse models of retinal degeneration
Francesca Doonan1, Maryanne Donovan, Thomas G Cotter
1Tumour Biology Laboratory, Biochemistry Department, Bioscience Research Institute, University College Cork, Cork, Ireland.
Abstract:
Apoptosis is the mode of cell death in retinitis pigmentosa, a group of retinal degenerative disorders primarily affecting rod photoreceptors. Although caspases have been demonstrated to play a central role in many incidences of apoptosis, accumulating evidence suggests that they may not be required for all forms of apoptotic cell death. The present study examined the mechanism of cell death in two in vivo models of photoreceptor apoptosis: the retinal degeneration (rd) mouse, a naturally occurring mutant model, and N-methyl-N-nitrosourea-induced retinal degeneration. Specifically, we examined the activation status of caspase-9, -8, -7, -3, and -2 and determined the caspase requirements for cytochrome c release, DNA fragmentation, and apoptosis-associated proteolysis of specific caspase substrates. We show that apoptosis in both in vivo models is independent of caspase-9, -8, -7, -3, and -2 activation. DNA fragmentation occurs in the absence of caspase-mediated ICAD (inhibitor of caspase-activated DNase) proteolysis, suggesting that an alternative endonuclease is responsible for DNA cleavage in these models. Importantly, we show that apoptosome activation is prevented because of an absence of mitochondrial cytochrome c release. Experiments performed using a cell-free system indicate that cytochrome c-dependent proteolysis and activation of caspase-9 can be restored in a neonatal cell-free system. However, we found that cytochrome c-dependent proteolysis and activation of caspase-9 could not be restored in an adult cell-free system because of an age-related decrease in the expression of Apaf-1 in the normal developing mouse retina. In the rd mouse, however, this age-related downregulation of apoptotic proteins was not observed, highlighting a critical feature of this model and the prevention of cytochrome c release as an apical event in caspase-independent apoptosis in this system.
Insights
Apoptosis in retinitis pigmentosa occurs independently of caspases. This study reveals that cytochrome c release is prevented, leading to caspase-independent cell death in retinal degeneration models.
Area of Science:
- Ophthalmology
- Cell Biology
- Molecular Biology
Background:
- Apoptosis, or programmed cell death, is central to retinitis pigmentosa, a group of retinal degenerative disorders affecting rod photoreceptors.
- While caspases are key mediators of apoptosis, their necessity in all forms of cell death is debated.
- Understanding the precise mechanisms of photoreceptor apoptosis is crucial for developing therapeutic strategies.
Purpose of the Study:
- To investigate the mechanism of photoreceptor apoptosis in two in vivo models: the retinal degeneration (rd) mouse and N-methyl-N-nitrosourea-induced retinal degeneration.
- To determine the role of caspases in cytochrome c release, DNA fragmentation, and substrate proteolysis during photoreceptor cell death.
- To explore the factors influencing caspase activation and apoptosome formation in retinal degeneration.
Main Methods:
- Assessed caspase activation (caspase-9, -8, -7, -3, -2) in rd mice and chemically induced retinal degeneration models.
- Evaluated cytochrome c release from mitochondria, DNA fragmentation, and caspase substrate cleavage.
- Utilized cell-free systems (neonatal and adult) to investigate the restoration of caspase activation and proteolysis.
- Examined the expression levels of Apaf-1 in developing and adult mouse retinas.
Main Results:
- Photoreceptor apoptosis in both models proceeded independently of caspase activation.
- DNA fragmentation occurred without caspase-mediated cleavage of ICAD (inhibitor of caspase-activated DNase), suggesting alternative endonucleases.
- Apoptosome activation was blocked due to the absence of mitochondrial cytochrome c release.
- While caspase activation could be restored in a neonatal cell-free system, it failed in an adult system due to decreased Apaf-1 expression.
- The rd mouse model did not exhibit age-related downregulation of apoptotic proteins, unlike the normal retina.
Conclusions:
- Retinal degeneration in the studied models involves caspase-independent apoptosis.
- The prevention of mitochondrial cytochrome c release is an apical event in this caspase-independent cell death pathway.
- Age-related changes in Apaf-1 expression may influence caspase activation capacity in adult retinas.
- The rd mouse model presents a unique system for studying caspase-independent apoptosis due to the absence of this age-related downregulation.
