Caspase-Independent Photoreceptor Apoptosis in Mouse Models of Retinal Degeneration

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.

Receptor interacting protein kinase mediates necrotic cone but not rod cell death in a mouse model of inherited degeneration

“…Indeed, morphological features of rod cell death are different in these two models: although rod cell death in rd10 mice is caused mostly by apoptosis, both apoptosis and necrosis are intermingled in retinal detachment (21,51), supporting the idea that different molecules may be involved in each condition. Although rods undergo apoptosis in inherited retinal degeneration, previous studies have shown that caspase activation is not observed in rd1 mice, which carry a viral insert and a nonsense mutation in exon 7 of the Pde6β gene (23). Taken together with our results, these data suggest that caspase and RIP kinase pathways may not be essential for rod cell death in RP models caused by Pde6β mutations.…”

“…However, attempts to inhibit caspases by pharmacologic or genetic interventions have failed to show effective protection against rod cell apoptosis (23,24). Recent studies have demonstrated that caspase-independent molecules, such as calpains and poly(ADP ribose) polymerases, which mediate not only apoptosis but also necrosis, are activated during rod cell death (25,26).…”

Multiple death pathways in retina-derived 661W cells following growth factor deprivation: crosstalk between caspases and calpains

“…30,41 They represent an invaluable tool for the study of cone PCD, given the detection problems that can arise in studying dynamic events in cells that account for a mere 2.8% of total photoreceptors in the mouse retina. 28 In spite of the fact that previous studies in our group ruled out the involvement of caspases in several in vitro and in vivo models of photoreceptor degeneration, [42][43][44] (data not shown) and -12 were active during apoptosis of 661W cells. This conflicting data can readily be explained if one considers the differences between the models employed.…”

Excessive Activation of Poly(ADP-Ribose) Polymerase Contributes to Inherited Photoreceptor Degeneration in the Retinal Degeneration 1 Mouse

“…Although apoptosis often involves caspases, these play only a minor role, if any, in the rd1 degeneration. For instance, relevant caspase activation is absent during the degeneration (Kim et al, 2002;Doonan et al, 2003) and interventions of the caspase cascade fail to preserve the rd1 photoreceptors (Yoshizawa et al, 2002;Zeiss et al, 2004).…”