Lysosomal membrane permeabilization and autophagy blockade contribute to photoreceptor cell death in a mouse model of retinitis pigmentosa

Abstract

Retinitis pigmentosa is a group of hereditary retinal dystrophies that normally result in photoreceptor cell death and vision loss both in animal models and in affected patients. The rd10 mouse, which carries a missense mutation in the Pde6b gene, has been used to characterize the underlying pathophysiology and develop therapies for this devastating and incurable disease. Here we show that increased photoreceptor cell death in the rd10 mouse retina is associated with calcium overload and calpain activation, both of which are observed before the appearance of signs of cell degeneration. These changes are accompanied by an increase in the activity of the lysosomal protease cathepsin B in the cytoplasm of photoreceptor cells, and a reduced colocalization of cathepsin B with lysosomal markers, suggesting that lysosomal membrane permeabilization occurs before the peak of cell death. Moreover, expression of the autophagosomal marker LC3-II (lipidated form of LC3) is reduced and autophagy flux is blocked in rd10 retinas before the onset of photoreceptor cell death. Interestingly, we found that cell death is increased by the induction of autophagy with rapamycin and inhibited by calpain and cathepsin inhibitors, both ex vivo and in vivo. Taken together, these data suggest that calpain-mediated lysosomal membrane permeabilization underlies the lysosomal dysfunction and downregulation of autophagy associated with photoreceptor cell death.

Alterations to retinal architecture prior to photoreceptor loss in a mouse model of retinitis pigmentosa

“…We know that cells respond to the mutation before any signs of degeneration in the ONL. For example, the rd10 retina presents signs of ER stress and calpain activation at P15 (Rodriguez-Muela et al, 2015). It is clear from the above studies that cell types outside the ONL are affected by the degeneration of photoreceptors.…”

Pharmacological modulation of autophagy: therapeutic potential and persisting obstacles

“…Consistent with this, the cone-restricted deletion of Atg5 results in a functional decline that is accompanied by the accumulation of damaged mitochondria, an increased sensitivity to light toxicity and glucose deprivation 266 . Moreover, the inhibition of autophagy reportedly contributes to photoreceptor cell loss in models of <m>retinitis pigmentosa</m> 267 . ATG5-dependent and BECN1-dependent LAP has been involved in a process known as the ‘visual cycle’, in which shedding photoreceptor outer segments are engulfed by the retinal pigment epithelium (RPE), and all- trans -retinoic acid is converted to 11- cis -retinal for photoreceptor regeneration.…”

Autophagy: A Role in the Apoptosis, Survival, Inflammation, and Development of the Retina

“…Rodríguez-Muela et al [39] found that autophagic flux was blocked in rd10 retinas before the onset of photoreceptor cell death, suggesting that autophagy blockade contributes to photoreceptor cell death. Moreover, Chen et al [29] discovered that the autophagosome marker LC3B-II was significantly increased in response to light exposure at a dose to cause retinal degeneration in wild-type mice but not in autophagy-deficient mice, suggesting that inadequate autophagy resulted in light-induced retinal degeneration.…”