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

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.

Autophagy promotes survival of retinal ganglion cells after optic nerve axotomy in mice.

Autophagy is an essential recycling pathway implicated in neurodegeneration either as a pro-survival or a pro-death mechanism. Its role after axonal injury is still uncertain. Axotomy of the optic nerve is a classical model of neurodegeneration. It induces retinal ganglion cell death, a process also occurring in glaucoma and other optic neuropathies. We analyzed autophagy induction and cell survival following optic nerve transection (ONT) in mice. Our results demonstrate activation of autophagy shortly after axotomy with autophagosome formation, upregulation of the autophagy regulator Atg5 and apoptotic death of 50% of the retinal ganglion cells (RGCs) after 5 days. Genetic downregulation of autophagy using knockout mice for Atg4B (another regulator of autophagy) or with specific deletion of Atg5 in retinal ganglion cells, using the Atg5(flox/flox) mice reduces cell survival after ONT, whereas pharmacological induction of autophagy in vivo increases the number of surviving cells. In conclusion, our data support that autophagy has a cytoprotective role in RGCs after traumatic injury and may provide a new therapeutic strategy to ameliorate retinal diseases.