Oxidative stress induces caspase-independent retinal apoptosis in vitro

Apoptosis is the mode of cell death in retinitis pigmentosa (RP), a heterogeneous group of retinal degenerations. The activation of the caspase proteases forms a pivotal step in the initiation and execution phase of apoptosis in many cells. Inhibition of caspases has been reported to prevent apoptosis in many model systems. However, we demonstrate the absence of caspase activation during retinal cell apoptosis in vitro which involves phosphatidylserine (PS) externalisation, DNA nicking and cell shrinkage. In addition, zVAD-fmk, DEVD-CHO and BD-fmk, inhibitors of the caspases, were unable to alter the characteristics or kinetics of apoptosis, implying that retinal cell death in vitro follows a caspase-independent pathway. We have previously demonstrated the ability of reactive oxygen species (ROS) to act as mediators of retinal cell apoptosis in vitro as well as the ability of antioxidants to prevent retinal cell apoptosis. Here we demonstrate the oxidative inactivation of caspases in this model of retinal apoptosis and provide evidence for an oxidative stress driven cell death pathway that does not involve caspase activity and which retains key features of apoptotic cell death. Furthermore, our data indicates that apoptotic events such as PS exposure, DNA nicking and cell shrinkage may occur independently of caspase activity.