mTOR-Mediated Dedifferentiation of the RPE Initiates Photoreceptor Degeneration

Abstract

 

Purpose:

 

Despite the central role of the retinal pigment epithelium (RPE) in numerous retinal degenerative diseases, little is known about adult RPE stress responses in vivo. We investigated the in vivo response of the adult murine RPE to diverse stressors in order to assess common features that might constitute a general RPE stress response.

 

Methods:

 

We created a model of chronic metabolic RPE stress by using cre/lox technology to ablate postnatally, and in an RPE-selective manner, the function of TFAM, which is essential for mitochondrial DNA replication and transcription. We also studied a model of acute oxidative damage to the RPE caused by intravenous injection of sodium iodate (15 mg/kg). The effects of these stressors on the RPE over time were assessed by histological, ultrastructural, and electrophysiological methods. Individual cellular pathways were evaluated by immunofluorescence in retinal sections, and by immunoblot analysis of RPE cells isolated at various time points. In vivo administration of specific pharmacological inhibitors was done to assess the role of particular signaling pathways.

 

Results:

 

We found that chronic metabolic stress of the mouse RPE triggers gradual dedifferentiation of the epithelium typified by reduction of RPE-characteristic proteins and cellular hypertrophy. Electroretinographic responses decrease and photoreceptors eventually degenerate, with cones affected more than rods. Abnormal RPE cell behavior, which eventually includes an epithelial to mesenchymal-like transition, is associated with increased glycolysis and activation of, and dependence upon, the HGF/c-Met pathway. RPE dedifferentiation and hypertrophy arise through stimulation of the AKT/mammalian target of rapamycin (mTOR) pathway. Acute oxidative damage to wild-type mice also causes RPE dedifferentiation and mTOR activation. Significantly, rapamycin treatment blunts key aspects of dedifferentiation and preserves photoreceptor function for both types of insult.

 

Conclusions:

 

These results reveal an in vivo response of the mature RPE to diverse stressors that prolongs RPE cell survival at the expense of epithelial attributes and photoreceptor function. Our findings provide a rationale for inhibition of the mTOR pathway as a therapeutic strategy for retinal degenerative diseases involving RPE stress.

 

Keywords: retinal pigment epithelium • stress response • EMT (epithelial mesenchymal transition)