Retinal pigment epithelial (RPE) cell dysfunction plays a central role in various retinal degenerative diseases, but knowledge is limited regarding the pathways responsible for adult RPE stress responses in vivo. RPE mitochondrial dysfunction has been implicated in the pathogenesis of several forms of retinal degeneration. Here we have shown that postnatal ablation of RPE mitochondrial oxidative phosphorylation in mice triggers gradual epithelium dedifferentiation, typified by reduction of RPE-characteristic proteins and cellular hypertrophy. The electrical response of the retina to light decreased and photoreceptors eventually degenerated. Abnormal RPE cell behavior was associated with increased glycolysis and activation of, and dependence upon, the hepatocyte growth factor/met proto-oncogene pathway. RPE dedifferentiation and hypertrophy arose through stimulation of the AKT/mammalian target of rapamycin (AKT/mTOR) pathway. Administration of an oxidant to wild-type mice also caused RPE dedifferentiation and mTOR activation. Importantly, treatment with the mTOR inhibitor rapamycin blunted key aspects of dedifferentiation and preserved photoreceptor function for both insults. 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 mTOR pathway inhibition as a therapeutic strategy for retinal degenerative diseases involving RPE stress. IntroductionThe retinal pigment epithelium (RPE) is a polarized, cuboidal epithelial cell layer situated in the outer retina between the photoreceptors and choroidal vasculature. The RPE supplies an estimated 60% of the glucose consumed by the neural retina (1) and performs a variety of other functions crucial for retinal homeostasis, including delivery of amino acids and docosahexaenoic acid for photoreceptor protein and membrane synthesis; transport, storage, and enzymatic conversion of retinoids essential for phototransduction; regulation of fluid and ion balance in the subretinal space; maintenance of the blood retinal barrier; secretion of growth factors; and phagocytosis of shed photoreceptor outer segment membranes (2). The RPE is a postmitotic tissue, so RPE cells must carry out these functions for the life of an individual.The retinal degenerative consequences of mutations in RPEexpressed genes illustrate the importance of the RPE for photoreceptor viability in humans. Mutations that impair production of the chromophore 11-cis retinal cause Leber congenital amaurosis, retinitis pigmentosa (RP), and allied disorders (3). Disruption of RPE phagocytosis causes RP and rod/cone dystrophy (4, 5). Mutations that affect ion channel function cause disease of the specialized retinal region necessary for high-acuity vision (the macula) (6) as well as RP (7), while mutations in genes encoding the RPE-secreted proteins TIMP3 (8) and EFEMP1 (9) cause lateronset macular disease.

Our study reveals surprising increases in BiP and to a lesser degree Chop in retinal degenerations arising from diverse causes. We propose that manipulation of ER stress responses may be helpful in treating many environmental and heritable forms of retinal degeneration.

Oxidative stress and photoreceptor apoptosis are prominent features of many forms of retinal degeneration (RD) for which there are currently no effective therapies. We previously observed that mesenchymal stem/stromal cells reduce apoptosis by being activated to secrete stanniocalcin-1 (STC-1), a multifunctional protein that reduces oxidative stress by upregulating mitochondrial uncoupling protein-2 (UCP-2). Therefore, we tested the hypothesis that intravitreal injection of STC-1 can rescue photoreceptors. We first tested STC-1 in the rhodopsin transgenic rat characterized by rapid photoreceptor loss. Intravitreal STC-1 decreased the loss of photoreceptor nuclei and transcripts and resulted in measurable retinal function when none is otherwise present in this rapid degeneration. We then tested STC-1 in the Royal College of Surgeons (RCS) rat characterized by a slower photoreceptor degeneration. Intravitreal STC-1 reduced the number of pyknotic nuclei in photoreceptors, delayed the loss of photoreceptor transcripts, and improved function of rod photoreceptors. Additionally, STC-1 upregulated UCP-2 and decreased levels of two protein adducts generated by reactive oxygen species (ROS). Microarrays from the two models demonstrated that STC-1 upregulated expression of a similar profile of genes for retinal development and function. The results suggested that intravitreal STC-1 is a promising therapy for various forms of RD including retinitis pigmentosa and atrophic age-related macular degeneration (AMD).

The unexpected lack of correlation of OKT testing with photoreceptor degeneration in the Tg rats emphasizes the need in behavioral therapeutic studies for careful analysis of visual thresholds of experimental animals prior to therapeutic intervention.