Neuro-protective action of RPE-derived Otx2 in a mouse model of AMD – NeuroprOtx

In age-related macular degeneration (AMD), dysfunction of retinal pigment epithelium (RPE) cells causes progressive accumulation of insoluble products at their external side. These deposits decrease RPE cell fitness and their ability to protect photoreceptor cells (PRs), leading to degeneration of PRs and loss of vision. AMD affects up to 30% of people over the age of 75 in western countries with the number of cases increasing with extended life expectancy and no cure exists. The disease affects the quality of daily living, impacts on the physical and mental health of patients and its social and economic costs are an increasing burden. AMD is recapitulated in a mouse model where expression of the Otx2 homeoprotein is blocked in the adult retina. In this model, lack of Otx2 primarily affects RPE function and subsequently leads to PR degeneration. When loss of Otx2 is specifically triggered in RPE cells, similar PR degeneration occurs, indicating a non-autonomous action of RPE-derived Otx2 on PRs. Conversely, RPE-restricted expression of Otx2 is sufficient to promote long-term PR survival. This firmly establishes the protective action of RPE-derived Otx2 on PRs. Yet, the mechanism by which RPE-derived Otx2 protects PRs is not known.

The recently discovered ability of Otx2 to transfer between cells in the brain and to exert neuro-protective activity dramatically extends the potential mode of action of this protein. As was shown for other homeodomain proteins such as Engrailed1/2, Partner 2 showed that extracellular Otx2 can be internalized into specific neurons and control their maturation and survival. In the mature retina, Otx2 transfer from RPE might be critical for the long-term protection of vulnerable PRs. This proposal aims to determine the contribution of transferred Otx2 of RPE origin to the neuro-protection of PRs. It will address the mechanism of the neuro-protective action on PRs and the therapeutic potential of transferred Otx2 in retinal degenerative disease models. Together, the results obtained should establish the physiological relevance of this novel mode of action of the Otx2 protein for the maintenance of a healthy retina and open up new avenues for the treatment of AMD.

The specific objectives are to demonstrate that Otx2 of RPE origin transfers to PRs, to show a functional role of this transfer, to analyze the mechanism of the neuro-protective action of Otx2 on PRs and to evaluate the therapeutic potential of Otx2 transfer in an animal AMD-related model. For these purposes, we shall use existing animals that allow conditional ablation of the Otx2 gene, the capture of extracellular Otx2 by conditional expression of anti-Otx2 single chain antibodies and develop a new mouse line to drive Otx2 ablation in PRs. We shall use in vivo and ex vivo cellular and molecular analyses to dissect the mechanisms of Otx2 neuro-protection. As a first step towards translational research, we shall test the potential of extra-cellular Otx2 together with identified co-factors, to reduce PR degeneration in mice recapitulating a human mutation of the Rpe65 gene.

This research timely bridges the expertise of two partners with complementary areas of interest and expertise (mouse genetics of the retina, retina degeneration, homeoprotein transfer, molecular biology, biochemistry, cell biology and physiology). The synergistic interaction of two recognized laboratories is supported by preliminary results. The work-plan allows independent progress in both labs and includes alternative strategies at all steps. The project has translational potential with potential high benefit for public health (AMD). The preservation of PRs is arguably a critical health issue, and significant progress in the field is likely to improve the quality of life of millions of people in the world.