Photoreceptor cKO of OTX2 Enhances OTX2 Intercellular Transfer in the Retina and Causes Photophobia

In the mature mouse retina, Otx2 is expressed in both Retinal Pigmented Epithelium (RPE) and photoreceptor (PR) cells, but Otx2 knockout in the RPE alone results in PR degeneration. To study the cell-autonomous function of OTX2 in PRs, we performed PR-specific Otx2 knockout (cKO) in adults. As expected, the protein disappears completely from PR nuclei but is still observed in PR inner and outer segments while its level concomitantly decreases in the RPE, suggesting a transfer of OTX2 from RPE to PRs in response to Otx2 ablation in PRs. The ability of OTX2 to transfer from RPE to PRs was verified by viral expression of tagged-OTX2 in the RPE. Transferred OTX2 distributed across the PR cytoplasm, suggesting functions distinct from nuclear transcription regulation functions. PR-specific Otx2 cKO did not alter the structure of the retina but impaired the translocation of photoreceptor arrestin-1 upon illumination changes, making mice photophobic. RNA-seq analyses following Otx2 knockout revealed down-regulation of genes involved in the cytoskeleton that might account for the arrestin-1 translocation defect, and of genes involved in extracellular matrix and signalling factors that may participate in the enhanced transfer of OTX2. Interestingly, several RPE-specific OTX2 target genes involved in melanogenesis were downregulated, lending weight to a decrease of OTX2 levels in the RPE following PR-specific Otx2 cKO. Our study reveals a new role of endogenous OTX2 in PR light adaptation and demonstrates the existence of OTX2 transfer from RPE to PR cells, which is increased upon PR-specific Otx2 ablation and might participate in PR neuroprotection.Significance statementOTX2 homeoprotein is expressed in retinal pigmented epithelium (RPE), photoreceptors (PRs) and bipolar cells. The function of endogenous PR OTX2, which, in contrast with RPE OTX2, is dispensable for PR maintenance, is unknown. We performed PR-specific Otx2 knockout and found that removal of endogenous PR OTX2 leads to impaired arrestin-1 translocation associated with photophobia, specific modifications of PR and RPE gene expression, and to increased transfer of OTX2 protein from the RPE to the PR cytoplasm. Thus, several PR activities, including light adaptation, rely on endogenous nuclear OTX2, while PR neuroprotection seems to require RPE OTX2, highlighting the importance of non-cell-autonomous OTX2 in the adult retina.