Mechanisms of photoreceptor protection upon targeting the <i>Nrl-Nr2e3</i> pathway.

Abstract

Acute knockout of the rod photoreceptor transcription factor <i>Nrl</i> delays retinal degeneration in multiple mouse models of blindness, but the downstream transcriptomic changes that mediate these therapeutic effects are unknown. Here, we show that acute <i>Nrl</i> knockout causes upregulation of a subset of cone genes in rods as well as downregulation of rod genes, including the rod-specific transcriptional repressor <i>Nr2e3</i>. We hypothesized that <i>Nr2e3</i> downregulation might mediate some of the therapeutic effects of <i>Nrl</i> knockout. Indeed, acute knockout of <i>Nr2e3</i> prevents photoreceptor degeneration and preserves visual function in mice with mutations in the catalytic subunit of the rod-specific phosphodiesterase (<i>Pde6b^rd10/rd10</i>). Upregulation of <i>Pde6c</i>, the cone-specific paralog of <i>Pde6b</i>, in <i>Nr2e3</i>-knockout rods is required to prevent degeneration in <i>Pde6b^rd10/rd10</i> mice, suggesting that this therapeutic effect is mediated, at least in part, by a gene-replacement mechanism. In contrast, acute <i>Nr2e3</i> knockout fails to prevent degeneration caused by loss- or gain-of-function mutations in Rhodopsin (<i>Rho^-/-</i> and <i>Rho^P23H/P23H</i>), whereas acute <i>Nrl</i> knockout delays degeneration in both models. Surprisingly, the therapeutic effect of acute <i>Nrl</i> knockout in <i>Pde6b^rd10/rd10</i> mice does not depend on <i>Pde6c</i> upregulation. These results suggest that acute <i>Nrl</i> knockout may exert its therapeutic effects via a mechanism independent of <i>Nr2e3</i> downregulation, perhaps by downregulating other rod genes. We conclude that acute <i>NRL</i> knockout may be a promising gene-independent strategy for preventing photoreceptor degeneration in human patients.