Receptor interacting protein kinase-mediated necrosis contributes to cone and rod photoreceptor degeneration in the retina lacking interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP) secreted by photoreceptors plays a pivotal role in photoreceptor survival with an unknown mechanism. A mutation in the human IRBP has been linked to retinitis pigmentosa, a progressive retinal degenerative disease. Mice lacking IRBP display severe early and progressive photoreceptor degeneration. However, the signaling pathway(s) leading to photoreceptor death in IRBP-deficient mice remains poorly understood. Here, we show that amounts of tumor necrosis factor-α (TNF-α) in the interphotoreceptor matrix and retinas of Irbp^-/- mice were increased more than 10-fold and fivefold, respectively, compared with those in wild-type mice. Moreover, TNF-α receptor 1, an important membrane death receptor that mediates both programmed apoptosis and necrosis, was also significantly increased in Irbp^-/- retina, and was colocalized with peanut agglutinin to the Irbp^-/- cone outer segments. Although these death signaling proteins were increased, the caspase-dependent and independent apoptotic pathways were mildly activated in the Irbp^-/- retinas, suggesting that other cell death mechanism(s) also contributes to the extensive photoreceptor degeneration in Irbp^-/- retina. We found that receptor interacting protein 1 and 3 (RIP1 and RIP3) kinases, the intracellular key mediators of TNF-induced cellular necrosis, were elevated at least threefold in the Irbp^-/- retinas. Moreover, pharmacological inhibition of RIP1 kinase significantly prevented cone and rod photoreceptor degeneration in Irbp^-/- mice. These results reveal that RIP kinase-mediated necrosis strongly contributes to cone and rod degeneration in Irbp^-/- mice, implicating the TNF-RIP pathway as a potential therapeutic target to prevent or delay photoreceptor degeneration in patients with retinitis pigmentosa caused by IRBP mutation.