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

Ischemia-reperfusion injury of the retina is linked to necroptosis via the ERK1/2-RIP3 pathway

PURPOSE

Ischemia-reperfusion (IR) injury is involved in the pathology of many retinal disorders since it contributes to the death of retinal neurons and the subsequent decline in vision. We determined the molecular patterns of some of the principal molecules involved in necroptosis and investigated whether IR retinal injury is associated with the extracellular signal-regulated kinase-1/2- receptor-interacting protein kinase 3 (ERK1/2-RIP3) pathway.

METHODS

The cellular and subcellular localization of molecules involved in the cell death pathway, including RAGE, ERK1/2, FLIP, and RIP3, was determined with immunohistochemistry of cryosections of IR-injured retina from 2-month-old Long Evans rats. The total and phosphorylated protein levels were analyzed with quantitative western blots. ERK1/2 activity was inhibited by intravitreal injection of U0126, a highly selective inhibitor of mitogen-activated protein kinase 1/2 (MEK1/2).

RESULTS

The IR-injured rat retinas expressed two RAGE isoforms with different intracellular localizations at early time points after injury. At that time point, frame inhibition of ERK activation decreased RIP3 accumulation and cell death. FLIP was detected in the IR-injured rat retinas at early time points after ischemia reperfusion.

CONCLUSIONS

We report that the necroptotic cell death mechanism is executed by an ERK1/2-RIP3 pathway in the retinal ganglion cells in early stages after IR injury. Inhibition of ERK1/2 activity increased retinal ganglion cell (RGC) survival indicating that targeting of this pathway within the initial 12 h after IR injury can be used to inhibit the necroptosis pathway. We also provide evidence that a novel RAGE isoform is expressed in the early stages in rat retinal RGCs.

Necroptosis, in vivo detection in experimental disease models

Over the last decade, our picture of cell death signals involved in experimental disease models totally shifted. Indeed, in addition to apoptosis, multiple forms of regulated necrosis have been associated with an increasing number of pathologies such as ischemia-reperfusion injury in brain, heart and kidney, inflammatory diseases, sepsis, retinal disorders, neurodegenerative diseases and infectious disorders. Especially necroptosis is currently attracting the attention of the scientific community. However, the in vivo identification of ongoing necroptosis in experimental disease conditions remains troublesome, mainly due to the lack of specific biomarkers. Initially, Receptor-Interacting Protein Kinase 1 (RIPK1) and RIPK3 kinase activity were uniquely associated with induction of necroptosis, however recent evidence suggests pleiotropic functions in cell death, inflammation and survival, obscuring a clear picture. In this review, we will present the last methodological advances for in vivo necroptosis identification and discuss past and recent data to provide an update of the so-called "necroptosis-associated pathologies".

RIP kinases: key decision makers in cell death and innate immunity

Innate immunity represents the first line of defence against invading pathogens. It consists of an initial inflammatory response that recruits white blood cells to the site of infection in an effort to destroy and eliminate the pathogen. Some pathogens replicate within host cells, and cell death by apoptosis is an important effector mechanism to remove the replication niche for such microbes. However, some microbes have evolved evasive strategies to block apoptosis, and in these cases host cells may employ further countermeasures, including an inflammatory form of cell death know as necroptosis. This review aims to highlight the importance of the RIP kinase family in controlling these various defence strategies. RIP1 is initially discussed as a key component of death receptor signalling and in the context of dictating whether a cell triggers a pathway of pro-inflammatory gene expression or cell death by apoptosis. The molecular and functional interplay of RIP1 and RIP3 is described, especially with respect to mediating necroptosis and as key mediators of inflammation. The function of RIP2, with particular emphasis on its role in NOD signalling, is also explored. Special attention is given to emphasizing the physiological and pathophysiological contexts for these various functions of RIP kinases.

Rescue of enzymatic function for disease-associated RPE65 proteins containing various missense mutations in non-active sites

Over 70 different missense mutations, including a dominant mutation, in RPE65 retinoid isomerase are associated with distinct forms of retinal degeneration; however, the disease mechanisms for most of these mutations have not been studied. Although some mutations have been shown to abolish enzyme activity, the molecular mechanisms leading to the loss of enzymatic function and retinal degeneration remain poorly understood. Here we show that the 26 S proteasome non-ATPase regulatory subunit 13 (PSMD13), a newly identified negative regulator of RPE65, plays a critical role in regulating pathogenicity of three mutations (L22P, T101I, and L408P) by mediating rapid degradation of mutated RPE65s via a ubiquitination- and proteasome-dependent non-lysosomal pathway. These mutant RPE65s were misfolded and formed aggregates or high molecular complexes via disulfide bonds. Interaction of PSMD13 with mutant RPE65s promoted degradation of misfolded but not properly folded mutant RPE65s. Many mutations, including L22P, T101I, and L408P, were mapped on non-active sites. Although their activities were very low, these mutant RPE65s were catalytically active and could be significantly rescued at low temperature, whereas mutant RPE65s with a distinct active site mutation could not be rescued under the same conditions. Sodium 4-phenylbutyrate and glycerol displayed a significant synergistic effect on the low temperature rescue of the mutant RPE65s by promoting proper folding, reducing aggregation, and increasing membrane association. Our results suggest that a low temperature eye mask and sodium 4-phenylbutyrate, a United States Food and Drug Administration-approved oral medicine, may provide a promising "protein repair therapy" that can enhance the efficacy of gene therapy by reducing the cytotoxic effect of misfolded mutant RPE65s.

Necrosis-dependent and independent signaling of the RIP kinases in inflammation

It is now widely accepted that some forms of necrosis are controlled by a dedicated signaling pathway triggered by various cell surface and intracellular receptors. This regulated form of necrosis is mediated by the kinase activity of receptor-interacting protein kinase 1 (RIP1/RIPK1) and/or RIP3/RIPK3. A number of studies using the RIP1 kinase inhibitor Necrostatin-1 (Nec-1) and its derivatives, or RIP3-deficient mice demonstrated that RIP1 and RIP3 are involved in various infectious and sterile inflammatory diseases. As a consequence, these specific phenotypes were construed to depend on necrosis. However, emerging evidence indicates that the RIP1 kinase activity and RIP3 can also control apoptosis and inflammatory cytokine production independent of necrosis. Therefore, we may need to re-interpret conclusions drawn based on loss of RIP1 or RIP3 functions in in vivo models. We propose that studies of RIP1 and RIP3 in different inflammatory responses need to consider cell death-dependent and independent mechanisms of the RIP kinases.