Inhibition of Retinal Detachment-Induced Apoptosis in Photoreceptors by a Small Peptide Inhibitor of the Fas Receptor

Photoreceptor cell death is the primary mechanism of vision loss after retinal detachment. The retinal pigment epithelium (RPE) provides the major metabolic and nutritional support for the photoreceptors.1,2 Numerous retinal diseases cause the separation of photoreceptors from the RPE, and this leads to the disruption of normal photoreceptor homeostasis.3–6 Interruption of this close relationship activates cell death in photoreceptors. Initially, a subpopulation of photoreceptors undergoes rapid cell death.7,8 This is followed by a slower, chronic loss of photoreceptors. Studies have demonstrated that photoreceptors die by apoptosis after separation from the RPE.9–12 Two major cell death pathways play critical role in activating apoptosis in photoreceptor cells, the Fas-mediated extrinsic death pathway and the mitochondrial intrinsic death pathway. Our previous studies have shown that photoreceptors activate both the Fas proapoptotic pathway and the intrinsic death pathway in a time-dependent fashion after retinal detachment.13,14 Fas pathway activation occurs when the Fas ligand (FasL) binds to the Fas receptor (Fas). This pathway is upstream of the intrinsic death pathway. Inhibition of Fas signaling with a neutralizing antibody or a small inhibitory RNA against the Fas transcript prevents the activation of the intrinsic cell death pathway and the separation-induced death of photoreceptors.14 Identifying novel means of inhibiting retinal Fas signaling may lead to the discovery of small molecules or biologicals that may be developed into new therapeutic agents. Recent studies have identified Met as a novel inhibitor of the Fas pathway. Met is an oncogene that encodes the tyrosine kinase receptor for hepatocyte growth factor.15 Met is a disulfide-linked heterodimer composed of an extracellular 50-kDa α chain and a transmembrane 145-kDa β chain. In normal mouse liver tissue, Met associates directly with Fas and prevents Fas activation by its sequestration.16 Studies have demonstrated that only the extracellular α chain domain of Met is required to form the Fas-Met complex.17 The extracellular α chain contains an N-terminal sequence motif, TyrLeuGlyAla (YLGA), which shows high similarity to the normal FasL.17 In cell cultures, α-Met or YLGA-containing peptides protect Jurkat cells from FasL-induced apoptosis.17 This inhibition is secondary to the sequestration of the Fas and decreasing the number of Fas molecules available to interact with FasL. We investigated the in vitro and in vivo effects of a small peptide containing the YLGA motif and the surrounding amino acids on the Fas proapoptotic pathway and its downstream targets in the 661W photoreceptor cell line and in a rodent model of retinal detachment. Treatment of 661W cells with a Fas-activating antibody induced caspase 8 activation, which was competitively blocked by the Met YLGA 12-mer (Met12). Similarly, the administration of Met12 during retinal detachment in rat eyes prevented Fas-induced extrinsic cell death pathway activation. Inhibition of Fas signaling by Met12 prevented activation of the intrinsic cell death pathway in detached photoreceptors. Finally, inhibition of the Fas signaling pathway by Met12 in detached rodent retinas reduced Fas-mediated apoptosis and increased photoreceptor survival.