Photoreceptor Protection by Adeno-Associated Virus–Mediated LEDGF Expression in the RCS Rat Model of Retinal Degeneration: Probing the Mechanism

Sequence changes in more than 50 genes have been identified to date as causing retinitis pigmentosa (RP),1 a progressive photoreceptor degeneration leading to blindness.2 These genes are involved in diverse functions, from the phototransduction cascade to protein folding, cell–cell interactions, intracellular trafficking, RNA splicing, proteasomal degradation, cell cycle control, and more.2 Despite this genetic heterogeneity, many forms of RP converge to a common mechanism of apoptosis in human diseases and in animal models.3,4 Therefore, a propitious treatment strategy for drug development for RP would be to target the apoptotic cascade generally rather than each specific causal mutation.5,6 Lens epithelium–derived growth factor (LEDGF), also known as PC4 and SFRS1 interacting protein 1 (Psip1), is a survival factor of the hepatoma-derived growth factor protein family,7 which becomes less sensitive to heat and proteolytic degradation by binding to heparin.8 LEDGF is a secreted protein that is naturally induced by stress signals, localizes to the nucleus, and, in turn, activates stress-related genes.9,10 Enhancing endogenous LEDGF supply, as well as exogenous administration of LEDGF, enhances the survival of retinal neurons under in vitro and in vivo conditions.11 LEDGF binds to promoter elements of many stress-related genes, including those for heat shock proteins (HSPs).11 HSPs are a group of proteins that are upregulated in response to stress and protect cells from damage subsequent to exposure to heat, ischemia, or toxins.12–14 Several groups have found that HSPs, including HSP25 and αβ-crystallin, exert a direct and negative effect on apoptosis.15,16 We previously reported that intravitreous injection of LEDGF promotes photoreceptor survival in light-damaged and Royal College of Surgeons (RCS) rats.17 The RCS rat is a model of progressive photoreceptor degeneration over the first few months of life.18 It harbors a mutation in the receptor tyrosine kinase Mertk gene19 that leads to photoreceptor cell loss from defective phagocytosis by the retinal pigment epithelium (RPE) and causes an abnormal accumulation of debris between the RPE and the neural retina. In this study, we asked whether the effectiveness of LEDGF could be increased by chronic expression in genetic animal models of retinal degeneration. For this purpose, we introduced the Ledgf gene to the retina of RCS rats via an adeno-associated virus (AAV) construct. In addition, conclusions drawn in our previous study were limited by use of the fusion protein GST-LEDGF, which complicated the interpretation, even though we found no effect of GST alone. We have circumvented this problem by using AAV-Ledgf. We found that LEDGF delivery via subretinal injection of the AAV-Ledgf construct was successful in preserving photoreceptors and retinal function in the RCS rat retina but not in P23H rhodopsin transgenic rats. The protection in RCS rats was not associated with substantial overexpression of small HSPs. However, providing the HSP27 gene (human orthologue of rat Hsp25) via an adenovirus vector to the RCS rat retina produced protection similar to that of AAV-Ledgf.