Photic Injury Promotes Photoreceptor Apoptosis via Nuclear and Mitochondrial Targeting of p75ICD, NRAGE and p53

Srinivasan, Bhooma., Photic injury promotes photoreceptor apoptosis via nuclear and mitochondrial targeting of p751ICD, NRAGE and p53. Doctor of Philosophy (Biomedical Sciences), May, 2003, 185 pp., 36 illustrations, bibliography, 256 titles. Retinal diseases involving photoreceptor cell degeneration such as Age-related Macular Degeneration (AMD) and Retinitis Pigmentosa (RP) remain some of the leading causes of blindness in the U.S. The changes in light-damaged retinas of laboratory animals resemble those in patients with degenerative retinal diseases, making photic injury widely accepted as a suitable model to study photoreceptor cell death. Intense light exposure to the retina leads to photoreceptor cell apoptosis but the molecular mechanisms remain known. Recent studies in our laboratory showed that photoreceptor cells express p76NTR, the low affinity neurotrophin receptor, and that p75NTR expression was upregulated in photoreceptor cells of Royal College of Surgeons (RCS) rats, a model of retinal degeneration. Earlier reports have also shown that photoreceptor cell death was decreased in p75NTR knock-out mice exposed to light as compared to wild type mice. These results suggest that p75NTR could be involved in photoreceptor cell death while signaling pathways employed by p75NTR in inducing cell death remain vague, the intracellular domain of p75NTR (p75ICD) has been shown to be sufficient to induce apoptosis, and adaptor proteins for p75NTR are necessary for cell death to occur. In this study, we have identified a novel signaling mechanism utilized by p75NTR in promoting apoptotic cell death of photoreceptor cells following photic injury. The mouse transgenic photoreceptor cell line (661w), exposed to intense light (1400fc) for 2-3h, exhibited increased expression of p75NTR, NRAGE (Neurotrophin receptor MAGE, a p75NTR adaptor protein), and TACE (Tumor necrosis factor-alpha converting enzyme, an alpha-secretase shown previously to promote shedding of several growth factors and cytokines including TNF-alpha). Furthermore, apoptotic cell death followed the increased levels of three proteins. Light-exposure also promoted the release of p75ICD by TACE, and increased association and nuclear translocation of p75ICD and NRAGE. The level of p53 was also increased in 661w cells following light exposure and binding of the p75ICD-NRAGE complex to p53 is required for translocation into the nucleus. P75ICD-NRAGE-p53 nuclear translocation appeared to be necessary for transcriptional activation of Bax and repression of Bc12. Moreover, mitochondrial targeting of p53 resulted in mitochondrial membrane permeability changes and release of cytochrome c. The use of TAPI, a TACE inhibitor, and of pifithrin, a p53 inhibitor, and overexpression of Bc12 suppressed apoptotic cell death of 661w cells after a light exposure. Our study shows that light exposure promotes apoptotic cell death of photoreceptor cells by activating novel signaling mechanisms involving the nuclear and mitochondrial targeting of p75NTR-NRAGE-p53. Our studies increase our understanding of the mechanisms that may be involved in photoreceptor degeneration and provide new molecular targets for the treatment of degenerative retinal diseases.