Multiple programmed cell death pathways are involved in N-methyl-N-nitrosourea-induced photoreceptor degeneration

• Published in: Graefe's archive for clinical and experimental ophthalmology 2015-05, Vol.253 (5), p.721-731
• Main Authors: Reisenhofer, Miriam, Balmer, Jasmin, Zulliger, Rahel, Enzmann, Volker
• Format: Article
• Language: English
• Subjects: Alkylating Agents; Animals; Apoptosis; Basic Science; Calpain - metabolism; Caspases - metabolism; Cell death; Disease Models, Animal; Heat-Shock Proteins - genetics; Humans; In Situ Nick-End Labeling; Injections, Intraperitoneal; Medicine; Medicine & Public Health; Methylnitrosourea; Mice; Mice, Inbred C57BL; Ophthalmology; Photoreceptor Cells, Vertebrate - drug effects; Photoreceptor Cells, Vertebrate - metabolism; Photoreceptor Cells, Vertebrate - ultrastructure; Real-Time Polymerase Chain Reaction; Retinal Degeneration - chemically induced; Retinal Degeneration - genetics; Retinal Degeneration - pathology; RNA, Messenger - genetics; Transcription Factor CHOP - genetics
• ISSN: 0721-832X; 1435-702X
• DOI: 10.1007/s00417-014-2906-x

Purpose To identify programmed cell death (PCD) pathways involved in N-methyl-N-nitrosourea (MNU)-induced photoreceptor (PR) degeneration. Methods Adult C57BL/6 mice received a single MNU i.p. injection (60 mg/kg bodyweight), and were observed over a period of 7 days. Degeneration was visualized by H&E overview staining and electron microscopy. PR cell death was measured by quantifying TUNEL-positive cells in the outer nuclear layer (ONL). Activity measurements of key PCD enzymes (calpain, caspases) were used to identify the involved cell death pathways. Furthermore, the expression level of C/EBP homologous protein (CHOP) and glucose-regulated protein 78 (GRP78), key players in endoplasmic reticulum (ER) stress-induced apoptosis, was analyzed using quantitative real-time PCR. Results A decrease in ONL thickness and the appearance of apoptotic PR nuclei could be detected beginning 3 days post-injection (PI). This was accompanied by an increase of TUNEL-positive cells. Significant upregulation of activated caspases (3, 9, 12) was found at different time periods after MNU injection. Additionally, several other players of nonconventional PCD pathways were also upregulated. Consequently, calpain activity increased in the ONL, with a maximum on day 7 PI and an upregulation of CHOP and GRP78 expression beginning on day 1 PI was found. Conclusions The data indicate that regular apoptosis is the major cause of MNU-induced PR cell death. However, alternative PCD pathways, including ER stress and calpain activation, are also involved. Knowledge about the mechanisms involved in this mouse model of PR degeneration could facilitate the design of putative combinatory therapeutic approaches.