article nature genetics • advance online publication 1 Evidence for two apoptotic pathways in light-induced retinal degeneration Wenshan Hao 1,6 *, Andreas Wenzel 2 *, Martin S. Obin 3 *, Ching-Kang Chen 1,6 *, Elliott Brill 4 , Nataliia V. Krasnoperova 4 , Pamela Eversole-Cire 1 , Yelena Kleyner 5 , Allen Taylor 3 , Melvin I. Simon 1 , Christian Grimm 2 , Charlotte E. Remé 2† & Janis Lem 4,5† *These authors contributed equally to this work. † These authors contributed equally to this work. Published online: 3 September 2002, doi:10.1038/ng984 Excessive phototransduction signaling is thought to be involved in light-induced and inherited retinal degenera- tion. Using knockout mice with defects in rhodopsin shut-off and transducin signaling, we show that two differ- ent pathways of photoreceptor-cell apoptosis are induced by light. Bright light induces apoptosis that is independent of transducin and accompanied by induction of the transcription factor AP-1. By contrast, low light induces an apoptotic pathway that requires transducin. We also provide evidence that additional genetic factors regulate sensitivity to light-induced damage. Our use of defined mouse mutants resolves some of the complexity underlying the mechanisms that regulate susceptibility to retinal degeneration. 1 Division of Biology, California Institute of Technology, Pasadena, California 91125, USA. 2 Laboratory of Retinal Cell Biology, University Hospital Zurich, CH-8091 Zurich, Switzerland. 3 Nutrition and Vision Research Laboratory, JMUSDA-HNRCA at Tufts University, Boston, Massachusetts 02111, USA. 4 Department of Ophthalmology and Molecular Cardiology Research Institute, Tufts-New England Medical Center, Boston, Massachusetts 02111, USA. 5 Program in Genetics, Tufts University School of Medicine, Boston, Massachusetts 02111, USA. 6 Present address: Department of Oncology, Wyeth Research, Pearl River, New York 10965, USA (W.H.); Department of Ophthalmology and Visual Sciences, University of Utah, Salt Lake City, Utah 84112, USA (C.- K.C.). Correspondence should be addressed to J.L. (e-mail: Jlem@lifespan.org). Introduction Retinal degeneration, whether a result of genetic defects or pro- longed light exposure, is generally characterized by apoptosis of photoreceptor cells 1–6 . But the molecular events that initiate the apoptotic cascade are poorly understood. Elucidating the molecu- lar mechanisms that trigger light-induced apoptosis might help to identify similar mechanisms in the inherited human retinal dystro- phies, retinitis pigmentosa and age-related macular degenerations. Although activation of the G-protein-coupled light receptor, rhodopsin, is required for light-induced retinal degeneration 7–10 , it is unclear whether apoptosis is triggered by rhodopsin activation alone or through a downstream signal transduction event. The ‘equivalent light’ hypothesis suggests that a subset of inherited reti- nal degenerations is mediated by excessive phototransduction sig- naling 11–13 , which mimics constant light activation even in the absence of light. The notion that retinal degeneration can be induced by excessive phototransduction signaling is consistent with the observation that exposure to continuous bright light induces photoreceptor-cell apoptosis in rodents 14 . We tested the role of phototransduction signaling in light- induced retinal degeneration using mice lacking the α-subunit of transducin (Gnat1 –/– ), a G-protein specific to rod photoreceptor cells 15 . This mutation disconnects phototransduction signaling from light-activated rhodopsin (Fig. 1a) but preserves normal retinal morphology 15 . We reasoned that if signaling mediated by transducin triggers photoreceptor-cell apoptosis, then mutant mice deficient in transducin should be resistant to light-induced degeneration. If, however, apoptosis is triggered by an event upstream of transducin, such as rhodopsin activation, then mutations prolonging rhodopsin activation (Fig. 1b) would be expected to increase susceptibility to light damage, irrespective of whether transducin is present or not (Fig. 1c). We also used mice with defects in rhodopsin signaling shutoff to examine mechanisms of light damage (Fig. 1b) because, unlike wildtype mice, these mice show light damage in normal, cyclic room light 2,3 . They provide a sensitive tool to assess the role of transducin signaling in retinal degenerative disease at low inten- sities of light. Rhodopsin activity is normally terminated by a two-step process in which phosphorylation of rhodopsin by rhodopsin kinase is followed by the binding of arrestin (also known as S-antigen) 16 . Transgenic mice with a null mutation in the gene encoding rhodopsin kinase 3 (Rhok –/– ) or arrestin 2 (Sag –/– ) show prolonged rhodopsin signaling (Fig. 1b). We there- fore examined Rhok –/– and Sag –/– mice on a transducin-null background to evaluate the role of transducin signaling in retinal degenerations induced by low intensities of light (Fig. 1c). Our results indicate that bright light triggers apoptosis of photoreceptor cells through a mechanism requiring activation of rhodopsin but not transducin signaling. The degeneration induced by bright light is accompanied by induction of the AP- 1 transcription factor. By contrast, photoreceptor-cell apopto- sis induced by low-intensity light in Rhok –/– and Sag –/– mice is © 2002 Nature Publishing Group http://www.nature.com/naturegenetics