COMMENTARY How to keep photoreceptors alive Alan Bird* Institute of Ophthalmology, University College London, London EC1V 9EL, United Kingdom O ver the last 40 years, there has been increasing success in the surgical treatment of retinal detachment in that the retinal reattachment could be achieved in a high proportion of cases but visual recovery was frequently poor. An explanation for this poor visual outcome was derived from work on experimental retinal detachment in which it was shown that photoreceptor cell death occurred because of a wave of apoptosis during the first few days of retinal detachment (1). It is recognized that apoptosis or programmed cell death is a genetically encoded potential of all cells (2). It is characterized by cleavage of most of the nuclear DNA into short but well organized chains of nucleosomes in multiples of 200 bp by an endogenous nonlysosomal nuclease (3, 4) and may be triggered by changes in the metabolic environment of the cell. The photoreceptor cells are closely approximated to the retinal pigment epithelium (Fig. 1) and depend on the retinal pigment epithelium for their metabolic sustenance. Physical separation of the two and loss of metabolic exchange as occurs in retinal detachment (Fig. 2) might have been supposed to cause a sequence of events that would inevitably induce cell loss that was not amenable to treatment. However, apoptosis may be manipulated by altering the metabolic environment, and it has been shown that brainderived growth factor injected into the eye reduces the rate of photoreceptor cell loss in experimental retinal detachment (5), although the precise means by which apoptosis was induced and the mechanism of the therapeutic effect were uncertain. In this issue of PNAS, Nakazawa et al. (6) describe convincing evidence that monocyte chemoattractant protein 1 (MCP-1) plays a critical role in inducing photoreceptor apoptosis in experimental retinal detachment in mice. It had been reported some years ago that levels of MCP-1 were high in the vitreous of patients with retinal detachment (7). MCP-1 seemed to be an attractive candidate as an inducer of cell loss because it had been proposed as playing a role in the pathogenesis of a variety of disorders of the central nervous system including Alzheimer’s disease. Nakazawa et al. showed that the level of MCP-1 in mice with retinal detachment was increased 10-fold in the vitreous when compared with normals and that its ex- www.pnas.org兾cgi兾doi兾10.1073兾pnas.0611014104 Fig. 1. Photomicrograph showing the close physical relationship between the photoreceptor cells of the retina and the retinal pigment epithelium (RPE). There is constant metabolic exchange between the two cell systems. (Magnification: ⫻1,200.) pression in Muller cells (a class of retinal glial cell) was up-regulated after 72 h of detachment, a time of maximum apoptosis. Apoptosis was reduced by injecting an anti-MCP-1 blocking fragment intravitreally. Apoptosis was also reduced by 80% in MCP-1 knockout mice with retinal detachment. In each case, suppression of apoptosis was accompanied by reduction of CD11b⫹ macrophage/microglial cells, invading cells that are found universally in retinal detachment. Interestingly, Nakazawa et al. provide evidence with both in vitro and in vivo experiments that the effect is not mediated by a direct effect of MCP-1 on photoreceptor cells; rather, it was mediated through the activated macrophage/microglial cells. This information is very important because there are clear therapeutic implications for the acute management of retinal detachment in humans. Hopefully it will be possible to transfer these findings into treatment strategies that can suppress the photoreceptor cell loss such patients often experience. These findings also have broad implications for retinal diseases, including diabetic retinopathy, retinal vascular occlusions, and retinal dystrophies. A brief summary of relevant work on the pathophysiology of retinal dystrophies and experimental therapeutics for these dystrophies will serve to put the work of Nakazawa et al. (6) in perspective. In both humans and animals, there has been increasing reason to believe that in Fig. 2. Diagram of a cross-section of the eye with retinal detachment (open arrow) and retina hole (solid arrow). Fluid exchange between the subretinal space and vitreous cavity through the hole compromises metabolic exchange between the retina and retinal pigment epithelium. inherited retinal diseases, the metabolic defect caused by the mutation does not cause cell death directly. This observation is evident clinically with respect to cone loss in patients with retinitis pigmentosa caused by mutations in the rhodopsin gene, which is expressed exclusively in rod photoreceptor cells. A similar situation exists in mice transfected with a mutant rhodopsin gene (8, 9). It is also illustrated in the setter dog with progressive atrophy of both the rod and cone photoreceptors, with phosphodiesterase activity being defective in rods but not in cones (10). Most striking are the observations on a chimera created of a pigmented mouse transfected with a mutant rhodopsin gene and a WT albino mouse. The setting was achieved by generating chimeric embryos composed of cells from both the albino and pigmented mouse lines. Although there was patchy distribution of cells from the pigmented and nonpigmented origin, the distribution of photoreceptor cell death followed exactly the same pattern as that seen in the pigmented rhodopsin mouse, implying that Author contributions: A.B. wrote the paper. The author declares no conflict of interest. See companion article on page 2425. *E-mail: [email protected]. © 2007 by The National Academy of Sciences of the USA PNAS 兩 February 13, 2007 兩 vol. 104 兩 no. 7 兩 2033–2034 the cells containing the mutant gene were no more likely to die than the cells with the WT rhodopsin gene. The only recorded variation between animals was that the proportions of mutant to WT populations determined the speed of degeneration. It was concluded that the disease was induced by the presence of the mutant photoreceptor cells, but cell death was somehow related to a change in the environment of the retina rather than a direct and cell-autonomous effect of the mutant gene. These observations stimulated further investigations of the cause of cell death in retinal dystrophies, and it became evident that cell loss is universally caused by apoptosis (11–13). This finding gave rise to the concept that modulating the risk of apoptosis by using growth factors may have therapeutic value. The first attempts were in Royal College of Surgeons rats in which the causative gene is expressed in the retinal pigment epithelium but causes death of photoreceptor cells. A single intravitreal injection of basic fibroblast growth factor caused prolongation of photoreceptor cell life (14). A series of trials were then undertaken in a light damage model (15) and genetically determined disease in rodents (16) using a variety of growth factors given as single intravitreal injections. These experiments showed variable rescue of photoreceptor cells depending on the neurokine used and the disease model and mode of delivery. Ciliary neutrophic factor (CNF) appeared to be consistently effective in a variety of models in rodent, cat, and dog. However, the rescue was inevitably short-lived given the limited exposure of the retina to the growth factor. With the advent of successful gene transfer into the retina (17, 18), it was possible to generate long-term treatment. A recombinant adeno-associated virus was used 1. Cook B, Lewis GP, Fisher SK, Adler R (1995) Invest Ophthalmol Visual Sci 36:990–996. 2. Wyllie AH (1980) Nature 284:555–556. 3. Raff MC (1992) Nature 356:397–400. 4. Wyllie AH, Beattie GJ, Hargreaves AD (1981) Histochem J 13:681–692. 5. Lewis GP, Linberg KA, Geller SF, Guerin CJ, Fisher SK (1999) Invest Ophthalmol Visual Sci 40:1530–1544. 6. Nakazawa T, Hisatomi T, Nakazawa C, Noda K, Maruyama K, She H, Matsubara A, Miyahara S, Nakao S, Yin Y, et al. (2007) Proc Natl Acad Sci USA 104:2425–2430. 7. Abu el-Asrar AM, Van Damme J, Put W, Veckeneer M, Dralands L, Billiau A, Missotten L (1997) Am J Ophthalmol 123:599–606. 8. Olsson J, Gordon JW, Pawlyk BS, Roof D, Hayes A, Molday RS, Mukai S, Cowley GS, Berson EL, Dryja TP (1992) Neuron 9:815–830. measured by their survival, was achieved after single injections into the subretinal compartment. However, there were unexpected side effects that appeared to be dose-related, including changes in rod photoreceptor nuclear morphology and a reduction in light sensitivity as measured by the electroretinogram. On the basis of these observations, it was decided to extend the work to humans. A phase I trial of prolonged treatment with CNF was undertaken in patients with severe retinitis pigmentosa by using a slow-release biological device consisting of cells transfected with the human CNF gene and sequestered within capsules that were surgically implanted into the vitreous cavity of the eye (20). As a safety trial, it was successful in showing no untoward effect. The severity of visual loss and the small number of subjects involved was believed to preclude any conclusions as to efficacy, but, surprisingly, some improvement in vision was recorded. Of seven eyes for which visual acuity could be tracked by conventional reading charts, three eyes reached and maintained improved acuities of 10–15 letters, equivalent to a two- to three-line improvement on standard Snellen acuity charts. These results are very encouraging because the primary objective was to slow degeneration rather than cause improvement in vision. Whether visual gain is a realistic objective will need to be tested in a phase II trial. For the clinician managing patients with retinal diseases, there have been only limited opportunities to influence the natural history of common diseases, and in disorders such as retinal detachment in which surgical treatment was available, the results with respect to visual recovery have frequently been disappointing. Similarly, laser treatment of diabetic retinopathy inevitably involves substantial retinal destruction to achieve therapeutic benefit that was often timelimited. We are now entering an era of biological treatment, including gene therapy and the manipulation of disease with neurokines, that is based on a better knowledge of pathologic mechanisms. The work of Nakazawa et al. (6) presents the potential for significant clinical applications by pointing to a novel therapeutic target. Limiting the retina’s exposure to MCP-1 could be beneficial in the context of many retinal disorders. With respect to inherited retinal dystrophies, this approach stands in contrast to gene-therapy approaches in which a separate genetic construct is likely be necessary for each disorder. After years of limited therapeutic opportunities, there are now a number of promising new approaches derived from experimental work, and it is likely that cooperative work between laboratory scientists and clinicians will transform treatment of retinal diseases over the next decade. The importance of these advances cannot be overemphasized because retinal diseases account for ⬎70% of severe vision loss in the Western world. 9. Naash MI, Hollyfield JG, al Ubaidi MR, Baehr W (1993) Proc Natl Acad Sci USA 90:5499–5503. 10. Suber ML, Pittler SJ, Qin N, Wright GC, Holcombe V, Lee RH, Craft CM, Lolley RN, Baehr W, Hurwitz RL (1993) Proc Natl Acad Sci USA 90:3968–3972. 11. Huang PC, Gaitan AE, Hao Y, Petters RM, Wong F (1993) Proc Natl Acad Sci USA 90:8484 – 8488. 12. Portera-Cailliau C, Sung CH, Nathans J, Adler R (1994) Proc Natl Acad Sci USA 91:974–978. 13. Adler R (1996) Arch Ophthalmol 114:79–83. 14. Faktorovich EG, Steinberg RH, Yasumura D, Matthes MT, LaVail MM (1990) Nature 347:83– 86. 15. LaVail MM, Unoki K, Yasumura D, Matthes MT, Yancopoulos GD, Steinberg RH (1992) Proc Natl Acad Sci USA 89:11249–11253. 16. LaVail MM, Yasumura D, Matthes MT, LauVillacorta C, Unoki K, Sung CH, Steinberg RH (1998) Invest Ophthalmol Visual Sci 39:592– 602. 17. Ali RR, Sarra GM, Stephens C, Alwis MD, Bainbridge JW, Munro PM, Fauser S, Reichel MB, Kinnon C, Hunt DM, et al. (2000) Nat Genet 25:306–310. 18. Acland GM, Aguirre GD, Ray J, Zhang Q, Aleman TS, Cideciyan AV, Pearce-Kelling SE, Anand V, Zeng Y, Maguire AM, et al. (2001) Nat Genet 28:92–95. 19. Bok D, Yasumura D, Matthes MT, Ruiz A, Duncan JL, Chappelow AV, Zolutukhin S, Hauswirth W, LaVail MM (2002) Exp Eye Res 74:719 –735. 20. Sieving PA, Caruso RC, Tao W, Coleman HR, Thompson DJ, Fullmer KR, Bush RA (2006) Proc Natl Acad Sci USA 103:3896–3901. to deliver minigenes that code for a secreted form of CNF under control of a chick ␤-actin promoter (19). Long-term, panretinal rescue of photoreceptors, as Monocyte chemoattractant protein-1 plays a critical role in inducing photoreceptor apoptosis. 2034 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0611014104 Bird