How to keep photoreceptors aliveAlan Bird*Institute of Ophthalmology, University College London, London EC1V 9EL, United Kingdom

Over the last 40 years, therehas been increasing success inthe surgical treatment of reti-nal detachment in that the

retinal reattachment could be achievedin a high proportion of cases but visualrecovery was frequently poor. An expla-nation for this poor visual outcome wasderived from work on experimental reti-nal detachment in which it was shownthat photoreceptor cell death occurredbecause of a wave of apoptosis duringthe first few days of retinal detachment(1). It is recognized that apoptosis orprogrammed cell death is a geneticallyencoded potential of all cells (2). It ischaracterized by cleavage of most of thenuclear DNA into short but well orga-nized chains of nucleosomes in multiplesof 200 bp by an endogenous nonlysoso-mal nuclease (3, 4) and may be trig-gered by changes in the metabolic envi-ronment of the cell. The photoreceptorcells are closely approximated to theretinal pigment epithelium (Fig. 1) anddepend on the retinal pigment epithe-lium for their metabolic sustenance.Physical separation of the two and lossof metabolic exchange as occurs in reti-nal detachment (Fig. 2) might have beensupposed to cause a sequence of eventsthat would inevitably induce cell lossthat was not amenable to treatment.However, apoptosis may be manipulatedby altering the metabolic environment,and it has been shown that brain-derived growth factor injected into theeye reduces the rate of photoreceptorcell loss in experimental retinal detach-ment (5), although the precise means bywhich apoptosis was induced and themechanism of the therapeutic effectwere uncertain.

In this issue of PNAS, Nakazawa etal. (6) describe convincing evidence thatmonocyte chemoattractant protein 1(MCP-1) plays a critical role in inducingphotoreceptor apoptosis in experimentalretinal detachment in mice. It had beenreported some years ago that levels ofMCP-1 were high in the vitreous of pa-tients with retinal detachment (7).MCP-1 seemed to be an attractive can-didate as an inducer of cell loss becauseit had been proposed as playing a rolein the pathogenesis of a variety of disor-ders of the central nervous system in-cluding Alzheimer’s disease. Nakazawaet al. showed that the level of MCP-1 inmice with retinal detachment was in-creased 10-fold in the vitreous whencompared with normals and that its ex-

pression in Muller cells (a class of reti-nal glial cell) was up-regulated after72 h of detachment, a time of maximumapoptosis. Apoptosis was reduced byinjecting an anti-MCP-1 blocking frag-ment intravitreally. Apoptosis was alsoreduced by 80% in MCP-1 knockoutmice with retinal detachment. In eachcase, suppression of apoptosis was ac-companied by reduction of CD11b�

macrophage/microglial cells, invadingcells that are found universally in retinaldetachment. Interestingly, Nakazawa etal. provide evidence with both in vitroand in vivo experiments that the effectis not mediated by a direct effect ofMCP-1 on photoreceptor cells; rather, itwas mediated through the activatedmacrophage/microglial cells. This infor-mation is very important because thereare clear therapeutic implications forthe acute management of retinal detach-ment in humans. Hopefully it will bepossible to transfer these findings intotreatment strategies that can suppress thephotoreceptor cell loss such patients oftenexperience.

These findings also have broad impli-cations for retinal diseases, includingdiabetic retinopathy, retinal vascular oc-clusions, and retinal dystrophies. A briefsummary of relevant work on the patho-physiology of retinal dystrophies andexperimental therapeutics for these dys-trophies will serve to put the work ofNakazawa et al. (6) in perspective. Inboth humans and animals, there hasbeen increasing reason to believe that in

inherited retinal diseases, the metabolicdefect caused by the mutation does notcause cell death directly. This observa-tion is evident clinically with respect tocone loss in patients with retinitis pig-mentosa caused by mutations in therhodopsin gene, which is expressed ex-clusively in rod photoreceptor cells. Asimilar situation exists in mice trans-fected with a mutant rhodopsin gene (8,9). It is also illustrated in the setter dogwith progressive atrophy of both the rodand cone photoreceptors, with phospho-diesterase activity being defective inrods but not in cones (10). Most strikingare the observations on a chimera cre-ated of a pigmented mouse transfectedwith a mutant rhodopsin gene and aWT albino mouse. The setting wasachieved by generating chimeric em-bryos composed of cells from both thealbino and pigmented mouse lines. Al-though there was patchy distribution ofcells from the pigmented and nonpig-mented origin, the distribution of photo-receptor cell death followed exactly thesame pattern as that seen in the pig-mented 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: alan.bird@ucl.ac.uk.

© 2007 by The National Academy of Sciences of the USA

Fig. 2. Diagram of a cross-section of the eye withretinal detachment (open arrow) and retina hole(solid arrow). Fluid exchange between the subreti-nal space and vitreous cavity through the holecompromises metabolic exchange between the ret-ina and retinal pigment epithelium.

Fig. 1. Photomicrograph showing the close phys-ical relationship between the photoreceptor cellsof the retina and the retinal pigment epithelium(RPE). There is constant metabolic exchange be-tween the two cell systems. (Magnification:�1,200.)

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the cells containing the mutant genewere no more likely to die than the cellswith the WT rhodopsin gene. The onlyrecorded variation between animals wasthat the proportions of mutant to WTpopulations determined the speed ofdegeneration. It was concluded that thedisease was induced by the presence ofthe mutant photoreceptor cells, but celldeath was somehow related to a changein the environment of the retina ratherthan a direct and cell-autonomous effectof the mutant gene.

These observations stimulated furtherinvestigations of the cause of cell deathin retinal dystrophies, and it becameevident that cell loss is universallycaused by apoptosis (11–13). This find-ing gave rise to the concept that modu-lating the risk of apoptosis by usinggrowth factors may have therapeuticvalue. The first attempts were in RoyalCollege of Surgeons rats in which thecausative gene is expressed in the retinalpigment epithelium but causes death ofphotoreceptor cells. A single intravitrealinjection of basic fibroblast growth fac-tor caused prolongation of photorecep-tor cell life (14). A series of trials werethen undertaken in a light damagemodel (15) and genetically determineddisease in rodents (16) using a variety ofgrowth factors given as single intravit-real injections. These experimentsshowed variable rescue of photoreceptorcells depending on the neurokine usedand the disease model and mode of de-livery. Ciliary neutrophic factor (CNF)appeared to be consistently effective ina variety of models in rodent, cat, anddog. However, the rescue was inevitablyshort-lived given the limited exposure ofthe retina to the growth factor. With theadvent of successful gene transfer intothe retina (17, 18), it was possible togenerate long-term treatment. A recom-binant adeno-associated virus was used

to deliver minigenes that code for a se-creted form of CNF under control of achick �-actin promoter (19). Long-term,panretinal rescue of photoreceptors, as

measured by their survival, was achievedafter single injections into the subretinalcompartment. However, there were un-expected side effects that appeared tobe dose-related, including changes inrod photoreceptor nuclear morphologyand a reduction in light sensitivity asmeasured by the electroretinogram.

On the basis of these observations, itwas decided to extend the work to hu-mans. A phase I trial of prolonged treat-ment with CNF was undertaken inpatients with severe retinitis pigmentosaby using a slow-release biological deviceconsisting of cells transfected with thehuman CNF gene and sequesteredwithin capsules that were surgically im-planted into the vitreous cavity of theeye (20). As a safety trial, it was suc-cessful in showing no untoward effect.The severity of visual loss and the smallnumber of subjects involved was be-lieved to preclude any conclusions as toefficacy, but, surprisingly, some im-provement in vision was recorded. Ofseven eyes for which visual acuity couldbe tracked by conventional reading

charts, three eyes reached and main-tained improved acuities of 10–15 let-ters, equivalent to a two- to three-lineimprovement on standard Snellen acuitycharts. These results are very encourag-ing because the primary objective was toslow degeneration rather than cause im-provement in vision. Whether visualgain is a realistic objective will need tobe tested in a phase II trial.

For the clinician managing patientswith retinal diseases, there have beenonly limited opportunities to influencethe natural history of common diseases,and in disorders such as retinal detach-ment in which surgical treatment wasavailable, the results with respect to vi-sual recovery have frequently been dis-appointing. Similarly, laser treatment ofdiabetic retinopathy inevitably involvessubstantial retinal destruction to achievetherapeutic benefit that was often time-limited. We are now entering an era ofbiological treatment, including genetherapy and the manipulation of diseasewith neurokines, that is based on a bet-ter knowledge of pathologic mecha-nisms. The work of Nakazawa et al. (6)presents the potential for significantclinical applications by pointing to anovel therapeutic target. Limiting theretina’s exposure to MCP-1 could bebeneficial in the context of many retinaldisorders. With respect to inherited reti-nal dystrophies, this approach stands incontrast to gene-therapy approaches inwhich a separate genetic construct islikely be necessary for each disorder.After years of limited therapeutic op-portunities, there are now a number ofpromising new approaches derived fromexperimental work, and it is likely thatcooperative work between laboratoryscientists and clinicians will transformtreatment of retinal diseases over thenext decade. The importance of theseadvances cannot be overemphasized be-cause retinal diseases account for �70%of severe vision loss in the Westernworld.

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Monocytechemoattractantprotein-1 playsa critical rolein inducing

photoreceptorapoptosis.

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