Rod-derived cone viability factor (RdCVF) is an inactive thioredoxin secreted by rod photoreceptors that protects cones from degeneration. Because the secondary loss of cones in retinitis pigmentosa (RP) leads to blindness, the administration of RdCVF is a promising therapy for this untreatable neurodegenerative disease. Here, we investigated the mechanism underlying the protective role of RdCVF in RP. We show that RdCVF acts through binding to Basigin-1 (BSG1), a transmembrane protein expressed specifically by photoreceptors. BSG1 binds to the glucose transporter GLUT1, resulting in increased glucose entry into cones. Increased glucose promotes cone survival by stimulation of aerobic glycolysis. Moreover, a missense mutation of RdCVF results in its inability to bind to BSG1, stimulate glucose uptake, and prevent secondary cone death in a model of RP. Our data uncover an entirely novel mechanism of neuroprotection through the stimulation of glucose metabolism.

Retinitis pigmentosa is an untreatable, inherited retinal disease that leads to blindness. The disease initiates with the loss of night vision due to rod photoreceptor degeneration, followed by irreversible, progressive loss of cone photoreceptor 1-3. Cone loss is responsible for the main visual handicap, as cones are essential for day and high-acuity vision 4. Their loss is indirect, as most genes associated with retinitis pigmentosa are not expressed by these cells. We previously showed that factors secreted from rods are essential for cone viability 5-8. Here we identified one such trophic factor by expression cloning and named it rod-derived cone viability factor (RdCVF). RdCVF is a truncated thioredoxin-like protein specifically expressed by photoreceptors. The identification of this protein offers new treatment possibilities for retinitis pigmentosa. We used a viability assay based on cone-enriched primary cultures from chicken embryos 9 for expression cloning. Unlike those of mammals, bird retinas are cone-dominated. Cones represent 60-80% of the total population in cultured cells 8. Once cultured, these cells degenerate over a few days, but adding conditioned medium from wild-type mouse retinal explants delays this loss 8. We carried out a screen to isolate factors that could support cone survival. We constructed a cDNA expression library from neural retinas of 5-week-old wild-type mice and we purified plasmid DNA from pools of 100 individual clones and used them to transfect COS-1 cells. We added conditioned medium from transfected COS cells to chicken retinal cultures seeded in 96-well plates. After 7 d of culture, we carried out an automated viability assay and we screened 2,100 pools, corresponding to 210,000 individual clones. Pool 939 contained twice as many living cells as the negative controls (Fig. 1). We isolated clone 939.09.08 by limiting dilution and found that it contained a 502-bp insert with an open reading frame encoding a putative polypeptide of 109 amino acids. We named this protein rod-derived cone viability factor (RdCVF, international patent no. PCT/EP 02/03810; Supplementary Fig. 1 online).

In retinitis pigmentosa (RP), a majority of causative mutations affect genes solely expressed in rods; however, cone degeneration inevitably follows rod cell loss. Following transplantation and in vitro studies, we demonstrated the role of photoreceptor cell paracrine interactions and identified a Rod-derived Cone Viability Factor (RdCVF), which increases cone survival. In order to establish the clinical relevance of such mechanism, we assessed the functional benefit afforded by the injection of this factor in a frequent type of rhodopsin mutation, the P23H rat. In this model of autosomal dominant RP, RdCVF expression decreases in parallel with primary rod degeneration, which is followed by cone loss. RdCVF protein injections induced an increase in cone cell number and, more important, a further increase in the corresponding electroretinogram (ERG). These results indicate that RdCVF can not only rescue cones but also preserve significantly their function. Interestingly, the higher amplitude of the functional versus the survival effect of RdCVF on cones indicates that RdCVF is acting more directly on cone function. The demonstration at the functional level of the therapeutic potential of RdCVF in the most frequent of dominant RP mutations paves the way toward the use of RdCVF for preserving central vision in many RP patients.