Following the initial hypothesis that cone maintenance is dependent on rod survival in a gene-independent process (Sahel et al, 1996) and the publication of the protective effect of cones resulting from photoreceptor transplantation in the rd1 mouse, a recessive model of retinitis pigmentosa (Mohand-Said et al., 1997; confirmed in Mohand-Said et al., 2000), the team of SPVN’s scientific founders started to investigate the molecular mechanisms of this protective effect. Their observations lead to co-culture experiments that revealed that the cone protection was linked to a diffusible molecule(s) secreted by rods (Mohand-Said et al., 1998).

A partial characterization, published in February 2003, showed that the protective molecule was a protein (Fintz et al., 2003). Rod-derived cone viability factor (RdCVF) was identified by high content screening of a retinal cDNA library using a functional assay on cone-enriched culture made of retina of chicken embryos, as reported in July 2004 (Leveillard et al., 2004).

The pharmacological activity of RdCVF for the treatment of RP was first demonstrated in the rd1 mice (Léveillard et al., 2004). RdCVF was shown to be expressed by all rod photoreceptors. In addition, the preferential localization of RdCVF in the cone extracellular matrix suggested that a cell surface RdCVF receptor is present on target cells (Léveillard et al., 2004). In 2006, it was discovered that some patients suffering from Leber congenital amaurosis carried variants of NXNL1, but not as a causative gene.

In May 2009, the in vivo protective effect of a synthetic human RdCVF protein on cones in the P23H dominant Rhodopsin mutant rat and their function was published (Yang et al., 2009).

In July 2010, IdV published the phenotype of the Nxnl1 mouse (Cronin et al., 2010). The loss of expression of NXNL1 in surgical specimens of retinal detachment (associated with photoreceptor damage and outer segment shortening) was reported in 2011 (Delyfer et al., 2011).

In 2010, the team at IdV launched several gene therapy internal studies and initiated a collaboration with John Flannery (UC Berkeley) to work on RdCVF delivery by using adenoassociated viral (AAV) vectors (fellowship Chateaubriand 2010-2011 to Leah Byrne) ( Sahel et al. 2013, Aït-Ali et al., 2015 ; Byrne et al., 2015).

Cone density and length of the cone outer segment were found to be higher in rd1 retina treated with AAV (Aït-Ali et al., 2015). Cone protection by RdCVF delivered by gene therapy has also been showed in the rat model of RP P23H (Sahel et al., 2013).

The protective effect on photoreceptors of RdCVFL was reported by IdV. IdV studied the function of the long form in the Nxnl1-/- KO mouse and showed that it was mainly involved as an anti-oxidant(e.g. light damage as shown below in figure 16) (Elachouri et al., 2015).

In June 2015, the team at IdV published the protective effect on cones of RdCVFL 2016 (Mei et al., 2016). In collaboration with John Flannery at Berkeley, the coinjection of two AAV7m8 vectors expressing respectively RdCVF and RdCVFL was demonstrated to be more effective and suggested that both factors should be combined to obtain a better treatment (Byrne et al. 2015).

The concept of metabolic and redox signaling in the retina (Leveillard and Ait-Ali, 2017; Leveillard and Sahel, 2017) and its mathematical model (Camacho et al., 2019) were further elaborated.

In February 2020, the outlines of RdCVF/RdCVFL therapy of retinitis pigmentosa have been published jointly by Institut de la Vision (Paris, France) and SPVN (Clerin et al., 2020).

Mechanism of action: RdCVFL protective activity on photoreceptors relies on RdCVF activity which increase glucose entry in cones and enhanced the reducing power of RdCVFL. RdCVF is a trophic factor only expressed by rods. It has been shown that its expression protects cones from degeneration by stimulation of aerobic glycolysis through the increase of glucose entry into the cells. RdCVF binds to its receptor basigin 1 expressed on the membrane of the cone. It forms a complex with the transporter of glucose (GLUT1) leading to the increased entry of glucose in cone, which stimulates the aerobic glycolysis with production of carbohydrates used for renewal of the cone outer segment to assure cone function (Aït-Ali et l., 2015, Leveillard, 2015). RdCVFL is a thioredoxin expressed by rods and cones which provides protection against oxidative stress. The reducing power of RdCVFL relies on the supply of nicotinamide adenine dinucleotide phosphate (NADPH) produced by pentose phosphate pathway (PPP). This pathway is dependent on the amount of glucose in cones (high quantity of glucose allows rederivation of glycolytic flux to PPP) (Byrne et al., 2015; Cronin et al., 2010; Elachouri et al., 2015).