GRF2 Is Crucial for Cone Photoreceptor Viability and Ribbon Synapse Formation in the Mouse Retina

Using constitutive GRF1/2 knockout mice, we showed previously that GRF2 is a key regulator of nuclear migration in retinal cone photoreceptors. To evaluate the functional relevance of that cellular process for two putative targets of the GEF activity of GRF2 (RAC1 and CDC42), here we compared the structural and functional retinal phenotypes resulting from conditional targeting of RAC1 or CDC42 in the cone photoreceptors of constitutive GRF2^KO and GRF2^WT mice. We observed that single RAC1 disruption did not cause any obvious morphological or physiological changes in the retinas of GRF2^WT mice, and did not modify either the phenotypic alterations previously described in the retinal photoreceptor layer of GRF2^KO mice. In contrast, the single ablation of CDC42 in the cone photoreceptors of GRF2^WT mice resulted in clear alterations of nuclear movement that, unlike those of the GRF2^KO retinas, were not accompanied by electrophysiological defects or slow, progressive cone cell degeneration. On the other hand, the concomitant disruption of GRF2 and CDC42 in the cone photoreceptors resulted, somewhat surprisingly, in a normalized pattern of nuclear positioning/movement, similar to that physiologically observed in GRF2^WT mice, along with worsened patterns of electrophysiological responses and faster rates of cell death/disappearance than those previously recorded in single GRF2^KO cone cells. Interestingly, the increased rates of cone cell apoptosis/death observed in single GRF2^KO and double-knockout GRF2^KO/CDC42^KO retinas correlated with the electron microscopic detection of significant ultrastructural alterations (flattening) of their retinal ribbon synapses that were not otherwise observed at all in single-knockout CDC42^KO retinas. Our observations identify GRF2 and CDC42 (but not RAC1) as key regulators of retinal processes controlling cone photoreceptor nuclear positioning and survival, and support the notion of GRF2 loss-of-function mutations as potential drivers of cone retinal dystrophies.