Early-onset, slow progression of cone photoreceptor dysfunction and degeneration in CNG channel subunit CNGB3 deficiency

Purpose. To investigate the progression of cone dysfunction and degeneration in CNG channel subunit CNGB3 deficiency. Methods. Retinal structure and function in CNGB3 ^-/- and wild-type (WT) mice were evaluated by electroretinography (ERG), lectin cytochemistry, and correlative Western blot analysis of cone-specific proteins. Cone and rod terminal integrity was assessed by electron microscopy and synaptic protein immunohistochemical distribution. Results. Cone ERG amplitudes (photopic b-wave) in CNGB3 ^-/- mice were reduced to approximately 50% of WT levels by postnatal day 15, decreasing further to approximately 30% of WT levels by 1 month and to approximately 20% by 12 months of age. Rod ERG responses (scotopic a-wave) were not affected in CNGB3 ^-/- mice. Average CNGB3 ^-/- cone densities were approximately 80% of WT levels at 1 month and declined slowly thereafter to only approximately 50% of WT levels by 12 months. Expression levels of M-opsin, cone transducin α-subunit, and cone arrestin in CNGB3 ^-/- mice were reduced by 50% to 60% by 1 month and declined to 35% to 45% of WT levels by 9 months. In addition, cone opsin mislocalized to the outer nuclear layer and the outer plexiform layer in the CNGB3 ^-/- retina. Cone and rod synaptic marker expression and terminal ultrastructure were normal in the CNGB3 ^-/- retina. Conclusions. These findings are consistent with an early-onset, slow progression of cone functional defects and cone loss in CNGB3 ^-/- mice, with the cone signaling deficits arising from disrupted phototransduction and cone loss rather than from synaptic defects.