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Retinal rod photoreceptor cells have double membrane discs located in their outer segments (ROS) that are continuously formed proximally from connecting cilia (CC) and phagocytized distally by the retinal pigmented epithelium. The major component of these rod discs, the light-sensitive visual pigment rhodopsin (Rho), consists of an opsin protein linked to 11- cis -retinal. The P23H mutation of rod opsin (P23H opsin) is the most common cause of human blinding autosomal dominant retinitis pigmentosa (adRP). A mouse model of adRP with this mutation ( Rho P23H/+  ) shows low levels of P23H opsin protein, partial misalignment of discs and progressive retinal degeneration. However, the impact of mutant P23H opsin on the formation of abnormal discs is unclear and it is still unknown whether this mutant pigment can mediate phototransduction. Using transretinal ERG recordings, we demonstrate that P23H mutant Rho can trigger phototransduction but  Rho P23H/P23H   rods are ∼17 000-fold less sensitive to light than  Rho  +/+  rods and produce abnormally fast photo-responses. By analyzing homozygous  Rho P23H/P23H   knock-in mice, we show that P23H opsin is transported to ciliary protrusions where it forms sagittally elongated discs. Transmission electron microscopy of postnatal day (PND) 14  Rho P23H/+   mouse retina revealed disordered sagittally oriented discs before the onset of retinal degeneration. Surprisingly, we also observed smaller, immature sagittally oriented discs in PND14  Rho +/   −  and  Rho +/+   mice that were not seen in older animals. These findings provide fundamental insights into the pathogenesis of the P23H mutant opsin and reveal a novel early sagittally aligned disc formation step in normal ROS disc expansion.

P23H opsin knock-in mice reveal a novel step in retinal rod disc morphogenesis