Prolonged Melanopsin-based Photoresponses Depend in Part on RPE65 and Cellular Retinaldehyde-binding Protein (CRALBP)

Purpose: Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin and can signal light continuously for many hours. Melanopsin is excited when its chromophore retinal absorbs a photon and becomes retinal, which must be reisomerized to retinal to regenerate photoexcitable melanopsin. Due to the great distance separating ipRGCs from the retinal pigment epithelium (RPE) whose retinoid cycle produces retinal, ipRGCs had been assumed to regenerate all melanopsin molecules autonomously. Surprisingly, we previously found that pharmacologically inhibiting the retinoid cycle rendered melanopsin-based responses to prolonged illumination less sustained, suggesting that the RPE may supply retinoids to help ipRGCs regenerate melanopsin during extended photostimulation. However, the specificity of those drugs is unclear. Here, we reexamined the role of the retinoid cycle, and tested whether the RPE-to-ipRGC transport of retinoids utilizes cellular retinaldehyde-binding protein (CRALBP), present throughout the RPE and Müller glia.

Methods: To measure melanopsin-mediated photoresponses in isolation, all animals were 8- to 12-month-old rod/cone-degenerate mice. We genetically knocked out RPE-specific 65 kDa protein (RPE65), a critical enzyme in the retinoid cycle. We also knocked out the CRALBP gene mainly in Foxg1-expressing Müller cells. We obtained multielectrode-array recordings from ipRGCs in a novel RPE-attached mouse retina preparation, and imaged pupillary light reflexes .

Results: Melanopsin-based ipRGC responses to prolonged light became less tonic in both knockout lines, and pupillary light reflexes were also less sustained in RPE65-knockout than control mice.

Conclusions: These results confirm that ipRGCs rely partly on the retinoid cycle to continuously regenerate melanopsin during prolonged photostimulation, and suggest that CRALBP in Müller glia likely transports retinal from the RPE to ipRGCs - this is the first proposed functional role for CRALBP in the inner retina.