Abstract
Efficient chromophore supply is paramount for the continuous function of vertebrate cone photoreceptors. It is well established that isomerization of all-trans- to 11-cis- retinoid in the retinal pigmented epithelium by RPE65 is a key reaction in this process. Mutations in RPE65 result in a disrupted chromophore supply, retinal degeneration, and blindness. Interestingly, RPE65 has recently been found to also be expressed in cone photoreceptors in several species, including mouse and human. However, the functional role of cone-expressed RPE65 has remained unknown. Here, we used loss and gain of function approaches to investigate this issue. First, we compared the function of cones from control and RPE65-deficient mice. Although we found that deletion of RPE65 partially suppressed cone dark adaptation, the interpretation of this result was complicated by the abnormal cone structure and function caused by the chromophore deficiency in the absence of RPE65 in the pigmented epithelium. As an alternative approach, we generated transgenic mice to express human RPE65 in the cones of mice where RPE65 expression is normally restricted to the pigmented epithelium. Comparison of control (RPE65-deficient) and transgenic (RPE65-expressing) cones revealed no morphological or functional changes, with only a slight delay in dark adaptation, possibly caused by the buffering of retinoids by RPE65. Together, our results do not provide any evidence for a functional role of RPE65 in mouse cones. Future studies will have to determine whether cone-expressed RPE65 plays a role in maintaining the long-term homeostasis of retinoids in cones and their function and survival, particularly in humans.
Highlights
Our nighttime and daytime visual functions are mediated by two major classes of retinal photoreceptor cells, the rods and cones, respectively
Our results demonstrate that the transgenic expression of human RPE65 in normal mouse cones does not enhance the recycling of 11-cis-retinal through the intraretinal visual cycle, it is still possible that this protein could play an important structural role in species whose cones express it naturally
As the chromophore released from photoreceptors after bleaching their visual pigment is all-trans-retinol (Vitamin A), its RPE65-driven reisomerization back to 11-cis- retinoid is critical for maintaining continuous photoreceptor function
Summary
Our nighttime and daytime visual functions are mediated by two major classes of retinal photoreceptor cells, the rods and cones, respectively. Activated pigments initiate a cascade of phototransduction reactions that generates a physiological response to light[2,3] Bleached photoreceptors restore their photon-catching function by continuous recycling of all-trans-retinal back to 11-cis-retinal via a process called the visual (retinoid) cycle. The classical retinal pigmented epithelium (RPE)-based visual cycle supplies 11-cis-retinal to both rods and cones, while a more recently discovered alternative visual cycle functions through Müller glial cells of the retina to deliver chromophore to cones[4,5,6,7]. Numerous naturally occurring mutations in the Rpe[65] gene result in a disrupted visual cycle and lead to a severe clinical disease called Type 2 Leber congenital amaurosis This blinding condition is characterized by the early loss of cone-mediated vision, sensory nystagmus, and the lack of electroretinographic (ERG) signals[14]. We used loss and gain of function approaches to investigate the proposed physiological function of cone RPE65 in the alternative visual cycle, as well as its possible role as a structural and functional component of the cone OS that might support the viability and signaling of these photoreceptors under conditions of severe visual chromophore deficiency
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