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Analysis of the visual cycle in cellular retinol-binding protein type I (CRBPI) knockout mice
John C Saari1, Maria Nawrot, Gregory G Garwin
1Departments of Ophthalmology, University of Washington, Seattle, Washington 98195-6485, USA. jsaari@u.washington.edu
Purpose:
To determine whether the visual cycle is affected in mice without a functional gene for cellular retinol-binding protein type I (CRBPI(-/-) mice).
Methods:
Visual-cycle retinoids and rhodopsin levels were analyzed in eyes of dark adapted (DA) CRBPI(-/-) and wild-type (wt) mice before and during recovery from a flash. The rate of dark adaptation was analyzed using electroretinography (ERG).
Results:
all-trans-retinyl esters were reduced to approximately 33% of wt levels in DA CRBPI(-/-) mice. Recovery from a flash in wt mice produced transient accumulations of all-trans-retinal and all-trans-retinyl ester, as the pulse of retinoid produced by the flash traversed the visual cycle. In CRBPI(-/-) mice, all-trans-retinal accumulated transiently, as in wt mice. However, all-trans-retinol also accumulated transiently in the neural retina, and the transient increase in all-trans-retinyl ester of the wt was reduced. Rates of 11-cis-retinal and rhodopsin formation were comparable in wt and CRBPI(-/-) mice. Dark adaptation was delayed by a factor of approximately two.
Conclusions:
The accumulation of all-trans-retinol in neural retina, in the absence of CRBPI and the reduced amount of retinyl esters in the RPE suggest that the binding protein participates in a process that drives diffusion of all-trans-retinol from photoreceptor cells to RPE, perhaps by delivering vitamin A to lecithin-retinol acyltransferase (LRAT) for esterification. Because the perturbation occurred upstream of a slow step of the visual cycle, there was no major impairment of the rate of visual pigment regeneration.