How Much Arrestin Is Good for Rod Photoreceptors?

Abstract

Abstract: : Purpose: To determine minimum arrestin expression levels necessary to keep photoreceptors healthy in normal cyclic light and to determine the mechanism of photoreceptor degeneration triggered by very high arrestin expression. Methods:Arrestin expression was measured by quantitative Western blot and normalized to rhodopsin expression levels in the same retina. The thickness of the outer nuclear layer (ONL) and the length of rod outer segments (OS) were used to determine the state of the photoreceptors in mice expressing WT and mutant phosphorylation–independent arrestin at the levels from 0.5% to 300% normal. Ratios of photoreceptors undergoing apoptosis were estimated as the frequency of TUNEL–positive nuclei in the ONL. Results: The morphology of the retinas of transgenic mice expressing phosphorylation–independent arrestin mutants on arrestin knockout (AKO) background at levels of 3%, 6%, 10%, and 46% of normal were indistinguishable from WT and arrestin+/– retinas for at least 6 months. Arrestin expression at the level of 0.5% failed to prevent the disappearance of OS and subsequent thinning of the ONL characteristic for AKO mice raised under cyclic light. The expression of phosphorylation–independent arrestin mutant at the level of 200% induced rapid retinal degeneration, although surviving rods (in sharp contrast to AKO mice) retained their OS for at least 4 months. Photoreceptors were found to undergo apoptotic cell death in these mice. The highest ratio of apoptotic nuclei in ONL was reached at around 7 weeks of age, and by 16–17 weeks their ONL was only 2–3 nuclei thick. Transgene expression on arrestin +/– and arrestin +/+ backgrounds progressively increases the rate of photoreceptor apoptosis as compared to AKO background. Conclusions: Arrestin expression levels as low as 3% of normal are sufficient to maintain photoreceptor health. High expression of transgenic phosphorylation–independent arrestin mutant (200% of normal) induces rapid retinal degeneration involving apoptotic death of photoreceptors. Increasing expression of endogenous WT arrestin in addition to mutant progressively exacerbates degeneration, suggesting that high arrestin expression per se, rather than phosphorylation–independent nature of the transgene, is responsible.

Keywords: photoreceptors • retinal degenerations: cell biology • signal transduction