According to the World Health Organization, age-dependent macular degeneration is the most common cause of blindness in industrialized countries. Fortunately, the fruit fly's compound eyes, each containing ~800 eye units, offer an exceptional model in which to uncover the molecular basis of retinal degeneration. Housed within each of these units are the eight photoreceptor cells that serve as the functional core of photosensation. Each photoreceptor cell contains an array of densely stacked membranes called rhabdomeres – the light-gathering structures where the rhodopsin (Rh) proteins, responsible for visual signalling, reside. In addition to light detection, Rh1 has been shown to be crucial during development for morphogenesis and differentiation of photoreceptor cells. To better understand the role of rhodopsin in retinal degeneration, Inga Kristaponyte, Yuan Hong, Haiqin Lu and Bih-Hwa Shieh from Vanderbilt University, USA, used microscopy to monitor the turnover of Rh1 in live Drosophila. Using fluorescently tagged Rh1, the team found that mutant flies that were unable to produce sufficient Rh1 displayed age-dependent degeneration and had fewer rhabdomeres. Although initially present, the rhabdomeres in mutant flies became smaller and eventually disappeared, confirming that Rh1 is important for the survival of photoreceptor cells in adulthood.
Signaling proteins in photosensation are commonly modulated by phosphorylation, which allows protein partners to couple transiently and tags proteins for degradation or recycling. Mutations that result in retinal degeneration have been shown to arise from the impairment of reversible phosphorylation of rhodopsin. In the vertebrate visual system, members of the arrestin (Arr) gene family are known to interact with phosphorylated and photoactivated rhodopsin. In the Drosophila visual system, Arr2 is crucial for the inactivation and degradation of photoactivated (phosphorylated) Rh1. Knowing that in vertebrates, arrestin binds phosphorylated rhodopsin to inactivate it and then targets it for internalization, recycling or degradation, Shieh's team wondered whether the same is true for the Drosophila visual system. Generating mutant flies with Rh1 proteins that either lacked the C-terminus or carried substitutions at the putative phosphorylation sites, the team tested whether phosphorylation is required for Arr2 to interact with rhodopsin. They found that, contrary to the situation in vertebrates, Drosophila Arr2 can still bind to unphosphorylated Rh1. Thus, phosphorylation of Rh1 at its C-terminus is not needed for the Arr2 interaction.
Given that flies with insufficient Rh1 display age-dependent retinal degeneration, the researchers decided to test whether they could protect photoreceptor cells from degeneration by expressing a phosphorylation-deficient form of Rh1, which cannot be degraded. As Arr2 and Rh1 interact, Shieh's team co-expressed fluorescently tagged arrestin and phosphorylation-deficient Rh1 in mutant flies that suffer the insect equivalent of macular degeneration. They then imaged the live retinas to find out where the proteins were located in the rhabdomere and whether they could protect the photoreceptors from damage. The team found that Arr2 moves from a uniformly cytoplasmic distribution in the photoreceptors to a subcellular localization, but only when bound to Rh1, and by co-expressing the two proteins, the researchers were able to completely block retinal degradation. This led them to conclude that the loss of Rh1 causes the loss of rhabdomeres, and the expression of Arr2 and Rh1 together protected the rhabdomeres from degradation.
Shieh's team propose that future therapies explore the use of kinase inhibitors that selectively reduce the phosphorylation of rhodopsin – and hope their work will offer novel alternatives in treating arrestin-dependent retinal degeneration.