All eyes on retinal degeneration

Feb 16, 2010

Research by Johns Hopkins sensory biologists studying fruit flies, has revealed a critical step in fly vision. Humans with problems in this same step suffer retinal dystrophies, which manifest as visual defects ranging from mild visual impairments to complete blindness. The article, published Jan. 26 in Current Biology paves the way for using the fruit fly to screen for therapies to treat human retinal degeneration.

Retinal dystrophies result from inherited defects in nearly every step of the so-called "visual cycle," a series of biochemical reactions known to occur in vertebrates, which recycles the molecule that enables light detection in eye cells. "Therapeutic approaches to tackle such retinal dystrophies are very limited," says Craig Montell, Ph.D., a professor of and of neuroscience at the Johns Hopkins University School of Medicine. "So it's useful to take advantage of simpler experimental model organisms, like , to tease apart complex systems like vision, then translate that to use in vertebrates."

This visual cycle previously was not thought to exist in invertebrate eyes. In fact, according to Montell, those who study fly vision long thought that as the molecules in the fly eye responsible for capturing photons of light can be regenerated by absorbing more light, they don't need a visual cycle for the cells to reuse the molecule.

Curious about whether one particular enzyme in the fly — pigment-cell-enriched dehydrogenase (PDH) — plays a role in the fly's ability to make the molecules that sense light, Montell and his research team generated flies carrying a mutation in the gene encoding PDH. They found the newly hatched flies lacking PDH to be totally normal in their ability to respond to light.

"It was a surprise. Initially the PDH looked dispensable as the visual responses were normal, but over time the pigment degraded," says Montell. "This led us to ask the question: If PDH doesn't make new light-sensing molecules, and flies can recycle them using light anyway, why are these flies losing their light-detecting molecules and consequently their sight?"

As it turns out, Montell and his team found PDH is required to help recycle the used light-capturing molecules in a previously unrecognized visual cycle in flies. Flies can recycle the molecules by absorbing light, but eventually the protein that holds the molecules in the cells needs to be replaced with new protein. When this happens the biochemical visual cycle is needed to regenerate the light sensing molecules. Over time, in pdh mutant flies, without a functional visual cycle, the used light-sensitive molecules were not regenerated causing cells in the retina to die, leading to loss.

To get an idea of how comparable the visual cycle is in flies and mammals, the team replaced the fly gene for PDH with a gene for a similar mammalian enzyme. These flies had normal electrical activity in cells of the retina in response to light, were able to maintain proper levels of light sensitive molecules, and had healthy retinas. This experiment showed the researchers that there are similarities in the visual cycles in mammals and flies.

"Flies are a good model in which to study and test new therapies for ," says Montell. "This research opens the door to using flies as a way to look for drugs to reduce human retinal degeneration due to defects in the visual cycle."

Explore further: 'Office life' of bacteria may be their weak spot

More information: Current Biology: www.cell.com/current-biology/

Provided by Johns Hopkins Medical Institutions

not rated yet
add to favorites email to friend print save as pdf

Related Stories

Protein protects against nerve degeneration

Nov 30, 2006

A protein called NMNAT protects against nerve cell degeneration in fruit flies and mice, said Baylor College of Medicine researchers in a report in the Public Library of Science Biology that appears online today.

Researchers id new class of photoreceptors

Apr 22, 2008

The identification of a new class of photoreceptors in the retina of fruit flies sheds light on the regulation of the pigments of the eye that confer color vision, researchers at New York University’s Center for Developmental ...

Recommended for you

Healthy humans make nice homes for viruses

Sep 16, 2014

The same viruses that make us sick can take up residence in and on the human body without provoking a sneeze, cough or other troublesome symptom, according to new research at Washington University School ...

Unraveling cell division

Sep 16, 2014

CRG researchers shed new light on mitosis. The study published in the Journal of Cell Biology describes how Topo 2 disentangles DNA molecules and is essential for proper cell division

User comments : 0