Research Sheds Light on a Key Component of the Visual System
March 26, 2008

Written by Dian Land
 
MADISON— Researchers at the UW School of Medicine and Public Health (SMPH) and colleagues at the College of Agriculture and Life Sciences (CALS) have zeroed in on a component of an important player in phototransduction—the primary molecular event underlying vision.

Understanding how this component--the gamma subunit of phosphodiasterase (PDE gamma)--works may lead in the future to the development of therapeutic strategies to treat a recently-categorized eye disease called bradyopsia, or slow vision, as well as other retinal disorders.

The work is an offshoot of years of research on G-proteins--switches at the heart of cell signaling systems--conducted in the laboratory of Arnold Ruoho, PhD, chair of the SMPH Department of Pharmacology and UWCCC member. The current research resulted from a productive collaboration between Lian-Wang Guo, PhD, an associate scientist in Ruoho’s lab, and Jikui Song, PhD, the nuclear magnetic resonance (NMR) team leader in the Center for Eukaryotic Structural Genomics under the direction of John Markey, PhD, CALS professor of biochemistry.

PDE gamma falls into the category of “intrinsically disordered proteins,” which generally are thought to be unstructured, or have rapidly shifting structures, making them difficult to study. The UW scientists used NMR spectroscopy, which provides information on the position of specific atoms within a molecule by using the magnetic properties of nuclei, to determine the structure of the subunit. This approach allowed them to “see” interactions between PDE gamma and other PDE subunits.

As reported in the Proceedings of the National Academy of Sciences on Feb. 5, 2008, the researchers found that despite being categorized as intrinsically disordered, PDE gamma is in fact partially structured, and may change shape to accommodate different partners with which it interacts.

“Like a molecular chameleon, the PDE gamma subunit is likely to adapt to its complex signaling environments,” Guo says. “We found that some structural elements pre-exist in this protein when it’s not attached to its partners.”

PDE is the central enzyme in the multi-step process of phototransduction, in which light is transformed into chemical and then electrical signals later interpreted by the brain. Light enters either rod (during darkness or low light) or cone (for day vision) photoreceptor cells in the retina. A photon of light is absorbed by a vitamin A-like molecule bound in rhodopsin, which resides in disc-like membrane structures in the cell. Rhodospin, a G-protein-coupled receptor, then undergoes a shape change that turns on transducin, a G-protein.

When transducin is activated, the GDP that usually binds to it is exchanged with GTP. Transducin then is able to interact with the PDE gamma subunit. This interaction removes the blocking effect PDE gamma usually exerts on PDE, freeing it to do its main job—to break down a messenger molecule called cyclic GMP, which photoreceptor cells depend on to convert chemical signals into electrical pulses through cyclic-GMP-controlled ion channels.

“In the dark, when there’s no phototransduction occurring, cyclic GMP levels are high because PDE is inactive since the gamma subunit is inhibiting its catalytic site. This opens cyclic- GMP-dependent ion channels to let calcium and sodium into the cell,” says Ruoho, also a member of the UW Eye Research Institute. “When the gamma unit is removed, PDE is activated, causing cyclic GMP levels to drop and channels to close.”

Visual perception requires rapid deactivation of light-stimulated responses, a process called recovery. In this process, RGS9, which regulates G-protein signaling, helps change GTP-bound transducin back to the GDP-bound inactive form. This unleashes PDE gamma from transducin to re-inhibit PDE, resulting in the cessation of PDE-catalyzed cyclic GMP breakdown.

The UW research may prove to be clinically relevant to people suffering from bradyopsia. Patients with this problem can’t see fast-moving objects, such as thrown balls during sporting events, and experience prolonged blindness upon driving from daylight into a darkened tunnel. The genetic defect causing this type of blindness has been traced to mutations in the gene that produces RGS9.

“Since PDE gamma can help turn off the signal in the absence of RGS9, pharmacologic up-regulation of the subunit in the retina of these patients, who do not have functional RGS9, could possibly restore their visual recovery,” says Guo.

Furthermore, if PDE gamma is not present in a rodent or human, the retina will not develop normally. The subunit may be useful in treating rapid retinal degeneration, a severe genetic disease.

“It’s all about achieving the right balance of cyclic GMP,” Guo says.