Chicago-A novel compound (AAQ, Photoswitch Therapeutics) may be able to bring vision to previously blind retinas by switching on remaining ganglion cells in the inner retina.

Russell N. Van Gelder, MD, PhD, described the status of preclinical investigations using chemicals to reanimate blind murine retinas in vivo and in vitro during a retina subspecialty day presentation at the annual meeting of the American Academy of Ophthalmology.

Several strategies have been developed that soon may be used clinically to reanimate retinas without photoreceptors in the outer retina, including implantation of electronic retinal prostheses, gene therapy, and stem cell replacement therapies, said Dr. Van Gelder, holder of the Boyd L. Bucey Memorial Chair, professor of ophthalmology, and chairman, Department of Ophthalmology, University of Washington School of Medicine, and director, University of Washington Medicine Eye Institute, Seattle.

Likewise, in a second study published by Masland et al., the Boston investigators achieved a similar result using melanopsin in retinal ganglion cells.

Induced light sensitivity

Dr. Van Gelder and colleagues are looking at a third approach to restoring vision using a small, photoactivatable molecule, AAQ, to induce light sensitivity in the remaining retinal ganglion cells in degenerated retinas.

He credits the work of his collaborators Dirk Trauner and Richard Kramer and colleagues (Nature Methods. 2008;5:331-337; Angew Chem Int Ed Engl. 2009;48:9097-101), initiated at the University of California, Berkeley, as the basis for his study.

"AAQ is a small molecule with three chemical moieties: a quaternary ammonium, which can block voltage-gated potassium channels; a light isomerizable azobenzene moiety; and a third group, acrylamide, which in principle may facilitate covalent attachment to the potassium channel," Dr. Van Gelder said.

When the compound is applied to neurons, it blocks the potassium channel, with resultant cell depolarization. When irradiated with 390-nm light, the channel is unblocked. After irradiation with longer-wavelength light, 500 nm, blockage and cell depolarization again result in cell firing. Drs. Kramer and Trauner applied the AAQ compound to hippocampal brain sections in vitro, and the compound produced light-dependent firing activity.

"This compound has turned the hippocampus into a crude retina," he said.

Activating cells

Based on the results, Dr. Van Gelder and his collaborators investigated whether they could use the molecule to produce vision in a blind retina by activating retinal ganglion cells and other non-photoreceptor cells in the preserved retina.

They used multi-electrode arrays to record from the blind retinas of young, blind mice. An array of 60 electrodes was placed near the ganglion cells in vitro, which allowed recording of potentials from several hundred ganglion cells simultaneously ex vivo, he explained.

He reported that 5 minutes after a low concentration of AAQ compound was administered, the retina fired briskly in all channels in response to short wavelength light. There was a delay of several seconds after exposure to light and cell firing continued long after the light was turned off. Switching the wavelengths of light used from short to long, however, rapidly turned ganglion cells back off.

"One injection of AAQ can induce photosensitivity for many hours ex vivo," Dr. Van Gelder said. "We can repetitively stimulate with an on-off stimulus for many cycles without fatiguing."

The investigators next used the rd/rd;opn4-/- mouse in vivo model, which lacks both melanopsin and rod and cone function. This mouse has no detectable pupillary light responses after 3 months of age. When 2 µl of the AAQ compound was injected into the vitreous cavity, after turning on the lights, recipient mice had a clearly visible pupillary light response.

"This result suggested that the reanimated retina is correctly interfacing with the brain circuits and the mechanism that controls the pupillary light response," Dr. Van Gelder said.

Can it be used to restore vision?

The big question remaining to be answered is whether AAQ can be used to restore vision. One problem is that the brightness of the light required to date in the animal studies with AAQ is difficult to achieve in normal, indoor situations. He explained that an adjunctive device would likely be necessary to scan a digital light processor of the two wavelengths across the ganglion cells to stimulate individual cells.

Another issue is that the technology is nonselective, i.e., most or all ganglion cells are reanimated. How the brain will interpret the reprogrammed ganglion cell firing patterns is presently unclear.

"These preliminary results led the investigators to conclude that AAQ confers light sensitivity to genetically blind mammalian retina in vitro, that temporal control of ganglion cell firing can be controlled by wavelength, and AAQ-conferred retinal photosensitivity is efficient for partial restoration of pupillary light responses," Dr. Van Gelder concluded.

FYI

Russell N. Van Gelder, MD, PhD
E-mail: russvg@u.washington.edu

Dr. Van Gelder has no financial interest in this topic.