A decade-old technique that allows researchers to control brain function in lab animals could partially restore sight to the blind.
In a trial sponsored by RetroSense Therapeutics, a startup company in Ann Arbor, Michigan, doctors will inject a harmless virus loaded with DNA from photoreceptive algae into the eyes of 15 patients suffering from retinitis pigmentosa. The experimental procedure represents the first human test of optogenetics, which is a technique that genetically modifies neurons to make them responsive to light. Doctors from the Retina Foundation of the Southwest will perform the procedure, and attempt to transfer the job duties of photoreceptor cells to different cells in the eye to restore sight.
Giving Cells a New Job
Before our brains build a visual image of our world, a chain of cells in our eyes must convert light into electrical signals that are processed in the brain. Photoreceptor cells in the retina represent the first link in this chain, and they are reactive to photons, or the fundamental particle of visible light. Retinitis pigmentosa causes these cells to degenerate, and patients with this condition lose peripheral and night vision and eventually go blind. The plan is to bypass these broken photoreceptor cells and make ganglion cells, which relay signals from the retina to the brain, sensitive to light.
Doctors will inject a virus carrying DNA instructions that will coax ganglion cells into producing a light-sensitive protein called channelrhodopsin — the same protein algae use to detect light. For a decade now, neuroscience researchers have been using this method to alter brain cells in lab animals in order to activate or shut down neural pathways with light. Typically, researchers control neural behavior by implanting fiber-optic cables that shine light onto the desired location in the brain. Testing optogenetic therapy in the eye is an ideal first human trial since the procedure doesn’t require implants or complicated surgery.
“All you really have to do is inject some sort of package to deliver these light sensitive molecules into the eye,” says Thomas Greenwell, a program director in retinal neuroscience at the National Eye Institute.
Will It Work in Humans?
Previous attempts to use optogenetics to restore vision in mice and monkeys were successful. However, Yi-Zhong Wang, director of the Jones Macular Function Lab at the Retina Foundation of the Southwest, cautioned against unfounded optimism until the results from the trial come in.
“The particular challenge here is looking at how the animal model will translate to the human model,” says Wang. “The best I can say is that it’s a very exciting trial, but the outcome is still unknown.”
One conceivable problem is that when the light-sensitive ganglion cells begin firing, the brain won’t recognize the signals, Wang says. These cells will be called upon to perform an entirely different task, and it’s unclear if they’re up for it. Another potential drawback is that the channelrhodopsin protein only responds to intense light, and light of a single color (blue). The light also needs to be quite bright to activate the light-sensitive molecules. Both factors may limit what test patients ultimately see.
“Expectations could be low. A person may see light and dark, or black-and-white blobs,” Greenwell says. “It’s still really in the early stages.”
Stem Cells and Artificial Circuits
Researchers have already demonstrated limited success using artificial circuits that connect to neurons in the eye to restore sight. One such device, the Argus II, uses a camera to transmit data to electrodes that convert the information into electric pulses that are sent to the brain. Another device being developed in Germany, replaces the camera with photodiodes that communicate with neurons.
Stem cells represent another promising solution to blindness. Researchers are experimenting with ways to use a patient’s own stem cells to repair damaged cells in the eye, or at least halt the process of degeneration. Greenwell says ensuring the stem cells integrate into the eye correctly is one of the challenges with this approach.