Transforming nerve cells into light-sensing cells aims to restore sight in some blind patients
SEEING THE LIGHT The brain’s window on the visual world is a multi-layered tissue at the back of the eye called the retina. Light-detecting rods and cones sit at the very back of the tissue. They pass information to the brain via bipolar cells and ganglion cells. Humans and some animals have sharp vision thanks to the fovea, a window in the retina that offers direct access to the cones.
A man who had been blind for 50 years allowed scientists to insert a tiny electrical probe into his eye.
The man’s eyesight had been destroyed and the photo-receptors, or light-gathering cells, at the back of his eye no longer worked. Those cells, known as rods and cones, are the basis of human vision. Without them, the world becomes gray and formless, though not completely black. The probe aimed for a different set of cells in the retina, the ganglion cells, which, along with the nearby bipolar cells, ferry visual information from the rods and cones to the brain.
No one knew whether those information-relaying cells still functioned when the rods and cones were out of service. As the scientists sent pulses of electricity to the ganglion cells, the man described seeing a small, faint candle flickering in the distance. That dim beacon was a sign that the ganglion cells could still send messages to the brain for translation into images.
That 1990s experiment and others like it sparked a new vision for researcher Zhuo-Hua Pan of Wayne State University in Detroit. He and his colleague Alexander Dizhoor wondered if, instead of tickling the cells with electricity, scientists could transform them to sense light and do what rods and cones no longer could.
The approach is part of a revolutionary new field called optogenetics. Optogeneticists use molecules from algae or other microorganisms that respond to light or create new molecules to do the same, and insert them into nerve cells that are normally impervious to light. By shining light of certain wavelengths on the molecules, researchers can control the activity of the nerve cells.
Optogenetics is a powerful tool for probing the inner workings of the brain. In mice, researchers have used optogenetics to study feeding behavior, map aggression circuits and even alter memories.
Channelrhodopsins form channels in a cell’s outer membrane. When certain wavelengths of light hit the protein, the channel opens and lets positively charged ions flow into the cell. That flow of energy is a nerve cell’s signal to talk to its neighbors and to the brain. Pan and Dizhoor immediately recognized its potential.
“We thought, ‘Wow! This is the molecule we’ve been waiting for,’ ” Pan says.
They lost little time packing a gene encoding a specific channelrhodopsin, ChR2, into a virus that could infect ganglion cells in blind mice. The researchers reported in Neuron in 2006 that the protein could make the cells light sensitive and send a message to the brain in response to blue light shone into the eyes of the mice.
A gaggle of ganglion cells
The experiment was just the first step toward restoring vision, though. Researchers have had to wrangle with the issue of which of the cells — ganglion or bipolar — might restore the most vision. Each type of cell has its pros and cons.
To understand the dilemma requires some clarity on how the eye works. Light enters the eye through the pupil and is focused on the retina, a thin, multilayer tissue in the back of the eye.
Light first encounters the retinal ganglion cells. These nerve cells have long tails that bundle together to form the optic nerve and send messages to the brain about what the eye detects. They aren’t normally light sensitive. Neither are the bipolar cells, the next layer of cells that light hits. Below both these layers, at the very back of the eye, are the light-detecting rods and cones. Bipolar cells collect light information from these photoreceptor cells and pass it to the ganglion cells, which send it on to the visual processing areas in the brain. Unlike mouse eyes, human eyes have a tiny window called the fovea where bipolar cells and ganglion cells sit off to the side, allowing light to shine directly on the photoreceptors.
The ganglion cells are easiest to reach, which makes them appealing for optogenetics. But human eyes contain about 20 different types of retinal ganglion cells, each of which may convey slightly different visual information to the brain.
Variety may spice up life, but it’s potentially the main strike against ganglion cells as a target for optogenetics. That’s because the viruses used to ferry optogenetic molecules cannot distinguish between the various ganglion cells. Optogeneticists and gene therapists favor viruses called adeno-associated viruses for delivering their cargo. The viruses come in a variety of packages that determine which types of cells they can infect, but no one has devised a package that will dock only with particular ganglion cell types.
The problem, then, is that optogenetic proteins could be made, and activated, in all 20 ganglion-cell varieties at the same time, including ones that send contradictory information to the brain, says Sahel, in Paris. “It’s like saying yes and no to the same thing,” he says.
Dizhoor has always thought the bipolar cells were the way to go. After all, they are the natural middlemen between the photoreceptors and the ganglion cells. If their connections with the ganglion cells still hold in degenerated retinas, activating the bipolar cells, which come in two major varieties, should give a less noisy picture of the world than 20 types of ganglion cells chattering at once.
Bipolar cells are described as either ON or OFF. ON bipolar cells are activated when light levels increase, like when you switch on a lamp in a dark room or walk outside into bright sunlight. OFF bipolar cells get excited when light levels decrease. In 2008, Roska and his colleagues put ChR2 into ON bipolar cells in blind mice, enabling them to see patterns about half as well as mice with normal sight.
So far, researchers haven’t demonstrated that targeting bipolar cells paints a clearer picture of the world than targeting ganglion cells does. Plus, bipolar cells are hard to reach. The viruses have to be injected under the retina, risking detaching the fragile tissue.
No matter which cells they target, researchers have gotten so good at using optogenetics to restore vision in blind mice that every experiment is virtually guaranteed to work, Roska says. New researchers in his lab often mistake blind mice that have had optogenetic therapy for normally sighted mice. Unfortunately, experience with mice doesn’t make moving the technology into humans any easier, he says. “You have to re-engineer everything you have.”