How to rewire the eye
Transforming nerve cells into light-sensing cells aims to restore sight in some blind patients
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 photoreceptors, 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.
After years of work with animals, researchers are now poised to insert optogenetic molecules into the retinal cells of people. The aim is to restore vision in those whose rods and cones don’t work.
“It makes sense that the organ that is light sensitive would benefit from [optogenetics] first,” says José-Alain Sahel, director of the Vision Institute in Paris. He is involved in one of two efforts to bring optogenetics out of the lab and into the eye clinic.
Studies in people could begin next year.
Circumvent the damage
Optogenetics is, at its heart, a gene therapy. Traditional gene therapy places a healthy copy of a mutated or damaged gene into the cells of a person with an inherited condition. The healthy copy is first packed into a virus. The virus delivers the gene to the “broken” cells and unloads its cargo. Once inside the cell, the gene produces functional copies of the proteins that the original mutations damaged, and the cell starts working again.
This type of gene therapy has famously been used to treat children born with faulty immune systems. It has also restored some vision in people with a rare type of inherited blindness called Leber congenital amaurosis .
That type of blindness, however, is the absolute best-case scenario for gene therapy, says neuroscientist Botond Roska. LCA patients eligible for gene therapy still have light-gathering rods and cones in their retinas but the cells don’t work properly because they have a mutation in a gene called RPE65 (one of a dozen gene mutations that can cause LCA). Introducing the normal version of the gene allows the rods and cones to function again. However, two studies published online this month in the New England Journal of Medicine suggest that even in patients who experience vision improvements after gene therapy for LCA, the photoreceptors continue to die and vision deteriorates over time (SN Online: 5/3/15). This could mean that, for long-term benefit, another approach is needed.
Most people with inherited blindness don’t even have the hope of temporary restoration. Mutations in any of more than 250 genes may lead to blindness, says John Flannery, a cell and molecular biologist at the University of California, Berkeley. Gene therapy is currently impractical or impossible for most of those diseases, he says.
Approximately 200,000 people in the United States have inherited retinal diseases that affect the rods and cones, according to estimates from the Foundation Fighting Blindness. Once those photoreceptors are gone, there’s no bringing them back, says Roska, of the Friedrich Miescher Institute for Biomedical Research in Basel, Switzerland.
The optogenetics approach that Pan and others are studying circumvents the missing photoreceptors. That means it differs from traditional gene therapy in important ways: It doesn’t fix broken genes, so the therapy should work regardless of which of the 250 genes are causing problems. And instead of trying to resurrect dead or damaged photoreceptors, the scientists aim to transform relay cells into ersatz photoreceptors.
Pan and Dizhoor began kicking around the idea of making bipolar and ganglion cells light sensitive in 2000. In principle it sounded simple: Just move the rods’ light-sensing protein, known as rhodopsin, to the other cells. But rhodopsin doesn’t work alone. It is part of a light-driven machine in the eye that has more than a dozen parts, says Dizhoor, a molecular biologist now at Salus University in Elkins Park, Pa. “Technically, it’s just unfeasible” to move that many cogs, he says. The researchers needed a simple molecule that could make ganglion and bipolar cells sensitive to light.
The breakthrough came two years later when scientists discovered a light-responsive protein called channelrhodopsin in a single-celled algae called Chlamydomonas reinhardtii.
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.
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.
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