Scientists see a day when damaged vision might be repaired with regrown retinas
This article first appeared in The San Diego Union Tribune, 5/1/02.
In 1780, Charles Bonnet - one of France's leading naturalists at the time - made a curious discovery. He had bungled an attempt to remove a salamander's eye, knocking out its lens and doing other serious damage. Time passed, and Bonnet was amazed to find that the animal's eye had nearly healed, complete with a restored lens. Since human eyes and those of other mammals cannot regenerate, the salamander's ability seemed magical.
Today, taking their cue from various fish and amphibians whose eyes can regenerate when injured, an increasing number of labs are investigating this phenomenon. Their research is directed particularly at the retina, the thin tissue that lines the back of the eye.
Since studies of newts and salamanders, frogs, goldfish, zebra fish and certain other lower vertebrates have shown that key retinal cells are capable of regrowth, the question being asked is, might a diseased or injured human retina be prodded to similarly repair itself? If so, "It could revolutionize the treatment of retinal diseases," noted Michael Young, a neurobiologist at the Schepens Eye Research Institute in Boston.
The retina, observes Young, is the only major eye tissue for which ophthalmologists lack effective repair approaches. Consequently, in the Western world, the large majority of debilitating vision disorders are retina related, the most common being macular degeneration, diabetic retinopathy, glaucoma and retina pigmentosa (a syndrome of inherited diseases).
"If the lens is damaged, an artificial lens can replace it," pointed out Young. "And corneal surgeons restore eyesight all the time with cornea transplants, one of the most successful types of transplants available."
But in the case of the retina, only minimal treatment is available. Scientists' new attention to retinal regeneration is due in large part to the recent emergence of stem cell biology.
First came the observation that, contrary to near-ancient dogma, the mammalian brain indeed contains stem cells - immature cells that can differentiate into a range of adult cells.
Then, in 2000, a University of Toronto team created a stir by both isolating stem cells from human, cow and mouse eyes and using them to generate specific retinal cells.
Their data suggested that retinal regrowth didn't occur naturally in higher vertebrates because of an inhibitory factor. In other words, perhaps the retina of mammals did possess regenerative power - just like Bonnet's salamander - but in latent form.
Still at an early exploratory stage, Young and his lab mates are attempting to transplant retinal stem cells into a mouse eye, in hope that they will replace damaged photoreceptor cells. The ultimate goal is to restore a mouse's vision.
"Those are the cells that are mainly lost in blinding diseases that affect the retina. The rest of the retinal system and connections to the brain are pretty much intact," said Young.
Photoreceptor cells, which consist of rods and cones, are one of nature's supreme gifts to animals. Their intricate features turn light energy entering the eye into chemical energy, which, channeled through the optic nerve to the brain, is further transformed into imagery. In macular degeneration, the leading cause of blindness in Americans age 55 and older, photoreceptors at the retina's center (the macula) die in vast numbers.
First, the Schepens team painstakingly extracts the retinas of baby green mice. That's right - green. Under ultraviolet light, their cells literally glow green because of a gene from a fluorescent jellyfish that has been added to their genome, resulting in green protein throughout the animal.
From the retinas, the researchers isolate green stem cells, or progenitors - cells that are a tad more mature than stem cells, yet not fully differentiated. They then inject these progenitors into various places in the eye of another specially engineered mouse, one that lacks working rod cells.
According to Young, when the cells are put into the vitreous chamber in front of the retina, they've been seen to migrate into the photoreceptor region.
"We can see that, when we put them in, we do get rods," said Young. "I think we're pretty confident that they are integrated and have the capacity to function, but we don't know if they are passing visual information to the mouse."
The research presents difficulties at every turn. Because a mouse eye is so small, simply injecting cells into it is tricky business, according to Young. Further damage to the eye can easily occur.
Also, it's hard to know if the cell transplantation has actually improved an animal's vision. Behavioral tests by a visiting mouse specialist will need to be done before the Schepens group definitively knows it's on track toward vision.
In work widely reported two years ago, Young and collaborators Fred Gage of Salk Institute and Henry Klassen of Childen's Hospital of Orange County took stem cells from the hippocampus of rat brains and put them into the eyes of rats with retinal degeneration. It was the first time brain stem cells from a mammal had been transplanted into an adult retina. It had some success.
Many of the injected cells did migrate and become integrated with existing photoreceptor layers. The study's downside, Young acknowledges, was that the cells did not appear to become functional photoreceptors. Nonetheless, the results were valuable.
"What struck us was that stem cells from a completely different part of the brain can recognize a different area and respond to that local environment," said Gage.
Cells from the hippocampus actually gained properties of a retinal cell. It told the researchers that a cell's fate "is determined to a large extent by the environment rather than exclusively by the intrinsic properties of cells," he noted.
Biologist Thomas Reh at the University of Washington, Seattle, is equally hopeful that stem cells can be used to repair the retina. But he's approaching the challenge quite differently. Instead of placing cells into the eye, his lab is trying to stimulate progenitor cells already present in the vicinity of the retina to migrate to the photoreceptor region and function as photoreceptors.
Last year, Reh and teammate Andrew Fischer reported a startling ability in glial cells that could help them achieve this goal. In chicks, glial cells - which are responsible for maintaining neural connections in the brain - are able to dedifferentiate, reverting back to a stem-like or progenitor-like form.
They can then mature anew into retinal cells. Nothing is more exciting to Reh than the possibility this presents for the human eye.
"Since glial cells are spread across the retina, what might be an interesting, sensible strategy is to somehow stimulate glia to undergo this process," he explained - to cajole glial cells to dedifferentiate and then form new photoreceptors.
Fish eyes are capable of a glial-to-photoreceptor cell change. Yet it happens only in limited fashion in birds, Reh said, while mammals show no evidence of it.
The present push in Reh's lab is to see if this phenomenon - glial cells switching roles and becoming photoreceptors - can be made to happen more robustly in birds, paving the way for the same conversion in humans.
To bolster the process, the scientists inject growth factor into the vitreous of a chick eye shortly after the eye has sustained injury. The growth factor "greatly stimulates the proliferation of cells. There's a very dramatic improvement," noted Reh.
But whether there are enough new cells, and whether they are functional and able to restore the bird's sight isn't yet known. As true of the Schepens mice, assessing a chick's vision isn't easy. Reh expects it will take a long time before the procedure truly works and can serve as restorative therapy.
The question becomes: What's the best route to retinal regeneration? Is it to transplant cells into the eye or to goad in-body cells into action?
"I hear people say that the job of the endogenous stem cell people is to put transplanters out of business, which makes perfect sense," said Young. "If you can get the body to fix itself, then you don't have to do a transplant, which is invasive and difficult and fraught with rejection problems. But can it fix itself?"
Although Young and Reh are best of friends, they cite problems with each other's chosen approach.
"One problem I see with endogenous stem cells is that one is trying to induce a cell in the body that should not be proliferating to proliferate. I think the potential for stem cells to give rise to tumors is a big problem with that approach," said Young.
"Another potential disadvantage is that growth factors used to stimulate the endogenous stem cells can cause new blood vessels, which itself is a form of blindness."
Reh, meanwhile, thinks transplantation could lead to a dead end. "I'm skeptical whether transplantation will work in a highly organized tissue like the retina. I'm skeptical that, in transplanting these cells and letting them crawl around, they'll organize so well as to give back vision."
Reh feels that, in general, the history of transplanting cells is a history "littered with failure."
But researchers in both camps also have reason to be hopeful. Although the retina contains many layers of photoreceptors, it might be that, in some cases only one layer needs replenishment for vision to be restored. In the complex landscape of the eye, any positive thinking helps.