Yale Medicine-Ophthalmology

Lessons from the lab

From the eyes of newts to the green glow of the jellyfish, clues to the mystery of sight.

Fifty years ago, a Yale scientist named Leon Stone made a remarkable discovery about the vision of newts. If the eye is dislodged and then reattached, the retina not surprisingly is destroyed. But in the newt, unlike humans, the cells of the retina grow back and sensitivity to light returns. Dr. Stone’s newts could regenerate retinal cells even when the eyes were completely removed and then put back.

Why should newts regain their sight while humans and other mammals lose it forever to injury or disease? What other clues from simpler organisms might there be to lead us to new treatments for vision loss? And on a much more basic level, how is it that we are able to convert the patterns of a million photons into images in our mind’s eye?

These questions drive the work of the Visual Neuroscience Program at Yale, where basic scientists pursue knowledge that may some day provide new and better treatments for retinal disorders. In addition to the explosion in molecular biology and genetics that promises to shed light on all human disease, basic research in ophthalmology benefits from an extremely rich base of knowledge generated over the past 150 years.

“More is known about the vision pathway than any other part of the nervous system,” says Colin Barnstable, D.Phil., professor of neuroscience and director of research at the Yale Eye Center. “Because vision is so important to us as humans, it has led to a fascination of how sight works. Scientists have been at it for centuries.”

Basic research at Yale is relevant to many of the degenerative diseases that rob sight. In their efforts to learn more about retinal disorders, researchers at Yale are focusing much of their attention on the photoreceptor cells, known as rods and cones, that process light energy. Macular degeneration involves the loss of cone photoreceptors in the central part of the visual field, and retinitis pigmentosa is caused by loss of rod photoreceptors that are used for night vision. Many forms of retinitis pigmentosa are caused by mutations in rod photoreceptor genes associated with the conversion of light energy into electrical nerve signals.

“We’re asking how a change in the protein leads to the death of the cell. Once we understand the sequence of events, we can design therapies to stop the death of the photoreceptor,” says Dr. Barnstable. Much of his research centers on transcription factors, proteins that regulate the expression of other genes in photoreceptors. Scientists around the world have isolated transcription factors in which mutations may be causing photoreceptors to degenerate. Dr. Barnstable and M.D./Ph.D. student Julian Martinez recently isolated a transcription factor named Erx that regulates the expression of rhodopsin, the light sensing molecule in rods. It is possible that mutations in Erx itself may cause some forms of retinitis pigmentosa. On a broader level understanding how genes like Erx work may lead to identification of new targets for drug or gene therapy.

A fluorescent marker
Five years ago, scientists at Columbia University found they could use an unusual substance produced by jellyfish to follow the activities of gene products within cells. This green fluorescent protein, or GFP as it is known to researchers today, showed enormous potential as a reporter—a detectable molecule that allows scientists to take a snapshot of a cell and determine the movement and function of proteins within it.

What was new about GFP was that for the first time one could study function without killing the cell, as previous reporter techniques based on antibodies had required. Instead of a snapshot, scientists now had a movie to study for clues to the inner working of genes and the proteins they expressed.

Yale scientist Thomas Hughes, Ph.D., was among the first to recognize the potential of the GFP reporter and to develop techniques for its use, in particular for the study of mammalian genes. “The [NIH-sponsored] Human Genome Project promises to reveal the sequence of every human gene by the year 2005,” says Dr. Hughes, associate professor of ophthalmology and visual science, and of neurobiology. “Having all the sequences is comparable to having all the books in a library. The problem is, we don’t have a card catalog. We don’t know what the genes do yet, just that they’re there.”

Dr. Hughes and his team have tacked GFP onto the proteins expressed by genes in cells from organisms as varied as yeast and human and watched them light up as they travel to the nucleus, the cell surface, or the cytoskeleton, for example. “By fusing GFP to these proteins,” he says, “we can learn where these genes are in the cell and how they are used—in a matter of two days instead of the months it used to take.”

Potential for transplantation
While a better understanding of transcription factors and gene products may someday help prevent degenerative diseases that destroy sight, another strategy is needed to help the large group of patients who have already lost the photoreceptor cells that make vision possible. One avenue being explored is the transplantation of cells to rebuild the damaged retina. For this to work, a source of photoreceptors needs to be identified.

Dr. Barnstable had long known of the work of Leon Stone with newts and wondered how the retina could regenerate. What genes might humans hold in common? Would it be possible, he wondered, to transform the pigmented cells that lie behind the retina and turn them into functioning photoreceptors?

Working in mice, the early results of research in this area have been encouraging. “So far,” Dr. Barnstable says, “we have been able to take these cells and turn them into retinal cells using a cocktail of growth factors. We also have a mouse strain in which a single mutation causes this to happen spontaneously.”

Understanding how to control this process might allow some degree of vision restoration for patients who have lost their sight. And while transforming pigment cells into functioning photoreceptors that can detect light might sound fantastic, it’s not that far-fetched. “The trillions of cells that make up the human body,” says Dr. Barnstable, “were initially derived from a single cell.” YM

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