Our eyes are especially demanding when it comes to energy: Along with our brain, they require a substantial amount of power to keep them functioning and healthy. Now a new study by the National Eye Institute suggests that because of their high-energy demands, our eyes function at high efficiency and with little reserve capacity, which scientists say may explain why they become vulnerable to degenerative diseases.
Better understanding of how cells in the eye become susceptible to degenerative diseases may point to biomarkers that could be used to identify people at risk, and also to develop potential therapies, said the study's lead author, Anand Swaroop, Ph.D., chief of NEI's Neurobiology-Neurodegeneration and Repair Laboratory.
In degenerative diseases such as retinitis pigmentosa and age-related macular degeneration, for example, photoreceptors are among the first cells to die. These are the cells in the retina that convert light into electrical signals that are sent to the brain. To get a handle on how energy requirements may factor into photoreceptor death, Dr. Swaroop and his colleagues focused on mitochondria, power plant structures within each cell that produce the energy needed for the cell to function. They studied mitochondrial function in the photoreceptors of mice both with and without retinal degenerative diseases. Their results were published in Investigative Ophthalmology & Visual Science (IOVS).
The team analyzed samples of mouse retina using a novel approach that involved directly measuring the mitochondrial oxygen consumption rate, an indicator of how efficiently mitochondria use oxygen and energy from nutrients to make energy that can be used by the cell. The novel technique involved mounting several retinal samples onto mesh and placing them onto a microplate for analysis. Previous methods involved inserting an electrode into a suspension of retinal cells, an approach that allowed for evaluation of only one retinal sample at a time and did not capture how the cells functioned within the context of tissue.
"We show for the first time that mitochondria within photoreceptors operate at 70 to 80 percent of their maximum capacity, with very little reserve, which suggests that the cells are in a perpetual state of high metabolic stress," Dr. Swaroop said. "It's like when a rubber band gets stretched all the way to the point where adding just a little more force breaks it." The findings suggest that photoreceptors would be particularly vulnerable should mitochondria encounter stress that disrupts energy production.
"Our data strongly support the use of mitochondrial oxygen consumption rate as an early biomarker of retinal disease, before the onset of overt degeneration," he said. However, this would also require a method to assess mitochondrial function in the intact eye, without having to even touch the retina.
Methods of detecting changes to mitochondrial health may already be in the pipeline.
Raul Covian, Ph.D., a study coauthor and a staff scientist at the National Heart, Lung and Blood Institute, said that his lab has been developing techniques that help determine the structure and connectivity of mitochondria in living cells of the heart. "Although the technical details would need to be worked out to apply these methods in an intact eye, in principle we could use these experimental approaches to study mitochondrial function in intact retinas."
Taking a different approach, Bruce Berkowitz, Ph.D., an NEI-funded researcher and professor of anatomy and cell biology, and ophthalmology at Wayne State University in Detroit, recently published a study, also in IOVS, in which he and his colleagues used a noninvasive MRI-based method to measure oxidative stress in the photoreceptors of mice with diabetic retinopathy. Oxidative stress is a harmful reaction that can occur as a byproduct of oxygen consumption in cells. "With this new approach we were able to confirm that photoreceptors are a major contributor to oxidative stress in diabetic retinopathy," Dr. Berkowitz said.
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