Previous studies have shown that the bet-amyloid proteins found in Alzheimer's disease also accumulate in the retinas of people with age-related macular degeneration. Now, a new study reveals insights that help to better understand how the retina becomes damaged in this way.
The researchers - from Southampton University in the United Kingdom - report their findings in the journal Experimental Eye Research.
Age-related macular degeneration (AMD) is a progressive eye disease that kills the light-sensitive cells (retinal photoreceptors) of the retina - the layer of tissue at the back of the eye.
AMD is a leading cause of vision loss in people aged 50 and older and affects some 50 million people worldwide.
Most of the damage in AMD occurs in the macula - an area near the center of the retina needed for seeing objects directly in front of us clearly and sharply. As AMD progresses, it gets harder to do everyday things like drive, read, do close work, use a computer, and recognize faces.
One change that occurs in the retina of people with AMD as the disease progresses is an increase in the number and size of fatty deposits called drusen.
In their study paper, the researchers explain that the causes of AMD are understood to be complex, with both genetic as well as environmental risks factors, and share similarities with Alzheimer's disease.
They note how recent studies have shed a lot of light on the genetic causes of AMD - although this is not matched by insights into the molecular mechanisms involved.
However, they also note that other studies have found aged and AMD retinas also show accumulation of the types of beta-amyloid proteins that are found in toxic plaques in the brains of people with Alzheimer's disease, and "for which there appears to be no clear genetic basis."
Amyloid proteins took less than 24 hours to enter retinal cells
Deposits of Alzheimer's-related beta-amyloid have been found in various parts of the retinas of people with AMD - including inside drusen and the light-sensitive cells.
- AMD has few symptoms in the early stages, so it is important to get your eyes checked regularly
- Not everyone with early AMD will develop late AMD
- AMD arises less often in people who take regular exercise, do not smoke, and have healthy diets that include green leafy vegetables and fish.
For their study, Dr. Arjuna Ratnayaka, a lecturer in vision sciences at Southampton, and colleagues used cell cultures and mouse models of AMD to investigate mechanisms of Alzheimer's beta-amyloid accumulation inside retinal cells.
They were particularly interested in the speed with which the proteins find their way inside the retinal cells.
The researchers found the retinal cells internalized the amyloid proteins within 24 hours of being exposed to them.
They also discovered that the amyloid proteins are retained inside the retinal cells, where they gradually impair a molecular mechanism reliant on the protein encoded by the MAP-2 gene. Among other things, MAP-2 mechanisms help to maintain important structures inside cells called microtubules.
Dr. Ratnayaka says they were surprised at the speed with which the amyloid proteins entered the cells, and he suggests the finding may help explain how a healthy retina can switch to a diseased, AMD retina.
The team is now planning to evaluate how the beta-amyloid proteins actually enter the retinal cells and set about causing internal damage. The hope is the continuing work will lead to measures to prevent or treat AMD.
"We know that AMD is caused by a combination of genetic, environmental and lifestyle risk factors, but this novel discovery could open up new possibilities to understand how the aging retina becomes damaged. Such advances are important if we are to develop better AMD treatments in the future."
Dr. Arjuna Ratnayaka