The researchers genetically removed the retinal neurons, which allowed the cortical axons to move into the brain earlier than they normally would.
"We were interested in what environmental molecular cues allow the retinal neurons to control the growth of cortical neurons," said Fox, who is also an associate professor of biological sciences in Virginia Tech's College of Science. "After years of screening potential mechanisms, we found aggrecan."
Aggrecan is a protein that has been well studied in cartilage, bones, and the spinal cord, where it is abundant after injuries. According to Fox, aggrecan may be able to isolate damaged areas of the spinal cord to stop inflammation and prevent further destruction. The downside, however, is that aggrecan inhibits axonal growth, which prevents further repair from taking place.
"Axons see this environment and either stop growing or turn around and grow in the opposite direction," said Fox.
Although it is less studied in the developing brain, aggrecan appears in abundance there. In the new study, the researchers found that retinal neurons control aggrecan in a region that receives ascending signals from retinal cells as well as descending signals from the cerebral cortex.
Once the retinal neurons have made connections, they cause the release of enzymes that break down the aggrecan, allowing cortical neurons to move in.
Fox said it is interesting that the retinal axons can grow in this region of the developing brain, despite the high levels of aggrecan. He suspects that it may be because retinal neurons express a receptor integrin that cortical axons do not express.
|Contact: Paula Byron|