First identified in 2012 by researchers from the Harvard Medical School, irisin is a messenger hormone that helps convert white fat into brown fat, which produces heat. It was named after Iris, the heavenly messenger of the gods in Greek mythology.
Irisin levels increase whenever the body performs intense physical activities. Its discoverers believe that the hormone plays a significant role in the numerous health benefits enjoyed by the body during sessions of aerobic exercise.
"Exercise can improve cognitive function and has been linked to the increased expression of brain-derived neurotrophic factor (BDNF)," explained Harvard researcher Christiane Wrann. (Related: Physical exercise reduces severity of Alzheimer’s symptoms.)
The new study took a three-pronged approach to investigate the role of irisin. In the first phase, the researchers acquired samples of cerebral tissue from human brain banks for analysis.
Post-mortem tissue evaluations showed that irisin molecules travel to the hippocampus, the region of the brain responsible for storing memories. Furthermore, the brains of patients with Alzheimer's disease have much lower concentrations of irisin in the hippocampus, which is a prime target of the neurodegenerative disease.
In the second phase, the researchers set up a mouse model and tested the ability of irisin to rescue the plasticity of synapses that connect neurons. They also evaluated the hormone's ability to preserve memory in the animals.
During this phase, the researchers deliberately disabled irisin in the brains of mice. They found that low levels of the hormone weakened both memory and synaptic function. On the other hand, increasing the levels of irisin in the brain improved the performance of the synapses and enhanced memory.
The third phase focused on the influence of aerobic exercise on irisin concentrations in the brain of mice. The researchers made the animals swim for 60 minutes five days each week. Meanwhile, they designated another group of mice that did not swim regularly as the control.
The researchers reported that, compared with the control group, the group which swam regularly produced detectable amounts of irisin, proving that irisin production is linked to exercise.
As part of their experiment, the researchers also infused beta-amyloid protein into the brains of the swimming group. The buildup of beta-amyloid plaques in the brain is linked to the onset of Alzheimer's disease.
The researchers found that the presence of exercise-induced irisin protected brain neurons against the harmful effects of beta-amyloid. When they administered a pharmaceutical drug that disables irisin to the animals, the mice lost the neuroprotective benefits they got from swimming. Similarly, irisin improved the memories of mice that swam often, but irisin-blocking drugs erased this improvement based on memory tests.
The results of the three-pronged study indicated that exercise-induced irisin offers protection against Alzheimer's disease and other forms of dementia. The study was published in the journal Nature Medicine.
"In the meantime, I would certainly encourage everyone to exercise, to promote brain function and overall health," said Columbia University researcher Ottavio Arancio, a co-author of the study.
"But that's not possible for many people, especially those with age-related conditions like heart disease, arthritis, or dementia."
That's why for their next study, Arancio and his colleagues are planning to investigate possible drugs that can increase the levels of irisin in the brain without the need for exercise.