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The first 3D human cell culture models of Alzheimer's disease (AD) that exhibit the two main symptoms of the diseasethe formation of amyloid beta deposits followed by tau tangleswere created previously. Researchers are now using their model to examine whether the exercise-induced muscle hormone irisin affects amyloid beta pathology.
The Massachusetts General Hospital (MGH)-led study found encouraging data showing that irisin-based therapeutics might aid in the treatment of AD, which were published in the journal Neuron. In numerous AD animal models, physical activity has been found to diminish amyloid beta deposition, although the underlying processes are still unknown. Exercise raises blood levels of the hormone irisin, which is generated from muscles and controls lipid and glucose metabolism in fat tissue and boosts energy expenditure by hastening the browning of white fat tissue. Studies have revealed that irisin is present in human and mouse brains and that its levels are reduced in patients with AD and in mouse models of the condition. To test whether irisin plays a causal role in the link between exercise and reduced amyloid beta, Se Hoon Choi, PhD and Eun Hee Kim, PhD, of the Genetics and Aging Research Unit at MGH, along with additional research colleagues applied the hormone to their 3D cell culture model of AD. First, we found that irisin treatment led to a remarkable reduction of amyloid beta pathology, said Choi. Second, we showed this effect of irisin was attributable to increased neprilysin activity owing to increased levels of neprilysin secreted from cells in the brain called astrocytes. Neprilysin is an amyloid betadegrading enzyme that has been found to be elevated in the brains of mice with AD that were exposed to exercise or other conditions leading to reduced amyloid beta. The researchers uncovered even more details about the mechanisms behind irisins link to reduced amyloid beta levels. For example, they identified integrin V/5 as the receptor that irisin binds to on astrocytes to trigger the cells to increase neprilysin levels. Furthermore, they discovered that irisins binding to this receptor causes reduced signaling of pathways involving two key proteins: extracellular signal-regulated kinase (ERK) and signal activator of transcription 3 (STAT3). Reduced ERK-STAT3 signaling was critical for irisin-induced enhancement of neprilysin. Previous studies have shown that in mice, irisin injected into the blood stream can make its way into the brain, making it potentially useful as a therapeutic. Our findings indicate that irisin is a major mediator of exercise-induced increases in neprilysin levels leading to reduced amyloid beta burden, suggesting a new target pathway for therapies aimed at the prevention and treatment of Alzheimers disease, said Rudolph Tanzi, PhD, a senior author of the study and director of the Genetics and Aging Research Unit. (ANI)
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