Five million Americans suffer from Alzheimer’s disease, but scientists still have more questions about it than answers. Arguments abound over whether the hallmark protein clusters that accumulate in the brain are a cause or an effect of the illness, and current treatments do not address the main problem that causes impaired thinking: broken synapses, the junctions that allow neurons to communicate with one another. Researchers are now zeroing in on a promising missing link: mitochondria, the cell components responsible for energy regulation. In October researchers at Columbia University reported that young mice predisposed to acquiring Alzheimer’s accumulate protein clusters in synaptic mitochondria and that these clusters directly impair synapse function.

The link between mitochondria and Alzheimer’s is not exactly new. In the 1990s studies suggested that in the diseased brains of people and mice, mitochondria do not produce and distribute energy normally. And as early as 1994 researchers at the University of Kentucky showed that amyloid-beta protein fragments, the type found in Alzheimer’s, interfere with mitochondrial function. But no one knew how, exactly, mitochondria were linked to synaptic problems, if at all.

To find out, Shirley ShiDu Yan and her colleagues at the Columbia University Medical Center genetically engineered mice to overproduce a compound that leads to the formation of amyloid-beta clusters. When the mice were at various ages, the researchers isolated mitochondria from their synapses and from other brain regions. When the mice were just four months old—well before they showed symptoms of the disease—their synaptic mitochondria had accumulated approx­imately five times more amyloid protein than nonsyn­aptic mitochondria had. The affected mitochondria could no longer provide the synapses with enough energy, which ultimately prevented the synapses from functioning—providing the first direct link between cellular injury caused by amyloid protein and the characteristic breakdown of neuronal communication that occurs in Alzheimer’s patients.

The findings could provide new treatment possibilities. In earlier research Yan reported that cyclosporin D, a drug used to treat patients with organ transplants and autoim­mune diseases, prevents amyloid-beta proteins from injur­ing mitochondria. Although the drug has serious side ef­fects, Yan hopes that she can develop a similar but safer compound that prevents synaptic problems early on. Sci­entists need to “stop the disease early, before neurons have already died,” she says.