"We found that insulin is not just produced in the pancreas, but also in the brain. And we discovered that insulin and its growth factors, which are necessary for the survival of brain cells, contribute to the progression of Alzheimer's," said senior author Suzanne de la Monte, a neuropathologist at Rhode Island Hospital and a professor of pathology at Brown Medical School. "This raises the possibility of a type 3 diabetes."
It was previously known that insulin resistance, a characteristic of diabetes, was tied to neurodegeneration, but, according to the researchers, this is the first study to prove evidence of a link between diabetes and Alzheimer's disease.
After studying a gene abnormality in rats that blocks insulin signaling in the brain, researchers found that insulin and IGF I and II are all expressed in neurons in several regions in the brain.
They also found that a drop in insulin production in the brain contributes to the degeneration of brain cells, an early symptom of Alzheimer's.
"These abnormalities do not correspond to type 1 or type 2 diabetes, but reflect a different and more complex disease process that originates in the CNS (central nervous system)," according to the research paper.
By looking at postmortem brain tissue from people diagnosed with Alzheimer's disease, the scientists discovered that growth factors are not produced at normal levels in the hippocampus - the part of the brain responsible for memory.
The absence of these growth factors, in turn, causes cells in other parts of the brain to die. The researchers found that insulin and IGF I were significantly reduced in the frontal cortex, hippocampus and hypothalamus - all areas that are affected by the progression of Alzheimer's. However, they did not see the same drop in insulin and IGF I in the cerebellum, which is generally not affected by Alzheimer's.
"Now that scientists have pinpointed insulin and its growth factors as contributors to Alzheimer's, this opens the way for targeted treatment to the brain and changes the way we view Alzheimer's disease," said de la Monte.