Insulin and its related proteins are produced in the brain, and according to researchers from Rhode Island Hospital and Brown Medical School, reduced levels of this combination can be linked to Alzheimer's disease. The findings are reported in the March issue of the Journal of Alzheimer's Disease.
Alzheimer's disease is a progressive form of presenile dementia that is similar to senile dementia except that it usually starts in the 40s or 50s. An estimated 4 million Americans suffer from the disease, for which there is no known cure.
"What we found is 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," says senior author Suzanne M. 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 has previously been known that insulin resistance, a characteristic of diabetes, is tied to neurodegeneration. While scientists have suspected a link between diabetes and Alzheimer's disease, this is the first study to provide evidence of that connection.
By 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.
Additionally, researchers determined 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)," the paper states.
By looking at postmortem brain tissue from people diagnosed with Alzheimer's disease, researchers 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. Reserachers 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. Conversely, in the cerebellum, which is generally not affected by Alzheimer's, scientists did not see the same drop in insulin and IGF I.
"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," de la Monte says.