In the study, brain deterioration was not related to the pancreas, which regulates insulin for the body. When pancreatic insulin is deficient or the body fails to respond to it, the result is Type 1 or Type 2 diabetes. Previous work by the researchers with postmortem brain tissue of Alzheimer's patients showed a strong link between insulin depletion in the brain and Alzheimer's disease, raising the possibility that Alzheimer's is a neuroendocrine disorder, or a Type 3 diabetes.
"We have demonstrated that a loss of insulin in the brain triggers the onset of Alzheimer's, probably because as the brain loses insulin, the cells that require insulin to function and survive also eventually die. The consequences are increased oxidative stress, brain deterioration, loss of cognitive function, and a buildup of plaques and tangles in the brain - all hallmarks of Alzheimer's, says senior author Suzanne M. de la Monte, MD, MPH, a neuropathologist at Rhode Island Hospital and a professor of pathology and clinical neuroscience at Brown Medical School in Providence, RI.
"We now know that if you specifically target insulin and its actions in the brain, you could develop new treatments for this disease," de la Monte says.
The study is published in the current issue (Volume 9, Issue 1) of the Journal of Alzheimer's Disease (http://www.
Researchers injected the brains of rats with Streptozotocin (STZ), a compound that when metabolized, destroys beta cells in pancreatic islets and produces diabetes. When injected directly into the brain, the treatment caused neurodegeneration, while the pancreatic islet cells remained intact. That is because insulin depletion produced by STZ was confined to the brain, just like what occurs in most cases of Alzheimer's.
"This study provides definitive evidence that impairments in insulin/IGF signaling and deficiencies in the corresponding growth factors can occur in the central nervous system (CNS) independent of Type 1 or Type 2 diabetes," the authors write.
As a result of the treatment, insulin and its IGF-I receptors were significantly reduced in the brain, triggering a cascade of neurodegeneration. Both insulin and IGF-I activate complex signaling pathways downstream, prompting energy metabolism and growth required for learning and memory, and inhibition of oxidative stress, which unchecked could trigger neurodegeneration. As insulin was depleted, neurons died and the brain dropped to half its size, a result of atrophy which is a prominent feature of Alzheimer's. At the same time, there was an increase in astrocytes and microglial cells, which are responsible for neuroinflammation, another critical and consistent feature of Alzheimer's and probably related to the increased amyloid deposition in the brain, the researchers say.
Also, there was increased activation of a kinase called GSK-3 beta. This kinase is overactive in Alzheimer's and triggers tau phosphorylation, which is known to be at the core of neurofibrillary tangles. The researchers had previously shown that tau is regulated by insulin and insulin-like growth factor (IGF-I). In the current research, they found that as insulin and IGF-I were depleted in the brain, the expression of GSK-3 beta increased, leading to oxidative stress and cell death.
While the link between insulin and tau had been established, researchers also looked at the connection between insulin and amyloid precursor protein gene expression, as increased levels could account for amyloid accumulation, or the buildup of plaques in the brain. They found that amyloid beta deposits in vessels and plaques did build up in the brain, and they suggest that these abnormalities occurred due to increased oxidative stress.
Another feature of Alzheimer's affected by impaired insulin signaling, acetylcholine deficiency, is linked to dementia and has long recognized as an early abnormality in Alzheimer's. The enzyme that makes acetylcholine, choline acetyltransferase (ChAT), was previously found to be regulated by insulin and IGF-1. In brains with Alzheimer's, impairment of insulin and IGF-I signaling mechanisms correlate with deficits in acetylcholine production. In this study, ChAT was markedly reduced in the experimental Alzheimer's model.
"Our previous work has shown that many of the important features of Alzheimer's - such as the accumulation of phosphorylated tau and the death of neurons - were somehow linked to insulin deficiency in the brain. This study shows that insulin is the controlling factor in all of these features of Alzheimer's disease," de la Monte says.
"The evidence suggests that impaired insulin and IGF signaling must be addressed in order to make significant progress in the treatment and prevention of Alzheimer's disease," she says.
This study was supported by grants from the National Institutes of Health.
Founded in 1863, Rhode Island Hospital (www.rhodeislandhospital.org) is a private, not-for-profit hospital and is the largest teaching hospital of Brown Medical School. A major trauma center for southeastern New England, the hospital is dedicated to being on the cutting edge of medicine and research. Rhode Island Hospital ranks 13th among independent hospitals who receive funding from the National Institutes of Health, with research awards of more than $27 million annually. Many of its physicians are recognized as leaders in their respective fields of oncology, cardiology, orthopedics and minimally invasive surgery. The hospital's pediatrics wing, Hasbro Children's Hospital, has pioneered numerous procedures and is at the forefront of fetal surgery, orthopedics and pediatric neurosurgery. Rhode Island Hospital is a founding member of the Lifespan health system.