Review of insulin and insulin-like growth factor expression, signaling, and malfunction in the central nervous system: Relevance to Alzheimer's disease

Interest in characterizing the role of impaired insulin actions in Alzheimer's disease (AD) and vascular dementia is growing exponentially. This review details what is currently known about insulin, insulin-like growth factor type I (IGF-I) and IGF-II proteins and their corresponding receptors in the brain, and delineates the major controversies pertaining to alterations in the expression and function of these molecules in AD. The various experimental animal models generated by over-expression, mutation, or depletion of genes that are critical to the insulin or IGF signaling cascades are summarized, noting the degrees to which they reproduce the histopathological, biochemical, molecular, or behavioral abnormalities associated with AD. Although no single model was determined to be truly representative of AD, depletion of the neuronal insulin receptor and intracerebroventricular injection of Streptozotocin reproduce a number of important aspects of AD-type neurodegeneration, and therefore provide supportive evidence that AD may be caused in part by neuronal insulin resistance, i.e. brain diabetes. The extant literature did not resolve whether the CNS insulin resistance in AD represents a local disease process, or complication/extension of peripheral insulin resistance, i.e. chronic hyperglycemia, hyperinsulinemia, and Type 2 diabetes mellitus. The available epidemiological data are largely inconclusive with regard to the contribution of Type 2 diabetes mellitus to cognitive impairment and AD-type neurodegeneration. A major conclusion drawn from this review is that there is a genuine need for thorough and comprehensive study of the neuropathological changes associated with diabetes mellitus, in the presence or absence of superimposed AD or vascular dementia. Strategies for intervention may depend entirely upon whether the CNS disease processes are mediated by peripheral, central, or both types of insulin resistance.