Cognitive impairments including those associated with metabolic syndrome, mild cognitive impairment, and Alzheimer's disease (AD), are increasingly associated with deficient insulin action in the central nervous system (CNS). Evidence exists for both decreased brain insulin levels and impaired brain insulin signaling (i.e., insulin resistance) as causes for this decreased insulin action. Intranasal insulin therapy can improve cognition in both healthy subjects as well as those suffering from cognitive impairments. However, being an apolipoprotein E4 (apoE4) carrier, female, or obese is associated with decreased CNS insulin action, including a decreased therapeutic response to CNS insulin. In this application, using mice expressing human E3 or E4 under control of the mouse apoE promoter, we will investigate the mechanisms by which the apoE genotype, sex, and diet can affect insulin action in the brain. We will examine the three main mechanisms that can underlie differences in CNS insulin action: 1) insulin transport, 2) insulin retention and degradation, and 3) insulin receptor activation. In SA1, we will assess brain pharmacokinetics, including measuring the transport and brain distribution of insulin after intravenous and intranasal administration. In SA2, insulin availability will be investigated by measuring the association of apoE with the insulin receptor and by measuring levels of insulin degradation. In SA3, insulin action at the receptor level will be investigated amongst the groups and we will investigate which factor (apoE4, sex, and diet) most predominantly impacts cognition in response to chronic intranasal insulin treatment. Overall, these studies will for the first time investigate the combinatorial roles and mechanisms involved in the effects of apoE isoform, sex, and diet on decreased CNS insulin activity and cognition.
Insulin in the brain improves learning and memory in healthy people as well as those with age-related cognitive decline, mild cognitive impairments, and Alzheimer's disease. However, those persons with the apoE4 isoform, who are female, or are overweight respond much less well to insulin. We will investigate the role and mechanisms underlying effects of apoE4, sex, and high fat diet on decreased insulin signaling in the brain.
