Loss of brain insulin production impairs learning and memory in female mice

Diabetes is a metabolic disorder characterized by dysfunctional insulin release and action, and it is a risk factor for Alzheimer’s disease, the most common form of dementia. Alterations in brain insulin signalling and metabolism have been linked with Alzheimer’s disease in multiple studies. It has been previously shown that the ancestral insulin gene, Ins2, is transcribed locally within the brain. Here we demonstrate that Ins2 mRNA is higher in females than males, and modulated by diet. Moreover, we demonstrate that the Ins2 protein is found in the hippocampus, a brain region with established roles in learning and memory. To specifically determine how insulin produced locally in the brain influences hippocampal function, specifically learning and memory, we used mice with germline Ins2 knockout (Ins2 ^-/-) and the normal complement of wildtype Ins1 alleles. Compensation from the Ins1 gene ensured normal glucose tolerance, normal insulin sensitivity, normal fasting insulin, and normal body weight under these conditions. We assessed visuo-spatial learning and memory in male and female Ins2 ^-/- and wild-type littermate control mice using the Morris water maze (MWM). Learning and memory performance of female Ins2 ^-/- mice was significantly impaired relative to wild-type mice, whereas the performance of male Ins2 ^-/- and wild-type mice did not differ. We profiled isolated hippocampi from female Ins2 ^-/- and littermate control mice using RNA sequencing to provide an unbiased analysis of gene expression differences that underlie these behavioural changes. Cyclin D1 (Ccnd1) was significantly reduced in Ins2 ^-/- mice, prompting us to examine adult neurogenesis using exogenous mitotic marker EdU and the immature neuronal marker doublecortin (DCX). Collectively, our data demonstrate female-specific role for brain-derived Ins2 on learning and memory function in mice.