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

Abstract

Diabetes is characterized by dysfunctional insulin release and action, and it is a risk factor for Alzheimers disease, the most common form of dementia. Alterations in brain insulin signalling and metabolism have been linked with Alzheimers disease. The ancestral insulin gene, Ins2, is transcribed locally within the brain. Here we demonstrate that hippocampal Ins2 mRNA is higher in females than males, and modulated by diet. To specifically determine how locally-produced insulin influences hippocampal function, 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. Using the Morris water maze, we found that visuo-spatial 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). Our data demonstrate female-specific roles for brain-derived Ins2 on learning and memory function in mice. Research in contextO_ST_ABSWhat is already known about this subject?C_ST_ABSO_LIAlzheimers disease and type 2 diabetes share common risk factors and metabolic dysfunction. C_LIO_LIInsulin and insulin signalling are known to have positive effects on neuronal health and cognition. C_LIO_LILocal insulin production has been reported in the brain, including the hippocampus, which plays critical roles in learning and memory, as well as Alzheimers pathophysiology. C_LI What is the key question?O_LIWhat are the consequences of deleting the Ins2 gene, which selectively prevents local brain insulin production, on learning and memory in mice? C_LI What are the new findings?O_LIHippocampal insulin production via the Ins2 gene is sexually dimorphic and modulated by diet. C_LIO_LIKnocking out brain insulin production in Ins2-/- mice does not have significant effects on glucose homeostasis in these diet and housing conditions. C_LIO_LIKnocking out brain insulin production in Ins2-/- mice results in learning and memory impairments in female but not male mice. C_LI How might this impact on clinical practice in the foreseeable future?O_LIThese pre-clinical observations demonstrate a causal role for insulin in learning and memory, but will require additional human studies before they can impact clinical practice. C_LI