Insulin receptor substrate proteins and neuroendocrine function

A family of insulin receptor substrate (IRS) proteins mediates the pleiotropic effects of insulin and insulin-like growth factor 1 (IGF-1) on cellular function by recruiting several intracellular signalling networks. Conventional murine knockout strategies have started to reveal distinct physiological roles for the IRS proteins. Deletion of Irsl produces a mild metabolic phenotype with compensated insulin resistance but also causes marked growth retardation. In contrast, mice lacking IRS-2 display nearly normal growth but develop diabetes owing to a combination of peripheral insulin resistance and β-cell failure. As well as the classical metabolic events regulated by insulin signalling pathways, studies in lower organisms have implicated insulin/IGF-1 signalling pathways in the control of food intake and reproductive function. Our analysis of IRS-2 knockout mice shows that female mice are infertile owing to defects in the hypothalamus, pituitary and gonad. IRS-2⁻¹ mice have small, anovulatory ovaries with reduced numbers of follicles. Levels of the pituitary hormones luteinizing hormone and prolactin and gonadal steroids are low in these animals. Pituitaries of IRS-2⁻¹ animals are decreased in size and contain reduced numbers of gonadotrophs. Additionally, IRS-2⁻¹ females display increased food intake and develop obesity, despite elevated leptin levels, suggesting abnormalities in hypothalamic function. Coupled with recent observations that brain-specific deletion of the insulin receptor causes a similar phenotype, these findings implicate IRS signalling pathways in the neuroendocrine regulation of reproduction and energy homeostasis.