The role of insulin receptor substrate 2 in hypothalamic and β cell function

Abstract

Insulin receptor substrate 2 (Irs2) plays complex roles in energy homeostasis. We generated mice lacking Irs2 in β cells and a population of hypothalamic neurons (RIPCreIrs2KO), in all neurons (NesCreIrs2KO), and in proopiomelanocortin neurons (POMCCreIrs2KO) to determine the role of Irs2 in the CNS and β cell. RIPCreIrs2KO mice displayed impaired glucose tolerance and reduced β cell mass. Overt diabetes did not ensue, because β cells escaping Cre-mediated recombination progressively populated islets. RIPCreIrs2KO and NesCreIrs2KO mice displayed hyperphagia, obesity, and increased body length, which suggests altered melanocortin action. POMCCreIrs2KO mice did not display this phenotype. RIPCreIrs2KO and NesCreIrs2KO mice retained leptin sensitivity, which suggests that CNS Irs2 pathways are not required for leptin action. NesCreIrs2KO and POMCCreIrs2KO mice did not display reduced β cell mass, but NesCreIrs2KO mice displayed mild abnormalities of glucose homeostasis. RIPCre neurons did not express POMC or neuropeptide Y. Insulin and a melanocortin agonist depolarized RIPCre neurons, whereas leptin was ineffective. Insulin hyperpolarized and leptin depolarized POMC neurons. Our findings demonstrate a critical role for IRS2 in β cell and hypothalamic function and provide insights into the role of RIPCre neurons, a distinct hypothalamic neuronal population, in growth and energy homeostasis. IntroductionInsulin regulates peripheral energy homeostasis by acting on multiple tissues to control carbohydrate, lipid, and protein metabolism (1). Gene targeting in mice has shown that β cell deletion of the insulin receptor causes reduced first-phase insulin release, reduced β cell insulin content, and progressive deterioration in glucose tolerance (2). Early studies of the effects of insulin in the CNS demonstrated a role for intracerebroventricularly administered insulin in the control of food intake and body weight (3). Mouse brain insulin receptor deletion causes mild hyperphagia and adiposity in female mice, diet-sensitive obesity, and defects in reproductive function (4). Results from studies in which insulinomimetics and insulin receptor antisense were centrally administered also support a role for CNS insulin signaling in energy homeostasis regulation (5, 6). Insulin signaling mechanisms therefore regulate β cell and CNS function, but it is unclear which postreceptor compo-

Pancreatic deletion of insulin receptor substrate 2 reduces beta and alpha cell mass and impairs glucose homeostasis in mice

“…Mouse breeding and genotyping strategies Mice with a floxed allele of Irs2 (Irs2 tm1With mice) [8] were intercrossed with Tg(Pdx1-cre)1Herr (Pdx1-cre) mice [13] to generate compound heterozygote mice. Double heterozygote mice were crossed with Irs2 tm1With mice to obtain wild-type, Irs2 tm1With/tm1With , Pdx1-cre and Irs2 tm1With/tm1With Pdx1-cre mice.…”

“…Wildtype, Cre transgenic and Irs2 tm1With/tm1With mice were phenotypically indistinguishable and mice of these genotypes were used as controls. RIPCre (B6.Cg-tg(Ins2-cre) 25Mgn/J) mice were maintained as previously described [8]. Genotyping of the mice was performed by PCR amplification of tail DNA as described previously [8].…”

“…RIPCre (B6.Cg-tg(Ins2-cre) 25Mgn/J) mice were maintained as previously described [8]. Genotyping of the mice was performed by PCR amplification of tail DNA as described previously [8].…”

“…Deletion of the insulin-like growth factor 1 receptor gene (Igf1r) likewise impairs insulin synthesis and secretion and combined deletion of the insulin receptor gene (Insr) and Igf1r causes marked beta cell failure [6,7]. Using similar techniques, we and others have recently generated 'beta cell-specific' Irs2-null mice that have utilised a rat insulin promoter 2 Cre recombinase transgenic mouse (RIPCre or B6.Cg-tg(Ins2-cre)25Mgn/J) to delete the floxed alleles of Irs2 [8][9][10][11]. Our studies and those of others have demonstrated that beta cell deletion of Irs2 reduces beta cell mass and impairs glucose tolerance.…”

“…However, the interpretation of all these studies with respect to the precise role of INSR and IRS2 signalling in beta cell function has been complicated by features of the RIPCre transgenic mouse used to delete the floxed Insr and Irs2 alleles. First, significant Cre recombinase expression in the hypothalamus resulted in the development of obesity, hyperleptinaemia and insulin resistance, which together per se will alter beta cell function [8][9][10]. Second, RIPCre mice themselves have been reported to display mild glucose intolerance, although it was unclear whether this was due to the influence of genetic background or the effects of Cre recombinase expression in beta cells [12].…”