Merly C. Vogt

Obesity-induced overexpression of miR-802 impairs glucose metabolism through silencing of Hnf1b

Insulin resistance represents a hallmark during the development of type 2 diabetes mellitus and in the pathogenesis of obesity-associated disturbances of glucose and lipid metabolism. MicroRNA (miRNA)-dependent post-transcriptional gene silencing has been recognized recently to control gene expression in disease development and progression, including that of insulin-resistant type 2 diabetes. The deregulation of miRNAs miR-143 (ref. 4), miR-181 (ref. 5), and miR-103 and miR-107 (ref. 6) alters hepatic insulin sensitivity. Here we report that the expression of miR-802 is increased in the liver of two obese mouse models and obese human subjects. Inducible transgenic overexpression of miR-802 in mice causes impaired glucose tolerance and attenuates insulin sensitivity, whereas reduction of miR-802 expression improves glucose tolerance and insulin action. We identify Hnf1b (also known as Tcf2) as a target of miR-802-dependent silencing, and show that short hairpin RNA (shRNA)-mediated reduction of Hnf1b in liver causes glucose intolerance, impairs insulin signalling and promotes hepatic gluconeogenesis. In turn, hepatic overexpression of Hnf1b improves insulin sensitivity in Leprdb/db mice. Thus, this study defines a critical role for deregulated expression of miR-802 in the development of obesity-associated impairment of glucose metabolism through targeting of Hnf1b, and assigns Hnf1b an unexpected role in the control of hepatic insulin sensitivity.

Neonatal Insulin Action Impairs Hypothalamic Neurocircuit Formation in Response to Maternal High-Fat Feeding

Maternal metabolic homeostasis exerts long-term effects on the offsprings health outcomes. Here, we demonstrate that maternal high-fat diet (HFD) feeding during lactation predisposes the offspring for obesity and impaired glucose homeostasis in mice, which is associated with an impairment of the hypothalamic melanocortin circuitry. Whereas the number and neuropeptide expression of anorexigenic proopiomelanocortin (POMC) and orexigenic agouti-related peptide (AgRP) neurons, electrophysiological properties of POMC neurons, and posttranslational processing of POMC remain unaffected in response to maternal HFD feeding during lactation, the formation of POMC and AgRP projections to hypothalamic target sites is severely impaired. Abrogating insulin action in POMC neurons of the offspring prevents altered POMC projections to the preautonomic paraventricular nucleus of the hypothalamus (PVH), pancreatic parasympathetic innervation, and impaired glucose-stimulated insulin secretion in response to maternal overnutrition. These experiments reveal a critical timing, when altered maternal metabolism disrupts metabolic homeostasis in the offspring via impairing neuronal projections, and show that abnormal insulin signaling contributes to this effect.

CNS insulin signaling in the control of energy homeostasis and glucose metabolism – from embryo to old age

Central nervous system (CNS) insulin signaling regulates energy and glucose homeostasis by acting on hypothalamic neurocircuits and higher brain circuits such as the dopaminergic system. However, overnutrition, obesity, and type 2 diabetes mellitus (T2DM) induce insulin resistance selectively in different regions of the brain, thereby impairing energy homeostasis and augmenting disease progression. Moreover, fetal hyperinsulinemia in response to maternal overnutrition, obesity, and diabetes disrupts hypothalamic neurocircuit development and predisposes to metabolic disorders later in life. In light of the current obesity and diabetes epidemic, we review the molecular basis of insulin action and resistance in the CNS, mechanisms which are causal to the development of these metabolic disorders, both in the neonate and in the adult.

AgRP Neurons Control Systemic Insulin Sensitivity via Myostatin Expression in Brown Adipose Tissue

Activation of Agouti-related peptide (AgRP) neurons potently promotes feeding, and chronically altering their activity also affects peripheral glucose homeostasis. We demonstrate that acute activation of AgRP neurons causes insulin resistance through impairment of insulin-stimulated glucose uptake into brown adipose tissue (BAT). AgRP neuron activation acutely reprograms gene expression in BAT toward a myogenic signature, including increased expression of myostatin. Interference with myostatin activity improves insulin sensitivity that was impaired by AgRP neurons activation. Optogenetic circuitry mapping reveals that feeding and insulin sensitivity are controlled by both distinct and overlapping projections. Stimulation of AgRP → LHA projections impairs insulin sensitivity and promotes feeding while activation of AgRP → anterior bed nucleus of the stria terminalis (aBNST)vl projections, distinct from AgRP → aBNSTdm projections controlling feeding, mediate the effect of AgRP neuron activation on BAT-myostatin expression and insulin sensitivity. Collectively, our results suggest that AgRP neurons in mice induce not only eating, but also insulin resistance by stimulating expression of muscle-related genes in BAT, revealing a mechanism by which these neurons rapidly coordinate hunger states with glucose homeostasis.

AgRP Innervation onto POMC Neurons Increases with Age and Is Accelerated with Chronic High-Fat Feeding in Male Mice

In many mammals, body weight increases continuously throughout adulthood until late middle age. The hormone leptin is necessary for maintaining body weight, in that high levels of leptin promote negative energy balance. As animals age, however, their increase in body weight is accompanied by a steady rise in circulating leptin levels, indicating the progressive development of counterregulatory mechanisms to antagonize leptins anorexigenic effects. Hypothalamic neurons coexpressing agouti-related peptide (AgRP) and neuropeptide Y are direct leptin targets. These neurons promote positive energy balance, and they inhibit anorexigenic proopiomelanocortin (POMC) neurons via direct neuropeptide action and release of γ-aminobutyric acid. We show here that AgRP and neuropeptide Y innvervation onto POMC neurons increases dramatically with age in male mice. This is associated with progressive increase of inhibitory postsynaptic currents and decrease of POMC firing rate with age. Neuronal activity is significantly attenuated in POMC neurons that receive a high density of AgRP puncta. These high-density AgRP inputs correlate with leptin levels in normal mice and are nearly absent in mice lacking leptin. The progression of increased AgRP innervation onto POMC somas is accelerated in hyperleptinemic, diet-induced obese mice. Together our study suggests that modulation of hypothalamic AgRP innervation constitutes one mechanism to counter the effects of the age-associated rise in leptin levels, thus sustaining body weight and fat mass at an elevated level in adulthood.

Perinatal Programming of Metabolic Diseases: Role of Insulin in the Development of Hypothalamic Neurocircuits

It is increasingly accepted that the metabolic future of an individual can be programmed as early as at developmental stages. For instance, offspring of diabetic mothers have a greater risk of becoming obese and diabetic later in life. Animal studies have demonstrated that hyperinsulinemia and/or hyperglycemia during perinatal life permanently impair the organization and long-term function of hypothalamic networks that control appetite and glucose homeostasis. This review summarizes the main findings regarding the key regulatory roles of perinatal insulin and glucose levels on hypothalamic development and on long-term programming of metabolic diseases reported in different rodent models.

Central insulin signaling modulates hypothalamus–pituitary–adrenal axis responsiveness

Objective Obesity is often accompanied by hyperactivity of the neuroendocrine stress axis and has been linked to an increased risk of psychiatric disorders. Insulin is reciprocally regulated with the stress hormone corticosterone (CORT), raising the possibility that insulin normally provides inhibitory tone to the hypothalamus-adrenal-pituitary (HPA) axis. Here we examined whether disrupting signaling via the insulin receptor (InsR) in hypothalamic subpopulations impacts the neuroendocrine response to acute psychological stress. Methods We used Nkx2.1-Cre, Sim1-Cre and Agrp-Cre transgenic driver lines to generate conditional knockouts of InsR signaling throughout the hypothalamus, paraventricular nucleus of the hypothalamus (PVH) and in neurons expressing Agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus (ARH), respectively. We used a combination of molecular, behavioral and neuroendocrine criteria to evaluate the consequences on HPA axis responsiveness. Results Endpoints related to body weight and glucose homeostasis were not altered in any of the conditional mutant lines. Consistent with observations in the neuronal Insr knockout mice (NIRKO), baseline levels of serum CORT were similar to controls in all three lines. In male mice with broad disruptions of InsR signals in Nkx2.1-expressing regions of the hypothalamus (IRNkx2.1 KO), we observed elevated arginine vasopressin (AVP) levels at baseline and heightened neuroendocrine responses to restraint stress. IRNkx2.1 KO males also exhibited increased anxiety-like behaviors in open field, marble burying, and stress-induced hyperthermia testing paradigms. HPA axis responsivity was not altered in IRSim1 KO males, in which InsR was disrupted in the PVH. In contrast to observations in the IRNkx2.1 KO males, disrupting InsR signals in ARH neurons expressing Agrp (IRAgrp KO) led to reduced AVP release in the median eminence (ME). Conclusions We find that central InsR signals modulate HPA responsivity to restraint stress. InsR signaling in AgRP/NPY neurons appears to promote AVP release, while signaling in other hypothalamic neuron(s) likely acts in an opposing fashion. Alterations in InsR signals in neurons that integrate metabolic and psychiatric information could contribute to the high co-morbidity of obesity and mental disorders.

Single and transient Ca2+ peaks in podocytes do not induce changes in glomerular filtration and perfusion

Chronic alterations in calcium (Ca2+) signalling in podocytes have been shown to cause proteinuria and progressive glomerular diseases. However, it is unclear whether short Ca2+ peaks influence glomerular biology and cause podocyte injury. Here we generated a DREADD (Designer Receptor Exclusively Activated by a Designer Drug) knock-in mouse line to manipulate intracellular Ca2+ levels. By mating to a podocyte-specific Cre driver we are able to investigate the impact of Ca2+ peaks on podocyte biology in living animals. Activation of the engineered G-protein coupled receptor with the synthetic compound clozapine-N-oxide (CNO) evoked a short and transient Ca2+ peak in podocytes immediately after CNO administration in vivo. Interestingly, this Ca2+ peak did neither affect glomerular perfusion nor filtration in the animals. Moreover, no obvious alterations in the glomerular morphology could be observed. Taken together, these in vivo findings suggest that chronic alterations and calcium overload rather than an induction of transient Ca2+ peaks contribute to podocyte disease.

Obesity exacerbates colitis-associated cancer via IL-6-regulated macrophage polarisation and CCL-20/CCR-6-mediated lymphocyte recruitment

Colorectal cancer (CRC) is one of the most lethal cancers worldwide in which the vast majority of cases exhibit little genetic risk but are associated with a sedentary lifestyle and obesity. Although the mechanisms underlying CRC and colitis-associated colorectal cancer (CAC) remain unclear, we hypothesised that obesity-induced inflammation predisposes to CAC development. Here, we show that diet-induced obesity accelerates chemically-induced CAC in mice via increased inflammation and immune cell recruitment. Obesity-induced interleukin-6 (IL-6) shifts macrophage polarisation towards tumour-promoting macrophages that produce the chemokine CC-chemokine-ligand-20 (CCL-20) in the CAC microenvironment. CCL-20 promotes CAC progression by recruiting CC-chemokine-receptor-6 (CCR-6)-expressing B cells and γδ T cells via chemotaxis. Compromised cell recruitment as well as inhibition of B and γδ T cells protects against CAC progression. Collectively, our data reveal a function for IL-6 in the CAC microenvironment via lymphocyte recruitment through the CCL-20/CCR-6 axis, thereby implicating a potential therapeutic intervention for human patients.Inflammation can be induced by obesity, and has been linked with onset of colorectal cancer (CAC). Here the authors show in mouse models that obesity-induced interleukin-6 alters macrophage function to enhance CCL-20/CCR-6-mediated recruitment of B cells and γδ T cells, thereby promoting gut inflammation and CAC progression.