Hindbrain insulin controls feeding behavior

OBJECTIVE

Pancreatic insulin was discovered a century ago, and this discovery led to the first lifesaving treatment for diabetes. While still controversial, nearly one hundred published reports suggest that insulin is also produced in the brain, with most focusing on hypothalamic or cortical insulin-producing cells. However, specific function for insulin produced within the brain remains poorly understood. Here we identify insulin expression in the hindbrain's dorsal vagal complex (DVC), and determine the role of this source of insulin in feeding and metabolism, as well as its response to diet-induced obesity in mice.

METHODS

To determine the contribution of Ins2-producing neurons to feeding behavior in mice, we used the cross of transgenic RipHER-cre mouse and channelrhodopsin-2 expressing animals, which allowed us to optogenetically stimulate neurons expressing Ins2 in vivo. To confirm the presence of insulin expression in Rip-labeled DVC cells, in situ hybridization was used. To ascertain the specific role of insulin in effects discovered via optogenetic stimulation a selective, CNS applied, insulin receptor antagonist was used. To understand the physiological contribution of insulin made in the hindbrain a virogenetic knockdown strategy was used.

RESULTS

Insulin gene expression and presence of insulin-promoter driven fluorescence in rat insulin promoter (Rip)-transgenic mice were detected in the hypothalamus, but also in the DVC. Insulin mRNA was present in nearly all fluorescently labeled cells in DVC. Diet-induced obesity in mice altered brain insulin gene expression, in a neuroanatomically divergent manner; while in the hypothalamus the expected obesity-induced reduction was found, in the DVC diet-induced obesity resulted in increased expression of the insulin gene. This led us to hypothesize a potentially divergent energy balance role of insulin in these two brain areas. To determine the acute impact of activating insulin-producing neurons in the DVC, optic stimulation of light-sensitive channelrhodopsin 2 in Rip-transgenic mice was utilized. Optogenetic photoactivation induced hyperphagia after acute activation of the DVC insulin neurons. This hyperphagia was blocked by central application of the insulin receptor antagonist S961, suggesting the feeding response was driven by insulin. To determine whether DVC insulin has a necessary contribution to feeding and metabolism, virogenetic insulin gene knockdown (KD) strategy, which allows for site-specific reduction of insulin gene expression in adult mice, was used. While chow-fed mice failed to reveal any changes of feeding or thermogenesis in response to the KD, mice challenged with a high-fat diet consumed less food. No changes in body weight were identified, possibly resulting from compensatory reduction in thermogenesis.

CONCLUSIONS

Together, our data suggest an important role for hindbrain insulin and insulin-producing cells in energy homeostasis.

δ-cells and β-cells are electrically coupled and regulate α-cell activity via somatostatin

Key points

We used a mouse expressing a light‐sensitive ion channel in β‐cells to understand how α‐cell activity is regulated by β‐cells.

Light activation of β‐cells triggered a suppression of α‐cell activity via gap junction‐dependent activation of δ‐cells.

Mathematical modelling of human islets suggests that 23% of the inhibitory effect of glucose on glucagon secretion is mediated by β‐cells via gap junction‐dependent activation of δ‐cells/somatostatin secretion.

Abstract

Glucagon, the body's principal hyperglycaemic hormone, is released from α‐cells of the pancreatic islet. Secretion of this hormone is dysregulated in type 2 diabetes mellitus but the mechanisms controlling secretion are not well understood. Regulation of glucagon secretion by factors secreted by neighbouring β‐ and δ‐cells (paracrine regulation) have been proposed to be important. In this study, we explored the importance of paracrine regulation by using an optogenetic strategy. Specific light‐induced activation of β‐cells in mouse islets expressing the light‐gated channelrhodopsin‐2 resulted in stimulation of electrical activity in δ‐cells but suppression of α‐cell activity. Activation of the δ‐cells was rapid and sensitive to the gap junction inhibitor carbenoxolone, whereas the effect on electrical activity in α‐cells was blocked by CYN 154806, an antagonist of the somatostatin‐2 receptor. These observations indicate that optogenetic activation of the β‐cells propagates to the δ‐cells via gap junctions, and the consequential stimulation of somatostatin secretion inhibits α‐cell electrical activity by a paracrine mechanism. To explore whether this pathway is important for regulating α‐cell activity and glucagon secretion in human islets, we constructed computational models of human islets. These models had detailed architectures based on human islets and consisted of a collection of >500 α‐, β‐ and δ‐cells. Simulations of these models revealed that this gap junctional/paracrine mechanism accounts for up to 23% of the suppression of glucagon secretion by high glucose.

We used a mouse expressing a light‐sensitive ion channel in β‐cells to understand how α‐cell activity is regulated by β‐cells.

Light activation of β‐cells triggered a suppression of α‐cell activity via gap junction‐dependent activation of δ‐cells.

Mathematical modelling of human islets suggests that 23% of the inhibitory effect of glucose on glucagon secretion is mediated by β‐cells via gap junction‐dependent activation of δ‐cells/somatostatin secretion.

Sox5 regulates beta-cell phenotype and is reduced in type 2 diabetes

Type 2 diabetes (T2D) is characterized by insulin resistance and impaired insulin secretion, but the mechanisms underlying insulin secretion failure are not completely understood. Here, we show that a set of co-expressed genes, which is enriched for genes with islet-selective open chromatin, is associated with T2D. These genes are perturbed in T2D and have a similar expression pattern to that of dedifferentiated islets. We identify Sox5 as a regulator of the module. Sox5 knockdown induces gene expression changes similar to those observed in T2D and diabetic animals and has profound effects on insulin secretion, including reduced depolarization-evoked Ca²⁺-influx and β-cell exocytosis. SOX5 overexpression reverses the expression perturbations observed in a mouse model of T2D, increases the expression of key β-cell genes and improves glucose-stimulated insulin secretion in human islets from donors with T2D. We suggest that human islets in T2D display changes reminiscent of dedifferentiation and highlight SOX5 as a regulator of β-cell phenotype and function.

Optogenetic Control of Pancreatic Islets

In light of the emerging diabetes epidemic, new experimental approaches in islet research are needed to elucidate the mechanisms behind pancreatic islet dysfunction and to facilitate the development of more effective therapies. Optogenetics has created numerous new experimental tools enabling us to gain insights into processes little was known about before. The spatial and temporal precision that it can achieve is also attractive for studying the cells of the pancreatic islet and we set out to explore the possibilities of this technology for our purposes. We here describe how to use the islets of an "optogenetic beta-cell" mouse line in islet batch incubations and Ca(2+) imaging experiments. This protocol enables light-induced insulin release and provides an all-optical solution to control and measure intracellular Ca(2+) levels in pancreatic beta-cells. The technique is easy to set up and provides a useful tool for controlling the activity of distinct islet cell populations.

Elevated Basal Insulin Secretion in Type 2 Diabetes Caused by Reduced Plasma Membrane Cholesterol

Elevated basal insulin secretion under fasting conditions together with insufficient stimulated insulin release is an important hallmark of type 2 diabetes, but the mechanisms controlling basal insulin secretion remain unclear. Membrane rafts exist in pancreatic islet cells and spatially organize membrane ion channels and proteins controlling exocytosis, which may contribute to the regulation of insulin secretion. Membrane rafts (cholesterol and sphingolipid containing microdomains) were dramatically reduced in human type 2 diabetic and diabetic Goto-Kakizaki (GK) rat islets when compared with healthy islets. Oxidation of membrane cholesterol markedly reduced microdomain staining intensity in healthy human islets, but was without effect in type 2 diabetic islets. Intriguingly, oxidation of cholesterol affected glucose-stimulated insulin secretion only modestly, whereas basal insulin release was elevated. This was accompanied by increased intracellular Ca²⁺ spike frequency and Ca²⁺ influx and explained by enhanced single Ca²⁺ channel activity. These results suggest that the reduced presence of membrane rafts could contribute to the elevated basal insulin secretion seen in type 2 diabetes.

Dual Effect of Rosuvastatin on Glucose Homeostasis Through Improved Insulin Sensitivity and Reduced Insulin Secretion

Statins are beneficial in the treatment of cardiovascular disease (CVD), but these lipid-lowering drugs are associated with increased incidence of new on-set diabetes. The cellular mechanisms behind the development of diabetes by statins are elusive. Here we have treated mice on normal diet (ND) and high fat diet (HFD) with rosuvastatin. Under ND rosuvastatin lowered blood glucose through improved insulin sensitivity and increased glucose uptake in adipose tissue. In vitro rosuvastatin reduced insulin secretion and insulin content in islets. In the beta cell Ca^2 + signaling was impaired and the density of granules at the plasma membrane was increased by rosuvastatin treatment. HFD mice developed insulin resistance and increased insulin secretion prior to administration of rosuvastatin. Treatment with rosuvastatin decreased the compensatory insulin secretion and increased glucose uptake. In conclusion, our data shows dual effects on glucose homeostasis by rosuvastatin where insulin sensitivity is improved, but beta cell function is impaired.

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Rosuvastatin lowered blood glucose in vivo most likely due to improved glucose uptake.

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Rosuvastatin reduced insulin content and impaired Ca^2 + signaling in beta cells leading to reduced insulin secretion.

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Dual effects of rosuvastatin in HFD mice though decreased compensatory insulin secretion and increased glucose uptake.

Statins are a group of drugs used to lower blood cholesterol in individuals with a risk of developing cardiovascular disease. It has been shown in several studies that statins increase the risk of developing type 2 diabetes. This increased risk has not yet been explained. We have investigated the effect of rosuvastatin on blood glucose regulation in mice. We found that rosuvastatin has a beneficial effect on glucose uptake in muscles which results in lowered blood glucose. However, in the insulin producing beta cells rosuvastatin altered normal cell function something that might increase the risk of developing type 2 diabetes.

Optogenetic control of insulin secretion in intact pancreatic islets with β-cell-specific expression of Channelrhodopsin-2

Insulin is secreted from the pancreatic β-cells in response to elevated glucose. In intact islets the capacity for insulin release is determined by a complex interplay between different cell types. This has made it difficult to specifically assess the role of β-cell defects to the insulin secretory impairment in type 2 diabetes. Here we describe a new approach, based on optogenetics, that enables specific investigation of β-cells in intact islets. We used transgenic mice expressing the light-sensitive cation channel Channelrhodopsin-2 (ChR2) under control of the insulin promoter. Glucose tolerance in vivo was assessed using intraperitoneal glucose tolerance tests, and glucose-induced insulin release was measured from static batch incubations. ChR2 localization was determined by fluorescence confocal microscopy. The effect of ChR2 stimulation with blue LED light was assessed using Ca(2+) imaging and static islet incubations. Light stimulation of islets from transgenic ChR2 mice triggered prompt increases in intracellular Ca(2+). Moreover, light stimulation enhanced insulin secretion in batch-incubated islets at low and intermediate but not at high glucose concentrations. Glucagon release was not affected. Beta-cells from mice rendered diabetic on a high-fat diet exhibited a 3.5-fold increase in light-induced Ca(2+) influx compared with mice on a control diet. Furthermore, light enhanced insulin release also at high glucose in these mice, suggesting that high-fat feeding leads to a compensatory potentiation of the Ca(2+) response in β-cells. The results demonstrate the usefulness and versatility of optogenetics for studying mechanisms of perturbed hormone secretion in diabetes with high time-resolution and cell-specificity.

The human L-type calcium channel Cav1.3 regulates insulin release and polymorphisms in CACNA1D associate with type 2 diabetes

AIMS/HYPOTHESIS

Voltage-gated calcium channels of the L-type have been shown to be essential for rodent pancreatic beta cell function, but data about their presence and regulation in humans are incomplete. We therefore sought to elucidate which L-type channel isoform is functionally important and its association with inherited diabetes-related phenotypes.

METHODS

Beta cells of human islets from cadaver donors were enriched using FACS to study the expression of the genes encoding voltage-gated calcium channel (Cav)1.2 and Cav1.3 by absolute quantitative PCR in whole human and rat islets, as well as in clonal cells. Single-cell exocytosis was monitored as increases in cell capacitance after treatment with small interfering (si)RNA against CACNA1D (which encodes Cav1.3). Three single nucleotide polymorphisms (SNPs) were genotyped in 8,987 non-diabetic and 2,830 type 2 diabetic individuals from Finland and Sweden and analysed for associations with type 2 diabetes and insulin phenotypes.

RESULTS

In FACS-enriched human beta cells, CACNA1D mRNA expression exceeded that of CACNA1C (which encodes Cav1.2) by approximately 60-fold and was decreased in islets from type 2 diabetes patients. The latter coincided with diminished secretion of insulin in vitro. CACNA1D siRNA reduced glucose-stimulated insulin release in INS-1 832/13 cells and exocytosis in human beta cells. Phenotype/genotype associations of three SNPs in the CACNA1D gene revealed an association between the C allele of the SNP rs312480 and reduced mRNA expression, as well as decreased insulin secretion in vivo, whereas both rs312486/G and rs9841978/G were associated with type 2 diabetes.

CONCLUSION/INTERPRETATION

We conclude that the L-type calcium channel Cav1.3 is important in human glucose-induced insulin secretion, and common variants in CACNA1D might contribute to type 2 diabetes.

Secreted frizzled-related protein 4 reduces insulin secretion and is overexpressed in type 2 diabetes

A plethora of candidate genes have been identified for complex polygenic disorders, but the underlying disease mechanisms remain largely unknown. We explored the pathophysiology of type 2 diabetes (T2D) by analyzing global gene expression in human pancreatic islets. A group of coexpressed genes (module), enriched for interleukin-1-related genes, was associated with T2D and reduced insulin secretion. One of the module genes that was highly overexpressed in islets from T2D patients is SFRP4, which encodes secreted frizzled-related protein 4. SFRP4 expression correlated with inflammatory markers, and its release from islets was stimulated by interleukin-1β. Elevated systemic SFRP4 caused reduced glucose tolerance through decreased islet expression of Ca(2+) channels and suppressed insulin exocytosis. SFRP4 thus provides a link between islet inflammation and impaired insulin secretion. Moreover, the protein was increased in serum from T2D patients several years before the diagnosis, suggesting that SFRP4 could be a potential biomarker for islet dysfunction in T2D.

Overexpression of alpha2A-adrenergic receptors contributes to type 2 diabetes

Several common genetic variations have been associated with type 2 diabetes, but the exact disease mechanisms are still poorly elucidated. Using congenic strains from the diabetic Goto-Kakizaki rat, we identified a 1.4-megabase genomic locus that was linked to impaired insulin granule docking at the plasma membrane and reduced beta cell exocytosis. In this locus, Adra2a, encoding the alpha2A-adrenergic receptor [alpha(2A)AR], was significantly overexpressed. Alpha(2A)AR mediates adrenergic suppression of insulin secretion. Pharmacological receptor antagonism, silencing of receptor expression, or blockade of downstream effectors rescued insulin secretion in congenic islets. Furthermore, we identified a single-nucleotide polymorphism in the human ADRA2A gene for which risk allele carriers exhibited overexpression of alpha(2A)AR, reduced insulin secretion, and increased type 2 diabetes risk. Human pancreatic islets from risk allele carriers exhibited reduced granule docking and secreted less insulin in response to glucose; both effects were counteracted by pharmacological alpha(2A)AR antagonists.

Glutaredoxin-1 mediates NADPH-dependent stimulation of calcium-dependent insulin secretion

Nicotinamide adenine dinucleotide phosphate (NADPH) enhances Ca(2+)-induced exocytosis in pancreatic beta-cells, an effect suggested to involve the cytosolic redox protein glutaredoxin-1 (GRX-1). We here detail the role of GRX-1 in NADPH-stimulated beta-cell exocytosis and glucose-stimulated insulin secretion. Silencing of GRX-1 by RNA interference reduced glucose-stimulated insulin secretion in both clonal INS-1 832/13 cells and primary rat islets. GRX-1 silencing did not affect cell viability or the intracellular redox environment, suggesting that GRX-1 regulates the exocytotic machinery by a local action. By contrast, knockdown of the related protein thioredoxin-1 (TRX-1) was ineffective. Confocal immunocytochemistry revealed that GRX-1 locates to the cell periphery, whereas TRX-1 expression is uniform. These data suggest that the distinct subcellular localizations of TRX-1 and GRX-1 result in differences in substrate specificities and actions on insulin secretion. Single-cell exocytosis was likewise suppressed by GRX-1 knockdown in both rat beta-cells and clonal 832/13 cells, whereas after overexpression exocytosis increased by approximately 40%. Intracellular addition of NADPH (0.1 mm) stimulated Ca(2+)-evoked exocytosis in both cell types. Interestingly, the stimulatory action of NADPH on the exocytotic machinery coincided with an approximately 30% inhibition in whole-cell Ca(2+) currents. After GRX-1 silencing, NADPH failed to amplify insulin release but still inhibited Ca(2+) currents in 832/13 cells. In conclusion, NADPH stimulates the exocytotic machinery in pancreatic beta-cells. This effect is mediated by the NADPH acceptor protein GRX-1 by a local redox reaction that accelerates beta-cell exocytosis and, in turn, insulin secretion.