Control Of Metabolic Homeostasis Research Articles

Understudied Proteins and Understudied Functions in the Model Bacterium Bacillus subtilis - a Major Challenge in Current Research.

Model organisms such as the Gram-positive bacterium Bacillus subtilis have been studied intensively for decades. However, even for model organisms no function has been identified for about one fourth of all proteins. It has recently been realized that such understudied proteins as well as poorly studied functions set a limitation to our understanding of the requirements for cellular life, and the Understudied Proteins Initiative has been launched. Of poorly studied proteins, those that are strongly expressed are likely to be important to the cell and should therefore be considered high priority in further studies. Since the functional analysis of unknown proteins can be extremely laborious, a minimal knowledge is required prior to targeted functional studies. In this review, we discuss strategies to obtain such a minimal annotation, e.g. from global interaction, expression or localization studies. We present a set of 41 highly expressed and poorly studied proteins of B. subtilis. Several of these proteins are thought or known to bind RNA and/or the ribosome, some may control the metabolism of B. subtilis, and another subset of particularly small proteins may act as regulatory elements to control the expression of downstream genes. Moreover, we discuss the challenges of poorly studied functions with a focus on RNA-binding proteins, amino acid transport and the control of metabolic homeostasis. The identification of the functions of the selected proteins will strongly advance our knowledge on B. subtilis, but also on other organisms since many of the proteins are conserved in many groups of bacteria.

Editorial introductions

Editorial introductions

Peptide-YY3-36/glucagon-like peptide-1 combination treatment of obese diabetic mice improves insulin sensitivity associated with recovered pancreatic β-cell function and synergistic activation of discrete hypothalamic and brainstem neuronal circuitries

ObjectiveObesity-linked type 2 diabetes (T2D) is a worldwide health concern and many novel approaches are being considered for its treatment and subsequent prevention of serious comorbidities. Co-administration of glucagon like peptide 1 (GLP-1) and peptide YY3-36 (PYY3-36) renders a synergistic decrease in energy intake in obese men. However, mechanistic details of the synergy between these peptide agonists and their effects on metabolic homeostasis remain relatively scarce. MethodsIn this study, we utilized long-acting analogues of GLP-1 and PYY3-36 (via Fc-peptide conjugation) to better characterize the synergistic pharmacological benefits of their co-administration on body weight and glycaemic regulation in obese and diabetic mouse models. Hyperinsulinemic-euglycemic clamps were used to measure weight-independent effects of Fc-PYY3-36 + Fc-GLP-1 on insulin action. Fluorescent light sheet microscopy analysis of whole brain was performed to assess activation of brain regions. ResultsCo-administration of long-acting Fc-IgG/peptide conjugates of Fc-GLP-1 and Fc-PYY3-36 (specific for PYY receptor-2 (Y2R)) resulted in profound weight loss, restored glucose homeostasis, and recovered endogenous β-cell function in two mouse models of obese T2D. Hyperinsulinemic-euglycemic clamps in C57BLKS/J db/db and diet-induced obese Y2R-deficient (Y2RKO) mice indicated Y2R is required for a weight-independent improvement in peripheral insulin sensitivity and enhanced hepatic glycogenesis. Brain cFos staining demonstrated distinct temporal activation of regions of the hypothalamus and hindbrain following Fc-PYY3-36 + Fc-GLP-1R agonist administration. ConclusionsThese results reveal a therapeutic approach for obesity/T2D that improved insulin sensitivity and restored endogenous β-cell function. These data also highlight the potential association between the gut–brain axis in control of metabolic homeostasis.

Editorial introductions

Editorial introductions

Central 5-HTR2C in the Control of Metabolic Homeostasis

The 5-hydroxytryptamine 2C receptor (5-HTR2C) is a class G protein-coupled receptor (GPCR) enriched in the hypothalamus and the brain stem, where it has been shown to regulate energy homeostasis, including feeding and glucose metabolism. Accordingly, 5-HTR2C has been the target of several anti-obesity drugs, though the associated side effects greatly curbed their clinical applications. Dissecting the specific neural circuits of 5-HTR2C-expressing neurons and the detailed molecular pathways of 5-HTR2C signaling in metabolic regulation will help to develop better therapeutic strategies towards metabolic disorders. In this review, we introduced the regulatory role of 5-HTR2C in feeding behavior and glucose metabolism, with particular focus on the molecular pathways, neural network, and its interaction with other metabolic hormones, such as leptin, ghrelin, insulin, and estrogens. Moreover, the latest progress in the clinical research on 5-HTR2C agonists was also discussed.

Reducing l-lactate release from hippocampal astrocytes by intracellular oxidation increases novelty induced activity in mice.

Astrocytes are in control of metabolic homeostasis in the brain and support and modulate neuronal function in various ways. Astrocyte‐derived l‐lactate (lactate) is thought to play a dual role as a metabolic and a signaling molecule in inter‐cellular communication. The biological significance of lactate release from astrocytes is poorly understood, largely because the tools to manipulate lactate levels in vivo are limited. We therefore developed new viral vectors for astrocyte‐specific expression of a mammalianized version of lactate oxidase (LOx) from Aerococcus viridans. LOx expression in astrocytes in vitro reduced their intracellular lactate levels as well as the release of lactate to the extracellular space. Selective expression of LOx in astrocytes of the dorsal hippocampus in mice resulted in increased locomotor activity in response to novel stimuli. Our findings suggest that a localized decreased intracellular lactate pool in hippocampal astrocytes could contribute to greater responsiveness to environmental novelty. We expect that use of this molecular tool to chronically limit astrocytic lactate release will significantly facilitate future studies into the roles and mechanisms of intercellular lactate communication in the brain.

Therapeutic potential of α7 nicotinic acetylcholine receptor agonists to combat obesity, diabetes, and inflammation.

The cholinergic anti-inflammatory reflex (CAIR) represents an important homeostatic regulatory mechanism for sensing and controlling the body's response to inflammatory stimuli. Vagovagal reflexes are an integral component of CAIR whose anti-inflammatory effects are mediated by acetylcholine (ACh) acting at α7 nicotinic acetylcholine receptors (α7nAChR) located on cells of the immune system. Recently, it is appreciated that CAIR and α7nAChR also participate in the control of metabolic homeostasis. This has led to the understanding that defective vagovagal reflex circuitry underlying CAIR might explain the coexistence of obesity, diabetes, and inflammation in the metabolic syndrome. Thus, there is renewed interest in the α7nAChR that mediates CAIR, particularly from the standpoint of therapeutics. Of special note is the recent finding that α7nAChR agonist GTS-21 acts at L-cells of the distal intestine to stimulate the release of two glucoregulatory and anorexigenic hormones: glucagon-like peptide-1 (GLP-1) and peptide YY (PYY). Furthermore, α7nAChR agonist PNU 282987 exerts trophic factor-like actions to support pancreatic β-cell survival under conditions of stress resembling diabetes. This review provides an overview of α7nAChR function as it pertains to CAIR, vagovagal reflexes, and metabolic homeostasis. We also consider the possible usefulness of α7nAChR agonists for treatment of obesity, diabetes, and inflammation.

Neuropeptide Control of Puberty: Beyond Kisspeptins.

Puberty is a fundamental developmental event in the lifespan of any individual, when sexual and somatic maturation is completed, and reproductive capacity is achieved. While the tempo of puberty is under strong genetic determination, it is also modulated by a wide array of internal and environmental cues, including, prominently, nutritional and metabolic signals. In the last decade, our understanding of the neurohormonal basis of normal puberty and its perturbations has enlarged considerably. This is illustrated by the elucidation of the essential roles of kisspeptins, encoded by the Kiss1 gene, in the hypothalamic circuits controlling puberty. Moreover, other neuropeptide pathways, convergent with kisspeptin signaling, have been pointed out as important coregulators of pubertal timing. These include the cotransmitters of Kiss1 neurons in the arcuate nucleus (ARC), neurokinin B, and dynorphin, as well as melanocortins, produced by ARC neurons expressing proopiomelanocortin, which are endowed with key roles also in the control of metabolic homeostasis. This neuropeptide setup seemingly participates, in a coordinated manner, in transmitting the regulatory actions of metabolic cues on pubertal maturation. In this function, cellular metabolic sensors, such as the AMP-activated protein kinase, and the fuel-sensing deacetylase, SIRT1, have also been shown recently to contribute to the metabolic regulation of puberty. Altogether, elucidation of the physiological roles of these signals and regulatory circuits will help uncover the intimacies of the brain control of puberty, and its alterations in conditions of metabolic stress, ranging from subnutrition to obesity.

Pancreatic β-cells detoxify H2O2 through the peroxiredoxin/thioredoxin antioxidant system

Oxidative stress is thought to promote pancreatic β-cell dysfunction and contribute to both type 1 and type 2 diabetes. Reactive oxygen species (ROS), such as superoxide and hydrogen peroxide, are mediators of oxidative stress that arise largely from electron leakage during oxidative phosphorylation. Reports that β-cells express low levels of antioxidant enzymes, including catalase and GSH peroxidases, have supported a model in which β-cells are ill-equipped to detoxify ROS. This hypothesis seems at odds with the essential role of β-cells in the control of metabolic homeostasis and organismal survival through exquisite coupling of oxidative phosphorylation, a prominent ROS-producing pathway, to insulin secretion. Using glucose oxidase to deliver H2O2 continuously over time and Amplex Red to measure extracellular H2O2 concentration, we found here that β-cells can remove micromolar levels of this oxidant. This detoxification pathway utilizes the peroxiredoxin/thioredoxin antioxidant system, as selective chemical inhibition or siRNA-mediated depletion of thioredoxin reductase sensitized β-cells to continuously generated H2O2 In contrast, when delivered as a bolus, H2O2 induced the DNA damage response, depleted cellular energy stores, and decreased β-cell viability independently of thioredoxin reductase inhibition. These findings show that β-cells have the capacity to detoxify micromolar levels of H2O2 through a thioredoxin reductase-dependent mechanism and are not as sensitive to oxidative damage as previously thought.

Regulation, Communication, and Functional Roles of Adipose Tissue-Resident CD4+ T Cells in the Control of Metabolic Homeostasis.

Evidence accumulated over the past few years has documented a critical role for adipose tissue (AT)-resident immune cells in the regulation of local and systemic metabolic homeostasis. In the lean state, visceral adipose tissue (VAT) is predominated by anti-inflammatory T-helper 2 (Th2) and regulatory T (Treg) cell subsets. As obesity progresses, the population of Th2 and Treg cells decreases while that of the T-helper 1 (Th1) and T-helper 17 (Th17) cells increases, leading to augmented inflammation and insulin resistance. Notably, recent studies also suggest a potential role of CD4+ T cells in the control of thermogenesis and energy homeostasis. In this review, we have summarized recent advances in understanding the characteristics and functional roles of AT CD4+ T cell subsets during obesity and energy expenditure. We have also discussed new findings on the crosstalk between CD4+ T cells and local antigen-presenting cells (APCs) including adipocytes, macrophages, and dendritic cells (DCs) to regulate AT function and metabolic homeostasis. Finally, we have highlighted the therapeutic potential of targeting CD4+ T cells as an effective strategy for the treatment of obesity and its associated metabolic diseases.

Liquid chromatography/mass spectrometry based detection and semi‐quantitative analysis of INSL5 in human and murine tissues

RationaleInsulin‐like peptide 5 (INSL5) is a hormone produced by enteroendocrine L‐cells in the colon that has recently been implicated in the control of metabolic homeostasis. However, research into its physiology has been hindered by the reported unreliability of commercially available immunoassays and additional detection assays would benefit this emerging field.MethodsPeptides from purified murine L‐cells and homogenates from both human and mouse colonic tissues were extracted by precipitating larger proteins with acetonitrile. Untargeted liquid chromatography/tandem mass spectrometry (LC/MS/MS) analyses, followed by database searching, were used to detect and identify various INSL5 gene derived peptides and characterise their precise sequence. A similar approach was developed to quantify INSL5 levels in primary intestinal culture supernatants after purification and concentration by solid‐phase extraction.ResultsMass spectral analysis of purified enteroendocrine cells and tissue homogenates identified the exact sequence of A and B chains of INSL5 endogenously expressed in L‐cells. Differences in the endogenously processed peptide and the Swissprot database entry were observed for murine INSL5, whereas the human sequence matched previous predictions from heterologous expression experiments. INSL5 was detected in the supernatant of human and mouse primary colonic cultures and concentrations increased after treatment with a known L‐cell stimulus.ConclusionsThe first LC/MS/MS‐based method capable of the detection and semi‐quantitative analysis of endogenous INSL5 using MS‐based techniques has been demonstrated. The methodology will enable the identification of stimulants for INSL5 secretion from murine and human primary colonic epithelial cultures.

Metabolic signaling functions of ER-mitochondria contact sites: role in metabolic diseases.

Beyond the maintenance of cellular homeostasis and the determination of cell fate, ER-mitochondria contact sites, defined as mitochondria-associated membranes (MAM), start to emerge as an important signaling hub that integrates nutrient and hormonal stimuli and adapts cellular metabolism. Here, we summarize the established structural and functional features of MAM and mainly focus on the latest breakthroughs highlighting a crucial role of organelle crosstalk in the control of metabolic homeostasis. Lastly, we discuss recent studies that have revealed the importance of MAM in not only metabolic diseases but also in other pathologies with disrupted metabolism, shedding light on potential common molecular mechanisms and leading hopefully to novel treatment strategies.

Adipose tissue macrophage in immune regulation of metabolism.

The prevalence of obesity and type 2 diabetes is escalating to an epidemic proportion worldwide. Obesity is known to be associated with a state of chronic, low-grade inflammation. Emerging lines of evidence have shown that both innate and adaptive immune responses play crucial roles in the control of metabolic homeostasis. Macrophages in adipose tissues are the essential effector cells in orchestrating metabolic inflammation, which is thought to promote the pathogenic progression of obesity and obesity-related disorders. Here we discuss our current understanding of the distinct modes of activation of adipose tissue macrophages, which can sense the metabolic cues and exert profound effects upon adipose homeostasis. Targeting macrophages in adipose tissues may provide new avenues for developing immunomodulation-based therapeutics against obesity and obesity-associated metabolic diseases.

Using a 3D Culture System to Differentiate Visceral Adipocytes In Vitro.

It has long been recognized that body fat distribution and regional adiposity play a major role in the control of metabolic homeostasis. However, the ability to study and compare the cell autonomous regulation and response of adipocytes from different fat depots has been hampered by the difficulty of inducing preadipocytes isolated from the visceral depot to differentiate into mature adipocytes in culture. Here, we present an easily created 3-dimensional (3D) culture system that can be used to differentiate preadipocytes from the visceral depot as robustly as those from the sc depot. The cells differentiated in these 3D collagen gels are mature adipocytes that retain depot-specific characteristics, as determined by imaging, gene expression, and functional assays. This 3D culture system therefore allows for study of the development and function of adipocytes from both depots in vitro and may ultimately lead to a greater understanding of site-specific functional differences of adipose tissues to metabolic dysregulation.

The Mitochondrial Permeability Transition Pore Regulator Cyclophilin D Exhibits Tissue‐Specific Control of Metabolic Homeostasis

The mitochondrial permeability transition pore (mPTP) is a key regulator of mitochondrial health, and the mPTP has been implicated in the pathogenesis of metabolic disease. Cyclophilin D (CypD) is a critical regulator that directly binds to mPTP constituents to facilitate pore opening. Mice lacking CypD in all tissues are protected from diet‐induced glucose intolerance; however, the tissue‐specific function of CypD and mPTP have not been ascertained. To determine the functional role of CypD in metabolic tissues that control glucose homeostasis, we generated liver‐specific and muscle‐specific CypD knockout mice (LKO and MKO, respectively; n=9‐11/gp). These mice were fed either normal chow (NC) or 45% high fat diet (HFD) and evaluated for metabolic phenotypes. We observed that MKO mice were statistically identical to floxed controls with respect to body weight gain or glucose tolerance when fed either diet. However, LKO mice fed HFD developed greater hepatic steatosis, adipose mass, glucose intolerance and pyruvate intolerance compared with flox control mice. These findings demonstrate that CypD has tissue‐specific roles in the context of metabolic physiology, and the loss of CypD in liver is sufficient to exacerbate whole‐body metabolic dysfunction with high fat feeding.Supported by NIH grant R01AR050429

AMP-Activated Protein Kinase (AMPK) Mediates Nutrient Regulation of Thioredoxin-Interacting Protein (TXNIP) in Pancreatic Beta-Cells

Thioredoxin-interacting protein (TXNIP) regulates critical biological processes including inflammation, stress and apoptosis. TXNIP is upregulated by glucose and is a critical mediator of hyperglycemia-induced beta-cell apoptosis in diabetes. In contrast, the saturated long-chain fatty acid palmitate, although toxic to the beta-cell, inhibits TXNIP expression. The mechanisms involved in the opposing effects of glucose and fatty acids on TXNIP expression are unknown. We found that both palmitate and oleate inhibited TXNIP in a rat beta-cell line and islets. Palmitate inhibition of TXNIP was independent of fatty acid beta-oxidation or esterification. AMP-activated protein kinase (AMPK) has an important role in cellular energy sensing and control of metabolic homeostasis; therefore we investigated its involvement in nutrient regulation of TXNIP. As expected, glucose inhibited whereas palmitate stimulated AMPK. Pharmacologic activators of AMPK mimicked fatty acids by inhibiting TXNIP. AMPK knockdown increased TXNIP expression in presence of high glucose with and without palmitate, indicating that nutrient (glucose and fatty acids) effects on TXNIP are mediated in part via modulation of AMPK activity. TXNIP is transcriptionally regulated by carbohydrate response element-binding protein (ChREBP). Palmitate inhibited glucose-stimulated ChREBP nuclear entry and recruitment to the Txnip promoter, thereby inhibiting Txnip transcription. We conclude that AMPK is an important regulator of Txnip transcription via modulation of ChREBP activity. The divergent effects of glucose and fatty acids on TXNIP expression result in part from their opposing effects on AMPK activity. In light of the important role of TXNIP in beta-cell apoptosis, its inhibition by fatty acids can be regarded as an adaptive/protective response to glucolipotoxicity. The finding that AMPK mediates nutrient regulation of TXNIP may have important implications for the pathophysiology and treatment of diabetes.

The Role of Sirtuins in the Control of Metabolic Homeostasis

Recently the function of the sirtuin family, named after their homology to the Saccharomyces cerevisiae gene silent information regulator 2 (Sir2), has received a lot of attention, as their beneficial impact on longevity was linked to their effects on metabolic control. All sirtuins require nicotinamide adenine dinucleotide (NAD(+)) for their deacetylase or ADP-ribosyl transferase activity, linking their function tightly to cellular energy levels. SIRT1, the founding member of the sirtuin family, modulates many aspects of glucose and lipid homeostasis in almost all key metabolic tissues. Other members including SIRT2, SIRT3, and SIRT4 are also implicated in various metabolic processes. Here, we review the recent data related to the role of sirtuins in the control of metabolic homeostasis and possible underlying molecular mechanisms.

Carboxy-terminal phosphorylation of SIRT1 by protein kinase CK2

Previous analyses of the sirtuin family of histone deacetylases and its most prominent member SIRT1 have focused primarily on the identification of cellular targets exploring the underlying molecular mechanisms of its implicated function in the control of metabolic homeostasis, differentiation, apoptosis and cell survival. So far, little is known about the regulation of SIRT1 itself. In the study presented herein, we assigned the main region of SIRT1 in vivo phosphorylation to amino acids 643–691 of the unique carboxy-terminal domain. Furthermore, we demonstrate that SIRT1 is a substrate for protein kinase CK2 both in vitro and in vivo. Both, deletion construct analyses and serine-to-alanine mutations identified SIRT1 Ser-659 and Ser-661 as major CK2 phosphorylation sites that are phosphorylated in vivo as well.

SIRTUIN 1: Regulating the regulator

Earlier analyses on the sirtuin family of histone deacetylases and its well-known member SIRT1 had their primary focus mostly on the identification of cellular targets exploring molecular mechanisms and functional networks in the control of metabolic homeostasis, differentiation, apoptosis and cell survival. However, only little is known about the regulation of SIRT1 itself, so far. Presently, SIRT1 is gaining increasing importance in the development of innovative treatment strategies for cancer, neurodegenerative disorders and metabolic disease. Based on differences in their catalytic activities, SIRT1 and the sirtuins in general, are insensitive to the classical class I and II HDAC inhibitors which are increasingly becoming part of treatment regimens for solid tumors and hematological malignancies. In this review we outline recent research advances on the regulation of SIRT1 which may provide the basis for the development of therapeutic inhibitors with improved specificity.

S12/5 Mitochondrial glutamate pathways and the control of metabolic homeostasis

S12/5 Mitochondrial glutamate pathways and the control of metabolic homeostasis