Featured ArticlesVolume 6 | No. 10 | October 2017
|Lorcaserin improves glycemic control via a melanocortin neurocircuit The obesity medication lorcaserin, a 5-hydroxytryptamine 2C receptor (5-HT2CR) agonist, improves glycemic control in association with weight loss in obese patients with type 2 diabetes. Burke and colleagues find that lorcaserin improves glycemic control in mouse models of T2D without altering energy balance or body weight. They reveal brain pro-opiomelanocortin (POMC) peptides as a necessary and sufficient neurochemical mediator of lorcaserin’s glucoregulatory effects.|
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Objective: The increasing prevalence of type 2 diabetes (T2D) and associated morbidity and mortality emphasizes the need for a more complete understanding of the mechanisms mediating glucose homeostasis to accelerate the identification of new medications. Recent reports indicate that the obesity medication lorcaserin, a 5-hydroxytryptamine (5-HT, serotonin) 2C receptor (5-HT2CR) agonist, improves glycemic control in association with weight loss in obese patients with T2D. Here we evaluate whether lorcaserin has an effect on glycemia without body weight loss and how this effect is achieved.
Methods: Murine models of common and genetic T2D were utilized to probe the direct effect of lorcaserin on glycemic control.
Results: Lorcaserin dose-dependently improves glycemic control in mouse models of T2D in the absence of reductions in food intake or body weight. Examining the mechanism of this effect, we reveal a necessary and sufficient neurochemical mediator of lorcaserin's glucoregulatory effects, brain pro-opiomelanocortin (POMC) peptides. To clarify further lorcaserin's therapeutic brain circuit, we examined the receptor target of POMC peptides. We demonstrate that lorcaserin requires functional melanocortin4 receptors on cholinergic preganglionic neurons (MC4RChAT) to exert its effects on glucose homeostasis. In contrast, MC4RChAT signaling did not impact lorcaserin's effects on feeding, indicating a divergence in the neurocircuitry underpinning lorcaserin's therapeutic glycemic and anorectic effects. Hyperinsulinemic-euglycemic clamp studies reveal that lorcaserin reduces hepatic glucose production, increases glucose disposal and improves insulin sensitivity.
Conclusions: These data suggest that lorcaserin's action within the brain represents a mechanistically novel treatment for T2D: findings of significance to a prevalent global disease.[Hide abstract]
|Mitochondrial uncoupling in the melanocortin system differentially regulates NPY and POMC neuronsBy using the original weight-loss inducing drug 2,4-dinitrophenol (DNP), Michael et al. demonstrate that chemical uncoupling of the melanocortin system promotes increased energy expenditure and weight loss through differentially regulating excitability of orexigenic neuropeptide Y (NPY) and anorexigenic proopiomelanocortin (POMC) neurons. DNP is known to cross the blood-brain-barrier, and the data presented support a key and novel mechanism by which chemical uncoupling agents targeting the melanocortin system, such as DNP, may offer new insight for future approaches to tackle obesity.|
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Objective: The mitochondrial uncoupling agent 2,4-dinitrophenol (DNP), historically used as a treatment for obesity, is known to cross the blood-brain-barrier, but its effects on central neural circuits controlling body weight are largely unknown. As hypothalamic melanocortin neuropeptide Y/agouti-related protein (NPY/AgRP) and pro-opiomelanocortin (POMC) neurons represent key central regulators of food intake and energy expenditure we investigated the effects of DNP on these neurons, food intake and energy expenditure.
Methods: C57BL/6 and melanocortin-4 receptor (MC4R) knock-out mice were administered DNP intracerebroventricularly (ICV) and the metabolic changes were characterized. The specific role of NPY and POMC neurons and the ionic mechanisms mediating the effects of uncoupling were examined with in vitro electrophysiology performed on NPY hrGFP or POMC eGFP mice.
Results: Here we show DNP-induced differential effects on melanocortin neurons including inhibiting orexigenic NPY and activating anorexigenic POMC neurons through independent ionic mechanisms coupled to mitochondrial function, consistent with an anorexigenic central effect. Central administration of DNP induced weight-loss, increased BAT thermogenesis and browning of white adipose tissue, and decreased food intake, effects that were absent in MC4R knock-out mice and blocked by the MC4R antagonist, AgRP.
Conclusions: These data show a novel central anti-obesity mechanism of action of DNP and highlight the potential for selective melanocortin mitochondrial uncoupling to target metabolic disorders.[Hide abstract]
|Peripheral cannabinoid-1 receptor blockade restores hypothalamic leptin signalingAccumulating evidence supports the pathogenic role of an overactive endocannabinoid/CB1 receptor (CB1R) system in obesity and the metabolic syndrome. Tam et al. find that peripheral CB1R blockade in mice with diet-induced obesity restores sensitivity to endogenous leptin, which elicits hypophagia via the re-activation of melanocortin signaling in the arcuate nucleus. |
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Objective: In visceral obesity, an overactive endocannabinoid/CB1 receptor (CB1R) system promotes increased caloric intake and decreases energy expenditure, which are mitigated by global or peripheral CB1R blockade. In mice with diet-induced obesity (DIO), inhibition of food intake by the peripherally restricted CB1R antagonist JD5037 could be attributed to endogenous leptin due to the rapid reversal of hyperleptinemia that maintains leptin resistance, but the signaling pathway engaged by leptin has remained to be determined.
Methods: We analyzed the hypothalamic circuitry targeted by leptin following chronic treatment of DIO mice with JD5037.
Results: Leptin treatment or an increase in endogenous leptin following fasting/refeeding induced STAT3 phosphorylation in neurons in the arcuate nucleus (ARC) in lean and JD5037-treated DIO mice, but not in vehicle-treated DIO animals. Co-localization of pSTAT3 in leptin-treated mice was significantly less common with NPY+ than with POMC+ ARC neurons. The hypophagic effect of JD5037 was absent in melanocortin-4 receptor (MC4R) deficient obese mice or DIO mice treated with a MC4R antagonist, but was maintained in NPY−/− mice kept on a high-fat diet.
Conclusions: Peripheral CB1R blockade in DIO restores sensitivity to endogenous leptin, which elicits hypophagia via the re-activation of melanocortin signaling in the ARC.[Hide abstract]
|Dwarfism and insulin resistance in male offspring caused by α1-adrenergic antagonism during pregnancyUsing the α1-adrenergic specific antagonist prazosin, Oelkrug et al. test the long-term consequences of maternal α1-blockade in pregnancy for the endocrine and metabolic phenotype of the adult offspring. They demonstrate that maternal α1-adrenergic blockade can constitute an epigenetic cause for dwarfism and insulin resistance.|
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Objective: Maternal and environmental factors control the epigenetic fetal programming of the embryo, thereby defining the susceptibility for metabolic or endocrine disorders in the offspring. Pharmacological interventions required as a consequence of gestational problems, e.g. hypertension, can potentially interfere with correct fetal programming. As epigenetic alterations are usually only revealed later in life and not detected in studies focusing on early perinatal outcomes, little is known about the long-term epigenetic effects of gestational drug treatments. We sought to test the consequences of maternal α1-adrenergic antagonism during pregnancy, which can occur e.g. during hypertension treatment, for the endocrine and metabolic phenotype of the offspring.
Methods: We treated C57BL/6NCrl female mice with the α1-adrenergic antagonist prazosin during pregnancy and analyzed the male and female offspring for endocrine and metabolic abnormalities.
Results: Our data revealed that maternal α1-adrenergic blockade caused dwarfism, elevated body temperature, and insulin resistance in male offspring, accompanied by reduced IGF-1 serum concentrations as the result of reduced hepatic growth hormone receptor (Ghr) expression. We subsequently identified increased CpG DNA methylation at the transcriptional start site of the alternative Ghr promotor caused by the maternal treatment, which showed a strong inverse correlation to hepatic Ghr expression.
Conclusions: Our results demonstrate that maternal α1-adrenergic blockade can constitute an epigenetic cause for dwarfism and insulin resistance. The findings are of immediate clinical relevance as combined α/β-adrenergic blockers are first-line treatment of maternal hypertension.[Hide abstract]
|ANGPTL8 promotes the ability of ANGPTL3 to bind and inhibit lipoprotein lipaseLipoprotein lipase (LPL) activity is critically regulated by several interacting proteins, including angiopoietin-like 3 (ANGPTL3). ANGPTL3 is thought to regulate triglyceride clearance by inhibiting LPL. The data of Chi and colleagues indicate that angiopoietin-like protein 8 (ANGPTL8) binds to ANGPTL3 and that this complex is necessary for ANGPTL3 to efficiently bind and inhibit LPL. Their studies provide mechanistic insight into the interactions of ANGPTL3 and ANGPTL8, providing critical information that could be used for development of therapeutics targeting ANGPTL8 or the interactions between ANGPTL8 and ANGPTL3.|
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Objective: Several members of the angiopoietin-like (ANGPTL) family of proteins, including ANGPTL3 and ANGPTL8, regulate lipoprotein lipase (LPL) activity. Deficiency in either ANGPTL3 or ANGPTL8 reduces plasma triglyceride levels and increases LPL activity, whereas overexpression of either protein does the opposite. Recent studies suggest that ANGPTL8 may functionally interact with ANGPTL3 to alter clearance of plasma triglycerides; however, the nature of this interaction has remained elusive. We tested the hypothesis that ANGPTL8 forms a complex with ANGPTL3 and that this complex is necessary for the inhibition of vascular LPL by ANGPTL3.
Methods: We analyzed the interactions of ANGPTL3 and ANGPTL8 with each other and with LPL using co-immunoprecipitation, western blotting, lipase activity assays, and the NanoBiT split-luciferase system. We also used adenovirus injection to overexpress ANGPTL3 in mice that lacked ANGPTL8.
Results: We found that ANGPTL3 or ANGPTL8 alone could only inhibit LPL at concentrations that far exceeded physiological levels, especially when LPL was bound to its endothelial cell receptor/transporter GPIHBP1 (glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1). Physical interaction was observed between ANGPTL3 and ANGPTL8 when the proteins were co-expressed, and co-expression with ANGPTL3 greatly enhanced the secretion of ANGPTL8. Importantly, ANGPTL3–ANGPTL8 complexes had a dramatically increased ability to inhibit LPL compared to either protein alone. Adenovirus experiments showed that 2-fold overexpression of ANGPTL3 significantly increased plasma triglycerides only in the presence of ANGPTL8. Protein interaction assays showed that ANGPTL8 greatly increased the ability of ANGPTL3 to bind LPL.
Conclusions: Together, these data indicate that ANGPTL8 binds to ANGPTL3 and that this complex is necessary for ANGPTL3 to efficiently bind and inhibit LPL.[Hide abstract]
|Adipocyte glucocorticoid receptor is important in lipolysis and insulin resistanceGlucocorticoids are produced by the adrenal cortex under the control of the hypothalamus and pituitary gland and have been implicated in multiple aspects of adipose tissue biology. Shen, Roh, et al. demonstrate that adipocyte glucocorticoid receptor (GR) participates in lipolysis but does not contribute to altered glucose homeostasis or insulin resistance in the setting of diet-induced obesity. Administration of dexamethasone, however, causes insulin resistance that depends upon the presence of GR in adipocytes.|
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Objective: The critical role of adipose tissue in energy and nutrient homeostasis is influenced by many external factors, including overnutrition, inflammation, and exogenous hormones. Prior studies have suggested that glucocorticoids (GCs) in particular are major drivers of physiological and pathophysiological changes in adipocytes. In order to determine whether these effects directly require the glucocorticoid receptor (GR) within adipocytes, we generated adipocyte-specific GR knockout (AGRKO) mice.
Methods: AGRKO and control mice were fed chow or high fat diet (HFD) for 14 weeks. Alternatively, AGRKO and control mice were injected with dexamethasone for two months. Glucose tolerance, insulin sensitivity, adiposity, lipolysis, thermogenesis, and insulin signaling were assessed.
Results: We find that obesity, insulin resistance, and dysglycemia associated with high fat feeding do not require an intact GR in the adipocyte. However, exogenous dexamethasone (Dex) promotes metabolic dysfunction in mice, and this effect is reduced in mice lacking GR in adipocytes. The ability of Dex to promote “whitening” of brown fat is also reduced in these animals. We also show that GR is required for β-adrenergic and cold stimulation-mediated lipolysis via expression of the key lipolytic enzyme ATGL.
Conclusions: Our data suggest that the GR plays a role in normal adipose physiology via effects on lipolysis and mediates at least some of the adverse effects of exogenous steroids on metabolic function. The data also indicate that intra-adipocyte GR plays less of a role than previously believed in the local and systemic pathology associated with overnutrition.[Hide abstract]
|Lipid nanoparticle delivery of glucagon receptor siRNA improves glucose homeostasisLipid nanoparticle (LNP) delivery of small interfering RNA (siRNA) effectively targets the liver and is in clinical trials for the treatment of various diseases. Neumann and colleagues compare the effectiveness of glucagon receptor (Gcgr)-siRNA delivered via LNPs to leptin in two mouse models of diabetes. Their results indicate that Gcgr siRNA encapsulated in LNPs is an effective therapy in mouse models of type 1 and type 2 diabetes.|
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Objective: Hyperglucagonemia is present in many forms of diabetes and contributes to hyperglycemia, and glucagon suppression can ameliorate diabetes in mice. Leptin, a glucagon suppressor, can also reverse diabetes in rodents. Lipid nanoparticle (LNP) delivery of small interfering RNA (siRNA) effectively targets the liver and is in clinical trials for the treatment of various diseases. We compared the effectiveness of glucagon receptor (Gcgr)-siRNA delivered via LNPs to leptin in two mouse models of diabetes.
Methods: Gcgr siRNA encapsulated into LNPs or leptin was administered to mice with diabetes due to injection of the β-cell toxin streptozotocin (STZ) alone or combined with high fat diet (HFD/STZ).
Results: In STZ-diabetic mice, a single injection of Gcgr siRNA lowered blood glucose levels for 3 weeks, improved glucose tolerance, and normalized plasma ketones levels, while leptin therapy normalized blood glucose levels, oral glucose tolerance, and plasma ketones, and suppressed lipid metabolism. In contrast, in HFD/STZ-diabetic mice, Gcgr siRNA lowered blood glucose levels for 2 months, improved oral glucose tolerance, and reduced HbA1c, while leptin had no beneficial effects.
Conclusions: While leptin may be more effective than Gcgr siRNA at normalizing both glucose and lipid metabolism in STZ diabetes, Gcgr siRNA is more effective at reducing blood glucose levels in HFD/STZ diabetes.[Hide abstract]
|Gαs regulates Glucagon-Like Peptide 1 Receptor-mediated cyclic AMP generationAfter activation, G protein coupled receptors associate with heterotrimeric G proteins at the plasma membrane to initiate second messenger signaling. Girada and colleagues suggest that cyclic AMP generation by internalized Glucagon-Like Peptide-1 Receptor takes place at the Rab5 endosomal compartment, where prolonged association of Galpha S subunit (Gαs) with the internalized receptor following orthosteric activation contributes to the sustained endosomal cyclic AMP generation that likely supports glucose stimulated insulin secretion in pancreatic beta cells.|
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Objective: Upon activation, G protein coupled receptors (GPCRs) associate with heterotrimeric G proteins at the plasma membrane to initiate second messenger signaling. Subsequently, the activated receptor experiences desensitization, internalization, and recycling back to the plasma membrane, or it undergoes lysosomal degradation. Recent reports highlight specific cases of persistent cyclic AMP generation by internalized GPCRs, although the functional significance and mechanistic details remain to be defined. Cyclic AMP generation from internalized Glucagon-Like Peptide-1 Receptor (GLP-1R) has previously been reported from our laboratory. This study aimed at deciphering the molecular mechanism by which internalized GLP-R supports sustained cyclic AMP generation upon receptor activation in pancreatic beta cells.
Methods: We studied the time course of cyclic AMP generation following GLP-1R activation with particular emphasis on defining the location where cyclic AMP is generated. Detection involved a novel GLP-1 conjugate coupled with immunofluorescence using specific endosomal markers. Finally, we employed co-immunoprecipitation as well as immunofluorescence to assess the protein–protein interactions that regulate GLP-1R mediated cyclic AMP generation at endosomes.
Results: Our data reveal that prolonged association of G protein α subunit Gαs with activated GLP-1R contributed to sustained cyclic AMP generation at Rab 5 endosomal compartment.
Conclusions: The findings provide the mechanism of endosomal cyclic AMP generation following GLP-1R activation. We identified the specific compartment that serves as an organizing center to generate endosomal cyclic AMP by internalized activated receptor complex.[Hide abstract]
|Macrophage alternative activation confers protection against lipotoxicity-induced cell deathIn lean individuals, adipose tissue macrophages (ATMs) have an alternatively activated (M2) phenotype that limits inflammation and sustains homeostasis. Dai et al. find that the signal transducer and activator of transcription 6 (Stat6) - peroxisome proliferator-activated receptor (Ppar) axis plays an important role in protecting macrophages against lipotoxicity-induced cellular dysfunction. This is mediated by transcriptional regulation of cell death/pro-survival genes. Dysregulation of M2 signaling increases susceptibility to palmitic acid-induced cell death, which contributes to the initiation of metabolic inflammation in white adipose tissue.|
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Objective: Alternative activation (M2) of adipose tissue resident macrophage (ATM) inhibits obesity-induced metabolic inflammation. The underlying mechanisms remain unclear. Recent studies have shown that dysregulated lipid homeostasis caused by increased lipolysis in white adipose tissue (WAT) in the obese state is a trigger of inflammatory responses. We investigated the role of M2 macrophages in lipotoxicity-induced inflammation.
Methods: We used microarray experiments to profile macrophage gene expression regulated by two M2 inducers, interleukin-4 (Il-4), and peroxisome proliferator-activated receptor delta/gamma (Pparδ/Pparγ) agonists. Functional validation studies were performed in bone marrow-derived macrophages and mice deprived of the signal transducer and activator of transcription 6 gene (Stat6; downstream effector of Il-4) or Pparδ/Pparγ genes (downstream effectors of Stat6). Palmitic acid (PA) and β-adrenergic agonist were employed to induce macrophage lipid loading in vitro and in vivo, respectively.
Results: Profiling of genes regulated by Il-4 or Pparδ/Pparγ agonists reveals that alternative activation promotes the cell survival program, while inhibiting that of inflammation-related cell death. Deletion of Stat6 or Pparδ/Pparγ increases the susceptibility of macrophages to PA-induced cell death. NLR family pyrin domain containing 3 (Nlrp3) inflammasome activation by PA in the presence of lipopolysaccharide is also increased in Stat6−/− macrophages and to a lesser extent, in Pparδ/γ−/− macrophages. In concert, β-adrenergic agonist-induced lipolysis results in higher levels of cell death and inflammatory markers in ATMs derived from myeloid-specific Pparδ/γ−/− or Stat6−/− mice.
Conclusions: Our data suggest that ATM cell death is closely linked to metabolic inflammation. Within WAT where concentrations of free fatty acids fluctuate, M2 polarization regulated by the Stat6-Ppar axis enhances ATM's tolerance to lipid-mediated stress, thereby maintaining the homeostatic state.[Hide abstract]
|Brown adipocytes can display a mammary basal myoepithelial cell phenotype in vivoLi, Li et al. demonstrate that classic brown adipocytes, and probably beige/brite adipocytes, are capable of showing a mammary basal myoepithelium phenotype but not a luminal secretory phenotype in vivo. They show that if cells that express uncoupling protein 1 are killed during lactation, the growth of offspring is reduced, suggesting the conversion of brown/beige adipocytes to mammary cells is functionally significant, even though numerically small.|
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Objective: Previous work has suggested that white adipocytes may also show a mammary luminal secretory cell phenotype during lactation. The capacity of brown and beige/brite adipocytes to display a mammary cell phenotype and the levels at which they demonstrate such phenotypes in vivo is currently unknown.
Methods: To investigate the putative adipocyte origin of mammary gland cells, we performed genetic lineage-labeling experiments in BAT and the mammary glands.
Results: These studies indicated that the classic brown adipocytes (Ucp1+) and subcutaneous beige/brite adipocytes (Ucp1−/+) were found in the mammary gland during lactation, when they exhibited a mammary myoepithelial phenotype. Up to 2.5% of the anterior dorsal interscapular mammary myoepithelial cell population had a brown adipocyte origin with an adipose and myoepithelial gene signature during lactation. Eliminating these cells, along with all the brown adipocytes, significantly slowed offspring growth, potentially demonstrating their functional importance. Additionally, we showed mammary epithelial lineage Mmtv+ and Krt14+ cells expressed brown adipocyte markers after weaning, demonstrating that mammary gland cells can display an adipose phenotype.
Conclusions: The identification of a brown adipocyte origin of mammary myoepithelial cells provides a novel perspective on the interrelationships between adipocytes and mammary cells with implications for our understanding of obesity and breast cancer.[Hide abstract]
|Cdkal1 regulates mitochondrial function in adipose tissuePolymorphic variants within the CDKAL1 locus are strongly associated with increased risk of developing type 2 diabetes by genome wide association studies (GWAS) and dozens of replication studies in diverse populations. Palmer and colleagues investigate the biological role of Cdkal1 in adipose tissue in vivo using a mouse model with adipocyte-specific knockout (A-KO) of Cdkal1. Their findings suggest that the type 2 diabetes GWAS candidate gene Cdkal1 has a functional role in regulating mitochondrial function in vivo.|
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Objective: Understanding how loci identified by genome wide association studies (GWAS) contribute to pathogenesis requires new mechanistic insights. Variants within CDKAL1 are strongly linked to an increased risk of developing type 2 diabetes and obesity. Investigations in mouse models have focused on the function of Cdkal1 as a tRNALys modifier and downstream effects of Cdkal1 loss on pro-insulin translational fidelity in pancreatic β−cells. However, Cdkal1 is broadly expressed in other metabolically relevant tissues, including adipose tissue. In addition, the Cdkal1 homolog Cdk5rap1 regulates mitochondrial protein translation and mitochondrial function in skeletal muscle. We tested whether adipocyte-specific Cdkal1 deletion alters systemic glucose homeostasis or adipose mitochondrial function independently of its effects on pro-insulin translation and insulin secretion.
Methods: We measured mRNA levels of type 2 diabetes GWAS genes, including Cdkal1, in adipose tissue from lean and obese mice. We then established a mouse model with adipocyte-specific Cdkal1 deletion. We examined the effects of adipose Cdkal1 deletion using indirect calorimetry on mice during a cold temperature challenge, as well as by measuring cellular and mitochondrial respiration in vitro. We also examined brown adipose tissue (BAT) mitochondrial morphology by electron microscopy. Utilizing co-immunoprecipitation followed by mass spectrometry, we performed interaction mapping to identify new CDKAL1 binding partners. Furthermore, we tested whether Cdkal1 loss in adipose tissue affects total protein levels or accurate Lys incorporation by tRNALys using quantitative mass spectrometry.
Results: We found that Cdkal1 mRNA levels are reduced in adipose tissue of obese mice. Using adipose-specific Cdkal1 KO mice (A-KO), we demonstrated that mitochondrial function is impaired in primary differentiated brown adipocytes and in isolated mitochondria from A-KO brown adipose tissue. A-KO mice displayed decreased energy expenditure during 4 °C cold challenge. Furthermore, mitochondrial morphology was highly abnormal in A-KO BAT. Surprisingly, we found that lysine codon representation was unchanged in Cdkal1 A-KO adipose tissue. We identified novel protein interactors of CDKAL1, including SLC25A4/ANT1, an inner mitochondrial membrane ADP/ATP translocator. ANT proteins can account for the UCP1-independent basal proton leak in BAT mitochondria. Cdkal1 A-KO mice had increased ANT1 protein levels in their white adipose tissue.
Conclusions: Cdkal1 is necessary for normal mitochondrial morphology and function in adipose tissue. These results suggest that the type 2 diabetes susceptibility gene CDKAL1 has novel functions in regulating mitochondrial activity.[Hide abstract]
|Activated macrophages control human adipocyte mitochondrial bioenergetics Recent data from mouse studies suggest that macrophages are modifiers of adipocyte energy metabolism and mitochondrial function. Keuper et al. report a direct action of IL10/TGFβ-activated macrophages on decreased mitochondrial gene expression and function of human white adipocytes, which is reflected in whole human white adipose tissue samples by the association of mitochondrial complex III (UQCRC2) and complex I (NDUFB8) gene expression levels with low CD40:CD163 ratio. Their data suggest that human white adipocytes in different inflammatory microenvironments demonstrate differential metabolic profiles.|
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Objective: Obesity-associated WAT inflammation is characterized by the accumulation and local activation of macrophages (MΦs), and recent data from mouse studies suggest that macrophages are modifiers of adipocyte energy metabolism and mitochondrial function. As mitochondrial dysfunction has been associated with obesity and the metabolic syndrome in humans, herein we aimed to delineate how human macrophages may affect energy metabolism of white adipocytes.
Methods: Human adipose tissue gene expression analysis for markers of macrophage activation and tissue inflammation (CD11c, CD40, CD163, CD206, CD80, MCP1, TNFα) in relationship to mitochondrial complex I (NDUFB8) and complex III (UQCRC2) was performed on subcutaneous WAT of 24 women (BMI 20–61 kg/m2). Guided by these results, the impact of secreted factors of LPS/IFNγ- and IL10/TGFβ-activated human macrophages (THP1, primary blood-derived) on mitochondrial function in human subcutaneous white adipocytes (SGBS, primary) was determined by extracellular flux analysis (Seahorse technology) and gene/protein expression.
Results: Stepwise regression analysis of human WAT gene expression data revealed that a linear combination of CD40 and CD163 was the strongest predictor for mitochondrial complex I (NDUFB8) and complex III (UQCRC2) levels, independent of BMI. IL10/TGFβ-activated MΦs displayed high CD163 and low CD40 expression and secreted factors that decreased UQCRC2 gene/protein expression and ATP-linked respiration in human white adipocytes. In contrast, LPS/IFNγ-activated MΦs showed high CD40 and low CD163 expression and secreted factors that enhanced adipocyte mitochondrial activity resulting in a total difference of 37% in ATP-linked respiration of white adipocytes (p = 0.0024) when comparing the effect of LPS/IFNγ- vs IL10/TGFβ-activated MΦs.
Conclusions: Our data demonstrate that macrophages modulate human adipocyte energy metabolism via an activation-dependent paracrine mechanism.[Hide abstract]
|Inhibition of cholinergic potentiation of insulin secretion: Protection by casein kinase 2 inhibitorAlthough cholinergic regulation of insulin release has been known for many years, the mechanisms by which acetylcholine stimulates insulin secretion are still debated. Doliba et al. show for the first time glucolipotoxic impairments of cholinergic potentiation of glucose-stimulated insulin secretion in isolated mouse and human islets. An inhibitor of casein kinase 2 (CK2) protects the glucose dependent acetylcholine potentiation of insulin secretion against glucolipotoxicity. The results strengthen the view that phosphorylation of β-cell M3-muscarinic receptors by CK2 is of pathophysiological and potential clinical relevance.|
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Objective: Chronic hyperlipidemia and hyperglycemia are characteristic features of type 2 diabetes (T2DM) that are thought to cause or contribute to β-cell dysfunction by “glucolipotoxicity.” Previously we have shown that acute treatment of pancreatic islets with fatty acids (FA) decreases acetylcholine-potentiated insulin secretion. This acetylcholine response is mediated by M3 muscarinic receptors, which play a key role in regulating β-cell function. Here we examine whether chronic FA exposure also inhibits acetylcholine-potentiated insulin secretion using mouse and human islets.
Methods: Islets were cultured for 3 or 4 days at different glucose concentration with 0.5 mM palmitic acid (PA) or a 2:1 mixture of PA and oleic acid (OA) at 1% albumin (PA/BSA molar ratio 3.3). Afterwards, the response to glucose and acetylcholine were studied in perifusion experiments.
Results: FA-induced impairment of insulin secretion and Ca2+ signaling depended strongly on the glucose concentrations of the culture medium. PA and OA in combination reduced acetylcholine potentiation of insulin secretion more than PA alone, both in mouse and human islets, with no evidence of a protective role of OA. In contrast, lipotoxicity was not observed with islets cultured for 3 days in medium containing less than 1 mM glucose and a mixture of glutamine and leucine (7 mM each). High glucose and FAs reduced endoplasmic reticulum (ER) Ca2+ storage capacity; however, preserving ER Ca2+ by blocking the IP3 receptor with xestospongin C did not protect islets from glucolipotoxic effects on insulin secretion. In contrast, an inhibitor of casein kinase 2 (CK2) protected the glucose dependent acetylcholine potentiation of insulin secretion in mouse and human islets against glucolipotoxicity.
Conclusions: These results show that chronic FA treatment decreases acetylcholine potentiation of insulin secretion and that this effect is strictly glucose dependent and might involve CK2 phosphorylation of β-cell M3 muscarinic receptors.[Hide abstract]
|Elevated hepatic DPP4 activity promotes insulin resistance and non-alcoholic fatty liver diseaseDipeptidyl peptidase 4 (DPP4) is a serine protease that cleaves a variety of substrates including incretin hormones, chemokines, growth factors, and neuropeptides. Baumeier and colleagues analyze the DPP4 activity in plasma of healthy subjects and patients with non-alcoholic fatty liver disease. They elucidate the effect of hepatocyte-specific Dpp4 overexpression on the development of insulin resistance and liver steatosis in mice under obese conditions.|
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Objective: Increased hepatic expression of dipeptidyl peptidase 4 (DPP4) is associated with non-alcoholic fatty liver disease (NAFLD). Whether this is causative for the development of NAFLD is not yet clarified. Here we investigate the effect of hepatic DPP4 overexpression on the development of liver steatosis in a mouse model of diet-induced obesity.
Methods: Plasma DPP4 activity of subjects with or without NAFLD was analyzed. Wild-type (WT) and liver-specific Dpp4 transgenic mice (Dpp4-Liv-Tg) were fed a high-fat diet and characterized for body weight, body composition, hepatic fat content and insulin sensitivity. In vitro experiments on HepG2 cells and primary mouse hepatocytes were conducted to validate cell autonomous effects of DPP4 on lipid storage and insulin sensitivity.
Results: Subjects suffering from insulin resistance and NAFLD show an increased plasma DPP4 activity when compared to healthy controls. Analysis of Dpp4-Liv-Tg mice revealed elevated systemic DPP4 activity and diminished active GLP-1 levels. They furthermore show increased body weight, fat mass, adipose tissue inflammation, hepatic steatosis, liver damage and hypercholesterolemia. These effects were accompanied by increased expression of PPARγ and CD36 as well as severe insulin resistance in the liver. In agreement, treatment of HepG2 cells and primary hepatocytes with physiological concentrations of DPP4 resulted in impaired insulin sensitivity independent of lipid content.
Conclusions: Our results give evidence that elevated expression of DPP4 in the liver promotes NAFLD and insulin resistance. This is linked to reduced levels of active GLP-1, but also to auto- and paracrine effects of DPP4 on hepatic insulin signaling.[Hide abstract]
|Intestinal SIRT3 overexpression in mice improves whole body glucose homeostasisThe most efficient treatment for type 2 diabetes and morbid obesity so far is surgical intervention, such as Roux-en-Y gastric bypass. Ramachandran et al. find that on high-fat diet Sirtuin 3 (SIRT3) overexpression protects the mice from developing glucose intolerance and insulin resistance. They suggest that an increase in the metabolic flux of enterocytes is sufficient to improve whole body glucose homeostasis in diet-induced obesity, independent of body weight, body composition, or fat distribution.|
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Objective: Intestinal metabolism might play a greater role in regulating whole body metabolism than previously believed. We aimed to enhance enterocyte metabolism in mice and investigate if it plays a role in diet-induced obesity (DIO) and its comorbidities.
Methods: Using the cre-loxP system, we overexpressed the mitochondrial NAD+ dependent protein deacetylase SIRT3 in enterocytes of mice (iSIRT3 mice). We chronically fed iSIRT3 mice and floxed-SIRT3 control (S3fl) mice a low-fat, control diet (CD) or a high-fat diet (HFD) and then phenotyped the mice.
Results: There were no genotype differences in any of the parameters tested when the mice were fed CD. Also, iSIRT3 mice were equally susceptible to the development of DIO as S3fl mice when fed HFD. They were, however, better able than S3fl mice to regulate their blood glucose levels in response to exogenous insulin and glucose, indicating that they were protected from developing insulin resistance. This improved glucose homeostasis was accompanied by an increase in enterocyte metabolic activity and an upregulation of ketogenic gene expression in the small intestine.
Conclusions: Enhancing enterocyte oxidative metabolism can improve whole body glucose homeostasis.[Hide abstract]
|Reduced renal sympathetic nerve activity contributes to improved glucose tolerance in Pomc knockout miceHypothalamic pro-opiomelanocortin (POMC) is a precursor polypeptide that is synthesized in the pituitary gland as well as the arcuate nucleus (Arc) of the hypothalamus. Chhabra et al. report that ArcPOMC is essential in maintaining basal renal sympathetic nerve activity (RSNA) in mice. They also demonstrate the critical function of RSNA in glucose reabsorption. Reduced RSNA in ArcPomc-/- mice as well as renal denervation in wildtype and diabetic db/db mice improves their glucose tolerance by elevating glycosuria via reduced proximal tubular GLUT2 levels. Elevated glycosuria is likely a mechanism for improving glucose tolerance after renal denervation in drug resistant hypertensive patients.|
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Objective: Hypothalamic arcuate nucleus-specific pro-opiomelanocortin deficient (ArcPomc−/−) mice exhibit improved glucose tolerance despite massive obesity and insulin resistance. We demonstrated previously that their improved glucose tolerance is due to elevated glycosuria. However, the underlying mechanisms that link glucose reabsorption in the kidney with ArcPomc remain unclear. Given the function of the hypothalamic melanocortin system in controlling sympathetic outflow, we hypothesized that reduced renal sympathetic nerve activity (RSNA) in ArcPomc−/− mice could explain their elevated glycosuria and consequent enhanced glucose tolerance.
Methods: We measured RSNA by multifiber recording directly from the nerves innervating the kidneys in ArcPomc−/− mice. To further validate the function of RSNA in glucose reabsorption, we denervated the kidneys of WT and diabetic db/db mice before measuring their glucose tolerance and urine glucose levels. Moreover, we performed western blot and immunohistochemistry to determine kidney GLUT2 and SGLT2 levels in either ArcPomc−/− mice or the renal-denervated mice.
Results: Consistent with our hypothesis, we found that basal RSNA was decreased in ArcPomc−/− mice relative to their wild type (WT) littermates. Remarkably, both WT and db/db mice exhibited elevated glycosuria and improved glucose tolerance after renal denervation. The elevated glycosuria in obese ArcPomc−/−, WT and db/db mice was due to reduced renal GLUT2 levels in the proximal tubules. Overall, we show that renal-denervated WT and diabetic mice recapitulate the phenotype of improved glucose tolerance and elevated glycosuria associated with reduced renal GLUT2 levels observed in obese ArcPomc−/− mice.
Conclusions: Hence, we conclude that ArcPomc is essential in maintaining basal RSNA and that elevated glycosuria is a possible mechanism to explain improved glucose tolerance after renal denervation in drug resistant hypertensive patients.[Hide abstract]
|Angiopoietin-like protein 4 is an exercise-induced hepatokine, regulated by glucagon and cAMPAngiopoietin-like protein-4 (ANGPTL4) is a secreted plasma protein expressed in adipose tissue and liver. Ingerslev, Hansen, and colleagues show that ANGPTL4 is released from the hepato-splanchnic bed but not the leg during exercise. The glucagon-to-insulin ratio is identified as an important regulator of ANGPTL4 plasma in humans, probably involving cAMP-PKA-driven hepatic ANGPTL4 expression.|
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Objective: Angiopoietin-like protein-4 (ANGPTL4) is a circulating protein that is highly expressed in liver and implicated in regulation of plasma triglyceride levels. Systemic ANGPTL4 increases during prolonged fasting and is suggested to be secreted from skeletal muscle following exercise.
Methods: We investigated the origin of exercise-induced ANGPTL4 in humans by measuring the arterial-to-venous difference over the leg and the hepato-splanchnic bed during an acute bout of exercise. Furthermore, the impact of the glucagon-to-insulin ratio on plasma ANGPTL4 was studied in healthy individuals. The regulation of ANGPTL4 was investigated in both hepatic and muscle cells.
Results: The hepato-splanchnic bed, but not the leg, contributed to exercise-induced plasma ANGPTL4. Further studies using hormone infusions revealed that the glucagon-to-insulin ratio is an important regulator of plasma ANGPTL4 as elevated glucagon in the absence of elevated insulin increased plasma ANGPTL4 in resting subjects, whereas infusion of somatostatin during exercise blunted the increase of both glucagon and ANGPTL4. Moreover, activation of the cAMP/PKA signaling cascade let to an increase in ANGPTL4 mRNA levels in hepatic cells, which was prevented by inhibition of PKA. In humans, muscle ANGPTL4 mRNA increased during fasting, with only a marginal further induction by exercise. In human muscle cells, no inhibitory effect of AMPK activation could be demonstrated on ANGPTL4 expression.
Conclusions: The data suggest that exercise-induced ANGPTL4 is secreted from the liver and driven by a glucagon-cAMP-PKA pathway in humans. These findings link the liver, insulin/glucagon, and lipid metabolism together, which could implicate a role of ANGPTL4 in metabolic diseases.[Hide abstract]
|Single-cell RNA-sequencing reveals a distinct population of proglucagon-expressing cellsEnteroendocrine preproglucagon-expressing PPG-cells secrete the gut hormones glucagon-like peptide 1 (GLP-1) and peptideYY (PYY) and are important regulators of glucose metabolism. Glass et al. find that upper small intestinal PPG-cells can be separated into at least 3 major clusters that exhibit differential expression of Gcg, Cck, Pyy, Gip, and Tph1. Receptor and ion channel expression profiles differ across the clusters, suggesting that these PPG-cell sub-populations likely contribute to the differential responsiveness of gut hormones to nutritional and local signals.|
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Objective: To identify sub-populations of intestinal preproglucagon-expressing (PPG) cells producing Glucagon-like Peptide-1, and their associated expression profiles of sensory receptors, thereby enabling the discovery of therapeutic strategies that target these cell populations for the treatment of diabetes and obesity.
Methods: We performed single cell RNA sequencing of PPG-cells purified by flow cytometry from the upper small intestine of 3 GLU-Venus mice. Cells from 2 mice were sequenced at low depth, and from the third mouse at high depth. High quality sequencing data from 234 PPG-cells were used to identify clusters by tSNE analysis. qPCR was performed to compare the longitudinal and crypt/villus locations of cluster-specific genes. Immunofluorescence and mass spectrometry were used to confirm protein expression.
Results: PPG-cells formed 3 major clusters: a group with typical characteristics of classical L-cells, including high expression of Gcg and Pyy (comprising 51% of all PPG-cells); a cell type overlapping with Gip-expressing K-cells (14%); and a unique cluster expressing Tph1 and Pzp that was predominantly located in proximal small intestine villi and co-produced 5-HT (35%). Expression of G-protein coupled receptors differed between clusters, suggesting the cell types are differentially regulated and would be differentially targetable.
Conclusions: Our findings support the emerging concept that many enteroendocrine cell populations are highly overlapping, with individual cells producing a range of peptides previously assigned to distinct cell types. Different receptor expression profiles across the clusters highlight potential drug targets to increase gut hormone secretion for the treatment of diabetes and obesity.[Hide abstract]
|Point mutation of Ffar1 abrogates fatty acid-dependent insulin secretionFree fatty acid receptor-1 (FFAR1) promotes long chain fatty acid-mediated augmentation of glucose-induced insulin secretion. Sabrautzki, Kaiser, Przemeck, and colleagues introduce a mouse model carrying the point mutation R258W in Ffar1, which abolishes the stimulation of insulin secretion in response to long chain fatty acids. They find that high fat diet feeding induces glucose intolerance in wild-type mice while mutant mice with dysfunctional FFAR1 remain glucose tolerant. |
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Objective: The fatty acid receptor 1 (FFAR1/GPR40) mediates fatty acid-dependent augmentation of glucose-induced insulin secretion (GIIS) in pancreatic β-cells. Genetically engineered Ffar1-knockout/congenic mice univocally displayed impaired fatty acid-mediated insulin secretion, but in vivo experiments delivered controversial results regarding the function of FFAR1 in glucose homeostasis and liver steatosis. This study presents a new coisogenic mouse model carrying a point mutation in Ffar1 with functional consequence. These mice reflect the situations in humans in which point mutations can lead to protein malfunction and disease development.
Methods: The Munich N-ethyl-N-nitrosourea (ENU) mutagenesis-derived F1 archive containing over 16,800 sperms and corresponding DNA samples was screened for mutations in the coding region of Ffar1. Two missense mutations (R258W and T146S) in the extracellular domain of the protein were chosen and homozygote mice were generated. The functional consequence of these mutations was examined in vitro in isolated islets and in vivo in chow diet and high fat diet fed mice.
Results: Palmitate, 50 μM, and the FFAR1 agonist TUG-469, 3 μM, stimulated insulin secretion in islets of Ffar1T146S/T146S mutant mice and of wild-type littermates, while in islets of Ffar1R258W/R258W mutant mice, these stimulatory effects were abolished. Insulin content and mRNA levels of Ffar1, Glp1r, Ins2, Slc2a2, Ppara, and Ppard were not significantly different between wild-type and Ffar1R258W/R258W mouse islets. Palmitate exposure, 600 μM, significantly increased Ppara mRNA levels in wild-type but not in Ffar1R258W/R258W mouse islets. On the contrary, Slc2a2 mRNA levels were significantly reduced in both wild-type and Ffar1R258W/R258W mouse islets after palmitate treatment. HFD feeding induced glucose intolerance in wild-type mice. Ffar1R258W/R258W mutant mice remained glucose tolerant although their body weight gain, liver steatosis, insulin resistance, and plasma insulin levels were not different from those of wild-type littermates. Worth mentioning, fasting plasma insulin levels were lower in Ffar1R258W/R258W mice.
Conclusions: A point mutation in Ffar1 abrogates the stimulatory effect of palmitate on GIIS, an effect that does not necessarily translate to HFD-induced glucose intolerance.[Hide abstract]
|Bidirectional manipulation of gene expression in adipocytes using CRISPRa and siRNAFunctional investigation of novel gene/protein targets associated with adipocyte differentiation or function heavily relies on efficient tools to manipulate gene expression in adipocytes in vitro. Lundh and colleagues report a detailed in vitro protocol using easily accessible tools to manipulate gene expression in adipocytes. The major advantage of this model is that once the core components of the CRISPRa SAM system are established, it is easy to use and allows for concurrent expression and silencing of virtually any gene of interest - either alone or in combination - in both pre- and mature adipocytes.|
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Objective: Functional investigation of novel gene/protein targets associated with adipocyte differentiation or function heavily relies on efficient and accessible tools to manipulate gene expression in adipocytes in vitro. Recent advances in gene-editing technologies such as CRISPR-Cas9 have not only eased gene editing but also greatly facilitated modulation of gene expression without altering the genome. Here, we aimed to develop and validate a competent in vitro adipocyte model of controllable functionality as well as multiplexed gene manipulation in adipocytes, using the CRISPRa “SAM” system and siRNAs to simultaneously overexpress and silence selected genes in the same cell populations.
Methods: We introduced a stable expression of dCas9-VP64 and MS2-P65, the core components of the CRIPSRa SAM system, in mesenchymal C3H/10T1/2 cells through viral delivery and used guide RNAs targeting Pparγ2, Prdm16, Zfp423, or Ucp1 to control the expression of key genes involved in adipocyte differentiation and function. We additionally co-transfected mature adipocytes with sgRNA plasmids and siRNA to simultaneously up-regulate and silence selected genes. Quantitative gene expression, oxygen consumption, fluorescence-activated cell sorting and immunocytochemistry served as validation proxies in pre- or mature adipocytes.
Results: CRISPRa SAM-mediated up-regulation of a key adipogenic gene, Pparγ2, was successfully achieved using selected sgRNAs targeting the Pparγ2 promoter region (i.e. up to 104 fold); this induction was long lasting and sufficient to promote adipogenesis. Furthermore, co-activation of Pparγ2 with either Prdm16 or Zfp423 transcripts drove distinct thermogenic gene expression patterns associated with increased or decreased oxygen consumption, respectively, mimicking typical characteristics of brite/beige or white cell lineages. Lastly, we demonstrated that up-regulation of endogenous genes in mature adipocytes was also easily and efficiently achieved using CRISPRa SAM, here exemplified by targeted Ucp1 overexpression (up to 4 × 103 fold), and that it was compatible with concomitant gene silencing using siRNA, allowing for bidirectional manipulation of gene expression in the same cell populations.
Conclusions: We demonstrate that the CRISPRa SAM system can be easily adopted and used to efficiently manipulate gene expression in pre- and mature adipocytes in vitro. Moreover, we describe a novel methodological approach combining the activation of endogenous genes and siRNA-mediated gene silencing, thus providing a powerful tool to functionally decipher genetic factors controlling adipogenesis and adipocyte functions.[Hide abstract]
|Evaluation of a melanocortin-4 receptor (MC4R) agonist (Setmelanotide) in MC4R deficiencyThe Melanocortin 4 receptor (MC4R), which is a seven-transmembrane domain G-protein coupled receptor (GPCR), has been considered as a potential drug target for the treatment of obesity. Setmelanotide is a synthetic cyclic peptide that binds to human MC4R with high affinity. Collet, Dubern, Mokrosinski, Connors, et al. report the comprehensive classification of all known human mutations in MC4R and show that individuals with MC4R deficiency as well as those expressing the wild type MC4R lose weight following Setmelanotide treatment. |
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Objective: Pro-opiomelanocortin (POMC)-derived peptides act on neurons expressing the Melanocortin 4 receptor (MC4R) to reduce body weight. Setmelanotide is a highly potent MC4R agonist that leads to weight loss in diet-induced obese animals and in obese individuals with complete POMC deficiency. While POMC deficiency is very rare, 1–5% of severely obese individuals harbor heterozygous mutations in MC4R. We sought to assess the efficacy of Setmelanotide in human MC4R deficiency.
Methods: We studied the effects of Setmelanotide on mutant MC4Rs in cells and the weight loss response to Setmelanotide administration in rodent studies and a human clinical trial. We annotated the functional status of 369 published MC4R variants.
Results: In cells, we showed that Setmelanotide is significantly more potent at MC4R than the endogenous ligand alpha-melanocyte stimulating hormone and can disproportionally rescue signaling by a subset of severely impaired MC4R mutants. Wild-type rodents appear more sensitive to Setmelanotide when compared to MC4R heterozygous deficient mice, while MC4R knockout mice fail to respond. In a 28-day Phase 1b clinical trial, Setmelanotide led to weight loss in obese MC4R variant carriers. Patients with POMC defects upstream of MC4R show significantly more weight loss with Setmelanotide than MC4R deficient patients or obese controls.
Conclusions: Setmelanotide led to weight loss in obese people with MC4R deficiency; however, further studies are justified to establish whether Setmelanotide can elicit clinically meaningful weight loss in a subset of the MC4R deficient obese population.[Hide abstract]
|Neonatal pancreatic pericytes support β-cell proliferationβ-cell proliferation rates decline with age and are significantly higher during the neonatal period than during adulthood. Epshtein and colleagues find that neonatal pancreatic pericytes secrete factors that promote β-cell proliferation. This finding could aid in developing improved protocols for β-cell expansion as a potential cure for diabetes.|
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Objective: The maintenance and expansion of β-cell mass rely on their proliferation, which reaches its peak in the neonatal stage. β-cell proliferation was found to rely on cells of the islet microenvironment. We hypothesized that pericytes, which are components of the islet vasculature, support neonatal β-cell proliferation.
Methods: To test our hypothesis, we combined in vivo and in vitro approaches. Briefly, we used a Diphtheria toxin-based transgenic mouse system to specifically deplete neonatal pancreatic pericytes in vivo. We further cultured neonatal pericytes isolated from the neonatal pancreas and combined the use of a β-cell line and primary cultured mouse β-cells.
Results: Our findings indicate that neonatal pancreatic pericytes are required and sufficient for β-cell proliferation. We observed impaired proliferation of neonatal β-cells upon in vivo depletion of pancreatic pericytes. Furthermore, exposure to pericyte-conditioned medium stimulated proliferation in cultured β-cells.
Conclusions: This study introduces pancreatic pericytes as regulators of neonatal β-cell proliferation. In addition to advancing current understanding of the physiological β-cell replication process, these findings could facilitate the development of protocols aimed at expending these cells as a potential cure for diabetes.[Hide abstract]