The glucagon-like peptide-1 receptor (GLP-1R) is a class B G protein-coupled receptor involved in the regulation of blood glucose levels and food intake. Stabilized agonists targeting GLP-1R are used in the treatment of type 2 diabetes and have recently become a breakthrough obesity therapy. Here, we revisit a classic article in Diabetes by Thorens et al. that described the cloning, sequencing, and functional expression of the human GLP-1R. The article also demonstrated that exendin4(1-39) was a full agonist of the human GLP-1R whereas exendin4(9-39) was a full antagonist. We discuss how the knowledge imparted by these studies has gone on to inform multiple strands of GLP-1R biology over the past three decades, including pharmacology, signaling, human genetics, structural biology, and chemical biology.

Glucagon is critical for the maintenance of blood glucose, however nutrient regulation of pancreatic α-cells remains poorly understood. Here, we identified a role of leucine, a well-known β-cell fuel, in the α-cell-intrinsic regulation of glucagon release. In islet perifusion assays, physiologic concentrations of leucine strongly inhibited alanine- and arginine-stimulated glucagon secretion from human and mouse islets under hypoglycemic conditions. Mechanistically, leucine dose-dependently reduced α-cell cAMP, independently of Ca2+, ATP/ADP, or fatty acid oxidation. Leucine also reduced α-cell cAMP in islets treated with somatostatin receptor 2 antagonists or diazoxide, compounds that limit paracrine signaling from β/δ-cells. Studies in dispersed mouse islets confirmed an α-cell-intrinsic effect. The inhibitory effect of leucine on cAMP was mimicked by glucose, α-ketoisocaproate, succinate, and the glutamate dehydrogenase activator BCH and blocked by cyanide, indicating a mechanism dependent on mitochondrial metabolism. Glucose dose-dependently reduced the impact of leucine on α-cell cAMP, indicating an overlap in function; however, leucine was still effective at suppressing glucagon secretion in the presence of elevated glucose, amino acids, and the incretin GIP. Taken together, these findings show that leucine plays an intrinsic role in limiting the α-cell secretory tone across the physiologic range of glucose levels, complementing the inhibitory paracrine actions of β/δ-cells.

Type 1 diabetes arises from the selective destruction of pancreatic β-cells by autoimmune mechanisms, and intracellular pathways driven by Janus kinase (JAK)-mediated phosphorylation of STAT isoforms (especially STAT1 and STAT2) are implicated as mediators of β-cell demise. Despite this, the molecular mechanisms that regulate JAK-STAT signaling in β-cells during the autoimmune attack remain only partially disclosed, and the factors acting to antagonize proinflammatory STAT1 signaling are uncertain. We have recently implicated signal regulatory protein α (SIRPα) in promoting β-cell viability in the face of ongoing islet autoimmunity and have now revealed that this protein controls the availability of a cytosolic lysine deacetylase, HDAC6, whose activity regulates the phosphorylation and activation of STAT1. We provide evidence that STAT1 serves as a substrate for HDAC6 in β-cells and that sequestration of HDAC6 by SIRPα in response to anti-inflammatory cytokines (e.g., IL-13) leads to increased STAT1 acetylation. This then impairs the ability of STAT1 to promote gene transcription in response to proinflammatory cytokines, including interferon-γ. We further found that SIRPα is lost from the β-cells of subjects with recent-onset type 1 diabetes under conditions when HDAC6 is retained and STAT1 levels are increased. On this basis, we report a previously unrecognized role for cytokine-induced regulation of STAT1 acetylation in the control of β-cell viability and propose that targeted inhibition of HDAC6 activity may represent a novel therapeutic modality to promote β-cell viability in the face of active islet autoimmunity.

Genetic studies of nontraditional glycemic biomarkers, glycated albumin and fructosamine, can shed light on unknown aspects of type 2 diabetes genetics and biology. We performed a multiphenotype genome-wide association study of glycated albumin and fructosamine from 7,395 White and 2,016 Black participants in the Atherosclerosis Risk in Communities (ARIC) study on common variants from genotyped/imputed data. We discovered two genome-wide significant loci, one mapping to a known type 2 diabetes gene (ARAP1/STARD10) and another mapping to a novel region (UGT1A complex of genes), using multiomics gene-mapping strategies in diabetes-relevant tissues. We identified additional loci that were ancestry- and sex-specific (e.g., PRKCA in African ancestry, FCGRT in European ancestry, TEX29 in males). Further, we implemented multiphenotype gene-burden tests on whole-exome sequence data from 6,590 White and 2,309 Black ARIC participants. Ten variant sets annotated to genes across different variant aggregation strategies were exome-wide significant only in multiancestry analysis, of which CD1D, EGFL7/AGPAT2, and MIR126 had notable enrichment of rare predicted loss of function variants in African ancestry despite smaller sample sizes. Overall, 8 of 14 discovered loci and genes were implicated to influence these biomarkers via glycemic pathways, and most of them were not previously implicated in studies of type 2 diabetes. This study illustrates improved locus discovery and potential effector gene discovery by leveraging joint patterns of related biomarkers across the entire allele frequency spectrum in multiancestry analysis. Future investigation of the loci and genes potentially acting through glycemic pathways may help us better understand the risk of developing type 2 diabetes.

Dopamine (DA) D2-like receptors in both the central nervous system (CNS) and the periphery are key modulators of metabolism. Moreover, disruption of D2-like receptor signaling is implicated in dysglycemia. Yet, the respective metabolic contributions of CNS versus peripheral D2-like receptors, including D2 (D2R) and D3 (D3R) receptors, remain poorly understood. To address this, we developed new pharmacological tools, D2-like receptor agonists with diminished and delayed blood-brain barrier capability, to selectively manipulate D2R/D3R signaling in the periphery. We designated bromocriptine methiodide (BrMeI), a quaternary methiodide analog of D2R/D3R agonist and diabetes drug bromocriptine, as our lead compound based on preservation of D2R/D3R binding and functional efficacy. We then used BrMeI and unmodified bromocriptine to dissect relative contributions of CNS versus peripheral D2R/D3R signaling in treating dysglycemia. Systemic administration of bromocriptine, with unrestricted access to CNS and peripheral targets, significantly improved both insulin sensitivity and glucose tolerance in obese, dysglycemic mice in vivo. In contrast, metabolic improvements were attenuated when access to bromocriptine was restricted either to the CNS through intracerebroventricular administration or delayed access to the CNS via BrMeI. Our findings demonstrate that the coordinated actions of both CNS and peripheral D2-like receptors are required for correcting dysglycemia. Ultimately, the development of a first-generation of drugs designed to selectively target the periphery provides a blueprint for dissecting mechanisms of central versus peripheral DA signaling and paves the way for novel strategies to treat dysglycemia.

Diabetes is a significant global public health issue with implications for vascular endothelial cells (ECs) dysfunction and the subsequent development and advancement of diabetes complications. This study aims to compare the cellular and molecular properties of the aorta in normal and streptozotocin (STZ)-induced diabetic mice, with a focus on elucidating potential mechanism underlying EC dysfunction. Here, we performed a single-cell RNA sequencing survey of 32,573 cells from the aorta of normal and STZ-induced diabetic mice. We found a compendium of 10 distinct cell types, mainly ECs, smooth muscle cells, fibroblast, pericyte, immune cells, and stromal cells. As the diabetes condition progressed, we observed a subpopulation of aortic ECs that exhibited significantly elevated expression of complement (C) molecule C1qa compared with their healthy counterparts. This increased expression of C1qa was found to induce reactive oxygen species (ROS) production, facilitate EC migration and increased permeability, and impair the vasodilation within the aortic segment of mice. Furthermore, AAV-Tie2-shRNA-C1qa was administered into diabetic mice by tail vein injection, showing that inhibition of C1qa in the endothelium led to a reduction in ROS production, decreased vascular permeability, and improved vasodilation. Collectively, these findings highlight the crucial involvement of C1qa in endothelial dysfunction associated with diabetes.

The T allele at rs7903146 in TCF7L2 increases the rate of conversion from prediabetes to type 2 diabetes. This has been associated with impaired β-cell function and with defective suppression of α-cell secretion by glucose. However, the temporal relationship of these abnormalities is uncertain. To study the longitudinal changes in islet function, we recruited 128 subjects, with 67 homozygous for the diabetes-associated allele (TT) at rs7903146 and 61 homozygous for the protective allele. Subjects were studied on two occasions, 3 years apart, using an oral 75-g glucose challenge. The oral minimal model was used to quantitate β-cell function; the glucagon secretion rate was estimated from deconvolution of glucagon concentrations. Glucose tolerance worsened in subjects with the TT genotype. This was accompanied by impaired postchallenge glucagon suppression but appropriate β-cell responsivity to rising glucose concentrations. These data suggest that α-cell abnormalities associated with the TT genotype (rs7903146) occur early and may precede β-cell dysfunction in people as they develop glucose intolerance and type 2 diabetes.

Macrophage (Mφ) plasticity is critical for normal wound repair; however, in type 2 diabetic wounds, Mφs persist in a low-grade inflammatory state that prevents the resolution of wound inflammation. Increased NLRP3 inflammasome activity has been shown in diabetic wound Mφs; however, the molecular mechanisms regulating NLRP3 expression and activity are unclear. Here, we identified that diabetic wound keratinocytes induce Nlrp3 gene expression in wound Mφs through IL-1 receptor-mediated signaling, resulting in enhanced inflammasome activation in the presence of pathogen-associated molecular patterns and damage-associated molecular patterns. We found that IL-1α is increased in human and murine wound diabetic keratinocytes compared with nondiabetic controls and directly induces Mφ Nlrp3 expression through IL-1 receptor signaling. Mechanistically, we report that the histone demethylase, JMJD3, is increased in wound Mφs late post-injury and is induced by IL-1α from diabetic wound keratinocytes, resulting in Nlrp3 transcriptional activation through an H3K27me3-mediated mechanism. Using genetically engineered mice deficient in JMJD3 in myeloid cells (Jmjd3f/flyz2Cre+), we demonstrate that JMJD3 controls Mφ-mediated Nlrp3 expression during diabetic wound healing. Thus, our data suggest a role for keratinocyte-mediated IL-1α/IL-1R signaling in driving enhanced NLRP3 inflammasome activity in wound Mφs. These data also highlight the importance of cell cross-talk in wound tissues and identify JMJD3 and the IL-1R signaling cascade as important upstream therapeutic targets for Mφ NLRP3 inflammasome hyperactivity in nonhealing diabetic wounds.

Diabetic encephalopathy (DE) is a severe complication of the central nervous system associated with diabetes. In this study, we investigated the regulatory role of mammalian target of rapamycin (mTOR) on nuclear factor κB (NF-κB) in mice with DE, and the neuroprotective effect and therapeutic mechanisms of luteolin, a natural flavonoid compound with anti-inflammatory, antioxidant, and neuroprotective properties. The results indicated that treatment with luteolin improved the degree of cognitive impairment in mice with DE. It also decreased the levels of phosphorylated mTOR, phosphorylated NF-κB, and histone deacetylase 2 (HDAC2) and increased the expression of brain-derived neurotrophic factor and synaptic-related proteins. Furthermore, protein-protein interaction and the Gene Ontology analysis revealed that luteolin was involved in the regulatory network of HDAC2 expression through the mTOR/NF-κB signaling cascade. Our bioinformatics and molecular docking results indicated that luteolin may also directly target HDAC2, as an HDAC2 inhibitor, to alleviate DE, complementing mTOR/NF-κB signaling inhibition. Analysis of luteolin's target proteins and their interactions suggest an effect on HDAC2 and cognition. In conclusion, HDAC2 and tau hyperphosphorylation are regulated by the mTOR/NF-κB signaling cascade in DE, and luteolin is found to reverse these effects, demonstrating its protective role in DE.

Insulin replacement therapy is indispensable in the treatment of type 1 and advanced type 2 diabetes. However, insulin's clinical application is challenging due to its narrow therapeutic index. To mitigate acute and chronic risks of glucose excursions, glucose-responsive insulin (GRI) has long been pursued for clinical application. By integrating GRI with glucose-sensitive elements, GRI is capable of releasing or activating insulin in response to plasma or interstitial glucose levels without external monitoring, thereby improving glycemic control and reducing hypoglycemic risk. In this Perspective, we first introduce the history of GRI development and then review major glucose-responsive components that can be leveraged to control insulin delivery. Subsequently, we highlight the recent advances in GRI delivery carriers and insulin analogs. Finally, we provide a look to the future and the challenges of clinical application of GRI.

Changes in microcirculation lead to the progression of organ pathology in diabetes. Although neuroimmune interactions contribute to a variety of conditions, it is still unclear whether abnormal neural activities affect microcirculation related to diabetes. Using laser speckle contrast imaging, we examined the skin of patients with type 2 diabetes and found that their microvascular perfusion was significantly compromised. This phenomenon was replicated in a high-fat diet-driven murine model of type 2 diabetes-like disease. In this setting, although both macrophages and mast cells were enriched in the skin, only mast cells and associated degranulation were critically required for the microvascular impairment. Sensory neurons exhibited enhanced TRPV1 activities, which triggered mast cells to degranulate and compromise skin microcirculation. Chemical and genetic ablation of TRPV1+ nociceptors robustly improved skin microcirculation status. Substance P (SP) is a neuropeptide and was elevated in the skin and sensory neurons in the context of type 2 diabetes. Exogenous administration of SP resulted in impaired skin microcirculation, whereas neuronal knockdown of SP dramatically prevented mast cell degranulation and consequently improved skin microcirculation. Overall, our findings indicate a neuron-mast cell axis underlying skin microcirculation disturbance in diabetes and shed light on neuroimmune therapeutics for diabetes-related complications.

Alterations in the structure, function, and microcirculation of the thalamus, a key brain region involved in pain pathways, have previously been demonstrated in patients with painless and painful diabetic peripheral neuropathy (DPN). However, thalamic neurotransmitter levels including γ-aminobutyric acid (GABA) (inhibitory neurotransmitter) and glutamate (excitatory neurotransmitter) in different DPN phenotypes are not known. We performed a magnetic resonance spectroscopy study and quantified GABA and glutamate levels within the thalamus, in a carefully characterized cohort of participants with painless and painful DPN. Participants with DPN (painful and painless combined) had a significantly lower GABA:H2O ratio compared with those without DPN (healthy volunteers [HV] and participants with diabetes without DPN [no DPN]). Participants with painless DPN had the lowest GABA:H2O ratio, which reached significance compared with HV and no DPN, but not painful DPN. There was no difference in GABA:H2O in painful DPN compared with all other groups. A significant correlation with GABA:H2O and neuropathy severity was also seen. This study demonstrates that lower levels of thalamic GABA in participants with painless DPN may reflect neuroplasticity due to reduced afferent pain impulses, whereas partially preserved levels of GABA in painful DPN may indicate that central GABAergic pathways are involved in the mechanisms of neuropathic pain in diabetes.

During diabetes progression, β-cell dysfunction due to loss of potassium channels sensitive to ATP, known as KATP channels, occurs, contributing to hyperglycemia. The aim of this study was to investigate if KATP channel expression or activity in the nervous system was altered in a high-fat diet (HFD)-fed mouse model of diet-induced obesity. Expression of two KATP channel subunits, Kcnj11 (Kir6.2) and Abcc8 (SUR1), were decreased in the peripheral and central nervous system of mice fed HFD, which was significantly correlated with mechanical paw-withdrawal thresholds. HFD mice had decreased antinociception to systemic morphine compared with control diet (CON) mice, which was expected because KATP channels are downstream targets of opioid receptors. Mechanical hypersensitivity in HFD mice was exacerbated after systemic treatment with glyburide or nateglinide, KATP channel antagonists clinically used to control blood glucose levels. Upregulation of SUR1 and Kir6.2, through an adenovirus delivered intrathecally, increased morphine antinociception in HFD mice. These data present a potential link between KATP channel function and neuropathy during early stages of diabetes. There is a need for increased knowledge of how diabetes affects structural and molecular changes in the nervous system, including ion channels, to lead to the progression of chronic pain and sensory issues.

Genetic determinants of interindividual differences in energy expenditure (EE) are largely unknown. Sphingolipids, such as ceramides, have been implicated in the regulation of human EE via mitochondrial uncoupling. In this study, we investigated whether genetic variants within enzymes involved in sphingolipid synthesis and degradation affect EE and insulin-related traits in a cohort of American Indians informative for 24-h EE and glucose disposal rates during a hyperinsulinemic-euglycemic clamp. Association analysis of 10,084 genetic variants within 28 genes involved in sphingolipid pathways identified a missense variant (rs267738, A>C, E115A) in exon 4 of CERS2 that was associated with higher sleeping EE (116 kcal/day) and increased rates of endogenous glucose production during basal (5%) and insulin-stimulated (43%) conditions, both indicators of hepatic insulin resistance. The rs267738 variant did not affect ceramide synthesis in HepG2 cells but resulted in a 30% decrease in basal mitochondrial respiration. In conclusion, we provide evidence that the CERS2 rs267738 missense variant may influence hepatic glucose production and postabsorptive sleeping metabolic rate.

Mouse models are extensively used in metabolic studies. However, inherent differences between the species, notably their blood glucose levels, hampered data translation into clinical settings. In this study, we confirmed GLUT1 to be the predominantly expressed glucose transporter in both adult and fetal human β-cells. In comparison, GLUT2 is detected in a small yet significant subpopulation of adult β-cells and is expressed to a greater extent in fetal β-cells. Notably, GLUT1/2 expression in INS+ cells from human stem cell-derived islet-like clusters (SC-islets) exhibited a closer resemblance to that observed in fetal islets. Transplantation of primary human islets or SC-islets, but not murine islets, lowered murine blood glucose to the human glycemic range, emphasizing the critical role of β-cells in establishing species-specific glycemia. We further demonstrate the functional requirements of GLUT1 and GLUT2 in glucose uptake and insulin secretion through chemically inhibiting GLUT1 in primary islets and SC-islets and genetically disrupting GLUT2 in SC-islets. Finally, we developed a mathematical model to predict changes in glucose uptake and insulin secretion as a function of GLUT1/2 expression. Collectively, our findings illustrate the crucial roles of GLUTs in human β-cells, and identify them as key components in establishing species-specific glycemic set points.