Intra-islet insulin suppresses glucagon release via GABA-GABAA receptor system

Glycine receptor activation promotes pancreatic islet cell proliferation via the PI3K/mTORC1/p70S6K pathway

Glycine and β-alanine activate glycine receptors (GlyRs), with glycine known to enhance insulin secretion from pancreatic islet β cells, primarily through GlyR activation. However, the effects of GlyR activation on β cell proliferation have not been examined. Here, we aim to investigate the potential proliferative effects of glycine and β-alanine on islets. In vitro experiments on mouse and human islets revealed that glycine and β-alanine, via GlyR activation, stimulated the proliferation of β cells and α cells, without affecting insulin or glucagon secretion. Further analysis indicated the involvement of the PI3K/mTORC1/p70S6K signaling pathway in this process. Inhibition of GlyRs and PI3K/mTORC1/p70S6K signaling attenuated proliferative effects of glycine and β-alanine. In vivo and ex vivo studies supported these findings, showing increased β and α cell mass after 12 weeks of oral administration of glycine and β-alanine, with no changes in insulin secretion or glucose homeostasis under normal conditions. However, during an acute insulin resistance induced by insulin receptor antagonist S961, glycine and β-alanine enhanced insulin secretion and reduced blood glucose levels by increasing β cell secretory capacity. These findings demonstrate glycine and β-alanine in vivo and in vitro promote islet cell proliferation via GlyR activation and the PI3K/mTORC1/p70S6K pathway, potentially providing a target to enhance islet capacity.

Targeting GABA signaling in type 1 diabetes and its complications- an update on the state of the art

Type 1 diabetes (T1D) is an autoimmune disease that leads to the progressive destruction of insulin-producing β cells, resulting in lifelong insulin dependence and a range of severe complications. Beyond conventional glycemic control, innovative therapeutic strategies are needed to address the underlying disease mechanisms. Recent research has highlighted gamma-aminobutyric acid (GABA) as a promising therapeutic target for T1D due to its dual role in modulating both β cell survival and immune response within pancreatic islets. GABA signaling supports β cell regeneration, inhibits α cell hyperactivity, and promotes α-to-β cell transdifferentiation, contributing to improved islet function. Moreover, GABA's influence extends to mitigating T1D complications, including nephropathy, neuropathy, and retinopathy, as well as regulating central nervous system pathways involved in glucose metabolism. This review consolidates the latest advances in GABA-related T1D therapies, covering animal preclinical and human clinical studies and examining the therapeutic potential of GABA receptor modulation, combination therapies, and dietary interventions. Emphasis is placed on the translational potential of GABA-based approaches to enhance β cell viability and counteract autoimmune processes in T1D. Our findings underscore the therapeutic promise of GABA signaling modulation as a novel approach for T1D treatment and encourage further investigation into this pathway's role in comprehensive diabetes management.

GABA and GLP-1 receptor agonist combination therapy modifies diabetes and Langerhans islet cytoarchitecture in a rat model of Wolfram syndrome

BACKGROUND AND AIM

Wolfram syndrome (WS) is a rare autosomal disorder caused by WFS1 gene mutations, currently lacking approved treatments. Preclinical and clinical reports suggest that diabetes medications, such as glucagon-like peptide-1 receptor agonist (GLP1-RA), slow WS-related diabetes and neurodegeneration, improving patient outcomes. Gamma-aminobutyric acid (GABA) has crucial role in pancreatic islet function and blood glucose regulation. However, its specific role in WS diabetic pathophysiology has never been explored. The aim of this study was to enhance the therapeutic efficacy of liraglutide in mitigating the progression of diabetes associated with WS through supplementation with GABA.

METHODS

In this study, 5-month-old glucose intolerant WS rats and their wild-type littermates where daily treated with GABA (1 g/kg/day), liraglutide (0.4 mg/kg/day), or a combination of both. During the four-month experimental period, the diabetic phenotype was closely monitored using intraperitoneal glucose tolerance tests (IPGTT) and corresponding hormone measurements via enzyme-linked immunoassay. Following the treatments, immunohistochemical staining was performed to examine the morphology, cellular distribution, and health of Langerhans islets.

RESULTS

Unlike in conventional diabetes models, GABA monotherapy alone had no significant effect on the diabetic phenotype in WS rats. In contrast, liraglutide monotherapy effectively delayed diabetes progression. Remarkably, the combined therapy of GABA and liraglutide reversed the diabetic phenotype, significantly enhancing glucose homeostasis, as well as insulin and C-peptide secretion. The combined treatment also increased β-cell mass and corrected the pancreatic Langerhans intra-islet ratio of α-, β-, and δ-cells. As a result, the overall morphology and cytoarchitecture of the pancreatic islets were fully restored, suggesting a potential role for these agents in preserving islet integrity. Additionally, both liraglutide and combination therapy increased the number of GAD (glutamic acid decarboxylase) 65/67-positive β-cells in WS rats, indicating an improvement in general β-cell health.

CONCLUSION

GABA monotherapy had no significant effect on the diabetic phenotype in WS rats, while liraglutide monotherapy effectively delayed diabetes progression. However, the combination therapy of GABA and liraglutide demonstrated a markedly superior effect, not only reversing the diabetic phenotype but also significantly enhancing glucose homeostasis, insulin and C-peptide secretion, and β-cell mass. This combined treatment led to a restoration of Langerhans islet architecture, correction of the endocrine cell proportions, and a notable increase in GAD65/67-positive β-cells, indicating improved β-cell health and function. These findings provide strong evidence supporting the evaluation of GABA and GLP-1 RAs as a combination therapy in clinical trials. Their synergistic effects may offer enhanced β-cell protection, promote functional recovery, and uncover novel therapeutic pathways for treating patients with WS.

Enteric GABAergic neuron-derived γ-aminobutyric acid initiates expression of Igfbp7 to sustain ILC3 homeostasis

Neuronal signals have emerged as critical factors that regulate group 3 innate lymphoid cell (ILC3) response and tissue homeostasis, but the molecular mechanisms underlying this regulation remain largely elusive. Here, we identified that the enteric GABAergic neuron-derived neurotransmitter γ-aminobutyric acid (GABA) inhibited proliferation and IL-17A production in ILC3s in a manner dependent on the GABA receptors Gabbr1 and Gabbr2. Conditional deletion of Gabbr1 or ablation of GABAergic neurons caused increased IL-17A production and aggravated colitis. Mechanistically, GABA suppressed the expression of the LIP isoform of the transcription factor C/EBP-β in ILC3s, which repressed the transcription of Igfbp7, which encodes the secreted factor Igfbp7. Autocrine Igfbp7 signaling through the receptor Igf1R inhibited ILC3 proliferation and IL-17A production. Suppression of signaling through the GABA-C/EBP-β-IGFBP7 pathway highly correlated with severity of intestinal inflammation in patients with inflammatory bowel disease (IBD). Collectively, our findings describe an important molecular mechanism underlying the maintenance of gut immune homeostasis.

Disease-modifying pharmacological treatments of type 1 diabetes: Molecular mechanisms, target checkpoints, and possible combinatorial treatments

After a century of extensive scientific investigations, there is still no curative or disease-modifying treatment available that can provide long-lasting remission for patients diagnosed with type 1 diabetes (T1D). Although T1D has historically been regarded as a classic autoimmune disorder targeting and destroying pancreatic islet β-cells, significant research has recently demonstrated that β-cells themselves also play a substantial role in the disease's progression, which could explain some of the unfavorable clinical outcomes. We offer a thorough review of scientific and clinical insights pertaining to molecular mechanisms behind pathogenesis and the different therapeutic interventions in T1D covering over 20 possible pharmaceutical intervention treatments. The interventions are categorized as immune therapies, treatments targeting islet endocrine dysfunctions, medications with dual modes of action in immune and islet endocrine cells, and combination treatments with a broader spectrum of activity. We suggest that these collective findings can provide a valuable platform to discover new combinatorial synergies in search of the curative disease-modifying intervention for T1D. SIGNIFICANCE STATEMENT: This research delves into the underlying causes of T1D and identifies critical mechanisms governing β-cell function in both healthy and diseased states. Thus, we identify specific pathways that could be manipulated by existing or new pharmacological interventions. These interventions fall into several categories: (1) immunomodifying therapies individually targeting immune cell processes, (2) interventions targeting β-cells, (3) compounds that act simultaneously on both immune cell and β-cell pathways, and (4) combinations of compounds simultaneously targeting immune and β-cell pathways.

The role of GABA in type 1 diabetes

Gamma aminobutyric acid (GABA) is synthesized from glutamate by glutamic decarboxylase (GAD). The entero-pancreatic biology of GABA, which is produced by pancreatic islets, GAD-expressing microbiota, enteric immune cells, or ingested through diet, supports an essential physiologic role of GABA in the health and disease. Outside the central nervous system (CNS), GABA is uniquely concentrated in pancreatic β-cells. They express GAD65, which is a type 1 diabetes (T1D) autoantigen. Glutamate constitutes 10% of the amino acids in dietary protein and is preeminently concentrated in human milk. GABA is enriched in many foods, such as tomato and fermented cheese, and is an over-the-counter supplement. Selected microbiota in the midgut have the enzymatic capacity to produce GABA. Intestinal microbiota interact with gut-associated lymphoid tissue to maintain host defenses and immune tolerance, which are implicated in autoimmune disease. Although GABA is a widely known inhibitory neurotransmitter, oral GABA does not cross the blood brain barrier. Three diabetes-related therapeutic actions are ascribed to GABA, namely, increasing pancreatic β-cell content, attenuating excess glucagon and tamping down T-cell immune destruction. These salutary actions have been observed in numerous rodent diabetes models that usually employed high or near-continuous GABA doses. Clinical studies, to date, have identified positive effects of oral GABA on peripheral blood mononuclear cell cytokine release and plasma glucagon. Going forward, it is reassuring that oral GABA therapy has been well-tolerated and devoid of serious adverse effects.

GABA-mediated inhibition of human CD4+ T cell functions is enhanced by insulin but impaired by high glucose levels

BACKGROUND

γ-aminobutyric acid (GABA), known as the main inhibitory neurotransmitter in the brain, exerts immunomodulatory functions by interaction with immune cells, including T cells. Metabolic programs of T cells are closely linked to their effector functions including proliferation, differentiation, and cytokine production. The physiological molecules glucose and insulin may provide environmental cues and guidance, but whether they coordinate to regulate GABA-mediated T cell immunomodulation is still being examined.

METHODS

CD4+ T cells that were isolated from blood samples from healthy individuals and from patients with type 1 diabetes (T1D) were activated in vitro. We carried out metabolic assays, multiple proximity extension assay (PEA), ELISA, qPCR, immunoblotting, immunofluorescence staining, flow cytometry analysis, MS-based proteomics, as well as electrophysiology and live-cell Ca2+ imaging.

FINDINGS

We demonstrate that GABA-mediated reduction of metabolic activity and the release of inflammatory proteins, including IFNγ and IL-10, were abolished in human CD4+ T cells from healthy individuals and patients with T1D when the glucose concentration was elevated above levels typically observed in healthy people. Insulin increased GABAA receptor-subunit ρ2 expression, enhanced the GABAA receptors-mediated currents and Ca2+ influx. GABA decreased, whereas insulin sustained, hexokinase activity and glycolysis in a glucose concentration-dependent manner.

INTERPRETATION

These findings support that metabolic factors, such as glucose and insulin, influence the GABA-mediated immunomodulation of human primary T cells effector functions.

FUNDING

The Swedish Children's Diabetes Foundation, The Swedish Diabetes Foundation, The Swedish Research Council 2018-02952, EXODIAB, The Ernfors Foundation, The Thurings Foundation and the Science for Life Laboratory.

Checkpoint kinase 2 controls insulin secretion and glucose homeostasis

After the discovery of insulin, a century ago, extensive work has been done to unravel the molecular network regulating insulin secretion. Here we performed a chemical screen and identified AZD7762, a compound that potentiates glucose-stimulated insulin secretion (GSIS) of a human β cell line, healthy and type 2 diabetic (T2D) human islets and primary cynomolgus macaque islets. In vivo studies in diabetic mouse models and cynomolgus macaques demonstrated that AZD7762 enhances GSIS and improves glucose tolerance. Furthermore, genetic manipulation confirmed that ablation of CHEK2 in human β cells results in increased insulin secretion. Consistently, high-fat-diet-fed Chk2-/- mice show elevated insulin secretion and improved glucose clearance. Finally, untargeted metabolic profiling demonstrated the key role of the CHEK2-PP2A-PLK1-G6PD-PPP pathway in insulin secretion. This study successfully identifies a previously unknown insulin secretion regulating pathway that is conserved across rodents, cynomolgus macaques and human β cells in both healthy and T2D conditions.

Insulin signaling and pharmacology in humans and in corals

Once thought to be a unique capability of the Langerhans islets in the pancreas of mammals, insulin (INS) signaling is now recognized as an evolutionarily ancient function going back to prokaryotes. INS is ubiquitously present not only in humans but also in unicellular eukaryotes, fungi, worms, and Drosophila. Remote homologue identification also supports the presence of INS and INS receptor in corals where the availability of glucose is largely dependent on the photosynthetic activity of the symbiotic algae. The cnidarian animal host of corals operates together with a 20,000-sized microbiome, in direct analogy to the human gut microbiome. In humans, aberrant INS signaling is the hallmark of metabolic disease, and is thought to play a major role in aging, and age-related diseases, such as Alzheimer's disease. We here would like to argue that a broader view of INS beyond its human homeostasis function may help us understand other organisms, and in turn, studying those non-model organisms may enable a novel view of the human INS signaling system. To this end, we here review INS signaling from a new angle, by drawing analogies between humans and corals at the molecular level.

A Supportive Role of Mesenchymal Stem Cells on Insulin-Producing Langerhans Islets with a Specific Emphasis on The Secretome

Type 1 Diabetes (T1D) is a chronic autoimmune disease characterized by a gradual destruction of insulin-producing β-cells in the endocrine pancreas due to innate and specific immune responses, leading to impaired glucose homeostasis. T1D patients usually require regular insulin injections after meals to maintain normal serum glucose levels. In severe cases, pancreas or Langerhans islet transplantation can assist in reaching a sufficient β-mass to normalize glucose homeostasis. The latter procedure is limited because of low donor availability, high islet loss, and immune rejection. There is still a need to develop new technologies to improve islet survival and implantation and to keep the islets functional. Mesenchymal stem cells (MSCs) are multipotent non-hematopoietic progenitor cells with high plasticity that can support human pancreatic islet function both in vitro and in vivo and islet co-transplantation with MSCs is more effective than islet transplantation alone in attenuating diabetes progression. The beneficial effect of MSCs on islet function is due to a combined effect on angiogenesis, suppression of immune responses, and secretion of growth factors essential for islet survival and function. In this review, various aspects of MSCs related to islet function and diabetes are described.

GABA and Combined GABA with GAD65-Alum Treatment Alters Th1 Cytokine Responses of PBMCs from Children with Recent-Onset Type 1 Diabetes

Type 1 diabetes (T1D) is an autoimmune disease culminating in the destruction of insulin-producing pancreatic cells. There is a need for the development of novel antigen-specific strategies to delay cell destruction, including combinatorial strategies that do not elicit systemic immunosuppression. Gamma-aminobutyric acid (GABA) is expressed by immune cells, β-cells, and gut bacteria and is immunomodulatory. Glutamic-acid decarboxylase 65 (GAD65), which catalyzes GABA from glutamate, is a T1D autoantigen. To test the efficacy of combinatorial GABA treatment with or without GAD65-immunization to dampen autoimmune responses, we enrolled recent-onset children with T1D in a one-year clinical trial (ClinicalTrials.gov NCT02002130) and examined T cell responses. We isolated peripheral blood mononuclear cells and evaluated cytokine responses following polyclonal activation and GAD65 rechallenge. Both GABA alone and GABA/GAD65-alum treatment inhibited Th1 cytokine responses over the 12-month study with both polyclonal and GAD65 restimulation. We also investigated whether patients with HLA-DR3-DQ2 and HLA-DR4-DQ8, the two highest-risk human leukocyte antigen (HLA) haplotypes in T1D, exhibited differences in response to GABA alone and GABA/GAD65-alum. HLA-DR4-DQ8 patients possessed a Th1-skewed response compared to HLA-DR3-DQ2 patients. We show that GABA and GABA/GAD65-alum present an attractive immunomodulatory treatment for children with T1D and that HLA haplotypes should be considered.

Emerging Anti-Diabetic Drugs for Beta-Cell Protection in Type 1 Diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disorder that damages beta cells in the pancreatic islets of Langerhans and results in hyperglycemia due to the loss of insulin. Exogenous insulin therapy can save lives but does not halt disease progression. Thus, an effective therapy may require beta-cell restoration and suppression of the autoimmune response. However, currently, there are no treatment options available that can halt T1D. Within the National Clinical Trial (NCT) database, a vast majority of over 3000 trials to treat T1D are devoted to insulin therapy. This review focuses on non-insulin pharmacological therapies. Many investigational new drugs fall under the category of immunomodulators, such as the recently FDA-approved CD-3 monoclonal antibody teplizumab. Four intriguing candidate drugs fall outside the category of immunomodulators, which are the focus of this review. Specifically, we discuss several non-immunomodulators that may have more direct action on beta cells, such as verapamil (a voltage-dependent calcium channel blocker), gamma aminobutyric acid (GABA, a major neurotransmitter with effects on beta cells), tauroursodeoxycholic acid (TUDCA, an endoplasmic reticulum chaperone), and volagidemab (a glucagon receptor antagonist). These emerging anti-diabetic drugs are expected to provide promising results in both beta-cell restoration and in suppressing cytokine-derived inflammation.

Modulation of transcription factors by small molecules in β-cell development and differentiation

Transcription factors regulate gene expression and play crucial roles in development and differentiation of pancreatic β-cell. The expression and/or activities of these transcription factors are reduced when β-cells are chronically exposed to hyperglycemia, which results in loss of β-cell function. Optimal expression of such transcription factors is required to maintain normal pancreatic development and β-cell function. Over many other methods of regenerating β-cells, using small molecules to activate transcription factors has gained insights, resulting in β-cells regeneration and survival. In this review, we discuss the broad spectrum of transcription factors regulating pancreatic β-cell development, differentiation and regulation of these factors in normal and pathological states. Also, we have presented set of potential pharmacological effects of natural and synthetic compounds on activities of transcription factor involved in pancreatic β-cell regeneration and survival. Exploring these compounds and their action on transcription factors responsible for pancreatic β-cell function and survival could be useful in providing new insights for development of small molecule modulators.

Metabolomics analysis of islet regeneration in partial pancreatectomy mice reveals increased levels of long-chain fatty acids and activated cAMP signaling pathway

Islet regeneration is a complex process involving multiple metabolic adaptions, but the specific characterization of the islet metabolome in relation to cell proliferation has not been established. This study aimed to investigate the metabolomic changes of regenerative islets from partial pancreatectomy (Ppx) mice and speculate underlying mechanisms. Islet samples were collected from C57/BL6 mice undergoing 70-80% Ppx or sham surgery, followed by analyses of glucose homeostasis, islet morphology, and untargeted metabolomics profiles using liquid chromatography-tandem mass spectrometry (LC-MS/MS). There is no difference in blood glucose and body weight between sham and Ppx mice. After surgery, the Ppx mice showed impaired glucose tolerance, increased Ki67 positive beta cells, and elevated beta-cell mass. LC-MS/MS analysis identified fourteen differentially changed metabolites in islets of Ppx mice, including long-chain fatty acids (e.g., docosahexaenoic acid) and amino acid derivatives (e.g., creatine). Pathway analysis based on the KEGG database revealed five significantly enriched signaling pathways including cAMP signaling pathway. Further immunostaining assay on pancreatic tissue sections showed the levels of p-CREB, a transcription factor downstream of cAMP, elevated in islets from Ppx mice. In conclusion, our results demonstrate that islet regeneration involves metabolic alterations in long-chain fatty acids and amino acid derivatives, as well as the activation of the cAMP signaling pathway.

14-3-3ζ Mediates GABAAR Activation by Interacting with BIG1

Most fast synaptic inhibitions in the mammalian brain are mediated by GABA_A receptors (GABA_ARs). An appropriate level of GABA_AR expression at the cell surface is essential for neurodevelopment and the efficacy of GABAergic synaptic transmission. We previously reported that brefeldin A-inhibited GDP/GTP exchange factor 1 (BIG1), a binding partner of GABA_ARs, plays an important role in trafficking GABA_ARs to the cell surface. However, its regulatory mechanisms remain unknown. In the present study, we identified a new cellular protein, 14-3-3ζ, which can interact with the β subunit of GABA_ARs and BIG1 both in vitro and in vivo and colocalizes in the soma, dendrites, and axons of hippocampal neurons. Overexpression of 14-3-3ζ-WT increased the surface expression of BIG1 in dendrites and axons, as well as the binding of BIG1 with GABA_AR. Depleted 14-3-3ζ with efficacious siRNA attenuated the interaction between BIG1 and GABA_ARs and resulted in significant decreases in the surface expression levels of BIG1 and GABA_AR. GABA_AR agonist treatment increased the expression levels of BIG1 and 14-3-3ζ on the surface, indicating that 14-3-3ζ is involved in regulating BIG1-mediated GABA_AR surface expression. Depletion of BIG1 or 14-3-3ζ significantly decreased GABA_AR expression at the cell surface and suppressed the GABA-gated influx of chloride ions. These data indicate that the combination of 14-3-3ζ and BIG1 is required for GABA_AR membrane expression. Our results provide a potential promising therapeutic target for neurological disorders involving GABAergic synaptic transmission.

GABA signalling in human pancreatic islets

The pancreatic islets are essential microorgans controlling the glucose level in the blood. The islets consist of different cell types which communicate with each other by means of auto- and paracrine interactions. One of the communication molecules produced by and released within the islets is γ-aminobutyric acid (GABA), a well-known inhibitor of neuronal excitability in the mammalian nervous system. Interestingly, GABA is also present in the blood in the nanomolar concentration range. Thus, GABA can affect not only islet function per se (e.g. hormone secretion) but also interactions between immune cells and the pancreatic islet cells in physiological conditions and in pathological states (particularly in type 1 diabetes). In the last decade the interest in GABA signalling in islets has increased. The broad research scope ranges from fundamental physiological studies at the molecular and cellular level to pathological implications and clinical trials. The aim of this mini-review is to outline the current status of the islet GABA field mostly in relation to human islets, to identify the gaps in the current knowledge and what clinical implications GABA signalling may have in islets.

γ-aminobutyric acid modulates α-cell hyperplasia but not β-cell regeneration induced by glucagon receptor antagonism in type 1 diabetic mice

AIMS

To investigate whether treatment with γ-aminobutyric acid (GABA) alone or in combination with glucagon receptor (GCGR) monoclonal antibody (mAb) exerted beneficial effects on β-cell mass and α-cell mass, and to explore the origins of the regenerated β-cells in mice with type 1 diabetes (T1D).

METHODS

Streptozotocin (STZ)-induced T1D mice were treated with intraperitoneal injection of GABA (250 μg/kg per day) and/or REMD 2.59 (a GCGR mAb, 5 mg/kg per week), or IgG dissolved in PBS for 8 weeks. Plasma hormone levels and islet cell morphology were evaluated by ELISA and immunofluorescence, respectively. The origins of the regenerated β-cells were analyzed by double-immunostaining, α-cell lineage-tracing and BrdU-tracing studies.

RESULTS

After the 8-week treatment, GABA or GCGR mAb alone or in combination ameliorated hyperglycemia in STZ-induced T1D mice. GCGR mAb upregulated plasma insulin level and increased β-cell mass, and GABA appeared to have similar effects in T1D mice. However, combination treatment did not reveal any additive or synergistic effect. Interestingly, the GCGR mAb-induced increment of plasma glucagon level and α-cell mass was attenuated by the combined treatment of GABA. In addition, duct-derived β-cell neogenesis and α-to-β cell conversion but not β-cell proliferation contributed to the increased β-cell mass in T1D mice.

CONCLUSION

These results suggested that GABA attenuated α-cell hyperplasia but did not potentiates β-cell regeneration induced by GCGR mAb in T1D mice. Our findings provide novel insights into a combination treatment strategy for β-cell regeneration in T1D.

Pancreatic Islet Cells Response to IFNγ Relies on Their Spatial Location within an Islet

Type 1 diabetes (T1D) is an auto-immune disease characterized by the progressive destruction of insulin-producing pancreatic beta cells. While beta cells are the target of the immune attack, the other islet endocrine cells, namely the alpha and delta cells, can also be affected by the inflammatory milieu. Here, using a flow cytometry-based strategy, we compared the impact of IFNγ, one of the main cytokines involved in T1D, on the three endocrine cell subsets isolated from C57BL/6 mouse islets. RNA-seq analyses revealed that alpha and delta cells exposed in vitro to IFNγ display a transcriptomic profile very similar to that of beta cells, with an increased expression of inflammation key genes such as MHC class I molecules, the CXCL10 chemokine and the programmed death-ligand 1 (PD-L1), three hallmarks of IFNγ signaling. Interestingly, at low IFNγ concentration, we observed two beta cell populations (responders and non-responders) based on PD-L1 protein expression. Our data indicate that this differential sensitivity relies on the location of the cells within the islet rather than on the existence of two different beta cells subsets. The same findings were corroborated by the in vivo analysis of pancreatic islets from the non-obese diabetic mouse model of T1D, showing more intense PD-L1 staining on endocrine cells close to immune infiltrate. Collectively, our work demonstrates that alpha and delta cells are as sensitive as beta cells to IFNγ, and suggests a gradual diffusion of the cytokine into an islet. These observations provide novel insights into the in situ inflammatory processes occurring in T1D progression.

A randomized trial of oral gamma aminobutyric acid (GABA) or the combination of GABA with glutamic acid decarboxylase (GAD) on pancreatic islet endocrine function in children with newly diagnosed type 1 diabetes

Gamma aminobutyric acid(GABA) is synthesized by glutamate decarboxylase(GAD) in β-cells. Regarding Type 1 diabetes(T1D), animal/islet-cell studies found that GABA promotes insulin secretion, inhibits α-cell glucagon and dampens immune inflammation, while GAD immunization may also preserve β-cells. We evaluated the safety and efficacy of oral GABA alone, or combination GABA with GAD, on the preservation of residual insulin secretion in recent-onset T1D. Herein we report a single-center, double-blind, one-year, randomized trial in 97 children conducted March 2015 to June 2019(NCT02002130). Using a 2:1 treatment:placebo ratio, interventions included oral GABA twice-daily(n = 41), or oral GABA plus two-doses GAD-alum(n = 25), versus placebo(n = 31). The primary outcome, preservation of fasting/meal-stimulated c-peptide, was not attained. Of the secondary outcomes, the combination GABA/GAD reduced fasting and meal-stimulated serum glucagon, while the safety/tolerability of GABA was confirmed. There were no clinically significant differences in glycemic control or diabetes antibody titers. Given the low GABA dose for this pediatric trial, future investigations using higher-dose or long-acting GABA formulations, either alone or with GAD-alum, could be considered, although GABA alone or in combination with GAD-alum did nor preserve beta-cell function in this trial.

The role of GABA in islet function

Gamma aminobutyric acid (GABA) is a non-proteinogenic amino acid and neurotransmitter that is produced in the islet at levels as high as in the brain. GABA is synthesized by the enzyme glutamic acid decarboxylase (GAD), of which the 65 kDa isoform (GAD65) is a major autoantigen in type 1 diabetes. Originally described to be released via synaptic-like microvesicles or from insulin secretory vesicles, beta cells are now understood to release substantial quantities of GABA directly from the cytosol via volume-regulated anion channels (VRAC). Once released, GABA influences the activity of multiple islet cell types through ionotropic GABAA receptors and metabotropic GABAB receptors. GABA also interfaces with cellular metabolism and ATP production via the GABA shunt pathway. Beta cells become depleted of GABA in type 1 diabetes (in remaining beta cells) and type 2 diabetes, suggesting that loss or reduction of islet GABA correlates with diabetes pathogenesis and may contribute to dysfunction of alpha, beta, and delta cells in diabetic individuals. While the function of GABA in the nervous system is well-understood, the description of the islet GABA system is clouded by differing reports describing multiple secretion pathways and effector functions. This review will discuss and attempt to unify the major experimental results from over 40 years of literature characterizing the role of GABA in the islet.

Betel quid: New insights into an ancient addiction

The use of areca nuts (areca) in the form of betel quids constitutes the fourth most common addiction in the world, associated with high risk for oral disease and cancer. Areca is a complex natural product, making it difficult to identify specific components associated with the addictive and carcinogenic properties. It is commonly believed that the muscarinic agonist arecoline is at the core of the addiction. However, muscarinic receptor activation is not generally believed to support drug-taking behaviour. Subjective accounts of areca use include descriptions of both sedative and stimulatory effects, consistent with the presence of multiple psychoactive agents. We have previously reported partial agonism of α4-containing nicotinic acetylcholine receptors by arecoline and subsequent inhibition of those receptors by whole areca broth. In the present study, we report the inhibition of nicotinic acetylcholine receptors and other types of neurotransmitter receptors with compounds of high molecular weight in areca and the ability of low molecular weight areca extract to activate GABA and glutamate receptors. We confirm the presence of a high concentration of GABA and glutamate in areca. Additionally, data also indicate the presence of a dopamine and serotonin transporter blocking activity in areca that could account for the reported stimulant and antidepressant activity. Our data suggest that toxic elements of high molecular weight may contribute to the oral health liability of betel quid use, while two distinct low molecular weight components may provide elements of reward, and the nicotinic activity of arecoline contributes to the physical dependence of addiction.

β subunits of GABAA receptors form proton-gated chloride channels: Insights into the molecular basis

Gamma-aminobutyric acid type A receptors (GABAARs) are ligand gated channels mediating inhibition in the central nervous system. Here, we identify a so far undescribed function of β-subunit homomers as proton-gated anion channels. Mutation of a single H267A in β3 subunits completely abolishes channel activation by protons. In molecular dynamic simulations of the β3 crystal structure protonation of H267 increased the formation of hydrogen bonds between H267 and E270 of the adjacent subunit leading to a pore stabilising ring formation and accumulation of Cl- within the transmembrane pore. Conversion of these residues in proton insensitive ρ1 subunits transfers proton-dependent gating, thus highlighting the role of this interaction in proton sensitivity. Activation of chloride and bicarbonate currents at physiological pH changes (pH50 is in the range 6- 6.3) and kinetic studies suggest a physiological role in neuronal and non-neuronal tissues that express beta subunits, and thus as potential novel drug target.

The influence of a tomato food matrix on the bioavailability and plasma kinetics of oral gamma-aminobutyric acid (GABA) and its precursor glutamate in healthy men

Gamma-aminobutyric acid (GABA) and its precursor glutamate play signaling roles in a range of tissues. Both function as neurotransmitters in the central nervous system, but they also modulate pancreatic and immune functioning, for example. Besides endogenous production, both compounds are found in food products, reaching relatively high levels in tomatoes. Recent studies in rodents suggest beneficial effects of oral GABA on glucose homeostasis and blood pressure. However, the bioavailability from food remains unknown. We studied the bioavailability of GABA and glutamate from tomatoes relative to a solution in water. After a fasting blood sample was taken, eleven healthy men randomly received 1 liter of 4 different drinks in a cross-over design with a one-week interval. The drinks were a solution of 888 mg L^-1 GABA, a solution of 3673 mg L^-1 glutamate, pureed fresh tomatoes and plain water as the control. Following intake, 18 blood samples were taken at intervals for 24 hours. Plasma GABA and glutamate concentrations were determined by ultra-pressure liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS). Fasting plasma GABA and glutamate concentrations were found to be 16.71 (SD 2.18) ng mL^-1 and 4626 (SD 1666) ng mL^-1, respectively. Fasting GABA levels were constant (5.8 CV%) between individuals, while fasting glutamate levels varied considerably (23.5 CV%). GABA from pureed tomatoes showed similar bioavailability to that of a solution in water. For glutamate, the absorption from pureed tomatoes occurred more slowly as seen from a longer t_max (0.98 ± 0.14 h vs. 0.41 ± 0.04 h, P = 0.003) and lower C_max (7815 ± 627 ng mL^-1vs. 16 420 ± 2778 ng mL^-1, P = 0.006). These data suggest that GABA is bioavailable from tomatoes, and that food products containing GABA could potentially induce health effects similar to those claimed for GABA supplements. The results merit further studies on the bioavailability of GABA from other food products and the health effects of GABA-rich diets. The clinical trial registry number is NCT04086108 (https://clinicaltrials.gov/ct2/show/NCT04303468).

α Cell dysfunction in islets from nondiabetic, glutamic acid decarboxylase autoantibody-positive individuals

BACKGROUNDMultiple islet autoantibodies (AAbs) predict the development of type 1 diabetes (T1D) and hyperglycemia within 10 years. By contrast, T1D develops in only approximately 15% of individuals who are positive for single AAbs (generally against glutamic acid decarboxylase [GADA]); hence, the single GADA+ state may represent an early stage of T1D.METHODSHere, we functionally, histologically, and molecularly phenotyped human islets from nondiabetic GADA+ and T1D donors.RESULTSSimilar to the few remaining β cells in the T1D islets, GADA+ donor islets demonstrated a preserved insulin secretory response. By contrast, α cell glucagon secretion was dysregulated in both GADA+ and T1D islets, with impaired glucose suppression of glucagon secretion. Single-cell RNA-Seq of GADA+ α cells revealed distinct abnormalities in glycolysis and oxidative phosphorylation pathways and a marked downregulation of cAMP-dependent protein kinase inhibitor β (PKIB), providing a molecular basis for the loss of glucose suppression and the increased effect of 3-isobutyl-1-methylxanthine (IBMX) observed in GADA+ donor islets.CONCLUSIONWe found that α cell dysfunction was present during the early stages of islet autoimmunity at a time when β cell mass was still normal, raising important questions about the role of early α cell dysfunction in the progression of T1D.FUNDINGThis work was supported by grants from the NIH (3UC4DK112217-01S1, U01DK123594-02, UC4DK112217, UC4DK112232, U01DK123716, and P30 DK019525) and the Vanderbilt Diabetes Research and Training Center (DK20593).

ZT-01: A novel somatostatin receptor 2 antagonist for restoring the glucagon response to hypoglycaemia in type 1 diabetes

AIM

To evaluate the pharmacokinetics and efficacy of a novel somatostatin receptor 2 antagonist, ZT-01, to stimulate glucagon release in rats with type 1 diabetes (T1D).

METHODS

The pharmacokinetics of ZT-01 and PRL-2903 were assessed following intraperitoneal or subcutaneous dosing at 10 mg/kg. We compared the efficacy of ZT-01 with PRL-2903 to prevent hypoglycaemia during an insulin bolus challenge and under hypoglycaemic clamp conditions.

RESULTS

Within 1 hour after intraperitoneal administration, ZT-01 achieved more than 10-fold higher plasma Cmax compared with PRL-2903. Twenty-four hour exposure was 4.7× and 11.3× higher with ZT-01 by the intraperitoneal and subcutaneous routes, respectively. The median time to reach hypoglycaemia of more than 3.0 mmol/L was 60, 70, and 125 minutes following vehicle, PRL-2903, or ZT-01 administration, respectively. Furthermore, rats receiving ZT-01 had significantly higher glucose nadirs following insulin administration compared with PRL-2903- and vehicle-treated rats. During the hypoglycaemic clamp, ZT-01 increased peak glucagon responses by ~4-fold over PRL-2903.

CONCLUSIONS

We conclude that ZT-01 may be effective in restoring glucagon responses and preventing the onset of hypoglycaemia in patients with T1D.

Autoantibodies directed against glutamate decarboxylase interfere with glucose-stimulated insulin secretion in dispersed rat islets

Islet autoantibodies, including autoantibodies directed against the 65kDa isoform of glutamate decarboxylase (GAD65Ab), are present in the majority of patients with newly diagnosed type 1 diabetes (T1D). Whereas these autoantibodies are historically viewed as an epiphenomenon of the autoimmune response with no significant pathogenic function, we consider in this study the possibility that they impact the major islet function, namely glucose-stimulated insulin secretion. Two human monoclonal GAD65Ab (GAD65 mAb) (b78 and b96.11) were investigated for uptake by live rat beta cells, subcellular localization and their effect on glucose-stimulated insulin secretion. The GAD65 mAbs were internalized by live pancreatic beta cells, where they localized to subcellular structures in an epitope-specific manner. Importantly, GAD65 mAb b78 inhibited, while GAD65 mAb b96.11 enhanced, glucose-stimulated insulin secretion (GSIS). These opposite effects on GSIS rule out non-specific effects of the antibodies and suggest that internalization of the antibody leads to epitope-specific interaction with intracellular machinery regulating insulin granule release. The most likely explanation for the alteration of GSIS by GAD65 Abs is via changes in GABA release due to inhibition or change in GAD65 enzyme activity. This is the first report indicating an active role of GAD65Ab in the pathogenesis of T1D.

Specific Expression of KCC2 in the α Cells of Normal and Type 1 Diabetes Model Mouse Pancreatic Islets

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter in the mature brain; however, it acts excitatory during development. This difference in action depends on the intracellular chloride ion concentration, primarily regulated by potassium chloride co-transporter2 (KCC2). Sufficient KCC2 expression results in its inhibitory action. GABA is also abundant in pancreatic islets, where it acts differentially on the islet cells, and is involved in carbohydrate metabolism. However, the mechanisms underlying the differential action remain unknown. We performed immunohistochemistry for glutamic acid decarboxylase (GAD), a synthetic enzyme for GABA, and KCC2 in normal adult islets. GAD was co-localized with insulin in β cells, whereas KCC2 was expressed in glucagon-positive α cells. These results are in line with previous observations that GABA decreases glucagon release but increases insulin release, and suggest that GABA and insulin may work together in reducing blood glucose levels under hyperglycemia. Next, we examined the streptozotocin-induced type1 diabetes mellitus mouse model. GAD and insulin expression levels were markedly decreased. KCC2 was expressed in glucagon-positive cells, whereas insulin- and somatostatin-positive cells were KCC2-negative. These findings suggest that in diabetes model, reduced GABA release may cause disinhibition of glucagon release, resulting in increased blood sugar levels and the maintenance of hyperglycemic state.

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

Targeting the Pancreatic α-Cell to Prevent Hypoglycemia in Type 1 Diabetes

Life-threatening hypoglycemia is a limiting factor in the management of type 1 diabetes. People with diabetes are prone to develop hypoglycemia because they lose physiological mechanisms that prevent plasma glucose levels from falling. Among these so-called counterregulatory responses, secretion of glucagon from pancreatic α-cells is preeminent. Glucagon, a hormone secreted in response to a lowering in glucose concentration, counteracts a further drop in glycemia by promoting gluconeogenesis and glycogenolysis in target tissues. In diabetes, however, α-cells do not respond appropriately to changes in glycemia and, thus, cannot mount a counterregulatory response. If the α-cell could be targeted therapeutically to restore its ability to prevent hypoglycemia, type 1 diabetes could be managed more efficiently and safely. Unfortunately, the mechanisms that allow the α-cell to respond to hypoglycemia have not been fully elucidated. We know even less about the pathophysiological mechanisms that cause α-cell dysfunction in diabetes. Based on published findings and unpublished observations, and taking into account its electrophysiological properties, we propose here a model of α-cell function that could explain its impairment in diabetes. Within this frame, we emphasize those elements that could be targeted pharmacologically with repurposed U.S. Food and Drug Administration-approved drugs to rescue α-cell function and restore glucose counterregulation in people with diabetes.

Pathways of Glucagon Secretion and Trafficking in the Pancreatic Alpha Cell: Novel Pathways, Proteins, and Targets for Hyperglucagonemia

Patients with diabetes mellitus exhibit hyperglucagonemia, or excess glucagon secretion, which may be the underlying cause of the hyperglycemia of diabetes. Defective alpha cell secretory responses to glucose and paracrine effectors in both Type 1 and Type 2 diabetes may drive the development of hyperglucagonemia. Therefore, uncovering the mechanisms that regulate glucagon secretion from the pancreatic alpha cell is critical for developing improved treatments for diabetes. In this review, we focus on aspects of alpha cell biology for possible mechanisms for alpha cell dysfunction in diabetes: proglucagon processing, intrinsic and paracrine control of glucagon secretion, secretory granule dynamics, and alterations in intracellular trafficking. We explore possible clues gleaned from these studies in how inhibition of glucagon secretion can be targeted as a treatment for diabetes mellitus.

The Role of Pancreatic Alpha Cells and Endothelial Cells in the Reduction of Oxidative Stress in Pseudoislets

Pancreatic beta cells have inadequate levels of antioxidant enzymes, and the damage induced by oxidative stress poses a challenge for their use in a therapy for patients with type 1 diabetes. It is known that the interaction of the pancreatic endocrine cells with support cells can improve their survival and lead to less vulnerability to oxidative stress. Here we investigated alpha (alpha TC-1), beta (INS1E) and endothelial (HUVEC) cells assembled into aggregates known as pseudoislets as a model of the pancreatic islets of Langerhans. We hypothesised that the coculture of alpha, beta and endothelial cells would be protective against oxidative stress. First, we showed that adding endothelial cells decreased the percentage of oxidative stress-positive cells. We then asked if the number of endothelial cells or the size (number of cells) of the pseudoislet could increase the protection against oxidative stress. However, no additional benefit was observed by those changes. On the other hand, we identified a potential supportive effect of the alpha cells in reducing oxidative stress in beta and endothelial cells. We were able to link this to the incretin glucagon-like peptide-1 (GLP-1) by showing that the absence of alpha cells in the pseudoislet caused increased oxidative stress, but the addition of GLP-1 could restore this. Together, these results provide important insights into the roles of alpha and endothelial cells in protecting against oxidative stress.

Insulin resistance and the role of gamma-aminobutyric acid

Insulin resistance (IR) is mentioned to be a disorder in insulin ability in insulin-target tissues. Skeletal muscle (SkM) and liver function are more affected by IR than other insulin target cells. SkM is the main site for the consumption of ingested glucose. An effective treatment for IR has two properties: An inhibition of β-cell death and a promotion of β-cell replication. Gamma-aminobutyric acid (GABA) can improve beta-cell mass and function. Multiple studies have shown that GABA decreases IR probably via increase in glucose transporter 4 (GLUT4) gene expression and prevention of gluconeogenesis pathway in the liver. This review focused on the general aspects of IR in skeletal muscle (SkM), liver; the cellular mechanism(s) lead to the development of IR in these organs, and the role of GABA to reduce insulin resistance.

The Role of the α Cell in the Pathogenesis of Diabetes: A World beyond the Mirror

Type 2 Diabetes Mellitus (T2DM) is one of the most prevalent chronic metabolic disorders, and insulin has been placed at the epicentre of its pathophysiological basis. However, the involvement of impaired alpha (α) cell function has been recognized as playing an essential role in several diseases, since hyperglucagonemia has been evidenced in both Type 1 and T2DM. This phenomenon has been attributed to intra-islet defects, like modifications in pancreatic α cell mass or dysfunction in glucagon's secretion. Emerging evidence has shown that chronic hyperglycaemia provokes changes in the Langerhans' islets cytoarchitecture, including α cell hyperplasia, pancreatic beta (β) cell dedifferentiation into glucagon-positive producing cells, and loss of paracrine and endocrine regulation due to β cell mass loss. Other abnormalities like α cell insulin resistance, sensor machinery dysfunction, or paradoxical ATP-sensitive potassium channels (KATP) opening have also been linked to glucagon hypersecretion. Recent clinical trials in phases 1 or 2 have shown new molecules with glucagon-antagonist properties with considerable effectiveness and acceptable safety profiles. Glucagon-like peptide-1 (GLP-1) agonists and Dipeptidyl Peptidase-4 inhibitors (DPP-4 inhibitors) have been shown to decrease glucagon secretion in T2DM, and their possible therapeutic role in T1DM means they are attractive as an insulin-adjuvant therapy.

GABAA receptors: structure, function, pharmacology, and related disorders

Background

γ-Aminobutyric acid sub-type A receptors (GABA_ARs) are the most prominent inhibitory neurotransmitter receptors in the CNS. They are a family of ligand-gated ion channel with significant physiological and therapeutic implications.

Main body

GABA_ARs are heteropentamers formed from a selection of 19 subunits: six α (alpha1-6), three β (beta1-3), three γ (gamma1-3), three ρ (rho1-3), and one each of the δ (delta), ε (epsilon), π (pi), and θ (theta) which result in the production of a considerable number of receptor isoforms. Each isoform exhibits distinct pharmacological and physiological properties. However, the majority of GABA_ARs are composed of two α subunits, two β subunits, and one γ subunit arranged as γ2β2α1β2α1 counterclockwise around the center. The mature receptor has a central chloride ion channel gated by GABA neurotransmitter and modulated by a variety of different drugs. Changes in GABA synthesis or release may have a significant effect on normal brain function. Furthermore, The molecular interactions and pharmacological effects caused by drugs are extremely complex. This is due to the structural heterogeneity of the receptors, and the existence of multiple allosteric binding sites as well as a wide range of ligands that can bind to them. Notably, dysfunction of the GABAergic system contributes to the development of several diseases. Therefore, understanding the relationship between GABA_A receptor deficits and CNS disorders thus has a significant impact on the discovery of disease pathogenesis and drug development.

Conclusion

To date, few reviews have discussed GABA_A receptors in detail. Accordingly, this review aims to summarize the current understanding of the structural, physiological, and pharmacological properties of GABA_ARs, as well as shedding light on the most common associated disorders.

Integrated Metabolomics and Proteomics Analyses in the Local Milieu of Islet Allografts in Rejection versus Tolerance

An understanding of the immune mechanisms that lead to rejection versus tolerance of allogeneic pancreatic islet grafts is of paramount importance, as it facilitates the development of innovative methods to improve the transplant outcome. Here, we used our established intraocular islet transplant model to gain novel insight into changes in the local metabolome and proteome within the islet allograft’s immediate microenvironment in association with immune-mediated rejection or tolerance. We performed integrated metabolomics and proteomics analyses in aqueous humor samples representative of the graft’s microenvironment under each transplant outcome. The results showed that several free amino acids, small primary amines, and soluble proteins related to the Warburg effect were upregulated or downregulated in association with either outcome. In general, the observed shifts in the local metabolite and protein profiles in association with rejection were consistent with established pro-inflammatory metabolic pathways and those observed in association with tolerance were immune regulatory. Taken together, the current findings further support the potential of metabolic reprogramming of immune cells towards immune regulation through targeted pharmacological and dietary interventions against specific metabolic pathways that promote the Warburg effect to prevent the rejection of transplanted islets and promote their immune tolerance.

Regulation of α-cell glucagon secretion: The role of second messengers

Glucagon is a potent glucose‐elevating hormone that is secreted by pancreatic α‐cells. While well‐controlled glucagon secretion plays an important role in maintaining systemic glucose homeostasis and preventing hypoglycaemia, it is increasingly apparent that defects in the regulation of glucagon secretion contribute to impaired counter‐regulation and hyperglycaemia in diabetes. It has therefore been proposed that pharmacological interventions targeting glucagon secretion/signalling can have great potential in improving glycaemic control of patients with diabetes. However, despite decades of research, a consensus on the precise mechanisms of glucose regulation of glucagon secretion is yet to be reached. Second messengers are a group of small intracellular molecules that relay extracellular signals to the intracellular signalling cascade, modulating cellular functions. There is a growing body of evidence that second messengers, such as cAMP and Ca²⁺, play critical roles in α‐cell glucose‐sensing and glucagon secretion. In this review, we discuss the impact of second messengers on α‐cell electrical activity, intracellular Ca²⁺ dynamics and cell exocytosis. We highlight the possibility that the interaction between different second messengers may play a key role in the glucose‐regulation of glucagon secretion.

Pancreatic β Cells Inhibit Glucagon Secretion from α Cells: An In Vitro Demonstration of α-β Cell Interaction

Interactions between endocrine α and β cells are critical to their secretory function in vivo. The interactions are highly regulated, although yet to be fully understood. In this study, we aim to assess the impact of α and β cell co-culture on hormone secretion. Mouse clonal cell lines α-TC6-1 (α cell line) and MIN-6 (β cell line) were cultured independently or in combination in a medium containing 5.5, 11.1, or 25 mM glucose, respectively. After 72 h, hormone release was measured using insulin and glucagon secretion assays, the cell distribution was visualized by inverted microscopy and an immunocytochemistry assay, and changes in gene expressions were assessed using the RT-PCR technique. The co-culture of the two cell lines caused a decrease in glucagon secretion from α-TC1-6 cells, while no effect on insulin secretion from MIN-6 cells was revealed. Both types of cells were randomly scattered throughout the culture flask, unlike in mice islets in vivo where β cells cluster in the core and α cells are localized at the periphery. During the α-β cell co-culture, the gene expression of glucagon (Gcg) decreased significantly. We conclude that islet β cells suppress glucagon secretion from α cells, apparently via direct cell-to-cell contact, of which the molecular mechanism needs further verification.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca2+ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Effect of Allopregnanolone on Spatial Memory and Synaptic Proteins in Animal Model of Metabolic Syndrome

Metabolic Syndrome (MetS) is considered a common disorder, especially with a sedentary lifestyle and unhealthy food consumption. Cognitive impairment is one of the MetS consequences that worsens the quality of life of the patients. The study aimed to assess the therapeutic effect of the neurosteroid Allopregnalonone on spatial memory and, therefore, the expression of two synaptic plasticity markers in the hippocampus. Thirty-two male rats were divided into four groups: control groups, MetS, and MetS + Allopregnalone. Spatial memory has been evaluated by the Y-maze task and blood pressure measured by the rat tail method. Biochemical evaluation of serum glucose, insulin, lipid profile, and hippocampal expression of Synaptophysin and Associated Protein 43 (GAP-43) were performed for assessing Allopregnanolone on serum and hippocampal markers. Allopregnanolone therapy improved working spatial memory, hypertension, and biochemical markers measured in the serum and hippocampus.

The Aging GABAergic System and Its Nutritional Support

Aging is associated with a decline in hormones and an associated decline in GABAergic function and calcium and ion current dysregulation. Neurosteroid hormones act as direct calcium channel blockers, or they can act indirectly on calcium channels through their interaction with GABA receptors. The calcium channel dysfunction associated with hormone loss further leads to an excitatory cell state, which can ultimately lead to cell death. The calcium theory of aging posits that cellular mechanisms, which maintain the homeostasis of cytosol Ca2+ concentration, play a key role in brain aging and that sustained changes in Ca2+ homeostasis provide the final common pathway for age-associated brain changes. There is a link between hormone loss and calcium dysregulation. Loss of calcium regulation associated with aging can lead to an excitatory cell state, primarily in the mitochondria and nerve cells, which can ultimately lead to cell death if not kept in check. A decline in GABAergic function can also be specifically tied to declines in progesterone, allopregnanolone, and DHEA levels associated with aging. This decline in GABAergic function associated with hormone loss ultimately affects GABAergic inhibition or excitement and calcium regulation throughout the body. In addition, declines in GABAergic function can also be tied to vitamin status and to toxic chemicals in the food supply. The decline in GABAergic function associated with aging has an effect on just about every body organ system. Nutritional support of the GABAergic system with supportive foods, vitamins, and GABA or similar GABA receptor ligands may address some of the GABAergic dysfunction associated with aging.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Functional maturation of immature β cells: A roadblock for stem cell therapy for type 1 diabetes

Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease caused by the specific destruction of pancreatic islet β cells and is characterized as the absolute insufficiency of insulin secretion. Current insulin replacement therapy supplies insulin in a non-physiological way and is associated with devastating complications. Experimental islet transplantation therapy has been proven to restore glucose homeostasis in people with severe T1DM. However, it is restricted by many factors such as severe shortage of donor sources, progressive loss of donor cells, high cost, etc. As pluripotent stem cells have the potential to give rise to all cells including islet β cells in the body, stem cell therapy for diabetes has attracted great attention in the academic community and the general public. Transplantation of islet β-like cells differentiated from human pluripotent stem cells (hPSCs) has the potential to be an excellent alternative to islet transplantation. In stem cell therapy, obtaining β cells with complete insulin secretion in vitro is crucial. However, after much research, it has been found that the β-like cells obtained by in vitro differentiation still have many defects, including lack of adult-type glucose stimulated insulin secretion, and multi-hormonal secretion, suggesting that in vitro culture does not allows for obtaining fully mature β-like cells for transplantation. A large number of studies have found that many transcription factors play important roles in the process of transforming immature to mature human islet β cells. Furthermore, PDX1, NKX6.1, SOX9, NGN3, PAX4, etc., are important in inducing hPSC differentiation in vitro. The absent or deficient expression of any of these key factors may lead to the islet development defect in vivo and the failure of stem cells to differentiate into genuine functional β-like cells in vitro. This article reviews β cell maturation in vivo and in vitro and the vital roles of key molecules in this process, in order to explore the current problems in stem cell therapy for diabetes.

Mechanisms for establishment of GABA signaling in adrenal medullary chromaffin cells

γ-Aminobutyric acid (GABA) is thought to play a paracrine role in adrenal medullary chromaffin (AMC) cells. Comparative physiological and immunocytochemical approaches were used to address the issue of how the paracrine function of GABA in AMC cells is established. GABA_A receptor Cl^- channel activities in AMC cells of rats and mice, where corticosterone is the major glucocorticoid, were much smaller than those in AMC cells of guinea-pigs and cattle, where cortisol is the major. The extent of enhancement of GABA_A receptor α3 subunit expression in rat pheochromocytoma (PC12) cells by cortisol was larger than that by corticosterone in parallel with their glucocorticoid activities. Thus, the species difference in GABA_A receptor expression may be ascribed to a difference in glucocorticoid activity between corticosterone and cortisol. GABA_A receptor Cl^- channel activity in mouse AMC cells was enhanced by allopregnanolone, as noted with that in guinea-pig AMC cells, and the enzymes involved in allopregnanolone production were immunohistochemically detected in the zona fasciculata in both mice and guinea pigs. The expression of glutamic acid decarboxylase 67 (GAD67), one of the GABA synthesizing enzymes, increased after birth, whereas GABA_A receptors already developed at birth. Stimulation of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors, but not nicotinic or muscarinic receptors, in PC12 cells, resulted in an increase in GAD67 expression in a protein-kinase A-dependent manner. The results indicate that glucocorticoid and PACAP are mainly responsible for the expressions of GABA_A receptors and GAD67 involved in GABA signaling in AMC cells, respectively.

The Potential Role of Pancreatic γ-Aminobutyric Acid (GABA) in Diabetes Mellitus: A Critical Reappraisal

Background

Diabetes mellitus (DM) is an endocrine disorder characterized by hyperglycemia, polyuria, polydipsia, and glucosuria. γ-aminobutyric acid (GABA) is an inhibitory neurotransmitter in the central nervous system (CNS) of humans and other mammals. GABA acts on two different receptors, which are GABA-_A and GABA-_B. Pancreatic β-cells synthesize GABA from glutamic acid by glutamic acid decarboxylase (GAD).

Aim

The objective of this study was to explore the potential role of pancreatic GABA on glycemic indices in DM.

Methods

Evidence from experimental, preclinical, and clinical studies are evaluated for bidirectional relationships between pancreatic GABA and blood glucose disorders. A multiplicity of search strategies took on and assumed included electronic database searches of Medline and Pubmed using MeSH terms, keywords and title words during the search.

Results

The pancreatic GABA signaling system has a role in the regulation of pancreatic hormone secretions, inhibition of immune response, improve β-cells survival, and change α cell into β-cell. Moreover, a GABA agonist improves the antidiabetic effects of metformin. In addition, benzodiazepine receptor agonists improve pancreatic β-cell functions through GABA dependent pathway or through modulation of pancreatic adenosine and glucagon-like peptide (GLP-1).

Conclusions

Pancreatic GABA improves islet cell function, glucose homeostasis, and autoimmunity in DM. Orally administered GABA is safe for humans, and acts on peripheral GABA receptors and represents a new therapeutic modality for both T1DM and T2DM. Besides, GABA-_A receptor agonist like benzodiazepines improves pancreatic β-cell function and insulin sensitivity through activation of GABA-_A receptors.

GLP-1 preserves β cell function via improvement on islet insulin signaling in high fat diet feeding mice

BACKGROUND

Numerous studies have shown that Glucagon like peptide-1 (GLP-1) treatment can protect β cell function, but the exact mechanism remains unclear. We hypothesized that GLP-1 may protect β cell function via its action on insulin signaling pathway.

METHODS

Mice were fed with high fat diet (HFD, 20 weeks) in the presence or absence of GLP-1 receptor agonist (exenatide) treatment. The islet structure was demonstrated by HE staining. Immunofluorescence antibodies targeting insulin and glucagon were used to illustrate α and β cell distribution. The insulin and glucagon abundance was measured by ELISA using pancreatic homogenates. The molecules involved in insulin signaling pathway (IRc, IRS1, IRS2, mTOR) in islet were examined with immunohistochemistry and immunoblotting. The effect of IRS1 silencing on mTOR and apoptosis were examined on NIT cells(β cell line)with immunoblotting and flow cytometry.

RESULTS

HE and immunofluorescence staining demonstrated that the normal structure of islet was deformed in HFD mice but preserved by exenatide. Insulin and glucagon contents were increased in islet and blood stream of HFD mice (HFD vs. Control, p<0.05) but resumed by exenatide. Meanwhile the expressions of IRc, IRS-1, mTOR from insulin signaling pathway and β cell apoptosis in the pancreas were significantly reduced (p<0.05) by HFD but reversed by exenatide.

CONCLUSION

Exenatide improved insulin signaling pathway that was suppressed by HFD in mice islet. Our results reveal a novel mechanism of the protective effects of GLP-1 on β cell function.

Impact of glucagon response on early postprandial glucose excursions irrespective of residual β-cell function in type 1 diabetes: A cross-sectional study using a mixed meal tolerance test

Aims/Introduction

Controlling postprandial glucose levels in patients with type 1 diabetes is challenging even under the adequate treatment of insulin injection. Recent studies showed that dysregulated glucagon secretion exacerbates hyperglycemia in type 2 diabetes patients, but little is known in type 1 diabetes patients. We investigated whether the glucagon response to a meal ingestion could influence the postprandial glucose excursion in patients with type 1 diabetes.

Materials and Methods

We enrolled 34 patients with type 1 diabetes and 23 patients with type 2 diabetes as controls. All patients underwent a liquid mixed meal tolerance test. We measured levels of plasma glucose, C‐peptide and glucagon at fasting (0 min), and 30, 60 and 120 min after meal ingestion. All type 1 diabetes patients received their usual basal insulin and two‐thirds of the necessary dose of the premeal bolus insulin.

Results

The levels of plasma glucagon were elevated and peaked 30 min after the mixed meal ingestion in both type 1 diabetes and type 2 diabetes patients. The glucagon increments from fasting to each time point (30, 60 and 120 min) in type 1 diabetes patients were comparable to those in type 2 diabetes patients. Among the type 1 diabetes patients, the glucagon response showed no differences between the subgroups based on diabetes duration (<5 vs ≥5 years) and fasting C‐peptide levels (<0.10 vs ≥0.10 nmol/L). The changes in plasma glucose from fasting to 30 min were positively correlated with those in glucagon, but not C‐peptide, irrespective of diabetes duration and fasting C‐peptide levels in patients with type 1 diabetes.

Conclusions

The dysregulated glucagon likely contributes to postprandial hyperglycemia independent of the residual β‐cell functions during the progression of type 1 diabetes.

γ-aminobutyric acid stimulates β-cell proliferation through the mTORC1/p70S6K pathway, an effect amplified by Ly49, a novel γ-aminobutyric acid type A receptor positive allosteric modulator

AIM

To examine the mechanism of action of γ-aminobutyric acid (GABA) on β-cell proliferation and investigate if co-treatment with Ly49, a novel GABA type A receptor positive allosteric modulator (GABA_A -R PAM), amplifies this effect.

METHODS

Human or mouse islets were co-treated for 4-5 days with GABA and selected receptor or cell signalling pathway modulators. Immunofluorescence was used to determine protein co-localization, cell number or proliferation, and islet size. Osmotic minipumps were surgically implanted in mice to assess Ly49 effects on pancreatic β-cells.

RESULTS

Amplification of GABA_A -R signalling enhanced GABA-stimulated β-cell proliferation in cultured mouse islets. Co-treatment of GABA with an inhibitor specific for PI3K, mTORC1/2, or p70S6K, abolished GABA-stimulated β-cell proliferation in mouse and human islets. Nuclear p-Akt^Ser473 and p-p70S6K^{Thr421/Ser424} expression in pancreatic β-cells was increased in GABA-treated mice compared with vehicle-treated mice, an effect augmented with GABA and Ly49 co-treatment. Mice co-treated with GABA and Ly49 exhibited enhanced β-cell area and proliferation compared with GABA-treated mice. Furthermore, S961 injection (an insulin receptor antagonist) resulted in enhanced plasma insulin in GABA and Ly49 co-treated mice compared with GABA-treated mice. Importantly, GABA co-treated with Ly49 increased β-cell proliferation in human islets providing a potential application for human subjects.

CONCLUSIONS

We show that GABA stimulates β-cell proliferation via the PI3K/mTORC1/p70S6K pathway in both mouse and human islets. Furthermore, we show that Ly49 enhances the β-cell regenerative effects of GABA, showing potential in the intervention of diabetes.