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

Type 1 Diabetes: A Guide to Autoimmune Mechanisms for Clinicians

Type 1 diabetes (T1D) results from the destruction of pancreatic beta cells by autoreactive T lymphocytes, leading to insulin deficiency and lifelong insulin dependence. It develops in genetically predisposed individuals, triggered by environmental or immunological factors. Although the exact causes of T1D remain unknown, the autoimmune pathogenesis of the disease is clearly indicated by the genetic risk conferred by allelic human leukocyte antigens (HLA), the almost obligatory presence of islet cell autoantibodies (AAbs) and immune cell infiltration of pancreatic islets from patients. At the same time, epidemiological data point to a role of environmental factors, notably enteroviral infections, in the disease, although precise causative links between specific pathogens and T1D have been difficult to establish. Studies of human pancreas organs from patients made available through repositories and the advent of high-dimensional high-throughput technologies for genomic and proteomic studies have significantly elucidated our understanding of the disease in recent years and provided mechanistic insights that can be exploited for innovative targeted therapeutic approaches. This short overview will summarise current salient knowledge on immune cell and beta cell dysfunction in T1D pathogenesis. PLAIN LANGUAGE SUMMARY: Type 1 diabetes (T1D) is a chronic disease where the body's own immune system attacks and destroys the insulin-producing beta cells in the pancreas. This leads to a lack of insulin, a hormone essential for regulating blood sugar, which means people with T1D need insulin for life. The disease can develop at any age but is most diagnosed in children and young adults. Despite advances in treatment, T1D still significantly reduces life expectancy, especially in countries with fewer healthcare resources. T1D develops in people with a genetic predisposition, often triggered by environmental factors such as viral infections or changes in the gut microbiome. The disease progresses silently through three stages: Stage 1: Autoantibodies to beta cell components appear, signalling the immune system is reacting against the pancreas, but there are no symptoms; Stage 2: Beta cell function starts to decline, but fasting blood sugar is still normal; Stage 3: Enough beta cells are destroyed that fasting blood sugar rises, and symptoms of diabetes appear. The risk of progressing from stage 1 to full-blown diabetes is about 35-50% within five years, and even higher from stage 2. Over 60 genes are linked to T1D risk, most of which affect how the immune system works. The strongest genetic risk comes from specific versions of histocompatibility genes, which help the immune system distinguish between the body's own cells and invaders. Some types of these genes make it easier for the immune system to mistakenly attack beta cells. However, 90% of people diagnosed with T1D have no family member with T1D, showing that genetics is only part of the story. Environmental factors also play a big role. For example, certain viral infections, especially with viruses infecting the intestine, are associated with a higher risk of developing T1D. The gut microbiome - the community of bacteria living in our intestines - also influences risk, with healthier, more diverse microbiomes appearing to offer some protection. In T1D, immune cells - especially so-called T lymphocytes - mistake beta cells in the pancreas for threats and destroy them. This process is called autoimmunity. The attack is often reflected by the presence of autoantibodies against proteins found in beta cells. Over time, as more beta cells are lost, the body can no longer produce enough insulin, leading to the symptoms of diabetes. Interestingly, not all people with T1D have the same pattern of disease. For example, children diagnosed before age 7 often have more aggressive disease, more autoantibodies, and stronger genetic risk factors than those diagnosed later. Much of our understanding of T1D has come from studying animal models, but new technologies now allow researchers to study human pancreas tissue and blood immune cells in greater detail. Scientists are also exploring how the gut microbiome, diet, and environmental exposures contribute to T1D risk and progression. Treatment currently focuses on replacing insulin, but researchers are working on therapies that target the immune system or aim to protect or replace beta cells. Strategies include immunotherapy, gene therapy, and even modifying the gut microbiome. The goal is to prevent or reverse the disease, not just manage its symptoms. In summary, T1D is a complex autoimmune disease influenced by both genes and the environment. It progresses silently before symptoms appear, and while insulin therapy is life-saving, new research is paving the way for treatments that could one day halt or even prevent the disease.

Long-term gamma-aminobutyric acid (GABA) treatment fails to regain beta-cell function in longstanding type 1 diabetes in a randomized trial

Gamma-amino butyric acid (GABA) has in experimental studies been found to promote beta-cell proliferation, enhance insulin secretion and reduce inflammation, positioning it as a candidate drug for type 1 diabetes (T1D) therapy. This phase I/II randomized controlled trial assessed the safety and efficacy of long-term treatment with Remygen® (Diamyd Medical), a controlled-release oral GABA formulation, as a potential beta-cell regenerative therapy in adults with long-standing T1D. Thirty-five male subjects with T1D (≥ 5 years) were randomized into three arms receiving the study drug(s) once daily for 6 months: GABA 200 mg (Arm 1), GABA 600 mg (Arm 2) and GABA 600 mg + alprazolam 0.5 mg for 3 months followed by GABA 600 mg alone for 3 months (Arm 3). Safety measures, hormonal counter-regulation during hypoglycemic clamps, fasting- and stimulated C-peptide levels, were assessed at multiple timepoints. Safety concerns included elevated aspartate aminotransferase (AST) in nine subjects, leading to the withdrawal of two subjects. Most elevations were, however, transient with no dose-differences. No effects were observed on fasting- or stimulated C-peptide levels, CGM metrics or HbA1c. Hypoglycemic hormonal counter-regulation was unaltered. To conclude, we found no clinical evidence of a beta-cell regenerative effect of GABA, but side effects were commonly observed.

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.

Active targeting of type 1 diabetes therapies to pancreatic beta cells using nanocarriers

Type 1 diabetes is an autoimmune disease characterised by the destruction of pancreatic beta cells, resulting in lifelong insulin dependence. Although exogenous insulin can maintain glycaemic control, this approach does not protect residual or replacement pancreatic beta cells from immune-mediated death. Current therapeutics designed to protect functional beta cell mass or promote beta cell proliferation and regeneration can have off-target effects, resulting in higher dose requirements and adverse side effects. Targeted drug delivery using nanocarriers has demonstrated potential for overcoming these limitations. The critical bottleneck limiting the development of beta cell-targeted therapies is a lack of highly specific beta cell markers. This review provides an overview of the use of nanocarriers for cell-targeted delivery and the current state of the field of beta cell targeting. Technologies such as systematic evolution of ligands by exponential enrichment (SELEX) aptamer selection, phage display screening, and omics datasets from human samples are highlighted as tools to identify novel beta cell-specific targets that can be combined with nanocarriers for targeted delivery of therapeutics. Ultimately, beta cell-targeted therapies using nanocarriers present a unique opportunity to develop tailored treatments for each stage of type 1 diabetes with the goal of providing individuals with treatment options that prevent further progression or reverse the course of the disease.

Role of γ-Aminobutyric Acid (GABA) as an Inhibitory Neurotransmitter in Diabetes Management: Mechanisms and Therapeutic Implications

GABA (γ-Aminobutyric Acid), a well-established inhibitory neurotransmitter in the central nervous system, has garnered considerable interest for its potential role in diabetes management, particularly due to its presence in pancreatic islets. This review aims to explore the therapeutic role of GABA in diabetes management and its potential mechanisms for antidiabetic effects. Relevant studies were searched across databases such as PubMed and ScienceDirect, applying strict eligibility criteria focused on GABA administration methods and diabetic models. The collective results showed that the administration of GABA in diabetic models resulted in remarkable enhancements in glucose and insulin homeostasis, favorable modifications in lipid profiles, and amelioration of dysfunctions across neural, hepatic, renal, and cardiac systems. The findings from the literature demonstrated that GABAergic signaling within pancreatic tissues can significantly contribute to the stimulation of β cell proliferation through the facilitation of a sustained trans-differentiation process, wherein glucagon-secreting α cells are converted into insulin-secreting β-like cells. In addition, activated GABAergic signaling can trigger the initiation of the PI3K/AKT signaling pathway within pancreatic tissues, leading to improved insulin signaling and maintained glucose homeostasis. GABAergic signaling can further function within hepatic tissues, promoting inhibitory effects on the expression of genes related to gluconeogenesis and lipogenesis. Moreover, GABA may enhance gut microbiota diversity by attenuating gut inflammation, attributable to its anti-inflammatory and immunomodulatory properties. Furthermore, the neuroprotective effects of GABA play a significant role in ameliorating neural disorders associated with diabetes by facilitating a substantial reduction in neuronal apoptosis. In conclusion, GABA emerges as a promising candidate for an antidiabetic agent; however, further research is highly encouraged to develop a rigorously designed framework that comprehensively identifies and optimizes the appropriate dosages and intervention methods for effectively managing and combating diabetes.

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.

Peripheral GABAA receptors - Physiological relevance and therapeutic implications

The role of γ- aminobutyric acid (GABA) and GABAA receptors is not only essential for neurotransmission in the central nervous system (CNS), but they are also involved in communication in various peripheral tissues such as the pancreas, liver, kidney, gastrointestinal tract, trachea, immune cells and blood vessels. GABAA receptors located outside the CNS ("peripheral GABAA receptors") enable both neuronal and non-neuronal GABA-ergic signaling in various physiological processes and are generally thought to have similar properties to the extrasynaptic receptors in the CNS. By activating these peripheral receptors, GABA and various GABAA receptor modulators, including drugs such as benzodiazepines and general anesthetics, may contribute to or otherwise affect the maintenance of general body homeostasis. However, the existing data in the literature on the role of non-neuronal GABA-ergic signaling in insulin secretion, glucose metabolism, renal function, intestinal motility, airway tone, immune response and blood pressure regulation are far from complete. In fact, they mainly focus on the identification of components for the local synthesis and utilization of GABA and on the expression repertoire of GABAA receptor subunits rather than on subunit composition, activation effects and (sub)cellular localization. A deeper understanding of how modulation of peripheral GABAA receptors can have significant therapeutic effects on a range of pathological conditions such as multiple sclerosis, diabetes, irritable bowel syndrome, asthma or hypertension could contribute to the development of more specific pharmacological strategies that would provide an alternative or complement to existing therapies. Selective GABAA receptor modulators with improved peripheral efficacy and reduced central side effects would therefore be highly desirable first-in-class drug candidates. This review updates recent advances unraveling the molecular components and cellular determinants of the GABA signaling machinery in peripheral organs, tissues and cells of both, humans and experimental animals.

Redosing with Intralymphatic GAD-Alum in the Treatment of Type 1 Diabetes: The DIAGNODE-B Pilot Trial

Immunotherapies aimed at preserving residual beta cell function in type 1 diabetes have been successful, although the effect has been limited, or raised safety concerns. Transient effects often observed may necessitate redosing to prolong the effect, although this is not always feasible or safe. Treatment with intralymphatic GAD-alum has been shown to be tolerable and safe in persons with type 1 diabetes and has shown significant efficacy to preserve C-peptide with associated clinical benefit in individuals with the human leukocyte antigen DR3DQ2 haplotype. To further explore the feasibility and advantages of redosing with intralymphatic GAD-alum, six participants who had previously received active treatment with intralymphatic GAD-alum and carried HLA DR3-DQ2 received one additional intralymphatic dose of 4 μg GAD-alum in the pilot trial DIAGNODE-B. The participants also received 2000 U/day vitamin D (Calciferol) supplementation for two months, starting one month prior to the GAD-alum injection. During the 12-month follow-up, residual beta cell function was estimated with Mixed-Meal Tolerance Tests, and clinical and immune responses were observed. C-peptide decreased minimally, and most patients showed stable HbA1c and IDAA1c. The mean % TIR increased while the mean daily insulin dose decreased at month 12 compared to the baseline. Redosing with GAD-alum seems to be safe and tolerable, and may prolong the disease modification elicited by the original GAD-alum treatment.

Immunotherapy for Type 1 Diabetes: Mechanistic Insights and Impact of Delivery Systems

Type 1 diabetes is an autoimmune disease characterized by the destruction of insulin-producing pancreatic β-cells, leading to hyperglycemia and various complications. Despite insulin replacement therapy, there is a need for therapies targeting the underlying autoimmune response. This review aims to explore the mechanistic insights into T1D pathogenesis and the impact of delivery systems on immunotherapy. Genetic predisposition and environmental factors contribute to T1D development, triggering an immune-mediated attack on β-cells. T cells, particularly CD4+ and CD8+ T cells, play a central role in β-cell destruction. Antigen- specific immunotherapy is a unique way to modify the immune system by targeting specific antigens (substances that trigger the immune system) for immunotherapy. It aims to restore immune tolerance by targeting autoantigens associated with T1D. Nanoparticle-based delivery systems offer precise antigen delivery, promoting immune tolerance induction. Various studies have demonstrated the efficacy of nanoparticle-mediated delivery of autoantigens and immunomodulatory agents in preclinical models, and several patents have been made in T1D. Combining antigen-specific immunotherapy with β-cell regeneration strategies presents a promising approach for T1D treatment. However, challenges remain in optimizing delivery systems for targeted immune modulation while ensuring safety and efficacy.

Decoding the Significance of Alpha Cell Function in the Pathophysiology of Type 1 Diabetes

Alpha cells in the pancreas, traditionally known for their role in secreting glucagon to regulate blood glucose levels, are gaining recognition for their involvement in the pathophysiology of type 1 diabetes (T1D). In T1D, autoimmune destruction of beta cells results in insulin deficiency, which in turn may dysregulate alpha cell function, leading to elevated glucagon levels and impaired glucose homeostasis. This dysfunction is characterized by inappropriate glucagon secretion, augmenting the risk of life-threatening hypoglycemia. Moreover, insulin deficiency and autoimmunity alter alpha cell physiological responses, further exacerbating T1D pathophysiology. Recent studies suggest that alpha cells undergo transdifferentiation and interact with beta cells through mechanisms involving gamma-aminobutyric acid (GABA) signaling. Despite these advances, the exact pathways and interactions remain poorly understood and are often debated. Understanding the precise role of alpha cells in T1D is crucial, as it opens up avenues for developing new therapeutic strategies for T1D. Potential strategies include targeting alpha cells to normalize glucagon secretion, utilizing glucagon receptor antagonists, enhancing GABA signaling, and employing glucagon-like peptide-1 (GLP-1) receptor agonists. These approaches aim to improve glycemic control and reduce the risk of hypoglycemic events in individuals with T1D. This review provides an overview of alpha cell function in T1D, highlighting the emerging focus on alpha cell dysfunction in the context of historically well-developed beta cell research.

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.

Shifting the paradigm of type 1 diabetes: a narrative review of disease modifying therapies

A new diagnosis of type 1 diabetes (T1D) may be accompanied by numerous lifelong financial, emotional, and physical challenges, thus advancements in therapies that can delay the onset of clinical disease are crucial. T1D is an autoimmune condition involving destruction of pancreatic beta cells leading to insulin deficiency, hyperglycemia, and long-term insulin dependence. The pathogenesis of T1D is classified into stages, with the first signal being the detection of autoantibodies without any glycemic changes. In the second stage, dysglycemia develops without symptoms, and in stage 3, symptoms of hyperglycemia become apparent, and at this time a clinical diagnosis of T1D is made. As a greater understanding of these stages of T1D have evolved, research efforts have been devoted to delaying the onset of clinical disease. To date, only one medication, teplizumab, has been approved by the Food and Drug Administration (FDA) for the treatment of stage 2 T1D. This narrative review present published trials and ongoing research on disease modifying therapies (DMT) in T1D, the mechanisms of action for each therapy, and the stages of T1D that these interventions are being studied.

Physiological and pathogenic T cell autoreactivity converge in type 1 diabetes

Autoimmune diseases result from autoantigen-mediated activation of adaptive immunity; intriguingly, autoantigen-specific T cells are also present in healthy donors. An assessment of dynamic changes of this autoreactive repertoire in both health and disease is thus warranted. Here we investigate the physiological versus pathogenic autoreactive processes in the context of Type 1 diabetes (T1D) and one of its landmark autoantigens, glutamic acid decarboxylase 65 (GAD65). Using single cell gene expression profiling and tandem T cell receptor (TCR) sequencing, we find that GAD65-specific true naïve cells are present in both health and disease, with GAD65-specific effector and memory responses showing similar ratios in healthy donors and patients. Deeper assessment of phenotype and TCR repertoire uncover differential features in GAD65-specific TCRs, including lower clonal sizes of healthy donor-derived clonotypes in patients. We thus propose a model whereby physiological autoimmunity against GAD65 is needed during early life, and that alterations of these physiological autoimmune processes in predisposed individuals trigger overt Type 1 diabetes.

Immunotherapy-Based Strategies for Treatment of Type 1 Diabetes

BACKGROUND

Type 1 diabetes (T1D) is more than an insulin-deficiency disease - it is an autoimmune disease, and the field is moving toward adopting disease-modifying immunotherapy as part of clinical care during T1D development.

SUMMARY

Recent successful immunotherapies as well as therapies that missed the mark are reviewed. T cell-directed therapies may allow for the greatest preservation of β cell function but also come with more side effects. Anti-cytokine therapies are very promising but likely need chronic administration. Antigen-specific therapies while safe have not produced meaningful results. Most successful trials have been conducted in adolescents and adults with stage 3 T1D (clinical T1D) with preserved C-peptide (up to 60% more compared to placebo) demonstrated 1-2 years post treatment. HbA1c and total insulin dose are less likely to be significantly different between treated and placebo groups because most participants in studies are meeting glycemic targets and because of the heterogeneous nature of these measures. In the prevention space (delaying progression from stage 2 to stage 3 T1D), the outcome is more discrete, and a T cell-directed therapy, teplizumab, has received FDA approval. Even negative studies with promising mechanistic and safety profiles have added value.

KEY MESSAGES

What is clear, a single administration or short course of an immunotherapy is unlikely to provide sustained freedom from exogenous insulin.

A novel class of oral, non-immunosuppressive, beta cell-targeting, TXNIP-inhibiting T1D drugs is emerging

Diabetes treatment options have improved dramatically over the last 100 years, however, close to 2 million individuals in the U.S. alone live with type 1 diabetes (T1D) and are still dependent on multiple daily insulin injections and/or continuous insulin infusion with a pump to stay alive and no oral medications are available. After decades of focusing on immunosuppressive/immunomodulatory approaches for T1D, it has now become apparent that at least after disease onset, this by itself may not be sufficient, and in order to be effective, therapies need to also address beta cell health. This Perspective article discusses the emergence of such a beta cell-targeting, novel class of oral T1D drugs targeting thioredoxin-interacting protein (TXNIP) and some very recent advances in this field that start to address this unmet medical need. It thereby focuses on repurposing of the antihypertensive drug, verapamil found to non-specifically inhibit TXNIP and on TIX100, a new chemical entity specifically developed as an oral anti-diabetic drug to inhibit TXNIP. Both have shown striking anti-diabetic effects in preclinical studies. Verapamil has also proven to be beneficial in adults and children with recent onset T1D, while TIX100 has just been cleared by the U.S. Food and Drug Administration (FDA) to proceed to clinical trials. Taken together, we propose that such non-immunosuppressive, adjunctive therapies to insulin, alone or in combination with immune modulatory approaches, are critical in order to achieve effective and durable disease-modifying treatments for T1D.

Unraveling the Potential of γ-Aminobutyric Acid: Insights into Its Biosynthesis and Biotechnological Applications

γ-Aminobutyric acid (GABA) is a widely distributed non-protein amino acid that serves as a crucial inhibitory neurotransmitter in the brain, regulating various physiological functions. As a result of its potential benefits, GABA has gained substantial interest in the functional food and pharmaceutical industries. The enzyme responsible for GABA production is glutamic acid decarboxylase (GAD), which catalyzes the irreversible decarboxylation of glutamate. Understanding the crystal structure and catalytic mechanism of GAD is pivotal in advancing our knowledge of GABA production. This article provides an overview of GAD's sources, structure, and catalytic mechanism, and explores strategies for enhancing GABA production through fermentation optimization, metabolic engineering, and genetic engineering. Furthermore, the effects of GABA on the physiological functions of animal organisms are also discussed. To meet the increasing demand for GABA, various strategies have been investigated to enhance its production, including optimizing fermentation conditions to facilitate GAD activity. Additionally, metabolic engineering techniques have been employed to increase the availability of glutamate as a precursor for GABA biosynthesis. By fine-tuning fermentation conditions and utilizing metabolic and genetic engineering techniques, it is possible to achieve higher yields of GABA, thus opening up new avenues for its application in functional foods and pharmaceuticals. Continuous research in this field holds immense promise for harnessing the potential of GABA in addressing various health-related challenges.

Is hepatic GABA transaminase a promising target for obesity and epilepsy treatments?

γ-Aminobutyric acid (GABA) transaminase (GABA-T) is a GABA-degrading enzyme that plays an essential role in regulating GABA levels and maintaining supplies of GABA. Although GABA in the mammalian brain was discovered 70 years ago, research on GABA and GABA-T has predominantly focused on the brain. Notwithstanding the high activity and expression of GABA-T in the liver, the exact functions of GABA-T in the liver remain unknown. This article reviews the up-to-date information on GABA-T in the liver. It presents recent findings on the role of liver GABA-T in food intake suppression and appetite regulation. Finally, the potential functions of liver GABA-T in other neurological diseases, natural GABA-T inhibitors, and future perspectives in this research area are discussed.

Glucagon-producing α-cell transcriptional identity and reprogramming towards insulin production

β-Cell replacement by in situ reprogramming of non-β-cells is a promising diabetes therapy. Following the observation that near-total β-cell ablation in adult mice triggers the reprogramming of pancreatic α-, δ-, and γ-cells into insulin (INS)-producing cells, recent studies are delving deep into the mechanisms controlling adult α-cell identity. Systematic analyses of the α-cell transcriptome and epigenome have started to pinpoint features that could be crucial for maintaining α-cell identity. Using different transgenic and chemical approaches, significant advances have been made in reprogramming α-cells in vivo into INS-secreting cells in mice. The recent reprogramming of human α-cells in vitro is an important step forward that must now be complemented with a comprehensive molecular dissection of the mechanisms controlling α-cell identity.

Supplementation with high-GABA-producing Lactobacillus plantarum L5 ameliorates essential tremor triggered by decreased gut bacteria-derived GABA

BACKGROUND

The γ-aminobutyric acid (GABA) hypothesis posits a role of GABA deficiency in the central nervous system in the pathogenesis and progression of essential tremor (ET). However, the specific causative factor for GABA deficiency is not clear. The gut microbiota in mammals has recently been considered as a significant source of GABA. Furthermore, the GABA-based signals originating from the intestine can be transmitted to the brain through the "enteric nervous system-vagus nerve-brain" axis. However, the plausible contribution of gut microbiota to ET seems inspiring but remains obscure.

METHODS

Fecal samples from patients with ET and healthy controls were examined by metagenomic sequencing to compare the composition of gut microbiota and the expression of genes involved in GABA biosynthesis. The impact of gut microbiota on ET was explored through transplantation of fecal microbiota from patients with ET into the murine ET model. Lactic acid bacteria producing high amounts of GABA were identified through whole-genome sequencing and ultra-performance liquid chromatography-tandem mass spectrometry. Subsequently, mice were treated with the high-GABA-producing strain Lactobacillus plantarum L5. Tremor severity, behavioral tests, pro-inflammatory cytokines, GABA concentration, and gut microbiota composition were examined in these mice.

RESULTS

The gut microbiota of patients with ET demonstrated an impaired GABA-producing capacity and a reduced fecal GABA concentration. Transplantation of the gut microbiota from patients with ET induced an extension of tremor duration and impaired mobility in the murine model of ET. L5 exhibited an augmented GABA-producing capacity, with the De Man-Rogosa-Sharpe culture broth containing 262 mg/l of GABA. In addition, administration of L5 significantly decreased the tremor severity and enhanced the movement capability and grasping ability of ET mice. In vivo mechanistic experiments indicated that L5 reshaped the gut microbial composition, supplemented the mucosa-associated microbiota with GABA-producing capacity, increased the GABA concentrations in the cerebellum, and diminished inflammation in the central nervous system.

CONCLUSIONS

These findings highlight that deficiency of GABA-producing gut microbes plays an essential role in the pathogenesis of ET and that L5 is a promising candidate for treating ET.

Preclinical models for Type 1 Diabetes Mellitus - A practical approach for research

Numerous preclinical models have been developed to advance biomedical research in type 1 diabetes mellitus (T1DM). They are essential for improving our knowledge of T1DM development and progression, allowing researchers to identify potential therapeutic targets and evaluate the effectiveness of new medications. A deeper comprehension of these models themselves is critical not only to determine the optimal strategies for their utilization but also to fully unlock their potential applications in both basic and translational research. Here, we will comprehensively summarize and discuss the applications, advantages, and limitations of the commonly used animal models for human T1DM and also overview the up-to-date human tissue bioengineering models for the investigation of T1DM. By combining these models with a better understanding of the pathophysiology of T1DM, we can enhance our insights into disease initiation and development, ultimately leading to improved therapeutic responses and outcomes.

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.

Efficacy of combination therapy with GABA, a DPP-4i and a PPI as an adjunct to insulin therapy in patients with type 1 diabetes

Introduction

The purpose of this retrospective clinic chart review study was to determine the potential of a combination therapy (CT) consisting of γ-aminobutyric acid (GABA), a dipeptidyl peptidase-4 inhibitor (DPP-4i), and a proton pump inhibitor (PPI) to improve glycemic control as an adjunct to insulin therapy in patients with type 1 diabetes (T1D).

Research design and methods

Nineteen patients with T1D on insulin therapy were treated with additional CT in oral form. Fasting blood glucose (FBG), HbA1c, insulin dose-adjusted HbA1c (IDA-A1c), daily insulin dose, insulin/weight ratio (IWR), and fasting plasma C-peptide were measured after 26-42 weeks of treatments.

Results

FBG, HbA1c, IDA-A1c, insulin dose and IWR were all significantly decreased while plasma C-peptide was significantly increased by the CT. Treatment outcomes were further analyzed by separation of the 19 patients into two groups. One group started on the CT within 12 months of insulin treatment (early therapy, 10 patients) and another group started on this therapy only after 12 months of insulin treatment (late therapy, 9 patients). FBG, IDA-A1c, insulin dose, and IWR decreased significantly in both the early and late CT groups, however to a better extent in the early therapy group. Moreover, plasma C-peptide increased significantly only in the early therapy group, and 7 of the 10 patients in this group were able to discontinue insulin treatment while maintaining good glycemic control to study end compared with none of the 9 patients in the late therapy group.

Conclusion

These results support the concept that the combination of GABA, a DPP-4i and a PPI as an adjunct to insulin therapy improves glycemic control in patients with T1D, and that the insulin dose required for glycemic control can be reduced or even eliminated in some patients receiving this novel therapy.

Verapamil improves One-Year C-Peptide Levels in Recent Onset Type-1 Diabetes: A Meta-Analysis

Meta-analysis studying the role of verapamil in improving C-peptide in people with recent-onset type-1 diabetes (T1DM) has not been conducted to date. We undertook this meta-analysis to address this knowledge gap. Electronic databases were systematically reviewed for RCTs having individuals with T1DM receiving verapamil in the treatment arm and placebo in the control arm over the standard of care. The primary outcome was to evaluate changes in the C-peptide area under the curve (AUC) at a one-year follow-up. Secondary outcomes were to assess alterations in C-peptide AUC, glycated hemoglobin (HbA1c), blood pressure, heart rate, and side effects at different time intervals over a one-year follow-up. From the initially screened 27 articles, data from two RCTs (112 patients) satisfied the inclusion criteria and were analyzed. Compared to placebo, C-peptide AUC in individuals receiving verapamil was not different at three months [MD 0.17 nmol/L (95%CI: -0.05-0.38); P = 0.13; I2 = 86%] but significantly higher at 1-year [MD 0.27 nmol/L (95%CI: 0.19-0.35); P < 0.01; I2 = 12%]. The verapamil arm showed similar changes in HbA1C at three months [MD 0.23% (95%CI: -0.43-0.90); P = 0.49; I2 = 88%] and 1-year [MD 0.18% (95% CI: -0.74 - 1.10); P = 0.70; I2 = 89%] compared to placebo. Occurrence of treatment-emergent adverse events [Risk ratio (RR) 1.90 (95%CI: 0.52-6.91); P = 0.33; I2 = 63%], serious adverse events [RR 1.40 (95%CI: 0.50-3.93); P = 0.53], constipation [RR4.11 (95%CI: 0.93-18.13); P = 0.06; I2 = 0%], headache [RR0.48 (95%CI: 0.16-1.43); P = 0.19; I2 = 0%], severe hypoglycemia [RR 0.87 (95%CI: 0.06 - 13.51); P = 0.92] were comparable across groups. Verapamil was well tolerated, and its use over one year was associated with significant improvements in C-peptide AUC though the HbA1c remained unchanged.

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.