Interrupting glucagon signaling decreases gluconeogenesis and the fractional extraction of amino acids by liver from blood, resulting in lower glycemia. The resulting hyperaminoacidemia stimulates α cell proliferation and glucagon secretion via a liver/α cell axis. We hypothesized that α cells detect and respond to circulating amino acids’ levels via a unique amino acid transporter repertoire. We found that Slc7a2/SLC7A2 is the most highly expressed cationic amino acid transporter in α cells, with its expression being 3-fold greater in α than β cells in both mouse and human. Employing cell culture, zebrafish, and knockout mouse models, we found that the cationic amino acid arginine and SLC7A2 are required for α cell proliferation in response to interrupted glucagon signaling. Ex vivo and in vivo assessment of islet function in Slc7a2–/– mice showed decreased arginine-stimulated glucagon and insulin secretion. We found that arginine activation of mTOR signaling and induction of the glutamine transporter SLC38A5 was dependent on SLC7A2, showing that the role of both in α cell proliferation is dependent on arginine transport and SLC7A2. Finally, we identified single nucleotide polymorphisms in SLC7A2 associated with HbA1c. Together, these data indicate a central role for SLC7A2 in amino acid–stimulated α cell proliferation and islet hormone secretion.

The glucose-dependent insulinotropic polypeptide receptor (GIPR) is a major therapeutic target in type 2 diabetes and obesity. Missense variation in GIPR could confer phenotypic effects through alterations to constitutive activity or functional responses to GIP or pharmacological agonists. In this study, we aimed to provide a deep understanding of the molecular mechanisms that underpin the cellular and physiological impacts of GIPR coding variation by studying 30 GIPR coding variants in cellular models and pancreatic islets. Many variants showed impaired GIP-induced cyclic adenosine monophosphate responses, and population-based association analysis highlighted that these loss-of-function variants decrease body mass index but increase glycemia. In many cases, reduced function was partly driven by reduced expression at the cell surface due to impaired stability and redirection toward proteasomal degradation. Molecular dynamics simulations suggest distinct variant-induced perturbations in inter- and intrahelical interactions, which interfere with receptor stability. This study highlights the mechanisms and consequences of GIPR coding variation, which may have implications for the therapeutic targeting of this receptor.

AIMS:Glucagon-like peptide-1 receptor (GLP-1R) agonists improve glycemic control and promote weight loss in diabetes and obesity but are also associated with gastrointestinal adverse events, including nausea and emesis in many patients. These concerns highlight the need for the development of novel GLP-1R agonists that minimize these side effects while maintaining beneficial metabolic outcomes. Here, we investigate the in vivo effects of exendin-4-Phe1 (Ex-Phe1), a GLP-1R biased agonist. MATERIALS AND METHODS:In three pre-clinical species, mice (n = 43), rats (n = 54), and musk shrews (n = 30), we examined in vivo glycemic control, feeding, and nausea/emesis following native Ex-4 and Ex-Phe1 administration. We also used cFos expression following Ex-4 and Ex-Phe1 administration to examine neural activation in regions involved in mediating nausea and emetic side effects of GLP-1R agonism. RESULTS:In vitro studies show Ex-Phe1 favors cAMP signaling with reduced β-arrestin recruitment. Compared to Ex-4, Ex-Phe1 produced fewer emetic episodes in musk shrews (Suncus murinus) and little to no pica, a proxy for nausea, in rats. Ex-Phe1 effects on food intake and body weight varied by species, while Ex-4 and Ex-Phe1 similarly enhanced glucose tolerance in all species. Ex-4 and Ex-Phe1 increased cFos expression within brain regions linked to nausea and emesis in all species. CONCLUSIONS:Collectively, Ex-Phe1 maintains glycemic benefits in all three species, but putatively blunts the ability of the CNS GLP-1R+ cells to drive anorexia and weight loss, as well as unwanted adverse events (nausea/emesis) in rats and musk shrews.

Internalisation of G protein-coupled receptors (GPCRs) can contribute to altered cellular responses by directing signalling from non-canonical locations, such as endosomes. If signalling processes are locally constrained, active receptors in different subcellular locations could produce different downstream effects. This phenomenon may be relevant to the optimal targeting of the glucagon-like peptide-1 receptor (GLP-1R), a type 2 diabetes and obesity target GPCR for which several ligands with varying internalisation tendency have been discovered. To investigate, we compared the signalling localisation effects of two prototypical GLP-1RAs with opposite signal bias and effects on GLP-1R trafficking: exendin-asp3 (ExD3), a full agonist that drives rapid internalisation, and exendin-phe1 (ExF1), which shows much slower internalisation. After using bioorthogonal labelling and fluorescent agonist conjugates to verify the divergent trafficking patterns of ExF1 and ExD3 in (3-cell lines and primary pancreatic islets, we used live cell biosensors to monitor signalling at different subcellular locations. This revealed that cAMP/PKA/ERK signalling in (3-cells is in fact distributed widely across the cell over short-(<5 min) and medium-term (up to 60 min) stimulation at pharmacological (>10 pM) concentrations, with no major differences in signal localisation that could be linked to internalised versus cell surface-bound GLP-1R. Moreover, washout experiments highlighted that, whilst fast-internalising ExD3 shows much greater accumulation and binding to GLP-1R in endosomes than slow-internalising ExF1, it is a rather inefficient driver of both cAMP production in (3-cells and insulin secretion from perfused rat pancreata. These data provide a greater understanding of the cellular effects of biased GLP-1R agonism.

Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are effective therapies for type 2 diabetes (T2D) and obesity, yet patient responses are variable, with GLP1R gene variation potentially linked to therapeutic outcomes. A GLP1R natural missense variant, A316T, protects against T2D and cardiovascular disease. Here, we generated and characterized a human GLP1R A316T mouse model. Human GLP1RA316T/A316T mice displayed lower fasting blood glucose versus wild-type littermates even under metabolic stress, as well as slower weight gain and alterations in islet cytoarchitecture, glucagon secretion, and liver metabolism under a high-fat, high-sucrose diet. This was however associated with blunted responses to pharmacological GLP-1RAs in vivo. Further investigations in β cell models demonstrated that human GLP1R A316T exhibits characteristics of constitutive activation but dampened GLP-1RA responses. Results are further supported by cryo-EM analyses and molecular dynamics simulations of GLP-1R A316T structure, collectively demonstrating that the A316T variant governs basal GLP-1R activity and pharmacological responses to GLP-1R-targeting therapies.

Agonists and antagonists of the glucose-dependent insulinotropic polypeptide receptor (GIPR) enhance body weight loss induced by glucagon-like peptide-1 receptor (GLP-1R) agonism. However, while GIPR agonism decreases body weight and food intake in a GLP-1R-independent manner via GABAergic GIPR+ neurons, it remains unclear whether GIPR antagonism affects energy metabolism via a similar mechanism. Here we show that the body weight and food intake effects of GIPR antagonism are eliminated in mice with global loss of either Gipr or Glp-1r but are preserved in mice with loss of Gipr in either GABAergic neurons of the central nervous system or peripherin-expressing neurons of the peripheral nervous system. Single-nucleus RNA-sequencing shows opposing effects of GIPR agonism and antagonism in the dorsal vagal complex, with antagonism, but not agonism, closely resembling GLP-1R signalling. Additionally, GIPR antagonism and GLP-1R agonism both regulate genes implicated in synaptic plasticity. Collectively, we show that GIPR agonism and antagonism decrease body weight via different mechanisms, with GIPR antagonism, unlike agonism, depending on functional GLP-1R signalling.

The incretin peptides glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 receptors coordinate β cell secretion that is proportional to nutrient intake. This effect permits consistent and restricted glucose excursions across a range of carbohydrate intake. The canonical signaling downstream of ligand-activated incretin receptors involves coupling to Gαs protein and generation of intracellular cAMP. However, recent reports have highlighted the importance of additional signaling nodes engaged by incretin receptors, including other G proteins and β-arrestin proteins. Here, the importance of Gαs signaling was tested in mice with conditional, postdevelopmental β cell deletion of Gnas (encoding Gαs) under physiological and pharmacological conditions. Deletion of Gαs/cAMP signaling induced immediate and profound hyperglycemia that responded minimally to incretin receptor agonists, a sulfonylurea, or bethanechol. While islet area and insulin content were not affected in Gnasβcell-/-, perifusion of isolated islets demonstrated impaired responses to glucose, incretins, acetylcholine, and IBMX In the absence of Gαs, incretin-stimulated insulin secretion was impaired but not absent, with some contribution from Gαq signaling. Collectively, these findings validate a central role for cAMP in mediating incretin signaling, but also demonstrate broad impairment of insulin secretion in the absence of Gαs that causes both fasting hyperglycemia and glucose intolerance.

BACKGROUND:Tirzepatide, a single-molecule dual glucose-dependent insulinotropic polypeptide (GIP)/glucagon-like peptide-1 (GLP-1) receptor (R) agonist, has shown superiority in the reduction of blood glucose and body weight, above selective GLP-1R agonists, but the contribution of GIP to these effects remains incompletely understood. OBJECTIVES:To characterize the preclinical and in-human effects of a long-acting GIPR agonist monotherapy in healthy participants and patients with type 2 diabetes (T2D). METHODS:A long-acting GIPR agonist (LY3537021) was characterized in vitro and in Long-Evans diet-induced obese rats and Wistar rats. Next, a phase 1, randomized, placebo-controlled, single ascending dose (SAD)/multiple ascending dose (MAD) study explored the safety, tolerability, pharmacokinetics, and pharmacodynamics of LY3537021 in healthy participants and participants with T2D in Singapore. RESULTS:In vitro, LY3537021 demonstrated potency greater than native GIP and selectivity for the GIPR. In vivo in rats, chronic treatment with LY3537021 resulted in weight loss and improved glycemic control during a glucose tolerance test. The phase 1 clinical study enrolled 85 healthy participants and patients with T2D (SAD, n = 47 [aged 25-64 years]; MAD, n = 38 [aged 25-69 years]; average baseline BMI was 25.9-27.0 kg/m2 across the arms). During the MAD part, dose-dependent decreases in mean body weight were observed in all LY3537021 dose groups, regardless of T2D status, and persisted at 35 days after the last dose. For example, participants with T2D treated with 25 mg of LY3537021 lost a mean of 3.14 kg of body weight compared with 0.36 kg in the placebo group (p < 0.05) at day 57. Transient reductions in fasting glucose were observed in these participants, but the reductions were not sustained and not significantly different from placebo at day 29. The time to maximum observed drug concentrations varied across cohorts (8-96 h), and the half-life was estimated at approximately 12 days for non-T2D and T2D cohorts with the 25-mg dose, supporting once-weekly administration. There was no delay in gastric emptying following a single subcutaneous dose of 0.3-25 mg LY3537021. LY3537021 was well tolerated with infrequent gastrointestinal adverse events. CONCLUSIONS:In vivo studies demonstrated that LY3537021 reduced body weight and improved glycemia during a glucose challenge in rats. The phase 1 study demonstrated that the long-acting GIPR agonist LY3537021 was well tolerated, induced weight loss, and improved glucose control in humans. These observations better define the therapeutic benefit of long-acting GIPR agonists and support a distinct contribution of GIP agonism to the benefits observed with multi-agonist peptides that act via the GIPR. Future studies are needed in more diverse populations and in cohorts with overweight/obesity to confirm these findings. CLINICALTRIALS:GOV: NCT04586907.

In combatting the obesity crisis, leveraging mechanisms that lower body weight is critical. The finding that treatment with tirzepatide, a glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) receptor agonist, produces profound weight loss highlights the value of activating the incretin receptors. Supporting this, recent studies have revealed mechanisms by which GIP receptor (GIPR) activation is beneficial in pancreatic islets, the central nervous system (CNS), and adipose tissue. Paradoxically, a hypothesis has emerged that GIPR antagonism could be an additional option in treating obesity. This concept stems from concern that GIP facilitates lipid uptake and storage in adipose tissue, although the lipid-buffering capacity of adipocytes versus other cell types is metabolically favorable. In this article, we highlight the natural physiology of the incretins, noting GIP as the primary incretin. In the CNS, GIPR agonism attenuates nausea and suppresses appetite, features that also help GLP-1 receptor agonism promote a negative energy balance. Further, we provide rationale that, in protecting against ectopic fat distribution and augmenting substrate utilization to promote insulin sensitivity, GIPR activity in adipose tissue is advantageous. Collectively, these attributes support GIPR agonism in the treatment of obesity and metabolic disease.

The glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) are important incretin receptors that are therapeutic targets for the treatment of type 2 diabetes and obesity. This study extensively characterised the metabolic phenotype of mice with global deletion of either the GLP-1R or GIPR side by side under identical conditions. Age-matched male wild-type (WT) C57Bl6NTac, GLP-1RKO or GIPRKO mice were placed on a high-fat or chow diet for 12 weeks, and a range of in vivo (weight gain, food intake, glucose tolerance, insulin tolerance, and whole-body energy metabolism) and ex vivo (white adipocyte lipolysis, brown adipose tissue and liver mitochondrial function, adipocyte and islet size, and hepatic steatosis) parameters were measured. While both WT and GLP-1RKO mice gained weight similarly on a HFD, obese high-fat-fed GLP-1RKO mice had altered glucose and insulin tolerance, and exhibited hepatic steatosis, highlighting the physiological importance of the GLP-1R in the regulation of blood glucose and lipid homoeostasis. In contrast, GIPRKO mice were partially resistant to diet-induced obesity compared to the WT mice, which was associated with a small reduction in food intake and intact epididymal and subcutaneous white adipocyte β-adrenoceptor-mediated lipolysis. Similarly, WT mice treated with a GIPR antagonist prevented weight gain due to a reduction in food intake on a HFD. These findings provide further support that the GLP-1R is important for normal glycaemic control, whereas the GIPR may play a role in the regulation of body weight.

α cells secrete proglucagon peptides to regulate nutrient metabolism. Recent findings support an α cell-to-β cell axis that is mediated by paracrine signaling through the glucagon receptor and glucagon-like peptide 1 (GLP-1) receptor in β cells. To address which proglucagon peptides stimulate insulin secretion, we developed an assay to quantify levels of GLP-1(7-36)NH2. We also generated three transgenic mouse lines that allow α cell-specific, inducible deletion of the genes for the two prohormone convertase enzymes that process proglucagon . Our studies reveal that both mouse and human islets contain GLP-1(7-36)NH2, but glucagon mediates α cell-to-β cell communication in mice. However, in the absence of normal production of glucagon, α cells up-regulate prohormone convertase 1 (PC1/3) to generate GLP-1 and enhance glucose tolerance. Human islets have substantially higher levels of GLP-1 than mice, which positively correlate with rates of insulin secretion. These studies show plasticity in proglucagon processing to support α cell-to-β cell communication.

OBJECTIVE:Glucagon is a crucial regulator of glucose and lipid metabolism as well as whole-body energy balance. Thus, modulation of glucagon receptor (GCGR) activity in the context of single-molecule multi-receptor co-agonists has become an emerging therapeutic target against obesity and obesity-associated metabolic dysfunction. To better elucidate the role of GCGR-signaling when paired with incretin receptor signaling or on its own, we developed, LY3324954, a GCGR agonist with improved potency and selectivity as compared to the native glucagon peptide. METHODS:LY3324954 was administered to DIO mice, rats, dogs, and monkeys to evaluate pharmacokinetic (PK) profile. Biweekly treatments were conducted in lean and DIO mice to characterize LY3324954-effects on glucose homeostasis and energy balance. Single dose studies were also conducted in liver Gcgr-deficient mice to establish receptor specificity. RESULTS:LY3324954 also exhibited extended PK profile in DIO mice, rats, dogs, and monkeys. When administered every 72 h, LY3324954 treatment stimulated transient glucose and insulin excursions in lean mice. In diet-induced obese mice, LY3324954 treatment stimulates energy expenditure, weight loss, and a reduction of adiposity in a dose-dependent manner. Benefit to whole-body lipid homeostasis was likewise observed in these mice. CONCLUSIONS:Taken together, these studies characterize a long-acting and potent GCGR-agonist and its regulation of glucose and lipid metabolism as well as whole-body energy balance following both acute and chronic treatment in mice.

Internalisation of G protein-coupled receptors (GPCRs) can contribute to altered cellular responses by directing signalling from non-canonical locations, such as endosomes. If signalling processes are locally constrained, active receptors in different subcellular locations could produce different downstream effects. This phenomenon may be relevant to the optimal targeting of the glucagon-like peptide-1 receptor (GLP-1R), a type 2 diabetes and obesity target GPCR for which several ligands with varying internalisation tendency have been discovered. To investigate, we compared the signalling localisation effects of two prototypical GLP-1RAs with opposite signal bias and effects on GLP-1R trafficking: exendin-asp3 (ExD3), a full agonist that drives rapid internalisation, and exendin-phe1 (ExF1), which shows much slower internalisation. After using bioorthogonal labelling and fluorescent agonist conjugates to verify the divergent trafficking patterns of ExF1 and ExD3 in β-cell lines and primary pancreatic islets, we used live cell biosensors to monitor signalling at different subcellular locations. This revealed that cAMP/PKA/ERK signalling in β-cells is in fact distributed widely across the cell over short- (<5 min) and medium-term (up to 60 min) stimulation at pharmacological (>10 pM) concentrations, with no major differences in signal localisation that could be linked to internalised versus cell surface-bound GLP-1R. Moreover, washout experiments highlighted that, whilst fast-internalising ExD3 shows much greater accumulation and binding to GLP-1R in endosomes than slow-internalising ExF1, it is a rather inefficient driver of both cAMP production in β-cells and insulin secretion from perfused rat pancreata. These data provide a greater understanding of the cellular effects of biased GLP-1R agonism.

Recent studies have found that GIPR agonism can enhance the metabolic efficacy of GLP-1R agonist treatment by promoting both weight-dependent and -independent improvements on systemic insulin sensitivity. These findings have prompted new investigations aimed at better understanding the broad metabolic benefit of GIPR activation. Herein, we determined whether GIPR agonism favorably influenced the pharmacologic efficacy of the insulin sensitizing thiazolidinedione, rosiglitazone in obese insulin resistant (IR) mice. Genetic and pharmacological approaches were employed to examine the role of GIPR signaling on rosiglitazone-induced weight gain, hyperphagia, and glycemic control. RNA-sequencing was conducted to uncover potential mechanisms by which GIPR activation influences energy balance and insulin sensitivity. In line with previous findings, treatment with rosiglitazone induced the mRNA expression of the GIPR in white and brown fat. However, obese GIPR null mice dosed with rosiglitazone displayed equivalent weight gain to that of wild-type (WT) animals. Strikingly, chronic treatment of obese IR WT animals with a long-acting GIPR agonist (LAGIPRA) prevented rosiglitazone-induced weight-gain and hyperphagia, and it enhanced the insulin-sensitivity effect of this thiazolidinedione. The systemic insulin sensitization was accompanied by increased glucose disposal in brown adipose tissue (BAT), which was underlined by the recruitment of metabolic and thermogenic genes. These findings suggest that GIPR agonism can counter the negative consequences of rosiglitazone treatment on body weight and adiposity, while at the same time improving its insulin sensitizing efficacy. Highlights · Body weight gain resulting from treatment with the insulin sensitizing thiazolidinedione rosiglitazone does not require the GIPR. · Treatment with a GIPR agonist prevents the hyperphagic and obesogenic activity of rosiglitazone. · Activation of the GIPR improves the insulin sensitizing actions of rosiglitazone.

Orally bioavailable, synthetic nonpeptide agonists (NPAs) of the glucagon-like peptide-1 receptor (GLP-1R) may offer an effective, scalable pharmacotherapy to address the metabolic disease epidemic. One of the first molecules in the emerging class of GLP-1R NPAs is orforglipron, which is in clinical development for treating type 2 diabetes and obesity. Here, we characterized the pharmacological properties of orforglipron in comparison with peptide-based GLP-1R agonists and other NPAs. Competition binding experiments using either [ 125 I]GLP-1(7-36)NH 2 or [ 3 H]orforglipron indicated that orforglipron is a high-affinity [inhibition constant ( K i ) = 1 nM], selective ligand of the human GLP-1R. Signal transduction assays showed that orforglipron has low intrinsic efficacy for effector activation and negligible β-arrestin recruitment. To evaluate GLP-1R engagement in vivo, mice expressing the human GLP-1R were administered orforglipron and subjected to a glucose tolerance test. Predicted receptor occupancy was calculated using the receptor K i value of orforglipron and its unbound concentration in vivo that reduces hyperglycemia. These experiments revealed that low GLP-1R occupancy by orforglipron is sufficient to yield a full biological response. Moreover, in a model where CRISPR-Cas9 gene editing was used to sensitize the rat GLP-1R ( Glp1r S33W ) to GLP-1R NPAs, target engagement by orforglipron in the pancreas and brain was consistent with peptide-based GLP-1R agonists. Diet-induced obesity in Glp1r S33W rats enabled studies showing weight loss in animals orally administered orforglipron versus subcutaneous injection of GLP-1R agonist semaglutide. Furthermore, crossover studies indicated oral orforglipron can sustain efficacy initiated by parenteral semaglutide. The pharmacological properties of orforglipron may inform targeting of other peptide receptors with NPAs.

AbstractGlucagon-like peptide-1 receptor agonists (GLP-1RAs) are effective therapies for type 2 diabetes (T2D) and obesity, yet patient responses are variable. Variation in the humanGlp1rgene might be directly linked to therapeutic responses. A naturally occurring missense variant, A316T, protects against T2D and cardiovascular disease. Here, we have generated and characterised a humanGlp1rA316T mouse model. HumanGlp1rA316T/A316Tmice displayed lower fasting blood glucoseversuswildtype littermates, even under metabolic stress, and exhibited alterations in islet cytoarchitecture and α/β-cell identity under a high-fat, high-sucrose diet. This was however associated with blunted responses to GLP-1RAsin vivo. Further investigations in rodent and human β-cell models demonstrated that humanGlp1rA316T exhibits characteristics of constitutive activation but dampened GLP-1RA responses. Results are further supported by cryo-EM analyses and molecular dynamics simulations of GLP-1R A316T structure, collectively demonstrating that the A316T variant governs basal GLP-1R activity and pharmacological responses to GLP-1R-targeting therapies.TeaserTheGlp1rA316T missense variant displays improved glucose tolerance but impaired pharmacological incretin responsesin vivo.