Biology subjects

Tomas, A.

Publications and source records attributed to Tomas, A..

At least 19 recordsLinked to original sources

bioRxiv2026

ZNK1 senses zinc and degrades zinc transporter mRNA in trypanosomes

Like other cells, parasitic and other trypanosomatids sense and regulate Zn2+ transport, but the mechanisms involved remained unknown. Here we identify a trypanosome RNA-binding protein which specifically eliminates ZIP3 Zn2+-transporter mRNA in Zn2+-replete conditions. We first demonstrated that Trypanosoma brucei ZIP3 mRNA abundance is subject to 3-untranslated region (3-UTR) and Zn2+-dependent negative control. A genome-wide RNA interference library screen, using a reporter associated with the ZIP3 3-UTR, identified Tb927.11.9510 as a candidate Zn2+-sensor. We name this protein Zinc Nuclear Knuckles 1 (ZNK1) since it localises to the nucleus and contains several Zn2+-knuckle motifs. ZNK1 is conserved among trypanosomatids, and a PIN-domain suggests a ribonuclease-based mechanism. We validate ZNK1 as a ZIP3 3-UTR dependent negative regulator and identify a GU-repeat motif in the ZIP3 3-UTR that is predictive of negative control by ZNK1. We use Cas9-editing to knockout ZNK1, and RNA-seq to assess the consequences, revealing highly specific accumulation of ZIP3 transcripts in znk1-null cells. We conclude that ZNK1 senses Zn2+-abundance and eliminates ZIP3 mRNA in a Zn2+-dependent manner. We suggest that trypanosomatid ZNK1 is an RNA-specific zinc finger nuclease that binds ZIP3 3-UTRs and degrades ZIP3 mRNA only when the tandem sensor modules are coordinated with Zn2+. Key pointsO_LITrypanosome Zinc Nuclear Knuckles 1 (ZNK1) is a zinc-sensor that eliminates zinc transporter mRNA. C_LIO_LIZNK1 negative control operates via the transporter mRNA 3-untranslated region. C_LIO_LIThe findings indicate that trypanosomatid ZNK1 is a conserved RNA-specific zinc finger nuclease. C_LI

microbiology↗

bioRxiv2026

Lipid droplet accumulation drives glutamine-dependent NLRP3 inflammasome activation

The NLRP3 inflammasome is implicated in the pathogenesis of many inflammatory and metabolic diseases. Although metabolic cues can shape NLRP3 inflammasome activation, the role of lipid metabolism in this response is poorly understood. Lipids are essential for energy and redox homeostasis, membrane biogenesis, and diverse cellular processes. However, excessive accumulation of lipids such as free fatty acids can be highly cytotoxic. Consequently, surplus free fatty acids are esterified into triglycerides and cholesteryl esters, which are then stored within lipid droplets (LDs). Interestingly, transcriptomic data analyses revealed that macrophages exposed to inflammasome-activating stimulus undergo a broad metabolic rewiring favouring lipid storage, suggesting a link between LDs and inflammasome response. Here, we show that LD accumulation actively promotes NLRP3 inflammasome activation. Increasing LD abundance, either by fatty acid supplementation or by inhibition of lipolysis, led to robust caspase-1 activation and IL-1{beta} release both in vitro and in vivo. Mechanistically, LD expansion promoted cellular reorganisation resulting in the formation of peri-droplet mitochondria, a distinct mitochondrial subpopulation which exhibited elevated membrane potential and increased ATP-linked respiration. Furthermore, LD-rich macrophages underwent metabolic reconfiguration whereby they exhibited a pronounced reliance on pyruvate and glutamine metabolism, but not fatty acid oxidation, to sustain mitochondrial bioenergetics. Surprisingly, inflammasome activation was found dependent predominantly on glutaminolysis, highlighting a key role for glutamine catabolism in LD-rich cells. Collectively, these findings identify a glutamine-dependent pathway as a key driver linking excess lipid storage to NLRP3 inflammasome, with profound implications in metabolic diseases.

immunology↗

bioRxiv2025

Glucagon-like peptide-1 receptor C-terminal tail phosphorylation determines signalling responses from pancreatic β-cells

The glucagon-like peptide-1 receptor (GLP-1R) is a class B G protein-coupled receptor that regulates glucose homeostasis by promoting cAMP production and insulin secretion in pancreatic {beta}-cells. Although receptor phosphorylation is known to influence GLP-1R signalling, its functional relevance in {beta}-cells remains incompletely understood. Here, we investigated the role of three C-terminal serine doublets (Ser441/442, Ser444/445, Ser451/452) in the human receptor using site-directed mutagenesis in INS-1 832/3 {beta}-cells deleted for endogenous GLP-1R expression. GLP-1 stimulation induced serine phosphorylation of wild-type GLP-1R, which was abolished in all individual doublet as well as triple doublet phospho-null mutants. Basal phosphorylation was detected in wild-type receptors under unstimulated conditions, suggesting dynamic cycling between phosphorylated and unphosphorylated states. All mutant receptors trafficked normally to the plasma membrane, but the triple alanine doublet (AA) mutant exhibited reduced GLP-1-induced internalisation and significantly decreased plasma membrane diffusion under vehicle conditions, indicating altered receptor dynamics even in the absence of agonist. Importantly, the Ser451/452 doublet was required for GRK2 recruitment to the active GLP-1R. Functionally, the triple AA mutant showed impaired GLP-1-induced cAMP generation with minimal change in {beta}-arrestin 2 recruitment, resulting in biased signalling. The cAMP defect was partially rescued by phosphodiesterase 4 (PDE4) inhibition with rolipram, implicating altered PDE engagement in the defective cAMP response. These findings demonstrate that both basal and agonist-induced GLP-1R phosphorylation are critical for receptor signalling in {beta}-cells, revealing a previously unappreciated role for basal phosphorylation in shaping receptor behaviour and establishing a positive link between GLP-1R phosphorylation and {beta}-cell signalling competence.

cell biology↗

bioRxiv2025

Binding Kinetics, Bias, Receptor Internalization and Effects on Insulin Secretion in vitro and in vivo of a Novel GLP-1R/GIPR Dual Agonist, HISHS-2001

The use of incretin analogues has emerged in recent years as an effective approach to achieve both enhanced insulin secretion and weight loss in type 2 diabetes (T2D) patients. Agonists which bind and stimulate multiple receptors have shown particular promise. However, off target effects, including nausea and diarrhoea, remain a complication of using these agents, and modified versions with optimized pharmacological profiles and/or biased signaling at the cognate receptors are increasingly sought. Here, we describe the synthesis and properties of a molecule which binds to both glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptors (GLP-1R and GIPR) to enhance insulin secretion. HISHS-2001 shows increased affinity at the GLP-1R, as well as a tendency towards reduced internalization and recycling at this receptor versus FDA-approved dual GLP-1R/GIPR agonist tirzepatide. HISHS-2001 also displayed significantly greater bias towards cAMP generation versus {beta}-arrestin 2 recruitment compared to tirzepatide. In contrast, Gs recruitment was lower versus tirzepatide at the GLP-1R, but higher at the GIPR. Administered to obese hyperglycaemic db/db mice, HISHS-2001 increased circulating insulin whilst lowering body weight and HbA1c with similar efficacy to tirzepatide at substantially lower doses. Thus, HISHS-2001 represents a novel dual receptor agonist with an improved pharmacological profile.

pharmacology and toxicology↗

bioRxiv2024

Human class B1 GPCR modulation by plasma membrane lipids

The class B1 G protein-coupled receptor (GPCR) subfamily is a class of receptors known for their regulatory roles in metabolism and neuronal activity and as important drug targets. Lipids play key functional roles in modulation of GPCR signalling, yet our understanding of the molecular level detail of specific lipid interactions with class B1 GPCRs remains limited. Here we present coarse-grained molecular dynamics (MD) simulations of the active and inactive states of 15 human class B1 family members and use aiida-gromacs to capture full provenance for the set-up of simulations in complex plasma membranes. Receptors exhibit state-dependent lipid interactions with the regulatory sterol cholesterol and phospholipid phosphatidylinositiol-3,4-bisphosphate (PIP2) at defined locations on the receptor surface. Global analysis of trends across the subfamily reveals conserved patterns of lipid interaction dynamics. The glycosphingolipid GM3 exerts a modulatory influence on the dynamics of class B1 extracellular domains in both simulations and in vitro time-resolved FRET assays.

molecular biology↗

bioRxiv2024

The physiological impact of an N-terminal Halo-tag on GIPR function in mice

AimsG protein-coupled receptors (GPCRs) modified with self-labelling enzymatic tags (e.g. Halo, SNAP, CLIP) have enabled the in vitro study of receptor expression and trafficking. We developed a GiprHalo/Halo mouse model to gain insights into endogenous glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) signalling, an important modulator of glucose and appetite homeostasis. MethodsGiprHalo/Halo mice were generated through ES cell-based gene targeting and physiologically characterised via body weight, oral glucose tolerance test and intraperitoneal glucose tolerance test analysis. Live cell imaging was used to measure cAMP responses in dispersed pancreatic islets from GiprWt/Wt and GiprHalo/Halo littermates in response to GIP. cAMP accumulation and surface receptor expression were assessed in AD293 cells transiently transfected with mouse and human untagged GIPR and Halo-GIPR. Endogenous pancreatic islet expression of Halo-GIPR was determined by immunohistochemistry. ResultsGiprHalo/Halo mice displayed comparable body weights and responses to oral glucose administration as GiprHalo/Wt and GiprWt/Wt littermates. However, GiprHalo/Halo mice only responded to high dose human GIP during intraperitoneal glucose tolerance testing, and cAMP responses to GIP were impaired in GiprHalo/Halo islets. Despite unimpaired surface expression, in vitro cAMP responses were lessened in AD293 cells expressing Halo-GIPR compared to untagged GIPR. Anti-Halo staining in mouse GiprHalo/Wt pancreatic islets displayed detectable signal. ConclusionsWhile the GiprHalo/Halo mouse model could be used to study endogenous GIPR expression, due to reduced responses we advise against its use for endogenous GIPR trafficking and signalling. Our study highlights the importance of carefully phenotyping mouse models in which modifications to endogenous proteins have been introduced.

cell biology↗

bioRxiv2024

In vivo functional profiling and structural characterisation of the human Glp1r A316T variant

Glucagon-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 human Glp1r gene 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 human Glp1r A316T mouse model. Human Glp1rA316T/A316T mice displayed lower fasting blood glucose versus wildtype littermates, even under metabolic stress, and exhibited alterations in islet cytoarchitecture and /{beta}-cell identity under a high-fat, high-sucrose diet. This was however associated with blunted responses to GLP-1RAs in vivo. Further investigations in rodent and human {beta}-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. TeaserThe Glp1r A316T missense variant displays improved glucose tolerance but impaired pharmacological incretin responses in vivo.

cell biology↗

bioRxiv2024

Molecular mapping and functional validation of GLP-1R cholesterol binding sites in pancreatic beta cells

G protein-coupled receptors (GPCRs) are integral membrane proteins which closely interact with their plasma membrane lipid microenvironment. Cholesterol is a lipid enriched at the plasma membrane with pivotal roles in the control of membrane fluidity and maintenance of membrane microarchitecture, directly impacting on GPCR stability, dynamics and function. Cholesterol extraction from pancreatic beta cells has previously been shown to disrupt the internalisation, clustering and cAMP responses of the glucagon-like peptide-1 receptor (GLP-1R), a class B1 GPCR with key roles in the control of blood glucose levels via the potentiation of insulin secretion in beta cells and weight reduction via the modulation of brain appetite control centres. Here, we unveil the detrimental effect of a high cholesterol diet on GLP-1R-dependent glucoregulation in vivo, and the improvement in GLP-1R function that a reduction in cholesterol synthesis using simvastatin exerts in pancreatic islets. We next identify and map sites of cholesterol high occupancy and residence time on active versus inactive GLP-1Rs using coarse-grained molecular dynamics (cgMD) simulations, followed by a screen of key residues selected from these sites and detailed analyses of the effects of mutating one of these residues, Val229, to alanine on GLP-1R interactions with cholesterol, plasma membrane behaviours, clustering, trafficking and signalling in pancreatic beta cells and primary islets, unveiling an improved insulin secretion profile for the V229A mutant receptor. This study 1) highlights the role of cholesterol in regulating GLP-1R responses in vivo; 2) provides a detailed map of GLP-1R - cholesterol binding sites in model membranes; 3) validates their functional relevance in beta cells; and 4) highlights their potential as locations for the rational design of novel allosteric modulators with the capacity to fine-tune GLP-1R responses.

cell biology↗

bioRxiv2024

An inter-organelle contact between endosomal GLP-1R, ER VAP-B, and the mitochondrial AKAP SPHKAP triggers PKA-dependent MIC19 phosphorylation and β-cell mitochondrial remodelling

Glucagon-like peptide-1 receptor (GLP-1R) agonists (GLP-1RAs) ameliorate mitochondrial health by increasing mitochondrial turnover in metabolically relevant tissues. Mitochondrial adaptation to metabolic stress is crucial to maintain pancreatic {beta}-cell function and prevent type 2 diabetes (T2D) progression. While the GLP-1R is well-known to stimulate cAMP production leading to Protein Kinase A (PKA) and Exchange Protein Activated by cyclic AMP 2 (Epac2) activation, there is a lack of understanding of the molecular mechanisms linking GLP-1R signalling with mitochondrial and {beta}-cell functional adaptation. Here, we present a comprehensive study in {beta}-cell lines and primary islets that demonstrates that, following GLP-1RA stimulation, GLP-1R-positive endosomes associate with the endoplasmic reticulum (ER) membrane contact site (MCS) tether VAPB at ER-mitochondria MCSs (ERMCSs), where active GLP-1R engages with SPHKAP, an A-kinase anchoring protein (AKAP) previously linked to T2D and adiposity risk in genome-wide association studies (GWAS). The inter-organelle complex formed by endosomal GLP-1R, ER VAPB and SPHKAP triggers a pool of ERMCS-localised cAMP/PKA signalling via the formation of a PKA-RI biomolecular condensate which leads to changes in mitochondrial contact site and cristae organising system (MICOS) complex phosphorylation, mitochondrial remodelling, and {beta}-cell functional adaptation, with important consequences for the regulation of {beta}-cell insulin secretion and survival to stress.

cell biology↗

bioRxiv2023

Control of human pancreatic beta cell kinome by GLP-1R biased agonism

AimTo determine the kinase activity profiles of human pancreatic beta cells downstream of GLP-1R balanced versus biased agonist stimulations. Materials and methodsThis study analysed the kinomic profiles of human EndoC-{beta}h1 cells following vehicle and glucagon-like peptide-1 receptor (GLP-1R) stimulation with the pharmacological agonist exendin-4, as well as exendin-4-based biased derivatives exendin-phe1 and exendin-asp3 for acute (10-minute) versus sustained (120-minute) responses, using PamChip(R) protein tyrosine kinase (PTK) and serine/threonine kinase (STK) assays. The raw data were filtered and normalised using BioNavigator. The kinase analyses were conducted with R, mainly including kinase-substrate mapping and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. ResultsThe present analysis reveals that kinomic responses are distinct for acute versus sustained GLP-1R agonist (GLP-1RA) exposure, with individual responses associated with agonists presenting specific bias profiles. According to pathway analysis, several kinases, including JNKs, PKCs, INSR and LKB1, are important GLP-1R signalling mediators, constituting potential targets for further research on biased GLP-1R downstream signalling. ConclusionResults from this study suggest that differentially biased exendin-phe1 and exendin-asp3 can modulate distinct kinase interaction networks. Further understanding of these mechanisms will have important implications for the selection of appropriate anti-T2D therapies with optimised downstream kinomic profiles.

cell biology↗

bioRxiv2022

An examination of the divergent spatiotemporal signaling of GLP-1R versus GIPR in pancreatic beta cells

The incretin receptors, glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR), are class B GPCRs and prime therapeutic targets for the treatment of type 2 diabetes (T2D) and obesity. They are expressed in pancreatic beta cells where they potentiate insulin release in response to food intake. Despite GIP being the main incretin in healthy individuals, GLP-1R has been favoured versus GIPR as a therapeutic target due to GIPR responses being blunted in T2D patients and the conflicting effects of GIPR agonists and antagonists in improving glucose tolerance and preventing weight gain. There is, however, a recently renewed interest in GIPR biology following the realisation that GIPR responses can be restored after an initial period of blood glucose normalization and the recent development of dual GLP-1R-GIPR agonists with superior capacity for the control of blood glucose levels and weight. The importance of GLP-1R trafficking and subcellular signaling in the control of receptor outputs is well established, but little is known about the pattern of spatiotemporal signaling from the GIPR in beta cells. Here we have directly compared the main trafficking and signaling characteristics of both receptors in pancreatic beta cells, finding striking differences in their propensities for internalization, recycling, and degradation, as well as plasma membrane versus endosomal activity, with potential implications for receptor-specific control of beta cell function.

cell biology↗

bioRxiv2022

Divergent acute versus prolonged in vivo GLP-1R responses in β-arrestin 2-deleted primary beta cells

The glucagon-like peptide-1 receptor (GLP-1R) is a major therapeutic target in type 2 diabetes (T2D) and obesity. Following activation, GLP-1Rs are rapidly desensitised by {beta}-arrestins, scaffolding proteins that terminate G protein interactions but also act as independent signalling mediators. While GLP-1R interacts with {beta}-arrestins 1 and 2, expression of the latter is greatly enhanced in beta cells, making this the most relevant isoform. Here, we have assessed in vivo glycaemic responses to the pharmacological GLP-1R agonist exendin-4 in adult beta cell-selective {beta}-arrestin 2 knockout (KO) mice. Lean female and high-fat, high-sucrose-fed KO mice of both sexes displayed worse acute responses versus control littermates, an effect that was inverted 6 hours post-agonist injection, resulting in prolonged in vivo cell-cell connectivity in KO islets implanted in mouse eyes. Similar effects were observed for the clinically relevant semaglutide and tirzepatide but not with exendin-phe1, an agonist biased away from {beta}-arrestin recruitment. Ex vivo acute cAMP was impaired, but overnight desensitisation was reduced in KO islets. The acute signalling defect was attributed to enhanced {beta}-arrestin 1 and phosphodiesterase (PDE) 4 activity in the absence of {beta}-arrestin 2, while the reduced desensitisation correlated with altered GLP-1R trafficking, involving impaired recycling and lysosomal targeting and increased trans-Golgi network (TGN) localisation and signalling, as well as reduced GLP-1R ubiquitination by the E3 ubiquitin ligase NEDD4. This study has unveiled fundamental aspects of the role of {beta}-arrestin 2 in regulating pharmacological GLP-1R responses with direct application to the rational design of improved GLP-1R-targeting therapeutics.

cell biology↗

bioRxiv2021

Expression of mini-G proteins specifically halt cognate GPCR trafficking and intracellular signalling

Mini-G proteins are engineered thermostable variants of G subunits designed to specifically stabilise G protein-coupled receptors (GPCRs) in their active conformation for structural analyses. Due to their smaller size and ease of use, they have become popular tools in recent years to assess specific GPCR behaviours in cells, both as reporters of receptor coupling to each G protein subtype and for in-cell assays designed to quantify compartmentalised receptor signalling from a range of subcellular locations. Here, we describe a previously unappreciated consequence of the co-expression of mini-G proteins with their cognate GPCRs, namely a profound disruption in GPCR trafficking and intracellular signalling caused by the co-expression of the specific mini-G subtype coupled to the affected receptor. We studied the Gs-coupled pancreatic beta cell class B GPCR glucagon-like peptide-1 receptor (GLP-1R) as a model to describe in detail the molecular consequences derived from this effect, including a complete halt in {beta}-arrestin-2 recruitment and receptor internalisation, despite near-normal levels of receptor GRK2 recruitment and lipid nanodomain segregation, as well as the disruption of endosomal GLP-1R signalling by mini-Gs co-expression. We also extend our analysis to a range of other prototypical GPCRs covering the spectrum of G subtype coupling preferences, to unveil a widely conserved phenomenon of GPCR internalisation blockage by specific mini-G proteins coupled to a particular receptor. Our results have important implications for the design of methods to assess intracellular GPCR signalling. We also present an alternative adapted bystander intracellular signalling assay for the GLP-1R in which we substitute the mini-Gs by a nanobody, Nb37, with specificity for active Gs:GPCR complexes and no deleterious effect on the capacity for GLP-1R internalisation.

cell biology↗

bioRxiv2021

Hepatocyte cholesterol content modulates glucagon receptor signalling

Glucagon decreases liver fat, and non-alcoholic fatty liver disease (NAFLD) is associated with hepatic glucagon resistance. Increasingly it is recognised that the function of G protein-coupled receptors can be regulated by their local plasma membrane lipid environment. The aim of this study was to evaluate the effects of experimentally modulating hepatocyte cholesterol content on the function of the glucagon receptor (GCGR). We found that glucagon-mediated cAMP production is inversely proportional to cholesterol content of human hepatoma and primary mouse hepatocytes after treatment with cholesterol-depleting and loading agents, with ligand internalisation showing the opposite trend. Mice fed a high cholesterol diet had increased hepatic cholesterol and a blunted hyperglycaemic response to glucagon, both of which were partially reversed by simvastatin. Molecular dynamics simulations identified potential membrane-exposed cholesterol binding sites on the GCGR. Overall, our data suggest that increased hepatocyte membrane cholesterol could directly contribute to glucagon resistance in NAFLD.

pharmacology and toxicology↗

bioRxiv2021

Glucose-dependent miR-125b is a negative regulator of β-cell function

Impaired pancreatic {beta}-cell function and insulin secretion are hallmarks of type 2 diabetes. MicroRNAs are short non-coding RNAs that silence gene expression, vital for the development and function of {beta}-cells. We have previously shown that {beta}-cell specific deletion of the important energy sensor AMP-activated protein kinase (AMPK) results in increased miR-125b-5p levels. Nevertheless, the function of this miRNA in {beta}-cells is unclear. We hypothesized that miR-125b-5p expression is regulated by glucose and that this miRNA mediates some of the deleterious effects of hyperglycaemia in {beta}-cells. Here we show that islet miR-125b-5p expression is up-regulated by glucose in an AMPK-dependent manner and that short-term miR-125b-5p overexpression impairs glucose stimulated insulin secretion (GSIS) in the mouse insulinoma MIN6 cells and in human islets. An unbiased high-throughput screen in MIN6 cells identified multiple miR-125b-5p targets, including the transporter of lysosomal hydrolases M6pr and the mitochondrial fission regulator Mtfp1. Inactivation of miR-125b-5p in the human {beta}-cell line EndoC{beta}-H1 shortened mitochondria and enhanced GSIS, whilst mice overexpressing miR-125b-5p selectively in {beta}-cells (MIR125B-Tg) were hyperglycaemic and glucose intolerant. MIR125B-Tg {beta}-cells contained enlarged lysosomal structures and showed reduced insulin content and secretion. Collectively, we identify miR-125b as a glucose-controlled regulator of organelle dynamics that modulates insulin secretion. Graphical abstract O_FIG_DISPLAY_L [Figure 1] M_FIG_DISPLAY C_FIG_DISPLAY

molecular biology↗

bioRxiv2021

Receptor Activity-Modifying Protein 2 (RAMP2) alters glucagon receptor trafficking in hepatocytes with functional effects on receptor signalling

ObjectivesReceptor Activity-Modifying Protein 2 (RAMP2) is a chaperone protein which allosterically binds to and interacts with the glucagon receptor (GCGR). The aims of this study were to investigate the effects of RAMP2 on GCGR trafficking and signalling in the liver, where glucagon is important for carbohydrate and lipid metabolism. MethodsSubcellular localisation of GCGR in the presence and absence of RAMP2 was investigated using confocal microscopy, trafficking assays and radioligand binding assays in human embryonic kidney (HEK293T) and human hepatoma (Huh7) cells. Mouse embryonic fibroblasts (MEFs) lacking Wiskott Aldrich Syndrome protein and scar homologue (WASH) complex were used to investigate the effect of a halt in recycling of internalised proteins on GCGR signalling in the absence of RAMP2. NanoBiT complementation and cyclic AMP assays were used to study the functional effect of RAMP2 on recruitment and activation of GCGR signalling mediators. Response to hepatic RAMP2 up-regulation in lean and obese adult mice using a bespoke adeno-associated viral vector was also studied. ResultsGCGR is predominantly localised at the plasma membrane in the absence of RAMP2 and exhibits remarkably slow internalisation in response to agonist stimulation. Rapid intracellular retention of glucagon-stimulated GCGR in cells lacking WASH complex indicates that activated GCGRs undergo continuous cycles of internalisation and recycling despite apparent GCGR plasma membrane localisation up to 40 minutes post-stimulation. Co-expression of RAMP2 induces GCGR internalisation both basally and in response to agonist-stimulation. The intracellular retention of GCGR in the presence of RAMP2 confers a bias away from {beta}-arrestin-2 recruitment coupled to increased activation of Gs proteins at endosomes. This is associated with increased short-term efficacy for glucagon-stimulated cAMP production, although long-term signalling is dampened by increased receptor lysosomal targeting for degradation. Despite these signalling effects, only minor disturbance of carbohydrate metabolism was observed in mice with up-regulated hepatic RAMP2. ConclusionsBy retaining GCGR intracellularly, RAMP2 alters the spatiotemporal pattern of GCGR signalling. Further exploration of the effects of RAMP2 on GCGR in vivo is warranted. Graphical abstractIcons sourced from [1] O_FIG_DISPLAY_L [Figure 1] M_FIG_DISPLAY C_FIG_DISPLAY

cell biology↗

bioRxiv2021

Evaluation of efficacy- versus affinity-driven agonism with biased GLP-1R ligands P5 and exendin-F1

The glucagon-like peptide-1 receptor (GLP-1R) is an important regulator of glucose homeostasis and has been successfully targeted for the treatment of type 2 diabetes. Recently described biased GLP-1R agonists with selective reductions in {beta}-arrestin versus G protein coupling show improved metabolic actions in vivo. However, two prototypical G protein-favouring GLP-1R agonists, P5 and exendin-F1, are reported to show divergent effects on insulin secretion. In this study we aimed to resolve this discrepancy by performing a side-by-side characterisation of these two ligands across a variety of in vitro and in vivo assays. Exendin-F1 showed reduced acute efficacy versus P5 for several readouts, including recruitment of mini-G proteins, G protein-coupled receptor kinases (GRKs) and {beta}-arrestin-2. Maximal responses were also lower for both GLP-1R internalisation and the presence of active GLP-1R-mini-Gs complexes in early endosomes with exendin-F1 treatment. In contrast, prolonged insulin secretion in vitro and sustained anti-hyperglycaemic efficacy in mice were both greater with exendin-F1 than with P5. We conclude that the particularly low acute efficacy of exendin-F1 and associated reductions in GLP-1R downregulation appear to be more important than preservation of endosomal signalling to allow sustained insulin secretion responses. This has implications for the ongoing development of affinity- versus efficacy-driven biased GLP-1R agonists as treatments for metabolic disease.

pharmacology and toxicology↗

bioRxiv2021

Acylation of the incretin peptide exendin-4 directly impacts GLP-1 receptor signalling and trafficking

The glucagon-like peptide-1 receptor (GLP-1R) is a class B G protein-coupled receptor and mainstay therapeutic target for the treatment of type 2 diabetes and obesity. Recent reports have highlighted how biased agonism at the GLP-1R affects sustained glucose-stimulated insulin secretion through avoidance of desensitisation and downregulation. A number of GLP-1R agonists (GLP-1RAs) feature a fatty acid moiety to promote albumin binding in order to prolong their pharmacokinetics, but the potential for these ligand changes to influence GLP-1R signalling has rarely been investigated beyond potency assessments for cyclic adenosine monophosphate (cAMP). In this work we directly compare the prototypical GLP-1RA exendin-4 with its C-terminally acylated analogue, exendin-4-C16, for their relative propensities to recruit and activate G proteins and {beta}-arrestins, endocytic and post-endocytic trafficking profiles, and interactions with model and cellular membranes. Both ligands had similar cAMP potency but the exendin-4-C16 showed [~]2.5-fold bias towards G protein recruitment and a [~]60% reduction in {beta}-arrestin-2 recruitment efficacy compared to exendin-4, as well as reduced GLP-1R endocytosis and preferential targeting towards recycling pathways. These effects were associated with a reduced ability to promote the movement of the GLP-1R extracellular domain, as determined using a conformational biosensor approach, and a [~]70% increase in insulin secretion. Interactions with plasma membrane lipids were enhanced by the acyl chain. Exendin-4-C16 showed extensive albumin binding and was highly effective for lowering of blood glucose in mice over at least 72 hours. Overall, our study highlights the importance of a broad approach to the evaluation of GLP-1RA pharmacology. Significance statementAcylation is a common strategy to enhance the pharmacokinetics of peptide-based drugs. Our work shows how acylation can also affect various other pharmacological parameters, including biased agonism, receptor trafficking and interactions with the plasma membrane, which may be therapeutically important.

pharmacology and toxicology↗