Functional studies of a glucagon receptor isolated from frog Rana tigrina rugulosa: implications on the molecular evolution of glucagon receptors in vertebrates

Glucagon and Its Receptors in the Mammalian Heart

Glucagon exerts effects on the mammalian heart. These effects include alterations in the force of contraction, beating rate, and changes in the cardiac conduction system axis. The cardiac effects of glucagon vary according to species, region, age, and concomitant disease. Depending on the species and region studied, the contractile effects of glucagon can be robust, modest, or even absent. Glucagon is detected in the mammalian heart and might act with an autocrine or paracrine effect on the cardiac glucagon receptors. The glucagon levels in the blood and glucagon receptor levels in the heart can change with disease or simultaneous drug application. Glucagon might signal via the glucagon receptors but, albeit less potently, glucagon might also signal via glucagon-like-peptide-1-receptors (GLP1-receptors). Glucagon receptors signal in a species- and region-dependent fashion. Small molecules or antibodies act as antagonists to glucagon receptors, which may become an additional treatment option for diabetes mellitus. Hence, a novel review of the role of glucagon and the glucagon receptors in the mammalian heart, with an eye on the mouse and human heart, appears relevant. Mouse hearts are addressed here because they can be easily genetically modified to generate mice that may serve as models for better studying the human glucagon receptor.

Signal Transduction Mechanisms for Glucagon-Induced Somatolactin Secretion and Gene Expression in Nile Tilapia (Oreochromis niloticus) Pituitary Cells

Glucagon (GCG) plays a stimulatory role in pituitary hormone regulation, although previous studies have not defined the molecular mechanism whereby GCG affects pituitary hormone secretion. To this end, we identified two distinct proglucagons, Gcga and Gcgb, as well as GCG receptors, Gcgra and Gcgrb, in Nile tilapia (Oreochromis niloticus). Using the cAMP response element (CRE)-luciferase reporter system, tilapia GCGa and GCGb could reciprocally activate the two GCG receptors expressed in human embryonic kidney 293 (HEK293) cells. Quantitative real-time PCR analysis revealed that differential expression of the Gcga and Gcgb and their cognate receptors Gcgra and Gcgrb was found in the various tissues of tilapia. In particular, the Gcgrb is abundantly expressed in the neurointermediate lobe (NIL) of the pituitary gland. In primary cultures of tilapia NIL cells, GCGb effectively stimulated SL release, with parallel rises in the mRNA levels, and co-incubation with the GCG antagonist prevented GCGb-stimulated SL release. In parallel experiments, GCGb treatment dose-dependently enhanced intracellular cyclic adenosine monophosphate (cAMP) accumulation with increasing inositol 1,4,5-trisphosphate (IP3) concentration and the resulting in transient increases of Ca²⁺ signals in the primary NIL cell culture. Using selective pharmacological approaches, the adenylyl cyclase (AC)/cAMP/protein kinase A (PKA) and phospholipase C (PLC)/IP3/Ca²⁺/calmodulin (CaM)/CaMK-II pathways were shown to be involved in GCGb-induced SL release and mRNA expression. Together, these results provide evidence for the first time that GCGb can act at the pituitary level to stimulate SL release and gene expression via GCGRb through the activation of the AC/cAMP/PKA and PLC/IP3/Ca²⁺/CaM/CaMK-II cascades.

Structural Refinement of Glucagon for Therapeutic Use

Glucagon counters insulin's effects on glucose metabolism and serves as a rescue medicine in the treatment of hypoglycemia. Acute hypoglycemia, a common occurrence in insulin-dependent diabetes, is the central obstacle to correcting high blood glucose, a primary cause of long-term microvascular complications. As a result, there has been a resurgence of interest in improved glucagon therapy, including nonconventional liquid formulations, alternative routes of administration, and novel analogs with optimized biophysical properties. These options collectively minimize the complexity of glucagon delivery and enable its application in ways not feasible with conventional emergency rescue kits. These advances have indirectly promoted the integrated use of glucagon agonism with other hormones in a manner that runs counter to the long-standing pursuit of glucagon antagonism. This review summarizes novel approaches to glucagon optimization, methods with potential application to the broader family of therapeutic peptides, where biophysical challenges may be encountered.

Diversification of the functions of proglucagon and glucagon receptor genes in fish

The teleost fish-specific genome duplication gave rise to a great number of species inhabiting diverse environments with different access to nutrients and life histories. This event produced duplicated gcg genes, gcga and gcgb, for proglucagon-derived peptides, glucagon and GLP-1 and duplicated gcgr receptor genes, gcgra and gcgrb, which play key roles connecting the consumption of nutrients with glucose metabolism. We conducted a systematic survey of the genomes from 28 species of fish (24 bony (Superclass Osteichthyes), 1 lobe-finned (Class Sarcoperygii), 1 cartilaginous (Superclass Chondrichthyes), and 2 jawless (Superclass Agnatha)) and find that almost all surveyed ray-finned fish contain gcga and gcgb genes with different coding potential and duplicated gcgr genes, gcgra and gcgrb that form two separate clades in the phylogenetic tree consistent with the accepted species phylogeny. All gcgb genes encoded only glucagon and GLP-1 and gcga genes encoded glucagon, GLP-1, and GLP-2, indicating that gcga was subfunctionalized to produce GLP-2. We find a single glp2r, but no glp1r suggesting that duplicated gcgrb was neofunctionalized to bind GLP-1, as demonstrated for the zebrafish gcgrb (Oren et al., 2016). In functional experiments with zebrafish gcgrb and GLP-1 from diverse fish we find that anglerfish GLP-1a, encoded by gcga, is less biologically active than the gcgb anglerfish GLP-1b paralog. But some other fish (zebrafish, salmon, and catfish) gcga GLP-1a display similar biological activities, indicating that the regulation of glucose metabolism by GLP-1 in ray-finned fish is species-specific. Searches of genomes in cartilaginous fish identified a proglucagon gene that encodes a novel GLP-3 peptide in addition to glucagon, GLP-1, and GLP-2, as well as a single gcgr, glp2r, and a new glucagon receptor-like receptor whose identity still needs to be confirmed. The sequence of the shark GLP-1 contained an N-terminal mammalian-like extension that in mammals undergoes a proteolytic cleavage to release biologically active GLP-1. Our results indicate that early in vertebrate evolution diverse regulatory mechanisms emerged for the control of glucose metabolism by proglucagon-derived peptides and their receptors and that in ray-finned fish they included subfunctionalization and neofunctionalization of these genes.

Structural Mapping and Functional Characterization of Zebrafish Class B G-Protein Coupled Receptor (GPCR) with Dual Ligand Selectivity towards GLP-1 and Glucagon

GLP-1 and glucagon regulate glucose metabolism through a network of metabolic pathways initiated upon binding to their specific receptors that belong to class B G-protein coupled receptors (GPCRs). The therapeutic potential of glucagon is currently being evaluated, while GLP-1 is already used in the treatment of type 2 diabetes and obesity. Development of a second generation of GLP-1 based therapeutics depends on a molecular and structural understanding of the interactions between the GLP-1 receptor (GLP-1R) and its ligand GLP-1. There is considerable sequence conservation between GLP-1 and glucagon and between the hGLP-1R and human glucagon receptor (hGCGR), yet each receptor recognizes only its own specific ligand. Glucagon receptors in fish and frogs also exhibit ligand selectivity only towards glucagon and not GLP-1. Based on competitive binding experiments and assays of increase in intracellular cAMP, we demonstrate here that a GPCR in zebrafish (Danio rerio) exhibits dual ligand selectivity towards GLP-1 and glucagon, a characteristic not found in mammals. Further, many structural features found in hGLP-1R and hGCGR are also found in this zebrafish GPCR (zfGPCR). We show this by mapping of its sequence and structural features onto the hGLP-1R and hGCGR based on their partial and complementary crystal structures. Thus, we propose that zfGPCR represents a dual GLP-1R/GCGR. The main differences between the three receptors are in their stalk regions that connect their N-terminal extracellular domains (NECDs) with their transmembrane domains and the absence of loop 3 in the NECD in zfGLP-1R/GCGR. These observations suggest that the interactions between GLP-1 and glucagon with loop 3 and the stalk regions may induce different conformational changes in hGLP-1R and hGCGR upon ligand binding and activation that lead to selective recognition of their native ligands.

Evolution of receptors for peptides similar to glucagon

The genes encoding the peptide precursors for glucagon (GCG), glucose-dependent insulinotropic peptide (GIP), and ortholog of exendin belong to the same family as shown by sequence similarity. The peptides similar to glucagon encoded by these genes signal through a closely related subfamily of G-protein coupled receptors. A total of five types of genes for receptors for these peptides have been identified, three for the products of GCG (GCGR, GLP1R, and GLP2R) and one each for the products of GIP (GIPR) and the ortholog of exendin (Grlr). Phylogenetic and genomic neighborhood analyses clearly show that these genes originated very early in vertebrate evolution and all were present in the common ancestor of tetrapods and bony fish. Despite their ancient origins, some of these genes are dispensable, with the Glp1r, Gipr, and Grlr being lost on the lineages leading to bony fish, birds, and mammals, respectively. The loss of the genes for these receptors may have been driving forces in the evolution of new functions for these peptides similar to glucagon.

Incretin hormones and the expanding families of glucagon-like sequences and their receptors

Peptide hormones encoded by the proglucagon (Gcg) and glucose-dependent insulinotropic polypeptide (Gip) genes are evolutionarily related glucagon-like sequences and act through a subfamily of G-protein-coupled receptors. A better understanding of the evolutionary history of these hormones and receptors should yield insight into their biological functions. The availability of a large number of near-complete vertebrate genome sequences is a powerful resource to address questions concerning the evolution of sequences; here, we utilize these resources to examine the evolution of glucagon-like sequences and their receptors. These studies led to the discovery of novel genes for a glucagon receptor-like receptor (Grlr) and a glucagon-like sequence (exendin) in vertebrates. Both exendin and GRLR have ancient origins, early in vertebrate evolution, but have been lost on the ancestral lineage leading to extant mammals. We also show that exendin and GRLR are both expressed in the brain of the chicken and Xenopus tropicals, results that suggest that the products of these genes function in this tissue. The lack of exendin or Grlr genes in mammals suggests that other genes may have acquired the functions of exendin and Grlr during mammalian evolution.

Insights into the evolution of proglucagon-derived peptides and receptors in fish and amphibians

Glucagon and the glucagon-like peptides (GLP-1 and GLP-2) share a common evolutionary origin and are triplication products of an ancestral glucagon exon. In mammals, a standard scenario is found where only a single proglucagon-derived peptide set exists. However, fish and amphibians have either multiple proglucagon genes or exons that are likely resultant of duplication events. Through phylogenetic analysis and examination of their respective functions, the proglucagon ligand-receptor pairs are believed to have evolved independently before acquiring specificity for one another. This review will provide a comprehensive overview of current knowledge of proglucagon-derived peptides and receptors, with particular focus on fish and amphibian species.

Evolution of genes for incretin hormones and their receptors

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are essential components in the regulation of blood glucose levels in mammals. These two incretins are produced by evolutionarily related genes and these hormones show similarity in sequence as both are glucagon-like sequences. Genes for these hormones have been identified in a number of diverse vertebrate species indicating that they originated prior to the earliest divergences of vertebrate species. However, analysis of functional and sequence data suggest that each of these hormones acquired incretin activity independently, and only since the divergence of tetrapods from fish. Not only are the hormones related, but so are their receptors. Like the hormones, the incretin action of the receptors is not a product of a shared common ancestral history, as the receptors for GLP-1 and GIP are not most closely related. Further study of the physiological functions of GLP-1 and GIP in additional vertebrates is required to better understand the origin of incretin action.

Evolution of new hormone function: loss and gain of a receptor

The vertebrate proglucagon gene encodes three glucagon-like sequences (glucagon, glucagon-like peptide-1 [GLP-1], and glucagon-like peptide 2 [GLP-2]) that have distinct functions in regulating metabolism in mammals. In contrast, glucagon and GLP-1 have similar physiological actions in fish, that of mammalian glucagon. We have identified sequences similar to receptors for proglucagon-derived peptides from the genomes of two fish (pufferfish and zebrafish), a frog (Xenopus tropicalis), and a bird (chicken). Phylogenetic analysis of the receptor sequences suggested an explanation for the divergent function of GLP-1 in fish and mammals. The phylogeny of our predicted and characterized receptors for proglucagon-derived peptides demonstrate that receptors for glucagon, GLP-1, and GLP-2 have an origin before the divergence of fish and mammals; however, fish have lost the gene encoding the GLP-1 class of receptors, and likely the incretin action of GLP-1. Receptors that bind GLP-1, but yield glucagon-like action, have been characterized in goldfish and zebrafish, and these sequences are most closely related to glucagon receptors. Both pufferfish and zebrafish have a second glucagon receptor-like gene that is most closely related to the characterized goldfish glucagon receptor. The phylogeny of glucagon receptor-like genes in fish indicates that a duplication of the glucagon receptor gene occurred on the ancestral fish lineage, and could explain the shared action of glucagon and GLP-1. We suggest that the binding specificity of one of the duplicated glucagon receptors has diverged, yielding receptors for GLP-1 and glucagon, but that ancestral downstream signaling has been maintained, resulting in both receptors retaining glucagon-stimulated downstream effects.

Localization and regulation of glucagon receptors in the chick eye and preproglucagon and glucagon receptor expression in the mouse eye

Myopia is a condition in which the eye is too long for the focal length of cornea and lens. Analysis of the messengers that are released by the retina to control axial eye growth in the animal model of the chicken revealed that glucagon-immunoreactive amacrine cells are involved in the retinal image processing that controls the growth of the sclera. It was found that the amount of retinal glucagon mRNA increased during treatment with positive lenses and pharmacological studies supported the idea that glucagon may act as a stop signal for eye growth. Glucagon exerts its regulatory effects by binding to a single type of glucagon receptor. In this study, we have sequenced the chicken glucagon receptor and compared its DNA and amino acid sequence with the human and mouse homologues. After sequencing about 80% of the receptor, we found a homology between 79.4 and 75.6% on cDNA level. At the protein level, about 73% of the amino acids were identical. Moreover, the cellular localization and regulation of the glucagon receptor in the chick retina was studied. In situ hybridization studies showed that many cells in the ganglion cell layer and inner nuclear layer, and some cells in the outer nuclear layer, express the receptor mRNA. Injection of the glucagon agonist Lys17,18,Glu21-glucagon induced a down-regulation of glucagon receptor mRNA content. Since the mouse would be an attractive mammalian model to study the biochemical and genetic basis of myopia, and because recent studies have demonstrated that form deprivation myopia can be induced, the expression of preproglucagon and glucagon receptor genes were also studied in the mouse retina and were found to be expressed.

Identification and characterization of a glucagon receptor from the goldfish Carassius auratus: implications for the evolution of the ligand specificity of glucagon receptors in vertebrates

The structural basis of ligand selectivity of G protein-coupled receptors for metabolic hormones has been an area of intense investigation, and yet it remains unresolved. One approach to delineating the mechanism of ligand-receptor interactions is to compare the ligand specificities of receptors expressed in species that emerged at different times within vertebrate evolution. In this paper we describe the isolation, functional, and phylogenetic characterization of the glucagon receptor from the goldfish Carassius auratus (Teleostei, order Cypriniformes), and compare its ligand specificity with that of the homologous rat receptor. Goldfish (gf) glucagon stimulated glucose production in a dose-dependent manner from isolated goldfish hepatocytes, resulting in 5-fold increase at 1 microm. The goldfish glucagon receptor (gfGlucR) shares 56, 51, 50, and 52% amino acid identities with frog Rana tigrina regulosa, mouse, rat, and human glucagon receptors, respectively. In competitive binding experiments, the recombinant gfGlucR displays high affinity toward goldfish, zebrafish, and human glucagons (IC(50) = 0.6, 9, and 13 nm, respectively) but not toward goldfish glucagon-like peptide-1 or human glucagon-like peptide-1 (7-36) amide. Whereas both goldfish and human glucagons stimulated dose-dependent increases in intracellular cAMP through the recombinant gfGlucR, the recombinant rat GlucR interacted only with human glucagon, analogous to the specificity of the previously characterized glucagon receptor from the frog R. tigrina regulosa. Our results demonstrate that the binding pocket of gfGlucR can accommodate a broad range of glucagon structures and that in the frogs and mammals, there is a structural switch to a more restrictive conformation of glucagon receptors.

Molecular determinants of glucagon receptor signaling

A 29-amino acid polypeptide hormone, glucagon has been one of the most prolific models in the study of hormone action. The key biologic function of glucagon is to counterbalance the actions of insulin and maintain a normal level of serum glucose. Diabetes mellitus can thus be considered a bihormonal disorder with an excess of glucagon contributing to the hyperglycemic state. The effects of glucagon are mediated by the glucagon receptor, which is itself a prototypical member of a distinct category called family B receptors within the G protein-coupled superfamily of seven-helical transmembrane receptors (GPCRs). At the structural level, the peptide ligands of family B receptors are highly homologous, in particular in the N-terminal region of the molecules. The mechanism by which highly homologous peptide ligands selectively recognize their receptors involves distinct molecular interactions that are gradually being elucidated. This review focuses on structural determinants of the glucagon receptor that are important for its activity with respect to interaction with its ligand and G proteins. Information about the glucagon receptor is presented within the context of what is known about other members of the family B GPCRs.

International Union of Pharmacology. XXXV. The glucagon receptor family

Peptide hormones within the secretin-glucagon family are expressed in endocrine cells of the pancreas and gastrointestinal epithelium and in specialized neurons in the brain, and subserve multiple biological functions, including regulation of growth, nutrient intake, and transit within the gut, and digestion, energy absorption, and energy assimilation. Glucagon, glucagon-like peptide-1, glucagon-like peptide-2, glucose-dependent insulinotropic peptide, growth hormone-releasing hormone and secretin are structurally related peptides that exert their actions through unique members of a structurally related G protein-coupled receptor class 2 family. This review discusses advances in our understanding of how these peptides exert their biological activities, with a focus on the biological actions and structural features of the cognate receptors. The receptors have been named after their parent and only physiologically relevant ligand, in line with the recommendations of the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR).

Identification of a proglucagon cDNA from Rana tigrina rugulosa that encodes two GLP-1s and that is alternatively spliced in a tissue-specific manner

Glucagon plays a pivotal role in the regulation of metabolism. A glucagon receptor has been previously characterized in the frog, Rana tigrina rugulosa, and the frog and human glucagon receptors have been shown to possess similar binding affinities toward human glucagon. To study the structural evolution of glucagon peptide and its receptor in vertebrates, in the current study, a proglucagon cDNA from the same frog species was cloned. Interestingly, in contrast to the mammalian proglucagons that contain only one GLP-1 peptide, the frog proglucagon cDNA encodes two GLP-1 peptides (GLP-1A and GLP-1B) in addition to a glucagon peptide and a glucagon-like peptide 2 (GLP-2). By reverse transcriptase-PCR (RT-PCR) analysis, the proglucagon gene expression was widely detected in the brain, colon, small intestine, liver, lung, and pancreas, suggesting that the proglucagon-derived peptides have diverse functions in frogs. Moreover, tissue-specific alternative mRNA splicing was observed in the brain, colon, and pancreas. In these tissues, proglucagon transcripts with a 135 bp in frame deletion encoding GLP-1A were found. This splicing event in R. tigrina rugulosa is novel because it deletes a GLP-1 encoding sequence instead of the GLP-2 observed in other vertebrates. These findings should enhance understanding of the proglucagon evolution, structure, and expression in vertebrates.

Molecular evolution of proglucagon

The vertebrate proglucagon gene encodes glucagon, and the two glucagon-like peptides GLP-1 and GLP-2. To better understand the origin and diversification of the distinct hormonal roles of the three glucagon-like sequences encoded by the proglucagon gene, we have examined the evolution of this gene. The structure of proglucagon has been largely maintained within vertebrates. Duplication of the proglucagon gene or duplications of sequences within the proglucagon gene are rare. All proglucagon gene duplications are likely to be the result of genome duplication events. Examination of the rates of amino acid sequence evolution of each hormone reveals that they have not evolved in a uniform manner. Each hormone has evolved in an episodic fashion, suggesting that the selective constraints acting upon the sequence vary between, and within, vertebrate classes. Changes in selection on a sequence often reflect changes in the function of the sequence, such as the change in function of GLP-1 from a glucagon-like hormone in fish to an incretin in mammals. We found that the GLP-2 sequence underwent rapid sequence evolution in the early mammal lineage, therefore we have concluded that mammalian GLP-2 has acquired a new biological function that is not found in other vertebrates. Comparisons of the hormone sequences show that many amino acid residues that are functionally important in mammalian hormones are not conserved through vertebrate evolution. This observation suggests that the sequences involved in hormone action change through evolution.

Evolution of receptors for proglucagon-derived peptides: isolation of frog glucagon receptors

The mammalian proglucagon gene encodes three glucagon-like sequences, glucagon, glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2). Each of these three functionally distinct proglucagon-derived peptides has a unique, but related, receptor. To better understand the origin of the unique physiological functions of each proglucagon-derived glucagon-like sequence we have cloned glucagon-like receptors from two species of frogs, Xenopus laevis and Rana pipiens. The cloned glucagon-like receptor sequences were found to be most closely related to glucagon receptors. To determine whether the evolutionary history of the receptors for proglucagon-derived peptides was the same as that inferred for the peptide hormones, we conducted a phylogenetic analysis using both parsimony and distance methods. We show that the evolutionary history of the receptors for glucagon-like sequences differ from the history of the glucagon-like sequences. The phylogeny of receptors for proglucagon-derived peptides is not monophyletic (i.e. they are not each other's closest relatives), as the receptor for the hormone glucose-dependent insulinotropic peptide (GIP) is more closely related to the glucagon receptor than either the GLP-1 or GLP-2 receptors. In contrast to the evolutionary origin of glucagon-like sequences, where glucagon is of most ancient origin, we found that the GLP-2 receptor has the most ancient origin. These observations suggest that the diversification of the glucagon-like sequences encoded by the proglucagon gene and of the receptors for these peptides occurred independently, and that either these hormones or their receptors have been recruited for new functions.

Characterization of an enhancer element in the proximal promoter of the mouse glucagon receptor gene

The 5'-flanking region of the mouse glucagon receptor has been previously cloned and two promoter regions were characterized. Functional analysis of the proximal promoter was now performed to characterize cis-acting element(s) regulating basal gene expression. Promoter analysis using deletion constructs in a rat cell line (CA-77) expressing the glucagon receptor, showed that the region from -64 to +127 relative to the proximal transcription start site was sufficient for maximal proximal promoter activity. A DNA sequence spanning the -28 to -16 region organized as an imperfect palindrome was demonstrated to be functional as a cis-acting enhancer. Constructs including several copies of this motif strongly increased activity of the heterologous thymidine kinase promoter. Gel mobility shift assays performed with different DNA fragments spanning this region confirmed that it specifically bound nuclear protein(s) from CA-77 cells, mouse MIN-6 cells or mouse liver. Mutations in the core sequence of this site impaired both reporter gene activity and nuclear protein binding. The palindrome is a novel DNA sequence with no homology to existing transcription factor binding site database. This is the first characterization of a functional cis-acting sequence into the proximal promoter of the mouse glucagon receptor that may support constitutive expression of the gene.