Expression of glucose-dependent insulinotropic polypeptide in the zebrafish

Molecular Characterization of Grass Carp GIPR and Effect of Nutrition States, Insulin, and Glucagon on Its Expression

GIP plays an important regulatory role in glucose and lipid metabolism. As the specific receptor, GIPR is involved in this physiological process. To assess the roles of GIPR in teleost, the GIPR gene was cloned from grass carp. The ORF of cloned GIPR gene was 1560 bp, encoding 519 amino acids. The grass carp GIPR was the G-protein-coupled receptor which contains seven predicted transmembrane domains. In addition, two predicted glycosylation sites were contained in the grass carp GIPR. The grass carp GIPR expression is in multiple tissues and is highly expressed in the kidney, brain regions, and visceral fat tissue. In the OGTT experiment, the GIPR expression is markedly decreased in the kidney, visceral fat, and brain by treatment with glucose for 1 and 3 h. In the fast and refeeding experiment, the GIPR expression in the kidney and visceral fat tissue was significantly induced in the fast groups. In addition, the GIPR expression levels were markedly decreased in the refeeding groups. In the present study, the visceral fat accumulation of grass carp was induced by overfed. The GIPR expression was significantly decreased in the brain, kidney, and visceral fat tissue of overfed grass carp. In primary hepatocytes, the GIPR expression was promoted by treatment with oleic acid and insulin. The GIPR mRNA levels were significantly reduced by treatment with glucose and glucagon in the grass carp primary hepatocytes. To our knowledge, this is the first time the biological role of GIPR is unveiled in teleost.

In vivo drug discovery for increasing incretin-expressing cells identifies DYRK inhibitors that reinforce the enteroendocrine system

Analogs of the incretin hormones Gip and Glp-1 are used to treat type 2 diabetes and obesity. Findings in experimental models suggest that manipulating several hormones simultaneously may be more effective. To identify small molecules that increase the number of incretin-expressing cells, we established a high-throughput in vivo chemical screen by using the gip promoter to drive the expression of luciferase in zebrafish. All hits increased the numbers of neurogenin 3-expressing enteroendocrine progenitors, Gip-expressing K-cells, and Glp-1-expressing L-cells. One of the hits, a dual-specificity tyrosine phosphorylation-regulated kinase (DYRK) inhibitor, additionally decreased glucose levels in both larval and juvenile fish. Knock-down experiments indicated that nfatc4, a downstream mediator of DYRKs, regulates incretin+ cell number in zebrafish, and that Dyrk1b regulates Glp-1 expression in an enteroendocrine cell line. DYRK inhibition also increased the number of incretin-expressing cells in diabetic mice, suggesting a conserved reinforcement of the enteroendocrine system, with possible implications for diabetes.

Glucose-Dependent Insulinotropic Polypeptide Suppresses Foam Cell Formation of Macrophages through Inhibition of the Cyclin-Dependent Kinase 5-CD36 Pathway

Glucose-dependent insulinotropic polypeptide (GIP) has been reported to have an atheroprotective property in animal models. However, the effect of GIP on macrophage foam cell formation, a crucial step of atherosclerosis, remains largely unknown. We investigated the effects of GIP on foam cell formation of, and CD36 expression in, macrophages extracted from GIP receptor-deficient (Gipr^−/−) and Gipr^+/+ mice and cultured human U937 macrophages by using an agonist for GIP receptor, [D-Ala²]GIP(1–42). Foam cell formation evaluated by esterification of free cholesterol to cholesteryl ester and CD36 gene expression in macrophages isolated from Gipr^+/+ mice infused subcutaneously with [D-Ala²]GIP(1–42) were significantly suppressed compared with vehicle-treated mice, while these beneficial effects were not observed in macrophages isolated from Gipr^−/− mice infused with [D-Ala²]GIP(1–42). When macrophages were isolated from Gipr^+/+ and Gipr^−/− mice, and then exposed to [D-Ala²]GIP(1–42), similar results were obtained. [D-Ala²]GIP(1–42) attenuated ox-LDL uptake of, and CD36 gene expression in, human U937 macrophages as well. Gene expression level of cyclin-dependent kinase 5 (Cdk5) was also suppressed by [D-Ala²]GIP(1–42) in U937 cells, which was corelated with that of CD36. A selective inhibitor of Cdk5, (R)-DRF053 mimicked the effects of [D-Ala²]GIP(1–42) in U937 cells. The present study suggests that GIP could inhibit foam cell formation of macrophages by suppressing the Cdk5-CD36 pathway via GIP receptor.

Molecular evolution of GIP and Exendin and their receptors

Glucose-dependent insulinotropic polypeptide (GIP) is a product of the Gip gene and acts as an incretin hormone in mammals. Gip is most closely related to the proglucagon (Gcg) and Exendin genes and diverged from these very early in vertebrate evolution. In mammals, GIP acts through its specific receptor, encoded by the Gipr gene, which belongs to a subfamily of 7-transmembrane G-protein coupled receptor (GPCR) genes that also includes those for the proglucagon-derived peptides (Gcgr, Glp1r, and Glp2r), and the receptor for Exendin (Grlr). Gip, Gipr, Exendin, and Grlr genes are found in species from most vertebrate classes. While most species that have a Gip gene also have a Gipr gene, two classes of vertebrates, cartilaginous fish and birds, retain conserved Gip genes but lack Gipr genes. This raises the possibility the GIP signals through other receptors in some vertebrates. Exendin genes and the gene for its receptor, Grlr, are also found in diverse vertebrates, with the notable exception of mammals. Both GIP and Exendin likely have important roles in vertebrate physiology, but their roles are either dispensable or can be replaced by other hormones.

Zebrafish models of diabetes-related CNS pathogenesis

Diabetes mellitus (DM) is a common metabolic disorder that affects multiple organ systems. DM also affects brain processes, contributing to various CNS disorders, including depression, anxiety and Alzheimer's disease. Despite active research in humans, rodent models and in-vitro systems, the pathogenetic link between DM and brain disorders remains poorly understood. Novel translational models and new model organisms are therefore essential to more fully study the impact of DM on CNS. The zebrafish (Danio rerio) is a powerful novel model species to study metabolic and CNS disorders. Here, we discuss how DM alters brain functions and behavior in zebrafish, and summarize their translational relevance to studying DM-related CNS pathogenesis in humans. We recognize the growing utility of zebrafish models in translational DM research, as they continue to improve our understanding of different brain pathologies associated with DM, and may foster the discovery of drugs that prevent or treat these diseases.

Evolution of the glucagon-like system across fish

In fishes, including the jawless lampreys, the most ancient lineage of extant vertebrates, plasma glucose levels are highly variable and regulation is more relaxed than in mammals. The regulation of glucose and lipid in fishes in common with mammals involves members of the glucagon (GCG)-like family of gastrointestinal peptides. In mammals, four peptides GCG, glucagon-like peptide 1 and 2 (GLP1 and GLP2) and glucose-dependent insulinotropic peptide (GIP) that activate four specific receptors exist. However, in lamprey and other fishes the glucagon-like family evolved differently and they retained additional gene family members (glucagon-related peptide, gcrp and its receptor, gcrpr) that are absent from mammals. In the present study, we analysed the evolution of the glucagon-like system in fish and characterized gene expression of the family members in the European sea bass (Dicentrarchus labrax) a teleost fish. Phylogenetic analysis revealed that multiple receptors and peptides of the glucagon-like family emerged early during the vertebrate radiation and evolved via lineage specific events. Synteny analysis suggested that family member gene loss is likely to be the result of a single gene deletion event. Lamprey was the only fish where a putative glp1r persisted and the presence of the receptor gene in the genomes of the elephant shark and coelacanth remains unresolved. In the coelacanth and elephant shark, unique proglucagon genes were acquired which in the former only encoded Gcg and Glp2 and in the latter, shared a similar structure to the teleost proglucagon gene but possessed an extra exon coding for Glp-like peptide that was most similar to Glp2. The variable tissue distribution of the gene transcripts encoding the ligands and receptors of the glucagon-like system in an advanced teleost, the European sea bass, suggested that, as occurs in mammals, they have acquired distinct functions. Statistically significant (p < .05) down-regulation of teleost proglucagon a in sea bass with modified plasma glucose levels confirmed the link between these peptides and metabolism. The tissue distribution of members of the glucagon-like system in sea bass and human suggests that evolution of the brain-gut-peptide regulatory loop diverged between teleosts and mammals despite the overall conservation and similarity of glucagon-like family members.

Evaluation of the insulinotropic and glucose-lowering actions of zebrafish GIP in mammalian systems: Evidence for involvement of the GLP-1 receptor

The insulinotropic properties of zebrafish GIP (zfGIP) were assessed in vitro using clonal pancreatic β-cell lines and isolated mouse islets and acute effects on glucose tolerance and insulin release in vivo were evaluated in mice. The peptide produced a dose-dependent increase in the rate of insulin release from BRIN-BD11 rat clonal β-cells at concentrations ≥30nM. Insulin release from 1.1 B4 human clonal β-cells and mouse islets was significantly increased by zfGIP (10nM and 1μM). The in vitro insulinotropic activity of zfGIP was decreased after incubating BRIN-BD11 cells with the GLP-1 receptor antagonist, exendin-4(9-39) (p<0.001) and the GIP receptor antagonist, GIP (6-30) Cex-K⁴⁰[Pal] (p<0.05) but the glucagon receptor antagonist [des-His¹,Pro⁴,Glu⁹]glucagon amide was without effect. zfGIP (10nM and 1μM) produced significant increases in cAMP concentration in CHL cells transfected with the human GLP-1 receptor but was without effect on HEK293 cells transfected with the human glucagon receptor. Conversely, zfGIP, but not human GIP, significantly stimulated insulin release from CRISPR/Cas9-engineered INS-1 clonal β-cells from which the GIP receptor had been deleted. Intraperitoneal administration of zfGIP (25 and 75nmol/kg body weight) to mice together with an intraperitoneal glucose load (18mmol/kg body weight) produced a significant decrease in plasma glucose concentrations concomitant with an increase in insulin concentrations. The study provides evidence that the insulinotropic action of zfGIP in mammalian systems involves activation of both the GLP-1 and the GIP receptors but not the glucagon receptor.

In vivo monitoring of intracellular Ca2+ dynamics in the pancreatic β-cells of zebrafish embryos

Assessing the response of pancreatic islet cells to glucose stimulation is important for understanding β-cell function. Zebrafish are a promising model for studies of metabolism in general, including stimulus-secretion coupling in the pancreas. We used transgenic zebrafish embryos expressing a genetically-encoded Ca²⁺ sensor in pancreatic β-cells to monitor a key step in glucose induced insulin secretion; the elevations of intracellular [Ca²⁺]_i. In vivo and ex vivo analyses of [Ca²⁺]_i demonstrate that β-cell responsiveness to glucose is well established in late embryogenesis and that embryonic β-cells also respond to free fatty acid and amino acid challenges. In vivo imaging of whole embryos further shows that indirect glucose administration, for example by yolk injection, results in a slow and asynchronous induction of β-cell [Ca²⁺]_i responses, while intravenous glucose injections cause immediate and islet-wide synchronized [Ca²⁺]_i fluctuations. Finally, we demonstrate that embryos with disrupted mutation of the Ca_V1.2 channel gene cacna1c are hyperglycemic and that this phenotype is associated with glucose-independent [Ca²⁺]_i fluctuation in β-cells. The data reveal a novel central role of cacna1c in β-cell specific stimulus-secretion coupling in zebrafish and demonstrate that the novel approach we propose - to monitor the [Ca²⁺]_i dynamics in embryonic β-cells in vivo - will help to expand the understanding of β-cell physiological functions in healthy and diseased states.

Obesity and the gastrointestinal tract: you are what you eat

Obesity represents a complex multifactorial syndrome that develops from interactions among genetic and environmental factors and is a leading cause of illness and death. The prevalence of obesity in the United States has increased dramatically since 1975. Although often ignored, the gastrointestinal tract, and the gastrointestinal regulatory peptides in particular, constitutes an ideal starting point for defining and investigating obesity as it represents the route by which all nutrients are ingested, processed, and absorbed. Another important factor to consider when evaluating the etiology of obesity is the capacity for all animals to store nutrients. Insulin is the most potent anabolic hormone, and it appears to have evolved from the need to maximize energy efficiency, obviating the requirement to continuously forage for food. Organisms expressing this important peptide possessed a distinct survival advantage and flourished. During the course of evolution, insulin biosynthesis translocated from the intestine to pancreatic islets, which necessitated a messenger from the intestine to complete the "enteroinsular axis." The eventual development of glucose-dependent insulinotropic polypeptide (GIP) and other incretins fulfilled this requirement. GIP appears to offer an additional survival benefit by not only stimulating intestinal glucose transport and maximally releasing insulin to facilitate nutrient storage but also by its insulin-mimetic properties, including enhanced uptake of glucose by adipocytes. This physiological redundancy offered by insulin and GIP ensured the survival of organisms during times when food was scarce. As food is no longer scarce, at least in the West, this survival advantage appears to have contributed to the current obesity epidemic.

The bHLH transcription factor Ascl1a is essential for the specification of the intestinal secretory cells and mediates Notch signaling in the zebrafish intestine

Notch signaling has a fundamental role in stem cell maintenance and in cell fate choice in the intestine of different species. Canonically, Notch signaling represses the expression of transcription factors of the achaete-scute like (ASCL) or atonal related protein (ARP) families. Identifying the ARP/ASCL genes expressed in the gastrointestinal tract is essential to build the regulatory cascade controlling the differentiation of gastrointestinal progenitors into the different intestinal cell types. The expression of the ARP/ASCL factors was analyzed in zebrafish to identify, among all the ARP/ASCL factors found in the zebrafish genome, those expressed in the gastrointestinal tract. ascl1a was found to be the earliest factor detected in the intestine. Loss-of-function analyses using the pia/ascl1a mutant, revealed that ascl1a is crucial for the differentiation of all secretory cells. Furthermore, we identify a battery of transcription factors expressed during secretory cell differentiation and downstream of ascl1a. Finally, we show that the repression of secretory cell fate by Notch signaling is mediated by the inhibition of ascl1a expression. In conclusion, this work identifies Ascl1a as a key regulator of the secretory cell lineage in the zebrafish intestine, playing the same role as Atoh1 in the mouse intestine. This highlights the diversity in the ARP/ASCL family members acting as cell fate determinants downstream from Notch signaling.

Discovery of a novel glucagon-like peptide (GCGL) and its receptor (GCGLR) in chickens: evidence for the existence of GCGL and GCGLR genes in nonmammalian vertebrates

Glucagon (GCG), glucagon-related peptides, and their receptors have been reported to play important roles including the regulation of glucose homeostasis, gastrointestinal activity, and food intake in vertebrates. In this study, we identified genes encoding a novel glucagon-like peptide (named GCGL) and its receptor (GCGLR) from adult chicken brain using RACE and/or RT-PCR. GCGL was predicted to encode a peptide of 29 amino acids (cGCGL(1-29)), which shares high amino acid sequence identity with mammalian and chicken GCG (62-66%). GCGLR is a receptor of 430 amino acids and shares relatively high amino acid sequence identity (53-55%) with the vertebrate GCG receptor (GCGR). Using a pGL3-CRE-luciferase reporter system, we demonstrated that synthetic cGCGL(1-29), but not its structurally related peptides, i.e. exendin-4 and GCG, could potently activate GCGLR (EC(50): 0.10 nm) expressed in Chinese hamster ovary cells, indicating that GCGLR can function as a GCGL-specific receptor. RT-PCR assay revealed that GCGL expression is mainly restricted to several tissues including various brain regions, spinal cord, and testes, whereas GCGLR mRNA is widely expressed in adult chicken tissues with abundant expression noted in the pituitary, spinal cord, and various brain regions. Using synteny analysis, GCGL and GCGLR genes were also identified in the genomes of fugu, tetraodon, tilapia, medaka, coelacanth, and Xenopus tropicalis. As a whole, the discovery of GCGL and GCGLR genes in chickens and other nonmammalian vertebrates clearly indicates a previously unidentified role of GCGL-GCGLR in nonmammalian vertebrates and provides important clues to the evolutionary history of GCG and GCGL genes in vertebrates.

Glucose metabolism in fish: a review

Teleost fishes represent a highly diverse group consisting of more than 20,000 species living across all aquatic environments. This group has significant economical, societal and environmental impacts, yet research efforts have concentrated primarily on salmonid and cyprinid species. This review examines carbohydrate/glucose metabolism and its regulation in these model species including the role of hormones and diet. Over the past decade, molecular tools have been used to address some of the downstream components of these processes and these are incorporated to better understand the roles played by carbohydrates and their regulatory paths. Glucose metabolism remains a contentious area as many fish species are traditionally considered glucose intolerant and, therefore, one might expect that the use and storage of glucose would be considered of minor importance. However, the actual picture is not so clear since the apparent intolerance of fish to carbohydrates is not evident in herbivorous and omnivorous species and even in carnivorous species, glucose is important for specific tissues and/or for specific activities. Thus, our aim is to up-date carbohydrate metabolism in fish, placing it to the context of these new experimental tools and its relationship to dietary intake. Finally, we suggest that new research directions ultimately will lead to a better understanding of these processes.

Evolutionary expression of glucose-dependent-insulinotropic polypeptide (GIP)

Glucose-dependent insulinotropic polypeptide (GIP) is a mammalian incretin hormone released into the circulation following nutrient ingestion. We examined the functional evolution of GIP and its relationship with insulin to delineate their respective roles in promoting nutrient efficiency. Expression patterns were examined in the sea lamprey (Petromyzon marinus), a basal vertebrate lacking a distinct pancreas, and in the zebrafish, Xenopus laevis, chicken, and mouse, organisms possessing extraintestinal pancreata. Although sea lamprey genomic analysis predicted a potential GIP-like gene, transcripts were not detected, and insulin expression was confined to the caudal pancreatic bud. GIP was detected in both the intestine and pancreas of the zebrafish and X. laevis. In contrast, GIP and insulin expression were limited to the intestine and pancreas, respectively, in chicken and mouse. Phylogenetic analysis of the glucagon-like ligands suggested proglucagon as the common ancestor, supporting the theory that GIP arose as a gene duplication of proglucagon. Insulin-secreting cells in the sea lamprey intestine may have obviated the need for an enteroinsular axis, and zebrafish may represent an evolutionary transition where GIP does not yet function as an incretin hormone. These observations are consistent with the hypothesis that GIP and insulin influence survival advantage by enhancing the efficiency of nutrient absorption and energy storage.

Evolutionary conservation of glucose-dependent insulinotropic polypeptide (GIP) gene regulation and the enteroinsular axis

Glucose-dependent insulinotropic polypeptide (GIP), an important component of the enteroinsular axis, is a potent stimulator of insulin secretion, functioning to maintain nutrient efficiency. Although well-characterized in mammals, little is known regarding GIP transcriptional regulation in Danio rerio (Dr). We previously demonstrated that DrGIP is expressed in the intestine and the pancreas, and we therefore cloned the Dr promoter to compare GIP transcriptional regulation in Dr and mammals. Although no significant homology was indentified between the highly conserved mammalian promoter and the DrGIP promoter, 1072-bp of the DrGIP promoter conferred tissue-specific expression in mammalian cell lines. Deletional analysis of the DrGIP promoter identified two regions that, when deleted, reduced transcription by 75% and 95%, respectively. Mutational analysis of the upstream region suggested involvement of an Nkx binding site, although we were unable to identify the factor binding to this site. The cis element in the downstream region was found to be a GATA binding site. Lastly, overexpression and shRNA experiments identified PAX4 as a potential repressor of DrGIP expression. These findings provide evidence that despite the identification of species-specific transcriptional regulators and differences in GIP expression patterns between D. rerio and mammals, a moderate degree of regulatory conservation appears to exist.

Differential processing of pro-glucose-dependent insulinotropic polypeptide in gut

Glucose-dependent insulinotropic polypeptide (GIP) is a hormone released from enteroendocrine K cells in response to meals. Posttranslational processing of the precursor protein pro-GIP at residue 65 by proprotein convertase subtilisin/kexin type 1 (PC1/3) in gut K cells gives rise to the established 42-amino-acid form of GIP (GIP(1-42)). However, the pro-GIP peptide sequence contains a consensus cleavage site for PC2 at residues 52-55 and we identified PC2 immunoreactivity in a subset of K cells, suggesting the potential existence of a COOH-terminal truncated GIP isoform, GIP(1-30). Indeed a subset of mouse and human K cells display GIP immunoreactivity with GIP antibodies directed to the mid portion of the peptide, but not with a COOH-terminal-directed GIP antibody, indicative of the presence of a truncated form of GIP. This population of cells represents approximately 5-15% of the total GIP-immunoreactive cells in mice, depending on the region of intestine, and is virtually absent in mice lacking PC2. Amidated GIP(1-30) and GIP(1-42) have comparable potency at stimulating somatostatin release in the perfused mouse stomach. Therefore, GIP(1-30) represents a naturally occurring, biologically active form of GIP.