Purification and sequence of rat oxyntomodulin

Development of a selective, sensitive and robust oxyntomodulin dual monoclonal antibody immunoassay

Background & Aim: Oxyntomodulin (Oxm) is a proglucagon-derived peptide agonist of both the GLP-1 and glucagon receptors and is a key regulator of gastric acid secretion and energy expenditure. Differential processing from proglucagon hinders assay immunoassay selectivity. Method & results: Antibody engineering was used to develop a sandwich immunoassay that selectively measures endogenous Oxm. The pre- and postprandial levels of Oxm from 19 healthy individuals over the course of 2 h were measured. Postprandial increases in Oxm occurred within minutes and levels significantly correlated with those obtained using previously published mass spectrometry assays. Conclusion: This sandwich immunoassay is appropriately sensitive and selective and is also amenable to high-throughput application for the reliable determination of endogenous levels of intact Oxm from human samples.

Hyperphagia and increased fat accumulation in two models of chronic CNS glucagon-like peptide-1 loss of function

Central administration of glucagon-like peptide-1 (GLP-1) causes a dose-dependent reduction in food intake, but the role of endogenous CNS GLP-1 in the regulation of energy balance remains unclear. Here, we tested the hypothesis that CNS GLP-1 activity is required for normal energy balance by using two independent methods to achieve chronic CNS GLP-1 loss of function in rats. Specifically, lentiviral-mediated expression of RNA interference was used to knock down nucleus of the solitary tract (NTS) preproglucagon (PPG), and chronic intracerebroventricular (ICV) infusion of the GLP-1 receptor (GLP-1r) antagonist exendin (9-39) (Ex9) was used to block CNS GLP-1r. NTS PPG knockdown caused hyperphagia and exacerbated high-fat diet (HFD)-induced fat accumulation and glucose intolerance. Moreover, in control virus-treated rats fed the HFD, NTS PPG expression levels correlated positively with fat mass. Chronic ICV Ex9 also caused hyperphagia; however, increased fat accumulation and glucose intolerance occurred regardless of diet. Collectively, these data provide the strongest evidence to date that CNS GLP-1 plays a physiologic role in the long-term regulation of energy balance. Moreover, they suggest that this role is distinct from that of circulating GLP-1 as a short-term satiation signal. Therefore, it may be possible to tailor GLP-1-based therapies for the prevention and/or treatment of obesity.

Role of glucocorticoids in tuning hindbrain stress integration

The nucleus of the solitary tract (NTS) is a critical integrative site for coordination of autonomic and endocrine stress responses. Stress-excitatory signals from the NTS are communicated by both catecholaminergic [norepinephrine (NE), epinephrine (E)] and noncatecholaminergic [e.g., glucagon-like peptide-1 (GLP-1)] neurons. Recent studies suggest that outputs of the NE/E and GLP-1 neurons of the NTS are selectively engaged during acute stress. This study was designed to test mechanisms of chronic stress integration in the paraventricular nucleus, focusing on the role of glucocorticoids. Our data indicate that chronic variable stress (CVS) causes downregulation of preproglucagon (GLP-1 precursor) mRNA in the NTS and reduction of GLP-1 innervation to the paraventricular nucleus of the hypothalamus. Glucocorticoids were necessary for preproglucagon (PPG) reduction in CVS animals and were sufficient to lower PPG mRNA in otherwise unstressed animals. The data are consistent with a glucocorticoid-mediated withdrawal of GLP-1 in key stress circuits. In contrast, expression of tyrosine hydroxylase mRNA, the rate-limiting enzyme in catecholamine synthesis, was increased by stress in a glucocorticoid-independent manner. These suggest differential roles of ascending catecholamine and GLP-1 systems in chronic stress, with withdrawal of GLP-1 involved in stress adaptation and enhanced NE/E capacity responsible for facilitation of responses to novel stress experiences.

Glucocorticoid regulation of preproglucagon transcription and RNA stability during stress

Stress elicits a synchronized response of the endocrine, sympathetic, and central nervous systems to preserve homeostasis and well-being. Glucagon-like peptide-1 (GLP-1), a primary posttranslational product of the preproglucagon (PPG) gene, activates both physical and psychological stress responses. The current study examined mechanisms regulating expression of PPG gene products in the hindbrain. Our results indicate that PPG mRNA decreases rapidly after exposure to acute stressors of multiple modalities. Reduced mRNA levels are accompanied by reduced GLP-1 immunoreactivity in the paraventricular nucleus of hypothalamus, suggesting release at PPG terminals. Stress-induced decrements in PPG mRNA were attenuated in adrenalectomized-corticosterone-replaced rats, suggesting that mRNA down-regulation is due at least in part to glucocorticoid secretion. In contrast, acute stress increased levels of PPG heteronuclear RNA (hnRNA) in a glucocorticoid-dependent manner, suggesting that decreases in PPG mRNA are due to increased degradation rather than reduced transcription. Glucocorticoid administration to unstressed rats is sufficient to cause decrements in PPG mRNA and increments in PPG hnRNA. These findings suggest that glucocorticoids deplete the pool of transcribed PPG mRNA and concurrently stimulate PPG gene transcription, with the latter allowing a mechanism for replenishment of PPG mRNA after stress cessation. The combination of rapid PPG mRNA depletion and initiation of PPG transcription within 30 min is consistent with a rapid action of glucocorticoids on GLP-1 bioavailability, resulting in a transient reduction in the capacity for neuropeptidergic excitation of stress responses.

The role of central glucagon-like peptide-1 in mediating the effects of visceral illness: differential effects in rats and mice

In rats, central administration of glucagon-like peptide-1 (GLP-1) elicits symptoms of visceral illness like those caused by the toxin lithium chloride (LiCl), including anorexia, conditioned taste aversion (CTA) formation, and neural activation in the hypothalamus and hindbrain including activation of brainstem preproglucagon cells. Most compellingly, pharmacological antagonists of the GLP-1 receptor (GLP-1R) block several effects of LiCl in rat. The major goal of these experiments was to further test the hypothesis that the central nervous system GLP-1 system is critical to the visceral illness actions of LiCl by using mice with a targeted disruption of the only described GLP-1R. First, we observed that, like the rat, LiCl activates preproglucagon neurons in wild-type mice. Second, GLP-1R -/- mice demonstrated normal anorexic and CTA responses to LiCl. To test the possibility that alternate GLP-1Rs mediate aversive effects, we examined the ability of GLP-1 to produce a CTA in GLP1R -/- mice. Although lateral ventricular GLP-1 produced a CTA in wild-type mice, it did not produce a CTA in GLP-1R -/- mice. Furthermore, the same GLP-1R antagonist that can block the aversive effects of LiCl in the rat failed to do so in the mouse. These results support the conclusion that in mouse, unlike in rat, GLP-1R signaling is not required for the visceral illness response to LiCl. Such species differences are an important consideration when comparing results from rat and mouse studies.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Neuropeptide families: evolutionary perspectives

Examination of neuropeptide families can provide information about phyletic relationships and evolutionary processes. In this article the oxytocin/vasopressin family, growth hormone releasing factor (GRF) superfamily and the substance P/tachykinin family have been considered in detail because they have been isolated from an extraordinarily diverse array of species from several vertebrate classes and invertebrate phyla. More important is that the nucleotide sequence of mRNA or cDNA encoding many of these peptides has been determined, which has allowed evolutionary distances to be estimated based on the DNA mutation rate. The origin of a given family lies in a primordial gene that arose many millions of years ago, and through time, exon duplication and insertion, gene duplication, point mutation and exon loss, the family developed into the forms that are now recognised. For example, in birds, GRF and pituitary adenylate cyclase activating peptide (PACAP) are encoded by the same gene, which probably arose as a result of exon duplication and tandem insertion of the ancestral GRF gene. In mammals GRF is the sole product on one gene, and PACAP is the product of a gene that also produces PACAP-related peptide (PRP), which is homologous to GRF. Thus it appears that between birds and mammals the GRF/PACAP gene duplicated: exon loss gave rise to the mammalian GRF gene, while mutation led to the formation of the mammalian PRP/PACAP gene. The neuropeptide Y superfamily is considered briefly, as is cionin, which is an invertebrate peptide that is closely related to the mammalian gastrin/cholecystokinin family.

Enteroglucagon

The gene encoding proglucagon, the biosynthetic precursor of glucagon, is expressed not only in the pancreatic islets but also in endocrine cells of the gastrointestinal mucosa. The proglucagon (PG)-derived peptides from the gut include glicentin (corresponding to PG 1-69); smaller amounts of oxyntomodulin (PG 33-69) and glicentin-related pancreatic polypeptide (GRPP, PG 1-30); glucagon-like peptide-1 (GLP-1, PG 78-107 amide); intervening peptide-2 (IP-2, PG 111-122 amide); and glucagon-like peptide-2 (GLP-2, PG 126-158). All are secreted into the blood in response to ingestion of carbohydrates and lipids. Only oxyntomodulin and GLP-1 have proven biological activity; oxyntomodulin possibly because it interacts (but with lower potency) with GLP-1 and glucagon receptors. GLP-1 is the most potent insulinotropic hormone known and functions as an incretin hormone. It also inhibits glucagon secretion and, therefore, lowers blood glucose. This effect is preserved in patients with non-insulin-dependent diabetes mellitus, in whom infusions of GLP-1 may completely normalize blood glucose. However, GLP-1 also potently inhibits gastrointestinal secretion and motility, and its physiological functions include mediation of the "ileal-brake" effect, i.e. the inhibition of upper gastrointestinal functions elicited by the presence of unabsorbed nutrients in the ileum. As such it may serve to regulate food intake.