Ghrelin

BACKGROUND

The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism.

SCOPE OF REVIEW

In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery.

MAJOR CONCLUSIONS

In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

The central nervous system sites mediating the orexigenic actions of ghrelin

The peptide hormone ghrelin is important for both homeostatic and hedonic eating behaviors, and its orexigenic actions occur mainly via binding to the only known ghrelin receptor, the growth hormone secretagogue receptor (GHSR). GHSRs are located in several distinct regions of the central nervous system. This review discusses those central nervous system sites that have been found to play critical roles in the orexigenic actions of ghrelin, including hypothalamic nuclei, the hippocampus, the amygdala, the caudal brain stem, and midbrain dopaminergic neurons. Hopefully, this review can be used as a stepping stone for the reader wanting to gain a clearer understanding of the central nervous system sites of direct ghrelin action on feeding behavior, and as inspiration for future studies to provide an even-more-detailed map of the neurocircuitry controlling eating and body weight.

GOAT induced ghrelin acylation regulates hedonic feeding

Ghrelin is an orexigenic hormone that regulates homeostatic and reward-related feeding behavior. Recent evidence indicates that acylation of ghrelin by the gut enzyme ghrelin O-acyl transferase (GOAT) is necessary to render ghrelin maximally active within its target tissues. Here we tested the hypothesis that GOAT activity modulates food motivation and food hedonics using behavioral pharmacology and mutant mice deficient for GOAT and the ghrelin receptor (GHSR). We evaluated operant responding following pharmacological administration of acyl-ghrelin and assessed the necessity of endogenous GOAT activity for operant responding in GOAT and GHSR-null mice. Hedonic-based feeding behavior also was examined in GOAT-KO and GHSR-null mice using a "Dessert Effect" protocol in which the intake of a palatable high fat diet "dessert" was assessed in calorically-sated mice. Pharmacological administration of acyl-ghrelin augmented operant responding; notably, this effect was dependent on intact GHSR signaling. GOAT-KO mice displayed attenuated operant responding and decreased hedonic feeding relative to controls. These behavioral results correlated with decreased expression of the orexin-1 receptor in reward-related brain regions in GOAT-KO mice. In summary, the ability of ghrelin to stimulate food motivation is dependent on intact GHSR signaling and modified by endogenous GOAT activity. Furthermore, GOAT activity is required for hedonic feeding behavior, an effect potentially mediated by forebrain orexin signaling. These data highlight the significance of the GOAT-ghrelin system for the mediation of food motivation and hedonic feeding.

Characterization of Kiss1 neurons using transgenic mouse models

Humans and mice with loss-of-function mutations of the genes encoding kisspeptins (Kiss1) or kisspeptin receptor (Kiss1r) are infertile due to hypogonadotropic hypogonadism. Within the hypothalamus, Kiss1 mRNA is expressed in the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus (Arc). In order to better study the different populations of kisspeptin cells we generated Kiss1-Cre transgenic mice. We obtained one line with Cre activity specifically within Kiss1 neurons (line J2-4), as assessed by generating mice with Cre-dependent expression of green fluorescent protein or β-galactosidase. Also, we demonstrated Kiss1 expression in the cerebral cortex and confirmed previous data showing Kiss1 mRNA in the medial nucleus of amygdala and anterodorsal preoptic nucleus. Kiss1 neurons were more concentrated towards the caudal levels of the Arc and higher leptin-responsivity was observed in the most caudal population of Arc Kiss1 neurons. No evidence for direct action of leptin in AVPV Kiss1 neurons was observed. Melanocortin fibers innervated subsets of Kiss1 neurons of the preoptic area and Arc, and both populations expressed melanocortin receptors type 4 (MC4R). Specifically in the preoptic area, 18-28% of Kiss1 neurons expressed MC4R. In the Arc, 90% of Kiss1 neurons were glutamatergic, 50% of which also were GABAergic. In the AVPV, 20% of Kiss1 neurons were glutamatergic whereas 75% were GABAergic. The differences observed between the Kiss1 neurons in the preoptic area and the Arc likely represent neuronal evidence for their differential roles in metabolism and reproduction.

Distribution and neurochemical characterization of protein kinase C-theta and -delta in the rodent hypothalamus

PKC-theta (PKC-θ), a member of the novel protein kinase C family (nPKC), regulates a wide variety of functions in the periphery. However, its presence and role in the CNS has remained largely unknown. Recently, we demonstrated the presence of PKC-θ in the arcuate hypothalamic nucleus (ARC) and knockdown of PKC-θ from the ARC protected mice from developing diet-induced obesity. Another isoform of the nPKC group, PKC-delta (PKC-δ), is expressed in several non-hypothalamic brain sites including the thalamus and hippocampus. Although PKC-δ has been implicated in regulating hypothalamic glucose homeostasis, its distribution in the hypothalamus has not previously been described. In the current study, we used immunohistochemistry to examine the distribution of PKC-θ and -δ immunoreactivity in rat and mouse hypothalamus. We found PKC-θ immunoreactive neurons in several hypothalamic nuclei including the ARC, lateral hypothalamic area, perifornical area and tuberomammillary nucleus. PKC-δ immunoreactive neurons were found in the paraventricular and supraoptic nuclei. Double-label immunohistochemisty in mice expressing green fluorescent protein either with the long form of leptin receptor (LepR-b) or in orexin (ORX) neurons indicated that PKC-θ is highly colocalized in lateral hypothalamic ORX neurons but not in lateral hypothalamic LepR-b neurons. Double-label immunohistochemistry in oxytocin-enhanced yellow fluorescent protein mice or arginine vasopressin-enhanced green fluorescent protein (AVP-EGFP) transgenic rats revealed a high degree of colocalization of PKC-δ within paraventricular and supraoptic oxytocin neurons but not the vasopressinergic neurons. We conclude that PKC-θ and -δ are expressed in different hypothalamic neuronal populations.

Expression of cone transducin, Gz alpha, and other G-protein alpha-subunit messenger ribonucleic acids in pancreatic islets

The G-proteins are a family of heterotrimeric guanine nucleotide-binding proteins that play important roles in signal transduction and whose expression is regulated in a tissue-specific manner. Here we have surveyed the expression of G-protein alpha-subunits in mouse pancreatic islets. Degenerate oligonucleotide primers corresponding to conserved primary sequences in known G alpha-subunits were used in a reverse transcriptase-polymerase chain reaction, and the amplified complementary DNA (cDNA) fragments were subcloned and sequenced. Over 100 clones were analyzed, from which we determined that islet cells express at least seven G alpha-subunits: G8 alpha, Gi1 alpha or Gi3 alpha, Gi2 alpha, G11 alpha, G14 alpha, Gz alpha, and Gt2 alpha (cone transducin). In particular, the identification of Gz alpha and Gt2 alpha was of interest in that previous studies had indicated that the expression of Gz alpha was restricted mainly to the brain, retina, and adrenal gland, whereas Gt2 alpha was expressed predominantly in retinal cone photoreceptors. By Western blot analysis, we estimated that the amount of Gz alpha protein present in mouse islets was about 40% of that in retina. To further investigate the expression of Gt2 alpha, mouse Gt2 alpha cDNA was cloned from a retinal library and sequenced. The cDNA was used as a probe for Northern blot analysis, and the results confirmed that mouse islets contained a substantial level of Gt2 alpha messenger RNA (mRNA), albeit less than that found in retina (approximately 5-fold lower). Gt2 alpha mRNA was also shown to be present in a clonal mouse pancreatic alpha-cell line (alpha TC1-6) as well as in adrenal gland, pituitary, and a clonal mouse anterior pituitary cell line (AtT20). In situ hybridization revealed that Gt2 alpha mRNA was expressed essentially throughout the islet, suggesting that it is normally expressed in the abundant islet beta-cells and possibly others. In situ analysis also showed that Gt2 alpha mRNA expressed in the pituitary was limited to the intermediate and anterior lobes. We conclude that islet cells express multiple G-proteins, including several that are normally expressed at high levels in certain neuronal cells.

Human G(olf) alpha: complementary deoxyribonucleic acid structure and expression in pancreatic islets and other tissues outside the olfactory neuroepithelium and central nervous system

G(olf) alpha is a G-protein originally believed to mediate signal transduction exclusively within the olfactory neuroepithelium and subsequently found to be a major stimulatory G-protein in the basal ganglia. Here we present evidence that G(olf) alpha is expressed in several other tissues. The human isoform of G(olf) alpha was isolated from two human insulinoma cDNA libraries. Comparison of the human sequence with rat G(olf) alpha shows 91% nucleotide identity (within the coding region) and 99% identity at the amino acid level. Northern and reverse transcriptase-polymerase chain reaction analyses indicated that G(olf) alpha is expressed in all human insulinomas examined thus far as well as in normal pancreatic islets. G(olf) alpha mRNA was also detected in testis, retina, brain, and liver. Western blot analysis of various mouse tissues demonstrated that the level of G(olf) alpha protein in islets is lower than that in the olfactory neuroepithelium and other parts of the brain; its expression in retina, lung, and spleen was moderately higher than that in islets, and its expression in testis approached that in olfactory neuroepithelium. G(olf) alpha was also detected by immunohistochemistry in mouse islets, human insulinomas, the epithelial lining of mouse epididymis, photoreceptor cells of mouse retina, and mouse lung alveoli. These findings suggest a role for G(olf) alpha in a diverse population of cells located outside the olfactory neuroepithelium and central nervous system.