Ghrelin cells replace insulin-producing beta cells in two mouse models of pancreas development

Purification, cDNA cloning, and characterization of ghrelin in channel catfish, Ictalurus punctatus

The ghrelin peptide and cDNA encoding precursor protein were isolated from the stomach of a channel catfish, Ictalurus punctatus. Catfish ghrelin is a 22-amino acid peptide with a sequence of GSSFLSPTQKPQNRGDRKPPRV. The third serine residue has been modified by n-decanoic acid and unsaturated fatty acids; however, an octanoylated form could not be identified. The carboxyl end of the peptide possessed an amide structure. A Gly-extended, non-amidated 23-amino acid ghrelin (ghrelin-Gly) was also isolated. Real-time quantitative PCR analysis revealed high levels of gene expression in the stomach and moderate levels in the pancreas and gall bladder. Intraperitoneal (IP) injection of ghrelin increased plasma GH levels in the catfish, but the effect of ghrelin-Gly was more potent than that of amidated ghrelin. Furthermore, IP injection with both amidated ghrelin and ghrelin-Gly caused a significant increase in pituitary GH mRNA expression over a 3-h period. These results indicate that ghrelin is present in catfish and stimulates GH gene expression and GH release in channel catfish.

NKX6 transcription factor activity is required for alpha- and beta-cell development in the pancreas

In diabetic individuals, the imbalance in glucose homeostasis is caused by loss or dysfunction of insulin-secreting beta-cells of the pancreatic islets. As successful generation of insulin-producing cells in vitro could constitute a cure for diabetes, recent studies have explored the molecular program that underlies beta-cell formation. From these studies, the homeodomain transcription factor NKX6.1 has proven to be a key player. In Nkx6.1 mutants, beta-cell numbers are selectively reduced, while other islet cell types develop normally. However, the molecular events downstream of NKX6.1, as well as the molecular pathways that ensure residual beta-cell formation in the absence of NKX6.1 are largely unknown. Here, we show that the Nkx6.1 paralog, Nkx6.2, is expressed during pancreas development and partially compensates for NKX6.1 function. Surprisingly, our analysis of Nkx6 compound mutant mice revealed a previously unrecognized requirement for NKX6 activity in alpha-cell formation. This finding suggests a more general role for NKX6 factors in endocrine cell differentiation than formerly suggested. Similar to NKX6 factors, the transcription factor MYT1 has recently been shown to regulate alpha- as well as beta-cell development. We demonstrate that expression of Myt1 depends on overall Nkx6 gene dose, and therefore identify Myt1 as a possible downstream target of Nkx6 genes in the endocrine differentiation pathway.

Glucagon inhibits ghrelin secretion in humans

OBJECTIVE

It is well known that i.m. glucagon administration stimulates GH and cortisol release in humans, although the mechanisms are unclear. These effects are similar to those described for ghrelin on somatotroph and corticotroph function. The aim of the present study was to investigate the role of ghrelin in mediating the stimulatory effects of glucagon and to evaluate the effect of glucagon on ghrelin secretion.

DESIGN AND METHODS

We studied the endocrine and metabolic response to i.m. glucagon administration in 24 subjects (14 men, 10 women; age 19-65 years; body mass index, 25.3 +/- 1 kg/m(2)), who were shown to have an intact anterior pituitary function as evaluated before enclosure.

RESULTS

Serum ghrelin concentrations fell significantly at 30, 60, 120 and 180 min after glucagon administration (means +/- s.e.m.; baseline, 377.9 +/- 34.5 pg/ml; nadir, 294.6 +/- 28.3 pg/ml (60 min); P < 0.01). Conversely, i.m. glucagon elicited an increase in GH (baseline, 1.5 +/- 0.4 microg/l; peak, 14.2 +/- 2.7 microg/l (180 min); P < 0.01) and cortisol concentrations (baseline, 452.6 +/- 35.2 nmol/l; peak, 622.1 +/- 44 nmol/l (180 min); P < 0.01). The changes in ghrelin concentration at both 120 and 180 min were still significant after correction for glucose and insulin (P < 0.05).

CONCLUSIONS

We show that i.m. glucagon decreases ghrelin significantly. Therefore, the already known stimulatory effects of i.m. glucagon on cortisol and GH are not mediated by a change in ghrelin concentrations. The mechanisms underlying the ghrelin suppression after i.m. glucagon are unlikely to include glucose or insulin variations and need to be further elucidated.

Association between endocrine pancreas and ductal system. More than an epiphenomenon of endocrine differentiation and development?

Traditional histological descriptions of the pancreas distinguish between the exocrine and the endocrine pancreas, as if they were two functionally distinct glands. This view has been proven incorrect and can be considered obsolete. Interactions between acinar and islet tissues have been well established through numerous studies that reveal the existence of anatomical and functional relationships between these compartments of the gland. Less attention, however, has traditionally been paid to the relationships occurring between the endocrine pancreas and the ductal system. Associations between islet tissue and ducts are considered by most researchers as only a transient epiphenomenon of endocrine development. This article reviews the evidence that has emerged in the last 10 years demonstrating the existence of stable, close, and systematic relationships between these two pancreatic compartments. Functional and pathophysiological implications are considered, and the existence of an "acinar-duct-islet" axis is put forward. The pancreas appears at present to be an integrated organ composed of three functionally related components of well-orchestrated endocrine and exocrine physiological responses.

Ghrelin: a gastric peptide that regulates food intake and energy homeostasis

Ghrelin, an endogenous ligand for the growth-hormone-secretagogue receptor, was isolated from human and rat stomach. It is a 28-amino acid peptide with a posttranslational acyl modification that is indispensable for its activity. In addition to stimulating growth-hormone secretion, food intake, and body weight gain, ghrelin also plays a role in a variety of other systems, including circulation, digestion, and cell proliferation. This review will focus on the discovery, structural characteristics, tissue distribution, and physiological functions of ghrelin, as well as the regulation of its expression and secretion.

The simultaneous loss of Arx and Pax4 genes promotes a somatostatin-producing cell fate specification at the expense of the alpha- and beta-cell lineages in the mouse endocrine pancreas

The specification of the different mouse pancreatic endocrine subtypes is determined by the concerted activities of transcription factors. However, the molecular mechanisms regulating endocrine fate allocation remain unclear. In the present study, we uncover the molecular consequences of the simultaneous depletion of Arx and Pax4 activity during pancreas development. Our findings reveal a so far unrecognized essential role of the paired-box-encoding Pax4 gene. Specifically, in the combined absence of Arx and Pax4, an early-onset loss of mature alpha- and beta-cells occurs in the endocrine pancreas, concomitantly with a virtually exclusive generation of somatostatin-producing cells. Furthermore, despite normal development of the PP-cells in the double-mutant embryos, an atypical expression of the pancreatic polypeptide (PP) hormone was observed in somatostatin-labelled cells after birth. Additional characterizations indicate that such an expression of PP was related to the onset of feeding, thereby unravelling an epigenetic control. Finally, our data provide evidence that both Arx and Pax4 act as transcriptional repressors that control the expression level of one another, thereby mediating proper endocrine fate allocation.

Harnessing the gut to treat diabetes

The gut contains one of the largest stem cell populations in the body, yet has been largely overlooked as a source of potentially therapeutic cells. The stem cells reside in the crypts located at the base of the protruding villi, reproduce themselves, and repopulate the gut lining as differentiated cells are sloughed off into the lumen. Some studies have demonstrated that gut stem cells can be isolated and maintained in culture, but the field is currently hampered by the lack of clear markers for these cells. Nevertheless, the relative accessibility of the cells and the similar pathways of differentiation of both intestinal and pancreatic endocrine cells make the gut an attractive potential source of cells to treat diabetes. In particular, it may be possible to recapitulate islet development by the introduction of specific factors to gut stem cells. Alternatively, gut endocrine cells might be coaxed to produce insulin and secrete it into the blood in a meal-responsive manner. Several investigations support the feasibility of both approaches as novel potential therapies for diabetes. Utilizing a patient's own gut cells to re-establish endogenous meal-regulated insulin secretion could represent an attractive approach to ultimately cure diabetes.

Ghrelin in Growth and Development

Exogenous administration of ghrelin increases caloric intake and stimulates growth hormone (GH) secretion, two effects that are mediated through binding of ghrelin to the GH secretagogue receptor (GHS-R). In addition, ghrelin is thought to inhibit adipogenesis by GHS-R-independent mechanisms. In adults, ghrelin is mainly produced by the stomach. In contrast, in the fetal and early postnatal period, ghrelin gene expression is abundant in the pancreas but not in the stomach. While knockout animal studies demonstrate that ghrelin is not required for perinatal development under normal nutritional conditions, the characteristics of ghrelin metabolism during fetal development suggest that ghrelin could contribute to the programming of mechanisms involved in energy balance, such as beta-cell maturation, orexigenic pathways and adipogenesis. In humans, ghrelin concentrations progressively decrease during childhood and adolescence, as well as with advancing puberty. In adolescents, similar to adults, ghrelin concentrations are inversely related to body mass index and to circulating insulin. One notable exception is the presence of elevated ghrelin concentrations in subjects with Prader-Willi syndrome, raising the possibility that ghrelin could be part of the etiology of excess food intake in this condition. These data raise a number of fascinating questions on the potential physiologic role of this hormone during growth and development.

Foxa2 is required for the differentiation of pancreatic alpha-cells

The differentiation of insulin-producing beta-cells has been investigated in great detail; however, little is known about the factors that delineate the second-most abundant endocrine lineage, the glucagon-producing alpha-cell. Here we utilize a novel YAC-based Foxa3Cre transgene to delete the winged helix transcription factor Foxa2 (formerly HNF-3beta) in the pancreatic primordium during midgestation. The resulting Foxa2(loxP/loxP); Foxa3Cre mice are severely hypoglycemic and die within the first week of life. Mutant mice are hypoglucagonemic secondary to a 90% reduction of glucagon expression. While the number of mature glucagon-positive alpha-cells is dramatically reduced, specification of alpha-cell progenitors is not affected by Foxa2 deficiency. By marker gene analysis, we show that the expression of the alpha-cell transcription factors Arx, Pax6, and Brn4 does not require Foxa2 in the transcriptional hierarchy governing alpha-cell differentiation.

Analysis of rat insulin II promoter-ghrelin transgenic mice and rat glucagon promoter-ghrelin transgenic mice

We developed and analyzed two types of transgenic mice: rat insulin II promoter-ghrelin transgenic (RIP-G Tg) and rat glucagon promoter-ghrelin transgenic mice (RGP-G Tg). The pancreatic tissue ghrelin concentration measured by C-terminal radioimmunoassay (RIA) and plasma desacyl ghrelin concentration of RIP-G Tg were about 1000 and 3.4 times higher than those of nontransgenic littermates, respectively. The pancreatic tissue n-octanoylated ghrelin concentration measured by N-terminal RIA and plasma n-octanoylated ghrelin concentration of RIP-G Tg were not distinguishable from those of nontransgenic littermates. RIP-G Tg showed suppression of glucose-stimulated insulin secretion. Arginine-stimulated insulin secretion, pancreatic insulin mRNA and peptide levels, beta cell mass, islet architecture, and GLUT2 and PDX-1 immunoreactivity in RIP-G Tg pancreas were not significantly different from those of nontransgenic littermates. Islet batch incubation study did not show suppression of insulin secretion of RIP-G Tg in vitro. The insulin tolerance test showed lower tendency of blood glucose levels in RIP-G Tg. Taking lower tendency of triglyceride level of RIP-G Tg into consideration, these results may indicate that the suppression of insulin secretion is likely due to the effect of desacyl ghrelin on insulin sensitivity. RGP-G Tg, in which the pancreatic tissue ghrelin concentration measured by C-RIA was about 50 times higher than that of nontransgenic littermates, showed no significant changes in insulin secretion, glucose metabolism, islet mass, and islet architecture. The present study raises the possibility that desacyl ghrelin may have influence on glucose metabolism.

Ghrelin: structure and function

Small synthetic molecules called growth hormone secretagogues (GHSs) stimulate the release of growth hormone (GH) from the pituitary. They act through the GHS-R, a G protein-coupled receptor whose ligand has only been discovered recently. Using a reverse pharmacology paradigm with a stable cell line expressing GHS-R, we purified an endogenous ligand for GHS-R from rat stomach and named it "ghrelin," after a word root ("ghre") in Proto-Indo-European languages meaning "grow." Ghrelin is a peptide hormone in which the third amino acid, usually a serine but in some species a threonine, is modified by a fatty acid; this modification is essential for ghrelin's activity. The discovery of ghrelin indicates that the release of GH from the pituitary might be regulated not only by hypothalamic GH-releasing hormone, but also by ghrelin derived from the stomach. In addition, ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin is orexigenic; it is secreted from the stomach and circulates in the bloodstream under fasting conditions, indicating that it transmits a hunger signal from the periphery to the central nervous system. Taking into account all these activities, ghrelin plays important roles for maintaining GH release and energy homeostasis in vertebrates.

Winged-helix transcription factors and pancreatic development

The forkhead gene family, named after the founding gene member in Drosophila, is characterized by a unique DNA-binding domain. This so-called forkhead box encodes a winged-helix DNA-binding motif, the name of which describes the structure of the domain when bound to DNA. The three Fox (forkhead box) group A genes, Foxa1, Foxa2 and Foxa3, are expressed in embryonic endoderm, the germ layer that gives rise to the digestive system, and contribute to the specification of the pancreas and the regulation of glucose homoeostasis. Deletion of the Foxa2 gene in pancreatic beta-cells in mice results in a phenotype resembling PHHI (persistent hyperinsulinaemic hypoglycaemia of infancy). Molecular analyses have demonstrated that Foxa2 is an important regulator of the genes encoding Sur1, Kir6.2 and Schad (short chain L-3-hydroxyacyl-CoA dehydrogenase), mutation of which causes PHHI in humans. Foxa1 was shown to be an essential activator of glucagon gene expression in vivo. An additional winged-helix protein, Foxo1, contributes to pancreatic beta-cell function by regulating the Pdx1 gene, which is required for pancreatic development in cooperation with Foxa2.

Concentration of the n-octanoylated active form of ghrelin in fetal and neonatal circulation

The octanoylation of Ser3 is essential for the biological function of ghrelin. We examined the concentrations of the n-octanoylated active-form ghrelin in cord and neonatal blood using an RIA system that specifically recognized n-octanoylated ghrelin, as well as a system that measured the total ghrelin concentration. Plasma levels of active ghrelin in cord blood ranged from 7.7 to 38.4 pmol/l and correlated excellently with those of total ghrelin (r = 0.81, p<0.0001). The active ghrelin/total ghrelin (A/T) ratio ranged from 0.038 to 0.12 (median 0.072). The active ghrelin concentrations negatively correlated with birth body weight (r = -0.34, p = 0.01) and IGF-1 concentrations (r = -0.40, p = 0.003), but did not correlate with growth hormone (GH) concentrations. A considerable level of active ghrelin was detected in premature newborns. Venous cord blood samples showed a significantly higher active ghrelin concentration (p = 0.03) and A/T ratio (p = 0.01) than those in the artery. In neonatal blood, active ghrelin concentrations ranged from 4.6 to 22.6 pmol/l and the A/T ratio ranged from 0.02 to 0.081. These results demonstrate the existence of active-form ghrelin in fetal and neonatal circulation and may suggest the energy supply-dependent regulation of ghrelin expression/secretion in utero.

Matrix metalloproteinases 2 and 9 are dispensable for pancreatic islet formation and function in vivo

Pancreatic islet formation is a highly regulated process that is initiated at the end of gestation in rodents. Endocrine precursor cells first form within the epithelium of duct-like structures and then delaminate from the epithelium, migrate, and cluster during the early stages of islet formation. The molecular mechanisms that regulate endocrine cell migration and islet formation are not well understood. Cell culture studies suggest that matrix metalloproteinases (MMPs) 2 and 9 are required for islet formation. To address whether MMP2 and MMP9 function are essential for endocrine cell migration and islet formation in vivo, we analyzed pancreas development in MMP2/MMP9 double-deficient mice. Our results show that islet architecture and function are unperturbed in these knockout mice, demonstrating that both MMP2 and MMP9 functions are dispensable for islet formation and endocrine cell differentiation. Our studies also show that a number of other MMPs are expressed at the time islet formation is initiated. This observation suggests that other MMPs may substitute for MMP2 and MMP9 loss in pancreatic tissue. However, islet formation is unaffected in transgenic mice with modified tissue inhibitor of metalloproteinase-1 (TIMP1) levels, suggesting that MMP activity may contribute little to islet morphogenesis in vivo.

Minireview: transcriptional regulation in pancreatic development

Considerable progress has been made in the understanding of the sequential activation of signal transduction pathways and the expression of transcription factors during pancreas development. Much of this understanding has been obtained by analyses of the phenotypes of mice in which the expression of key genes has been disrupted (knockout mice). Knockout of the genes for Pdx1, Hlxb9, Isl1, or Hex results in an arrest of pancreas development at a very early stage (embryonic d 8-9). Disruption of genes encoding components of the Notch signaling pathway, e.g. Hes1 or neurogenin-3, abrogates development of the endocrine pancreas (islets of Langerhans). Disruption of transcription factor genes expressed more downstream in the developmental cascade (Beta2/NeuroD, Pax4, NKx2.2, and Nkx6.1) curtails the formation of insulin-producing beta-cells. An understanding of the importance of transcription factor genes during pancreas development has provided insights into the pathogenesis of diabetes, in which the mass of insulin-producing beta-cells is reduced.

Roles for ghrelin in the regulation of appetite and body weight

PURPOSE OF REVIEW

Ghrelin, the only known circulating appetite stimulant, has garnered widespread scientific interest and is currently featured in more than 2.3 new publications per day. Antagonists and agonists of its receptor are vigorously being developed by pharmaceutical companies to treat obesity and wasting conditions respectively. Here, the authors summarize the current state of knowledge regarding ghrelin's roles in energy homeostasis.

RECENT FINDINGS

Ghrelin is an acylated peptide produced primarily by the stomach and proximal small intestine. Circulating levels sharply increase before, and decrease after, meals. These and other findings implicate ghrelin in pre-meal hunger and meal initiation. Moreover, ghrelin satisfies established criteria for an adiposity-associated hormone involved in long-term body weight regulation. Blood levels correlate with energy stores and display compensatory changes in response to alterations of those stores. Ghrelin influences neuronal activity in brain areas critical to energy homeostasis. Excessive ghrelin signaling durably increases food intake and decreases energy expenditure, thereby promoting weight gain. Conversely, acute ghrelin blockade in adult animals reduces food intake and body weight, although the effects of lifelong genetic deletions are very subtle. Overproduction of ghrelin is etiologically implicated in Prader-Willi syndrome, whereas defective secretion may contribute to weight loss after gastric bypass surgery.

SUMMARY

Ghrelin appears to participate in mealtime hunger and meal initiation, as well as in long-term energy balance. Whether these roles are sufficiently important that ghrelin blockade will prove to be an effective antiobesity modality is a pivotal question that should soon be answered as ghrelin receptor antagonists are developed. Ghrelin agonists hold promise in the treatment of wasting conditions, for which few medications currently exist.

Ultrastructure of islet ghrelin cells in the human fetus

Ghrelin is a peptide hormone predominantly produced in the stomach. Ghrelin expression has also been reported in other tissues including the pancreas. We have reported that ghrelin cells constitute a novel endocrine cell type in the human and the developing rat islets. The cells are most numerous pre- and neonatally and, in humans, constitute 10% of all islet cells from mid-gestation to birth. Since gastric ghrelin expression is low before birth, the islets may be the main source of circulating ghrelin during this time. In the present investigation, we have performed an ultrastructural analysis of pancreatic ghrelin cells in human fetuses by using transmission electron microscopy and immunogold labelling. In addition, morphometrical analysis of secretory granules size was performed. Our data provide evidence for the unique ultrastructural features of ghrelin cells versus other islet cells. Notably, the secretory granules of ghrelin cells were of small size with a mean dense-core diameter of 110 nm. We conclude that ghrelin cells constitute a novel islet cell type, distinct from the previously hormonally characterised islet cell types.

Ghrelin: more than a natural GH secretagogue and/or an orexigenic factor

Ghrelin, an acylated peptide produced predominantly by the stomach, has been discovered to be a natural ligand of the growth hormone secretagogue receptor type 1a (GHS-R1a). Ghrelin has recently attracted considerable interest as a new orexigenic factor. However, ghrelin exerts several other neuroendocrine, metabolic and also nonendocrine actions that are explained by the widespread distribution of ghrelin and GHS-R expression. The likely existence of GHS-R subtypes and evidence that the neuroendocrine actions, but not all the other actions, of ghrelin depend on its acylation in serine-3 revealed a system whose complexity had not been completely explored by studying synthetic GHS. Ghrelin secretion is mainly regulated by metabolic signals and, in turn, the modulatory action of ghrelin on the control of food intake and energy metabolism seems to be among its most important biological actions. However, according to a recent study, ghrelin-null mice are neither anorectics nor dwarfs and this evidence clearly depicts a remarkable difference from leptin null mice. Nevertheless, the original and fascinating story of ghrelin, as well as its potential pathophysiological implications in endocrinology and internal medicine, is not definitively cancelled by these data as GHS-R1a null aged mice show significant alterations in body composition and growth, in glucose metabolism, cardiac function and contextual memory. Besides potential clinical implications for natural or synthetic ghrelin analogues acting as agonists or antagonists, there are several open questions awaiting an answer. How many ghrelin receptor subtypes exist? Is ghrelin 'the' or just 'a' GHS-R ligand? That is, are there other natural GHS-R ligands? Is there a functional balance between acylated and unacylated ghrelin forms, potentially with different actions? Within the next few years suitable answers to these questions will probably be found, making it possible to gain a better knowledge of ghrelin's potential clinical perspectives.

Epithelial hedgehog signals pattern the intestinal crypt-villus axis

Morphological development of the small intestinal mucosa involves the stepwise remodeling of a smooth-surfaced endodermal tube to form finger-like luminal projections (villi) and flask-shaped invaginations (crypts). These remodeling processes are orchestrated by instructive signals that pass bidirectionally between the epithelium and underlying mesenchyme. Sonic (Shh) and Indian (Ihh) hedgehog are expressed in the epithelium throughout these morphogenic events, and mice lacking either factor exhibit intestinal abnormalities. To examine the combined role of Shh and Ihh in intestinal morphogenesis, we generated transgenic mice expressing the pan-hedgehog inhibitor, Hhip (hedgehog interacting protein) in the epithelium. We demonstrate that hedgehog (Hh) signaling in the neonatal intestine is paracrine, from epithelium to Ptch1-expressing subepithelial myofibroblasts (ISEMFs) and smooth muscle cells (SMCs). Strong inhibition of this signal compromises epithelial remodeling and villus formation. Surprisingly, modest attenuation of Hh also perturbs villus patterning. Desmin-positive smooth muscle progenitors are expanded, and ISEMFs are mislocalized. This mesenchymal change secondarily affects the epithelium: Tcf4/beta-catenin target gene activity is enhanced, proliferation is increased, and ectopic precrypt structures form on villus tips. Thus, through a combined Hh signal to underlying ISEMFs, the epithelium patterns the crypt-villus axis, ensuring the proper size and location of the emerging precrypt compartment.

Clinical endocrinology and metabolism. Ghrelin, a novel growth-hormone-releasing and appetite-stimulating peptide from stomach

Recent identification of novel appetite-regulating hormones has revealed the complex interactions of these humoral factors in the regulation of feeding behavior in mammals. One of these hormones, ghrelin, a natural ligand of the orphan receptor GHS-R, purified from stomach, is able to stimulate growth hormone release from pituitary cells. Ghrelin is a 28 amino acid peptide containing an n-octanoylated serine 3 residue that is essential for its activity. Ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, the region known to control food intake. As an orexigenic peptide, ghrelin is therefore an endogenous regulator of feeding behavior from the peripheral tissues to the central nervous system.

Endogenous ghrelin in pancreatic islets restricts insulin release by attenuating Ca2+ signaling in beta-cells: implication in the glycemic control in rodents

Ghrelin, isolated from the human and rat stomach, is the endogenous ligand for the growth hormone (GH) secretagogue receptor, which is expressed in a variety of tissues, including the pancreatic islets. It has been shown that low plasma ghrelin levels correlates with elevated fasting insulin levels and type 2 diabetes. Here we show a physiological role of endogenous ghrelin in the regulation of insulin release and blood glucose in rodents. Acylated ghrelin, the active form of the peptide, was detected in the pancreatic islets. Counteraction of endogenous ghrelin by intraperitoneal injection of specific GH secretagogue receptor antagonists markedly lowered fasting glucose concentrations, attenuated plasma glucose elevation, and enhanced insulin responses during the glucose tolerance test (GTT). Conversely, intraperitoneal exogenous ghrelin GH-independently elevated fasting glucose concentrations, enhanced plasma glucose elevation, and attenuated insulin responses during GTT. Neither GH secretagogue receptor antagonist nor ghrelin affected the profiles of the insulin tolerance test. In isolated islets, GH secretagogue receptor blockade and antiserum against acylated ghrelin markedly enhanced glucose-induced increases in insulin release and intracellular Ca2+ concentration ([Ca2+]i), whereas ghrelin at a relatively high concentration (10 nmol/l) suppressed insulin release. In single beta-cells, ghrelin attenuated glucose-induced first-phase and oscillatory [Ca2+]i increases via the GH secretagogue receptor and in a pertussis toxin-sensitive manner. Ghrelin also increased tetraethylammonium-sensitive delayed outward K+ currents in single beta-cells. These findings reveal that endogenous ghrelin in islets acts on beta-cells to restrict glucose-induced insulin release at least partly via attenuation of Ca2+ signaling, and that this insulinostatic action may be implicated in the upward control of blood glucose. This function of ghrelin, together with inducing GH release and feeding, suggests that ghrelin underlies the integrative regulation of energy homeostasis.

GDF11 modulates NGN3+ islet progenitor cell number and promotes beta-cell differentiation in pancreas development

Identification of endogenous signals that regulate expansion and maturation of organ-specific progenitor cells is a major goal in studies of organ development. Here we provide evidence that growth differentiation factor 11 (GDF11), a member of the TGF-beta ligand family, governs the number and maturation of islet progenitor cells in mouse pancreas development. Gdf11 is expressed in embryonic pancreatic epithelium during formation of islet progenitor cells that express neurogenin 3. Mice deficient for Gdf11 harbor increased numbers of NGN3+ cells, revealing that GDF11 negatively regulates production of islet progenitor cells. Despite a marked expansion of these NGN3+ islet progenitors, mice lacking Gdf11 have reduced beta-cell numbers and evidence of arrested beta-cell development, indicating that GDF11 is also required for beta-cell maturation. Similar precursor and islet cell phenotypes are observed in mice deficient for SMAD2, an intracellular signaling factor activated by TGF-beta signals. Our data suggest that Gdf11 and Smad2 regulate islet cell differentiation in parallel to the Notch pathway, which previously has been shown to control development of NGN3+ cells. Thus, our studies reveal mechanisms by which GDF11 regulates the production and maturation of islet progenitor cells in pancreas development.

PAX4 gene variations predispose to ketosis-prone diabetes

Ketosis-prone diabetes (KPD) is a rare form of type 2 diabetes, mostly observed in subjects of west African origin (west Africans and African-Americans), characterized by fulminant and phasic insulin dependence, but lacking markers of autoimmunity observed in type 1 diabetes. PAX4 is a transcription factor essential for the development of insulin-producing pancreatic beta-cells. Recently, a missense mutation (Arg121Trp) of PAX4 has been implicated in early and insulin deficient type 2 diabetes in Japanese subjects. The phenotype similarities between KPD and Japanese carriers of Arg121Trp have prompted us to investigate the role of PAX4 in KPD. We have screened 101 KPD subjects and we have found a new variant in the PAX4 gene (Arg133Trp), specific to the population of west African ancestry, and which predisposes to KPD under a recessive model. Homozygous Arg133Trp PAX4 carriers were found in 4% of subjects with KPD but not in 355 controls or 147 subjects with common type 2 or type 1 diabetes. In vitro, the Arg133Trp variant showed a decreased transcriptional repression of target gene promoters in an alpha-TC1.6 cell line. In addition, one KPD patient was heterozygous for a rare PAX4 variant (Arg37Trp) that was not found in controls and that showed a more severe biochemical phenotype than Arg133Trp. Clinical investigation of the homozygous Arg133Trp carriers and of the Arg37Trp carrier demonstrated a more severe alteration in insulin secretory reserve, during a glucagon-stimulation test, compared to other KPD subjects. Together these data provide the first evidence that ethnic-specific gene variants may contribute to the predisposition to this particular form of diabetes and suggest that KPD, like maturity onset diabetes of the young, is a rare, phenotypically defined but genetically heterogeneous form of type 2 diabetes.

Ghrelin gene expression is markedly higher in fetal pancreas compared with fetal stomach: effect of maternal fasting

Ghrelin is an orexigenic peptide secreted mainly by the stomach in adult rats. Ghrelin concentrations increase with fasting and decrease after food intake. Ghrelin is also present in the placenta and in the fetal stomach, but the role of fetal ghrelin remains unclear. In this study, we compared changes in plasma ghrelin, insulin, and glucose concentrations and in ghrelin gene expression in stomach, pancreas, and placenta in response to fasting and feeding in adult nonpregnant rats and in 20-d pregnant dams and their fetuses. Plasma total ghrelin concentrations were three times higher in the fetus than in the dam but did not increase in response to fasting. In contrast to total ghrelin, plasma active ghrelin concentrations wee 50% lower in the fetus compared with the adult pregnant rat. Ghrelin mRNA and total ghrelin were markedly elevated in the fetal pancreas and six to seven times greater than in the fetal stomach but were not affected by fasting. In contrast, fetal pancreas and stomach active ghrelin concentrations increased two to three times after maternal fasting. Ghrelin receptor mRNA was present in all fetal pancreas samples. Placenta ghrelin gene expression was detectable but low. These data raise the possibility that in the fetus, in contrast to the adult, the pancreas and not the stomach is a major source of circulating immunoreactive ghrelin. Furthermore, the presence of a strong ghrelin gene expression and of ghrelin receptor mRNA in the fetal pancreas is intriguing and suggests that ghrelin may play an important role in beta-cell development.

Gut-derived signaling molecules and vagal afferents in the control of glucose and energy homeostasis

PURPOSE OF REVIEW

The control of glucose and energy homeostasis, including feeding behaviour, is tightly regulated by gut-derived peptidic and nonpeptidic endocrine mediators, autonomic nervous signals, as well as nutrients such as glucose. We will review recent findings on the role of the gastrointestinal tract innervation and of portal vein glucose sensors; we will review selected data on the action of gastrointestinally released hormones.

RECENT FINDINGS

The involvement of mechanosensory vagal afferents in postprandial meal termination has been clarified using mouse models with selective impairments of genes required for development of mechanosensory fibres. These activate central glucogen-like peptide-1/glucogen-like peptide-2 containing ascending pathways linking the visceroceptive brainstem neurons to hypothalamic nuclei. Mucosal terminals comprise the chemosensory vagal afferents responsive to postprandially released gastrointestinal hormones. The mechanism by which the hepatoportal glucose sensor stimulates glucose utilization by muscles was demonstrated, using genetically modified mice, to be insulin-independent but to require GLUT4 and AMP-kinase. This sensor is a key site of glucogen-like peptide-1 action and plays a critical role in triggering first phase insulin secretion. PeptideYY and ghrelin target intracerebral receptors as they are bidirectionally transported across the blood brain barrier. The anorectic functions of peripherally released peptideYY may however be mediated both via vagal afferents and intracerebral Y2 receptors in the brainstem and arcuate nucleus.

SUMMARY

These recent findings demonstrate that the use of improved anatomical and physiological techniques and animal models with targeted gene modifications lead to an improved understanding of the complex role of gastrointestinal signals in the control of energy homeostasis.

Ghrelin, hypothalamus-pituitary-adrenal (HPA) axis and Cushing's syndrome

Ghrelin, a peptide predominantly produced by the stomach, has been discovered as a natural ligand of the GH Secretagogue receptor type 1a (GHS-R1a), known as specific for synthetic GHS. Ghrelin has recently attracted considerable interest as a new orexigenic factor. However, ghrelin exerts pleiotropic actions that are explained by the widespread distribution of ghrelin and GHS-R expression. Besides strong stimulation of GH secretion, the neuroendocrine ghrelin actions also include significant stimulation of both lactotroph and corticotroph secretion; all these actions depend on acylation of ghrelin in serine-3 that allows binding and activation of the GHS-R1a. However, GHS-R subtypes are likely to exist; they also bind unacylated ghrelin that is, in fact, the most abundant circulating form and exerts some biological actions. Ghrelin secretion is mainly regulated by metabolic signals, namely inhibited by feeding, glucose and insulin while stimulated by energy restriction. The role of glucocorticoids on ghrelin synthesis and secretion is still unclear although morning ghrelin levels have been found reduced in some patients with Cushing's syndrome; this, however, would simply reflect its negative association to body mass. Ghrelin, like synthetic GHS, stimulates ACTH and cortisol secretion in normal subjects and this effect is generally sensitive to the negative glucocorticoid feedback. It is remarkable that, despite hypercortisolism, ghrelin as well as synthetic GHS display marked increase in their stimulatory effect on ACTH and cortisol secretion in patients with Cushing's disease. This is even more intriguing considering that the GH response to ghrelin and GHS is markedly reduced by glucocorticoid excess. It has been demonstrated that the ACTH-releasing effect of ghrelin and GHS is purely mediated at the central level in physiological conditions; its enhancement in the presence of ACTH-secreting tumours is, instead, likely to reflect direct action on GHS receptors present on the neoplastic tissues. In fact, peculiar ACTH hyperresponsiveness to ghrelin and GHS has been observed also in ectopic ACTH-secreting tumours.