Ghrelin is dispensable for embryonic pancreatic islet development and differentiation. ACTA ACUST UNITED AC. 2009;157:51-56. [PMID: 19268691 DOI: 10.1016/j.regpep.2009.02.013]

Ghrelin deletion and conditional ghrelin cell ablation increase pancreatic islet size in mice

Ghrelin exerts key effects on islet hormone secretion to regulate blood glucose levels. Here, we sought to determine whether ghrelin's effects on islets extend to the alteration of islet size and β cell mass. We demonstrate that reducing ghrelin - by ghrelin gene knockout (GKO), conditional ghrelin cell ablation, or high-fat diet (HFD) feeding - was associated with increased mean islet size (up to 62%), percentage of large islets (up to 854%), and β cell cross-sectional area (up to 51%). In GKO mice, these effects were more apparent in 10- to 12-week-old mice than in 4-week-old mice. Higher β cell numbers from decreased β cell apoptosis drove the increase in β cell cross-sectional area. Conditional ghrelin cell ablation in adult mice increased the β cell number per islet by 40% within 4 weeks. A negative correlation between islet size and plasma ghrelin in HFD-fed plus chow-fed WT mice, together with even larger islet sizes in HFD-fed GKO mice than in HFD-fed WT mice, suggests that reduced ghrelin was not solely responsible for diet-induced obesity-associated islet enlargement. Single-cell transcriptomics revealed changes in gene expression in several GKO islet cell types, including upregulation of Manf, Dnajc3, and Gnas expression in β cells, which supports decreased β cell apoptosis and/or increased β cell proliferation. These effects of ghrelin reduction on islet morphology might prove useful when designing new therapies for diabetes.

Development and Characteristics of Pancreatic Epsilon Cells

Pancreatic endocrine cells expressing the ghrelin gene and producing the ghrelin hormone were first identified in 2002. These cells, named ε cells, were recognized as the fifth type of endocrine cells. Differentiation of ε cells is induced by various transcription factors, including Nk2 homeobox 2, paired box proteins Pax-4 and Pax6, and the aristaless-related homeobox. Ghrelin is generally considered to be a "hunger hormone" that stimulates the appetite and is produced mainly by the stomach. Although the population of ε cells is small in adults, they play important roles in regulating other endocrine cells, especially β cells, by releasing ghrelin. However, the roles of ghrelin in β cells are complex. Ghrelin contributes to increased blood glucose levels by suppressing insulin release from β cells and is also involved in the growth and proliferation of β cells and the prevention of β cell apoptosis. Despite increasing evidence and clarification of the mechanisms of ε cells over the last 20 years, many questions remain to be answered. In this review, we present the current evidence for the participation of ε cells in differentiation and clarify their characteristics by focusing on the roles of ghrelin.

Role of ghrelin in pancreatic development and function

Ghrelin is a gastric peptide with anabolic functions. It acutely stimulates growth hormone (GH) secretion from the anterior pituitary glands and modulates hypothalamic circuits that control food intake and energy expenditure. Besides its central activity, ghrelin is also involved in the regulation of pancreatic development and physiology. Particularly, several studies highlighted the ability of ghrelin to sustain β-cell viability and proliferation. Furthermore, ghrelin seems to exert inhibitory effects on pancreatic acinar and endocrine secretory functions. Due to its pleiotropic activity on energy metabolism, ghrelin has become a topic of great interest for experimental research focused on type II diabetes and obesity. The aim of this review is to illustrate the complex and not fully understood interplay between ghrelin, pancreas and glucose homeostasis.

Multipotent pancreas progenitors: Inconclusive but pivotal topic

The establishment of multipotent pancreas progenitors (MPP) should have a significant impact not only on the ontology of the pancreas, but also for the translational research of glucose-responding endocrine β-cells. Deficiency of the latter may lead to the pandemic type 1 or type 2 diabetes mellitus, a metabolic disorder. An ideal treatment of which would potentially be the replacement of destroyed or failed β-cells, by restoring function of endogenous pancreatic endocrine cells or by transplantation of donor islets or in vitro generated insulin-secreting cells. Thus, considerable research efforts have been devoted to identify MPP candidates in the pre- and post-natal pancreas for the endogenous neogenesis or regeneration of endocrine insulin-secreting cells. In order to advance this inconclusive but critical field, we here review the emerging concepts, recent literature and newest developments of potential MPP and propose measures that would assist its forward progression.

Implications of ghrelin and hexarelin in diabetes and diabetes-associated heart diseases

Ghrelin and its synthetic analog hexarelin are specific ligands of growth hormone secretagogue (GHS) receptor. GHS have strong growth hormone-releasing effect and other neuroendocrine activities such as stimulatory effects on prolactin and adrenocorticotropic hormone secretion. Recently, several studies have reported other beneficial functions of GHS that are independent of GH. Ghrelin and hexarelin, for examples, have been shown to exert GH-independent cardiovascular activity. Hexarelin has been reported to regulate peroxisome proliferator-activated receptor gamma (PPAR-γ) in macrophages and adipocytes. PPAR-γ is an important regulator of adipogenesis, lipid metabolism, and insulin sensitization. Ghrelin also shows protective effects on beta cells against lipotoxicity through activation of phosphatidylinositol-3 kinase/protein kinase B, c-Jun N-terminal kinase (JNK) inhibition, and nuclear exclusion of forkhead box protein O1. Acylated ghrelin (AG) and unacylated ghrelin (UAG) administration reduces glucose levels and increases insulin-producing beta cell number, and insulin secretion in pancreatectomized rats and in newborn rats treated with streptozotocin, suggesting a possible role of GHS in pancreatic regeneration. Therefore, the discovery of GHS has opened many new perspectives in endocrine, metabolic, and cardiovascular research areas, suggesting the possible therapeutic application in diabetes and diabetic complications especially diabetic cardiomyopathy. Here, we review the physiological roles of ghrelin and hexarelin in the protection and regeneration of beta cells and their roles in the regulation of insulin release, glucose, and fat metabolism and present their potential therapeutic effects in the treatment of diabetes and diabetic-associated heart diseases.

Pancreatic stem cells remain unresolved

Diabetes mellitus is caused by absolute (type 1) or relative (type 2) deficiency of insulin-secreting islet β cells. An ideal treatment of diabetes would, therefore, be to replace the lost or deficient β cells, by transplantation of donated islets or differentiated endocrine cells or by regeneration of endogenous islet cells. Due to their ability of unlimited proliferation and differentiation into all functional lineages in our body, including β cells, embryonic stem cells and induced pluripotent stem cells are ideally placed as cell sources for a diabetic transplantation therapy. Unfortunately, the inability to generate functional differentiated islet cells from pluripotent stem cells and the poor availability of donor islets have severely restricted the broad clinical use of the replacement therapy. Therefore, endogenous sources that can be directed to becoming insulin-secreting cells are actively sought after. In particular, any cell types in the developing or adult pancreas that may act as pancreatic stem cells (PSC) would provide an alternative renewable source for endogenous regeneration. In this review, we will summarize the latest progress and knowledge of such PSC, and discuss ways that facilitate the future development of this often controversial, but crucial research.

Different ghrelin localisation in adult human and rat endocrine pancreas

Ghrelin is an endocrine peptide that has been identified in gastric oxyntic glands and that induces growth hormone secretion in the pituitary gland. This growth hormone secretagogue is expressed in many tissues such as stomach, pituitary gland, thyroid, testis, placenta and pancreas. Initial studies of ghrelin focused on its role as a circulating orexigenic signal. However, ghrelin has also been found to be involved in the modulation of glucose homeostasis. Although a number of studies have reported ghrelin expression in developing pancreas, the location of ghrelin-immunoreactive cells in adult pancreas (epsilon cells) remains controversial. In this study, we have analysed the distribution of pancreatic epsilon cells in adult human and rat islets by immunohistochemistry and in situ hybridisation. In humans, our immunohistochemical analysis has shown that ghrelin is expressed in glucagon-secreting cells, whereas in rats, it is present in insulin-secreting cells. Similar observations have been revealed by in situ hybridisation.

Ghrelin expression in the mouse pancreas defines a unique multipotent progenitor population

Pancreatic islet cells provide the major source of counteractive endocrine hormones required for maintaining glucose homeostasis; severe health problems result when these cell types are insufficiently active or reduced in number. Therefore, the process of islet endocrine cell lineage allocation is critical to ensure there is a correct balance of islet cell types. There are four endocrine cell types within the adult islet, including the glucagon-producing alpha cells, insulin-producing beta cells, somatostatin-producing delta cells and pancreatic polypeptide-producing PP cells. A fifth islet cell type, the ghrelin-producing epsilon cells, is primarily found during gestational development. Although hormone expression is generally assumed to mark the final entry to a determined cell state, we demonstrate in this study that ghrelin-expressing epsilon cells within the mouse pancreas do not represent a terminally differentiated endocrine population. Ghrelin cells give rise to significant numbers of alpha and PP cells and rare beta cells in the adult islet. Furthermore, pancreatic ghrelin-producing cells are maintained in pancreata lacking the essential endocrine lineage regulator Neurogenin3, and retain the ability to contribute to cells within the pancreatic ductal and exocrine lineages. These results demonstrate that the islet ghrelin-expressing epsilon cells represent a multi-potent progenitor cell population that delineates a major subgrouping of the islet endocrine cell populations. These studies also provide evidence that many of hormone-producing cells within the adult islet represent heterogeneous populations based on their ontogeny, which could have broader implications on the regulation of islet cell ratios and their ability to effectively respond to fluctuations in the metabolic environment during development.

Similar developmental patterns of ghrelin- and glucagon-expressing cells in the human pancreas

The pancreas appears to be a major source of ghrelin during fetal development, but the ontogeny of ghrelin cells in the human pancreas and their developmental relationship with α- and β-cells remain largely unknown. In the present study, we examined the dynamics of ghrelin cell growth, colocalization of ghrelin with major pancreatic hormones and defined the similarities and differences among developmental patterns of ghrelin-, glucagon- and insulin-expressing cells in the human pancreas. To this end, paraffin-embedded pancreatic tissue sections from human embryos and fetuses were assessed by immunohistochemistry. Ghrelin-positive cells were first detected in the pancreas of 11-week-old fetuses. With advancing gestational age, both ghrelin- and glucagon-expressing cells were increasingly observed at the periphery of the developing islets, whereas insulin-containing cells were typically found in the islet core. Double immunohistochemistry showed that ghrelin-expressing cells were clearly separate from insulin-, somatostatin- and pancreatic polypeptide-containing cells. In contrast, cells coexpressing ghrelin and glucagon were sporadically detected during both the early and late fetal periods. Furthermore, morphometric analysis revealed a similar trend in the volume density of ghrelin- and glucagon-positive cells, and a contrasting pattern in β-cell density at specific time points during the development of the human pancreas. This study demonstrates that the developmental pattern of ghrelin cells, although clearly distinct, is quite similar to that of glucagon-expressing cells. The obtained findings indicate a close lineage relationship between these cell populations, a functional relationship between their secretory products and an auto/paracrine mode of ghrelin-glucagon interaction in pancreatic development.

The endocrine pancreas: insights into development, differentiation, and diabetes

In the developing embryo, appropriate patterning of the endoderm fated to become pancreas requires the spatial and temporal coordination of soluble factors secreted by the surrounding tissues. Once pancreatic progenitor cells are specified in the developing gut tube epithelium, epithelial-mesenchymal interactions, as well as a cascade of transcription factors, subsequently delineate three distinct lineages, including endocrine, exocrine, and ductal cells. Simultaneous morphological changes, including branching, vascularization, and proximal organ development, also influence the process of specification and differentiation. Decades of research using mouse genetics have uncovered many of the key factors involved in pancreatic cell fate decisions. When pancreas development or islet cell functions go awry, due to mutations in genes important for proper organogenesis and development, the result can lead to a common pancreatic affliction, diabetes mellitus. Current treatments for diabetes are adequate but not curative. Therefore, researchers are utilizing the current understanding of normal embryonic pancreas development in vivo, to direct embryonic stem cells toward a pancreatic fate with the goal of transplanting these in vitro generated 'islets' into patients. Mimicking development in vitro has proven difficult; however, significant progress has been made and the current differentiation protocols are becoming more efficient. The continued partnership between developmental biologists and stem cell researchers will guarantee that the in vitro generation of insulin-producing β cells is a possible therapeutic option for the treatment of diabetes.

Neuroendocrine and metabolic activities of ghrelin gene products

Acylated ghrelin (AG) is a 28 amino acid gastric peptide a natural ligand for the growth hormone secretagogue (GHS) receptor type 1a (GHS-R1a), endowed with GH-secreting and orexigenic properties. Besides, ghrelin exerts several peripheral metabolic actions, including modulation of glucose homeostasis and stimulation of adipogenesis. Notably, AG administration causes hyperglycemia in rodents as in humans. Ghrelin pleiotropy is supported by a widespread expression of the ghrelin gene, of GHS-R1a and other unknown ghrelin binding sites. The existence of alternative receptors for AG, of several natural ligands for GHS-R1a and of acylation-independent ghrelin non-neuroendocrine activities, suggests that there might be a complex 'ghrelin system' not yet completely explored. Moreover, the patho-physiological implications of unacylated ghrelin (UAG), and obestatin (Ob), the other two ghrelin gene-derived peptides, need to be clarified. Within the next few years, we may better understand the 'ghrelin system', where we might envisage clinical applications.

Molecular cloning and expression analysis of porcine ghrelin o-acyltransferase

The peptide hormone ghrelin is secreted in the stomach, with unique N-octanoylation at serine 3, which is a requirement for its functionality. These functions include growth hormone release, appetite stimulation, gastrointestinal motility, glucose regulation, and cell proliferation. The enzyme responsible for ghrelin acylation was recently identified as ghrelin O-acyltransferase (GOAT). In this study, porcine GOAT was cloned and characterized. A full-length cDNA of GOAT of 2013 bp was obtained, which included a 70-bp 5' UTR, a 635-bp 3' UTR, and a 1308-bp open reading frame encoding a protein of 415 amino acids. The GOAT and ghrelin mRNAs are co-expressed in stomach, pancreas, and duodenum at high levels. GOAT was also detected in liver, lung, brain, testis, spleen, kidney, heart, muscle, lipid, and ovary. Our results provide an important basis for further research on GOAT function and the relationship between ghrelin and GOAT.

Pancreatic islet plasticity: interspecies comparison of islet architecture and composition

The pancreatic islet displays diverse patterns of endocrine cell arrangement. The prototypic islet, with insulin-secreting beta-cells forming the core surrounded by other endocrine cells in the periphery, is largely based on studies of normal rodent islets. Recent reports on large animals, including humans, show a difference in islet architecture, in which the endocrine cells are randomly distributed throughout the islet. This particular species difference has raised concerns regarding the interpretation of data based on rodent studies to humans. On the other hand, further variations have been reported in marsupials and some nonhuman primates, which possess an inverted ratio of beta-cells to other endocrine cells. This review discusses the striking plasticity of islet architecture and cellular composition among various species including changes in response to metabolic states within a single species. We propose that this plasticity reflects evolutionary acquired adaptation induced by altered physiological conditions, rather than inherent disparities between species.

Nkx2.2 activates the ghrelin promoter in pancreatic islet cells

Nkx2.2 is an essential regulator of pancreatic endocrine differentiation. Nkx2.2-null mice are completely devoid of beta-ells and have a large reduction of alpha- and PP cells. In the place of these islet populations, there is a corresponding increase in the ghrelin-positive epsilon-cells. Molecular studies have indicated that Nkx2.2 functions as an activator and repressor to regulate islet cell fate decisions. To determine whether Nkx2.2 is solely important for islet cell fate decisions or also has the capability to control ghrelin at the promoter level, we studied the transcriptional regulation of the ghrelin promoter within the pancreas, in vitro and in vivo. These studies demonstrate that both of the previously identified transcriptional start sites in the ghrelin promoter are active within the embryonic pancreas; however, the long transcript is preferentially up-regulated in the Nkx2.2-null pancreas. We also show that the promoter region between -619 and -488 bp upstream of the translational start site is necessary for repression of ghrelin in alphaTC1 and betaTC6 cells. Surprisingly, we also show that Nkx2.2 is able to bind to and activate the ghrelin promoter in several cell lines that do or do not express endogenous ghrelin. Together, these results suggest that the up-regulation of ghrelin expression in the Nkx2.2-null mice is not due to loss of repression of the ghrelin promoter in the nonghrelin islet populations. Furthermore, Nkx2.2 may contribute to the activation of ghrelin in mature islet epsilon-cells.

Islet formation during the neonatal development in mice

The islet of Langerhans is a unique micro-organ within the exocrine pancreas, which is composed of insulin-secreting beta-cells, glucagon-secreting alpha-cells, somatostatin-secreting delta-cells, pancreatic polypeptide-secreting PP cells and ghrelin-secreting epsilon-cells. Islets also contain non-endocrine cell types such as endothelial cells. However, the mechanism(s) of islet formation is poorly understood due to technical difficulties in capturing this dynamic event in situ. We have developed a method to monitor beta-cell proliferation and islet formation in the intact pancreas using transgenic mice in which the beta-cells are specifically tagged with a fluorescent protein. Endocrine cells proliferate contiguously, forming branched cord-like structures in both embryos and neonates. Our study has revealed long stretches of interconnected islets located along large blood vessels in the neonatal pancreas. Alpha-cells span the elongated islet-like structures, which we hypothesize represent sites of fission and facilitate the eventual formation of discrete islets. We propose that islet formation occurs by a process of fission following contiguous endocrine cell proliferation, rather than by local aggregation or fusion of isolated beta-cells and islets. Mathematical modeling of the fission process in the neonatal islet formation is also presented.