InteractiveFly: GeneBrief

Limostatin: Biological Overview | References

BIOLOGICAL OVERVIEW

Decretins, hormones induced by fasting that suppress insulin production and secretion, have been postulated from classical human metabolic studies. From genetic screens, this study identified Drosophila Limostatin (Lst), a peptide hormone that suppresses insulin secretion. Lst is induced by nutrient restriction in gut-associated endocrine cells. limostatin deficiency leads to hyperinsulinemia, hypoglycemia, and excess adiposity. A conserved 15-residue polypeptide encoded by limostatin suppresses secretion by insulin-producing cells. Targeted knockdown of CG9918, a Drosophila ortholog of mammalian Neuromedin U receptors (NMURs), in insulin-producing cells phenocopied limostatin deficiency and attenuated insulin suppression by purified Lst, suggesting CG9918 encodes an Lst receptor. Human NMUR1 is expressed in islet β cells, and purified NMU suppressed insulin secretion from human islets. A human mutant NMU variant that co-segregates with familial early-onset obesity and hyperinsulinemia failed to suppress insulin secretion. The study proposes Lst as an index member of an ancient hormone class called decretins, which suppress insulin output (Alfa, 2015).

The coupling of hormonal responses to nutrient availability is fundamental for metabolic control. In mammals, regulated secretion of insulin from pancreatic b cells is a principal hormonal response orchestrating metabolic homeostasis. Circulating insulin levels constitute a dynamic metabolic switch, signaling the fed state and nutrient storage (anabolic pathways) when elevated, or starvation and nutrient mobilization (catabolic path ways) when decreased. Thus, insulin secretion must be precisely tuned to the nutritional state of the animal. Increased circulating glucose stimulates b cell depolarization and insulin secretion. In concert with glucose, gut-derived incretin hormones amplify glucose-stimulated insulin secretion (GSIS) in response to ingested carbohydrates, thereby tuning insulin output to the feeding state of the host (Alfa, 2015).

While the incretin effect on insulin secretion during feeding is well-documented, counter-regulatory mechanisms that suppress insulin secretion during or after starvation are incompletely understood. Classical starvation experiments in humans and other mammals revealed that sustained fasting profoundly alters the dynamics of insulin production and secretion, resulting in impaired glucose tolerance, relative insulin deficits, and 'starvation diabetes'. Remarkably, starvation-induced suppression of GSIS was not reverted by normalizing circulating glucose levels, suggesting that the dampening effect of starvation on insulin secretion perdures and is uncoupled from blood glucose and macronutrient concentrations. Based on these observations, it has been postulated that hormonal signals induced by fasting may actively attenuate insulin secretion suggested that enteroendocrine 'decretin' hormones may constrain the release of insulin to prevent hypoglycemia. This concept is further supported by recent studies identifying a G protein that suppresses insulin secretion from pancreatic b cell. Thus, after nutrient restriction, decretin hormones could signal through G protein-coupled receptors (GPCRs) to attenuate GSIS from b cells (Alfa, 2015).

The discovery of hormonal pathways regulating metabolism in mammals presents a formidable challenge. However, progress has revealed conserved mechanisms of metabolic regulation by insulin and glucagon-like peptides in Drosophila, providing a powerful genetic model to address unresolved questions relevant to mammalian metabolism. Similar to mammals, secretion of Drosophila insulin-like peptides (Ilps) from neuroendocrine cells in the brain regulates glucose homeostasis and nutrient stores in the fly. Ilp secretion from insulin-producing cells (IPCs) is responsive to circulating glucose and macronutrients and is suppressed upon nutrient withdrawal. Notably, recent studies have identified hormonal and GPCR-linked mechanisms regulating the secretion of Ilps from IPCs, suggesting further conservation of pathways regulating insulin secretion in the fly (Alfa, 2015).

In mammals, the incretin hormones gastric inhibitory peptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted by enteroendocrine cells following a meal and enhance glucose-stimulated insulin production and secretion from pancreatic b cells. Thus, It was postulated that a decretin hormone would have the 'opposite' hallmarks of incretins. Specifically, a decretin (1) derives from an enteroendocrine source that is sensitive to nutrient availability, (2) is responsive to fasting or carbohydrate deficiency, and (3) suppresses insulin production and secretion from insulin-producing cells. However, like incretins, the action of decretins on insulin secretion would be manifest during feeding, when a stimulus for secretion is present (Alfa, 2015).

This study identifed a secreted hormone, Limostatin (Lst), that suppresses insulin secretion following starvation in Drosophila. lst is regulated by starvation, and flies deficient for lst display phenotypes consistent with hyperinsulinemia. Lst production was shown to be localized to glucose-sensing, endocrine corpora cardiaca (CC) cells associated with the gut, and show that lst is suppressed by carbohydrate feeding. Using calcium imaging and in vitro insulin secretion assays, a 15-aa Lst peptide (Lst-15) was identified that is sufficient to suppress activity of IPCs and Ilp secretion. An orphan GPCR was identified in IPCs as a candidate Lst receptor. Moreover, Neuromedin U (NMU) is likely a functional mammalian ortholog of Lst that inhibits islet b cell insulin secretion. These results establish a decretin signaling pathway that suppresses insulin output in Drosophila (Alfa, 2015).

Limostatin is a peptide hormone induced by carbohydrate restriction from endocrine cells associated with the gut that suppresses insulin production and release by insulin-producing cells. Thus, Drosophila Lst fulfills the functional criteria for a decretin and serves as an index member of this hormone class in metazoans. Results here also show that Lst signaling from corpora cardica cells may be mediated by the GPCR encoded by CG9918 in insulin-producing cells. In addition, the results reveal cellular and molecular features of a cell-cell signaling system in Drosophila with likely homologies to a mammalian entero-insular axis (Alfa, 2015).

Reduction of nutrient-derived secretogogues, like glucose, is a primary mechanism for attenuating insulin output during starvation in humans and flies. Consistent with this, it was found that circulating Ilp2HF levels were reduced to a similar degree in lst mutant or control flies during prolonged fasting. Therefore, lst was dispensable for Ilp2 reduction during fasting. However, lst mutants upon refeeding or during subsequent ad libitum feeding had enhanced circulating Ilp2HF levels compared to controls, findings that demonstrate a requirement for Lst to restrict insulin output in fed flies. Thus, while induced by nutrient restriction, Lst decretin function was revealed by nutrient challenge. This linkage of feeding to decretin regulation of insulin output is reminiscent of incretin regulation and action (Campbell, 2013; Alfa, 2015 and references therein).

Recent studies have demonstrated functional conservation in Drosophila of fundamental hormonal systems for metabolic regulation in mammals, including insulin, glucagon, and leptin (Rajan, 2012). This study used Drosophila to identify a hormonal regulator of insulin output, glucose, and lipid metabolism without an identified antecedent mammalian ortholog -- emphasizing the possibility for work on flies to presage endocrine hormone discovery in mammals. Gain of Lst function in these studies led to reduced insulin signaling, and hyperglycemia, consistent with prior work. By contrast, loss of Lst function led to excessive insulin production and secretion, hypoglycemia, and elevated triglycerides, phenotypes consistent with the recognized anabolic functions of insulin signaling in metazoans, and with the few prior metabolic studies of flies with insulin excess (Alfa, 2015).

Prior studies show that somatostatin and galanin are mammalian gastrointestinal hormones that can suppress insulin secretion. Somatostatin-28 (SST-28) is a peptide derivative of the pro-somatostatin gene that is expressed widely, including in gastrointestinal cells and pancreatic islet cells. Islet somatostatin signaling is thought to be principally paracrine, rather than endocrine, and serum SST-28 concentrations increase post-prandially. Galanin is an orexigenic neuropeptide produced throughout the CNS and in peripheral neurons and has been reported to inhibit insulin secretion. Unlike enteroendocrine-derived hormones that act systemically, galanin is secreted from intrapancreatic autonomic nerve terminals and is thought to exert local effects. In addition, Galanin synthesis and secretion are increased by feeding and dietary fat. Thus, like incretins, output of SST- 28 and galanin are induced by feeding, but in contrast to incretins, these peptides suppress insulin secretion. Further studies are needed to assess the roles of these peptide regulators in the modulation of insulin secretion during fasting (Alfa, 2015).

While sequence-based searches did not identify vertebrate orthologs of Lst, this study found that the postulated Lst receptor in IPCs, encoded by CG9918, is most similar to the GPCRs NMUR1 and NMUR2. In rodents, NMU signaling may be a central regulator of satiety and feeding behavior, and this role may be conserved in other organisms. In addition, NMU mutant mice have increased adiposity and hyperinsulinemia, but a direct role for NMU in regulating insulin secretion by insulin-producing cells was not identified. In rodents, the central effects of NMU on satiety are thought to be mediated by the receptor NMUR2; however, hyperphagia, hyperinsulinemia, and obesity were not reported in NMUR2 mutant mice. Together, these studies suggest that a subset of phenotypes observed in NMU mutant mice may instead reflect the activity of NMU on peripheral tissues like pancreatic islets, but this has not been previously shown. Notably, humans harboring the NMU R165W allele displayed obesity and elevated insulin C-peptide levels, without evident hyperphagia -- further suggesting that the central and peripheral effects of NMU reflect distinct pathways that may be uncoupled. This study has shown that NMU is produced abundantly in human foregut organs and suppresses insulin secretion from pancreatic b cells, supporting the view that NMU has important functions outside the CNS in regulating metabolism. Thus, like the incretin GLP-1, NMU may have dual central and peripheral signaling functions in the regulating metabolism. Demonstration that NMU is a mammalian decretin will require further studies on NMU regulation and robust methods to measure circulating NMU levels in fasting and re-feeding. In summary, these findings should invigorate searches for mammalian decretins with possible roles in both physiological and pathological settings (Alfa, 2015).

Transgenic overexpression of neuromedin U promotes leanness and hypophagia in mice

Recent work has shown that neuromedin U (NmU), a peptide initially identified as a smooth muscle contractor, may play a role in regulating food intake and energy homeostasis. To further evaluate this putative function, this study measured food intake, body weight, energy expenditure and glucose homeostasis in transgenic mice that ubiquitously overexpress murine proNmU. NmU transgenic mice were lighter and had less somatic and liver fat, were hypophagic, and had improved insulin sensitivity as judged by an intraperitoneal insulin tolerance test. Transgenic mice had higher levels of hypothalamic NPY, POMC and MCH mRNA. There was no difference in O2 consumption between genotypes; however, NmU transgenic mice displayed a modest increase in respiratory quotient during food deprivation and refeeding. There were no behavioral disturbances in the NmU transgenic mice that could account for the results (e.g. changes in locomotor activity). When placed on a high-fat diet, transgenic mice remained lighter than wild-type mice and ate less, but gained weight at a rate similar to wild-type mice. Despite the increased weight gain with high-fat feeding, glucose tolerance was significantly improved in the transgenic mice. These findings support the hypothesized role of NmU as an endogenous anorexigenic peptide (Kowalski, 2005).

Neuromedin U has a novel anorexigenic effect independent of the leptin signaling pathway

Neuromedin U (NMU) is a hypothalamic neuropeptide that regulates body weight and composition. This study shows that mice lacking the gene encoding NMU (Nmu^-/- mice) develop obesity. Nmu(-/-) mice showed increased body weight and adiposity, hyperphagia, and decreased locomotor activity and energy expenditure. Obese Nmu^-/- mice developed hyperleptinemia, hyperinsulinemia, late-onset hyperglycemia and hyperlipidemia. Notably, however, treatment with exogenous leptin was effective in reducing body weight in obese Nmu^-/- mice. In addition, central leptin administration did not affect NMU gene expression in the hypothalamus of rats. These results indicate that NMU plays an important role in the regulation of feeding behavior and energy metabolism independent of the leptin signaling pathway. These characteristic functions of NMU may provide new insight for understanding the pathophysiological basis of obesity (Hanada, 2004).

Identification of receptors for neuromedin U and its role in feeding

Neuromedin U (NMU) is a neuropeptide with potent activity on smooth muscle which was isolated first from porcine spinal cord and later from other species. It is widely distributed in the gut and central nervous system. Peripheral activities of NMU include stimulation of smooth muscle, increase of blood pressure, alteration of ion transport in the gut, control of local blood flow and regulation of adrenocortical function. An NMU receptor has not been molecularly identified. This study showed that the previously described orphan G-protein-coupled receptor FM-3 and a newly discovered one (FM-4) are cognate receptors for NMU. FM-3, designated NMU1R, is abundantly expressed in peripheral tissues whereas FM-4, designated NMU2R, is expressed in specific regions of the brain. NMU is expressed in the ventromedial hypothalamus in the rat brain, and its level is significantly reduced following fasting. Intracerebroventricular administration of NMU markedly suppresses food intake in rats. These findings provide a molecular basis for the biochemical activities of NMU and may indicate that NMU is involved in the central control of feeding (Howard, 2000).

Search PubMed for articles about Drosophila Limostatin

Alfa, R.W., Park, S., Skelly, K.R., Poffenberger, G., Jain, N., Gu, X., Kockel, L., Wang, J., Liu, Y., Powers, A.C. and Kim, S.K. (2015). Suppression of insulin production and secretion by a Decretin hormone. Cell Metab 21: 323-333. PubMed ID: 25651184

Hanada, R., Teranishi, H., Pearson, J. T., Kurokawa, M., Hosoda, H., Fukushima, N., Fukue, Y., Serino, R., Fujihara, H., Ueta, Y., Ikawa, M., Okabe, M., Murakami, N., Shirai, M., Yoshimatsu, H., Kangawa, K. and Kojima, M. (2004). Neuromedin U has a novel anorexigenic effect independent of the leptin signaling pathway. Nat Med 10: 1067-1073. PubMed ID: 15448684

Howard, A. D., et al. (2000). Identification of receptors for neuromedin U and its role in feeding. Nature 406: 70-74. PubMed ID: 10894543

Kowalski, T. J., Spar, B. D., Markowitz, L., Maguire, M., Golovko, A., Yang, S., Farley, C., Cook, J. A., Tetzloff, G., Hoos, L., Del Vecchio, R. A., Kazdoba, T. M., McCool, M. F., Hwa, J. J., Hyde, L. A., Davis, H., Vassileva, G., Hedrick, J. A. and Gustafson, E. L. (2005). Transgenic overexpression of neuromedin U promotes leanness and hypophagia in mice. J Endocrinol 185: 151-164. PubMed ID: 15817836

Rajan, A. and Perrimon, N. (2012). Drosophila cytokine unpaired 2 regulates physiological homeostasis by remotely controlling insulin secretion. Cell 151: 123-137. PubMed ID: 23021220

date revised: 5 August 2021

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