Secretin: structure of the precursor and tissue distribution of the mRNA

Secretin: a hormone for HCO3- homeostasis

Secretin is a key hormone of the intestinal phase of digestion which activates pancreatic, bile duct and Brunner gland HCO3- secretion. Recently, the secretin receptor (SCTR) was also found in the basolateral membrane of the beta-intercalated cell (B-IC) of the collecting duct. Experimental addition of secretin triggers a pronounced activation of urinary HCO3- excretion, which is fully dependent on key functional proteins of the B-IC, namely apical pendrin and CFTR and the basolateral SCTR. Recent studies demonstrated that the SCTR knock-out mouse is unable to respond to an acute base load. Here, SCTR KO mice could not rapidly increase urine base excretion, developed prolonged metabolic alkalosis and exhibited marked compensatory hypoventilation. Here, we review the physiological effects of secretin with distinct focus on how secretin activates renal HCO3- excretion. We describe its new function as a hormone for HCO3- homeostasis.

A gut-brain axis mediates sodium appetite via gastrointestinal peptide regulation on a medulla-hypothalamic circuit

Salt homeostasis is orchestrated by both neural circuits and peripheral endocrine factors. The colon is one of the primary sites for electrolyte absorption, while its potential role in modulating sodium intake remains unclear. Here, we revealed that a gastrointestinal hormone, secretin, is released from colon endocrine cells under body sodium deficiency and is indispensable for inducing salt appetite. As the neural substrate, circulating secretin activates specific receptors in the nucleus of the solitary tracts, which further activates the downstream paraventricular nucleus of the hypothalamus, resulting in enhanced sodium intake. These results demonstrated a previously unrecognized gut-brain pathway for the timely regulation of sodium homeostasis.

Modulating effects of RAMPs on signaling profiles of the glucagon receptor family

Receptor activity-modulating proteins (RAMPs) are accessory molecules that form complexes with specific G protein-coupled receptors (GPCRs) and modulate their functions. It is established that RAMP interacts with the glucagon receptor family of GPCRs but the underlying mechanism is poorly understood. In this study, we used a bioluminescence resonance energy transfer (BRET) approach to comprehensively investigate such interactions. In conjunction with cAMP accumulation, Gα_q activation and β-arrestin1/2 recruitment assays, we not only verified the GPCR–RAMP pairs previously reported, but also identified new patterns of GPCR–RAMP interaction. While RAMP1 was able to modify the three signaling events elicited by both glucagon receptor (GCGR) and glucagon-like peptide-1 receptor (GLP-1R), and RAMP2 mainly affected β-arrestin1/2 recruitment by GCGR, GLP-1R and glucagon-like peptide-2 receptor, RAMP3 showed a widespread negative impact on all the family members except for growth hormone-releasing hormone receptor covering the three pathways. Our results suggest that RAMP modulates both G protein dependent and independent signal transduction among the glucagon receptor family members in a receptor-specific manner. Mapping such interactions provides new insights into the role of RAMP in ligand recognition and receptor activation.

Comparative analysis of enteroendocrine cells and their hormones between mouse intestinal organoids and native tissues

Endocrine cells in the gastrointestinal tract secrete multiple hormones to maintain homeostasis in the body. In the present study, we generated intestinal organoids from the duodenum, jejunum, and ileum of Neurogenin 3 (Ngn3)-EGFP mice and examined how enteroendocrine cells (EECs) within organoid cultures resemble native epithelial cells in the gut. Transcriptome analysis of EGFP-positive cells from Ngn3-EGFP organoids showed gene expression pattern comparable to EECs in vivo. We also compared mRNAs of five major hormones, namely, ghrelin (Ghrl), cholecystokinin (Cck), Gip, secretin (Sct), and glucagon (Gcg) in organoids and small intestine along the longitudinal axis and found that expression patterns of these hormones in organoids were similar to those in native tissues. These findings suggest that an intestinal organoid culture system can be utilized as a suitable model to study enteroendocrine cell functions in vitro.

A genome-wide transcriptomic analysis of protein-coding genes in human blood cells

Blood is the predominant source for molecular analyses in humans, both in clinical and research settings. It is the target for many therapeutic strategies, emphasizing the need for comprehensive molecular maps of the cells constituting human blood. In this study, we performed a genome-wide transcriptomic analysis of protein-coding genes in sorted blood immune cell populations to characterize the expression levels of each individual gene across the blood cell types. All data are presented in an interactive, open-access Blood Atlas as part of the Human Protein Atlas and are integrated with expression profiles across all major tissues to provide spatial classification of all protein-coding genes. This allows for a genome-wide exploration of the expression profiles across human immune cell populations and all major human tissues and organs.

Will medications that mimic gut hormones or target their receptors eventually replace bariatric surgery?

Bariatric surgery is currently the most effective therapeutic modality through which sustained beneficial effects on weight loss and metabolic improvement are achieved. During recent years, indications for bariatric surgery have been expanded to include cases of poorly controlled type 2 (T2DM) diabetes mellitus in lesser extremes of body weight. A spectrum of the beneficial effects of surgery is attributed to robust changes of postprandial gut peptide responses that are observed post operatively. Consolidated knowledge regarding gut peptide physiology as well as emerging new evidence shedding light on the mode of action of previously overlooked gut hormones provide appealing potential obesity and T2DM therapeutic perspectives. The accumulation of evidence from the effect of exogenous administration of native gut peptides alone or in combinations to humans as well as the development of mimetic agents exerting agonistic effects on combinations of gut hormone receptors pave the way for future integrated gut peptide-based treatments, which may mimic the effects of bariatric surgery.

Non-adrenergic control of lipolysis and thermogenesis in adipose tissues

The enormous plasticity of adipose tissues, to rapidly adapt to altered physiological states of energy demand, is under neuronal and endocrine control. In energy balance, lipolysis of triacylglycerols and re-esterification of free fatty acids are opposing processes operating in parallel at identical rates, thus allowing a more dynamic transition from anabolism to catabolism, and vice versa. In response to alterations in the state of energy balance, one of the two processes predominates, enabling the efficient mobilization or storage of energy in a negative or positive energy balance, respectively. The release of noradrenaline from the sympathetic nervous system activates lipolysis in a depot-specific manner by initiating the canonical adrenergic receptor-G_s-protein-adenylyl cyclase-cyclic adenosine monophosphate-protein kinase A pathway, targeting proteins of the lipolytic machinery associated with the interface of the lipid droplets. In brown and brite adipocytes, lipolysis stimulated by this signaling pathway is a prerequisite for the activation of non-shivering thermogenesis. Free fatty acids released by lipolysis are direct activators of uncoupling protein 1-mediated leak respiration. Thus, pro- and anti-lipolytic mediators are bona fide modulators of thermogenesis in brown and brite adipocytes. In this Review, we discuss adrenergic and non-adrenergic mechanisms controlling lipolysis and thermogenesis and provide a comprehensive overview of pro- and anti-lipolytic mediators.

Secretin, at the hub of water-salt homeostasis

Water and salt metabolism are tightly regulated processes. Maintaining this milieu intérieur within narrow limits is critical for normal physiological processes to take place. Disturbances to this balance can result in disease and even death. Some of the better-characterized regulators of water and salt homeostasis include angiotensin II, aldosterone, arginine vasopressin, and oxytocin. Although secretin (SCT) was first described >100 years ago, little is known about the role of this classic gastrointestinal hormone in the maintenance of water-salt homeostasis. In recent years, increasing body of evidence suggested that SCT and its receptor play important roles in the central nervous system and kidney to ensure that the mammalian extracellular fluid osmolarity is kept within a healthy range. In this review, we focus on recent advances in our understanding of the molecular, cellular, and network mechanisms by which SCT and its receptor mediate the control of osmotic homeostasis. Implications of hormonal cross talk and receptor-receptor interaction are highlighted.

Allostery and Biased Agonism at Class B G Protein-Coupled Receptors

Class B G protein-coupled receptors (GPCRs) respond to paracrine or endocrine peptide hormones involved in control of bone homeostasis, glucose regulation, satiety, and gastro-intestinal function, as well as pain transmission. These receptors are targets for existing drugs that treat osteoporosis, hypercalcaemia, Paget's disease, type II diabetes, and obesity and are being actively pursued as targets for numerous other diseases. Exploitation of class B receptors has been limited by difficulties with small molecule drug discovery and development and an under appreciation of factors governing optimal therapeutic efficacy. Recently, there has been increasing awareness of novel attributes of GPCR function that offer new opportunity for drug development. These include the presence of allosteric binding sites on the receptor that can be exploited as drug binding pockets and the ability of individual drugs to enrich subpopulations of receptor conformations to selectively control signaling, a phenomenon termed biased agonism. In this review, current knowledge of biased signaling and small molecule allostery within class B GPCRs is discussed, highlighting areas that have progressed significantly over the past decade, in addition to those that remain largely unexplored with respect to these phenomena.

The physiological roles of secretin and its receptor

Secretin is secreted by S cells in the small intestine and affects the function of a number of organ systems. Secretin receptors (SR) are expressed in the basolateral domain of several cell types. In addition to regulating the secretion of a number of epithelia (e.g., in the pancreas and biliary epithelium in the liver), secretin exerts trophic effects in several cell types. In this article, we will provide a comprehensive review on the multiple roles of secretin and SR signaling in the regulation of epithelial functions in various organ systems with particular emphasis in the liver. We will discuss the role of secretin and its receptor in health and biliary disease pathogenesis. Finally, we propose future areas of research for the further evaluation of the secretin/secretin receptor axis in liver pathophysiology.

Eating behaviors, diet quality, and gastrointestinal symptoms in children with autism spectrum disorders: a brief review

Children with autism spectrum disorders (ASD) and their caregivers face unique challenges in the children's daily eating routines and food intake patterns. The aim of this brief review is to describe eating behaviors of children with ASD, including increased food neophobia and food selectivity, and review findings on children's diet quality, and gastrointestinal (GI) symptoms. Advancing knowledge about the interrelationships between these nutrition-related domains in children with ASD is expected to have important implications for clinical nursing practice and caregiver care.

Brain meets body: the blood-brain barrier as an endocrine interface

The blood-brain barrier (BBB) separates the central nervous system (CNS) from the peripheral tissues. However, this does not prevent hormones from entering the brain, but shifts the main control of entry to the BBB. In general, steroid hormones cross the BBB by transmembrane diffusion, a nonsaturable process resulting in brain levels that reflect blood levels, whereas thyroid hormones and many peptides and regulatory proteins cross using transporters, a saturable process resulting in brain levels that reflect blood levels and transporter characteristics. Protein binding, brain-to-blood transport, and pharmacokinetics modulate BBB penetration. Some hormones have the opposite effect within the CNS than they do in the periphery, suggesting that these hormones cross the BBB to act as their own counterregulators. The cells making up the BBB are also endocrine like, both responding to circulating substances and secreting substances into the circulation and CNS. By dividing a hormone's receptors into central and peripheral pools, the former of which may not be part of the hormone's negative feed back loop, the BBB fosters the development of variable hormone resistance syndromes, as exemplified by evidence that altered insulin action in the CNS can contribute to Alzheimer's disease. In summary, the BBB acts as a regulatory interface in an endocrine-like, humoral-based communication between the CNS and peripheral tissues.

Effect of secretin on preadipocyte, differentiating and mature adipocyte functions

OBJECTIVE

Several gastrointestinal peptides are now recognized to have target functions beyond the intestinal wall, including effects on adipocytes. Secretin (SEC), one of the first identified, has not been evaluated in this context.

METHODS

Using cultured 3T3-L1 preadipocytes, adipocytes and primary rat adipocytes we evaluated the effect of SEC on cell proliferation, mitochondrial activity, differentiation, triglyceride (TG) synthesis, lipolysis as well expression of the SEC receptor (SCTR) in rodent and human adipose tissues.

RESULTS

In preadipocytes, SEC significantly increased mitochondrial activity (115%; P<0.01), thymidine incorporation (149.7%; P<0.05) and C/EBPβ expression (123.4%; P<0.05). During standard differentiation, SCTR mRNA increased up to a maximum of ninefold (P<0.001). In human adipose tissue, SCTR correlated with body mass index and plasma insulin, and SCTR mRNA expression was also detected in rat adipose tissues. SEC supplementation during differentiation enhanced TG accumulation (+138%; P<0.01). In mature adipocytes, SEC increased fatty acid (FA) uptake (186%; P<0.01), adiponectin and monocyte chemotactic protein-1 secretion (+142% and +149%, respectively; P<0.05) and mRNA expression of PPARγ (+206%; P<0.01), FABP4 (+164%; P<0.001), DGAT-1 (+144%; P<0.01), adiponectin (+138%; P<0.001) and CD36 (+149%; P<0.05). In primary rat adipocytes, SEC also increased FA uptake (137%; P<0.05). Pretreatment with a SEC antagonist impaired SEC-induced FA uptake and cAMP accumulation. SEC treatment simultaneously stimulated lipolysis measured as glycerol release in 3T3-L1 adipocytes and rat adipose tissue.

CONCLUSION

The present results suggest that SEC is a potent modulator of adipocyte functions, demonstrating overall a role in enhanced substrate cycling.

Characterization of chicken secretin (SCT) and secretin receptor (SCTR) genes: a novel secretin-like peptide (SCT-LP) and secretin encoded in a single gene

Secretin and the secretin receptor have been reported to play an important role in regulating pancreatic water and bicarbonate secretion in mammals; however, little is known about their expression, structure, and biological functions in non-mammalian vertebrates including birds. In this study, the full-length cDNAs encoding secretin and secretin receptor have first been cloned from duodenum of adult chickens. The putative chicken secretin receptor (cSCTR) is 449 amino acids in length and shares high sequence identity (58-63%) with its mammalian counterparts. Interestingly, chicken secretin cDNA encodes not only the secretin peptide (cSCT), but also a novel secretin-like peptide (cSCT-LP), which shares high amino acid identity with chicken (56%) and mammalian (48-52%) secretin. Using a pGL3-CRE-luciferase reporter system, we further demonstrated that both cSCT (EC(50): 0.31nM) and cSCT-LP (EC(50): 1.10nM), but not other structurally-related peptides, could potently activate cSCTR expressed in CHO cells, suggesting that both peptides may function as potential ligands for cSCTR. Using RT-PCR, the expression of secretin and secretin receptor in adult chicken tissues was also examined. Secretin was detected to be predominantly expressed in small intestine, while the mRNA expression of cSCTR was restricted to several tissues including gastrointestinal tract, liver, testis, pancreas and several brain regions. Collectively, results from present study not only established a molecular basis to elucidate the physiological roles of SCT, SCT-LP and SCTR in chickens, but also provide critical insights into structural and functional changes of secretin and its receptor during vertebrate evolution.

Secretin: Should we revisit its metabolic outcomes?

Metabolic pathologies such as Type 2 Diabetes have become a major health problem for worldwide populations. Unfortunately, efforts to cure and especially to prevent these significant global problems have so far been met with disappointment. Recently, the involvement of the gut-derived hormonal dysregulation in the development of obesity-related disturbances has been intensively studied. For instance, studies of gut-derived peptides such as peptide YY 3-36, glucagon-like peptide-1, oxyntomodulin and, more recently, ghrelin have significantly improved our understanding of mechanisms underlying weight and metabolic regulation. Even though early reports of the existence of secretin, the first peptide hormone to be described, date back as far as 1825, so much and yet so little is still known about its physiological role in mammals, including humans. However, recent years have provided a better understanding of how the release of secretin is regulated by enteral secretagogues. On the other hand, most basic questions about its role in the post-prandial regulation of metabolic functions in normal and pathophysiological conditions remain to be elucidated. The present work intends to review the physiology of secretin along with its central and peripheral outcomes on metabolic functions.

Multiple actions of secretin in the human body

The discovery of secretin initiated the field of endocrinology. Over the past century, multiple gastrointestinal functions of secretin have been extensively studied, and it was discovered that the principal function of this peptide in the gastrointestinal system is to facilitate digestion and to provide protection. In view of the late identification of secretin and the secretin receptor in various tissues, including the central nervous system, the pleiotropic functions of secretin have more recently been an area of intense focus. Secretin is a classical hormone, and recent studies clearly showed secretin's involvement in neural and neuroendocrine pathways, although the neuroactivity and neural regulation of its release are yet to be elucidated. This chapter reviews our current understanding of the pleiotropic actions of secretin with a special focus on the hormonal and neural interdependent pathways that mediate these actions.

Immunohistochemical distribution of glucose-dependent insulinotropic polypeptide in the adult rat brain

We have previously demonstrated that glucose-dependent insulinotropic polypeptide (GIP; gastric inhibitory polypeptide) is present in the adult rat hippocampus. This finding leads to the conclusion that all members of the secretin-glucagon family of gastrointestinal regulatory polypeptides can be found in the brain. To investigate the localization of GIP-producing cells, we used immunohistochemistry on sections of the adult rat brain. High levels of GIP immunoreactivity were observed in the olfactory bulb, hippocampus, and Purkinje cells in the cerebellum. Moreover, a moderate but distinct GIP immunoreactivity was observed in the cerebral cortex, amygdala, substantia nigra, and lateral septal nucleus as well as in several nuclei in the thalamus, hypothalamus, and brainstem. GIP immunoreactivity was frequently found to colocalize with the neuronal marker NeuN but never with the glial marker glial fibrillary acidic protein. Thus, GIP appears to be mainly neuronal to its distribution. This widespread distribution of GIP-immunoreactive cells suggests the involvement of GIP in various neuronal functions and suggests that GIP may act as a neurotransmitter or neuromodulator. This is the first characterization of the anatomical distribution of GIP-immunoreactive cells in the rat brain providing an anatomical framework for future investigations regarding the functions of GIP in the central nervous system.

The role of secretin in the cerebellum

There is increasing interest in the role played by secretin in the central nervous system. Recent evidence suggests that this peptide is widely expressed in the brain but some areas, notably the cerebellum, show a prominent expression of the peptide and its specific receptor. In this article we summarize our current understanding of the expression pattern and action of secretin in the cerebellum. We discuss the findings supporting the endogenous release of this peptide from Purkinje neurons and its role as a retrograde messenger modulating GABAergic synaptic transmission via multiple mechanisms. In addition, we would like to propose other possible, but still to be confirmed, functions of secretin in the cerebellum..

Transient expression of secretin in serotoninergic neurons of mouse brain during development

Existence of the gastro-intestinal peptide secretin in the CNS has been a matter of debate, and contrasting results have been reported, altogether indicating that the CNS is not a major site of production of this peptide. A thorough analysis was conducted in brain of transgenic mice in which the expression of the early region of simian virus 40 large T antigen (Tag) is under control of the rat secretin gene promoter. We studied Tag expression in the brains of E14-P90 transgenic mice as well as secretin mRNA and protein expression in transgenic and control CD1 mice at corresponding developmental stages. We show here a perfect correspondence of Tag and secretin mRNA expression in the mesencephalon of transgenic and normal mice between E14 and birth. In embryos, Tag is also expressed in the spinal cord, as well as in several areas of the peripheral nervous system. Localization of Tag in P0-P90 animals becomes restricted to a single compact cellular mass in mesencephalon at the level of the dorsal raphe, raphe magnus and lateral paragigantocellular nuclei. Neurons of these nuclei display secretin mRNA from E14 to birth, in both control CD1 and transgenic mice. Approximately half of these secretin-expressing neurons are immunoreactive for serotonin (5HT) and/or tryptophan hydroxylase. These results demonstrate that the secretin gene is transiently expressed in mouse serotoninergic mesencephalic neurons during development. In addition our data suggest a trophic role for secretin on neurons known to be involved in multiple superior functions in the normal brain, and lost in neurodegenerative disorders.

Sudden onset of colitis after ablation of secretin-expressing lymphocytes in transgenic mice

Though secretin mRNA was demonstrated in mouse lymphoid organs, its role in the immune system is unknown. Here, secretin gene-expressing cells were ablated by ganciclovir infusion in mice transgenic for the rat secretin promoter (Sec) directing the expression of herpesvirus thymidine kinase (Sec-HSVTK). Thymus, spleen, blood, and colon were investigated by histology. Lymphoid cells were extracted and quantified, and CD19+ B-cells and CD3+, CD103+, CD4+, and CD8+ T-cells were analyzed by flow cytometry. Protein extracts from spleen and thymus were assayed for secretin by Western blotting, and isolated lymphocytes were investigated for HSVTK, secretin, and secretin receptor (Sec-R) mRNA by reverse transcription-polymerase chain reaction (RT-PCR). Ablation of secretin-expressing cells produced severe colitis with morphological features similar to those observed in graft-versus-host (GVH) disease. Profound lymphoid depletion was observed in spleen, thymus, and peripheral blood. The relative percentage of B- and T-cell subsets were unaffected. Analysis of colonic lymphocytes revealed a marked depletion of CD4+ T lymphocytes. Colitis and lymphoid depletion were not reversed by secretin cotreatment. Immunoblot analysis of protein extracts from spleen and thymus identified secretin-like immmunoreactant. RT-PCR of lymphocyte mRNA from spleen and thymus identified secretin and secretin receptor transcripts. We conclude that GVH-like colitis in ganciclovir-treated Sec-HSVTK mice arises from depletion of secretin gene-expressing lymphoid cells and not from the failure of secretin production.

Secretin: hypothalamic distribution and hypothesized neuroregulatory role in autism

1. This study aims (1) to determine whether secretin is synthesized centrally, specifically by the HPA axis and (2) to discuss, on the basis of the findings in this and previous studies, secretin's possible neuroregulatory role in autism. 2. An immunocytochemical technique with single-cell resolution was performed in 12 age/weight-matched male rats pretreated with stereotaxic microinjection of colchicine (0.6 microg/kg) or vehicle into the lateral ventricle. Following 2-day survival, rats were anesthetized and perfused for immunocytochemistry. Brain segments were blocked and alternate frozen 30-microm sections incubated in rabbit antibodies against secretin, vasoactive intestinal peptide, glucagon, or pituitary-adenylate-cyclase-activating peptide. Adjacent sections were processed for Nissl stain. Preadsorption studies were performed with members of the secretin peptide family to demonstrate primary antibody specificity. 3. Specificity of secretin immunoreactivity (ir) was verified by clear-cut preadsorption control data and relatively high concentrations and distinct topographic localization of secretin ir to paraventricular/supraoptic and intercalated hypothalamic nuclei. Secretin levels were upregulated by colchicine, an exemplar of homeostatic stressors, as compared with low constitutive expression in untreated rats. 4. This study provides the first direct immunocytochemical demonstration of secretinergic immunoreactivity in the forebrain and offers evidence that the hypothalamus, like the gut, is capable of synthesizing secretin. Secretin's dual expression by gut and brain secretin cells, as well as its overlapping central distribution with other stress-adaptation neurohormones, especially oxytocin, indicates that it is stress-sensitive. A neuroregulatory relationship between the peripheral and central stress response systems is suggested, as is a dual role for secretin in conditioning both of those stress-adaptation systems. Colchicine-induced upregulation of secretin indicates that secretin may be synthesized on demand in response to stress, a possible mechanism of action that may underlie secretin's role in autism.

Antiaging methods and medicines for the memory

The interaction of bedside and basic science has led to the identification ofa short list of pathological proteins as causal in Alzheimer's disease. AI3P has received the most attention, and work with animal models has reinforced the evidence that overproduction of ABP causes cognitive impairments. Animal models are now being used to discover and develop unique therapeutics directed at reversing the deleterious effects of ABP. These models strongly suggest that established Alzheimer's disease might be reversible, not just preventable. Animal models are also demonstrating that other peptides and proteins can enhance or impair cognitive function. These peptides and proteins add further to the list of possible therapeutic candidates. Approaches such as these, and not the commercial antiaging remedies that have no scientific basis, will eventually provide medicine for memory enhancement.

Secretin Controls Anion Secretion in the Rat Epididymis in an Autocrine/Paracrine Fashion1

There is growing evidence that secretin, the first hormone discovered in our history, has functions in the brain other than in the gastrointestinal tract. This article reports for the first time that secretin and its receptor mRNAs are produced in distinct cell types within the epididymis. To test if secretin affects electrolyte transport in the epididymis, we measured short-circuit current (Isc) in cultured epididymal epithelia and found secretin dose-dependently stimulated Isc. Ion substitution experiments and use of pharmacological agents inferred that the stimulated Isc is a result of concurrent electrogenic chloride and bicarbonate secretion. It is further shown that secretin and pituitary adenylate cyclase-activating polypeptide (PACAP) function via totally different mechanisms: 1) PACAP works only from the apical side of the epithelium to stimulate chloride and not bicarbonate secretion, while secretin acts on the apical and basolateral sides to stimulate chloride and bicarbonate secretion. 2) the stimulation by PACAP but not secretin requires local prostaglandin synthesis. By immunocytochemical staining, secretin is localized in the principal cells of the initial segment and caput epididymidis, whereas secretin receptor is present in the principal cells of the proximal as well as the distal part of the epididymis. This pattern of distribution appears to be consistent with the idea that secretin is secreted by the proximal epididymis and acts on the proximal and distal epididymis in an autocrine and paracrine fashion. Its function is to control secretion of electrolytes and water.

Secretin activation of chromogranin A gene transcription. Identification of the signaling pathways in cis and in trans

Secretin evokes catecholamine secretion from PC12 pheochromocytoma cells. We tested whether secretin activates transcription of the major vesicular core protein chromogranin A (CgA). Secretin stimulated both endogenous CgA gene transcription (approximately 4-6-fold) as well as transfected CgA promoter activity (approximately 8-10-fold; EC50, approximately 7 nm) in PC12 cells. Studies on CgA promoter 5'-deletion mutant/luciferase reporter constructs, point mutations of the CgA cAMP response element (CRE), and their transfer to a heterologous promoter implicated CRE in cis as both necessary and sufficient for secretin-stimulated CgA gene transcription. Secretin-induced CgA gene transcription was inhibited/abolished by cytosolic Ca2+ chelation, chemical blockade of phospholipase C, protein kinase A (PKA), or mitogen-activated protein (MAP) kinase extracellular signal regulated kinase (ERK) 1/2 and the expression of dominant negative mutants of ERK1/2, CRE binding protein (CREB) kinase RSK2, or CREB. Secretin also augmented (approximately 4-fold) phosphorylation of ERK1/2. Trans-activation (approximately 21-fold) of GAL4-CREB fusion protein by secretin indicates involvement of CREB in secretin signaling to gene transcription. Electrophoretic mobility shift assays also identified CREB as the mediator of secretin-induced CgA gene transcription, and pCREB supershifts indicated Ser-133 as the active CREB moiety in vitro. This conclusion was reinforced in vivo by results of chromatin pCREB immunoprecipitation assays. We conclude that secretin signals to CgA gene transcription through the CRE domain in cis and through cAMP, Ca2+, PKA, MAP kinase, and the transcription factor CREB in trans. Thus, multiple signal transduction pathways seem to subserve the function of stimulus-transcription coupling after this peptidergic stimulus to chromaffin cells.

Evidence on the presence of secretin cells in the gastric antral and oxyntic mucosa

Secretin is released from upper small intestinal mucosa to drive pancreatic secretion of fluid and bicarbonate and inhibit gastric acid secretion. Recently, we found that, in isolated, vascularly perfused rat stomach model, the inhibition of acid secretion by pituitary adenylate cyclase activating polypeptide (PACAP) was mediated in part via local release of secretin. However, the presence of secretin-producing cells and mRNA in gastric mucosa, particularly in oxyntic mucosa, has not been established. The present study was carried out to establish the presence of secretin cells by immunohistochemical and mRNA by biochemical methods in gastric mucosa. Secretin cells were identified in antral mucosa (27.8 +/- 2.0 cells/mm(2)) and corpus (4.7 +/- 0.5 cells/mm(2)). They were distinguishable, through double immunostaining, from gastrin and somatostatin cells in the antrum and from somatostatin cells in the corpus. The results of reverse transcription (RT)-PCR and Southern blot indicated that a secretin gene transcript of 454 bp was present in the mRNA extracts of both antral and corpus mucosae. The results indicated that secretin mRNA is present in gastric mucosa. In conclusion, secretin-producing cells and mRNA are present in gastric mucosa and the locally released secretin may exert a paracrine effect to inhibit acid secretion.

International Union of Pharmacology. XXXV. The glucagon receptor family

Peptide hormones within the secretin-glucagon family are expressed in endocrine cells of the pancreas and gastrointestinal epithelium and in specialized neurons in the brain, and subserve multiple biological functions, including regulation of growth, nutrient intake, and transit within the gut, and digestion, energy absorption, and energy assimilation. Glucagon, glucagon-like peptide-1, glucagon-like peptide-2, glucose-dependent insulinotropic peptide, growth hormone-releasing hormone and secretin are structurally related peptides that exert their actions through unique members of a structurally related G protein-coupled receptor class 2 family. This review discusses advances in our understanding of how these peptides exert their biological activities, with a focus on the biological actions and structural features of the cognate receptors. The receptors have been named after their parent and only physiologically relevant ligand, in line with the recommendations of the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR).

Differential transport of a secretin analog across the blood-brain and blood-cerebrospinal fluid barriers of the mouse

Secretin is a gastrointestinal peptide belonging to the vasoactive intestinal peptide (VIP)/glucagon/pituitary adenylate cyclase-activating polypeptide (PACAP) family recently suggested to have therapeutic effects in autism. A direct effect on brain would require secretin to cross the blood-brain barrier (BBB), an ability other members of the VIP/PACAP family have. Herein, we examined whether a secretin analog (SA) radioactively labeled with (131)I (I-SA) could cross the BBB of 4-week-old mice. We found I-SA was rapidly cleared from serum with fragments not precipitating with acid appearing in brain and serum. Levels of radioactivity were corrected to reflect only intact I-SA as estimated by acid precipitation. After i.v. injection, I-SA was taken up by brain at a modest rate of 0.9 to 1.5 microl/g-mm. Capillary depletion, brain perfusion, and high-performance liquid chromatography were used to confirm the passage of intact I-SA across the BBB. I-SA entered every brain region, with the highest uptake into the hypothalamus and cerebrospinal fluid (CSF). Unlabeled SA (10 microg/mouse) did not inhibit uptake by brain but did inhibit clearance from blood and uptake by the CSF, colon, kidney, and liver. The decreased clearance of I-SA from blood increased the percentage of the i.v. injected dose taken up per brain (%Inj/g) from about 0.118 to 0.295%Inj/g. In conclusion, SA crosses the vascular barrier by a nonsaturable process and the choroid plexus by a saturable process in amounts that for other members of its family produce central nervous system (CNS) effects. This passage provides a pathway through which peripherally administered SA could affect the CNS.

Secretin as a neuropeptide

The role of secretin as a classical hormone in the gastrointestinal system is well-established. The recent debate on the use of secretin as a potential therapeutic treatment for autistic patients urges a better understanding of the neuroactive functions of secretin. Indeed, there is an increasing body of evidence pointing to the direction that, in addition to other peptides in the secretin/glucagon superfamily, secretin is also a neuropeptide. The purpose of this review is to discuss the recent data for supporting the neurocrine roles of secretin in rodents. By in situ hybridization and immunostaining, secretin was found to be expressed in distinct neuronal populations within the cerebellum and cerebral cortex, whereas the receptor transcript was found throughout the brain. In the rat cerebellum, secretin functions as a retrograde messenger to facilitate GABA transmission, indicating that it can modulate motor and other functions. In summary, the recent data support strongly the neuropeptide role of secretin, although the secretin-autism link remains to be clarified in the future.

The human secretin gene: fine structure in 11p15.5 and sequence variation in patients with autism

Secretin is a peptide hormone involved in digestion that has been studied as a potential therapeutic agent in patients with autism. We characterized the human secretin locus to determine whether mutations in this gene might play a role in a fraction of autism patients. While the secretin gene (SCT) was not found to be mutated in the majority of autistic patients, rare heterozygous sequence variants were identified in three patients. We also investigated length variation in a variable number of tandem repeats (VNTR) immediately upstream of SCT and found no significant differences in length between patients with autism and normal controls. SCT is located on 11p15.5, adjacent to DRD4 and HRAS. This region has been reported to be associated with both autism and attention deficit hyperactivity disorder (ADHD). Although imprinting is a characteristic of some genes in the vicinity, we could find no evidence for methylation of SCT in lymphoblast cells from patients or control individuals.

Secretin facilitates GABA transmission in the cerebellum

Secretin was the first hormone discovered in human history, and yet, its function as a neuropeptide has been overlooked in the past. The recent discovery of the potential use of secretin in treating autistic patients, together with the conflicting reports on its effectiveness, urges an in-depth investigation of this issue. We show here that in the rat cerebellar cortex, mRNAs encoding secretin are localized in the Purkinje cells, whereas those of its receptor are found in both Purkinje cells and GABAergic interneurons. Immunoreactivity for secretin is localized in the soma and dendrites of Purkinje cells. In addition, secretin facilitates evoked, spontaneous, and miniature IPSCs recorded from Purkinje cells. We propose that secretin is released from the somatodendritic region of Purkinje cells and serves as a retrograde messenger modulating GABAergic afferent activity.

Secretin facilitates GABA transmission in the cerebellum

Secretin was the first hormone discovered in human history, and yet, its function as a neuropeptide has been overlooked in the past. The recent discovery of the potential use of secretin in treating autistic patients, together with the conflicting reports on its effectiveness, urges an in-depth investigation of this issue. We show here that in the rat cerebellar cortex, mRNAs encoding secretin are localized in the Purkinje cells, whereas those of its receptor are found in both Purkinje cells and GABAergic interneurons. Immunoreactivity for secretin is localized in the soma and dendrites of Purkinje cells. In addition, secretin facilitates evoked, spontaneous, and miniature IPSCs recorded from Purkinje cells. We propose that secretin is released from the somatodendritic region of Purkinje cells and serves as a retrograde messenger modulating GABAergic afferent activity.

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.

Expression and motor effects of secretin in small and large intestine of the rat

Immunocytochemistry and in situ hybridization revealed abundant secretin expressing cells on duodenal villi with a gradual decrease throughout the small intestines of the rat. They were absent in pancreas, stomach and colon. Secretin caused relaxation of rat intestinal longitudinal muscle in vitro. Studies on colon revealed that the secretin-evoked response was unaffected by apamin, tetrodotoxin, L-NAME, VIP or PACAP pretreatment; secretin itself caused desensitization. Addition of VIP or PACAP when the secretin-evoked relaxation was maximal evoked a further relaxation suggesting the presence of distinct receptors. Secretin causes relaxation via activation of secretin receptors located on the smooth muscle and not via any of the related VIP/PACAP receptors.

Secretin, glucagon, gastric inhibitory polypeptide, parathyroid hormone, and related peptides in the regulation of the hypothalamus- pituitary-adrenal axis

Secretin, glucagon, gastric inhibitory polypeptide (GIP), and parathyroid hormone (PTH) belong, together with vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase (AC)-activating polypeptide, to a family of peptides (the VIP-secretin-glucagon family), which also includes growth hormone-releasing hormone and exendins. All the members of this peptide family possess a remarkable amino-acid sequence homology, and bind to G-protein-coupled receptors, whose signaling mechanism primarily involves AC/protein kinase A and phospholipase C/protein kinase C cascades. VIP and pituitary AC-activating polypeptide play a role in the regulation of the hypothalamus-pituitary-adrenal (HPA) axis, and in this review we survey findings that also other members of the VIP-secretin-glucagon family may have the same function. Secretin and secretin receptors are expressed in the hypothalamus and pituitary gland, and secretin inhibits adrenocorticotropic hormone (ACTH) release. No evidence is available for the presence of secretin receptors in adrenal glands, but secretin selectively depresses the glucocorticoid response to ACTH of dispersed zona fasciculata-reticularis (ZF/R) cells. Glucagon and glucagon-like peptide-1 are contained in the hypothalamus, and all the components of the HPA axis are provided with glucagon and glucagons-like-1 receptors. These peptides exert a short-term inhibitory effect on stress-induced pituitary ACTH release and depress the ZF/R cell response to ACTH by inhibiting the AC/protein kinase A cascade; they also stimulate hypothalamic arginine-vasopressin release. GIP receptors are present in the ZF/R of the normal adrenals, and are particularly abundant in some types of adrenocortical adenomas and hyperplasias. GIP, through the activation of the AC/protein kinase A cascade, evokes a sizeable glucocorticoid secretagogue effect, leading to the identification of a food/GIP-dependent Cushing's syndrome. PTH and PTH-related protein are expressed in the hypothalamus and pituitary gland, and PTH and PTH-related protein receptors in all the components of the HPA axis. Both peptides enhance ACTH and arginine-vasopressin release, as well as stimulate aldosterone and glucocorticoid secretion of dispersed zona glomerulosa and ZF/R cells, respectively. The involvement of growth hormone-releasing hormone and exendins in the functional regulation of the HPA axis has not yet been extensively investigated.

COOH-terminally extended secretins are potent stimulants of pancreatic secretion

Posttranslational processing of preprosecretin generates several COOH-terminally extended forms of secretin and alpha-carboxyl amidated secretin. We used synthetic canine secretin analogs with COOH-terminal -amide, -Gly, or -Gly-Lys-Arg to examine the effects of COOH-terminal extensions of secretin on bioactivity and detection in RIA. Synthetic products were purified by reverse-phase and ion-exchange HPLC and characterized by reverse-phase isocratic HPLC and amino acid, sequence, and mass spectral analyses. Secretin and secretin-Gly were noted to coelute during reverse-phase HPLC. In RIA using eight different antisera raised against amidated secretin, COOH-terminally extended secretins had little or no cross-reactivity. Bioactivity was assessed by measuring pancreatic responses in anesthetized rats. Amidated canine and porcine secretins were equipotent. Secretin-Gly and secretin-Gly-Lys-Arg had potencies of 81 +/- 9% (P > 0.05) and 176 +/- 13% (P < 0.01), respectively, compared with amidated secretin, and the response to secretin-Gly-Lys-Arg lasted significantly longer. These data demonstrate that 1) amidated secretin and secretin-Gly are not separable under some chromatographic conditions, 2) current RIA may not detect bioactive COOH-terminally extended forms of secretin in tissue extracts or blood, and 3) the secretin receptor mediating stimulation of pancreatic secretion recognizes both amidated and COOH-terminally extended secretins.

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.

Purification, cDNA sequence, and tissue distribution of rat uroguanylin

Guanylin, a peptide purified from rat jejunum, is thought to regulate water and electrolyte balance in the intestine. We show here, using a combination of Northern blots, Western blots, and functional assays, that guanylin and its receptor (GCC) are not distributed in parallel within the rat intestine. To investigate the possibility that there might be a second intestinal peptide that serves as a ligand for GCC, we assayed tissue extracts for the ability to stimulate cyclic GMP synthesis in a GCC-expression cell line. Duodenal extracts display a peak of biological activity that is not present in colon and that does not comigrate with guanylin or proguanylin. The activity co-purifies with a novel peptide (TIATDECELCINVACTGC) that has high homology with uroguanylin, a peptide initially purified from human and opossum urine. A rat uroguanylin cDNA clone was found to encode a propeptide whose C-terminus corresponds to our purified peptide. Northern blots with probes generated from this clone reveal that prouroguanylin mRNA is strongly expressed in proximal small intestine, but virtually absent from colon, corroborating our biochemical measurements. Taken together, these studies demonstrate an intestinal origin for uroguanylin, and show that within the intestine its distribution is complementary to that of guanylin.

Two alternative processing pathways for a preprohormone: a bioactive form of secretin

An N-terminally 9-residue elongated form of secretin, secretin-(-9 to 27) amide, was isolated from porcine intestinal tissue and characterized. Current knowledge about peptide processing sites does not allow unambiguous prediction of the signal peptide cleavage site in preprosecretin but suggests cleavage in the region of residues -10 to -14 counted upstream from the N terminus of the hormone. However, the structure of the isolated peptide suggests that the cleavage between the signal peptide and the N-terminal propeptide occurs at the C-terminal side of residue -10. Moreover, the isolated peptide demonstrates that secretin can be fully processed C-terminally prior to the final N-terminal cleavage. The results from this report, and those from earlier studies, where C-terminally elongated variants were isolated, show that the processing of the secretin precursor may proceed by one of two alternative pathways, in which either of the two ends is processed first. The bioactivity of the N-terminally extended peptide on exocrine pancreatic secretion was lower than that of secretin, indicating the importance of the finally processed free N terminus of the hormone for interaction with secretin receptors.

Identification and localization of secretin and secretin receptor mRNAs in rat testis

Secretin is a well-conserved member of the growth hormone-releasing hormone (GHRH) family of peptides expressed in brain, gut and gonads. To determine whether secretin may also play a physiological role in testis, we examined the level and cellular distribution of secretin and secretin receptor gene expression in rat testis. RNAs from total testis, Sertoli, germ and Leydig cells were amplified by comparative reverse transcription-polymerase chain reaction (RT-PCR). Southern blot analysis of the PCR products indicated secretin and secretin receptor mRNA expression primarily in germ cells. Sequence comparisons of cloned secretin and secretin receptor PCR products showed 100% identity with the previously reported sequences. To localize secretin and secretin receptor mRNAs at the cellular level within testis,in situ hybridization was performed. Specific hybridization to secretin mRNA was observed in low abundance in many germ cell types, but was heaviest over step 19 spermatids in stages VII and VIII tubules. Secretin receptor mRNA was detected in approximately the same cell types as was secretin mRNA, except that labeling was greatest in round spermatids (steps 6-8). Since the patterns of gene expression of secretin and its receptor overlap, these data suggest that there may be an intrinsic secretin system in testis.

Studies in transgenic mice reveal potential relationships between secretin-producing cells and other endocrine cell types

We have produced transgenic mice expressing fusion genes consisting of 1.6 kilobase pairs of the secretin gene 5' flanking region to direct the expression of human growth hormone (hGH) or simian virus 40 large T antigen to secretin-producing cells. Analysis of different mouse tissues for hGH transcripts revealed expression in each of the major secretin-producing tissues, namely the intestine and endocrine pancrease. Multiple label immunohistochemistry demonstrated that the transgene was correctly directed to secretin cells in the intestinal tract, including a previously unrecognized population of secretin cells in the colon of adult and developing mice. In the small intestine, subpopulations of hGH-containing cells frequently coexpressed substance P, serotonin, and cholecystokinin, whereas in the colon, cells expressing hGH frequently coexpressed glucagon, peptide YY, or neurotensin. Transgenic mice expressing large T antigen in secretin cells developed poorly differentiated neuroendocrine tumors of the small intestine, well differentiated colonic tumors containing glucagon-expressing cells, and insulin-producing tumors in pancreas. These studies indicate that the major cis-regulatory sequences necessary for secretin expression in enteroendocrine cells and fetal islets are localized with 1.6 kilobase pairs of the transcriptional start site. Coexpression of reporter transgenes with several gastrointestinal hormones suggests a potential relationships between secretin cells and other enteroendocrine cell types, as well as pancreatic beta cells.

Induction of ethanol dependence increases signal peptidase mRNA levels in rat brain

Differential Northern blot hybridization was used as a screening tool to identify mRNAs that respond quantitatively to the induction of ethanol dependence. Adult male rats were treated with repeated, high doses of ethanol for 4 consecutive days. This regimen resulted in the development of tolerance and dependence upon ethanol. RNA isolated from the ethanol-dependent rat brains was used to construct a cDNA library. One cDNA was identified that hybridized to a mRNA which increased in rat brain during the ethanol treatment. Sequence analysis of the cDNA indicated that it recognized a mRNA in rat brain which was very similar to that which encodes the 18 kDa subunit of canine signal peptidase. The rat signal peptidase mRNA was observed to increase in brain nearly 2-fold within 48 h after the initiation of ethanol treatment. Ethanol did not significantly alter beta-actin mRNA levels during the treatment period. These results support the existence of an ethanol-responsive signal peptidase mRNA in rat brain.

Novel hirudin variants from the leech Hirudinaria manillensis. Amino acid sequence, cDNA cloning and genomic organization

Novel hirudin variants isolated from the leech Hirudinaria manillensis, a leech more specialized for mammalian parasitism, are described. Isolation of antithrombin polypeptides was performed by ion-exchange chromatographies followed by an affinity chromatography step on immobilized thrombin. The major active component, antithrombin polypeptide peak 2 (HM2) and a second polypeptide, named HM1, were purified to homogeneity and their complete amino acid sequences were determined. The protein structure of the two hirudin variants include 64 amino acids with 6 cysteine residues at highly conserved positions. Comparison of the amino acid sequences of HM1 and HM2 with other known hirudins shows differences mainly in the central part and in the C-terminal region of the polypeptides. Particularly significant is the lack of a sulfated tyrosine residue in the C-terminal portion of the molecule which is replaced by aspartic acid. Polymerase chain reaction cloning techniques were used to isolate and characterize the cDNAs and determine the genomic structures of these hirudin-like polypeptides. The cDNA clones coding for the two variants indicate the expression of pre-hirudins of 84 amino acids where the first 20 residues constitute the signal peptide required for extracellular secretion. The leader sequence appears to be highly conserved for both isoforms and shares a complete similarity with the partial hirudin variant 2 (HV2) signal peptide sequence previously reported. The HM1 and HM2 gene fragments show the presence of four exons: the first one corresponding to a 20-amino-acid signal peptide while the other three exons share the full primary structure of the antithrombin polypeptides. HM2 was also efficiently produced in recombinant Escherichia coli by expressing a periplasmic construction containing the synthetic gene.

Glucose-dependent insulinotropic peptide: structure of the precursor and tissue-specific expression in rat

Glucose-dependent insulinotropic peptide (GIP) is a 42-amino acid gastrointestinal regulatory peptide that stimulates insulin secretion from pancreatic beta cells in the presence of glucose. Approximately 7.8 x 10(5) recombinant clones of a neonatal rat intestinal cDNA library were screened by using plaque hybridization, and three clones were identified and sequenced with the dideoxynucleotide chain-termination method. The translated amino acid sequence deduced from the nucleotide sequence of the cDNA indicated that rat GIP was derived by proteolytic processing of a 144-amino acid precursor polypeptide. The mature peptide is flanked by a 43-amino acid NH2-terminal peptide that contains a 21-amino acid signal peptide and by a 59-amino acid COOH-terminal peptide. Analysis of the nucleotide and amino acid sequence of rat GIP revealed only two substitutions from the known human GIP peptide. The use of high-stringency RNA blot-hybridization analysis of total RNA extracted from various organs demonstrated expression of the GIP gene in the duodenum and jejunum and, to a lesser extent, in the ileum. In addition, expression of the GIP gene was observed in the submandibular salivary gland both by RNA analysis and RIA. In response to duodenal perfusion of a 20% Lipomul meal for 60 min, duodenal mucosal GIP mRNA concentrations increased by 42.8% and 48.2% at 30 and 60 min, respectively.

Identification of a transcriptional enhancer important for enteroendocrine and pancreatic islet cell-specific expression of the secretin gene

It is well established that the gene encoding the hormone secretin is expressed in a specific enteroendocrine cell, the S cell. We now show that the secretin gene is transiently expressed in insulin-producing B cells of the developing pancreatic islets in addition to the intestine. Furthermore, secretin is produced by most established islet cell lines. In order to identify and characterize the regulatory elements within the secretin gene that control tissue-specific expression, we have introduced secretin reporter gene constructions into the secretin-producing HIT and STC-1 cell lines as well as the nonexpressing INR1-G9 glucagonoma line. Analysis of deletion mutants revealed that sequences between 174 and 53 bp upstream from the transcriptional start site are required for maximal expression in secretin-producing cells. This positive element functioned independently of position and orientation. Further deletions into the enhancer resulted in a stepwise loss of transcriptional activity, suggesting the presence of several discrete control elements. The sequence CAGCTG within the secretin enhancer closely resembles that of the core of the B-cell-specific enhancer in the insulin gene. Point mutations introduced into this putative element led to greater than 85% reduction in transcriptional activity. Gel mobility shift assays suggested that a factor in B cells closely related or identical to proteins that bind to the insulin enhancer interacts with the CAGCTG motif in the secretin gene.

Identification of a transcriptional enhancer important for enteroendocrine and pancreatic islet cell-specific expression of the secretin gene

It is well established that the gene encoding the hormone secretin is expressed in a specific enteroendocrine cell, the S cell. We now show that the secretin gene is transiently expressed in insulin-producing B cells of the developing pancreatic islets in addition to the intestine. Furthermore, secretin is produced by most established islet cell lines. In order to identify and characterize the regulatory elements within the secretin gene that control tissue-specific expression, we have introduced secretin reporter gene constructions into the secretin-producing HIT and STC-1 cell lines as well as the nonexpressing INR1-G9 glucagonoma line. Analysis of deletion mutants revealed that sequences between 174 and 53 bp upstream from the transcriptional start site are required for maximal expression in secretin-producing cells. This positive element functioned independently of position and orientation. Further deletions into the enhancer resulted in a stepwise loss of transcriptional activity, suggesting the presence of several discrete control elements. The sequence CAGCTG within the secretin enhancer closely resembles that of the core of the B-cell-specific enhancer in the insulin gene. Point mutations introduced into this putative element led to greater than 85% reduction in transcriptional activity. Gel mobility shift assays suggested that a factor in B cells closely related or identical to proteins that bind to the insulin enhancer interacts with the CAGCTG motif in the secretin gene.

The secretin gene: evolutionary history, alternative splicing, and developmental regulation

The gene encoding the hormone secretin has been isolated and structurally characterized. The transcriptional unit is divided into four exons spanning 813 nucleotides. Comparison of the rat secretin gene to the other members of the glucagon-secretin gene family reveals that similarities are restricted to the exons encoding the biologically active peptides. Analysis of RNA from porcine intestine indicates that at least two transcripts are generated from the porcine secretin gene as a result of differential splicing. The longer and more abundant transcript appears to be identical to a previously isolated cDNA, which encodes a precursor that includes a 72-amino acid C-terminal extension peptide. The shorter transcript does not contain the third exon and, as a result, encodes only 44 residues beyond the C terminus of secretin. The amino acid sequence deduced from the shorter transcript is identical to a precursor form of secretin recently isolated from porcine duodenum [Gafvelin, G., Jornvall, H. & Mutt, V. (1990) Proc. Natl. Acad. Sci. USA 87, 6781-6785]. Developmental studies reveal that both secretin mRNA and peptide levels in the intestine are highest just before birth, prior to the onset of gastric acid secretion and feeding. This observation implies that secretin biosynthesis in developing animals is controlled independently of the principal factors known to regulate secretin release in adult animals.