Circadian regulation of uroguanylin and guanylin in the rat intestine

Receptor Guanylyl Cyclase C and Cyclic GMP in Health and Disease: Perspectives and Therapeutic Opportunities

Receptor Guanylyl Cyclase C (GC-C) was initially characterized as an important regulator of intestinal fluid and ion homeostasis. Recent findings demonstrate that GC-C is also causally linked to intestinal inflammation, dysbiosis, and tumorigenesis. These advances have been fueled in part by identifying mutations or changes in gene expression in GC-C or its ligands, that disrupt the delicate balance of intracellular cGMP levels and are associated with a wide range of clinical phenotypes. In this review, we highlight aspects of the current knowledge of the GC-C signaling pathway in homeostasis and disease, emphasizing recent advances in the field. The review summarizes extra gastrointestinal functions for GC-C signaling, such as appetite control, energy expenditure, visceral nociception, and behavioral processes. Recent research has expanded the homeostatic role of GC-C and implicated it in regulating the ion-microbiome-immune axis, which acts as a mechanistic driver in inflammatory bowel disease. The development of transgenic and knockout mouse models allowed for in-depth studies of GC-C and its relationship to whole-animal physiology. A deeper understanding of the various aspects of GC-C biology and their relationships with pathologies such as inflammatory bowel disease, colorectal cancer, and obesity can be leveraged to devise novel therapeutics.

Mouse knockout of guanylyl cyclase C: Recognition memory deficits in the absence of activity changes

Guanylyl cyclase C (GC-C) is found in brain regions where dopamine is expressed. We characterized a mouse in which GC-C was knocked out (KO) that was reported to be a model of attention deficit hyperactivity disorder (ADHD). We re-examined this model and controlled for litter effects, used 16 to 23 mice per genotype per sex and assessed an array of behavioral and neurochemical outcomes. GC-C KO mice showed no phenotypic differences from wild-type mice on most behavioral tests, or on striatal or hippocampal monoamines, and notably no evidence of an ADHD-like phenotype. KO mice were impaired on novel object recognition, had decreased tactile startle but not acoustic startle, and females had increased latency on cued training trials in the Morris water maze, but not hidden platform spatial learning trials. Open-field activity showed small differences in females but not males. The data indicate that the GC-C KO mouse with proper controls and sample sizes has a moderate cognitive and startle phenotype but has no ADHD-like phenotype.

Dietary salt regulates uroguanylin expression and signaling activity in the kidney, but not in the intestine

The peptide uroguanylin (Ugn) is expressed at significant levels only in intestine and kidney, and is stored in both tissues primarily (perhaps exclusively) as intact prouroguanylin (proUgn). Intravascular infusion of either Ugn or proUgn evokes well-characterized natriuretic responses in rodents. Furthermore, Ugn knockout mice display hypertension and salt handling deficits, indicating that the Na(+) excretory mechanisms triggered when the peptides are infused into anesthetized animals are likely to operate under normal physiological conditions, and contribute to electrolyte homeostasis in conscious animals. Here, we provide strong corroborative evidence for this hypothesis, by demonstrating that UU gnV (the rate of urinary Ugn excretion) approximately doubled in conscious, unrestrained rats consuming a high-salt diet, and decreased by ~15% after salt restriction. These changes in UU gnV were not associated with altered plasma proUgn levels (shown here to be an accurate index of intestinal proUgn secretion). Furthermore, enteric Ugn mRNA levels were unaffected by salt intake, whereas renal Ugn mRNA levels increased sharply during periods of increased dietary salt consumption. Together, these data suggest that diet-evoked Ugn signals originate within the kidney, rather than the intestine, thus strengthening a growing body of evidence against a widely cited hypothesis that Ugn serves as the mediator of an entero-renal natriuretic signaling axis, while underscoring a likely intrarenal natriuretic role for the peptide. The data further suggest that intrarenal Ugn signaling is preferentially engaged when salt intake is elevated, and plays only a minor role when salt intake is restricted.

Pharmacology and clinical potential of guanylyl cyclase C agonists in the treatment of ulcerative colitis

Agonists of the transmembrane intestinal receptor guanylyl cyclase C (GCC) have recently attracted interest as promising human therapeutics. Peptide ligands that can specifically induce GCC signaling in the intestine include endogenous hormones guanylin and uroguanylin, diarrheagenic bacterial enterotoxins (ST), and synthetic drugs linaclotide, plecanatide, and SP-333. These agonists bind to GCC at intestinal epithelial surfaces and activate the receptor’s intracellular catalytic domain, an event initiating discrete biological responses upon conversion of guanosine-5′-triphosphate to cyclic guanosine monophosphate. A principal action of GCC agonists in the colon is the promotion of mucosal homeostasis and its dependent barrier function. Herein, GCC agonists are being developed as new medications to treat inflammatory bowel diseases, pathological conditions characterized by mucosal barrier hyperpermeability, abnormal immune reactions, and chronic local inflammation. This review will present important concepts underlying the pharmacology and therapeutic utility of GCC agonists for patients with ulcerative colitis, one of the most prevalent inflammatory bowel disease disorders.

The multiple and enigmatic roles of guanylyl cyclase C in intestinal homeostasis

Guanylyl cyclase C (GC-C) is predominantly expressed in intestinal epithelial cells and serves as the receptor for the gastrointestinal hormones guanylin and uroguanylin, and the heat-stable enterotoxin, the causative agent for Travellers' Diarrhea. Activation of GC-C results in an increase in intracellular levels of cGMP, which can regulate fluid and ion secretion, colon cell proliferation, and the gut immune system. This review highlights recent findings arising from studies in the GC-C knock-out mouse, along with enigmatic results obtained from the first descriptions of human disease caused by mutations in the GC-C gene. We provide some insight into these new findings and comment on areas of future study, which may enhance our knowledge of this evolutionarily conserved receptor and signaling system.

Circadian regulation of electrolyte absorption in the rat colon

The intestinal transport of nutrients exhibits distinct diurnal rhythmicity, and the enterocytes harbor a circadian clock. However, temporal regulation of the genes involved in colonic ion transport, i.e., ion transporters and channels operating in absorption and secretion, remains poorly understood. To address this issue, we assessed the 24-h profiles of expression of genes encoding the sodium pump (subunits Atp1a1 and Atp1b1), channels (α-, β-, and γ-subunits of Enac and Cftr), transporters (Dra, Ae1, Nkcc1, Kcc1, and Nhe3), and the Na(+)/H(+) exchanger (NHE) regulatory factor (Nherf1) in rat colonic mucosa. Furthermore, we investigated temporal changes in the spatial localization of the clock genes Per1, Per2, and Bmal1 and the genes encoding ion transporters and channels along the crypt axis. In rats fed ad libitum, the expression of Atp1a1, γEnac, Dra, Ae1, Nhe3, and Nherf1 showed circadian variation with maximal expression at circadian time 12, i.e., at the beginning of the subjective night. The peak γEnac expression coincided with the rise in plasma aldosterone. Restricted feeding phase advanced the expression of Dra, Ae1, Nherf, and γEnac and decreased expression of Atp1a1. The genes Atp1b1, Cftr, αEnac, βEnac, Nkcc1, and Kcc1 did not show any diurnal variations in mRNA levels. A low-salt diet upregulated the expression of βEnac and γEnac during the subjective night but did not affect expression of αEnac. Similarly, colonic electrogenic Na(+) transport was much higher during the subjective night than the subjective day. These findings indicate that the transporters and channels operating in NaCl absorption undergo diurnal regulation and suggest a role of an intestinal clock in the coordination of colonic NaCl absorption.

Role for the membrane receptor guanylyl cyclase-C in attention deficiency and hyperactive behavior

Midbrain dopamine neurons regulate many important behavioral processes, and their dysfunctions are associated with several human neuropsychiatric disorders such as attention deficit hyperactivity disorder (ADHD) and schizophrenia. Here, we report that these neurons in mice selectively express guanylyl cyclase-C (GC-C), a membrane receptor previously thought to be expressed mainly in the intestine. GC-C activation potentiates the excitatory responses mediated by glutamate and acetylcholine receptors via the activity of guanosine 3',5'-monophosphate-dependent protein kinase (PKG). Mice in which GC-C has been knocked out exhibit hyperactivity and attention deficits. Moreover, their behavioral phenotypes are reversed by ADHD therapeutics and a PKG activator. These results indicate important behavioral and physiological functions for the GC-C/PKG signaling pathway within the brain and suggest new therapeutic targets for neuropsychiatric disorders related to the malfunctions of midbrain dopamine neurons.

The rat kidney contains high levels of prouroguanylin (the uroguanylin precursor) but does not express GC-C (the enteric uroguanylin receptor)

The peptide uroguanylin (Ugn) regulates enteric and renal electrolyte transport. Previous studies have shown that Ugn and its receptor GC-C (a ligand-activated guanylate cyclase) are abundant in the intestine. Less is known about Ugn and GC-C expression in the kidney. Here, we identify a 9.4-kDa polypeptide in rat kidney extracts that appears, based on its biochemical and immunological properties, to be authentic prouroguanylin (proUgn). This propeptide is relatively plentiful in the kidney (~16% of intestinal levels), whereas its mRNA is marginally present (<1% of intestinal levels), and free Ugn peptide levels are below detection limits (<0.4% of renal proUgn levels). The paucity of preproUgn-encoding mRNA and free Ugn peptide raises the possibility that the kidney might absorb intact proUgn from plasma, where the concentration of propeptide greatly exceeds that of Ugn. However, immunocytochemical analysis reveals that renal proUgn is found exclusively in distal tubular segments, sites previously shown not to accumulate radiolabeled proUgn after intravascular infusions. Thus proUgn appears to be synthesized within the kidney, but the factors that determine its abundance (rates of transcription, translation, processing, and secretion) must be balanced quite differently than in the gut. Surprisingly, we also find negligible expression of GC-C in the rat kidney, a result confirmed both by RT-PCR and by functional assays that measure Ugn-activated cGMP synthesis. Taken together, these data provide evidence for an intrarenal Ugn system that differs from the well-described intestinal system in its regulatory mechanisms and in the receptor targeted by the peptide.

Role of clock genes in gastrointestinal motility

Biological rhythms coordinate the timing of our internal bodily functions. Colonic motility follows a rhythm as well: most people will have a bowel movement in the morning and rarely during the night. Recent work provides a potential mechanism for this observation: the mouse colon possesses a functional circadian clock as well as a subset of rhythmically expressed genes that may directly impact on colonic motility. Furthermore, measures of colonic motility such as the colonic tissue contractile response to acetylcholine, stool output, and intracolonic pressure changes vary as a function of the time of day, but these variations are attenuated in mice with disrupted clock function. These laboratory findings are supported by clinical observations. Gastrointestinal symptoms such as diarrhea or constipation are prevalent among shift workers and time-zone travelers, both of which are conditions associated with disruptions in biological rhythms. This review will discuss new insights into the role of clock genes in colonic motility and their potential clinical relevance.

Pressed for time: the circadian clock and hypertension

Hypertension is a major risk factor for cardiovascular disease and death. The "silent" rise of blood pressure that occurs over time is largely asymptomatic. However, its impact is deafening-causing and exacerbating cardiovascular disease, end-organ damage, and death. The present article addresses recent observations from human and animal studies that provide new insights into how the circadian clock regulates blood pressure, contributes to hypertension, and ultimately evolves vascular disease. Further, the molecular components of the circadian clock and their relationship with locomotor activity, metabolic control, fluid balance, and vascular resistance are discussed with an emphasis on how these novel, circadian clock-controlled mechanisms contribute to hypertension.

Uroguanylin, an intestinal natriuretic peptide, is delivered to the kidney as an unprocessed propeptide

Orally delivered salt stimulates renal salt excretion more effectively than does iv delivered salt. Although the mechanisms that underlie this "postprandial natriuresis" are poorly understood, the peptide uroguanylin (UGn) is thought to be a key mediator. However, the lack of selective assays for UGn gene products has hindered rigorous testing of this hypothesis. Using peptide-specific assays, we now report surprisingly little UGn in rat intestine or plasma. In contrast, prouroguanylin (proUGn), the presumed-inactive precursor of UGn, is plentiful (at least 40 times more abundant than UGn) in both intestine and plasma. The intestine is the likely source of the circulating proUGn because: 1) the proUGn portal to systemic ratio is approximately two under normal conditions, and 2) systemic proUGn levels decrease rapidly after intestinal resection. Together, these data suggest that proUGn itself is actively involved in enterorenal signaling. This is strongly supported by our observation that iv infusion of proUGn at a physiological concentration produces a long-lasting renal natriuresis, whereas previously reported natriuretic effects of UGn have required supraphysiological concentrations. Thus, our data point to proUGn as an endocrine (i.e. circulating) mediator of postprandial natriuresis, and suggest that the propeptide is secreted intact from the intestine into the circulation and processed to an active form at an extravascular site.

Duodenal bicarbonate secretion in rats: stimulation by intra-arterial and luminal guanylin and uroguanylin

AIM

Uroguanylin and guanylin are endogenous ligands for guanylate cyclase C, an upstream regulator of the cystic fibrosis transmembrane resistance (CFTR) anion channel, and both peptides increase intestinal anion export in vitro. We have compared the effects of close intra-arterial and luminal administration of uroguanylin and guanylin on duodenal bicarbonate secretion in vivo and studied the interactions with melatonin and cholinergic stimulation.

METHODS

Lewis x Dark Agouti rats were anaesthetized and a segment of the proximal duodenum with intact blood supply was cannulated in situ. Mucosal bicarbonate secretion (pH stat) was continuously recorded and peptides were infused intra-arterially or added to the luminal perfusate.

RESULTS

Intra-arterial (50-1000 pmol kg(-1) h(-1)) as well as luminal administration (50-500 nmol L(-1)) of guanylin or uroguanylin caused dose-dependent increases in the duodenal secretion. Luminal administration induced more rapidly appearing rises in secretion and the two peptides induced secretory responses of similar shape and magnitude. The melatonin MT(2)-selective antagonist luzindole (600 nmol kg(-1)) significantly depressed the response to intra-arterial guanylins but did not affect secretion induced by luminal guanylins. Similarly, the muscarinic antagonist atropine (0.75 micromol kg(-1) followed by 0.15 micromol kg(-1) h(-1)) abolished the response to intra-arterial uroguanylin but caused only slight suppression of the response to luminal uroguanylin.

CONCLUSIONS

Intra-arterial as well as luminal uroguanylin and guanylin are potent stimuli of duodenal mucosal bicarbonate secretion in vivo. The response to luminal guanylins reflects an action at apical receptors. Stimulation by parenteral guanylins, in contrast, is under cholinergic influence and interacts with melatonin produced by mucosal enteroendocrine cells.

Circadian oscillation of innate immunity components in mouse small intestine

The digestive system is a major port of entry for pathogens. To detect and combat pathogens, the innate immunity in the gut utilizes pattern recognition receptors, such as Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD) proteins, and broad-spectrum anti-bacterial polypeptides, such as defensins. We have previously shown that mouse enteric defensins (cryptdins) oscillate around the circadian cycle and peak at the end of the dark phase suggesting control by the biological clock. As the core mechanism of the biological clock has never been studied in the small intestine, our objective was to determine whether the biological clock is functional in mouse jejunum and examine whether mTlr and mNod2 mRNAs, similarly to cryptdins, oscillate throughout the circadian cycle. Mouse jejunum and Paneth-enriched crypt base cells were isolated around the circadian day and the levels of clock (mClock, mBmal1, mPer1, mPer2, mCry1) and innate immunity component (mTlr2, mTlr3, mTlr4, mTlr5, mTlr9, mNod2) genes were measured by real-time PCR. Analysis of mouse jejunum and Paneth-enriched crypt base cells revealed that all clock genes exhibited circadian oscillation. Similarly to cryptdins, mTlr2, mTlr3, mTlr4, mTlr5 displayed circadian rhythmicity in mouse jejunum. Although no circadian oscillation could be detected for mTlr9 and mNod2 in the whole jejunum, these genes oscillated in Paneth-enriched crypt base cells. In addition, mTlr3 exhibited the highest expression level. As the clock regulates intestinal motility and function, resetting of the clock in the small intestine may help not only to restore activity but also to gain better protection against pathogens.

Heterogeneous expression of melatonin receptor MT1 mRNA in the rat intestine under control and fasting conditions

Melatonin is found in mammalian central nervous system and various peripheral tissues including gastrointestinal tract (GIT) where it participates in the regulation of intestinal motility, blood flow, immunomodulation, ion transport, cell proliferation and scavenging of free radicals. Some of these effects are achieved via melatonin binding to specific receptors, MT1 and MT2. As no thorough study on the expression of these receptors in the GIT has yet been done, the aim of this study was to determine the MT1 mRNA expression in the rat intestine under both control and fasting conditions. Our results suggest that MT1 mRNA is present in epithelial as well as subepithelial layer, with higher expression in the latter in all intestinal segments studied. The highest signal of the MT1 transcript along the rostro-caudal intestinal axis was found both in epithelial and subepithelial layers of the duodenum. Nevertheless, duodenal MT1 mRNA expression did not reach the level found in pituitary gland. In a 12:12-hr light:dark cycle a MT1 receptor expression in the subepithelial layer of rat distal colon did not manifest a significant diurnal rhythm. Short-term fasting increased the expression of MT1 transcript in the subepithelial layer of both the small and large intestine. During long-term fasting the increase persisted only in distal colon while a return to control levels was observed in small intestinal segments. In conclusion we demonstrated heterogeneous expression of MT1 receptor in the rat intestine and showed that its expression is up-regulated by nutritional deprivation.

The emergence of ErbB2 expression in cultured rat hepatocytes correlates with enhanced and diversified EGF-mediated signaling

The proliferative effects of EGF in liver have been extensively investigated in cultured hepatocytes. We studied the effects of EGF, insulin, and other growth regulators on the expression, interaction, and signaling of ErbB receptors in primary cultures of adult rat hepatocytes. Using immunological methods and ErbB tyrosine kinase inhibitors, we analyzed the expression and signaling patterns of the ErbB kinases over 120 h of culture. Basal and EGF-stimulated protein tyrosine phosphorylation increased as cells adapted in vitro. EGF receptor (EGFr) expression declined in the first 24 h, whereas ErbB3 expression rose. Although ErbB2 was not present in freshly isolated hepatocytes, EGF and insulin independently induced ErbB2 while suppressing ErbB3 expression. Low concentrations of EGF and insulin synergistically stimulated ErbB2 expression and DNA synthesis. The greatest increase in ErbB2, which is normally expressed by fetal and neonatal hepatocytes, occurred shortly before the onset of DNA synthesis (> 40 h). EGF promoted EGFr and ErbB2 coassociation, stimulating tyrosine phosphorylation of both proteins. In contrast, heregulin beta1 (HRG-beta1) did not promote ErbB2 and ErbB3 coassociation. A selective tyrphostin inhibitor of ErbB2 suppressed EGF-stimulated DNA synthesis, but maximum suppression required the blockade of the EGFr kinase as well. Maximal EGF stimulation of DNA synthesis in vitro depends on the induction of ErbB2 and involves an EGFr-ErbB2 heterodimer. The ability of insulin to induce ErbB2 suggests both a mechanism for the synergy between insulin and EGF and a possible metabolic control of ErbB2 in vivo.

Preclinical relevance of dosing time for the therapeutic index of gemcitabine-cisplatin

The relevance of gemcitabine timing for chronotherapeutic optimisation was investigated. Healthy mice received multiple doses of gemcitabine (120, 160 or 200 mg kg⁻¹ injection (inj)⁻¹) at one of six circadian times (3, 7, 11, 15, 19 or 23 h after light onset – HALO) on days 1, 4, 7 and 10 or a single dose of gemcitabine (400 mg kg⁻¹) at 11 or 23 HALO±cisplatin (5 mg kg⁻¹ at 1 min, 4 or 8 h later). Mice bearing Glasgow osteosarcoma received multiple doses of gemcitabine (200 mg kg⁻¹ inj⁻¹) at 11 or 23 HALO±cisplatin (5 mg kg⁻¹ inj⁻¹ at 1 min or 4 h later) on days of 10, 13, 16 and 19 following tumour inoculation. A circadian rhythm in body weight loss was statistically validated, with 1030 HALO corresponding to the least toxic time (95% CL, 0800 to 1300). Gemcitabine dosing produced least body weight loss and least neutropenia after injection at 11 vs 23 HALO, whether the drug was given alone or with cisplatin (P=0.001). Gemcitabine–cisplatin tolerability was improved by dosing gemcitabine at 11 HALO and CDDP at 15 HALO (P<0.001). The administration of this schedule to tumour-bearing mice increased median survival three-fold as compared to treatments where both drugs were given simultaneously at 11 or 23 HALO (P=0.02). The optimal schedule would correspond to the delivery of gemcitabine upon awakening and cisplatin near mid-activity in cancer patients.

Glycosylation of the receptor guanylate cyclase C: role in ligand binding and catalytic activity

GC-C (guanylate cyclase C) is the receptor for heat-stable enterotoxins, guanylin and uroguanylin peptides. Ligand binding to the extracellular domain of GC-C activates the guanylate cyclase domain leading to accumulation of cGMP. GC-C is expressed as differentially glycosylated forms in HEK-293 cells (human embryonic kidney-293 cells). In the present study, we show that the 145 kDa form of GC-C contains sialic acid and galactose residues and is present on the PM (plasma membrane) of cells, whereas the 130 kDa form is a high mannose form that is resident in the endoplasmic reticulum and serves as the precursor for the PM-associated form. Ligand-binding affinities of the differentially glycosylated forms are similar, indicating that glycosylation of GC-C does not play a role in direct ligand interaction. However, ligand-stimulated guanylate cyclase activity was observed only for the fully mature form of the receptor present on the PM, suggesting that glycosylation had a role to play in imparting a conformation to the receptor that allows ligand stimulation. Treatment of cells at 20 degrees C led to intracellular accumulation of a mature glycosylated form of GC-C that now showed ligand-stimulated guanylate cyclase activity, indicating that localization of GC-C was not critical for its catalytic activity. To determine if complex glycosylation was required for ligand-stimulated activation of GC-C, the receptor was expressed in HEK-293 cells that were deficient in N -acetylglucosaminyltransferase 1. This minimally glycosylated form of the receptor was expressed on the cell surface and could bind a ligand with an affinity comparable with the 145 kDa form of the receptor. However, this form of the receptor was poorly activated by the ligand. Therefore our studies indicate a novel role for glycosidic modification of GC-C during its biosynthesis, in imparting subtle conformational changes in the receptor that allow for ligand-mediated activation and perhaps regulation of basal activity.

Impact of commensal microbiota on murine gastrointestinal tract gene ontologies

The gastrointestinal tract (GIT) of eukaryotes is colonized by a vast number of bacteria, where the commensal microbiota play an important role in defining the healthy gut. To investigate the influence of commensal bacteria on multiple regions of the host GIT transcriptome, the gene expression profiles of the corpus, jejunum, descending colon, and rectum of conventional (n = 3) and germ-free mice (n = 3) were examined using the Affymetrix Mu74Av2 GeneChip. Differentially regulated genes were identified using the global error assessment model, and a novel method of Gene Ontology (GO) clustering was used to identify significantly modulated biological functions. The microbiota modify the greatest number of genes in the jejunum (267 genes with an alpha < 0.001) and the fewest in the rectum (137 genes with an alpha < 0.001). Clustering genes by GO biological process and molecular function annotations revealed that, despite the large number of differentially regulated genes, the residential microbiota most significantly modified genes involved in such biological processes as immune function and water transport all along the length of the mouse GIT. Additionally, region-specific communication between the host and microbiota were identified in the corpus and jejunum, where tissue kallikrein and apoptosis regulator activities were modulated, respectively. These findings identify important interactions between the microbiota and the mouse gut tissue transcriptome and, furthermore, suggest that interactions between the microbial population and host GIT are implicated in the coordination of region-specific functions.

Site-specific effects of dietary salt intake on guanylin and uroguanylin mRNA expression in rat intestine

Guanylin and uroguanylin are newly discovered intestinal peptides that have been shown to affect NaCl transport in both the intestine and kidney. The present study tests the hypothesis that guanylin and uroguanylin mRNA expression in each major region of the intestine is regulated by NaCl intake. Semiquantitative multiplex RT-PCR analysis was used to determine the molecular expression of guanylin and uroguanylin in the duodenum, jejunum, ileum, and colon in rats maintained on low (LS), normal (NS), or high (HS) NaCl intake for 4 days. LS intake reduced the expression of uroguanylin, and to a lesser degree, guanylin mRNA in all intestinal segments compared to NS intake. The duodenum was the site of the greatest decrease for both. In contrast, HS intake significantly increased the expression of guanylin mRNA only in the duodenum and jejunum and had minimal effect on uroguanylin mRNA. The minimum time required for altered gene expression was determined by delivering an oral NaCl challenge directly to the gastrointestinal tract by oro-gastric administration to LS or NS animals. In LS rats, NaCl oro-gastric administration significantly increased mRNA expression of both peptides in all intestinal segments. Furthermore, the increases in guanylin and uroguanylin mRNA were detected within 4 h and plateaued by 8 h. Conversely, acute oro-gastric administration of the same NaCl solution to NS rats caused elevations of guanylin mRNA only in the duodenum and jejunum, and of uroguanylin mRNA only in the ileum and colon. In conclusion, the data demonstrate that variations in NaCl intake lead to intestinal segment-specific changes in guanylin and uroguanylin mRNA expression.

Molecular bases of circadian rhythms

Circadian rhythms are found in most eukaryotes and some prokaryotes. The mechanism by which organisms maintain these roughly 24-h rhythms in the absence of environmental stimuli has long been a mystery and has recently been the subject of intense research. In the past few years, we have seen explosive progress in the understanding of the molecular basis of circadian rhythms in model systems ranging from cyanobacteria to mammals. This review attempts to outline these primarily genetic and biochemical findings and encompasses work done in cyanobacteria, Neurospora, higher plants, Drosophila, and rodents. Although actual clock components do not seem to be conserved between kingdoms, central clock mechanisms are conserved. Somewhat paradoxically, clock components that are conserved between species can be used in diverse ways. The different uses of common components may reflect the important role that the circadian clock plays in adaptation of species to particular environmental niches.

Increases in guanylin and uroguanylin in a mouse model of osmotic diarrhea are guanylate cyclase C-independent

BACKGROUND & AIMS

Guanylin and uroguanylin are peptide hormones that are homologous to the diarrhea-causing Escherichia coli enterotoxins. These secretagogues are released from the intestinal epithelia into the intestinal lumen and systemic circulation and bind to the receptor guanylate cyclase C (GC-C). We hypothesized that a hypertonic diet would result in osmotic diarrhea and cause a compensatory down-regulation of guanylin/uroguanylin.

METHODS

Gut-to-carcass weights were used to measure fluid accumulation in the intestine. Northern and/or Western analysis was used to determine the levels of guanylin, uroguanylin, and GC-C in mice with osmotic diarrhea.

RESULTS

Wild-type mice fed a polyethylene glycol or lactose-based diet developed weight loss, diarrhea, and an increased gut-to-carcass ratio. Unexpectedly, 2 days on either diet resulted in increased guanylin/uroguanylin RNA and prohormone throughout the intestine, elevated uroguanylin RNA, and prohormone levels in the kidney and increased levels of circulating prouroguanylin. GC-C-deficient mice given the lactose diet reacted with higher gut-to-carcass ratios. Although they did not develop diarrhea, GC-C-sufficient and -deficient mice on the lactose diet responded with elevated levels of guanylin and uroguanylin RNA and protein. A polyethylene glycol drinking water solution resulted in diarrhea, higher gut-to-carcass ratios, and induction of guanylin and uroguanylin in both GC-C heterozygous and null animals.

CONCLUSIONS

We conclude that this model of osmotic diarrhea results in a GC-C-independent increase in intestinal fluid accumulation, in levels of these peptide ligands in the epithelia of the intestine, and in prouroguanylin in the kidney and blood.

High salt intake increases uroguanylin expression in mouse kidney

The intestinal peptides, guanylin and uroguanylin, may have an important role in the endocrine control of renal function. Both peptides and their receptor, guanylyl cyclase C (GC-C), are also expressed within the kidney, suggesting that they may act locally in an autocrine/paracrine fashion. However, their physiological regulation within the kidney has not been studied. To begin to address this issue, we evaluated the distribution of uroguanylin and guanylin messenger RNA (mRNA) in the mouse nephron and the regulation of renal expression by changes in dietary salt/water intake. Expression was determined in 1) wild-type mice, 2) two strains of receptor-guanylyl cyclase-deficient mice (ANP-receptor-deficient, GC-A-/-, and GC-C-deficient mice); and 3) cultured renal epithelial (M-1) cells, by RT-PCR, Northern blotting and immunocytochemistry. Renal uroguanylin messenger RNA expression was higher than guanylin and had a different distribution pattern, with highest levels in the proximal tubules, whereas guanylin was mainly expressed in the collecting ducts. Uroguanylin expression was significantly lower in GC-C-/- mice than in GC-A-/- and wild-types, suggesting that absence of a receptor was able to down-regulate ligand expression. Salt-loading (1% NaCl in drinking water) increased uroguanylin-mRNA expression by >1.8-fold but had no effect on guanylin expression. Uroguanylin but not guanylin transcripts were detected in M-1 cells and increased in response to hypertonic media (+NaCl or mannitol). Our results indicate that high-salt intake increases uroguanylin but not guanylin expression in the mouse kidney. The synthesis of these peptides by tubular epithelium may contribute to the local control of renal function and its adaptation to dietary salt.

Prouroguanylin overproduction and localization in the intestine of zinc-deficient rats

Identification of the upregulation of preprouroguanylin mRNA in the rat small intestine during zinc deficiency provides a potential mechanistic link between production of the intestinal hormone uroguanylin and the diarrhea that may accompany zinc deficiency. In the current study, in situ hybridization demonstrated that the number of preprouroguanylin mRNA-expressing cells was significantly higher in zinc-deficient rats than in zinc-adequate rats. Immunohistochemical studies, with a uroguanylin peptide affinity-purified antibody, demonstrated that immunoreactivity was localized to the tips of villi of the duodenum and jejunum in zinc-adequate rats. However, positive cells were scattered throughout the villus of zinc-deficient rats. A subset of cells, perhaps enterochromaffin cells, exhibited the predominant staining, whereas no specific staining was found in goblet cells or lymphocytes of the lamina propria. Western blotting demonstrated that the expression of prouroguanylin in both duodenum and jejunum was elevated by dietary zinc depletion. These results show that dietary zinc deficiency upregulates prouroguanylin in intestinal cells, which is consistent with a role for uroguanylin in the etiology of diarrhea observed in human zinc deficiency.

Expression of GC-C, a receptor-guanylate cyclase, and its endogenous ligands uroguanylin and guanylin along the rostrocaudal axis of the intestine

Members of the receptor-guanylate cyclase (rGC) family possess an intracellular catalytic domain that is regulated by an extracellular receptor domain. GC-C, an intestinally expressed rGC, was initially cloned by homology as an orphan receptor. The search for its ligands has yielded three candidates: STa (a bacterial toxin that causes traveler's diarrhea) and the endogenous peptides uroguanylin and guanylin. Here, by performing Northern and Western blots, and by measuring [125I]STa binding and STa-dependent elevation of cGMP levels, we investigate whether the distribution of GC-C matches that of its endogenous ligands in the rat intestine. We establish that 1) uroguanylin is essentially restricted to small bowel; 2) guanylin is very low in proximal small bowel, increasing to prominent levels in distal small bowel and throughout colon; 3) GC-C messenger RNA and STa-binding sites are uniformly expressed throughout the intestine; and 4) GC-C-mediated cGMP synthesis peaks at the proximal and distal extremes of the intestine (duodenum and colon), but is nearly absent in the middle (ileum). These observations suggest that GC-C's activity may be posttranslationally regulated, demonstrate that the distribution of GC-C is appropriate to mediate the actions of both uroguanylin and guanylin, and help to refine current hypotheses about the physiological role(s) of these peptides.

Biological clocks and the digestive system

Circadian rhythms play a major role in regulating the digestive systems of many organisms. Cell proliferation, migration, differentiation, and even structure vary as a function of time of day in many different digestive organs (i.e., stomach, gut, liver, and pancreas) and cell types, resulting in regionally specific temporal variations in protein and gene expression. Feeding and light set the hands of the digestive clock(s). However, the clockwork has a genetic basis. During the last 10 years, new developments have emerged in our understanding of how cells keep time. Surprisingly, clock genes in mammals are expressed not only in specialized time keepers in the brain, but also in peripheral organs, suggesting that the ability to keep time may also belong to cells within the digestive system. This article reviews several classic examples of circadian variation in the digestive system, with an emphasis on rhythms in cell proliferation, function, and structure. It also briefly summarizes several new ideas about how cells in the brain and possibly the digestive system keep time.

Expression of guanylin is downregulated in mouse and human intestinal adenomas

Guanylin is a pro-secretory hormone that is expressed in intestinal epithelia. Previously, we mapped the guanylin gene to mouse and human chromosomal regions containing multiple intestinal tumor-modifying loci. Here, we investigate whether guanylin expression is downregulated in precancerous human and mouse intestinal adenomas and whether diminished guanylin expression increases adenoma susceptibility in an animal model of intestinal cancer, the multiple intestinal neoplasia (Min) mouse. In situ hybridization analysis indicated diminished guanylin expression in both mouse and human adenomas. Northern analysis of mouse intestinal tissues showed strain-specific levels of guanylin expression but no correlation with the resistance or susceptibility of each strain to adenoma formation. Similarly, cDNA sequence analysis indicated no inactivating mutations or polymorphisms common to either the high or low adenoma-risk groups. Nonetheless, we have shown that significant loss of guanylin RNA in adenomas of mouse and human is a marker of intestinal epithelial cell transformation.