Disruption of the guanylyl cyclase-C gene leads to a paradoxical phenotype of viable but heat-stable enterotoxin-resistant mice

A uroguanylin-GUCY2C endocrine axis regulates feeding in mice

Intestinal enteroendocrine cells are critical to central regulation of caloric consumption, since they activate hypothalamic circuits that decrease appetite and thereby restrict meal size by secreting hormones in response to nutrients in the gut. Although guanylyl cyclase and downstream cGMP are essential regulators of centrally regulated feeding behavior in invertebrates, the role of this primordial signaling mechanism in mammalian appetite regulation has eluded definition. In intestinal epithelial cells, guanylyl cyclase 2C (GUCY2C) is a transmembrane receptor that makes cGMP in response to the paracrine hormones guanylin and uroguanylin, which regulate epithelial cell dynamics along the crypt-villus axis. Here, we show that silencing of GUCY2C in mice disrupts satiation, resulting in hyperphagia and subsequent obesity and metabolic syndrome. This defined an appetite-regulating uroguanylin-GUCY2C endocrine axis, which we confirmed by showing that nutrient intake induces intestinal prouroguanylin secretion into the circulation. The prohormone signal is selectively decoded in the hypothalamus by proteolytic liberation of uroguanylin, inducing GUCY2C signaling and consequent activation of downstream anorexigenic pathways. Thus, evolutionary diversification of primitive guanylyl cyclase signaling pathways allows GUCY2C to coordinate endocrine regulation of central food acquisition pathways with paracrine control of intestinal homeostasis. Moreover, the uroguanylin-GUCY2C endocrine axis may provide a therapeutic target to control appetite, obesity, and metabolic syndrome.

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.

Linaclotide, a synthetic guanylate cyclase C agonist, for the treatment of functional gastrointestinal disorders associated with constipation

Chronic constipation (CC) and irritable bowel syndrome with constipation (IBS-C) are two functional gastrointestinal disorders that are associated with constipation. CC and IBS-C affect approximately 20% of the general population including the elderly, impairing quality of life. Patients not responding to over-the-counter treatments require effective and safe long-term therapies. Some treatments introduced in the last decade have been associated with side effects that led to withdrawal from the US market (e.g., tegaserod) or intolerance to treatment (e.g., nausea in patients treated with lubiprostone). Linaclotide is a novel drug, with a unique mechanism of action, low bioavailability and local action in the intestinal epithelial cells. It is currently being developed for patients with CC and IBS-C. From animal studies to human pharmacodynamic Phase Ib trials, and a comprehensive program of Phase IIb and III trials in health and disease, linaclotide demonstrates long-term efficacy and safety in CC and IBS-C.

Murine guanylate cyclase C regulates colonic injury and inflammation

Guanylate cyclase C (GUCY2C or GC-C) and its ligands, guanylin (GUCA2A or Gn) and uroguanylin (GUCA2B or Ugn), are expressed in intestinal epithelial cells and regulate ion secretion, intestinal barrier function, and epithelial monolayer homeostasis via cGMP-dependent signaling pathways. The aim of this study was to determine whether GC-C and its ligands direct the course of intestinal inflammation. In this article, we show that dextran sodium sulfate (DSS)-induced clinical disease and histological damage to the colonic mucosa were significantly less severe in GC-C(-/-) mice and moderately reduced in Gn(-/-) animals. Relative to wild-type controls, GC-C(-/-) and Gn(-/-) mice had reduced apoptosis and increased proliferation of intestinal epithelial cells during DSS colitis. Basal and DSS-induced production of resistin-like molecule β (RELMβ) was substantially diminished in GC-C(-/-) mice. RELMβ is thought to stimulate cytokine production in macrophages in this disease model and, consistent with this, TNF-α and IFN-γ production was minimal in GC-C(-/-) animals. RELMβ and cytokine levels were similar to wild-type in Gn(-/-) mice, however. Colonic instillation of recombinant RELMβ by enema into GC-C(-/-) mice restores sensitivity to DSS-mediated mucosal injury. These findings demonstrate a novel role for GC-C signaling in facilitating mucosal wounding and inflammation, and further suggest that this may be mediated, in part, through control of RELMβ production.

Transmembrane guanylate cyclase in intestinal pathophysiology

PURPOSE OF REVIEW

Production of cyclic guanosine monophosphate (cGMP) by guanylate cyclase is of critical importance to gastrointestinal physiology. Tight regulation of cGMP concentration is necessary for proper intestinal secretion and intestinal epithelial cell proliferative and apoptotic homeostasis. This review focuses on recent work detailing the role of a subset of transmembrane guanylate cyclases in the pathophysiology of intestinal secretory and motility disorders and intestinal epithelial cell transformation. Also considered is the potential for therapeutic manipulation of intestinal guanylate cyclase/cGMP signaling for the correction of chronic constipation and gastrointestinal cancer.

RECENT FINDINGS

Recent work in mice and humans suggests a role for transmembrane guanylate cyclases in intestinal fluid secretion as well as hormonal enteric-renal signaling which mediates postprandial natriuresis. Transmembrane guanylate cyclases are also important in gastrointestinal transit rate and motility. Ongoing clinical trials have found that guanylate cyclase activating peptides are safe and effective in the treatment of constipation-predominant irritable bowel syndrome and chronic constipation. In addition, accumulating evidence indicates that membrane-associated guanylate cyclase receptors regulate intestinal epithelial cell homeostatic proliferation and apoptosis as well as gastrointestinal malignancy. The anticancer activity of cGMP signaling in animal studies suggests additional therapeutic applications for guanylate cyclase agonists.

SUMMARY

Progress toward understanding gastrointestinal transmembrane guanylate cyclase/cGMP physiology has recently accelerated due to definitive in-vitro studies and work using gene-targeted animal models and has facilitated the development of safe and effective drugs designed to regulate cGMP production in the intestine. Current work should be directed toward a detailed understanding of cGMP effector pathways and the manner in which subcellular concentrations of cGMP regulate them to influence intestinal health and disease.

Regulation and therapeutic targeting of peptide-activated receptor guanylyl cyclases

Cyclic GMP is a ubiquitous second messenger that regulates a wide array of physiologic processes such as blood pressure, long bone growth, intestinal fluid secretion, phototransduction and lipolysis. Soluble and single-membrane-spanning enzymes called guanylyl cyclases (GC) synthesize cGMP. In humans, the latter group consists of GC-A, GC-B, GC-C, GC-E and GC-F, which are also known as NPR-A, NPR-B, StaR, Ret1-GC and Ret2-GC, respectively. Membrane GCs are activated by peptide ligands such as atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), C-type natriuretic peptide (CNP), guanylin, uroguanylin, heat stable enterotoxin and GC-activating proteins. Nesiritide and carperitide are clinically approved peptide-based drugs that activate GC-A. CD-NP is an experimental heart failure drug that primarily activates GC-B but also activates GC-A at high concentrations and is resistant to degradation. Inactivating mutations in GC-B cause acromesomelic dysplasia type Maroteaux dwarfism and chromosomal mutations that increase CNP concentrations are associated with Marfanoid-like skeletal overgrowth. Pump-based CNP infusions increase skeletal growth in a mouse model of the most common type of human dwarfism, which supports CNP/GC-B-based therapies for short stature diseases. Linaclotide is a peptide activator of GC-C that stimulates intestinal motility and is in late-stage clinical trials for the treatment of chronic constipation. This review discusses the discovery of cGMP, guanylyl cyclases, the general characteristics and therapeutic applications of GC-A, GC-B and GC-C, and emphasizes the regulation of transmembrane guanylyl cyclases by phosphorylation and ATP.

Natriuretic and antikaliuretic effects of uroguanylin and prouroguanylin in the rat

The peptide uroguanylin (Ugn) is stored and released as a propeptide (proUgn) by enterochromaffin cells in the intestine, and converted to Ugn and other metabolites in the renal tubules. Both proUgn and Ugn are natriuretic, although the response to proUgn is thought to depend on its conversion to Ugn within nephrons. To assess the efficiency of intrarenal conversion of proUgn to Ugn, we measured urinary Ugn excretion in rats following intravenous infusions of proUgn or Ugn. Infusion of 2 and 10 nmol proUgn/kg body wt increased plasma proUgn concentration from 2.2 ± 0.3 to 5.6 ± 1.3 pmol/ml and to 37 ± 9.6 pmol/ml, respectively. No proUgn was detected in urine before, during, or after proUgn infusions. These two proUgn infusion doses resulted in total Ugn recovery in urine of 162 ± 64 and 206 ± 39 pmol/kg body wt (9 and 2% of the infused amount, respectively). By contrast, the same molar amounts of Ugn resulted in 1,009 ± 477 and 5,352 ± 2,133 pmol/kg body wt of Ugn in urine (recoveries of ∼50%). Unexpectedly, comparisons of natriuretic dose-response curves for each peptide showed proUgn to be about five times more potent than Ugn, despite the relatively modest amount of Ugn generated from infused proUgn. In addition, both peptides were antikaliuretic at low doses, but in this case Ugn showed greater potency than proUgn. These data do not support Ugn as the primary active principle of proUgn for regulation of renal sodium excretion. Instead, an alternative peptide fragment produced from proUgn may be responsible for natriuretic activity in the kidney, whereas Ugn itself may play an antikaliuretic role.

Linaclotide, through activation of guanylate cyclase C, acts locally in the gastrointestinal tract to elicit enhanced intestinal secretion and transit

Linaclotide is a first-in-class, orally administered 14-amino acid peptide that is in development for the treatment of irritable bowel syndrome with constipation and chronic constipation. We have characterized the solution structure of linaclotide, the in vitro binding and agonist activity to guanylate cyclase C receptors, the stability of linaclotide under conditions mimicking the gastric environment, oral bioavailability, and the pharmacodynamic effects in rat models of gastrointestinal transit and intestinal secretion. Nuclear magnetic resonance spectroscopy analysis determined that the molecular structure of linaclotide is stabilized by three intramolecular disulfide bridges. Linaclotide exhibited high affinity and pH-independent binding (K(i): 1.23-1.64 nM) to guanylate cyclase C receptors on human colon carcinoma T84 cells and concomitantly, linaclotide binding resulted in a significant, concentration-dependent accumulation of intracellular cyclic guanosine-3', 5'-monophosphate (cGMP) (EC₅₀:99 nM). Linaclotide was stable after 3 h incubation in simulated gastric fluid (pH 1) and similarly, was completely resistant to hydrolysis by pepsin. Pharmacokinetic analysis of linaclotide showed very low oral bioavailability (0.1%). Orally administered linaclotide elicited a significant, dose-dependent increase in gastrointestinal transit rates in rats at doses of ≥5 μg/kg. Exposure of surgically ligated small intestinal loops to linaclotide induced a significant increase in fluid secretion, accompanied by a significant increase in intraluminal cGMP levels. These results suggest that the guanylate cyclase C agonist linaclotide elicits potent pharmacological responses locally in the gastrointestinal tract, and that orally administered guanylate cyclase C agonists may be capable of improving bowel habits in patients suffering from irritable bowel syndrome with constipation and chronic constipation.

Cure and curse: E. coli heat-stable enterotoxin and its receptor guanylyl cyclase C

Enterotoxigenic Escherichia coli (ETEC) associated diarrhea is responsible for roughly half a million deaths per year, the majority taking place in developing countries. The main agent responsible for these diseases is the bacterial heat-stable enterotoxin STa. STa is secreted by ETEC and after secretion binds to the intestinal receptor guanylyl cyclase C (GC-C), thus triggering a signaling cascade that eventually leads to the release of electrolytes and water in the intestine. Additionally, GC-C is a specific marker for colorectal carcinoma and STa is suggested to have an inhibitory effect on intestinal carcinogenesis. To understand the conformational events involved in ligand binding to GC-C and to devise therapeutic strategies to treat both diarrheal diseases and colorectal cancer, it is paramount to obtain structural information on the receptor ligand system. Here we summarize the currently available structural data and report on physiological consequences of STa binding to GC-C in intestinal epithelia and colorectal carcinoma cells.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

Bacterial heat-stable enterotoxins: translation of pathogenic peptides into novel targeted diagnostics and therapeutics

Heat-stable toxins (STs) produced by enterotoxigenic bacteria cause endemic and traveler’s diarrhea by binding to and activating the intestinal receptor guanylyl cyclase C (GC-C). Advances in understanding the biology of GC-C have extended ST from a diarrheagenic peptide to a novel therapeutic agent. Here, we summarize the physiological and pathophysiological role of GC-C in fluid-electrolyte regulation and intestinal crypt-villus homeostasis, as well as describe translational opportunities offered by STs, reflecting the unique characteristics of GC-C, in treating irritable bowel syndrome and chronic constipation, and in preventing and treating colorectal cancer.

Lack of guanylate cyclase C results in increased mortality in mice following liver injury

Background

Guanylate Cyclase C (GC-C) expression in the intestine plays a role in the regulation of fluid and ion transport, as well as epithelial cell apoptosis and proliferation. In the adult rat liver, GC-C expression is increased in response to injury. We hypothesized that GC-C is required for repair/recovery from liver injury.

Methods

We subjected wild type (WT) and GC-C deficient mice to acute liver injury with a single injection of the hepatotoxin carbon tetrachloride. Changes in the level of expression of GC-C and its ligands uroguanylin and guanylin were quantified by real-time PCR. Liver morphology, and hepatocyte necrosis, apoptosis and proliferation, were examined at 1-3 days post-injury in mice on a mixed genetic background. Survival was followed for 14 days after carbon tetrachloride injection in wild type and GC-C deficient mice on both a mixed genetic background and on an inbred C57BL6/J background.

Results

GC-C deficient mice on the mixed genetic background nearly all died (median survival of 5 days) following carbon tetrachloride injection while WT littermates experienced only 35% mortality. Elevated levels of TUNEL-positive hepatocyte death on post-injury day 1, increased apoptosis on day 2, and increased areas of centrilobular necrosis on days 2 and 3, were evident in livers from GC-C null mice compared to WT. Collectively these data suggest increased hepatocyte death in the GC-C null mice in the early time period after injury. This corresponds temporally with increased expression of GC-C and its ligands guanylin and uroguanylin in post-injury WT mouse liver. The hepatocyte proliferative response to injury was the same in both genotypes. In contrast, there was no difference in survival between GC-C null and WT mice on the inbred C57BL/6 J background in response to acute liver injury.

Conclusions

Signalling via GC-C promotes hepatocyte survival in vivo and is required for effective recovery from acute toxic injury to the liver in a strain-specific manner.

Linaclotide is a potent and selective guanylate cyclase C agonist that elicits pharmacological effects locally in the gastrointestinal tract

AIMS

Linaclotide is an orally administered 14-amino acid peptide being developed for the treatment of constipation-predominant irritable bowel syndrome (IBS-C) and chronic constipation. We determined the stability of linaclotide in the intestine, measured the oral bioavailability, and investigated whether the pharmacodynamic effects elicited in rodent models of gastrointestinal function are mechanistically linked to the activation of intestinal guanylate cyclase C (GC-C).

MAIN METHODS

Linaclotide binding to intestinal mucosal membranes was assessed in competitive binding assays. Stability and oral bioavailability of linaclotide were measured in small intestinal fluid and serum, respectively, and models of gastrointestinal function were conducted using wild type (wt) and GC-C null mice.

KEY FINDINGS

Linaclotide inhibited in vitro [(125)I]-STa binding to intestinal mucosal membranes from wt mice in a concentration-dependent manner. In contrast, [(125)I]-STa binding to these membranes from GC-C null mice was significantly decreased. After incubation in vitro in jejunal fluid for 30 min, linaclotide was completely degraded. Pharmacokinetic analysis showed very low oral bioavailability (0.10%). In intestinal secretion and transit models, linaclotide exhibited significant pharmacological effects in wt, but not in GC-C null mice: induction of increased fluid secretion into surgically ligated jejunal loops was accompanied by the secretion of elevated levels of cyclic guanosine-3',5'-monophosphate and accelerated gastrointestinal transit.

SIGNIFICANCE

Linaclotide is a potent and selective GC-C agonist that elicits pharmacological effects locally in the gastrointestinal tract. This pharmacological profile suggests that orally administered linaclotide may be capable of improving the abdominal symptoms and bowel habits of patients suffering from IBS-C and chronic constipation.

Heat-stable enterotoxin of enterotoxigenic Escherichia coli as a vaccine target

Enterotoxigenic Escherichia coli (ETEC) is responsible for 280 million to 400 million episodes of diarrhea and about 380,000 deaths annually. Epidemiological data suggest that ETEC strains which secrete heat-stable toxin (ST), alone or in combination with heat-labile toxin (LT), induce the most severe disease among children in developing countries. This makes ST an attractive target for inclusion in an ETEC vaccine. ST is released upon colonization of the small intestine and activates the guanylate cyclase C receptor, causing profuse diarrhea. To generate a successful toxoid, ST must be made immunogenic and nontoxic. Due to its small size, ST is nonimmunogenic in its natural form but becomes immunogenic when coupled to an appropriate large-molecular-weight carrier. This has been successfully achieved with several carriers, using either chemical conjugation or recombinant fusion techniques. Coupling of ST to a carrier may reduce toxicity, but further reduction by mutagenesis is desired to obtain a safe vaccine. More than 30 ST mutants with effects on toxicity have been reported. Some of these mutants, however, have lost the ability to elicit neutralizing immune responses to the native toxin. Due to the small size of ST, separating toxicity from antigenicity is a particular challenge that must be met. Another obstacle to vaccine development is possible cross-reactivity between anti-ST antibodies and the endogenous ligands guanylin and uroguanylin, caused by structural similarity to ST. Here we review the molecular and biological properties of ST and discuss strategies for developing an ETEC vaccine that incorporates immunogenic and nontoxic derivatives of the ST toxin.

Linaclotide - a secretagogue and antihyperalgesic agent - what next?

Ongoing clinical trials suggest that linaclotide, a first-in-class, 14-amino acid peptide guanylate cyclase-C (GC-C) receptor agonist and intestinal secretagogue is an effective treatment for chronic constipation. A study in this issue of the Journal suggests that linaclotide also has antihyperalgesic effects in three common rat models of inflammation- and stress-induced hypersensitivity (i.e., acute trinitrobenzene sulfonic acid colitis, water avoidance stress [WAS], and restraint-induced stress) but not in naïve animals. In mice, linaclotide at least partly reduces hyperalgesia via GC-C receptors. Dose-effect relationships of linaclotide were complicated and non-linear. This viewpoint discusses human clinical trials with linaclotide and the results of this study. Potential mechanisms and clinical significance of these findings are explored. Collectively, these data suggest that GC-C receptors exert other, as yet poorly understood, effects on gastrointestinal sensitivity in conditions associated with inflammation and/or stress-induced increased intestinal permeability. However, the data need to be confirmed in humans and in long-term animal models. Further studies are also necessary to elucidate the mechanisms as these effects cannot be explained by linaclotide's known effects on epithelial GC-C receptors.

Novel functions of photoreceptor guanylate cyclases revealed by targeted deletion

Targeted deletion of membrane guanylate cyclases (GCs) has yielded new information concerning their function. Here, we summarize briefly recent results of laboratory generated non-photoreceptor GC knockouts characterized by complex phenotypes affecting the vasculature, heart, brain, kidney, and other tissues. The main emphasis of the review, however, addresses the two GCs expressed in retinal photoreceptors, termed GC-E and GC-F. Naturally occurring GC-E (GUCY2D) null alleles in human and chicken are associated with an early onset blinding disorder, termed "Leber congenital amaurosis type 1" (LCA-1), characterized by extinguished scotopic and photopic ERGs, and retina degeneration. In mouse, a GC-E null genotype produces a recessive cone dystrophy, while rods remain functional. Rod function is supported by the presence of GC-F (Gucy2f), a close relative of GC-E. Deletion of Gucy2f has very little effect on rod and cone physiology and survival. However, a GC-E/GC-F double knockout (GCdko) phenotypically resembles human LCA-1 with extinguished ERGs and rod/cone degeneration. In GCdko rods, PDE6 and GCAPs are absent in outer segments. In contrast, GC-E(-/-) cones lack proteins of the entire phototransduction cascade. These results suggest that GC-E may participate in transport of peripheral membrane proteins from the endoplasmic reticulum (ER) to the outer segments.

Receptor guanylyl cyclase C (GC-C): regulation and signal transduction

Receptor guanylyl cyclase C (GC-C) is the target for the gastrointestinal hormones, guanylin, and uroguanylin as well as the bacterial heat-stable enterotoxins. The major site of expression of GC-C is in the gastrointestinal tract, although this receptor and its ligands play a role in ion secretion in other tissues as well. GC-C shares the domain organization seen in other members of the family of receptor guanylyl cyclases, though subtle differences highlight some of the unique features of GC-C. Gene knock outs in mice for GC-C or its ligands do not lead to embryonic lethality, but modulate responses of these mice to stable toxin peptides, dietary intake of salts, and development and differentiation of intestinal cells. It is clear that there is much to learn in future about the role of this evolutionarily conserved receptor, and its properties in intestinal and extra-intestinal tissues.

Activation of guanylate cyclase C signaling pathway protects intestinal epithelial cells from acute radiation-induced apoptosis

Uroguanylin (UGN) is a peptide hormone that binds to and activates the intestinal epithelial cell (IEC) transmembrane receptor guanylate cyclase C (GC-C), which in turn increases intracellular cGMP. Gene targeting of murine UGN or GC-C results in significantly lower levels of cGMP in IECs. On the basis of effects of cGMP in nonintestinal systems, we hypothesized that loss of GC-C activation would increase intestinal epithelial apoptosis following radiation-induced injury. We first compared apoptosis from the proximal jejunum of C57BL/6 wild-type (WT) and GC-C knockout (KO) mice 3 h after they received 5 Gy of gamma-irradiation. We then investigated whether supplementation via intraperitoneal injection of 1 mM 8BrcGMP would mitigate radiation-induced apoptosis in these experimental animals. Identical experiments were performed in BALB/c UGN WT and KO mice. Apoptosis was assessed by quantitating morphological indications of cell death, terminal dUTP nick-end labeling, and cleaved caspase 3 immunohistochemistry. Both UGN KO and GC-C KO mice were more susceptible than their WT littermates in this in vivo model of apoptotic injury. Furthermore, cGMP supplementation in both GC-C and UGN KO animals ameliorated radiation-induced apoptosis. Neither WT strain demonstrated significant alteration in apoptotic susceptibility as a result of cGMP supplementation before radiation injury. These in vivo findings demonstrate increased radiosensitivity of IECs in UGN and GC-C KO mice and a role for cGMP as a primary downstream mediator of GC-C activation in the protection of these IECs from radiation-induced apoptosis.

Function and dysfunction of mammalian membrane guanylyl cyclase receptors: lessons from genetic mouse models and implications for human diseases

Besides soluble guanylyl cyclase (GC), the receptor for NO, there are seven plasma membrane forms of guanylyl cyclase (GC) receptors, enzymes that synthesize the second-messenger cyclic GMP (cGMP). All membrane GCs (GC-A to GC-G) share a basic topology, which consists of an extracellular ligand binding domain, a short transmembrane region, and an intracellular domain that contains the catalytic (GC) region. Although the presence of the extracellular domain suggests that all these enzymes function as receptors, specific ligands have been identified for only four of them (GC-A through GC-D). GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure and volume homeostasis and also local antihypertrophic and antifibrotic actions in the heart. GC-B, the specific receptor for C-type natriuretic peptide, has a critical role in endochondral ossification. GC-C mediates the effects of guanylin and uroguanylin on intestinal electrolyte and water transport and epithelial cell growth and differentiation. GC-E and GC-F are colocalized within the same photoreceptor cells of the retina and have an important role in phototransduction. Finally, GC-D and GC-G appear to be pseudogenes in the human. In rodents, GC-D is exclusively expressed in the olfactory neuroepithelium, with chemosensory functions. GC-G is the last member of the membrane GC form to be identified. No other mammalian transmembrane GCs are predicted on the basis of gene sequence repositories. In contrast to the other orphan receptor GCs, GC-G has a broad tissue distribution in rodents, including the lung, intestine, kidney, skeletal muscle, and sperm, raising the possibility that there is another yet to be discovered family of cGMP-generating ligands. This chapter reviews the structure and functions of membrane GCs, with special focus on the insights gained to date from genetically modified mice and the role of alterations of these ligand/receptor systems in human diseases.

Guanylyl cyclase C as a biomarker for targeted imaging and therapy of metastatic colorectal cancer

The guanylyl cyclase C (GCC) receptor posseses several well-established properties ideal for use as a biomarker in gastrointestinal malignancies. The GCC receptor is constitutively expressed in the apical membranes of the intestine and its expression is universally preserved in primary colorectal tumors and their metastases. Moreover, receptor binding is retained by GCC’s cognate ligand, the bacterial enterotoxin ST, even after conjugation to functional moieties. Selective tumor, but not gastrointestinal, uptake of ST in mice bearing GCC-expressing colon cancer xenografts demonstrates the potential of exploiting ST–GCC interaction for diagnostic imaging and targeted therapy of metastatic colorectal cancer. We expect this specific targeting provided by ST–GCC interaction to improve diagnosis, staging and management of colorectal cancer metastases, and ultimately prolong patient survival in this disease.

Sex modulates intestinal transformation by the tumor-suppressor GCC

BACKGROUND AND AIMS

Ovarian hormones oppose colorectal cancer, although mechanisms remain undefined. Similarly, the most commonly lost gene products in intestinal neoplasia include guanylin and uroguanylin, paracrine hormones for guanylyl cyclase C (GCC), which recently emerged as a tumor suppressor. However, the molecular intersection between intestinal paracrine and systemic sex hormones opposing intestinal neoplasia has not been explored.

METHODS

Intestinal tumorigenesis was quantified in wild type (Gcc(+/+)) and GCC-deficient (Gcc(-/-)) mice carrying mutations in adenomatous polyposis coli (Apc) (Apc(Min/+)) or exposed to the carcinogen azoxymethane (AOM). Proliferation of epithelial cells was examined employing cell cycle markers.

RESULTS

Deletion of Gcc increased tumor multiplicity and growth in colons and small intestines, respectively, of Apc(Min) (/+) mice. While changes in multiplicity and growth increased tumor burden, females exhibited approximately 60% (p= 0.040) of the burden in males. Similarly, female Gcc(-/-) mice treated with AOM exhibited approximately 40% (p= 0.048) of the burden in males. Moreover, Gcc deletion promoted epithelial cell proliferation, quantified by increases in beta-catenin, cMyc, cyclin D1, and phosphorylated retinoblastoma protein (pRb), in males but not females.

CONCLUSION

There is a previously unappreciated interaction between sex and GCC signaling restricting crypt cell proliferation. Thus, the invariable loss of guanylin and uroguanylin resulting in tumorigenesis is mitigated in females by hormonal components of the ovarian axis. In the context of the universal overexpression of GCC by tumors, these observations highlight the combination of GCC paracrine and ovarian hormones for targeted prevention and therapy of colorectal cancer.

Guanylyl cyclase C-induced immunotherapeutic responses opposing tumor metastases without autoimmunity

BACKGROUND

One of the greatest impediments to cancer immunotherapy is the paucity of antigens that are tumor specific, sufficiently immunogenic, and shared among patients. Mucosa-restricted antigens that are expressed by tumor cells represent a novel class of vaccine targets that are characterized by immunologic privilege, which limits systemic tolerance to those antigens, and immunologic partitioning, which shields mucosae from systemic autoimmune responses. Here we defined the immunogenicity and antitumor efficacy of guanylyl cyclase C (GCC), a protein that is normally restricted to intestinal mucosa and universally expressed by metastatic colorectal cancer.

METHODS

BALB/c mice (n = 197) were immunized with recombinant GCC-expressing viral vectors before (prophylactic) or after (therapeutic) a lethal challenge of GCC-expressing mouse colon cancer cells, and antitumor efficacy was monitored by quantifying metastasis and survival. Induction of autoimmunity was monitored by histopathology. Induction of GCC-specific B-cell and CD4(+) and CD8(+) T-cell responses were determined by enzyme-linked immunosorbent assay and ELISpot, respectively. Tolerance to GCC was quantified by comparing responses in GCC-deficient (n = 45) and wild-type (n = 69) C57BL/6 mice. Statistical tests were two-sided.

RESULTS

Immunization with GCC-expressing viral vectors reduced the formation of metastases to liver (control vs GCC: mean = 30.4 vs 3.55 nodules, difference = 26.9 nodules, 95% confidence interval [CI] = 8.47 to 45.3 nodules; P = .008) and lung (control vs GCC: mean = 263 vs 55.7 nodules, difference = 207, 95% CI = 163 to 251; P < .001) and extended the median survival of mice with established lung metastases following therapeutic immunization (control vs GCC: 29 vs 38 days, P = .024), without autoimmunity. Antitumor efficacy reflected asymmetrical tolerance that was characterized by CD8(+) T-cell, but not CD4(+) T-cell or antibody, responses.

CONCLUSIONS

Immunologic partitioning together with immunologic privilege highlight the potential of mucosa-restricted antigens, particularly GCC, as therapeutic targets for metastatic cancer.

Enterotoxin preconditioning restores calcium-sensing receptor-mediated cytostasis in colon cancer cells

Guanylyl cyclase C (GCC), the receptor for diarrheagenic bacterial heat-stable enterotoxins (STs), inhibits colorectal cancer cell proliferation by co-opting Ca(2+) as the intracellular messenger. Similarly, extracellular Ca(2+) (Ca(2+)(o)) opposes proliferation and induces terminal differentiation in intestinal epithelial cells. In that context, human colon cancer cells develop a phenotype characterized by insensitivity to cytostasis imposed by Ca(2+)(o). Here, preconditioning with ST, mediated by GCC signaling through cyclic nucleotide-gated channels, restored Ca(2+)(o)-dependent cytostasis, reflecting posttranscriptional regulation of calcium-sensing receptors (CaRs). ST-induced GCC signaling deployed CaRs to the surface of human colon cancer cells, whereas elimination of GCC signaling in mice nearly abolished CaR expression in enterocytes. Moreover, ST-induced Ca(2+)(o)-dependent cytostasis was abrogated by CaR-specific antisense oligonucleotides. Importantly, following ST preconditioning, newly expressed CaRs at the cell surface represented tumor cell receptor targets for antiproliferative signaling by CaR agonists. Since expression of the endogenous paracrine hormones for GCC is uniformly lost early in carcinogenesis, these observations offer a mechanistic explanation for the Ca(2+)(o)-resistant phenotype of colon cancer cells. Restoration of antitumorigenic CaR signaling by GCC ligand replacement therapy represents a previously unrecognized paradigm for the prevention and treatment of human colorectal cancer employing dietary Ca(2+) supplementation.

Homeostatic control of the crypt-villus axis by the bacterial enterotoxin receptor guanylyl cyclase C restricts the proliferating compartment in intestine

Guanylyl cyclase C (GC-C), the receptor for diarrheagenic enterotoxins and the paracrine ligands guanylin and uroguanylin, regulates intestinal secretion. Beyond volume homeostasis, its importance in modulating cancer cell proliferation and its uniform dysregulation early in colon carcinogenesis, reflecting loss of ligand expression, suggests a role for GC-C in organizing the crypt-villus axis. Here, eliminating GC-C expression in mice increased crypt length along a decreasing rostral-caudal gradient by disrupting component homeostatic processes. Crypt expansion reflected hyperplasia of the proliferating compartment with reciprocal increases in rapidly cycling progenitor cells and reductions in differentiated cells of the secretory lineage, including Paneth and goblet cells, but not enteroendocrine cells. GC-C signaling regulated proliferation by restricting the cell cycle at the G(1)/S transition. Moreover, crypt expansion in GC-C(-/-) mice was associated with adaptive increases in cell migration and apoptosis. Reciprocal alterations in proliferation and differentiation resulting in expansion associated with adaptive responses in migration and apoptosis suggest that GC-C coordinates component processes maintaining homeostasis of the crypt progenitor compartment. In the context of uniform loss of GC-C signaling during tumorigenesis, dysregulation of those homeostatic processes may contribute to mechanisms underlying colon cancer.

Guanylyl cyclase C suppresses intestinal tumorigenesis by restricting proliferation and maintaining genomic integrity

BACKGROUND AND AIMS

The most commonly lost gene products in colorectal carcinogenesis include guanylin and uroguanylin, endogenous ligands for guanylyl cyclase C (GCC). Beyond intestinal fluid balance, GCC mediates diarrhea induced by bacterial enterotoxins, and an inverse relationship exists between enterotoxigenic Escherichia coli infections producing the exogenous GCC ligand ST and colorectal cancer worldwide. However, the role of GCC in neoplasia remains obscure.

METHODS

Intestinal tumorigenesis was examined in wild-type (Gcc(+/+)) and GCC-deficient (Gcc(-/-)) mice carrying mutations in Apc (Apc(Min/+)) or exposed to the carcinogen azoxymethane. Markers of DNA damage, loss of Apc heterozygosity, and beta-catenin mutations were used to assess genomic integrity. Hyperproliferation was explored using Ki67 and cell cycle markers. Apoptosis was quantified by transferase biotin-dUTP nick end labeling analysis.

RESULTS

In colons of Apc(Min/+) mice, deletion of Gcc increased tumor incidence and multiplicity, reflecting uncoupling of loss of genomic integrity and compensatory apoptosis. Conversely, in the small intestine, elimination of Gcc increased tumorigenesis by enhancing proliferation without altering genomic integrity. Moreover, these distinct but mutually reinforcing mechanisms collaborate in azoxymethane-exposed mice, and deletion of Gcc increased tumor initiation and growth associated with hypermutation and hyperproliferation, respectively, in conjunction with attenuated apoptosis.

CONCLUSIONS

GCC suppresses tumor initiation and growth by maintaining genomic integrity and restricting proliferation. This previously unrecognized role of GCC in inhibiting tumorigenesis, together with the invariant disruption in guanylin and uroguanylin expression early in carcinogenesis, and the uniform over-expression of GCC by tumors, underscores the potential of oral administration of GCC ligands for targeted prevention and therapy of colorectal cancer.

An enteroendocrine cell-based model for a quiescent intestinal stem cell niche

We have shown that the kinetics of conversion of intestinal crypt cell populations to a partially or wholly mutant phenotype are consistent with a model in which each crypt contains an infrequently dividing 'deep' stem cell that is the progenitor of several more frequently dividing 'proximate' stem cells. An assumption of our model is that each deep stem cell exists in a growth inhibitory niche. We have used information from the literature to develop a model for a quiescent intestinal stem cell niche. This niche is postulated to be primarily defined by an enteroendocrine cell type that maintains stem cell quiescence by secretion of growth inhibitory peptides such as somatostatin and guanylin/uroguanylin. Consistent with this model, there is evidence that the proteins postulated as defining a growth-inhibitory stem cell niche can act as intestinal tumour suppressors. Confirmation that a growth-inhibitory niche does exist would have important implications for our understanding of intestinal homeostasis and tumorigenesis.

Contribution of comparative fish studies to general endocrinology: structure and function of some osmoregulatory hormones

Fish endocrinologists are commonly motivated to pursue their research driven by their own interests in these aquatic animals. However, the data obtained in fish studies not only satisfy their own interests but often contribute more generally to the studies of other vertebrates, including mammals. The life of fishes is characterized by the aquatic habitat, which demands many physiological adjustments distinct from the terrestrial life. Among them, body fluid regulation is of particular importance as the body fluids are exposed to media of varying salinities only across the thin respiratory epithelia of the gills. Endocrine systems play pivotal roles in the homeostatic control of body fluid balance. Judging from the habitat-dependent control mechanisms, some osmoregulatory hormones of fish should have undergone functional and molecular evolution during the ecological transition to the terrestrial life. In fact, water-regulating hormones such as vasopressin are essential for survival on the land, whereas ion-regulating hormones such as natriuretic peptides, guanylins and adrenomedullins are diversified and exhibit more critical functions in aquatic species. In this short review, we introduce some examples illustrating how comparative fish studies contribute to general endocrinology by taking advantage of such differences between fishes and tetrapods. In a functional context, fish studies often afford a deeper understanding of the essential actions of a hormone across vertebrate taxa. Using the natriuretic peptide family as an example, we suggest that more functional studies on fishes will bring similar rewards of understanding. At the molecular level, recent establishment of genome databases in fishes and mammals brings clues to the evolutionary history of hormone molecules via a comparative genomic approach. Because of the functional and molecular diversification of ion-regulating hormones in fishes, this approach sometimes leads to the discovery of new hormones in tetrapods as exemplified by adrenomedullin 2.

Membrane guanylyl cyclase receptors: an update

Recent studies have demonstrated key roles for several membrane guanylyl cyclase receptors in the regulation of cell hyperplasia, hypertrophy, migration and extracellular matrix production, all of which having an impact on clinically relevant diseases, including tissue remodeling after injury. Additionally, cell differentiation, and even tumor progression, can be profoundly influenced by one or more of these receptors. Some of these receptors also mediate important communication between the heart and intestine, and the kidney to regulate blood volume and Na+ balance.

Uroguanylin and guanylin regulate transport of mouse cortical collecting duct independent of guanylate cyclase C

BACKGROUND

Electrolyte and water homeostasis mostly depend on differentially regulated intestinal and renal transport. Guanylin and uroguanylin were proposed as first hormones linking intestinal with renal electrolyte and water transport, which is disturbed in pathophysiology. Guanylate cyclase C is the intestinal receptor for these peptides, but in guanylate cyclase C-deficient mice renal effects are retained. Unlike for the intestine the sites of renal actions and cellular mechanisms of guanylin peptides are still unclear.

METHODS

After first data on proximal tubular effects in this study their effects are examined in detail in mouse cortical collecting duct (CCD). Effects of guanylin peptides on principal cells of isolated mouse CCD were studied by slow whole-cell patch-clamp analysis, reverse transcription-polymerase chain reaction (RT-PCR), and microfluorimetric measurements of intracellular Ca2+.

RESULTS

Guanylin peptides depolarized or hyperpolarized principal cells. Whereas 8-Br-cyclic guanosine monophosphate (8-Br-cGMP) hyperpolarized, 8-Br-cyclic adenosine monophosphate (8-Br-cAMP) depolarized principal cells. All effects of guanylin peptides were inhibited by Ba2+. Hyperpolarizations were blocked by clotrimazole or protein kinase G (PKG) inhibition, suggesting an involvement of basolateral Ca2+- and cGMP-dependent K+ channels. Effects remained in CCD isolated from guanylate cyclase C-deficient mice. Depolarizations were inhibited by arachidonic acid or inhibition of phospholipase A2 (PLA2), but not by protein kinase A (PKA) inhibition. Conclusion. These results suggest the existence of two signaling pathways for guanylin peptides in principal cells of mouse CCD. One pathway is cGMP- and PKG-dependent but not mediated by guanylate cyclase C, the second involves PLA2 and arachidonic acid. The first pathway most likely leads to an activation of the basolateral K+-conductance while the latter probably results in decreased activity of ROMK channels in the luminal membrane.

Cellular effects of guanylin and uroguanylin

Ingestion of a salty meal induces secretion of guanylin (GN) and uroguanylin (UGN) into the intestinal lumen, where they inhibit Na+ absorption and induce Cl-, HCO3-, and water secretion. Simultaneously, these hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis. GN and UGN therefore participate in the prevention of hypernatremia and hypervolemia after salty meals. The signaling pathway of GN and UGN in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C), which leads to cGMP-dependent inhibition of Na+/H+ exchange and activation of the cystic fibrosis transmembrane regulator. In GC-C-deficient mice, GN and UGN still produce renal natriuresis, kaliuresis, and diuresis, suggesting different signaling pathways in the kidney compared with the intestine. Signaling pathways for GN and UGN in the kidney differ along the various nephron segments. In proximal tubule cells, a cGMP- and GC-C-dependent signaling was demonstrated for both peptides. In addition, UGN activates a pertussis toxin-sensitive G-protein-coupled receptor. A similar dual signaling pathway is also known for atrial natriuretic peptide. Recently, a cGMP-independent signaling pathway for GN and UGN was also shown in principal cells of the human and mouse cortical collecting duct. Because GN and UGN activate different signaling pathways in specific organs and even within the kidney, this review focuses on more recent findings on cellular effects and signaling mechanisms of these peptides and their pathophysiologic implications in the intestine and the kidney.

Lack of guanylyl cyclase C, the receptor for Escherichia coli heat-stable enterotoxin, results in reduced polyp formation and increased apoptosis in the multiple intestinal neoplasia (Min) mouse model

Guanylyl cyclase C (GC-C), a transmembrane receptor for bacterial heat-stable enterotoxin and the mammalian peptides guanylin and uroguanylin, mediates intestinal ion secretion and affects intestinal cell growth via cyclic GMP signaling. In intestinal tumors, GC-C expression is maintained while guanylin and uroguanylin expression is lost, suggesting a role for GC-C activation in tumor formation or growth. We show by in situ hybridization that GC-C expression is retained in adenomas from multiple intestinal neoplasia (Apc(Min/+)) mice. In order to determine the in vivo role of GC-C in intestinal tumorigenesis, we generated Apc(Min/+) mice homozygous for a targeted deletion of the gene encoding GC-C and hypothesized that these mice would have increased tumor multiplicity and size compared to wild-type Apc(Min/+) mice on the same genetic background. In contrast, the absence of GC-C resulted in a reduction of median polyp number by 55%. There was no change in the median diameter of polyps, suggesting no effect on tumor growth. Somatic loss of the wild-type Apc allele, an initiating event in intestinal tumorigenesis, also occurred in polyps from GC-C-deficient Apc(Min/+) mice. We have found increased levels of apoptosis as well as increased caspase-3 and caspase-7 gene expression in the intestines of GC-C-deficient Apc(Min/+) mice compared with Apc(Min/+) mice. We propose that these alterations are a possible compensatory mechanism by which loss of GC-C signaling also affects tumorigenesis.

Diverse Forms of Guanylyl Cyclases in Medaka Fish – Their Genomic Structure and Phylogenetic Relationships to those in Vertebrates and Invertebrates

Fish species such as medaka fish, fugu, and zebrafish contain more guanylyl cyclases (GCs) than do mammals. These GCs can be divided into two types: soluble GCs and membrane GCs. The latter are further divided into four subfamilies: (i) natriuretic peptide receptors, (ii) STa/guanylin receptors, (iii) sensory-organ-specific membrane GCs, and (iv) orphan receptors. Phylogenetic analyses of medaka fish GCs, along with those of fugu and zebrafish, suggest that medaka fish is a much closer relative to fugu than to zebrafish. Analyses of nucleotide data available on a web site (http://www.ncbi. nlm.nih.gov/) of GCs from a range of organisms from bacteria to vertebrates suggest that gene duplication, and possibly chromosomal duplication, play important roles in the divergence of GCs. In particular, the membrane GC genes were generated by chromosomal duplication before the divergence of tetrapods and teleosts.

Escherichia coli Heat Stable (STa) Enterotoxin and the Upper Small Intestine: Lack of Evidence in Vivo for Net Fluid Secretion

Heat stable (STa) enterotoxin from E. coli reduced fluid absorption in vivo in the perfused jejunum of the anaesthetized rat in Krebs-phosphate buffer containing lactate and glucose (nutrient buffer), in glucose saline and in glucose free saline. Bicarbonate ion enhanced fluid absorption of 98 +/- 7 (6) microl/cm/h was very significantly (P < 0.0001) reduced by STa to 19 +/- 4 (6) microl/cm/h, but net secretion was not found. When impermeant MES substituted for bicarbonate ion, net fluid absorption of 29 +/- 3 (6) microl/cm/h was less (P < 0.01) than the values for phosphate buffer and bicarbonate buffer. With STa in MES buffer, fluid absorption of 3 +/- 2 (6) microl/cm/h was less than (P < 0.001) that in the absence of STa and not significantly different from zero net fluid absorption. E. coli STa did not cause net fluid secretion in vivo under any of the above circumstances. Neither bumetanide nor NPPB when co-perfused with STa restored the rate of fluid absorption. In experiments with zero sodium ion-containing perfusates, STa further reduced fluid absorption modestly by 20 microl/cm/h. Perfusion of ethyl-isopropyl-amiloride (EIPA) with STa in zero sodium ion buffers prevented the small increment in fluid entry into the lumen caused by STa, indicating that the STa effect was attributable to residual sodium ion and fluid uptake that zero sodium-ion perfusates did not eradicate. These experiments, using a technique that directly measures mass transport of fluid into and out of the in vivo proximal jejunum, do not support the concept that E. coli STa acts by stimulating a secretory response.

Cardiac and intestinal natriuretic peptides: insights from genetically modified mice

Since the original discovery of atrial natriuretic peptide (ANP) more than two decades ago, the application of gene targeting technology in mice has provided new insights into the diverse physiological functions of natriuretic peptides and their membrane guanylyl cyclase (GC) receptors. Disruption of the genes for ANP or its receptor, GC-A, demonstrated that this system is not only essential for the maintenance of normal blood pressure and volume, but in addition exerts local antihypertrophic effects in the heart. Disruption of the genes encoding B-type (BNP) or C-type natriuretic peptides (CNP) or the CNP-receptor, GC-B, demonstrated that these "natriuretic" peptides are in fact unlikely to physiologically regulate renal sodium excretion but instead exert important autocrine/paracrine cGMP-mediated effects on cellular proliferation and differentiation in various tissues. Notably, the intestinal peptide uroguanylin, which activates a third guanylyl cyclase receptor (GC-C), exerts diuretic/natriuretic activity and links the intestine and kidney in an endocrine way to modulate renal function in response to oral salt load. Reviewed here is the physiology of cardiac and intestinal natriuretic peptides and their guanylyl cyclase receptors, with special focus on the information gained to date from genetically modified mice.

Enterotoxin/Guanylin Receptor Type Guanylyl Cyclases in Non-Mammalian Vertebrates

Cyclic GMP is a ubiquitous intracellular second messenger produced by guanylyl cyclases (GCs). The enterotoxin/guanylin receptor type membrane GC (designated as GC-C in mammals) is activated by exogenous ligands such as heat-stable enterotoxins (STa), small peptides secreted by some pathogenic strains of Escherichia coli which cause severe secretory diarrhea and also activated by endogenous ligands such as guanylin and uroguanylin. The STa/guanylin receptor type membrane GC, as well as other type membrane GCs, is composed of an extracellular domain, a single transmembrane domain, and an intracellular region comprising a kinase-like domain and a catalytic domain. The STa/guanylin receptor type membrane GC is identified in various vertebrates including fishes, amphibians, reptiles, and birds, implying that it serves some important and undefined physiological roles in the intestine of non-mammalian vertebrates, e.g. the regulation of water and salt absorption. In mammals, only a single membrane GC (GC-C) is known to be the STa/guanylin receptor. On the contrary, two membrane GC cDNAs are cloned from the intestine of the European eel Anguilla anguilla (GC-C1 and GC-C2) and the medaka fish Oryzias latipes (OlGC6 and OlGC9). OlGC6 and OlGC9 are structurally distinct and show different ligand responsibility. Accumulated evidences indicate that the transcriptional regulatory mechanism of the human GC-C gene is different from that of the corresponding medaka fish GC gene; the human GC-C gene is regulated by Cdx2 and/or HNF-4, and the medaka fish OlGC6 gene is regulated by OlPC4, which is a medaka fish homologue of the mammalian transcriptional positive co-factor 4 (PC4). Furthermore, the transcriptional regulatory mechanism of the OlGC9 gene is different from those of both the OlGC6 and human GC-C genes, indicating that the study on these two medaka fish GCs will be useful for further understanding of the STa/guanylin receptor type membrane GC in the vertebrates.

Regulation of nitric oxide and soluble guanylyl cyclase

Since the discoveries that have verified nitric oxide (NO) as an endogenously produced cell signaling molecule, research surrounding its production and mechanisms of action have been studied at an exponentially increasing rate. NO is produced by a family of enzymes termed the NO synthases (NOS), which are regulated independently by various stimuli. Once produced, NO can solicit numerous biological events by reacting with various metals, thiols, and oxygen species to modify proteins, DNA and lipids. One of the most biologically relevant actions of NO is its binding to the heme moiety in the heterodimeric enzyme, soluble guanylyl cyclase (sGC). Activation of sGC by NO results in the production of the second messenger molecule, 3',5'-cyclic guanosine monophosphate (cGMP), which can regulate numerous physiological events such as vasodilatation and neurotransmission. Here we will review the synthesis and fate of NO, and discuss the activation and regulation of the NO receptor, sGC.

A novel role for uroguanylin in the regulation of sodium balance

Uroguanylin is a peptide hormone that regulates sodium excretion by the kidney when excess NaCl is consumed. A new study demonstrates that mice deficient in uroguanylin have blunted urinary sodium excretion responses to oral sodium loads in addition to elevated blood pressure (see related article beginning on page 1244). A physiological role for uroguanylin is discussed, linking the intestine and kidney in an endocrine axis for the maintenance of sodium balance.

Structure, Regulation, and Function of Mammalian Membrane Guanylyl Cyclase Receptors, With a Focus on Guanylyl Cyclase-A

Besides soluble guanylyl cyclase (GC), the receptor for NO, there are at least seven plasma membrane enzymes that synthesize the second-messenger cGMP. All membrane GCs (GC-A through GC-G) share a basic topology, which consists of an extracellular ligand binding domain, a short transmembrane region, and an intracellular domain that contains the catalytic (GC) region. Although the presence of the extracellular domain suggests that all these enzymes function as receptors, specific ligands have been identified for only three of them (GC-A through GC-C). GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure and volume homeostasis and also local antihypertrophic actions in the heart. GC-B is a specific receptor for C-type natriuretic peptide, having more of a paracrine function in vascular regeneration and endochondral ossification. GC-C mediates the effects of guanylin and uroguanylin on intestinal electrolyte and water transport and on epithelial cell growth and differentiation. GC-E and GC-F are colocalized within the same photoreceptor cells of the retina and have an important role in phototransduction. Finally, the functions of GC-D (located in the olfactory neuroepithelium) and GC-G (expressed in highest amounts in lung, intestine, and skeletal muscle) are completely unknown. This review discusses the structure and functions of membrane GCs, with special emphasis on the physiological endocrine and cardiac functions of GC-A, the regulation of hormone-dependent GC-A activity, and the relevance of alterations of the atrial natriuretic peptide/GC-A system to cardiovascular diseases.

Promoter analysis of a medaka fish intestinal guanylyl cyclase gene

We characterized the promoter activity of a medaka fish intestinal guanylyl cyclase gene, OlGC6, by assay of enzyme activity in response to various promoter-luciferase fusion gene constructs introduced into CACO-2 cells and medaka fish embryos. A transient transfection assay of the various fusion gene constructs showed that the nucleotides between -98 and -89 in the 5'-flanking region of the OlGC6 gene are essential for transcription of the OlGC6 gene in CACO-2, and that the OlGC6 gene fragment between -98 and +50 is sufficient to drive gene expression in the medaka fish intestine. An electrophoretic mobility shift assay and ultraviolet (UV) cross-linking experiments demonstrated that a nuclear protein from CACO-2 cells and the adult medaka fish intestinal cells binds specifically to the AGACCTTTGC nucleotides in the regulatory element.

E. coli heat-stable enterotoxin and guanylyl cyclase C: new functions and unsuspected actions

Some E. coli cause diarrhea by elaborating heat-labile and heat-stable (ST) enterotoxins which stimulate intestinal secretion. E. coli ST's are small peptides which bind to intestinal luminal epithelial cell receptors. The ST receptor, one of a family of receptor-cyclases called guanylyl cyclase C (GC-C), is a membrane spanning protein containing an extracellular binding domain and intracellular protein kinase and catalytic domains. The intestine synthesizes and secretes homologous peptides, guanylin and uroguanylin. The kidney also synthesizes uroguanylin. ST, guanylin or uroguanylin binding to GC-C results in increased cGMP, phosphorylation of the CFTR Cl- channel and secretion. Proguanylin and prouroguanylin circulate in blood and bind to receptors in intestine, kidney, liver, brain etc. In the kidney, they stimulate the excretion of Na+ and K+. Study of GC-C "knock-out" mice reveal that GC-C is important to intestinal salt and water secretion, duodenal bicarbonate secretion, recovery from CCl4-induced liver injury, and to intestinal polyp formation in Min mice lacking GC-C.

Targeted inactivation of the mouse guanylin gene results in altered dynamics of colonic epithelial proliferation

Heat-stable enterotoxin (STa), elaborated by enterotoxigenic Echerichia coli, is a worldwide cause of secretory diarrhea in infants and travelers. Both STa and guanylin, a peptide structurally similar to STa, increase intracellular cGMP levels after binding to the same intestinal receptor, guanylate cyclase C (GC-C). Distinct from its role as an intestinal secretagogue, guanylin may also have a role in intestinal proliferation, as guanylin expression is lost in intestinal adenomas. To determine the function of guanylin in intestinal epithelia, guanylin null mice were generated using a Cre/loxP-based targeting vector. Guanylin null mice grew normally, were fertile and showed no signs of malabsorption. However, the levels of cGMP in colonic mucosa of guanylin null mice were significantly reduced. The colonic epithelial cell migration rate was increased and increased numbers of colonocytes expressing proliferating cell nuclear antigen (PCNA) were present in crypts of guanylin null mice as well. The apoptotic index was similar in guanylin null mice and littermate controls. We conclude from these studies that loss of guanylin results in increased proliferation of colonic epithelia. We speculate that the increase in colonocyte number is related to decreased levels of cGMP and that this increase in proliferation plays a role in susceptibility to intestinal adenoma formation and/or progression.

Expression of the receptor guanylyl cyclase C and its ligands in reproductive tissues of the rat: a potential role for a novel signaling pathway in the epididymis

Guanylyl cyclase C (GC-C) is a membrane-associated form of guanylyl cyclase and serves as the receptor for the heat-stable enterotoxin (ST) peptide and endogenous ligands guanylin, uroguanylin, and lymphoguanylin. The major site of expression of GC-C is the intestinal epithelial cell, although GC-C is also expressed in extraintestinal tissue such as the kidney, airway epithelium, perinatal liver, stomach, brain, and adrenal glands. Binding of ligands to GC-C leads to accumulation of intracellular cGMP, the activation of protein kinases G and A, and phosphorylation of the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel that regulates salt and water secretion. We examined the expression of GC-C and its ligands in various tissues of the reproductive tract of the rat. Using reverse transcriptase and the polymerase chain reaction, we demonstrated the presence of GC-C, uroguanylin, and guanylin mRNA in both male and female reproductive organs. Western blot analysis using a monoclonal antibody to GC-C revealed the presence of differentially glycosylated forms of GC-C in the caput and cauda epididymis. Exogenous addition of uroguanylin to minced epididymal tissue resulted in cGMP accumulation, suggesting an autocrine or endocrine activation of GC-C in this tissue. Immunohistochemical analyses demonstrated expression of GC-C in the tubular epithelial cells of both the caput epididymis and cauda epididymis. Our results suggest that the GC-C signaling pathway could converge on CFTR in the epididymis and perhaps control fluid and ion balance for optimal sperm maturation and storage in this tissue.

A novel PDZ protein regulates the activity of guanylyl cyclase C, the heat-stable enterotoxin receptor

Secretory diarrhea is the leading cause of infectious diarrhea in humans. Secretory diarrhea may be caused by binding of heat-stable enterotoxins to the intestinal receptor guanylyl cyclase C (GCC). Activation of GCC catalyzes the formation of cGMP, initiating a signaling cascade that opens the cystic fibrosis transmembrane conductance regulator chloride channel at the apical cell surface. To identify proteins that regulate the trafficking or function of GCC, we used the unique COOH terminus of GCC as the "bait" to screen a human intestinal yeast two-hybrid library. We identified a novel protein, IKEPP (intestinal and kidney-enriched PDZ protein) that associates with the COOH terminus of GCC in biochemical assays and by co-immunoprecipitation. IKEPP is expressed in the intestinal epithelium, where it is preferentially accumulated at the apical surface. The GCC-IKEPP interaction is not required for the efficient targeting of GCC to the apical cell surface. Rather, the association with IKEPP significantly inhibits heat-stable enterotoxin-mediated activation of GCC. Our findings are the first to identify a regulatory protein that associates with GCC to modulate the catalytic activity of the enzyme and provides new insights in mechanisms that regulate GCC activity in response to bacterial toxin.

Structure and function of the heat-stable enterotoxin receptor/guanylyl cyclase C

Guanylyl cyclase C (GC-C) was found to function as the principal receptor for heat-stable enterotoxins (STa), major causative factors in E. coli-induced secretory diarrhea. GC-C is enriched in intestinal epithelium, but was also detected in other epithelial tissues. The enzyme belongs to the family of receptor guanylyl cyclases, and consists of an extracellular receptor domain, a single transmembrane domain, a kinase homology domain, and a catalytic domain. GC-C is modified by N-linked glycosylation and, at least in the small intestine, by proteolysis, resulting in a STa receptor that is coupled non-covalently to the intracellular domain. So far two endogenous ligands of mammalian GC-C have been identified i.e. the small cysteine-rich peptides guanylin and uroguanylin. The guanylins are released in an auto- or paracrine fashion into the intestinal lumen but may also function as endocrine hormones in gut-kidney communication and as regulators of ion transport in extra-intestinal epithelia. They are thought to activate GC-C by inducing a conformational change in the extracellular portion of the homotrimeric GC-C complex, which allows two of the three intracellular catalytic domains to dimerize and form two active catalytic clefts. In the intestine, activation of GC-C results in a dual action: stimulation of Cl and HCO3 secretion, through the opening of apical CFTR Cl channels; and inhibition of Na absorption, through blockade of an apical Na/H exchanger. The principal effector of the GC-C effect on ion transport is cGMP dependent protein kinase type II, which together with GC-C and the ion transporters, may form a supramolecular complex at the apical border of epithelial cells.

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.

The guanylyl cyclase family at Y2K

During the 1980s the purification, cloning, and expression of various forms of guanylyl cyclase (GC) revealed that they served as receptors for extracellular signals. Seven membrane forms, which presumably exist as homodimers, and four subunits of apparent heterodimers (commonly referred to as the soluble forms) are known, but in animals such as nematodes, much larger numbers of GCs are expressed. The number of transmembrane segments (none, one, or multiple) divide the GC family into three groups. Those with no or one transmembrane segment bind nitric oxide/carbon monoxide (NO/CO) or peptides. There are no known ligands for the multiple transmembrane segment class of GCs. Mutational and structural analyses support a model where catalysis requires a shared substrate binding site between the subunits, whether homomeric or heteromeric in nature. Because some cyclases or cyclase ligand genes lack specific GC inhibitors, disruption of either has been used to define the functions of individual cyclases, as well as to define human genetic disease counterparts.

Effect of E. coli heat-stable enterotoxin on colonic transport in guanylyl cyclase C receptor-deficient mice

We studied the functional importance of the colonic guanylyl cyclase C (GCC) receptor in GCC receptor-deficient mice. Mice were anesthetized with pentobarbital sodium, and colon segments were studied in Ussing chambers in HCO3- Ringer under short-circuit conditions. Receptor-deficient mouse proximal colon exhibited similar net Na+ absorption, lower net Cl- absorption, and a negative residual ion flux (J(R)), indicating net HCO3- absorption compared with that in normal mice. In normal mouse proximal colon, mucosal addition of 50 nM Escherichia coli heat-stable enterotoxin (STa) increased the serosal-to-mucosal flux of Cl- (J(s-->m)(Cl)) and decreased net Cl- flux (J(net)(Cl)) accompanied by increases in short-circuit current (I(sc)), potential difference (PD), and tissue conductance (G). Serosal STa had no effect. In distal colon neither mucosal nor serosal STa affected ion transport. In receptor-deficient mice, neither mucosal nor serosal 500 nM STa affected electrolyte transport in proximal or distal colon. In these mice, 1 mM 8-bromo-cGMP produced changes in proximal colon J(s-->m)(Cl) and J(net)(Cl), I(sc), PD, G, and J(R) similar to mucosal STa addition in normal mice. We conclude that the GCC receptor is necessary in the mouse proximal colon for a secretory response to mucosal STa.

IS1414, an Escherichia coli insertion sequence with a heat-stable enterotoxin gene embedded in a transposase-like gene

Enteroaggregative Escherichia coli (EAEC) heat-stable enterotoxin 1 (EAST1) was originally discovered in EAEC but has also been associated with enterotoxigenic E. coli (ETEC). Multiple genomic restriction fragments from each of three ETEC strains of human origin showed homology with an EAST1 gene probe. A single hybridizing fragment was detected on the plasmid of ETEC strain 27D that also encodes heat-stable enterotoxin Ib and colonization factor antigen I. We isolated and characterized this fragment, showing that it (i) carries an allele of astA nearly identical to that originally reported from EAEC 17-2 and (ii) expressed enterotoxic activity. Sequence analysis of the toxin coding region revealed that astA is completely embedded within a 1,209-bp open reading frame (ORF1), whose coding sequence is on the same strand but in the -1 reading frame in reference to the toxin gene. In vitro expression of the predicted M(r)- approximately 46,000 protein product of ORF1 was demonstrated. ORF1 is highly similar to transposase genes of IS285 from Yersinia pestis, IS1356 from Burkholderia cepacia, and ISRm3 from Rhizobium meliloti. It is bounded by 30-bp imperfect inverted repeat sequences and flanked by 8-bp direct repeats. Based on these structural features, pathognomonic of a regular insertion sequence, this element was designated IS1414. Preliminary experiments to show IS1414 translocation were unsuccessful. Overlapping genes of the type suggested by the IS1414 core region have heretofore not been described in bacteria. It seems to offer a most efficient mechanism for intragenomic and horizontal dissemination of EAST1.