Novel Complex GUCY2D Mutation in Japanese Family with Cone-Rod Dystrophy

PURPOSE

All mutations in the retinal guanylate cyclase gene (GUCY2D) that causes autosomal dominant cone-rod dystrophy (CORD) are associated with an amino acid substitution in codon 838. A novel heterozygous complex missense mutation of I915T and G917R in the GUCY2D gene was found in a Japanese family with autosomal dominant CORD. The clinical features associated with this mutation were described.

METHODS

Blood samples were collected from 27 patients with cone-rod or cone dystrophies and from 11 patients with macular dystrophy. Genomic DNA was extracted from peripheral leukocytes. All 18 coding exons of the GUCY2D gene were directly sequenced. The PCR product carrying a novel mutation was subcloned, and each allele was sequenced. A complete ophthalmologic examination was performed in members of the family with the novel mutation.

RESULTS

A novel heterozygous complex missense mutation of T2817C and G2822C that would predict I915T and G917R amino acid substitutions, respectively, was found in an autosomal dominant CORD family. The two nucleotide changes were located on the same allele, and segregated with the disease. Two other known missense mutations of R838H and R838C were found in two other CORD families. The clinical phenotype associated with the novel mutation was similar to that with the Arg838 mutations.

CONCLUSIONS

A heterozygous complex mutation of I915T and G917R in the GUCY2D gene caused autosomal dominant CORD, indicating that a heterozygous mutation that does not include a codon 838 substitution can lead to this ocular phenotype.

Functional analyses of mutant recessive GUCY2D alleles identified in Leber congenital amaurosis patients: protein domain comparisons and dominant negative effects

PURPOSE

Recessive mutations in GUCY2D, the gene encoding the retinal guanylyl cyclase protein, RetGC-1, have been shown to cause Leber Congenital Amaurosis (LCA), a severe retinal dystrophy. The purpose of this study was to determine the functional consequences of selected mutations in GUCY2Dlinked to LCA. The mutations investigated in this study map to the catalytic domain (P858S, L954P) and the extracellular domain (C105Y, L325P) of RetGC-1.

METHODS

All four mutations were introduced into the in vitro expression plasmid, pRC-CMV human RetGC-1, and expressed in HEK-293 cells. We assayed the abilities of the mutant cyclases to generate cGMP (basal activity), and to be activated by guanylyl cyclase activating proteins (GCAP-1 and GCAP-2). Additionally, we co-expressed the catalytic domain mutations (P858S and L954P) with a wild-type allele to test for dominant negative effects on wild-type RetGC-1.

RESULTS

The P858S and L954P mutations, both in highly conserved residues of the catalytic domain of RetGC-1, severely impair basal, GCAP-1, and GCAP-2 stimulated catalytic activity of the enzyme. In addition, when co-expressed with the wild-type allele, both catalytic domain mutations act as dominant negative proteins and reduce the activity of wild-type RetGC-1. The basal activities of the C105Y and L325P mutants are unaltered, but GCAP-1 and GCAP-2 stimulated cyclase activities are reduced approximately 50%.

CONCLUSIONS

GUCY2D mutations from LCA patients have distinct functional consequences on RetGC-1 catalytic activity in vitro. Our analyses showed that the catalytic domain mutations cause a marked reduction in cyclase activity, while the extracellular domain mutations moderately reduce activity. The catalytic domain mutant alleles cause dominant negative effects, indicating that the functionality of RetGC-1 is compromised even in heterozygotes. This is consistent with abnormalities in cone electroretinograms (ERGs) detected in obligate heterozygous GUCY2D parents that carry the L954P mutation.

A role for guanylate cyclase C in acid-stimulated duodenal mucosal bicarbonate secretion

Luminal acidification provides the strongest physiological stimulus for duodenal HCO3- secretion. Various neurohumoral mechanisms are believed to play a role in acid-stimulated HCO3- secretion. Previous studies in the rat and human duodenum have shown that guanylin and Escherichia coli heat-stable toxin, both ligands of the transmembrane guanylyl cyclase receptor [guanylate cyclase C (GC-C)], are potent stimulators for duodenal HCO3- secretion. We postulated that the GC-C receptor plays an important role in acid-stimulated HCO3- secretion. In vivo perfusion studies performed in wild-type (WT) and GC-C knockout (KO) mice indicated that acid-stimulated duodenal HCO3- secretion was significantly decreased in the GC-C KO animals compared with the WT counterparts. Pretreatment with PD-98059, an MEK inhibitor, resulted in attenuation of duodenal HCO3- secretion in response to acid stimulation in the WT mice with no further effect in the KO mice. In vitro cGMP generation studies demonstrated a significant and comparable increase in cGMP levels on acid exposure in the duodenum of both WT and KO mice. In addition, a rapid, time-dependent phosphorylation of ERK was observed with acid exposure in the duodenum of WT mice, whereas a marked attenuation in ERK phosphorylation was observed in the KO animals despite equivalent levels of ERK in both groups of animals. On the basis of these studies, we conclude that transmembrane GC-C is a key mediator of acid-stimulated duodenal HCO3- secretion. Furthermore, ERK phosphorylation may be an important intracellular mediator of duodenal HCO3- secretion.

The role of gelatinase in hepatic metastasis of colorectal cancer

PURPOSE

To determine the role of gelatinase in the hepatic metastatic process of colorectal cancer, we correlated gelatinolytic activity in colorectal tumor tissue both to the presence of intravasated colorectal epithelial cells and to the formation of liver metastasis.

EXPERIMENTAL DESIGN

The gelatinolytic activity was analyzed in tumor tissue samples from 68 colorectal cancer patients by using gelatin substrate zymography. The presence of intravasated colorectal epithelial cells was defined as detection of guanylyl cyclase C mRNA in blood sampled from drainage vein of a tumor-bearing colorectal segment.

RESULTS

Forty of 68 patients were noted to have guanylyl cyclase C mRNA expression in their drainage venous blood. Fifteen patients were noted to have liver metastasis at the time of surgery, and another 15 patients developed liver metastasis during median follow-up period of 53 months. Either individual or total gelatinolytic activity in colorectal tumor tissue failed to predict the presence of intravasated colorectal epithelial cells in the drainage venous blood or formation of liver metastasis. Presence of both intravasated colorectal epithelial cells and high total gelatinolytic activity in colorectal tumor tissue, however, is a strong predictor of liver metastasis (P = 0.004).

CONCLUSION

Our data suggested that gelatinolytic activity in colorectal tumor tissues may facilitate the hepatic metastatic process in the steps after intravasation but not during or before intravasation.

Atrial natriuretic peptide induces natriuretic peptide receptor-cGMP-dependent protein kinase interaction

Circulating natriuretic peptides such as atrial natriuretic peptide (ANP) counterbalance the effects of hypertension and inhibit cardiac hypertrophy by activating cGMP-dependent protein kinase (PKG). Natriuretic peptide binding to type I receptors (NPRA and NPRB) activates their intrinsic guanylyl cyclase activity, resulting in a rapid increase in cytosolic cGMP that subsequently activates PKG. Phosphorylation of the receptor by an unknown serine/threonine kinase is required before ligand binding can activate the cyclase. While searching for downstream PKG partners using a yeast two-hybrid screen of a human heart cDNA library, we unexpectedly found an upstream association with NPRA. PKG is a serine/threonine kinase capable of phosphorylating NPRA in vitro; however, regulation of NPRA by PKG has not been previously reported. Here we show that PKG is recruited to the plasma membrane following ANP treatment, an effect that can be blocked by pharmacological inhibition of PKG activation. Furthermore, PKG participates in a ligand-dependent gain-of-function loop that significantly increases the intrinsic cyclase activity of the receptor. PKG translocation is ANP-dependent but not nitric oxide-dependent. Our results suggest that anchoring of PKG to NPRA is a key event after ligand binding that determines distal effects. As such, the NPRA-PKG association may represent a novel mechanism for compartmentation of cGMP-mediated signaling and regulation of receptor sensitivity.

Cytokeratin 20 and guanylyl cyclase C mRNA is largely present in lymph node and liver specimens of colorectal cancer patients

The aim of our prospective study was to detect circulating epithelial cells (CEC) indicating the presence of disseminated tumor cells (DTC) in tissues affected by lymphatic and hematogenic colorectal cancer metastasis. DTC were tracked in lymph node, liver or bone marrow samples of 245 colorectal cancer patients using 2 independent RT-PCR assays for cytokeratin 20 (CK20) and guanylylcyclase C (GCC) that demonstrated a sensitivity of 1 colorectal cancer cell in 10(6) nucleated hematopoietic cells. CK20 mRNA was detected in 79% of lymph nodes, 35% of both liver lobes and 11% of bone marrow samples. GCC mRNA was found in 68% of lymph nodes, 60% of both liver lobes and 6% of bone marrow specimens. Both markers were recorded in 63% of lymph nodes, 45% of at least 1 liver lobe and 1% of bone marrow samples. There was no significant difference when comparing lymph node samples tested positive for both markers in patients with (N1/2; 65%) and without (N0; 56%) nodal involvement. The same was true when comparing the percentages of patients with and without clinically overt distant metastasis who were positive for both markers in at least 1 liver lobe (62% vs. 41%) or in bone marrow (4% vs. 0%). A score denoting the cumulative sum of tests indicating presence of CK20 and GCC mRNA in the liver was significantly related with UICC classification (p = 0.039). However, addition of lymph node results to this score decreased the correlation. The high incidence of clinically inconspicuous lymph node and liver samples tested positive for both markers emphasizes the function of these organs as primary filters for epithelial cells possibly shed from colorectal carcinomas. The potential prognostic significance of these findings warrants verification, especially regarding the importance of CEC or DTC resident in the liver of colorectal cancer patients.

Cellular refractoriness to the heat-stable enterotoxin peptide is associated with alterations in levels of the differentially glycosylated forms of guanylyl cyclase C

The heat-stable enterotoxin peptides (ST) produced by enterotoxigenic Escherichia coli are one of the major causes of transitory diarrhea in the developing world. Toxin binding to its receptor, guanylyl cyclase C (GC-C), results in receptor activation and the production of high intracellular levels of cGMP. GC-C is expressed in two differentially glycosylated forms in intestinal epithelial cells. Prolonged exposure of human colonic cell lines to ST peptides induces cellular refractoriness to the ST peptide, in terms of intracellular cGMP accumulation. We have investigated the mechanism of cellular desensitization in human colonic Caco2 cells, and observe that exposure of cells to ST leads to a time and dose-dependent inability of cells to respond to the peptide in terms of GC-C stimulation, both in whole cells and membranes prepared from desensitized cells. This is concomitant with a 50% reduction in ST-binding activity in desensitized cells. Desensitization was correlated with a loss of the plasma membrane-associated, hyperglycosylated 145 kDa form of GC-C, while the predominant 130 kDa form, localized both on the plasma membrane and the endoplasmic reticulum, continued to be present in ST-treated cells. Desensitized cells recovered ST-responsiveness on removal of the ST peptide, which was correlated with a reappearance of the 145 kDa form on the cell surface, following processing of the endoplasmic reticulum-associated pool of the 130 kDa form. Selective internalization of the 145 kDa form of the receptor was required for cellular desensitization, as ST-treatment of cells at 4 degrees C did not lead to refractoriness. We therefore show a novel means of regulation of cellular responsiveness to the ST peptide, whereby altering cellular levels of the differentially glycosylated forms of GC-C can lead to differential ligand-mediated activation of the receptor.

Interaction of guanylyl cyclase C with SH3 domain of Src tyrosine kinase. Yet another mechanism for desensitization

Protein-protein interactions mediated by the Src homology 3 (SH3) domain have been implicated in the regulation of receptor functions for subcellular localization of proteins and the reorganization of cytoskeleton. The experiments described in this article begin to identify the interaction of the SH3 domain of Src tyrosine kinase with the guanylyl cyclase C receptor after activation with Escherichia coli heat-stable enterotoxin (ST). Only one of two post-translationally modified forms of guanylyl cyclase C from T84 colonic carcinoma cells bind to GST-SH3 fusion protein of Src and Hck tyrosine kinases. Interestingly, the GST-Src-SH3 fusion protein showed 2-fold more affinity to native guanylyl cyclase C in solution than the GST-Hck-SH3 fusion protein. The affinity of the GST-Src-SH3 fusion protein to guanylyl cyclase C increased on desensitization of receptor in vivo. An in vitro cyclase assay in the presence of GST-Src-SH3 fusion protein indicated inhibition of the catalytic activity of guanylyl cyclase C. The catalytic domain recombinant protein (GST-GCD) of guanylyl cyclase C could pull-down a 60-kDa protein that reacted with Src tyrosine antibody and also showed autophosphorylation. These data suggest that SH3 domain-mediated protein-protein interaction with the catalytic domain of guanylyl cyclase C inhibited the cyclase activity and that such an interaction, possibly mediated by Src tyrosine kinase or additional proteins, might be pivotal for the desensitization phenomenon of the guanylyl cyclase C receptor.

Selective induction of CD8+CD4- thymocyte apoptosis mediated by the B-subunit of Escherichia coli heat-labile enterotoxin

Receptor-binding by the B-subunit of Escherichia coli heat-labile enterotoxin (EtxB) induces apoptosis of peripheral CD8(+), but not CD4(+) T-cells. Given that peripheral CD8(+) and CD4(+) T cells arise from a common developmental pathway in the thymus, we investigated the effects of EtxB on different thymocyte populations. We show that the acquisition of sensitivity to EtxB-mediated cell death arises following transition of CD4(+)CD8(+) double positive cells into the CD4(-)CD8(+) pathway. Maturation of T cells into CD4(-)CD8(+) single positive cells is associated with upregulated expression of receptors for EtxB.

Bacterial enterotoxins are associated with resistance to colon cancer

One half million patients suffer from colorectal cancer in industrialized nations, yet this disease exhibits a low incidence in under-developed countries. This geographic imbalance suggests an environmental contribution to the resistance of endemic populations to intestinal neoplasia. A common epidemiological characteristic of these colon cancer-spared regions is the prevalence of enterotoxigenic bacteria associated with diarrheal disease. Here, a bacterial heat-stable enterotoxin was demonstrated to suppress colon cancer cell proliferation by a guanylyl cyclase C-mediated signaling cascade. The heat-stable enterotoxin suppressed proliferation by increasing intracellular cGMP, an effect mimicked by the cell-permeant analog 8-br-cGMP. The antiproliferative effects of the enterotoxin and 8-br-cGMP were reversed by L-cis-diltiazem, a cyclic nucleotide-gated channel inhibitor, as well as by removal of extracellular Ca(2+), or chelation of intracellular Ca(2+). In fact, both the enterotoxin and 8-br-cGMP induced an L-cis-diltiazem-sensitive conductance, promoting Ca(2+) influx and inhibition of DNA synthesis in colon cancer cells. Induction of this previously unrecognized antiproliferative signaling pathway by bacterial enterotoxin could contribute to the resistance of endemic populations to intestinal neoplasia, and offers a paradigm for targeted prevention and therapy of primary and metastatic colorectal cancer.

Olfactory signal transduction in the mouse septal organ

The septal organ, a distinct chemosensory organ observed in the mammalian nose, is essentially a small island of olfactory neuroepithelium located bilaterally at the ventral base of the nasal septum. Virtually nothing is known about its physiological properties and function. To understand the nature of the sensory neurons in this area, we studied the mechanisms underlying olfactory signal transduction in these neurons. The majority of the sensory neurons in the septal organ express olfactory-specific G-protein and adenylyl cyclase type III, suggesting that the cAMP signaling pathway plays a critical role in the septal organ as in the main olfactory epithelium (MOE). This is further supported by patch-clamp recordings from individual dendritic knobs of the sensory neurons in the septal organ. Odorant responses can be mimicked by an adenylyl cyclase activator and a phosphodiesterase inhibitor, and these responses can be blocked by an adenylyl cyclase inhibitor. There is a small subset of cells in the septal organ expressing a cGMP-stimulated phosphodiesterase (phosphodiesterase 2), a marker for the guanylyl cyclase-D subtype sensory neurons identified in the MOE. The results indicate that the septal organ resembles the MOE in major olfactory signal transduction pathways, odorant response properties, and projection to the main olfactory bulb. Molecular and functional analysis of the septal organ, which constitutes approximately 1% of the olfactory epithelium, will provide new insights into the organization of the mammalian olfactory system and the unique function this enigmatic organ may serve.

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.

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.

The micro domain responsible for ligand-binding of guanylyl cyclase C

Guanylyl cyclase C (GC-C), a member of membrane-bound guanylyl cyclases, is a receptor protein for guanylin and uroguanylin. The binding of a ligand to the extracellular domain of GC-C (ECD(GC-C)) triggers signal transduction, resulting in the regulation of intestinal fluids and electrolytes. A previous study proposed that a ligand-binding site on GC-C is localized near the transmembrane region. To further investigate the mechanism by which GC-C is activated, the C-terminal polypeptide (Met341-Gln407) of ECD(GC-C) (the micro domain), which includes the ligand-binding site, was over-expressed in Escherichia coli and its ligand-binding ability was examined. The micro domain showed ligand-binding activity (IC(50)=1 x 10(-8) M). This result clearly indicates that a ligand-binding site is located in close proximity to the membrane-bound region, and that the micro domain is capable of independently binding the ligand, without assistance from other domains. The use of this micro binding domain in the study of interactions between GC-C and ligands could be a useful tool and could lead to a better understanding of GC-C signal transduction.

The electrolyte/fluid secretion stimulatory peptides guanylin and uroguanylin and their common functional coupling proteins in the rat pancreas: a correlative study of expression and cell-specific localization

INTRODUCTION

Guanylin and uroguanylin are intestinal or urine peptides that stimulate epithelial electrolyte/fluid secretion by activating the cystic fibrosis gene product (CFTR).

AIMS

Because CFTR is essentially involved in the electrolyte secretion of the pancreatic duct cells, the rat pancreas was investigated for expression and cell-specific localization of guanylin and uroguanylin; expression of major components of the guanylin signaling pathway, i.e., the guanylin/uroguanylin receptor guanylate cyclase C (GC-C), cGMP-dependent protein kinase II, and CFTR; and expression of the epithelial Cl-/HCO3- exchanger AE2.

METHODOLOGY AND RESULTS

Reverse transcriptase polymerase chain reaction analyses revealed high expression of guanylin, uroguanylin, and the functional coupling proteins in the rat pancreas. At the cellular level, guanylin and uroguanylin were localized by immunohistochemistry to the centroacinar cells and proximal duct cells of the exocrine pancreas. The guanylin/uroguanylin receptor GC-C, cGKII, CFTR, and AE2 were all found in the same segments of the ductal system, where they were confined to the apical membrane of centroacinar cells and proximal duct epithelial cells, a circumstance suggesting that both peptides may act through the ductal lumen.

CONCLUSION

In view of the well-known functional significance of guanylin and uroguanylin, the presence and cell-specific expression not only of the both peptides but also of their common functional coupling proteins implicates a regulatory function of these peptides in the electrolyte/fluid secretion within the rat exocrine pancreas.

Ectopic expression of guanylyl cyclase C in adenocarcinomas of the esophagus and stomach

Guanylyl cyclase C (GC-C), a receptor specifically expressed in cells originating from differentiated intestinal epithelium, is a marker and therapeutic target for colorectal cancer metastases. Intestinal metaplasia, in which epithelial cells assume histological and molecular characteristics of differentiated intestinal enterocytes, is a common precursor to adenocarcinomas of the esophagus and stomach. Thus, those tumors, tissues adjacent to them, and their associated regional lymph nodes were assessed for GC-C expression by reverse transcription coupled with the PCR. GC-C mRNA was detected in five of five and eight of nine esophageal and gastric adenocarcinomas, respectively. Also, GC-C mRNA was detected in three of five and six of seven tissues adjacent to, but not histologically involved in, esophageal and gastric adenocarcinomas, respectively, reflecting molecular changes associated with neoplastic transformation preceding histopathological changes. In contrast, three normal gastric specimens did not express GC-C. Furthermore, GC-C mRNA was detected in 1 of 1 lymph node containing tumor cells by histopathology from a patient with gastric adenocarcinoma and in 3 of 11 lymph nodes, all of which were free of tumor cells by histopathology, from a patient with a gastroesophageal junction tumor. This is the first demonstration that GC-C is ectopically expressed by primary and metastatic adenocarcinomas of the esophagus and stomach and suggests that GC-C may be a sensitive and specific clinical marker and target for adenocarcinomas of the upper gastrointestinal tract.

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.

[Micrometastases in colonic cancers: diagnostic methods and prognostic elements]

Micrometastasis are defined by the existence of cells or groups of cells in target organs. In the particular cas of colon cancers, although lymph node involvement is frequent, metastatic medullary involvement (while rarely at the origin of identified metastasis) can also be observed. Furthermore, micrometastatics cells can be identified in the circulating blood. This research relies on recent technics of immunocytochemistry with image analysis or molecular biology technics (generally PCR or RT-PCR). It is essential to have a specific reliable marker of metastatic cells. The prognostic value of identifying micrometastasis in organs also remains to be defined.

Guanylin, uroguanylin, and heat-stable euterotoxin activate guanylate cyclase C and/or a pertussis toxin-sensitive G protein in human proximal tubule cells

Membrane guanylate cyclase C (GC-C) is the receptor for guanylin, uroguanylin, and heat-stable enterotoxin (STa) in the intestine. GC-C-deficient mice show resistance to STa in intestine but saluretic and diuretic effects of uroguanylin and STa are not disturbed. Here we describe the cellular effects of these peptides using immortalized human kidney epithelial (IHKE-1) cells with properties of the proximal tubule, analyzed with the slow-whole-cell patch clamp technique. Uroguanylin (10 or 100 nm) either hyperpolarized or depolarized membrane voltages (V(m)). Guanylin and STa (both 10 or 100 nm), as well as 8-Br-cGMP (100 microm), depolarized V(m). All peptide effects were absent in the presence of 1 mm Ba(2+). Uroguanylin and guanylin changed V(m) pH dependently. Pertussis toxin (1 microg/ml, 24 h) inhibited hyperpolarizations caused by uroguanylin. Depolarizations caused by guanylin and uroguanylin were blocked by the tyrosine kinase inhibitor, genistein (10 microm). All three peptides increased cellular cGMP. mRNA for GC-C was detected in IHKE-1 cells and in isolated human proximal tubules. In IHKE-1 cells GC-C was also detected by immunostaining. These findings suggest that GC-C is probably the receptor for guanylin and STa. For uroguanylin two distinct signaling pathways exist in IHKE-1 cells, one involves GC-C and cGMP as second messenger, the other is cGMP-independent and connected to a pertussis toxin-sensitive G protein.

The atrial natriuretic peptide receptor (NPR-A/GC-A) is dephosphorylated by distinct microcystin-sensitive and magnesium-dependent protein phosphatases

Natriuretic peptide receptor (NPR)-A is the primary signaling receptor for atrial natriuretic peptide and brain natriuretic peptide. Ligand binding to NPR-A rapidly activates its guanylyl cyclase domain, but its rate of cGMP synthesis declines with time. This waning of activity is called homologous desensitization and is mediated in part by receptor dephosphorylation. Here, we characterize two distinct NPR-A phosphatase activities. The serine/threonine protein phosphatase inhibitor, microcystin, inhibited the desensitization of NPR-A in membrane guanylyl cyclase assays in the absence of magnesium. EDTA also inhibited the desensitization, whereas MgCl(2) stimulated the desensitization. Because the effects of microcystin and EDTA were additive, and microcystin did not block the magnesium-dependent desensitization, the targets for these agents appear to be distinct. Incubation of membranes at 37 degrees C stimulated the dephosphorylation of NPR-A, and microcystin blocked the temperature-dependent dephosphorylation. The addition of MgCl(2) or MnCl(2), but not CaCl(2), further stimulated the dephosphorylation of NPR-A, and microcystin failed to inhibit this process. The desensitization required changes in the phosphorylation state of NPR-A because the guanylyl cyclase activity of a receptor variant containing glutamate substitutions at all six phosphorylation sites was unaffected by MgCl(2), EDTA, or microcystin. Together, these data indicate that NPR-A is regulated by two distinct phosphatases, possibly including a member of the protein phosphatase 2C family. Finally, we observed that the desensitization of NPR-A in membranes from mouse kidneys and NIH3T3 cells was increased by prior exposure to atrial natriuretic peptide, suggesting that hormone binding enhances receptor dephosphorylation.

A comparative molecular field analysis (COMFA) of the structural determinants of heat-stable enterotoxins mediating activation of guanylyl cyclase C

The heat-stable enterotoxin binds to and activates guanylyl cyclase C (GC-C), regulating fluid and electrolyte secretion in intestinal epithelial cells. A COMFA model was developed to predict the primary interactions between GC-C agonists and their receptor. This model predicts that the amide backbone of Cys(5)-Cys(6)-Glu(7)-Leu(8), the beta carbon atoms of Cys(5)-Cys(6), and the side chains of Pro(12), Ala(13), and Ala(15) comprise the primary interactions of GC-C agonists with the receptor surface.

In vivo imaging of human colon cancer xenografts in immunodeficient mice using a guanylyl cyclase C--specific ligand

Guanylyl cyclase C (GC-C) is a transmembrane receptor expressed by human intestinal cells and primary and metastatic colorectal adenocarcinomas but not by extraintestinal tissues or tumors. The Escherichia coli heat-stable enterotoxin analog, STa (5--18), is a 14--amino acid peptide that selectively binds to the extracellular domain of GC-C with subnanomolar affinity. This study examined the utility of a radiolabeled conjugate of STa (5--18) to selectively target and image extraintestinal human colon cancer xenografts in vivo in nude mice.

METHODS

The STa conjugate, ethoxyethyl-mercaptoacetamidoadipoylglycylglycine-STa (5--18) (NC100586), was synthesized and labeled with (99m)Tc to produce (99m)Tc-NC100586. This compound was intravenously administered to nude mice bearing human colon cancer xenografts, and specific targeting was evaluated by biodistribution and gamma camera imaging.

RESULTS

In CD-1 nude mice, biodistribution and scintigraphic imaging analyses showed selective uptake of (99m)Tc-NC100586 into human colon cancer xenografts that express GC-C but not into normal tissues that do not express GC-C. Similarly, (99m)Tc-NC100586 injected intravenously into CD-1 nude mice with human colon cancer hepatic metastases selectively accumulated in those metastases, and about 5-mm foci of tumor cells were visualized after ex vivo imaging of excised livers. Accumulation of (99m)Tc-NC100586 in human colon cancer xenografts reflected binding to GC-C because (99m)Tc-NC100588, an inactive analog that does not bind to GC-C, did not selectively accumulate in cancer xenografts compared with normal tissues. Also, coadministration of excess unlabeled STa (5--18) prevented accumulation of (99m)Tc-NC100586 in human colon cancer xenografts. Furthermore, (99m)Tc-NC100586 did not selectively accumulate in Lewis lung tumor xenografts, which do not express GC-C.

CONCLUSION

This study showed that intravenously administered STa (5--18) selectively recognizes and binds to GC-C expressed by human colon cancer cells in vivo. Also shown was the ability to exploit this selective interaction to target imaging agents to extraintestinal human colon tumors in nude mice. These results suggest the utility of STa and GC-C for the development of novel targeted imaging and therapeutic agents with high specificity for metastatic colorectal tumors in humans.

In vitro binding characteristics and affinity for sulfatide of Escherichia coli STb enterotoxin

It has previously been demonstrated that sulfatide (3'-sulfogalactosyl-ceramide), present at the surface of epithelial cells of the small intestine of pigs, interacts with the thermostable enterotoxin b (STb) produced by ETEC, and that this molecule is implicated in the mechanism of action of the toxin. However, few things are known about the affinity and physical characteristics of the interaction between these two macromolecules. In this study, using a microtiter plate binding assay (MPBA), we showed that STb toxin has a strong specificity for sulfatide and that this binding is dose-dependent and saturable. A very weak binding occurred with galactosyl-ceramide whereas attachment to 3'-sulfolactosyl-ceramide corresponded to 76% of the binding to sulfatide. STb toxin was shown to possess a lectin-like property; a significative binding was observed when a terminal beta-galactose was present in the glycosphingolipids tested and an increased binding was observed in presence of a sulfate group in position 3 on the galactose. These findings suggest that a sulfated galactosyl residue seems to represent the epitope recognized by the toxin. The reaction between sulfatide and STb toxin is also time and temperature dependent and is not affected by pH. The interaction was not inhibited by free sugars, sulfated polymers, glycolipids or free ions, but was partly inhibited by high concentrations of charged sugars. STb-sulfatide binding process is a low affinity interaction, as demonstrated by the determined Kd of 2-6 +/- 1.5 microM.

Biochemistry and physiology of the natriuretic peptide receptor guanylyl cyclases

Guanylyl cyclases (GC) exist as soluble and particulate, membrane-associated enzymes which catalyse the conversion of GTP to cGMP, an intracellular signalling molecule. Several membrane forms of the enzyme have been identified up to now. Some of them serve as receptors for the natriuretic peptides, a family of peptides which includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP), three peptides known to play important roles in renal and cardiovascular physiology. These are transmembrane proteins composed of a single transmembrane domain, a variable extracellular natriuretic peptide-binding domain, and a more conserved intracellular kinase homology domain (KHD) and catalytic domain. GC-A, the receptor for ANP and BNP, also named natriuretic peptide receptor-A or -1 (NPR-A or NPR- 1), has been studied widely. Its mode of activation by peptide ligands and mechanisms of regulation serve as prototypes for understanding the function of other particulate GC. Activation of this enzyme by its ligand is a complex process requiring oligomerization, ligand binding, KHD phosphorylation and ATP binding. Gene knockout and genetic segregation studies have provided strong evidence for the importance of GC-A in the regulation of blood pressure and heart and renal functions. GC-B is the main receptor for CNP, the latter having a more paracrine role at the vascular and venous levels. The structure and regulation of GC-B is similar to that of GC-A. This chapter reviews the structure and roles of GC-A and GC-B in blood pressure regulation and cardiac and renal pathophysiology.