Two novel rat guanylin molecules, guanylin-94 and guanylin-16, do not increase cyclic GMP production in T84 cells

Characterisation of proguanylin expressing cells in the intestine - evidence for constitutive luminal secretion

Guanylin, a peptide implicated in regulation of intestinal fluid secretion, is expressed in the mucosa, but the exact cellular origin remains controversial. In a new transgenic mouse model fluorescent reporter protein expression driven by the proguanylin promoter was observed throughout the small intestine and colon in goblet and Paneth(-like) cells and, except in duodenum, in mature enterocytes. In Ussing chamber experiments employing both human and mouse intestinal tissue, proguanylin was released predominantly in the luminal direction. Measurements of proguanylin expression and secretion in cell lines and organoids indicated that secretion is largely constitutive and requires ER to Golgi transport but was not acutely regulated by salt or other stimuli. Using a newly-developed proguanylin assay, we found plasma levels to be raised in humans after total gastrectomy or intestinal transplantation, but largely unresponsive to nutrient ingestion. By LC-MS/MS we identified processed forms in tissue and luminal extracts, but in plasma we only detected full-length proguanylin. Our transgenic approach provides information about the cellular origins of proguanylin, complementing previous immunohistochemical and in-situ hybridisation results. The identification of processed forms of proguanylin in the intestinal lumen but not in plasma supports the notion that the primary site of action is the gut itself.

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.

Release of guanylin immunoreactivity from the isolated vascularly perfused rat colon

The intestinal peptide guanylin regulates the electrolyte/water transport in the intestinal epithelium. The aim of the present study was to investigate the mechanisms that modulate its secretion in the isolated vascularly perfused rat colon by using a specific guanylin RIA. Intraarterial infusion of bethanechol (10(-4) M) or bombesin (10(-7) M) elicited a significant 6-fold increase in the release of guanylin immunoreactivity (G-IR) in the lumen. Bombesin-stimulated G-IR secretion was strongly reduced by tetrodotoxin, whereas atropine had no effect. VIP (10(-7) M) induced a moderate release of G-IR, whereas substance P, calcitonin gene-related peptide, peptide YY, somatostatin, and neurotensin were without effect. Dimethyl-PGE2 (1.4 x 10(-5) M) or interleukin-1beta (2.5 x 10(-10) M) induced a 3-fold increase in G-IR in the lumen, whereas the degranulator compound bromolasalocid did not stimulate guanylin secretion. Forskolin (10(-5) M) or sodium nitroprusside (10(-4)-10(-3) M) induced a significant release of G-IR. In contrast, PMA (10(-7) M) or ionophore A23187 (10(-6) M) did not modify basal secretion of G-IR. Upon stimulation of guanylin release with bombesin or bethanechol, an increase in G-IR in the portal effluent was also detected. The release of G-IR in the portal effluent was 40-fold lower than that of G-IR into the luminal perfusate. Additionally, analysis with gel chromatography revealed that the immunoreactive material released in the lumen or in the portal effluent coeluted with the 15-amino acid peptide originally isolated from rat intestine. In conclusion, the present data suggest that the enteric nervous system and immune cells may modulate guanylin release from the rat colon. The release of guanylin in the lumen and portal effluent suggests that this peptide may exert both luminal/paracrine and hormonal effects.

Regulated, side-directed secretion of proguanylin from isolated rat colonic mucosa

Guanylin, an activator of the guanylyl cyclase C receptor in the apical membrane of intestinal epithelium, modulates intestinal fluid and electrolyte transport. The bioactive 15-amino acid peptide originally isolated from rat intestine represents the C-terminal part of a longer, 115-residue prepropeptide. The aim of the present study was to characterize the direction and molecular form in which guanylin is secreted from the colonic mucosa, as well as the mechanisms that trigger its secretion. Isolated rat colonic mucosa was mounted in Ussing chambers, allowing the separate determination of apical and basolateral release. After HPLC purification, two different molecular forms of guanylin were identified in the apical incubation media by combining a bioassay for guanylyl cyclase C activation, a specific guanylin enzyme-linked immunosorbent assay and mass spectrometry, as well as sequence analysis: a bioactive form coeluting with synthetic 15-residue guanylin and the 94-residue propeptide, guanylin-22-115. The basal concentration of proguanylin at the apical side of epithelia was about 15-fold higher, compared with that of the small, bioactive peptide. In the basolateral incubation media, no proguanylin and only very low amounts of bioactive guanylin were detected. Incubation with carbachol led to a significant increase of about 7-fold in the release of proguanylin to both sides of the isolated epithelia. On the apical side, a concomitant increase of the small, bioactive peptide was observed; whereas, on the basolateral side, its concentration remained unchanged. Vasoactive intestinal peptide or the NO-donor S-nitroso-N-acetylpenicillamine did not affect guanylin secretion. Our results suggest that, in the intestine, guanylin is secreted mainly to the luminal side of the epithelium. The peptide is released as a 94-residue propeptide, which is then processed to a smaller, bioactive form (luminocrine secretion). Carbachol stimulates the release of proguanylin to both sides of the intestinal mucosa, but a parallel increase in the bioactive C-terminal derivative only occurs on the apical side. In vivo, the basolateral release could be a source of circulating proguanylin, which might be processed proteolytically to the active peptide in distant target tissues (endocrine secretion).

Role of the prosequence of guanylin

Guanylin is a guanylyl cyclase (GC)-activating peptide that is mainly secreted as the corresponding prohormone of 94 amino acid residues. In this study, we show that the originally isolated 15-residue guanylin, representing the COOH-terminal part of the prohormone, is released from the prohormone by cleavage of an Asp-Pro amide bond under conditions applied during the isolation procedures. Thus, the 15-residue guanylin is probably a non-native, chemically induced GC-activating peptide. This guanylin molecule contains two disulfide bonds that are absolutely necessary for receptor activation. We demonstrate that the folding of the reduced 15-residue guanylin results almost completely in the formation of the two inactive disulfide isomers. In contrast, the reduced form of proguanylin containing the entire prosequence folds to a product with the native cysteine connectivity. Because proguanylin lacking the 31 NH2-terminal residues of the prosequence folds only to a minor extent to guanylin with the native disulfide bonds, it is evident that this NH2-terminal region contributes significantly to the correct disulfide-coupled folding. Structural studies using CD and NMR spectroscopy show that native proguanylin contains a considerable amount of alpha-helical and, to a lesser extent, beta-sheet structural elements. In addition, a close proximity of the NH2- and the COOH-terminal regions was found by NOESY. It appears that this interaction is important for the constitution of the correct conformation and provides an explanation of the minor guanylyl cyclase activity of proguanylin by shielding the bioactive COOH-terminal domain from the receptor.

One peptide, two topologies: structure and interconversion dynamics of human uroguanylin isomers

The peptide hormone uroguanylin stimulates chloride secretion via activation of intestinal guanylyl cyclase C (GC-C). It is characterized by two disulfide bonds in a 1-3/2-4 pattern that causes the existence of two topological stereoisomers of which only one induces intracellular cGMP elevation. To obtain an unambiguous structure-function relationship of the isomers, we determined the solution structure of the separated uroguanylin isoforms using NMR spectroscopy. Both isomers adopt well-defined structures that correspond to those of the isomers of the related peptide guanylin. Furthermore, the structure of the GC-C-activating uroguanylin isomer A closely resembles the structure of the agonistic Escherichia coli heat-stable enterotoxin. Compared with guanylin isomers, the conformational interconversion of uroguanylin isomers is retarded significantly. As judged from chromatography and NMR spectroscopy, both uroguanylin isoforms are stable at low temperatures, but are subject to a slow pH-dependent mutual isomerization at 37 degrees C with an equilibrium isomer ratio of approximately 1:1. The conformational exchange is most likely under the sterical control of the carboxy-terminal leucine. These results imply that GC-C is activated by ligands exhibiting the molecular framework corresponding to the structure of uroguanylin isomer A.

Topological isomers of human uroguanylin: interconversion between biologically active and inactive isomers

The solution structures of the two compounds of human uroguanylin (I and II), which were generated during disulfide bond forming reaction, were found to be topological isomers by 1H-nuclear magnetic resonance spectroscopy. These isomers are interconvertible in aqueous media at rates which vary with the pH and temperature of the solution. Because compound I is active in the cGMP producing assay, but compound II is not, this interconversion may be useful for evaluating the activity of human uroguanylin both in vivo and in vitro.

Urine and plasma levels of uroguanylin and its molecular forms in renal diseases

Uroguanylin activates the intestinal and possibly the renal guanylate cyclase C receptor, and stimulates Cl- secretion. We developed a sensitive radioimmunoassay (RIA) for human uroguanylin and measured its concentration in the urine and plasma. Twenty-four-hour urinary excretion of immunoreactive (ir-) uroguanylin for persons with a high-salt diet (10 g/day) was 137.8 +/- 14.4 pmol/day, significantly higher than that for persons with a low-salt diet (7 g/day, 95.1 +/- 16.3 pmol/day, P < 0.05). There were significantly positive correlations between the urinary excretion of ir-uroguanylin and Na+, Cl-, K+ or cyclic GMP (cGMP). We demonstrated the presence of messenger RNA of guanylate cyclase C in the medulla of human kidney. The concentration of plasma ir-uroguanylin significantly correlated with that of serum creatinine (r = 0.71, P < 0.001). Biologically active uroguanylin-16 accounted for 99% of the endogenous uroguanylin molecules in normal urine and 60% in plasma, the remainder being the 10 kDa precursor. The precursor content increased in the urine and plasma as the severity of renal impairment increased. These findings suggest that bioactive uroguanylin-16 is involved in the regulation of electrolyte homeostasis and that the kidney participates in the metabolism and excretion of uroguanylin.

Purification, cDNA sequence, and tissue distribution of rat uroguanylin

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