Guanylin peptides: cyclic GMP signaling mechanisms

Phase I double-blind, placebo-controlled trial of dolcanatide (SP-333) 27 mg to explore colorectal bioactivity in healthy volunteers

Guanylyl cyclase C (GUCY2C) is a tumor-suppressing receptor silenced by loss of expression of the luminocrine hormones guanylin and uroguanylin early in colorectal carcinogenesis. This observation suggests oral replacement with a GUCY2C agonist may be an effective targeted chemoprevention agent. Previous studies revealed that linaclotide, an oral GUCY2C agonist formulated for gastric release, did not persist to activate guanylyl cyclase signaling in the distal rectum. Dolcanatide is an investigational oral uroguanylin analog, substituted with select D amino acids, for enhanced stability and extended persistence to activate GUCY2C in small and large intestine. However, the ability of oral dolcanatide to induce a pharmacodynamic (PD) response by activating GUCY2C in epithelial cells of the colorectum in humans remains undefined. Here, we demonstrate that administration of oral dolcanatide 27 mg daily for 7 d to healthy volunteers did not activate GUCY2C, quantified as accumulation of its product cyclic GMP, in epithelial cells of the distal rectum. These data reveal that the enhanced stability of dolcanatide, with persistence along the rostral-caudal axis of the small and large intestine, is inadequate to regulate GUCY2C across the colorectum to prevent tumorigenesis. These results highlight the importance of developing a GUCY2C agonist for cancer prevention formulated for release and activity targeted to the colorectum.

Plecanatide-mediated activation of guanylate cyclase-C suppresses inflammation-induced colorectal carcinogenesis in Apc+/Min-FCCC mice

AIM

To evaluate the effect of orally administered plecanatide on colorectal dysplasia in Apc^+/Min-FCCC mice with dextran sodium sulfate (DSS)-induced inflammation.

METHODS

Inflammation driven colorectal carcinogenesis was induced in Apc^+/Min-FCCC mice by administering DSS in their drinking water. Mice were fed a diet supplemented with plecanatide (0-20 ppm) and its effect on the multiplicity of histopathologically confirmed polypoid, flat and indeterminate dysplasia was evaluated. Plecanatide-mediated activation of guanylate cyclase-C (GC-C) signaling was assessed in colon tissues by measuring cyclic guanosine monophosphate (cGMP) by ELISA, protein kinase G-II and vasodilator stimulated phosphoprotein by immunoblotting. Ki-67, c-myc and cyclin D1 were used as markers of proliferation. Cellular levels and localization of β-catenin in colon tissues were assessed by immunoblotting and immunohistochemistry, respectively. Uroguanylin (UG) and GC-C transcript levels were measured by quantitative reverse transcription polymerase chain reaction (RT-PCR). A mouse cytokine array panel was used to detect cytokines in the supernatant of colon explant cultures.

RESULTS

Oral treatment of Apc^+/MinFCCC mice with plecanatide produced a statistically significant reduction in the formation of inflammation-driven polypoid, flat and indeterminate dysplasias. This anti-carcinogenic activity of plecanatide was accompanied by activation of cGMP/GC-C signaling mediated inhibition of Wnt/β-catenin signaling and reduced proliferation. Plecanatide also decreased secretion of pro-inflammatory cytokines (IL-6, IL1 TNF), chemokines (MIP-1, IP-10) and growth factors (GCSF and GMCSF) from colon explants derived from mice with acute DSS-induced inflammation. The effect of plecanatide-mediated inhibition of inflammation/dysplasia on endogenous expression of UG and GC-C transcripts was measured in intestinal tissues. Although GC-C expression was not altered appreciably, a statistically significant increase in the level of UG transcripts was detected in the proximal small intestine and colon, potentially due to a reduction in intestinal inflammation and/or neoplasia. Taken together, these results suggest that reductions in endogenous UG, accompanied by dysregulation in GC-C signaling, may be an early event in inflammation-promoted colorectal neoplasia; an event that can potentially be ameliorated by prophylactic intervention with plecanatide.

CONCLUSION

This study provides the first evidence that orally administered plecanatide reduces the multiplicity of inflammation-driven colonic dysplasia in mice, demonstrating the utility for developing GC-C agonists as chemopreventive agents.

Molecular Physiology of Membrane Guanylyl Cyclase Receptors

cGMP controls many cellular functions ranging from growth, viability, and differentiation to contractility, secretion, and ion transport. The mammalian genome encodes seven transmembrane guanylyl cyclases (GCs), GC-A to GC-G, which mainly modulate submembrane cGMP microdomains. These GCs share a unique topology comprising an extracellular domain, a short transmembrane region, and an intracellular COOH-terminal catalytic (cGMP synthesizing) region. GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure/volume and energy balance. GC-B is activated by C-type natriuretic peptide, stimulating endochondral ossification in autocrine way. GC-C mediates the paracrine effects of guanylins on intestinal ion transport and epithelial turnover. GC-E and GC-F are expressed in photoreceptor cells of the retina, and their activation by intracellular Ca(2+)-regulated proteins is essential for vision. Finally, in the rodent system two olfactorial GCs, GC-D and GC-G, are activated by low concentrations of CO2and by peptidergic (guanylins) and nonpeptidergic odorants as well as by coolness, which has implications for social behaviors. In the past years advances in human and mouse genetics as well as the development of sensitive biosensors monitoring the spatiotemporal dynamics of cGMP in living cells have provided novel relevant information about this receptor family. This increased our understanding of the mechanisms of signal transduction, regulation, and (dys)function of the membrane GCs, clarified their relevance for genetic and acquired diseases and, importantly, has revealed novel targets for therapies. The present review aims to illustrate these different features of membrane GCs and the main open questions in this field.

Intestinal and renal guanylin peptides system in hypertensive obese mice

Guanylin (GN), uroguanylin (UGN) and the GC-C receptor have been associated with two endocrine axes: the salt and water homeostasis regulating enterorenal axis and the recently described appetite-regulating UGN/GC-C extraintestinal axis. The present work assessed the mRNA expression levels of GN peptides system (GPS) in a model of diet-induced obesity. Male C57BL/6J mice were submitted to either a high-fat high-simple carbohydrate diet (obese) or a normal diet (control). The renal and intestinal GN, UGN and GC-C receptor mRNA expression were evaluated by reverse transcriptase quantitative polymerase chain reaction in both groups, during normo-saline (NS) and high-saline (HS) diet. The diet-induced obesity was accompanied by glucose intolerance and insulin resistance as well as by a significant increase in blood pressure. During NS diet, obese mice presented reduced mRNA expression of GN in ileum and colon, UGN in duodenum, ileum and colon and GC-C in duodenum, jejunum, ileum and colon. This was accompanied by increased UGN mRNA expression in renal cortex. During HS diet, obese mice presented reduced mRNA expression of GN in jejunum as well as reduced mRNA expression of UGN and GC-C in duodenum, jejunum and colon. The data obtained suggest that, in a mouse model of diet-induced obesity, a down-regulation of intestinal mRNA expression of GN, UGN and its GC-C receptor is accompanied by a compensatory increase of renal UGN mRNA expression. We hypothesize that the decrease in gene expression levels of intestinal GPS may contribute to the development of hypertension and obesity during hypercaloric diet intake.

Guanylate cyclase C-mediated antinociceptive effects of linaclotide in rodent models of visceral pain

BACKGROUND Linaclotide is a novel, orally administered investigational drug currently in clinical development for the treatment of constipation-predominant irritable bowel syndrome (IBS-C) and chronic idiopathic constipation. Visceral hyperalgesia is a major pathophysiological mechanism in IBS-C. Therefore, we investigated the anti-nociceptive properties of linaclotide in rodent models of inflammatory and non-inflammatory visceral pain and determined whether these pharmacological effects are linked to the activation of guanylate cyclase C (GC-C). METHODS Orally administered linaclotide was evaluated in non-inflammatory acute partial restraint stress (PRS) and acute water avoidance stress (WAS) models in Wistar rats, and in a trinitrobenzene sulfonic acid (TNBS)-induced inflammatory model in Wistar rats and GC-C null mice. KEY RESULTS In TNBS-induced colonic allodynia, linaclotide significantly decreased the number of abdominal contractions in response to colorectal distension without affecting the colonic wall elasticity change in response to distending pressures after TNBS. However, linaclotide had no effect on visceral sensitivity under basal conditions. In addition, linaclotide significantly decreased colonic hypersensitivity in the PRS and WAS models. In wild type (wt) and GC-C null mice, the instillation of TNBS induced similar hyperalgesia and allodynia. However, in post-inflammatory conditions linaclotide significantly reduced hypersensitivity only in wt mice, but not in GC-C null mice. CONCLUSIONS & INFERENCES These findings indicate that linaclotide has potent anti-nociceptive effects in several mechanistically different rodent models of visceral hypersensitivity and that these pharmacological properties of linaclotide are exerted through the activation of the GC-C receptor. Therefore, linaclotide may be capable of decreasing abdominal pain in patients suffering from IBS-C.

Role of Disulfide Bonds for the Structure and Folding of Proguanylin

The intestinal peptide hormone guanylin circulates mainly as its corresponding prohormone of 94 amino acids and is the first identified endogenous ligand of intestinal guanylyl cyclase C. While the prohormone is biologically inactive, it is processed to the fully functional form with 15 amino acid residues corresponding to the COOH terminus of the precursor protein. In addition to this inactivation of the hormone region, the prosequence makes an essential contribution to the disulfide-coupled folding of the hormone. On the basis of the recently determined solution structure of proguanylin, explanations for these functions of the prosequence were found, indicating that interstrand contacts between the NH2-terminal beta-hairpin of the prosequence and the COOH-terminal hormone region are crucial for formation of the correct disulfide bonds of guanylin. To further investigate the role of individual disulfide bonds upon stabilization of the overall three-dimensional structure of proguanylin and to test the assumption of a direct effect of the prosequence on the structure of the hormone region, we studied the cysteine double mutant proteins proguanylin-C48S/C61S and proguanylin-C86S/C94S. Disulfide determination as well as CD and NMR spectroscopy of proguanylin-C48S/C61S reveals an essential function of the Cys48-Cys61 disulfide bond for the stability of the hydrophobic core and thereby for the stability of the overall three-dimensional structure of proguanylin. Furthermore, sequence specific resonance assignment of the second disulfide deletion mutant, proguanylin-C86S/C94S, and comparison of the NMR spectra of this protein with those of the wild-type protein demonstrate that the rigid helical core structure of proguanylin is not affected by the mutation. Additionally, analysis of the interstrand contacts between the termini reveals a direct effect of the prosequence on the conformation of the hormone region. On the basis of these results, we propose a cooperative mechanism that leads to formation of the correct disulfide pattern of guanylin.

Innovative techniques and applications in histochemistry and cell biology

Recent studies documenting novel histochemical methods and applications in cell biology and in other areas of the life sciences have again rendered insights into structure and functions of tissues, cells, and cellular components to the level of proteins and genes. Particularly, sophisticated microscopic techniques have proved to be able to significantly advance our knowledge. Findings of recent investigations representing this progress are summarized in the present review.

Occurrence and localization of uroguanylin in the aging human prostate

Uroguanylin, a peptide hormone highly expressed in the gastrointestinal tract, is implicated in the regulation of epithelial salt and water transport processes. Since little is known about a possible role of uroguanylin in the reproductive system, we investigated for the first time the occurrence of this peptide in the human prostate using specimens of benign prostatic hyperplasia. Northern blot analyses detected a single uroguanylin transcript of approximately 600 bp in prostate RNA. The uroguanylin expression was further investigated by reverse transcriptase polymerase chain reaction of prostate RNA with uroguanylin-specific primers. Sequencing of the fragments obtained indicated the presence of a uroguanylin molecule with a sequence identical to its intestinal counterpart. Furthermore, in situ hybridization and immunohistochemistry revealed that uroguanylin mRNA and peptide are confined to epithelial cells of the prostate glands. Comparison with the distribution pattern of immunoreactivity for prostate-specific antigen (PSA) showed a high degree of colocalization of uroguanylin- and PSA-immunoreactive cells. In addition, by western blotting techniques we detected the presence of high molecular weight uroguanylin-immunoreactive material in prostatic fluid. In conclusion, our study indicates that the human prostate glands synthesize and secrete (pro-)uroguanylin. We hypothesize that this hormone may play a novel role in the male reproductive tract.

Native and recombinant proguanylin feature identical biophysical properties and are monomeric in solution

Guanylin, an intestinal peptide hormone and endogenous ligand of guanylyl cyclase C, is produced as the corresponding prohormone proguanylin. The mature hormone consists of 15 amino acid residues, representing the COOH-terminal part of the prohormone comprised of 94 amino acid residues. Here we report the recombinant expression and purification of proguanylin with its native disulfide connectivity, as well as the biophysical characterization of the recombinant and native protein. The comparison of recombinant and native proguanylin revealed identical biophysical and structural properties, as deduced from CZE, HPLC, and mass spectrometry, as well as NMR spectroscopy and CD spectroscopy at various temperatures and pH values. Exhaustive analytical ultracentrifugation studies were employed for protein concentrations up to the millimolar range to determine the association state of recombinant as well as native proguanylin, revealing both proteins to be monomeric at the applied solution conditions. As a result, a former identified close proximity between the termini of proguanylin is due to intramolecular interactions.