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

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

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

Guanylin and uroguanylin mRNA expression is increased following Roux-en-Y gastric bypass, but guanylins do not play a significant role in body weight regulation and glycemic control

AIM

To determine whether intestinal expression of guanylate cyclase activator 2A (GUCA2A) and guanylate cyclase activator 2B (GUCA2B) genes is regulated in obese humans following Roux-en-Y gastric bypass (RYGB), and to evaluate the corresponding guanylin (GN) and uroguanylin (UGN) peptides for potentially contributing to the beneficial metabolic effects of RYGB.

METHODS

Enteroendocrine cells were harvested peri- and post-RYGB, and GUCA2A/GUCA2B mRNA expression was compared. GN, UGN and their prohormones (proGN, proUGN) were administered subcutaneously in normal-weight mice to evaluate effects on food intake and glucose regulation. The effect of pro-UGN or UGN overexpression, using adeno-associated virus (AAV) vectors, was assessed in diet-induced obese (DIO) mice. Intracerebroventricular administration of GN and UGN was performed in rats for assessment of putative centrally mediated effects on food intake. GN and UGN, as well as their prohormones, were evaluated for effects on glucose-stimulated insulin secretion (GSIS) in rat pancreatic islets and perfused rat pancreas.

RESULTS

GUCA2A and GUCA2B mRNA expression was significantly upregulated in enteroendocrine cells after RYGB. Peripheral administration of guanylins or prohormones did not influence food intake, oral glucose tolerance, and GSIS. Central administration of GN and UGN did not affect food intake in rats. Chronic AVV-mediated overexpression of UGN and proUGN had no effect on body weight or glucose homeostasis in DIO mice.

CONCLUSION

GN and UGN, as well as their prohormones, do not seem to play a significant role in body weight regulation and glycemic control, suggesting that guanylin-family peptides do not show promise as targets for the treatment of obesity or diabetes.

Guanylin and uroguanylin are produced by mouse intestinal epithelial cells of columnar and secretory lineage

Guanylin (GN) and uroguanylin (UGN), through activation of guanylyl cyclase C (GCC), serve to control intestinal fluid homeostasis. Both peptides are produced in the intestinal epithelium, but their cellular origin has not been fully charted. Using quantitative PCR and an improved in situ hybridization technique (RNAscope), we have assessed the expression of GN (Guca2a), UGN (Guca2b), and GCC (Gucy2c) in mouse intestine. In the crypts of Lieberkühn, expression of Guca2a and Guca2b was restricted to cells of secretory lineage, at the crypt's base, and to a region above, previously identified as a common origin of cellular differentiation. In this compartment, comparatively uniform levels of Guca2a and Guca2b expression were observed throughout the length of the gut. In contrast, Guca2a and Guca2b expression in the villus-surface region was more variable, and reflected the distinct, but overlapping expression pattern observed previously. Accordingly, in jejunum and ileum, Guca2a and Guca2b were abundantly expressed by enterocytes, whereas in colon only Guca2a transcript was found in the surface region. In duodenum, only low levels of Guca2b transcript were observed in columnar cells, and Guca2a expression was restricted entirely to cells of the secretory lineage. Gucy2c was shown to be expressed relatively uniformly along the rostrocaudal and crypt-villus axes and was also found in the duodenal glands. Our study reveals novel aspects of the cellular localization of the GCC signaling axis that, apart from its role in the regulation of fluid balance, link it to pH regulation, cell cycle control, and host defense.

The Physiology and Pathophysiology of Pancreatic Ductal Secretion: The Background for Clinicians

The human exocrine pancreas consists of 2 main cell types: acinar and ductal cells. These exocrine cells interact closely to contribute to the secretion of pancreatic juice. The most important ion in terms of the pancreatic ductal secretion is HCO3. In fact, duct cells produce an alkaline fluid that may contain up to 140 mM NaHCO3, which is essential for normal digestion. This article provides an overview of the basics of pancreatic ductal physiology and pathophysiology. In the first part of the article, we discuss the ductal electrolyte and fluid transporters and their regulation. The central role of cystic fibrosis transmembrane conductance regulator (CFTR) is highlighted, which is much more than just a Cl channel. We also review the role of pancreatic ducts in severe debilitating diseases such as cystic fibrosis (caused by various genetic defects of cftr), pancreatitis, and diabetes mellitus. Stimulation of ductal secretion in cystic fibrosis and pancreatitis may have beneficial effects in their treatment.

Acid-base transport in pancreatic cancer: molecular mechanisms and clinical potential

Solid tumors are characterized by a microenvironment that is highly acidic, while intracellular pH (pHi) is normal or even elevated. This is the result of elevated metabolic rates in the highly proliferative cancer cells, in conjunction with often greatly increased rates of net cellular acid extrusion. Studies in various cancers have suggested that while the acid extrusion mechanisms employed are generally the same as those in healthy cells, the specific transporters upregulated vary with the cancer type. The main such transporters include Na(+)/H(+) exchangers, various HCO3(-) transporters, H(+) pumps, and lactate-H(+) cotransporters. The mechanisms leading to their dysregulation in cancer are incompletely understood but include changes in transporter expression levels, trafficking and membrane localization, and posttranslational modifications. In turn, accumulating evidence has revealed that in addition to supporting their elevated metabolic rate, their increased acid efflux capacity endows the cancer cells with increased capacity for invasiveness, proliferation, and chemotherapy resistance. The pancreatic duct exhibits an enormous capacity for acid-base transport, rendering pHi dysregulation a potentially very important topic in pancreatic ductal adenocarcinoma (PDAC). PDAC - accounting for about 90% of all pancreatic cancers - has one of the highest cancer mortality rates known, and new diagnostic and treatment options are highly needed. However, very little is known about whether pH regulation is altered in PDAC and, if so, the possible role of this in cancer development. Here, we review current models for pancreatic acid-base transport and pH homeostasis and summarize current views on acid-base dysregulation in cancer, focusing where possible on the few studies to date in PDAC. Finally, we present new data-mining analyses of acid-base transporter expression changes in PDAC and discuss essential directions for future work.

Uroguanylin inhibits H-ATPase activity and surface expression in renal distal tubules by a PKG-dependent pathway

Cumulative evidence suggests that guanylin peptides play an important role on electrolyte homeostasis. We have previously reported that uroguanylin (UGN) inhibits bicarbonate reabsorption in a renal distal tubule. In the present study, we tested the hypothesis that the bicarbonaturic effect of UGN is at least in part attributable to inhibition of H(+)-ATPase-mediated hydrogen secretion in the distal nephron. By in vivo stationary microperfusion experiments, we were able to show that UGN inhibits H(+)-ATPase activity by a PKG-dependent pathway because KT5823 (PKG inhibitor) abolished the UGN effect on distal bicarbonate reabsorption and H89 (PKA inhibitor) was unable to prevent it. The in vivo results were confirmed by the in vitro experiments, where we used fluorescence microscopy to measure intracellular pH (pHi) recovery after an acid pulse with NH4Cl. By this technique, we observed that UGN and 8 bromoguanosine-cGMP (8Br-cGMP) inhibited H(+)-ATPase-dependent pHi recovery and that the UGN inhibitory effect was abolished in the presence of the PKG inhibitor. In addition, by using RT-PCR technique, we verified that Madin-Darby canine kidney (MDCK)-C11 cells express guanylate cyclase-C. Besides, UGN stimulated an increase of both cGMP content and PKG activity but was unable to increase the production of cellular cAMP content and PKA activity. Furthermore, we found that UGN reduced cell surface abundance of H+-ATPase B1 subunit in MDCK-C11 and that this effect was abolished by the PKG inhibitor. Taken together, our data suggest that UGN inhibits H(+)-ATPase activity and surface expression in renal distal cells by a cGMP/PKG-dependent pathway.

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

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

Guanylin-immunoreactive cells in the female and male rat adenohypophysis and their changes under various physiological and experimental conditions

The peptide guanylin, first isolated from rat small intestine, is involved in the regulation of water-electrolyte transport between the intracellular and extracellular compartments of the epithelia. The main sites of guanylin expression are the intestinal, airway, or exocrine gland ductal epithelia where guanylin acts in a paracrine/luminocrine fashion. Because guanylin also circulates in the blood, sources of this peptide were sought in endocrine glands. Our group has already demonstrated the presence of guanylin-immunoreactive cells in the pars tuberalis of male rat adenohypophysis. In this study, we investigated whether guanylin-immunoreactive cells exist also in the adenohypophysial pars distalis and whether their appearance or distribution correlates with various physiological conditions in female rats or alters after gonadectomy in both sexes. These studies revealed that the rat pars distalis contains two guanylin-immunoreactive cell types, gonadotrophic cells, whose number varied notably during the estrous cycle, reached a peak in the proestrous phase, and increased consistently during pregnancy, in lactating animals, and after gonadectomy, and folliculo-stellate cells, a discrete number of which were found only in female rats at the estrous phase. These findings suggest that guanylin is involved in regulating gonadotrophic cell function. They also add important information on the controversially discussed functions of folliculo-stellate cells.

Expression of guanylin and uroguanylin mRNA in human nasal mucosa and nasal polyps

OBJECTIVES

Guanylin and uroguanylin are small, heat-stable peptides that were originally isolated from the small intestine and from urine, respectively. Functionally, it has been proposed that these peptides can regulate ion and water transport in various fluid-transporting epithelia. In the present study we evaluated the presence of mRNAs for human guanylin and uroguanylin in human inferior turbinate mucosa and nasal polyps.

MATERIAL AND METHODS

The expression and localization of mRNAs for both peptides were investigated in inferior turbinate tissues and nasal polyps using reverse transcriptase polymerase chain reaction and in situ hybridization.

RESULTS

mRNAs for both peptides were detected in human turbinate mucosa and nasal polyps. In situ hybridization revealed that they were localized in the epithelial layer and submucosal glands of inferior turbinate mucosa and in the epithelial layer of nasal polyps. However, their expression was noted at low levels in the superficial epithelium of nasal polyps, compared with that of inferior turbinate mucosa.

CONCLUSIONS

These results indicate that guanylin and uroguanylin in the nasal mucosa may participate in normal physiological processes, but also suggest that altered expression of these genes in nasal polyps may cause impaired electrolyte and water transport across the epithelial cells.