A novel role for uroguanylin in the regulation of sodium balance

Phosphate as a Signaling Molecule

Phosphorus plays a vital role in diverse biological processes including intracellular signaling, membrane integrity, and skeletal biomineralization; therefore, the regulation of phosphorus homeostasis is essential to the well-being of the organism. Cells and whole organisms respond to changes in inorganic phosphorus (Pi) concentrations in their environment by adjusting Pi uptake and altering biochemical processes in cells (local effects) and distant organs (endocrine effects). Unicellular organisms, such as bacteria and yeast, express specific Pi-binding proteins on the plasma membrane that respond to changes in ambient Pi availability and transduce intracellular signals that regulate the expression of genes involved in cellular Pi uptake. Multicellular organisms, including humans, respond at a cellular level to adapt to changes in extracellular Pi concentrations and also have endocrine pathways which integrate signals from various organs (e.g., intestine, kidneys, parathyroid glands, bone) to regulate serum Pi concentrations and whole-body phosphorus balance. In mammals, alterations in the concentrations of extracellular Pi modulate type III sodium-phosphate cotransporter activity on the plasma membrane, and trigger changes in cellular function. In addition, elevated extracellular Pi induces activation of fibroblast growth factor receptor, Raf/mitogen-activated protein kinase/ERK kinase (MEK)/extracellular signal-regulated kinase (ERK) and Akt pathways, which modulate gene expression in various mammalian cell types. Excessive Pi exposure, especially in patients with chronic kidney disease, leads to endothelial dysfunction, accelerated vascular calcification, and impaired insulin secretion.

Dietary salt regulates uroguanylin expression and signaling activity in the kidney, but not in the intestine

The peptide uroguanylin (Ugn) is expressed at significant levels only in intestine and kidney, and is stored in both tissues primarily (perhaps exclusively) as intact prouroguanylin (proUgn). Intravascular infusion of either Ugn or proUgn evokes well-characterized natriuretic responses in rodents. Furthermore, Ugn knockout mice display hypertension and salt handling deficits, indicating that the Na(+) excretory mechanisms triggered when the peptides are infused into anesthetized animals are likely to operate under normal physiological conditions, and contribute to electrolyte homeostasis in conscious animals. Here, we provide strong corroborative evidence for this hypothesis, by demonstrating that UU gnV (the rate of urinary Ugn excretion) approximately doubled in conscious, unrestrained rats consuming a high-salt diet, and decreased by ~15% after salt restriction. These changes in UU gnV were not associated with altered plasma proUgn levels (shown here to be an accurate index of intestinal proUgn secretion). Furthermore, enteric Ugn mRNA levels were unaffected by salt intake, whereas renal Ugn mRNA levels increased sharply during periods of increased dietary salt consumption. Together, these data suggest that diet-evoked Ugn signals originate within the kidney, rather than the intestine, thus strengthening a growing body of evidence against a widely cited hypothesis that Ugn serves as the mediator of an entero-renal natriuretic signaling axis, while underscoring a likely intrarenal natriuretic role for the peptide. The data further suggest that intrarenal Ugn signaling is preferentially engaged when salt intake is elevated, and plays only a minor role when salt intake is restricted.

Molecular and neuronal homology between the olfactory systems of zebrafish and mouse

Studies of the two major olfactory organs of rodents, the olfactory mucosa (OM) and the vomeronasal organ (VNO), unraveled the molecular basis of smell in vertebrates. However, some vertebrates lack a VNO. Here we generated and analyzed the olfactory transcriptome of the zebrafish and compared it to the olfactory transcriptomes of mouse to investigate the evolutionary and molecular relationship between single and dual olfactory systems. Our analyses revealed a high degree of molecular conservation, with orthologs of mouse olfactory cell-specific markers and all but one of their chemosensory receptor classes expressed in the single zebrafish olfactory organ. Zebrafish chemosensory receptor genes are expressed across a large dynamic range and their RNA abundance correlates positively with the number of neurons expressing that RNA. Thus we estimate the relative proportions of neuronal sub-types expressing different chemosensory receptors. Receptor repertoire size drives the absolute abundance of different classes of neurons, but we find similar underlying patterns in both species. Finally, we identified novel marker genes that characterize rare neuronal populations in both mouse and zebrafish. In sum, we find that the molecular and cellular mechanisms underpinning olfaction in teleosts and mammals are similar despite 430 million years of evolutionary divergence.

Pendrin, a novel transcriptional target of the uroguanylin system

Guanylin (GN) and uroguanylin (UGN) are low-molecular-weight peptide hormones produced mainly in the intestinal mucosa in response to oral salt load. GN and UGN (guanylin peptides) induce secretion of electrolytes and water in both intestine and kidney. Thought to act as "intestinal natriuretic factors", GN and UGN modulate renal salt secretion by both endocrine mechanisms (linking the digestive system and kidney) and paracrine/autocrine (intrarenal) mechanisms. The cellular function of GN and UGN in intestine and proximal tubule is mediated by guanylyl cyclase C (GC-C)-, cGMP-, and G protein-dependent pathways, whereas, in principal cells of the cortical collecting duct (CCD), these peptide hormones act via GC-C-independent signaling through phospholipase A2 (PLA2). The Cl(-)/HCO(-)3 exchanger pendrin (SLC26A4), encoded by the PDS gene, is expressed in non-α intercalated cells of the CCD. Pendrin is essential for CCD bicarbonate secretion and is also involved in NaCl balance and blood pressure regulation. Our recent studies have provided evidence that pendrin-mediated anion exchange in the CCD is regulated at the transcriptional level by UGN. UGN exerts an inhibitory effect on the pendrin gene promoter likely via heat shock factor 1 (HSF1) action at a defined heat shock element (HSE) site. Recent studies have unraveled novel roles for guanylin peptides in several organ systems including involvement in appetite regulation, olfactory function, cell proliferation and differentiation, inflammation, and reproductive function. Both the guanylin system and pendrin have also been implicated in airway function. Future molecular research into the receptors and signal transduction pathways involved in the action of guanylin peptides and the pendrin anion exchanger in the kidney and other organs, and into the links between them, may facilitate discovery of new therapies for hypertension, heart failure, hepatic failure and other fluid retention syndromes, as well as for diverse diseases such as obesity, asthma, and cancer.

Abdominal contributions to cardiorenal dysfunction in congestive heart failure

Current pathophysiological models of congestive heart failure unsatisfactorily explain the detrimental link between congestion and cardiorenal function. Abdominal congestion (i.e., splanchnic venous and interstitial congestion) manifests in a substantial number of patients with advanced congestive heart failure, yet is poorly defined. Compromised capacitance function of the splanchnic vasculature and deficient abdominal lymph flow resulting in interstitial edema might both be implied in the occurrence of increased cardiac filling pressures and renal dysfunction. Indeed, increased intra-abdominal pressure, as an extreme marker of abdominal congestion, is correlated with renal dysfunction in advanced congestive heart failure. Intriguing findings provide preliminary evidence that alterations in the liver and spleen contribute to systemic congestion in heart failure. Finally, gut-derived hormones might influence sodium homeostasis, whereas entrance of bowel toxins into the circulatory system, as a result of impaired intestinal barrier function secondary to congestion, might further depress cardiac as well as renal function. Those toxins are mainly produced by micro-organisms in the gut lumen, with presumably important alterations in advanced heart failure, especially when renal function is depressed. Therefore, in this state-of-the-art review, we explore the crosstalk between the abdomen, heart, and kidneys in congestive heart failure. This might offer new diagnostic opportunities as well as treatment strategies to achieve decongestion in heart failure, especially when abdominal congestion is present. Among those currently under investigation are paracentesis, ultrafiltration, peritoneal dialysis, oral sodium binders, vasodilator therapy, renal sympathetic denervation and agents targeting the gut microbiota.

The pendrin anion exchanger gene is transcriptionally regulated by uroguanylin: a novel enterorenal link

The pendrin/SLC26A4 Cl(-)/HCO(3)(-) exchanger, encoded by the PDS gene, is expressed in cortical collecting duct (CCD) non-A intercalated cells. Pendrin is essential for CCD bicarbonate secretion and is also involved in NaCl balance and blood pressure regulation. The intestinal peptide uroguanylin (UGN) is produced in response to oral salt load and can function as an "intestinal natriuretic hormone." We aimed to investigate whether UGN modulates pendrin activity and to explore the molecular mechanisms responsible for this modulation. Injection of UGN into mice resulted in decreased pendrin mRNA and protein expression in the kidney. UGN decreased endogenous pendrin mRNA levels in HEK293 cells. A 4.2-kb human PDS (hPDS) promoter sequence and consecutive 5' deletion products were cloned into luciferase reporter vectors and transiently transfected into HEK293 cells. Exposure of transfected cells to UGN decreased hPDS promoter activity. This UGN-induced effect on the hPDS promoter occurred within a 52-bp region encompassing a single heat shock element (HSE). The effect of UGN on the promoter was abolished when the HSE located between nt -1119 and -1115 was absent or was mutated. Furthermore, treatment of HEK293 cells with heat shock factor 1 (HSF1) small interfering RNA (siRNA) reversed the UGN-induced decrease in endogenous PDS mRNA level. In conclusion, pendrin-mediated Cl(-)/HCO(3)(-) exchange in the renal tubule may be regulated transcriptionally by the peptide hormone UGN. UGN exerts its inhibitory activity on the hPDS promoter likely via HSF1 action at a defined HSE site. These data define a novel signaling pathway involved in the enterorenal axis controlling electrolyte and water homeostasis.

Transcriptional regulation of the pendrin gene

Pendrin (SLC26A4), a Cl(-)/anion exchanger encoded by the gene PDS, is highly expressed in the kidney, thyroid and inner ear epithelia and is essential for bicarbonate secretion/chloride reabsorption, iodide accumulation and endolymph ion balance, respectively. The molecular mechanisms controlling pendrin activity in renal, thyroid and inner ear epithelia have been the subject of recent studies. The effects of ambient pH, the hormone aldosterone and the peptide uroguanylin (UGN; the "intestinal natriuretic hormone"), known modulators of electrolyte balance, on transcription of the pendrin gene, have been investigated. Luciferase reporter plasmids containing different length fragments of the human PDS (hPDS) promoter were transfected into renal HEK293, thyroid LA2, and inner ear VOT36 epithelial cells. Acidic pH decreased and alkaline pH increased hPDS promoter activity in transfected HEK293 and VOT36, but not in LA2 cells. Aldosterone reduced hPDS promoter activity in HEK293 but had no effect in LA2 and VOT36 cells. These pH and aldosterone-induced effects on the hPDS promoter occurred within 96-bp and 89-bp regions, respectively, which likely contain distinct response elements to these modulators. Injection of UGN into mice resulted in decreased pendrin mRNA and protein expression in the kidney. Exposure of transfected HEK293 to UGN decreased hPDS promoter activity. The findings provided evidence for the presence of a UGN response element within the 96-bp region overlapping with the pH response element on the hPDS promoter. Pendrin is also expressed in airway epithelium. The cytokins interleukin 4 (IL-4) and interleukin-13 (IL-13), known regulators of airway surface function, have been shown to increase hPDS promoter activity by a STAT6-dependent mechanism. In conclusion, systemic pH, the hormone aldosterone, and the peptide UGN influence renal tubular pendrin gene expression and, perhaps, pendrin-mediated Cl(-)/HCO(3)(-) exchange at the transcriptional level. Pendrin-driven anion transport in the endolymph and at the airway surface may be regulated transcriptionally by systemic pH and IL-3/IL-4, respectively. The distinct response elements and the corresponding transcription factors mediating the effect of these modulators on the PDS promoter remain to be identified and characterized.

Guanylin peptide family: history, interactions with ANP, and new pharmacological perspectives

The guanylin family of peptides has 3 subclasses of peptides containing either 3 intramolecular disulfide bonds found in bacterial heat-stable enterotoxins (ST), or 2 disulfides observed in guanylin and uroguanylin, or a single disulfide exemplified by lymphoguanylin. These peptides bind to and activate cell-surface receptors that have intrinsic guanylate cyclase (GC) activity. These hormones are synthesized in the intestine and released both luminally and into the circulation, and are also produced within the kidney. Stimulation of renal target cells by guanylin peptides in vivo or ex vivo elicits a long-lived diuresis, natriuresis, and kaliuresis by both cGMP-dependent and independent mechanisms. Uroguanylin may act as a hormone in a novel endocrine axis linking the digestive system and kidney as well as a paracrine system intrarenally to increase sodium excretion in the postprandial period. This highly integrated and redundant mechanism allows the organism to maintain sodium balance by eliminating excess sodium in the urine. In addition, small concentrations of the atrial natriuretic peptide (ANP) can synergize with low concentrations of both guanylin or uroguanylin, which do not induce natriuresis per se, to promote significant natriuresis. Interestingly, the activation of the particulate guanylate cyclase receptors by natriuretic peptides can promote relaxation of animal and human penile erectile tissue and increase intracavernosal pressure to induce penile erection. These peptides can be prototypes for new drugs to treat erectile dysfunction, especially in patients with endothelial and nitrergic dysfunction, such as in diabetes.

Defying the stereotype: non-canonical roles of the Peptide hormones guanylin and uroguanylin

The peptide hormones uroguanylin and guanylin have been traditionally thought to be mediators of fluid-ion homeostasis in the vertebrate intestine. They serve as ligands for receptor guanylyl cyclase C (GC-C), and both receptor and ligands are expressed predominantly in the intestine. Ligand binding to GC-C results in increased cyclic GMP production in the cell which governs downstream signaling. In the last decade, a significant amount of research has unraveled novel functions for this class of peptide hormones, in addition to their action as intestinal secretagogues. An additional receptor for uroguanylin, receptor guanylyl cyclase D, has also been identified. Thus, unconventional roles of these peptides in regulating renal filtration, olfaction, reproduction, and cell proliferation have begun to be elucidated in detail. These varied effects suggest that these peptide hormones act in an autocrine, paracrine as well as endocrine manner to regulate diverse cellular processes.

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.

Activation of a novel natriuretic endocrine system in humans with heart failure

Proguanylin and prouroguanylin are the inactive precursors of guanylin and uroguanylin, natriuretic peptides involved in the regulation of sodium balance. Urinary uroguanylin levels have been found previously to be elevated in patients with HF (heart failure). The aim of the present study was to investigate whether plasma proguanylin and prouroguanylin levels are increased in patients with HF and to evaluate their relationship with cardiac and renal function. In this prospective observational study, we recruited 243 patients with HF (151 men) and 72 healthy controls. In patients with HF, plasma levels of proguanylin [median, 7.2 (range, 0.9-79.0) microg/l] and prouroguanylin [8.3 (1.7-53.0 microg/l)] were both significantly (P<0.0005) higher compared with levels in healthy controls [5.5 (0.4-22.3 microg/l) for proguanylin and 6.3 (2.5-16.9) microg/l for prouroguanylin]. In patients with HF, increased age, a history of hypertension, diabetes and atrial fibrillation, use of diuretics, a higher NYHA (New York Heart Association) class and a lower eGFR (estimated glomerular filtration rate) were significant univariate predictors of proguanylin and prouroguanylin levels. In multivariate analysis, a history of hypertension and low eGFR both had strong independent associations with proguanylin and prouroguanylin levels. Proguanylin and prouroguanylin varied significantly between NYHA class with a trend of increasing plasma concentrations with worsening severity of symptoms. In conclusion, plasma proguanylin and prouroguanylin are elevated in patients with HF. Elevated plasma proguanylin and prouroguanylin levels are associated with hypertension, renal impairment and increasing severity of HF. This novel endocrine system may contribute to the pathophysiology of HF.

Guanylin-like peptides, guanylate cyclase and osmoregulation in the European eel (Anguilla anguilla)

Three guanylin-like peptides, guanylin, uroguanylin and renoguanylin and two guanylate cyclase type C (GC-C) receptor isoforms were cloned and sequenced from the European eel (Anguilla anguilla). All peptides and both receptors (GC-C1 and GC-C2) were predominantly expressed within the intestine and kidney of both sexually immature yellow, and sexually maturing, migratory silver eels. The derived amino acid sequences for the pre-prohormones and guanylate cyclase isoforms had structural features in common with sequences previously reported for guanylin-like peptides and guanylate cyclases from teleost fish and other species in general. The highest sequence homologies for the prohormones were found within the active, 15-16 amino acid C-terminal peptide domain, whereas the guanylate cyclase receptors exhibited highest homology throughout the transmembrane domain and intracellular region of the protein comprising the kinase homology, oligomerisation/coiled-coil and catalytic domains. In both yellow and silver eels, seawater (SW) acclimation induced sustained increases in the expression of uroguanylin and GC-C1 mRNAs within the intestine but no significant changes were found in the abundance of mRNAs for guanylin, renoguanylin or GC-C2. Likewise there were no significant changes in expression of any of the prohormone or receptor mRNAs within the renal kidney following transfer to SW. The results suggest that uroguanylin and GC-C1 are key components of a cGMP signalling system that may play an important role within intestinal enterocytes for the regulation of salt and water absorption in the SW-acclimated eel.

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.

Circulating prouroguanylin is processed to its active natriuretic form exclusively within the renal tubules

The intestine and kidney are linked by a mechanism that increases salt excretion in response to salt intake. The peptide uroguanylin (UGn) is thought to mediate this signaling axis. Therefore, it was surprising to find (as reported in a companion publication) that UGn is stored in the intestine and circulates in the plasma almost exclusively in the form of its biologically inactive propeptide precursor, prouroguanylin (proUGn), and, furthermore, that infused proUGn leads to natriuretic activity. Here, we investigate the fate of circulating proUGn. Kinetic studies show rapid renal clearance of radiolabeled propeptide. Radiolabel accumulates at high specific activity in kidney (relative to other organs) and urine (relative to plasma). The principal metabolites found in kidney homogenates are free cysteine and methionine. In contrast, urine contains cysteine, methionine, and three other radioactive peaks, one comigrating with authentic rat UGn15. Interestingly, proUGn is not converted to these or other metabolites in plasma, indicating that circulating proUGn is not processed before entering the kidney. Therefore, our findings suggest that proUGn is the true endocrine agent released in response to salt intake and that the response of the kidney is dependent on conversion of the propeptide to an active form after it reaches the renal tubules. Furthermore, proUGn metabolites (other than small amounts of cysteine and methionine) are not returned to the circulation from the kidney or any other organ. Thus, to respond to proUGn released from the gut, any target organ must use a local mechanism for production of active peptide.

Guanylin peptides and colorectal cancer (CRC)

Agonists of guanylyl-C receptor, such as guanylin/uroguanylin, are correlated not only with the intestinal cell epithelial physiology but also with the colorectal cancer tumorigenesis. Activation of the second intracellular messenger cyclic guanosine monophosphate by guanylyl cyclase-C receptor results in a complex intracellular signalling cascade involving the phosphodiesterase, the ion channels and the protein kinase. After an analytical review of relevant new knowledge, new diagnostic and therapeutic approaches for colorectal cancer are discussed.

The proximal convoluted tubule is a target for the uroguanylin-regulated natriuretic response

OBJECTIVES AND METHODS

Guanylin and uroguanylin are peptides synthesized in the intestine and kidney that are postulated to have both paracrine and endocrine functions, forming a potential enteric-renal link to coordinate salt ingestion with natriuresis. To explore the in vivo role of guanylin and uroguanylin in the regulation of sodium excretion, we used gene-targeted mice in which the uroguanylin, guanylin or the peptide receptor guanylate cyclase C gene expression had been ablated.

RESULTS

Metabolic balance studies demonstrated that there was impaired excretion of a sodium load in uroguanylin (but not in guanylin or guanylate cyclase C) knockout mice. Uroguanylin-dependent natriuresis occurred without an increase in circulating prouroguanylin. A distinct morphological phenotype was present in the proximal convoluted tubules of uroguanylin knockout animals after an enteral salt loading. Marked vacuolization of the proximal convoluted tubule epithelial cells was observed by using light and electron microscopy. There was also a change in the distribution of the sodium hydrogen exchanger 3 (NHE3) after an enteral salt loading. In wild-type animals, there was a partial redistribution of NHE3 from the villus fraction to the less accessible submicrovillus membrane compartment, but this effect was less apparent in uroguanylin knockout animals, presumably resulting in greater Na/H exchange.

CONCLUSIONS

Together, these findings further establish a role for uroguanylin in fluid homeostasis and support a role for uroguanylin as an integral component of a signaling mechanism that mediates changes in Na excretion in response to an enteral salt loading. Proximal tubular NHE3 activity is a possible target for uroguanylin-mediated changes in Na excretion.

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.

Links between dietary salt intake, renal salt handling, blood pressure, and cardiovascular diseases

Epidemiological, migration, intervention, and genetic studies in humans and animals provide very strong evidence of a causal link between high salt intake and high blood pressure. The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt. From an evolutionary viewpoint, the human species is adapted to ingest and excrete <1 g of salt per day, at least 10 times less than the average values currently observed in industrialized and urbanized countries. Independent of the rise in blood pressure, dietary salt also increases cardiac left ventricular mass, arterial thickness and stiffness, the incidence of strokes, and the severity of cardiac failure. Thus chronic exposure to a high-salt diet appears to be a major factor involved in the frequent occurrence of hypertension and cardiovascular diseases in human populations.

Role of uroguanylin, a Peptide with natriuretic activity, in rats with experimental nephrotic syndrome

Uroguanylin induces natriuresis and diuresis in vivo as well as in vitro and is found mainly in the intestine and the kidney. However, the roles of uroguanylin in nephrotic syndrome, which is associated with sodium and water retention, have not been determined. Therefore, changes in the urine and plasma concentration of immunoreactive uroguanylin (ir-uroguanylin) and its mRNA expression in the kidney and intestine were examined using rats with puromycin aminonucleoside (PAN)-induced nephrosis. Male Sprague-Dawley rats were separated into control and nephrotic groups, and then the urinary excretion of sodium, protein, and ir-uroguanylin was examined over time. The plasma levels and renal and intestinal mRNA expression of uroguanylin at the periods of sodium retention and remarkable natriuresis also were evaluated. The sequential changes of urinary ir-uroguanylin excretion in the nephrotic group were similar to those of urinary sodium excretion. When the urinary excretion of ir-uroguanylin and sodium peaked, the plasma level of ir-uroguanylin also increased compared with that of the control group. Uroguanylin mRNA expression in the kidney increased during the period of sodium retention and then decreased during the period of remarkable natriuresis. Uroguanylin mRNA expression in the small intestines of control and nephrotic rats were identical. However, in a unilateral PAN-induced proteinuria, uroguanylin expression significantly increased in the PAN-perfused kidney compared with that in the opposite kidney. Considering the natriuretic effect of uroguanylin, these results suggested that uroguanylin plays an important role as a natriuretic factor in nephrotic syndrome via both the circulation and the kidney itself.

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.