Porcine guanylin and uroguanylin: cDNA sequences, deduced amino acid sequences, and biological activity of the chemically synthesized peptides

Current understanding of guanylin peptides actions

Guanylin peptides (GPs) family includes guanylin (GN), uroguanylin (UGN), lymphoguanylin, and recently discovered renoguanylin. This growing family is proposed to be intestinal natriuretic peptides. After ingestion of a salty meal, GN and UGN are secreted into the intestinal lumen, where they inhibit sodium absorption and induce anion and water secretion. At the same conditions, those hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis and therefore prevent hypernatremia and hypervolemia after salty meals. In the intestine, a well-known receptor for GPs is guanylate cyclase C (GC-C) whose activation increases intracellular concentration of cGMP. However, in the kidney of GC-C-deficient mice, effects of GPs are unaltered, which could be by new cGMP-independent signaling pathway (G-protein-coupled receptor). This is not unusual as atrial natriuretic peptide also activates two different types of receptors: guanylate cylcase A and clearance receptor which is also G-protein coupled receptor. Physiological role of GPs in other organs (liver, pancreas, lung, sweat glands, and male reproductive system) needs to be discovered. However, it is known that they are involved in pathological conditions like cystic fibrosis, asthma, intestinal tumors, kidney and heart failure, obesity, and metabolic syndrome.

Renal electrolyte effects of guanylin and uroguanylin

PURPOSE OF REVIEW

Guanylin peptides are secreted from the intestine and influence electrolyte and water transport in intestine and kidney, suggesting that these peptides act as intestinal natriuretic peptides. This review presents recent research on renal guanylin and uroguanylin effects.

RECENT FINDINGS

After salty meals guanylin peptides are produced in the intestine activating anion secretion and inhibiting sodium absorption. In the kidney guanylin peptides induce saluresis and diuresis. The signaling of guanylin peptides in the intestine is well known, involving guanylate cyclase C and increases in cellular cGMP concentrations. As in the intestine in proximal tubule cells a cGMP and guanylate cyclase C-dependent signaling pathway exists. In guanylate cyclase C-deficient mice, renal effects are unaltered, which could be by explained by recently described new cGMP-independent signaling pathways. In proximal tubules, Uroguanylin activates a pertussis toxin-sensitive receptor. Another cGMP-independent signaling pathway of guanylin peptides involving phospholipase A2 and arachidonic acid is shown for principal cells of human and mouse cortical collecting ducts.

SUMMARY

Mechanisms and sites of renal actions of guanylin peptides are still not completely understood. Renal receptors for guanylin peptides are probably G-protein-coupled. The influences of guanylin peptides on natriuresis, kaliuresis, and diuresis are complex and only further detailed studies will allow a complete understanding of the function of these peptides.

Mechanisms of actions of guanylin peptides in the kidney

After a salty meal, stimulation of salt excretion via the kidney is a possible mechanism to prevent hypernatremia and hypervolemia. Besides the well known hormonal regulators of salt and water excretion in the distal nephron, arginine vasopressin and aldosterone, guanylin (GN) peptides produced in the intestine were proposed to be intestinal natriuretic peptides. These peptides inhibit Na+ absorption in the intestine and induce natriuresis, kaliuresis and diuresis in the kidney. The signaling pathway of GN peptides in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C) and increase the cellular concentration of cGMP which leads to secretion of Cl-, HCO3- and water into the intestinal lumen and to inhibition of Na+ absorption. Guanylin peptides are filtered in the glomerulus, and additionally synthesized and excreted by tubular cells. They activate receptors located in the luminal membrane of the tubular cells along the nephron. In GC-C deficient mice renal effects of GN peptides are retained. In human, rat, and opossum proximal tubule cells, a cGMP-dependent signaling was demonstrated, but in addition GN peptides apparently also activate a PT-sensitive G-protein coupled receptor. A similar dual signaling pathway is also known for other natriuretic peptides like atrial natriuretic peptide. A cGMP-independent signaling pathway of GN peptides is also shown for principal cells of the human cortical collecting duct where the final hormonal regulation of electrolyte homeostasis takes place. This review will focus on the current knowledge on renal actions of GN peptides and specifically address novel GC-C- and cGMP-independent signaling mechanisms.

Mechanisms of guanylin action via cyclic GMP in the kidney

Guanylin, uroguanylin, and lymphoguanylin are small peptides that activate cell-surface guanylate cyclase receptors and influence cellular function via intracellular cGMP. Guanylins activate two receptors, GC-C and OK-GC, which are expressed in intestine and/or kidney. Elevation of cGMP in the intestine elicits an increase in electrolyte and water secretion. Activation of renal receptors by uroguanylin stimulates urine flow and excretion of sodium, chloride, and potassium. Intracellular cGMP pathways for guanylins include activation of PKG-II and/or indirect stimulation of PKA-II. The result is activation of CFTR and/or C1C-2 channel proteins to enhance the electrogenic secretion of chloride and bicarbonate. Similar cellular mechanisms may be involved in the renal responses to guanylin peptides. Uroguanylin serves as an intestinal natriuretic hormone in postprandial states, thus linking the digestive and renal organ systems in a novel endocrine axis. Therefore, uroguanylin participates in the complex physiological processes underlying the saliuresis that is elicited by a salty meal.