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

Afferent activity to necklace glomeruli is dependent on external stimuli

Background

The main olfactory epithelium (MOE) is a complex organ containing several functionally distinct subpopulations of sensory neurons. One such subpopulation is distinguished by its expression of the guanylyl cyclase GC-D. The axons of GC-D-expressing (GC-D+) neurons innervate 9–15 "necklace" glomeruli encircling the caudal main olfactory bulb (MOB). Chemosensory stimuli for GC-D+ neurons include two natriuretic peptides, uroguanylin and guanylin, and CO₂. However, the biologically-relevant source of these chemostimuli is unclear: uroguanylin is both excreted in urine, a rich source of olfactory stimuli for rodents, and expressed in human nasal epithelium; CO₂ is present in both inspired and expired air.

Findings

To determine whether the principal source of chemostimuli for GC-D+ neurons is external or internal to the nose, we assessed the consequences of removing external chemostimuli for afferent activity to the necklace glomeruli. To do so, we performed unilateral naris occlusions in Gucy2d-Mapt-lacZ ^+/- mice [which express a β-galactosidase (β-gal) reporter specifically in GC-D+ neurons] followed by immunohistochemistry for β-gal and a glomerular marker of afferent activity, tyrosine hydroxylase (TH). We observed a dramatic decrease in TH immunostaining, consistent with reduced or absent afferent activity, in both necklace and non-necklace glomeruli ipsilateral to the occluded naris.

Conclusion

Like other MOB glomeruli, necklace glomeruli exhibit a large decrease in afferent activity upon removal of external stimuli. Thus, we conclude that activity in GC-D+ neurons, which specifically innervate necklace glomeruli, is not dependent on internal stimuli. Instead, GC-D+ neurons, like other OSNs in the MOE, primarily sense the external world.

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.

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.

Expression and distribution of ion transport mRNAs in human nasal mucosa and nasal polyps

CONCLUSIONS

Our results indicate that the electrogenic kidney Na(+)/HCO(3)(-) cotransporter (kNBC), KCl cotransporter (KCC1 and -4) and Ca(2+)-activated Cl(-) channel (CaCC1, -2, -3) mRNAs are expressed in normal nasal mucosa and nasal polyp, suggesting that altered expression of all CaCC mRNAs in nasal polyp may cause impaired electrolyte and water transport across the epithelial cells.

OBJECTIVE

Electrolyte transport by nasal epithelia has been suggested to be important for controlling the quantity and composition of the nasal fluid and may play an important role in the development of nasal polyps. Transepithelial transport of ions and water in various fluid-transporting epithelia is strictly dependent on the localization of specific membrane proteins in the polarized epithelial cells. In this study we investigated the expression and distribution of mRNA transcripts for kNBC, pancreatic NBC, KCC1, -2, -3, -4 and CaCC1, -2, -3 gene families in human nasal mucosa and nasal polyp.

MATERIAL AND METHODS

The expression and localization of these gene families were investigated in inferior turbinate tissues and nasal polyp using reverse transcriptase polymerase chain reaction (RT-PCR), semiquantitative RT-PCR and in situ hybridization.

RESULTS

mRNAs for kNBC, KCC1 and -4 and all the CaCC families (CaCC1, -2 and -3) are expressed in human turbinate mucosa and nasal polyp. The expression levels of kNBC and KCC1 and -4 mRNAs did not differ between nasal mucosa and nasal polyp. However, the expression levels of all the CaCC genes were significantly decreased in nasal polyp. In situ hybridization revealed that the expression of these genes was mainly localized in the epithelial layer and submucosal glands of inferior turbinate mucosa and in the epithelial layer of nasal polyp.