Background/Aims: Renal tubular Mg²⁺ reabsorption is mediated predominantly by the tight junction channel protein claudin-16 which is encoded by the gene CLDN16. Hypermagnesemia decreases, whereas hypomagnesemia increases Mg²⁺ reabsorption. This study examines the role of claudin-16 in the adaptive response of the kidney to Mg²⁺ availability. Methods/Results: Mice received a low-, normal- or high Mg²⁺ diet for up to 3 days. Mg²⁺-loaded animals displayed hypermagnesemia with increasing urine Mg²⁺/Ca²⁺ levels paralleled by a decrease in claudin-16 protein and mRNA in the kidney. Mg²⁺- deprived animals developed hypomagnesemia with decreasing urine Mg²⁺/Ca²⁺ levels associated with an increase in claudin-16 protein and mRNA abundance. Mg²⁺ depletion markedly increased and Mg²⁺ load decreased endogenous claudin-16 mRNA levels in calcium-sensing receptor-transfected HEK293 cells compared with native HEK293 cells. The effect of Mg²⁺ availability on the human CLDN16 (hCLDN16) gene promoter was examined. Using a 2.5kb hCLDN16 5′-flanking DNA sequence, we show that magnesium depletion increases and Mg²⁺ load decreases hCLDN16 promoter activity in transfected HEK293 cells. Conclusions: Changes in Mg²⁺ availability may influence claudin-16 mediated Mg²⁺ transport at the transcriptional level. The possible involvement of the cell membrane bound Ca²⁺/Mg²⁺ sensing receptor or the potential role of a hypothetical Mg²⁺ response element on the CLDN16 promoter in the Mg²⁺-induced response remains to be explored.

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.

New Findings r What is the central question of this study?In the kidney, the bulk of the filtered Mg 2+ is reabsorbed in the thick ascending limb by paracellular conductance, mediated by the tight junction protein, claudin-16, which is encoded by the gene CLDN16.

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.

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