Plasticity of intercalated cell polarity: effect of metabolic acidosis

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.

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.

Adaptation of intercalated cells along the collecting duct to systemic acid/base changes

Collecting duct intercalated cells respond to short-term acid/base perturbations by rapidly shuttling H(+)-ATPase to and from the plasma membrane. Purkerson et al. provide information on the regulation of the anion transporters during chronic acidosis and acute recovery (alkalosis). They found that the major mechanism for both acute and chronic states is regulation of both the H(+)-ATPase and the anion exchangers plus changes in the overall expression level of these anion transporters in chronic adaptation.

Molecular mechanisms of epithelial cell-specific expression and regulation of the human anion exchanger (pendrin) gene

Pendrin, 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, iodide accumulation, and endolymph ion balance, respectively. This study aimed to define promoter regulatory elements essential for renal, thyroid, and inner ear epithelial cell-specific expression of human PDS (hPDS) and to explore the effect of ambient pH and aldosterone on hPDS promoter activity. Endogenous pendrin mRNA and protein were detected in renal HEK293, thyroid LA2, and inner ear VOT36 epithelial cell lines, but not in the fibroblast cell line, NIH3T3. A 4.2-kb hPDS 5'-flanking DNA sequence and consecutive 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the above cell lines. Distinct differences in expression/activity of deduced positive/negative regulatory elements within the hPDS promoter between HEK293, LA2, and VOT36 cells were demonstrated, with only basal activity in NIH3T3 cells. Acidic pH (7.0-7.1) decreased and alkaline pH (7.6-7.7) increased hPDS promoter activity in transfected HEK293 and VOT36, but not in LA2 cells. Aldosterone (10(-8) M) 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. Acidic pH and aldosterone decreased, and alkaline pH increased, endogenous pendrin mRNA level in HEK293 cells. In conclusion, pendrin-mediated HCO3(-) secretion in the renal tubule and anion transport in the endolymph may be regulated transcriptionally by systemic pH and aldosterone.

Replication of segment-specific and intercalated cells in the mouse renal collecting system

The renal collecting system (CS) is composed of segment-specific (SS) and intercalated (IC) cells. The latter comprise at least two subtypes (type A and non-type A IC). The origin and maintenance of cellular heterogeneity in the CS is unclear. Among other hypotheses, it was proposed that one subtype of IC cells represents a stem cell population from which all cell types in the CS may arise. In the present study, we tested this stem cell hypothesis for the adult kidney by assessing DNA synthesis as a marker for cell replication. SS and IC cells were identified by their characteristic expressions of sodium- (epithelial sodium channel, Na-K-ATPase), water- (aquaporin-2) and acid/base- (H+ -ATPase, anion exchanger AE1) transporting proteins. Immunostaining for bromodeoxyuridine (BrdU) and for the proliferating cell nuclear antigen (PCNA) was used to reveal DNA synthesis in CS epithelium. BrdU- and PCNA-immunostaining as well as mitotic figures were seen in all subtypes of CS cells. Dividing cells retained the cell-type specific expression of marker molecules. Treatment of mice with bumetanide combined with a high oral salt intake, which increases the tubular salt load in the CS, profoundly increased the DNA-synthesis rate in SS and non-type A IC cells, but reduced it in type A IC cells. Thus, our data show that DNA synthesis and cell replication occur in each cell lineage of the CS and in differentiated cells. The replication rate in each cell type can be differently modulated by functional stimulation. Independent proliferation of each cell lineage might contribute to maintain the cellular heterogeneity of the CS of the adult kidney and may also add to the adaptation of the CS to altered functional requirements.

Endothelin and nitric oxide mediate adaptation of the cortical collecting duct to metabolic acidosis

Endothelin (ET) and nitric oxide (NO) modulate ion transport in the kidney. In this study, we defined the function of ET receptor subtypes and the NO guanylate cyclase signaling pathway in mediating the adaptation of the rabbit cortical collecting duct (CCD) to metabolic acidosis. CCDs were perfused in vitro and incubated for 3 h at pH 6.8, and bicarbonate transport or cell pH was measured before and after acid incubation. Luminal chloride was reversibly removed to isolate H(+) and HCO(3)(-) secretory fluxes and to raise the pH of beta-intercalated cells. Acid incubation caused reversal of polarity of net HCO(3)(-) transport from secretion to absorption, comprised of a 40% increase in H(+) secretion and a 75% decrease in HCO(3)(-) secretion. The ET(B) receptor antagonist BQ-788, as well as the NO synthase inhibitor, N(G)-nitro-l-arginine methyl ester (l-NAME), attenuated the adaptive decrease in HCO(3)(-) secretion by 40%, but only BQ-788 inhibited the adaptive increase in H(+) secretion. There was no effect of inactive d-NAME or the ET(A) receptor antagonist BQ-123. Both BQ-788 and l-NAME inhibited the acid-induced inactivation (endocytosis) of the apical Cl(-)/HCO(3)(-) exchanger. The guanylate cyclase inhibitor LY-83583 and cGMP-dependent protein kinase inhibitor KT-5823 affected HCO(3)(-) transport similarly to l-NAME. These data indicate that signaling via the ET(B) receptor regulates the adaptation of the CCD to metabolic acidosis and that the NO guanylate cyclase component of ET(B) receptor signaling mediates downregulation of Cl(-)/HCO(3)(-) exchange and HCO(3)(-) secretion.

Nitric oxide production modulates cyclosporin A-induced distal renal tubular acidosis in the rat

Cyclosporine A (CsA) causes distal renal tubular acidosis (dRTA) in humans and rodents. Because mice deficient in nitric-oxide (NO) synthase develop acidosis, we examined how NO production modulated H+ excretion during acid loading and CsA treatment in a rat model. Rats received CsA, L-arginine (L-Arg), or N omega-nitro-L-arginine methyl ester (L-NAME), or combinations of CsA and L-NAME or L-Arg, followed by NH4Cl (acute acid load). In vehicle-treated rats, NH4Cl loading reduced serum and urine (HCO3-) and urine pH, which was associated with increases in serum [K+] and [Cl-] and urine NH3 excretion. Similar to CsA (7.5 mg/kg), L-NAME impaired H+ excretion of NH4Cl-loaded animals. The combination CsA and L-NAME reduced H+ excretion to a larger extent than either drug alone. In contrast, administration of L-Arg ameliorated the effect of CsA on H+ excretion. Urine pH after NH4Cl was 5.80 +/- 0.09, 6.11 +/- 0.13*, 6.37 +/- 0.16*, and 5.77 +/- 0.09 in the vehicle, CsA, CsA + L-NAME and CsA + L-Arg groups, respectively (*P < 0.05). The effect of CsA and alteration of NO synthesis were mediated at least in part by changes in bicarbonate absorption in perfused cortical collecting ducts. CsA or L-NAME reduced net HCO3- absorption, and, when combined, completely inhibited it. CsA + L-Arg restored HCO3- absorption to near control levels. Administration of CsA along with L-NAME reduced NO production to below levels observed with either drug alone. These results suggest that CsA causes dRTA by inhibiting H+ pumps in the distal nephron. Inhibition of NO synthesis may be one of the mechanisms underlying the CsA effect.

Regulated expression of pendrin in rat kidney in response to chronic NH4Cl or NaHCO3 loading

The anion exchanger pendrin is present in the apical plasma membrane of type B and non-A-non-B intercalated cells of the cortical collecting duct (CCD) and connecting tubule and is involved in HCO(3)(-) secretion. In this study, we investigated whether the abundance and subcellular localization of pendrin are regulated in response to experimental metabolic acidosis and alkalosis with maintained water and sodium intake. NH(4)Cl loading (0.033 mmol NH(4)Cl/g body wt for 7 days) dramatically reduced pendrin abundance to 22 +/- 4% of control values (n = 6, P < 0.005). Immunoperoxidase labeling for pendrin showed reduced intensity in NH(4)Cl-loaded animals compared with control animals. Moreover, double-label laser confocal microscopy revealed a reduction in the fraction of cells in the CCD exhibiting pendrin labeling to 65% of the control value (n = 6, P < 0.005). Conversely, NaHCO(3) loading (0.033 mmol NaHCO(3)/g body wt for 7 days) induced a significant increase in pendrin expression to 153 +/- 11% of control values (n = 6, P < 0.01) with no change in the fraction of cells expressing pendrin. Immunoelectron microscopy revealed no major changes in the subcellular distribution, with abundant labeling in both the apical plasma membrane and the intracellular vesicles in all conditions. These results indicate that changes in pendrin protein expression play a key role in the well-established regulation of HCO(3)(-) secretion in the CCD in response to chronic changes in acid-base balance and suggest that regulation of pendrin expression may be clinically important in the correction of acid-base disturbances.