Dopamine acutely decreases apical membrane Na/H exchanger NHE3 protein in mouse renal proximal tubule

Biomarkers of acute renal injury and renal failure

Acute renal failure (ARF) is a frequent problem in the intensive care unit and is associated with a high mortality. Early recognition could help clinical management, but current indices lack sufficient predictive value for ARF. Therefore, there might be a need for biomarkers in detecting renal tubular injury and/or dysfunction at an early stage before a decline in glomerular filtration rate is noted by an increased serum creatinine. A MEDLINE/PubMed search was performed, including all articles about biomarkers for ARF. All publication types, human and animal studies, or subsets were searched in English language. An extraction of relevant articles was made for the purpose of this narrative review. These biomarkers include tubular enzymes (alpha- and pi-glutathione S-transferase, N-acetyl-glucosaminidase, alkaline phosphatase, gamma-glutamyl transpeptidase, Ala-(Leu-Gly)-aminopeptidase, and fructose-1,6-biphosphatase), low-molecular weight urinary proteins (alpha1- and beta2-microglobulin, retinol-binding protein, adenosine deaminase-binding protein, and cystatin C), Na+/H+ exchanger, neutrophil gelatinase-associated lipocalin, cysteine-rich protein 61, kidney injury molecule 1, urinary interleukins/adhesion molecules, and markers of glomerular filtration such as proatrial natriuretic peptide (1-98) and cystatin C. These biomarkers, detected in urine or serum shortly after tubular injury, have been suggested to contribute to prediction of ARF and need for renal replacement therapy. However, excretion of these biomarkers may also increase after reversible and mild dysfunction and may not necessarily be associated with persistent or irreversible damage. Large prospective studies in human are needed to demonstrate an improved outcome of biomarker-driven management of the patient at risk for ARF.

Na+/H+ exchangers in renal regulation of acid-base balance

The kidney plays key roles in extracellular fluid pH homeostasis by reclaiming bicarbonate (HCO(3)(-)) filtered at the glomerulus and generating the consumed HCO(3)(-) by secreting protons (H(+)) into the urine (renal acidification). Sodium-proton exchangers (NHEs) are ubiquitous transmembrane proteins mediating the countertransport of Na(+) and H(+) across lipid bilayers. In mammals, NHEs participate in the regulation of cell pH, volume, and intracellular sodium concentration, as well as in transepithelial ion transport. Five of the 10 isoforms (NHE1-4 and NHE8) are expressed at the plasma membrane of renal epithelial cells. The best-studied isoform for acid-base homeostasis is NHE3, which mediates both HCO(3)(-) absorption and H(+) excretion in the renal tubule. This article reviews some important aspects of NHEs in the kidney, with special emphasis on the role of renal NHE3 in the maintenance of acid-base balance.

Cardiac glycoside downregulates NHE3 activity and expression in LLC-PK1 cells

Ouabain, a cardiotonic steroid and a specific inhibitor of the Na(+)-K(+)-ATPase, has been shown to significantly inhibit transcellular Na(+) transport without altering the intracellular Na(+) concentration ([Na(+)](i)) in the epithelial cells derived from the renal proximal tubules. We therefore studied whether ouabain affects the activity and expression of Na(+)/H(+) exchanger isoform 3 (NHE3) representing the major route of apical Na(+) reabsorption in LLC-PK(1) cells. Chronic basolateral, but not apical, exposure to low-concentration ouabain (50 and 100 nM) did not change [Na(+)](i) but significantly reduced NHE3 activity, NHE3 protein, and mRNA expression. Inhibition of c-Src or phosphoinositide 3-kinase (PI3K) with PP2 or wortmannin, respectively, abolished ouabain-induced downregulation of NHE3 activity and mRNA expression. In caveolin-1 knockdown LLC-PK(1) cells, ouabain failed to downregulate NHE3 mRNA expression and NHE3 promoter activity. Ouabain response elements were mapped to a region between -450 and -1,194 nt, where decreased binding of thyroid hormone receptor (TR) and Sp1 to their cognate cis-elements was documented in vitro and in vivo by protein/DNA array analysis, EMSA, supershift, and chromatin immunoprecipitation. These data suggest that, in LLC-PK(1) cells, ouabain-induced signaling through the Na(+)-K(+)-ATPase-Src pathway results in decreased Sp1 and TR DNA binding activity and consequently in decreased expression and activity of NHE3. These novel findings may represent the underlying mechanism of cardiotonic steroid-mediated renal compensatory response to volume expansion and/or hypertension.

Shank2 associates with and regulates Na+/H+ exchanger 3

Na+/H+ exchanger 3 (NHE3) plays a pivotal role in transepithelial Na+ and HCO3(-) absorption across a wide range of epithelia in the digestive and renal-genitourinary systems. Accumulating evidence suggests that PDZ-based adaptor proteins play an important role in regulating the trafficking and activity of NHE3. A search for NHE3-binding modular proteins using yeast two-hybrid assays led us to the PDZ-based adaptor Shank2. The interaction between Shank2 and NHE3 was further confirmed by immunoprecipitation and surface plasmon resonance studies. When expressed in PS120/NHE3 cells, Shank2 increased the membrane expression and basal activity of NHE3 and attenuated the cAMP-dependent inhibition of NHE3 activity. Furthermore, knock-down of native Shank2 expression in Caco-2 epithelial cells by RNA interference decreased NHE3 protein expression as well as activity but amplified the inhibitory effect of cAMP on NHE3. These results indicate that Shank2 is a novel NHE3 interacting protein that is involved in the fine regulation of transepithelial salt and water transport through affecting NHE3 expression and activity.

Blunted renal dopaminergic system activity in puromycin aminonucleoside-induced nephrotic syndrome

BACKGROUND

A primary tubular sodium handling abnormality has been implicated in the edema formation of nephrotic syndrome. Dopamine synthesized by renal proximal tubules behaves as an endogenous natriuretic hormone by activating D(1)-like receptors as a paracrine/autocrine substance.

METHODS

We examined the time courses of the urinary excretion of sodium, protein and dopamine in puromycin aminonucleoside (PAN)-treated and control rats. The rats were sacrificed during greatest sodium retention (day 7) as well as during negative sodium balance (day 14) for the evaluation of renal aromatic l-amino acid decarboxylase (AADC) activity, the enzyme responsible for the synthesis of renal dopamine. Also, the influence of volume expansion (VE) and the effects of the D(1)-like agonist fenoldopam (10 microg/kg bw/min) on natriuresis and on proximal tubular Na(+),K(+)-ATPase activity were examined on day 7.

RESULTS

The daily urinary excretion of dopamine was decreased in PAN-treated rats, from day 5 and beyond. This was accompanied by a marked decrease in the renal AADC activity, on days 7 and 14. During VE, the fenoldopam-induced decrease in proximal tubular Na(+),K(+)-ATPase activity was more pronounced in PAN-treated rats than in controls. However, the urinary sodium excretion during fenoldopam infusion was markedly increased in control rats but was not altered in PAN-treated animals.

CONCLUSION

PAN nephrosis is associated with a blunted renal dopaminergic system activity which may contribute to enhance the proximal tubular Na(+),K(+)-ATPase activity. However, the lack of renal dopamine appears not to be related with the overall renal sodium retention in a state of proteinuria.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

Use of phospho-specific antibodies to determine the phosphorylation of endogenous Na+/H+ exchanger NHE3 at PKA consensus sites

Transfection studies using mutant constructs have implicated one or both protein kinase A (PKA) consensus phosphorylation sites [serines 552 and 605 in rat Na+/H+ exchanger type 3 (NHE3)] as critical for mediating inhibition of NHE3 in response to several stimuli including dopamine. However, whether one or both of these sites is actually phosphorylated in endogenous NHE3 in proximal tubule cells is unknown. The purpose of this study was to generate phosphospecific antibodies so that the state of phosphorylation of these serine residues in endogenous NHE3 could be assessed in vitro and in vivo. To this end, polyclonal and monoclonal phosphospecific peptide antibodies were generated against each PKA consensus site. Phosphospecificity was established by ELISA and Western blot assays. We then used these antibodies in vitro to evaluate the effect of dopamine on phosphorylation of the corresponding PKA sites (serines 560 and 613) in NHE3 endogenously expressed in opossum kidney cells. Baseline phosphorylation of both sites was detected that was significantly increased by dopamine. Next, we determined the baseline phosphorylation state of each serine in rat kidney NHE3 in vivo. We found that serine 552 of NHE3 is phosphorylated to a much greater extent than serine 605 at baseline in vivo. Moreover, we detected a distinct subcellular localization for NHE3 phosphorylated at serine 552 compared with total NHE3. Specifically, NHE3 phosphorylated at serine 552 localized to the coated pit region of the brush-border membrane, where NHE3 is inactive, while total NHE3 was found throughout the brush-border membrane. These findings strongly suggest that phosphorylation of NHE3 plays a role in its subcellular trafficking in vivo. In conclusion, we successfully generated phosphospecific antibodies that should be useful to assess the phosphorylation of endogenous NHE3 at its two PKA consensus sites under a variety of physiological conditions in vitro and in vivo.

cGMP inhibition of Na+/H+ antiporter 3 (NHE3) requires PDZ domain adapter NHERF2, a broad specificity protein kinase G-anchoring protein

Electroneutral NaCl absorption mediated by Na+/H+ exchanger 3 (NHE3) is important in intestinal and renal functions related to water/Na+ homeostasis. cGMP inhibits NHE3 in intact epithelia. However, unexpectedly it failed to inhibit NHE3 stably transfected in PS120 cells, even upon co-expression of cGMP-dependent protein kinase type II (cGKII). Additional co-expression of NHERF2, the tandem PDZ domain adapter protein involved in cAMP inhibition of NHE3, restored cGMP as well as cAMP inhibition, whereas NHERF1 solely restored cAMP inhibition. In vitro conditions were identified in which NHERF2 but not NHERF1 bound cGKII. The NHERF2 PDZ2 C terminus, which binds NHE3, also bound cGKII. A non-myristoylated mutant of cGKII did not support cGMP inhibition of NHE3. Although cGKI also bound NHERF2 in vitro, it did not evoke inhibition of NHE3 unless a myristoylation site was added. These results show that NHERF2, acting as a novel protein kinase G-anchoring protein, is required for cGMP inhibition of NHE3 and that cGKII must be bound both to the plasma membrane by its myristoyl anchor and to NHERF2 to inhibit NHE3.

PKC-alpha-mediated remodeling of the actin cytoskeleton is involved in constitutive albumin uptake by proximal tubule cells

One key role of the renal proximal tubule is the reabsorption of proteins from the glomerular filtrate by constitutive receptor-mediated endocytosis. In the opossum kidney (OK) renal proximal tubule cell line, inhibition of protein kinase C (PKC) reduces albumin uptake, although the isoforms involved and mechanisms by which this occurs have not been identified. We used pharmacological and molecular approaches to investigate the role of PKC-alpha in albumin endocytosis. We found that albumin uptake in OK cells was inhibited by the pan-PKC blocker bisindolylmaleimide-1 and the isoform-specific PKC blockers Go-6976 and 2',3,3',4,4'-hexahydroxy-1,1'-biphenyl-6,6'-dimethanol dimethyl ether, indicating a role for PKC-alpha. Overexpression of a kinase deficient PKC-alpha(K368R) but not wild-type PKC-alpha significantly reduced albumin endocytosis. Western blot analysis of fractionated cells showed an increased association of PKC-alpha-green fluorescent protein with the membrane fraction within 10-20 min of exposure to albumin. We used phalloidin to demonstrate that albumin induces the formation of clusters of actin at the apical surface of OK cells and that these clusters correspond to the location of albumin uptake. These clusters were not present in cells grown in the absence of albumin. In cells treated either with PKC inhibitors or overexpressing kinase-deficient PKC-alpha(K368R) this actin cluster formation was significantly reduced. This study identifies a role for PKC-alpha in constitutive albumin uptake in OK cells by mediating assembly of actin microfilaments at the apical membrane.

Activation of dopamine D1-like receptors induces acute internalization of the renal Na+/phosphate cotransporter NaPi-IIa in mouse kidney and OK cells

The Na(+)/phosphate cotransporter NaPi-IIa (SLC34A1) is the major transporter mediating the reabsorption of P(i) in the proximal tubule. Expression and activity of NaPi-IIa is regulated by several factors, including parathyroid hormone, dopamine, metabolic acidosis, and dietary P(i) intake. Dopamine induces natriuresis and phosphaturia in vivo, and its actions on several Na(+)-transporting systems such as NHE3 and Na(+)-K(+)-ATPase have been investigated in detail. Using freshly isolated mouse kidney slices, perfused proximal tubules, and cultured renal epithelial cells, we examined the acute effects of dopamine on NaPi-IIa expression and localization. Incubation of isolated kidney slices with the selective D(1)-like receptor agonists fenoldopam (10 microM) and SKF-38393 (10 microM) for 1 h induced NaPi-IIa internalization and reduced expression of NaPi-IIa in the brush border membrane (BBM). The D(2)-like selective agonist quinpirole (1 microM) had no effect. The D(1) and D(2) agonists did not affect the renal Na(+)/sulfate cotransporter NaSi in the BBM of the proximal tubule. Studies with isolated perfused proximal tubules demonstrated that activation of luminal, but not basolateral, D(1)-like receptors caused NaPi-IIa internalization. In kidney slices, inhibition of PKC (1 microM chelerythrine) or ERK1/2 (20 microM PD-098089) pathways did not prevent the fenoldopam-induced internalization. Inhibition with the PKA blocker H-89 (10 microM) abolished the effect of fenoldopam. Immunoblot demonstrated a reduction of NaPi-IIa protein in BBMs from kidney slices treated with fenoldopam. Incubation of opossum kidney cells transfected with NaPi-IIa-green fluorescent protein chimera shifted fluorescence from the apical membrane to an intracellular pool. In summary, dopamine induces internalization of NaPi-IIa by activation of luminal D(1)-like receptors, an effect that is mediated by PKA.

Phosphatonin washout in Hyp mice proximal tubules: evidence for posttranscriptional regulation

X-linked hypophosphatemia is the most common inherited form of rickets. It is characterized by renal phosphate wasting, leading to hypophosphatemia and an inappropriately normal or low serum level of 1,25(OH)2 vitamin D. Previous studies have pointed to a circulating factor or phosphatonin-inhibiting phosphate transport by decreasing mRNA of the proximal tubule NaP(i) cotransporter NaPi-2A. The present study examined the hypothesis that there was also posttranscriptional regulation of the NaPi-2A cotransporter in Hyp mice proximal tubules and whether the phosphate transport defect in Hyp mice persisted when they were studied in vitro. We found that the rate of phosphate transport in Hyp mice was <50% that in C57/B6 control mice. While phosphate transport remained stable during incubation with time in C57/B6 mice proximal tubules, it increased from 0.46 +/- 0.47 to 1.83 +/- 0.40 pmol x mm(-1) x min(-1) in Hyp proximal tubules (P < 0.01) consistent with phosphatonin washout in Hyp proximal tubules perfused in vitro. This time-dependent increase in phosphate transport was still observed in the presence of cycloheximide. There was also a reduction of proximal tubule apical NaPi-2A expression from Hyp mice compared with C57/B6 mice using single-tubule immunohistochemistry. Using immunohistochemistry, we demonstrate an increase in apical expression of the NaPi-2A transporter in proximal tubules perfused in vitro in Hyp mice even in the presence of bath cycloheximide. The increase in apical expression of the NaPi-2A transporter in proximal tubules perfused in vitro in Hyp mice was blocked by colchicine. These data are consistent with a rapidly reversible posttranscriptional defect in Hyp mice causing a reduction in phosphate transport.