Characterization of a long-term rat mTAL cell line

miRNA-23a modulates sodium-hydrogen exchanger 1 expression: studies in medullary thick ascending limb of salt-induced hypertensive rats

BACKGROUND

The kidney is the main organ in the pathophysiology of essential hypertension. Although most bicarbonate reabsorption occurs in the proximal tubule, the medullary thick ascending limb (mTAL) of the nephron also maintains acid-base balance by contributing to 25% of bicarbonate reabsorption. A crucial element in this regulation is the sodium-hydrogen exchanger 1 (NHE1), a ubiquitous membrane protein controlling intracellular pH, where proton extrusion is driven by the inward sodium flux. MicroRNA (miRNA) expression of hypertensive patients significantly differs from that of normotensive subjects. The aim of this study was to determine the functional role of miRNA alterations at the mTAL level.

METHODS

By miRNA microarray analysis, we identified miRNA expression profiles in isolated mTALs from high sodium intake-induced hypertensive rats (HSD) versus their normotensive counterparts (NSD). In vitro validation was carried out in rat mTAL cells.

RESULTS

Five miRNAs involved in the onset of salt-sensitive hypertension were identified, including miR-23a, which was bioinformatically predicted to target NHE1 mRNA. Data demonstrated that miRNA-23a is downregulated in the mTAL of HSD rats while NHE1 is upregulated. Consistently, transfection of an miRNA-23a mimic in an mTAL cell line, using a viral vector, resulted in NHE1 downregulation.

CONCLUSION

NHE1, a protein involved in sodium reabsorption at the mTAL level and blood pressure regulation, is upregulated in our model. This was due to a downregulation of miRNA-23a. Expression levels of this miRNA are influenced by high sodium intake in the mTALs of rats. The downregulation of miRNA-23a in humans affected by essential hypertension corroborate our data and point to the potential role of miRNA-23a in the regulation of mTAL function following high salt intake.

Activation of the calcium sensing receptor increases claudin-14 expression via a PLC -p38-Sp1 pathway

Activation of the basolateral calcium sensing receptor (CaSR) in the renal tubular thick ascending limb (TAL) increases claudin‐14 expression, which reduces paracellular calcium (Ca²⁺) permeability, thus increasing urinary Ca²⁺ excretion. However, the upstream signaling pathway contributing to altered CLDN14 gene expression is unknown. To delineate this pathway, we identified and then cloned the CaSR responsive region including the promoter of mouse Cldn14 into a luciferase reporter vector. This 1500 bp sequence upstream of the 5′ UTR of Cldn14 variant 1, conferred increased reporter activity in the presence of high extracellular Ca²⁺ (5 mM) relative to a lower (0.5 mM) concentration. Assessment of Cldn14 reporter activity in response to increased extracellular Ca²⁺ in the presence or absence of specific inhibitors confirmed signaling through PLC and p38, but not JNK. Overexpression of SP1 attenuated Cldn14 reporter activity in response to CasR signaling. SP1 is expressed in the TAL and phosphorylation was attenuated by CaSR signaling. Finally, activating mutations in the CaSR increased Cldn14 reporter activity while a dominant negative mutation in the CaSR inhibited it. Together, these studies suggest that basolateral activation of the CASR leads to increased Cldn14 expression via a PLC‐ stimulated p38 pathway that prevents Sp1 mediated repression.

Single cell transcriptional and chromatin accessibility profiling redefine cellular heterogeneity in the adult human kidney

The integration of single cell transcriptome and chromatin accessibility datasets enables a deeper understanding of cell heterogeneity. We performed single nucleus ATAC (snATAC-seq) and RNA (snRNA-seq) sequencing to generate paired, cell-type-specific chromatin accessibility and transcriptional profiles of the adult human kidney. We demonstrate that snATAC-seq is comparable to snRNA-seq in the assignment of cell identity and can further refine our understanding of functional heterogeneity in the nephron. The majority of differentially accessible chromatin regions are localized to promoters and a significant proportion are closely associated with differentially expressed genes. Cell-type-specific enrichment of transcription factor binding motifs implicates the activation of NF-κB that promotes VCAM1 expression and drives transition between a subpopulation of proximal tubule epithelial cells. Our multi-omics approach improves the ability to detect unique cell states within the kidney and redefines cellular heterogeneity in the proximal tubule and thick ascending limb.

Salt stress in the renal tubules is linked to TAL-specific expression of uromodulin and an upregulation of heat shock genes

Previously, our comprehensive cardiovascular characterization study validated Uromodulin as a blood pressure gene. Uromodulin is a glycoprotein exclusively synthesized at the thick ascending limb of the loop of Henle and is encoded by the Umod gene. Umod-/- mice have significantly lower blood pressure than Umod+/+ mice, are resistant to salt-induced changes in blood pressure, and show a leftward shift in pressure-natriuresis curves reflecting changes of sodium reabsorption. Salt stress triggers transcription factors and genes that alter renal sodium reabsorption. To date there are no studies on renal transcriptome responses to salt stress. Here we aimed use RNA-Seq to delineate salt stress pathways in tubules isolated from Umod+/+ mice (a model of sodium retention) and Umod-/- mice (a model of sodium depletion) ± 300 mosmol sodium chloride ( n = 3 per group). In response to salt stress, the tubules of Umod+/+ mice displayed an upregulation of heat shock transcripts. The greatest changes occurred in the expression of: Hspa1a (Log2 fold change 4.35, P = 2.48 e-12) and Hspa1b (Log2 fold change 4.05, P = 2.48 e-12). This response was absent in tubules of Umod-/- mice. Interestingly, seven of the genes discordantly expressed in the Umod-/- tubules were electrolyte transporters. Our results are the first to show that salt stress in renal tubules alters the transcriptome, increasing the expression of heat shock genes. This direction of effect in Umod+/+ tubules suggest the difference is due to the presence of Umod facilitating greater sodium entry into the tubule cell reflecting a specific response to salt stress.

A perfusion chamber for monitoring transepithelial NaCl transport in an in vitro model of the renal tubule

Transepithelial electrical measurements in the renal tubule have provided a better understanding of how kidney regulates electrolyte and water homeostasis through the reabsorption of molecules and ions (e.g., H₂ O and NaCl). While experiments and measurement techniques using native tissue are difficult to prepare and to reproduce, cell cultures conducted largely with the Ussing chamber lack the effect of fluid shear stress which is a key physiological stimulus in the renal tubule. To overcome these limitations, we present a modular perfusion chamber for long-term culture of renal epithelial cells under flow that allows the continuous and simultaneous monitoring of both transepithelial electrical parameters and transepithelial NaCl transport. The latter is obtained from electrical conductivity measurements since Na⁺ and Cl^- are the ions that contribute most to the electrical conductivity of a standard physiological solution. The system was validated with epithelial monolayers of raTAL and NRK-52E cells that were characterized electrophysiologically for 5 days under different flow conditions (i.e., apical perfusion, basal, or both). In addition, apical to basal chemical gradients of NaCl (140/70 and 70/140 mM) were imposed in order to demonstrate the feasibility of this methodology for quantifying and monitoring in real time the transepithelial reabsorption of NaCl, which is a primary function of the renal tubule.

A variant in a cis-regulatory element enhances claudin-14 expression and is associated with pediatric-onset hypercalciuria and kidney stones

The greatest risk factor for kidney stones is hypercalciuria, the etiology of which is largely unknown. A recent genome-wide association study (GWAS) linked hypercalciuria and kidney stones to a claudin-14 (CLDN14) risk haplotype. However, the underlying molecular mechanism was not delineated. Recently, renal CLDN14 expression was found to increase in response to increased plasma calcium, thereby inducing calciuria. We hypothesized therefore that some children with hypercalciuria and kidney stones harbor a CLDN14 variant that inappropriately increases gene expression. To test this hypothesis, we sequenced the CLDN14 risk haplotype in a cohort of children with idiopathic hypercalciuria and kidney stones. An intronic SNP was more frequent in affected children. Dual luciferase and cell-based assays demonstrated increased reporter or CLDN14 expression when this polymorphism was introduced. In silico studies predicted the SNP introduced a novel insulinoma-associated 1 (INSM1) transcription factor binding site. Consistent with this, repeating the dual luciferase assay in the presence of INSM1 further increased reporter expression. Our data suggest that children with the INSM1 binding site within the CLDN14 risk haplotype have a higher likelihood of hypercalciuria and kidney stones. Enhanced CLDN14 expression may play a role in the pathophysiology of their hypercalciuria.

Plasma Membrane Targeting of Endogenous NKCC2 in COS7 Cells Bypasses Functional Golgi Cisternae and Complex N-Glycosylation

Na⁺K⁺2Cl⁻ co-transporters (NKCCs) effect the electroneutral movement of Na⁺-K⁺ and 2Cl⁻ ions across the plasma membrane of vertebrate cells. There are two known NKCC isoforms, NKCC1 (Slc12a2) and NKCC2 (Slc12a1). NKCC1 is a ubiquitously expressed transporter involved in cell volume regulation, Cl⁻ homeostasis and epithelial salt secretion, whereas NKCC2 is abundantly expressed in kidney epithelial cells of the thick ascending loop of Henle, where it plays key roles in NaCl reabsorption and electrolyte homeostasis. Although NKCC1 and NKCC2 co-transport the same ions with identical stoichiometry, NKCC1 actively co-transports water whereas NKCC2 does not. There is growing evidence showing that NKCC2 is expressed outside the kidney, but its function in extra-renal tissues remains unknown. The present study shows molecular and functional evidence of endogenous NKCC2 expression in COS7 cells, a widely used mammalian cell model. Endogenous NKCC2 is primarily found in recycling endosomes, Golgi cisternae, Golgi-derived vesicles, and to a lesser extent in the endoplasmic reticulum. Unlike NKCC1, NKCC2 is minimally hybrid/complex N-glycosylated under basal conditions and yet it is trafficked to the plasma membrane region of hyper-osmotically challenged cells through mechanisms that require minimal complex N-glycosylation or functional Golgi cisternae. Control COS7 cells exposed to slightly hyperosmotic (~6.7%) solutions for 16 h were not shrunken, suggesting that either one or both NKCC1 and NKCC2 may participate in cell volume recovery. However, NKCC2 targeted to the plasma membrane region or transient over-expression of NKCC2 failed to rescue NKCC1 in COS7 cells where NKCC1 had been silenced. Further, COS7 cells in which NKCC1, but not NKCC2, was silenced exhibited reduced cell size compared to control cells. Altogether, these results suggest that NKCC2 does not participate in cell volume recovery and therefore, NKCC1 and NKCC2 are functionally different Na⁺K⁺2Cl⁻ co-transporters.

PGE(2) EP(3) receptor downregulates COX-2 expression in the medullary thick ascending limb induced by hypertonic NaCl

We tested the hypothesis that inhibition of EP3 receptors enhances cyclooxygenase (COX)-2 expression in the thick ascending limb (TAL) induced by hypertonic stimuli. COX-2 protein expression in the outer medulla increased approximately twofold in mice given free access to 1% NaCl in the drinking water for 3 days. The increase was associated with an approximate threefold elevation in COX-2 mRNA accumulation and an increase in PGE2 production by isolated medullary (m)TAL tubules from 77.3 ± 8.4 to 165.7 ± 10.8 pg/mg protein. Moreover, administration of NS-398 abolished the increase in PGE2 production induced by 1% NaCl. EP3 receptor mRNA levels also increased approximately twofold in the outer medulla of mice that ingested 1% NaCl. The selective EP3 receptor antagonist L-798106 increased COX-2 mRNA by twofold in mTAL tubules, and the elevation in COX-2 protein induced by 1% NaCl increased an additional 50% in mice given L-798106. COX-2 mRNA in primary mTAL cells increased twofold in response to media made hypertonic by the addition of NaCl (400 mosmol/kg H2O). L-798106 increased COX-2 mRNA twofold in isotonic media and fourfold in cells exposed to 400 mosmol/kg H2O. PGE2 production by mTAL cells increased from 79.3 ± 4.6 to 286.7 ± 6.3 pg/mg protein after challenge with 400 mosmol/kg H2O and was inhibited in cells transiently transfected with a lentivirus short hairpin RNA construct targeting exon 5 of COX-2 to silence COX-2. Collectively, the data suggest that local hypertonicity in the mTAL is associated with an increase in COX-2 expression concomitant with elevated EP3 receptor expression, which limits COX-2 activity in this segment of the nephron.

Validation of Uromodulin as a Candidate Gene for Human Essential Hypertension

A recent genome-wide association study identified a locus on chromosome 16 in the promoter region of the uromodulin ( UMOD ) gene that is associated with hypertension. Here, we examined the hypertension signal with functional studies in Umod knockout (KO) mice. Systolic blood pressure was significantly lower in KO versus wild-type (WT) mice under basal conditions (KO: 116.6±0.3 mm Hg versus WT: 136.2±0.4 mm Hg; P <0.0001). Administration of 2% NaCl did not alter systolic blood pressure in KO mice, whereas it increased in WT mice by ≈33%, P <0.001. The average 24-hour urinary sodium excretion in the KO was greater than that of WT mice ( P <0.001). Chronic renal function curves demonstrate a leftward shift in KO mice, suggesting that the relationship between UMOD and blood pressure is affected by sodium. Creatinine clearance was increased during salt loading with 2% NaCl in the KO mice, leading to augmented filtered Na + excretion and further Na + loss. The difference in sodium uptake that exists between WT and KO strains was explored at the molecular level. Urinary tumor necrosis factor-α levels were significantly higher in KO mice compared with WT mice ( P <0.0001). Stimulation of primary thick ascending limb of the loop of Henle cells with exogenous tumor necrosis factor-α caused a reduction in NKCC2A expression ( P <0.001) with a concurrent rise in the levels of UMOD mRNA ( P <0.001). Collectively, we demonstrate that UMOD regulates sodium uptake in the thick ascending limb of the loop of Henle by modulating the effect of tumor necrosis factor-α on NKCC2A expression, making UMOD an important determinant of blood pressure control.

Annexin A2 mediates apical trafficking of renal Na⁺-K⁺-2Cl⁻ cotransporter

The furosemide-sensitive Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) is responsible for urine concentration and helps maintain systemic salt homeostasis. Its activity depends on trafficking to, and insertion into, the apical membrane, as well as on phosphorylation of conserved N-terminal serine and threonine residues. Vasopressin (AVP) signaling via PKA and other kinases activates NKCC2. Association of NKCC2 with lipid rafts facilitates its AVP-induced apical translocation and activation at the surface. Lipid raft microdomains typically serve as platforms for membrane proteins to facilitate their interactions with other proteins, but little is known about partners that interact with NKCC2. Yeast two-hybrid screening identified an interaction between NKCC2 and the cytosolic protein, annexin A2 (AnxA2). Annexins mediate lipid raft-dependent trafficking of transmembrane proteins, including the AVP-regulated water channel, aquaporin 2. Here, we demonstrate that AnxA2, which binds to phospholipids in a Ca(2+)-dependent manner and may organize microdomains, is codistributed with NKCC2 to promote its apical translocation in response to AVP stimulation and low chloride hypotonic stress. NKCC2 and AnxA2 interact in a phosphorylation-dependent manner. Phosphomimetic AnxA2 carrying a mutant phosphoacceptor (AnxA2-Y24D-GFP) enhanced surface expression and raft association of NKCC2 by 5-fold upon low chloride hypotonic stimulation, whereas AnxA2-Y24A-GFP and PKC-dependent AnxA2-S26D-GFP did not. As the AnxA2 effect involved only nonphosphorylated NKCC2, it appears to affect NKCC2 trafficking. Overexpression or knockdown experiments further supported the role of AnxA2 in the apical translocation and surface expression of NKCC2. In summary, this study identifies AnxA2 as a lipid raft-associated trafficking factor for NKCC2 and provides mechanistic insight into the regulation of this essential cotransporter.

NKCC2A and NFAT5 regulate renal TNF production induced by hypertonic NaCl intake

Pathways that contribute to TNF production by the kidney are not well defined. Mice given 1% NaCl in the drinking water for 3 days exhibited a 2.5-fold increase in urinary, but not plasma, TNF levels compared with mice given tap water. Since furosemide attenuated the increase in TNF levels, we hypothesized that hypertonic NaCl intake increases renal TNF production by a pathway involving the Na(+)-K(+)-2Cl(-) cotransporter (NKCC2). A 2.5-fold increase in NKCC2A mRNA accumulation was observed in medullary thick ascending limb (mTAL) tubules from mice given 1% NaCl; a concomitant 2-fold increase in nuclear factor of activated T cells 5 (NFAT5) mRNA and protein expression was observed in the outer medulla. Urinary TNF levels were reduced in mice given 1% NaCl after an intrarenal injection of a lentivirus construct designed to specifically knockdown NKCC2A (EGFP-N2A-ex4); plasma levels of TNF did not change after injection of EGFP-N2A-ex4. Intrarenal injection of EGFP-N2A-ex4 also inhibited the increase of NFAT5 mRNA abundance in the outer medulla of mice given 1% NaCl. TNF production by primary cultures of mTAL cells increased approximately sixfold in response to an increase in osmolality to 400 mosmol/kgH2O produced with NaCl and was inhibited in cells transiently transfected with a dnNFAT5 construct. Transduction of cells with EGFP-N2A-ex4 also prevented increases in TNF mRNA and protein production in response to high NaCl concentration and reduced transcriptional activity of a NFAT5 promoter construct. Since NKCC2A expression is restricted to the TAL, NKCC2A-dependent activation of NFAT5 is part of a pathway by which the TAL produces TNF in response to hypertonic NaCl intake.

β-Adrenergic receptor stimulation increases surface NKCC2 expression in rat thick ascending limbs in a process inhibited by phosphodiesterase 4

The thick ascending limb of the loop of Henle (THAL) reabsorbs ∼30% of the filtered NaCl in a process mediated by the apical Na-K-2Cl cotransporter NKCC2. Stimulation of β-adrenergic receptors in the THAL enhances NaCl reabsorption and increases intracellular cAMP. We found that intracellular cAMP stimulates NKCC2 trafficking to the apical membrane via protein kinase A (PKA). Several cAMP-specific phosphodiesterases (PDE) have been identified in rat THALs, and PDE4 decreases cAMP generated by β-adrenergic stimulation in other cells. However, it is not known whether β-adrenergic receptors activation stimulates NKCC2 trafficking. Thus we hypothesized that β-adrenergic receptor stimulation enhances THAL apical membrane NKCC2 expression via the PKA pathway and PDE4 blunts this effect. THAL suspensions were obtained from Sprague-Dawley rats, and surface NKCC2 expression was measured by surface biotinylation and Western blot. Incubation of THALs with the β-adrenergic receptor agonist isoproterenol at 0.5 and 1.0 μM increased surface NKCC2 by 17 ± 1 and 29 ± 5% respectively (P < 0.05). Preventing cAMP degradation with 3-isobutyl-methylxanthine (IBMX; a nonselective phosphodiesterase inhibitor) enhanced isoproterenol-stimulated surface NKCC2 expression to 51 ± 7% (P < 0.05 vs. isoproterenol). The β-adrenergic receptor antagonist propranolol or the PKA inhibitor H-89 completely blocked isoproterenol + IBMX-induced increase on surface NKCC2, while propranolol or H-89 alone had no effect. Selective inhibition of PDE4 with rolipram (20 μM) potentiated the effect of isoproterenol on surface NKCC2 and increased cAMP levels. We concluded that β-adrenergic receptor stimulation enhances surface NKCC2 expression in the THALs via PKA and PDE4 blunts this effect.

Group VIA phospholipase A2 is a target for vasopressin signaling in the thick ascending limb

Na+-K+-2Cl− cotransporter (NKCC2)-mediated NaCl reabsorption in the thick ascending limb (TAL) is stimulated by AVP via V2 receptor/PKA/cAMP signaling. This process is antagonized by locally produced eicosanoids such as 20-HETE or prostaglandin E2, which are synthesized in a phospholipase A2-dependent reaction cascade. Using microarray-based gene expression analysis, we found evidence for an AVP-dependent downregulation of the calcium-independent isoform of PLA2, iPLA2β, in the outer medulla of rats. In the present study, we therefore examined the contribution of iPLA2β to NKCC2 regulation. Immunoreactive iPLA2β protein was detected in cultured mTAL cells as well as in the entire TAL of rodents and humans with the exception of the macula densa. Administration of the V2 receptor-selective agonist desmopressin (5 ng/h; 3 days) to AVP-deficient diabetes insipidus rats increased outer medullary phosphorylated NKCC2 (pNKCC2) levels more than twofold in association with a marked reduction in iPLA2β abundance (−65%; P < 0.05), thus confirming microarray results. Inhibition of iPLA2β in Sprague-Dawley rats with FKGK 11 (0.5 μM) or in mTAL cells with FKGK 11 (10 μM) or ( S)-bromoenol lactone (5 μM) for 1 h markedly increased pNKCC2 levels without affecting total NKCC2 expression. Collectively, these data indicate that iPLA2β acts as an inhibitory modulator of NKCC2 activity and suggest that downregulation of iPLA2β may be a relevant step in AVP-mediated urine concentration.

Increased expression of NAD(P)H oxidase subunit p67(phox) in the renal medulla contributes to excess oxidative stress and salt-sensitive hypertension

NAD(P)H oxidase has been shown to be important in the development of salt-sensitive hypertension. Here, we show that the expression of a subunit of NAD(P)H oxidase, p67(phox), was increased in response to a high-salt diet in the outer renal medulla of the Dahl salt-sensitive (SS) rat, an animal model for human salt-sensitive hypertension. The higher expression of p67(phox), not the other subunits observed, was associated with higher NAD(P)H oxidase activity and salt sensitivity in SS rats compared with a salt-resistant strain. Genetic mutations of the SS allele of p67(phox) were found in the promoter region and contributed to higher promoter activity than that of the salt-resistant strain. To verify the importance of p67(phox), we disrupted p67(phox) in SS rats using zinc-finger nucleases. These rats exhibited a significant reduction of salt-sensitive hypertension and renal medullary oxidative stress and injury. p67(phox) could represent a target for salt-sensitive hypertension therapy.

Tumor necrosis factor-alpha is an endogenous inhibitor of Na+-K+-2Cl- cotransporter (NKCC2) isoform A in the thick ascending limb

The effects of TNF gene deletion on renal Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) expression and activity were determined. Outer medulla from TNF(-/-) mice exhibited a twofold increase in total NKCC2 protein expression compared with wild-type (WT) mice. This increase was not observed in TNF(-/-) mice treated with recombinant human TNF (hTNF) for 7 days. Administration of hTNF had no effect on total NKCC2 expression in WT mice. A fourfold increase in NKCC2A mRNA accumulation was observed in outer medulla from TNF(-/-) compared with WT mice; NKCC2F and NKCC2B mRNA accumulation was similar between genotypes. The increase in NKCC2A mRNA accumulation was attenuated when TNF(-/-) mice were treated with hTNF. Bumetanide-sensitive O(2) consumption, an in vitro correlate of NKCC2 activity, was 2.8 ± 0.2 nmol·min(-1)·mg(-1) in medullary thick ascending limb tubules from WT, representing ∼40% of total O(2) consumption, whereas, in medullary thick ascending limb tubules from TNF(-/-) mice, it was 5.6 ± 0.3 nmol·min(-1)·mg(-1), representing ∼60% of total O(2) consumption. Administration of hTNF to TNF(-/-) mice restored the bumetanide-sensitive component to ∼30% of total O(2) consumption. Ambient urine osmolality was higher in TNF(-/-) compared with WT mice (2,072 ± 104 vs. 1,696 ± 153 mosmol/kgH(2)O, P < 0.05). The diluting ability of the kidney, assessed by measuring urine osmolality before and after 1 h of water loading also was greater in TNF(-/-) compared with WT mice (174 ± 38 and 465 ± 81 mosmol/kgH(2)O, respectively, P < 0.01). Collectively, these findings suggest that TNF plays a role as an endogenous inhibitor of NKCC2 expression and function.

Differential regulation of NFAT5 by NKCC2 isoforms in medullary thick ascending limb (mTAL) cells

The effects of Na(+)-K(+)-2Cl(-) cotransporter type 2 (NKCC2) isoforms on the regulation of nuclear factor of activated T cells isoform 5 (NFAT5) were determined in mouse medullary thick ascending limb (mTAL) cells exposed to high NaCl concentration. Primary cultures of mTAL cells and freshly isolated mTAL tubules, both derived from the outer medulla (outer stripe>inner stripe), express NKCC2 isoforms A and F. The relative expression of NKCC2A mRNA was approximately twofold greater than NKCC2F in these preparations. The abundance of NKCC2A mRNA, but not NKCC2F mRNA, increased approximately twofold when mTAL cells were exposed for 2 h to a change in osmolality from 300 to 500 mosmol/kgH₂O, produced with NaCl. Total NKCC2 protein expression also increased. Moreover, a 2.5-fold increase in NFAT5 mRNA accumulation was observed after cells were exposed to 500 mosmol/kgH₂O for 4 h. Laser-scanning cytometry detected a twofold increase in endogenous NFAT5 protein expression in response to high NaCl concentration. Pretreatment with the loop diuretic bumetanide dramatically reduced transcriptional activity of the NFAT5-specific reporter construct TonE-Luc in mTAL cells exposed to high NaCl. Transient transfection of mTAL cells with shRNA vectors targeting NKCC2A prevented increases in NFAT5 mRNA abundance and protein expression and inhibited NFAT5 transcriptional activity in response to hypertonic stress. Silencing of NKCC2F mRNA did not affect NFAT5 mRNA accumulation but partially inhibited NFAT5 transcriptional activity. These findings suggest that NKCC2A and NKCC2F exhibit differential effects on NFAT5 expression and transcriptional activity in response to hypertonicity produced by high NaCl concentration.

Renal medullary microRNAs in Dahl salt-sensitive rats: miR-29b regulates several collagens and related genes

MicroRNAs are endogenous repressors of gene expression. We examined microRNAs in the renal medulla of Dahl salt-sensitive rats and consomic SS-13(BN) rats. Salt-induced hypertension and renal injury in Dahl salt-sensitive rats, particularly medullary interstitial fibrosis, have been shown previously to be substantially attenuated in SS-13(BN) rats. Of 377 microRNAs examined, 5 were found to be differentially expressed between Dahl salt-sensitive rats and consomic SS-13(BN) rats receiving a high-salt diet. Real-time PCR analysis demonstrated that high-salt diets induced substantial upregulation of miR-29b in the renal medulla of SS-13(BN) rats but not in SS rats. miR-29b was predicted to regulate 20 collagen genes, matrix metalloproteinase 2 (Mmp2), integrin beta1 (Itgb1), and other genes related to the extracellular matrix. Expression of 9 collagen genes and Mmp2 was upregulated by a high-salt diet in the renal medulla of SS rats, but not in SS-13(BN) rats, an expression pattern opposite to miR-29b. Knockdown of miR-29b in the kidneys of SS-13(BN) rats resulted in upregulation of several collagen genes. miR-29b reduced expression levels of several collagen genes and Itgb1 in cultured rat renal medullary epithelial cells. Moreover, miR-29b suppressed the activity of luciferase when the reporter gene was linked to a 3'-untranslated segment of collagen genes Col1a1, Col3a1, Col4a1, Col5a1, Col5a2, Col5a3, Col7a1, Col8a1, Mmp2, or Itgb1 but not Col12a1. The result demonstrated broad effects of miR-29b on a large number of collagens and genes related to the extracellular matrix and suggested involvement of miR-29b in the protection from renal medullary injury in SS-13(BN) rats.

Expression and function of NFAT5 in medullary thick ascending limb (mTAL) cells

The contribution of nuclear factor of activated T cells 5 (NFAT5) to the regulation of tumor necrosis factor-alpha (TNF) production in medullary thick ascending limb (mTAL) cells is unclear. RT-PCR analysis was performed on primary cultures of mouse mTAL cells and freshly isolated mTAL tubules to determine which NFAT isoforms are present in this nephron segment. Primer pairs were designed, based on published sequences for mouse NFAT1-5, to produce fragments of approximately 200 bp. Analysis of PCR products by gel electrophoresis and subsequent DNA sequencing indicated that cells and tubules contained mRNA for all five NFAT isoforms. The relative expression of NFAT isoforms was then determined using quantitative real-time RT-PCR. The data indicate that NFAT isoforms 5 >/= 1 are the predominant isoforms present in mTAL cells and tubules. Western blot analysis demonstrated constitutive expression of NFAT5 in nuclear extracts from mTAL tubules and primary culture cells; expression in mTAL cells also was detected by immunofluorescence. Expression of NFAT5 was increased in mTAL cells transiently transfected with an NFAT5 overexpression vector (pcDNA3.1-NFAT5), resulting in increased basal and calcium-sensing receptor (CaR)-mediated TNF production. Transient transfection of mTAL cells with a small hairpin RNA vector that targeted exon 8 of NFAT5 (U6-N5 ex8) significantly inhibited TNF promoter activity. Transient transfection with U6-N5 ex8 also reduced nuclear expression of NFAT5, TNF mRNA accumulation, and attenuated CaR-mediated activation of Cl(-) entry into polarized mTAL cells. Collectively, these data suggest that activation of NFAT5 is part of a TNF-dependent pathway that inhibits apical Cl(-) influx in the mTAL after activation of CaR.

Calcium-sensing receptor signaling pathways in medullary thick ascending limb cells mediate COX-2-derived PGE2 production: functional significance

We determined the functional implications of calcium-sensing receptor (CaR)-dependent, Gq- and Gi-coupled signaling cascades, which work in a coordinated manner to regulate activity of nuclear factor of activated T cells and tumor necrosis factor (TNF)-alpha gene transcription that cause expression of cyclooxygenase (COX)-2-derived prostaglandin E2 (PGE2) synthesis by rat medullary thick ascending limb cells (mTAL). Interruption of Gq, Gi, protein kinase C (PKC), or calcineurin (CaN) activities abolished CaR-mediated COX-2 expression and PGE2 synthesis. We tested the hypothesis that these pathways contribute to the effects of CaR activation on ion transport in mTAL cells. Ouabain-sensitive O2 consumption, an in vitro correlate of ion transport in the mTAL, was inhibited by approximately 70% in cells treated for 6 h with extracellular Ca2+ (1.2 mM), an effect prevented in mTAL cells transiently transfected with a dominant negative CaR overexpression construct (R796W), indicating that the effect was initiated by stimulation of the CaR. Pretreatment with the COX-2-selective inhibitor, NS-398 (1 microM), reversed CaR-activated decreases in ouabain-sensitive O2 consumption by approximately 60%, but did not alter basal levels of ouabain-sensitive O2 consumption. Similarly, inhibition of either Gq, Gi, PKC, or CaN, which are components of the mechanism associated with CaR-stimulated COX-2-derived PGE2 synthesis, reversed the inhibitory effects of CaR on O2 consumption without affecting basal O2 consumption. Our findings identified signaling elements required for CaR-mediated TNF production that are integral components regulating mTAL function via a mechanism involving COX-2 expression and PGE2 production.

PGE2 inhibits basolateral 50 pS potassium channels in the thick ascending limb of the rat kidney

To study the inhibition of the inwardly rectifying basolateral 50 pS potassium channels by PGE(2) we performed patch-clamp studies on the basolateral membrane of the rat kidney thick ascending limb. PGE(2)'s effect was mimicked by the selective EP1- and EP3-receptor agonist, sulprostone, but was prevented by inhibiting protein kinase-C with calphostin-C. The mitogen-activated protein kinase inhibitor PD98059 (ERK) or SB203580 (p38) increased basal channel activity; however, while neither alone prevented the inhibitory effect of PGE(2), but using both of them together completely abolished PGE(2)'s effect on channel activity. Treatment with PGE(2) stimulated phosphorylation of both p38 and ERK in primary cultures of medullary thick ascending limb cells. The PGE(2)-mediated mitogen-activated protein kinase activation was not affected by indomethacin, but was completely blocked by calphostin-C. These studies show that inhibition of basolateral 50 pS potassium channels by PGE(2) is mediated by protein kinase-C, which in turn stimulates mitogen-activated protein kinases in the thick ascending limb of the rat kidney.