Acid pH increases the stability of BSC1/NKCC2 mRNA in the medullary thick ascending limb

Fluxomics reveals cellular and molecular basis of increased renal ammoniagenesis

The kidney plays a critical role in excreting ammonia during metabolic acidosis and liver failure. The mechanisms behind this process have been poorly explored. The present study combines results of in vivo experiments of increased total ammoniagenesis with systems biology modeling, in which eight rats were fed an amino acid-rich diet (HD group) and eight a normal chow diet (AL group). We developed a method based on elementary mode analysis to study changes in amino acid flux occurring across the kidney in increased ammoniagenesis. Elementary modes represent minimal feasible metabolic paths in steady state. The model was used to predict amino acid fluxes in healthy and pre-hyperammonemic conditions, which were compared to experimental fluxes in rats. First, we found that total renal ammoniagenesis increased from 264 ± 68 to 612 ± 87 nmol (100 g body weight)^-1 min^-1 in the HD group (P = 0.021) and a concomitated upregulation of NKCC2 ammonia and other transporters in the kidney. In the kidney metabolic model, the best predictions were obtained with ammonia transport as an objective. Other objectives resulting in a fair correlation with the measured fluxes (correlation coefficient >0.5) were growth, protein uptake, urea excretion, and lysine and phenylalanine transport. These predictions were improved when specific gene expression data were considered in HD conditions, suggesting a role for the mitochondrial glycine pathway. Further studies are needed to determine if regulation through the mitochondrial glycine pathway and ammonia transporters can be modulated and how to use the kidney as a therapeutic target in hyperammonemia.

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na⁺–K⁺–Cl⁻ cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na⁺, K⁺, and Cl⁻ ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na⁺, Cl⁻, Ca²⁺, Mg²⁺, and HCO₃⁻ loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na⁺–K⁺–Cl⁻ cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Impact of blood collection and processing on peripheral blood gene expression profiling in type 1 diabetes

BACKGROUND

The natural history of type 1 diabetes (T1D) is challenging to investigate, especially as pre-diabetic individuals are difficult to identify. Numerous T1D consortia have been established to collect whole blood for gene expression analysis from individuals with or at risk to develop T1D. However, with no universally accepted protocol for their collection, differences in sample processing may lead to variances in the results. Here, we examined whether the choice of blood collection tube and RNA extraction kit leads to differences in the expression of genes that are changed during the progression of T1D, and if these differences could be minimized by measuring gene expression directly from the lysate of whole blood.

RESULTS

Microarray analysis showed that the expression of 901 genes is highly influenced by sample processing using the PAXgene versus the Tempus system. These included a significant number of lymphocyte-specific genes and genes whose expression has been reported to differ in the peripheral blood of at-risk and T1D patients compared to controls. We showed that artificial changes in gene expression occur when control and T1D samples were processed differently. The sample processing-dependent differences in gene expression were largely due to loss of transcripts during the RNA extraction step using the PAXgene system. The majority of differences were not observed when gene expression was measured in whole blood lysates prepared from blood collected in PAXgene and Tempus tubes.

CONCLUSION

We showed that the gene expression profile of samples processed using the Tempus system is more accurate than that of samples processed using the PAXgene system. Variation in sample processing can result in misleading changes in gene expression. However, these differences can be minimized by measuring gene expression directly in whole blood lysates.

Klotho/fibroblast growth factor 23- and PTH-independent estrogen receptor-α-mediated direct downregulation of NaPi-IIa by estrogen in the mouse kidney

Estrogen treatment causes renal phosphate (Pi) wasting and hypophosphatemia in rats and humans; however, the signaling mechanisms mediating this effect are still poorly understood. To determine the specific roles of estrogen receptor isoforms (ERα and ERβ) and the Klotho pathway in mediating these effects, we studied the effects of estrogen on renal Pi handling in female mice with null mutations of ERα or ERβ or Klotho and their wild type (WT) using balance studies in metabolic cages. Estrogen treatment of WT and ERβ knockout (KO) mice caused a significant reduction in food intake along with increased renal phosphate wasting. The latter resulted from a significant downregulation of NaPi-IIa and NaPi-IIc protein abundance. The mRNA expression levels of both transporters were unchanged in estrogen-treated mice. These effects on both food intake and renal Pi handling were absent in ERα KO mice. Estrogen treatment of Klotho KO mice or parathyroid hormone (PTH)-depleted thyroparathyroidectomized mice exhibited a significant downregulation of NaPi-IIa with no change in the abundance of NaPi-IIc. Estrogen treatment of a cell line (U20S) stably coexpressing both ERα and ERβ caused a significant downregulation of NaPi-IIa protein when transiently transfected with a plasmid containing full-length or open-reading frame (ORF) 3'-untranslated region (UTR) but not 5'-UTR ORF of mouse NaPi-IIa transcript. In conclusion, estrogen causes phosphaturia and hypophosphatemia in mice. These effects result from downregulation of NaPi-IIa and NaPi-IIc proteins in the proximal tubule through the activation of ERα. The downregulation of NaPi-IIa by estrogen involves 3'-UTR of its mRNA and is independent of Klotho/fibroblast growth factor 23 and PTH signaling pathways.

Hepcidin as a Major Component of Renal Antibacterial Defenses against Uropathogenic Escherichia coli

The iron-regulatory peptide hepcidin exhibits antimicrobial activity. Having previously shown hepcidin expression in the kidney, we addressed its role in urinary tract infection (UTI), which remains largely unknown. Experimental UTI was induced in wild-type (WT) and hepcidin-knockout (Hepc-/-) mice using the uropathogenic Escherichia coli CFT073 strain. Compared with infected WT mice, infected Hepc-/- mice showed a dramatic increase in renal bacterial load. Moreover, bacterial invasion was significantly dampened by the pretreatment of WT mice with hepcidin. Infected Hepc-/- mice exhibited decreased iron accumulation in the renal medulla and significant attenuation of the renal inflammatory response. Notably, we demonstrated in vitro bacteriostatic activity of hepcidin against CFT073. Furthermore, CFT073 repressed renal hepcidin, both in vivo and in cultured renal cells, and reduced phosphorylation of SMAD kinase in vivo, suggesting a bacterial strategy to escape the antimicrobial activities of hepcidin. In conclusion, we provide new mechanisms by which hepcidin contributes to renal host defense and suggest that targeting hepcidin offers a strategy to prevent bacterial invasion.

Estrogen directly and specifically downregulates NaPi-IIa through the activation of both estrogen receptor isoforms (ERα and ERβ) in rat kidney proximal tubule

We have previously demonstrated that estrogen (E2) downregulates phosphate transporter NaPi-IIa and causes phosphaturia and hypophosphatemia in ovariectomized rats. In the present study, we examined whether E2 directly targets NaPi-IIa in the proximal tubule (PT) and studied the respective roles of estrogen receptor isoforms (ERα and ERβ) in the downregulation of NaPi-IIa using both in vivo and an in vitro expression systems. We found that estrogen specifically downregulates NaPi-IIa but not NaPi-IIc or Pit2 in the kidney cortex. Proximal tubules incubated in a "shake" suspension with E2 for 24 h exhibited a dose-dependent decrease in NaPi-IIa protein abundance. Results from OVX rats treated with specific agonists for either ERα [4,4',4″;-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, PPT] or ERβ [4,4',4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, DPN] or both (PPT + DPN), indicated that only the latter caused a sharp downregulation of NaPi-IIa, along with significant phosphaturia and hypophosphatemia. Lastly, heterologous expression studies demonstrated that estrogen downregulated NaPi-IIa only in U20S cells expressing both ERα and ERβ, but not in cells expressing either receptor alone. In conclusion, these studies demonstrate that rat PT cells express both ERα and ERβ and that E2 induces phosphaturia by directly and specifically targeting NaPi-IIa in the PT cells. This effect is mediated via a mechanism involving coactivation of both ERα and ERβ, which likely form a functional heterodimer complex in the rat kidney proximal tubule.

The role of the renal ammonia transporter Rhcg in metabolic responses to dietary protein

High dietary protein imposes a metabolic acid load requiring excretion and buffering by the kidney. Impaired acid excretion in CKD, with potential metabolic acidosis, may contribute to the progression of CKD. Here, we investigated the renal adaptive response of acid excretory pathways in mice to high-protein diets containing normal or low amounts of acid-producing sulfur amino acids (SAA) and examined how this adaption requires the RhCG ammonia transporter. Diets rich in SAA stimulated expression of enzymes and transporters involved in mediating NH4 (+) reabsorption in the thick ascending limb of the loop of Henle. The SAA-rich diet increased diuresis paralleled by downregulation of aquaporin-2 (AQP2) water channels. The absence of Rhcg transiently reduced NH4 (+) excretion, stimulated the ammoniagenic pathway more strongly, and further enhanced diuresis by exacerbating the downregulation of the Na(+)/K(+)/2Cl(-) cotransporter (NKCC2) and AQP2, with less phosphorylation of AQP2 at serine 256. The high protein acid load affected bone turnover, as indicated by higher Ca(2+) and deoxypyridinoline excretion, phenomena exaggerated in the absence of Rhcg. In animals receiving a high-protein diet with low SAA content, the kidney excreted alkaline urine, with low levels of NH4 (+) and no change in bone metabolism. Thus, the acid load associated with high-protein diets causes a concerted response of various nephron segments to excrete acid, mostly in the form of NH4 (+), that requires Rhcg. Furthermore, bone metabolism is altered by a high-protein acidogenic diet, presumably to buffer the acid load.

Physiology and pathophysiology of SLC12A1/2 transporters

The electroneutral Na(+)-K(+)-Cl(-) cotransporters NKCC1 (encoded by the SLC12A2 gene) and NKCC2 (SLC12A1 gene) belong to the Na(+)-dependent subgroup of solute carrier 12 (SLC12) family of transporters. They mediate the electroneutral movement of Na(+) and K(+), tightly coupled to the movement of Cl(-) across cell membranes. As they use the energy of the ion gradients generated by the Na(+)/K(+)-ATPase to transport Na(+), K(+), and Cl(-) from the outside to the inside of a cell, they are considered secondary active transport mechanisms. NKCC-mediated transport occurs in a 1Na(+), 1K(+), and 2Cl(-) ratio, although NKCC1 has been shown to sometimes mediate partial reactions. Both transporters are blocked by bumetanide and furosemide, drugs which are commonly used in clinical medicine. NKCC2 is the molecular target of loop diuretics as it is expressed on the apical membrane of thick ascending limb of Henle epithelial cells, where it mediates NaCl reabsorption. NKCC1, in contrast, is found on the basolateral membrane of Cl(-) secretory epithelial cells, as well as in a variety of non-epithelial cells, where it mediates cell volume regulation and participates in Cl(-) homeostasis. Following their molecular identification two decades ago, much has been learned about their biophysical properties, their mode of operation, their regulation by kinases and phosphatases, and their physiological relevance. However, despite this tremendous amount of new information, there are still so many gaps in our knowledge. This review summarizes information that constitutes consensus in the field, but it also discusses current points of controversy and highlights many unanswered questions.

Molecular mechanisms of acid-base sensing by the kidney

A major function of the kidney is to collaborate with the respiratory system to maintain systemic acid-base status within limits compatible with normal cell and organ function. It achieves this by regulating the excretion and recovery of bicarbonate (mainly in the proximal tubule) and the secretion of buffered protons (mainly in the distal tubule and collecting duct). How proximal tubular cells and distal professional proton transporting (intercalated) cells sense and respond to changes in pH, bicarbonate, and CO(2) status is a question that has intrigued many generations of renal physiologists. Over the past few years, however, some candidate molecular pH sensors have been identified, including acid/alkali-sensing receptors (GPR4, InsR-RR), kinases (Pyk2, ErbB1/2), pH-sensitive ion channels (ASICs, TASK, ROMK), and the bicarbonate-stimulated adenylyl cyclase (sAC). Some acid-sensing mechanisms in other tissues, such as CAII-PDK2L1 in taste buds, might also have similar roles to play in the kidney. Finally, the function of a variety of additional membrane channels and transporters is altered by pH variations both within and outside the cell, and the expression of several metabolic enzymes are altered by acid-base status in parts of the nephron. Thus, it is possible that a master pH sensor will never be identified. Rather, the kidney seems equipped with a battery of molecules that scan the epithelial cell environment to mount a coordinated physiologic response that maintains acid-base homeostasis. This review collates current knowledge on renal acid-base sensing in the context of a whole organ sensing and response process.

Thick ascending limb: the Na(+):K (+):2Cl (-) co-transporter, NKCC2, and the calcium-sensing receptor, CaSR

The thick ascending limb of Henle's loop is a nephron segment that is vital to the formation of dilute and concentrated urine. This ability is accomplished by a consortium of functionally coupled proteins consisting of the apical Na(+):K(+):2Cl(-) co-transporter, the K(+) channel, and basolateral Cl(-) channel that mediate electroneutral salt absorption. In thick ascending limbs, salt absorption is importantly regulated by the calcium-sensing receptor. Genetic or pharmacological disruption impairing the function of any of these proteins results in Bartter syndrome. The thick ascending limb is also an important site of Ca(2+) and Mg(2+) absorption. Calcium-sensing receptor activation inhibits cellular Ca(2+) absorption induced by parathyroid hormone, as well as passive paracellular Ca(2+) transport. The present review discusses these functions and their genetic and molecular regulation.

Renal response to metabolic acidosis: role of mRNA stabilization

The renal response to metabolic acidosis is mediated, in part, by increased expression of the genes encoding key enzymes of glutamine catabolism and various ion transporters that contribute to the increased synthesis and excretion of ammonium ions and the net production and release of bicarbonate ions. The resulting adaptations facilitate the excretion of acid and partially restore systemic acid-base balance. Much of this response may be mediated by selective stabilization of the mRNAs that encode the responsive proteins. For example, the glutaminase mRNA contains a direct repeat of 8-nt AU sequences that function as a pH-response element (pHRE). This element is both necessary and sufficient to impart a pH-responsive stabilization to chimeric mRNAs. The pHRE also binds multiple RNA-binding proteins, including zeta-crystallin (zeta-cryst), AU-factor 1 (AUF1), and HuR. The onset of acidosis initiates an endoplasmic reticulum (ER)-stress response that leads to the formation of cytoplasmic stress granules. zeta-cryst is transiently recruited to the stress granules, and concurrently, HuR is translocated from the nucleus to the cytoplasm. On the basis of the cumulative data, a mechanism for the stabilization of selective mRNAs is proposed. This hypothesis suggests multiple experiments that should define better how cells in the kidney sense very slight changes in intracellular pH and mediate this essential adaptive response.

Regulation of ROMK (Kir 1.1) Channel Expression in Kidney Thick Ascending Limb by Hypertonicity: Role of TonEBP and MAPK Pathways

The present study assessed the mechanisms by which hypertonicity caused by NaCl enhances the renal outer medullary potassium channel (ROMK) mRNA abundance in rat kidney medullary thick ascending limb (MTAL) and in cultured mouse TAL cells. Using the run-off technique, we observed that the ROMK gene transcription rate in nuclei isolated from MTAL fragments was enhanced approximately 40% by a high NaCl medium. In MTAL fragments, hypertonicity (450 mosm) caused by NaCl, not by mannitol or urea, enhanced both ROMK mRNA abundance and tonicity-responsive enhancer binding protein (TonEBP) total abundance and nuclear localization. In an immortalized mouse TAL cell culture in which ROMK is apically expressed, hypertonicity caused by both NaCl and mannitol, not urea, enhanced both ROMK mRNA abundance and TonEBP total abundance and nuclear localization. Confocal microscopy confirmed an increased nuclear translocation of TonEBP in response to NaCl-induced hypertonicity. Finally, inhibition of the p38 MAPK pathway by SB203580 and of the ERK pathway by PD98059 abolished the NaCl-induced stimulation of TonEBP and ROMK. These results establish that mRNA expression of ROMK is augmented in the MTAL by NaCl-induced hypertonicity through stimulation of ROMK gene transcription, and that TonEBP and the p38 MAPK and ERK pathways are involved in this effect.

Metabolic acidosis has dual effects on sodium handling by rat kidney

Chronic metabolic acidosis (CMA) is associated with decreased NaCl reabsorption in the proximal tubule (PT). However, the effect of CMA on Na(+) transport in the distal tubule (DT) and collecting duct (CD) is poorly understood. Rats were placed in metabolic cages and had access to water (control), 0.28 M NH(4)Cl, or 0.28 M KCl solutions in a pair-feeding protocol for 5 days (5d). Metabolic acidosis developed within 24 h in NH(4)Cl-, but not in KCl-loaded rats. Interestingly, NH(4)Cl- but not KCl-loaded rats exhibited a significant natriuresis after 24 h of treatment. Urinary Na(+) excretion increased from 1.94 to 2.97 meq/24 h (P < 0.001) and returned to below baseline level (1.67 meq/l) after 5d of CMA. The protein abundance of the cortical Na-Cl cotransporter (NCC) remained unchanged at 24 h, but increased significantly (P < 0.01) after 5d of CMA. The protein abundance of alpha-, beta-, and gamma-subunits of the epithelial Na(+) channel (ENaC) in the cortex decreased sharply during the first 24 h and then returned to baseline levels after 5d of CMA. Interestingly, Sgk1 expression decreased after 24 h (-31%, P < 0.05) and then returned to baseline after 5d of CMA. Nedd4-2 expression was not altered during CMA. CMA enhanced serum aldosterone levels by 54% and increased the expression of aldosterone synthase in the adrenal gland by 134% after 5d of CMA. In conclusion, metabolic acidosis has dual effects on urinary Na(+) excretion. The early natriuresis results from decreased Na(+) reabsorption in the PT and Sgk1-related decreased ENaC activity in the DT and CD. Aldosterone-induced upregulation of NCC, Sgk1, and ENaC likely contributes to the antinatriuretic phase of metabolic acidosis. This adaptation prevents Na(+) wasting and volume depletion during chronic acid insult.

Acid-base status determines the renal expression of Ca2+ and Mg2+ transport proteins

Chronic metabolic acidosis results in renal Ca2+ and Mg2+ wasting, whereas chronic metabolic alkalosis is known to exert the reverse effects. It was hypothesized that these adaptations are mediated at least in part by the renal Ca2+ and Mg2+ transport proteins. The aim of this study, therefore, was to determine the effect of systemic acid-base status on renal expression of the epithelial Ca2+ channel TRPV5, the Ca2+-binding protein calbindin-D28K, and the epithelial Mg2+ channel TRPM6 in relation to Ca2+ and Mg2+ excretion. Chronic metabolic acidosis that was induced by NH4Cl loading or administration of the carbonic anhydrase inhibitor acetazolamide for 6 d enhanced calciuresis accompanied by decreased renal TRPV5 and calbindin-D28K mRNA and protein abundance in wild-type mice. In contrast, metabolic acidosis did not affect Ca2+ excretion in TRPV5 knockout (TRPV5-/-) mice, in which active Ca2+ reabsorption is effectively abolished. This demonstrates that downregulation of renal Ca2+ transport proteins is responsible for the hypercalciuria. Conversely, chronic metabolic alkalosis that was induced by NaHCO3 administration for 6 d increased the expression of Ca2+ transport proteins accompanied by diminished urine Ca2+ excretion in wild-type mice. However, this Ca2+-sparing action persisted in TRPV5-/- mice, suggesting that additional mechanisms apart from upregulation of active Ca2+ transport contribute to the hypocalciuria. Furthermore, chronic metabolic acidosis decreased renal TRPM6 expression, increased Mg2+ excretion, and decreased serum Mg2+ concentration, whereas chronic metabolic alkalosis resulted in the exact opposite effects. In conclusion, these data suggest that regulation of Ca2+ and Mg2+ transport proteins contributes importantly to the effects of acid-base status on renal divalent handling.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Glutamine metabolism: Role in acid-base balance*

The intent of this review is to provide a broad overview of the interorgan metabolism of glutamine and to discuss in more detail its role in acid-base balance. Muscle, adipose tissue, and the lungs are the primary sites of glutamine synthesis and release. During normal acid-base balance, the small intestine and the liver are the major sites of glutamine utilization. The periportal hepatocytes catabolize glutamine and convert ammonium and bicarbonate ions to urea. In contrast, the perivenous hepatocytes are capable of synthesizing glutamine. During metabolic acidosis, the kidney becomes the major site of glutamine extraction and catabolism. This process generates ammonium ions that are excreted in the urine to facilitate the excretion of acids and bicarbonate ions that are transported to the blood to partially compensate the acidosis. The increased renal extraction of glutamine is balanced by an increased release from muscle and liver and by a decreased utilization in the intestine. During chronic acidosis, this adaptation is sustained, in part, by increased renal expression of genes that encode various transport proteins and key enzymes of glutamine metabolism. The increased levels of phosphoenolpyruvate carboxykinase result from increased transcription, while the increase in glutaminase and glutamate dehydrogenase activities result from stabilization of their respective mRNAs. Where feasible, this review draws upon data obtained from studies in humans. Studies conducted in model animals are discussed where available data from humans is either lacking or not firmly established. Because there are quantitative differences in tissue utilization and synthesis of glutamine in different mammals, the review will focus more on common principles than on quantification.