H+-ATPase B1 subunit localizes to thick ascending limb and distal convoluted tubule of rodent and human kidney

β3-Adrenoceptor as a new player in the sympathetic regulation of the renal acid-base homeostasis

Efferent sympathetic nerve fibers regulate several renal functions activating norepinephrine receptors on tubular epithelial cells. Of the beta-adrenoceptors (β-ARs), we previously demonstrated the renal expression of β3-AR in the thick ascending limb (TAL), the distal convoluted tubule (DCT), and the collecting duct (CD), where it participates in salt and water reabsorption. Here, for the first time, we reported β3-AR expression in the CD intercalated cells (ICCs), where it regulates acid-base homeostasis. Co-localization of β3-AR with either proton pump H+-ATPase or Cl-/HCO3 - exchanger pendrin revealed β3-AR expression in type A, type B, non-A, and non-B ICCs in the mouse kidney. We aimed to unveil the possible regulatory role of β3-AR in renal acid-base homeostasis, in particular in modulating the expression, subcellular localization, and activity of the renal H+-ATPase, a key player in this process. The abundance of H+-ATPase was significantly decreased in the kidneys of β3-AR-/- compared with those of β3-AR+/+ mice. In particular, H+-ATPase reduction was observed not only in the CD but also in the TAL and DCT, which contribute to acid-base transport in the kidney. Interestingly, we found that in in vivo, the absence of β3-AR reduced the kidneys' ability to excrete excess proton in the urine during an acid challenge. Using ex vivo stimulation of mouse kidney slices, we proved that the β3-AR activation promoted H+-ATPase apical expression in the epithelial cells of β3-AR-expressing nephron segments, and this was prevented by β3-AR antagonism or PKA inhibition. Moreover, we assessed the effect of β3-AR stimulation on H+-ATPase activity by measuring the intracellular pH recovery after an acid load in β3-AR-expressing mouse renal cells. Importantly, β3-AR agonism induced a 2.5-fold increase in H+-ATPase activity, and this effect was effectively prevented by β3-AR antagonism or by inhibiting either H+-ATPase or PKA. Of note, in urine samples from patients treated with a β3-AR agonist, we found that β3-AR stimulation increased the urinary excretion of H+-ATPase, likely indicating its apical accumulation in tubular cells. These findings demonstrate that β3-AR activity positively regulates the expression, plasma membrane localization, and activity of H+-ATPase, elucidating a novel physiological role of β3-AR in the sympathetic control of renal acid-base homeostasis.

The B1 H + -ATPase ( Atp6v1b1 ) Subunit in Non-Type A Intercalated Cells is Required for Driving Pendrin Activity and the Renal Defense Against Alkalosis

SIGNIFICANCE STATEMENT

In the kidney, the B1 H + -ATPase subunit is mostly expressed in intercalated cells (IC). Its importance in acid-secreting type A ICs is evident in patients with inborn distal renal tubular acidosis and ATP6V1B1 mutations. However, the protein is also highly expressed in alkali-secreting non-type A ICs where its function is incompletely understood. We demonstrate in Atp6v1b1 knock out mice that the B1 subunit is critical for the renal response to defend against alkalosis during an alkali load or chronic furosemide treatment. These findings highlight the importance of non-type A ICs in maintaining acid-base balance in response to metabolic challenges or commonly used diuretics.

BACKGROUND

Non-type A ICs in the collecting duct system express the luminal Cl - /HCO 3- exchanger pendrin and apical and/or basolateral H + -ATPases containing the B1 subunit isoform. Non-type A ICs excrete bicarbonate during metabolic alkalosis. Mutations in the B1 subunit (ATP6V1B1) cause distal renal tubular acidosis due to its role in acid secretory type A ICs. The function of B1 in non-type A ICs has remained elusive.

METHODS

We examined the responses of Atp6v1b1-/- and Atp6v1b1+/+ mice to an alkali load and to chronic treatment with furosemide.

RESULTS

An alkali load or 1 week of furosemide resulted in a more pronounced hypokalemic alkalosis in male ATP6v1b1-/- versus Atp6v1b1+/+ mice that could not be compensated by respiration. Total pendrin expression and activity in non-type A ICs of ex vivo microperfused cortical collecting ducts were reduced, and β2 -adrenergic stimulation of pendrin activity was blunted in ATP6v1b1-/- mice. Basolateral H + -ATPase activity was strongly reduced, although the basolateral expression of the B2 isoform was increased. Ligation assays for H + -ATPase subunits indicated impaired assembly of V 0 and V 1 H + -ATPase domains. During chronic furosemide treatment, ATP6v1b1-/- mice also showed polyuria and hyperchloremia versus Atp6v1b1+/+ . The expression of pendrin, the water channel AQP2, and subunits of the epithelial sodium channel ENaC were reduced.

CONCLUSIONS

Our data demonstrate a critical role of H + -ATPases in non-type A ICs function protecting against alkalosis and reveal a hitherto unrecognized need of basolateral B1 isoform for a proper H + -ATPase complexes assembly and ability to be stimulated.

Generation of Atp6v1g3-Cre mice for investigation of intercalated cells and the collecting duct

Kidney intercalated cells (ICs) maintain acid-base homeostasis and recent studies have demonstrated that they function in the kidney's innate defense. To study kidney innate immune function, ICs have been enriched using vacuolar ATPase (V-ATPase) B1 subunit (Atp6v1b1)-Cre (B1-Cre) mice. Although Atp6v1b1 is considered kidney specific, it is expressed in multiple organ systems, both in mice and humans, raising the possibility of off-target effects when using the Cre-lox system. We have recently shown using single-cell RNA sequencing that the gene that codes for the V-ATPase G3 subunit (mouse gene: Atp6v1g3; human gene: ATP6V1G3; protein abbreviation: G3) mRNA is selectively enriched in human kidney ICs. In this study, we generated Atp6v1g3-Cre (G3-Cre) reporter mice using CRISPR/CAS technology and crossed them with Tdtomatoflox/flox mice. The resultant G3-Cre+Tdt+ progeny was evaluated for kidney specificity in multiple tissues and found to be highly specific to kidney cells with minimal or no expression in other organs evaluated compared with B1-Cre mice. Tdt+ cells were flow sorted and were enriched for IC marker genes on RT-PCR analysis. Next, we crossed these mice to ihCD59 mice to generate an IC depletion mouse model (G3-Cre+ihCD59+/+). ICs were depleted in these mice using intermedilysin, which resulted in lower blood pH, suggestive of a distal renal tubular acidosis phenotype. The G3-Cre mice were healthy, bred normally, and produce regular-sized litter. Thus, this new "IC reporter" mice can be a useful tool to study ICs.NEW & NOTEWORTHY This study details the development, validation, and experimental use of a new mouse model to study the collecting duct and intercalated cells. Kidney intercalated cells are a cell type increasingly recognized to be important in several human diseases including kidney infections, acid-base disorders, and acute kidney injury.

Claudin-19 localizes to the thick ascending limb where its expression is required for junctional claudin-16 localization

The kidney is critical for mineral homeostasis. Calcium and magnesium reabsorption in the renal thick ascending limb (TAL) involves claudin-16 (CLDN16) and claudin-19 (CLDN19) and pathogenic variants in either gene lead to familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) with severe calcium and magnesium wasting. While both CLDN16 and CLDN19 localize to the TAL, varying expression patterns in the renal tubule have been reported using different antibodies. We, therefore, studied the localization of CLDN19 in the kidneys of wild-type and Cldn19-deleted mice using three anti-CLDN19 antibodies and examined the role of Cldn19 deletion on CLDN16 and CLDN10 localization. We find that CLDN19 localizes to basolateral membrane domains of the medullary and cortical TAL but only to the tight junction of TALs in the outer stripe of outer medulla and cortex, where it colocalizes with CLDN16. Furthermore, in TALs from Cldn19-deleted mice, CLDN16 is expressed in basolateral membrane domains but not at the tight junction. In contrast, Cldn19 ablation does not change CLDN10 localization. These findings directly implicate CLDN19 in regulating permeability in the TAL by allowing junctional insertion of CLDN16 and may explain the shared renal phenotypic characteristics in FHHNC patients.

The pathophysiology of distal renal tubular acidosis

The kidneys have a central role in the control of acid-base homeostasis owing to bicarbonate reabsorption and production of ammonia and ammonium in the proximal tubule and active acid secretion along the collecting duct. Impaired acid excretion by the collecting duct system causes distal renal tubular acidosis (dRTA), which is characterized by the failure to acidify urine below pH 5.5. This defect originates from reduced function of acid-secretory type A intercalated cells. Inherited forms of dRTA are caused by variants in SLC4A1, ATP6V1B1, ATP6V0A4, FOXI1, WDR72 and probably in other genes that are yet to be discovered. Inheritance of dRTA follows autosomal-dominant and -recessive patterns. Acquired forms of dRTA are caused by various types of autoimmune diseases or adverse effects of some drugs. Incomplete dRTA is frequently found in patients with and without kidney stone disease. These patients fail to appropriately acidify their urine when challenged, suggesting that incomplete dRTA may represent an intermediate state in the spectrum of the ability to excrete acids. Unrecognized or insufficiently treated dRTA can cause rickets and failure to thrive in children, osteomalacia in adults, nephrolithiasis and nephrocalcinosis. Electrolyte disorders are also often present and poorly controlled dRTA can increase the risk of developing chronic kidney disease.

NHE3 in the thick ascending limb is required for sustained but not acute furosemide-induced urinary acidification

Na+/H+ exchanger isoform 3 (NHE3) facilitates Na+ reabsorption and H+ secretion by the kidneys. Despite stronger NHE3 abundance in the thick ascending limb (TAL) compared with the S1 and S2 segments of the proximal tubule, the role of NHE3 in the TAL is poorly understood. To investigate the role of NHE3 in the TAL, we generated and phenotyped TAL-specific NHE3 knockout (NHE3TAL-KO) mice. Compared with control mice, NHE3TAL-KO mice did not show significant differences in body weight, blood pH, or plasma Na+, K+, or Cl- levels. Fluid intake trended to be higher and urine osmolality was significantly lower in NHE3TAL-KO mice. Despite a similar glomerular filtration rate, NHE3TAL-KO mice had a greater urinary K+-to-creatinine ratio. One proposed role of NHE3 relates to furosemide-induced urinary acidification. Acute bolus treatment with furosemide under anesthesia did not result in differences in the dose dependence of urinary flow rate, Cl- excretion, or maximal urinary acidification between genotypes; however, in contrast with control mice, urinary pH returned immediately toward baseline levels in NHE3TAL-KO mice. Chronic furosemide treatment reduced urine osmolality similarly in both genotypes but metabolic alkalosis, hypokalemia, and calciuresis were absent in NHE3TAL-KO mice. Compared with vehicle, chronic furosemide treatment resulted in greater Na+-K+-2Cl- abundance regardless of genotype. Na+-phosphate cotransporter 2a abundance was also greater in furosemide-treated control mice compared with vehicle treatment, an effect that was absent in NHE3TAL-KO mice. In summary, NHE3 in the TAL plays a role in the sustained acidification effect of furosemide. Consistent with this, long-term treatment with furosemide did not result in metabolic alkalosis in NHE3TAL-KO mice.NEW & NOTEWORTHY Na+/H+ exchanger isoform 3 (NHE3) is very abundant in the thick ascending limb (TAL) compared with the proximal tubule. Much has been learned about the role of NHE3 in the proximal tubule; however, the function of NHE3 in the TAL remains elusive. A novel mouse model that lacks NHE3 selectively in the TAL not only shows a phenotype under baseline conditions but also identifies that NHE3 is required for sustained but not acute furosemide-induced urinary acidification.

High dietary potassium causes ubiquitin-dependent degradation of the kidney sodium-chloride cotransporter

The thiazide-sensitive sodium-chloride cotransporter (NCC) in the renal distal convoluted tubule (DCT) plays a critical role in regulating blood pressure (BP) and K⁺ homeostasis. During hyperkalemia, reduced NCC phosphorylation and total NCC abundance facilitate downstream electrogenic K⁺ secretion and BP reduction. However, the mechanism for the K⁺-dependent reduction in total NCC levels is unknown. Here, we show that NCC levels were reduced in ex vivo renal tubules incubated in a high-K⁺ medium for 24–48 h. This reduction was independent of NCC transcription, but was prevented using inhibitors of the proteasome (MG132) or lysosome (chloroquine). Ex vivo, high K⁺ increased NCC ubiquitylation, but inhibition of the ubiquitin conjugation pathway prevented the high K⁺-mediated reduction in NCC protein. In tubules incubated in high K⁺ media ex vivo or in the renal cortex of mice fed a high K⁺ diet for 4 days, the abundance and phosphorylation of heat shock protein 70 (Hsp70), a key regulator of ubiquitin-dependent protein degradation and protein folding, were decreased. Conversely, in similar samples the expression of PP1α, known to dephosphorylate Hsp70, was also increased. NCC coimmunoprecipitated with Hsp70 and PP1α, and inhibiting their actions prevented the high K⁺-mediated reduction in total NCC levels. In conclusion, we show that hyperkalemia drives NCC ubiquitylation and degradation via a PP1α-dependent process facilitated by Hsp70. This mechanism facilitates K⁺-dependent reductions in NCC to protect plasma K⁺ homeostasis and potentially reduces BP.

Differential localization patterns of claudin 10, 16, and 19 in human, mouse, and rat renal tubular epithelia

Functional properties of the paracellular pathway depend critically on the set of claudins (CLDN) expressed at the tight junction. Two syndromes are causally linked to loss-of-function mutations of claudins: hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia (HELIX) syndrome caused by genetic variations in the CLDN10 gene and familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by genetic variations in the CLDN16 or CLDN19 genes. All three genes are expressed in the kidney, particularly in the thick ascending limb (TAL). However, localization of these claudins in humans and rodents remains to be delineated in detail. We studied the segmental and subcellular expression of CLDN10, CLDN16, and CLDN19 in both paraffin-embedded and frozen kidney sections from the adult human, mouse, and rat using immunohistochemistry and immunofluorescence, respectively. Here, CLDN10 was present in a subset of medullary and cortical TAL cells, localizing to basolateral domains and tight junctions in human and rodent kidneys. Weak expression was detected at the tight junction of proximal tubular cells. CLDN16 was primarily expressed in a subset of TAL cells in the cortex and outer stripe of outer medulla, restricted to basolateral domains and tight junctional structures in both human and rodent kidneys. CLDN19 predominantly colocalized with CLDN16 in tight junctions and basolateral domains of the TAL but was also found in basolateral and junctional domains in more distal sites. CLDN10 expression at tight junctions almost never overlapped with that of CLND16 and CLDN19, consistent with distinct junctional pathways with different permeation profiles in both human and rodent kidneys.NEW & NOTEWORTHY This study used immunohistochemistry and immunofluorescence to investigate the distribution of claudin 10, 16, and 19 in the human, mouse, and rat kidney. The findings showed distinct junctional pathways in both human and rodent kidneys, supporting the existence of different permeation profiles in all species investigated.

Distal renal tubular acidosis: a systematic approach from diagnosis to treatment

Renal tubular acidosis (RTA) comprises a group of disorders in which excretion of hydrogen ions or reabsorption of filtered HCO₃ is impaired, leading to chronic metabolic acidosis with normal anion gap. In the current review, the focus is placed on the most common type of RTA, Type 1 RTA or Distal RTA (dRTA), which is a rare chronic genetic disorder characterized by an inability of the distal nephron to secrete hydrogen ions in the presence of metabolic acidosis. Over the years, knowledge of the molecular mechanisms behind acid secretion has improved, thereby greatly helping the diagnosis of dRTA. The primary or inherited form of dRTA is mostly diagnosed in infancy, childhood, or young adulthood, while the acquired secondary form, as a consequence of other disorders or medications, can happen at any age, although it is more commonly seen in adults. dRTA is not as “benign” as previously assumed, and can have several, highly variable long-term consequences. The present review indeed reports and summarizes both clinical symptoms and diagnosis, long-term outcomes, genetic inheritance, epidemiology and current treatment options, with the aim of shedding more light onto this rare disorder. Being a chronic condition, dRTA also deserves attention in the transition between pediatric and adult nephrology care, and as a rare disease it has a place in the European and Italian rare nephrological diseases network.

Differential diagnosis of perinatal Bartter, Bartter and Gitelman syndromes

The common finding of hypokalemic alkalosis in several unrelated disorders may confound the early diagnosis of salt-losing tubulopathy (SLT). Antenatal Bartter syndrome (BS) must be considered in idiopathic early-onset polyhydramnios. Fetal megabladder in BS may allow its distinction from third-trimester polyhydramnios that occurs in congenital chloride diarrhea (CCD). Fetal megacolon occurs in CCD while fecal chloride >90 mEq/L in infants is diagnostic. Failure-to-thrive, polydipsia and polyuria in early childhood are the hallmarks of classic BS. Unlike BS, there is low urinary chloride in hypokalemic alkalosis of intractable emesis and cystic fibrosis. Rarely, renal salt wasting may result from cystinosis, Dent disease, disorders of paracellular claudin-10b and Kir4.1 potassium-channel deficiency. Acquired BS may result from calcimimetic up-regulation of a calcium-sensing receptor or autoantibody inactivation of sodium chloride co-transporters in Sjögren syndrome. A relatively common event of heterozygous gene mutations for Gitelman syndrome increases the likelihood of its random occurrence in certain diseases of adult onset. Finally, diuretic abuse is the most common differential diagnosis of SLT. Unlike the persistent elevation in BS, urinary chloride concentration losses waxes and wanes on day-to-day assessment in patients with diuretic misuse.

Urinary Extracellular Vesicles and Salt-Losing Tubulopathies: A Proteomic Approach

Renal tubular cells release urinary extracellular vesicles (uEV) that are considered a promising source of molecular markers for renal dysfunction and injury. We investigated uEV proteomes of patients with hereditary salt-losing tubulopathies (SLTs), focusing on those caused by Gitelman and Bartter (BS) syndromes, to provide potential markers for differential diagnosis. Second morning urine was collected from patients with genetically proven SLTs and uEV were isolated by the ultracentrifugation-based protocol. The uEV proteome was run through a diagonal bidimensional electrophoresis (16BAC/SDS-PAGE), to improve hydrophobic protein resolution. Sixteen differential spots from the proteome of two variants (BS2 and BS3) were analysed by nLC-ESI-MS/MS after in-gel tryptic digestion. A total of 167 protein species were identified from 7 BS2 spots and 9 BS3 spot. Most of these proteins were membrane-associated proteins, in particular transmembrane proteins, and were related to typical renal functions. The differential content of some uEV was then validated by immunoblotting. Our work suggests that uEV proteomics represents a promising strategy for the identification of differential SLT proteins. This could play a role in understanding the pathophysiological disease mechanisms and may support the recognition of different syndromes.

Dietary Na+ intake in healthy humans changes the urine extracellular vesicle prostasin abundance while the vesicle excretion rate, NCC, and ENaC are not altered

Low Na+ intake activates aldosterone signaling, which increases renal Na+ reabsorption through increased apical activity of the NaCl cotransporter (NCC) and the epithelial Na+ channel (ENaC). Na+ transporter proteins are excreted in urine as an integral part of cell-derived extracellular vesicles (uEVs). It was hypothesized that Na+ transport protein levels in uEVs from healthy humans reflect their physiological regulation by aldosterone. Urine and plasma samples from 10 healthy men (median age: 22.8 yr) were collected after 5 days on a low-Na+ (70 mmol/day) diet and 5 days on a high-Na+ (250 mmol/day) diet. uEVs were isolated by ultracentrifugation and analyzed by Western blot analysis for EV markers (CD9, CD63, and ALIX), transport proteins (Na+-K+-ATPase α1-subunit, NCC, ENaC α- and γ-subunits, and aquaporin 2), and the ENaC-cleaving protease prostasin. Plasma renin and aldosterone concentrations increased during the low-Na+ diet. uEV size and concentration were not different between diets by tunable resistive pulse sensing. EV markers ALIX and CD9 increased with the low-Na+ diet, whereas CD63 and aquaporin 2 excretion were unchanged. Full-length ENaC γ-subunits were generally not detectable in uEVs, whereas ENaC α-subunits, NCC, and phosphorylated NCC were consistently detected but not changed by Na+ intake. Prostasin increased with low Na+ in uEVs. uEV excretion of transporters was not correlated with blood pressure, urinary Na+ and K+ excretion, plasma renin, or aldosterone. In conclusion, apical Na+ transporter proteins and proteases were excreted in uEVs, and while the excretion rate and size of uEVs were not affected, EV markers and prostasin increased in response to the low-Na+ diet.

Nephrotic syndrome is associated with increased plasma K+ concentration, intestinal K+ losses, and attenuated urinary K+ excretion: a study in rats and humans

The present study tested the hypotheses that nephrotic syndrome (NS) leads to renal K⁺ loss because of augmented epithelial Na⁺ channel (ENaC) activity followed by downregulation of renal K⁺ secretory pathways by suppressed aldosterone. The hypotheses were addressed by determining K⁺ balance and kidney abundance of K⁺ and Na⁺ transporter proteins in puromycin aminonucleoside (PAN)-induced rat nephrosis. The effects of amiloride and angiotensin II type 1 receptor and mineralocorticoid receptor (MR) antagonists were tested. Glucocorticoid-dependent MR activation was tested by suppression of endogenous glucocorticoid with dexamethasone. Urine and plasma samples were obtained from pediatric patients with NS in acute and remission phases. PAN-induced nephrotic rats had ENaC-dependent Na⁺ retention and displayed lower renal K⁺ excretion but elevated intestinal K⁺ secretion that resulted in less cumulated K⁺ in NS. Aldosterone was suppressed at day 8. The NS-associated changes in intestinal, but not renal, K⁺ handling responded to suppression of corticosterone, whereas angiotensin II type 1 receptor and MR blockers and amiloride had no effect on urine K⁺ excretion during NS. In PAN-induced nephrosis, kidney protein abundance of the renal outer medullary K⁺ channel and γ-ENaC were unchanged, whereas the Na⁺-Cl^- cotransporter was suppressed and Na⁺-K⁺-ATPase increased. Pediatric patients with acute NS displayed suppressed urine Na⁺-to-K⁺ ratios compared with remission and elevated plasma K⁺ concentration, whereas fractional K⁺ excretion did not differ. Acute NS is associated with less cumulated K⁺ in a rat model, whereas patients with acute NS have elevated plasma K⁺ and normal renal fractional K⁺ excretion. In NS rats, K⁺ balance is not coupled to ENaC activity but results from opposite changes in renal and fecal K⁺ excretion with a contribution from corticosteroid MR-driven colonic secretion.

ERA-EDTA fellowship, a 'bonne opportunité': the scientific and human experience of a fellow

As a fellow of the ERA-EDTA long-term fellowship programme, I spent two fantastic years as a post-doc in Prof. D. Eladari’s laboratory at Inserm U970, Paris-Cardiovascular Research Centre. It was a highly formative and productive scientific experience. On a personal level, immersion into the French society and the international environment of the laboratory were added bonuses that enriched my experience. I am honoured to report my experience here from the ERA-EDTA fellowship programme. I hope this will inspire young fellows to apply for such a programme and pursue their career in science. Good mentorship, a passion for scientific investigation and determination are required.