The DCT (distal convoluted tubule) is the site of microregulation of water reabsorption and ion handling in the kidneys, which is mainly under the control of aldosterone. Aldosterone binds to and activates mineralocorticoid receptors, which ultimately lead to increased sodium reabsorption in the distal part of the nephron. Impairment of mineralocorticoid signal transduction results in resistance to aldosterone and mineralocorticoids, and, therefore, causes disturbances in electrolyte balance. Pseudohypoaldosteronism type II (PHAII) or familial hyperkalemic hypertension (FHHt) is a rare, autosomal dominant syndrome characterized by hypertension, hyperkalemia, metabolic acidosis, elevated or low aldosterone levels, and decreased plasma renin activity. PHAII is caused by mutations in the WNK isoforms (with no lysine kinase), which regulate the Na-Cl and Na-K-Cl cotransporters (NCC and NKCC2, respectively) and the renal outer medullary potassium (ROMK) channel in the DCT. This review focuses on new candidate genes such as KLHL3 and Cullin3, which are instrumental to unraveling novel signal transductions pathways involving NCC, to better understand the cause of PHAII along with the molecular mechanisms governing the pathophysiology of PHAII and its clinical manifestations.

The furosemide/fludrocortisone test can be used for incomplete distal renal tubular acidosis screening in stone formers, but an abnormal furosemide/fludrocortisone test result needs confirmation by ammonium chloride testing. Our data furthermore indicate that incomplete distal renal tubular acidosis can reliably be excluded in stone formers by use of nonprovocative clinical parameters.

Background: The WNK-dependent STE20/SPS1-related proline/alanine-rich kinase (SPAK) regulates the renal thiazide sensitive NaCl cotransporter (NCC) and the renal furosemide sensitive Na⁺,K⁺,2Cl^- cotransporter (NKCC2) and thus participates in the regulation of renal salt excretion, extracellular fluid volume and blood pressure. Inhibition of NCC leads to anticalciuria. Moreover, NCC is also expressed in osteoblasts where it is implicated in the regulation of bone mineralization. Osteoblasts further influence mineral metabolism by releasing the phosphaturic hormone FGF23. The present study explored, whether SPAK participates in the regulation of calcium-phosphate homeostasis. Methods: FGF23 serum levels and phosphate homeostasis were analyzed in gene targeted mice expressing SPAK resistant to WNK-dependent activation (spak^tg/tg) and in mice expressing wild type SPAK (spak^wt/wt). Results: Serum FGF23 level was significantly higher, urinary phosphate excretion significantly larger and serum phosphate concentration significantly lower in spak^tg/tg mice than in spak^wt/wt mice. Urinary calcium excretion was significantly decreased in spaktg/tg mice. Serum levels of calcitriol and PTH were not significantly different between the genotypes. Bone density was significantly increased in spak^tg/tg mice compared to spak^wt/wt mice. Treatment of spak^wt/wt mice with HCT increased FGF23 serum levels, and led to phosphaturia and hypophosphatemia. Conclusions: SPAK is a strong regulator of FGF23 formation, bone mineralization and renal Ca²⁺ and phosphate excretion.

Background: The oxidative stress-responsive kinase 1 (OSR1) participates in the WNK-(with no K) kinase dependent regulation of renal salt excretion and blood pressure. Little is known, however, about the role of OSR1 in the regulation of further renal transport systems. The present study analyzed the effect of OSR1 on NaPiIIa, the major renal tubular phosphate transporter. Methods: Immunohistochemistry and confocal microscopy were employed to determine renal localization of OSR1 and NaPiIIa. To elucidate the effect of OSR on NaPiIIa activity, cRNA encoding NaPiIIa was injected into Xenopus oocytes with or without additional injection of cRNA encoding OSR1, and phosphate transport was estimated from phosphateinduced currents determined with dual electrode voltage clamp. To elucidate the in vivo significance of OSR1 serum phosphate and hormone concentrations as well as urinary phosphate output of mice carrying one allele of WNK-resistant OSR1 (osr1tg/⁺) were compared to the respective values of wild type mice (osr1^+/+). Results: NaPiIIa and OSR1 were both expressed in proximal renal tubule cells. Coexpression of OSR1 significantly up-regulated phosphate-induced currents in NaPiIIa-expressing Xenopus oocytes. Despite decreased serum phosphate concentration urinary phosphate excretion was significantly increased and NaPiIIa protein abundance in the brush border membrane significantly reduced in osr1tg/⁺ mice as compared to osr1^+/+ mice. Serum PTH and calcitriol levels were similar in osr1tg/⁺ mice and in osr1^+/+ mice, serum FGF23 concentration was, however, significantly higher in osr1tg/⁺ mice than in osr1^+/+ mice. Conclusions: OSR1 is expressed in proximal renal tubules and participates in the regulation of FGF23 release and renal tubular phosphate transport.

Glucocorticoid (GC) treatment of inflammatory disorders, such as inflammatory bowel disease, causes deranged metabolism, in part by enhanced intestinal resorption of glucose. However, the underlying molecular mechanism is poorly understood. Hence, we investigated transcriptional control of genes reported to be involved in glucose uptake in the small intestine after GC treatment and determined effects of GC on electrogenic glucose transport from transepithelial currents. GR(villinCre) mice lacking the GC receptor (GR) in enterocytes served to identify the target cell of GC treatment and the requirement of the GR itself; GR(dim) mice impaired in dimerization and DNA binding of the GR were used to determine the underlying molecular mechanism. Our findings revealed that oral administration of dexamethasone to wild-type mice for 3 d increased mRNA expression of serum- and GC-inducible kinase 1, sodium-coupled glucose transporter 1, and Na(+)/H(+) exchanger 3, as well as electrogenic glucose transport in the small intestine. In contrast, GR(villinCre) mice did not respond to GC treatment, neither with regard to gene activation nor to glucose transport. GR(dim) mice were also refractory to GC, because dexamethasone treatment failed to increase both, gene expression and electrogenic glucose transport. In addition, the rise in blood glucose levels normally observed after GC administration was attenuated in both mutant mouse strains. We conclude that enhanced glucose transport in vivo primarily depends on gene regulation by the dimerized GR in enterocytes, and that this mechanism contributes to GC-induced hyperglycemia.

Klotho, a transmembrane protein, protease, and hormone mainly expressed in renal tissue counteracts aging. Overexpression of Klotho substantially prolongs the life span. Klotho deficiency leads to excessive formation of 1,25(OH)(2)D(3), growth deficit, accelerated aging, and early death. Aging is frequently paralleled by dehydration, which is considered to accelerate the development of age-related disorders. The present study explored the possibility that dehydration influences Klotho expression. Klotho transcript levels were determined by RT-PCR, and Klotho protein abundance was detected by Western blotting in renal tissue from hydrated and 36-h-dehydrated mice as well as in human embryonic kidney (HEK293) cells. Dehydration was followed by a significant decline of renal Klotho transcript levels and protein abundance, accompanied by an increase in plasma osmolarity as well as plasma ADH, aldosterone, and 1,25(OH)(2)D(3) levels. Antidiuretic hormone (ADH; 50 nM) and aldosterone (1 μM) significantly decreased Klotho transcription and protein expression in HEK293 cells. In conclusion, the present observations disclose a powerful effect of dehydration on Klotho expression, an effect at least partially mediated by enhanced release of ADH and aldosterone.

A heterozygous mutation (c.643C.A; p.Q215X) in the monocarboxylate transporter 12-encoding gene MCT12 (also known as SLC16A12) that mediates creatine transport was recently identified as the cause of a syndrome with juvenile cataracts, microcornea, and glucosuria in a single family. Whereas the MCT12 mutation cosegregated with the eye phenotype, poor correlation with the glucosuria phenotype did not support a pathogenic role of the mutation in the kidney. Here, we examined MCT12 in the kidney and found that it resides on basolateral membranes of proximal tubules. Patients with MCT12 mutation exhibited reduced plasma levels and increased fractional excretion of guanidinoacetate, but normal creatine levels, suggesting that MCT12 may function as a guanidinoacetate transporter in vivo. However, functional studies in Xenopus oocytes revealed that MCT12 transports creatine but not its precursor, guanidinoacetate. Genetic analysis revealed a separate, undescribed heterozygous mutation (c.265G.A; p.A89T) in the sodium/glucose cotransporter 2-encoding gene SGLT2 (also known as SLC5A2) in the family that segregated with the renal glucosuria phenotype. When overexpressed in HEK293 cells, the mutant SGLT2 transporter did not efficiently translocate to the plasma membrane, and displayed greatly reduced transport activity. In summary, our data indicate that MCT12 functions as a basolateral exit pathway for creatine in the proximal tubule. Heterozygous mutation of MCT12 affects systemic levels and renal handling of guanidinoacetate, possibly through an indirect mechanism. Furthermore, our data reveal a digenic syndrome in the index family, with simultaneous MCT12 and SGLT2 mutation. Thus, glucosuria is not part of the MCT12 mutation syndrome.

The thiazide-sensitive NaCl cotransporter (NCC), located apically in distal convoluted tubule epithelia, regulates the fine-tuning of renal sodium excretion. Three isoforms of NCC are generated through alternative splicing of the transcript, of which the third isoform has been the most extensively investigated in pathophysiological conditions. The aim of this study was to investigate the effect of different anti-hypertensive treatments on the abundance and phosphorylation of all three NCC isoforms in urinary extracellular vesicles (uEVs) of essential hypertensive patients. In uEVs isolated from patients ( = 23) before and after hydrochlorothiazide or valsartan treatment, the abundance and phosphorylation of the NCC isoforms was determined. Additionally, clinical biochemistry and blood pressure of the patients was assessed. Our results show that NCC detected in human uEVs has a glycosylated and oligomeric structure, comparable to NCC present in human kidney membrane fractions. Despite the inhibitory action of hydrochlorothiazide on NCC activity, immunoblot analysis of uEVs showed significantly increased abundance of NCC isoforms 1 and 2 (NCC), total NCC (NCC), and the phosphorylated form of total NCC (pNCC-T55/T60) in essential hypertensive patients treated with hydrochlorothiazide but not with valsartan. This study highlights that NCC, NCC, and pNCC-T55/T60 are upregulated by hydrochlorothiazide, and the increase in NCC abundance in uEVs of essential hypertensive patients correlates with the blood pressure response to hydrochlorothiazide.