The kidney has an important role in the regulation of acid-base homeostasis. Renal ammonium production and excretion are essential for net acid excretion under basal conditions and during metabolic acidosis. Ammonium is secreted into the urine by the collecting duct, a distal nephron segment where ammonium transport is believed to occur by non-ionic NH(3) diffusion coupled to H(+) secretion. Here we show that this process is largely dependent on the Rhesus factor Rhcg. Mice lacking Rhcg have abnormal urinary acidification due to impaired ammonium excretion on acid loading-a feature of distal renal tubular acidosis. In vitro microperfused collecting ducts of Rhcg(-/-) acid-loaded mice show reduced apical permeability to NH(3) and impaired transepithelial NH(3) transport. Furthermore, Rhcg is localized in epididymal epithelial cells and is required for normal fertility and epididymal fluid pH. We anticipate a critical role for Rhcg in ammonium handling and pH homeostasis both in the kidney and the male reproductive tract.

No abstract

The distal convoluted tubule (DCT) plays an essential role in the reabsorption of NaCl by the kidney, a process that can be inhibited by thiazide diuretics. Parvalbumin (PV), a Ca 2؉ -binding protein that plays a role in muscle fibers and neurons, is selectively expressed in the DCT, where its role remains unknown. We therefore investigated the renal phenotype of PV knockout mice (Pvalb ؊/؊ ) vs. wild-type (Pvalb ؉/؉ ) littermates. PV colocalized with the thiazidesensitive Na ؉ -Cl ؊ cotransporter (NCC) in the early DCT. The Pvalb ؊/؊ mice showed increased diuresis and kaliuresis at baseline with higher aldosterone levels and lower lithium clearance. Acute furosemide administration increased diuresis and natriuresis/kaliuresis, but, surprisingly, did not increase calciuria in Pvalb ؊/؊ mice. NaCl supplementation of Pvalb ؊/؊ mice increased calciuria at baseline and after furosemide. The Pvalb ؊/؊ mice showed no significant diuretic response to hydrochlorothiazide, but an accentuated hypocalciuria. A decreased expression of NCC was detected in the early DCT of Pvalb ؊/؊ kidneys in the absence of ultrastructural and apoptotic changes. The PV-deficient mice had a positive Ca 2؉ balance and increased bone mineral density. Studies in mouse DCT cells showed that endogenous NCC expression is Ca 2؉ -dependent and can be modulated by the levels of PV expression. These results suggest that PV regulates the expression of NCC by modulating intracellular Ca 2؉ signaling in response to ATP in DCT cells. They also provide insights into the Ca 2؉ -sparing action of thiazides and the pathophysiology of distal tubulopathies.distal convoluted tubule ͉ kidney ͉ salt-losing nephropathy ͉ sodium-chloride cotransport P arvalbumin (PV) belongs to the superfamily of EF-hand Ca 2ϩ -binding proteins that play a role in multiple cellular processes, including gene transcription, ion transport, protein phosphorylation, and enzymatic activities (1). These proteins possess well conserved helix-loop-helix motifs that bind Ca 2ϩ ions with high affinity, leading to conformational changes. The conformational plasticity and the cell-specific expression of these Ca 2ϩ sensor or buffer proteins contribute to the versatility of Ca 2ϩ signaling (2). PV is a 109-aa cytosolic protein that contains a pair of functional EF-hand motifs forming a stable unit that binds two Ca 2ϩ ions (3). This Ca 2ϩ buffer is expressed in a restricted number of vertebrate tissues, including fast-contracting/relaxing skeletal muscle fibers and GABA neurons in the brain (4). The generation of PV knockout (Pvalb Ϫ/Ϫ ) mice confirmed the important role played by PV in muscle and brain (5). The fast muscles of Pvalb Ϫ/Ϫ mice exhibit a decreased relaxation rate of the twitch (5), suggesting that PV facilitates Ca 2ϩ diffusion from myofibrils to the sarcoplasmic reticulum (6). The lack of PV in the brain induces changes in short-term synaptic plasticity and modified network properties, resulting in increased susceptibility to epileptic seizures (7). Although no human disease is ass...

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Magnesium (Mg) homeostasis is critical for metabolism. However, the genetic determinants of the renal handling of Mg, which is crucial for Mg homeostasis, and the potential influence on metabolic traits in the general population are unknown. We obtained plasma and urine parameters from 9099 individuals from seven cohorts, and conducted a genome-wide meta-analysis of Mg homeostasis. We identified two loci associated with urinary magnesium (uMg), rs3824347 (4.4×10) near , which encodes an epithelial Mg channel, and rs35929 (2.1×10), a variant of , which encodes a GTP-binding protein. Together, these loci account for 2.3% of the variation in 24-hour uMg excretion. In human kidney cells, ARL15 regulated TRPM6-mediated currents. In zebrafish, dietary Mg regulated the expression of the highly conserved ortholog, and knockdown resulted in renal Mg wasting and metabolic disturbances. Finally, rs35929 modified the association of uMg with fasting insulin and fat mass in a general population. In conclusion, this combined observational and experimental approach uncovered a gene-environment interaction linking Mg deficiency to insulin resistance and obesity.

The nature and importance of genetic factors regulating the differential handling of Ca and Mg by the renal tubule in the general population are poorly defined. We conducted a genome-wide meta-analysis of urinary magnesium-to-calcium ratio to identify associated common genetic variants. We included 9320 adults of European descent from four genetic isolates and three urban cohorts. Urinary magnesium and calcium concentrations were measured centrally in spot urine, and each study conducted linear regression analysis of urinary magnesium-to-calcium ratio on ~2.5 million single-nucleotide polymorphisms (SNPs) using an additive model. We investigated, in mouse, the renal expression profile of the top candidate gene and its variation upon changes in dietary magnesium. The genome-wide analysis evidenced a top locus (rs172639, p = 1.7 × 10), encompassing CLDN14, the gene coding for claudin-14, that was genome-wide significant when using urinary magnesium-to-calcium ratio, but not either one taken separately. In mouse, claudin-14 is expressed in the distal nephron segments specifically handling magnesium, and its expression is regulated by chronic changes in dietary magnesium content. A genome-wide approach identified common variants in the CLDN14 gene exerting a robust influence on the differential excretion of Mg over Ca in urine. These data highlight the power of urinary electrolyte ratios to unravel genetic determinants of renal tubular function. Coupled with mouse experiments, these results support a major role for claudin-14, a gene associated with kidney stones, in the differential paracellular handling of divalent cations by the renal tubule.

I nherited renal tubulopathies are rare diseases often diagnosed in children, particularly those with autosomal recessive transmission. Although some tubulopathies are diagnosed in adulthood, including recessive diseases with potentially mild presentation (e.g., Gitelman syndrome), slowly progressive dominant diseases (e.g., autosomal dominant tubulointerstitial kidney disease), and diseases with variable severity (e.g., Dent disease), the prevalence of tubulopathies in adults remains mostly unknown. 1-3 In addition, the clinical presentation may be atypical or insidious, complicating and delaying the diagnosis of tubular dysfunction. 4 Advances in understanding renal tubular solute transport systems has been achieved through the elucidation of

We herein expand the clinical and mutational spectrum of HDR syndrome, illustrating considerable inter- and intrafamilial phenotypic variability. Diagnosis of HDR should be considered in any patient with hypoparathyroidism and deafness, whether associated with renal abnormalities or not. HDR diagnosis is established through identification of a mutation in the GATA-3 gene.