Hendrica Belge

A role for Rhesus factor Rhcg in renal ammonium excretion and male fertility

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 NH3 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 NH3 and impaired transepithelial NH3 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.

Renal expression of parvalbumin is critical for NaCl handling and response to diuretics

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 Ca2+-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 thiazide-sensitive 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 Ca2+ balance and increased bone mineral density. Studies in mouse DCT cells showed that endogenous NCC expression is Ca2+-dependent and can be modulated by the levels of PV expression. These results suggest that PV regulates the expression of NCC by modulating intracellular Ca2+ signaling in response to ATP in DCT cells. They also provide insights into the Ca2+-sparing action of thiazides and the pathophysiology of distal tubulopathies.

The rhesus protein RhCG : a new perspective in ammonium transport and distal urinary acidification.

Urinary acidification is a complex process requiring the coordinated action of enzymes and transport proteins and resulting in the removal of acid and the regeneration of bicarbonate. Proton secretion is mediated by luminal H(+)-ATPases and requires the parallel movement of NH₃, and its protonation to NH₄(+), to provide sufficient buffering. It has been long assumed that ammonia secretion is a passive process occurring by means of simple diffusion driven by the urinary trapping of ammonium. However, new data indicate that mammalian cells possess specific membrane proteins from the family of rhesus proteins involved in ammonia/μm permeability. Rhesus proteins were first identified in yeast and later also in plants, algae, and mammals. In rodents, RhBG and RhCG are expressed in the collecting duct, whereas in humans only RhCG was detected. Their expression increases with maturation of the kidney and accelerates after birth in parallel with other acid-base transport proteins. Deletion of RhBG in mice had no effect on renal ammonium excretion, whereas RhCG deficiency reduces renal ammonium secretion strongly, causes metabolic acidosis in acid-challenged mice, and impairs restoration of normal acid-base status. Microperfusion experiments or functional reconstitution in liposomes demonstrates that ammonia is the most likely substrate of RhCG. Similarly, crystal structures of human RhCG and the homologous bacterial AmtB protein suggest that these proteins may form gas channels.

Clinical and mutational spectrum of hypoparathyroidism, deafness and renal dysplasia syndrome

Background Hypoparathyroidism, deafness and renal dysplasia (HDR) syndrome is a rare autosomal dominant disorder, secondary to mutations in the GATA-3 gene. Due to its wide range of penetrance and expressivity, the disease may not always be recognized. We herein describe clinical and genetic features of patients with HDR syndrome, highlighting diagnostic clues. Methods Medical records of eight patients from five unrelated families exhibiting GATA-3 mutations were reviewed retrospectively, in conjunction with all previously reported cases. Results HDR syndrome was diagnosed in eight patients between the ages of 18 and 60 years. Sensorineural deafness was consistently diagnosed, ranging from clinical hearing loss since infancy in seven patients to deafness detected only by audiometry in adulthood in one single patient. Hypoparathyroidism was present in six patients (with hypocalcaemia and inaugural seizures in two out of six). Renal abnormalities observed in six patients were diverse and of dysplastic nature. Three patients displayed nephrotic-range proteinuria and reached end-stage renal disease (ESRD) between the ages of 19 and 61 years, whilst lesions of focal and segmental glomerulosclerosis were histologically demonstrated in one of them. Interestingly, phenotype severity differed significantly between a mother and son within one family. Five new mutations of GATA-3 were identified, including three missense mutations affecting zinc finger motifs [NM_001002295.1: c.856A>G (p.N286D) and c.1017C>G (p.C339W)] or the conserved linker region [c.896G>A (p.R299G)], and two splicing mutations (c.924+4_924+19del and c.1051-2A>G). Review of 115 previously reported cases of GATA-3 mutations showed hypoparathyroidism and deafness in 95% of patients, and renal abnormalities in only 60%. Overall, 10% of patients had reached ESRD. Conclusions 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.

Paradoxical response to furosemide in uromodulin-associated kidney disease

Mutations in the UMOD gene coding for uromodulin cause autosomal dominant tubulointerstitial kidney disease. Uromodulin is known to regulate transport processes in the thick ascending limb, but it remains unknown whether UMOD mutations are associated with functional tubular alterations in the early phase of the disease. The responses to furosemide and to a water deprivation test were compared in a 32-year-old female patient carrying the pathogenic UMOD mutation p.C217G and her unaffected 31-year-old sister. A single dose of furosemide induced an intense headache with exaggerated decrease in blood pressure (Δsyst: 30 versus 20 mmHg; Δdiast: 18 versus 5 mmHg) and body weight (Δ2.6 kg versus Δ0.9 kg over 3 h) in the proband versus unaffected sib. The diuretic response and the fall in urine osmolality were also more important and detected earlier in the affected sib. Water deprivation led to increased plasma osmolality and urine concentration in both siblings; however, the response to desmopressin was attenuated in the affected sib. These data reveal that mutations of uromodulin cause specific transport alterations, including exaggerated response to furosemide and a failure to maximally concentrate urine, in the early phase of the disease.

Genome-Wide Meta-Analysis Unravels Interactions between Magnesium Homeostasis and Metabolic Phenotypes.

Magnesium (Mg2+) homeostasis is critical for metabolism. However, the genetic determinants of the renal handling of Mg2+, which is crucial for Mg2+ 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 Mg2+ homeostasis. We identified two loci associated with urinary magnesium (uMg), rs3824347 (P=4.4×10-13) near TRPM6, which encodes an epithelial Mg2+ channel, and rs35929 (P=2.1×10-11), a variant of ARL15, 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 Mg2+ regulated the expression of the highly conserved ARL15 ortholog arl15b, and arl15b knockdown resulted in renal Mg2+ wasting and metabolic disturbances. Finally, ARL15 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 Mg2+ deficiency to insulin resistance and obesity.