Update on vitamin D and evaluation of vitamin D status.

Knowledge about vitamin D has greatly improved during the last few years. Vitamin D cannot any more be considered as exclusively necessary to prevent ricket/osteomalacia. Its role in the prevention of some osteoporotic fractures in the elderly (in association with calcium nutrition) is now well demonstrated and many epidemiologic and laboratory data argue for a role in the prevention of several diseases or anomalies (cancer, auto-immune diseases, cardiovascular events, sarcopenia...). A few intervention studies confirming some of these effects also exist. Vitamin D status can easily be assessed by measuring serum 25 hydroxy vitamin D (25OHD) level. However, many experts have claimed that the population-based reference values for 25OHD are too low and that the cut-off value below which vitamin D insufficiency can be present is somewhere between 20 and 40ng/mL with a clear tendency to target values above 30ng/mL (75nmol/L). The main consequences are that vitamin D insufficiency is highly frequent whereas the currently recommended supplementation doses are not sufficient

Claudin Loss-of-Function Disrupts Tight Junctions and Impairs Amelogenesis

Claudins are a family of proteins that forms paracellular barriers and pores determining tight junctions (TJ) permeability. Claudin-16 and -19 are pore forming TJ proteins allowing calcium and magnesium reabsorption in the thick ascending limb of Henle's loop (TAL). Loss-of-function mutations in the encoding genes, initially identified to cause Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis (FHHNC), were recently shown to be also involved in Amelogenesis Imperfecta (AI). In addition, both claudins were expressed in the murine tooth germ and Claudin-16 knockout (KO) mice displayed abnormal enamel formation. Claudin-3, an ubiquitous claudin expressed in epithelia including kidney, acts as a barrier-forming tight junction protein. We determined that, similarly to claudin-16 and claudin-19, claudin-3 was expressed in the tooth germ, more precisely in the TJ located at the apical end of secretory ameloblasts. The observation of Claudin-3 KO teeth revealed enamel defects associated to impaired TJ structure at the secretory ends of ameloblasts and accumulation of matrix proteins in the forming enamel. Thus, claudin-3 protein loss-of-function disturbs amelogenesis similarly to claudin-16 loss-of-function, highlighting the importance of claudin proteins for the TJ structure. These findings unravel that loss-of-function of either pore or barrier-forming TJ proteins leads to enamel defects. Hence, the major structural function of claudin proteins appears essential for amelogenesis

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 NH(3), and its protonation to NH(4)(+), 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.Kidney International advance online publication, 6 October 2010; doi:10.1038/ki.2010.386

Gitelman-Like Syndrome Caused by Pathogenic Variants in mtDNA

Background: Gitelman syndrome is the most frequent hereditary salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. Gitelman syndrome is caused by biallelic pathogenic variants in SLC12A3, encoding the Na+-Cl− cotransporter (NCC) expressed in the distal convoluted tubule. Pathogenic variants of CLCNKB, HNF1B, FXYD2, or KCNJ10 may result in the same renal phenotype of Gitelman syndrome, as they can lead to reduced NCC activity. For approximately 10 percent of patients with a Gitelman syndrome phenotype, the genotype is unknown. Methods: We identified mitochondrial DNA (mtDNA) variants in three families with Gitelman-like electrolyte abnormalities, then investigated 156 families for variants in MT-TI and MT-TF, which encode the transfer RNAs for phenylalanine and isoleucine. Mitochondrial respiratory chain function was assessed in patient fibroblasts. Mitochondrial dysfunction was induced in NCC-expressing HEK293 cells to assess the effect on thiazide-sensitive 22Na+ transport. Results: Genetic investigations revealed four mtDNA variants in 13 families: m.591C>T (n=7), m.616T>C (n=1), m.643A>G (n=1) (all in MT-TF), and m.4291T>C (n=4, in MT-TI). Variants were near homoplasmic in affected individuals. All variants were classified as pathogenic, except for m.643A>G, which was classified as a variant of uncertain significance. Importantly, affected members of six families with an MT-TF variant additionally suffered from progressive chronic kidney disease. Dysfunction of oxidative phosphorylation complex IV and reduced maximal mitochondrial respiratory capacity were found in patient fibroblasts. In vitro pharmacological inhibition of complex IV, mimicking the effect of the mtDNA variants, inhibited NCC phosphorylation and NCC-mediated sodium uptake. Conclusion: Pathogenic mtDNA variants in MT-TF and MT-TI can cause a Gitelman-like syndrome. Genetic investigation of mtDNA should be considered in patients with unexplained Gitelman syndrome-like tubulopathies

Parathyroid hormone and phosphate homeostasis in patients with Bartter and Gitelman syndrome:an international cross-sectional study

Background:Small cohort studies have reported high parathyroid hormone (PTH) levels in patients with Bartter syndrome and lower serum phosphate levels have anecdotally been reported in patients with Gitelman syndrome. In this cross-sectional study, we assessed PTH and phosphate homeostasis in a large cohort of patients with salt-losing tubulopathies. Methods:Clinical and laboratory data of 589 patients with Bartter and Gitelman syndrome were provided by members of the European Rare Kidney Diseases Reference Network (ERKNet) and the European Society for Paediatric Nephrology (ESPN). Results:A total of 285 patients with Bartter syndrome and 304 patients with Gitelman syndrome were included for analysis. Patients with Bartter syndrome type I and II had the highest median PTH level (7.5 pmol/L) and 56% had hyperparathyroidism (PTH &gt;7.0 pmol/L). Serum calcium was slightly lower in Bartter syndrome type I and II patients with hyperparathyroidism (2.42 versus 2.49 mmol/L; P = .038) compared to those with normal PTH levels and correlated inversely with PTH (rs −0.253; P = .009). Serum phosphate and urinary phosphate excretion did not correlate with PTH. Overall, 22% of patients had low serum phosphate levels (phosphate—standard deviation score &lt; −2), with the highest prevalence in patients with Bartter syndrome type III (32%). Serum phosphate correlated with tubular maximum reabsorption of phosphate/glomerular filtration rate (TmP/GFR) (rs 0.699; P &lt; .001), suggesting renal phosphate wasting. Conclusions:Hyperparathyroidism is frequent in patients with Bartter syndrome type I and II. Low serum phosphate is observed in a significant number of patients with Bartter and Gitelman syndrome and appears associated with renal phosphate wasting.</p

Parathyroid hormone and phosphate homeostasis in patients with Bartter and Gitelman syndrome:an international cross-sectional study

Background:Small cohort studies have reported high parathyroid hormone (PTH) levels in patients with Bartter syndrome and lower serum phosphate levels have anecdotally been reported in patients with Gitelman syndrome. In this cross-sectional study, we assessed PTH and phosphate homeostasis in a large cohort of patients with salt-losing tubulopathies. Methods:Clinical and laboratory data of 589 patients with Bartter and Gitelman syndrome were provided by members of the European Rare Kidney Diseases Reference Network (ERKNet) and the European Society for Paediatric Nephrology (ESPN). Results:A total of 285 patients with Bartter syndrome and 304 patients with Gitelman syndrome were included for analysis. Patients with Bartter syndrome type I and II had the highest median PTH level (7.5 pmol/L) and 56% had hyperparathyroidism (PTH &gt;7.0 pmol/L). Serum calcium was slightly lower in Bartter syndrome type I and II patients with hyperparathyroidism (2.42 versus 2.49 mmol/L; P = .038) compared to those with normal PTH levels and correlated inversely with PTH (rs −0.253; P = .009). Serum phosphate and urinary phosphate excretion did not correlate with PTH. Overall, 22% of patients had low serum phosphate levels (phosphate—standard deviation score &lt; −2), with the highest prevalence in patients with Bartter syndrome type III (32%). Serum phosphate correlated with tubular maximum reabsorption of phosphate/glomerular filtration rate (TmP/GFR) (rs 0.699; P &lt; .001), suggesting renal phosphate wasting. Conclusions:Hyperparathyroidism is frequent in patients with Bartter syndrome type I and II. Low serum phosphate is observed in a significant number of patients with Bartter and Gitelman syndrome and appears associated with renal phosphate wasting.</p

Nephrocalcinosis (enamel renal syndrome) caused by autosomal recessive FAM20A mutations

Calcium homeostasis requires regulated cellular and interstitial systems interacting to modulate the activity and movement of this ion. Disruption of these systems in the kidney results in nephrocalcinosis and nephrolithiasis, important medical problems whose pathogenesis is incompletely understood

Rôle du Calcium/polycation-sensing receptor >> CaSR dans la régulation de la calcémie indépendamment de l'hormone parathyroïdienne

Le récepteur du calcium (CaSR) exprimé par les cellules principales des parathyroïdes permet d'adapter en permanence la sécrétion de PTH à la valeur de calcémie. Cependant, CaSR est exprimé par de nombreux organes, dont le rein, dans lesquels son rôle reste mal défini. Le but de cette étude était de déterminer le rôle éventuel du CaSR extraparathyroïdien dans le contrôle de la calcémie. En condition basale, les rats thyroparathyroidectomisés traités par T4 et PTH à dose fixe avaient une calcémie normale. Le traitement par véhicule ne modifiait pas la calcémie, les autres concentrations ioniques plasmatiques, le débit de filtration glomérulaire, les débits d'urine ou d'excrétion ionique urinaire. Au contraire, chez les rats traités par NPS2143, la calciurie diminuait significativement le premier jour de traitement créant un bilan calcique positif. La calcémie s'élevait significativement dès le deuxième jour de traitement comparativement à celle des rats témoins. Le traitement de BLA microperfusées in vitro par le NPS2143 (1 M) entrainait une augmentation de la réabsorption de Ca et de la perméabilité transépithéliale au Ca (+ 50 %, p<0,03) sans modifier le transport de NaCl ou la Vte. L'inhibition du CaSR extraparathyroïdien provoque une augmentation spécifique de la perméabilité au Ca de la BLAc et de la réabsorption tubulaire rénale de Ca, induit un bilan calcique positif et augmente la calcémie à l'état stable, en l'absence de toute variation de la sécrétion de PTH. CaSR extraparathyroïdien est un déterminant direct de la calcémie, indépendamment de son effet sur la sécrétion de PTH.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF