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The majority of calcium filtered by the glomerulus is reabsorbed along the nephron. Most is reabsorbed from the proximal tubule (> 60%) via a paracellular pathway composed of the tight junction proteins claudins-2 and -12, a process driven by sodium and consequently water reabsorption. The thick ascending limb reabsorbs the next greatest amount of calcium (20-25%), also by a paracellular pathway composed of claudins-16 and -19. This pathway is regulated by the CaSR, whose activity increases the expression of claudin-14, a protein that blocks paracellular calcium reabsorption. The fine tuning of urinary calcium excretion occurs in the distal convoluted and connecting tubule by a transcellular pathway composed of the apical calcium channel TRPV5, the calcium shuttling protein calbindin-D.sub.28K and the basolateral proteins PMCA1b and the sodium calcium exchanger, NCX. Not surprisingly, mutations in a subset of these genes cause monogenic disorders with hypercalciuria as a part of the phenotype. More commonly, 'idiopathic' hypercalciuria is encountered clinically with genetic variations in CLDN14, the CASR and TRPV5 associating with kidney stones and increased urinary calcium excretion. An understanding of the molecular pathways conferring kidney tubular calcium reabsorption is employed in this review to help explain how dietary and medical interventions for this disorder lower urinary calcium excretion., Author(s): Mallory L Downie [sup.1] [sup.2] , R. Todd Alexander [sup.3] [sup.4] [sup.5] Author Affiliations: (1) grid.83440.3b, 0000000121901201, Department of Renal Medicine, University College London, , London, UK (2) grid.424537.3, [...]

The kidney plays a pivotal role in regulating calcium levels within the body. Approximately 98% of the filtered calcium is reabsorbed in the nephron, and this process is tightly controlled to maintain calcium homeostasis, which is required to facilitate optimal bone mineralization, preserve serum calcium levels within a narrow range, and support intracellular signalling mechanisms. The maintenance of these functions is attributed to a delicate balance achieved by various calcium channels, transporters, and calcium-binding proteins in renal cells. Perturbation of this balance due to deficiency or dysfunction of calcium channels and calcium-binding proteins can lead to severe complications. For example, polycystic kidney disease is linked to aberrant calcium transport and signalling. Furthermore, dysregulation of calcium levels can promote the formation of kidney stones. This Review provides an updated description of the key aspects of calcium handling in the kidney, focusing on the function of various calcium channels and the physiological stimuli that control these channels or are communicated through them. A discussion of the role of calcium as an intracellular second messenger and the pathophysiology of renal calcium dysregulation, as well as a summary of gaps in knowledge and future prospects, are also included. Calcium reabsorption along the nephron is essential for calcium homeostasis and whole-body electrolyte balance. Here, Staruschenko et al. highlight signalling pathways and molecules involved in renal calcium handling in health and disease, and discuss progress in the integration of systems-level and molecular understanding of calcium transport and regulation. Key points: The kidney has an essential role in the maintenance of serum calcium levels owing to its careful regulation of reabsorption along the nephron, which contributes to whole-body calcium balance. Physiologically relevant mechanisms that control calcium channels in the kidney include endocrine and paracrine signals and physical factors such as fluid flow. Tight control of calcium influx and intracellular calcium levels maintains intracellular calcium concentrations within the physiological range, despite acute and chronic variation in calcium intake. Failure to reabsorb calcium from renal tubules results in hypercalciuria and an increased risk of kidney stones. Aberrant calcium signalling in particular renal cell types is a hallmark of renal diseases such as polycystic kidney disease and focal segmental glomerulosclerosis. [ABSTRACT FROM AUTHOR]

The major risk factor for kidney stone disease is idiopathic hypercalciuria. Recent evidence implicates a role for defective calcium reabsorption in the renal proximal tubule. We hypothesized that claudin-2, a paracellular cation channel protein, mediates proximal tubule calcium reabsorption. We found that claudin-2-null mice have hypercalciuria due to a primary defect in renal tubule calcium transport and papillary nephrocalcinosis that resembles the intratubular plugs in kidney stone formers. Our findings suggest that a proximal tubule defect in calcium reabsorption predisposes to papillary calcification, providing support for the vas washdown hypothesis. Claudin-2-null mice were also found to have increased net intestinal calcium absorption, but reduced paracellular calcium permeability in the colon, suggesting that this was due to reduced intestinal calcium secretion. Common genetic variants in the claudin-2 gene were associated with decreased tissue expression of claudin-2 and increased risk of kidney stones in 2 large population-based studies. Finally, we describe a family in which males with a rare missense variant in claudin-2 have marked hypercalciuria and kidney stone disease. Our findings indicate that claudin-2 is a key regulator of calcium excretion and a potential target for therapies to prevent kidney stones., Introduction Kidney stone disease is common, with a lifetime risk in the United States of about 19% in men and 9% in women (1). Studies of families (2), twin concordance [...]

FAM111A gene mutations cause Kenney–Caffey syndrome (KCS) and Osteocraniostenosis (OCS), conditions characterized by short stature, low serum ionized calcium (Ca2+), low parathyroid hormone (PTH), and bony abnormalities. The molecular mechanism mediating this phenotype is unknown. The c‐terminal domain of FAM111A harbors all the known disease‐causing variations and encodes a domain with high homology to serine proteases. However, whether this serine protease domain contributes to the maintenance of Ca2+ homeostasis is not known. We hypothesized the disruption of the serine protease domain of FAM111A would disrupt Ca2+ homeostasis. To test this hypothesis, we generated with CRISPR/Cas9, mice with a frameshift insertion (c.1450insA) or large deletion (c.1253‐1464del) mutation in the Fam111a serine protease domain. Serum‐ionized Ca2+ and PTH levels were not significantly different between wild type, heterozygous, or homozygous Fam111a mutant mice. Additionally, there were no significant differences in fecal or urine Ca2+ excretion, intestinal Ca2+ absorption or overall Ca2+ balance. Only female homozygous (c.1450insA), but not heterozygous mice displayed differences in bone microarchitecture and mineral density compared to wild‐type animals. We conclude that frameshift mutations that disrupt the c‐terminal serine protease domain do not induce a KCS or OCS phenotype in mice nor alter Ca2+ homeostasis. [ABSTRACT FROM AUTHOR]

This article explores the molecular mechanisms of loop diuretics and their effects on renal calcium and magnesium transport. Loop diuretics are commonly used to treat fluid retention in patients with heart failure, kidney disease, and cirrhosis. They primarily act on the thick ascending limb of the loop of Henle by inhibiting the NKCC2 cotransporter, resulting in increased urine volume and salt wasting. Loop diuretics also affect the reabsorption of calcium and magnesium in the kidney, leading to their loss in urine. The article discusses the role of various transporters and channels involved in calcium and magnesium reabsorption in the renal tubules. It also highlights the differential effects of loop diuretics on calcium and magnesium handling in Bartter syndrome, a genetic disorder affecting renal salt and mineral transport. Additionally, the article presents new findings suggesting that furosemide, a commonly used loop diuretic, may have beneficial effects on calcium excretion in patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC). Further research is needed to explore the therapeutic potential of furosemide in these patients. [Extracted from the article]

A higher concentration of calcium in breast milk than blood favors paracellular calcium absorption enabling growth during postnatal development. We aimed to determine whether suckling animals have greater intestinal calcium permeability to maximize absorption and to identify the underlying molecular mechanism. We examined intestinal claudin expression at different ages in mice and in human intestinal epithelial (Caco-2) cells in response to hormones or human milk. We also measured intestinal calcium permeability in wildtype, Cldn2 and Cldn12 KO mice and Caco-2 cells in response to hormones or human milk. Bone mineralization in mice was assessed by μCT. Calcium permeability across the jejunum and ileum of mice were 2-fold greater at 2 wk than 2 mo postnatal age. At 2 wk, Cldn2 and Cldn12 expression were greater, but only Cldn2 KO mice had decreased calcium permeability compared to wildtype. This translated to decreased bone volume, cross-sectional thickness, and tissue mineral density of femurs. Weaning from breast milk led to a 50% decrease in Cldn2 expression in the jejunum and ileum. Epidermal growth factor (EGF) in breast milk specifically increased only CLDN2 expression and calcium permeability in Caco-2 cells. These data support intestinal permeability to calcium, conferred by claudin-2, being greater in suckling mice and being driven by EGF in breast milk. Loss of the CLDN2 pathway leads to suboptimal bone mineralization at 2 wk of life. Overall, EGF-mediated control of intestinal claudin-2 expression contributes to maximal intestinal calcium absorption in suckling animals. Graphical Abstract Epidermal growth factor in milk increases the paracellular permeability to calcium across the small intestine via increasing the intestinal claudin-2 abundance in suckling mammals. [ABSTRACT FROM AUTHOR]

Background Data guiding the timing of dialysis initiation in children are limited. We sought to determine current practice and secular trends in Canada with respect to the timing of dialysis initiation in children based on estimated glomerular filtration rate (eGFR). Methods This observational study included incident chronic dialysis patients aged [less than or equal to]21 years identified from the Canadian Organ Replacement Register who started dialysis in Canada between January 2001 and December 2010 at any of the nine participating Canadian centers (n = 583). Youth were categorized utilizing CKiD Schwartz eGFR into [greater than or equal to]10.5 (higher) or Results Median eGFR at dialysis initiation was 8.1 (interquartile range 5.4-11.0) ml/min/1.73 m.sup.2. Overall, 29 % of the patients started dialysis with an eGFR of [greater than or equal to]10.5 ml/min/1.73 m.sup.2. The proportion of children starting with higher eGFR increased from 27.3 % in 2001 to 35.4 % in 2010 (p = 0.04) and differed by treatment facility (12-70 %; p = 0.0001). Factors associated with higher eGFR at dialysis initiation in the adjusted regression model were female sex [odds ratio (OR) 1.48; 95 % confidence interval (CI) 1.02-2.14], genetic cause of end-stage kidney disease (OR 2.77; 95 % CI 1.37-5.58) and living [greater than or equal to]50 km from treatment facility (OR 1.47; 95 % CI 1.01-2.14). Conclusions One-third of the children were found to have initiated dialysis with an eGFR [greater than or equal to]10.5 ml/min/1.73 m.sup.2, however significant practice variation exists with respect to timing of dialysis initiation by treatment facility. More data is required to evaluate the clinical implications of this practice variation., Author(s): Allison B. Dart[sup.1] , Michael Zappitelli[sup.2] , Manish M. Sood[sup.3] , R. Todd Alexander[sup.4] , Steven Arora[sup.5] , Robin L. Erickson[sup.6] , Kristine Kroeker[sup.1] , Andrea Soo[sup.7] , Braden [...]

Central to the maintenance of calcium homeostasis is the regulated reabsorption of calcium along the nephron. To this end, parathyroid hormone (PTH) is released from the parathyroid gland in response to lowered plasma calcium levels. This hormone acts through the PTH 1 receptor along the nephron to increase urinary phosphate excretion and decrease urinary calcium excretion. In the proximal tubule, PTH inhibits phosphate reabsorption by reducing the abundance of sodium phosphate cotransporters in the apical membrane. PTH likely decreases calcium reabsorption from the proximal tubule, by reducing the reabsorption of sodium, an event necessary for the paracellular movement of calcium across this segment. In the thick ascending limb (TAL), PTH increases calcium permeability and may increase the electrical driving force thereby increasing calcium reabsorption in the TAL. Finally, in the distal convolution, PTH acts to increase transcellular calcium reabsorption by increasing the activity and abundance of the apically expressed calcium channel TRPV5. [ABSTRACT FROM AUTHOR]

Calcium ([Ca.sup.2+]) is a key constituent in a myriad of physiological processes from intracellular signalling to the miner alization of bone. As a consequence, [Ca.sup.2+] is maintained within narrow limits when circulating in plasma. This is accomplished via regulated interplay between intestinal absorption, renal tubular reabsorption, and exchange with bone. Many studies have focused on the highly regulated active transcellular transport pathways for [Ca.sup.2+] from the duodenum of the intestine and the distal nephron of the kidney. However, comparatively little work has examined the molecular constituents creating the paracellular shunt across intestinal and renal epithelium, the transport pathway responsible for the majority of transepithelial [Ca.sup.2+] flux. More specifically, passive paracellular [Ca.sup.2+] absorption occurs across the majority of the intestine in addition to the renal proximal tubule and thick ascending limb of Henle's loop. Importantly, recent studies demonstrated that [Ca.sup.2+] transport through the paracellular shunt is significantly regulated. Therefore, we have summarized the evidence for different modes of paracellular [Ca.sup.2+] flux across renal and intestinal epithelia and highlighted recent molecular insights into both the mechanism of secondarily active paracellular [Ca.sup.2+] movement and the identity of claudins that permit the passage of [Ca.sup.2+] through the tight junction of these epithelia. Key words: kidney, NHE3, claudins, solvent drag. Le calcium ([Ca.sup.2+]) est un important constituant d'une myriade de processus physiologiques allant de la signalisation intracellulaire a la mineralisation de l'os. Consequemment, le [Ca.sup.2+] est maintenu a l'interieur de limites etroites lorsqu'il circule dans le plasma. Cela s'accomplit grace a l'influence reciproque regulee entre l'absorption intestinale, la reabsorption renale et l'echange avec l'os. Plusieurs etudes se sont concentrees sur la voie hautement regulee du transport actif transellulaire du [Ca.sup.2+] du duodenum et du nephron distal. Cependant, en comparaison, peu d'etudes ont examine les constituants moleculaires qui creent une derivation para-cellulaire a travers l'epithelium intestinal et renal, la voie de transport responsable de la majorite du flux de [Ca.sup.2+] trans-epithelial. Plus specifiquement, l'absorption para-cellulaire passive de [Ca.sup.2+] survient le long de la plus grande partie de l'intestin en plus du tubule proximal du rein et de la branche ascendante large de l'anse de Henle. Fait important, des etudes recentes demontrent que le transport de [Ca.sup.2+] a travers la derivation para-cellulaire est sujet a une importante regulation. Les auteurs ont donc fait la synthese des indices qui suggerent qu'il existe differents modes de flux para-cellulaires de [Ca.sup.2+] a travers l'epithelium du rein et de l'intestin, et souligne les recentes percees sur les plans du mecanisme moleculaire du mouvement para-cellulaire de [Ca.sup.2+] secondairement actif et de l'identite des claudines qui permettent le passage du [Ca.sup.2+] a travers les jonctions serrees de ces epitheliums. [Traduit par la Redaction] Mots-cles: rein, NHE3, claudines, deplacement de solvant., Introduction Calcium ([Ca.sup.2+]) participates in a diverse array of physiological processes, including neural transmission, muscle contraction, blood coagulation, intracellular signal transduction as a second messenger, and the mineralization of bone [...]

The 61st annual meeting of the Canadian Society for Molecular Biosciences (CSMB) focused on the topic of Membrane Proteins in Health and Disease. The meeting was held at the Banff [...]

Large proton fluxes accompany cell migration, but their precise role remains unclear. We studied pH regulation during the course of chemokinesis and chemotaxis in human neutrophils stimulated by attractant peptides. Activation of cell motility by chemoattractants was accompanied by a marked increase in metabolic acid generation, attributable to energy consumption by the contractile machinery and to stimulation of the NADPH oxidase and the ancillary hexose monophosphate shunt. Despite the increase in acid production, the cytosol underwent a sizable alkalinization, caused by acceleration of [Na.sup.+]/[H.sup.+] exchange. The development of the alkalinization mirrored the increase in the rate of cell migration, suggesting a causal relationship. However, elimination of [Na.sup.+]/[H.sup.+] exchange by omission of external [Na.sup.+] or by addition of potent inhibitors was without effect on either chemokinesis or chemotaxis, provided the cytosolic pH remained near neutrality. At more acidic levels, cell motility was progressively inhibited. These observations suggest that [Na.sup.+]/[H.sup.+] exchange plays a permissive role in cell motility but is not required for the initiation or development of the migratory response. Chemokinesis also was found to be exquisitely sensitive to extracellular acidification. This property may account for the inability of neutrophils to access abscesses and solid tumors that have been reported to have inordinately low pH. proton transport; cell motility; cytoskeleton; leukocyte

Hyperosmotic shrinkage induces multiple cellular responses, including activation of volume-regulatory ion transport, cytoskeletal reorganization, and cell death. Here we investigated the possible roles of ezrin/radixin/moesin (ERM) proteins in these events. Osmotic shrinkage of Ehrlich Lettre ascites cells elicited the formation of long microvillus-like protrusions, rapid translocation of endogenous ERM proteins and green fluorescent protein-tagged ezrin to the cortical region including these protrusions, and [Thr.sup.567/564/558] (ezrin/radixin/ moesin) phosphorylation of cortical ERM proteins. Reduced cell volume appeared to be the critical parameter in hypertonicity-induced ERM protein activation, whereas alterations in extracellular ionic strength or intracellular pH were not involved. A shrinkage-induced increase in the level of membrane-associated phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)[P.sub.2]] appeared to play an important role in ERM protein activation, which was prevented after PtdIns(4,5)[P.sub.2] depletion by expression of the synaptojanin-2 phosphatase domain. While expression of constitutively active RhoA increased basal ERM phosphorylation, the Rho-Rho kinase pathway did not appear to be involved in shrinkage-induced ERM protein phosphorylation, which was also unaffected by the inhibition or absence of [Na.sup.+]/[H.sup.+] exchanger isoform (NHE1). Ezrin knockdown by small interfering RNA increased shrinkage-induced NHE1 activity, reduced basal and shrinkage-induced Rho activity, and attenuated the shrinkage-induced formation of microvillus-like protrusions. Hyperosmolarity-induced cell death was unaltered by ezrin knockdown or after phosphatidylinositol 3-kinase (PI3K) inhibition. In conclusion, ERM proteins are activated by osmotic shrinkage in a PtdIns(4,5)[P.sub.2]-dependent, NHE1-independent manner. This in turn mitigates the shrinkage-induced activation of NHE1, augments Rho activity, and may also contribute to F-actin rearrangement. In contrast, no evidence was found for the involvement of an NHE1-ezrin-PI3K-PKB pathway in counteracting shrinkage-induced cell death. RhoA; [Na.sup.+]/[H.sup.+] exchanger 1; cell volume; cytoskeleton; phosphatidylinositol 4,5-bisphosphate

Proximal tubular reabsorption of filtered sodium by the sodium/ hydrogen exchanger isoform 3 (NHE3), located on the apical membrane, is fundamental to the maintenance of systemic volume and pH homeostasis. NHE3 is finely regulated by a variety of hormones and by changes in ionic composition and volume, likely requiring redistribution of the exchangers. We analyzed the subcellular distribution and dynamics of the exchangers by generating an epithelial line expressing NHE3 tagged with an exofacial epitope, which enabled us to monitor exchanger mobility and traffic in intact cells. Using determinations of fluorescence recovery after photobleaching in combination with dynamic measurements of subcellular distribution, we found that, in renal epithelial cells, NHE3 exists in four distinct subcompartments: a virtually immobile subpopulation that is retained on the apical membrane by interaction with the actin cytoskeleton in a manner that depends on the sustained activity of Rho GTPases; a mobile subpopulation on the apical membrane, which can be readily internalized; and two intracellular compartments that can be differentiated by their rate of exchange with the apical pool of NHE3. We provide evidence that detachment of the immobile fraction from its cytoskeletal anchorage leads to rapid internalization. These observations suggest that modulation of the mobile fraction of NHE3 on the apical membrane can alter the number of functional exchangers on the cell surface and, consequently, the rate of transepithelial ion transport. Regulation of the interaction of NHE3 with the actin cytoskeleton can therefore provide a new mode of regulation of sodium and hydrogen transport. fluorescence recovery after photobleaching | pH regulation | sodium proton exchange | hypertension | Madin-Darby canine kidney cells