2138 Pediatr Nephrol (2011) 26:2133–2142 Fig. 2 Molecular model of paracellin-1/claudin-16. Claudin-16 has four predicted transmembrane domains with intracellular N- and C- termini. Acidic residues in the first extracellular domain that participate in cation permeation (as evidenced by neutralizing mutations that preserve normal expression and trafficking but reduce paracellular Na permeability [17]), probably by affecting pore electronegativity [20], are shaded red; those that do not appear to be important for permeation are shaded yellow. FHHNC mutations that preserve paracellin-1 expression and trafficking but impair permeabil- ity are shaded blue [17]; those that abolish expression or cause miss- trafficking are mostly uninformative of paracellin-1 function and are not shown. Serine-217 (purple) is phosphorylated by PKA and thereby facilitates trafficking to the tight junction [45]. Cysteines located at the intracellular end of the second and fourth transmem- brane domains and shaded brown are homologous to those that are palmitoylated and participate in tight junction trafficking in other claudins [61]. A C-terminal PDZ-binding motif (TRV) that is required for binding to ZO1 and tight junction trafficking is shaded green [62] expression of claudin-16 in vitro increases Na permeability [17], while overexpression of claudin-19 decreases Cl permeability [39]. Furthermore, claudin-16 and -19 physi- cally interact to form heteromultimers, and coexpression of both has additive effects to increase PNa/PCl [39]. Also consistent with this, in in vitro perfused TALH tubules from claudin-16 knockdown mice, PNa/PCl was found to be decreased twofold with no change in PNa/PMg [38]. Since a high TAL PNa/PCl is necessary to generate the transepithelial dilution potential, loss-of-function mutations in claudin-16 and -19 would be predicted to abrogate the electrical driving force for Ca and Mg reabsorption in the TAL. [48], but subsequently localized by immunohistochemistry to the thin descending and thick ascending limbs of the loop of Henle, and the proximal convoluted tubule [49]. In vitro, it seems to behave as a paracellular cation barrier [48]. However, exactly how claudin-14 mutations cause hypercalciuria or kidney stones remains to be determined. Role of other Claudins in renal tubule function Knowledge of the role of claudins other than 16 and 19 in renal function is limited, and comes mainly from inferences from localization studies, and analysis of knockout mice (Table 1). Claudin mutations and kidney stones Claudin-2 Recently, Thorleifsson et al. reported an association between common synonymous variants in the claudin- 14 gene and both increased risk of kidney stones and lower bone mineral density [47]. These variants were also associated with increased urine calcium excretion and decreased serum total CO2, suggesting that the effects on both kidney stones and bone mineral density are mediated by hypercalciuria and metabolic acidosis. Claudin-14 was initially reported to be expressed in the collecting duct Claudin-2 is expressed in the proximal tubule (with highest levels in the late proximal tubule) and the early thin descending limb [50, 51]. In vitro studies have shown that it functions as a cation-selective paracellular pore (Na:Cl permeability ratio of 8:1) [21, 52]. Late proximal tubule fluid has a relatively high chloride concentration (due to transcellular reabsorption of Na with bicarbonate and organic solutes). This drives passive reabsorption of chloride and generates a lumen-positive electrical potential.