Targeted deletion of murineCldn16identifies extra- and intrarenal compensatory mechanisms of Ca²⁺and Mg²⁺wasting

Abstract

Claudin-16 (CLDN16) is critical for renal paracellular epithelial transport of Ca(2+) and Mg(2+) in the thick ascending loop of Henle. To gain novel insights into the role of CLDN16 in renal Ca(2+) and Mg(2+) homeostasis and the pathological mechanisms underlying a human disease associated with CLDN16 dysfunction [familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), OMIM 248250], we generated a mouse model of CLDN16 deficiency. Similar to patients, CLDN16-deficient mice displayed hypercalciuria and hypomagnesemia. Contrary to FHHNC patients, nephrocalcinosis was absent in our model, indicating the existence of compensatory pathways in ion handling in this model. In line with the renal loss of Ca(2+), compensatory mechanisms like parathyroid hormone and 1,25(OH)(2)D(3) were significantly elevated. Also, gene expression profiling revealed transcriptional upregulation of several Ca(2+) and Mg(2+) transport systems including Trpv5, Trpm6, and calbindin-D9k. Induced gene expression was also seen for the transcripts of two putative Mg(2+) transport proteins, Cnnm2 and Atp13a4. Moreover, urinary pH was significantly lower when compared with wild-type mice. Taken together, our findings demonstrate that loss of CLDN16 activity leads to specific alterations in Ca(2+) and Mg(2+) homeostasis and that CLDN16-deficient mice represent a useful model to further elucidate pathways involved in renal Ca(2+) and Mg(2+) handling.

Deletion of claudin-10 ( Cldn10 ) in the thick ascending limb impairs paracellular sodium permeability and leads to hypermagnesemia and nephrocalcinosis

“…The deposits are seen along the outer stripe of the outer medulla and in the inner stripe of the outer medulla at the transition to the inner medulla, in which the TAL is located and the hyperreabsorption of Ca 2+ takes place. The hyperreabsorption of divalent cations in mice deficient in claudin-10 is in opposition to the loss of divalent cations seen in mouse models of claudin-16 deficiency and in human patients with mutation in CLDN16 or CLDN19 (8,(11)(12)(13)(14). This finding indicates that claudins in the TAL have functions that differentially affect paracellular cation transport in this segment.…”

“…The importance of claudin-16 and -19 in this tissue is documented by mutations in CLDN16 and CLDN19, which cause familial hypomagnesemia, hypercalciuria, and nephrocalcinosis, an autosomal recessive disorder that leads to end-stage renal disease (8,11). The relevance of CLDN16 for paracellular reabsorption of Mg 2+ and Ca 2+ was confirmed in mouse models with targeted gene disruption (12)(13)(14). In addition, claudin-14, expressed in the TAL of mice on a highcalcium diet, was identified as negative regulator of claudin-16 function (15), and sequence variants in CLDN14 have been associated with human kidney stone disease (16).…”

Mosaic expression of claudins in thick ascending limbs of Henle results in spatial separation of paracellular Na + and Mg 2+ transport

“…Cldn16 deficiency in mice resulted in a small but significant reduction of paracellular Mg 2+ permeability in the TAL (15,16). In the present study, we directly correlated claudin expression and ion permeability in TAL segments of untreated wild-type mice, allowing the analysis of claudin function in a natural background.…”

“…In contrast, the importance of both cldn16 and cldn19 for intact Ca 2+ and Mg 2+ reabsorption was demonstrated in several studies using deficiency mouse models (15,16,18) and is emphasized by the finding that patients with defects in CLDN16 or CLDN19 suffer from FHHNC (13,14). However, whether cldn16 and cldn19 themselves are constituents of the paracellular channel for divalent cations is still controversial.…”

“…Likewise, cldn16 knockout in mice resulted in hypercalciuria and hypomagnesemia with severely diminished Ca 2+ and Mg 2+ permeabilities in the TAL (15). In a cldn16 knockdown approach mice also developed renal wasting of Ca 2+ and Mg 2+ which was accompanied by a nonselective decrease in cation permeability (16).…”