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Claudin-16 (CLDN16) is critical for renal paracellular epithelial transport of [Ca.sup.2+] and [Mg.sup.2+] in the thick ascending loop of Henle. To gain novel insights into the role of CLDN16 in renal [Ca.sup.2+] and [Mg.sup.2+] homeostasis and the pathological mechanisms underlying a human disease associated with CLDNI6 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.sup.2+], compensatory mechanisms like parathyroid hormone and 1,25[(OH).sub.2][D.sub.3] were significantly elevated. Also, gene expression profiling revealed transcriptional upregulation of several [Ca.sup.2+] and [Mg.sup.2+] transport systems including Trpv5, Trpm6, and calbindin-D9k. Induced gene expression was also seen for the transcripts of two putative [Mg.sup.2+] transport proteins, Cnnm2 and Atpl3a4. Moreover, urinary pH was significantly lower when compared with wild-type mice. Taken together, our findings demonstrate that loss of CLDNI6 activity leads to specific alterations in [Ca.sup.2+] and [Mg.sup.2+] homeostasis and that CLDN16-deficient mice represent a useful model to further elucidate pathways involved in renal [Ca.sup.2+] and [Mg.sup.2+] handling. magnesium; calcium; kidney; tight junction; transgenic animals doi: 10.1152/ajprenal.00499.2009.

Functional properties of the paracellular pathway depend critically on the set of claudins (CLDN) expressed at the tight junction. Two syndromes are causally linked to loss-of-function mutations of claudins: hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia (HELIX) syndrome caused by genetic variations in the CLDN10 gene and familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by genetic variations in the CLDN16 or CLDN19 genes. All three genes are expressed in the kidney, particularly in the thick ascending limb (TAL). However, localization of these claudins in humans and rodents remains to be delineated in detail. We studied the segmental and subcellular expression of CLDN10, CLDN16, and CLDN19 in both paraffin-embedded and frozen kidney sections from the adult human, mouse, and rat using immunohistochemistry and immunofluorescence, respectively. Here, CLDN10 was present in a subset of medullary and cortical TAL cells, localizing to basolateral domains and tight junctions in human and rodent kidneys. Weak expression was detected at the tight junction of proximal tubular cells. CLDN16 was primarily expressed in a subset of TAL cells in the cortex and outer stripe of outer medulla, restricted to basolateral domains and tight junctional structures in both human and rodent kidneys. CLDN19 predominantly colocalized with CLDN16 in tight junctions and basolateral domains of the TAL but was also found in basolateral and junctional domains in more distal sites. CLDN10 expression at tight junctions almost never overlapped with that of CLND16 and CLDN19, consistent with distinct junctional pathways with different permeation profiles in both human and rodent kidneys. [ABSTRACT FROM AUTHOR]

Claudin-16 (CLDN 16) is critical for renal paracellular epithelial transport of Ca2+ and Mg2+ in the thick ascending loop of Henle. To gain novel insights into the role of CLDN16 in renal Ca2+ and Mg2+ 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 Io 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 Ca2+, compensation mechanisms like parathyroid hormone and 1,25(OH)2D3 were significantly elevated. Also, gene expression profiling revealed transcriptional upregulation of several Ca2+ and Mg2+ transport systems including Trpv5, Trpm6. and calbindin-D9k. Induced gene expression was also seen for the transcripts of two putative Mg2+ 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 Ca2+ and Mg2+ homeostasis and that CLDN16-deficient mice represent a useful model to further elucidate pathways involved in renal Ca2+ and Mg2+ handling. [ABSTRACT FROM AUTHOR]