Proximal Tubule‐Specific Knockout of Claudin‐2 Causes Hypercalciuria in Mice

Idiopathic hypercalciuria is the major risk factor for kidney stone disease. Constitutive global knockout in mice of claudin‐2 ( Cldn2 ), which encodes a paracellular cation channel expressed at the tight junction in proximal renal tubule and intestinal crypt epithelial cells, causes hypercalciuria and nephrocalcinosis, due to increased intestinal absorption and/or decreased renal tubule reabsorption of calcium. Furthermore, human mutations in CLDN2 are associated with kidney stone disease. It remains unknown if claudin‐2 in the kidney or intestines is most important for calcium homeostasis. To test the hypothesis that the loss of renal claudin‐2 is sufficient to cause hypercalciuria, we generated inducible renal tubule‐specific Cldn2 knockout mice. Cldn2 floxed mice were first generated by homologous recombination, then crossed to renal tubule‐specific, doxycycline‐inducible Cre transgenic mice ( Pax8‐rtTA / LC1‐cre ). Renal tubule‐specific Cldn2 deletion was induced at 3‐5 weeks of age by treatment with 2 mg/mL doxycycline in 2% sucrose in the drinking water for 1 week. Urine calcium/creatinine ratio increased 5‐fold from baseline to week 1 in Cldn2 floxed‐ Pax8‐rtTA / LC1‐cre mice treated with doxycycline, but not in doxycycline‐treated Cldn2 floxed mice missing either Pax8‐rtTA or LC1‐cre , nor in mice treated with vehicle alone. Our results show that the loss of renal tubule claudin‐2 is sufficient to cause hypercalciuria and suggests that paracellular calcium transport in the proximal tubule is a key determinant of urinary calcium excretion.