The loss of Cldn12 and functional Trpv6 leads to compensatory upregulation of colonic Trpv5 and reduced bone mineral density.

Calcium (Ca²⁺) homeostasis is tightly regulated by the coordinated actions of the intestine, kidneys and bone. Ca²⁺ transport occurs via either a paracellular pathway, which depends on the expression of select claudins, including claudin-12 (CLDN12) in the tight junction, or a transcellular pathway, involving apical influx through TRPV6. While disruption of paracellular or transcellular pathways impairs transepithelial Ca²⁺ flux, Cldn12-deficient mice and Trpv6 mutant (Trpv6D541A/D541A) mice do not display alterations in Ca²⁺ balance as reported previously, perhaps because one pathway compensates for the other. To test this hypothesis, we generated a double knockout mouse (DKO, Cldn12-/-/ Trpv6D541A/D541A) to assess Ca²⁺ homeostasis using metabolic cage balance studies, quantitative real-time PCR, and micro-computed tomography (Micro-CT). Despite lacking these key Ca²⁺ transport mechanisms, DKO mice maintained normal blood Ca²⁺. However, PTH and calcitriol were significantly elevated, as were renal Cyp27b1 and Cyp24a1 expression, consistent with hormonal compensation. The proximal colon and cecum exhibited significant compensatory changes in Ca²⁺-regulatory gene expression, including upregulation of the Ca²⁺ buffer S100g and basolateral extrusion mechanism Atp2b1. Notably, the Ca²⁺ channel Trpv5, which is absent from the intestine, became detectable in the proximal colon and cecum, but not in the distal colon or small intestine of DKO mice. Micro-CT of bone revealed reduced trabecular bone volume and thickness, indicating increased bone resorption as a secondary compensatory mechanism. This study highlights the adaptive plasticity of Ca²⁺-regulatory mechanisms and suggests a role for TRPV5 in reabsorbing Ca²⁺ from the proximal colon and cecum when upstream mechanisms of Ca²⁺ absorption are impaired.