Introduction: Calcium transport is regulated by active transcellular and passive paracellular pathways which are responsible for calcium (re)absorption in the intestine and kidney. The activity of passive transport is determined by the permeability of tight junction. In general, the permeability of tight junction is inversely proportional with the expression of tight junction genes. Claudin and occludin among the tight junction genes are concerned directly with permeability of the tight junction. In this study, we examined the effect of reduced dietary supply of calcium and vitamin D on paracellular gene expression in the calbindin-D9k (CaBP-D9k), -D28k (CaBP-D28k), and -D9k/D28k knockout (KO) mice.
Methods: The tissue-specific mRNA and protein expressions of tight junction genes in the duodenum and kidney, the main organs for calcium (re)absorbtion, were examined using real-time PCR and western blot analysis. The localization of tight junction genes was also investigated by immunohistochemistry.
Results: The expression levels of tight junction genes showed similar patterns in CaBP-D9k and -D28k single KO mice. The most of tight junction genes in the intestine were less expressed in CaBP-D9k KO mice than wild-type (WT), while they were over expressed in the kidney. When the calcium or vitamin D deficient diet was given, this difference was accelerated. In addition, both calcium and vitamin D deficient diet in the KO mice showed additive effects on the tissue-specific regulation of tight junction gene expression. Immunohistochmical staining results indicated that the tight junction genes were mainly localized in apical site of paracellular region in both intestine and kidney.
Conclusions: The ablation of CaBPs and/or reduced dietary calcium resulted in the decreased expression of paracellular barrier genes, therefore, this may lead to acceleration of the paracellular transport of calcium.
Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.