Diet-dependent hypercalciuria in transgenic mice with reduced CLC5 chloride channel expression

Abstract

Dent's disease is an X-linked inherited disorder characterized by hypercalciuria, nephrocalcinosis, nephrolithiasis, low molecular weight proteinuria, Fanconi's syndrome, and renal failure. It is caused by inactivating mutations in CLC5, a member of the CLC voltage-gated chloride channel family. CLC5 is known to be expressed in the endosomal compartment of the renal proximal tubule, where it may be required for endosomal acidification and trafficking. Although the Fanconi's syndrome and low molecular weight proteinuria in Dent's disease can be explained by disruption of endosomal function in this nephron segment, the pathogenesis of the hypercalciuria in this disease is unknown. We have generated transgenic mice (RZ) with reduced CLC5 expression by introduction of an antisense ribozyme targeted against CLC5. RZ mice are markedly hypercalciuric compared with nontransgenic control mice, at a time when their serum electrolytes and renal function are otherwise normal. This suggests that hypercalciuria in Dent's disease is a direct consequence of CLC5 hypofunction and is not attributable to a gain of function by mutant CLC5, an effect of modifier genes, or a secondary result of nonspecific renal injury. Surprisingly, hypercalciuria in RZ mice is abolished by dietary calcium deprivation, suggesting that the hypercalciuria may be attributable to gastrointestinal hyperabsorption of calcium rather than a renal calcium leak.

The Proximal Tubule and Albuminuria

“…Mutations in ClC-5 in apical endosomes in three different mouse models, as seen in Dent disease, have demonstrated defective receptor-mediated endocytosis and fluid-phase endocytosis, deficient endosomal acidification, decreased internalization of the sodium-phosphate cotransporter 2 and NHE-3, and proteinuria. [13][14][15] Figure 1. Albumin filtration across the glomerulus is greater than previously thought and reclaimed by the PTC, especially S1 cells.…”

Molecular Mechanisms of Primary Hypercalciuria

“…To examine the biologic effects of decreased CLC-5, Luyckx et al (86) designed a hammerhead ribozyme specific for CLC-5 and produced transgenic knockdown mice (RZ). By Western blotting, kidney membranes from RZ mice had an 80% reduction in levels of CLC-5.…”

Molecular Cloning and Characterization of an Intracellular Chloride Channel in the Proximal Tubule Cell Line, LLC-PK1

“…The occurrence of a Fanconi-like syndrome may be explained by internalization of apical membrane sodium-coupled solute transporters into endosomal vesicles but failure to recycle these transporters back to the surface. The hypercalciuria in this disease appears to be absorptive in origin (18) and may be due to abnormal regulation of proximal tubule 25-hydroxyvitamin D 1-hydroxylase activity. How this might be possible, given that the 1-hydroxylase is located on the inner membrane of mitochondria, remains unknown.…”