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Loss of CLC‐5 reduces endocytic capacity and alters cholesterol distribution in the proximal tubule

Dent disease is a progressive X‐linked disorder affecting the kidney proximal tubule (PT) caused by loss of function of the Cl−/H+ exchanger, CLC‐5. Early symptoms include low molecular weight (LMW) proteinuria resulting from inefficient reabsorption of filtered proteins by the PT. However, the role of CLC‐5 in this process in unclear. Knockout of Clc‐5 in mice recapitulates the LMW proteinuria observed in human disease and causes decreased PT levels of megalin and cubilin, the receptors that bind to and recover filtered proteins. How loss of CLC‐5 leads to reduced receptor expression remains unknown. We used a previously developed opossum kidney (OK) cell culture model that recapitulates morphologic and functional features of the PT in vivo to study the role of Clc‐5 in the endocytic pathway. In Clc‐5 siRNA knockdown (KD) and CRISPR/Cas9 mediated Clc‐5 knockout (KO) cells, there was a significant decrease in endocytic capacity that was rescued by heterologous expression of wild‐type human CLC‐5. Additionally, we used CRISPR/Cas9 technology to generate Clc‐5 knockout mice and confirmed LMW proteinuria in the KO. Heterozygous females also have reduced PT albumin uptake and megalin expression. Previous gene expression studies in Clcn5 KO mice suggest there are alterations in cholesterol and lipid metabolic enzymes, and elevated cholesterol levels have been shown to alter recycling endosome organization and impair fast recycling. We hypothesize that loss of CLC‐5 disrupts cholesterol homeostasis, resulting in reduced recycling and concomitantly increased degradation of megalin. Consistent with this, we observed an accumulation and redistribution of cholesterol in Clc‐5 KD OK cells. We are currently working to quantify cholesterol in Clc‐5 KO OK cells. In female mice with only one functional copy of Clc‐5, we observed a dramatic a change in the distribution of cholesterol in the PT when compared to WT mice. Current studies are focused on quantifying the trafficking kinetics and distribution of megalin in the PT in the absence of CLC‐5 and determining whether altered cholesterol metabolism contributes to the reduced endocytic capacity of the PT and resulting LMW proteinuria in Dent disease.