A Drosophila model for Dent’s disease type 1 revealed impaired endoplasmic reticulum export of Cubilin as pathogenic mechanism

INTRODUCTION: Pathogenic variants in the CLCN5 gene encoding the chloride-hydrogen exchanger ClC-5 cause Dent's disease type 1, a genetic disorder of the endolysosomal pathway in the proximal tubules of the kidneys. A hallmark of this disease is the downregulation of the protein uptake receptor consisting of megalin, cubilin and amnionless, causing low-molecular-weight proteinuria. Why these receptors are downregulated is not fully understood. METHODS: mice. RESULTS: Upon depletion of ClC-c, the fly ortholog of CLCN5, Cubilin was lost from the plasma membrane of nephrocytes, leading to a strong decrease in albumin uptake and ectopic slit diaphragms. Importantly, Cubilin exhibited a strong accumulation in the endoplasmic reticulum, while its binding partner Amnionless was overall reduced. This was accompanied by a fragmentation of the endoplasmic reticulum (ER) morphology and an increase in ER exit sites and associated Golgi stacks. Additionally, the actin and microtubular cytoskeleton as well as recycling endosomes showed a strong cortical accumulation, whereas cholesterol-enriched autophagic compartments emerged in the perinuclear area. Similar phenotypes were observed upon silencing of the C subunit of the vacuolar proton-ATPase (V-ATPase), suggesting they might depend on defects in acidification and glycosylation in the Golgi apparatus. Amnionless loss and ER retention of cubilin was confirmed in ClC-5 knockout mice, underscoring the relevance of this pathomechanism for Dent's disease. CONCLUSIONS: Our findings suggest that in addition to its endosomal role ClC-c/ClC-5 acts in the secretory pathway to promote surface trafficking of the Cubilin/Amnionless complex. This function may partly explain low-molecular-weight proteinuria in patients with Dent's disease.