Protection of Cystinotic Mice by Kidney-Specific Megalin Ablation Supports an Endocytosis-Based Mechanism for Nephropathic Cystinosis Progression
Authors: Janssens Virginie , Gaide Chevronnay Héloïse P. , Marie Sandrine , Vincent Marie-Françoise , Van Der Smissen Patrick , Nevo Nathalie , Vainio Seppo , Nielsen Rikke , Christensen Erik I. , Jouret François , Antignac Corinne , Pierreux Christophe E. , Courtoy Pierre J. 
Published: September 2019

Significance Statement
Nephropathic cystinosis is the result of deletion or inactivating mutations of the gene encoding the lysosomal cystine transporter cystinosin, but the extent to which disease progression depends on cystine accumulation or transport-independent effects of cystinosin is unknown. Cysteamine, the current treatment to prevent cystine accumulation, delays progression to renal failure but does not correct the Fanconi syndrome nor does it provide a cure. The authors demonstrate that suppression of endocytosis in kidney proximal tubular cells of cystinosin-deficient mice by genetic excision of megalin/Lrp2 largely prevents cystine accumulation and can help preserve kidney structure and proximal tubular cell differentiation. These observations stress the importance of cystine accumulation in disease progression and provide proof of concept for exploring novel strategies aiming at blocking the megalin pathway.

Background
Deletions or inactivating mutations of the cystinosin gene CTNS lead to cystine accumulation and crystals at acidic pH in patients with nephropathic cystinosis, a rare lysosomal storage disease and the main cause of hereditary renal Fanconi syndrome. Early use of oral cysteamine to prevent cystine accumulation slows progression of nephropathic cystinosis but it is a demanding treatment and not a cure. The source of cystine accumulating in kidney proximal tubular cells and cystine’s role in disease progression are unknown.

Methods
To investigate whether receptor-mediated endocytosis by the megalin/LRP2 pathway of ultrafiltrated, disulfide-rich plasma proteins could be a source of cystine in proximal tubular cells, we used a mouse model of cystinosis in which conditional excision of floxed megalin/LRP2 alleles in proximal tubular cells of cystinotic mice was achieved by a Cre-LoxP strategy using Wnt4-CRE. We evaluated mice aged 6–9 months for kidney cystine levels and crystals; histopathology, with emphasis on swan-neck lesions and proximal-tubular-cell apoptosis and proliferation (turnover); and proximal-tubular-cell expression of the major apical transporters sodium-phosphate cotransporter 2A (NaPi-IIa) and sodium-glucose cotransporter-2 (SGLT-2).

Results

Wnt4-CRE–driven megalin/LRP2 ablation in cystinotic mice efficiently blocked kidney cystine accumulation, thereby preventing lysosomal deformations and crystal deposition in proximal tubular cells. Swan-neck lesions were largely prevented and proximal-tubular-cell turnover was normalized. Apical expression of the two cotransporters was also preserved.

Conclusions
These observations support a key role of the megalin/LRP2 pathway in the progression of nephropathic cystinosis and provide a proof of concept for the pathway as a therapeutic target.