Protection of Cystinotic Mice by Kidney-Specific Megalin Ablation Supports an Endocytosis-Based Mechanism for Nephropathic Cystinosis Progression.

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.

Current medical aspects of pantethine.

Pantethine, the stable disulfide form of pantetheine, is the major precursor of coenzyme A, which plays a central role in the metabolism of lipids and carbohydrates. Coenzyme A is a cofactor in over 70 enzymatic pathways, including fatty acid oxidation, carbohydrate metabolism, pyruvate degradation, amino acid catabolism, haem synthesis, acetylcholine synthesis, phase II detoxification, acetylation, etc. Pantethine has beneficial effects in vascular disease, it able to decrease the hyperlipidaemia, moderate the platelet function and prevent the lipid-peroxidation. Moreover its neuro-endocrinological regulating role, its good influence on cataract and cystinosis are also proved. This molecule is a well-tolerated therapeutic agent; the frequency of its side-effect is very low and mild. Based on these preclinical and clinical data, it could be recommended using this compound as adjuvant therapy.

Strict cysteamine dose regimen is required to prevent nocturnal cystine accumulation in cystinosis.

Cystinosis is an autosomal recessive disorder, caused by mutations in the lysosomal cystine carrier cystinosin, encoded by the CTNS gene. The disease generally manifests with Fanconi syndrome during the first year of life and progresses towards end stage renal disease before the age of 10 years. Cysteamine depletes intralysosomal cystine content, postpones the deterioration of renal function and the occurrence of extra-renal organ damage. Based on the pharmacokinetic data, patients with cystinosis are advised to use cysteamine every 6 h. The aim of this study was (1) to evaluate the cysteamine dose regimen in Dutch patients with cystinosis and (2) to determine morning polymorphonuclear (PMN) leukocyte cystine content 6 h vs 9 h after the last evening cysteamine dose. Only 5/22 of Dutch cystinosis patients ingested cysteamine every 6 h. Morning (8 a.m.) PMN cystine content in 11 examined patients was elevated 9 h after 12.5-15 mg/kg evening cysteamine dose compared to the value 6 h after the ingestion of the same dose (0.73+/-0.81 nmol vs 0.44+/-0.52 nmol cystine/mg protein, p =0.02). In conclusion, only the minority of Dutch cystinosis patients follows the recommended strict cysteamine dose regimen. We provide evidence that cysteamine has to be administered every 6 h, including the night, as it has much better effect for maintaining low PMN cystine levels.