Cystinosin is a component of the vacuolar H+-ATPase-ragulator-rag complex controlling mammalian target of rapamycin complex 1 signaling

Abstract

Cystinosis is a rare autosomal recessive storage disorder characterizedbydefective lysosomal efflux of cystine due tomutations in the CTNS gene encoding the lysosomal cystine transporter, cystinosin. Lysosomal cystine accumulation leads to crystal formation and functional impairment ofmultiple organs.Moreover, cystinosis is the most common inherited cause of renal Fanconi syndrome in children. Oral cysteamine therapy delays diseaseprogression by reducing intracellular cystine levels.However, because cysteamine does not correct all complications of cystinosis, including Fanconi syndrome, we hypothesized that cystinosin could have novel roles in addition totransportingcystineoutof the lysosome.By coimmunoprecipitation experiments andmass spectrometry, we found cystinosin interacts with almost all components of vacuolar H+-ATPase and the Ragulator complex and with the smallGTPases Ras-related GTP-binding protein A (RagA) and RagC. Furthermore, the mammalian target of rapamycin complex 1 (mTORC1) pathway was downregulated in proximal tubular cell lines derived from Ctns2/2 mice. Decrease of lysosomal cystine levels by cysteamine did not rescue mTORC1 activation in these cells, suggesting that the downregulation of mTORC1 is due to the absence of cystinosin rather than to the accumulation of cystine. Our results show a dual role for cystinosin as a cystine transporter and as a component of the mTORC1 pathway, and provide an explanation for the appearance of Fanconi syndrome in cystinosis. Furthermore, this study highlights the need to develop new treatments not dependent on lysosomal cystine depletion alone for this devastating disease.