Impairment of chaperone‐mediated autophagy leads to selective lysosomal degradation defects in the lysosomal storage disease cystinosis

Abstract

Metabolite accumulation in lysosomal storage disorders (LSDs) results in impaired cell function and multi-systemic disease. Although substrate reduction and lysosomal overload-decreasing therapies can ameliorate disease progression, the significance of lysosomal overload-independent mechanisms in the development of cellular dysfunction is unknown for most LSDs. Here, we identify a mechanism of impaired chaperone-mediated autophagy (CMA) in cystinosis, a LSD caused by defects in the cystine transporter cystinosin (CTNS) and characterized by cystine lysosomal accumulation. We show that, different from other LSDs, autophagosome number is increased, but macroautophagic flux is not impaired in cystinosis while mTOR activity is not affected. Conversely, the expression and localization of the CMA receptor LAMP2A are abnormal in CTNS-deficient cells and degradation of the CMA substrate GAPDH is defective in Ctns−/− mice. Importantly, cysteamine treatment, despite decreasing lysosomal overload, did not correct defective CMA in Ctns−/− mice or LAMP2A mislocalization in cystinotic cells, which was rescued by CTNS expression instead, suggesting that cystinosin is important for CMA activity. In conclusion, CMA impairment contributes to cell malfunction in cystinosis, highlighting the need for treatments complementary to current therapies that are based on decreasing lysosomal overload.

Molecular Basis of Cystinosis: Geographic Distribution, Functional Consequences of Mutations in the <b><i>CTNS</i></b> Gene, and Potential for Repair

“…Another role of CTNS, in addition to its potential involvement in mTORC1 signalling, is the altered chaperone mediated autophagy, as shown by two studies from Napolitano et al [37] and Zhang et al [36]. They observed impaired chaperone mediated autophagy (CMA) in cystinotic mouse skin fibroblasts presenting dislocation of LAMP2A (CMA receptor) and accumulation of GAPDH (CMA substrate).…”

“…This was also true for CTNS-LKG, which is an alternative splice variant of CTNS, primarily targeted to the plasma membrane and other lysosomal/endosomal vesicles. Based on these results, it was suggested that CTNS is a necessary cofactor for LAMP2A trafficking [36,37].…”

Functional and Molecular Insights of Hydrogen Sulfide Signaling and Protein Sulfhydration

“…Metabolite accumulation in lysosomal storage disorders (LSDs) results in impaired cell function and multi-systemic disease [62]. Although substrate reduction and lysosomal overload-decreasing therapies can ameliorate disease progression, the significance of lysosomal overload-independent mechanisms in the development of cellular dysfunction is unknown for most LSDs.…”

Autophagy in renal diseases

“…-The yeast analogue of cystinosin, ERS1, interacts with the yeast analogue of the lysosome, regulating the yeast TOR signalling [80,81] -↑ mitophagy levels in cystinosis fibroblasts and proximal tubular cells [82,83] -↑ levels of autophagosome markers LC3-II and SQSTM1/p62 in cystinosis cells, suggestive for a disrupted autophagic flux [84] -↓ expression and abnormal localisation of LAMP-2A in cystinosis cells [86] Conclusion: Cystinosis is associated with altered autophagy, but the exact mechanisms are unknown.…”

“…However, a decreased expression and abnormal localisation of LAMP-2A, the lysosomal receptor responsible for CMA, was described. Correspondingly, CMA was impaired in cystinosin-deficient cells, as demonstrated by measurement of the degradation of GAPDH, one of the CMA substrates, and this impairment could not be corrected by cysteamine treatment [86].…”