Molecular and Cellular Mechanisms of the Lysosomal Storage Disease Cystinosis

SUMMARY Lysosomal function is crucial for cell homeostasis, autophagy, nutrient sensing, apoptosis and tissue remodeling. In lysosomal storage disorders (LSDs), characterized by genetic defects leading to anomalous accumulation of metabolites in lysosomes, cells are affected by lysosomal malfunction frequently leading to cell death. Cystinosis is a lysosomal storage disorder resulting from defects in the cystine transporter cystinosin (CTNS). Increased levels of intra-lysosomal cystine lead to cell malfunction and progressive tissue deterioration, which is especially manifested in kidneys. As with most LSDs, this leads to a slow but irreversible deterioration, organ dysfunction and early death. Patients with nephropathic cystinosis develop proximal tubule cell dedifferentiation, Fanconi syndrome and progressive renal injury, which are not corrected by the current therapy, cysteamine. Thus, cell malfunction and tissue failure occur despite cystine depletion, suggesting that cystine accumulation is not the only cause of all the defects observed in cystinosis. We recently revealed a defective mechanism of chaperone-mediated autophagy (CMA) in cystinosis. Defective CMA is directly linked to human disease, including kidney pathologies and neurological disorders. CMA defects in cystinosis are caused by mislocalization and downregulation of the only lysosomal CMA receptor, LAMP2A. Defective CMA activity correlates with high susceptibility to cell death in cystinosis. Importantly, the defect was not rescued by cystine depleting therapies supporting that it is independent of lysosomal overload. Our data highlight that CMA impairment is an important contributor to the pathogenesis of cystinosis and underline the need for new treatments to complement cystine depletion therapies. Our research plan aims to elucidate the molecular and cellular mechanisms leading to abnormal CMA activity in cystinosis. We also propose translational approaches that utilize small-molecule activators of CMA to improve cellular function in cystinosis. Our Specific Aims are: Aim 1: To understand the molecular basis of the regulation of LAMP2A function in cystinosis. To this end, we will study the interplay between the CTNS protein and the CMA receptor LAMP2A and elucidate the mechanisms that mediate LAMP2A trafficking and destabilization at the lysosomal membrane in cystinosis. Aim 2: To determine the molecular basis of the regulation of CMA activity and proximal tubule cell function in cystinosis. We will study the mechanisms mediated by CTNS to regulate CMA function and will test the hypothesis that the rescue of LAMP2A expression and CMA activity improves the function of proximal tubule cells from cystinotic patients. Aim 3: To utilize small-molecule CMA activators in vivo to improve renal function in cystinotic mice. We will correct cellular and renal function in cystinotic mice using CMA activators, alone, or in combination with cysteamine. Our research is highly significant because it aims to elucidate molecular mechanisms associated with a devastating human pathology and will help develop new therapies for the treatment of cystinosis and other human diseases.