THE ROLE OF MUCOLIPIN-1 IN PATHOGENESIS OF THE LYSOSOMAL STORAGE DISEASE MUCOLIPIDOSIS TYPE IV

Lysosomal storage diseases (LSDs) are a group of inherited disorders that are caused by the defective activity of lysosomal proteins, resulting in the intracellular accumulation of undigested lysosomal metabolites. Mucolipidosis type IV (MLIV) is a neurodegenerative, recessive disease that results in the accumulation of undigested material in most tissue types. MLIV is caused by mutations in the MCOLN1 gene, encoding the transient receptor potential (TRP) cation channel family member mucolipin-1 (TRP-ML1). While previous work has provided insight into the role of this protein in progression of MLIV, a detailed knowledge of TRP-ML1 function and subsequent role in MLIV pathogenesis remains unclear. The aim of this project was to gain a better understanding of TRP-ML1 function and to provide further insight into the molecular mechanisms behind MLIV pathogenesis. Since TRP-ML1 is lysosomally localized and is subject to proteolysis, I wanted to first examine the trafficking and characterize the processing of this protein. TRP-ML1 undergoes cleavage within its first extracellular loop during its biosynthetic delivery to lysosomes. The lysosomal delivery of TRP-ML1 is impaired by depletion of the Adaptor Protein (AP) complex AP-1, while proteolysis remains unaffected in cells lacking AP-1, indicating that cleavage may occur in a pre-lysosomal compartment. Current models of MLIV pathogenesis suggest that TRP-ML1 directly regulates the postendocytic membrane trafficking by mediating interactions between late endosomes and lysosomes. Acute down-regulation of TRP-ML1 reveals that the lysosomal delivery and degradation of various markers is unperturbed, arguing against this model for MLIV pathogenesis. However, LDL-derived cholesterol ester hydrolysis in MLIV fibroblasts was found to be selectively impaired, suggesting that loss of TRP-ML1 alters lysosomal hydrolytic activity and has a cumulative effect on lysosome function.My findings support a role for TRP-ML1 in maintenance of the ionic balance that is critical for proper lysosome function. This work is significant because it shifts focus away from the concept that TRP-ML1 only functions to regulate specific membrane trafficking events along the endocytic pathway. These results contribute to a more complete understanding of the physiological role of TRP-ML1 and broaden current understanding of MLIV progression, providing the basis for potential therapeutic treatment strategies