Uncovering the role of mucolipin‐1 in the pathogenesis of the lysosomal storage disease mucolipidosis type IV

Mucolipin‐1 (ML1) is a member of the transient receptor potential ion channel superfamily that is preferentially localized to late endosomes and lysosomes. Mutations in the ML1 gene result in the autosomal recessive lysosomal storage disease mucolipidosis type IV. ML1 is suggested to be a nonspecific cation channel whose activity is modulated by both Ca 2+ and pH, indicating that this protein may be involved in trafficking or fusion events between endosomes and lysosomes. However, recent studies by Soyombo et al . ( J. Biol. Chem . (2006) 281 :–7301) suggest that ML1 preferentially conducts monovalent cations including H + and that lysosomes from ML1‐deficient fibroblasts accumulate more acridine orange (AO) than control fibroblasts. Furthermore, cells lacking ML1 showed a marked reduction in lipid hydrolysis. To test whether ML1 regulates specific membrane trafficking events or if trafficking is delayed as a secondary result of chronic abnormal lipid turnover, we developed an siRNA‐based approach to acutely knock down ML1. Lipid inclusions accumulated in ML1 knockdown cells over a six day period. In addition, cells lacking ML1 demonstrated increased accumulation of AO similar to patient fibroblasts. Interestingly, we found no defects in transferrin recycling or epidermal growth factor degradation kinetics in cells lacking ML1. Our data support a model in which absence of ML1 channel activity leads to a selective defect in lipid catabolism rather than to a disruption in membrane delivery to lysosomes.