Lysosomal trafficking functions of mucolipin-I in murine macrophages

Background: Mucolipidosis Type IV is currently characterized as a lysosomal storage disorder with defects that include corneal clouding, achlorhydria and psychomotor retardation. MCOLNI, the gene responsible for this disease, encodes the protein mucolipin-I that belongs to the Transient Receptor Potential family of proteins and has been shown to function as a non-selective cation channel whose activity is modulated by pH. Two cell biological defects that have been described in MLIV fibroblasts are a hyperacidification of lysosomes and a delay in the exit of lipids from lysosomes. Results: We show that mucolipin-I localizes to lysosomal compartments in RAW264.7 mouse macrophages that show subcompartmental accumulations of endocytosed molecules. Using stable RNAi clones, we show that mucolipin- 1 is required for the exit of lipids from these compartments, for the transport of endocytosed molecules to terminal lysosomes, and for the transport of the Major Histocompatibility Complex II to the plasma membrane. Conclusion: Mucolipin-I functions in the efficient exit of molecules, destined for various cellular organelles, from lysosomal compartments.