Determining the mechanism of pathogenesis of mucolipidosis type IV and related lysosomal storage disorders for development of novel therapies

Mucolipidosis type IV (MLIV) is a rare, autosomal recessive, neurodegenerative, lysosomal storage disorder. MLIV is caused by mutations in a gene (MCOLN1) encoding a TRP channel family member known as Mucolipin 1 or TRPML1. TRPML1 is a lysosomal transmembrane protein that appears to be required for normal lysosomal pH regulation, recycling of molecules and membrane reorganisation including lysosomal biogenesis, fusion and exocytosis. The exact function of the channel is unknown but it is permeable to multiple ions including Ca2+, Na+ and K+, possibly also Fe2+ and Zn2+. How normal TRPML1 function regulates lysosomal processes is not clearly understood. Mutations in the MCOLN1 gene can lead to complete loss of TRPML1 function, partial loss of function or mislocalisation, all of which lead to lysosomal dysfunction, lysosomal lipid storage and ultimately neurodegeneration. The disease processes that lead to neurodegeneration are poorly understood and at present no therapy exists for MLIV. We have discovered that TRPML1 results in regulating lysosomal Ca2+ homeostasis that is the opposite of the Ca2+ dysregulation associated with Niemann-Pick type C disease (NPC). Our findings indicate that disrupted function of TRPML1 leads to enhanced Ca2+ release via the NAADP receptor, recently shown to be the lysosomal two-pore channel TPC2. This indicates that TRPML1 is not the NAADP receptor as suggested by others, indeed NAADP mediated Ca2+ release is enhanced with multiple NAADP induced lysosomal Ca2+ release events occurring in TRPML1 null cells compared to single releases in normal cells. This phenotype appears to be responsible for the cellular dysfunction associated with MLIV disease cells, enhanced lysosomal fusion, defective endocytosis and potentially even altered lysosomal pH. Several of these phenotypes are normalised by the NAADP receptor specific antagonist Ned-19. These findings illustrate that the NAADP receptor is central to MLIV disease pathology and may be a novel candidate for disease therapy.