Activating Mutations Reveal a Role of TRPML1 in Lysosomal Exocytosis

The contents of lysosomes undergo exocytosis (lysosomal exocytosis) in response to an increase of intracellular Ca2+. Emerging evidence suggests that lysosomal exocytosis plays important roles in a variety of cell biological functions including neurotransmitter release, neurite outgrowth, and plasma membrane repair. The putative Ca2+ channel in the lysosome that mediates intralysosomal Ca2+ release, however, has not been identified. The mucolipin TRP (TRPML) proteins are a family of endolysosomal cation channels with genetically established importance in man and rodent. Mutations of human TRPML1 cause type IV mucolipidosis, a devastating pediatric neurodegenerative disease. In this study, we found that although TRPML1-mediated currents can only be recorded in late endosome and lysosome (LEL) using the lysosome patch clamp technique, several proline substitutions in TRPML1 (such as TRPML1V432P) display gain-of-function (GOF) constitutive activities at both the plasma membrane (PM) and endolysosomal membranes. Although wild-type TRPML1 localized exclusively in LEL and were barely detectable in the PM, the GOF mutants were not restricted to LEL compartments, and most significantly, exhibited significant surface expression. As a Ca2+-permeable channel, the constitutive Ca2+ permeability due to Pro substitutions may allow TRPML1 proteins traffic to the PM via Ca2+-dependent lysosomal exocytosis, resulting in the surface expression and whole cell currents of TRPML1. Consistent with the hypothesis, surface staining of lysosome-associated membrane protein-1 (Lamp-1) was dramatically increased in cells expressing GOF TRPML1 channels. Interestingly, the extent of the lysosomal exocytosis appeared to be correlated with the degree of channel gain-of-function of TRPML1 mutants. Our results suggest that upon unidentified cellular stimulations, TRPML1 mediates intralysosomal Ca2+ release to trigger lysosomal exocytosis. Currently we are investigating whether inhibiting exocytosis could reduce surface expression of GOF TRPML1 mutants, and whether stimulating exocytosis could enhance surface expression of TRPML1.