Lipid storage disorders block lysosomal trafficking by inhibiting a TRP channel and lysosomal calcium release

Lysosomal lipid accumulation, defects in membrane trafficking and altered Ca 2+ homoeostasis are common features in many lysosomal storage diseases. Mucolipin transient receptor potential channel 1 (TRPML1) is the principle Ca 2+ channel in the lysosome. Here we show that TRPML1-mediated lysosomal Ca 2+ release, measured using a genetically encoded Ca 2+ indicator (GCaMP3) attached directly to TRPML1 and elicited by a potent membrane-permeable synthetic agonist, is dramatically reduced in Niemann–Pick (NP) disease cells. Sphingomyelins (SMs) are plasma membrane lipids that undergo sphingomyelinase (SMase)-mediated hydrolysis in the lysosomes of normal cells, but accumulate distinctively in lysosomes of NP cells. Patch-clamp analyses revealed that TRPML1 channel activity is inhibited by SMs, but potentiated by SMases. In NP-type C cells, increasing TRPML1's expression or activity was sufficient to correct the trafficking defects and reduce lysosome storage and cholesterol accumulation. We propose that abnormal accumulation of luminal lipids causes secondary lysosome storage by blocking TRPML1- and Ca 2+ -dependent lysosomal trafficking. Accumulation of lysosomal lipids is a feature of Niemann'-Picks (NP) disease, but how these lipids contribute to the disease is unclear. In this study, calcium released via the lysosomal TRPML1 channel is shown to be reduced in NP-type C cells, and sphingomyelins are found to inhibit the channel's activity.