The transient receptor potential cation channelmucolipin 1 (TRPML1) channel is a conduit for lysosomal calcium efflux, and channel activity may be affected by lysosomal contents. The lysosomes of retinal pigmented epithelial (RPE) cells are particularly susceptible to build-up of lysosomal waste products because they must degrade the outer segments phagocytosed daily from adjacent photoreceptors; incomplete degradation leads to accumulation of lipid waste in lysosomes. This study asks whether stimulation of TRPML1 can release lysosomal calciuminRPE cells andwhether such release is affectedby lysosomal accumulations.The TRPM LagonistML-SA1 raised cytoplasmic calcium levels in mouse RPE cells, hesRPE cells, and ARPE-19 cells; this increase was rapid, robust, reversible, and reproducible. The increase was not altered by extracellular calcium removal or by thapsigargin but was eliminated by lysosomal rupture with glycyl-L-phenylalanine-b-naphthylamide. Treatment with desipramine toinhibit acidsphingomyelinase orYM201636 to inhibitPIKfyve also reducedthe cytoplasmic calcium increase triggered by ML-SA1, whereas RPE cells from TRPML-/- mice showed no response to ML-SA1. Cotreatmentwith chloroquine and U18666A induced formation of neutral, autofluorescent lipid in RPE lysosomes and decreased lysosomalCa2+ release.LysosomalCa2+ releasewas also impaired in RPEcells from the ATP-binding cassette, subfamily A, member 4-/-mouse model of Stargardt's retinal dystrophy. Neither TRPML1 mRNA nor total lysosomal calcium levels were altered in these models,suggesting a more direct effect on the channel. In summary, stimulation of TRPML1 elevates cytoplasmic calciumlevels in RPE cells, but this response is reduced by lysosomal accumulation.-Gomez, N. M.,Lu,W.Lim, J. C.,Kiselyov, K.,Campagno, K.E.,Grishchuk,Y., Slaugenhaupt, S. A., Pfeffer, B., Fliesler, S. J., Mitchell, C. H. Robust lysosomal calcium signaling through channel TRPML1 is impaired by lysosomal lipid accumulation. FASEB J. 32, 782-794 (2018). www.fasebj.org. © FASEB.