Lysosomes shape IP3-evoked Ca2+ signals by selectively sequestering Ca2+ released from the ER

Cristina I. Lopez Sanjurjo, Stephen C. Tovey, David L. Prole, Colin W. Taylor. Department of Pharmacology. University of Cambridge, Tennis Court Road, Cambridge, CB2 1PD, UK

Most intracellular Ca2+ signals result from opening of Ca2+ channels in the plasma membrane or ER, and they are reversed by active transport across these membranes or by shuffling Ca2+ into mitochondria. Ca2+ channels in lysosomes contribute to endo-lysosomal trafficking and Ca2+ signalling, but the role of lysosomal Ca2+ uptake in Ca2+ signalling is unexplored.

Inhibition of lysosomal Ca2+ uptake by dissipating the H+ gradient (bafilomycin A1), perforating lysosomal membranes (glycyl-L-phenylalanine 2-naphthylamide, GPN) or lysosome fusion (vacuolin) increased the Ca2+ signals evoked by receptors that stimulate inositol 1,4,5-trisphosphate (IP3) formation (1.93 ± 0.2, 1.48 ± 0.1 and 1.45 ± 0.1-fold increase to maximal CCh in the presence of bafilomycin A1, GPN and vacuolin, respectively). Bafilomycin A1 amplified the Ca2+ signals evoked by photolysis of caged IP3 (1.3 ± 0.05-fold increase) or by inhibition of ER Ca2+ pumps (1.8 ± 0.2-fold increase), and it slowed recovery from them (t1/2 18.9 ± 1.4 and 45.6 ± 7.9 s for control and bafilomycin A1-treated cells, respectively). Ca2+ signals evoked by store-operated Ca2+ entry were unaffected by bafilomycin A1 (488 ± 28 and 381 ± 68 nM increase in intracellular Ca2+ in response to 30 mM extracellular CaCl2 in control and bafilomycin A1-treated cells). Video-imaging with total internal reflection fluorescence microscopy revealed that lysosomes were motile and remained intimately associated with ER. Close association of lysosomes with ER allows them selectively to accumulate Ca2+ released by IP3 receptors.