IP3 Receptors Preferentially Associate with ER-Lysosome Contact Sites and Selectively Deliver Ca2+ to Lysosomes

Inositol 1,4,5-trisphosphate (IP 3 ) receptors (IP 3 Rs) allow extracellular stimuli to redistribute Ca 2+ from the ER to cytosol or other organelles. We show, using small interfering RNA (siRNA) and vacuolar H + -ATPase (V-ATPase) inhibitors, that lysosomes sequester Ca 2+ released by all IP 3 R subtypes, but not Ca 2+ entering cells through store-operated Ca 2+ entry (SOCE). A low-affinity Ca 2+ sensor targeted to lysosomal membranes reports large, local increases in cytosolic [Ca 2+ ] during IP 3 -evoked Ca 2+ release, but not during SOCE. Most lysosomes associate with endoplasmic reticulum (ER) and dwell at regions populated by IP 3 R clusters, but IP 3 Rs do not assemble ER-lysosome contacts. Increasing lysosomal pH does not immediately prevent Ca 2+ uptake, but it causes lysosomes to slowly redistribute and enlarge, reduces their association with IP 3 Rs, and disrupts Ca 2+ exchange with ER. In a "piston-like" fashion, ER concentrates cytosolic Ca 2+ and delivers it, through large-conductance IP 3 Rs, to a low-affinity lysosomal uptake system. The involvement of IP 3 Rs allows extracellular stimuli to regulate Ca 2+ exchange between the ER and lysosomes.