IP 3 Receptors Preferentially Associate with ER-Lysosome Contact Sites and Selectively Deliver Ca 2+ to Lysosomes.

Inositol 1,4,5-trisphosphate (IP3 ) receptors (IP3 Rs) allow extracellular stimuli to redistribute Ca2+ 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 Ca2+ released by all IP3 R subtypes, but not Ca2+ entering cells through store-operated Ca2+ entry (SOCE). A low-affinity Ca2+ sensor targeted to lysosomal membranes reports large, local increases in cytosolic [Ca2+ ] during IP3 -evoked Ca2+ release, but not during SOCE. Most lysosomes associate with endoplasmic reticulum (ER) and dwell at regions populated by IP3 R clusters, but IP3 Rs do not assemble ER-lysosome contacts. Increasing lysosomal pH does not immediately prevent Ca2+ uptake, but it causes lysosomes to slowly redistribute and enlarge, reduces their association with IP3 Rs, and disrupts Ca2+ exchange with ER. In a "piston-like" fashion, ER concentrates cytosolic Ca2+ and delivers it, through large-conductance IP3 Rs, to a low-affinity lysosomal uptake system. The involvement of IP3 Rs allows extracellular stimuli to regulate Ca2+ exchange between the ER and lysosomes.