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