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

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