| Abstrakt: |
Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) 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 IP3R 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 IP3R clusters, but IP3Rs 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 IP3Rs, and disrupts Ca2+exchange with ER. In a “piston-like” fashion, ER concentrates cytosolic Ca2+and delivers it, through large-conductance IP3Rs, to a low-affinity lysosomal uptake system. The involvement of IP3Rs allows extracellular stimuli to regulate Ca2+exchange between the ER and lysosomes. |
