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

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.

Atakpa P ，Thillaiappan NB ，Mataragka S ，Prole DL ，Taylor CW ... -  《Cell Reports》

IP(3) receptors and store-operated Ca(2+) entry: a license to fill.

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are widely expressed intracellular Ca2+ channels that evoke large local increases in cytosolic Ca2+ concentration. By depleting the ER of Ca2+, IP3Rs also activate store-operated Ca2+ entry (SOCE). Immobile IP3Rs close to the plasma membrane (PM) are the only IP3Rs that respond to physiological stimuli. The association of these 'licensed' IP3Rs with the ER-PM junctions where STIM interacts with Orai PM Ca2+ channels may define the autonomous functional unit for SOCE. Ca2+ entering cells through SOCE can be delivered directly to specific effectors, or it may reach them only after the Ca2+ has been sequestered by the ER and then released through IP3Rs. This 'tunnelling' of Ca2+ through the ER to IP3Rs selectively delivers Ca2+ to different effectors.

Taylor CW ，Machaca K 《-》

The endoplasmic reticulum, not the pH gradient, drives calcium refilling of lysosomes.

Garrity AG ，Wang W ，Collier CM ，Levey SA ，Gao Q ，Xu H ... -  《eLife》

IP(3) receptors and Ca(2+) entry.

Inositol 1,4,5-trisphosphate receptors (IP3R) are the most widely expressed intracellular Ca2+ release channels. Their activation by IP3 and Ca2+ allows Ca2+ to pass rapidly from the ER lumen to the cytosol. The resulting increase in cytosolic [Ca2+] may directly regulate cytosolic effectors or fuel Ca2+ uptake by other organelles, while the decrease in ER luminal [Ca2+] stimulates store-operated Ca2+ entry (SOCE). We are close to understanding the structural basis of both IP3R activation, and the interactions between the ER Ca2+-sensor, STIM, and the plasma membrane Ca2+ channel, Orai, that lead to SOCE. IP3Rs are the usual means through which extracellular stimuli, through ER Ca2+ release, stimulate SOCE. Here, we review evidence that the IP3Rs most likely to respond to IP3 are optimally placed to allow regulation of SOCE. We also consider evidence that IP3Rs may regulate SOCE downstream of their ability to deplete ER Ca2+ stores. Finally, we review evidence that IP3Rs in the plasma membrane can also directly mediate Ca2+ entry in some cells.

Thillaiappan NB ，Chakraborty P ，Hasan G ，Taylor CW ... -  《-》

GPN does not release lysosomal Ca(2+) but evokes Ca(2+) release from the ER by increasing the cytosolic pH independently of cathepsin C.

The dipeptide glycyl-l-phenylalanine 2-naphthylamide (GPN) is widely used to perturb lysosomes because its cleavage by the lysosomal enzyme cathepsin C is proposed to rupture lysosomal membranes. We show that GPN evokes a sustained increase in lysosomal pH (pHly), and transient increases in cytosolic pH (pHcyt) and Ca2+ concentration ([Ca2+]c). None of these effects require cathepsin C, nor are they accompanied by rupture of lysosomes, but they are mimicked by structurally unrelated weak bases. GPN-evoked increases in [Ca2+]c require Ca2+ within the endoplasmic reticulum (ER), but they are not mediated by ER Ca2+ channels amplifying Ca2+ release from lysosomes. GPN increases [Ca2+]c by increasing pHcyt, which then directly stimulates Ca2+ release from the ER. We conclude that physiologically relevant increases in pHcyt stimulate Ca2+ release from the ER in a manner that is independent of IP3 and ryanodine receptors, and that GPN does not selectively target lysosomes.

Atakpa P ，van Marrewijk LM ，Apta-Smith M ，Chakraborty S ，Taylor CW ... -  《-》