Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA095

Poster Communications

Ca2+ tunneling as a selective intracellular signaling pathway.

R. Courjaret1, M. Dib1, K. Machaca1

1. Research, WCM-Q, Doha, Qatar.

Intracellular calcium is the most ubiquitous second messenger and is central to key events such as fertilization and cell death. The Ccells have only two main sources connected to the cytosol through ion channels to create intracellular Ca2+ signals: the endoplasmic reticulum (ER) and the extracellular space. Because of the multitude of Ca2+targets in a cell, the signals have to be very precisely encoded in time and space. The relative localization of the Ca2+source and of the Ca2+ targets are therefore of key importance. In the case of Store Operated Ca2+entry (SOCE), effectors have to be located in the immediate vicinity of the Ca2+ source as SOCE cannot activate efficiently targets away from the plasma membrane (PM). One way to deliver Ca2+ from SOCE to distant targets is to use the tunneling process where the ER cisterns are used as a pipeline taking Ca2+ from the SOCE microdomain and releasing it through IP3Rs to distant effectors. Here we used HeLa cells to study the effect on specific intracellular targets of various Ca2+ mobilizing pathways: SOCE, ER release and tunneling. In HeLa cells mitochondria localize away from the PM and constitute therefore a candidate of choice for tunneling. When Ca2+ is released from the stores following the opening ofstimulation IP3Rs, the mitochondrial Ca2+ levels follow s the rise in cytosolic Ca2+. In contrast, the mitochondria did not respond to Ca2+ influx through while SOCE has little effect. Surprisingly, during tunneling, the large cytoplasmic rise observed was not associated with a significant change in mitochondrial Ca2+. Detailed analysis of the relative localization of the mitochondria and of the ER during store depletion failed to explain the differences in mitochondrial signaling. The kinetics of the cytosolic Ca2+ increase induced by tunneling was however slower than ER release, presumably due to the slow pumping in the ER by the SERCA as compared to Ca2+ release through the IP3Rs. Conversely, when the target was a Ca2+-activated potassium channel, or the PM itself, tunneling was extremely effective in delivering Ca2+ to these sites, whereas and SOCE itself behaved as a poor activator. In contrastOn the opposite, when NFAT translocation was used as a reference for Ca2+ signaling, SOCE was extremely potent while Ca2+ release was ineffective and tunneling only induced a slight enhancement of the SOCE effect. We conclude that Ca2+ entering through SOCE to be release by IP3Rs is preferentially released close to the source (i.e the PM) and generates a weaker rise deep in the cell unable to reach the required conditions to trigger Ca2+ increase in the mitochondria. Recent findings suggest that active IP3Rs indeed located close to the PM surroundingand avoid the SOCE clusters(1). It is therefore clear the spatio-temporal properties of the Ca2+ increase induced by tunneling contribute to activate selective intracellular Ca2+ targets.

Where applicable, experiments conform with Society ethical requirements