The Wellcome Prize Lecture

In eukaryotic cells, hormones and neurotransmitters that engage the phosphoinositide pathway evoke a biphasic increase in intracellular free Ca 2+ concentration: an initial transient release of Ca 2+ from intracellular stores is followed by a sustained phase of Ca 2+ influx. This influx is generally store‐dependent and is required for controlling a host of Ca 2+ ‐dependent processes ranging from exocytosis to cell growth and proliferation. In many cell types, store‐operated Ca 2+ entry is manifest as a non‐voltage‐gated Ca 2+ current called I CRAC (Ca 2+ release‐activated Ca 2+ current). Just how store emptying activates CRAC channels remains unclear, and some of our recent experiments that address this issue will be described. No less important from a physiological perspective is the weak Ca 2+ buffer paradox: whereas macroscopic (whole cell) I CRAC can be measured routinely in the presence of strong intracellular Ca 2+ buffer, the current is generally not detectable under physiological conditions of weak buffering following store emptying with the second messenger Ins P 3 . In this review, I describe some of our experiments aimed at understanding just why Ins P 3 is ineffective under these conditions and which lead us to conclude that respiring mitochondria are essential for the activation of I CRAC in weak intracellular Ca 2+ buffer. Mitochondrial Ca 2+ uptake also increases the dynamic range over which Ins P 3 functions as the second messenger that controls Ca 2+ influx. Finally, we find that Ca 2+ ‐dependent slow inactivation of Ca 2+ influx, a widespread but poorly understood phenomenon that helps shape the profile of an intracellular Ca 2+ signal, is regulated by mitochondrial Ca 2+ buffering. Thus, by enabling macroscopic store‐operated Ca 2+ current to activate and then by controlling its extent and duration, mitochondria play a crucial role in all stages of store‐operated Ca 2+ influx. Store‐operated Ca 2+ entry reflects therefore a dynamic interplay between endoplasmic reticulum, mitochondria and plasma membrane.