Calcium signalling, coming of age

Calcium occupies a very special position in the general area of signal transduction. One striking feature that comes immediately to mind is its versatility. Calcium is by far the most versatile of all carriers of information: The list of processes it modulates is so long and diversified that it could be confidently stated that hardly anything of importance in the vital cycle of cells escapes calcium attention. Secondly, calcium has the unique ability to act both as a first messenger (a number of cells contain a calcium receptor on their surface) and as an intracellular second messenger liberated in response to the interaction of other first messengers with cells. One could also add that the concept of a regulator of cell activity was in effect born with calcium, at the time when cell biology was hesitantly moving its first steps: The landmark experiment of Sidney Ringer on calcium and heart muscle contraction is actually now nearly 120 years old. The progress in the area of calcium signalling in the last few years has been very remarkable: This series of mini-reviews, which by necessity is selective, has singled out some topics where particular significant advances have been made, opening new avenues for future expansions of the field.