CALCIUM SIGNALING: ENCODING AND DECODING

Calcium is a universal second messenger that controls many cellular reactions. Considering its pleiotropic role, it seems evident that the cells, to get a specific message in the proper time and manner, need precise and efficient mechanisms to encode and decode Ca2+ signals. Generally, extracellular stimuli are converted in a transient increase in cytosolic Ca2+ concentration, [Ca2+]c, which, in turn, modulates cell function. In the last two decades, improvements in the development of probes and instrumentation for Ca2+ imaging have led to the discovery that the coordinated action of different players is responsible for a complex spatio-temporal organization of the Ca2+ signal. It is intriguing to observe that cells can encode and discriminate Ca2+ signals not only according to their magnitude but also according to their localization (microdomains) and shape; i.e., cells can discriminate between sustained and oscillatory signals. Even more, in the case of oscillations, messages can be read differently according to the frequency of the oscillatory signals. The mechanisms by which cells decode Ca2+ signals are now explored in numerous laboratories. This article focuses on the autoregulation properties of the Ca2+ signals. It will show that Ca2+ itself is central in the regulation of the Ca2+ signal. It will also show that it can act as a first and second messenger and that it can modulate the activity and the availability of the other players in the signaling operation.