Functional Interactions in Ca2+ Signaling over Different Time and Distance Scales

Cytosolic Ca2+ plays a central role in the regulation of numerous aspects of cellular activity in virtually all cell types. Much of this versatility arises through the diverse mechanisms by which Ca2+ signals are generated and transmitted to act over very different time and distance scales. For example, Ca2+ can act in a very fast and highly localized manner, as in the triggering of neurotransmitter release within microseconds of Ca2+ entry through voltage-gated channels closely apposed to active release zones (Neher 1998); or it can evoke slower responses involving global Ca2+ elevations throughout the cell, as in the generation of Ca2+-dependent Cl− currents by Ca2+ waves that take many seconds to sweep across Xenopus oocytes (Parker and Yao 1994). A common starting point in considering these differences is that a Ca2+ in the cytosol can move only by passive diffusion, a process that is further modified by the presence of endogenous mobile and immobile Ca2+ buffers, so that precipitous Ca2+ gradients (microdomains) exist around an open Ca2+ channel. The concentration near the channel mouth may be 100 μM or more, whereas concentrations as close as 1 or 2 μm fall below 1 μM (Naraghi and Neher 1997; Rios and Stern 1997; Neher 1998). Therefore, Ca2+ has only a restricted “range of action,” in the order of 5 μm (Allbritton et al. 1992). Furthermore, the mean distance a Ca2+ diffuses decreases as a square root function of time. Whereas the Ca2+ concentration at the mouth of a Ca2+ channel will track almost instantly the opening and closing of the pore, the kinetics are slowed by ∼0.1 ms at a distance of 100 nm, 10 ms at 1 μm, and by 1s at 10 μm (assuming an apparent diffusion coefficient of 17 μm2s−1 for Ca2+ in cytosol). The characteristics and specificity of Ca2+ signaling systems can, therefore, be tuned by using sensors with differing affinities located at different distances from a Ca2+ source. A Ca2+-activated effector with low (tens of micromolars) affinity located close to a Ca2+ channel will signal rapidly, whereas an effector with higher affinity situated further away will respond more slowly, but will integrate Ca2+ signals across a wider area. On the basis of passive diffusion alone, one would expect the action of Ca2+ entering the cell to be restricted to a subplasmalemmal shell a few micrometers thick. However, cells possess further mechanisms that allow these local signals to be amplified and propagated globally throughout the cell interior. Both of the major families of Ca2+ channel that mediate Ca2+ release from intracellular Ca2+ stores (inositol trisphosphate receptors [IP3R] and ryanodine receptors [RyR]) display a property of Ca2+-induced Ca2+ release (CICR), so that long-range signaling is possible through the generation of actively propagating Ca2+ waves that travel at velocities of a few tens of micrometers/second by successive cycles of Ca2+ release, diffusion, and CICR (Berridge 1997). Here also, the spatial organization of Ca2+ signaling plays a crucial role. Ca2+ release channels are not usually distributed homogeneously, but are clustered at functional release sites spaced a few micrometers apart, so that Ca2+ waves propagate in a saltatory manner, jumping from site to site. Microdomains of Ca2+ exist within the wavefront and, in some circumstances, individual release sites can be activated in isolation to produce local, “elementary” Ca2+ signals (Parker et al. 1996; Berridge 1997). On a more macroscopic scale, subcellular variations in the properties and density of release sites, as well as in the distribution of other Ca2+ handling systems such as mitochondria, dictate the spatial organization of global Ca2+ signals; i.e., where Ca2+ waves arise in the cell, what direction they travel in, and whether they propagate robustly or become abortive and fizzle out. Progress in understanding the complex spatiotemporal organization of Ca2+ signaling has been greatly facilitated by the development of efficient fluorescent Ca2+ indicators together with confocal imaging systems with improved temporal ( 2 orders of magnitude.