Calcium signaling: a tale for all seasons

Abstract

An experiment performed in London nearly 120 years ago, which by today's standards would be considered unacceptably sloppy, marked the beginning of the calcium (Ca(2+)) signaling saga. Sidney Ringer [Ringer, S. (1883) J. Physiol. 4, 29-43] was studying the contraction of isolated rat hearts. In earlier experiments, Ringer had suspended them in a saline medium for which he admitted to having used London tap water, which is hard: The hearts contracted beautifully. When he proceeded to replace the tap water with distilled water, he made a startling finding: The beating of the hearts became progressively weaker, and stopped altogether after about 20 min. To maintain contraction, he found it necessary to add Ca(2+) salts to the suspension medium. Thus, Ringer had serendipitously discovered that Ca(2+), hitherto exclusively considered as a structural element, was active in a tissue that has nothing to do with bone or teeth, and performed there a completely novel function: It carried the signal that initiated heart contraction. It was a landmark observation, which should have immediately aroused wide interest. Unexpectedly, however, for decades it attracted no particular attention. Occasionally, farsighted pioneers argued forcefully for a messenger role of Ca(2+), offering compelling experimental evidence. Among them, one could quote L. V. Heilbrunn [Heilbrunn, L. V. (1940) Physiol. Zool. 13, 88-94], who contracted frog muscle fibers by applying Ca(2+) salts to their cut ends, but not to their surfaces. Heilbrunn correctly concluded that Ca(2+) had diffused from the cut ends to the internal contractile elements to elicit their contraction. One could also quote K. Bailey [Bailey, K. (1942) Biochem. J. 36, 121-139], who showed that the ATPase activity of myosin was strongly activated by Ca(2+) (but not by Mg(2+)), and concluded that the liberation of Ca(2+) in the neighborhood of the myosin controlled muscle contraction. Clearly, enough evidence was there, but only a handful of people had the vision to see it and to foresee its far-reaching implications. Perhaps no better example of clairvoyance can be offered than the quip by O. Loewy in 1959: "Ja Kalzium, das ist alles!"

Figure 1

Decoding of the Ca^{2 +} signal by conformational changes in EF hand proteins (CaM). CaM interacts with a 26-residue binding domain (red peptide, top right) of a skeletal muscle myosin light chain kinase termed M13. CaM (left) has bound Ca²⁺ (yellow shares) to its four EF hands. It has already undergone the change that has made its surface more hydrophobic, but it still is in the fully extended conformation. The interaction with M13 collapses it to a hairpin shape that engulfs the binding peptide.

Figure 2

The Ca²⁺ transporters of animal cell membranes. Plasma membrane (PM) channels are gated by potential, by ligands (e.g., neurotransmitters), or by the emptying of Ca²⁺ stores. Channels in the ER/SR are opened by InsP₃ or cADPr (the cADPr channel is sensitive to ryanodine, and is thus called ryanodine receptor, RyR). The ER/SR channels are shown with a large domain protruding into the cytosol. ATPase (pumps) are found in the PM (PMCA) and in the ER/SR (SERCA). The nuclear envelope, which is an extension of ER, contains the same transporters of the latter. NCXs are located in the PM (NCX) and in the inner mitochondria membrane (MNCX). A uniporter driven by the internal negative potential (−180 Mv) transports Ca²⁺ into mitochondria. A Ca²⁺ pump has also been described in the Golgi (not shown). Ca²⁺-binding proteins are represented with the dumbbell shape typical of CaM.

Figure 3

Crystal structure of the SERCA pump. The structure of the pump is in the Ca²⁺ bound (E1) form. Details of the structure and of the predicted motions of the cytosolic domains are discussed in the text. Some important residues are indicated, including K400, which is the site of interaction with the regulatory protein phospholamban (PLN).

Figure 4

The microdomain concept of mitochondrial Ca²⁺ transport. Ca²⁺ penetrating from outside or released from the ER generates restricted domains of high Ca²⁺ concentration (20 μM or more), adequate to activate the low affinity Ca²⁺ uptake system of neighboring mitochondria. The Ca²⁺-releasing agonist shown is InsP₃; however, other agonists acting on different channels (e.g., cADPr) also generate the Ca²⁺ hotspots.