Generic Signals and Specific Outcomes: Signaling through Ca2+, Calcineurin, and NF-AT

Why are drugs such as cyclosporin A and FK506 effective and specific immunosuppressants when their target, calcineurin is present in all tissues? A counterintuitive answer to this question arises from the unusual mechanism of action of these drugs (Liu et al. 1991xLiu, J, Farmer, J.D, Lane, W.S, Friedman, J, Weissman, I, and Schreiber, S.L. Cell. 1991; 66: 807–815Abstract | Full Text PDF | PubMed | Scopus (2779)See all ReferencesLiu et al. 1991). At therapeutic concentrations, cyclosporin and FK506 bind to their intracellular receptors, cyclophilin and FKBP, respectively, forming inhibitory complexes that bind calcineurin and block its phosphatase activity. Since the levels of calcineurin vary widely between different cell types (Klee 1998xKlee, C.B. J. Biol. Chem. 1998; 273: 13367–13370Crossref | PubMed | Scopus (643)See all ReferencesKlee 1998 and references therein) low concentrations of these drugs would be expected to have a far greater effect on cells with lower cytoplasmic concentrations of calcineurin than cells with higher concentrations of calcineurin. In accord with this hypothesis, varying the concentration of either calcineurin, FKBP, or cyclophilin by transfection shifts the dose–response curve to cyclosporin and FK506 in the directions predicted by the laws of mass action. For example, transfection of calcineurin A and B chains renders cells resistant to cyclosporin. Based on published affinities and intracellular concentrations of FKBP, simple calculations predict that 10 nM FK506 would be expected to inhibit about 70% of calcineurin activity in lymphocytes, which have about 5000 molecules of calcineurin per cell, but only 4% of the calcineurin activity of hippocampal cells or cardiac muscle cells, which have about 200,000 molecules of calcineurin per cell (Klee 1998xKlee, C.B. J. Biol. Chem. 1998; 273: 13367–13370Crossref | PubMed | Scopus (643)See all ReferencesKlee 1998 and references therein). These considerations are compatible with the finding that higher concentrations of cyclosporin or FK506, often in the range of 1 μ M as opposed to 10 nM for lymphocytes, are required for exploring the actions of calcineurin in memory (Winder et al. 1998xWinder, D.G, Mansuy, I.M, Osman, M, Moallem, T.M, and Kandel, E.R. Cell. 1998; 92: 25–37Abstract | Full Text | Full Text PDF | PubMed | Scopus (257)See all ReferencesWinder et al. 1998) and myocardial function (17xMolkentin, J.D, Lu, J.R, Antos, C.L, Markham, B, Richardson, J, Robbins, J, Grant, S.R, and Olson, E.N. Cell. 1998; 93: 215–228Abstract | Full Text | Full Text PDF | PubMed | Scopus (1819)See all References, 21xSussman, M.A, Lim, H.W, Gude, N, Taigen, T, Olson, E.N, Robbins, J, Colbert, M.C, Gualberto, A, Wieczorek, D.F, and Molkentin, J.D. Science. 1998; 281: 1690–1693Crossref | PubMed | Scopus (347)See all References). Furthermore, in certain cell types the levels of calcineurin may actually exceed the concentration of FKBP or cyclophilin, making them resistant to even the highest drug concentrations.Although the mechanisms outlined above are only a few of possibly many that impose biologic and pharmacologic specificity on generic calcium signals, they provide concrete examples of ways that generic signals can be channeled into specific responses. Similar mechanisms for achieving specificity are likely to apply to the MAP kinase, JNK, and other signaling pathways that appear to be ubiquitous and also interface with the Ca2+/calcineurin/NF-AT pathway.