Preface [Hot Topic: Regulators of Calcium Signaling and Cellular Function (Executive Editor: Andreas H. Guse)]

Ca2+ signaling is one of the fundamental intracellular signaling mechanisms. It can be found in almost any cell type regardless of its phylogenetic origin. Accordingly a wide range of cellular functions is regulated by changes in the free cytosolic and / or nucleoplasmic Ca2+ concentration. Among these are contraction of muscle cells, fertilization of oocytes, exocytosis, proliferation of lymphocytes and many others. Since intracellular Ca2+ signaling is essential for these regulated cell functions, almost any exchange of information and cooperation between individual cells in multicellular organisms is based on intracellular Ca2+ signaling processes. Changes in the free cytosolic and / or nucleoplasmic Ca2+ concentration are achieved by different mechanisms: Ca2+ release from intracellular stores and Ca2+ inflow from the extracellular space increase the free cytosolic Ca2+ concentration, while Ca2+ extrusion by calcium pumps in both intracellular and plasma membranes and binding of Ca2+ ions to Ca2+ binding proteins reduces the free cytosolic Ca2+ concentration. In this special issue, experts in the field review recent findings in various areas of Ca2+ signaling. Articles are devoted to biological activity, metabolism and pharmacology of the Ca2+ mobilizing second messengers D-myoinositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate. Additional reviews focus on localized Ca2+ signals, which likely represent the origin of almost any global Ca2+ signal, on Ca2+ entry mechanisms related to the depletion of Ca2+ stores, and last, but not least on the role of Ca2+ pumps, both in plasma membranes and in the cell interior. It was a great pleasure for me to compile this special issue. Hopefully, it will develop into a valuable source for many colleagues within the calcium signaling field, but also for those who intend to enter this fascinating topic.