Current Molecular Medicine - Volume 4, Issue 3, 2004

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.

It is generally believed that multiple Ca2+ stores are present in cells, a notion that has now been made substantive by the discovery of multiple Ca2+ mobilizing messengers. Cyclic ADP-ribose (cADPR) and nicotinic acid dinucleotide phosphate (NAADP) are two such messengers that are derived from NAD and NADP, respectively. A wide variety of cells, from plants to mammals, including human, have been shown to be responsive to these two novel Ca2+ messengers. Not only are their structures and mechanisms of action different, their targeted Ca2+ stores are also distinct and separable. This article explores the implications of the multiplicity of Ca2+ stores in cellular signaling. Special emphasis will be put on the recent progress in the understanding of the physiological functions of NAADP.

Ca2+ ions are involved in the regulation of many diverse functions in animal and plant cells, e.g. muscle contraction, secretion of neurotransmitters, hormones and enzymes, fertilization of oocytes, and lymphocyte activation and proliferation. The intracellular Ca2+ concentration can be increased by different molecular mechanisms, such as Ca2+ influx from the extracellular space or Ca2+ release from intracellular Ca2+ stores. Release from intracellular Ca2+ stores is accomplished by the small molecular compounds D-myo-inositol 1,4,5-trisphosphate (InsP3), cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will focus on the effects of cADPR in different cells and tissues, the mechanisms of cADPR-mediated Ca2+ release and Ca2+ entry, extracellular effects of cADPR, and the role of cADPR in a cell system studied in detail, human T-lymphocytes.

Cyclic ADP-ribose is an important calcium mobilizing metabolite produced by the ADP-ribosyl cyclase (cyclases) family of enzymes. Three evolutionarily conserved ADP-ribosyl cyclase superfamily members have been identified, one from the invertebrate Aplysia californica and two from mammalian tissues, CD38 and CD157. CD38 regulates calcium signaling in a number of cell types, and it was recently shown that cyclic ADP-ribose produced by CD38 modulates calcium mobilization induced upon chemokine receptor engagement. Excitingly, because immunocytes deficient in CD38 are unable to migrate to inflammatory sites in vivo, this enzyme has now become an attractive target for drug development. To rationally design inhibitors it is critical to understand the mechanism(s) by which CD38 catalyzes the transformation of its substrate NAD+ into cyclic ADP-ribose. Likewise, it is necessary to identify the CD38 substrate-binding site. Importantly, significant progress has been made in these two areas and much is now known about the structure and enzymology of CD38 and the other ADP-ribosyl cyclase superfamily members. In this review, we will outline the critical data demonstrating a role for CD38 in regulating calcium mobilization in mammalian cells. We will also describe the crystallographic data and site-directed mutagenesis studies that have helped to elucidate the CD38 structure and the identification of its active site and key catalytic residues. Finally, we will address the important advances in our understanding of the kinetic and molecular mechanisms that control cyclic ADP-ribose production by CD38.

Local Ca2+ rises and propagated Ca2+ signals represent different patterns that are differentially decoded for fine tuning cellular signalling. This Ca2+ concentration plasticity is absolutely required to allow adaptation to different needs of the cells ranging from contraction or increased learning to proliferation and cell death. A wide diversity of molecular structures and specific location of Ca2+ signalling molecules confer spatial and temporal versatility to the Ca2+ changes allowing specific cellular responses to be elicited. Various types of local Ca2+ signals have been described. Ca2+ spikes correspond to Ca2+ signals spanning several micrometers but displaying limited propagation into a cell leading to regulation of cellular functions in one particular zone of this cell. This is of particular relevance in cells presenting distinct morphological specializations, i.e. apical versus basal sites or dendritic versus somatic / axonal sites. More stereotyped elementary Ca2+ events (denominated Ca2+ sparks or Ca2+ puffs depending on the type of endoplasmic reticulum Ca2+ release channel involved) are highly confined and nonpropagated Ca2+ rises which are observed in the close neighbouring of the Ca2+ channels. These elementary Ca2+ events play a major role in controlling cellular excitability. Elementary Ca2+ events involve Ca2+ release channels such as the ryanodine receptors (RyRs) and the inositol 1,4,5-trisphosphate receptors (InsP3Rs). The molecular bases underlying the various local Ca2+ release events will be discussed by reviewing the channels and particularly the different isoforms of RyRs and InsP3Rs and their role in inducing localized Ca2+ responses. These calcium release events are controlled by various second messengers and are regulated by Ca2+ channel-associated proteins, intra-luminal Ca2+ content of the endoplasmic reticulum (ER) and other Ca2+ organelles. We will discuss on how the control of local cellular Ca2+ content may account for cellular functions in physiological and physiopathological conditions.

The formation and degradation of the second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] are of great metabolic importance, because of its role in the mediation of calcium release from intracellular stores. The concentration of Ins(1,4,5)P3 in the cell is regulated by three signaling enzymes: phospholipase C isoforms release Ins(1,4,5)P3 from the plasma membrane by hydrolysis of phosphatidyl inositol 4,5-bisphosphate, whereas inositol phosphate 5-phosphatases remove it by dephosphorylation and a group of inositol phosphate kinases eliminate it by further phosphorylation at its 3- or 6-hydroxy group. The latter group is formed by the three isoforms of Ins(1,4,5)P3 3-kinase (IP3K) and inositol phosphate multikinase. In this article the tissue specific gene expression, molecular structure, role in calcium oscillations, regulation by calcium calmodulin, by phosphorylation and by intracellular localization of the IP3K isoforms are discussed. Another important aspect is the evolution of diverse inositol phosphate metabolizing enzymes from a eukaryotic founder by different mechanisms of gene diversification. Finally the role of IPMK in calcium signaling will be elucidated in more detail.

In a variety of cell types, activation of phospholipase C-linked receptors results in the generation of intracellular Ca2+ signals comprised of components of both intracellular Ca2+ release, and enhanced entry of Ca2+ across the plasma membrane. This entry of Ca2+ occurs by either of two general mechanisms: the release of stored Ca2+ can activate, by an unknown mechanism, storeoperated channels in the plasma membrane, a process known as capacitative calcium entry. Alternatively, second messengers generated at the plasma membrane can activate Ca2+ channels more directly, a non-capacitative calcium entry process. This review summarizes current knowledge of the underlying signaling mechanisms and the nature of the channel molecules responsible for these two general categories of regulated Ca2+ entry.

Cyclic ADP-ribose (cADPR) is a signaling molecule that has been shown to regulate calcium mobilization from intracellular stores in a wide variety of biological systems (reviewed in [1-3]). Synthesis of structural analogs of cADPR has provided insights into structure-activity relationships as well as produced pharmacological research tools with useful properties such as, hydrolysis-resistance and cell permeability. The first generation of cADPR analogs was synthesized by a chemo-enzymatic approach that took advantage of the broad substrate specificity of Aplysia ADP-ribosyl cyclase. Analogs synthesized by this approach provided useful structure-activity information, including the importance of the 8-position of the adenine in determining agonistic or antagonistic activity and of the 3'-hydroxyl group of the southern ribose for activity. Hydrolysis resistant analogs were generated by replacing the southern ribose with a carbocyclic structure or by replacing the adenine ring with 7-deazaor 3-deaza-adenine. Approaches to synthesize cADPR analogs by total chemical approaches have been recently reported. These approaches allow the synthesis of analogs with stable linkages between N1 of adenine and the northern ribose (or surrogate) that are not possible with the enzymatic strategy. This review will focus on the synthesis and properties of analogs that have been shown to have utility in dissecting the role of cADPR in calcium signaling.

Release of Ca2+ from intracellular stores can occur by different intracellular messengers such as InsP3, cADPR and NAADP. Although in some cells messengers may operate on different stores, there are also Ca2+ stores with sensitivities for all three of these messengers. It is well documented, that InsP3- and cADPR-sensitive Ca2+ stores are involved in the activation of “storeoperated Ca2+ channels” (SOCC). It has not yet been unequivocally shown, however, if Ca2+ release from stores, which respond to NAADP but not to InsP3 or cADPR, also generate signals which lead to “store-operated Ca2+ entry”. Neither localization nor the mechanism of coupling to the plasma membrane of those InsP3- and cADPR-sensitive Ca2+ stores which activate SOCCs is yet clear. In this review localization and properties of InsP3-, cADPR- and NAADP-sensitive Ca2+ pools and their mutual interactions are discussed. Differential sensitivities of Ca2+ release mechanisms to InsP3, cADPR and NAADP have consequences on Ca2+ release, Ca2+ oscillations, propagation of Ca2+ waves and on activation of SOCC. Possible interaction of InsP3R and cADPR with candidates of SOCCs (TRP channels) and mechanisms involved in the regulation of SOCCs (activation-deactivation) will be elaborated.

Calcium entering the cell from the outside or from intracellular organelles eventually must be returned to the extracellular milieu or to intracellular storage organelles. The two major systems capable of pumping Ca2+ against its large concentration gradient out of the cell or into the sarco / endoplasmatic reticulum are the plasma membrane Ca2+ ATPases (PMCAs) and the sarco / endoplasmic reticulum Ca2+ ATPases (SERCAs), respectively. In mammals, multigene families code for these Ca2+ pumps and additional isoform subtypes are generated via alternative splicing. PMCA and SERCA isoforms show developmental-, tissue- and cell type-specific patterns of expression. Different PMCA and SERCA isoforms are characterized by different regulatory and kinetic properties that likely are optimized for the distinct functional tasks fulfilled by each pump in setting resting cytosolic or intra-organellar Ca2+ levels, and in shaping intracellular Ca2+ signals with spatial and temporal resolution. The loss or malfunction of specific Ca2+ pump isoforms is associated with defects such as deafness, ataxia or heart failure. Understanding the involvement of different Ca2+ pump isoforms in the pathogenesis of disease allows their identification as therapeutic targets for the development of selective strategies to prevent or combat the progression of these disorders.