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Current Molecular Medicine  Volume 4, Number 3, 2004

 

Contents

 

Regulators of Calcium Signaling and Cellular Function

Executive Editor: Andreas H. Guse

 

Multiplicity of Ca²⁺ Messengers and Ca²⁺ Stores: A Perspective from Cyclic ADP-Ribose and NAADP Pp.227-237

Hon Cheung Lee

[Abstract]

 

Regulation of Calcium Signaling by the Second Messenger Cyclic Adenosine Diphosphoribose (cADPR) Pp.239-248

Andreas H. Guse

[Abstract]

 

Structure and Enzymology of ADP-ribosyl Cyclases: Conserved Enzymes that Produce Multiple Calcium Mobilizing Metabolites Pp.249-261

Francis Schuber and Frances E. Lund

[Abstract]

 

Local Ca²⁺ Signals in Cellular Signalling Pp.263-275

N. Macrez and J. Mironneau

[Abstract]

 

The Families of Kinases Removing the Ca²⁺ Releasing Second Messenger Ins(1,4,5)P₃ Pp.277-290

Marcus M. Nalaskowski and Georg W. Mayr

[Abstract]

 

Mechanisms of Phospholipase C-Regulated Calcium Entry Pp.291-301

Gary S. Bird, Omar Aziz, Jean-Philippe Lievremont, Barbara J. Wedel, Mohamed Trebak, Guillermo Vazquez and James W. Putney Jr.

[Abstract]

 

Medicinal Chemistry and Pharmacology of Cyclic ADP-Ribose Pp.303-311

Barry V.L. Potter and Timothy F. Walseth

[Abstract]

 

Inositol 1,4,5-Trisphosphate and its Co-players in the Concert of Ca²⁺ Signalling – New Faces in the Line Up Pp.313-322

Irene Schulz and Elmar Krause

[Abstract]

 

Calcium Pumps of Plasma Membrane and Cell Interior Pp.323-335

Emanuel E. Strehler and Marek Treiman

[Abstract]

 

Abstracts

 

[Back to top] Multiplicity of Ca²⁺ Messengers and Ca²⁺ Stores: A Perspective from Cyclic ADP-Ribose and NAADP

Hon Cheung Lee

 

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.

 

[Back to top] Regulation of Calcium Signaling by the Second Messenger Cyclic Adenosine Diphosphoribose (cADPR)

Andreas H. Guse

 

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.

 

[Back to top] Structure and Enzymology of ADP-ribosyl Cyclases: Conserved Enzymes that Produce Multiple Calcium Mobilizing Metabolites

Francis Schuber and Frances E. Lund

 

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.

 

[Back to top] Local Ca²⁺ Signals in Cellular Signalling

N. Macrez and J. Mironneau

 

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.

 

[Back to top] The Families of Kinases Removing the Ca²⁺ Releasing Second Messenger Ins(1,4,5)P₃

Marcus M. Nalaskowski and Georg W. Mayr

 

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.

 

[Back to top] Mechanisms of Phospholipase C-Regulated Calcium Entry

Gary S. Bird, Omar Aziz, Jean-Philippe Lievremont, Barbara J. Wedel, Mohamed Trebak, Guillermo Vazquez and James W. Putney Jr.

 

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.

 

[Back to top] Medicinal Chemistry and Pharmacology of Cyclic ADP-Ribose

Barry V.L. Potter and Timothy F. Walseth

 

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.

 

[Back to top] Inositol 1,4,5-Trisphosphate and its Co-players in the Concert of Ca²⁺ Signalling – New Faces in the Line Up

Irene Schulz and Elmar Krause

 

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.

 

[Back to top] Calcium Pumps of Plasma Membrane and Cell Interior

Emanuel E. Strehler and Marek Treiman

 

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.