Cyclic ADP-ribose-dependent Ca2+ release is modulated by free [Ca2+] in the scallop sarcoplasmic reticulum

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

From contraction to gene expression: nanojunctions of the sarco/endoplasmic reticulum deliver site- and function-specific calcium signals

Calcium signals determine, for example, smooth muscle contraction and changes in gene expression. How calcium signals select for these processes is enigmatic. We build on the "panjunctional sarcoplasmic reticulum" hypothesis, describing our view that different calcium pumps and release channels, with different kinetics and affinities for calcium, are strategically positioned within nanojunctions of the SR and help demarcate their respective cytoplasmic nanodomains. SERCA2b and RyR1 are preferentially targeted to the sarcoplasmic reticulum (SR) proximal to the plasma membrane (PM), i.e., to the superficial buffer barrier formed by PM-SR nanojunctions, and support vasodilation. In marked contrast, SERCA2a may be entirely restricted to the deep, perinuclear SR and may supply calcium to this sub-compartment in support of vasoconstriction. RyR3 is also preferentially targeted to the perinuclear SR, where its clusters associate with lysosome-SR nanojunctions. The distribution of RyR2 is more widespread and extends from this region to the wider cell. Therefore, perinuclear RyR3s most likely support the initiation of global calcium waves at L-SR junctions, which subsequently propagate by calcium-induced calcium release via RyR2 in order to elicit contraction. Data also suggest that unique SERCA and RyR are preferentially targeted to invaginations of the nuclear membrane. Site- and function-specific calcium signals may thus arise to modulate stimulus-response coupling and transcriptional cascades.

Calcium is required for coelomocyte activation in earthworms

The role of calcium signaling in activation of both innate and adaptive immunity is basically important, however, the evolutionary aspects are not clarified yet. Currently limited data are available about calcium levels of coelomocytes, cellular mediators of earthworm immunity. We aimed to observe basal and induced Ca(2+) levels of coelomocyte subgroups after various stimulations in Eisenia fetida and Allolobophora caliginosa using a Ca(2+)-sensitive dye. E. fetida chloragocytes had the highest basal Ca(2+) levels among subpopulations; however there was no detectable Ca(2+) influx after any stimuli, while coelomocytes showed strong Ca(2+) increase after ionomycin treatment, which could be attenuated using phorbol ester. A. caliginosa coelomocytes showed a weak response to ionophore, while chloragocytes, similar to those in E. fetida, exhibited no changes after this stimulation. Intracellular calcium is mainly stored in the endoplasmic reticulum of coelomocytes as proved by thapsigargin treatments. Among several mitogens only phytohemagglutinin caused increased Ca(2+) level in E. fetida coelomocytes, but not in A. caliginosa coelomocytes. Moreover, the chemoattractant fMLP revealed calcium influx of Eisenia coelomocytes. For the first time we observed various basal Ca(2+) levels and sensibility to Ca(2+) influx inducers (including mitogens and chemoattractant) of coelomocyte subgroups using flow cytometry. These observations suggest that Ca(2+) influx and signal transduction may play crucial roles in the innate immunity of the earthworm.

The role of intracellular ion channels in regulating cytoplasmic calciumin pulmonary arterial mmooth muscle: which store and where?

The mobilisation of intracellular Ca(2+) stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. Moreover, different spatiotemporal Ca(2+) signalling patterns may trigger differential gene expression while mediating the same functional response. These facts alone serve to highlight the importance of the growing body of evidence in support of the view that different Ca(2+) storing organelles may be selected by the discrete or co-ordinated actions of multiple Ca(2+) mobilising messengers. In this respect, it is generally accepted that sarcoplasmic reticulum stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca(2+) mobilising pyridine nucleotides in arterial smooth muscle, namely cyclic adenosine diphosphate-ribose and nicotinic acid adenine dinucleotide phosphate. This review will, therefore, focus on the role of these novel Ca(2+) mobilising messengers in pulmonary arterial smooth muscle, with particular reference to hypoxic pulmonary vasoconstriction.

A flow cytometric approach to study intracellular-free Ca2+ in Crassostrea gigas haemocytes

Bivalve haemocytes are essential in defence mechanisms including phagocytosis. They also produce molecules including hydrolytic enzymes and antimicrobial peptides that contribute to pathogen destruction. Although haemocyte activities have been extensively studied, relatively little is known about the intracellular signalling pathways that are evoked during haemocyte activation and especially the role of calcium. Flow cytometry has been used for the first time to define the effect of cell incubation in haemolymph and artificial sea water (ASW) on Pacific oyster, Crassostrea gigas, haemocytes. Cell viability, enzymatic activities (esterases and aminopeptidases), phagocytosis and granulocyte percentage were analysed. Viability and some activities were different in haemolymph and ASW. Cytoplasmic-free calcium in circulating haemocytes was then investigated by flow cytometry in both media using a calcium probe (Fluo-3/AM). To explore calcium homeostasis, different calcium modulators were tested. The calcium chelator Bapta/AM (10 microM) reduced significantly the percentage of Fluo-3-positive cells in ASW. In addition, ryanodine (5 microM) induced a significant enhancement of the percentage of Fluo-3 positive cells in haemolymph and in ASW. Flow cytometry may be used to study calcium movements in C. gigas haemocytes, but several haemocyte incubation media need to be tested in order to confirm results. The objective of the study should be considered before selecting a particular experimental medium.

Pyridine nucleotides and calcium signalling in arterial smooth muscle: from cell physiology to pharmacology

It is generally accepted that the mobilisation of intracellular Ca2+ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the spatiotemporal pattern of different Ca2+ signals that elicit such responses may also contribute to the regulation of, for example, differential gene expression. These findings, among others, demonstrate the importance of discrete spatiotemporal Ca2+ signalling patterns and the mechanisms that underpin them. Of fundamental importance in this respect is the realisation that different Ca2+ storing organelles may be selected by the discrete or coordinated actions of multiple Ca2+ mobilising messengers. When considering such messengers, it is generally accepted that sarcoplasmic reticulum (SR) stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca2+ mobilising pyridine nucleotides in arterial smooth muscle, namely, cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will therefore focus on these novel mechanisms of calcium signalling and their likely therapeutic potential.

CD38/cyclic ADP-ribose signaling: role in the regulation of calcium homeostasis in airway smooth muscle

The contractility of airway smooth muscle cells is dependent on dynamic changes in the concentration of intracellular calcium. Signaling molecules such as inositol 1,4,5-trisphosphate and cyclic ADP-ribose play pivotal roles in the control of intracellular calcium concentration. Alterations in the processes involved in the regulation of intracellular calcium concentration contribute to the pathogenesis of airway diseases such as asthma. Recent studies have identified cyclic ADP-ribose as a calcium-mobilizing second messenger in airway smooth muscle cells, and modulation of the pathway involved in its metabolism results in altered calcium homeostasis and may contribute to airway hyperresponsiveness. In this review, we describe the basic mechanisms underlying the dynamics of calcium regulation and the role of CD38/cADPR, a novel pathway, in the context of airway smooth muscle function and its contribution to airway diseases such as asthma.

Free radical-dependent Ca2+ signaling: role of Ca2+-induced Ca2+ release

Previously we have shown that Fe3+/ascorbate-induced Ca2+ release from scallop sarcoplasmic reticulum (SR) is due to Ca2+-channel gating by free radicals. This study is aimed at demonstrating that Ca2+-induced Ca2+ release (CICR) plays a role in this kind of Ca2+ release. Scallop SR vesicles were incubated with fluo-3 and exposed to Fe3+/ascorbate. Fluorimetric recordings showed massive Ca2+ release, with maximum rate and 50% release occurring at 30 min after exposure. Conversely, the use of the probe for reactive oxygen species dihydrorhodamine or the assay of malondialdehyde allowed oxyradical production to be traced for approximately 5 min only. Hence, although Ca2+ release started just after exposure to Fe3+/ascorbate, most release occurred after free radical exhaustion. Ruthenium red addition after Fe3+/ascorbate slowed down the Ca2+ release, whereas cyclic adenosine 5'-diphosphoribose addition accelerated it, indicating that the free radical-induced Ca2+ release from SR vesicles triggers a mechanism of CICR that dramatically increases the initial effect.