Extracellular Application of Nicotinic Acid Adenine Dinucleotide Phosphate Induces Ca2+ Signaling in Astrocytes in Situ

Abstract

Astrocytes are in intimate contact with neurons, in particular with synapses, and are able to sense, react to, and even influence neuronal activity. The astrocytic response to neuronal activity can be most readily detected by observing changes in the intracellular Ca 2ϩ concentration mediated via transmitter receptors (e.g. glutamate, GABA (␥-aminobutyric acid), or acetylcholine) or via other messengers, such as nitric oxide. Ca 2ϩ increases can occur by Ca 2ϩ influx via the plasma membrane as found for the nitric oxide-mediated neuron-glia signaling in the cerebellum (1). Ca 2ϩ can furthermore be released from intracellular stores by activation of metabotropic receptors such as GABA B receptors, which mediate neuron-glia signaling in the hippocampus (2). Indeed, astrocytes express a variety of receptors linked to the Ca 2ϩ mobilization from intracellular stores (3). To produce specific signals with one and the same messenger (Ca 2ϩ ), several second messenger pathways and different sources of Ca 2ϩ are involved. It seems important, at the current stage, to decipher the astrocytic Ca 2ϩ code, since this would lead to further understanding of how these cells can react to and influence the neuronal network.The best investigated second messenger linking activation of metabotropic receptors and Ca 2ϩ release is inositol 1,4,5-trisphosphate (IP 3 ). 2The importance of other intracellular messengers, such as cyclic ADPribose (cADPR), is less well understood (4) or still hypothetical as for the two sphingomyelin metabolites sphingosine 1-phosphate (5) and sphingosylphosphorylcholine (6). Almost a decade ago, a novel intracellular Ca 2ϩ -releasing second messenger was identified in sea urchin eggs, nicotinic acid adenine dinucleotide phosphate (NAADP ؉ ) (7), which binds to an unknown receptor. Since its discovery, there is increasing evidence that NAADP ϩ also has a physiological role in Ca 2ϩ signal transduction in vertebrates, including mammalian cells (8). In the vertebrate nervous system, intracellular Ca 2ϩ release by NAADP ϩ has only been demonstrated from rat brain microsome preparations (9) and in frog motoneurons (10). NAADP ϩ -specific binding sites are found in gray and white matter of rat brain, but the cellular specificity of the binding site has not yet been determined (11). The major way of NAADP ϩ synthesis in mammalian tissue takes place under acidic conditions and in the presence of nicotinic acid from NADP ϩ by the type II transmembrane glycoprotein ADP-ribosyl cyclase CD38. In a neutral or alkaline environment, CD38 exclusively catalyzes the conversion of NAD ϩ into cADPR (12). Apart from cell membrane localization, CD38 has been located to various intracellular organelle membranes and can moreover be internalized by endocytosis (13). Therefore, NAADP ϩ synthesis most likely takes place in membranes of intracellular acidic organelles such as lysosomes or late endosomes, whereas CD38 in the plasma membrane synthesizes cADPR. Regarding the mammalian central nervous system, CD38 expression was demonstr...

Recruitment of NAADP-sensitive acidic Ca2+ stores by glutamate

“…Despite this biochemical data, functional evidence for NAADP signalling in mammalian neurons is scant. In addition to possible extracellular effects of NAADP [49], we have shown intracellular delivery of NAADP in to cortical neurons and PC12 cells mediates Ca 2+ signals that are inhibited by acid store blockers but readily demonstrable following depletion of endoplasmic reticulum Ca 2+ stores with thapsigargin [50;51]. We also confirmed the presence of NAADP receptors in the former preparation using a novel cell permeable acetoxymethyl ester of NAADP [52].…”

Autocrine/Paracrine Function of Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) for Glucose Homeostasis in Pancreatic β-Cells and Adipocytes

“…This is consistent with the findings of Billington et al (24), showing that NAADP is efficiently transported in a Na ϩ -and Ca 2ϩ -dependent manner in RBL-2H3 cells, resulting in an elevation in intracellular Ca 2ϩ . That extracellular application of NAADP is effective in evoking Ca 2ϩ responses in glial and neuronal cells (21)(22)(23) has also been reported, although the response may have been mediated by the adenosine receptors or by purinergic receptors. The impermeability reported previously could be due to a non-conducive condition used.…”

“…In addition to targeting intracellular Ca 2ϩ stores, extracellular application of NAADP has also been reported to induce intracellular Ca 2ϩ changes, possibly via activation of surface adenosine receptors or purinergic receptors (21)(22)(23). That NAADP can be efficiently transported into cells has been shown in a rat basophilic cell line and a neuroblastoma cell line, resulting in elevation of intracellular Ca 2ϩ (24).…”

Emerging Functions of Extracellular Pyridine Nucleotides

“…On the other hand, contractions were markedly attenuated by pharmacological inhibition of P2X receptors, suggesting that NADP(H) may also be able to act directly on purinergic receptors (26). Furthermore, 2 different reports have proposed a role of extracellular NAADP in the activation of purinergic receptors (27,28), although it has also been shown that NAADP and cADPR can be transported into cells (see below).…”

“…The predominantly extracellular or intravesicular location of CD38 raises the question of whether extracellularly produced cADPR and NAADP can cross the plasma membrane (29). Surprisingly, it has been shown that both extracellular cADPR and NAADP enter cells and raise intracellular Ca 2+ concentrations (9,27,30). The entry of cADPR into cells is mediated by CD38 itself as well as by members of both the equilibrative (ENT2) and the concentrative nucleoside transporter (CNT2, CNT3, and especially cs-csg) families, as determined by both pharmacology and molecular biology (9).…”