Gilles Charpentier

Generation of Specific Ca2+ Signals from Ca2+ Stores and Endocytosis by Differential Coupling to Messengers

It remains unclear how different intracellular stores could interact and be recruited by Ca(2+)-releasing messengers to generate agonist-specific Ca(2+) signatures. In addition, refilling of acidic stores such as lysosomes and secretory granules occurs through endocytosis, but this has never been investigated with regard to specific Ca(2+) signatures. In pancreatic acinar cells, acetylcholine (ACh), cholecystokinin (CCK), and the messengers cyclic ADP-ribose (cADPR), nicotinic acid adenine dinucleotide phosphate (NAADP), and inositol 1,4,5-trisphosphate (IP(3)) evoke repetitive local Ca(2+) spikes in the apical pole. Our work reveals that local Ca(2+) spikes evoked by different agonists all require interaction of acid Ca(2+) stores and the endoplasmic reticulum (ER), but in different proportions. CCK and ACh recruit Ca(2+) from lysosomes and from zymogen granules through different mechanisms; CCK uses NAADP and cADPR, respectively, and ACh uses Ca(2+) and IP(3), respectively. Here, we provide pharmacological evidence demonstrating that endocytosis is crucial for the generation of repetitive local Ca(2+) spikes evoked by the agonists and by NAADP and IP(3). We find that cADPR-evoked repetitive local Ca(2+) spikes are particularly dependent on the ER. We propose that multiple Ca(2+)-releasing messengers determine specific agonist-elicited Ca(2+) signatures by controlling the balance among different acidic Ca(2+) stores, endocytosis, and the ER.

Co-ordination of Ca(2+) signalling in mammalian cells by the new Ca(2+)-releasing messenger NAADP.

Ca2+ signalling is one of the most important means in mammalian cells of relaying the action of hormones and neurotransmitters. The great diversity of agonist-induced Ca2+ signatures, visualized by optical imaging techniques, can be explained by the production of intracellular messengers triggering Ca2+ release from internal stores and/or by different coupling of Ca2+ release to Ca2+ entry. Several messengers, such as inositol trisphosphate and cyclic ADP-ribose, have been identified to date. More recent studies have reported the important role of a newly discovered Ca2+ releasing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). These studies have shown important interactions of these messengers in the generation of specific Ca2+ signals. NAADP acts at a very low concentration and seems to have a key role in sensitising cyclic ADP-ribose and inositol trisphosphate receptors. These points will be discussed in the present review.

The Ca2+-releasing messenger NAADP, a new player in the nervous system

Many physiological processes are controlled by a great diversity of Ca2+ signals. Within cell, Ca2+ signals depend upon Ca2+ entry and/or Ca2+ release from internal Ca2+ stores. The control of Ca2+-store mobilization is ensured by a family of messengers comprising inositol 1,4,5 trisphosphate, cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). From recent works, new concepts have emerged where activation of the cells by outside stimuli, acting at the plasma membrane, results in the synthesis of multiple Ca2+-releasing messengers which may interact and shape complex Ca2+ signals in the cytosol as well as in the nucleus. This contribution will cover the most recent advances on NAADP signalling with some emphasis on neurons.

Induction of Na+ channel voltage sensitivity in Xenopus oocytes depends on Ca2+ mobilization

An unusual inward current which is slowly elicited in the Xenopus oocyte membrane during sustained depolarization is reportedly carried by Na+. It is thought that Na+ selective channels are in some way induced to become voltage‐sensitive by the depolarization. Earlier studies report that the induction process involves a phospholipase C and a protein kinase C as well as calcium ions. The present work investigated the origins of this calcium in the oocyte. We show that injection of the powerful Ca2+ chelator (BAPTA) in the oocyte, before induction of the Na+ channels, prevented the appearance of the Na+ current, confirming an important role for [Ca2+]i. However, in oocytes perfused with Ca2+‐free medium, induction of the channels could still be obtained, indicating that induction did not depend upon the entry of external Ca2+. Downmodulation of Ca2+ release from inositol 1,4,5‐trisphosphate (InsP3)‐sensitive stores with caffeine and with a low molecular weight heparin resulted in decreased or no Na+ currents. The results are discussed in terms of the contributions from other endogenous calcium‐dependent conductances which can influence the Na+ current amplitudes and time courses. The results presented support the idea that intracellular Ca2+ increase principally due to Ca2+ released from InsP3‐sensitive stores is needed by the enzyme systems to produce the depolarization‐induced activation of the Na+ conductance in the Xenopus oocyte. J Cell Physiol 178:258–266, 1999.