The Roles of Ca2+ and cAMP in the Nematocyst Discharge of the Sea Anemone Tentacle

Abstract

The phylum Cnidarians are aquatic animals, including jellyfish, hydra, sea anemones, and corals. They are the simplest metazoans having a nervous system and are diploblastic. Cnidarians are obligate predators that capture prey using specialized stinging cells called cnidocytes. The cnidocyte contains a nematocyst, which is a capsule containing an inverted, hollow tubule. Prey contacting the tentacle triggers the nematocyst tubule to rapidly evert; a process called discharge. The everting tubule penetrates and envenomates the prey, which is carried to the mouth by tentacle movements. Generally both chemical and mechanical stimuli are needed to trigger discharge. The cnidocytes, in sea anemones, are surrounded by two or more supporting cells that have chemoreceptors and possibly contact-sensitive mechanoreceptors (CSMs). Therefore, the cnidocyte/supporting cell complex (CSCC) controls nematocyst discharge. This project explores the chemoreceptor signaling pathway controlling nematocyst discharge from tentacles of the sea anemone, Aiptasia pallida. Chemoreceptors respond to N-acetylated sugars, such as N-acetylneuraminic acid (NANA), which occur on prey surfaces. When stimulated, this chemosensory pathway sensitizes CSMs to trigger nematocyst discharge in response to physical contact. Indirect evidence suggested that the NANA chemosensory pathway acts through the intracellular second-messenger, cyclic-AMP (cAMP). We now show that NANA dose-dependently increases in situ cAMP levels in the ectodermal layer of tentacles from A. pallida, but has no effect on the endodermal cAMP content. In addition, NANA activates cAMP-dependent protein kinase (PKA) in whole tentacle homogenates. High levels of extracellular Mg2+ are commonly used to anesthetize excised tentacles and to block discharge. We find that high levels of Mg2+ block the NANAstimulated cAMP increase. This supports the fact that high Mg2+ levels inhibit nematocyst discharge, but calls into question published findings in which NANAinduced changes in stereociliary bundle length of excised, Mg2+ -anesthetized tentacles are attributed to cAMP. We also find that NANA stimulates calcium influx into isolated tentacle ectodermal cells and that the influx is sensitive to various L-type calcium channel blockers, including dihydropyridines. The coincidence of the desensitization region of the nematocyst discharge curve with those NANA concentrations that most stimulate calcium influx, suggests a role of NANA-stimulated calcium influx in desensitization