N-acetylneuraminic acid (NANA) stimulates in situ cyclic AMP production in tentacles of sea anemone (Aiptasia pallida): Possible role in chemosensitization of nematocyst discharge

Abstract

Cnidocytes, the stinging cells of cnidarians, optimally discharge nematocysts in response to combined physical contact and stimulation of specific chemoreceptors. In the tentacles of certain sea anemones, the primary chemoreceptors bind N-acetylated sugars, such as N-acetylneuraminic acid (NANA). Sensitization with NANA predisposes contact-sensitive mechanoreceptors (CSMs) to trigger discharge in response to physical contact. In the ectoderm of sea anemone tentacles, cnidocyte/supporting cell complexes (CSCCs) control and trigger nematocyst discharge. Previous findings have implicated cyclic AMP (cAMP) as a second messenger in NANA-sensitized nematocyst discharge. However, no reports have directly demonstrated that the cAMP content of tentacles changes in response to NANA stimulation. We now show that NANA elevates in situ cAMP levels in a dose-dependent manner in the ectoderm of tentacles from the sea anemone Aiptasia pallida. However, the endoderm of tentacles shows no detectable cAMP response to NANA. The effect of NANA on the cAMP content of the ectoderm is biphasic. Micromolar NANA increases the in situ cAMP level, with a maximal response occurring at 1.8×10-5moll-1 NANA. At higher NANA concentrations, the cAMP content decreases to that of controls. Because the cAMP dose/response curve to NANA coincides precisely with the dose/response curves of NANA-sensitized nematocyst discharge and nematocyst-mediated adhesive force, a second-messenger role for cAMP in NANA-sensitized nematocyst discharge is strongly suggested. The addition of isobutyl-1-methylxanthine (IBMX) to the medium with sea anemones increases tissue cAMP levels both in the absence and in the presence of NANA. However, anesthetizing anemones in sea water containing high levels of Mg2+ blocks the NANA-stimulated cAMP response of the ectoderm. In addition, our results suggest that NANA-stimulated cAMP may activate endogenous cAMP-dependent protein kinase (PKA) in broken cell preparations of tentacles. Thus, NANA-stimulated cAMP may function as a second messenger in the NANA chemosensory signaling pathway controlling nematocyst discharge.