A vesicular Na+/Ca2+ exchanger in coral calcifying cells.

The calcium carbonate skeletons of corals provide the underlying structure of coral reefs; however, the cellular mechanisms responsible for coral calcification remain poorly understood. In osteoblasts from vertebrate animals, a Na+/Ca2+ exchanger (NCX) present in the plasma membrane transports Ca2+ to the site of bone formation. The aims of this study were to establish whether NCX exists in corals and its localization within coral cells, which are essential first steps to investigate its potential involvement in calcification. Data mining identified genes encoding for NCX proteins in multiple coral species, a subset of which were more closely related to NCXs from vertebrates (NCXA). We cloned NCXA from Acropora yongei (AyNCXA), which, unexpectedly, contained a peptide signal that targets proteins to vesicles from the secretory pathway. AyNCXA subcellular localization was confirmed by heterologous expression of fluorescently tagged AyNCXA protein in sea urchin embryos, which localized together with known markers of intracellular vesicles. Finally, immunolabeling of coral tissues with specific antibodies revealed AyNCXA was present throughout coral tissue. AyNCXA was especially abundant in calcifying cells, where it exhibited a subcellular localization pattern consistent with intracellular vesicles. Altogether, our results demonstrate AyNCXA is present in vesicles in coral calcifying cells, where potential functions include intracellular Ca2+ homeostasis and Ca2+ transport to the growing skeleton as part of an intracellular calcification mechanism.

Identification of a molecular pH sensor in coral.

Maintaining stable intracellular pH (pHi) is essential for homeostasis, and requires the ability to both sense pH changes that may result from internal and external sources, and to regulate downstream compensatory pH pathways. Here we identified the cAMP-producing enzyme soluble adenylyl cyclase (sAC) as the first molecular pH sensor in corals. sAC protein was detected throughout coral tissues, including those involved in symbiosis and calcification. Application of a sAC-specific inhibitor caused significant and reversible pHi acidosis in isolated coral cells under both dark and light conditions, indicating sAC is essential for sensing and regulating pHi perturbations caused by respiration and photosynthesis. Furthermore, pHi regulation during external acidification was also dependent on sAC activity. Thus, sAC is a sensor and regulator of pH disturbances from both metabolic and external origin in corals. Since sAC is present in all coral cell types, and the cAMP pathway can regulate virtually every aspect of cell physiology through post-translational modifications of proteins, sAC is likely to trigger multiple homeostatic mechanisms in response to pH disturbances. This is also the first evidence that sAC modulates pHi in any non-mammalian animal. Since corals are basal metazoans, our results indicate this function is evolutionarily conserved across animals.

Established and potential physiological roles of bicarbonate-sensing soluble adenylyl cyclase (sAC) in aquatic animals.

Soluble adenylyl cyclase (sAC) is a recently recognized source of the signaling molecule cyclic AMP (cAMP) that is genetically and biochemically distinct from the classic G-protein-regulated transmembrane adenylyl cyclases (tmACs). Mammalian sAC is distributed throughout the cytoplasm and it may be present in the nucleus and inside mitochondria. sAC activity is directly stimulated by HCO3(-), and sAC has been confirmed to be a HCO3(-) sensor in a variety of mammalian cell types. In addition, sAC can functionally associate with carbonic anhydrases to act as a de facto sensor of pH and CO2. The two catalytic domains of sAC are related to HCO3(-)-regulated adenylyl cyclases from cyanobacteria, suggesting the cAMP pathway is an evolutionarily conserved mechanism for sensing CO2 levels and/or acid/base conditions. Reports of sAC in aquatic animals are still limited but are rapidly accumulating. In shark gills, sAC senses blood alkalosis and triggers compensatory H(+) absorption. In the intestine of bony fishes, sAC modulates NaCl and water absorption. And in sea urchin sperm, sAC may participate in the initiation of flagellar movement and in the acrosome reaction. Bioinformatics and RT-PCR results reveal that sAC orthologs are present in most animal phyla. This review summarizes the current knowledge on the physiological roles of sAC in aquatic animals and suggests additional functions in which sAC may be involved.