Introduction

Coral reefs are among the most diverse ecosystems on the planet1, yet have been in decline over the last several decades due in large part to human activity2,3. However, our ability to predict how corals will respond to pollution, eutrophication, climate change and increasingly acidified oceans is hampered by the limited information on fundamental mechanisms of coral cell biology2. Furthermore, as members of the basal metazoan phylum Cnidaria, corals are likely to provide important clues into the evolution of multicellular animals. Interestingly, recent sequencing efforts have revealed that cnidarians have maintained genome complexity similar to that of vertebrates, unlike the more common invertebrate model species Drosophila melanogaster and Caenorhabditis elegans4,5,6,7. One such conserved pathway depends on cyclic adenosine monophosphate (cAMP), a ubiquitous signal transduction molecule that is involved in the regulation of virtually every aspect of both prokaryotic and eukaryotic physiology including metabolism, cell division, ion transport, vesicle trafficking and gene expression8. Adenylyl cyclases, the enzymes that catalyze the production of cAMP, are present throughout Bacteria, Archaea and Eukarya and may therefore be conserved in corals as well. Eukaryotes have two distinct types of adenylyl cyclases: the classic G-protein regulated transmembrane adenylyl cyclases (tmACs) (reviewed in9) and the recently discovered bicarbonate-stimulated soluble adenylyl cyclase (sAC)10. Mammals have nine tmAC isoforms; though they differ in their tissue distribution, expression during development and fine kinetic and regulatory characteristics, all are located at the cell membrane11. On the other hand, sAC can be found throughout the cytoplasm, in mitochondria and in the nucleus12. While tmACs first appeared in metazoans13, sAC and sAC-related enzymes have been shown to produce cAMP in response to bicarbonate in a variety of organisms ranging from cyanobacteria to chloroflexi bacteria, sea urchins and fish12,14,15,16,17, indicating that these enzymes are evolutionarily conserved and thus likely to be active in corals.

The cAMP pathway may regulate calcification in corals18 and it has also been implicated as a component of coral stress responses in a few transcriptomic studies19,20,21. However, there remains a distinct lack of basic information on the presence, structure and activity of cAMP related proteins, the dynamics of cAMP levels in vivo and the physiological triggers and consequences of this signal cascade in corals. A solid foundation on the basic biochemistry of cAMP production is a prerequisite for the development of mechanistic models. Additionally, characterizing the effects of mammalian-specific pharmacological drugs on coral cAMP production is necessary for designing and interpreting physiological experiments. For example, forskolin is a potent activator of tmACs in vertebrates22,23 that has been used in a few coral studies with the aim to manipulating cAMP levels18,24. However, while forskolin stimulates tmACs in some invertebrates such as mollusks25, it does not have any effect on some cnidarians26,27 and its effect on corals has not been characterized.

The aim of this work was to perform an initial characterization of cAMP production in the coral holobiont. Our results demonstrate that coral tissues, but not zooxanthellae or bacteria-harboring mucus, have an unprecedentedly high capacity for producing cAMP that is consistent with the presence of both tmACs and sAC. Furthermore, cAMP levels in live corals fluctuate in a diel manner, suggesting that the cAMP pathway may play a role in light-enhanced calcification, circadian rhythms and metabolic communication between corals and symbiotic zooxanthellae.

Results

Coral adenylyl cyclase homologs

Several putative tmAC and sAC genes were identified through BLAST searches of the available coral genomic and transcriptomic databases (Acropora digitifera and Pocillopora damicornis, respectively; Figure 1). These searches revealed the presence of several tmAC isoforms (four in A. digitifera and five in P. damicornis) and at least two sAC genes (both from A. digitifera; Figure 1). Coral tmAC genes clustered into four different groups, which were most similar to human tmACs 3 (coral type b), 5 (coral type a), or 9 (coral type c), with the fourth not grouping as closely with other tmACs (coral group d; Figure 1). The two coral sAC genes were most similar to anemone sAC, followed by sAC from other invertebrates, including the sea urchin, sea squirt and acorn worm (Figure 1).

Figure 1
figure 1

Phylogenetic tree of putative adenylyl cyclases identified in A. digitifera and P. damicornis.

Coral tmAC isoforms are colored in red and coral sAC isoforms are colored in blue.

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Diurnal fluctuations of endogenous cAMP in live corals

Endogenous cAMP levels in coral tissue were highest during the day (Figure 2). Within one hour after dark (7:45 pm), endogenous cAMP decreased from the previous time point (6:40 pm), although this difference was not statistically significant (Figure 2). A minimum of cAMP was reached after 10 hr of darkness (5:00 am, p = 0.01; Figure 2). No cAMP was detected in coral-free populations of Symbiodinium (either freshly isolated from corals or cultured) when prepared in the same manner as the coral tissue homogenates (resuspended and vortexed in 0.1 N HCl), while cAMP in coral mucus was ~25-fold lower than in tissue. Therefore, the majority of cAMP measured from coral fragments is likely derived from cells of the cnidarian host and not the associated microorganisms.

Figure 2
figure 2

Daily fluctuations of endogenous cAMP levels in the coral P. damicornis normalized to total protein.

N = 9 (3 fragments per time point per day on 3 different days spanning 2 months). Error bars indicate SEM; asterisk indicates statistical significance (p < 0.01).

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Characterization of cAMP production by corals

Basal production of cAMP from coral tissue homogenates ranged from 17,000 pmol mg−1 min−1 in A. youngei to ~30,000 pmol mg−1 min−1 in P. damicornis (Table 1). This production was stimulated by bicarbonate, with a half maximal effect concentration (EC50) of ~10 mM bicarbonate for both coral species (Figure 3). To achieve maximal stimulation, 40 mM bicarbonate was used for subsequent experiments. Treatment with 40 mM bicarbonate significantly increased cAMP production in both A. youngei and P. damicornis by ~1.4 and ~1.6-fold, respectively (paired t-test: p = 0.024 and repeated measures [RM] one-way ANOVA [p < 0.0001] with Tukey post-hoc: α = 0.05, p < 0.001, respectively; Table 1). To validate cAMP measurements, we performed a subset of experiments and measured cAMP production using both ELISA and a radioactivity-based two-column assay28. Both methods used for measuring cAMP production by coral homogenates yielded similar values (Figure 4A,B). Bicarbonate-stimulated production of cAMP was abolished in the presence of 50 μM KH7 (Figure 4A,B), a specific inhibitor of sAC-like adenylyl cyclases that is inert towards mammalian tmACs at this concentration12,29. KH7 reduced basal cAMP production by ~20% as measured both by ELISA and the 2-column assay; however, this result was statistically significant only in the latter case (RM one-way ANOVA: p = 0.0006, Tukey post-hoc: α = 0.05, p < 0.01; Figure 3A,B). Production of cAMP in all subsequent experiments was measured using ELISA.

Table 1 Production of cAMP from coral tissue (Pocillopora damicornis and Acropora youngei) or zooxanthellae (mean ± SEM) determined by ELISA. Letters indicate statistically significant groups as determined by ANOVA and Tukey post-hoc test (P. damicornis) or t-test (A. youngei)
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Figure 3
figure 3

Dose response curve of the effect of bicarbonate on cAMP production in coral tissue homogenates (P. damicornis, N = 5; A. youngei, N = 3).

Lines were drawn from a non-linear curve fit model. Error bars indicate SEM.

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Figure 4
figure 4

Production of cAMP in P. damicornis tissue homogenates as measured by A) ELISA (N = 5) or B) 2-column assay (N = 3) or C) freshly isolated zooxanthellae (N = 5) as measured by ELISA.

Note different scale for panel C. Treatments: Control = 40 mM NaCl; KH7 = 40 mM NaCl + 50 μM KH7; Bicarb = 40 mM NaHCO3; Bicarb + KH7 = 40 mM NaHCO3 + 50 μM KH7. Symbols indicate statistically significant differences. Error bars indicate SEM.

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Production of cAMP from freshly isolated or cultured zooxanthellae could not be detected after cells were homogenized by vortexing or sonication. Zooxanthellae cAMP production was detected when algal cells were homogenized by glass bead beating and was ~1000-fold lower than cAMP production by coral tissues (30 vs. 30,000 pmol cAMP mg−1 min−1; Table 1, Figure 4C). Production of cAMP by zooxanthellae was not responsive to bicarbonate or KH7 (Figure 4C). The total amount of cAMP produced by the mucus of one coral fragment was ~106-fold lower than coral tissue from similarly sized fragments (~10 vs. 107 pmol). Production of cAMP in mucus was not stimulated by 40 mM NaHCO3, neither was it inhibited by 50 μM KH7 (N = 3).

Treatment of coral homogenates with 2.5 mM CaCl2 significantly decreased cAMP production relative to controls (RM one-way ANOVA, p = 0.0087; Tukey post-hoc: α = 0.05, p < 0.01). However, 40 mM NaHCO3 still stimulated cAMP production by ~2.7 fold in the presence of 2.5 mM CaCl2 (relative to 40 mM NaCl + 2.5 CaCl2) (Table 1, Figure 5A). Treatment with 50 μM forsoklin, a potent activator of vertebrate tmACs, had no effect on basal cAMP production relative to the DMSO control (Table 1, Figure 5B).

Figure 5
figure 5

Cyclic AMP production in P. damicornis tissue homogenates in response to A) bicarbonate and/or calcium (N = 6) or B) forskolin (N = 4).

Treatments: Control = 40 mM NaCl; Bicarb = 40 mM NaHCO3; Ca2+ = 2.5 mM CaCl2; Bicarb + Ca2+ = 40 mM NaHCO3 with 2.5 mM CaCl2; 50 μM forskolin. Letters indicate statistically significant differences. Error bars indicate SEM.

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Discussion

This is the first time, to our knowledge, that endogenous cAMP or adenylyl cyclase activity has been measured in any coral. We found that endogenous cAMP levels were significantly lower at night than during the day, though this decrease appears to take some time, as levels were lower but not significantly so after 45 minutes of darkness. It is possible that more time is required for cAMP levels to reach a minimum, particularly given that it takes isolated symbiotic coral cells at least 35 minutes for pH to reach a stable minimum after dark is initiated30. Coral endogenous cAMP levels were several fold higher than cAMP levels in any other cnidarian examined to date, including anemones (7–20 fold31), hydra (2–15 fold32,33) and the sea pansy (5–13 fold34). In addition, corals had higher basal adenylyl cyclase activity than other cnidarians, ranging from ~200 to over 105-fold higher26,35,36. To the best of our knowledge, coral cAMP production rates measured here are higher than any other animal observed to date. It is important to note that corals are host to a variety of microorganisms that could potentially contribute to cAMP production, including endosymbtiotic zooxanthellae, prokaryotes, fungi and viruses (i.e. the coral holobiont37). However, given that cAMP production from isolated zooxanthellae and coral mucus was 103-fold and 105-fold lower, respectively, than whole tissue homogenates, we can rule out any significant contribution from these microorganisms.

Our bioinformatic analysis revealed that A. digitifera has at least four tmAC and two sAC isoforms, while P. damicornis has at least five tmACs but no sAC mRNA transcripts. However, further characterization of coral adenylyl cyclase activity was consistent with the presence of active forms of both tmACs and sAC in both species. Adenylyl cyclase activity in A. youngei and P. damicornis was stimulated by bicarbonate and the sAC inhibitor KH7 abolished this stimulation. These responses indicate that both coral species have one or more active forms of sAC and that the absence of a sAC ortholog in the P. damicornis transcriptome database is likely due to incomplete coverage.

The physiological role of sAC in corals remains an outstanding question, but it likely acts as a pH/CO2/HCO3 sensor as it does in other animals. For example, mammalian sAC is stimulated by bicarbonate to trigger sperm flagellar movement29; it also regulates embryo development, epithelial salt and fluid transport, gene transcription and acid-base homeostasis in epididymis and kidney, among other processes (reviewed in16). Knowledge on the roles of sAC in aquatic organisms is much more limited; however, they seem to be similar to those in mammals: sAC regulates sperm function in sea urchin15,38, epithelial salt transport across the intestine of bony fish39,40 and modulates acid-base homeostasis in shark gills41. There are many potential roles for sAC as a pH/CO2/HCO3 sensor in corals. For example, photosynthetic activity by symbiotic zooxanthellae increases pH in zooxanthellae-hosting gastrodermal cells42, in the sub-calicoblastic medium43 and in the coelenteron42; photosynthesis also significantly increases bicarbonate concentration in tissues44. Calcification in zooxanthellate corals is stimulated by light, a phenomenon referred to as ‘light-enhanced calcification’45. The mechanism is unknown, but it is thought to be related to availability of energy from photosynthesis, production of photosynthates that serve as substrates for the organic matrix of the skeleton45, increased oxygen to support aerobic metabolism, titration of protons produced by calcification46 and/or by associated changes in pH and [HCO3−]42,43,44. The presence of bicarbonate-sensitive adenylyl cyclase activity in corals demonstrated here, as well as the potentially light-driven changes in endogenous cAMP, point to sAC as a promising possibility for sensing acid-base disturbances in corals and triggering light enhanced calcification and other homeostatic physiological responses.

Unlike tmACs from vertebrate animals22, adenylyl cyclase activity in coral homogenates was insensitive to forskolin. This lack of response has been observed in other invertebrates, including the sea pansy26, hydra27 and sea urchins47, suggesting that the structure of invertebrate cyclases is different from those in vertebrates. However, forskolin does have biological effects on corals and other cnidarians, including inhibition of coral metamorphosis24 and sensitization of mechanoreceptors for nematocyst discharge in anemones48, a process that is stimulated by cAMP31,35,48,49. It is important to note, however, that neither study measured the effect of forskolin on cAMP levels, so whether the responses observed are the result of forskolin stimulation of tmACs is not known. In any case, it is clear that future research cannot rely on inhibitors and activators of adenylyl cyclases developed for mammals without first verifying their effects on corals.

The inhibitory effect of calcium on cAMP production in corals was consistent with tmAC activity. Moreover, bicarbonate stimulation was retained in the presence of calcium, indicating that calcium inhibition was specific to tmACs and not sAC. In mammals, calcium ions significantly affect the activity of tmACs, but the effect differs depending on the concentration and the isoform involved. For example, micromolar concentrations of calcium activate some tmACs50,51,52 but inhibit others50,52, while millimolar concentrations inhibit all mammalian tmAC isoforms50,51,52,53. Here we found that coral adenylyl cyclase activity was inhibited by millimolar concentrations of calcium. It remains to be determined how the four individual isoforms of coral tmACs identified here respond to different levels of calcium and it is possible that each may respond differently, particularly to lower (micromolar) concentrations of calcium. The interaction between adenylyl cyclases and calcium may be particularly important for coral calcification, which requires calcium uptake from the surrounding seawater and transport across the epithelia18,54. This process involves both calcium ion channels55 and Ca-ATPases56,57 and leads to an increasing concentration gradient of calcium in coral tissue going from the outer oral ectoderm, through the oral and aboral endoderm, to the skeleton-facing calicoblastic ectoderm58. Interestingly, the activity of these calcium transport enzymes is not regulated at the level of transcription59, suggesting that they are post-translationally modulated.

The cAMP pathway has been somewhat characterized in other cnidarians: it is involved in the regulation of mechanoreceptors and nematocyst discharge in anemones31,35,48; promotion of regeneration in anemones60 and hydra27,61; promotion of cilia beating and feeding in anemones35 but inhibition of feeding in hydra33,62; progression of metamorphosis in jellyfish63; and modulation of muscle contractions34 and bioluminescence64 in sea pansies. Many of these processes may also be regulated through the cAMP pathway in corals and there are several additional and possibly coral-specific roles open to discovery. For example, a variety of coral stressors (e.g. temperature, allelochemicals) have been shown to cause upregulation of cAMP-dependent transcription factors such as CREB19,20, ATF-220, ATF-419,20 and CRE modulators19, as well as PKAs21, the upstream mechanisms and downstream effects of which are unknown. The diurnal fluctuations of cAMP in corals observed here suggest that coral sAC may be involved in regulating acid-base homeostasis during photosynthesis and light-enhanced calcification. Coral gametogenesis and gamete release, which are coordinated to daily and annual light cycles65, are other physiological processes potentially modulated by adenylyl cyclase activity (for example, via light-sensitive GPCRs or cryptochromes and downstream effects linked to circadian rhythms).

In summary, our study demonstrates that corals have a uniquely high capacity for cAMP production that appears to involve both tmACs and sAC and that cAMP levels in host coral tissues fluctuate on a diel basis. These results make the cAMP pathway in corals a promising new avenue of research. If we are to predict how corals will respond to environmental perturbations, including but not limited to eutrophication, rising sea surface temperatures and ocean acidification2,66, it is essential to understand the cellular mechanisms regulating coral physiology.

Methods

Bioinformatics

Homologs for adenylyl cyclases in corals were identified by querying mammalian tmAC and sAC nucleotide sequences via BLASTn versus the Acropora digitifera genome7 and a Pocillopora damicornis transcriptome database (http://cnidarians.bu.edu/PocilloporaBase/cgi-bin/index.cgi). Putative coral adenylyl cylases were aligned with representatives from various phyla using Clustal W2 and a phylogenetic tree of the alignment was generated using the neighbor joining method67.

Specimens

Pocillopora damicornis fragments were obtained from coral colonies growing at aquaria at Rutgers University or at the Birch Aquarium at Scripps. Acropora youngei fragments were obtained from Dimitri Deheyn at the Scripps Institution of Oceanography. All fragments were from different colonies. Coral tissue was removed from the skeleton and homogenized using an airbrush (80 psi) or by scraping with a toothbrush into 100 mM Tris (pH 7.5) supplemented with a protease inhibitor cocktail (Sigma-Aldrich). Protein concentration in each homogenate was measured by the method of Bradford in triplicate. Zooxanthellae were removed via differential centrifugation (1000 × g for 2 min) and the supernatant was divided into aliquots and frozen at −20°C. All cAMP assays were performed with first thaw homogenates, since samples subjected to more than one freeze-thaw lost HCO3--stimulation of cAMP production. Mucus release from A. youngei fragments was stimulated by brief exposure to air and was collected in a sterile syringe. Mucus was concentrated by transferring strands and clumps of mucus to a new tube with 500 μL of 0.2 μm filtered seawater. Mucus was then broken up by passing it through a small pipet tip multiple times, causing sheering and was assayed for cAMP production and protein content immediately. Because protein content within the mucus was undetectable, specific activity could not be calculated.

cAMP levels in live corals

At least one week before sampling, P. damicornis fragments of ~5 mm in length were severed from coral colonies and placed in individual stands. Corals were maintained under a 12:12 hr light:dark cycle. At different times throughout the day, coral fragments were removed from the tank by hand, immediately immersed in 200 μL 0.1 N HCl, vortexed for 15 sec and stored at −20°C. This treatment bursts the cells and causes the release of cytoplasmic molecules (including cAMP) into the surrounding 0.1 N HCl, which was then used in the ELISA (acetylated protocol) for cAMP quantification. Total protein concentration in each sample was measured by the method of Bradford and used to standardize cAMP concentration. Values of cAMP are an average from nine coral fragments at each time point, which were sampled in groups of three per sampling time on three different days over a two-month period. The effect of time of day was analyzed as a fixed effect in a linear mixed-effects model fit by maximum likelihood test with "sampling day" treated as a random effect using R 2.11.168.

Assays for cAMP-producing activity

Coral tissue homogenates were incubated in “cyclase assay buffer” consisting of 100 mM Tris (pH 7.5), 0.5 mM isobutylmethylxanthine (IBMX), 1 mM dithiothreitol (DTT), 20 mM creatine phosphate (CP), 100 U mL-1 creatine phosphokinase (CPK), 5 mM MgCl2 and 5 mM ATP for 30 min at room temperature on an orbital shaker (300 rpm). Treatments were performed in triplicate for each homogenate along with one reaction to determine the initial concentration of cAMP (t = 0). Treatments included a control (40 mM NaCl), increasing concentrations of NaHCO3, 2.5 mM CaCl2, DMSO, 50 μM forskolin (in DMSO), or 50 μM KH7 (in DMSO). NaHCO3 solutions were prepared fresh before addition to the assay. Reactions were stopped with the addition of one volume of 0.2 N HCl. Production of cAMP was measured by ELISA using the acetylated protocol of the Direct cAMP ELISA kit (Enzo Life Sciences).

Freshly isolated zooxanthellae pellets were resuspended in 100 mM Tris (pH 7.5), sonicated on ice and assayed for cAMP production as described above. However, these samples did not produce any detectable cAMP. Additional zooxanthellae pellets were combined with glass beads in Tris buffer and homogenized using a bead beater. These homogenates were then assayed for cAMP production as described above and had detectable cAMP production (data shown in ‘Results’). Since zooxanthellae cAMP production could only be detected when the cells were prepared with bead beating, this method was used to determine if zooxanthellae cAMP production was sensitive to bicarbonate or KH7. Protein concentration in each homogenate was determined by the Bradford method. All cAMP production rates are reported in pmol cAMP produced per mg total protein per minute.

Selected treatments with coral tissue homogenates were repeated using a radioactivity-based two-column adenylyl cyclase assay according to the method of Salomon28. Homogenates were incubated in cyclase assay buffer supplemented with ~1,000,000 cpm [α-32P]ATP (Perkin Elmer) and ~5,000 cpm [3H]cAMP (Perkin Elmer). Reactions were performed in 100 μL for 30 min at room temperature and stopped with 150 μL 1.5% SDS. Product [32P]cAMP was separated from substrate [α-32P]ATP by sequential column chromatography over dowex 50WX4-400 resin (Fluka) followed by aluminium oxide resin (Sigma). Product [32P]cAMP was eluted from dowex directly onto the alumina by water and cAMP was eluted from alumina by 0.1 M imidazole (pH = 7.3).