Identification of multiple isoforms of the cAMP-dependent protein kinase catalytic subunit in the bivalve mollusc Mytilus galloprovincialis

Evaluating bivalve cytoprotective responses and their regulatory pathways in a climate change scenario

Temperature is a relevant abiotic factor affecting physiological performance and distribution of marine animals in natural environments. The changes in global seawater temperatures make it necessary to understand how molecular mechanisms operate under the cumulative effects of global climate change and chemical pollution to promote/hamper environmental acclimatization. Marine mussels are excellent model organisms to infer the impacts of those anthropogenic threats on coastal ecosystems. In this study, Mediterranean mussels (Mytilus galloprovincialis) were exposed to different concentrations of the metal copper (Cu as CuCl₂: 2.5, 5, 10, 20, 40 μg/L) or the antibiotic oxytetracycline (OTC: 0.1, 1, 10, 100, 1000 μg/L) at increasing seawater temperatures (16 °C, 20 °C, 24 °C). Transcriptional modulation of a 70-kDa heat shock protein (HSP70) and of the ABC transporter P-glycoprotein (P-gp, encoded by the ABCB gene) was assessed along with the cAMP/PKA signaling pathway regulating both gene expressions. At the physiological temperature of mussels (16 °C), Cu and OTC induced bimodal changes of cAMP levels and PKA activities in gills of exposed animals. A correlation between OTC- or Cu- induced changes of PKA activity and expression of hsp70 and ABCB was observed. Temperature increases (up to 24 °C) altered ABCB and hsp70 responses to the pollutants and disrupted their relationship with cAMP/PKA modulation, leading to loss of correlation between the biological endpoints. On the whole, the results indicate that temperature may impair the effects of inorganic and organic chemicals on the cAMP/PKA signaling pathway of mussels, in turn altering key molecular mediators of physiological plasticity and cytoprotection.

Adrenergic signaling in teleost fish liver, a challenging path

Adrenergic receptors or adrenoceptors (ARs) belong to the huge family of G-protein coupled receptors (GPCRs) that have been well characterized in mammals primarily because of their importance as therapeutic drug targets. ARs are found across vertebrates and this review examines the path to identify and characterize these receptors in fish with emphasis on hepatic metabolism. The absence of reliable and specific pharmacological agents led investigators to define the fish hepatic AR system as relying solely on a β2-AR, cAMP-dependent signaling transduction pathway. The use of calcium-radiometric imaging, purified membranes for ligand-binding studies, and perifused rather than static cultured fish hepatocytes, unequivocally demonstrated that both α1- and β2-AR signaling systems existed in the fish liver consistent with studies in mammals. Additionally, the use of molecular tools and phylogenetic analysis clearly demonstrated the existence of multiple AR-types and -subtypes in hepatic and other tissues of a number of fish species. This review also examines the use of β-blockers as pharmaceuticals and how these drugs that are now in the aquatic environment may be impacting aquatic species including fish and some invertebrates. Clearly there is a large conservation of structure and function within the AR system of vertebrates but there remain a number of key questions that need to be addressed before a clear understanding of these systems can be resolved.

Cyclic-AMP mediated regulation of ABCB mRNA expression in mussel haemocytes

Background

The multixenobiotic resistance system (MXR) allows aquatic organisms to cope with their habitat despite high pollution levels by over-expressing membrane and intracellular transporters, including the P-glycoprotein (Pgp). In mammals transcription of the ABCB1 gene encoding Pgp is under cAMP/PKA-mediated regulation; whether this is true in mollusks is not fully clarified.

Methodology/Principal Findings

cAMP/PKA regulation and ABCB mRNA expression were assessed in haemocytes from Mediterranean mussels (Mytilus galloprovincialis) exposed in vivo for 1 week to 0.3 ng/L fluoxetine (FX) alone or in combination with 0.3 ng/L propranolol (PROP). FX significantly decreased cAMP levels and PKA activity, and induced ABCB mRNA down-regulation. FX effects were abolished in the presence of PROP. In vitro experiments using haemocytes treated with physiological agonists (noradrenaline and serotonin) and pharmacological modulators (PROP, forskolin, dbcAMP, and H89) of the cAMP/PKA system were performed to obtain clear evidence about the involvement of the signaling pathway in the transcriptional regulation of ABCB. Serotonin (5-HT) decreased cAMP levels, PKA activity and ABCB mRNA expression but increased the mRNA levels for a putative 5-HT₁ receptor. Interestingly, 5-HT₁ was also over-expressed after in vivo exposures to FX. 5-HT effects were counteracted by PROP. Forskolin and dbcAMP increased PKA activity as well as ABCB mRNA expression; the latter effect was abolished in the presence of the PKA inhibitor H89.

Conclusions

This study provides the first direct evidence for the cAMP/PKA-mediated regulation of ABCB transcription in mussels.

Evidence of anti-proliferative activities in blue mussel (Mytilus edulis) by-products

Shellfish waste components contain significant levels of high quality protein and are therefore a potential source for biofunctional high-value peptides. The feasibility of applying a pilot scale enzymatic hydrolysis process to whole Mytilus edulis and, by fractionation, recover hydrolysates presenting a biological activity of interest, was evaluated. Fractions were tested on four immortalized cancerous cell lines: A549, BT549, HCT15 and PC3. The 50 kDa fraction, enriched in peptides, presented anti-proliferative activity with all cell lines and results suggest a bioactive molecule synergy within the fraction. At a protein concentration of 44 µg/mL, the 50 kDa fraction induced a mortality of 90% for PC3, 89% for A549, 85% for HCT15 and of 81% for BT549 cell lines. At the low protein concentration of only 11 µg/mL the 50 kDa fraction still entails a cell mortality of 76% for A549 and 87% for PC3 cell lines. The 50 kDa fraction contains 56% of proteins, 3% of lipids and 6% of minerals on a dry weight basis and the lowest levels detected of taurine and methionine and highest levels of threonine, proline and glycine amino acids. The enzymatic hydrolysis process suggests that Mytilus edulis by-products should be viewed as high-valued products with strong potential as anti-proliferative agent and promising active ingredients in functional foods.

Differential distribution of cAMP-dependent protein kinase isoforms in various tissues of the bivalve mollusc Mytilus galloprovincialis

The cAMP signalling pathway is involved in the regulation of basic physiological processes in bivalve molluscs. We had previously identified and characterized two isoforms of cAMP-dependent protein kinase (PKA) from the sea mussel Mytilus galloprovincialis that differ at their regulatory (R) subunit, namely, R(myt1) or R(myt2). Here we investigated the immunohistochemical expression of both PKA isoforms in various mussel tissues. R(myt1) and R(myt2) displayed a complementary subcellular localization. In general, R(myt1) was found to be uniformly distributed in the cytoplasm of most cell types, whereas R(myt2) appears to be localized only in the cell periphery and associated with certain cellular structures, such as the cilia of labial palps and gill filaments. Thus, both PKA isoforms appear to be non-redundant, but they have specific functions. R(myt1) was the main isoform present in catch muscle fibers, which suggests that PKA(myt1) may be the isoform involved in the regulation of the catch state. Conversely, R(myt2) was the only isoform detected in the cilia of gill filaments, indicating that PKA(myt2) could mediate the effects of cAMP on the ciliary beat frequency.

Cyclic AMP signaling in bivalve molluscs: an overview

The cyclic AMP (cAMP)-dependent signaling accounts for the control of cellular cascades involved in many physiological functions, and a wealth of information is available on the cAMP system that operates in mammalian cells. Nevertheless, cAMP has a central role also in nonmammalian vertebrates and invertebrates. The present review aims at examining the information available on bivalve molluscs, from the first studies carried out in the early 1980s to the last progresses made in the present days. The major focus is on the structural and operational characteristics of the main actors of the signaling pathway, i.e., adenylyl cyclase, G proteins, and protein kinase A, and on the role played by the cyclic nucleotide on smooth muscle, heart, gills, gonads, and metabolism regulation. Moreover, recent evidence regarding the cAMP system as a target of environmental stress factors are discussed. It will become clear that cAMP does play a wide and important role in bivalve physiology. Several issues have been sufficiently clarified, although investigated only in a few model species. However, further fundamental aspects remain unknown, mainly regarding molecular features and interactions with other signaling pathways, thus requiring further elucidation.