p63 in Mytilus galloprovincialis and p53 family members in the phylum Mollusca

High-throughput quantification of protein structural change reveals potential mechanisms of temperature adaptation in Mytilus mussels

Background

Temperature exerts a strong influence on protein evolution: species living in thermally distinct environments often exhibit adaptive differences in protein structure and function. However, previous research on protein temperature adaptation has focused on small numbers of proteins and on proteins adapted to extreme temperatures. Consequently, less is known about the types and quantity of evolutionary change that occurs to proteins when organisms adapt to small shifts in environmental temperature. In this study, these uncertainties were addressed by developing software that enabled comparison of structural changes associated with temperature adaptation (hydrogen bonding, salt bridge formation, and amino acid use) among large numbers of proteins from warm- and cold-adapted species of marine mussels, Mytilus galloprovincialis and Mytilus trossulus, respectively.

Results

Small differences in habitat temperature that characterize the evolutionary history of Mytilus mussels were sufficient to cause protein structural changes consistent with temperature adaptation. Hydrogen bonds and salt bridges that increase stability and protect against heat-induced denaturation were more abundant in proteins from warm-adapted M. galloprovincialis compared with proteins from cold-adapted M. trossulus. These structural changes were related to deviations in the use of polar and charged amino acids that facilitate formation of hydrogen bonds and salt bridges within proteins, respectively. Enzymes, in particular those within antioxidant and cell death pathways, were over-represented among proteins with the most hydrogen bonds and salt bridges in warm-adapted M. galloprovincialis. Unlike extremophile proteins, temperature adaptation in Mytilus proteins did not involve substantial changes in the number of hydrophobic or large volume amino acids, nor in the content of glycine or proline.

Conclusions

Small shifts in organism temperature tolerance, such as that needed to cope with climate warming, may result from structural and functional changes to a small percentage of the proteome. Proteins in which function is dependent on large conformational change, notably enzymes, may be particularly sensitive to temperature perturbation and represent foci for natural selection. Protein temperature adaptation can occur through different types and frequencies of structural change, and adaptive mechanisms used to cope with small shifts in habitat temperature appear different from mechanisms used to retain protein function at temperature extremes.

The immune response of the scallop Argopecten purpuratus is associated with changes in the host microbiota structure and diversity

All organisms live in close association with a variety of microorganisms called microbiota. Furthermore, several studies support a fundamental role of the microbiota on the host health and homeostasis. In this context, the aim of this work was to determine the structure and diversity of the microbiota associated with the scallop Argopecten purpuratus, and to assess changes in community composition and diversity during the host immune response. To do this, adult scallops were immune challenged and sampled after 24 and 48 h. Activation of the immune response was established by transcript overexpression of several scallop immune response genes in hemocytes and gills, and confirmed by protein detection of the antimicrobial peptide big defensin in gills of Vibrio-injected scallops at 24 h post-challenge. Then, the major bacterial community profile present in individual scallops was assessed by denaturing gradient gel electrophoresis (DGGE) of 16S rDNA genes and dendrogram analyses, which indicated a clear clade differentiation of the bacterial communities noticeable at 48 h post-challenge. Finally, the microbiota structure and diversity from pools of scallops were characterized using 16S deep amplicon sequencing. The results revealed an overall modulation of the microbiota abundance and diversity according to scallop immune status, allowing for prediction of some changes in the functional potential of the microbial community. Overall, the present study showed that changes in the structure and diversity of bacterial communities associated with the scallop A. purpuratus are detected after the activation of the host immune response. Now, the relevance of microbial balance disruption in the immune capacity of the scallop remains to be elucidated.

Immunity in mussels: An overview of molecular components and mechanisms with a focus on the functional defenses

Bivalves' immunity has received much more attention in the last decade, which resulted to a valuable growth in the availability of its molecular components. Such data availability coupled with the economical importance of these organisms aimed to shift the increase in the number of immunological and stress-related studies. Unfortunately, the crowd of generated data deciphering the involved physiological processes, investigators' differential conceptualization and the aimed objectives, has complicated the sensu stricto outlining of immune-related mechanisms. Overall, this review tried to compiles a summary about the molecular components of the mussels' immune response, surveying an overview of the mussels' functional immunity through gathering the most recent-related topics of bivalves' immunity as apoptosis and autophagy which deserves a great attention as stress-related mechanisms, the disseminated neoplasia as outbreak transmissible disease, not only within the same specie but also among different species, the hematopoiesis as topic that still generating interesting debate in the scientific community, the mucosal immunity described as the interface where host-pathogen interactions would occurs and determinate the late immune response, and innate immune memory and transgenerational priming, which described as very recent research topic with extensive applications in shellfish farming industry.

Exposure to tritiated water at an elevated temperature: Genotoxic and transcriptomic effects in marine mussels (M. galloprovincialis)

Temperature is an abiotic factor of particular concern for assessing the potential impacts of radionuclides on marine species. This is particularly true for tritium, which is discharged as tritiated water (HTO) in the process of cooling nuclear institutions. Additionally, with sea surface temperatures forecast to rise 0.5-3.5 °C in the next 30-100 years, determining the interaction of elevated temperature with radiological exposure has never been more relevant. We assessed the tissue-specific accumulation, transcriptional expression of key genes, and genotoxicity of tritiated water to marine mussels at either 15 or 25 °C, over a 7 day time course with sampling after 1 h, 12 h, 3 d and 7d. The activity concentration used (15 MBq L^-1) resulted in tritium accumulation that varied with both time and temperature, but consistently produced dose rates (calculated using the ERICA tool) of <20 Gy h^-1, i.e. considerably below the recommended guidelines of the IAEA and EURATOM. Despite this, there was significant induction of DNA strand breaks (as measured by the comet assay), which also showed a temperature-dependent time shift. At 15 °C, DNA damage was only significantly elevated after 7 d, in contrast to 25 °C where a similar response was observed after only 3 d. The transcription profiles of two isoforms of hsp70, hsp90, mt20, p53 and rad51 indicated potential mechanisms behind this temperature-induced acceleration of genotoxicity, which may be the result of compromised defence. Specifically, genes involved in protein folding, DNA double strand break repair and cell cycle checkpoint control were upregulated after 3 d HTO exposure at 15 °C, but significantly downregulated when the same exposure occurred at 25 °C. This study is the first to investigate temperature effects on radiation-induced genotoxicity in an ecologically relevant marine invertebrate, Mytilus galloprovincialis. From an ecological perspective, our study suggests that mussels (or similar marine species) exposed to increased temperature and HTO may have a compromised ability to defend against genotoxic stress.

p63 gene structure in the phylum mollusca

Roles of p53 family ancestor (p63) in the organisms' response to stressful environmental conditions (mainly pollution) have been studied among molluscs, especially in the genus Mytilus, within the last 15 years. Nevertheless, information about gene structure of this regulatory gene in molluscs is scarce. Here we report the first complete genomic structure of the p53 family orthologue in the mollusc Mediterranean mussel Mytilus galloprovincialis and confirm its similarity to vertebrate p63 gene. Our searches within the available molluscan genomes (Aplysia californica, Lottia gigantea, Crassostrea gigas and Biomphalaria glabrata), found only one p53 family member present in a single copy per haploid genome. Comparative analysis of those orthologues, additionally confirmed the conserved p63 gene structure. Conserved p63 gene structure can be a helpful tool to complement or/and revise gene annotations of any future p63 genomic sequence records in molluscs, but also in other animal phyla. Knowledge of the correct gene structure will enable better prediction of possible protein isoforms and their functions. Our analyses also pointed out possible mis-annotations of the p63 gene in sequenced molluscan genomes and stressed the value of manual inspection (based on alignments of cDNA and protein onto the genome sequence) for a reliable and complete gene annotation.

Biomarkers and transcription levels of cancer-related genes in cockles Cerastoderma edule from Galicia (NW Spain) with disseminated neoplasia

Disseminated neoplasia (DN) is a pathological condition reported for several species of marine bivalves throughout the world, but its aetiology has not yet been satisfactorily explained. It has been suggested that chemical contamination could be a factor contributing to neoplasia. The aim of the present study was to compare cell and tissue biomarkers and the transcription level of cancer-related genes in cockles (Cerastoderma edule) affected by DN with those of healthy cockles in relation to chemical contaminant burdens. For this, cockles were collected from a natural bed in Cambados (Ria de Arousa, Galicia) in May 2009. The prevalence of DN was 12.36% and 3 degrees of DN severity were distinguished. No significant differences in metal accumulation, non-specific inflammatory responses and parasites were observed between healthy and DN-affected cockles. Lysosomal membrane stability was significantly reduced in cockles affected by DN, which indicates a poorer health condition. Very low frequencies of micronuclei were recorded and no significant differences were detected between DN severity groups. Haemolymph analyses showed a higher frequency of mitotic figures and binucleated cells in cockles affected by moderate and heavy DN than in healthy ones. Neoplastic animals showed significantly higher transcription levels of p53 and ras than healthy cockles and mutational alterations in ras gene sequence were detected. Low concentrations of metals, polycyclic aromatic hydrocarbons, polychlorinated biphenyls and phthalate esters were measured in cockles from Cambados. In conclusion, cockles affected by DN suffer a general stress situation and have altered patterns of cancer-related gene transcription. Further studies are in progress to elucidate mechanisms of carcinogenesis in this species.

Genes of the mitochondrial apoptotic pathway in Mytilus galloprovincialis

Bivalves play vital roles in marine, brackish, freshwater and terrestrial habitats. In recent years, these ecosystems have become affected through anthropogenic activities. The ecological success of marine bivalves is based on the ability to modify their physiological functions in response to environmental changes. One of the most important mechanisms involved in adaptive responses to environmental and biological stresses is apoptosis, which has been scarcely studied in mollusks, although the final consequence of this process, DNA fragmentation, has been frequently used for pollution monitoring. Environmental stressors induce apoptosis in molluscan cells via an intrinsic pathway. Many of the proteins involved in vertebrate apoptosis have been recognized in model invertebrates; however, this process might not be universally conserved. Mytilus galloprovincialis is presented here as a new model to study the linkage between molecular mechanisms that mediate apoptosis and marine bivalve ecological adaptations. Therefore, it is strictly necessary to identify the key elements involved in bivalve apoptosis. In the present study, six mitochondrial apoptotic-related genes were characterized, and their gene expression profiles following UV irradiation were evaluated. This is the first step for the development of potential biomarkers to assess the biological responses of marine organisms to stress. The results confirmed that apoptosis and, more specifically, the expression of the genes involved in this process can be used to assess the biological responses of marine organisms to stress.

Transcriptional responses of cancer-related genes in turbot Scophthalmus maximus and mussels Mytilus edulis exposed to heavy fuel oil no. 6 and styrene

Recent spills in European waters have released polycyclic aromatic hydrocarbons, important components of heavy fuel oil, and the hydrocarbon styrene. Heavy fuel oil and styrene are classified as potentially genotoxic and carcinogenic. Here we investigate transcription of genes involved in cancer development in the liver of juvenile turbots and in the digestive gland of mussels exposed to heavy fuel oil and to styrene and after a recovery period. In turbot, oil produced a significant up-regulation of p53 and gadd45α after 14 days exposure. cyclin G1 was up-regulated after 7 days treatment with styrene. In mussels, ras was down-regulated in both treatments after the recovery periods. No mutations in ras hotspots were detected in exposed mussels. gadd45α was up-regulated after the recovery period of the styrene experiment. Overall, transcriptional responses differed in mussels compared to turbot. Turbot responded to hydrocarbon exposure by triggering cell cycle arrest (p53) and DNA repair (gadd45α).

The role of p63 in cancer, stem cells and cancer stem cells

The transcription factor p63 has important functions in tumorigenesis, epidermal differentiation and stem cell self-renewal. The TP63 gene encodes multiple protein isoforms that have different or even antagonistic roles in these processes. The balance of p63 isoforms, together with the presence or absence of the other p53 family members, p73 and p53, has a striking biological impact. There is increasing evidence that interactions between p53-family members, whether cooperative or antagonistic, are involved in various cell processes. This review summarizes the current understanding of the role of p63 in tumorigenesis, metastasis, cell migration and senescence. In particular, recent data indicate important roles in adult stem cell and cancer stem cell regulation and in the response of cancer cells to therapy.

Promoter complexity and tissue-specific expression of stress response components in Mytilus galloprovincialis, a sessile marine invertebrate species

The mechanisms of stress tolerance in sessile animals, such as molluscs, can offer fundamental insights into the adaptation of organisms for a wide range of environmental challenges. One of the best studied processes at the molecular level relevant to stress tolerance is the heat shock response in the genus Mytilus. We focus on the upstream region of Mytilus galloprovincialis Hsp90 genes and their structural and functional associations, using comparative genomics and network inference. Sequence comparison of this region provides novel evidence that the transcription of Hsp90 is regulated via a dense region of transcription factor binding sites, also containing a region with similarity to the Gamera family of LINE-like repetitive sequences and a genus-specific element of unknown function. Furthermore, we infer a set of gene networks from tissue-specific expression data, and specifically extract an Hsp class-associated network, with 174 genes and 2,226 associations, exhibiting a complex pattern of expression across multiple tissue types. Our results (i) suggest that the heat shock response in the genus Mytilus is regulated by an unexpectedly complex upstream region, and (ii) provide new directions for the use of the heat shock process as a biosensor system for environmental monitoring.

Variations in p53-like cDNA sequence are correlated with mussel haemic neoplasia: A potential molecular-level tool for biomonitoring

Several bivalve species, including mussels (Mytilus spp.) and clams (Mya spp.), are susceptible to a leukemia-like disease called haemic neoplasia that has been known to decimate whole populations. Previous studies of molecular processes associated with late stages of this disease have implicated analogs of the p53 tumour suppressor protein family in disease etiology. We detected synonymous single nucleotide polymorphisms (SNPs) in the coding region sequence of p53-like cDNA from Mytilus trossulus (bay mussel) that differ between normal and neoplastic haemolymph. SNPs were located at positions 182, 392 and 821 bp. Most (94%) of the late leukemic animals sampled from cages in Burrard Inlet (British Columbia, Canada) had the same p53-like genotype, C182T G392G C821T, whereas 75% of the healthy animals were homozygous at positions C182C and T821T, independent of the genotype at the 392 bp position. As well, we detected an increased number of allelic variants in the leukemic animals that may arise from separate somatic mutation events in haemocyte precursors or from additional p53-like gene copies in polyploidy. Therefore, detection of these SNPs may provide a useful genetic biomarker for efficient monitoring of mussel population health.