Apoptosis and pattern of Bcl‐2 and Bax expression in the alimentary tract during the colonial blastogenetic cycle ofBotryllus schlosseri(Urochordata, Ascidiacea)

Multiple Forms of Neural Cell Death in the Cyclical Brain Degeneration of A Colonial Chordate

Human neuronal loss occurs through different cellular mechanisms, mainly studied in vitro. Here, we characterized neuronal death in B. schlosseri, a marine colonial tunicate that shares substantial genomic homology with mammals and has a life history in which controlled neurodegeneration happens simultaneously in the brains of adult zooids during a cyclical phase named takeover. Using an ultrastructural and transcriptomic approach, we described neuronal death forms in adult zooids before and during the takeover phase while comparing adult zooids in takeover with their buds where brains are refining their structure. At takeover, we found in neurons clear morphologic signs of apoptosis (i.e., chromatin condensation, lobed nuclei), necrosis (swollen cytoplasm) and autophagy (autophagosomes, autolysosomes and degradative multilamellar bodies). These results were confirmed by transcriptomic analyses that highlighted the specific genes involved in these cell death pathways. Moreover, the presence of tubulovesicular structures in the brain medulla alongside the over-expression of prion disease genes in late cycle suggested a cell-to-cell, prion-like propagation recalling the conformational disorders typical of some human neurodegenerative diseases. We suggest that improved understanding of how neuronal alterations are regulated in the repeated degeneration-regeneration program of B. schlosseri may yield mechanistic insights relevant to the study of human neurodegenerative diseases.

Searching for the Origin and the Differentiation of Haemocytes before and after Larval Settlement of the Colonial Ascidian Botryllus schlosseri: An Ultrastructural Viewpoint

The colonial ascidian Botryllus schlosseri possesses an innate immunity, which plays fundamental roles in its survival, adaptability, worldwide spread and ecological success. Three lines of differentiation pathways of circulating haemocytes are known to be present in the haemolymph, starting from undifferentiated haemoblasts: (i) the phagocytic line (hyaline amoebocytes and macrophage-like cells), (ii) the cytotoxic line (granular amoebocytes and morula cells) and (iii) the storage cell line (pigment cells and nephrocytes). Many questions remain about their origin, and thus, observations during various stages of development were undertaken in this study. Haemocytes were detected beginning from the early tailbud embryo stage. Haemoblasts were always present and morula cells were the first differentiated haemocytes detected. In both the next stage, just before hatching, and the swimming tadpole larva stage, hyaline amoebocytes and pigment cells were also recognisable. Some morula cells containing active phenoloxidase migrated from the haemolymph into the tunic after having crossed the epidermis, and this behaviour could be related to the preparation of a defensive function for spatial competition. During larval metamorphosis, macrophage-like cells appeared with their phagosomes positive to acid phosphatase activity and containing apoptotic cells from tail tissue degeneration. After metamorphosis, in the filter-feeding oozoid stage, nephrocytes involved in nitrogen catabolism finally appeared. In both the subendostylar sinus and the peripheral blind-sac vessels (ampullae), clusters of haemoblasts were recognisable, some of which showed incipient specialisations, considering the hypothesis of the presence of putative niches of haemolymph stem cells.

Protection from Oxidative Stress in Immunocytes of the Colonial Ascidian Botryllus schlosseri: Transcript Characterization and Expression Studies

Botryllus schlosseri is a cosmopolitan colonial ascidian that undergoes cyclical generation changes, or take-overs, during which adult zooids are resorbed and replaced by their buds. At take-over, adult tissues undergo diffuse apoptosis and effete cells are massively ingested by circulating phagocytes, with a consequent increase in oxygen consumption and in production of reactive oxygen species (ROS). The latter are responsible for the death of phagocytes involved in the clearance of apoptotic cells and corpses by phagocytosis-induced apoptosis. However, the majority of phagocytes and hemocytes do not die, even if they experience oxidative stress. This fact suggests the presence of detoxification mechanisms assuring their protection. To test this assumption, we searched for transcripts of genes involved in detoxification in the transcriptome of B. schlosseri. We identified and characterized transcripts for Cu/Zn superoxide dismutase (SOD), γ-glutamyl-cysteine ligase modulatory subunit (GCLM), glutathione synthase (GS), and two glutathione peroxidases (i.e., GPx3 and GPx5), all involved in protection from ROS. We also carried out a phylogenetic analysis of the putative amino acid sequences, confirming their similarity to their vertebrate counterparts, and studied the location of their mRNAs by in situ hybridization on hemocyte monolayers. We also analyzed gene transcription during the colonial blastogenetic cycle, which is the interval of time between one take-over and the next, by qRT-PCR. In addition, we investigated the effects of cadmium (Cd), an inducer of oxidative stress, on gene transcription. Our results indicated that i) antioxidant gene expression is modulated in the course of the blastogenetic cycle and upon exposure to Cd, and ii) hemocytes synthesize both enzymatic and nonenzymatic antioxidants, in line with the idea that they represent a major detoxification system for ascidians.

Recurrent phagocytosis-induced apoptosis in the cyclical generation change of the compound ascidian Botryllus schlosseri

Colonies of the marine, filter-feeding ascidian Botryllus schlosseri undergo cyclical generation changes or takeovers. These events are characterised by the progressive resorption of adult zooids and their replacement by their buds that grow to adult size, open their siphons and start filtering. During the take-over, tissues of adult zooids undergo extensive apoptosis; circulating, spreading phagocytes enter the effete tissues, ingest dying cells acquiring a giant size and a round morphology. Then, phagocytes re-enter the circulation where they represent a considerable fraction (more than 20%) of circulating haemocytes. In this study, we evidence that most of these circulating phagocytes show morphological and biochemical signs of apoptosis. Accordingly, these phagocytes express transcripts of orthologues of the apoptosis-related genes Bax, AIF1 and PARP1. Electron microscopy shows that giant phagocytes contain apoptotic phagocytes inside their own phagocytic vacuole. The transcript of the orthologues of the anti-apoptotic gene IAP7 was detected only in spreading phagocytes, mostly abundant in phases far from the take-over. Therefore, the presented data suggest that, at take-over, phagocytes undergo phagocytosis-induced apoptosis (PIA). In mammals, PIA is assumed to be a process assuring the killing and the complete elimination of microbes, by promoting the disposal of terminally differentiated phagocytes and the resolution of infection. In B. schlosseri, PIA assumes a so far undescribed role, being required for the control of asexual development and colony homeostasis.

Transcriptome dynamics in the asexual cycle of the chordate Botryllus schlosseri

Background

We performed an analysis of the transcriptome during the blastogenesis of the chordate Botryllus schlosseri, focusing in particular on genes involved in cell death by apoptosis. The tunicate B. schlosseri is an ascidian forming colonies characterized by the coexistence of three blastogenetic generations: filter-feeding adults, buds on adults, and budlets on buds. Cyclically, adult tissues undergo apoptosis and are progressively resorbed and replaced by their buds originated by asexual reproduction. This is a feature of colonial tunicates, the only known chordates that can reproduce asexually.

Results

Thanks to a newly developed web-based platform (http://botryllus.cribi.unipd.it), we compared the transcriptomes of the mid-cycle, the pre-take-over, and the take-over phases of the colonial blastogenetic cycle. The platform is equipped with programs for comparative analysis and allows to select the statistical stringency. We enriched the genome annotation with 11,337 new genes; 581 transcripts were resolved as complete open reading frames, translated in silico into amino acid sequences and then aligned onto the non-redundant sequence database. Significant differentially expressed genes were classified within the gene ontology categories. Among them, we recognized genes involved in apoptosis activation, de-activation, and regulation.

Conclusions

With the current work, we contributed to the improvement of the first released B. schlosseri genome assembly and offer an overview of the transcriptome changes during the blastogenetic cycle, showing up- and down-regulated genes. These results are important for the comprehension of the events underlying colony growth and regression, cell proliferation, colony homeostasis, and competition among different generations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2598-1) contains supplementary material, which is available to authorized users.

Characterization and transcription studies of a phytochelatin synthase gene from the solitary tunicate Ciona intestinalis exposed to cadmium

The major thiol-containing molecules involved in controlling the level of intracellular ROS in eukaryotes, acting as a nonenzymatic detoxification system, are metallothioneins (MTs), glutathione (GSH) and phytochelatins (PCs). Both MTs and GSH are well-known in the animal kingdom. PC was considered a prerogative of the plant kingdom but, in 2001, a phytochelatin synthase (PCS) gene was described in the nematode Caenorhabditis elegans; additional genes encoding this enzyme were later described in the earthworm Eisenia fetida and in the parasitic nematode Schistosoma mansoni but scanty data are available, up to now, for Deuterostomes. Here, we describe the molecular characteristics and transcription pattern, in the presence of Cd, of a PCS gene from the invertebrate chordate Ciona intestinalis, a ubiquitous solitary tunicate and demonstrate the presence of PCs in tissue extracts. We also studied mRNA localization by in situ hybridization. In addition, we analyzed the behavior of hemocytes and tunic cells consequent to Cd exposure as well as the transcription pattern of the Ciona orthologous for proliferating cell nuclear antigen (PCNA), usually considered a proliferation marker, and observed that cell proliferation occurs after 96h of Cd treatment. This matches the hypothesis of Cd-induced cell proliferation, as already suggested by previous data on the expression of a metallothionein gene in the same animal.

Looking for putative phenoloxidases of compound ascidians: haemocyanin-like proteins in Polyandrocarpa misakiensis and Botryllus schlosseri

Phenoloxidases (POs) and haemocyanins constitute a family of copper-containing proteins widely distributed among invertebrates. Both of them are able, under appropriate conditions, to convert polyphenols to quinones and induce cytotoxicity through the production of reactive oxygen species, a fundamental event in many immune responses. In ascidians, PO activity has been described and studied in both solitary and colonial species and the enzyme is involved in inflammatory and cytotoxic reactions against foreign cells or molecules, and in the formation of the cytotoxic foci which characterise the nonfusion reaction of botryllids. Expressed genes for two putative POs (CiPO1 and CiPO2) have been recently identified in C. intestinalis. In the present study, we determined the cDNA sequences of two haemocyanin-like proteins from two colonial ascidians: Botryllus schlosseri from the Mediterranean Sea and Polyandrocarpa misakiensis from Japan. Multiple sequence alignments evidenced the similarity between the above sequences and crustacean proPOs whereas the analysis of the three-dimensional structure reveals high similarity with arthropod haemocyanins which share common precursors with arthropod proPOs. Botryllus HLP grouped in the same cluster with Ciona POs, whereas Polyandrocarpa HLP clustered with arthropod haemocyanins; all of them share the full conservation of the six histidines at the two copper-binding sites as well as of other motifs, also found in arthropod haemocyanin subunits, involved in the regulation of enzyme activity. In situ hybridisation indicated that the genes are transcribed inside morula cells, a characteristic haemocyte type in ascidians where PO activity is located, at the beginning of their differentiation. These results represent a first attempt to identify candidate molecules responsible of the PO activity in compound ascidians.

Natural apoptosis during the blastogenetic cycle of the colonial ascidian Botryllus schlosseri: a morphological analysis

Colonies of the compound ascidian Botryllus schiosseri undergo regular generation changes, during which adult zooids are progressively resorbed and replaced by growing buds. The generation change, or take-over, is characterized by massive cell death by apoptosis, as indicated by nuclear condensation, activation of caspases, overexpression of molecules recognized by antibodies against mammalian Bax, Fas, and FasL, changes in the expression of surface molecules by senescent cells of zooid tissues, and recruitment of circulating phagocytes in zooid tissues which ensure the complete clearing of dying cells. The entire process lasts 24-36 h at 20 degrees C and has been subdivided, on the basis of the degree of contraction of old zooids, into four substages. In the present work, we carried out a detailed morphological analysis of the events occurring in zooid tissues during the take-over substages. Results Indicate that traces of apoptosis can be found in the epidermis, peribranchial epithelium, and heart in the late substage but are easily found in the branchial basket 2-4 h after the beginning of the generation change, thus confirming the antero-posterior progression of cell death, at least in the alimentary system.

Hovering between death and life: natural apoptosis and phagocytes in the blastogenetic cycle of the colonial ascidian Botryllus schlosseri

Colonies of the compound ascidian Botryllus schlosseri undergo recurrent generation changes during which massive, natural apoptosis occurs in zooid tissues: for this reason the species is emerging as an interesting model of invertebrate chordate, phylogenetically related to vertebrates, for studies of apoptosis during development. In the present work, we carried out a series of morphological, cytofluorimetrical and biochemical analyses, useful for a better characterization of Botryllus apoptosis. Results are consistent with the following viewpoints: (i) both intrinsic and extrinsic pathways, probably connected by the BH3-only protein Bid, are involved in cell death induction; (ii) phagocytes, once loaded with senescent cells, frequently undergo apoptosis, probably as a consequence of oxidative stress caused by prolonged respiratory burst, and (iii) senescent phagocytes are easily recognized and ingested by other phagocytes, responsible for their clearance. In addition, results suggest the conservation of apoptosis induction mechanisms throughout chordate evolution.