In VitroCulture of Mesenchymal Lineage Cells Established from the Colonial Tunicate Botryllus primigenus

Stress granule related-genes during blastogenetic cycle of two colonial ascidians: Botryllus schlosseri compared to Botryllus primigenus

BACKGROUND

Colonial ascidians are chordates known to reproduce both sexually and asexually. In the present study, we used the Mediterranean species Botryllus schlosseri and the Japanese Botryllus primigenus to investigate the possible role of tiar, ttp, and g3bp in the periodical renewal of the colonies, defined by generation changes or takeovers. In this scenario, the above genes, which codify key components for the formation of stress granules, storing specific mRNAs, can play a pivotal role, allowing the regulation of processes such as stress responses, cell proliferation, and stem cell development.

RESULTS

We characterized tiarx1, ttp, and g3bp2 sequences in B. schlosseri and B. primigenus. Then, we analyzed gene transcription by in situ hybridization in hemolymph cells and colony tissues, and we proceeded with quantification of the gene transcription by quantitative real‐time PCR, during the phases of the colonial blastogenetic cycle.

CONCLUSIONS

Our results allowed us to assign a role in the development of the new colonial generations to the studied genes.

Sixty years of experimental studies on the blastogenesis of the colonial tunicate Botryllus schlosseri

In the second half of the eighteenth century, Schlosser and Ellis described the colonial ascidian Botryllus schlosseri garnering the interest of scientists around the world. In the 1950's scientists began to study B. schlosseri and soon recognized it as an important model organism for the study of developmental biology and comparative immunology. In this review, we summarize the history of B. schlosseri studies and experiments performed to characterize the colony life cycle and bud development. We describe experiments performed to analyze variations in bud productivity, zooid growth and bilateral asymmetry (i.e., the situs viscerum), and discuss zooid and bud removal experiments that were used to study the cross-talk between consecutive blastogenetic generations and vascular budding. We also summarize experiments that demonstrated that the ability of two distinct colonies to fuse or reject is controlled by a single polymorphic gene locus (BHF) with multiple, codominantly expressed alleles. Finally, we describe how the ability to fuse and create chimeras was used to show that within a chimera somatic and germline stem cells compete to populate niches and regenerate tissue or germline organs. Starting from the results of these 60 years of study, we can now use new technological advances to expand the study of B. schlosseri traits and understand functional relationships between its genome and life history phenotypes.

Tunicate cytostatic factor TC14-3 induces a polycomb group gene and histone modification through Ca(2+) binding and protein dimerization

Background

As many invertebrate species have multipotent cells that undergo cell growth and differentiation during regeneration and budding, many unique and interesting homeostatic factors are expected to exist in those animals. However, our understanding of such factors and global mechanisms remains very poor. Single zooids of the tunicate, Polyandrocarpa misakiensis, can give off as many as 40 buds during the life span. Bud development proceeds by means of transdifferentiation of very limited number of cells and tissues. TC14-3 is one of several different but closely related polypeptides isolated from P. misakiensis. It acts as a cytostatic factor that regulates proliferation, adhesion, and differentiation of multipotent cells, although the molecular mechanism remains uncertain. The Polycomb group (PcG) genes are involved in epigenetic control of genomic activity in mammals. In invertebrates except Drosophila, PcG and histone methylation have not been studied so extensively, and genome-wide gene regulation is poorly understood.

Results

When Phe⁶⁵ of TC14-3 was mutated to an acidic amino acid, the resultant mutant protein failed to dimerize. The replacement of Thr⁶⁹ with Arg⁶⁹ made dimers unstable. When Glu¹⁰⁶ was changed to Gly¹⁰⁶, the resultant mutant protein completely lost Ca²⁺ binding. All these mutant proteins lacked cytostatic activity, indicating the requirement of protein dimerization and calcium for the activity. Polyandrocarpa Eed, a component of PcG, is highly expressed during budding, like TC14-3. When wild-type and mutant TC14-3s were applied in vivo and in vitro to Polyandrocarpa cells, only wild-type TC14-3 could induce Eed without affecting histone methyltransferase gene expression. Eed-expressing cells underwent trimethylation of histone H3 lysine27. PmEed knockdown by RNA interference rescued cultured cells from the growth-inhibitory effects of TC14-3.

Conclusion

These results show that in P. misakiensis, the cytostatic activity of TC14-3 is mediated by PmEed and resultant histone modification, and that the gene expression requires both the protein dimerization and Ca²⁺-binding of TC14-3. This system consisting of a humoral factor, PcG, and histone methylation would contribute to the homeostatic regulation of cell growth and terminal differentiation of invertebrate multipotent cells.

Role of Vasa, Piwi, and Myc-expressing coelomic cells in gonad regeneration of the colonial tunicate, Botryllus primigenus

In the colonial tunicate, Botryllus primigenus Oka, gonads consist of indifferent germline precursor cells, the primordial testis and ovary, and mature gonads, of which the immature gonad components can be reconstructed de novo in vascular buds that arise from the common vascular system, although the mechanism is uncertain. In this study, we investigated how and what kinds of cells regenerated the gonad components. We found that few Vasa-positive cells in the hemocoel entered the growing vascular bud, where their number increased, and finally developed exclusively into female germ cells. Simultaneously, small cell aggregates consisting of Vasa(-) and Vasa(±) cells appeared de novo in the lateral body cavity of developing vascular buds. Double fluorescent in situ hybridization showed that these cell aggregates were both Piwi- and Myc-positive. They could form germline precursor cells and a primordial testis and ovary that strongly expressed Vasa. Myc knockdown by RNA interference conspicuously lowered Piwi expression and resulted in the loss of germline precursor cells without affecting Vasa(+) oocyte formation. Myc may contribute to gonad tissue formation via Piwi maintenance. When human recombinant BMP 4 was injected in the test vessel, coelomic Piwi(+) cells were induced to express Vasa in the blood. We conclude, therefore, that in vascular buds of B. primigenus, female germ cells can develop from homing Vasa(+) cells in the blood, and that other gonad components can arise from coelomic Vasa(-)/Piwi(+)/Myc(+) cells.

Cell cultures from marine invertebrates: new insights for capturing endless stemness

Despite several decades of extensive research efforts, there is yet no single permanent cell line available from marine invertebrates as these cells stop dividing in vitro within 24-72 h after their isolation, starting cellular quiescence. This ubiquitous quiescent state should be modified in a way that at least some of the quiescent cells will become pluripotent, so they will have the ability to divide and become immortal. Following the above need, this essay introduces the rationale that the discipline of marine invertebrates' cell culture should gain from applying of two research routes, relevant to mammalian systems but less explored in the marine arena. The first is the use of adult stem cells (ASC) from marine organisms. Many marine invertebrate taxa maintain large pools of ASC in adulthood. Ample evidence attests that these cells from sponges, cnidarians, flatworms, crustaceans, mollusks, echinoderms, and ascidians play important roles in maintenance, regeneration, and asexual cloning, actively proliferating in vivo, resembling the vertebrates' cancer stem cells features. The second route is to target resting somatic cell constituents, manipulating them in the same way as has recently been performed on mammalian induced pluripotent stem (iPS) cells. While "iPS cells" are the outcome of an experimental manipulation, ASC are natural and rather frequent in a number of marine invertebrates. Above two cell categories reveal that there are more than a few types of seeds (cells) waiting to be sowed in the right soil (in vitro environmental conditions) for acquiring stemness and immortality. This rationale carries the potential to revolutionize the discipline of marine invertebrate cell cultures. When cultured "correctly," ASC and "iPS cells" from marine invertebrates may stay in their primitive stage and proliferate without differentiating into cells lineages, harnessing the stem cell's inherent abilities of self-replication versus differentiated progenies, toward the development of immortal cell lines.

Cell proliferation dynamics of somatic and germline tissues during zooidal life span in the colonial tunicate Botryllus primigenus

Botryllus primigenus is a colonial tunicate in which three successive generations develop synchronously. To identify proliferation centers and possible adult stem cells during asexual reproduction, somatic and germline cells were labeled with 5-bromo-2'-deoxyuridine (BrdU). In the youngest generation, multipotent epithelial cells exhibited an average labeling index (LI) of 30% 24 hr after BrdU injection. In the middle generation, the LI of organ rudiments decreased gradually and reached zero by the beginning of the eldest generation. Exceptionally, cells of specialized tissues such as the pharyngeal inner longitudinal vessel and the posterior end of the endostyle continued DNA synthesis and mitosis even in the eldest generation. Proliferating somatic and germline cells of younger generations expressed a Botryllus myc homolog (BpMyc), but adult tissues did not. This result strongly suggests that in B. primigenus undifferentiated progenitor cells are discernible from possible adult stem cells by the presence or absence of BpMyc.