Regulation of the number of cell division rounds by tissue-specific transcription factors and Cdk inhibitor during ascidian embryogenesis

Zic-r.b controls cell numbers in Ciona embryos by activating CDKN1B

The control of cell numbers and the establishment of cell types are two processes that are essential in early embryonic development. We have a reasonable understanding of how these processes occur individually, but we have considerably less sophisticated understanding of how these processes are linked. Tunicates have fixed cell lineages with predictable cell cycles, making them well suited to investigate these processes. In the ascidian Ciona, we show that the transcription factor Zic-r.b, known to be involved in establishing several cell types in early development also activates the expression of the cell cycle inhibitor CDKN1B. Zic-r.b is a major missing component of the cell division clock establishing specific cell numbers. We also show that a larvacean homolog of Zic-r.b is expressed one cell cycle earlier than its Ciona counterpart. The early expression in larvaceans may explain why they have half as many notochord cells as ascidians and may illustrate a general mechanism to evolve changes in morphology.

Regulators specifying cell fate activate cell cycle regulator genes to determine cell numbers in ascidian larval tissues

In animal development, most cell types stop dividing before terminal differentiation; thus, cell cycle control is tightly linked to cell differentiation programmes. In ascidian embryos, cell lineages do not vary among individuals, and rounds of the cell cycle are determined according to cell lineages. Notochord and muscle cells stop dividing after eight or nine rounds of cell division depending on their lineages. In the present study, we showed that a Cdk inhibitor, Cdkn1.b, is responsible for stopping cell cycle progression in these lineages. Cdkn1.b is also necessary for epidermal cells to stop dividing. In contrast, mesenchymal and endodermal cells continue to divide even after hatching, and Myc is responsible for maintaining cell cycle progression in these tissues. Expression of Cdkn1.b in notochord and muscle is controlled by transcription factors that specify the developmental fate of notochord and muscle. Likewise, expression of Myc in mesenchyme and endoderm is under control of transcription factors that specify the developmental fate of mesenchyme and endoderm. Thus, cell fate specification and cell cycle control are linked by these transcription factors.

Different strategies for tissue scaling in dwarf tailbud embryos revealed by single-cell analysis

The tailbud stage is part of the organogenesis period-an evolutionarily conserved developmental period among chordates that is essential for determining the characteristics of the chordate body plan. When the volume of the egg is artificially decreased by cutting, ascidians produce a normal-looking but miniature (dwarf) tailbud embryo. Although cell lineages during ascidian embryogenesis are invariant, the number of cell divisions in the dwarf embryo is altered by a different mechanism in each tissue (Yamada and Nishida, 1999). Here, to elucidate the size-regulation strategies of the Ciona robusta dwarf tailbud embryo, we compared anatomical structure, developmental speed, and cell number/volume in each tissue between dwarf and wild type (WT) embryos. To do this, we constructed a 3D virtual mid-tailbud embryo (Nakamura et al., 2012). We could make a Ciona dwarf tailbud embryo from eggs with a diameter over 108 ​μm (correspond to ​> ​40% of the wild type egg volume). The timings of cleavage (~St. 12) and subsequent morphogenesis were nearly the same but blastomeres of animal hemisphere slightly delayed the timing of mitosis in the early cleavage period. Intriguingly, the tissue-to-tissue volume ratios of dwarf tailbud embryos were similar to those of wild type embryos suggesting that the ratio of tissue volumes is essential for maintaining the proper shape of the tailbud embryo. The number of cells in the epidermis, nervous system, and mesenchyme was significantly reduced in the dwarf embryos whereas the cell volume distribution of these tissues was similar in the dwarf and wild type. In contrast, the number of cells in the notochord, muscle, heart, and endoderm were maintained in the dwarf embryos; cell volumes were significantly reduced. Neither parameter changed in germline precursors. These results indicate that each tissue uses different scaling strategies to coordinate cell number and cell volume in accordance with the embryo size.

Dynein-Mediated Regional Cell Division Reorientation Shapes a Tailbud Embryo

Regulation of cell division orientation controls the spatial distribution of cells during development and is essential for one-directional tissue transformation, such as elongation. However, little is known about whether it plays a role in other types of tissue morphogenesis. Using an ascidian Halocynthia roretzi, we found that differently oriented cell divisions in the epidermis of the future trunk (anterior) and tail (posterior) regions create an hourglass-like epithelial bending between the two regions to shape the tailbud embryo. Our results show that posterior epidermal cells are polarized with dynein protein anteriorly localized, undergo dynein-dependent spindle rotation, and divide along the anteroposterior axis. This cell division facilitates constriction around the embryo's circumference only in the posterior region and epithelial bending formation. Our findings, therefore, provide an important insight into the role of oriented cell division in tissue morphogenesis.

Regulation and evolution of muscle development in tunicates

For more than a century, studies on tunicate muscle formation have revealed many principles of cell fate specification, gene regulation, morphogenesis, and evolution. Here, we review the key studies that have probed the development of all the various muscle cell types in a wide variety of tunicate species. We seize this occasion to explore the implications and questions raised by these findings in the broader context of muscle evolution in chordates.

An FGF-driven feed-forward circuit patterns the cardiopharyngeal mesoderm in space and time

In embryos, multipotent progenitors divide to produce distinct progeny and express their full potential. In vertebrates, multipotent cardiopharyngeal progenitors produce second-heart-field-derived cardiomyocytes, and branchiomeric skeletal head muscles. However, the mechanisms underlying these early fate choices remain largely elusive. The tunicate Ciona emerged as an attractive model to study early cardiopharyngeal development at high resolution: through two asymmetric and oriented divisions, defined cardiopharyngeal progenitors produce distinct first and second heart precursors, and pharyngeal muscle (aka atrial siphon muscle, ASM) precursors. Here, we demonstrate that differential FGF-MAPK signaling distinguishes between heart and ASM precursors. We characterize a feed-forward circuit that promotes the successive activations of essential ASM determinants, Hand-related, Tbx1/10 and Ebf. Finally, we show that coupling FGF-MAPK restriction and cardiopharyngeal network deployment with cell divisions defines the timing of gene expression and permits the emergence of diverse cell types from multipotent progenitors.

T-Box Genes and Developmental Gene Regulatory Networks in Ascidians

Ascidians are invertebrate chordates with a biphasic life cycle characterized by a dual body plan that displays simplified versions of chordate structures, such as a premetamorphic 40-cell notochord topped by a dorsal nerve cord and postmetamorphic pharyngeal slits. These relatively simple chordates are characterized by rapid development, compact genomes and ease of transgenesis, and thus provide the opportunity to rapidly characterize the genomic organization, developmental function, and transcriptional regulation of evolutionarily conserved gene families. This review summarizes the current knowledge on members of the T-box family of transcription factors in Ciona and other ascidians. In both chordate and nonchordate animals, these genes control a variety of morphogenetic processes, and their mutations are responsible for malformations and developmental defects in organisms ranging from flies to humans. In ascidians, T-box transcription factors are required for the formation and specialization of essential structures, including notochord, muscle, heart, and differentiated neurons. In recent years, the experimental advantages offered by ascidian embryos have allowed the rapid accumulation of a wealth of information on the molecular mechanisms that regulate the expression of T-box genes. These studies have also elucidated the strategies employed by these transcription factors to orchestrate the appropriate spatial and temporal deployment of the numerous target genes that they control.

New targeted therapies for breast cancer: A focus on tumor microenvironmental signals and chemoresistant breast cancers

Breast cancer is the most frequent female malignancy worldwide. Current strategies in breast cancer therapy, including classical chemotherapy, hormone therapy, and targeted therapies, are usually associated with chemoresistance and serious adverse effects. Advances in our understanding of changes affecting the interactome in advanced and chemoresistant breast tumors have provided novel therapeutic targets, including, cyclin dependent kinases, mammalian target of rapamycin, Notch, Wnt and Shh. Inhibitors of these molecules recently entered clinical trials in mono- and combination therapy in metastatic and chemo-resistant breast cancers. Anticancer epigenetic drugs, mainly histone deacetylase inhibitors and DNA methyltransferase inhibitors, also entered clinical trials. Because of the complexity and heterogeneity of breast cancer, the future in therapy lies in the application of individualized tailored regimens. Emerging therapeutic targets and the implications for personalized-based therapy development in breast cancer are herein discussed.