[Seminar] Establishment of body axes: Animal-vegetal and left-right axis specification in ascidian eggs and embryos by Hiroki Nishida
Title: Establishment of body axes: Animal-vegetal and left-right axis specification in ascidian eggs and embryos
Speaker: Professor. Hiroki Nishida
Institution: Department of Biological Sciences, Graduate School of Science, Osaka University
Ascidians are marine invertebrate chordates and the closest relative of vertebrates in phylogeny. Both share basic body plan, fate map, and morphogenetic processes. I am going to talk about our recent analyses on axis establishment during ascidian embryogenesis.
Animal-vegetal axis: The animal-vegetal (A-V) axis is already set in unfertilized eggs. It plays crucial roles to coordinate germ layer formation. In chordates, ectoderm, mesoderm, and endoderm are arranged along the axis in this order. However, it has not been well studied how the A-V axis is set. In the ascidian, Halocynthia roretzi, unfertilized eggs are already polarized along the axis in terms of cellular components, such as mitochondria and localized maternal RNAs. The polarization takes place during oocyte maturation. The oocytes within the gonad have the germinal vesicle (GV) closely to the future animal pole. We showed that the eccentric position of the GV directs orientation of the A-V axis. The GV of full-grown oocytes were experimentally translocated to the opposite pole by centrifugal force. In these GV-reversed oocytes, every aspect that designates the A-V polarity was reversed, namely, the cortical allocation of the meiotic spindle, position of polar body emission, polarized distribution of mitochondria and the postplasmic/PEM mRNA, direction of the cortical flow during oocyte maturation, cleavage pattern, and future fate map in relation to germ layer formation during embryogenesis. Therefore, the eccentric position of the GV triggers subsequent polarizing events, and establishes the A-V axis in matured eggs and developing embryos. This is the first report that the A-V axis was experimentally and totally reversed in animal oocytes before fertilization.
Left-right axis: Stereotyped left–right asymmetry both in external and internal organization is found in various animals. Left-right symmetry is broken by the neurula rotation in ascidians. Neurula embryos rotate along the anterior–posterior axis in a counterclockwise direction, and the rotation stops when the left side of the embryo is oriented downwards, resulting in contact of the left-side epidermis with the vitelline membrane at the bottom of perivitelline space. Then, such contact induces the expression of nodal and its downstream Pitx2 gene in the left-side epidermis. This is a novel mechanism in which rotation of whole embryos provides the initial cue for breaking left–right symmetry. Epidermal monocilia, which appear at the neurula rotation stage, generate the driving force for rotation. Epidermis cilia moved in a wavy and serpentine way like sperm flagella. They moved very slowly, at 1/7 Hertz, consistent with the low angular velocity of neurula rotation (ca. 43°/min). We also showed that a chemical signal from the vitelline membrane promotes nodal gene expression since the treatment of devitellinated neurulae with an extract of the vitelline membrane promoted nodal expression on both sides. These signal molecules are proteins but not sugars. By mass spectrometry, we selected 48 candidate proteins. It is of evolutionary interest that ascidians use ciliary movements to break embryonic left–right symmetry, like in many vertebrates. Meanwhile, ascidian embryos rotate as a whole, similar to embryos of non-vertebrate deuterostomes, such as echinoderm, hemichordate, and amphioxus, while swimming.
Hiroki Nishida completed his undergraduate work at Tohoku University, then he received his PhD from Kyoto University in 1987 for his studies on cell lineage during ascidian embryogenesis. He served as assistant professor at Kobe University in 1988 and then associate processor of a laboratory at Tokyo Institute of Technology in 1991. Since 2003, he has been professor in the Department of Biological Sciences, Osaka University. Dr. Nishida’s scientific accomplishments have been recognized by several awards such as the Zoological Society Prize (2003), Osaka Science Prize (2004), Inoue Prize for Science (2005). Nishida also acts on the editorial board of a number of journals and is a section editor for Development Genes, and Evolution. The main focus of his current studies is to understand mechanisms of establishment of embryonic axes, cell fate specification, and morphogenesis during ascidian and appendicularian embryogenesis.