Brief hypo-osmotic shock causes test cell death, prevents neurula rotation, and disrupts left-right asymmetry in Ciona intestinalis

Bilaterally Asymmetric Helical Myofibrils in Ascidian Tadpole Larvae

The locomotor system is highly bilateral at the macroscopic level. Homochirality of biological molecules is fully compatible with the bilateral body. However, whether and how single-handed cells contribute to the bilateral locomotor system is obscure. Here, exploiting the small number of cells in the swimming tadpole larva of the ascidian Ciona, we analyzed morphology of the tail at cellular and subcellular scales. Quantitative phase-contrast X-ray tomographic microscopy revealed a high-density midline structure ventral to the notochord in the tail. Muscle cell nuclei on each side of the notochord were roughly bilaterally aligned. However, fluorescence microscopy detected left-right asymmetry of myofibril inclination relative to the longitudinal axis of the tail. Zernike phase-contrast X-ray tomographic microscopy revealed the presence of left-handed helices of myofibrils in muscle cells on both sides. Therefore, the locomotor system of ascidian larvae harbors symmetry-breaking left-handed helical cells, while maintaining bilaterally symmetrical cell alignment. These results suggest that bilateral animals can override cellular homochirality to generate the bilateral locomotor systems at the supracellular scale.

Wavy movements of epidermis monocilia drive the neurula rotation that determines left-right asymmetry in ascidian embryos

Tadpole larvae of the ascidian, Halocynthia roretzi, show morphological left-right asymmetry in the brain structures and the orientation of tail bending within the vitelline membrane. Neurula embryos rotate along the anterior-posterior axis in a counterclockwise direction, and then this rotation stops when the left side of the embryo is oriented downwards. Contact of the left-side epidermis with the vitelline membrane promotes nodal gene expression 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. Here we show that epidermal monocilia, which appear at the neurula rotation stage, generate the driving force for rotation. A ciliary protein, Arl13b, fused with Venus YFP was used for live imaging of ciliary movements. Although overexpression of wild-type Arl13b fusion protein resulted in aberrant movements of the cilia and abrogation of neurula rotation, mutant Arl13b fusion protein, in which the GTPase and coiled-coil domains were removed, did not affect the normal ciliary movements and neurula rotation. Epidermis cilia moved in a wavy and serpentine way like sperm flagella but not in a rotational way or beating way with effective stroke and recovery stroke. They moved very slowly, at 1/7 Hz, consistent with the low angular velocity of neurula rotation (ca. 43°/min). The tips of most cilia pointed in the opposite direction of embryonic rotation. Similar motility was also observed in Ciona robusta embryos. When embryos were treated with a dynein inhibitor, Ciliobrevin D, both ciliary movements and neurula rotation were abrogated, showing that ciliary movements drive neurula rotation in Halocynthia. The drug also inhibited Ciona neurula rotation. Our observations suggest that the driving force of rotation is generated using the vitelline membrane as a substrate but not by making a water current around the embryo. 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.

Vitelline membrane proteins promote left-sided nodal expression after neurula rotation in the ascidian, Halocynthia roretzi

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 the ascidian, Halocynthia roretzi. 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. Vitelline membrane is required for the promotion of nodal expression. Here, we showed that a chemical signal from the vitelline membrane promotes nodal gene expression, but mechanical stimulus at the point of contact is unnecessary since the treatment of devitellinated neurulae with an extract of the vitelline membrane promoted nodal expression on both sides. The signal molecules are already present in the vitelline membranes of unfertilized eggs. These signal molecules are proteins but not sugars. Specific fractions in gel filtration chromatography had the nodal promoting activity. By mass spectrometry, we selected 48 candidate proteins. Proteins that contain both a zona pellucida (ZP) domain and epidermal growth factor (EGF) repeats were enriched in the candidates of the nodal inducing molecules. Six of the ZP proteins had multiple EGF repeats that are only found in ascidian ZP proteins. These were considered to be the most viable candidates of the nodal-inducing molecules. Signal molecules are anchored to the entire vitelline membrane, and contact sites of signal-receiving cells are spatially and mechanically controlled by the neurula rotation. In this context, ascidians are unusual with respect to mechanisms for specification of the left-right axis. By suppressing formation of epidermis monocilia, we also showed that epidermal cilia drive the neurula rotation but are dispensable for sensing the signal from the vitelline membrane.

Establishment of lateral organ asymmetries in the invertebrate chordate, Ciona intestinalis

BACKGROUND

The evolutionary emergence and diversification of the chordates appear to involve dramatic changes in organ morphogenesis along the left/right axis. However, the ancestral chordate mechanism for establishing lateral asymmetry remains ambiguous. Additionally, links between the initial establishment of lateral asymmetry and subsequent asymmetries in organ morphogenesis are poorly characterized.

RESULTS

To explore asymmetric organ morphogenesis during chordate evolution, we have begun to characterize left/right patterning of the heart and endodermal organs in an invertebrate chordate, Ciona intestinalis. Here, we show that Ciona has a laterally asymmetric, right-sided heart. Our data indicate that cardiac lateral asymmetry requires H+/K+ ion flux, but is independent of Nodal signaling. Our pharmacological inhibitor studies show that ion flux is required for polarization of epidermal cilia and neurula rotation and suggest that ion flux functions synergistically with chorion contact to drive cardiac laterality. Live imaging analysis revealed that larval heart progenitor cells undergo a lateral shift without displaying any migratory behaviors. Furthermore, we find that this passive shift corresponds with the emergence of lateral asymmetry in the endoderm, which is also ion flux dependent.

CONCLUSIONS

Our data suggest that ion flux promotes laterally asymmetric morphogenesis of the larval endoderm rudiment leading to a passive, Nodal-independent shift in the position of associated heart progenitor cells. These findings help to refine hypotheses regarding ancestral chordate left/right patterning mechanisms and how they have diverged within invertebrate and vertebrate chordate lineages.

The CNS connectome of a tadpole larva of Ciona intestinalis (L.) highlights sidedness in the brain of a chordate sibling

Left-right asymmetries in brains are usually minor or cryptic. We report brain asymmetries in the tiny, dorsal tubular nervous system of the ascidian tadpole larva, Ciona intestinalis. Chordate in body plan and development, the larva provides an outstanding example of brain asymmetry. Although early neural development is well studied, detailed cellular organization of the swimming larva's CNS remains unreported. Using serial-section EM we document the synaptic connectome of the larva's 177 CNS neurons. These formed 6618 synapses including 1772 neuromuscular junctions, augmented by 1206 gap junctions. Neurons are unipolar with at most a single dendrite, and few synapses. Some synapses are unpolarised, others form reciprocal or serial motifs; 922 were polyadic. Axo-axonal synapses predominate. Most neurons have ciliary organelles, and many features lack structural specialization. Despite equal cell numbers on both sides, neuron identities and pathways differ left/right. Brain vesicle asymmetries include a right ocellus and left coronet cells.