Axis Determination in Zebrafish—flipping our understanding upside down
Jean-Daniel Fauny, Bernard Thisse and Christine Thisse. 2009. The entire zebrafish blastula-gastrula margin acts as an organizer dependent on the ratio of Nodal to BMP activity. Development. 136, 3811-3819.
Fu-I Lu, Christine Thisse, and Bernard Thisse. 2011. Identification and mechanism of regulation of the zebrafish dorsal determinant. PNAS 108:38. P15876-15880.
Briefly tell us a bit about yourself, your career path over the years, and specifically what brought you to begin working on axis determination in zebrafish.
To help set up our discussions today, can you describe further the benefits of having the ratio of BMP and Nodal signaling pathways control expression rather than through independent control of their absolute levels of stimulation?
Because the development of anterior/posterior identity relies on the ratio, and not the absolute concentrations, of Nodal and BMP expression, why can’t BMP levels adjust when Nodal concentrations are altered systemically? Is there no “check” mechanism for BMP?
In addition, in the 128 cell stage embryo, why does single stimulation of Nodal mRNA at the animal pole blastomere lead to ectopic tail structures and single stimulation of BMP RNA has no effect? If the organization is concentration dependent, why wouldn’t stimulation of BMP lead to any morphological changes? (2009, figure 3).
How far, in the anterior direction and in the posterior direction, from the blastula-gastrula margin do the cells possess the organizer activity described at the lateral margin? Do the structures that the cells develop into become less defined as the grafts move away from the midline?
Katharine von Herrmann
How would you expect the frequency of the ectopic structures to differ if the grafts were 4 discrete organizers as opposed to one continuous organizer?
Since there is the idea that the entire zebrafish margin acts as a continuous and global organizer, do you think that this could be due to the presence of a “global morphogen” whose concentration gradient regulates the levels of BMP and Nodal along the embryonic margin? If not then what mechanism initiates and maintains this ratio?
Lastly, it seems like there might be some applications to understanding these control mechanisms for regenerative medicine. Knowing what you now know, could you ever “grow” an embryo from a cluster of cells in a plate?
In Identification and mechanism of regulation of the zebrafish dorsal determinant, it was found that Wnt3a functions similarly to Wnt8a, both required for posterior mesoderm formation and “able to activate the maternal β-catenin pathway”, but Wnt3a is not a maternal determinant. After the mid-blastula transition does the role of Wnt8a stop or change, turning its job over to other genes such as Wnt3a?
Amphibians require Wnt11/5a (which are two noncanonical Wnts) in order to activate the canonical pathway. The same pathway is activated by Wnt8a (a canonical Wnt) in zebrafish. In both models, activation of the canonical pathway is necessary, seemingly more so than the exact means by which it is achieved. Would you expect alternative forms of activation to yield the same phenotype? For example, would you expect local application of Wnt8a (since it has been characterized as a strong activator of the canonical pathway) to rescue the wt phenotype in an amphibian Wnt11/5a knockdown?
Can you please elaborate on the role of Sfrp1a and Frzb during blastula and gastrula stages of specification, and specifically comment on what you propose activates Sfrp1a and Frzb only in the blastula stage as opposed to the gastrula stage?
It has been identified that Wnt8a plays a significant role in axis determination. Is there an interaction between Wnt and the Nodal/BMP ratio in embryonic development? How do you see these different signaling pathways working together? More specifically, how is the transition between the early BMP/Nodal and the later Wnt/β-catenin achieved? Is this change synchronous or do different structures switch control at various times?
Gwen Huynh and Emily Frontierer
You mention that Wnt8a has different effects on the maternal and zygotic beta catenin pathways. Does this mean there are differences in the function of these pathways or is it just a temporal change since the zygotic genome is not immediately active after fertilization?
Given the importance of the Balbiani body in Wnt8a mRNA accumulation and its revealing of the first symmetry in vertebrate oocytes, has there been any experiments about a possible complete removal of the Balbiani or taking a part of this tissue and transferring it to a different part of the body?
The PNAS paper shows that while “the molecular mechanism responsible for the initial radial symmetry breaking (an mRNA encoding a Wnt ligand, asymmetrically transported along microtubules, stimulating the canonical pathway) is conserved between amphibians and fish, different regulatory factors are used for these two classes of vertebrates” (Lu et al. p15879). You further state that “considering how essential this initial break of radial symmetry is for the establishment of the vertebrate body plan, we find these differences very surprising. Such a fundamental process would have been expected to exhibit a high degree of conservation of both the mechanisms and the specific determinants involved” (Lu et al. p15879). It seems clearly impossible that this is a result of convergent evolution. Do you have any hypotheses that might explain the result? Moreover, What are the evolutionary benefits of having only one dorsal determinant yet continuous organizing properties in the gastrula?
Dan McCune and Chelsea Moriarty
Currently in your opinion what are the most pressing questions in the study of axis determination, and what steps is your lab taking to address these questions?