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J Theor Biol 2014 Jul 21;353:67-77. doi: 10.1016/j.jtbi.2014.03.015.
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Two different network topologies yield bistability in models of mesoderm and anterior mesendoderm specification in amphibians.

Brown LE , King JR , Loose M .

Understanding the Gene Regulatory Networks (GRNs) that underlie development is a major question for systems biology. The establishment of the germ layers is amongst the earliest events of development and has been characterised in numerous model systems. The establishment of the mesoderm is best characterised in the frog Xenopus laevis and has been well studied both experimentally and mathematically. However, the Xenopus network has significant differences from that in mouse and humans, including the presence of multiple copies of two key genes in the network, Mix and Nodal. The axolotl, a urodele amphibian, provides a model with all the benefits of amphibian embryology but crucially only a single Mix and Nodal gene required for the specification of the mesoderm. Remarkably, the number of genes within the network is not the only difference. The interaction between Mix and Brachyury, two transcription factors involved in the establishment of the endoderm and mesoderm respectively, is not conserved. While Mix represses Brachyury in Xenopus, it activates Brachyury in axolotl. Thus, whilst the topology of the networks in the two species differs, both are able to form mesoderm and endoderm in vivo. Based on current knowledge of the structure of the mesendoderm GRN we develop deterministic models that describe the time evolution of transcription factors in a single axolotl cell and compare numerical simulations with previous results from Xenopus. The models are shown to have stable steady states corresponding to mesoderm and anterior mesendoderm, with the in vitro model showing how the concentration of Activin can determine cell fate, while the in vivo model shows that β-catenin concentration can determine cell fate. Moreover, our analysis suggests that additional components must be important in the axolotl network in the specification of the full range of tissues.

PubMed ID: 24650939
PMC ID: PMC4029075
Article link: J Theor Biol

Species referenced: Xenopus laevis
Genes referenced: bmpr1a chrd.1 ctnnb1 fgf4 fgf8 grn gsc hoxb4 hoxb9 hoxc6 hoxd1 lhx1 mixer ncam1 nodal nodal1 nodal2 nodal5.2 nodal5.4 nodal6 sia1 sox17a tbx2 tbxt vegt ventx1 zic3

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References [+] :
Bourillot, A changing morphogen gradient is interpreted by continuous transduction flow. 2002, Pubmed, Xenbase