Click here to close
Hello! We notice that you are using Internet Explorer, which is not supported by Xenbase and may cause the site to display incorrectly.
We suggest using a current version of Chrome,
FireFox, or Safari.
Two different network topologies yield bistability in models of mesoderm and anteriormesendoderm specification in amphibians.
Brown LE
,
King JR
,
Loose M
.
???displayArticle.abstract???
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 mesendodermGRN 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 anteriormesendoderm, 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.
Fig. 1. A comparison of the axolotl and Xenopus mesendoderm GRNs. (a) The axolotl mesendodermGRN and (b) the simplified Xenopus mesendodermGRN. Arrow and bar heads represent, respectively, activation and repression. The âAâ indicates that an input is, in Boolean terms, an âANDâ gate. The âSâ indicates a synergy between the two transcription factors, i.e. β-catenin activates Nodal1 and this activation is enhanced by Nodal autoregulation. Otherwise, multiple inputs consisting of only one type (repression or activation) correspond to an âORâ gate. When both types are present, the repression and activation inputs are treated as two âORâ gates coupled by an âANDâ gate. Red lines show interactions which are the same in both networks and blue lines show those which differ. In (b) solid lines indicate experimentally verified links and dashed lines indicate links which are inferred from the Xenopus mesendoderm network, and which need to be verified experimentally. (c) Table summarising the main differences between the axolotl and Xenopus mesendoderm GRNs. Row 1: At least 6 Nodal genes are found in Xenopus, compared with 2 Nodal genes in axolotl. Row 2: There are seven Mix genes in Xenopus and one Mix gene in axolotl. Row 3: VegT acts to activate expression of Nodal, Mix and Brachyury in Xenopus, but in axolotl VegT does not activate these genes. Row 4: Siamois is a gene found in Xenopus but not axolotl. Row 5: In Xenopus, β-catenin acts in two different ways on Nodal: β-catenin enhances Nodal autoregulation of Xnr1 and Xnr2, and the expression of Xnr5 and Xnr6 is activated by β-catenin in the presence of VegT. In axolotl, Nodal1 can be activated by β-catenin alone and we also assume that it can enhance Nodal autoregulation. Row 6: Mix and Brachyury mutually repress each other in Xenopus, but, in axolotl, Mix is required for the expression of Brachyury. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this paper.)
Fig. 2. Mesoderm and anterior mesendoderm induction by Activin in animal cap explants (48 h after animal caps explants are cut from embryo). (A) Axolotl animal caps injected with 1pg Activin mRNA induce mesoderm, and 25pg of Activin induces anterior mesendoderm. (B) qPCR analysis of Brachyury, Mix, Sox17, Goosecoid, FGF8 and NCAM expression in animal caps.
Fig. 3. The axolotl in vitro network: Nodal signalling is simulated by bathing either whole or dissociated animal caps in Activin. Note that this network is identical to the simplified Xenopus in vitro network, except that Mix is required here for the expression of Brachyury.
Fig. 4. (a) Steady state solutions to (7) plotted against λAM,B for A=5. Thick solid lines represent the mesoderm steady state, thin solid lines represent the anterior mesendoderm steady state and dashed lines represent the unstable steady state. Fold bifurcations mark the appearance and the disappearance of the steady states. (b) Solution structure in terms of the bifurcation parameters λAM,B and λA,M, these representing the folds that determine the maximum rates of production of Brachyury and Mix in response to activation by Activin. (c) Solution structure in terms of the bifurcation parameters λAM,B and λM,G, these representing the folds that determine the maximum rates of production of Brachyury in response to activation by Activin and Goosecoid in response to Mix. Unless otherwise stated, parameters were chosen as in Table 2.
Fig. 5. Numerical solutions of the axolotl in vitro model. The responses of Brachyury (thin solid line), Mix (dashed line) and Goosecoid (dot-dashed line) are shown. Parameters were chosen as in Table 2.
Fig. 6. Numerical solutions of the axolotl in vitro model as functions of A for (a) A=4, (b) A=4.5, (c) A=6. The responses of Brachyury (thin solid line), Mix (dashed line) and Goosecoid (dot-dashed line) are shown. Parameters were chosen as in Table 2.
Fig. 7. Numerical solutions of the Xenopus in vitro model (Middleton et al., 2009) and the axolotl in vitro model (7), in the absence of Goosecoid, as functions of A. The responses of Brachyury (thin solid line), Mix (dashed line) and Goosecoid (dot-dashed line) are shown. Parameters used are given in Table 2 for the axolotl model and as given in Middleton et al. (2009) for the Xenopus. Values of λM,G, θG,G and λXM,B are higher in the axolotl model than in the Xenopus model, corresponding to higher rates of production of Mix and Brachyury and a higher threshold for Goosecoid negative autoregulation.
Fig. 8. Numerical solutions to (9) subject to initial conditions (10) with C0 as shown above. For sufficiently large C0, N tends to Nâ, first overshooting this value. Parameters were chosen as in Table 3.
Fig. 9. Numerical solutions of the axolotl model as functions of C0, for various values of Ï. The response of Brachyury (thin solid line), eFGF (dotted line), Mix (dashed line), Goosecoid (dot-dashed line) and Nodal (blue solid line) are shown in response to an initial concentration of β-catenin. Parameters were chosen as in Table 3. (a) T=0.5, (b) T=1, (c) T=8 and (d) T=100. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this paper.)
Fig. 10. Gene regulatory networks which yield bistability. (A) Mutual negative regulation between X and Y drives differentiation of a cell to express either X or Y, with the indirect repression of X by Y (via Z) being dispensible. (B) An alternative network, where the indirect repression of X by Y (via Z) is necessary to drive differentiation of a cell to express Y.
Bourillot,
A changing morphogen gradient is interpreted by continuous transduction flow.
2002, Pubmed,
Xenbase
Bourillot,
A changing morphogen gradient is interpreted by continuous transduction flow.
2002,
Pubmed
,
Xenbase
Cho,
Molecular nature of Spemann's organizer: the role of the Xenopus homeobox gene goosecoid.
1991,
Pubmed
,
Xenbase
Clements,
Mode of action of VegT in mesoderm and endoderm formation.
1999,
Pubmed
,
Xenbase
Davidson,
A genomic regulatory network for development.
2002,
Pubmed
Delaune,
Neural induction in Xenopus requires early FGF signalling in addition to BMP inhibition.
2005,
Pubmed
,
Xenbase
Dixon,
Axolotl Nanog activity in mouse embryonic stem cells demonstrates that ground state pluripotency is conserved from urodele amphibians to mammals.
2010,
Pubmed
Green,
Graded changes in dose of a Xenopus activin A homologue elicit stepwise transitions in embryonic cell fate.
1990,
Pubmed
,
Xenbase
Guo,
A human Mix-like homeobox gene MIXL shows functional similarity to Xenopus Mix.1.
2002,
Pubmed
,
Xenbase
Gurdon,
Activin signalling and response to a morphogen gradient.
1994,
Pubmed
,
Xenbase
Gurdon,
An experimental system for analyzing response to a morphogen gradient.
1996,
Pubmed
,
Xenbase
Gurdon,
Single cells can sense their position in a morphogen gradient.
1999,
Pubmed
,
Xenbase
Hellsten,
The genome of the Western clawed frog Xenopus tropicalis.
2010,
Pubmed
,
Xenbase
Isaacs,
eFGF regulates Xbra expression during Xenopus gastrulation.
1994,
Pubmed
,
Xenbase
Johnson,
Evolution of the germ line-soma relationship in vertebrate embryos.
2011,
Pubmed
Johnson,
Evolution of predetermined germ cells in vertebrate embryos: implications for macroevolution.
2003,
Pubmed
Kaneda,
Gastrulation and pre-gastrulation morphogenesis, inductions, and gene expression: similarities and dissimilarities between urodelean and anuran embryos.
2012,
Pubmed
,
Xenbase
Kofron,
Mesoderm induction in Xenopus is a zygotic event regulated by maternal VegT via TGFbeta growth factors.
1999,
Pubmed
,
Xenbase
Koide,
Xenopus as a model system to study transcriptional regulatory networks.
2005,
Pubmed
,
Xenbase
Latinkic,
Goosecoid and mix.1 repress Brachyury expression and are required for head formation in Xenopus.
1999,
Pubmed
,
Xenbase
Lee,
Timing of endogenous activin-like signals and regional specification of the Xenopus embryo.
2001,
Pubmed
,
Xenbase
Lemaire,
A role for the vegetally expressed Xenopus gene Mix.1 in endoderm formation and in the restriction of mesoderm to the marginal zone.
1998,
Pubmed
,
Xenbase
Loose,
A genetic regulatory network for Xenopus mesendoderm formation.
2004,
Pubmed
,
Xenbase
Maduro,
Endomesoderm specification in Caenorhabditis elegans and other nematodes.
2006,
Pubmed
Middleton,
Bistability in a model of mesoderm and anterior mesendoderm specification in Xenopus laevis.
2009,
Pubmed
,
Xenbase
Nath,
RNA of AmVegT, the axolotl orthologue of the Xenopus meso-endodermal determinant, is not localized in the oocyte.
2007,
Pubmed
,
Xenbase
Papin,
Gradual refinement of activin-induced thresholds requires protein synthesis.
2000,
Pubmed
,
Xenbase
Saka,
A mechanism for the sharp transition of morphogen gradient interpretation in Xenopus.
2007,
Pubmed
,
Xenbase
Schohl,
A role for maternal beta-catenin in early mesoderm induction in Xenopus.
2003,
Pubmed
,
Xenbase
Schohl,
Beta-catenin, MAPK and Smad signaling during early Xenopus development.
2002,
Pubmed
,
Xenbase
Swiers,
A conserved mechanism for vertebrate mesoderm specification in urodele amphibians and mammals.
2010,
Pubmed
,
Xenbase
Takahashi,
Two novel nodal-related genes initiate early inductive events in Xenopus Nieuwkoop center.
2000,
Pubmed
,
Xenbase
Weaver,
Move it or lose it: axis specification in Xenopus.
2004,
Pubmed
,
Xenbase
Zhang,
The role of maternal VegT in establishing the primary germ layers in Xenopus embryos.
1998,
Pubmed
,
Xenbase
Zhou,
Nodal is a novel TGF-beta-like gene expressed in the mouse node during gastrulation.
1993,
Pubmed
,
Xenbase