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Formation of the definitive endoderm in mouse is a Smad2-dependent process
K D Tremblay1, P A Hoodless, E K Bikoff
1Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138, USA.
Abstract:
TGFbeta growth factors specify cell fate and establish the body plan during early vertebrate development. Diverse cellular responses are elicited via interactions with specific cell surface receptor kinases that in turn activate Smad effector proteins. Smad2-dependent signals arising in the extraembryonic tissues of early mouse embryos serve to restrict the site of primitive streak formation and establish anteroposterior identity in the epiblast. Here we have generated chimeric embryos using lacZ-marked Smad2-deficient ES cells. Smad2 mutant cells extensively colonize ectodermal and mesodermal populations without disturbing normal development, but are not recruited into the definitive endoderm lineage during gastrulation. These experiments provide the first evidence that TGFbeta signaling pathways are required for specification of the definitive endoderm lineage in mammals and identify Smad2 as a key mediator that directs epiblast derivatives towards an endodermal as opposed to a mesodermal fate. In largely Smad2-deficient chimeras, asymmetric nodal gene expression is maintained and expression of pitx2, a nodal target, is also unaffected. These results strongly suggest that other Smad(s) act downstream of Nodal signals in mesodermal populations. We found Smad2 and Smad3 transcripts both broadly expressed in derivatives of the epiblast. However, Smad2 and not Smad3 mRNA is expressed in the visceral endoderm, potentially explaining why the primary defect in Smad2 mutant embryos originates in this cell population.
Insights
Transforming growth factor beta (TGFbeta) signaling is crucial for early development. Smad2 is essential for specifying definitive endoderm in mouse embryos, directing cell fate decisions during gastrulation.
Area of Science:
- Developmental Biology
- Cell Signaling
- Genetics
Background:
- Transforming growth factor beta (TGFbeta) signaling pathways regulate cell fate and body plan establishment in vertebrate development.
- Smad proteins act as key mediators in TGFbeta signaling cascades, translating extracellular signals into intracellular responses.
- Smad2-dependent signals in early mouse embryos influence primitive streak formation and anteroposterior patterning.
Purpose of the Study:
- To investigate the role of Smad2 in the specification of the definitive endoderm lineage during mammalian gastrulation.
- To determine if Smad2 is required for directing epiblast derivatives towards an endodermal fate.
- To explore the involvement of other Smad proteins in mesodermal populations downstream of Nodal signals.
Main Methods:
- Generation of chimeric mouse embryos using lacZ-marked Smad2-deficient embryonic stem (ES) cells.
- Analysis of cell colonization and lineage recruitment in chimeric embryos during gastrulation.
- Assessment of gene expression patterns, including Nodal and Pitx2, in Smad2-deficient cells and tissues.
Main Results:
- Smad2-deficient ES cells extensively colonized ectodermal and mesodermal lineages but were not recruited into the definitive endoderm.
- TGFbeta signaling, mediated by Smad2, is essential for the specification of the definitive endoderm in mammals.
- Asymmetric Nodal gene expression and Pitx2 expression remained unaffected in Smad2-deficient chimeras, suggesting alternative Smad involvement in mesodermal populations.
- Smad2, but not Smad3, mRNA expression was detected in the visceral endoderm, correlating with the primary developmental defect observed.
Conclusions:
- Smad2 is a critical mediator of TGFbeta signaling required for definitive endoderm specification in mouse embryos.
- Smad2 plays a key role in directing epiblast cell fate towards endoderm rather than mesoderm.
- These findings highlight distinct roles for Smad proteins in different embryonic lineages and signaling pathways.