Biomedical subjects

Daniel B Constam

Publications and source records attributed to Daniel B Constam.

7 recordsLinked to original sources

PubMed2006

Nodal specifies embryonic visceral endoderm and sustains pluripotent cells in the epiblast before overt axial patterning.

Anteroposterior (AP) polarity in the mammalian embryo is specified during gastrulation when naive progenitor cells in the primitive ectoderm are recruited into the primitive streak to form mesoderm and endoderm. At the opposite pole, this process is inhibited by signals previously induced in distal visceral endoderm (DVE). Both DVE and primitive streak formation, and hence positioning of the AP axis, rely on the TGFbeta family member Nodal and its proprotein convertases Furin and Pace4. Here, we show that Nodal and Furin are initially co-expressed in the primitive endoderm together with a subset of DVE markers such as Lefty1 and Hex. However, with the appearance of extra-embryonic ectoderm (ExE), DVE formation is transiently inhibited. During this stage, Nodal activity is essential to specify embryonic VE and restrict the expression of Furin to the extra-embryonic region. Activation of Nodal is also necessary to maintain determinants of pluripotency such as Oct4, Nanog and Foxd3 during implantation, and to stimulate elongation of the egg cylinder, before inducing DVE and germ layer formation. We conclude that Nodal is already activated in primitive endoderm, but induces a functional DVE only after promoting the expansion of embryonic VE and pluripotent progenitor cells in the epiblast.

Animals↗

PubMed2006

The nodal precursor acting via activin receptors induces mesoderm by maintaining a source of its convertases and BMP4.

During early mouse development, the subtilisin-like proprotein convertases (SPC) Furin and PACE4 pattern the primitive ectoderm and visceral endoderm, presumably by activating the TGFss-related Nodal precursor. Here, mutation of the SPC motif provides direct evidence that Nodal processing is essential to specify anterior visceral endoderm and mesendoderm. Surprisingly, however, the Nodal precursor binds and activates activin receptors to maintain expression of Furin, PACE4, and Bmp4 in extraembryonic ectoderm at a distance from the Nodal source. In return, Bmp4 induces Wnt3, which amplifies Nodal expression in the epiblast and mediates induction of mesoderm. We conclude that uncleaved Nodal sustains the extraembryonic source of proprotein convertases and Bmp4 to amplify Nodal signaling in two nonredundant feedback loops with dual timescales and to localize primitive streak formation at the posterior pole. Based on mathematical modeling, we discuss how these sequential loops control cell fate.

Activin Receptors↗

PubMed2005

Nodal stability determines signaling range.

Secreted TGFbeta proteins of the Nodal family pattern the vertebrate body axes and induce mesoderm and endoderm . Nodal proteins can act as morphogens , but the mechanisms regulating their activity and signaling range are poorly understood. In particular, it has been unclear how inefficient processing or rapid turnover of the Nodal protein influences autocrine and paracrine signaling properties . Here, we evaluate the role of Nodal processing and stability in tissue culture and zebrafish embryos. Removal of the pro domain potentiates autocrine signaling but reduces Nodal stability and signaling range. Insertion of an N-glycosylation site present in several related TGFbeta proteins increases the stability of mature Nodal. The stabilized form of Nodal acts at a longer range than the wild-type form. These results suggest that increased proteolytic maturation of Nodal potentiates autocrine signaling, whereas increased Nodal stability extends paracrine signaling.

Amino Acid Sequence↗

PubMed2004

Nodal protein processing and fibroblast growth factor 4 synergize to maintain a trophoblast stem cell microenvironment.

Before implantation in the uterus, mammalian embryos set aside trophoblast stem cells that are maintained in the extraembryonic ectoderm (ExE) during gastrulation to generate the fetal portion of the placenta. Their proliferation depends on diffusible signals from neighboring cells in the epiblast, including fibroblast growth factor 4 (Fgf4). Here, we show that Fgf4 expression is induced by the transforming growth factor beta-related protein Nodal. Together with Fgf4, Nodal also acts directly on neighboring ExE to sustain a microenvironment that inhibits precocious differentiation of trophoblast stem cells. Because the ExE itself produces the proteases Furin and PACE4 to activate Nodal, it represents the first example, to our knowledge, of a stem cell compartment that actively maintains its own microenvironment.

Animals↗

PubMed2004

Asymmetric Nodal expression in the mouse is governed by the combinatorial activities of two distinct regulatory elements.

In all vertebrates, invariant left/right (L/R) positioning and organization of the internal viscera is controlled by a conserved pathway. Nodal, a member of the TGFbeta superfamily is a critical upstream component responsible for initiating L/R axis determination. Asymmetric Nodal expression in the node preceeds and foreshadows morphological L/R asymmetry. Here we address the mechanism of Nodal activation in the left LPM by studying the function of a novel enhancer element, the AIE. We show this element is exclusively active in cells of the left lateral plate mesoderm (LPM) and is not itself responding to Nodal asymmetry. To test the hypothesis that this element may initiate asymmetric Nodal expression in the LPM, we deleted it from the mouse germ line. Mice homozygous for the AIE deletion (Nodal(deltaaie/deltaaie)) show no defects. However, we find that the AIE contributes to regulating the level of asymmetric Nodal activity; analysis of transheterozygous embryos (Nodal(deltaaie/null)) shows reduced Nodal expression in the left LPM associated with a low penetrance of L/R defects. Our findings point to the existence of two independent pathways that control Nodal expression in the left LPM.

Alleles↗

PubMed2004

A gene network establishing polarity in the early mouse embryo.

In mammalian embryos, molecular cross-talk with extraembryonic tissues is essential to elaborate the primary body axes. Here, we review a series of reciprocal interactions that occur shortly after implantation in the uterus, and discuss how they are integrated in a complex signaling network to establish antero-posterior and dorso-ventral polarity. At the heart of this signaling network is the TGFbeta-related protein Nodal which acts on extraembryonic tissues to induce positive and negative feedback regulators at opposite poles of the egg cylinder. This likely results in an activity gradient which is instrumental to pattern the embryo proper.

Animals↗

PubMed2002

Extraembryonic proteases regulate Nodal signalling during gastrulation.

During gastrulation, a cascade of inductive tissue interactions converts pre-existing polarity in the mammalian embryo into antero-posterior pattern. This process is triggered by Nodal, a protein related to transforming growth factor-beta (TFG-beta) that is expressed in the epiblast and visceral endoderm, and its co-receptor Cripto, which is induced downstream of Nodal. Here we show that the proprotein convertases Spc1 and Spc4 (also known as Furin and Pace4, respectively) are expressed in adjacent extraembryonic ectoderm. They stimulate Nodal maturation after its secretion and are required in vivo for Nodal signalling. Embryo explants deprived of extraembryonic ectoderm phenocopy Spc1(-/-); Spc4(-/-) double mutants in that endogenous Nodal fails to induce Cripto. But recombinant mature Nodal, unlike uncleaved precursor, can efficiently rescue Cripto expression. Cripto is also expressed in explants treated with bone morphogenetic protein 4 (BMP4). This indicates that Nodal may induce Cripto through both a signalling pathway in the embryo and induction of Bmp4 in the extraembryonic ectoderm. A lack of Spc1 and Spc4 affects both pathways because these proteases also stimulate induction of Bmp4.

Animals↗