Lefty

Abstract: Maintenance of pluripotency is crucial to the mammalian embryo's ability to generate the extra-embryonic and embryonic tissues that are needed for intrauterine survival and foetal development. The recent establishment of embryonic stem cells from human blastocysts (hESCs) provides an opportunity to identify the factors supporting pluripotency at early stages of human development. Using this in vitro model, we have recently shown that Nodal can block neuronal differentiation, suggesting that TGFβ family members are involved in cell fate decisions of hESCs, including preservation of their pluripotency. Here, we report that Activin/Nodal signalling through Smad2/3 activation is necessary to maintain the pluripotent status of hESCs. Inhibition of Activin/Nodal signalling by follistatin and by overexpression of Lefty or Cerberus-Short, or by the Activin receptor inhibitor SB431542, precipitates hESC differentiation. Nevertheless, neither Nodal nor Activin is sufficient to sustain long-term hESC growth in a chemically defined medium without serum. Recent studies have shown that FGF2 can also maintain long-term expression of pluripotency markers, and we find that inhibition of the FGF signalling pathway by the tyrosine kinase inhibitor SU5402 causes hESC differentiation. However, this effect of FGF on hESC pluripotency depends on Activin/Nodal signalling, because it is blocked by SB431542. Finally, long-term maintenance of in-vitro pluripotency can be achieved with a combination of Activin or Nodal plus FGF2 in the absence of feeder-cell layers, conditioned medium or Serum Replacer. These findings suggest that the Activin/Nodal pathway maintains pluripotency through mechanism(s) in which FGF acts as a competence factor and therefore provide further evidence of distinct mechanisms for preservation of pluripotency in mouse and human ESCs.

Abstract: ▪ Abstract TGFs signals belonging to the Nodal family set up the embryonic axes, induce mesoderm and endoderm, pattern the nervous system, and determine left-right asymmetry in vertebrates. Nodal signaling activates a canonical TGFs pathway involving activin receptors, Smad2 transcription factors, and FoxH1 coactivators. In addition, Nodal signaling is dependent on coreceptors of the EGF-CFC family and antagonized by the Lefty and Cerberus families of secreted factors. Additional modulators of Nodal signaling include convertases that regulate the generation of the mature signal, and factors such as Arkadia and DRAP1 that regulate the cellular responses to the signal. Complex regulatory cascades and autoregulatory loops coordinate Nodal signaling during early development. Nodals have concentration-dependent roles and can act both locally and at a distance. These studies demonstrate that Nodal signaling is modulated at almost every level to precisely orchestrate tissue patterning during vertebrate embryogen...