Researchers have discovered a new pathway in stem cell differentiation.


Stem cells, which have the potential to turn into any kind of cell, offer the tantalizing possibility of generating new tissues for organ replacements, stroke victims and patients of many other diseases. Now, scientists at the Salk Institute have uncovered details about stem cell growth that could help improve regenerative therapies.

While previous studies have shown that two key cellular processes, called Wnt and Activin, were needed for stem cells to grow into specific mature cells, no one knew exactly how these pathways worked together. Now, the details of how Wnt and Activin influence each other offer guidance for improving stem cell therapies. The new work also reveals more about certain cancers that arise when these processes go astray, for example, when the Wnt signaling step becomes inappropriately reactivated, as happens in most colon cancers.  The opensource study is published in the journal Molecular Cell.

The current study found that the mechanisms of these two pathways are complementary and activate the transcription, or turning on, of about 200 genes essential for stem cells to differentiate. These genes are among the first steps that prompt stem cells to begin to change, or differentiate, into specific tissues, particularly ones that will eventually form the digestive and respiratory tracts, including intestines, lung, pancreas, thyroid and liver, state the team.

The researchers found that Wnt loads up the cellular machinery needed to begin the copying and activation of genes. Activin, meanwhile, boosts the process further: it increases the speed and efficiency by which the cellular machinery moves to copy the gene. Whereas Wnt treatment alone enhances the expression of developmental genes by a factor of 20-fold, further treatment with Activin boosts the signal to 150-fold or higher. The team also found that the order of the signaling is equally important, because Activin could not turn on these genes unless the cells were first exposed to the Wnt signal.

The team expand that when Wnt gets the ball rolling and Activin amplifies the signal.  This is a particularly clear example of how two different pathways, working through two different mechanisms, can cooperate to activate the same genes.  The new data finding adds to a growing picture that the transcription process is much more dynamic than previous thought.  The researchers add that now the medical community understand stem cell differentiation at a much finer level by seeing how these cellular signals transmit their effects in the cells.  Understanding these details is important for developing more robust stem cell protocols and optimizing the efficiency of stem cell therapies.

When the lab looked closer at the genes that both pathways activated, they were surprised to find that the pathways were further connected to a third process, which is known to control tissue growth and organ size. The central protein in this new pathway, called Yap, acted specifically at these genes to counteract the effects of the Activin.

The results showed that the opposing effects of Activin and Yap are exerted at a late step in transcription, the elongation phase.  The team note that not a lot is known about the signaling networks in normal or cancer cells specifically affect the elongation stage of transcription, so it was a real bonus to find that it is targeted by two pathways in stem cells.

Previous studies have shown that both the Wnt and Activin signaling processes operate differently in cancer, compared to stem cells. Wnt, in particular, is turned on very early in human colon cancer in nearly 90 percent of cases. The aberrant behaviour of the Activin process, meanwhile, is tied to the metastasis of many cancers.

The team state that there is great interest in developing transcription-based inhibitors of the Wnt pathway, because these would have strong anticancer activity for many tumor types.  The researchers conclude that because the environment of stem cells and cancer cells are quite distinct, and different target genes are involved, it will be interesting to see how the synergy and regulation that we have defined in stem cells operates in the cells of a tumour.

Source:  The Salk Institute for Biological Studies 

 

The Wnt3a/β-catenin and Activin/SMAD2,3 signaling pathways synergize to induce endodermal differentiation of human embryonic stem cells; however, the underlying mechanism is not well understood. Using ChIP-seq and GRO-seq analyses, we show here that Wnt3a-induced β-catenin:LEF-1 enhancers recruit cohesin to direct enhancer-promoter looping and activate mesendodermal (ME) lineage genes. Moreover, we find that LEF-1 and other hESC enhancers recruit RNAPII complexes (eRNAPII) that are highly phosphorylated at Ser5, but not Ser7. Wnt3a signaling further increases Ser5P-RNAPII at LEF-1 sites and ME gene promoters, indicating that elongation remains limiting. However, subsequent Activin/SMAD2,3 signaling selectively increases transcription elongation, P-TEFb occupancy, and Ser7P-RNAPII levels at these genes. Finally, we show that the Hippo regulator, YAP, functions with TEAD to regulate binding of the NELF negative elongation factor and block SMAD2,3 induction of ME genes. Thus, the Wnt3a/β-catenin and Activin/SMAD2,3 pathways act in concert to counteract YAP repression and upregulate ME genes during early hESC differentiation.  SMADs and YAP Compete to Control Elongation of β-Catenin:LEF-1-Recruited RNAPII during hESC Differentiation.  Jones et al 2015.

The Wnt3a/β-catenin and Activin/SMAD2,3 signaling pathways synergize to induce endodermal differentiation of human embryonic stem cells; however, the underlying mechanism is not well understood. Using ChIP-seq and GRO-seq analyses, we show here that Wnt3a-induced β-catenin:LEF-1 enhancers recruit cohesin to direct enhancer-promoter looping and activate mesendodermal (ME) lineage genes. Moreover, we find that LEF-1 and other hESC enhancers recruit RNAPII complexes (eRNAPII) that are highly phosphorylated at Ser5, but not Ser7. Wnt3a signaling further increases Ser5P-RNAPII at LEF-1 sites and ME gene promoters, indicating that elongation remains limiting. However, subsequent Activin/SMAD2,3 signaling selectively increases transcription elongation, P-TEFb occupancy, and Ser7P-RNAPII levels at these genes. Finally, we show that the Hippo regulator, YAP, functions with TEAD to regulate binding of the NELF negative elongation factor and block SMAD2,3 induction of ME genes. Thus, the Wnt3a/β-catenin and Activin/SMAD2,3 pathways act in concert to counteract YAP repression and upregulate ME genes during early hESC differentiation. SMADs and YAP Compete to Control Elongation of β-Catenin:LEF-1-Recruited RNAPII during hESC Differentiation. Jones et al 2015.

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