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Molecular switches for gene expression in the embryo

Developmental biologists are still fascinated by the following question: how is it possible for a single fertilised egg cell to develop into a complete organism with all its different tissues and cell types? Prof. Dr. Andreas Hecht and his team at the University of Freiburg are investigating one of seven known fundamental signalling systems that are involved in almost all embryonic development processes and that determine the fate of cells. The scientists are mainly interested in gaining insights into why the same molecules initiate different genetic programmes and hence development pathways, depending on the cell type where they have their effect.

Mammals consist of hundreds of different cell types. Normally, everything is located in a predestined place, and does the predetermined thing at the predetermined time during development. But how are the destinies of the different precursor cells determined in the embryo? How do cells know whether they will become liver, muscle or brain cells? Developmental biologists now know that closely interacting signalling molecules and signalling pathways control the formation of tissue identity during embryonic development. But they only know of seven major systems that have an effect on almost all cell types and exert these regulatory functions. “How is it possible that the same small set of tools leads to different reactions in different contexts?“ asks Prof. Dr. Andreas Hecht from the Institute of Molecular Medicine and Cell Research at the University of Freiburg. “We have shed light onto this question through our close investigations of one of these signalling systems over the last few years.“

Determination is made quite early during development

The signalling pathway induced by Wnt: a number of molecular steps lead to the activation of ß-catenin, which then enters the cell nucleus where it can bind to DNA. © Prof. Dr. Andreas Hecht

The model used by Hecht and his team is the so-called Wnt/ß-catenin signalling pathway. A key constituent of this pathway is the signalling molecule Wnt that can dock to the membrane of a cell, where it is bound by a receptor complex. Molecules that interact with the receptor proteins translate the message and activate a signalling pathway to transmit the message into the cell. The final recipient of this message is ß-catenin, a protein whose activity is normally inhibited. However, a Wnt signal resolves the inhibition, enabling ß-catenin to enter the cell nucleus and affect the transcription rate of DNA. This is how a Wnt signal can switch genetic programmes in cells on and off. These genetic programmes determine the cell type that a particular cell becomes. The Wnt/ß-catenin system is extremely important as early as embryonic development. For example, in amphibians and mice, the system helps determine the cells that eventually become the head and tail of the spherical embryo. However, kidneys, intestines, spinal column and the haematopoietic system also only develop correctly under the influence of Wnt and related proteins.

"In principle, there is no single organ in adult organisms whose development is not in some way or other influenced by the Wnt/ß-catenin pathway," said Hecht. The pathway is also of major medical relevance. The system regulates the self-renewal and differentiation ability of stem cells and might therefore become important for regenerative medicine. On the other hand, mutations of several genes in this system have been found to be associated with the development of certain diseases. Therefore, the question as to why the system induces different genetic programmes in certain stem cells or precursor tissue is also important in this context. Hecht and his team have focused intensively on answering this question over the last few years. They mainly used mouse cell lines that are highly suited to experiments. They looked more closely at three cell types: embryonic stem cells, neural cells with a stem-cell-like character and muscle precursor cells (myoblasts).

Is it the genes themselves?

A Wnt signal can activate the gene Axin2 in a neural cell, but the gene Brachyury cannot. Why is this so? © Prof. Dr. Andreas Hecht

"We have investigated different genes in these three cell types. It is known that these genes can be switched on by the Wnt/ß-catenin signalling pathway," said Hecht explaining that they investigated the gene Cdx1, which is, amongst other things, important for the correct development of parts of the skeleton and the spinal cord, T/Brachyury, which plays an important role in the development of mesoderm cells which later give rise to muscles and bones, and Axin2, a negative regulator of ß-catenin. It is known that Axin2 is activated by Wnt signals in all three cell types investigated. The researchers found that this was not the case for the two other genes. When the scientists stimulated their cell cultures with Wnt signalling molecules, the embryonic stem cells activated all three genes; however, the myoblasts and the neural cells only led to the activation of Axin2. This was evidence for the fact that the Wnt/ß-catenin signalling cascade functioned in each of the three cell types. On the other hand, the scientists had reached the decisive point that raises the important question: do the same signalling molecules activate certain genes only in specific cell types? "If the signalling pathway functions in principle in all three cell types, then the different activability must be directly associated with the affected genes themselves," said Hecht.

The reason for this might be that ß-catenin requires helper molecules (T-cell factors, TCFs) to bind to DNA. The TCFs are kinds of adapters through which the signalling molecule docks to the regulatory sequences of genes. Hecht and his team were able to show that the four representatives of these adapter genes do not bind arbitrarily to the regulatory elements of the Wnt/ß-catenin target genes. Depending on the gene under consideration, only certain TCFs are used. One of these T-cell factors, TCF4, also occurs in a broad range of different variants. For example, ß-catenin is only able to activate the gene Cdx2 together with a specific variant. "This might have different consequences," said Hecht explaining that cells are believed to produce different TCF variants, which is why they react so differently to Wnt signals. Another possibility might be that the cells carry different epigenetic labels on their genes, small methyl or acetyl groups on histones, the proteins that package the DNA. Depending on the type of labelling and hence packaging, genes labelled in this way can no longer be transcribed and remain inactive. A Wnt signal that would normally activate these genes, would in this case have no effect.

Faulty control in cancer?

Hecht and his team have indeed been able to show that certain genes are only bound by ß-catenin and the TCFs when this is made possible through their particular epigenetic labels. These labels therefore enable a cell to control the genes that are switched on or off upon receiving a Wnt signal. “It is known that, in contrast to differentiated cell types, embryonic stem cells still have a very plastic and highly dynamic DNA packaging,” said Hecht. “As the organism develops, this leads to certain epigenetic labelling patterns that subsequently remain stable and enable or prevent the expression of genes.” This might be the reason why embryonic stem cells activated all three aforementioned genes in the experiments carried out by the Freiburg researchers, and why neural cells and myoblasts only activated Axin2.

In future, Hecht and his team plan to investigate the mechanisms that control the packaging of DNA. In addition, they also hope to gain deeper insights into the interaction between different TCF variants and the DNA of certain regulatory elements. Furthermore, they are also part of a planned Freiburg collaborative research centre. If funding is provided for the planned collaborative research centre, the Freiburg researchers hope to focus on the faulty control of target genes of the Wnt/ß-catenin signalling pathway in tumours. It is known that epigenetic mechanisms also play a role in the development of cancer and contribute to tumour cells losing the characteristics of normal cells.

Further information:
Prof. Dr. Andreas Hecht
Institute of Molecular Medicine and Cell Research
Stefan-Meier-Str. 17 / 2. floor
D-79104 Freiburg, Germany
Tel.: +49-761/203 9608
Fax: +49-761/203 9602
E-mail: andreas.hecht(at)mol-med.uni-freiburg.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/molecular-switches-for-gene-expression-in-the-embryo