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We have a new section in our blog. Our member Noelia Muñoz talks about her doctoral thesis and how it contributed to the field of cardiovascular research and tissue repair

Noelia Muñoz Martín

From the question of how heart development is regulated to Cell Competition as a tissue-damaged repair tool

My name is Noelia Muñoz Martín, I am a scientist and did my doctoral thesis at the National Center of Cardiovascular Research (CNIC) in Madrid. Sharing my work and results with people with a different background has always been a challenge that I like to face. For this reason, I would like to share here one of the projects I carried out when I was working as a PhD candidate.

I consider science the best tool we have to answer many of the questions that human beings ask themselves. Specifically, in my doctoral thesis, we questioned about how the process of heart formation is regulated during embryonic development. The idea is simple, if you know how something is built, you will know how to repair it when needed. The embryonic development of the heart is a very complex process, which is regulated by multiple mechanisms. We decided to focus on the regulation carried out by the transcription factor Myc. Transcription factors are important biological regulators because they are proteins which bind to DNA and indicate which subset of genes should be activated, driving the cell to acquire a specific identity.

Now you might think, why Myc and not another protein? The last studies that had been carried out on the function of this transcription factor in the developing heart dated from 1993 and pointed to an important role in controlling cardiac cell division. If the cells cannot divide, the heart does not acquire the appropriate size and the embryos die at early developmental stages. These results were obtained using embryos in which Myc’s function was completely eliminated, affecting the heart, but also the formation of other organs. Therefore, it was difficult to conclude whether the effects described in those studies were directly driven by Myc or secondary to other defects.

Thanks to current genetic engineering technology we were able to modify mouse embryos by eliminating Myc only in the heart and not in the whole embryo. Why did we use this strategy to study the function of a transcription factor? This is based on the following idea, a good way to know the importance of something is deleting it and studying its consequences, so that’s what we did. We observed that all the embryos without Myc in the heart were born in the expected proportion. Therefore, this observation indicated that Myc was not essential for cardiac development. In addition, we followed the mice and evaluated heart function throughout their adult life. The results showed that there was no difference between an unmodified mouse and a mouse in which Myc had been removed from the heart.

How was that possible? One of the things that research work teaches you is to keep asking questions and not to stop looking for answers, so we decided to see whether Myc was actually present in the heart of unmodified embryos. In an organism, all the cells have the same DNA, so the same genes, but not all of them are activated, that depends on the function that each cell has.  If a gene is active, it gives rise to a protein that carry out a function. Using antibodies, we can detect the presence of a target protein in the tissue of interest. Thus, we used this method to look for the presence of Myc protein in the developing heart. No matter how much we looked, we were not able to detect any Myc in cardiac embryonic cells. This means that the gene producing Myc protein is not activated in cardiac cells during development because it does not play any role there. Consequently, we had to redefine what had been assumed until now and this result will be helpful for other research groups to answer more questions.

Moreover, I was working in Dr. Miguel Torres’ laboratory where people are experts in a fascinating process called Cell Competition. This mechanism allows cells to compare with each other detecting and eliminating those cells that are less competent. This process ensures that the organism is formed from the most fit cells, the “winners”. The reason for mentioning this process is not to make the reading more complicated, but the cells that win this competition turn out to be those with the highest amount of Myc. Thanks to the work of Dr. Cristina Villa del Campo, we knew that Cell Competition can artificially be induced in the embryonic heart without being detrimental for the embryo. In this work, the authors described how the cells with the highest level of Myc expand and induce the death of the neighbors with lower levels. In summary, we know that under normal conditions Myc does nothing in the development of the heart, but if we artificially activate it in a small population, it will give an advantage to those cells over the others.

Could we use that advantage to rescue other defects? Reading about other research studies, we found out that there was another transcription factor, very similar to Myc, whose name is Mycn. In addition, the elimination of Mycn in the heart resulted lethal at early developmental stages. When two proteins are very similar, it is possible that they have also similar functions, so we thought Myc might be able to replace the functions of Mycn in the heart.

Working together with Dr. Villa del Campo, we used genetic engineering tools to generate mouse embryos in which we combined the loss of Mycn function throughout the heart with the presence of Myc in a portion of cardiac cells. The results we found were surprising, the modified embryos not only did not die in early stages, but they successfully reached birth. We also discovered that this result was possible due to Cell Competicion. Cells with Myc expanded throughout the heart during development, dividing and inducing the death of those that did not have neither Myc nor Mycn transcription factors. Thus, Myc-driven Cell Competition rescued the defects caused by the loss of Mycn in the heart.

In conclusion, this work made us to redefine the functions of Myc in heart development and pointed to Cell Competition as a powerful tool for tissue repair, which in the future might be used as a therapy. At the same time, it is showing how important it is not to stop asking questions and how those questions can lead you to unexpected discoveries.

If you would like more information or to see some nice images you can find the article here: https://dev.biologists.org/content/146/3/dev170753?rss=1

 

Confocal microscopy image of a mouse embryo. Heart labelled in red and cells with Myc in green. No overlay detected between red and green signals.

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