First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping researchers promote themselves alongside their papers. Tanja Müller is first author on ‘ The neuronal transcription factor MEIS2 is a calpain-2 protease target’, published in JCS. Tanja conducted the research described in this article while a PhD student in Prof. Dr Dorothea Schulte's lab at the Institute of Neurology (Edinger Institute), University Hospital Frankfurt, Germany. She is now a postdoc and co-lead of the Translational Neurooncology Junior Group in the lab of Dr Ann-Christin Hau at the Translational Neurooncology Group at the University Hospital Frankfurt, Germany. Her current research focuses on the identification of invasion-promoting genes in high-grade glial tumors.

Tanja Müller

How would you explain the main findings of your paper in lay terms?

For a cell to be fully functional, certain gene products must be produced at exactly the right time, in the right quantity and in the right type of cell. Transcription factors (TFs) are special proteins that are responsible for switching on certain genes. Close control of TF activity is crucial, especially within stem cell populations, to prevent cell loss and uncontrolled differentiation. MEIS2 is a TF expressed in the stem cell niche within the adult brain, where it is required to turn on neuron-specific genes that transform stem cells into neurons. Only a certain proportion of these stem cells should become neurons and they should only do this when external signals direct the cells to differentiate. In order to ensure precise but also rapid activation of the TF MEIS2, the stem cells have developed an exciting mechanism that we describe within our article. MEIS2 is continuously produced by the stem cells, but it cannot fulfil its function because it is constantly cleaved by a protein-cutting protease called calpain-2. Leaving the stem cell niche heralds the beginning of neuronal differentiation for some of the stem cells. During this process, signals that arrive externally at the cell surface lead internally to the inactivation of calpain-2 and to chemical modifications on MEIS2, protecting the TF from further cleavage. MEIS2 is thus stabilized and finally able to bind to neuron-specific genes, setting off the neuronal differentiation of the stem cell.

Were there any specific challenges associated with this project? If so, how did you overcome them?

The phenotype of the stem cell population as having low MEIS2 protein stability, in contrast to the differentiated neuronal progeny, which have increased MEIS2 stability, was a consistent observation within our laboratory. To figure out what caused the low stability of MEIS2 within the stem cells was quite challenging. Initially, I suspected MEIS2 to be a target for degradation by the proteasome. This idea was fueled by experiments using a commonly used proteasome inhibitor, which resulted in increased MEIS2 protein levels. However, all the additional experiments that I performed trying to further validate proteasomal degradation of MEIS2 showed negative results. Those consistently negative results led me to doubt my working hypothesis. I asked myself why I was able to see an effect on MEIS2 protein when using this specific inhibitor but not with other approaches, and I finally realized that the inhibitor also has an inhibitory effect on calpain proteases. Extensive literature research and computer-based analysis of MEIS2 protein structure revealed more indications that further pointed towards this new direction. I changed my hypothesis from MEIS2 being a proteasome target to MEIS2 being cleaved by calpain proteases. Intensive, unbiased literature research helped me to develop an alternative hypothesis that I was able to experimentally validate, and I finally wrapped up this study that we have now published in the Journal of Cell Science.

When doing the research, did you have a particular result or ‘eureka’ moment that has stuck with you?

After I could experimentally demonstrate that MEIS2 becomes cleaved by the protease calpain-2, I went through some older immunoblot images searching for any signs of MEIS2 cleavage products that might have been overlooked before. On nearly every immunoblot I did previously, I found MEIS2-immunopositive bands that were smaller than full-length MEIS2. Before I developed the hypothesis that MEIS2 might be subject to calpain-2 protease cleavage, I misinterpreted those bands, thinking they arose from the non-specific binding of the antibody that I had used. This insight became a ‘eureka’ moment for me. It taught me that it is extremely important to keep an unbiased view of the results of every single experiment I am performing. The truth in science is always determined by observational facts.

Why did you choose Journal of Cell Science for your paper?

I am very happy that my project was published here, as Journal of Cell Science is a prestigious journal known to publish high-quality peer-reviewed articles. The scope of JCS matches this project, as it provides a novel perspective to understanding regulatory mechanisms in adult neuronal stem cell biology. By publishing in JCS, I hope to reach a broad audience within the cell biology community.

Immunofluorescence image of a primary adult neural stem cell-derived neuron. Even though the neurogenic transcription factor Meis2 is already expressed within the V-SVZ stem cell population, it is rather instable and barely detectable at the protein level. Upon neuronal differentiation, MEIS2 protein stabilizes and localizes to the cell nucleus of doublecortin positive neurons.

Immunofluorescence image of a primary adult neural stem cell-derived neuron. Even though the neurogenic transcription factor Meis2 is already expressed within the V-SVZ stem cell population, it is rather instable and barely detectable at the protein level. Upon neuronal differentiation, MEIS2 protein stabilizes and localizes to the cell nucleus of doublecortin positive neurons.

What motivated you to pursue a career in science, and what have been the most interesting moments on the path that led you to where you are now?

There is nothing more satisfying than proving a theory that you have developed in your head. The rising tension when you are waiting through the endless minutes until you finally get the results of that one experiment that shows that you were right – I am addicted to that feeling.

Who are your role models in science? Why?

There are many people that I admire for their scientific achievements but there is no one person I have as a role model. I focus on finding my very personal role within the scientific community rather than trying to re-create someone else's career path.

What's next for you?

Currently, I work as a postdoctoral researcher and co-lead of the Translational Neurooncology Group at the University Hospital in Frankfurt, Germany. Our junior group has now started its second year. The next steps in my academic career include recruiting a PhD student for my project and raising further third-party funds. Even though I have clear goals that I would like to achieve, I stay open-minded and give my very best every day. Let's see where this path will take me!

Tell us something interesting about yourself that wouldn't be on your CV

During the Covid-19 pandemic I spent 2 semesters taking a correspondence course in patent law alongside my research job. I became interested in this subject after I dove into the spectacular CRISPR/Cas patent dispute between the University of California in Berkley and the Broad Institute. In the end, the court decision was made based on lab book entries of both parties. This is a reminder for every scientist to document everything accurately within their lab books.

What is your current research within the Translational Neurooncology Group about?

Within the Translational Neurooncology group, we have focused our research on high-grade gliomas, especially glioblastomas, as the most common and aggressive primary brain tumors in adults. Over the past few decades, only a little progress has been made towards providing better treatment options for glioma patients. Survival rates remain low, with less than 5% of patients surviving 5 years. Until now preclinical models fail to fully recapitulate glioma pathophysiology, preventing efficient translation from the lab into successful therapies in the clinic. Our junior group is located at the interface between the Neurosurgery, Neurooncology and Neuropathology departments at Frankfurt Cancer Institute. Here, our group establishes advanced preclinical glioma models enabling a more truthful recapitulation of glioma tumor pathobiology. We apply our models for clinical as well as basic research projects where I personally focus on the identification of novel invasion-relevant genes by combining different (epi)genetic analysis and screening methods.

Tanja Müller’s contact details: Translational Neurooncology Group at the University Hospital Frankfurt, Frankfurt, Germany.

E-mail: ta.mueller@med.uni-frankfurt.de

Müller
,
T.
,
Reichlmeir
,
M.
,
Hau
,
A.-C.
,
Wittig
,
I.
and
Schulte
,
D.
(
2024
).
The neuronal transcription factor MEIS2 is a calpain-2 protease target
.
J. Cell Sci.
137
,
jcs261482
.