PMH, astrocytic cells and a new cell division mechanism

"Hormones should be banned!", one of our eminent neuroscientist colleagues at the University of Newark or New York once exclaimed at his group's usual afternoon tea chats, when asked about his beautiful daughters. Anyone who has children knows immediately what happened. The time when little girls and their behavior change can make life difficult for everyone for a while.

Let's leave it at that, hormones in a casting - which, who knows why, is called with its English word in Hungary, too - would be put into the bad guys' group. But as some bad guys turn out later to be not so bad after all, it's best to accept now that the role of hormones is vital.

Take thyroid hormones (PMH), for example. It is known from our secondary school studies that the most common cause of goiter (struma), once known as a folk disease, is iodine deficiency, which is essential for the normal functioning of the thyroid gland. This gland produces a pre-hormone called thyroxine (T4), which is converted into triiodothyronine (T3) by cleaving iodine, which binds to receptors and has a biological effect on them. PMH, stored in thyroid stores and bound to proteins, is released into the bloodstream by thyroid-stimulating hormone (TSH) produced in the pituitary gland to regulate the activity of all cells in the body. Including nerve cells and glial cells.

There is therefore a compelling reason why the Molecular Cell Metabolism research group led by Balázs Gereben at our institute is investigating the molecular regulatory mechanisms of PMH signaling at the tissue and cellular level. The neuronal regulation of thyroid hormone homeostasis is particularly complex, as the brain is both a regulator and a target of PMH. 

Petra Mohácsik PhD, first author of this paper published in the prestigious Journal of Biological Chemistry, has undertaken to help us understand the complex world of thyroid hormones and what is needed to understand the significance of the main findings of their newly published study.

- There are two basic cell types of the nervous system the neurons and the more abundant glial cells. The astrocytic cell is one of the most important subtypes of glial cells and the most abundant in the brain. Why did you study these if PMH affects both cell types? 

- Although PMH is known to be an important regulator of neuronal energy use, cell division, and differentiation, it requires the prohormone thyroxine (T4) to be activated to T3. The process is catalyzed by the selenoenzyme deiodase type two (D2), which is present in the brain almost exclusively in astrocytic cells. These cells are present in all areas of the brain, in addition to the tanicyte cells of the hypothalamus. Not by chance have they always been in the focus of our interest! Astrocytes transmit metabolic signals to neurons, including hormones, and can regulate the amount of active hormone reaching neurons, even in a brain-region-dependent manner. However, hormones affect both neurons and astrocytes, and changes in hormone levels trigger a response in them. It is fair to say that these cells are both the source and the target cells for thyroid hormone in the brain. 

- You could convince me not only that it made sense to start the studies on astrocytes, but also that these cells are not only many but also special. Are there any other characteristics unique to them that are relevant to these studies?

- Not just any! Under certain conditions, such as stroke or traumatic brain injury, these cells can regain their stem cell-like properties, divide, and interfere at the injury site. The two main players in the cell division/differentiation pathway we have now identified, the cell cycle regulatory proteins MSI1 and D2, are mainly located/expressed in astrocytes. This justifies our hypothesis that the newly identified pathway may also play a role in primary brain tumors originating from glial cells!

- The experiments were first performed in cell culture. To what extent can we expect the results of the in vivo experiment to be the same?

- In cell culture, under more controllable conditions, without the complexity of a living animal organism, it is easier to change the elements of a regulatory system and to gain more information about how it works more quickly. It soon became clear that the regulation acts as a molecular limbo between the inhibitory regulatory protein MSI1 and the target protein D2. By decreasing the level of proliferation-stimulating MSI1, the amount of D2 enzyme, which catalyzes T3 production to promote differentiation in cells, increased, and the level of active thyroid hormone in cells was higher while increasing the amount of MSI1 resulted in a decrease in D2 levels. 

We also confirmed that MSI1-induced D2 regulation occurs through the regulatory region of the unusually long mRNA of the D2 molecule. Since everything worked as we theorized in the cellular experiments, we were confident, that we could confirm this mechanism in the living animal.

- How could you modulate/alter the MSI1-D2 pathway in vivo - was there any other observable effect of reducing MS1 levels?

- In vivo, MSI1 levels were markedly reduced using a brain-hypophyseal KO (gene knockout) mouse model. In the cerebral cortex of these mice, D2 enzyme activity was increased, resulting in higher tissue thyroid hormone levels. This was examined by measuring PMH-sensitive gene expression. 

- What does the statement in the article that "the slowing of astrocytic cell division was dependent on the presence of T3" mean? Was there division without the presence of T3, just half as fast?

- In primary cell cultures of astrocytic cells isolated from the brains of control mice and MSI1 KO mice, the number of dividing cells was lower in MSI1-deficient cultures and D2 activity was higher compared to control.

The higher amount of active T3 produced by more D2 enzymes may be a key player in slowing down cell division. T3 treatment of primary astrocyte cultures from control mice also resulted in fewer dividing cells, leading us to conclude that MSI1 can influence T3 levels and regulate cell division/proliferation via the regulation of D2.

- What do you consider the most significant result of your work?

- MSI1-D2-T3 is a novel molecular pathway regulating brain cell division and cell differentiation. It is also a novel mechanism of brain thyroid hormone regulation, that contributes to the complex regulatory circuit that decides whether a cell divides through antagonism between cell division and cell differentiation.

We want to gain a deeper understanding of the role of this new pathway in the brain, as both MSI1 and D2 proteins are structurally highly conserved, both are present in the glial compartment of the mammalian brain and may be regulators of several processes. 

- How will you continue your experiments?

- I suppose it is not unexpected that our interest has now turned to tumor biology, as levels of proliferation-promoting MSI1 are elevated in many tumors, including primary brain tumors such as glioblastoma, which has a poor prognosis. We wanted to investigate whether there is a link between tumor MSI1 and D2 levels. If so, we want to reveal its effect on the tumor growth and spread. We have already received the necessary TUKEB approval. Under the supervision of Dr. Tamás Mezei and Dr. László Sípos, the collection of human glioblastoma samples has started at the Department of Neurosurgery and Neurointerventional Medicine at Semmelweis University. 

- The role of the first author is always highlighted in an article, as it is in the work. What was your part?

- The most important thing was the idea itself. It came to me from another piece of work. I was looking at the mRNA of the D2 enzyme using software when the name the Musashi protein came up. When I looked it up on the Internet, I found a picture of a Japanese warrior, Miyamoto Musashi, who was said to be invincible, appeared, fighting duels with two swords. The protein was named after him because of the molecule's double RNA binding site. 

Since what I had described about the mechanism of action of the MSI1 protein seemed to fit in with our existing picture of D2 regulation, I started preliminary studies. At first, I did the experiments exclusively, but as the work became more promising, more colleagues joined the project and we could move forward more quickly. I want to thank Emese Halmos and Beáta Dorogházi for their help, and our lab manager, Dr. Balázs Gereben.

Seeing the encouraging results, we started in vivo testing and began organizing the collection of human glioblastoma samples. 

Immediately, as the number of samples reaches the required level, we will start the experiments. We look forward to seeing if the results obtained on human samples match those obtained in animal studies. Because if the MSI1-D2 relationship is similar here, a new chapter can begin, with the development of new typing and possibly treatment options!