Most of the current research involving stem cells has to do with injecting or implanting them with the hope that there will be positive effects, mostly expected from bystander effects due to the secretion of hormones and other molecules. As we learned from Prof. Berninger in Part I of his interview, there is no real evidence that stem cells such as MSCs (mesenchymal stem cells, often used in clinical trials) integrating into the brain and taking on the function of neurons.
So although these methods could perhaps produce some positive results, Prof. Berninger is searching for a better way to promote tissue repair. He is looking for a way to really regenerate neuronal tissue in the brain after an injury. In the second part of our interview, Prof. Berninger describes some other approaches to stem cell research and explains his approach to finding out if the generation of new neurons can actually be promoted.
Can the brain’s natural capacity promote healing?
Strokemark: Your focus is on the possibility of using endogenous cells in the brain for repair. In other words, using cells that originate in the brain to help the brain heal. This is quite different than what is happening in most clinical trials right now where different lines of stem cells are implanted. It sounds like with your approach we could use the brain’s natural capacity to boost its own healing. Is that true?
Prof. Berninger: There are some restricted parts of the brain like the hippocampus (the area of the brain involved with learning and memory) where stem cells give rise to specific neurons. However, our research aims at recruiting cells that are not stem cells for brain repair.
We’re interested in finding out whether it might be possible to induce other cells than neurons in the brain and convert them into neurons. There are a lot of non-neuronal cells in the brain (called glial cells). We think we could trick these cells to change their identity so they become neurons. This is not trying to make naturally occurring stem cells turn into neurons because, in most parts of the brain, there are no such stem cells. But we have these glial cells present, and they share some features with stem cells. Glial cells are part of healthy neural tissue. They surround neurons and support and protect them.
Stem cells are not a tabula rasa. They are not cells that have no features whatsoever. Stem cells actually do have glial features. That’s why we reasoned that if these glial cells we have in the brain are so similar to stem cells, perhaps it would be possible, at some stage, to make them behave like stem cells and finally convert them into neurons. This is our field of research, and our approach has actually gained quite some momentum.
Is this research just theory?
Strokemark: So is your research still in the theoretical stage or has it been tested in animal experiments or cell cultures?
Prof. Berninger: We started testing this about ten years ago in cell cultures.
Research from other labs and ours also describe the reprogramming in animals. We call this process reprogramming because it involves changing the genetic programs that underlie cell identity.
We still have to overcome some very important challenges before we can say this works. But there is some indication that it does. However, the question still remains whether this direct conversion approach or the grafting approach that is used in most stem cell research is more effective. But I think, eventually converting cells in the brain into neurons will be one way to go. The different possibilities stem cells offer (implanting and reprogramming) should be tested rigorously.
Strokemark: How does the process of converting cells work? Would someone need to take medication or receive an injection in their brain?
Prof. Berninger: Currently, we are using viral vectors (engineered viruses) that introduce the genes that code the needed proteins into the cells. These cells eventually convert their identities. Of course, in the long run, we would like to get rid of the viral vectors because of the potential dangers they can pose (e.g., possibility of cancer). So some people are now experimenting with small molecules, but I think there are also many safety issues to be dealt with.
It certainly works in cell cultures, and recently researchers claimed that the use of small molecules can cause a massive conversion of glial cells into neurons in living mice.
Strokemark: Does that also have non-beneficial side effects?
Prof. Berninger: I don’t think they looked at side effects in that study. One might also expect non-beneficial side effects from using very powerful, small molecule compounds. But again this is still in the proof of principle phase.
The main problem at this stage is to demonstrate that you can convert a cell in the brain with a specific identity into another cell with a specific identity. And then this newly generated cell can potentially integrate in a meaningful way into the brain’s circuit.
Strokemark: That sounds promising when you think in particular about the glial scars that happen in the brain after injury.
Prof. Berninger: Absolutely. The idea is that maybe cells from a glial scar could be converted into cells that would benefit the damaged brain.
What are some of the issues involved with this method of regenerating cells?
Strokemark: Is this age-dependent? I know that there are no human studies yet, but from a theoretical perspective, do you think this is dependent on the age of cells or of the patient?
Prof. Berninger: This we don’t know yet. One thing we know for sure is that if the cells are already mature and differentiated, it is certainly tougher. This means converting cells might be more difficult when a person is an adult or even an adolescent. But overall, the evidence that we have is that age, per se, is not a showstopper.
Strokemark: If the cells change identity and then do something else, what about the original function of the cell? It was there for a reason, and now what takes over the function of the original cell type?
Prof. Berninger: In the current state, this process shouldn’t be used for generating a massive number of cells, but rather for generating cells that are missing in tissue that is still working to some degree. The hope is that homeostatic mechanisms kick in. Let me explain. We know, for example, that the production of glial cells in people is actually really low. But when a glial cell dies, other cells nearby start to divide and can replace it by filling in the gap. There is the hope that this would also happen in reprogramming. When we convert a glial cell into a neuron, and then other glial cells divide and replace the missing glia.
But again, this is still in the experimental phase, and we need to learn several lessons at a time. We need to learn how to generate an authentic neuron, and we need to learn how this affects the tissue.
There is a study with quite impressive findings recently published in the journal Nature. Interestingly, in contrast to mammals, some fish have the ability to restore damaged tissue in the retina by converting glial cells into retinal stem cells. A group of researchers attempted to replicate this process in mice. And they actually managed to convert mammalian glial cells into retinal stem cells that divided and then differentiated into another cell type. Using this method, they were able to restore some vision in blind mice.
So, you don’t just make a one-to-one conversion. You make the cell divide first so that you have many cells produced from one starting cell. And these cells can then be differentiated. So even if you don’t replace this one glial cell, there is no great loss. With this method, one would get more output although only one cell was invested.
What does the future look like for this method?
Strokemark: What do you think the timeline is for this to happen, how much research needs to be done to start to see clinical trials?
Prof. Berninger: I think research-wise, the next 5 to 10 years will revolutionize what we can do. But I cannot predict when this will move into clinical application.
Strokemark: Let’s come back to the sources of stem cells that exist in the human brain. What is their actual function? Is it to migrate to damaged areas and help to form the glial scar and/or to replace neurons?
Prof. Berninger: Originally this neurogenesis (production of new neurons in the brain tissue) was observed in very few brain areas in mice. Then the idea emerged that you could mobilize and recruit these cells to the damaged brain areas. Indeed, there were reports that this can be the case. But the problem here is that we are talking about mouse brains that have considerably smaller distances between where stem cells reside and the sites of damage. The cells would have to migrate much longer distances in the human brain, so this might be too much to hope for. I don’t think there is any evidence that this could work.
Can patients help promote their own cell regeneration?
Strokemark: Is there something a patient could do to boost the stem cells that already exist in the brain or get those cells to help the damage?
Prof. Berninger: Currently, there is no real way to mobilize them outside of the niches where they are. I don’t think that you can really target them in any way.
I wouldn’t exclude, though, that some rehabilitation activity might have some positive effect on that. Running in mice stimulates neurogenesis, but we don’t know if this is the case for humans.
It is not entirely understood why running may produce new neurons. There is evidence these new neurons might be very critical for our mood regulation. By enhancing neuron generation to improve mood and life quality, a positive cycle can be created. If you are feeling better, you might be motivated to invest more time in your activities and exercises. I would consider exercises as a possibility to help with your recovery.
Is there any proof that new neurons can be produced?
Strokemark: Is there any way to prove that neurogenesis happens in humans?
Prof. Berninger: The problem is that it is very difficult to detect neurogenesis in humans because we cannot apply the same techniques as with mice. In mice, we can inject a retrovirus which labels the neurons. But obviously, we can’t do this in humans.
So what we have to do is take post-mortem material, and then use methods from histochemistry (a way to stain and visualize cells in the tissue) to demonstrate that cells are regenerated.
There is one indirect technique. When a cell divides, it has to generate new DNA molecules. One group of researchers of the Karolinska Institute in Sweden used C 14 labeling, as used to determine the age of things (e.g., bones in archaeology), and applied this to the brains of patients to see when the neurons were born. Actually, these researchers saw very high numbers of newborn neurons in the hippocampus area (responsible for learning and memory) of the brain and to a lesser degree also in the striatum (deep brain area with various functions). It’s a solid method and would indicate that there is neurogenesis in humans.
Offering hope for the future
So although there is still a lot to learn about getting the brain to help itself generate new neurons, there is a lot of evidence that suggests repair from within could work. Researchers like Prof. Berninger continue to explore possible avenues to promote real healing of brain tissue.
We would like to thank Dr. Berninger for taking his time to talk with us and for shedding some light on his research, the research of others, and the possible future direction of stem cell treatment.
Prof. Berninger's work deviates from other stem cell research by employing a method that uses cells already existing in the brain.