Dr. Kriegstein is the John Bowes Distinguished Professor in Stem Cell and Tissue Biology and Founding Director of the Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research at UCSF. His research focuses on the way in which neural stem and progenitor cells in the embryonic brain produce neurons, and ways in which this information can be used for cell based therapies to treat diseases of the nervous system. His lab found that radial glial cells are neuronal stem cells in the developing brain, and also identified a second type of precursor cell produced by radial glial cells that is responsible for generating specific neuronal subtypes. He has recently begun to characterize the progenitor cells within the developing human brain, to determine the genetic profiles of specific progenitor populations, and to explore how these cells contribute to the huge expansion of neuron number that characterizes human cerebral cortex.
The following has been paraphrased from an interview with Prof. Arnold Kriegstein on April 3rd, 2018.
(Click here for the full audio version)
Your lab has played a pivotal role in our understanding of radial glial cells, could you give an overview of them and their implications for neurodegenerative diseases?
We discovered that these cells were neural progenitor cells (cells that develop into the neurons and glial cells that makeup the brain), which was important because up until 20 years ago nobody knew which cell types produce neurons. We showed that these radial glial cells, which were thought to just have a passive role in guiding neurons as they migrate into the cortex during embryonic development, were the neural progenitors themselves, dividing and generating neurons and the supporting glial cells.
That is the early stages of development; neurodegeneration is the opposite end of the spectrum. We don’t know yet if there are any of these radial glial cells in the adult brain of humans. It has been shown in mice and rats that throughout life there are certain areas of the brain where radial glial cells persist and continue to make neurons, even in aged animals. It was assumed that the same was true in people and there were a few studies suggesting that the hippocampus continued to make radial glial cells, even in older human beings. That was very encouraging because it suggested that if we found a way to harvest this regenerative potential we might be able to treat certain conditions where cells die, like stroke or Alzheimer’s disease. That fed the last two decades of research on adult neurogenesis. But, I was recently involved in a study here at UCSF suggesting that in a human that doesn’t happen, it’s discouraging but it does seem like once you reach adolescence the human hippocampus stops producing new neurons.
Do you see any new techniques emerging that might allow us to restart this process?
This finding, that neurogenesis stops in humans but not in other animals, raises questions of why and how this happens. Understanding these differences might allow us to change this sequence of events and recover some of this ability. It seems like it is going to be a harder path to travel than people thought, but it is something we are starting to look at.
What I am interested in is what happens to these radial glial cells after early brain development. What seems to happen is that they turn into a certain type of astrocyte (the most numerous and diverse glial cell), which closely resembles the radial glial cells and are present all over the brain throughout life. We and others have been trying to tweak these astrocytes in the adult brain to see if they can start generating new neurons again. It could be an interesting approach to treat stroke and neurodegenerative diseases.
Could you talk about your work on brain organoids and their future role as models of brain disorders?
Brain organoids have attracted a lot of attention and excitement recently, but they still have quite a long way to go. They are derived from stem cells, we then use growth factors or synthetic analogs to drive them to become forebrain cells, which are cortical (outer layer of the cerebrum) regions of the developing human brain. Then we give time for the cells to self-aggregate, once this happens they start behaving as though they were a developing embryonic brain. That is what is so exciting about this, it is a human organ in a dish, and we can do this for other organs as well. But it is at a very early stage, these are fetal or embryonic tissue, and they don’t represent the actual organ because they are missing many critical parts. There is no vasculature, no blood cells, no microglia (the immune cells of the brain), and they are often missing other glial cells that are made later in embryonic development. They are primitive and incomplete; nonetheless, they are human and have already been used to model some early stage diseases.
When you look into the future of these organoids, do you ever see it getting to the point when we can grow human brains in a lab?
I think it would be very difficult, but not impossible to engineer these organoids to get closer and closer to the real thing. Many scientists are working towards that goal, I imagine that 10 or 20 years down the road they will be much better than people can imagine today.
But there is another way of producing human organs that is almost doable now – growing human organs in other animals. Some of these approaches cross ethical lines, but the technology is already there. It has already been done in mice; you can take a developing mouse embryo, at a very early stage, and inject a rat stem cell into it. The rat cell gets incorporated into the embryo, in a random way, so some organs will be made up of both mouse and rat cells. However, if we knockout the genes responsible for making, for example, the liver from that embryo, then the rat cells will replace the missing mouse cells and form a rat liver in the mouse. That has already been done, and the same thing could be done with the brain, or parts of the brain. We could knock out the mouse cerebellum and it would grow a rat cerebellum.
You could then imagine doing the same thing with human cells, the problem is that there is a mismatch in developmental time and size, so I don’t think you would have a viable creature. But if you did the same thing with a pig or a monkey you could potentially grow a human brain in a non-human animal. This raises all kinds of ethical issues, and the science is not entirely there, but the blueprint is already available and it is much closer to reality than full brain organoid models.
There is a lot of communication that happens between cells as they develop and mature and live out their lives, how complex is this ‘language’ that cells ‘speak’?
When we develop organoids, after a certain period of time, they are left to develop on their own, and the way they do this is by signaling to each other. Signaling centers develop and they produce morphogens (signaling molecules) that allow the cells to send signals across gradients. Those gradients often dictate the effects of the environment on the cells and start to change the fate of the cells based on the signals they receive. There is a huge amount of that kind of interaction that occurs. Some signals diffuse from one cell to another, some result from direct contact with other cells, and in the nervous system there are also electrical signals between cells. Additionally, cells also have gap junctions that allow them to exchange small molecules. So there are many ways cells communicate and a lot of signals that can get exchanged, all of that has huge implications for the function of the organ. It also opens up the possibility of mis-signaling from environmental toxins or cues that misdirect signals.
There seems to be a lot of anthropomorphizing that happens when people talk about cells. They refer to mother cells and daughter cells, and cells talking to each other etc. Does this come from a feeling that cells are alive and possibly even conscious?
They are definitely alive, in many ways they are autonomous and they do their own thing in their environment, which you can think of as their world. But they are really just components; they are parts of a greater whole. In a multi-cellular organ like the brain, each element has its own lineage, it has a mother cell that produces daughter cells, and eventually a mature cell then migrates to a different part of the organ. So cells have an entire life cycle and growth pattern that resembles what happens in an organism as an organ matures. Cells are parts of a bigger organism, but when we look at them in our experiments we can think of them as little individuals. We try to track where they come from, where they’re going, and how they get there. In many ways it does resemble what happens to people as they go through life, but in a more tractable way.
What excites you the most about the future of this field?
I’m happy that I’m alive at this time in the history of science. The technologies that we now have available are unbelievable. We have decoded the genome of all living organisms which allows us to better examine who we are, how we got to be this way, and what makes us unique. We can also do single cell gene sequencing now to look at differences from one cell to another and from healthy individuals to people with diseases. We are just beginning to get a glimpse of what accounts for the vulnerability of certain cells that lead to diseases. Then there is the gene editing revolution that started a few years ago that allows us to add or remove genes and correct mutations. We have an almost god-like control now over the genome, this just emerged by breakthroughs in the last decade or so. The kind of experiments and insights that we now have are really mind boggling.
(Added April 8th) Could you also comment on this recent article in Cell Stem Cell titled: Human Hippocampal Neurogenesis Persists throughout Aging?
The new Cell Stem Cell article is interesting and will ensure that the topic of adult neurogenesis remains controversial, but in my view the data are very misleading. The conclusions are based on just a few so-called ‘markers’ that are interpreted as being definitive for identification of neuron progenitors and young neurons. The problem is that these markers are not specific. They are also expressed in non-neuronal lineages, more so in humans than in mice, which may have made the paper more compelling to those who are not familiar with the species differences. Supporting the notion that the cells are falsely identified, is that they do not morphologically resemble the neurons they are supposed to represent. It is much more likely that they are mis-identified glial cells that are known to divide in the adult hippocampus. Unfortunately, it will take additional studies and further investment in time and effort to make this clear. It is always very difficult to make a definitive argument for a negative result.
Click here to find out more about the work of Prof. Arnold Kriegstein or watch this video below to learn more about human brain development.