Assembling the brain
Nearly one in five Americans lives with a mental illness. Unfortunately there’s a limited set of options for treating psychiatric disorders. One reason for that is that these disorders are still defined based on people’s behavior or invisible internal states — things like depressed mood or hallucinations.
But of course, all our thoughts and behaviors are governed by our brains. And there’s a lot of research that makes it clear that many disorders, including schizophrenia, autism, and probably depression, may have their origin during early-stage brain development. The problem is that we still don’t know which brain circuits specifically are responsible for these disorders — or how they got that way.
Studying human brain circuits as they develop is — obviously — challenging. But what if we could rewind the clock and follow the development of neurological circuits in real time? Believe it or not, new technologies may soon make this possible.
Today's guest is Sergiu Pasca, Kenneth T. Norris, Jr. Professor of Psychiatry and Behavioral Sciences at Stanford University School of Medicine and Bonnie Uytengsu and Family Director of the Stanford Brain Organogenesis Program at the Wu Tsai Neurosciences Institute.
Pasca and his team have developed techniques to create tiny models of a patient's brain tissue in the lab — models called brain organoids and assembloids. They can watch these models grow in lab dishes from a few cells into complex circuits. And they can even transplant them into rats to see how they integrate into a working brain.
While all this may sound like science fiction, these techniques are fueling a revolution in scientists' ability to observe human brain development in real time, trace the origins of psychiatric disorders and — hopefully — develop new treatments.
Listen to the full episode below, or SUBSCRIBE on Apple Podcasts, Spotify, Google Podcasts, Amazon Music or Stitcher. (More options)
Further Reading
- Reverse engineering human brain by growing neural circuits in the lab | Wu Tsai Neuro
- Human brain cells transplanted into rat brains hold promise for neuropsychiatric research | News Center | Stanford Medicine
- Sergiu P. Pasca: How we're reverse engineering the human brain in the lab | TED Talk
- Assembloid models usher in a new era of brain science | Stanford Medicine
- Human Brains Are Hard to Study. Sergiu Paşca Grows Useful Substitutes. | Quanta Magazine
Episode Credits
This episode was produced by Michael Osborne, with production assistance by Morgan Honaker, and hosted by Nicholas Weiler. Art by Aimee Garza.
Episode Transcript
Nicholas Weiler:
This is from our Neurons to Yours, a podcast from the Wu side Neurosciences Institute at Stanford University. On this show, we crisscrossed scientific disciplines to bring you to the frontiers of brain science. I'm your host, Nicholas Weiler.
Here's the sound we created to introduce today's episode. Perhaps that is the sound of the brain assembling itself.
Nearly one in five Americans lives with a mental illness. One in five. And unfortunately, there's just a limited set of options for treating psychiatric disorders. One big reason for that is that these disorders are still defined based on people's behavior, or invisible internal states. Things like depressed mood or hallucinations.
But of course, all our thoughts and behaviors are governed by our brains. And there's a lot of research that makes it clear many disorders, including schizophrenia, autism, probably depression, may have their origin during early-stage brain development.
In other words, subtle deviations in how brain circuits are set down in utero, in the womb, can trigger a domino effect that sets the stage for disorders that only become apparent later in life.
The problem is that, one, we still don't know which brain circuits specifically are responsible for these disorders. And two, how they got that way in the first place.
Studying human brain circuits as they develop is obviously challenging. What if we could rewind the clock, and follow the development of neurological circuits in real time? Believe it or not, new technologies may soon make this possible.
Researchers here at Stanford have developed techniques to create tiny models of a patient's brain tissue in the lab, models they call brain organoids and assembloids. They can watch these models grow in lab dishes from a few cells into complex circuits, and they can even transplant them into rats to see how they integrate into a working brain.
While all this may sound like science fiction, these techniques are fueling a revolution in scientist's ability to observe human brain development in real time, trace the origins of psychiatric disorders, and hopefully develop new treatments.
That brings us to today's guest, a leading pioneer in this growing field.
Sergiu Pasca:
I'm Sergiu Pasca, I'm a professor of psychiatry and behavioral sciences, and I'm also the director of the Stanford Brain Organogenesis Program in the [inaudible 00:03:37] Neuroscience Institute.
Nicholas Weiler:
Well, Sergio, thanks so much for coming on the show.
Sergiu Pasca:
Thank you for having me.
Nicholas Weiler:
So it seems like we are at a really exciting time in neuroscience, where we're starting to be able to study what causes psychiatric disorders like autism and schizophrenia in a precise, rigorous way that could potentially lead to better treatments for these conditions. And your work is right in the middle of this.
Because what you've done is create a new way to grow simple models of the developing human brain in a lab. And what you've done is actually pretty complex. So that we don't get tangled up in all this, I was thinking it might be helpful for me to give it a shot first, to try to summarize and explain the context for the amazing work you've done. And then I want to hear from you if this big picture that I'm outlining is right.
Does that sound good?
Sergiu Pasca:
Sounds perfect.
Nicholas Weiler:
Okay. So it seems really clear that certain neurological disorders like autism and schizophrenia have their origins in the developing brain, in utero, where brain circuits are not wiring up the way they're supposed to. But the process of how the brain builds itself is so complicated, and it's so hard to study the developing human brain, that it's been hard to figure out exactly what's going wrong.
But recently advances in studying the human genome have led to growing evidence about specific genetic changes that could be driving these miswired circuits.
At the same time, this amazing stem cell technology came along that lets you take people's skin cells and essentially reboot them to this early developmental state, so you can tell them to become brain cells or whatever you want. And we also have the ability to precisely target and modify specific genes using tools like CRISPR.
So what your team has done, combining all these technologies, is develop ways to grow tiny models of people's brain circuits in the lab derived from their skin cells. And then you can rewind development and watch it from the start. You can look at how particular genes and mutations in people with developmental disorders change, how their brain circuits are wired up, and you can even start to look at whether you can fix these misfired circuits by modifying particular genes.
So that's a lot, but I mean, do I basically have this transformation, right?
Sergiu Pasca:
Yeah, absolutely. That's exactly right. We should write a review together. Actually, you should write the review of the...
Nicholas Weiler:
So I mean, are there elaboration on what I said that you would want to add to?
Sergiu Pasca:
No, I think it's an exciting time, as you mentioned, for neuroscience. Because we can finally start to get access to stages of human brain development that were previously inaccessible.
And the reality is that most of brain development is inaccessible to direct study. And by direct study I really mean accessing cells and molecules in a way that you can both probe and manipulate those cells. Not just watching them on imaging at a distance, in a way that you can manipulate them into experiments with causal variances that come out of that.
At the same time, we can finally see the promise of being able to demystify some of the psychiatric disorders. I mean, most psychiatric disorders are quite mysterious. I mean, something happens in the brain, obviously, and then it causes these highly dysfunctional behaviors. Think about autism spectrum disorders or think about schizophrenia and hallucinations.
And yet, all of these incredibly complex behaviors do arise in the brain, at the level of cells and circuits. And I think you are already describing, but I think the technology that we've been developing so far truly gives us access to the multiple levels that underline brain function. Going from genes to cells to circuits, and all the way to behavior.
Nicholas Weiler:
One word I want to come back to is the word that you use, model. Because this is something that's so crucial in science, that we're trying to understand something incredibly complex, the human brain and how it builds itself from a few simple cells during embryonic development, and during infancy in childhood and into our twenties, really.
But we've lacked the ability to model that in this kind of way in the lab. And so these tiny little brain circuits that you're growing, that I think are often smaller than your pinky fingernail, that's what they are. They're a model.
Sergiu Pasca:
Absolutely. They're models. And as George Boggs famously said, "All models are wrong, but some are useful." And so we cannot model the entire human brain. What we can do is, we can model specific processes, each human brain development. And hopefully now even in early stages, postnatal brain development. But they're still models at the end of the day.
Nicholas Weiler:
I want to get back to some more details on the experiments that you're now doing in the lab. But first, I did want to sort of step back. You're a physician, right?
Sergiu Pasca:
I'm a physician by training, yes. I don't see patients anymore, but deep inside I remain a medical doctor, I think.
Nicholas Weiler:
Right. Once a physician, always a physician. Well, if I remember correctly, you got into this question about how brains develop, and how psychiatric conditions begin because of some experiences working with families with autism while you're in medical school.
Sergiu Pasca:
Yeah, absolutely. I mean, my expertise still remains autism and autism spectrum disorders. And when I saw some of the first patients with autism, when I was early in medical school, I was just shocked. I was shocked by the complexity of the disease. I was shocked by just how little was known about the disease. And I think an encounter with a few parents of some of these children had a very big influence on me emotionally as well.
And then I spent the summer at the Max Planck in Frankfurt for Brain Research, and it was a transformative experience for me. And I thought, if you really want to understand the psychiatric disorders, we need functional access to neurons from patients. And obviously that was not a possibility at that time.
Nicholas Weiler:
Right, because these disorders are developing exactly at a time when the human brain is at its most mysterious and inaccessible, right? You can't just record from it.
Sergiu Pasca:
Exactly, and that's one reason. And as I was finishing medical school, Yamanaka made this transformative discovery that you could take any somatic cell type and push it back in time to turn it into stem cells. And suddenly the possibility that you could actually make neurons from patients in a non-invasive way became possible.
And that essentially what changed my trajectory. And I came to Stanford, and build some of the first early models, making neurons from patients with genetic forms of autism.
Nicholas Weiler:
Right. And so now you can grow these whole circuits to actually see how those stem cells grow into brain circuits, and what might be going wrong.
Sergiu Pasca:
Exactly. And it been a process. Interestingly, has followed the developmental path of the brain. If you were to think about the human brain, how it develops, there are very clear stages. Initially you specify and you generate cells across parts of the nervous system. Then cells start to migrate to find their final position. Then they start to extend axons and form circuits, and then of course, behavior arises as an emergent property of some of those circuits.
And what we've actually done for the past almost 15 years or so, was exactly that. Then these models became slightly more complex when we discovered that you can actually get remarkable self-organization when you put them in three dimensional cultures. When you simply don't let them sit at the bottom of a dish.
And that comes with remarkable advances. You can keep those cultures for very long. You can make multiple brain regions. But they were still just cells in part of the nervous system.
Nicholas Weiler:
Right.
Sergiu Pasca:
And so to move into the next stage of development, which is migration of cells, then we introduce the first assembloid. Where you make two parts of the nervous system that interact in one way. And then when you put them together, cells start to migrate in the way that you would expect them to migrate and for. Small circuits. And so we've been assembling ever more complex circuits.
Nicholas Weiler:
So you can actually start to model specific wiring diagrams, essentially.
Sergiu Pasca:
Exactly. And of course, the final stage for this was, can they participate to behavior? But as you can imagine, there's no behavior in a dish. And so that has really been a challenge.
Nicholas Weiler:
I was going to say, And so that's why you've gone ahead and recently were able to transplant some of these circuits that you grew in the lab into a rat.
Sergiu Pasca:
Right, exactly. I mean, for multiple reasons. One was certainly because we wanted to know whether the defects that we identify for any disease are going to be present in an unenviable setting as well. I mean, you can imagine that, let's say in a form of autism, you identified there're fewer synopsis. Or blunted [inaudible 00:12:19].
But does that matter? Once the cells are embedded into a complex circuit, maybe there's compensation. Maybe there is a growth factor or a nutrient that will just rescue that phenotype.
So demonstrating that those phenotypes really matter for disease in living circuits is, I think, a very important next step for the field. And the other reason is truly because we have a lot of drugs that we need to test moving forward.
And if there's not a viable animal model for that specific disease, the only other alternative is to use a primate. And it's not easy to build a primate model for many of these diseases either.
Nicholas Weiler:
Sure.
Sergiu Pasca:
And so having a chimeric model, where you have human cells embedded into the circuitry of an animal, allows you sometimes to test in default the effect of some drugs in cells derived from patients.
Nicholas Weiler:
And chimeric there means just having human and, say, rat tissues in the same brain.
Sergiu Pasca:
Exactly. I mean, that's not the most fortunate term. I think it sometimes can be confusing because it has mythological origin or implications. We refer generally to transplanting some of the cells into the rat or the mouse nervous system.
Nicholas Weiler:
So I mean, in other words, basically if your goal is to model a disorder or a condition like autism that's really defined by its behavioral symptoms, you'd better be able to show how the circuits that you think are responsible are actually affecting an animal's behavior.
Sergiu Pasca:
Sure. But we have to be careful. Because there are limitations on the repertoire of behaviors in these animals.
Nicholas Weiler:
They're not the same as human patients, of course.
Sergiu Pasca:
Right. But I think what we can certainly do, that we can effectively embed human neurons into the circuit, we can create units of human tissue that, those integrate information from the rat from rat senses even, can participate to behavior. And I think it's a viable way in which you can test whether a mutation that causes disease causes defects into this context of a circuit.
And I think there's going to be way more work to do in order to build them as useful behavioral readouts. But certainly that it's the goal. I mean, let's not forget, all psychiatric disorders are behaviorally defined. None of the psychiatric disorders require a pathological assessment or a biomarker.
Nicholas Weiler:
And so that's the thing that needs to get connected to some specific defect or difference in the brain circuitry, in order to understand how to maybe treat them.
Sergiu Pasca:
Exactly.
Nicholas Weiler:
So it must be really exciting to finally see the development of these brain circuits in a clear way.
Sergiu Pasca:
Yes. You can look at, are some of these mutations changing the type of cells that you're making? Are they changing the way which they migrate and find their final position? Or is it changing the electrophysiological properties of these neurons, the number of synapses that they're making? How big or small the neurons are.
And one of the most remarkable things that we've discovered by maintaining some of these cultures now in a dish for a very long time is that the timing of development is really well conserved. Even en vitro.
Nicholas Weiler:
So in your models, the timing of development matches what we see in a real, developing human brain?
Sergiu Pasca:
Exactly. I mean, we've maintained the longest cultures that have ever been reported, going 800, 900 days and beyond. In principle, we can keep them indefinitely.
And then you can ask the fundamental questions. Sure, you made cultures that are 800 days in a dish, but are they the equivalent of still fetal periods like mid-gestation, late gestation, early postnatal? And in a series of three studies, we've actually discovered the same thing.
Which is timing of development matches or replaced to a large extent in vivo development. And to such an extent that when you look transcriptionally at the chromatin level, once the cells reach nine months of keeping them in a dish, they actually transition to a postnatal signature. And that tells us that they have some sort of molecular path that keeps track of time, even in the absence of information in utero or birth itself.
Nicholas Weiler:
That's amazing.
Sergiu Pasca:
I mean, the beauty of this model is that once you start development, you unleash a program of development that goals progresses at a certain timing.
Now, that is not to say that all aspects of maturation are actually recapitulated. In fact, one of the reasons why we wanted to do the transplantation it's because there were limitations for some of the cultures. I mean, neurons would not have some of the electrophysiological properties that they would expect. They would not have [inaudible 00:16:49].
Nicholas Weiler:
But it wasn't matching up, exactly.
Sergiu Pasca:
Yeah. They were not matching. They would have a signature that there are there in time, but there would be a mismatch in some of the properties of the neurons. Clearly indicating that they knew where they were, but they were unable to completely start a program of maturation.
And I think that's what the transplantation essentially does. If you take human cortical organoids and you keep that for 250 days in a dish, or you put them for 250 days in a rat, neurons that are in the rat will be six to eightfold larger in size.
Nicholas Weiler:
Yeah. It's so exciting to be at that stage where we can start to take these circuits apart and see what makes them work or not work.
But I think coming back to the transplant experiments, I mean, I think still, some people may still feel a little creeped out by this idea of putting human brain circuits into rat brains. And I know this is something that you've thought a lot about and taken a lot of care into your work. Could you talk to me a little bit about some of what you've done to address the question of how to make this ethical, and the implications of the work overall?
Sergiu Pasca:
Sure. As you can imagine, the more complex the models are becoming, the more uncomfortable we sort of feel. And there's precisely the dilemma. Psychiatric disorders are, to a large extent, human conditions. And we know that there are limitations in modeling them with animal models because some of those human characteristics are missing.
So we certainly want models of the human brain that are as human as possible, but the more human, the more uncomfortable we sort of feel about them. So that's precisely the dilemma.
But psychiatric disorders represent a huge burden to society. I mean, almost one in five individuals suffer from a condition. We have no cures for any of these conditions. We have treatments that help us manage them. But true cures for psychiatric disorders have been very difficult to develop.
And we have a responsibility, as a scientific community and I think as a society, to find solutions for these conditions. And transplantation models, they've raise a number of ethical concerns moving forward.
And we've been spending a lot of time discussing this. I mean, we usually have these conversations before we start this experiment. Not just with the panels, not just with the animal review panels or with the ethics panel, but really as part of the community here with other faculty. And so we've been discussing at length, some of the implications of this work.
And I think in vitro, there are very few reasons to believe that there are advanced properties of the human brain that arise, like cognition or consciousness. That is not conceivable. You could argue that once some of these human cells are transplanted into the rat, and in this latest publication that we've had, we've shown that we can actually grow a unit of human cortex to be about a third of a rat's hemisphere.
Nicholas Weiler:
Wow.
Sergiu Pasca:
To be integrated with the thalamus. You can actually run reward conditioning tasks in this animal.
Nicholas Weiler:
It really integrates into the animal's brain.
Sergiu Pasca:
It integrates into the animal brains. But even there, and I think this links back to the discussion that we've had before, the timing of development is very different. I mean, even when you put human cells into the rat, they will still develop at their own pace.
If it takes a week to make the rat cortex, it takes at least 20 weeks to make just all the neurons in the human cortex. There's a huge discrepancy between the two species. And just because the human cells are in the rat, they're not maturing at a faster pace.
That perhaps offers some comfort about the level of integration that we do allow. Because there is a self-limiting feature of the system.
Nicholas Weiler:
Yeah. And I think, as you said before, I mean, you were also very careful to show that it's not like you're humanizing the rat in terms of its behavior. I mean, it's almost more like the human circuits are becoming part of the rat brain.
Sergiu Pasca:
And we spent a lot of effort making sure, first of all, that the animals are not suffering. They don't have seizures, for instance. But at the same time, we've also run a comprehensive series of behavioral tasks, and we have not seen differences between transplant.
And we were, just to make it clear, we were not expecting augmentation. On the contrary, we were hoping that there would be a deterioration of some of the functions, because you suddenly now have an extra mass of tissue into the brain of the rat.
Nicholas Weiler:
Right. And as you said, we sort of have this ethical imperative. Because there are real people suffering from these conditions. And for the most part, we have very little to offer. And that sort of brings me to the last question I wanted to ask you.
Which is, all of this is still very new of course, and it's hard to predict the future. But if you could think back to young Sergiu attending medical school and being frustrated about how little you could say to the parents of patients with autism, what do you hope that someone like you going into medical school, say 10 years from now, will be able to understand thanks to having this kind of model where we can study what's actually happening in these conditions?
Sergiu Pasca:
At this point, and I didn't feel this way even a few years back, I am convinced that we will see treatment for some of these conditions in the next five to 10 years.
The more we understand these conditions, the more many of the other therapeutic tools that already existed for cancer and broader conditions will be applicable for psychiatric diseases. And I think we're slowly going to see demystification of some of these conditions.
I mean, psychiatric disorders are, to a large extent, mysterious. And I think we will see examples of genetically-defined forms of autism where we will understand some of the detailed molecular mechanisms, and we'll find a way of restoring them, and perhaps improve some of the outcomes in those patients.
I mean, it still won't be easy, but I think there is hope. Which 15 years ago was not something that I could offer in any way.
Nicholas Weiler:
Yeah. Well, thank you so much, Sergiu. This has been such a fascinating conversation.
Sergiu Pasca:
Oh, thank you so much. Thank you. It's been great. Thank you.
Nicholas Weiler:
Thanks so much again to our guest, Sergiu Pasca. There's a lot to unpack in this story, and I strongly encourage you to check out the links provided in the show notes.
This episode was produced by Michael Osborne, with production assistance by Morgan Honacher. Amy Garza designed our show art. I'm Nicholas Weiler, see you next time.
