a16z Podcast - Don't Call it a Brain in a Dish!

Episode Date: May 24, 2020

Our understanding of the human brain and its disorders has always been limited by our lack of access to living, human, developing brain tissue. For the first time, that's changing. In this episode, Sergiu Pasca, Professor of Behavioral Science at Stanford, talks with a16'z General Partner Vijay Pande and Hanne Tidnam about the wild new tech that's pioneering a whole new approach to understanding the brain: brain organoids.So what are brain organoids, what are the scientific breakthroughs that lead to their creation, and how can we use them best? The conversation starts with the existing models we have used to learn about the living brain, from genetic studies to autopsies to primates—and what this new model now brings us: the ability to study the human brain, both how it develops and what goes wrong in certain disorders, with human brain tissue "alive" in a dish. We talk about what these organoids can and can’t do; what they’re good for understanding and where that understanding becomes limited; why calling these “brains in a dish” or “mini-brains” isn’t the right terminology at all; and finally, how far can this new tool and model be taken now and in the future, leading us closer towards understanding psychology itself on a molecular level.Image: Brain organoids derived in the Pasca Lab at Stanford University.

Hi, and welcome to the A16Z podcast. I'm Hannah, and in this episode, General Partner Vijay Pande and I talk with Sergio Pashka, Professor of Behavioral Science at Stanford, all about a new technology we have for understanding brain disorders, the wild and very sci-fi new frontier of brain organoids. So what are brain organoids? How are they developed and how can we use them? The conversation starts with the essential problem that we've never had real access to the tissue and actions of the developing brain or even a living normal brain and the problems with all of our existing models for understanding it from genetic studies to autopsies to primates we look at those models we've relied on in the past and what this new model of brain organoids now brings us allowing us to study the human

brain both how it develops and what goes wrong in certain disorders with living human brain cells in a dish for the very first time we talk about what these organoids can do and can't what they're good for understanding and where that understanding becomes limited and why calling these mini brains or brains in a dish isn't the right terminology at all. And finally, how far this new tool and model might be taken now and in the future and how it will lead us closer towards one day even perhaps understanding psychology itself on a molecular level. We're here today to talk about understanding brain disorders and some of the new tools we're developing for how to do so. So let's start where we actually are in that. Are we actually anywhere significantly more

advanced than we were in the days of hysteria? You know, thinking about things like labeling these sort of conditions, societal conditions that we had no clue. Like, where are we actually right now? Psychiatric disorders are still behaviorally defined. And there are very few biomarkers that are considered reliable for diagnosis. The truth is that our understanding, of psychiatric disorder is actually quite limited. I often like to joke that I suffer from an oncology envy syndrome, which is essentially this deep frustration that you feel, as you see just like how fast cancer research has gone in the last few decades, from really like no treatments whatsoever to almost completely curing certain forms of cancers. And if you look careful

at it, you realize that one of the reasons for this ink, incredible progress is that oncology has really made use of the revolution in molecular biology. And it has done so because it actually has access to tissue, to the tissue of interest. We know almost nothing about how the human brain develops because it's completely inaccessible. And so, again, we are defining psychiatric disorders based on combinations of behaviors, presence or absence or certain patterns of behavior. We've made, I guess, a lot of progress into classifying these disorders and reclassifying them. But the truth is that our molecular understanding of psychiatric disorders or brain disorders more broadly is very limited.

And probably behind any other branch of medicine, which I think is reflected in the therapeutics that we have. And the complexity. I mean, it's fun to think about, like, you know, in the 80s, molecular biology was this hot new term. I mean, you're talking about something almost like molecular psychology, taking this big sort of emergent phenotype that is, you know, a behavioral and then trying to connect it, not just at the tissue level, not just at the cellular level, but all the way from molecular level, that is a hard thing to do. It's hard to imagine like if someone has schizophrenia or severe depression, what's the target to hit?

Right. You said something really interesting about just never being held back by not really having the tissue. And you mean by that, you know, that we, the first time we get to look at the tissue is after somebody who has suffered from a psychiatric disorder has died, right? That is our primary tool at the moment. Yes. And there are a number of challenges associated with studying post-mortem tissue from patients. Of course, the obvious one is the fact that the tissue is not alive. For me, as a neuroscientist, it is really important to be able to record electrical signals from cells, to really look at how they're communicating with each other. But at the same time, another limitation is actually the

availability of tissue. I mean, if you were to just think, for instance, about autism spectrum disorder's which is very common one in 60 or so individuals and there is even an autism brain bank but the number of brains that we have in a brain

bank is really in the hundreds not in the thousands for a disorder that is like so common and it's probably for adults too right and it's right another limitation is actually the age of this individuals but very often also the cause of death because in most of this case is actually traumatic

death and most of the psychiatric patients will take many, many medications and undergo various therapeutic interventions across their lifespan. We don't know, for instance, how is that influencing what we're seeing in post-mortem tissue? So you're getting a very small amount of information that may not even be accurate? Or very anecdotal. Or very anecdotal, yes. And that's the only real tool that you have at the moment besides behavior? Well, I think, of course, there are imaging studies that you could use, For instance, MRI and functional MRIs. The problem with those studies is that you don't really get the molecular resolution.

You don't get to really study the tissue. An alternative, which has been, you know, used in the last decade or so, has been to model many of the disorders with animals. Right. And that has been quite an exciting field that was primarily accelerated by identifying genes associated with psychiatric disorders. But I think we always have to be aware of the differences between, between species, right?

Even in how the brain, the structure of the brain, the fact that there are millions of years that separates us in evolution, that the behavioral repertoire is very different across species. Of course, there, the behavioral repertoire is much closer to that of humans. But as you can imagine, again, the limitation there is how scalable, is that how many primates can we really use for this type of studies and who can afford to do this experiment on a large scale? The truth is that most of the psychiatric disorders have a very complex genetics.

It is very rare like one single gene or one single variant, but very often a combination of this. And it's not just obviously about the genes, but what are the cells and the circuits that are affected by this? And I think that only once we start to understand some of the molecular machinery behind the psychiatric disorders, can we, as it happened in, I guess in the cancer field, start thinking about therapies that have been designed? for specific disorders rather than identified by chance. Because many of the drugs that we have for psychiatric disorders today have actually been identified by chance. Yeah, most, right?

And not only identified by chance, but the method of action even after the fact is poorly understood. Absolutely. Yeah, we're using them and even that we don't quite understand why. It's purely empirical. It's empirical. So what are some of the major shifts that we're beginning to see where we are beginning to engineer a better model

or a better mapping of what's actually going on behind the scenes? There are a number of important discoveries that have happened over the last few decades that I think brought us in this unique position right now to start asking questions about human brain disorders with human patient cells. And the first one is probably even the fact that we were able to maintain some stem cells in a dish, which was done in the 80s. For a long time, we were unable to really maintain pluripotent stem cells in a dish. So living cells?

cells, embryonic stem cells maintained in a dish. Another paradigm shift has happened 12 to 13 years ago when it was shown that development is really not a one-way street. Cells don't just differentiate and never can go back, but there are ways of actually pushing them back in time to resemble those prolipotent stem cells from which every cell that we have is made. Essentially, you can take skin cells from any individual, put them in a dish, and then just overexpress a series of genes that are important for a cell to be pluripotent. And that is sufficient to actually push that cell in time to look like a pluripotent stem cell. So this is... It's like time travel for cells.

But I think what that... Or fountain of youths. Or fountain, yeah. But I think what that allows us to do is essentially be able to obtain in a non-invasive way, right? Because this is not invasively by getting a skin biopsy or a blood sample, obtain pluripotent stem cells from any individual. And by pluripotent, and the name already implies, that means that they can become multiple cell types, including brain tissue. And so that is where it becomes about studying the brain. Right. So I think the third series of breakthroughs has to do with us slowly learning how we can instruct this pluripotent stem cells to become brain-like tissue in a dish.

So it comes from cells from an adult that then become under-franchated. Right. And then sort of the fun begins. It's kind of a mind-blowing, so to speak, to take like a skin cell or a blood cell, turn it into a brain cell, neuron cell, with others. And then developmental starts again. Starts again, absolutely. The human cerebral cortex has a diversity of cell types, which are arranged in layers.

But those layers are not born all at the same time. They're born in a specific sequence. And initially you get layers of deep layers, like layer five and six, and slowly you get upper layers. Now, if you start to recapitulate that process in a dish, then you start seeing that you don't have to provide all the cues all the time. You just have to provide the initial cues. And once the process starts, the progenitor cells start making first deep layer neurons,

and then slowly start to make upper layer neurons. But doesn't it take years as long as it does in the human brain? Exactly. And it's, well, if you think about the human cerebral cortex, all the neurons in our cerebral cortex are made by 27 weeks of gestation. Okay. So that's second part of gestation. But that's still a very long time for cultures.

Yeah. And essentially, long for research. Precisely. And this is exactly what motivated us to even develop new methods. Because initial methods were keeping cells at the, on a flat surface, a dish on a plastic flat surface and so what happens when you start to keep cultures this cultures the cells for like you know 10 weeks 12 weeks it doesn't seem like they'd be very happy they're not very happy and essentially what they do is they're still peeling off and my frustration

initially was that we couldn't go farther in development yeah you could see that they were progressing but we did they were just like peeling off nowhere to go yeah yeah so so what was the magic what was the secret essentially i mean if i don't know if we can call this innovation but around i think the summer of 2011 or so, I thought that maybe would be easier to just move the cells to a plate that doesn't allow the cells to attach at all. It's essentially a plastic dish that is coated with a substance that doesn't allow the cells to sit down. Because they cannot sit down, then essentially they form a ball of cells, a sphere. To each other. Exactly. And it's interesting enough that then they don't really... They're not bothered. Yeah, they're not bothered. So you can keep

them for very long periods of time. And in fact, we've kept some of the longest, if not the longest cultures, ever maintained, which went on for 800 days and beyond. Initially, we just thought, oh, let's just like do that so we can keep them a little bit longer. But then we realized that that enabled the sales to self-organized. Because they're in a three-dimensional culture, they start to interact very differently, one with each other. And they can just develop and progress in development for much longer. So you see more. They're doing more. You can recapit you. Exactly. You You allow them actually to, you know, essentially develop as they would. Kind of getting out of their way.

Right. Exactly. Exactly. You know, what's interesting about this is that we're, in many ways, the science here is just replicating the natural aspect of developmental biology. That development of biology starts with, you know, originally these stem cells. And these stem cells have to then differentiate into becoming all different types of tissue. The fact that actually we can go backwards is interesting. And also the fact that actually, in some ways,

the experiments will recapitulate that developmental biology is very intriguing. Yes. I mean, if we really work to think like how the human brain comes together, there are a group of stem cells, which are cells that are capable of turning into other cell types under appropriate conditions, and those cells start organizing and forming the brain over a very long period of time. The human brain literally takes hundreds and hundreds of days, and then years afterwards to mature and come together.

And that, I think, is one of the most exciting possibilities for the cellular reprogramming technologies is that they allow us to essentially recapitulate in a dish some of the cellar processes in a way that allows us to study them and, of course, to do this in the context of patients. It really does make me think about when you say, well, the original black box is the human brain, right? It is truly that we see nothing inside it,

except for studying the behaviors that are as a result of that. And actually, you know, the amateur developmental biologist in me likes to point out that all this organization that goes on kind of recapitulates evolution to some degree, too. And so, you know, we have, the brain has all this complexity of lower brain and then other parts and eventually cerebral cortex gets added through evolution. That seems to be what you're talking about in terms of it being developed. So we recapitulate in developmental biology. So now the issue is like since if we care about these disorders,

and probably most of these disorders are not going to be in the lower sort of animal brains. Absolutely. You know, we have to be able to build models that have them. Right. So now this is what some media have started to refer to as brain in a dish or organoids or we've heard mini brains as well. Those are all attempts to try to describe this model

of how we can begin to understand more from live human brain cells. Can we talk about, are all of those labels, do they all, to get at something different? There's still a little bit of confusion. the field about the terminology, but some of these terms are definitely inaccurate, and I think they do not reflect the science. Probably even the term organoid is not really the most appropriate. And that's a more generic term, because you can have liver, organoids, or cardiac. Absolutely, yes. An organoid, broadly speaking, is a three-dimensional cell culture that

self-organizes and recapitulates some aspects of organ function, but not all. But actually the majority of the organoid field is about cells that are derived directly from patients. So, for instance, you take a biopsy, a gut biopsy or a liver biopsy, and you just want to maintain those cells. So you keep them in a three-dimensional culture that's also called an organoid. I mean, organoid, of course, as the terms implies, is organ-like. Because they don't have organ functionality. No, they don't. And most of them don't also recapitulate many of the structures.

And I think there is a little bit of confusion. But in the organoid field, like, there are, I think, are a lot of. lung and liver organoids that have some recapitulation of function. Right. So in that sense, maybe that's appropriate. But you're saying on the brain side, I mean, what aspect of function? All of the organoids recapitulate some features of the organ. Of course, none of them recapitulates all the features of the organ.

Yes. Like neurons in brain organoids fire action potentials. They make synapses with each other. So they do communicate. But of course, it is not like the brain. Most of this brain organoids resemble very specific brain region. The brain is not a homogeneous organ, it's not like the liver. It does matter which part you actually probe.

I didn't realize that you could make a little visual cortex or a little hippocampus or a little... Yes. Well, actually, paradoxically, the easiest structure to make is the cerebral cortex. It turns out as kind of like the default is the one that has the least number of instructive signals that you provide. But then you can, you know, and we've shown this, you can derive multiple brain regions. You can make a mid-brain. You can make a spinal cord. It can make a striatum.

You can make a hypothalamus and guide their differentiation with essentially small molecules. These brain organoids, where you're using them to try and understand more about these brain disorders, how are we getting more information out of them? What is that beginning to teach us? I think one of the first things is that they recapitulate many aspects of human brain function. Again, by maintaining them for very long periods of time, we show, for instance, that cell types are generated in a sequence, in the same sequence as they would in the brain. So, for instance, you don't just get neurons in the brain.

You also get glial cells, which for a very long time were thought to be like unimportant cells, they just like glue the brain together. But they come late in development, especially in humans, for instance, astrocytes are essential for neurons to even form synapses with each other. But we know very little about them. Now, if you keep this organized for long enough, and it does take 20 to 30 weeks. But once you do that, you start seeing, for instance, astrocytes also appear at the right time. and they actually mature.

In fact, one of the most surprising things that we've discovered by maintaining these cultures for a very long time and comparing them to primary tissue, to tissue obtained from actual patients, is that the cells do, they know the maturation time point. And for instance, they know when birth should happen. Oh, my God. That's crazy.

Is there a moment? I'm imagining all of you in the lab at this point, you're seeing these developments and hoping that you might start to see these different types of cells form? Yeah. And then suddenly one day they did these, is that like, was it like a birth? No, I think it was like a slow process like first even. And for me personally, it was like difficult to accept.

As a physician, I always kind of like thought as like birth as being like this dramatic event to the brain that triggers a series of processes. But, you know, birth in humans is actually not even happening when it should happen. Yeah, it should happen three months later. It should happen much, much later. Yeah, exactly. But there are limitations because of head size. And so it's not really about the birth itself. It's just about the maturation.

the brain. But you must have been amazed to see the maturation reach that level. If you think about glial cells, astrocytes in particular are very different before birth and after birth. In astrocytes, as they approach nine to ten months of keeping them in a dish, they slowly, not suddenly, slowly transition to a postnatal signature, as if there is some sort of program that once they started, it just progresses. And we know that not just at a gene expression level, but at the chromatin level, epigenetic level, and even functional level, many of the cellar processes are, and again, is it surprising? Sure, it's surprising for us, but if you think from a point of view of developing a brain, the mechanisms for making a brain

must be very robust because you have to make the same brain over and over again. And of course, there are a lot of differences between our brains, but to a large extent, they're quite similar, right, structurally and functionally. Well, yeah, and it's a funny thing about biology because at some level it's amazing, anything works. And then another level, like, it doesn't take much. Like, if we're talking about a very different context. We're talking about plants or something like that. We're just planting seeds and letting them go. Yes. I mean, that's amazing in its own right. And in a sense, these iPS cells or these stem cells are seeds of sorts. Absolutely. And then we put them together.

They interact with each other. The cell to cell communication creates this developmental profile. There's probably a cellular clock. And it's just, it's just a machine going tick, tick, tick, tick, tick, rolling along. As long as you just don't get in its way and create an environment that's kind of close enough to what it needs to be. But we're talking about development of, you know, sort of normal development in a way. How does this begin to connect to abnormal development and our understanding of when things are going wrong?

Well, I think the fact that we can even model in a dish in a non-invasive way, right, because it doesn't involve like taking anybody's tissue, brain tissue, right? I think the greatest opportunity is in accessing this stages of brain development that were previously inaccessible, that are likely related to disease.

Just to give you an example, a very specific example about modeling disease and asking questions about disease, the human cortex doesn't have just cells that excite other cells. It also has about 20% of the cells that inhibit other cells, other neurons. So there is a very clear balance.

Yes, precisely, between excitation and inhibition in the cortex. And that is very important because if you think, for instance, about epilepsy, right? In epilepsy, there's more excitation or there's not enough inhibition. This balance just goes awry. It's also thought that, to a large extent, this balance also goes awry in autism. So very often autism is thought to be a disease of excitation to inhibition imbalance. But here's an interesting fact about the developing of the human cortex.

All of this inhibitory cortical neurons, all of these neurons that put a break, are actually not born. in the cortex in the brain, but they're born literally inches away from the cortex in another part of the brain and sometimes around mid gestation start all of them to migrate one by one and they go up to reach the cortex and populate that region.

So which region do they originate from? So they come from the ventral forebrain in a region called MGE, and they have to literally, again, and one by one migrate and reach the cortex. They don't just crawl on a surface. What they do is they have a very peculiar way of movement to the cells, and they have a very long process that they point towards the direction which they want to move.

And at one point, their cell body almost breaks into the nucleus stays behind, and then almost like a muscle contraction, the nucleus is pulled up about 30 microns. So that's how they move? That's how they move. They move in a very peculiar way. And they do this every, you know, three hours or so. It's like an inchworm almost. Kind of, yeah, yeah, exactly.

And so we started watching to see what, again, it was unclear also how it really happens in humans. Because this is one of those stages, which is completely inaccessible. Yes, yes. Do you think that recapitulates evolution in some sense, or is it, I mean, because it's odd, but that, I mean, we had to tack on this brain on top of another brain. Yes. Well, I think it has to do with the fact that this cells, because they're so different. require different cues when they're, like, developed. It's interesting, like, one of the most surprising things for students

is just to learn that most of the cells in the brain are not born in the place in which they reside. You just assume that they're like there. The movement. Precisely. If anything, a rule of development is that cells have to move from where they were born to reach. In inches, inches.

And this happens, actually, there is evidence in rodents. this migration stops before birth. I was just going to say, does it continue as the brain grows? Yes. In humans, interestingly, there is like recent evidence that shows that in humans, in primates, but in humans particularly, this continues up to the second year of life in humans.

So towards the prefrontal cortex, there is a population of internos that continues to migrate. In motion. Right. So we know very little about this. So a number of years ago, we wanted to really model this complicated process in a dish. And we did so with a new approach, which we often call second-generation organoids or assembloids,

which essentially involve deriving these two brain regions separately in a dish from pluripotent stem cells. So, for instance, making the cortex that has all the excitatory cells, and then making this ventral forebrain regions that has all the intern neurons, developing separately, we put different cocktails of small molecules. And at one point, at the right point, we essentially put them together in a tube. essentially at the bottom of a tube, we leave them to sit close to each other overnight, and next day they essentially are fused to each other. Oh my God.

I mean, were there lightning bolts? Mary Shelley must be like rolling over in her grave. Well, I think what was really surprising because we really thought is going to be really difficult to actually do this experiment, that we're going to have to develop all kinds of like engineering tools to like stick them to each other until somebody in the lab came and said, look, it's very simple. You put them at the bottom of the tube. We leave them overnight the next day, they're essentially fused.

but I think what is even more surprising is what they do after because you can color the cells differently and you can watch them under a microscope and you start seeing over the coming weeks that those inhibitory cells that are migratory start to all move towards the other side and the migration is actually quite specific because the other cells don't care

and even if you fuse different combinations even if you put two spheres to organize or ventral they don't move so there's something that are attracted to. And so they move onto the other side. And once they arrive, they even change their shapes. They become very complex. And they start making synapses with excitatory cells. So that's the moments where this, where energy starts being transmitted. So the cells, like, initially they're not connected to each other. They're not synaptically connected. But once they arrive onto the other side, they start connecting to the other cells. Now, one of the things that

we discovered, then I think this brings us back to how can we use this model is that we, we, been studying for a while a form of autism and epilepsy called Timothy syndrome. And this disease is very, very rare. There are only probably a couple dozen patients all around the world. What makes them really unique is that they only have a point mutation. That means one single letter in their entire genome is changed in a calcium channel. And it's been known that this calcium channel is important for cells to migrate. And so we thought, could it be that in patients, this mutation affects this cellar process? We, again, recruited patients that have this disease, which are very rare. We brought them to Stanford. We got skin cells from these patients. We took those skin cells

and we turned them back in time to make them look and behave like pluripotent stem cells. And then we took those pluripotent stem cells and guided them to become either cortex or ventral forebrain. and then put them together. And what we noticed is that the patient cells had the cells jump much more often. So they would just engage in these jumps prematurely, so to speak, and every single time they would jump, they would actually jump a shorter distance.

So they wouldn't do a very good job. They don't move very well. Now, the good thing is that there are drugs that can modulate this channel, and once you add them and you block the activity of the channel, we showed that within essentially a few hours, you can completely restore this abnormal cell behavior. And the reason why I think some of these technologies are so exciting is because they offer us access to certain cell states, cell behaviors that would be very difficult to access otherwise.

And so understand some of these molecular, subtle molecular mechanisms in that context. So one of the big limitations seems like that you're kind of constrained to the time of human development. Is that for the foreseeable future, you're stuck with like, this pacing, or would the next kind of iteration of engineering this process be speeding that up somehow? No, I agree. One of the limitations is obviously how long this cultures really take. But I think that's both a limitation and I think an opportunity, because now that we know that in a dish, some of this timing is recapitulated, it would allow us to really try to understand the molecular machinery. What is this clock? And I'm hopeful that once we're going to identify

what that clock really is, it would allow us to maybe accelerate or decelerate. I love that idea that there's almost like an invisible clock going on here that Right, and we don't even know what the mechanism are. We don't know what the molecular mechanism is. And especially in the context of development of an organism, you might want certain things to time out with other things. You have to like have the bodies being developed while the brain's being developed. But in the context of just the brain, and maybe those constraints go away and you can imagine speeding it up. Right, right.

It's also very clear that the human brain has slowed down in general, like even versus other primates, slow down its development. Really? So many parts of the brain are actually developing at a much slower rate through a process that is called neotony, essentially just putting a break on brain development. Yeah.

What would be the, what's the selective pressure towards? The main, I think, hypothesis is that this allows the human brain. We have enough brain to do the basic functions, but we also have enough brain that we can allow to slow down its development and allow for social learning. So if you just think about the prefrontal cortex, that is the last one to myelinate,

to mature, and it's thought that because we're allowing social learning to happen more than any other species, more than any other primate. And so there are very clear mechanism. And we know this is true. I mean, this is very well conserved across species. Yeah. It's funny one of the parenting books I've read talked about actually took this analogy of development mirroring evolution one step further, which I talked about after they get out of the womb, the babies are first on a little caveman stage and then an old village stage and you probably see this. I can totally relate to that. And it's, I mean, caveman to villager is the social development of that frontal cortex.

Yes. That has to happen and maybe probably needs the stimulus for it to be done right. Right. But because of our social group, we can allow to just like have our children develop for much longer. And this is species specific just to make it. I mean, we, for instance, you know, if you take chimp, a chimpanzee derived stem cells and you compare them side by side, the brain organites, they do develop at different paces. So the chimp finishes earlier development and the human continues. So this is recapitulated in vitro as it is in vivo.

So the question is like, what is the molecular mechanism? How can we figure out? Because I think as you were mentioning, of course, that is a challenge because today we have to keep hundreds of days of culture. What are some of the other big challenges that you think would unlock a whole new level of understanding for us? One mistake very often in the field is to really think that this model is really the answer to anything.

I mean, I think as George Box famously said, all models are wrong, but some are useful. And so I think the idea is, like, what is it useful? I think very often people confuse, oh, are they fully recapitulating the brain? Of course not. The question is, what is the question that you're trying to answer? What process, what disease are you really trying to answer? And I think, you know, there are a lot of things that are missing. There's, like, for instance, no vascularization.

Many of the immune cells are missing. For instance, all the microglia are not. They can be at it, but I think it's fair to look at this as an incomplete model that can be tuned in a way and engineered. so that you can ask questions. So if you were to translate that into brain disorders, what types of brain disorders can we then study usefully with this and what not? I think the neurodevelopmental disorders,

so the ones that arise because of the human brain developing in some unusual way or in an abnormal way, I think those are the primary disorders that we can study. The ones that, for instance, have a later onset, such as Alzheimer's disease, neurodegenerative disorders, I think those are a little bit more challenging to study right now. And of course, the ones that have an immune component as well, as the immune system has to be brought into. I mean, there are going to be ways, I think.

What about other things like schizophrenia? Schizophrenia, I think there are multiple genetic forms of schizophrenia right now that I think could be very useful. I think if we say broadly schizophrenia, I think that's going to be difficult because schizophrenia is again, right. And that's why we never really say, oh, we're modeling schizophrenia. because schizophrenia is a combination of behaviors. We don't have behaviors in a dish.

What we're modeling is the molecular biology behind some of this patient's genetic makeup. Well, you know, something that I've seen in other areas is that it's common outside of sort of neuroscience that just even a cellular phenotype can recapitulate disease enough that at least you can see how that cell responds to small molecules. Absolutely. And that is predictive of whether it's going to work in animals or humans. So if cells are enough to predict phenotypes in some areas, you can imagine this is one step beyond. Precisely. It's a group of cells. It's more complex.

It's not all the way to a full organ or a full organism. Yes. But that, you know, it seems very reasonable that when you go up a level of sophistication, you can get to a lot more. Right. And our approach has been precisely to try to identify genetic forms of disease. Some of them are actually very common. So for instance, there is a large deletion of chromosome 22, which is present in 1% of all patients with schizophrenia. These patients have a 40% chance of getting schizophrenia lifelong versus 1% in the general population.

So studying some of this patients that have a high susceptibility to developing this disease would probably represent at the end a window into maybe other forms. Because I think it's also important that I think we started the conversation with this. All psychiatric disorders are behaviorally defined. We are ultimately going to have to define them in a different way biologically. The way we've discovered that the fact that cancer is in the pancreas, it doesn't necessarily mean that it's going to have the same treatment, right? Yeah, well, cancer is moving from tissue of origin to like a pathway.

The pathway. Right. So I think the same thing is probably going to be for brain disorders. It's probably going to be more circuit-based, maybe even cellular-based in some of those disorders. But we're going to have to accept that autism is not one disease, but it's a group of disorders. Yes. That makes a result. Yeah.

But a group of disorders. So the DSM will look totally different. it will have that. The molecular DSM. Yes, it will look very different. You know, part of what really intrigues me about this, too, is that you can basically nudge the systems to build these things, and that it's repeatable, and you can build upon your past successes. I mean, these are the hallmarks of an engineering process. I'm curious how you see that bearing out, what is the future looks like? How far do you think you can push the engineering of these things? I mean, I actually think a lot about this, like this day, because I never really wanted to become a tool developer.

I kind of like became one by chance. And I just wanted to understand disease and turned out to start using this new methods. And initially I also thought that we really need an engineering approach. But the engineering approach actually is not working that well here if you think about it. Because an engineer, and I see this actually when I have students that have an engineering background, They want to understand every single part of the system before they would put it together. The beauty, I think, about biology is very often

that a lot of this information is encoded in the genome. So once you start the process, the cells actually find each other in a meaningful way. And actually, very often, we do reverse engineering in the sense that we make parts and we put them together and we see that they connect in a specific way and then we start asking,

how did they actually connect? Because we had no idea before. It's almost which type of engineering because you could have an electric engineer get hung up on the fundamental physics and we're going to get stuck there or they can sort of accept that these are working blocks and they're somewhat black box working blocks.

Absolutely. But then let's see what you can do to put them together. Exactly. And reverse engineering is I think exactly the right discipline sounds like for where we are in terms of the time of this. And very often also our disease modeling has actually moved in that direction. I mean initially very often we would say,

okay, here's a disease, a psychiatric disease, let's try to model it. But now very often we say, what is an aspect of human brain function that nobody has had access to. Let's try to model it in a dish and then say, okay, here are interneurons moving into the other side. What are the molecular machinery behind it? And very often we would identify disease-related genes. And so in that case, we can say, oh, now we have the perfect acid in the first system.

But if you would have tried to do that a priori would have been very difficult, right? Because you wouldn't have had the tool. Now we can make more defined brain regions. And when we assemble them, we can actually get cells projecting at a long distance, connecting, and forming small circuits. Well, and that is what I would expect from that type of engineering approach, because if you had to do this purely as rolling dice and seeing what comes out, it's going to be really hard to start building these more complex structures versus sort of getting better year after year. Precisely. Yes. So I have to ask, if now we're in this new world of sort of beginning to grow essentially small human brains or versions of brains that.

have some functionality to them, they're firing, they're doing some stuff. How do we begin to think about sort of some of the ethical and legal issues around that? I mean, is there a line at which you can say clearly like, no, this is a thousand miles away from consciousness? Or if you let that organoid grow in a dish for, I don't know, five years, like would you begin to be approaching? Like, how do you start thinking about what's actually happening there in terms of consciousness? Of course, there are a lot of discussions in the field. And what is clear is that this structures that we're building in a dish are very different than the actual human brain. First of all, because they're not a miniature version of the human brain.

That's why we don't like the term mini-brain because it's inaccurate. It's a slice. It's a chunk. But there are parts. There are incomplete parts of the human brain. And there are another very important aspect of human brain that is also missing here, which is actually sensory experience. We know that the sensory experience, everything that comes through our senses during development.

Absolutely, that shapes the circuits. We know that that's critical periods. But is there no sensory input at all coming in to these cells? I mean, we can build all kinds of ways of actually stimulating, but there are no sensory inputs. So that is the reason why overall we don't really think that there are major ethical concerns with the cultures maintained in the dish. Now, there are some discussions, some ethical discussions around the transplantations

of some of these cultures into animals. So, for instance, one can take them and transplant them into a rodent and integrate them into the circuit of a rodent, in which case, for instance, they may receive some sensory input. They may participate to some of the circuit functions. So I think many of the discussions are around that. And of course, what is really considered not acceptable at this point is, for instance, transplantations into primates. We also, I think, have to put this into the broader context of how are we going to model psychiatric disorders. And we called this broadly surrogate. brain surrogates, because they're all kind of like trying to mimic the human brain. And I think it's very clear that we need better model for understanding psychiatric disorders. We need model that resemble more and more of the human brain. And obviously, the closer they resemble the human brain, the more uncomfortable we also feel with anything resembling the human brain. So I think that's why I think ethicists and lawyers are in this, because, of course,

there are also legal implications, right, versus the cells come from human individuals. What are the limitations to what are really the applications for which they can be used for? And so I think those are discussions that are active in the field at this point. And I think the long-term implications is kind of amazing because we're talking about this very much in the context of scientific discovery and disease. And that's one direction. Just as a technology, it's also just kind of amazing. The fact that we can engineer silicon allows us to have chips. So the transistor is cute, but sort of having that engineering aspect of it is what allows us that microprocessors.

So this reminds me in the early days of that in some ways or you're putting these things together and you're seeing what they can do. I can imagine that they could start, especially, you know, since on the silicon side, we're spending so much energy on the silicon side doing neural nets. Right. You know, and running machine learning. It's interesting to think what you could do with this. Then that gets really kind of very science fictiony, like could you create a little visual quartz? and use that to do computation. Could you do this and that? And there it's going to be a really interesting challenge on the bioethics side to figure out, you know, what makes sense. Right, right.

You can see a path. Absolutely. And one can think that once we put the cells together and they connect to each other, they form small networks. Yes. Of source, there is some sort of computation. Yes. There's like some information that is transmitted among themselves. So I guess one question is like how complex it is, how similar is that to the human brain? Yes. But we do know that sensory input and many other external information do shape that developing. So I think that addresses some of the ethical concerns when we're maintaining them purely in vitro.

But again, one can really see it as some sort of computing device, so to speak, which is biological. So I want to wrap up by just asking kind of what the next near-term exciting steps are here and then what the wildest ones that you can imagine is when we have this new tool, this new way of understanding the brain,

new model appropriate for some things, not for others, but what is the sort of like the next major evolution that you see? I think the, I mean, the next step is really applying it broadly, right? I think it's still early days in developing some of these technologies and applying it, but we really need to see it at work more broadly and maybe bringing psychiatry into a new era, like a molecular year, into what we would often refer to as molecular psychiatry. So I think on the long term, that's what I see the most exciting avenue. you. But of course, the models are going to get more complex in many ways. We're likely going to be

able to build ever more complex circuits. And I think some of the transplantation studies are also going to be critical in that regard. And then, of course, the implications about just understanding how the human brain function. I think how and what makes it so unique. There's a lot of interesting biology. We know very little about the actual human brain, not just the biology, but also the computing. How does it come together and computes? Do you think that it could bring us closer to understanding the nature of consciousness? I would be satisfied with just understanding the biology of one single neuron. I think that would be exciting for me.

It's really intriguing time right now. You think about one analogy I often make is that birds and fighter jets both have wings, but they fly very differently. And maybe the human version was inspired by the natural one, but that we can go beyond. But to go beyond, it's useful for us to really understand. people to probe deeply and then figure out what we want to do there. And the Go Beyond might be using neurons as an analogy as it is in machine learning right now in neural networks. Or it could be actually as literal neurons. That back and forth is really interesting. And one of the most

exciting things for me about the field of neuroscience is where it sits right now in that sort of hub between understanding the brain and understanding intelligence, understanding learning and both at this sort of macro scale, which we talked about and the micro. Those connections, I think, will be useful sort of in many disciplines for years to come. Thank you so much for joining us on the A16D podcast. Thank you so much for having me.