From Our Neurons to Yours

Big Ideas: How see-through brains could transform neuroscience | Guosong Hong

• Nicholas Weiler, Wu Tsai Neurosciences Institute • Season 9 • Episode 5

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What if we could make the brain see-through? 

It sounds like science fiction, but it could revolutionize how we study the brain. 

Today on the show, we're talking with Guosong Hong, a faculty scholar here at the Wu Tsai Neurosciences Institute who has a unique reputation for developing creative techniques that literally shed light on the brain—from using fluorescent nanomaterials and focused ultrasound to create a virtual flashlight inside the skull, to discovering a common food dye that temporarily makes skin, muscle, and even parts of the brain transparent. 

Now, Guosong and colleagues are taking this work to the next level through a Wu Tsai Neuro Big Ideas grant, genetically engineering mice to have see-through brains from birth. 

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Nicholas Weiler (00:10):

This is From Our Neurons to Yours, a podcast from the Wu Tsai Neurosciences Institute at Stanford University, bringing you to the frontiers of brain science. I'm your host, Nicholas Weiler.

(00:25):

Today on the show, transparent brains. Okay, so this may sound like a crazy science fictional idea, but what if we could make the brain see through? So many techniques in neuroscience today depend on getting light in and out of the brain. Scientists use high-powered microscopes to watch brain cells light up as they fire. They use lasers to turn specific neurons on or off, mapping how different regions connect and communicate. But of course, light doesn't penetrate very deep into biological tissue. Typically less than a millimeter. It scatters, it bends, it gets absorbed. That's just physics. Or is it? Actually, we know some tissues are already transparent, like the belly of a glass frog or the jelly within our eyes.

(01:15):

As wild as it sounds, my guest today, Guosong Hong, thinks he knows how to make the brain transparent. Guosong is a faculty scholar here at the Wu Tsai Neurosciences Institute based in the Department of Material Science and Engineering. He has a unique reputation for developing creative techniques for literally shedding light on the brain. Things like using fluorescent nanomaterials and focused ultrasound beams to create a virtual flashlight inside the skull.

(01:44):

In the past year, his work received a lot of attention because of his discovery of a common food dye that can temporarily make skin, muscle, even parts of the brain transparent. Now, Guosong and colleagues are taking this work to the next level, genetically engineering mice to have see-through brains from birth. This is part of a big ideas in neuroscience initiative from Wu Tsai Neuro, our flagship program fostering ambitious collaborations to transform our understanding of the brain that we'll be covering on the show over the next few months.

(02:14):

Before we get into the how and the why of building transparent brains, I wanted to talk to Guosong about the interesting path he's taken to get to this work. So I started by asking him to share how he went from being a chemist studying the properties of materials to one of our premier neuroscience toolmakers.

Guosong Hong (02:33):

I've always had the habit of seeing the world through the lens of physics and chemistry. And for a long time, that way of thinking was focused almost entirely on the external world, such as molecules, materials, and structures made of these molecules and materials. But that kind of view changed when I was a graduate student in chemistry at Stanford. So I remember that was around 2013-ish. I read about President Obama's announcement of BRAIN Initiative. And then he said in the announcement that as humans, we can identify galaxies light years away. We can study particles more than an atom, but we still haven't unlocked the mystery of the three pounds of matter that exists between our years. So I see this contrast in this quote that the main focus of my study and my training up to that point had been things like studying particles more than an atom or studying these nanomaterials made of tens and hundreds in atoms.

(03:39):

They're really small, but these are not part of our body. But it is this need to unlock a mystery of the three pounds of matter that sits between our ears that drove me to dive a little bit deeper by using tools we have developed based on physics and chemistry and understand the brain. So after reading this announcement, I suddenly realized that although physics and chemistry have been incredibly successful at helping us understand the world around us, those same principles and tools could also be turned in an inward way toward our internal world, which is the three pounds of matter that sits between our ears.

(04:21):

And that realization, which is brain is also a physical system governed by the same laws of physics was what set me on this journey into neurobiology and into neuroengineering, which I developed tools to understand the brain and nervous system better.

Nicholas Weiler (04:41):

So many of the tools that you've been developing are about getting light deep into the brain, making it possible to have some kind of visual access or optical access, as we might say in science, getting light into the brain. Why is it so important to get light deep into the brain?

Guosong Hong (05:00):

Yeah. But let me start by saying that the need to get the light in the brain and then the difficulty of getting light in the brain, any median tissue is just another example of how some fundamental understanding in physics could help make things possible in biology that one seems unimaginable. There are two physical insights and two physical reasons behind this. The first is actually very basic question. So why isn't the brain transparent to begin with or why aren't most of the animals tissues transparent to begin with?

Nicholas Weiler (05:37):

I was thinking about that after I read about your transparent tissue. I was like, "Why aren't we transparent?"

Guosong Hong (05:43):

Yeah, why aren't we transparent, right? So biologic tissue is actually made of many different molecules and structures. And each of these molecules and structures has its own optical properties that we as physicists call them refractive index. And the refractive index is just a way to describe how fast the light travels through that particular material. Since our body is made of so many different structures and so many different molecules, and when light, you can imagine light is trying to pass through this mixture of components with different refractive indexes.

(06:16):

Imagine light in the pigment of my skin and tries to penetrate it through the skin with a lot of cells and a lot of different subset of structures. They all have different refractive indexes. This instant light gets bent and redirected in completely random ways. And then instead of traveling straight in free space, if you imagine light travel in air, we always picture this light ray as a straight line, but in the biological tissue no longer travel straight.

(06:43):

It actually scatters. It gets bent and got redirected in random ways, and that's because we have different components with different reflective. In this, it's each one of which is going to change the direction and speed of light in a slightly different way.

Nicholas Weiler (06:57):

I mean, the example I always think of is mixing a salad dressing or something where you have water-based solutions and oil-based solutions. And maybe you can make one where all the ingredients are transparent on their own, but when you mix them together, suddenly you can't see through it. There are all these bubbles and things that are scattering the light everywhere.

Guosong Hong (07:16):

Exactly.

Nicholas Weiler (07:17):

So same idea in the brain. Same idea in all of our tissues. This is why we are not transparent. I want to come back to the question of why. It would be very cool to be able to create a mouse, which is I think the goal here with a transparent brain, but why does neuroscience need a transparent brain?

Guosong Hong (07:34):

Yeah. Well, so neuroscientists have created a lot of amazing tools to study the brain, and many of these tools rely on use of light. So for example, people use optogenetics, which is shining light on the brain tissue, and then those specific neurons that express opsins will respond to light, and then they will change their activity in response to the illumination. And then neuroscientists can start asking the question, well, how does manipulation of this particular type of neurons and their projections change the specific function of the brain?

Nicholas Weiler (08:16):

Yeah. Optogenetics is something that we've touched on a few times on the show, but I don't know that we've ever done a deep dive into optogenetics. Karl Deisseroth and Ed Boyden and others developed this technology in the early 2000s that has really transformed neuroscience, letting neuroscientists genetically engineer particular neurons or groups of neurons to make them sensitive to light.

(08:39):

So you can hit them with a beam of a particular color of light and turn the neurons on or turn the neurons off, really exploring what is that neuron doing? What is that kind of neuron doing? And that's been really incredibly valuable for the field. We can go into some examples maybe, but what's the limitation there that you've been trying to address with these technologies?

Guosong Hong (09:01):

Well, as you said, optogenetics really has beautifully and powerfully shaped modern neuroscience studies. The beauty actualizes the use of visible light, which is within the visible spectrum. It has different colors going from the blue to red to interrogate nervous system. So in order to get the light into the deep brain, for example, if there is a target of a cluster of neurons sitting a few millimeters down in the [inaudible 00:09:28] which is already pretty deep, it's too deep for visible light to get in there by shining light on the outside of the brain.

(09:36):

And then because of that challenge, people have to get through this overlying tissue. So remove the tissue in an invasive way by removing the scalp, by performing craniotomy to open up a hole in the skull. Sometimes it involves removing part of the cortex or even other parts of the brain, deep into the brain by inserting an optical fiber into the brain until it stops right above the region of interest.

(10:02):

And then now light can be delivered precisely and exactly at the region where you want to shine the light onto the neurons. And then this is exactly because otherwise light will be lost significantly. It travels through the tissue because it's opaque, so it's going to attenuate to light significantly. You won't be able to have the light deliver so precisely and so efficiently into the deep brain region.

(10:27):

But now you see there is a very invasive procedure that damages this overlying tissues. So if you want to get into five millimeters in the brain, that entire four millimeter and a half of the tissue sitting above will have to be damaged by this optical fiber going through it physically. And then this neural tissue or this molecule mediums of tissue could contain many neurons that participate in this intricate functions of brain you're trying to understand and trying to study, but we could make the brain transparent. We no longer need such invasive procedures.

Nicholas Weiler (11:02):

Well, this strikes me as having two really important corollaries, which is, one, as you were saying, you're damaging some of the tissue and you don't really know was that tissue important to the thing that you were trying to study? If you're trying to go down and understand what happens when particular neurons deep in a brain region called the basal ganglia are involved in creating Parkinson's symptoms, as people have done, and they're able to show, well, these neurons, if you stimulate them, they make Parkinson's symptoms worse. And these neurons, if you stimulate them, they make Parkinson's symptoms better.

(11:37):

You need to be able to get the resolution to get down there and do that. But you do that by cutting through all this tissue that's above it. And who knows, maybe that tissue is important to the thing you're studying. And the second thing, for the moment, optogenetics and a lot of these techniques and a lot of these tools, both getting images and stimulating brain circuits, is really restricted to animal models in the lab like mice.

(12:01):

We're not really talking at the moment about doing a lot of optogenetics in humans. Part of the reason is that no one wants to have parts of their brain cut away. It's just not going to happen. That's not useful. So it's great in mice. It does have this problem of being invasive. If you want to get image somewhere deep in the brain, you've got to cut the tissue away above it. So with a transparent brain, you wouldn't have to do any of that. The other thing that strikes me as being potentially really valuable about this is that you could do imaging all over the brain. You don't have to restrict yourself to just one spot. And it's becoming increasingly clear that for all kinds of important things that we do, from visual perception to language, to you name it, the whole brain is involved in almost everything that we're doing. So the ability to image across the brain in different regions at the same time seems like it could be another big advantage of this.

Guosong Hong (12:54):

Exactly.

Nicholas Weiler (13:05):

So before we get into some of the how, like how you would create a transparent brain, I want to dwell just a little bit more on some of the specific kinds of experiments that would be possible here. So I mentioned before with optogenetics, people have done things like looking deep into the brain, looking at which kinds of neurons are involved in a model of Parkinson's disease, looking at epilepsy and seeing what kinds of cells are involved. To you, what are some of the sort of ideal experiments that might be possible if we were able to see deeper into the brain?

Guosong Hong (13:43):

Yeah. One of the experiments you kind of alluded to earlier is that, well, can we make the entire volume of the brain transparent? But then imagine this is going to be in a live mouse that grows, develops, and ages just as normally as a typical non-transparent mouse. And then what we could do as neuroscientists is that we may be able to first map a lot of projections and also their activity via the fluorescence microscopy in three dimensions. And then this is not just a 3D.

(14:19):

This could potential also be 4D with the fourth dimension being time. Now we're talking about at any time during a really long, evolving process of brain development or aging, we can first map this three-dimensional information with very high resolution. This is getting down to single neurons and there's also this glial cells as well. And in the meantime, we can also track this process over time.

(14:45):

And this gives us a 4D map to understand how development works and how the brain changes in response to maybe there could be a drug, this could be a particular task, this could be a learning process, it could be aging, this could be neurodegeneration. We can actually get this four-dimensional map throughout the entire brain by making the brain transparent in a live animal.

Nicholas Weiler (15:06):

That's amazing. Yeah, I hadn't thought about that. Not only is it not invasive, but because you don't have to pick a specific spot, you could just watch the whole brain over time.

Guosong Hong (15:15):

Exactly, exactly.

Nicholas Weiler (15:16):

Well, I want to get into a little bit more detail about the team that's come together for the transparent brain and how you're planning to approach this, what does that looks like. There's one thing that's still sticking in my mind that I think we should touch on first, which is we've talked about a lot of the science that's gone on with optogenetics, which has given us a lot of new insight into how different kinds of neurons are interacting with each other, what are the microcircuits, how are they connected across long distances in the brain? And most of this work, as we said, has been done in research animals, mainly mice in the lab, which is great, but listeners might also be wondering, are there direct applications for technologies like this for humans?

(15:59):

And so since we've talked about this kind of technology being potentially non-invasive, do you see this maybe not in the near term, but in the long term as having direct applications to humans might be one day temporarily be making our brains transparent to take a look inside?

Guosong Hong (16:18):

Well, this is a great question. I appreciate you asking me that. So I think the physical principle behind this finding is really agnostic of applications. In other words, this finding really enables a lot of different applications, including those mentioned by you in humans. So maybe not so closely related to the brain per se, but I just wanted to use this time to maybe give a few examples since the publication or work slightly more than a year ago. Well, since the publication, many groups have already used this tartrazine which is the dye molecule described now working on other dye monitors.

Nicholas Weiler (16:55):

Sorry, this is the technique for making tissues transparent using this food dye.

Guosong Hong (17:00):

Yeah, using the food dye. And then we're very excited to see that a lot of people not only reproduce the results, but also extended them off in the creative ways involving the human use. Here, I'm just going to give you some example.

Nicholas Weiler (17:11):

Yeah, please.

Guosong Hong (17:12):

So there's a ophthalmologist, Cynthia Toth's lab at Duke University who applied this method to temporarily make the sclera, which is a wide part of the eye, including the human eye transparent. And this will allow the ophthalmologist to see deeper into the eye much more easy. So imagine that you can actually apply this to a person with some degeneration in the vena. And now typically you have the images through the only transparent part of the eye which is the men's and there's a cornea, but now you can actually image sideways through this sclera by making a transient reversely transparent by applying this method.

(17:47):

They have already published several beautiful papers on this. I think this is really pointing to a very exciting future of this apply to humans. And another example I mentioned, this is actually once again from actually some other labs, but a different set of labs, including labs in the US, in China, and also in Europe. They have applied this dye-enabled transparency approach to very human-ready imaging techniques such as OCT. This is optical coherence tomography, which is very widely used in the clinic.

(18:21):

And also photoacoustic microscopy. And they have found that, well, you can actually use this approach to make the skin transparent. You can use this approach to make the muscle tissue, connective tissue transparent as well. And recently I found there is a group from University of Houston, they use this approach to monitor the embryonic development. Well, now it's still just in rodents, but this really points to a very exciting future for women's health in the near future for-

Nicholas Weiler (18:48):

So you're talking about temporarily making a piece of the belly transparent so you can check on the fetus.

Guosong Hong (18:55):

That's right.

Nicholas Weiler (18:55):

So all of this sounds like creating windows, right? Like creating a window in the eye.

Guosong Hong (18:59):

Creating the literal window.

Nicholas Weiler (19:01):

Seeing the ticket. But without actually damaging the tissue, right? You're just putting this dye on it and the dye makes the tissue transparent.

Guosong Hong (19:08):

Exactly. If I may, just want one other thing. So this physical principle actually is also applicable to a lot of the FDA approved contrast agents. Conventionally, people think this dye molecules are contrast agents for imaging, just increasing the contrast, for example, the lumen of the vessel.

Nicholas Weiler (19:23):

So this is things you would inject into the bloodstream so that you can do imaging.

Guosong Hong (19:27):

Yeah, exactly. But now my lab has revealed in the recent paper is that we can actually repurpose these contrast agents as clearing agents based on the physical principle we have found. And then because these agents already have the established safety and regulatory profiles and that they're known to be safe to be applied in humans even with an injection. But now what do we do is we either apply the topical on the skin or we inject that into the skin or into the muscle and potentially maybe you can also inject into the brain in the future and they can actually make it transparent at a particular wavelength span. A lot of these bands are actually in the knee infrared, and now you can see deeper at that particular wavelength into the body.

Nicholas Weiler (20:09):

That's incredible. So these contrast agents are usually there to make things darker in a sense on imaging?

Guosong Hong (20:15):

Right. Stand out.

Nicholas Weiler (20:15):

To stand out, but you found that you can use them to make things clearer so that you can see deeper.

Guosong Hong (20:21):

Yeah, exactly. Which is also very counterintuitive, but I think this is the beauty of physics behind it.

Nicholas Weiler (20:27):

So speaking of physics, it's very hard to talk about physics on an audio show. We need diagrams, but it's really fascinating stuff. So let's take a shot. Let's see if we can explain something about how this transparency is working, specifically these food dyes that you've found. I think it was dye that they use in Doritos chips, this sort of orangey colored dye.

Guosong Hong (20:49):

Right.

Nicholas Weiler (20:49):

I heard you talking with Russ Altman about this last year, and he was like, "If I put this on my tongue, does it make my tongue transparent?" You're like, "I guess."

Guosong Hong (20:56):

If it penetrates enough into the tongue tissue, it will make the whatever level you have penetrates in transparent.

Nicholas Weiler (21:02):

Check it out. And next time you're eating Doritos, maybe your tongue is a little bit transparent, now you know. So here's my understanding of this. So we talked a little bit before about refractive index. This is basically the idea that light goes through different materials at different speeds. And if you mix materials with different refractive indices, that causes the light to scatter. So that's the thing that you're trying to do to resolve to create transparency. Basically, you want to match the refractive indices of the materials. And in this case, we're talking about the watery stuff in our cells and the fatty stuff in our cells. Is that basically what you've been able to do?

Guosong Hong (21:41):

Yeah. Maybe I can add a little bit of more information to this.

Nicholas Weiler (21:44):

Sure.

Guosong Hong (21:45):

When I was in high school, I tend to think refraction is just a physical constant that describes the most intrinsic and the most unchangeable property of a material. For example, water has index 1.33. That's just a physical constant, just like the density of water. So we don't really think this can be changed.

(22:04):

But the fascinating thing is that you can actually change how light propagates through a material by adding in some material into it. There's a fundamental principle called the Kramers-Kronig relations. This was found about almost exactly 100 years ago in 1926. And this principle tells us that if you change how much your material absorbs light to the one wavelength, for example, in the blue, you inevitably change how light travels, how fast it travels and how much it bends at other wavelengths, for example, in their red tongue. So this is really mind-boggling-

Nicholas Weiler (22:39):

It is.

Guosong Hong (22:39):

Because if you think about it, so two wavelengths, you typically think the different wavelengths in the rainbow is completely independent, but the physics tells us that if you change how much it absorbs that one wavelength is going to change how much it refracts light at the different wavelength. And then by applying this principle, we can change how much scattering you have in the system, as you said, like a mixture of oil and the water looks milky.

(23:05):

And then you can just add the biomolecules including tartrazine within into the aqueous component of this milky mixture, such that the entire thing apparently becomes much more red, but in the meantime, it also becomes much more transparent because of this physical principle because you're basically homogenizing the difference of reflecting this between water and oil without changing their fundamental chemical principle, chemical properties.

Nicholas Weiler (23:31):

Because we don't want to change the liquids in our cells. That would be bad.

Guosong Hong (23:34):

Right, exactly.

Nicholas Weiler (23:35):

So you're saying you can adjust their refractive index in the ultraviolet and change their transparency in the visual spectrum?

Guosong Hong (23:43):

In the longer wavelength, which is the visual spectrum.

Nicholas Weiler (23:45):

That's magic. You can call it physics. I call it magic. That's amazing.

(23:47):

Okay. So as you said, you discovered that there's this common fluid dye that's approved for use in humans, for consumption by humans. You can make skin transparent, and I'll put links in the show notes so people can see the images. There are some amazing images of mice with their stomachs made transparent, like one of these glass frogs that have a totally transparent stomach.

(24:28):

And you've talked about some of the applications people are already finding for this, making windows, looking into the eye, looking at a fetus in a pregnant animal. So how do you transition from that technology that's temporary, that's using this dye that you're applying, that you can wash it off and it goes back to normal. That's not exactly what you're planning for this Big Ideas project where you're talking about creating animals with transparent brains. What's the next step that you all are planning to take?

Guosong Hong (24:52):

Yeah. Well, so when we were applying this transient and reversible transparency approach, we realized some challenges when applying this technology. One of them is you have to find a way to deliver this dye into the body and how deep we can see via this transparent window highly depends on how deep this molecule gets into the body. Apparently, if it relies on just diffusion, it creates a profile where it is closer to the source, you get a lot of the transparency, but the deeper you go, the less transparent it becomes.

Nicholas Weiler (25:21):

So you still have the surface problem.

Guosong Hong (25:23):

Right. It's a surface problem. And then we realize that, well, can we make these diet molecules from inside the body? And then we create a more uniform transparency throughout a large piece of the tissue.

Nicholas Weiler (25:36):

Oh, so can you get the body itself to create some molecule that will have the same effect?

Guosong Hong (25:42):

Yeah, exactly. And there are actually two pieces of information that are quite encouraging. One of them is, although tartrazine is a synthetic dye, we know biology itself creates a lot of pigments in plants, in flowers from the cells within the plants, within the flowers. It is possible to identify the synthetic pathway in these biological systems and learn from these pathways. We produce these pathways in a mammalian cell such that mammals can make these pigments to change refractory and excellent achieve transparency as well. So this is the first insight.

(26:18):

The second insight comes from our collaborators on the team, especially Lauren O'Connell's lab in biology. So a couple of years ago, I remember her lab published a very interesting paper in science, and then they studied the physiological basis of this very, really remarkable transparency in glass frogs.

Nicholas Weiler (26:34):

So, again, these are these frogs that have totally transparent stomachs? You can see all of their organs, you can see their heart speeding?

Guosong Hong (26:39):

Exactly. That's right. And then we actually started talking after we published this paper and I asked her, "Well, do you think there is a particular molecule inside this glass frog cells that do the same trick as a homogenizing refracting next across the body?" We did some literature search together and we found that although the relevant literature sparse, some studies did hypothesize that to reduce the refractory index heterogeneity, which is exact the same physics as we discussed earlier in these organisms contributes to the suppressed scattering.

(27:15):

And then this is done potentially through this endogenous production of high refractive index molecules that act as intrinsic optical purine agents. And some of this identified molecules are antifreeze proteins, crystallines, and some other molecules as well.

Nicholas Weiler (27:35):

I mean, that's kind of amazing. I mean, we know about these glass frogs. We've known about them for a long time, but no one really knows exactly how they're transparent. It sounds like you found some evidence that maybe they're doing something similar to what you've been talking about, creating these proteins that minimize the difference in refractive index between different parts of the different components of the tissue.

(27:55):

It strikes me, again, I think this came up in your conversation with Russ on Future of Everything last year, we also have transparent tissues. We have our eyeball. We can see through our eye because it's transparent inside. So there's precedent, I guess, for the body creating its own transparent tissues. And so your plan then I gather is for you and Lauren and Xiaoke Chen, who's the other member of your team, to come up with ways to mimic that, to say, "Well, if a glass frog can make its stomach transparent, if our eyeballs can make the vitreous fluid in the middle transparent, how can we learn from that to make the brain transparent genetically?"

Guosong Hong (28:35):

Exactly. I'm so glad you mentioned the lens because I really feel like nature has known this for billions of years and biology has already leveraged the same principle to do this optical trick. And we're just starting to be so privileged to learn about these things because the lens actually represents one of the only inherent to transparent tissue in mammals. And then the lens achieves transparency through the bench packing of a high refractive index crystalline proteins as well. And turns out that including this crystalline molecules and high freeze proteins in the glass frogs and also zebrafish, they all derive their high refractive indices from their strong UV absorption.

(29:17):

And this is the exact same results of the squamous chronic relations we mentioned earlier. By having very strong UV absorption, you achieve this index rays of the aqueous component in the cell and that increased refractive index levels the index difference between aqueous component and the lipid bridge components, thus having the transparency. And a lot of times actually these two results actually come from the same physical principle, but actually fulfilling different purposes. On one hand, he should express these molecules to prevent the biology from the UV radiation. That's why they need to have very strong EV absorption.

Nicholas Weiler (29:58):

So absorb the UV rays don't let it damage the tissue?

Guosong Hong (30:01):

Damage. it doesn't damage the DNA, the genetic material in the body. But in the meantime, it's very strong EV absorption leads to the high refractive index and that in turn leads to transparency. That serves the purpose of having the lens being transparent for us to see things with this focus in power to see things sharply and also give rise to the transparency in glass box.

Nicholas Weiler (30:24):

So the same thing protects our retinas from UV light and makes them transparent. Wow. So for all that, it still sounds like this is going to be a very big challenging project. What do you see as some of the big challenges and what's your hope for when we'll get to see mice with transparent brains running around?

Guosong Hong (30:46):

Well, this is very challenging. This is actually also the most challenging part of this project or this idea is exactly what we appreciate the most about the Big Ideas program. So challenging ideas like this often do not get funded at all through the more conventional mechanisms, but this Big Ideas program also together with the Stanford's forward-looking environment makes us feel extremely encouraged to pursue this kind of work.

(31:14):

Some of the challenges include... Well, so we are working with Xiaoke Chen's lab to find a way to deliver these transgenes that either encode these high refractive index proteins or encode the enzymes that make this very absorbing pigment molecules in the mammalian cells. His lab has done a lot of pioneering working packing all these transgenes in AVs. I believe his lab is also working Lauren's lab to deliver transgenes into [inaudible 00:31:45] tissue. We're trying to learn from his expertise and work with his lab to deliver these transgenes into the mammalian tissue.

(31:54):

But one challenge I can foresee is the expression level. Can we express enough of these molecules without damaging the cells, without altering the physiology of the cells? If we wanted to achieve 100% complete transparency, it may be impossible. It cannot be done without changing the fundamental physiology of the cells.

(32:20):

But one thing I think is possible is that we may be able to achieve partial transparency, either in just a subset of cells or just in the specific region of the body, in a specific region of the brain in particular without significantly compromising the systemic physiology of the entire body or of the brain.

Nicholas Weiler (32:44):

It's going to be about finding that balance somehow.

Guosong Hong (32:47):

Exactly. And then another way of doing this is we don't go for 100% transparency. We maybe go for 20% reduction of the scattering, just partially enhancing the translucency of the tissue so that previously we see about less than one millimeter of the tissue of the brain, which is just stop short of the edge of the cortex. But now we can see maybe two millimeters, which is not the entire brain, but it would get us to a lot of the interesting regions of the brain conventionally cannot be studied non-invasively.

Nicholas Weiler (33:24):

Again, in mice, the human brain is not anything altogether.

Guosong Hong (33:28):

Yeah, exactly.

Nicholas Weiler (33:29):

Okay. Let's take it step by step. At least we can make it easier to see deeper. If not a completely transparent brain, we'll get partway there. And then maybe over time we can find ways to make it more and more transparent or to focus the efforts on different areas. And then of course, there are all the other technologies you've been developing. So maybe all of these things can work together hand in hand. You do a little bit with transparency, you'll do a little bit with nanoparticles and you do a little bit with ultrasound and so on.

Guosong Hong (33:58):

Yeah. So we really wanted to make this useful and helpful to the neuroscience colleagues and also the bigger neuroscience community to enable their specific research projects.

Nicholas Weiler (34:12):

Well, we'll have you back on to talk about that once... I know it'll take some time, but really excited to see where all of this is going. And thank you for taking the time to come on and explain to us why you're doing this, how it's going to work, and what we hope to see in the future.

(34:32):

Thanks again so much to our guest, Guosong Hong. He's a Wu Tsai Neurosciences Institute faculty scholar, and an assistant professor of material science and engineering. On this Big Ideas and neuroscience project, he's working with Lauren O'Connell in the Department of Biology and Xiaoke Chen in the Department of Neurobiology. To read more about their work and the Transparent Brain Big Ideas Project, check out the links in the show notes.

(34:56):

If you enjoyed this episode, be sure to subscribe for more conversations from the frontiers of brain science. We also love hearing from listeners. If you have thoughts about the show or questions about the brain you'd like to hear us discuss in a future episode, send us an email at neuronspodcast@stanford.edu, or leave us a comment on your favorite podcast platform. While you're at it, give us a rating and share the show with your friends. This may seem like a small thing, and I know everyone asks this, but it's tremendously valuable for us to be able to bring more listeners to the frontiers of brain science. Next, time on From Our Neurons to Yours.

Kathleen Poston (35:34):

I'm trained as a movement disorders Parkinson's disease neurologist. The fact is that by the time somebody has the clinical symptoms, the pathological progression is actually rather advanced.

Nicholas Weiler (35:51):

From Our Neurons to Yours is produced by Michael Osborne at 14th Street Studios with sound designed by Mark Bell. Our social media strategy is by Julia Diaz and Nathan Collins provided additional editing. Our logo was designed by Aimee Garza. I'm Nicholas Weiler. Until next time.