Using micropipette aspiration in Xenopus laevis oocyte nuclei, authors show the nucleolar granular component behaves as a liquid while the dense fibrillar component and fibrillar center exhibit RNA-dependent viscoelastic, partially solid-like properties; RNase A fluidizes the DFC and alters interfacial tensions.
0:00Welcome to Base by Base, the papercast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. Yeah, really glad to be back for another deep dive.
0:11So to start us off today, I want you to imagine for a 2nd that you could, like, shrink down. Smaller than a grain of sand smaller than a bacterium, and you step directly inside the nucleus of a living cell.
0:23It's pretty wild thought experiment. Right, and your intuition might tell you that it would just be this uniform watery soup in there, but it's not. No, definitely not. Instead, you'd actually find a space that's organized into distinct membraneless droplets.
0:38They're just kind of floating and interacting, much like oil suspended in water. Exactly, yeah. Now think about this. What really happens when we physically reach inside a living cell and pull on these microscopic droplets? How could measuring their squishiness change our fundamental understanding of how the cell's most vital manufacturing plants operate?
0:58I mean, that is the big question, right? It really challenges the very limits of how we interact with the microscopic world. We're moving beyond just observing the interior of a cell to actually feeling its physical material properties.
1:13Today we celebrate the work of Holly H. Cheng. Clifford P. Brangwin and their collaborative teams across Princeton University and Rutgers University, who have advanced our understanding of biomolecular compensates and cellular material properties.
1:26Yeah, and just to give everyone the publication context here, this deep dive is actually based on a research article that was published in PNAS in May of 2025. So we really need to set the stage here by talking about cellular architecture because, you know, most of us learn in high school biology, that a cell is full of these compartmentalized organelles.
1:46Like the mitochondria or the nucleus? Exactly. And they're all neatly wrapped up in their own protective membranes. But there is a whole other category of structures in there called membraneless organelles.
1:56Yeah, the scientific term for them is biomolecular condensates. Right. And they're essentially these dense, crowded assemblies of proteins and RNAs that phase separate from their surroundings. It's similar to a vinaigrette dressing separating into oil and vinegar.
2:12And that phase separation is just a brilliant evolutionary strategy. Because it allows the cell to concentrate specific biochemical reactions in these distinct, highly crowded locations. Without needing a physical wall, right?
2:25Exactly. You don't need a lipid membrane to keep everything contained. So molecules can still flow in and out. But here is the massive challenge in biology right now. We actually don't know the true material properties of these condensates while they're functioning inside living cell.
2:41Meaning we don't know if they're like thick or thin. Right. We don't know their true viscosity or their elasticity. Which feels surprising. I mean, considering how advanced microscopy has become, why is it so hard to just, you know, measure how thick or stretchy one of these microscopic droplets is?
2:55Well, the problem really lies in our traditional methods. Historically, we've had to rely on indirect measurements. A really common technique is something called FRA, which stands for fluorescence recovery after photobleaching.
3:08Okay, let's unpack that. How does FRA actually work? So you take a living cell, and you overexpress a specific protein inside of it, and then you attach a glowing fluorescent tag to that protein. Then you basically blast a tiny section of the condensate with a laser to bleach the glow out of it.
3:27Oh, wow. So you're making a dark spot. Exactly. And finally, you just use a microscope to watch how fast the unbleached glowing proteins from the surrounding area swim into that dark spot to fill it back in.
3:38So you're basically measuring the swim speed of a single type of protein to guess how thick the surrounding fluid is. Right. And that is the fundamental flaw. Tracking the diffusion of one single protein species just doesn't give you the bulk realology.
3:51The overall thickness. Yeah, the overall physical thickness and stretchiness of the entire condensate. Because a condensate might contain 100s of different proteins and RNA molecules interacting in these really complex networks.
4:04So it would be like trying to figure out the consistency of a thick chunky stew by only measuring how fast a single carrot sinks to the bottom. That's great analogy. And furthermore, when you overexpress fluorescent proteins to track them, You risk artificially altering the very material properties you're trying to measure in the 1st place.
4:23Right, you're changing the environment just by observing it. And I imagine the alternative, like taking these proteins out of the cell and putting them in a test tube isn't much better. Not really, no.
4:34And vitro models are just vast simplifications. When you recreate a condensate artificially in a test tube, it lacks the true cellular context. It doesn't have the continuous energy usage or the complex mixture of surrounding molecules or the active biological processes of a living organism.
4:50And this puzzle goes way beyond just basic biology. I mean, this matters globally because the physical material state of these condensates, whether they're acting like a runny liquid or a solid, rigid gel, is directly tied to human health.
5:04Oh, absolutely. When these material states go awry, like when a liquid droplet aberrantly solidifies into a clump, we see direct links to devastating human diseases. We are talking about severe neurodegenerative conditions like ALS.
5:19Right, which is driven by abnormal liquid to solid transitions of FUS proteins. Yeah, and Alzheimer's disease is heavily linked to similar physical tangles involving the tao protein. So if we look at the broader implications here, understanding how and why these droplets maintain their specific physical states or, you know, why they fail to do so over time, could be the key to unlocking entirely new therapeutic pathways.
5:43Which is huge for diseases that currently have very few, if any, effective treatments. Exactly. Which brings us to the specific target of this study. Because if you want to understand these droplets, you might as well go after the biggest prize in the nucleus, which is the nucleolus.
5:56Yeah, the largest cellular condensate vouphar. It is literally the factory responsible for ribosome biogenesis. For everyone listening, ribosomes are the molecular machines that manufacture every single protein in your body.
6:10And the nucleolus has this monumental task. It has to gather and assemble roughly 80 different proteins and 4 massive ribosomal RNAs just to build one of these machines. It's an incredibly complex, highly coordinated assembly line.
6:24The timing just has to be perfect, and to manage that assembly line, the nucleolus is multiphasic. Meaning it has distinct internal layers. Exactly. Yes. And to help visualize this. Let's start with a classic analogy.
6:36Visually under a microscope. The nucleolus looks a bit like a microscopic job walker candy, featuring 3 concentric layers. The innermost core is called the fibriller center, or FC. Surrounding that core is a middle layer called the dense fibriller component or DFC.
6:52And enveloping both of those is a thick outer shell called the granular component, or GC. But here is the problem with the jawbreaker analogy. A jawbreaker is solid sugar all the way through. And as we'll see, the nucleolis is definitely not that simple.
7:07It is more like a bizarre reverse engineered candy, maybe a thick liquid syrup on the outside that hides a rubbery gummy center. Yeah, that's a much more accurate way to think about the mechanics we're going to discuss.
7:19Okay, let's untack this. How do you even begin to poke and prod a microscopic jawbreaker buried inside a cell? Like, how do you grab something that small without destroying the cell entirely? Well, it requires a very strategic choice of a model organism.
7:33So the research team didn't use typical human cells grown in a dish? They actually utilize the usytes. The unfertilized eggs of Xenopaslavis. Which is the African clawed frog. Why a frog egg? Because these single cell eggs are absolutely massive.
7:47I mean, a typical human cell is maybe 10 to 20 micrometers across. These frog eggs reach one to one. 3 millimeters in diameter. Oh, wow. So you can literally see a single cell with the naked eye. You can.
8:00And because the cell is so large, it's nucleus, which biologists call a germinal vesicle, is correspondingly huge. Meaning the target, the nucleolus inside that nucleus, is blown up to a manageable scale as well.
8:12Exactly. The nucleoli in these frog usites are around 10 micrometers across. For a cellular structure, that is an incredibly rare, highly accessible target. Crucially, they are structurally in compositionally very similar to mammalian nucleoli.
8:30But their massive size makes them uniquely suited for this kind of direct physical manipulation. That makes total sense. And to get a crystal clear look at what they were doing, the team actually physically isolated the huge nucleus, pulling it out of the opaque cellular fluid, and placing it into a drop of transparent mineral oil.
8:48I just want to pause and appreciate the mechanical skill required to do that. They fish out a nucleus, drop it in oil, and then deploy what I consider the hero technology of this study. Micro pipe head aspiration or MPA.
8:58It's fantastic technique. They literally took a microscopic glass tube, inserted it directly into that isolated nucleus, brought it right up against the surface of our microscopic nuclear candy, and applied a tiny, precisely controlled vacuum to it.
9:13Yeah, an adapting micropipet aspiration for a condensate inside a nucleus is a phenomenal technical leap. Because this technique has been used for decades to test mechanical properties of whole cells, like red blood cells, but doing it on a membraneless droplet is a different physical challenge entirely.
9:31I am genuinely amazed this worked. Wait, if you apply a vacuum to a liquid droplish. shouldn't it just pop or get instantly sucked away, like a milkshake through a straw? What's fascinating here is how carefully they controlled the physics to prevent exactly that scenario.
9:44They performed a highly sensitive procedure known as a creep aspiration test. Okay, so not a sudden vacuum. No, not at all. They applied very low, constant sustained pressures over time. We are talking incredibly gentle suction in the range of 5 to 20 Pascals.
9:59To give you a sense of scale. A Pascal is a microscopic unit of pressure. If you take a crisp dollar bill and lay it flat on a table. The weight of that dollar bill pushing down on the table is roughly one Pascal.
10:12So they are applying the suction equivalent of just a few dollar bills. Right. And under this incredibly gentle stress, they filmed through the microscope to measure how the different internal layers of the nucleolus deformed and crept into the glass pipe head over time.
10:27And they used fluorescent tags to tell the layers apart. Yeah. To ensure they could tell the layers apart while stretching them, the outer shell, the granular component, was tagged with a red fluorescent protein, so it appeared magenta on the screen, and the inner layer, the dense fibriller component, was tagged with a green fluorescent protein.
10:45And there is a critical detail here about the physics of the glass tube itself. The researchers had to verify that the droplet wasn't just sticking to and wetting the inside of the glass tube as it was pulled in.
10:55Oh absolutely. So they used a fluorescent dye in the surrounding nuclear fluid. and were able to observe a microscopic lubricating layer of fluid between the nucleolus and the glass wall. It was a true non-wedding physical test.
11:09And that non-wedding confirmation is just absolutely critical for the mathematical models they use to interpret the data. If the material of the droplet wets the glass. The friction changes entirely, and your measurements are ruined.
11:22But by confirming it was non-wedding, they could confidently calculate the true viscosity and the true elasticity simply based on how far the magenta and green materials moved into the tube over a set period of time.
11:35Okay, let's get into the actual results. When they turn on that gentle vacuum and pulled on the outer layer, the magenta outer shell, the GC, what happened? Well, they observed that it behaved exactly like a classic Newtoni liquid.
11:46What does that mean in practical terms? So a Newtonian liquid is something like water or honey. When you apply a constant stress or force to it, it flows at a constant linear rate. It does not spring back into its original shape when you remove the stress.
12:01Okay, so it just kept flowing. Yeah, the outer layer of the nucleolus flowed smoothly and steadily into the pipette at a nearly linear rate over time. They conclusively proved that the outer shell of this rygosome factory is fundamentally a true liquid.
12:15And they put hard numbers on that fluidity, didn't they? They did. The data shows the outer GC layer has a viscosity of roughly 220 Pascal seconds. Which is pretty thick, right? Yeah, to put that in perspective, water has a viscosity of about 1000th of a Pascal second.
12:31Honey is about 10. So this outer cellular layer is quite thick. essentially 22 times more viscous than honey. They also measured its interfacial tension, the force holding the droplet together against the surrounding fluid at about one.
12:457 micro Newtons per meter. Okay, so we have a thick honey like outer shell. But what about the inner green layer, the DFC, because this is where the physical behavior gets really strange. Right. So when the vacuum force reached that inner green layer, the material did not flow linearly at all.
13:00It behaved like a visco elastic solid. I love this part. How do we visualize visco elastic solid? Well, in their physical modeling, the inner layer specifically fit what is known as a Kelvin Void model.
13:11You can imagine this mechanically as a shock absorber paired with a coil spring. Okay. When subjected to a constant pulling force. The material stretches outward into the tube, but the further it stretches, the harder the internal springs pull back against the vacuum.
13:27Oh, wow. Eventually it reaches a constant state of strain. It stretches to a certain point, slows down, and then just stops moving entirely, even with the vacuum still on. So it acts more like a rubbery gel than a liquid.
13:39Like if you apply a gentle vacuum to a piece of Jell-O. It'll bulge into the vacuum tube, but eventually the internal structural bonds of the gelatin will pull back with an equal amount of force against the vacuum, and it will start moving.
13:52That is exactly what the middle of the nucleolus is doing. It has solid like properties. Exactly. The structural contrast within this single droplet is stark. You have a cellular factory that is a highly viscous liquid on the outside, and a rubbery vicuolastic solid on the inside.
14:09Which is a profound physical gradient. It is. But the researchers push the boundary further. They wanted to uncover the biological mechanism causing this gradient. They wanted to know why the inner layer was acting like a solid gel.
14:22Here's where it gets really interesting because they had a hypothesis that the solid nature of the interlayer was being driven by the RNA molecules being produced inside of it. So they designed an incredibly clever experiment.
14:34Really? They took an enzyme called RNA's A, which acts like a pair of molecular scissors that specifically seeks out and chops up RNA molecules, and they injected it straight into the isolated nucleus.
14:47And the consequences of introducing those molecular scissors were dramatic. By destroying the RNA, they instantly fluidize the inner layer. Wait, really? It's just melted. Yeah, it completely lost its solid, elastic characteristics.
14:59It physically melted from a rubbery gel into a flowing liquid. And you could actually see this physical change happening in real time through the microscope. Because before they added the RNA, the green inner droplets were sort of irregular and clumpy, you know, maintaining their awkward shapes, after the RNA was destroyed, those clumpy structures quickly melted into perfect spheres.
15:22Right, minimizing their surface area, just like liquid oil droplets naturally do in water. That's incredible. And the researchers actually quantified this dramatic change in fluidity. Using a metric called inverse capillary velocity.
15:35This is essentially a measure of how fast 2 separate droplets are able to fuse together when they touch. Okay. Before the RNA was destroyed. It took an agonizing amount of time for the solid like inner droplets to merge.
15:48Their internal structures were fighting the fusion. Their inverse capillary of velocity was around 300 seconds per micrometer. So 2 droplets touch, and they just sit there for minutes slowly morphing together.
15:58But after the RNA's A injection. The fusion metric dropped by 2 orders of magnitude, plunging to roughly 12nd per micrometer. Oh, wow. Yeah, the merging process spit up from minutes to mirror seconds. Two droplets would touch and almost instantly snap together into a single larger sphere.
16:15Furthermore, when they ran the micropipet aspiration test a 2nd time on this RNA depleted inner layer, it no longer stopped stretching. It flowed continuously into the vacuum tube, just like the liquid outer shell.
16:27To give everyone a strong visual analogy for what is happening mechanically here. Imagine that native inner layer as a bowl of thick Jell-O. But that Jell-O is held firmly together by an incredibly intricate microscopic hairnet woven throughout the entire bowl.
16:43That hairnet represents the RNA molecules. It provides a physical, structural scaffold that gives the whole thing an elastic, solid property. When the researchers injected the RNAA, it was like taking tiny scissors and snipping that entire hairnet into 1000s of little pieces.
16:58Without the structural support of the RNA net, the whole system collapses into a runny fluid. That perfectly illustrates the mechanism of entanglement. And this raises an essential biological question.
17:10I mean, why does the cell orchestrate this? Why would a cell intentionally build the inner core of its vital ribosome factory out of a solid RNA entangled gel while carefully maintaining the outer shell as a flowing liquid?
17:25So what does this all mean? Like for how the factory actually operates? Well, if we look at the biological function, We have to remember what is happening in that inner layer. This is where transcription occurs.
17:35The cell is actively manufacturing extremely long, newly transcribed, ribosomal, RNAs. Right, the raw materials. Yeah. And we're talking about massive molecules, many 1000s of genetic bases long, because they're incredibly long impacts so densely together in a confined space, they physically entangle with one another.
17:54They literally have no room to avoid getting tangled. That physical entanglement is the hairnet creating the solid state. It's like throwing a 100 long, unwrapped extension cords into a small cardboard box.
18:05They are going to naturally tangle up and form a solid knotted mass that is incredibly hard to pull apart. Exactly. And this solid RNA network is not a mistake. It acts as a kinetic trap. It is a strict functional requirement for the cellular assembly line.
18:20Because ribosoned by Genesis is a highly sequential process. These long, raw RNA transcripts need to be precisely chopped, chemically modified, and folded in very specific ways before they are ready to be combined with proteins.
18:34Ah, I see. The solid gel physically holds onto them. It traps the unprocessed raw RNAs, preventing them from just floating away into the liquid outer layer until they have been properly chopped, folded, and assembled into pre-ribosomes.
18:49Right. It enforces strict quality control on the assembly line. Only once the RNAs are properly processed, you know, chopped into smaller pieces and folded correctly. Do they physically untangle from the solid gel?
19:00And then what? Once they're untangled, the local material fluidizes. allowing them to easily transition outward into the liquid granular component, where the final stages of ribosome assembly take place.
19:10The physical material properties of the droplet are actively regulating the timing and the sequence of the biochemical reactions. It is absolute architectural genius by the cell. It uses the literal physical state of matter solid versus liquid, as a sorting and timing mechanism for its manufacturing process.
19:28It really is a beautiful demonstration of how biology leverages soft matter physics. However, as with all groundbreaking science, we do have to consider the boundaries of the experiment. Of course. The researchers are very transparent about the study's limitations.
19:44For one, their mathematical modeling treated the inner layer as a single continuous material. But we know from the visual structure of the nucleolis that there is actually a 3rd innermost layer buried deep in there, the fibriller center, or FC, where the transcription of the RNA actually begins.
20:02Right. And the researchers noted that this innermost FC layer might be even more solid than the middle layer we've been discussing. As the raw RNAs are transcribed and begin moving outward, getting processed along the way, there is likely a continuous subtle gradient of visco elasticity, getting progressively more fluid as you move away from the center.
20:20So treating it as one block was just a model limitation. Yeah, treating the inner region is one continuous block of material was a necessary mathematical simplification for the physical modeling, but the reality is likely an even more nuanced physical ingredient.
20:34There is also a limitation regarding the timing of the observations, right? Because the micropipet aspiration test they use to confirm the outer layer was a Newtonian fluid took place over a roughly 102nd observation window.
20:48Exactly. And because of that specific time frame, The team acknowledges they might have missed ultra fast dynamics. It is entirely possible that on time scales much shorter than 10 seconds. Even the liquid outer layer exhibits some minor fleeting elastic properties before flowing like a pure liquid.
21:05And there's the incredible question of active real-time tuning. The interior of a cell isn't a passive environment. We know from other research that cells can actively change the physical viscosity of their cytoplasm based on temperature drops or sudden changes in energy availability.
21:21A fascinating future step would be to investigate how the nucleolist actively tunes its own viscosity. Does the inner core get stiffer or more fluid depending on the cell's ATP levels? Can the cell dynamically adjust the squishiness of this factory to intentionally speed up or slow down ribosome production in response to environmental stress?
21:41During the experiments, the researchers actually noticed that the viscosity of the outer layer seemed to change depending on how long the nucleus had been isolated from the cell's active metabolism. Wow, that observation strongly hints that this physical tuning is an active, ongoing process in a living cell.
21:58It really does. It is incredible to think about the cell, not just as a bag of chemicals, but as this dynamic squishy machine that actively manipulates its own physical state of matter to control its chemistry.
22:09So, to distill all of this down for you listening, the nucleolus, the cell's vital ribism factory isn't just a simple liquid droplet floating in the nucleus. It is defined by a complex physical material gradient.
22:21It transitions from a solid, tangled RNA core to a fluid, liquid outer shell. Yes. And this specific physical architecture is absolutely essential for choreographing the precise step-by-step manufacturing of the cell's molecular machines.
22:36The physical structure acts as a kinetic trap for the raw materials. The physical structure itself is the fundamental regulator of the biological function. And this leaves us with an incredibly profound thought to ponder.
22:48We know that aberrant physical changes in these droplets are linked to devastating human diseases. We know that cancer and cellular aging are frequently associated with misshapen nucleola and defective ribosome production.
23:00What does this mean for how we might one day treat aging or cancer by physically tuning the squishiness of our own cellular droplets? We might eventually move beyond just blocking chemical receptors and start designing therapies that act as microscopic fabric softeners or stiffeners to repair the cells internal architecture.
23:18That is a fascinating path forward. This episode was based on an open access article under the CCBY 4.0 license. You can find a direct link to the paper and the license in our episode description. If you enjoyed this, follow or subscribe in your podcast app and leave a 5 star rating.
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