🔬 condensed matter

Condensate-mediated shape transformations of cellular membranes by capillary forces

This study combines live-cell imaging, in vitro reconstitution, and simulations to demonstrate that condensate-mediated capillary forces drive distinct membrane shape transformations (tubes, sheets, and cups) governed by interfacial tension and hysteresis, thereby providing a mechanism for the temporal control of intracellular organelle morphogenesis.

Original authors: Lukas Hauer, Katharina Sporbeck, Joseph F. McKenna, Dmytro Puchkov, Alexander I. May, Lorenzo Frigerio, Roland L. Knorr, Amir H. Bahrami

Published 2026-03-18

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CC BY 4.0

Original authors: Lukas Hauer, Katharina Sporbeck, Joseph F. McKenna, Dmytro Puchkov, Alexander I. May, Lorenzo Frigerio, Roland L. Knorr, Amir H. Bahrami

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a cell as a bustling city. Inside this city, there are two main types of structures: organelles (like the vacuoles in plant cells, which are essentially giant storage bubbles) and condensates.

Think of condensates as "liquid droplets" or "oil slicks" floating inside the cell. They are made of proteins and RNA that have clumped together, much like oil separating from water. These droplets are not just sitting there; they are active, dynamic, and they interact with the cell's membranes (the walls of the storage bubbles).

This paper is about a fascinating discovery: These liquid droplets can physically reshape the cell's walls, turning them into tubes, flat sheets, or cup-like bowls.

Here is the story of how they figured this out, explained with some everyday analogies:

1. The Setup: The "Wetting" Effect

Imagine you have a balloon (the cell membrane) and you place a drop of water on it. If the water "wets" the balloon, it spreads out. But if the water is very "tense" (like a tight rubber band), it tries to stay in a perfect sphere.

In this study, the researchers found that these condensates act like that water drop. They sit right on the edge of the cell's storage bubble (the vacuole). The "tension" of the surface where the liquid droplet meets the air (or the cell's fluid) creates a force called capillary force.

Think of it like a tightrope walker. The tension in the rope pulls on the walker. Similarly, the tension of the condensate pulls on the cell membrane, trying to reshape it.

2. The Three Shapes: Tubes, Sheets, and Cups

Depending on how "tight" that surface tension is, the membrane changes shape into one of three things:

Tubes: Like long, thin straws. These happen when the tension is low.
Sheets: Like flat, open pancakes or sheets of paper. These happen when the tension is high. The tension pulls the membrane flat to minimize the "cost" of the interface.
Cups: Like little bowls or saucers. These happen when the tension is low, but the membrane has already been through a transformation.

3. The "History" Matters (The Hysteresis Loop)

This is the most surprising part of the paper. The shape of the membrane doesn't just depend on the current tension; it depends on what happened before.

Imagine you are folding a piece of paper:

If you start with a tube (straw) and increase the tension, it flattens out into a sheet (pancake).
If you then decrease the tension, the sheet doesn't go back to being a tube. Instead, it curls up into a cup (bowl).

The paper calls this hysteresis. It's like a ratchet mechanism: once you go from Tube → Sheet → Cup, you can't easily go back to the Tube just by changing the tension. You have to "reset" the system.

The Analogy: Think of a molded clay pot.

If you have a long clay snake (tube) and you press it flat, it becomes a pancake (sheet).
If you let the pressure off, it doesn't snap back into a snake. It might curl up into a bowl (cup).
To get the snake back, you have to completely unroll and re-roll the clay. You can't just wiggle it.

4. How They Figured It Out

The researchers used a "three-pronged" approach, like a detective using three different tools:

Looking at Real Plants: They used high-powered microscopes to watch living plant seeds. They saw these "cups" (which they call "bulbs") forming inside the plant's storage bubbles. They realized these cups were sitting right where the liquid droplets touched the membrane.
Building a Model in a Lab: Since looking inside a tiny plant cell is hard, they built a giant version in a dish. They made giant bubbles (GUVs) filled with a mixture that separates into two liquids (like oil and water). They watched the bubble's skin change shape as they tweaked the chemistry. It worked exactly like the real plants!
Computer Simulations: They used supercomputers to create a virtual model of the membrane. This allowed them to calculate the "energy" required for each shape. They found that high tension makes the "flat sheet" the most energy-efficient shape, while low tension favors the "cup."

5. Why Does This Matter?

Why should we care if a plant cell makes a cup or a tube?

Plant Growth: These "cups" (or bulbs) are crucial for how plants store nutrients and how they wake up from dormancy (like a seed sprouting). The paper suggests that by changing the "tension" of these liquid droplets, the plant can control when and how these structures form.
General Biology: This isn't just about plants. Cells in humans use similar liquid droplets to organize their insides. This research suggests that liquid droplets are a universal tool cells use to sculpt their own architecture.

The Big Takeaway

Cells aren't just static bags of soup. They are dynamic, shape-shifting entities. By using liquid droplets as tools, cells can pull and push their membranes to create tubes, sheets, and cups.

The most important lesson? Shape is history. The form a cell takes right now depends on the journey it took to get there. You can't just look at a cell and guess its shape based on current conditions; you have to know its past to understand its future.

1. Problem Statement

Cellular organization relies heavily on two mechanisms: membrane-enclosed organelles and membraneless biomolecular condensates formed via liquid-liquid phase separation (LLPS). While the equilibrium shapes of isolated membranes (tubes, sheets, cups) are well-understood through elastic deformation theory, the dynamic interplay between condensates and membranes remains unclear.

The Gap: Previous studies focused on equilibrium conditions or specific biological contexts (e.g., autophagosome closure). However, the intracellular environment is inherently non-equilibrium. It is unknown how condensate wetting, driven by interfacial tension (capillary forces), dynamically reshapes membranes into specific morphologies (tubes, sheets, cups) and whether these transitions are reversible or hysteretic.
Specific Context: In plant vacuoles, condensates of storage proteins wet the tonoplast (vacuolar membrane), forming complex structures like "bulbs" (cup-like invaginations) and sheets. The physical mechanism driving the formation and stability of these structures, particularly the role of interfacial tension and energy barriers, had not been systematically quantified.

2. Methodology

The authors employed a multi-scale, integrated approach combining in vivo imaging, in vitro reconstitution, and computational modeling to decouple physicochemical forces from complex biological factors.

In Planta Live-Cell Imaging:
- System: Arabidopsis thaliana embryo cotyledons expressing the tonoplast marker TPK-GFP.
- Observation: Confocal microscopy was used to visualize condensates (stained with Neutral Red) and tonoplast-derived structures during seed maturation.
- Advanced Imaging: High-pressure freezing and Scanning Electron Microscopy (SEM) were used to preserve condensates for structural analysis, as chemical fixation caused them to dissolve.
In Vitro Reconstitution (Biomimetic Model):
- System: Giant Unilamellar Vesicles (GUVs) encapsulating a phase-separating aqueous polymer solution (Dextran/PEG).
- Control: Interfacial tension ( $\Sigma$ ) was tuned by adjusting osmotic conditions (hyperosmotic quenching) to control the polymer concentration and phase separation.
- Microscopy: Confocal microscopy and Stimulated Emission Depletion (STED) microscopy (approx. 50 nm resolution) were used to resolve the bilayer structure of the formed membranes (distinguishing single vs. double bilayers).
Computational Modeling (In Silico):
- Model: A triangulated elastic vesicle model based on Helfrich bending energy, coupled with a flat liquid-liquid interface.
- Parameters: The system was defined by the reduced volume ( $v = 6\sqrt{\pi}V/A^{3/2}$ ) and reduced interfacial tension ( $\sigma = \Sigma R^2 / 2\kappa$ ).
- Simulation Techniques:
  - Monte Carlo (MC) Minimization: Used with simulated annealing to map the free energy landscape and identify metastable states and energy barriers ( $H_1$ for tube-to-sheet, $H_2$ for sheet-to-cup).
  - MC Simulations: Used to study thermally driven transition pathways and kinetics (transition times).

3. Key Contributions

Discovery of Interfacial Membrane Morphologies: First identification and characterization of three distinct membrane morphologies (tubes, sheets, and cups) localized specifically at condensate-liquid interfaces in both plant vacuoles and synthetic GUVs.
Quantification of Capillary Forces: Established that interfacial tension ( $\Sigma$ ) is the primary driver of shape selection, acting as a control parameter that dictates the stability of specific morphologies.
Energy Landscape Mapping: Calculated the free energy barriers governing transitions between shapes, revealing that condensate wetting creates a rugged energy landscape with metastable states.
Demonstration of Hysteresis: Proved that shape transformations are history-dependent. The system exhibits tube-to-cup hysteresis, meaning the final shape depends on the temporal evolution of interfacial tension, not just its instantaneous value.

4. Key Results

Morphological Classification:
- Tubes: Observed at low interfacial tension or as initial precursors.
- Sheets: Flattened, planar, double-bilayer structures.
- Cups: Spherical, invaginated, double-bilayer structures (resembling "bulbs" in seeds).
- Structure: STED microscopy confirmed that sheets and cups consist of two closely apposed bilayers (separated by ~100–200 nm), stabilized by electrostatic repulsion, rather than a single folded membrane.
Role of Interfacial Tension ( $\Sigma$ ):
- High $\Sigma$ : Favors the formation of sheets. High tension minimizes the interfacial area penalty by flattening the membrane against the interface.
- Low $\Sigma$ : Favors cups and tubes. Lower tension allows membrane bending energy to dominate, leading to curved structures.
- Crossover: A critical transition point exists around $\Sigma \approx 5$ µN/m where the frequency of sheets and cups is equal.
Energy Barriers and Metastability:
- The transition from Tube $\to$ Sheet involves barrier $H_1$ , and Sheet $\to$ Cup involves barrier $H_2$ .
- Increasing $\Sigma$ reduces $H_1$ (making tube-to-sheet spontaneous) but increases $H_2$ (stabilizing sheets against cup formation).
- Conversely, low $\Sigma$ results in negligible $H_2$ , allowing sheets to spontaneously collapse into cups.
Hysteresis and Kinetics:
- Pathway: To form a cup, the system must follow the path: Tube $\to$ (increase $\Sigma$ ) $\to$ Sheet $\to$ (decrease $\Sigma$ ) $\to$ Cup.
- Irreversibility: Direct transition from Tube to Cup is kinetically blocked by high energy barriers. Once a cup forms, it cannot revert to a tube simply by lowering tension; it must revert to a sheet first.
- Timescales: Transition times ( $\tau$ ) scale exponentially with energy barriers. Tube-to-sheet transitions are fast at high $\Sigma$ , while sheet-to-cup transitions are slow at high $\Sigma$ .
Contact Angle Robustness:
- Theoretical analysis showed that while contact angle ( $\theta$ ) affects free energy, the morphological stability trends (High $\Sigma$ $\to$ Sheets) are robust to variations in $\theta$ (tested range 60°–120°).

5. Significance

Mechanistic Insight into Plant Biology: The study provides a physical explanation for the formation of "bulbs" in plant vacuoles, linking them to the dynamic modulation of condensate surface properties during seed maturation and germination.
General Physical Principle: It establishes a general mechanism where capillary forces from biomolecular condensates can remodel cellular membranes. This extends beyond plant vacuoles to other organelles like the Golgi apparatus and autophagosomes, where condensate-membrane contacts are observed.
Non-Equilibrium Dynamics: The work highlights that intracellular organization is governed by non-equilibrium processes. The history of the system (temporal changes in condensate properties due to metabolic or environmental shifts) dictates the final morphology, challenging purely equilibrium-based models.
Therapeutic/Engineering Implications: Understanding how to control membrane shape via interfacial tension could inform the design of synthetic cells or therapeutic strategies targeting organelle remodeling in disease states.

In summary, the paper demonstrates that temporal control of condensate surface tension acts as a switch for membrane morphogenesis, driving a hysteretic cascade from tubes to sheets to cups, thereby providing a fundamental physical basis for the dynamic organization of the cell interior.