A glioma cell sitting on a glass dish has no instructions. Nothing outside it is telling it which way to go, where to bulge, where to flatten. And yet, watched under the right microscope, it gets on with the business of moving anyway: a broad sheet of membrane swells out at one edge, the cell tips forward, and off it crawls. For decades that has been one of the maddening puzzles of cell biology. We know cells reshape themselves in response to chemical signals from their surroundings. What we have struggled to explain is how they do it when there are no signals at all.
Now a team at the Nara Institute of Science and Technology in Japan thinks it has caught the culprit in the act. Reporting in EMBO Reports, Kio Yagami, Naoyuki Inagaki and their colleagues describe small assemblies of actin filaments that scoot around inside the cell like independent objects, bumping into the membrane, pushing it outward, and piling up wherever the cell starts to bulge. They have called them self-propelled treadmilling actin filaments, or SpTAs, and the name is doing a lot of work.
Actin is the protein that gives a cell its internal scaffolding, the stuff that pushes against the membrane from the inside whenever a cell changes shape. Every time a white blood cell wraps itself around a bacterium, or a neuron reaches out to wire itself to a neighbour, or a cancer cell forces its way through tissue to spread, actin is doing the shoving. The textbook story has it that external cues switch on regulator proteins, which in turn decide where actin grows. Tidy enough. The trouble is that cells throw out protrusions even when nobody has flipped the switch.
Particles, not waves
The team worked with a line of human glioma cells that happen to polarise and crawl entirely on their own, no prompting required. Using a clutch of high-resolution techniques (total internal reflection microscopy to watch the underside of the cell, epifluorescence to see right through it, confocal deconvolution for the three-dimensional picture), they watched bundles and meshworks of actin emerge, drift in seemingly random directions, and vanish. Some were linear bundles. Others were broad meshworks. They swapped back and forth, bundles spawning from meshworks and branching off one another.
What made these structures interesting was the way they moved. Biologists had described something a bit like this before, calling them actin “waves” and modelling them as chemical reactions rippling across the cell, the way a Mexican wave travels round a stadium without any single person leaving their seat. The SpTAs did not behave like that. They moved as discrete things, individual travellers that changed direction at random and carried their material along with them. If that sounds familiar to a physicist, it should. It is more or less the definition of a self-propelled particle, the kind of active-matter object that has kept soft-matter labs busy for years.
The engine turns out to be treadmilling, a property actin has had all along. New building blocks, carrying a molecule of ATP, lock onto the front of a filament. Older ones, having burned through their fuel, fall off the back. Add at the front, shed at the rear, consume energy in between, and the whole assembly creeps forward without any one molecule actually going very far. A linker protein called shootin1b helps by pinning the filaments loosely to the membrane, which stops them sliding backwards and lets the forward growth count for something. Knock shootin1b out and the bundles slow right down.
“We discovered SpTA and established the assembly of actin filaments as a novel class of biological active particles, solving the mystery of biological self-organization and providing new insights into how molecular-scale motion orchestrates complex higher-order organization,” says Yagami.
Trapped at the corners
That is the first half of the story. The second half is what happens when one of these wandering bundles reaches the edge. Its growing end is pointing outward, so when it arrives it shoves the membrane into a little spike or sheet, a nascent filopodium or lamellipodium. Then comes the clever bit. Other SpTAs, arriving later, do not just bounce off. They get caught at the protrusion and slide along the membrane, fetching up at the bulge and adding their actin to it. The protrusion grows because it is good at catching the very things that make protrusions grow.
To check the idea, the team grew cells on tiny triangular islands of adhesive protein, forcing them into a fixed shape, and found actin heaping up at the sharp corners. The sharper the corner, the more it accumulated; a 30-degree point gathered far more than a gentle 180-degree edge. A mathematical model built entirely from their measured numbers, the bundles’ speed, length, lifetime and tendency to turn, reproduced the same pattern without anyone having to write the corners in by hand. It is, the researchers note, exactly how self-propelled particles behave when they collide with a boundary. They get stuck in the tight spots.
This piling-up is its own kind of feedback loop. A protrusion forms, traps more SpTAs, grows larger, traps still more. Out of that, with no master plan and no outside signal, the cell assembles a single dominant leading edge and commits to a direction. Spontaneous order, bootstrapped from molecular fidgeting.
What gives the work its reach is the bridge it builds. Active-matter physicists have spent years showing, in simulation and in synthetic systems, that self-propelled particles trapped inside a flexible vesicle will deform it in ways that look uncannily like a living cell. Here is a living cell apparently doing precisely that, with actin bundles standing in for the particles. “We expect our findings to serve as a bridge connecting modern biology and modern physics to tackle the enduring puzzle of self-organization,” says Inagaki. Whether the same trick explains microvilli, the spines on a neuron, the invading feet of a tumour cell, all of them built from outward-growing actin, is the obvious next question, and one the team is careful to leave open. Cells, it seems, have been doing physics all along. We are only now learning to read the equations.
DOI / Source: https://doi.org/10.1038/s44319-026-00804-6
Frequently Asked Questions
How can a cell change shape without any signal telling it to?
The Nara team found that small assemblies of actin filaments move around inside the cell on their own, powered by their own growth and decay rather than by external chemical cues. When these mobile bundles reach the membrane they push it outward, and they tend to collect wherever a bulge is already forming, so the cell can build a protrusion and pick a direction with no outside instruction at all. It suggests that some of what looks like decision-making in a cell is really self-organising physics.
What does it mean to call actin a self-propelled particle?
Physicists use the term for objects that draw energy from their surroundings and convert it into directed motion, with a random wobble to their path, like microscopic swimmers. The researchers showed that actin bundles fit this description: they consume ATP, travel as discrete units, and change direction at random, rather than spreading as the chemical waves biologists had previously assumed. That reframing is what lets cell biology borrow tools from active-matter physics.
How does the bundle actually move if the molecules barely budge?
Through a process called treadmilling. Fresh actin subunits attach to the front of a filament while spent ones drop off the back, so the structure as a whole creeps forward even though no single molecule travels far. A linker protein called shootin1b tethers the filaments loosely to the membrane to keep them from slipping backwards, which is why removing it slows the bundles down.
Why does the shape of a cell decide where actin piles up?
Because self-propelled particles get trapped when they hit a boundary, and the tighter the boundary, the harder they are to escape. When cells were grown on triangular patches, actin gathered most densely at the sharpest corners, and a model built from the bundles’ measured behaviour reproduced the effect without being told to. The same trapping could explain why many cell structures, from filopodia to invading cancer protrusions, form where they do.
