Fibers and Glasses are Competing Material States in FUS Protein Condensation

Inside the brain, certain proteins can behave like oil droplets in water, clustering together to form dense, liquid-like drops called condensates. These structures are vital for healthy cell function, helping to organize the complex chemistry of life. However, when these proteins go wrong, they can transform into rigid, thread-like structures known as amyloid fibers. These fibers are the hallmark of devastating neurodegenerative diseases, such as ALS and fronto-temporal dementia, where they accumulate and damage brain tissue. For years, scientists have debated how these dangerous fibers form. A prevailing idea was that the liquid drops themselves might be the birthplace of these fibers, perhaps hardening from the inside out as they age. Understanding exactly where and how this transformation happens is crucial, because it could reveal new ways to stop the disease before it starts.

A team of researchers has now watched this process unfold in real time, using a protein called FUS as a model. They discovered that the story is more complex than a simple transformation from liquid to solid. Instead of growing inside the dense drops, the fibers actually sprout and grow in the thin, watery fluid that surrounds them. As the fibers pull protein out of this surrounding fluid to build themselves, the dense drops nearby begin to shrink and eventually turn into a different kind of solid: a glassy, jammed state that is distinct from the fibers. The study reveals that these two solid forms—the dangerous fibers and the glassy drops—are competitors. They do not turn into one another; rather, they fight for the same building blocks, and the aging of the drops can actually slow down the formation of the fibers.

To see this clearly, the researchers created a controlled environment in a lab dish. They mixed a fluorescently tagged version of the FUS protein with a special dye that lights up only when it binds to the rigid, thread-like fibers. They then added tiny fragments of existing fibers, acting as seeds, to jump-start the process. Using a powerful microscope, they watched what happened over several hours. Instead of seeing the fibers burst out from the center of the liquid drops, they observed the seeds landing on the surface of the drops. From there, thick fibers began to grow outward, extending into the surrounding watery space. Over time, a shell of these fibers formed around the drops, while the drops themselves started to shrink and crumple.

The team wanted to know exactly where the protein for these growing fibers was coming from. Did the fibers suck material directly from the liquid drops they were touching, or were they pulling it from the wider, thinner fluid around everything? To find out, they performed a clever experiment. They took a fiber growing out of a drop and used a laser to bleach a small spot on it, turning that specific section dark. If the fiber were growing by adding new material to its base near the drop, the dark spot would move away as the fiber lengthened. But if the fiber were growing at its tip by pulling in new material from the surrounding fluid, the dark spot would stay in the same place relative to the drop. When they watched, the dark spot remained fixed. This proved that the fibers were elongating by recruiting protein from the dilute fluid around them, not by eating into the drop itself.

Further measurements confirmed just how dominant this process was. By tracking the volume of the drops that were touching fibers versus those that were not, the researchers calculated that about 96 percent of the protein used to build the fibers came from the surrounding fluid, while only about 4 percent came directly from the drops. As the fibers consumed protein from the fluid, the concentration of protein in that fluid dropped. This caused the nearby drops to shrink, as they tried to maintain their balance by releasing more protein into the fluid. It was a dynamic system where the drops acted as a reservoir, slowly feeding the fiber growth from the outside.

However, the story took a turn when the researchers considered what happens as these drops get older. They knew that over time, these liquid drops can lose their fluidity and become "glassy," a state where the molecules inside get stuck and jammed, unable to move freely. They wondered if this glassy state would help or hinder the formation of fibers. To test this, they prepared three sets of samples and let the drops sit for different amounts of time: one hour, twenty-four hours, and forty-eight hours. The longer they waited, the more the drops turned into a glassy, arrested solid. When they added the fiber seeds to these aged samples, the results were striking. In the fresh, one-hour samples, the fibers grew rapidly and consumed most of the drops. But in the samples that had aged for twenty-four and forty-eight hours, the fiber growth was significantly slower, and many of the glassy drops remained intact.

The researchers concluded that the glassy state acts as a barrier. Because the molecules inside the aged drops are jammed and cannot move easily, they cannot release their protein into the surrounding fluid fast enough to feed the growing fibers. This creates a competition: the system has two paths to a solid state. One path leads to the formation of amyloid fibers, which requires a steady supply of protein from the fluid. The other path leads to the drops themselves hardening into a glassy solid, which cuts off that supply. The two states are distinct; the researchers confirmed this by adding a chemical detergent that dissolves the glassy drops but leaves the fibers untouched. The glassy drops vanished, while the fibers remained, proving they are different materials.

This work changes the understanding of how these diseases might progress. It suggests that the formation of toxic fibers and the hardening of cellular drops are not a single, linear process where one turns into the other. Instead, they are two separate, competing outcomes. The liquid drops do not simply age into fibers; they can age into a glassy state that actually blocks the fibers from forming. This discovery highlights a delicate balance in the cell's chemistry, where the speed at which drops release their contents determines whether the system tips toward the formation of disease-causing fibers or remains in a stalled, glassy state. By mapping these competing pathways, the study provides a clearer picture of the physical rules that govern protein behavior in the brain, offering new angles for understanding how these diseases take hold.