Mapping the physical forces that propel proteins forward

Scientists at Oregon Health & Science University have discovered a previously unknown system of internal “trade winds” that help cells rapidly move essential proteins to the front of the cell, reshaping the way researchers understand cell migration, cancer spread and wound healing.

The discovery, published today in Nature Communications, reshapes what researchers thought they knew about how cells direct proteins to the right place at the right time.

For decades, biology textbooks have taught that free-floating proteins inside cells move primarily by diffusion, drifting randomly until they reach their destination. But the new study shows that cells don’t leave this to chance. Instead, they create targeted currents of fluid that propel necessary proteins toward the tip of the cell, where movement and repair begin.

Accidental discovery in a great discovery

Study co-corresponding authors Catherine (Cathy) Galbraith, Ph.D., and James (Jim) Galbraith, Ph.D., associate professors in the OHSU Biomedical Engineering Department and Discovery Engine Investigators at the OHSU Knight Cancer Institute, trace the discovery to an unexpected lesson years ago in neurobiology class. Massachusetts.

It actually started as an unexpected find. We just did an experiment with students in the classroom.”

Catherine (Cathy) Galbraith, PhD, Study Corresponding Author and Associate Professor, Department of Biomedical Engineering, Oregon Health & Science University

They were using a laser to make proteins invisible in a stripe on the back of a living cell—a standard technique for tracking how materials move inside cells—when something unexpected happened: a second small, dark line appeared at the leading edge that the cell stretches as it moves.

“We did it for fun and then realized that this gave us a way to measure something that couldn’t be measured before,” he said.

This unexpected dark line turned out to be a wave of soluble actin—one of the key proteins that helps cells move—quickly pushed to the leading edge of the cell. Until now, scientists assumed that actin mostly got there by diffusion, drifting into the cell randomly. But Galbraith’s experiments revealed something else entirely.

“We realized that the cartoon models in the textbooks were missing a huge piece,” Jim said. “There had to be some kind of flow in the cell that pushes things forward. Cells really do ‘go with the flow’.”

Understanding cancer cell migration

Cathy and Jim were recruited to OHSU in 2013 by the National Institutes of Health, where they had worked with Nobel Laureate Eric Betzig, Ph.D., at the Janelia Research Campus of the Howard Hughes Medical Institute, to develop live-cell ultra-resolution microscopy.

Using custom imaging analyses, the scientists discovered that the cells actively generate directional fluid flows, which the team compares to atmospheric rivers. These currents push actin and other proteins forward much faster than diffusion could.

“We found that the cell can actually push on the back and target where it’s sending that stuff,” Jim said. “If you squeeze half a sponge, the water only goes to that half. That’s basically what the cell does.”

This internal flow is nonspecific, meaning it scans many types of proteins simultaneously.

The result is a fast, efficient delivery system that fuels protrusion, adhesion, and rapid shape changes, all critical processes for cell movement, immune response, and tissue repair. The published findings confirm that these flows occur within a specialized compartment at the cell front, separated from the rest of the cell by an actin-myosin condensate barrier that acts as a physical wall and targets the flows to advancing regions along the cell edge.

To image the currents, the team developed a reverse version of a standard fluorescence technique. Instead of using lasers to beam the light away, they activated fluorescent molecules at a single point and watched how they moved.

The team called one of the key experiments FLOP, or Fluorescence Leaving the Original Point.

“It wasn’t a failure at all,” Kathy said. “It was the opposite. It’s anything but a failure, because it worked.” The team’s discovery may explain why some cancer cells move so aggressively.

“We know that these highly invasive cells have this very cool mechanism to push proteins very, very quickly to where they’re needed at the front of the cell,” Jim said. “All cells have basically the same parts inside them, just like a Porsche and a Volkswagen have many of the same parts, but when those parts are assembled into the final machine, they behave and function very differently.”

By understanding these differences, researchers hope to identify new ways to disrupt the way cancer cells migrate.

“If you can understand the differences, you can target future treatments based on how cancer cells and normal cells work differently,” he said.

Collaboration on key findings

The project brought together engineering, physics, microscopy and cell biology, with key contributions from collaborators at the Janelia Research Campus in Virginia, including experts in fluorescence correlation spectroscopy and 3D superresolution imaging.

“The instruments we needed are not available in most places,” Cathy said. “Janelia had a unique facility that allowed us to test and confirm what we were seeing.”

Much of the work was based on advanced imaging techniques developed at Janelia—among them iPALM, an interferometric method for resolving nanometer-scale structures that the Galbraiths helped develop.

“The iPALM allowed us to physically see the apartments,” said Jim. “There is no other light-based technique that could do this.”

The researchers say the study reveals a “pseudoorganelle,” or a functional compartment that isn’t surrounded by a membrane but still shapes how the cell behaves.

“Just as small changes in the jet stream can change the weather, small changes in these cellular winds could change how diseases start or progress,” Cathy said.

The team believes the project opens up new directions for cancer research, drug delivery, tissue repair and synthetic biology.

“All you had to do was look,” Kathy said. “The fluxes have been there all along. Now we know how cells use them.”