The filopodial scaffold polyphosphate dictates cell adhesion-versus-invasion decisions
This study identifies inorganic polyphosphate as a structural scaffold for filopodia that stabilizes cell adhesion by recruiting IRSp53, and demonstrates that restoring reduced polyP levels in cancer cells suppresses invasion and reverses metastatic gene signatures, implicating it as a primordial tumor suppressor.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Cells are not static bricks in a wall; they are active, moving entities that constantly probe their surroundings. To do this, they extend tiny, finger-like projections from their surface called filopodia. These structures act as sensory antennae, helping cells stick to surfaces, sense chemical signals, and decide whether to stay put or move on. The balance between sticking down and moving away is critical for healthy tissue formation, but when this balance tips too far toward movement, it can lead to the uncontrolled spreading of cancer. For decades, scientists have known that these filopodia are built from a scaffold of protein fibers, but a key component holding this structure together has remained hidden.
Researchers at the University of Michigan have now identified a long-overlooked molecule that acts as a structural glue for these cellular fingers. This molecule is inorganic polyphosphate, a simple chain of phosphates found in every living thing. The team discovered that polyphosphate accumulates along the cell membrane, specifically within these finger-like projections, where it serves as a scaffold to organize and stabilize the proteins that build them. When polyphosphate levels are high, the cellular fingers become long, sturdy, and stable, causing the cell to adhere firmly to its environment. When polyphosphate levels drop, these fingers become short, wobbly, and fleeting, prompting the cell to detach and migrate.
The study began by observing normal mouse and human cells under a microscope. The researchers used a special glowing probe that binds to polyphosphate to see where it lived inside the cell. They found that while much of the molecule floated freely in the cell's interior, distinct bright spots lined the plasma membrane and ran along the length of the filopodia. To test if this molecule was actually doing something important, the scientists genetically engineered cells to produce extra polyphosphate. These cells sprouted far more filopodia than usual, and the ones they did form were significantly longer. Crucially, these new projections did not flicker in and out of existence; they remained stable for long periods. Conversely, when the researchers reduced the amount of polyphosphate in the cells, the filopodia became sparse and highly dynamic, constantly assembling and disassembling. This behavior suggested that polyphosphate acts as a stabilizer, locking the cellular machinery in place to promote adhesion rather than movement.
To understand how this molecule works, the team looked at the proteins that build these structures. One key player is a protein called IRSp53, which helps bend the cell membrane to form the initial tip of a filopodium. In a test tube, the researchers mixed purified IRSp53 with polyphosphate chains. Without the polyphosphate, the protein remained dissolved and inactive. However, the moment they added long chains of polyphosphate, the proteins clumped together into liquid-like droplets. This process, known as phase separation, allowed the proteins to organize into dense clusters. The researchers found that polyphosphate chains longer than a certain length were required to trigger this effect; shorter chains had no impact. This suggests that polyphosphate acts as a molecular scaffold, gathering IRSp53 molecules together to kickstart the formation of the cellular finger.
The team then recreated this process outside of a living cell using synthetic bubbles made of lipids, which mimic the cell membrane. When they added polyphosphate to these bubbles, it successfully recruited the IRSp53 protein to the surface. When they added the full set of proteins needed to build a filopodium, the bubbles with polyphosphate on their surface began to sprout long, inward-pointing projections, while the control bubbles remained smooth. This confirmed that polyphosphate alone is sufficient to recruit the necessary machinery and drive the formation of these structures.
The most striking findings emerged when the researchers looked at cancer cells. They compared normal human cells to a version of those cells that had been transformed to behave like aggressive, metastatic cancer. The cancer cells showed a dramatic reduction in polyphosphate levels, and what remained was largely trapped inside the nucleus rather than lining the membrane. This loss of membrane-associated polyphosphate correlated with a loss of stable filopodia and a gain in invasive behavior. To test if this was a cause or a consequence, the researchers used lipid nanoparticles—tiny fat bubbles—to deliver polyphosphate directly into the cancer cells. This treatment restored the molecule to the cell membrane, stabilized the filopodia, and significantly reduced the cells' ability to invade surrounding tissue.
The researchers took this a step further by studying three-dimensional clusters of tumor cells, known as organoids, grown from aggressive breast cancer models. These organoids naturally invade the surrounding gel, mimicking how tumors spread in the body. When treated with the polyphosphate-delivering nanoparticles, the organoids stopped invading. Instead of sending out jagged, invasive tendrils, they remained round and compact. Genetic analysis of these treated organoids revealed that the restoration of polyphosphate turned off genes associated with movement and invasion while turning on genes linked to cell adhesion and stability.
The study also addressed why previous attempts to use polyphosphate as a cancer treatment had yielded mixed results. The researchers noted that simply adding polyphosphate to the cell culture media often failed because the cells would react by depleting their own internal stores. By using lipid nanoparticles to deliver the molecule directly into the cell, they bypassed this defensive mechanism and successfully replenished the internal supply. The findings suggest that the loss of membrane-bound polyphosphate is a fundamental step in the transformation of a normal cell into a metastatic one. By restoring this ancient, simple molecule, the researchers were able to reverse the invasive behavior of aggressive cancer cells, pointing to a potential new strategy for halting the spread of disease. The work highlights how a basic, inorganic polymer, conserved across billions of years of evolution, plays a decisive role in the complex decision-making processes of modern cells.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.