Heterotypic Condensate Formation by Prion Proteins and Pro-Inflammatory S100A9
This study demonstrates that the pro-inflammatory protein S100A9 promotes the aggregation of prion proteins by forming heterotypic liquid-liquid phase separation condensates, which subsequently accelerate the formation of amyloid structures under high molecular crowding conditions.
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 This is an AI-generated explanation of a preprint that has not been peer-reviewed.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
Inside every living cell, there is a constant, bustling activity where proteins move, fold, and interact to keep life running. Sometimes, these proteins gather together in specific spots, forming droplets that float freely without a surrounding membrane, much like oil droplets in water. Scientists call this process liquid-liquid phase separation. While this natural behavior helps organize cellular work, things can go wrong when these droplets become too stable or transform into solid, sticky clumps. This transition from a fluid drop to a solid mass is a hallmark of several devastating brain diseases, including Alzheimer's and Parkinson's, where proteins misfold and accumulate into harmful structures known as amyloids. For years, researchers have focused on how single types of proteins cause these problems, but a growing body of evidence suggests that different proteins might mix together inside these droplets, accelerating the damage in ways we are only beginning to understand.
A team of researchers at Vilnius University in Lithuania recently investigated a specific pairing that had been observed to interact under harsh laboratory conditions: the prion protein, which is central to prion diseases, and S100A9, a protein involved in inflammation. They wanted to know if these two proteins could form mixed droplets together in a more natural environment and, if so, how that mixture would affect the formation of harmful clumps. To test this, they created a crowded laboratory environment using a substance called polyethylene glycol to mimic the dense conditions found inside cells. They mixed the two proteins together and watched closely to see if they would separate into liquid droplets and how those droplets would change over time.
The researchers found that under neutral conditions, which are closer to the environment inside the human body, the prion protein and S100A9 did indeed form mixed droplets. When the two proteins were present together, they created a significantly larger number of these droplets compared to when either protein was alone. Using a fluorescent tag to track the S100A9, the team observed that this inflammatory protein was incorporated into the droplets formed by the prion protein. However, the mixture was not uniform; some droplets contained only prion protein, while others held a combination of both. This suggests that the presence of the inflammatory protein actively encourages the prion protein to gather into these liquid clusters.
What happened next was perhaps the most significant discovery. While the prion protein alone could form fibril-like structures under these specific crowding conditions, the presence of S100A9 led to the formation of even more stable and abundant aggregates. In standard laboratory settings, scientists often have to use strong chemical denaturants to force prion proteins to clump together. Here, the researchers found that the mere presence of S100A9 allowed the prion protein to form stable, solid aggregates without those harsh chemicals, and these mixed aggregates were more resistant to breaking apart than those formed by the prion protein alone. The team measured the internal structure of these clumps and found they were rich in beta-sheets, a specific arrangement of protein strands that characterizes the rigid, harmful fibers seen in neurodegenerative diseases.
The study suggests that the inflammatory protein S100A9 may play a critical, previously underappreciated role in the early stages of prion diseases. By helping to form these mixed liquid droplets, S100A9 appears to lower the barrier for prion proteins to clump together, effectively jump-starting the process that leads to solid, disease-causing aggregates. The researchers propose that in the human body, where both proteins are found in the brain and spinal fluid, this interaction could be a key factor in how these disorders begin and progress. While the work was conducted in a test tube, the findings point to a new pathway for understanding how inflammation and protein misfolding might work together to drive neurodegeneration, offering a fresh perspective on why these diseases are so difficult to stop.
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