RhoBAST-FLIM enables high-fidelity and lifetime-assisted super-resolution RNA imaging
This paper introduces RhoBAST-FLIM, a versatile imaging platform that leverages fluorescence lifetime differences between free and bound dyes to achieve high-fidelity, background-suppressed, and super-resolution visualization of RNA and cell-surface proteins in living cells.
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 This is an AI-generated explanation of a preprint that has not been peer-reviewed.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, a vast library of instructions is constantly being read, copied, and moved to build the machinery of life. These instructions are carried by molecules called RNA, which act as messengers and managers within the microscopic city of the cell. For decades, scientists have struggled to watch these RNA molecules move in real time without disturbing them. The most common tools for this task rely on attaching a glowing tag to the RNA and then shining a light to see where it goes. However, this method has a significant flaw: the glowing tag is often mixed with loose, unattached dye floating freely in the cell. Because the loose dye glows just as brightly as the tagged RNA, it creates a fog of background light that makes it difficult to tell exactly where the RNA is or how much of it is present. This is like trying to spot a single lit candle in a room filled with thousands of other flickering lights; the signal gets lost in the noise.
A team of researchers at Heidelberg University has developed a new way to cut through this fog, allowing for a much clearer view of RNA as it moves and functions inside living cells. Instead of just measuring how bright the light is, their new method, called RhoBAST-FLIM, measures how long the light lasts after the molecule is excited. This technique, known as fluorescence lifetime imaging, works like a stopwatch for light. When the dye molecule is floating freely, it glows for a very brief moment before fading. But when it binds to the specific RNA tag, its glow lingers for a noticeably longer time. By timing this difference, the researchers can mathematically separate the signal of the RNA from the background noise of the loose dye, revealing the true location of the RNA with high precision.
The researchers began by designing a library of new dye molecules based on a class of compounds called rhodamines. They tested many variations to find the ones that showed the biggest difference in how long they glowed when bound to the RNA tag versus when they were free. They discovered that certain modified dyes, particularly those with specific chemical groups attached, bound tightly to the RNA tag and changed their glowing duration significantly. Some of these dyes glowed for about 0.9 nanoseconds when free, but stretched to over 4.5 nanoseconds when attached to the RNA. This large difference allowed the imaging system to distinguish the two states clearly, even when they were mixed together in the same tiny space.
Using these optimized dyes, the team demonstrated that they could image various types of RNA in living human cells with a single tag. In previous methods, scientists often had to attach multiple tags to an RNA molecule to make it bright enough to see, which could interfere with the RNA's natural behavior. With this new lifetime-based approach, a single tag was sufficient because the system could ignore the background light so effectively. They successfully tracked different RNA molecules, including those that stay in the nucleus and others that move into the cytoplasm, showing that the method works reliably across different cellular environments. The researchers found that the glowing duration of the RNA-bound dye remained consistent regardless of where the RNA was located, proving that the signal was a stable signature of the RNA itself rather than an artifact of the cell's surroundings.
Beyond simply seeing where the RNA is, this new technique also allows for super-resolution imaging, which reveals details far smaller than what standard microscopes can show. Standard super-resolution methods often require intense beams of light to force the molecules to switch off, which can damage the cell or bleach the dye. The researchers combined their lifetime measurements with a technique called tauSTED. This method uses the difference in glowing duration to sharpen the image without needing such high power. When they applied this to RNA molecules located in tiny structures called Cajal bodies inside the nucleus, the new method revealed intricate internal structures that were previously blurry or invisible. They could see how the RNA was organized within these compartments, showing distinct patterns that other imaging techniques missed.
The versatility of this approach was further demonstrated by applying it to proteins on the surface of cells. By attaching the RNA-binding tag to a different molecule that recognizes a specific protein, the researchers could use the same lifetime trick to image cell-surface proteins. They found that dyes which were too permeable for RNA imaging worked perfectly here because they stayed outside the cell, binding only to the surface proteins. This showed that the method could be adapted for different biological targets by simply changing the recognition part of the tag while keeping the powerful lifetime-detection system the same.
The study confirms that measuring the duration of light, rather than just its brightness, offers a more faithful way to observe the molecular world inside living cells. By distinguishing between the RNA-bound dye and the free-floating dye, the researchers have removed a major source of uncertainty in cellular imaging. This allows scientists to see RNA and proteins with higher fidelity and sharper detail, all while using less light to protect the delicate living systems they are studying. The work establishes a new platform where the physical properties of light itself become a tool for sorting signal from noise, opening the door to observing the dynamic life of cells with unprecedented clarity.
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