iG6PSnFR: A genetically encoded fluorescent sensor for observing glucose-6-phosphate dynamics in living preparations

Inside every living cell, a complex network of chemical reactions constantly converts food into the energy required to keep life moving. At the very start of this process lies a molecule called glucose-6-phosphate. It acts as a central hub, a critical junction where a cell decides whether to burn sugar for immediate power, store it for later, or use it to build new structures. For decades, scientists have struggled to watch this molecule in action. Traditional methods required freezing cells or breaking them apart to take a snapshot, which meant they could never see how the levels of this vital fuel changed in real time as a cell responded to its environment. Without a way to observe these dynamics, the precise moment a cell shifts its metabolic gears remained a mystery.

To solve this, researchers have developed a new tool that allows them to watch glucose-6-phosphate move and change within living organisms. They created a genetically encoded sensor, a tiny protein that can be programmed into the DNA of a cell. When this sensor encounters glucose-6-phosphate, it changes its brightness, glowing either brighter or dimmer depending on the specific version used. This allows scientists to see the invisible chemical shifts as they happen, turning the cell's internal chemistry into a visible light show that can be filmed and measured.

The team behind this work, led by scientists at the Howard Hughes Medical Institute and Yale School of Medicine, designed two distinct versions of this sensor. One version, which they named iG6PSnFR, lights up when it binds to glucose-6-phosphate, signaling an increase in the molecule's presence. The other version, called iG6PSnFR-D, does the opposite; it dims when it encounters the molecule. By creating both an "on" switch and an "off" switch, the researchers ensured they could detect changes in concentration regardless of whether the levels were rising or falling. They built these sensors by taking a protein that naturally grabs onto glucose-6-phosphate and inserting a light-emitting protein into its structure. When the binding protein snaps shut around the target molecule, it twists the light-emitting part, changing how much light it gives off.

The researchers first tested these sensors in a controlled laboratory setting using purified proteins. They confirmed that the sensors reacted specifically to glucose-6-phosphate and ignored other similar sugar molecules that might be present in a cell. They also measured how quickly the sensors responded, finding that they could detect changes in mere milliseconds, fast enough to keep up with the rapid pace of cellular metabolism. Crucially, they discovered that the sensors were not easily confused by changes in the levels of inorganic phosphate, a common chemical in cells that often interferes with other types of measurements. This specificity meant the sensors could provide a clear, accurate reading of glucose-6-phosphate levels even in the crowded chemical environment of a living cell.

With the sensors validated in the lab, the team moved to living cells to see how they performed in a real biological context. They introduced the sensors into human cells grown in a dish and then subjected those cells to various metabolic challenges. When they blocked the cells from taking in sugar, the sensors showed a rapid drop in glucose-6-phosphate levels, exactly as expected. When they added a drug that stops the cells from processing sugar, the sensors again showed a quick decline. Conversely, when they reintroduced sugar to cells that had been starved, the sensors lit up, showing that the cells were successfully converting the sugar into glucose-6-phosphate. These experiments proved that the sensors could track the ebb and flow of this molecule as the cells reacted to their environment.

The researchers then took the work a step further, testing the sensors in neurons, the specialized cells that make up the nervous system. Neurons are known to be highly active and require a constant supply of energy. When the team stimulated these neurons with electrical signals, the sensors revealed an immediate drop in glucose-6-phosphate levels, indicating that the cells were rapidly using up their fuel reserves to power the electrical activity. This drop was followed by a recovery as the cells replenished their supply. By watching these changes in real time, the team could see the direct link between neuronal activity and the consumption of metabolic fuel, a process that happens too quickly to be seen with older methods.

To see if this technology worked in a whole living animal, the researchers turned to the tiny roundworm C. elegans. They engineered the worms to produce the sensor in specific sensory neurons that respond to changes in salt concentration. When the worms were exposed to a sudden increase in salt, the sensors in the active neurons showed a decrease in glucose-6-phosphate, while the sensors in the inactive neurons showed an increase. This demonstrated that even in a complex, moving animal, the sensors could distinguish between different metabolic states in different parts of the nervous system. The results confirmed that the tool could reveal how metabolism shifts in response to sensory input in a living organism.

The team also applied the sensors to pancreatic islets, clusters of cells in the pancreas that are responsible for releasing insulin. These cells must sense sugar levels in the blood and respond by releasing insulin to help the body use that sugar. Using the sensors, the researchers watched what happened when they increased the sugar concentration around these cells. They saw that the levels of glucose-6-phosphate rose quickly, followed by a rise in other metabolic signals and a surge in calcium, which triggers the release of insulin. This sequence of events, captured in real time, provided a clear picture of how the cells sense sugar and translate that signal into a hormonal response.

Finally, the researchers looked at the liver, a large organ that plays a major role in managing the body's sugar and energy stores. The liver is not uniform; cells near the blood vessels that bring fresh oxygen-rich blood behave differently than cells near the center of the organ. By injecting the sensors into the livers of live mice, the team was able to map the distribution of glucose-6-phosphate across the organ. They found that cells near the center of the liver, which are more involved in storing energy, maintained higher levels of glucose-6-phosphate than cells near the blood vessels, which are more focused on breaking down sugar for energy. This spatial map, visible for the first time, showed that metabolic activity varies significantly across different regions of the same organ.

The development of these sensors marks a significant step forward in the ability to observe cellular metabolism. By providing a way to watch glucose-6-phosphate change in real time, in living cells and whole animals, the researchers have opened a new window into how cells manage their energy. The ability to see these processes as they happen, rather than just inferring them from static snapshots, offers a clearer understanding of how cells make decisions about fuel use. This tool allows scientists to observe the immediate consequences of metabolic shifts, whether in a neuron firing an electrical signal, a pancreatic cell releasing insulin, or a liver cell managing energy storage. As these sensors are refined and applied to new questions, they promise to reveal the dynamic nature of metabolism in health and disease, turning the invisible chemistry of life into a visible, understandable story.