ATPLyzer An advanced ratiometric multi-colour biosensor for long-term monitoring of ATP dynamics

This paper introduces ATPLyzer, a robust, genetically encoded, ratiometric multi-colour biosensor with a unique matryoshka design that enables minimally invasive, long-term, and single-wavelength excitation monitoring of intracellular ATP dynamics in living cells.

The Gist

Imagine your body is a bustling city. In this city, ATP (Adenosine Triphosphate) is the electricity that keeps the lights on, the trains running, and the factories working. Without it, the city goes dark, and life stops.

Scientists have long wanted a way to watch this "electricity" in real-time inside living cells to understand how the city runs, why it sometimes fails (like in diseases), and how it recovers. However, the tools they had before were like old, flickering flashlights: they were hard to read, got tired quickly (photobleached), and often gave confusing numbers because they depended on how bright the flashlight was held, not just the light itself.

This paper introduces a new, super-smart tool called ATPLyzer. Think of it as a self-calibrating, dual-lens smart camera that can take a perfect photo of the city's power grid, no matter how the camera moves or how the lighting changes.

Here is a simple breakdown of how it works and what the scientists found:

1. The "Matryoshka" Design (The Russian Doll)

The biggest innovation is how ATPLyzer is built. The scientists used a design they call "Matryoshka," named after those famous Russian nesting dolls.

• The Outer Doll (The Reporter): This is a glowing green protein that changes its brightness when it grabs an ATP molecule. It's like a mood ring that changes color based on how much "power" is around.
• The Inner Doll (The Reference): Hidden inside the outer doll is a second, different glowing protein (orange or red). This one is a "stubborn" doll; it glows at a constant level no matter what happens. It doesn't care about ATP.

Why is this cool?
Imagine you are trying to measure the volume of a song, but your speaker volume keeps turning up and down randomly. It's hard to tell if the song is getting louder or if the volume knob just moved.

• Old Tools: Only had the "song" (the ATP sensor). If the speaker volume changed, you couldn't trust the reading.
• ATPLyzer: Has both the "song" and a "metronome" (the reference). By comparing the song's volume to the metronome's steady beat, the computer can instantly calculate the true volume, ignoring any glitches in the speaker. This makes the measurement incredibly accurate and stable over long periods.

2. Single-Lens Magic

Old tools often needed two different colored lights (like a red flashlight and a blue flashlight) to work, which is like trying to juggle two flashlights at once. ATPLyzer is special because it can be lit up by just one single color of light. This is easier on the delicate living cells (less "energy intake") and allows scientists to watch the cells for much longer without hurting them.

3. The "Lock and Key" Mechanism

The sensor uses a tiny piece of a bacterial machine (an enzyme) as its "lock."

• The Key: ATP is the key.
• The Lock: When ATP fits into the lock, the whole structure of the sensor snaps shut, changing the shape of the green "mood ring."
• The Result: The green light dims. The more ATP there is, the dimmer the green light gets (relative to the steady orange/red light).
• Specificity: The scientists tested this with ADP (a "broken" key that looks similar but doesn't work). The sensor ignored the broken key completely, proving it only cares about the real ATP.

4. Testing in the "Wild" (Inside Bacteria)

The team tested ATPLyzer inside E. coli bacteria (tiny single-celled organisms). They treated the bacteria like a city going through different seasons:

• Starvation: They took away the food (carbon source). The sensor showed the bacteria's "power grid" dropping, just as expected.
• Feast: They added sugar (glucose). The sensor showed a rapid spike in power as the bacteria started eating and growing.
• Sabotage: They added a chemical (CCCP) that breaks the power plant. The sensor immediately detected the crash in energy.

Crucially, the sensor worked perfectly even when the bacteria were growing, dividing, and changing size. It could tell the difference between a cell that was just "dim" because it was small and a cell that was actually "low on power."

The Big Takeaway

ATPLyzer is a robust, long-lasting, and highly accurate tool for watching how living cells manage their energy.

• Before: Scientists were looking at a blurry, flickering image of cell energy.
• Now: They have a high-definition, self-correcting video camera.

This tool will help researchers understand everything from how cancer cells steal energy to how neurons fire in the brain, all by giving them a clear, reliable window into the cell's power supply. It's a major step forward in making the invisible world of cellular energy visible and understandable.

Technical Summary

1. Problem Statement

Adenosine triphosphate (ATP) is the primary energy currency of the cell, and its dynamics are critical for understanding metabolism, signaling, and pathological states (e.g., cancer, neurodegeneration). While genetically encoded biosensors exist to monitor ATP, current technologies face significant limitations:

• FRET-based sensors (e.g., ATeam): Require dual fluorophores and dual excitation wavelengths. This introduces complexity, spectral overlap issues, higher susceptibility to photobleaching, and slower response kinetics due to the need for sequential imaging.
• Single-FP intensity sensors (e.g., Perceval, ATPQueen): While they offer high sensitivity and fast kinetics, they rely on single-wavelength excitation and intensity changes. This makes them highly susceptible to artifacts caused by variations in protein expression levels, cell thickness, and photobleaching, complicating quantitative ratiometric analysis.
• Lack of Robust Referencing: There is a need for a biosensor that combines the high sensitivity and fast kinetics of single-FP sensors with the robustness of ratiometric measurements, without the complexity of FRET.

2. Methodology

The authors developed ATPLyzer, a novel genetically encoded biosensor based on a "Matryoshka" (nesting doll) design.

• Molecular Design:
  • Core Mechanism: The sensor utilizes the ATP-binding $\epsilon$-subunit of FoF1-ATP synthase (from Bacillus subtilis and PS3) as the recognition element.
  • Matryoshka Architecture: A circularly permuted Green Fluorescent Protein (cpGFP) acts as the responsive reporter. A Large Stokes Shift (LSS) reference fluorophore (either LSSmOrange or LSSmApple) is embedded within the synthetic linker region of the cpGFP.
  • Optical Advantage: The LSS reference fluorophore has an excitation spectrum that overlaps with the cpGFP but emits at a significantly different wavelength (~100 nm separation). This allows single-wavelength excitation (460 nm for GO, 453 nm for GA) to excite both fluorophores simultaneously, enabling true ratiometric readouts without sequential scanning.
• Variants Created:
  • Affinity Tuning: Variants were engineered with different dissociation constants ($K_D$) to cover micromolar (ATPLyzer 7$\mu$) and millimolar (ATPLyzer 1.7 m) ATP ranges.
  • Controls: Non-responsive (n.r.) mutants were created to validate specificity.
• Characterization Techniques:
  • In Vitro: Purified proteins were subjected to fluorimetric titrations with ATP and ADP to determine $K_D$, specificity, and dynamic range.
  • Structural Analysis: Small-Angle X-ray Scattering (SAXS) coupled with Size-Exclusion Chromatography (SEC-SAXS) was used to determine the solution structure of the apo and ATP-bound states. Molecular docking and AlphaFold3 modeling were employed to understand the conformational changes.
  • In Vivo: The biosensors were expressed in Escherichia coli (XL1-Blue). Cultures were subjected to starvation, glucose supplementation, and treatment with the protonophore CCCP (to dissipate the proton motive force and inhibit ATP synthesis) while monitoring fluorescence ratios in a plate reader.

3. Key Contributions

• Novel Matryoshka Design: Successfully adapted the Matryoshka concept to ATP sensing, creating the first ratiometric ATP biosensor that uses a single excitation wavelength.
• Multi-Color Capability: Developed two distinct color variants (Green-Orange and Green-Apple), allowing for multiplexing and optimization of signal-to-noise ratios for different experimental setups.
• Overcoming FRET Limitations: Demonstrated that a single-excitation, dual-emission ratiometric sensor can achieve the robustness of FRET sensors while retaining the fast kinetics and simplicity of single-FP sensors.
• Structural Insight: Provided experimental SAXS data and structural models elucidating how ATP binding induces a conformational change in the $\epsilon$-subunit (trapping helices in a compact state) that modulates the cpGFP fluorescence, explaining the mechanism of action and ADP insensitivity.

4. Key Results

• Spectroscopic Performance:
  • ATPLyzer exhibits an inverse sensing mode: ATP binding causes a decrease in cpGFP fluorescence intensity, while the LSS reference fluorescence remains constant.
  • Specificity: The sensors show high specificity for ATP over ADP. No significant fluorescence change was observed upon ADP titration.
  • Affinity:
    • ATPLyzer 7$\mu$: $K_D \approx 7$–11 $\mu$M (suitable for physiological intracellular ATP levels).
    • ATPLyzer 1.7 m: $K_D \approx 1.7$ mM (suitable for high-concentration environments).
• Structural Findings:
  • SAXS data revealed that the biosensor is monomeric in solution.
  • In the apo state, the $\epsilon$-subunit helices are relaxed and flexible.
  • Upon ATP binding, the helices adopt a compact, static conformation.
  • Molecular docking suggests that ATP bridges positively charged arginine residues (R92, R99, R122) on the helices, a mechanism that ADP cannot replicate due to charge distribution differences, explaining the lack of ADP response.
• In Vivo Dynamics in E. coli:
  • The biosensor functioned robustly at 37°C without affecting bacterial growth.
  • Starvation: A transient increase in the ratiometric signal (indicating ATP drop) was observed upon carbon starvation.
  • Glucose Rescue: Upon glucose addition, the ratio decreased (indicating ATP rise) as cells resumed growth.
  • CCCP Inhibition: Addition of CCCP caused a rapid, reversible shift in the ratio, confirming the sensor's ability to detect acute disruption of ATP synthesis.
  • Controls: Wild-type and non-responsive variants showed no specific response to metabolic perturbations, validating that the signal is ATP-dependent.

5. Significance

ATPLyzer represents a significant advancement in metabolic imaging tools. By combining ratiometric accuracy (normalizing for expression levels and photobleaching) with single-wavelength excitation (simplifying imaging and doubling time resolution compared to FRET), it offers a superior tool for long-term, minimally invasive monitoring of ATP dynamics.

Its ability to function across a wide range of ATP concentrations (via tunable affinity variants) and its proven stability in live bacterial cells make it a versatile platform for studying metabolic flux, cellular stress responses, and energy homeostasis in health and disease. The multi-color capability further expands its utility for complex, multi-parameter imaging experiments.