SurpHer: a genetically encoded ratiometric sensor for dynamic extracellular pH imaging

The authors developed SurpHer, a genetically encoded, ratiometric biosensor that enables real-time, dynamic imaging of extracellular pH gradients at the surface of individual living cells, particularly for studying tumor microenvironments.

The Gist

Imagine a cell as a busy house. While we often focus on what's happening inside the house (the cell's interior), the air right outside the front door—the extracellular environment—is just as important. One specific thing about this "outside air" is its acidity, or pH. Just like how the weather outside can change from sunny to stormy, the acidity around a cell can shift rapidly, and these changes tell the cell how to behave, especially when it comes to diseases like cancer.

The problem is that scientists didn't have a good way to "see" these rapid changes happening right at the cell's surface in real-time. They needed a tool that could act like a weather station, but one that could stick to the cell and report the acidity levels instantly.

Enter "SurpHer."

Think of SurpHer as a high-tech, two-color badge that scientists designed to stick onto the outside of a cell. Here is how it works:

1. The Two Colors: The badge is made of two parts. One part is a "smart" light that changes its color depending on how acidic the air is (like a mood ring that turns different colors based on the temperature). The other part is a "steady" light that never changes, no matter what the weather is like.
2. The Comparison: By comparing the color of the "smart" light to the "steady" light, scientists can get a precise reading of the acidity. This is called a "ratiometric" sensor, which is just a fancy way of saying it uses a built-in reference to make sure the reading is accurate, even if the light gets dim or the camera moves.
3. The Design Hunt: The scientists didn't just guess the design; they built a massive library of different "badge" combinations and tested them one by one. They were looking for the perfect fit that would stick firmly to the cell's surface and work well in the pH range where most biological action happens (between 6 and 7.8).
4. The Winner: They found the perfect match: a combination they named SurpHer. It sticks reliably to the outside of various human cells (like HEK293T, PANC-1, and MDA-MB231) and instantly flashes different color ratios as the acidity changes.

What did they do with it?

The researchers took cells that had this SurpHer badge permanently attached and put them in a special, tiny water-flow system (a microfluidic platform) designed to mimic the tricky environment inside a tumor. Because SurpHer is so sensitive, they were able to watch a "time-lapse movie" of the acidity gradients forming around the tumor cells.

In short:
The paper introduces SurpHer, a custom-built, glowing sensor that acts like a real-time acidity meter for the outside of living cells. It allows scientists to watch and measure how the "weather" (pH) changes right at the doorstep of cells, specifically helping them understand the acidic conditions found in tumor environments.

Technical Summary

Technical Summary: SurpHer – A Genetically Encoded Ratiometric Sensor for Dynamic Extracellular pH Imaging

Problem Statement
Extracellular pH ($pH_e$) is a critical microenvironmental factor that significantly influences cell physiology and disease progression. Despite its importance, there is a distinct lack of quantitative biosensors capable of capturing dynamic changes in $pH_e$ specifically at the surface of individual living cells. Existing tools often fail to provide the necessary spatial resolution, dynamic range, or ratiometric stability required for precise interrogation of the pericellular environment in diverse biological contexts.

Methodology
To address this gap, the authors developed a genetically encoded, ratiometric extracellular pH biosensor through a systematic screening process. The core strategy involved constructing a modular library of membrane-display designs. These constructs fused SEpHluorin, a pH-sensitive fluorophore, with a pH-stable reference fluorophore to enable ratiometric measurements that correct for variations in expression levels and optical path length.

The screening process focused on identifying optimal configurations for cell-surface localization. Through this systematic evaluation, the authors identified a specific construct: a fusion of mKate2 (the reference fluorophore) and SEpHluorin, designated as SurpHer. This construct was designed to anchor to the cell membrane, ensuring that the pH sensing occurs strictly in the extracellular space.

Key Contributions and Results
The primary contribution of this work is the successful development and validation of SurpHer. The key findings include:

• Ratiometric Performance: SurpHer exhibits robust dynamic ratiometric responses across a physiologically relevant extracellular pH range of 6.0 to 7.8.
• Localization and Stability: The sensor demonstrates reliable membrane localization, ensuring that the signal originates from the cell surface rather than intracellular compartments.
• Versatility: The sensor was validated across diverse human cell types, including HEK293T, PANC-1, and MDA-MB-231 cells, showing consistent extracellular pH responsiveness in each.
• Functional Imaging: In a specific application, MDA-MB-231 cells were stably integrated with SurpHer. These cells were utilized in a microfluidic platform designed to model tumor microenvironments. This setup enabled time-course imaging of $pH_e$ gradients, demonstrating the sensor's capability to track dynamic pH changes in real-time.

Significance
The paper claims that SurpHer provides a practical tool for the real-time interrogation of the pericellular pH environment of tumor cells. Beyond its specific application in cancer research, the work establishes a generalizable strategy for probing microenvironmental pH dynamics. By offering a genetically encoded, ratiometric solution, SurpHer enables researchers to quantitatively assess how extracellular pH fluctuates across diverse biological contexts, thereby facilitating a deeper understanding of the role of pH in cell physiology and disease mechanisms.