Quantitative comparison of fluorescent reporters by FCS excitation scan

This paper introduces and validates a fluorescence correlation spectroscopy (FCS) based excitation scan assay to quantitatively compare the brightness and photostability of various fluorescent reporters in living systems, demonstrating that mNeonGreen outperforms mEGFP and providing a broadly applicable method for optimizing fluorescent tag selection.

The Gist

Imagine you are a photographer trying to take the perfect picture of a tiny, moving firefly inside a dark, foggy forest. To do this, you need a flashlight (your microscope) and a camera (your detector). But here's the problem: not all fireflies glow with the same intensity, and not all flashlights work well in the fog. Some fireflies might burn out if you shine the light too brightly, and some might be so dim that you can't see them at all without blinding the rest of the forest.

This paper is essentially a "Firefly Report Card" for scientists who study living cells. The authors, Falk Schneider and his team, wanted to figure out which "glowing tags" (fluorescent proteins) are the best for taking clear, bright pictures of living things without hurting them or burning out the tag.

Here is the breakdown of their study using simple analogies:

1. The Problem: Choosing the Right "Glow Stick"

Scientists use special glowing proteins to tag molecules inside cells so they can watch them move. Think of these proteins as glow sticks attached to tiny toys.

• The Challenge: There are hundreds of different colors and types of glow sticks. Some are super bright but burn out quickly. Some are dim but last forever. Some work great in a clear room (a petri dish) but get lost in the fog (inside a living animal).
• The Old Way: Scientists used to guess which glow stick was best based on how bright it looked in a test tube. But a glow stick in a test tube behaves differently than one inside a messy, crowded, warm cell. It's like testing a car engine in a garage versus driving it up a steep mountain; the performance changes.

2. The Solution: The "FCS Excitation Scan" (The Brightness Test)

The team developed a new, smarter way to test these glow sticks. They call it an FCS Excitation Scan.

• The Analogy: Imagine you have a dimmer switch on a light bulb. You slowly turn the light up from "off" to "blindingly bright."
  • The Test: They shine a laser (the dimmer switch) on the glowing tag at different power levels.
  • The Measurement: They count how many "photons" (particles of light) come out for every single molecule.
  • The Goal: They want to find the "Sweet Spot." This is the point where the tag is bright enough to give a clear picture, but the laser isn't so strong that it burns the tag out or hurts the living cell.

They call this the "Usable Brightness." It's not about how bright the tag could be if you blasted it with maximum power (which would destroy it); it's about how bright it is when you use just the right amount of power to get a good photo without causing damage.

3. The Results: Who Won the Race?

The team tested 10 different fluorescent proteins (the "glow sticks") in two different environments:

1. In a Petri Dish (HEK Cells): Like a clear, controlled room.
2. Inside a Baby Zebrafish: Like a deep, foggy forest.

The Winners:

• The Green Champion: mNeonGreen was the clear winner. It was significantly brighter than the old standard (mEGFP) in both the petri dish and the zebrafish. It's like finding a glow stick that is twice as bright as the others but doesn't burn out any faster.
• The New Contenders: They also tested "StayGold" variants. These are like indestructible glow sticks. They aren't necessarily the absolute brightest, but they don't fade away when you shine the light on them for a long time. They are perfect for long movies of cells.
• The "Chemigenetic" Tags: These are glow sticks where you can swap the bulb. The team found that some of these (like HALO and SNAP tags with specific dyes) were incredibly bright, even brighter than the best protein tags. However, they were a bit trickier to use because sometimes unattached dye floated around and confused the camera.

4. The "Fog" Factor (Depth)

One of the coolest parts of the study was testing how deep they could see inside the zebrafish.

• The Analogy: If you try to see a blue light through thick fog, it gets scattered and disappears quickly. A red light, however, cuts through the fog better.
• The Finding: Even though the "Red" glow sticks cut through the fog better, the "Green" champion (mNeonGreen) was still so incredibly bright at the start that it remained visible deeper in the fish than the red ones, despite the fog.
• Lesson: If you start with a super-bright source, you can see further, even if the light scatters a bit.

5. Why This Matters

Before this paper, scientists were often guessing which glow stick to use. They might pick a dim one and wonder why their pictures were grainy, or pick a bright one that killed the cell.

This paper provides a standardized rulebook. It tells scientists:

• "If you are looking at something in a petri dish, use mNeonGreen."
• "If you are filming a long movie of a cell, use StayGold."
• "If you need to see deep inside a tissue, start with the brightest option available."

The Bottom Line

The authors built a "test drive" for fluorescent proteins. Instead of just looking at the specs on the box, they actually drove the cars (proteins) on the real roads (living cells and fish) to see which ones handle the terrain best. Their conclusion? mNeonGreen is currently the best all-around car for the job, but StayGold is the best for long road trips.

This helps researchers stop wasting time and money on the wrong tools and start getting clearer, better pictures of life in action.

Technical Summary

1. Problem Statement

Selecting the optimal fluorescent reporter (fluorescent proteins or chemigenetic tags) is critical for quantitative microscopy, particularly for Fluorescence Fluctuation Spectroscopy (FCS) and single-molecule imaging. While parameters like photostability and photo-toxicity are relatively easy to assess, fluorescent brightness (photons per molecule per time) is elusive to measure accurately in living biological environments.

• Limitations of current methods: Traditional brightness assessments rely on in vitro spectrophotometry (extinction coefficients and quantum yields) or relative intensity comparisons in polycistronic expression systems. These often fail to account for complex biological variables (pH, salinity, cellular milieu) and detector wavelength efficiencies.
• The Gap: There is a lack of objective, quantitative methods to compare the "usable" brightness of diverse reporters (FPs, organic dyes, chemigenetic tags) under biologically relevant conditions (e.g., in vivo, at specific temperatures, and at imaging depths).

2. Methodology

The authors developed and validated an FCS Excitation Scan assay to quantitatively determine reporter performance.

• Core Principle: The method involves recording FCS data at a single focal spot while systematically varying the incident laser power.
  • Data Acquisition: Fluorescence intensity fluctuations are recorded for fluorophores diffusing through the confocal observation volume.
  • Metrics Extracted: From the autocorrelation curves, two key parameters are derived:
    1. Molecular Brightness (cpm): Counts per molecule, derived from the correlation amplitude and intensity moments.
    2. Transit Time: The average time a molecule takes to traverse the observation volume.
• The "Excitation Scan" Analysis:
  • Saturation Model: Brightness (cpm) vs. Laser Power is fitted to a saturation curve.
  • Linear Model: Transit time vs. Laser Power is fitted to a linear model to detect deviations caused by photobleaching (decreasing transit time) or optical saturation (increasing transit time).
  • Definition of "Usable Brightness": The authors define a robust metric for "usable brightness" as the point where the system operates in the linear regime, specifically at 10% of the inflection point of the saturation curve. If photobleaching occurs before this point, the limit is defined where transit time drops by <20%.
• Experimental Systems:
  • In Vitro: Aqueous solutions of standard dyes (Alexa Fluor 488, Rhodamine B, Atto655) at 28°C and 37°C.
  • Cell Culture: HEK293T cells expressing Nuclear Localization Signal (NLS)-tagged reporters.
  • In Vivo: Zebrafish embryos (~20 hpf) with NLS-tagged reporters in the hindbrain (imaging depth ~60 µm).
• Reporters Tested:
  • 10 Fluorescent Proteins (FPs) across the spectrum (e.g., mNeonGreen, mEGFP, mCherry, miRFP670nano3, StayGold variants).
  • Chemigenetic tags: HALO and SNAP fused to organic dyes (Janelia Fluor 646, SNAP-Cell 505-Star, etc.).
  • StayGold variants (mSG, mSG2, mBaoJin, SG-E138D).

3. Key Contributions

1. Novel Quantitative Pipeline: Introduction of an FCS-based excitation scan that determines "usable brightness" and "usable laser power" directly in living systems, bypassing the need for internal standards.
2. Standardized Comparison Framework: A unified method to compare FPs, organic dyes, and chemigenetic reporters on the same instrument under identical conditions.
3. Definition of "Usable" vs. "Maximum" Brightness: Distinguishing between the theoretical maximum brightness (saturated regime) and the practical brightness achievable in the linear regime without inducing photobleaching or saturation artifacts.
4. Validation Across Models: Demonstrating that performance in HEK293T cells correlates strongly with performance in zebrafish embryos (excluding blue-shifted FPs due to tissue scattering), suggesting cell culture can serve as a reliable proxy for in vivo screening.

4. Key Results

• Green FPs: mNeonGreen significantly outperformed mEGFP in both HEK cells and zebrafish embryos.
  • In embryos: mNeonGreen was 2.3x brighter than mEGFP.
  • In HEK cells: mNeonGreen was 1.8x brighter than mEGFP.
  • Crucially, mNeonGreen achieved this higher signal-to-noise ratio (SNR) at similar laser powers, reducing the light dose required for experiments.
• Blue FPs: mCerulean3 outperformed mTurquoise2 but required higher laser power. Blue FPs showed significantly reduced performance in zebrafish embryos due to tissue absorption and scattering.
• Red/Orange FPs:
  • miRFP670nano3 showed the highest brightness in the red regime, though it required higher light doses.
  • Despite higher scattering for red light, mNeonGreen (green) still provided higher absolute photon counts at depths of 60–70 µm compared to the red-shifted miRFP670nano3, indicating that initial brightness is the dominant factor for deep-tissue FCS.
• Chemigenetic Tags:
  • Organic dyes (e.g., Janelia Fluor 646, SNAP-Cell 505-Star) generally exhibited higher usable brightness than FPs.
  • NLS-HALO-JF646 and NLS-fSNAP-647SiR were brighter than miRFP670nano3 and required less excitation power.
  • NLS-fSNAP-505-Star was even brighter than mNeonGreen in HEK cells.
• StayGold Variants:
  • mSG and mSG2 demonstrated exceptional performance, with usable brightness values (~14.9 kHz and ~14.6 kHz) exceeding both standard FPs and organic dyes tested.
  • They exhibited superior photostability; unlike mNeonGreen, their transit time did not decrease with increasing laser power, indicating minimal photobleaching during the scan.
• Temperature Effects: A decrease in usable brightness was observed for all dyes when moving from 28°C to 37°C, highlighting the necessity of characterizing probes at the specific experimental temperature.

5. Significance

• Experimental Design: The study provides a data-driven resource for researchers to select the optimal fluorophore for specific applications (e.g., deep-tissue imaging vs. high-speed dynamics), moving beyond literature values which may not reflect in vivo conditions.
• Quantitative Rigor: By defining "usable brightness," the method ensures that quantitative measurements (like diffusion coefficients) are performed in the linear regime, avoiding artifacts from saturation or bleaching.
• Broad Applicability: The pipeline is applicable to any FCS-capable microscope and can be extended to new fluorophores as they are developed.
• Biological Insight: The findings confirm that while red-shifted probes reduce scattering, the superior intrinsic brightness of probes like mNeonGreen and StayGold variants often outweighs scattering losses in deep-tissue FCS, provided the probe is sufficiently bright to begin with.

In conclusion, this paper establishes a robust, quantitative framework for evaluating fluorescent reporters, identifying mNeonGreen and StayGold variants (mSG/mSG2) as top performers for live-cell and in vivo FCS applications, and validating cell culture as a predictive model for in vivo performance.