Atto 643 Carboxy Selectively Labels Astrocytes with Minimal Oligodendrocyte Cross-Reactivity

This study identifies Atto 643 carboxy as a highly specific far-red fluorescent probe that selectively labels astrocytes in both in vivo and acute brain slice preparations while minimizing the oligodendrocyte and neuronal cross-reactivity commonly observed with the widely used dye sulforhodamine 101 (SR101).

The Gist

Imagine the brain as a bustling, high-tech city. In this city, there are three main types of workers: Neurons (the electricians who send messages), Oligodendrocytes (the construction crew that wraps wires in insulation), and Astrocytes (the janitors and support staff who keep the streets clean, manage traffic, and feed the workers).

For a long time, scientists trying to study the "janitors" (astrocytes) had a problem. They used a special glowing spray paint called SR101 to tag them. But this paint was messy. It didn't just stick to the janitors; it also accidentally tagged the construction crew (oligodendrocytes) and even some of the electricians (neurons). It was like trying to take a photo of a specific group of people at a concert, but the camera flash was so bright it blinded everyone else in the crowd, making it impossible to tell who was who.

The New Solution: A Precision Laser Pointer

In this new study, researchers found a new, smarter spray paint called Atto 643 carboxy.

Think of SR101 as a spray can of glitter. It covers everything in a wide area, looking cool but making a huge mess.
Atto 643 carboxy is more like a high-tech, laser-guided marker. It only sticks to the specific "janitors" (astrocytes) and ignores everyone else.

How They Tested It

The researchers did a few clever experiments to prove their new marker was better:

1. The "ID Check" Test: They used mice that were genetically programmed to wear a green vest if they were astrocytes. When they sprayed the old paint (SR101), many cells glowed red but didn't have the green vest (meaning they weren't astrocytes). But when they used the new paint (Atto 643), almost every red cell also had the green vest. It was a perfect match.
2. The "Construction Site" Test: They looked at the insulation workers (oligodendrocytes). The old paint stuck to them about 35% of the time. The new paint? It barely touched them (less than 3%).
3. The "Insulation" Test: Sometimes the old paint would leak onto the insulation itself (myelin sheaths), making the wires look like they were glowing. The new paint stayed strictly on the cells and never leaked onto the insulation.
4. The "Deep Dive" Test: They used the new paint to look deep inside the brain while the mice were still alive. The new paint glowed brightly even 700 microns deep (like looking through a thick fog), allowing scientists to see the intricate, star-shaped branches of the astrocytes clearly.

Why Does It Work?

The researchers discovered that the new paint works because it uses a specific "door" on the astrocytes to get inside. This door is called OATP1C1.

• Analogy: Imagine the astrocytes have a special keyhole. The old paint (SR101) was a master key that could jam into many different locks (astrocytes, oligodendrocytes, etc.). The new paint (Atto 643) is a custom-made key that only fits the astrocyte's keyhole. If you block that keyhole with a piece of gum (a chemical inhibitor), the paint can't get in, proving it uses that specific door.

Why Does This Matter?

This is a big deal for brain science. Because the new paint is so precise, scientists can now:

• Study how astrocytes help the brain think and heal without worrying about "noise" from other cells.
• Watch how these cells move and change shape in real-time, like watching a live movie instead of a blurry photo.
• Combine this paint with other colors to see how astrocytes talk to neurons and blood vessels simultaneously.

In short: The researchers found a "magic marker" that finally lets us see the brain's support staff clearly, without the messy background noise that confused us for years. This helps us understand how the brain's city actually runs.

Technical Summary

1. Problem Statement

Astrocytes are critical regulators of brain homeostasis, yet studying them requires highly specific labeling tools. The current gold standard for chemical labeling of astrocytes is Sulforhodamine 101 (SR101). While SR101 is widely used due to its ease of application and compatibility with live imaging, it suffers from significant limitations:

• Off-target labeling: It labels 20–40% of oligodendrocytes in cortical layers, likely due to dye leakage via gap junctions.
• Non-specific uptake: It can infiltrate myelin sheaths after prolonged incubation and enters neurons via synaptic activity-dependent endocytosis.
• Regional limitations: It non-specifically labels hippocampal pyramidal neurons.

These cross-reactivities complicate the interpretation of astrocyte morphology, dynamics, and function, particularly in studies involving tripartite synapses or neurovascular coupling. There is an unmet need for a small-molecule probe that offers high specificity for astrocytes without these confounding factors.

2. Methodology

The authors employed a combination of chemical synthesis, stereotaxic surgery, transgenic mouse models, viral vectors, and advanced microscopy to evaluate a new far-red fluorescent dye, Atto 643 carboxy.

• Chemical Screening: The team tested various functionalized forms of Atto 643 (carboxy, azide, biotin) and the structurally related Atto 647N carboxy to identify the specific moiety required for astrocyte uptake.
• Animal Models:
  • Wild-type C57BL/6 mice: For initial screening and in vivo imaging.
  • Transgenic Mice:
    • Aldh1l1-EGFP: To identify astrocytes.
    • Tmem119-EGFP: To identify microglia.
    • NG2-DsRed: To identify NG2 glia and pericytes.
    • AAV9-hSyn-EGFP: To label neurons specifically.
  • Viral Labeling: Olig001-CBh-GFP virus was used to selectively label oligodendrocytes for cross-reactivity testing.
• Surgical & Imaging Protocols:
  • Intracerebral Injection: Dyes (50 µM) were injected into the caudate-putamen (CPu), hippocampus, cortex, and brainstem using stereotaxic coordinates.
  • Acute Brain Slices: Prepared 30 minutes post-injection for two-photon microscopy.
  • In Vivo Imaging: Performed through cranial windows in the barrel cortex using head-fixed two-photon microscopy.
  • Mechanism Study: Co-injection of L-thyroxine (T4), an OATP1C1 transporter inhibitor, was used to test the uptake mechanism.
  • Myelin Visualization: Third Harmonic Generation (THG) imaging was used as a dye-free method to visualize myelin sheaths and compare dye localization.

3. Key Contributions

• Discovery of a Specific Probe: Identification of Atto 643 carboxy as a far-red fluorescent dye that selectively labels astrocytes with minimal off-target effects.
• Structural Specificity: Demonstration that the carboxy functional group is essential for astrocyte uptake; other functional groups (azide, biotin) or the parent Atto 647N did not label astrocytes.
• Mechanistic Insight: Evidence suggesting uptake is mediated by the organic anion-transporting polypeptide 1C1 (OATP1C1), similar to SR101 but with improved specificity.
• Superior Performance: Comprehensive quantitative comparison showing Atto 643 carboxy outperforms SR101 in specificity across multiple cell types and brain regions.

4. Key Results

A. Astrocyte Specificity and Morphology

• Atto 643 carboxy robustly labeled cells with classic astrocytic morphology (soma, primary branches, fine branchlets, leaflets) and perivascular endfeet.
• Colocalization with Aldh1l1-EGFP:
  • Atto 643 carboxy: ~91.8% (CPu) and ~83.1% (Cortex) of labeled cells were astrocytes.
  • SR101: Only ~35.7% (CPu) and ~52.0% (Cortex) were astrocytes.
• Non-Astrocyte Exclusion:
  • Neurons: <1.0% colocalization (vs. significant uptake by SR101 in hippocampus).
  • Microglia: <0.4% colocalization.
  • NG2 Glia: ~1.3% colocalization.

B. Oligodendrocyte and Myelin Specificity

• Oligodendrocytes: Atto 643 carboxy showed minimal overlap with GFP-labeled oligodendrocytes (2.9%) compared to SR101 (35.8%).
• Myelin Sheaths:
  • At 30 minutes, neither dye labeled myelin.
  • At 2 hours, SR101 showed prominent myelin sheath labeling.
  • Atto 643 carboxy showed no myelin labeling even at 2 hours.
  • THG imaging confirmed that SR101 signals overlapped with myelin structures, while Atto 643 carboxy signals did not.

C. Regional Specificity

• Hippocampus: Atto 643 carboxy labeled protoplasmic astrocytes in CA1, CA3, and DG without labeling pyramidal neuron somata or dendrites. In contrast, SR101 heavily labeled CA1 pyramidal neurons.
• Brainstem: No labeling was observed in the brainstem, consistent with the known distribution of the OATP1C1 transporter.

D. In Vivo Imaging

• Atto 643 carboxy enabled high-resolution two-photon imaging of astrocytes in the living brain up to 700 µm depth below the pial surface.
• The signal remained sharp, allowing visualization of fine processes and vessel-associated endfeet.

E. Mechanism of Uptake

• Co-administration of L-thyroxine (T4) significantly reduced Atto 643 carboxy uptake in the hippocampus, confirming that the dye relies on the OATP1C1 transporter for entry, similar to SR101.

5. Significance

This study establishes Atto 643 carboxy as a superior chemical tool for astrocyte research compared to the long-standing SR101 standard. Its significance lies in:

1. High Fidelity: It eliminates the major confounders of oligodendrocyte and neuronal cross-labeling, allowing researchers to study astrocyte structure and function in isolation.
2. Multiplexing Capability: Its far-red emission spectrum allows for simultaneous imaging with green (e.g., GCaMP, GFP) and other fluorophores, facilitating complex studies of neurovascular coupling and tripartite synapses.
3. Versatility: It is effective in both acute brain slices and in vivo preparations, supporting deep-tissue imaging.
4. Broad Applicability: It offers a non-genetic, cost-effective solution for astrocyte labeling in species or models where genetic manipulation is difficult.

Limitations: The dye is not compatible with standard fixation protocols (limiting immunohistochemistry) and may require repeated administration for long-term imaging sessions, similar to SR101. However, its specificity makes it a powerful immediate tool for dissecting astrocyte dynamics in intact brain tissue.