Lineage tracing reveals atoh7 positive and negative retinal ganglion cell populations in the zebrafish retina

This study utilizes a novel atoh7:iCre;ubi:Switch lineage tracing system in zebrafish to demonstrate that while 79% of retinal ganglion cells originate from atoh7-positive progenitors, a significant atoh7-negative RGC population exists, providing a model to investigate RGC survival mechanisms and cell fate plasticity.

The Gist

Imagine the retina of your eye as a bustling, high-tech city. This city is built by a master architect named Atoh7. In the past, scientists knew that Atoh7 was crucial for building the city's most important delivery trucks: the Retinal Ganglion Cells (RGCs). Without Atoh7, the city would lose almost all its delivery trucks, causing the city to go dark (blindness).

However, there was a confusing mystery: In mice, scientists found that only about half of the delivery trucks were actually built by Atoh7. The other half were built by someone else, yet they still needed Atoh7 to survive. It was like a factory where the boss (Atoh7) was firing half the workers, but the remaining workers were still needed to keep the other half of the factory running.

This paper asks: Does this same mystery exist in zebrafish? And if so, can we use these fish to figure out how the "non-Atoh7" trucks survive?

Here is the breakdown of their discovery, using simple analogies:

1. The "Permanent Tattoo" Tool

The biggest problem with studying Atoh7 is that it's a "flash in the pan." It shows up during construction, does its job, and then vanishes. By the time the city is finished, the architect is gone, so you can't tell which buildings he built.

To solve this, the researchers created a special genetic tool for zebrafish. Think of it as a permanent tattoo machine.

• They gave the fish a gene that says: "If you ever meet Atoh7, get a red tattoo (mCherry) that never fades."
• Even after Atoh7 disappears, the red tattoo stays. This allows the scientists to look at a fully grown fish and say, "Ah, this cell was built by Atoh7," and "This cell was built by someone else."

2. The Big Discovery: Two Types of Delivery Trucks

When they looked at the zebrafish retinas, they found something fascinating. Just like in mice, the delivery trucks (RGCs) came in two flavors:

• The "Atoh7-Positive" Trucks: About 79% of the trucks were built by the Atoh7 architect.
• The "Atoh7-Negative" Trucks: About 21% of the trucks were built by a different architect, but they still needed Atoh7 to stay alive.

Why is this a big deal?
In mice, only 55% of trucks were Atoh7-built. In zebrafish, it's 79%. This means zebrafish are an even better model for studying this mystery because they have a huge population of "Atoh7-negative" trucks that still depend on Atoh7. It's like finding a whole new neighborhood of people who rely on a leader they never met.

3. The "Fate Switch" Mystery

The researchers then looked at what happens when the Atoh7 architect is missing (in mutant fish).

• The Result: As expected, almost all the delivery trucks vanished.
• The Twist: The "Atoh7-negative" trucks didn't just survive; they seemed to change their minds. The researchers found that when Atoh7 is gone, some cells that usually become other types of workers (like "Amacrine cells," which are like the city's security guards) suddenly start acting like they were built by Atoh7.
• The Analogy: Imagine a construction site where the boss is fired. Suddenly, the people who were supposed to be building windows start building doors instead. The total number of doors doesn't change, but the source of the doors has shifted. This suggests that without Atoh7, the cells get confused and switch their career paths.

4. A Surprise in the Brain

The researchers also looked outside the eye. They found that Atoh7 isn't just a "retina architect." It also shows up in other parts of the brain, specifically in areas related to hearing and smell.

• The Analogy: They thought Atoh7 was only the architect for the "Eye City," but they found he was also sketching blueprints for the "Ear Village" and "Nose Town." This opens up new doors for studying how Atoh7 affects the whole brain, not just vision.

The Bottom Line

This paper is like finding a new map for a treasure hunt.

1. We have a tool: A permanent tattoo that lets us track which cells were built by Atoh7.
2. We have a model: Zebrafish have a massive population of "Atoh7-negative" cells that still need Atoh7 to survive.
3. The Goal: Now that we can see these two groups clearly, scientists can finally ask: How does the Atoh7 group keep the non-Atoh7 group alive?

Understanding this could be the key to saving vision in humans. If we can figure out the secret signal that Atoh7 sends to keep the "strangers" alive, we might be able to treat diseases where those cells die, even if we can't bring Atoh7 back.

Technical Summary

1. Problem Statement

The transcription factor Atoh7 (Atonal homolog 7) is transiently expressed during retinal development and is essential for the genesis and survival of Retinal Ganglion Cells (RGCs). While Atoh7-expressing progenitors give rise to all seven major retinal cell types, the loss of Atoh7 results in a >95% loss of RGCs, with other cell types largely unaffected.

• The Paradox: In mice, lineage tracing revealed that only ~55% of mature RGCs originate from the Atoh7+ lineage, yet Atoh7 mutants lose nearly all RGCs. This suggests a cell non-autonomous mechanism where Atoh7+ cells are required for the survival of Atoh7- RGCs.
• The Gap: While Atoh7 function is conserved in zebrafish, it was previously unknown if zebrafish possess an Atoh7-negative RGC population. Furthermore, the full retinal lineage composition in zebrafish (compared to mice) and the presence of Atoh7+ cells outside the retina remained uncharacterized in adult stages.

2. Methodology

The authors developed a genetic lineage tracing system to permanently label cells that transiently express atoh7.

• Transgenic Generation:
  • Created a new transgenic line: atoh7:iCre, where the atoh7 promoter drives the expression of improved Cre recombinase (iCre).
  • Crossed this with the established ubi:Switch line (LoxP-eGFP-LoxP-mCherry).
  • Mechanism: In cells where atoh7 is expressed, Cre excises the eGFP stop cassette, permanently switching expression to mCherry. This allows the lineage to be tracked even after atoh7 expression ceases.
• Validation:
  • Validated the spatiotemporal expression of the transgene against endogenous atoh7 mRNA using time-lapse confocal microscopy (29–48 hours post-fertilization).
  • Confirmed no "leaky" expression in single-transgenic controls.
• Experimental Groups:
  • Wild Type (WT): To map the atoh7 lineage in normal development.
  • atoh7 mutants (lakritz): To assess lineage shifts and cell survival in the absence of Atoh7 protein.
• Imaging & Analysis:
  • Used immunohistochemistry (IHC) on cryosections and retinal flatmounts at 5 days post-fertilization (dpf) and 3 months post-fertilization (mpf).
  • Markers: Rbpms2 (RGCs), Pax6 (Amacrine cells/RGCs), Prox1 (Bipolar cells), Zpr1 (Cones), WGA (Rods), and DAPI (nuclei).
  • Cleared Tissue Imaging: Used Accu-OptiClearing and light-sheet/confocal microscopy to visualize the atoh7 lineage in the whole adult brain.
  • Quantification: Cell counts and lineage proportions were calculated for WT and mutants.

3. Key Contributions

1. Generation of atoh7:iCre Line: The first permanent lineage tracing tool for atoh7 in zebrafish, capable of tracking cells from embryogenesis into adulthood.
2. Discovery of Atoh7-Negative RGCs: Definitive proof that zebrafish, like mice, contain a population of RGCs that do not originate from the atoh7 lineage.
3. Comparative Lineage Mapping: Provided the first comprehensive quantification of the atoh7 lineage contribution to all seven retinal cell types in zebrafish, allowing direct comparison with mouse data.
4. Extra-Retinal Discovery: Identified novel atoh7+ cell populations in the adult zebrafish central nervous system (CNS) outside the visual system.

4. Key Results

A. RGC Lineage Composition

• Wild Type: Approximately 79% of RGCs in the zebrafish retina are derived from the atoh7+ lineage, while 21% are atoh7-negative.
  • Comparison: This is a higher proportion of atoh7+ RGCs than in mice (~55%).
• Stability: The ratio of atoh7+ to atoh7- RGCs remains consistent from larval stages (5 dpf) through adulthood (3 mpf), indicating that new RGCs generated at the ciliary marginal zone (CMZ) maintain the same lineage proportions.
• Mutant Phenotype: In atoh7 mutants, 100% of RGCs are lost (both atoh7+ and atoh7- populations), confirming the non-autonomous survival requirement.

B. Other Retinal Cell Types (Wild Type vs. Mutant)

• Amacrine Cells (Pax6+):
  • WT: 40% atoh7+, 60% atoh7-.
  • Mutant: The proportion shifts significantly to 54% atoh7+ and 46% atoh7-.
  • Interpretation: While the total number of amacrine cells remains unchanged, there is a fate switch where progenitors that would normally become atoh7- amacrine cells are redirected to the atoh7+ lineage in the absence of Atoh7.
• Bipolar Cells (Prox1+):
  • WT: 8% atoh7+, 92% atoh7-. (Much higher atoh7+ contribution than in mice, where it is <0.1%).
  • Mutant: The proportion remains stable (~9% atoh7+), but the total number of bipolar cells increases significantly compared to WT. This suggests the extra bipolar cells arise from the atoh7- progenitor pool.
• Photoreceptors:
  • Rods: Mostly atoh7- (77% atoh7-).
  • Cones: Mostly atoh7+ (75% atoh7+).
  • Note: The high atoh7+ contribution to cones aligns with the high contribution to RGCs, likely due to the shared early mitotic division of these precursors in zebrafish.
• Müller Glia & Horizontal Cells: Showed minimal atoh7+ contribution (MG: ~0.5%) and no significant lineage shifts in mutants.

C. Extra-Retinal Expression

• Using cleared whole-brain imaging, the authors detected atoh7+ cells in:
  • The deep central forebrain (potentially olfactory processing).
  • The superficial ventral hindbrain (potentially auditory processing).
• This suggests atoh7 plays a role in broader sensory system development beyond the retina.

5. Significance

• Model for RGC Survival: The discovery of a robust atoh7- RGC population in zebrafish validates the zebrafish as a superior model for studying the cell non-autonomous mechanisms of RGC survival. Researchers can now investigate how atoh7+ cells support the survival of atoh7- RGCs.
• Evolutionary Insights: The data highlights species-specific differences in retinal development. The higher proportion of atoh7+ RGCs and cones in zebrafish (diurnal, cone-rich) compared to mice (nocturnal, rod-rich) suggests that lineage allocation is adapted to ecological visual needs.
• New Tool for CNS Research: The atoh7:iCre line provides a unique tool for targeted gene manipulation and lineage tracing in the zebrafish CNS, particularly for studying auditory and olfactory development.
• Fate Switching Mechanism: The results in atoh7 mutants support the hypothesis that in the absence of Atoh7, multipotent progenitors undergo a fate switch toward later-born cell types (e.g., amacrine and bipolar cells), rather than simply failing to differentiate.