In vivo interrogation of transcriptional and epigenetic regulators of lung epithelial regeneration

This study introduces SAGE, a versatile in vivo AAV-based platform for high-throughput genetic interrogation, which was used to identify Kat8 as essential for alveolar repair and to map transcription factor dependencies that distinguish between reparative and pathological epithelial trajectories in viral lung injury.

The Gist

Imagine your lungs are a bustling city made of tiny, delicate houses called alveoli. These houses are where your blood picks up oxygen. When a virus (like the flu) attacks, it's like a storm that smashes the roofs off these houses.

To survive, the city needs to rebuild quickly. The "construction workers" are special cells called AT2 cells. Their job is to multiply and turn into new roof tiles (AT1 cells) to seal the damage. But sometimes, the construction goes wrong. The workers get confused, the buildings become misshapen, and instead of a healthy city, you end up with a scarred, fibrotic mess (like pulmonary fibrosis).

For a long time, scientists didn't have a good way to watch this construction crew work while they were actually building inside a living body. They had to take the workers out of the city to study them, which changed how they behaved.

This paper introduces a revolutionary new tool called SAGE that lets scientists watch the construction crew in real-time, inside the living lung.

The Magic Tool: SAGE

Think of SAGE as a high-tech, invisible drone swarm.

• The Delivery System: Scientists use a harmless virus (AAV) as a delivery truck to drop these drones into the mouse's lungs.
• The Sticky Note: Unlike normal viruses that just float around and eventually wash away, SAGE is designed to "stick" permanently into the cell's DNA. It's like a permanent sticky note attached to the cell's instruction manual.
• The Mission: Each drone carries a specific "delete" command (a CRISPR guide). It tells the cell to turn off one specific gene (a specific instruction in the manual) to see what happens.
• The Result: If turning off that gene stops the construction crew from working, the cell dies or gets sick. If the gene isn't important, the cell keeps building. By counting which cells survive, scientists can figure out exactly which genes are essential for repair.

What They Discovered

Using this drone swarm, the researchers ran two types of experiments: a "bulk" check (looking at the whole crew) and a "high-definition" check (looking at every single worker individually).

1. The "Glue" That Holds It Together (Kat8 and the NSL Complex)

They found that a specific protein called Kat8 is absolutely critical.

• The Analogy: Imagine the construction workers are trying to read a blueprint, but the pages are stuck together. Kat8 is the tool that unsticks the pages so the workers can read the instructions.
• The Discovery: Without Kat8 (specifically working with a team called the NSL complex), the workers get confused. They stop building new roofs and instead start panicking. They turn into "senescent" cells—zombie workers that just sit there, shouting stress signals that tell the body to build scar tissue instead of healthy lung tissue. This explains why some viral infections lead to permanent lung damage.

2. The Two Paths of Recovery

The high-definition scan revealed that when the lungs are injured, the construction workers don't just take one path. They split into two distinct groups:

• Path A (The Hero Path): These workers follow the blueprint perfectly. They repair the damage and turn into healthy new roof tiles. This is the DATP-like-1 state.
• Path B (The Villain Path): These workers get overwhelmed by stress and inflammation. They stop trying to be roof tiles and turn into something weird and misshapen, looking like the wrong type of cell (basaloid cells). This is the DATP-like-2 state.
• The Connection: The "Villain Path" looks exactly like the cells found in human patients with severe lung scarring (fibrosis).

3. The Alarm System (NF-κB)

Why do some workers take the "Villain Path"? The study found that inflammation is the culprit.

• The Analogy: Think of inflammation as a loud, blaring alarm siren.
• The Discovery: When the alarm (specifically a pathway called NF-κB) is too loud, it scares the construction workers. Instead of focusing on building, they panic and switch to the "Villain Path," creating scar tissue. If you can calm the alarm, the workers are more likely to take the "Hero Path" and heal the lung properly.

Why This Matters

This paper is a game-changer for three reasons:

1. New Tool: SAGE is the first time we can do this kind of massive, gene-by-gene screening inside a living lung. It's like moving from studying a map of a city to actually walking the streets while the construction is happening.
2. New Targets: They found that the NSL complex (the "glue") and NF-κB (the "alarm") are key players. This gives doctors new targets for drugs. Maybe we can make a drug that calms the alarm or helps the glue stick, preventing lung scarring after a virus.
3. Understanding Fibrosis: They proved that the scary, scar-causing cells seen in human fibrosis patients can be recreated in mice. This means we can now test cures for lung scarring much faster and more accurately.

In short: The researchers built a spy network to watch how lungs heal. They found that if the construction crew gets too stressed by inflammation, they stop fixing the roof and start building a mess. By understanding the specific switches that control this, we might be able to stop lung scarring in the future.

Technical Summary

1. The Problem

Effective repair of the lung alveoli following viral injury (e.g., Influenza A) is critical for survival. This process relies on Alveolar Type 2 (AT2) cells acting as progenitors to proliferate and differentiate into Alveolar Type 1 (AT1) cells. However, the molecular mechanisms governing this regeneration are not fully understood.

• Limitations of Current Models: Existing genetic screening tools often rely on in vitro organoids or cell lines, which fail to recapitulate the complex in vivo tissue microenvironment, extrinsic signaling cues, and mechanical forces present during regeneration.
• Technical Gap: While CRISPR screening is powerful, establishing a high-throughput, pooled genetic screen in the mammalian lung in vivo has been hindered by two main AAV (Adeno-Associated Virus) limitations:
  1. Transient Expression: Standard AAV genomes remain episomal and are diluted during cell division, preventing long-term tracking of perturbations in regenerating tissues.
  2. Low Multiplicity of Infection (MOI) Efficiency: Achieving efficient, cell-type-specific transduction at low MOI is difficult, which is a prerequisite for pooled screens to avoid confounding effects from multiple gRNAs per cell.
• Knowledge Gap: It remains unclear how specific transcription factors (TFs) and epigenetic regulators dictate the fate of AT2 cells, particularly regarding the emergence of distinct transitional states that lead to either successful repair or pathological fibrosis (e.g., basaloid states seen in human pulmonary fibrosis).

2. Methodology

The authors developed a novel platform called SAGE (Stable AAV Genomic IntEgration) to overcome these barriers.

• SAGE Platform Development:

  • Vector Design: They engineered an AAV9-based vector (the most efficient serotype for AT2 transduction) fused with the Sleeping Beauty (SB) transposon system.
  • Integration Mechanism: The system uses a transposase (SBase) to integrate the viral genome (containing the gRNA and a reporter) into the host DNA at TA dinucleotides.
  • Stabilization: To prevent "remobilization" (cutting and re-inserting the transgene), they embedded the SB inverted terminal repeats (IRs) within a chimeric intron. This ensures that once integration occurs, the transposase expression is lost, locking the gRNA into the genome.
  • Validation: They confirmed high integration efficiency (~35% of organoids were uniformly mScarlet+) and stable gene editing in vivo using an Sftpc-targeting gRNA in Sftpc-CreER; LSL-Cas9-eGFP mice.

• Screening Strategy:

  • Model: Influenza A Virus (IAV) infection in mice to induce acute lung injury and subsequent regeneration.
  • Libraries:
    • Chromatin Modulators (CMs): 173 genes (histone acetyltransferases, deacetylases, etc.).
    • Transcription Factors (TFs): 118 TFs enriched in alveolar lineages.
  • Readouts:
    1. SAGE-Perturb (Bulk): Sequencing gRNA abundance in sorted AT2s at 28 days post-infection (dpi) to identify essential genes (depletion indicates a failure to regenerate).
    2. SAGE-Perturb-seq (Single-cell): Integrating SAGE with 10x Genomics 5' scRNA-seq to link gRNA identity to transcriptomic states across four time points (0, 7, 14, 28 dpi).

• Analysis:

  • Used HOTSPOT to identify co-varying gene modules (programs) rather than discrete cell clusters.
  • Applied trajectory inference (Slingshot) to map differentiation paths.
  • Compared mouse data with human pulmonary fibrosis (PF) single-cell datasets.

3. Key Contributions

1. First In Vivo Lung Genetic Screen: Established the first scalable, pooled CRISPR screening platform for the mammalian lung epithelium that supports both bulk phenotypic screens and single-cell resolution (Perturb-seq).
2. Discovery of Kat8/NSL Complex: Identified Kat8 (a histone acetyltransferase) as an essential regulator of alveolar repair, specifically functioning through the Non-Specific Lethal (NSL) complex, not the MSL complex.
3. Resolution of Transitional States: Defined two distinct, independent AT2-derived transitional states:
   • DATP-like-1: A reparative state leading to AT1 differentiation.
   • DATP-like-2: A pathological, stress-primed state resembling the basaloid population found in human pulmonary fibrosis.
4. Regulatory Logic of Pathology: Mapped the genetic drivers of these states, revealing that NF-κB signaling and inflammatory cues drive the emergence of the pathological (basaloid) state, while specific TFs (e.g., Nkx2-1, Max) bias cells toward repair or pathology.

4. Key Results

• Platform Validation: SAGE achieved stable genomic integration in ~35% of transduced AT2s. Bulk screens showed high reproducibility (Pearson's r > 0.92) and successfully depleted gRNAs targeting essential genes (e.g., Copa, Prpf8).
• Epigenetic Regulators: The CM screen highlighted histone acetylation as critical. Kat8, Kat5, Taf1, and Hdac3 were strongly depleted. Inhibiting BET proteins (Brd2/4) or HDAC3 severely impaired organoid formation.
• Kat8 and the NSL Complex:
  • Single-cell analysis revealed that loss of Kat8 or other NSL components (but not MSL components) caused AT2s to accumulate in a subpopulation (AT2-2) characterized by senescence-like features (upregulation of Cdkn1a, Gadd45) and a profibrotic secretome (upregulation of Gdf15, Pdgfa).
  • This suggests NSL dysfunction blocks differentiation and promotes maladaptive remodeling.
• Transcription Factor Atlas:
  • Analysis of 118 TF knockouts revealed four functional groups: AT2 retention, AT1-biased, aberrant-biased, and mixed-fated.
  • Nkx2-1 knockout led to an expansion of aberrant, secretory-like cells and a failure to differentiate into AT1s.
  • Max (a partner of Myc) knockout biased cells toward the reparative DATP-like-1 state, suggesting Myc activity drives the pathological state.
• Two Distinct Trajectories:
  • Trajectory analysis showed that AT2s can independently enter either DATP-like-1 (reparative) or DATP-like-2 (pathological/basaloid).
  • DATP-like-2 cells share gene signatures with human basaloid cells (KRT5+/KRT17+) found in pulmonary fibrosis, including high expression of senescence markers (Cdkn2a) and stress response genes.
• Inflammation Drives Pathology:
  • NF-κB signaling (both canonical Rela and non-canonical Relb) was significantly elevated in DATP-like-2 cells and human basaloid populations.
  • Knockout of Relb prevented the emergence of the pathological state, favoring AT1 differentiation.
  • This links elevated inflammation directly to the maladaptive epithelial remodeling seen in fibrosis.

5. Significance

• Therapeutic Insight: The study identifies NF-κB signaling and the NSL complex as potential therapeutic targets. Modulating these pathways could potentially steer regeneration away from fibrosis and toward functional repair.
• Disease Modeling: It demonstrates that the basaloid state, previously thought to be unique to chronic human fibrosis, can be recapitulated in a mouse model of acute viral injury under specific genetic perturbations. This validates the IAV model for studying early fibrotic mechanisms.
• Methodological Advance: SAGE provides a versatile framework for functional genomics in regenerating tissues, scalable to hundreds of genes and applicable to other organs. It bridges the gap between in vitro screens and complex in vivo physiology.
• Mechanistic Clarity: The work resolves the "black box" of AT2 fate decisions, showing that the balance between regeneration and fibrosis is determined by a competition between lineage-determining TFs (like Nkx2-1) and inflammatory stress signals (NF-κB).

In summary, this paper provides a comprehensive map of the genetic circuitry controlling lung repair, revealing how inflammation and specific epigenetic regulators dictate whether tissue regeneration succeeds or fails into fibrosis.