Mitotic bookmarking by Prox1 preserves mammalian neuronal lineage identity memory via promoting timely H3K27me3 restoration

This study reveals that the transcription factor Prox1 acts as a mitotic bookmark in mouse hippocampal neural stem cells by retaining on chromatin during division to ensure the timely restoration of H3K27me3 repression on alternative lineage genes, thereby preserving dentate gyrus neuronal identity and preventing developmental defects.

The Gist

Imagine your brain is a massive, bustling city being built from scratch. To construct this city, you start with a small group of "master builders" (stem cells). These builders divide rapidly, creating millions of new workers who eventually become specific types of neurons—some become the "librarians" of the hippocampus (the Dentate Gyrus), while others become the "archivists" (the Cornu Ammonis).

The big challenge? Every time a master builder divides, the blueprints for the city are temporarily packed away into tight bundles (chromosomes) to be sorted. During this chaotic moment of division, the "construction crew" (transcription factors) usually gets kicked off the blueprints. If the crew forgets which building they were supposed to construct, the next generation of workers might get confused and build a library where an archive should be. This would cause a "lineage identity crisis."

This paper discovers how the brain solves this problem using a specific protein called Prox1. Here is the story of how it works, broken down into simple analogies:

1. The "Mitotic Bookmark" (The Sticky Note)

Think of the DNA during cell division as a book that is being slammed shut and shaken. Usually, the readers (proteins) have to let go. But Prox1 is special. It acts like a super-sticky bookmark that refuses to let go of the book even while it's being shaken.

• The Discovery: The researchers found that in the Dentate Gyrus (DG) area of the brain, Prox1 stays glued to the DNA during cell division. However, in the neighboring CA area, Prox1 lets go. This "bookmark" tells the new cells: "Hey, we are building a Dentate Gyrus library, not a CA archive!"

2. The "Identity War" (Prox1 vs. Fezf2)

Imagine two rival construction managers: Prox1 (who wants to build a DG library) and Fezf2 (who wants to build a CA archive). They are fighting over the same plot of land (specific genes).

• The Problem: Fezf2 is a strong competitor. If Prox1 lets go of the land during cell division, Fezf2 might sneak in and take over, turning the library into an archive.
• The Solution: Prox1 doesn't just hold the land; it holds it through the division. By staying attached (mitotic retention), Prox1 ensures that when the cell finishes dividing and opens the book again, Prox1 is already there, ready to say, "No, we are building a library," before Fezf2 can even show up.

3. The "Molecular Glue" (Coiled-Coils)

How does Prox1 stick so tightly? The researchers found that Prox1 has a special "molecular glue" made of coiled-coil structures (think of them like sturdy, twisted rope knots).

• The Experiment: The scientists created a mutant version of Prox1 where they cut these rope knots. This mutant could still do its job before the cell divided, but it couldn't stick to the DNA during division.
• The Result: Without the rope knots, the "bookmark" fell off. The cells got confused, started expressing the wrong genes (building archives instead of libraries), and the brain development went wrong. This proved that the "sticking" ability is the most critical part.

4. The "Security Guard" (PRC2 and H3K27me3)

Prox1 doesn't just sit there; it brings a security guard with it. This guard is a complex called PRC2.

• The Job: PRC2 puts a "Do Not Disturb" sign (a chemical tag called H3K27me3) on the genes that would turn the cell into a CA archive.
• The Timing: When a cell divides, these "Do Not Disturb" signs get washed away (diluted) because the DNA is copied. It takes time to put them back.
• The Magic: Because Prox1 stays attached during division, it immediately calls PRC2 back to the site the moment the cell finishes dividing. This ensures the "Do Not Disturb" signs are put back up fast, preventing the wrong genes from turning on. If Prox1 isn't there, the signs are put back too slowly, and the wrong genes wake up, causing chaos.

5. The Real-World Consequence

To prove this wasn't just a theory, the scientists made mice with the "cut rope knot" version of Prox1 (the mutant that can't bookmark).

• The Outcome: These mice had severe brain defects. Their Dentate Gyrus (the library) shrank and didn't form correctly. They also had trouble with spatial memory (getting lost easily), which is exactly what happens when the hippocampus doesn't work right.

The Big Picture

This paper tells us that memory isn't just about what you learn; it's about how cells remember who they are.

Just as you might use a sticky note to remember which page you were on in a book so you don't lose your place, cells use Prox1 as a mitotic bookmark to remember their identity. Without this simple but brilliant mechanism, the brain's complex architecture would collapse into confusion, and we wouldn't be able to form the specific memories that make us who we are.

In short: Prox1 is the brain's ultimate "Keep Calm and Carry On" sticker, ensuring that even when cells divide and everything gets chaotic, the blueprint for who they are remains safe and sound.

Technical Summary

1. Problem Statement

The mammalian brain is generated from a limited pool of neural stem cells (NSCs) that undergo mitotic divisions to produce diverse neuronal subtypes. A fundamental biological question remains: How is specific neuronal lineage identity faithfully transmitted across mitosis?
During mitosis, chromatin condenses, and the transcriptional machinery is largely displaced or degraded, halting transcription. It is unclear how specific gene regulatory programs defining unique lineage identities (e.g., Dentate Gyrus vs. Cornu Ammonis in the hippocampus) are preserved through cell division and re-established in daughter cells. While "mitotic bookmarking" (retention of transcription factors on mitotic chromosomes) has been observed in cultured cells and Drosophila, its role and mechanism in preserving neuronal lineage identity in the developing mammalian brain were previously unexplored.

2. Methodology

The authors employed a multidisciplinary approach combining in vivo mouse genetics, advanced genomics, and biophysical assays:

• Genetic Models:
  • Conditional Knockouts (cKO): Used Nestin-CreERT2 and Emx1-Cre to induce lineage-specific deletion of Prox1 at different developmental stages.
  • Conditional Knock-in (cKI): Generated a specific Prox1 mutant mouse line (Prox1.6mKI) where six hydrophobic/aromatic residues in the N2 domain were mutated to serine. This mutation specifically impairs mitotic retention while preserving interphase transcriptional function and condensate formation.
• Cell Sorting & Multi-omics:
  • Developed a novel pipeline to purify mitotic vs. interphase NSCs from embryonic mouse brains using pH3 (phospho-histone H3) immunolabeling and FACS.
  • Performed low-input CUT&Tag-seq on purified mitotic and interphase NSCs to map transcription factor binding and histone modifications (H3K27me3, H3K4me3) with high resolution.
  • Conducted RNA-seq and single-cell RNA-seq (scRNA-seq) to analyze transcriptomic changes upon Prox1 depletion or mutation.
• Biophysical & Molecular Assays:
  • FRAP (Fluorescence Recovery After Photobleaching): To assess the dynamics of Prox1 condensates.
  • Co-immunoprecipitation (CoIP): To verify interactions between Prox1 and PRC2 subunits (EZH2, SUZ12).
  • In Utero Electroporation (IUE): To overexpress wild-type or mutant Prox1 variants in specific hippocampal lineages for functional rescue or loss-of-function assays.
• Behavioral Assays: Morris Water Maze tests to assess spatial learning and memory deficits in adult mice.

3. Key Contributions

• Identification of Prox1 as a Mitotic Bookmark: The study identifies Prox1 as the first bona fide mitotic bookmark in the developing mammalian brain, specifically retaining on mitotic chromosomes in Dentate Gyrus (DG) NSCs but not in Cornu Ammonis (CA) NSCs.
• Mechanism of Mitotic Retention: The authors elucidate that coiled-coil (CC) mediated strong condensate formation (rather than weak intrinsically disordered region interactions) is the structural basis for Prox1's retention on mitotic chromosomes.
• Functional Decoupling: By creating the Prox1.6m mutant, the authors successfully separated the protein's mitotic retention function from its interphase transcriptional function, proving that retention is essential for lineage fidelity independent of general transcriptional activity.
• Epigenetic Mechanism: The study reveals that Prox1 bookmarking facilitates the timely restoration of H3K27me3 (a repressive mark) at bivalent gene loci after DNA replication, preventing lineage identity crisis.

4. Key Results

A. Prox1 Defines DG Identity via Mitotic Retention

• Phenotype: Prox1 cKO mice exhibit severe DG malformation (shrinking to ~25% of normal size) and spatial memory deficits.
• Lineage Switch: Loss of Prox1 leads to the downregulation of DG markers (e.g., Calb2, Hopx) and the upregulation of CA identity genes (e.g., Fezf2, FOXG1, Ctip2), indicating a DG-to-CA lineage transition.
• Mitotic Retention: Prox1 is retained on mitotic chromosomes in DG NSCs (specifically at pericentromeric heterochromatin) but is absent from CA NSC chromosomes.

B. Prox1 Bookmarks the Fezf2 Locus

• Target Identification: CUT&Tag analysis revealed that Prox1 binds to and retains on the Fezf2 locus (a master regulator of CA identity) during mitosis.
• Competition: Fezf2 and Prox1 compete for binding to the same loci. Fezf2 suppresses Prox1 expression. However, Prox1's ability to remain bound during mitosis allows it to "win" the competition in the daughter cells (early G1), ensuring Fezf2 remains silenced in the DG lineage.
• Mutual Repression: Prox1 and Fezf2 act as mutual repressors to define DG and CA identities, respectively.

C. Structural Basis: Coiled-Coil Condensates

• Domain Mapping: The N2 domain of Prox1 contains two redundant motifs (M1, M2) essential for self-association and condensate formation.
• Strong vs. Weak Condensation: Prox1 forms dynamic condensates in interphase. However, mitotic retention requires "strong" condensates mediated by structured coiled-coil (CC) motifs, not weak interactions mediated by intrinsically disordered regions (IDRs).
• The 6m Mutant: The Prox1.6m mutant retains normal interphase condensate formation and chromatin binding but fails to retain on mitotic chromosomes.

D. Mechanism: Timely H3K27me3 Restoration

• PRC2 Recruitment: Prox1 recruits the Polycomb Repressive Complex 2 (PRC2) subunits (EZH2, SUZ12) to form co-condensates.
• Replication Dilution: During DNA replication, H3K27me3 levels are diluted by 50%. In bivalent genes like Fezf2 (which carry both active H3K4me3 and repressive H3K27me3), slow restoration of H3K27me3 can lead to ectopic expression.
• The Bookmarking Advantage: Prox1's mitotic retention ensures the immediate recruitment of PRC2 to the Fezf2 locus in early G1, facilitating rapid H3K27me3 restoration before H3K4me3 can dominate. This prevents the ectopic expression of CA genes in DG neurons.

E. Physiological Significance

• Lethality: Heterozygous Prox1.6mKI mice die shortly after birth, indicating that mitotic bookmarking is critical for early development.
• DG Defects: NSC-specific Prox1.6mcKI mice (lacking mitotic retention but retaining interphase function) exhibit severe DG hypoplasia and ectopic expression of Fezf2 in DG neurons, confirming that mitotic bookmarking is the specific mechanism safeguarding lineage identity.

5. Significance

• Fundamental Principle: This work establishes mitotic bookmarking as a fundamental mechanism for preserving neuronal lineage identity memory in mammalian brain development, moving beyond the previously known roles in Drosophila or cultured cells.
• Epigenetic Memory: It provides a mechanistic link between transcription factor retention, phase separation (condensates), and the kinetics of histone modification restoration (H3K27me3) to explain how epigenetic memory survives cell division.
• Disease Relevance: The findings suggest that defects in mitotic bookmarking could underlie neurodevelopmental disorders where neuronal lineage specification fails.
• Technical Innovation: The development of a pipeline to isolate mitotic NSCs from mouse brains for low-input omics (BISMIB adaptation) opens new avenues for studying epigenetic memory in mammalian development.

In summary, the paper demonstrates that Prox1 acts as a mitotic bookmark via coiled-coil-mediated condensates to recruit PRC2, ensuring the rapid re-establishment of repressive H3K27me3 marks on the Fezf2 locus after mitosis, thereby preventing lineage identity crisis and ensuring proper hippocampal development.