EYA1/EYA2 and EYA3/EYA4 act as stage-specific SIX cofactors in embryonic and adult regenerative skeletal myogenesis

This study demonstrates that EYA1/EYA2 and EYA3/EYA4 function as stage-specific SIX cofactors, where EYA3 and EYA4 are dispensable for embryonic myogenic stem cell formation but essential for adult muscle regeneration, with EYA4 being critical for satellite cell maintenance and EYA3/EYA4 acting redundantly to drive myoblast fusion via the regulation of Myomixer, Follistatin, and Noggin.

The Gist

Imagine your body's muscles are like a bustling construction site. To build and repair these structures, you need a team of specialized workers. In this story, the SIX proteins are the foremen who direct the work, but they can't do it alone. They need EYA proteins to act as their essential assistants or "co-pilots." Without these co-pilots, the foremen can't get the job done.

This paper investigates two specific pairs of these co-pilots: EYA1/EYA2 and EYA3/EYA4. The researchers wanted to see which pair works during the "construction phase" of a baby (embryonic development) and which pair is needed for "renovations" later in life (adult muscle repair).

Here is what they found, broken down into simple parts:

1. The Adult Repair Crew (Muscle Regeneration)

When an adult muscle gets injured (like a torn muscle from exercise or an accident), the body wakes up dormant "stem cells" (the reserve workers) to fix the damage. The researchers tested what happens when they remove the EYA3 and EYA4 co-pilots from these stem cells.

• The EYA3 Worker: When they removed just EYA3, the construction site kept running smoothly. The number of workers and the size of the repaired muscle fibers looked normal. It seems EYA3 isn't strictly necessary for the repair crew to function.
• The EYA4 Worker: When they removed just EYA4, the repair crew struggled. The number of workers dropped, and the new muscle fibers were smaller than they should be.
• The Double Trouble (Removing Both): When they removed both EYA3 and EYA4, the situation got much worse. The muscle repair was severely damaged, and even after 30 days, the new muscle fibers looked strange and didn't form correctly.

The "Fusion" Problem:
To understand why the double-mutant crew failed, the scientists grew these cells in a lab dish. They found that the cells could still multiply (hire more workers), but they couldn't fuse.

• Analogy: Imagine a team of bricklayers who can lay bricks perfectly but refuse to join hands to build a single, solid wall. Instead, they stay as separate, tiny piles of bricks.
• The Cause: The scientists looked at the cells' instruction manuals (their genetic code) and found that three specific "glue" instructions—Myomixer, Follistatin, and Noggin—were missing. Without these instructions, the cells couldn't stick together to form a strong muscle fiber.

2. The Embryonic Construction Crew (Development)

The researchers also looked at how these proteins work when a baby is forming in the womb (specifically at day 18.5 of development).

• EYA1 and EYA2: These are the heavy lifters for building the body from scratch. When the researchers removed both EYA1 and EYA2, the baby mice were missing their leg muscles entirely, and their face muscles were very small. It's as if the construction site for the limbs and face was abandoned before it even started.
• EYA3 and EYA4: In contrast, when they removed EYA3 and EYA4 from the developing fetus, the face muscles formed just fine, and the stem cells (the reserve workers) were present and ready. This suggests that while EYA3 and EYA4 are critical for fixing muscles later in life, they aren't the main drivers for building the initial muscle framework in the womb.

The Big Picture

The paper concludes that these EYA proteins act as stage-specific co-pilots:

• EYA1/EYA2 are the essential team for building the muscle factory during embryonic development.
• EYA3/EYA4 are the essential team for repairing the factory in adults. Specifically, EYA4 is the most important for adult repair, and without it (and EYA3), the repair crew loses the ability to "glue" new muscle cells together, leaving the muscle weak and fragmented.

Technical Summary

Technical Summary: Stage-Specific Roles of EYA Co-factors in Skeletal Myogenesis

Problem Statement
The Eya (eyes absent) family of proteins functions as essential transcriptional co-activators for the Six family of transcription factors, playing critical roles in organogenesis and tissue maintenance. While the developmental roles of Eya1 and Eya2 in embryonic myogenesis are established, the specific contributions of Eya3 and Eya4 in adult skeletal muscle regeneration and the maintenance of the PAX7+ satellite cell pool remain unclear. This study addresses the gap in understanding how these specific Eya isoforms function as stage-specific SIX cofactors during both embryonic development and adult muscle repair.

Methodology
The researchers employed a multi-faceted approach combining in vivo genetic models with ex vivo cell culture and transcriptomic analysis:

• Genetic Models: Single and compound mutant mice were generated, including satellite cell-specific Eya4 mutants and global Eya3 mutants. Double mutants (Eya3^-/-;Eya4^-/-) were also analyzed.
• Regeneration Assay: Muscle regeneration was induced via Notexin injury in the Tibialis Anterior (TA) muscle. Kinetic analyses were performed at 4, 14, and 30 days post-injury to assess PAX7+ cell numbers and myofiber cross-sectional area (CSA).
• Ex Vivo Culture: Satellite cells were isolated and cultured. Eya4 deletion was induced via Ad-Cre-mediated recombination to create single Eya3 mutants and double mutants for functional assays.
• Functional Assays: Proliferation and fusion capabilities were measured in cultured cells.
• Transcriptomics: RNA sequencing was performed on mutant cells to identify differentially expressed genes.
• Embryonic Analysis: E18.5 fetuses from Eya1^-/-;Eya2^-/-, Eya3^-/-, Eya4^-/-, and Eya3^-/-;Eya4^-/- genotypes were examined for limb and craniofacial muscle development and PAX7+ cell presence.

Key Results

• Adult Regeneration Phenotypes:
  • Eya3 single mutants showed no significant defects in regeneration kinetics, PAX7+ cell numbers, or myofiber CSA at any time point.
  • Eya4 single mutants exhibited decreased PAX7+ cell numbers and reduced myofiber CSA.
  • Eya3/Eya4 double mutants displayed a synergistic worsening of the phenotype, with further reductions in regenerative parameters and altered myofiber morphology at 30 days post-injury.
• Cellular Mechanisms:
  • Ex vivo analysis revealed that while single Eya3 mutants proliferated and differentiated normally, double mutant cells maintained normal proliferation rates but failed to fuse.
  • Transcriptome analysis of double mutant cells identified a severe downregulation of fusion-critical genes, specifically Myomixer, Follistatin, and Noggin.
• Embryonic Development:
  • Eya1^-/-;Eya2^-/- fetuses confirmed the absence of distal limb muscles and reduced craniofacial muscles.
  • In contrast, Eya3^-/-;Eya4^-/- fetuses showed preserved craniofacial myogenesis and the presence of PAX7+ myogenic stem cells, indicating that Eya3 and Eya4 are not strictly required for the initial genesis of these stem cells during embryogenesis.

Significance and Claims
The paper establishes that Eya3 and Eya4 act as redundant but essential SIX cofactors specifically during adult muscle regeneration, rather than during the initial embryonic formation of the satellite cell pool. The study demonstrates that the failure of muscle regeneration in the absence of both isoforms is driven by a specific defect in myoblast fusion, mediated by the downregulation of key fusogenic and regulatory genes (Myomixer, Follistatin, Noggin). The authors conclude that while Eya1 and Eya2 are critical for embryonic myogenesis, Eya3 and Eya4 are the primary drivers of the regenerative capacity of adult skeletal muscle, highlighting a stage-specific functional divergence within the Eya family.