Tiny Hearts and Big Answers | Dr. Eman Akam-Baxter | TEDxYouth@DoverSherbornHighSchool

Key Concepts:

• Myocardial Infarction (Heart Attack): Blockage of blood flow to the heart, leading to cell death and scar tissue formation.
• Scar Tissue: Primarily composed of collagen, it replaces damaged heart muscle but lacks contractile function.
• Collagen Cross-linking: Chemical bonds between collagen molecules that provide stability to scar tissue.
• Zebrafish Regeneration: The ability of zebrafish to degrade scar tissue and regenerate heart muscle after injury.
• Molecular Imaging Probe (TMRO): A fluorescent molecule designed to bind to collagen cross-links, allowing visualization and quantification.
• Three Rs of Ethical Animal Research: Replacement, Reduction, and Refinement.
• Pyrodinoline (PYD): A specific type of collagen cross-link associated with permanent, non-degradable scar tissue.
• Fibrotic Diseases: A class of diseases characterized by excessive scarring in various organs.

1. The Problem: Scar Tissue After Heart Attacks

• Heart attacks (myocardial infarctions) occur when plaque blocks blood flow to the heart, causing cell death.
• The body repairs the damage by laying down scar tissue, primarily composed of collagen.
• Scar tissue is non-contractile, impairing heart function. The healthy heart muscle has to work harder to compensate for the scar tissue.
• This leads to heart failure, a major cause of death in the US (one person every 33 seconds dies as a result of a cardiovascular disease).
• In most mammals, including humans, heart injuries result in permanent scar tissue formation.

2. The Zebrafish Solution: Scar Degradation and Regeneration

• Zebrafish can completely degrade heart scar tissue and regenerate functional heart muscle.
• Both mammalian and zebrafish scar tissue are primarily made of collagen.
• The key question: What allows zebrafish to degrade scar tissue while mammals cannot?
• Initial hypothesis: Differences in the degree of collagen cross-linking.

3. Overcoming Research Challenges: Molecular Imaging with TMRO

• Traditional methods require large amounts of tissue, impractical for small zebrafish hearts (zebra fish heart is about a fifth of the size of a grain of rice).
• Ethical considerations (the three Rs: Replacement, Reduction, and Refinement) necessitate minimizing animal use.
• Solution: Development of a molecular imaging probe called TMRO.
  • TMRO is a fluorescent molecule that binds specifically to collagen cross-links.
  • It acts like "sniffer dogs" trained to find collagen cross-links.
  • Allows visualization and quantification of collagen cross-linking in vivo.
• A library of TMRO probes with varying binding rates and extents was developed and tested in vitro.
• Control probes (non-binding) were used to confirm specificity.

4. Validating TMRO in Healthy and Injured Animals

• In healthy mice, TMRO accumulates in the aorta (rich in collagen) but not in the heart muscle (ventricle).
• Similar results in healthy zebrafish: TMRO accumulates in the bulbous arteriosus (similar to the aorta) but not in the ventricle.
• In infarcted mice (mice that have undergone a heart attack), TMRO accumulates in the scar tissue within the ventricle.
• These experiments validated TMRO as a tool to study collagen cross-linking in heart injury.

5. Unveiling the Mechanism: The Role of Pyrodinoline (PYD)

• The extent of collagen cross-linking was found to be similar between zebrafish and mice.
• The critical difference lies in the type of collagen cross-linking.
• Mammalian scar tissue has "locked" cross-links, while zebrafish scar tissue has "open" cross-links that can be degraded.
• The "locking" is due to the presence of pyrodinoline (PYD), a specific collagen cross-link.
• PYD is abundant in mammalian scar tissue but almost absent in zebrafish scar tissue.
• PYD inherently has a fluorescent property that allows it to be detected even at teeny tiny quantities.

6. Implications for Fibrotic Diseases

• The findings are relevant to a broader class of diseases called fibrotic diseases, characterized by excessive scarring in various organs.
• Fibrotic diseases account for 30-50% of deaths in the industrialized world.
• The goal is to inhibit the formation of PYD cross-links, allowing scar tissue to be degraded and restoring organ function.

7. Conclusion

• By studying zebrafish regeneration and developing molecular imaging tools like TMRO, researchers have identified a key difference in collagen cross-linking that determines scar degradability.
• The absence of PYD cross-links in zebrafish allows for scar tissue degradation and heart regeneration.
• This discovery has significant implications for treating heart attacks and other fibrotic diseases by targeting PYD formation.
• The research exemplifies how studying living systems and developing chemistry-based technologies can lead to new paths of healing.