Cryo-ET reveals nanoscale thick filamentdisorganization in MYH7 P710R hypertrophiccardiomyopathy cardiomyocytes

Using cryo-electron tomography, this study reveals that the MYH7 P710R mutation in hypertrophic cardiomyopathy causes nanoscale disorganization of thick filament hexagonal packing and ribosome infiltration within cardiomyocyte sarcomeres, bridging the gap between molecular myosin dysfunction and cellular structural disarray.

The Gist

The Big Picture: A Heart in Disarray

Imagine your heart is a massive, high-performance factory. Inside this factory, there are millions of tiny workers (cells) called cardiomyocytes. Their job is to squeeze and pump blood. To do this, they are packed with microscopic machines called sarcomeres, which act like the factory's assembly lines.

These assembly lines are made of two main types of ropes:

1. Thin filaments (like guide rails).
2. Thick filaments (the heavy-duty motors that pull the rails to create the squeeze).

In a healthy heart, these ropes are perfectly organized, packed in neat, parallel rows like soldiers standing at attention or logs stacked in a perfect hexagon. This order allows the heart to pump efficiently.

Hypertrophic Cardiomyopathy (HCM) is a genetic heart disease where this factory starts to break down. The heart muscle gets too thick (hypertrophy), and the workers start to get confused and disorganized. This can lead to heart failure or sudden cardiac arrest.

The Mystery: What Goes Wrong?

Scientists know that in HCM, a specific gene called MYH7 often has a typo (mutation). This gene is the blueprint for the "motor" part of the assembly line. One specific typo, called P710R, causes a severe form of the disease that often hits children.

We know this mutation makes the motor pull too hard. But here is the big question: How does a tiny change in a single protein cause the entire factory floor to become a chaotic mess?

Until now, we couldn't see the answer clearly. Traditional microscopes were like looking at a factory through a foggy window or a blurry security camera. You could see the building was messy, but you couldn't see the individual workers tripping over each other.

The New Tool: The "Super-Microscope"

This paper introduces a new way of looking at the heart cells using a technique called Cryo-Electron Tomography (cryo-ET).

• The Analogy: Imagine taking a photo of a busy city street.
  • Old way: You take a picture of a frozen, plastic model of the street. It's static, and the process of making the model might have squished the buildings.
  • New way (Cryo-ET): You flash-freeze the actual street in a split second (keeping it in its natural, wet state) and then take a 3D X-ray movie of it. You can see every car, every person, and every puddle exactly as they were in real life, down to the nanometer scale.

The researchers used this "super-microscope" on heart cells grown from stem cells (which act like a test version of a human heart).

The Discovery: The "Hexagon" Breaks

When they looked at the healthy cells, the thick filaments (the motors) were packed in a perfect hexagonal pattern. It was like a honeycomb or a perfectly stacked pile of logs.

But when they looked at the cells with the P710R mutation, the pattern was broken.

• The Finding: The motors were no longer standing in neat rows. They were tilted, twisted, and scattered.
• The Analogy: Imagine a marching band where everyone is supposed to be in a perfect grid. In the mutated cells, the band members are spinning in circles, leaning on each other, and losing their formation. Even though they are still trying to march, the whole group is stumbling.

This "sub-sarcomeric disarray" (disorder inside the tiny machine) happens before the heart gets visibly thick. It suggests that the chaos starts at the microscopic level and eventually causes the whole heart to fail.

The Surprise Guest: Ribosomes

The researchers also noticed something strange happening in the messy areas. They found a lot of ribosomes (tiny machines that build proteins) hanging out in the disordered zones.

• The Analogy: Imagine a construction site where the bricks are falling off the wall. You would expect to see a lot of bricklayers (ribosomes) standing there, trying to fix the wall or perhaps accidentally knocking it down further.
• The Meaning: This suggests that the cell is trying to repair the broken machinery by building new parts right where the damage is. However, because the foundation is shaky, the new parts might just add to the chaos. It's a "fix-it" attempt that might be making the problem worse.

Why This Matters

This study is a breakthrough because it connects the dots between the tiny genetic typo and the big heart failure.

1. It explains the "How": It shows that a mutation doesn't just make the heart pump harder; it physically breaks the structural order of the cell.
2. It catches the problem early: This disorder happens in the "baby" heart cells (stem cells) before the heart even gets thick. This means we might be able to detect or treat the disease much earlier than before.
3. New Targets: By seeing that ribosomes are gathering in the mess, scientists now have a new clue. Maybe we can develop drugs that help the cell organize these repairs better, rather than just trying to stop the heart from pumping so hard.

Summary

Think of the heart as a well-oiled machine. This paper used a super-powerful, 3D, freeze-frame camera to see that a tiny genetic glitch causes the machine's gears to fall out of alignment. The machine tries to fix itself by bringing in more repair crews, but the chaos spreads until the whole engine fails. Now that we can see exactly how the gears are misaligned, we have a better map for how to fix the engine.

Technical Summary

1. Problem Statement

Hypertrophic Cardiomyopathy (HCM) is the most common monogenic inherited heart disease, often caused by mutations in the $\beta$-cardiac myosin gene (MYH7). While it is established that MYH7 mutations (such as P710R) lead to increased force production at the molecular level and pathological hypertrophy (thickening) and disarray at the tissue level, the intermediate mechanisms linking these scales are poorly understood.

• The Gap: Previous studies relied on conventional electron microscopy (EM) of resin-embedded samples, which lacks molecular resolution and introduces artifacts (fixation, dehydration). Conversely, biochemical studies provide molecular details but lack cellular context.
• The Question: How do molecular-scale perturbations in myosin activity propagate to cause nanoscale disorganization of sarcomeres and subsequent cellular/tissue-level pathology in HCM?

2. Methodology

The authors developed an integrated workflow combining human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), micropatterning, and cryo-electron tomography (cryo-ET) to visualize structures in a near-native, hydrated state.

• Cell Model:
  • Used an isogenic pair of hiPSC-CMs: Wild-type (WT) control and a mutant line expressing the severe pediatric-onset MYH7 P710R mutation.
  • Cells were differentiated using standard protocols and seeded onto micropatterned EM grids (17 $\times$ 118 $\mu$m rectangles). This patterning forces cells into an elongated shape mimicking in vivo cardiomyocytes and promotes maturation.
• Sample Preparation (Cryo-FIB):
  • Cells were vitrified via plunge freezing.
  • Cryo-Focused Ion Beam (FIB) milling was used to thin the vitrified cells into electron-transparent lamellae (~200 nm thick), specifically targeting regions adjacent to the nucleus.
• Data Acquisition:
  • Imaging was performed on a Thermo Fisher Krios G2 equipped with a Falcon 4i direct detector.
  • Tilt-series were collected using a dose-symmetric scheme (50–60° tilt range).
• Image Processing:
  • Data was motion-corrected (MotionCor2) and reconstructed (AreTomo).
  • Denoising: CryoCare deep learning models were used to enhance signal-to-noise ratios.
  • Segmentation: Neural networks (ORS Dragonfly) and IMOD were used to segment and visualize specific features, particularly myosin thick filaments.

3. Key Contributions

• Novel Imaging Modality: Successfully applied cryo-ET to hiPSC-CMs to visualize sarcomeric ultrastructure at the nanometer scale without the artifacts of chemical fixation or resin embedding.
• Discovery of Sub-Sarcomeric Disarray: Identified a specific type of disorder in the hexagonal packing of thick filaments within individual sarcomeres, distinct from the larger-scale myofibril disarray seen in conventional EM.
• Ribosome Infiltration: Discovered a correlation between sarcomeric disorder and the local accumulation of ribosomes, suggesting active local translation or remodeling in disordered regions.

4. Key Results

• Thick Filament Disorganization:
  • In WT cells, thick filaments exhibited a highly ordered, parallel hexagonal packing.
  • In P710R mutant cells, there was a significant increase in the angular dispersion of thick filaments relative to the average sarcomere orientation.
  • Quantification: The average dispersion angle was significantly higher in mutants compared to controls ($p = 5.7 \times 10^{-10}$). This indicates that the P710R mutation disrupts the precise alignment of myosin filaments at the sub-sarcomeric level.
• Ribosome Localization:
  • Disordered sarcomeric regions in P710R cells showed a striking up-regulation of ribosomes compared to the organized regions of WT cells.
  • Ribosomes were also observed in disordered regions of WT cells, though less frequently, suggesting that ribosome infiltration is a general response to sarcomeric instability, potentially facilitating local protein turnover or repair.
• Organelle Preservation: The workflow successfully preserved and visualized various organelles (Z-discs, mitochondria, nuclear pores, glycogen granules) in their native state, confirming the viability of the micropatterning and cryo-FIB approach for complex cellular analysis.

5. Significance and Implications

• Mechanistic Insight: The study provides the first direct evidence that MYH7 mutations cause nanoscale disorganization of thick filaments. This suggests that the "disarray" characteristic of HCM begins at the molecular level (thick filament misalignment) and propagates upward to the myofibril and tissue levels, preceding overt hypertrophy.
• Developmental Context: Since hiPSC-CMs resemble fetal cardiomyocytes, these findings imply that mutation-driven structural defects emerge early during cardiomyocyte maturation, potentially driving the disease process before clinical symptoms appear.
• Proteostasis Hypothesis: The presence of ribosomes in disordered regions supports a model where sarcomeric instability triggers local translation or proteolytic remodeling. This links mechanical failure to cellular repair mechanisms.
• Therapeutic Relevance: Understanding that force production changes lead to structural disarray reinforces the potential of drugs like mavacamten (which modulates myosin activity) to restore structural order, not just reduce force.
• Methodological Advance: The established workflow demonstrates that cryo-ET is a powerful tool for studying genetic heart diseases in human cell models, bridging the gap between molecular biophysics and cellular pathology.

In summary, this paper utilizes advanced cryo-ET to reveal that the P710R MYH7 mutation disrupts the fundamental nanoscale architecture of the sarcomere, providing a structural basis for the cellular disarray observed in Hypertrophic Cardiomyopathy.