PARP6-dependent vimentin ADP-ribosylation prevents myofibroblast activation in cardiac fibrosis

This study identifies PARP6 as a cardioprotective enzyme that suppresses cardiac fibrosis by ADP-ribosylating vimentin to inhibit RhoA-mediated actin stress fiber formation and myofibroblast activation.

The Gist

The Big Picture: A Broken Brake in the Heart

Imagine your heart is a busy city. When the heart gets sick (heart failure), the "construction crews" in the city (called fibroblasts) go into overdrive. They start building too much "concrete" (scar tissue) and tightening the streets too much. This process is called cardiac fibrosis. It makes the heart stiff, unable to pump blood properly, and eventually leads to heart failure.

For a long time, scientists knew that this happened, but they didn't fully understand how the construction crews decided to start working so hard.

This paper discovers a specific "brake pedal" that usually keeps these crews in check. The brake is a protein called PARP6. When the brake is working, the heart stays flexible. When the brake breaks or disappears, the construction crews go wild, and the heart becomes stiff and scarred.

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The Main Characters

1. The Construction Crew (Fibroblasts): These are cells that normally fix small injuries. In heart failure, they get confused and turn into "super-workers" (myofibroblasts) that build too much scar tissue.
2. The Scaffolding (Vimentin): Think of this as the internal steel beams inside the construction crew's building. It gives the cell its shape and helps it move.
3. The Brake (PARP6): This is a special enzyme (a molecular machine) that acts like a supervisor. Its job is to put a tiny "sticky note" (called ADP-ribosylation) on the steel beams (Vimentin).
4. The Engine (RhoA): This is a protein that acts like the gas pedal. When it's active, it tells the cell to tighten up, pull hard, and build scar tissue.

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How the Story Unfolds

1. The Brake is Missing in Sick Hearts

The researchers looked at hearts from people who had died of heart failure. They found that the PARP6 brake was almost completely missing. At the same time, the hearts were covered in scar tissue. It was like finding a car with no brakes and a pile of concrete in the driveway.

2. Testing the Theory with Mice

To prove that the missing brake caused the problem, they created mice that had only half a dose of the PARP6 brake (like having a car with a slightly worn-out brake pedal).

• Result: Even with just a little less brake, these mice developed heart failure. Their hearts got bigger, stiffer, and full of scar tissue. This proved that PARP6 is essential for keeping the heart healthy.

3. The Mechanism: How the Brake Works

This is the coolest part of the discovery. How does PARP6 stop the construction crew?

• The Sticky Note: PARP6 puts a chemical "sticky note" (ADP-ribosylation) on the Vimentin steel beams.
• The Disconnection: When Vimentin has this sticky note, it acts like a magnet that repels the Engine (RhoA). The Engine can't get close to the steel beams, so it can't turn on.
• The Chaos: When PARP6 is missing, the sticky note disappears. Now, the Engine (RhoA) can grab onto the Steel Beams (Vimentin) tightly.
• The Result: Once connected, the Engine revs up. It tells the cell to tighten its muscles (actin stress fibers), pull harder, and start building scar tissue.

Analogy: Imagine Vimentin is a steering wheel. PARP6 puts a piece of tape on the steering wheel. As long as the tape is there, the driver (RhoA) can't get a good grip, so the car (the cell) stays calm. If you peel off the tape (remove PARP6), the driver gets a perfect grip, spins the wheel, and drives the car off a cliff into a pile of concrete.

4. The Domino Effect

Once the Engine (RhoA) is turned on, it triggers a chain reaction:

1. It activates a protein called ROCK.
2. ROCK activates LIMK.
3. LIMK stops a "cleanup crew" called Cofilin from working.
4. Because the cleanup crew is stopped, the cell builds massive, unbreakable bundles of muscle fibers (actin stress fibers).
5. The cell becomes a stiff, scar-building machine.

Why This Matters

This discovery is a game-changer for two reasons:

1. New Understanding: It shows that heart failure isn't just about the heart muscle cells dying; it's also about the "construction crew" getting out of control because a specific chemical brake (PARP6) failed.
2. New Treatments: Currently, doctors try to treat heart failure with drugs that lower blood pressure or help the heart pump. This paper suggests a new strategy: Don't just treat the symptoms; fix the brake.
   • Caution: You wouldn't want to give a drug that stops PARP6 (that would make fibrosis worse). Instead, scientists might look for drugs that boost PARP6 activity or block the "Engine" (RhoA) to stop the construction crews from going crazy.

Summary in One Sentence

The heart has a built-in safety switch (PARP6) that puts a chemical tag on its internal scaffolding to stop it from getting too stiff; when this switch breaks, the heart's repair cells go into overdrive, turning the heart into a rigid, scarred block that can't pump blood.

Technical Summary

1. Problem Statement

Cardiac fibrosis is a primary driver of adverse heart remodeling and heart failure, characterized by the excessive activation of cardiac fibroblasts into myofibroblasts. This process involves cytoskeletal reorganization, specifically the assembly of actin stress fibers, and the deposition of extracellular matrix (ECM). While post-translational modifications (PTMs) like phosphorylation are well-studied, the role of ADP-ribosylation (specifically mono-ADP-ribosylation or MARylation) in regulating fibroblast activation and cardiac fibrosis remains poorly defined. Current research focuses heavily on PARP1-mediated PARylation in cardiomyocytes, leaving a gap in understanding how mono-ADP-ribosyltransferases (mono-ARTs) like PARP6 regulate cytoskeletal dynamics in fibroblasts.

2. Methodology

The study employed a multi-disciplinary approach combining clinical sample analysis, in vivo mouse models, and extensive in vitro cellular assays:

• Clinical Samples: mRNA and protein expression of various mono-ARTs (PARP3, 6, 8, 9, 10) were quantified in failing human heart tissues compared to non-failing controls.
• In Vivo Models:
  • Isoproterenol (ISO) Model: Induced cardiac dysfunction in wild-type mice to observe PARP6 expression changes.
  • Genetic Models: Generated global Parp6 heterozygous knockout (Parp6^+/-) mice using Cre-loxP recombination. These mice were subjected to echocardiography, histology (Masson's trichrome, WGA staining), and gene expression analysis to assess cardiac remodeling.
• Cell Culture Models:
  • Cell Lines: AC16 cardiomyoblasts and Vimentin-null (Vim^-/-) Mouse Embryonic Fibroblasts (MEFs).
  • Primary Cells: Murine primary cardiac fibroblasts.
  • Interventions: Used pharmacological inhibitors (AZ0108 for PARP6, Talazoparib for PARP1/2, Y-27632 for ROCK, Rhosin for RhoA) and siRNA-mediated knockdowns (Vimentin, RhoA, ROCK, LIMK, Cofilin, GEF-H1).
  • Overexpression: Doxycycline-inducible expression of Wild-Type (WT) PARP6 and catalytically inactive mutants (Y508A).
• Omics and Proteomics:
  • ADP-ribosylome Analysis: Mass spectrometry (LC-MS/MS) using macrodomain enrichment (Af1521/eAf1521) to identify PARP6 substrates and modification sites.
  • Quantitative Proteomics: To analyze global protein abundance changes upon PARP6 inhibition.
• Imaging and Biochemistry:
  • Super-resolution microscopy and confocal imaging for cytoskeletal visualization (F-actin, Vimentin).
  • Immunoprecipitation (IP) and Rhotekin-RBD pulldowns to assess protein-protein interactions (Vimentin-RhoA) and RhoA activation states (RhoA-GTP).
  • Western blotting for signaling pathway markers (p-cofilin, α-SMA, ECM proteins).

3. Key Contributions and Findings

A. PARP6 is Downregulated in Failing Hearts

• Observation: PARP6 mRNA and protein levels were significantly reduced in failing human hearts and in ISO-induced failing mouse hearts.
• In Vivo Phenotype: Parp6^+/- mice (haploinsufficiency) spontaneously developed cardiac hypertrophy, interstitial fibrosis, cardiomyocyte enlargement, and contractile dysfunction (reduced ejection fraction) by 6–7 months of age. They also exhibited increased lung and spleen weights, indicating systemic remodeling.

B. PARP6 is a Cysteine-Specific Writer of Vimentin ADP-ribosylation

• Substrate Identification: Mass spectrometry revealed that PARP6 preferentially modifies cysteine residues on cytoskeletal proteins.
• Primary Target: Vimentin was identified as the most abundant target. The primary modification site was Cys328, with secondary sites at Arg207 and Arg196.
• Structural Impact: PARP6-dependent ADP-ribosylation of Vimentin did not disrupt the bulk filament architecture or lysosomal positioning, suggesting the modification acts as a signaling switch rather than a structural destabilizer.

C. Mechanism: The PARP6-Vimentin-RhoA Axis

• Vimentin-RhoA Interaction: PARP6-mediated ADP-ribosylation of Vimentin prevents the formation of the Vimentin-RhoA complex. When PARP6 is inhibited or Vimentin is knocked down, Vimentin binds more strongly to RhoA.
• RhoA Hyperactivation: Loss of PARP6 activity leads to increased levels of active RhoA-GTP.
• Downstream Signaling: Hyperactive RhoA triggers the RhoA-ROCK-LIMK-cofilin pathway.
  • ROCK phosphorylates LIMK.
  • LIMK phosphorylates Cofilin (at Ser3), inhibiting its actin-severing activity.
  • This results in the accumulation of actin stress fibers.
• Myofibroblast Activation: The accumulation of actin stress fibers increases cellular traction forces, driving the differentiation of fibroblasts into myofibroblasts (marked by increased α-SMA, Fibronectin, and Collagens). This occurs via a TGF-β-independent mechanism in cell culture, driven primarily by mechanotransduction.

D. Genetic Validation

• In Parp6^+/- mouse hearts, there was a marked increase in phosphorylated cofilin (p-cofilin), confirming the activation of the RhoA-ROCK-LIMK pathway in vivo.
• Pharmacological inhibition of PARP6 in primary cardiac fibroblasts recapitulated the Parp6^+/- phenotype (increased stress fibers, migration, and ECM production), which was rescued by inhibiting RhoA or ROCK.

4. Significance and Implications

• Novel Regulatory Mechanism: The study defines a previously unknown PARP6-Vimentin-RhoA axis that acts as a "brake" on fibroblast activation. It establishes ADP-ribosylation as a critical regulator of cytoskeletal mechanics and mechanotransduction.
• Therapeutic Insight:
  • Cardioprotection: PARP6 acts as a cardioprotective enzyme; its loss promotes fibrosis.
  • Caution with Inhibitors: While PARP inhibitors are used in cancer, this study suggests that systemic inhibition of PARP6 (or off-target effects of PARP1/2 inhibitors) could inadvertently promote cardiac fibrosis by releasing the brake on RhoA-driven myofibroblast activation.
  • Targeting the Pathway: The convergence on the RhoA-ROCK-LIMK pathway highlights these enzymes as potential therapeutic targets to prevent fibrosis, specifically by maintaining the balance of Vimentin ADP-ribosylation.
• Metabolic Link: Since ADP-ribosylation consumes NAD+, the study links metabolic stress (NAD+ depletion) and PARP6 downregulation to the loss of cytoskeletal regulation, providing a mechanistic link between metabolic state and cardiac fibrosis.

Conclusion

The paper demonstrates that PARP6 prevents cardiac fibrosis by ADP-ribosylating Vimentin at Cys328. This modification disrupts the interaction between Vimentin and RhoA, thereby preventing RhoA hyperactivation, actin stress fiber accumulation, and subsequent myofibroblast differentiation. The loss of PARP6, observed in failing hearts, removes this protective mechanism, driving pathological cardiac remodeling.