PARP16 protects against cardiac hypertrophic response by ADP-ribosylation-dependent inhibition of NFAT transcription factor

This study demonstrates that PARP16 protects against cardiac hypertrophy and heart failure by ADP-ribosylating and inhibiting the transcriptional activity of NFAT1, a mechanism whose loss leads to pathological cardiac remodeling.

The Gist

Imagine your heart as a bustling, high-performance factory. Its workers are the heart muscle cells (cardiomyocytes), and their job is to pump blood tirelessly. Sometimes, due to stress, high blood pressure, or disease, the factory gets overwhelmed. The workers try to cope by getting bigger and building more machinery. This is called cardiac hypertrophy. While it starts as a helpful attempt to handle the load, if it goes on too long, the factory gets clogged, the walls get too thick or too weak, and the whole system starts to fail. This is heart failure.

For a long time, scientists knew that the factory had a "safety switch" to stop this overgrowth, but they didn't know exactly how it worked. This paper introduces a new hero: a tiny protein called PARP16.

The Hero: PARP16 (The "Brake Pedal")

Think of PARP16 as a specialized brake pedal for the heart factory. Under normal conditions, it sits quietly, making sure the workers don't get too big or too excited.

However, when the heart is under stress (like in heart failure), the paper found that this brake pedal gets stolen or broken. The levels of PARP16 drop significantly in failing human hearts and in mice with heart stress. Without this brake, the factory goes into overdrive.

The Villain: NFAT1 (The "Overzealous Foreman")

Inside the factory, there is a foreman named NFAT1. His job is to tell the workers to grow and build more.

• Normally: PARP16 keeps a close eye on NFAT1. It puts a little "handcuff" on him (a chemical tag called ADP-ribosylation) that stops him from running around the factory shouting, "Grow! Grow! Grow!"
• When PARP16 is missing: The handcuffs are gone. NFAT1 runs wild, moving into the control room (the cell nucleus) and screaming orders to the workers. The workers get huge, the factory walls get thick and scarred (fibrosis), and the heart stops pumping efficiently.

The Experiment: Testing the Theory

The scientists played detective using mice to prove this story:

1. Removing the Brake (Knockout Mice): They genetically engineered mice that had no PARP16 at all.
   • Result: Just like removing a car's brakes, the mice's hearts grew too big, became weak, and started failing. The "foreman" NFAT1 was running wild.
2. Adding Extra Brakes (Transgenic Mice): They made mice that had extra PARP16.
   • Result: When these mice were stressed with a drug that usually causes heart failure, their hearts stayed healthy. The extra PARP16 kept the NFAT1 foreman in check.
3. The Chemical Tag: Using advanced microscopes and chemical analysis, they found exactly where PARP16 puts the handcuffs on NFAT1. It attaches a tiny chemical tag to two specific spots (E398 and T533) on the NFAT1 protein. This tag is the signal that says, "Stop! Do not enter the control room."

The Solution: Fixing the Problem

The most exciting part of the study is the potential cure. The scientists tried to fix the broken hearts of the PARP16-deficient mice by giving them a drug that blocks NFAT1 directly (like putting a gag on the overzealous foreman).

• Result: Even without the PARP16 brake, blocking the foreman stopped the heart failure. The heart function improved, and the structural damage was reduced.

The Big Picture

This paper tells us that PARP16 is a critical protector of the heart. It works by chemically tagging and silencing a protein (NFAT1) that causes the heart to grow dangerously large.

Why does this matter?
Currently, treatments for heart failure are like trying to fix a leaky roof with duct tape—they manage symptoms but don't fix the root cause. This study suggests a new strategy:

• We could develop drugs that boost PARP16 levels in the heart.
• Or, we could develop drugs that mimic the chemical tag PARP16 puts on NFAT1, effectively putting the handcuffs back on the foreman even if the brake pedal is broken.

In short, this research identifies a new "master switch" that could help us stop heart failure before it destroys the factory, offering hope for a more targeted and effective treatment in the future.

Technical Summary

1. Problem Statement

Heart failure (HF) and cardiac hypertrophy are major causes of global mortality, driven by pathological cardiac remodeling. While post-translational modifications (PTMs) like phosphorylation and ubiquitination are known to regulate cardiac stress responses, the role of mono-ADP-ribosylation in cardiac pathophysiology remains poorly understood. Specifically, the function of PARP16, an endoplasmic reticulum (ER)-localized mono-ADP-ribosyltransferase known for regulating the Unfolded Protein Response (UPR), has not been explored in the context of cardiac hypertrophy and heart failure. Previous literature suggested conflicting roles for PARP family members in the heart, necessitating a clear definition of PARP16's specific contribution to cardiac health.

2. Methodology

The study employed a multi-faceted approach combining clinical analysis, genetic mouse models, in vitro cellular assays, and advanced computational biology:

• Clinical and Experimental Models:
  • Human Samples: Analyzed PARP16 expression in heart septum samples from patients with dilated cardiomyopathy (DCM) and preoperative heart failure compared to non-failing controls.
  • In Vivo Models: Generated and characterized four distinct mouse models:
    1. Whole-body PARP16 Knockout (KO).
    2. Cardiac-specific PARP16 Knockout (csKO) using Myh6-Cre.
    3. Tamoxifen-inducible cardiac-specific PARP16 Knockout (icsKO) to rule out developmental compensation.
    4. Cardiac-specific PARP16 Transgenic (csTG) mice for overexpression.
  • Stress Induction: Used Isoproterenol (ISO) and Phenylephrine (PE) to induce cardiac stress/hypertrophy in mice and neonatal rat cardiomyocytes (NRCMs).
• Phenotypic Assessment:
  • Echocardiography: Measured fractional shortening, left ventricular internal diameter (LVID), and wall thickness.
  • Histology: Used Wheat Germ Agglutinin (WGA) for cardiomyocyte cross-sectional area and Masson's Trichrome for fibrosis.
  • Molecular Biology: RT-qPCR, immunoblotting, and immunoprecipitation to assess gene expression, protein levels, and interactions.
• Mechanistic Investigations:
  • RNA Sequencing (RNA-seq): Performed on KO hearts to identify dysregulated pathways.
  • Mass Spectrometry (MS): Identified specific ADP-ribosylation sites on NFAT1.
  • Computational Modeling: Utilized AlphaFold2/3 for structure prediction, ClusPro/HDOCK for protein-protein docking, and 1000 ns all-atom Molecular Dynamics (MD) simulations to analyze the PARP16-NAD-NFAT1 ternary complex.
  • Functional Assays: Luciferase reporter assays for NFAT transcriptional activity and rescue experiments using the NFAT inhibitor VIVIT.

3. Key Contributions

• Discovery of PARP16 Downregulation: Established that PARP16 expression is significantly reduced in human heart failure samples and in mouse models of cardiac stress (ISO/PE treatment).
• Genetic Validation: Demonstrated that PARP16 deficiency (both constitutive and inducible) is sufficient to cause eccentric hypertrophy, fibrosis, and systolic dysfunction, while overexpression protects against stress-induced remodeling.
• Novel Mechanism: Identified NFAT1 (Nuclear Factor of Activated T-cells 1) as the direct substrate of PARP16 in the heart.
• Site-Specific Modification: Mapped two specific mono-ADP-ribosylation sites on NFAT1 (E398 and T533 within the Rel Homology Domain) that are critical for PARP16-mediated inhibition of NFAT1 activity.
• Therapeutic Insight: Showed that pharmacological inhibition of NFAT activation rescues the cardiac dysfunction caused by PARP16 deficiency.

4. Key Results

• Phenotypic Consequences of Depletion:
  • PARP16 KO, csKO, and icsKO mice exhibited increased heart weight-to-body weight ratios, enlarged LVID, reduced fractional shortening, increased cardiomyocyte size, and interstitial fibrosis.
  • These mice showed upregulation of fetal genes (ANP, BNP, $\beta$-MHC).
  • Conversely, PARP16 csTG mice maintained normal cardiac physiology under basal conditions and were protected from ISO-induced hypertrophy and dysfunction.
• Transcriptomic and Signaling Analysis:
  • RNA-seq revealed upregulation of hypertrophy and dilated cardiomyopathy gene signatures in KO hearts.
  • NFAT1 signaling was hyperactivated in PARP16-deficient hearts, evidenced by increased nuclear translocation of NFAT1, reduced phosphorylation (activation state), and elevated transcriptional activity.
  • Crucially, canonical UPR pathways were not significantly altered, indicating the cardiac phenotype is independent of the ER stress response.
• Molecular Interaction and Modification:
  • Interaction: Co-immunoprecipitation confirmed a physical interaction between PARP16 and NFAT1.
  • Catalytic Dependence: The catalytically inactive PARP16 mutant (Y254A) failed to suppress NFAT1 activity, confirming the role of ADP-ribosylation.
  • Mass Spec & MD Simulations: MS identified ADP-ribosylation at E398 and T533 of NFAT1. MD simulations showed these residues are in close proximity to the PARP16 catalytic site and NAD+, forming stable hydrogen bonds and van der Waals contacts.
  • Functional Validation: Mutating E398 and T533 (E398Q, T533A, and double mutant) rendered NFAT1 resistant to PARP16-mediated suppression, confirming these sites are essential for regulation.
• Rescue Experiment: Treatment of PARP16-deficient mice and cardiomyocytes with VIVIT (an NFAT inhibitor) reversed the hypertrophic phenotype, improved fractional shortening, and normalized NFAT activity, proving that NFAT1 hyperactivation is the primary driver of the pathology.

5. Significance

• Novel Regulatory Axis: This study defines a new regulatory axis where PARP16 acts as a "brake" on cardiac hypertrophy by mono-ADP-ribosylating and inhibiting the pro-hypertrophic transcription factor NFAT1.
• Context-Dependent Function: It clarifies a discrepancy in the literature regarding PARP16's role; while a previous study suggested PARP16 promotes hypertrophy via the IRE1$\alpha$-GATA4 pathway, this study demonstrates that in the context of direct NFAT1 regulation, PARP16 is protective. The difference is attributed to the use of genetic deletion models (KO) versus transient knockdown/overexpression in prior studies.
• Therapeutic Potential: The findings suggest that maintaining or enhancing PARP16 levels, or targeting the downstream NFAT1 activation, could be a viable therapeutic strategy for treating heart failure and pathological cardiac remodeling.
• PTM Expansion: It highlights mono-ADP-ribosylation as a critical, previously underappreciated PTM in cardiovascular disease, expanding the landscape of post-translational regulation in the heart beyond phosphorylation and acetylation.