Neprilysin inhibition reduces microtubule detyrosination in cardiomyocytes through a cGMP-PRKG1-VASH1 axis

This study identifies a cGMP-PRKG1-VASH1 signaling axis activated by neprilysin inhibition that phosphorylates VASH1 to suppress microtubule detyrosination, thereby providing a mechanistic basis for the therapeutic benefits of LCZ696 in heart failure.

The Gist

The Big Picture: Fixing a Stiff Heart

Imagine your heart muscle cells are like a busy construction site. To keep the building (the heart) strong and flexible, it needs a sturdy scaffolding system made of tiny rods called microtubules.

In a healthy heart, these rods are flexible and can be easily rearranged. But in a failing heart (heart failure), these rods get "stuck" in a rigid state. Scientists call this detyrosination. Think of it like the scaffolding rods getting covered in a layer of super-hard, sticky cement. This makes the heart muscle stiff, unable to squeeze and relax properly, which leads to heart failure.

The researchers wanted to know: Can a popular heart failure drug help remove this "cement" and make the heart flexible again?

The Drug: The "Double-Action" Hero

The study focused on LCZ696 (often known by the brand name Entresto). This drug is actually a "two-in-one" package containing two active ingredients:

1. Valsartan: Blocks a hormone that tightens blood vessels.
2. Sacubitrilat: Stops an enzyme (neprilysin) from breaking down "relaxing" hormones.

The team tested these ingredients on human heart cells grown in a lab to see which one could clean off that sticky "cement" (reduce detyrosination).

The Surprise: While both ingredients stopped the heart cells from getting too big (hypertrophy), only Sacubitrilat successfully removed the sticky cement from the microtubules. Valsartan alone didn't do it.

The Mechanism: The "Relaxation Chain Reaction"

How did Sacubitrilat do this? The researchers discovered a specific chain of events, like a row of dominoes falling:

1. The Signal: Sacubitrilat stops the breakdown of "relaxing" hormones (natriuretic peptides). This is like turning up the volume on a "calm down" radio station.
2. The Messenger: These hormones trigger the production of a chemical messenger called cGMP. Think of cGMP as a courier delivering an urgent message: "We need to loosen up!"
3. The Worker: This message activates a worker protein called PRKG1. Imagine PRKG1 as a specialized mechanic with a wrench.
4. The Target: The mechanic (PRKG1) goes to the enzyme responsible for making the rods sticky, called VASH1.
5. The Fix: The mechanic puts a "tag" (a phosphate group) on VASH1. This is like putting a "Do Not Touch" sign or a "Stop" sticker on the enzyme. Once tagged, VASH1 can no longer stick to the microtubules or apply the sticky cement.

The Result: Without the sticky cement, the microtubules stay flexible, and the heart muscle can contract and relax more easily.

The "Switch" Analogy

To prove this, the scientists played a game of "switches" in the lab:

• They created a version of the enzyme (VASH1) that couldn't be tagged by the mechanic. This version kept making the heart stiff.
• They created a "fake" version (VASH1-7E) that looked like it had already been tagged (a "phosphomimic"). This fake version refused to stick to the rods, keeping them flexible.

This confirmed that the "tagging" process is the key to unlocking the heart's flexibility.

Why This Matters

This study explains why a drug like LCZ696 works so well for heart failure patients. It's not just about lowering blood pressure; it's about physically remodeling the heart's internal structure to make it less stiff and more efficient.

In short: The drug turns on a signal that tells a specific enzyme to stop making the heart's internal scaffolding stiff. It's like finding the secret switch that turns a rigid, frozen heart back into a flexible, pumping machine.

Technical Summary

1. Problem Statement

• Clinical Context: Elevated levels of detyrosinated $\alpha$-tubulin (dTyr-tub) are a consistent pathological hallmark across all forms of heart failure (HFrEF, HFpEF, HCM, DCM). High dTyr-tub levels increase cytoskeletal stiffness, impairing cardiomyocyte contractility and contributing to disease progression.
• Therapeutic Gap: While reducing dTyr-tub levels (via VASH1 knockdown or TTL overexpression) has been shown to improve cardiac function, the mechanism by which clinically approved heart failure therapies, specifically the angiotensin receptor-neprilysin inhibitor (ARNI) LCZ696 (sacubitril/valsartan), might modulate this pathway was unknown.
• Specific Question: Does the neprilysin inhibitor component (sacubitrilat) or the angiotensin receptor blocker (valsartan) of LCZ696 influence microtubule detyrosination in cardiomyocytes, and if so, through what molecular mechanism?

2. Methodology

The study utilized a multi-faceted approach combining human cell models, molecular biology, and in vitro biophysics:

• Cell Model: Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Hypertrophy was induced using Endothelin-1 (ET1).
• Genetic Manipulation:
  • Knockout (KO) Lines: Generation of hiPSC lines deficient in SVBP (reduces dTyr-tub), TTL (increases dTyr-tub), and VASH1 (using CRISPR/Cas9).
  • Knockdown (KD): siRNA-mediated knockdown of PRKG1.
  • Overexpression: Lentiviral transduction of VASH1 mutants (Wild-Type, Phosphomimetic VASH1-7E, and Non-phosphorylatable VASH1-7A) into VASH1-KO hiPSC-CMs.
• Pharmacological Interventions: Treatment with Sacubitrilat (sac), Valsartan (val), CNP (C-type natriuretic peptide), and the PRKG1 inhibitor RP-8-Br-PET-cGMPS (RP8).
• Omics & Signaling:
  • RNA Sequencing (RNA-seq): To identify dysregulated pathways in ET1-treated cells and those rescued by sacubitrilat.
  • cGMP Measurement: ELISA and FRET imaging (using cGi500 sensor) to quantify intracellular cGMP levels.
• Biochemical & Biophysical Assays:
  • In Vitro Kinase Assay: Using ATP$\gamma$S and specific antibodies to detect phosphorylation of VASH1 by PRKG1A.
  • TIRF Microscopy: Total Internal Reflection Fluorescence microscopy to visualize single-molecule binding of VASH1-SVBP complexes to microtubules.
  • Immunofluorescence: To assess microtubule detyrosination/tyrosination status in cells and in vitro.

3. Key Contributions

This study establishes a novel signaling axis linking neprilysin inhibition to cytoskeletal remodeling:

1. Discovery of Specificity: Identified that sacubitrilat (but not valsartan) specifically prevents ET1-induced microtubule detyrosination, despite both drugs preventing cellular hypertrophy.
2. Mechanistic Pathway: Defined the cGMP-PRKG1-VASH1 axis. Sacubitrilat increases natriuretic peptides $\rightarrow$ activates guanylate cyclase $\rightarrow$ increases cGMP $\rightarrow$ activates PRKG1 $\rightarrow$ phosphorylates VASH1.
3. Regulatory Mechanism: Demonstrated that phosphorylation of VASH1 by PRKG1 acts as a "brake" on detyrosination. Phosphorylation (or phosphomimicry) prevents the VASH1-SVBP complex from binding to microtubules, thereby inhibiting the removal of the C-terminal tyrosine.
4. Therapeutic Insight: Provides a molecular explanation for the superior efficacy of ARNI therapy in heart failure, suggesting that cytoskeletal softening via dTyr-tub reduction is a key therapeutic mechanism.

4. Key Results

• Differential Effects of LCZ696 Components:
  • ET1 induced a 2.5-fold increase in cell volume and a >1.5-fold increase in dTyr-tub levels.
  • Both sacubitrilat and valsartan prevented cell volume expansion (hypertrophy).
  • Crucially, only sacubitrilat prevented the accumulation of dTyr-tub. The combination treatment (sac+val) did not reduce dTyr-tub in this specific in vitro model, suggesting valsartan may mask the effect of sacubitrilat under ET1 conditions, or that the effect is strictly dependent on the neprilysin inhibition pathway.
• Role of PRKG1:
  • Inhibition of PRKG1 (via RP8) or knockdown of PRKG1 mRNA/protein increased dTyr-tub levels, confirming PRKG1 is a negative regulator of detyrosination.
  • Sacubitrilat treatment increased intracellular cGMP levels (approx. 2-fold) and phosphorylation of the PRKG1 target VASP.
• Phosphorylation of VASH1:
  • In Vitro Kinase Assay: PRKG1A phosphorylated the C-terminal domain of VASH1 at seven specific serine residues (S313, S323, S324, S330, S331, S337, S344).
  • Binding Affinity: TIRF microscopy showed that the wild-type VASH1-SVBP complex binds to microtubules and induces detyrosination. In contrast, the phosphomimetic mutant (VASH1-7E) failed to bind to microtubules and could not induce detyrosination.
  • Cellular Validation: In VASH1-KO hiPSC-CMs, overexpression of VASH1-7E resulted in significantly lower dTyr-tub levels compared to the non-phosphorylatable mutant (VASH1-7A), confirming that phosphorylation inhibits the enzyme's activity in a cellular context.
• Transcriptomic Analysis:
  • RNA-seq revealed that sacubitrilat normalized ET1-induced upregulation of hypertrophic genes (e.g., CSRP3, DES, MYH7) and restored expression of genes related to cell division and microtubule organization.

5. Significance

• Mechanistic Insight: The study elucidates a previously unknown mechanism of action for neprilysin inhibitors. Beyond hemodynamic effects, they directly modulate the cardiomyocyte cytoskeleton to reduce stiffness.
• Target Validation: It validates VASH1 as a critical therapeutic target. The study suggests that mimicking the phosphorylation state of VASH1 (or inhibiting VASH1 activity) could be a strategy to treat heart failure.
• Clinical Relevance: The findings support the clinical superiority of ARNI (LCZ696) over angiotensin receptor blockers alone in heart failure, attributing part of this benefit to the restoration of normal microtubule dynamics via the cGMP-PRKG1 pathway.
• Future Directions: The identification of the specific phosphorylation sites on VASH1 opens avenues for developing small molecules that specifically inhibit VASH1-SVBP binding or enhance PRKG1-mediated phosphorylation to treat heart failure.