Myeloid DRP1 Sulfenylation Drives Reparative Macrophage Polarization and Neovascularization in Ischemic Muscle

This study identifies cysteine sulfenylation of the mitochondrial fission protein DRP1 as a critical redox-sensing mechanism that links ischemia-induced ROS to reparative macrophage polarization and neovascularization, thereby promoting blood flow recovery in peripheral artery disease.

The Gist

The Big Picture: Fixing a Blocked Highway

Imagine your body's blood vessels are a network of highways. In a condition called Peripheral Artery Disease (PAD), a major highway (an artery) gets blocked, cutting off traffic (blood flow) to a destination (like a leg muscle). This causes the muscle to starve and potentially die.

To save the muscle, the body tries to build new detour roads (new blood vessels) to bypass the blockage. This process is called revascularization.

The paper asks: Who are the construction workers building these detours, and how do they know what to do?

The Construction Crew: Macrophages

The main construction workers are immune cells called Macrophages. Think of them as a dual-purpose crew:

1. The Demolition Crew (M1 type): They show up first to clear debris and fight infection. They are loud, aggressive, and use a lot of energy (sugar/glycolysis).
2. The Construction Crew (M2 type): They show up later to build new roads and repair the damage. They are calm, efficient, and use a steady fuel source (oxygen/oxidative metabolism).

For a leg to heal, the body needs to switch the crew from "Demolition" mode to "Construction" mode quickly. If they stay in "Demolition" mode too long, the leg gets damaged and new roads never get built.

The Foreman: DRP1

Inside every cell, there is a tiny machine called DRP1. Think of DRP1 as the Foreman of the cell's power plant (the mitochondria).

• Normally, the Foreman knows when to split the power plant into smaller, efficient units (fission) to get the job done.
• The researchers already knew that in a blocked leg, this Foreman (DRP1) gets activated to help the Construction Crew (M2 macrophages) build new roads.
• The Mystery: They didn't know how the Foreman got the signal to start working. Was it a phone call? A text message? A chemical switch?

The Discovery: The "Redox Switch" (Sulfenylation)

The researchers discovered that the signal isn't a phone call; it's a chemical spark.

When blood flow stops, the body produces a tiny bit of Reactive Oxygen Species (ROS). Think of ROS not as "rust" that damages things, but as a flashlight beam or a spark that turns things on.

1. The Spark: In a healthy healing process, this "spark" hits the Foreman (DRP1).
2. The Switch: The spark hits a specific spot on the Foreman called Cysteine 631. It attaches a tiny chemical tag to it. The scientists call this Sulfenylation.
3. The Result: This tag acts like a "Start" button. It tells the Foreman to split the power plant, which changes the cell's fuel source from "sugar-burning" (Demolition) to "oxygen-burning" (Construction). The cell switches from M1 (Demolition) to M2 (Construction).

The Experiment: What happens when the switch is broken?

To prove this, the scientists created a group of mice with a "broken switch." They genetically engineered mice where the Foreman (DRP1) had a spot that couldn't accept the "spark" (the Cysteine was replaced with Alanine).

• The Result: When these mice had a blocked leg, their Foreman couldn't feel the spark.
• The Consequence:
  • The power plant stayed fused and inefficient.
  • The cells kept burning sugar (Demolition mode) instead of switching to construction.
  • The "Demolition Crew" (M1) stayed angry and aggressive.
  • The "Construction Crew" (M2) never showed up.
  • Outcome: The mice couldn't build new roads. Their legs remained starved, and the damage got worse.

The Analogy: The Car Engine

Imagine your car engine (the cell) has a turbocharger (DRP1).

• Normal Healing: When you need to speed up (heal), a sensor detects a signal and sprays a special fuel additive (Sulfenylation) into the turbo. The turbo spins, the engine shifts gears, and the car accelerates smoothly.
• Broken Switch: In the mutant mice, the sensor is broken. Even though the signal is there, the additive doesn't stick. The turbo doesn't spin. The engine stays in "idling" or "stalling" mode. The car (the leg) never gets moving.

Why This Matters

This paper is a breakthrough because:

1. It solves a mystery: It explains how the body knows to switch from fighting inflammation to building new blood vessels.
2. It finds a new target: Currently, there are no good drugs to help people with severe PAD grow new blood vessels. This research suggests that if we can design a drug that mimics this "spark" (or protects the switch), we might be able to force the body to repair itself.
3. It changes the view on "Oxidation": We often think of oxidation (rust) as bad. This study shows that a controlled amount of oxidation is actually a vital signal for healing.

In short: The body uses a tiny chemical "spark" to flip a switch on a cellular foreman. This switch tells the immune system to stop fighting and start building. If that switch is broken, the leg can't heal. Fixing the switch could be the key to saving limbs.

Technical Summary

1. Problem Statement

Peripheral Artery Disease (PAD), particularly in the context of critical limb ischemia, is characterized by impaired endogenous revascularization, chronic inflammation, and tissue damage. While the recruitment of monocytes/macrophages and their subsequent polarization from pro-inflammatory (M1) to reparative (M2) phenotypes are known to be crucial for vascular regeneration, the molecular mechanisms governing this fate decision remain incompletely understood.

Previous studies established that:

• Reactive Oxygen Species (ROS) are signaling molecules required for ischemia-induced neovascularization.
• The mitochondrial fission protein DRP1 (Dynamin-related protein 1) is essential for reparative macrophage polarization and metabolic reprogramming.
• DRP1 activation in ischemic macrophages occurs independently of canonical phosphorylation (Ser616/Ser637), suggesting a non-canonical regulatory mechanism.

The Knowledge Gap: The specific redox-dependent mechanism that activates DRP1 in ischemic macrophages and how this activation links ROS signaling to metabolic reprogramming and angiogenesis was unknown.

2. Methodology

The study employed a combination of in vivo mouse models, in vitro cellular assays, and advanced molecular techniques:

• Animal Models:
  • Hindlimb Ischemia (HLI): A standard mouse model of PAD created by ligating and resecting the femoral artery.
  • CRISPR/Cas9 Knock-in Mice: Generation of "redox-dead" mice where the redox-sensitive cysteine residue in DRP1 (Cys631 in mice; Cys644 in humans) was mutated to Alanine (Drp1C631A or Drp1C/A).
  • Bone Marrow Transplantation (BMT): Lethally irradiated Wild-Type (WT) recipient mice were reconstituted with bone marrow from either WT or Drp1C/A donors. This isolated the effect of myeloid-specific DRP1 sulfenylation.
• In Vitro Models:
  • Hypoxia-Serum Starvation (HSS): Bone Marrow-Derived Macrophages (BMDMs) from WT and Drp1C/A mice were subjected to HSS to mimic ischemic stress.
• Key Assays:
  • Perfusion & Histology: Laser Doppler blood flow (LDBF) and Laser Speckle Contrast Imaging for perfusion recovery; CD31 staining for capillary density.
  • Flow Cytometry (FACS): Immunophenotyping of immune cells (Ly6G neutrophils, Ly6C monocytes, F4/80 macrophages, CD80 M1, CD206 M2) in ischemic muscle.
  • Redox Detection: DCP-Bio1 assay to specifically detect and pull down sulfenylated proteins (Cys-OH formation).
  • Mitochondrial Dynamics: Seahorse XF analysis for Oxygen Consumption Rate (OCR) and Extracellular Acidification Rate (ECAR); MitoSOX for mitochondrial ROS; microscopy for mitochondrial morphology.
  • Molecular Analysis: Western blotting for signaling pathways (p-AMPK, p-NF-κB) and qRT-PCR for gene expression (M1/M2 markers).

3. Key Contributions

• Discovery of a Novel Redox Switch: The study identifies cysteine sulfenylation (Cys-OH) at residue Cys631 of DRP1 as the critical redox modification that activates DRP1 in ischemic macrophages, operating independently of canonical phosphorylation.
• Mechanistic Link: It establishes a direct causal link between ischemia-induced ROS, DRP1 sulfenylation, mitochondrial fission, and the metabolic reprogramming required for macrophage polarization.
• Therapeutic Target: It identifies DRP1 sulfenylation as a novel therapeutic target to enhance revascularization in PAD by promoting reparative macrophage phenotypes.

4. Detailed Results

A. DRP1 Sulfenylation is Essential for Revascularization

• In Vivo Observation: Following HLI, DRP1 sulfenylation in bone marrow-derived cells peaked at Day 1 and declined by Day 3.
• Functional Deficit: Drp1C/A BM chimeric mice (lacking DRP1 sulfenylation) exhibited significantly impaired limb perfusion recovery and reduced CD31+ capillary density in ischemic muscles compared to WT controls.

B. Impact on Macrophage Polarization and Immune Infiltration

• Phenotypic Skewing: The loss of DRP1 sulfenylation caused a shift in macrophage polarization:
  • Decreased: Anti-inflammatory, pro-angiogenic M2-like macrophages (F4/80+CD206+).
  • Increased: Pro-inflammatory M1-like macrophages (F4/80+CD80+).
• Neutrophil Accumulation: Drp1C/A mice showed enhanced accumulation of Ly6G+ neutrophils at Day 3, suggesting a failure in the clearance of apoptotic neutrophils (efferocytosis), a process usually required to trigger the M2 switch.
• Monocyte Recruitment: Monocyte recruitment (Ly6C+) was unaffected, indicating DRP1 sulfenylation regulates macrophage fate rather than recruitment.

C. Mechanism: Mitochondrial Dynamics and Metabolic Reprogramming

• Mitochondrial Fission: Under HSS, WT macrophages underwent rapid mitochondrial fission. In contrast, Drp1C/A macrophages displayed mitochondrial hyperfusion (elongated mitochondria) and failed to undergo fission.
• ROS Production: Despite normal basal respiration, Drp1C/A macrophages exhibited excessive mitochondrial ROS (mitoROS) production under ischemic stress.
• Metabolic Shift:
  • Glycolysis: Drp1C/A macrophages showed increased glycolytic flux (high ECAR).
  • Signaling Pathways: There was suppressed AMPK activation (normally anti-inflammatory/M2-promoting) and enhanced NF-κB phosphorylation (pro-inflammatory/M1-promoting).
• Gene Expression: Drp1C/A macrophages upregulated M1 genes (Nos2, Ptgs2, Tnfα) and downregulated M2 genes (Retnla).

5. Significance and Conclusion

This study redefines the understanding of macrophage-mediated tissue repair in ischemic disease.

• Novel Mechanism: It uncovers that DRP1 is activated via sulfenylation (a redox modification) rather than just phosphorylation in the context of ischemia. This modification acts as a "redox sensor" that translates ischemic ROS signals into mitochondrial fission.
• Pathophysiological Insight: The failure of this redox switch leads to mitochondrial dysfunction (hyperfusion, excess mitoROS), which drives a metabolic shift toward glycolysis and sustained pro-inflammatory M1 polarization. This creates a vicious cycle of chronic inflammation and failed angiogenesis.
• Therapeutic Implication: Targeting the DRP1 sulfenylation pathway could offer a new strategy to treat PAD. By preserving or mimicking this redox modification, therapies could potentially force macrophages into a reparative M2 state, resolve inflammation, and restore blood flow to ischemic limbs.

Limitations Noted: The precise upstream source of ROS (e.g., NOX2 vs. mitochondrial) driving the sulfenylation in vivo requires further cell-specific dissection, and the downstream molecular links between fission and metabolic reprogramming need further elucidation.