Chemical activation of mitophagy via the N-degron pathway alleviates mitochondrial neuropathies

The study presents ATB1071, an orally bioavailable chemical N-degron that activates p62-mediated mitophagy through both Parkin-dependent and independent pathways, demonstrating therapeutic efficacy in alleviating mitochondrial neuropathies in Leigh syndrome and cerebral ischemia-reperfusion mouse models.

The Gist

Imagine your body is a bustling city, and inside every building (cell), there are tiny power plants called mitochondria. These power plants generate the electricity your city needs to run. But sometimes, these power plants get damaged, start leaking toxic smoke (free radicals), and stop producing power. If you don't clean them out, they can cause the whole city to shut down, leading to diseases like stroke, Alzheimer's, or rare genetic disorders like Leigh Syndrome.

The city has a sanitation crew called mitophagy (literally "mitochondria eating"). Their job is to find the broken power plants, tag them, and haul them away to the recycling center (the lysosome) to be destroyed.

However, in many diseases, the sanitation crew is either asleep, confused, or the "broken" tags aren't being put on the right buildings. Scientists have tried to wake them up with drugs, but most of those drugs are like shouting "Clean everything!" which is too messy and can hurt the healthy buildings too.

The New Solution: A Smart "Wanted" Poster

This paper introduces a new, clever drug called ATB1071. Instead of shouting "Clean everything," ATB1071 acts like a smart, chemical "Wanted" poster.

Here is how it works, using a simple analogy:

1. The Problem: The broken mitochondria are hiding. The city's main security guard (a protein called Parkin) sometimes fails to see them, or the "broken" signal is too weak.
2. The Secret Weapon: The researchers found a special protein called p62. Think of p62 as the foreman of the sanitation crew. Usually, p62 needs a specific "badge" (called an N-degron) to know it's time to work.
3. The Magic Drug (ATB1071): ATB1071 is a tiny molecule that acts as a fake badge. When it enters the cell, it sticks to the p62 foreman. This wakes p62 up and tells it, "Hey! Time to go to work!"
4. The Cleanup: Once woken up, p62 doesn't just clean randomly. It looks for specific "eat me" signals on the broken mitochondria. The paper found two main signals:
   • NIPSNAP1/2: These are like "Help!" flags that pop up on damaged mitochondria.
   • EBP1: This is another signal that appears when mitochondria are really hurt (like during a stroke).
5. The Result: The p62 foreman grabs the broken mitochondria, wraps them in a trash bag (autophagosome), and sends them to the recycling center.

Why This is a Big Deal

The researchers tested this drug on two very different "disaster scenarios":

• Scenario A: The Genetic Glitch (Leigh Syndrome)
  They used mice with a genetic defect that makes their power plants fail from birth. These mice usually die young and have trouble walking.

  • The Result: When they gave the mice ATB1071, the drug cleaned out the broken power plants. The mice lived 30% longer, walked better, and had less brain inflammation. It was like giving a broken-down car a new engine and a tune-up.

• Scenario B: The Sudden Disaster (Stroke)
  They simulated a stroke (ischemia-reperfusion injury) where blood flow is cut off and then restored, causing a massive explosion of damage.

  • The Result: In normal mice, ATB1071 saved a huge chunk of the brain from dying and helped them recover their memory and movement.
  • The Catch: When they tested the drug on mice that lacked the EBP1 signal, the drug didn't work. This proved that the drug needs that specific "Wanted" signal to know what to clean. It's not a magic wand that fixes everything blindly; it's a smart sniper that only hits the damaged targets.

The Safety Check

Before a drug can be used on humans, it must be safe.

• Brain Access: The drug is small enough to cross the "border control" between the blood and the brain (the Blood-Brain Barrier). About 68% of the drug gets into the brain, which is excellent.
• Safety: In tests on mice and rats, the drug was very safe. The dose needed to fix the problem was tiny compared to the dose that would cause harm. It didn't cause cancer or heart problems in the tests.

The Bottom Line

This paper presents a new way to treat diseases caused by broken power plants in our cells. Instead of forcing the whole body to clean itself (which is dangerous), this drug acts as a smart key that unlocks the cell's natural cleaning crew specifically for the broken parts.

It's like having a specialized janitor who only mops up the spilled coffee in the kitchen, leaving the rest of the house perfectly clean, rather than flooding the whole house with water to wash the floor. This approach could lead to new treatments for strokes, neurodegenerative diseases, and rare genetic disorders.

Technical Summary

1. Problem Statement

Mitochondrial dysfunction is a central driver of neurodegenerative disorders (e.g., Leigh Syndrome), ischemic stroke, and aging. While pharmacological activation of mitophagy (selective autophagy of mitochondria) is a promising therapeutic strategy, current approaches face significant hurdles:

• Lack of Selectivity: Many inducers (e.g., rapamycin, urolithin A) act via global mTOR inhibition, affecting healthy mitochondria and causing metabolic dysregulation.
• Mechanistic Limitations: Existing compounds often fail to discriminate between damaged and healthy organelles or rely on pathways (like PINK1/Parkin) that may be compromised in specific disease contexts.
• Clinical Translation: No mitophagy inducer has yet demonstrated clinical efficacy for mitochondria-associated diseases, partly due to poor blood-brain barrier (BBB) penetration and safety concerns.

The authors aimed to develop a small molecule that selectively activates p62-mediated mitophagy via the N-degron pathway without interfering with global autophagy or mTOR signaling, specifically targeting damaged mitochondria in neurological contexts.

2. Methodology

The study employed a multi-disciplinary approach combining structural biology, cell biology, and in vivo animal models:

• Compound Design & Screening:
  • Developed ATB1071, an optimized peptidomimetic small molecule (chemical N-degron) designed to bind the ZZ zinc-finger domain of the autophagy receptor p62/SQSTM1.
  • Used structure-activity relationship (SAR) analysis and molecular docking (MAESTRO) to optimize binding to p62 residues (Asp129, Asp149, Arg139, Lys141).
• In Vitro Mechanistic Studies:
  • Cell Models: HeLa cells (expressing mt-mKeima reporter), SH-SY5Y neuroblastoma cells, and primary neurons.
  • Assays: Flow cytometry for mitophagy quantification (mt-mKeima pH shift), immunoblotting for protein turnover (TOM20, MTCO2, LC3), and co-immunoprecipitation (Co-IP) to map protein interactions.
  • Genetic Manipulation: Used siRNA/shRNA knockdown and CRISPR/Cas9 knockout (p62-, ATE1-, NIPSNAP1/2-, EBP1-deficient) to validate pathway dependencies.
• In Vivo Disease Models:
  • Leigh Syndrome (LS) Model: Ndufs4-/- mice (deficient in Complex I), treated with ATB1071 (10 mg/kg, IP, every other day) starting at postnatal day 10.
  • Cerebral Ischemia-Reperfusion (IR) Model: Wild-type and neuron-specific Ebp1 knockout (Ebp1 CKO) mice subjected to Middle Cerebral Artery Occlusion (MCAO).
• Pharmacokinetics (PK) & Toxicology:
  • Evaluated oral bioavailability, brain-to-plasma ratios, half-life, and safety (NOAEL) in mice, rats, and dogs.

3. Key Contributions & Mechanisms

The paper establishes a novel mechanism for selective mitophagy activation:

1. N-Degron Pathway as a Regulator: The study confirms that p62 functions as an N-recognin. The N-terminal arginine (Nt-Arg) generated by the enzyme ATE1 binds to the p62 ZZ domain, triggering p62 self-polymerization (via the PB1 domain) and autophagy activation.
2. ATB1071 Mechanism of Action:
   • ATB1071 mimics the Nt-Arg degron, binding directly to the p62 ZZ domain.
   • This binding induces conformational changes that promote p62 oligomerization and recruitment of LC3+ phagophores.
   • Selectivity: ATB1071 does not induce mitophagy under basal conditions but selectively targets damaged mitochondria (depolarized by OXPHOS inhibitors or genetic defects).
3. Dual Pathways for Mitophagy:
   • Parkin-Independent: In Ndufs4-/- models (LS), ATB1071 recruits p62 to damaged mitochondria via NIPSNAP1 and NIPSNAP2, which act as "eat-me" signals on the outer mitochondrial membrane (OMM). This pathway does not require Parkin or ubiquitination of NIPSNAPs.
   • Parkin-Dependent: In Cerebral IR injury, ATB1071 enhances mitophagy via the EBP1/PA2G4 substrate. EBP1 translocates to the OMM, is ubiquitinated by Parkin, and recruits p62.

4. Key Results

In Vitro Findings

• ATB1071 induced robust mitophagy in cells treated with mitochondrial stressors (rotenone, oligomycin, MPP+) but had minimal effect on healthy mitochondria.
• It activated p62 polymerization and LC3 lipidation without altering AMPK phosphorylation or mTOR inhibition, confirming it acts independently of the canonical AMPK-mTOR axis.
• Knockdown of ATE1, p62, NIPSNAP1/2, or EBP1 significantly abrogated ATB1071-induced mitophagy, validating the specific molecular dependencies.

In Vivo Findings: Leigh Syndrome (Ndufs4-/- Mice)

• Mitophagy Induction: ATB1071 treatment increased p62 oligomers and LC3+ puncta on mitochondria in the brain (cerebellum and olfactory bulb).
• Neuroprotection:
  • Reduced neuroinflammation (decreased GFAP+ astrocytes and Iba1+ microglia).
  • Downregulated inflammatory gene expression (e.g., Il1b, Cd14).
  • Restored mitochondrial membrane potential (MMP) and ATP levels.
• Functional Improvement:
  • Extended median survival by ~31% and maximal lifespan by ~64%.
  • Improved neuromuscular coordination (rotarod latency increased ~3-fold) and grip strength.
  • Delayed onset of clasping behavior and reduced hair loss associated with skin inflammation.

In Vivo Findings: Cerebral Ischemia-Reperfusion (IR)

• Dependence on EBP1: In Ebp1 CKO mice, the neuroprotective effects of ATB1071 were severely blunted.
• Therapeutic Efficacy (Wild-Type):
  • Reduced infarct volume by ~85%.
  • Reduced neuronal death (TUNEL+) in the hippocampal CA1 region by ~91%.
  • Restored motor function (rotarod, grip strength) and cognitive function (Y-maze working memory).
• Synergy: Reintroduction of EBP1 in Ebp1 CKO mice restored ATB1071 efficacy, confirming EBP1 is the critical scaffold for this pathway in IR injury.

Pharmacokinetics & Safety

• Bioavailability: Oral bioavailability of 70.4%; Brain-to-plasma ratio of 2.12 (67.9% BBB penetration).
• Safety: High therapeutic window with a NOAEL of 300 mg/kg in rats (TIW dosing) and no genotoxicity or cardiotoxicity observed.
• Dosing: Brain concentrations sufficient for mitophagy induction (~2.7 µM) are achievable with a 3 mg/kg oral dose.

5. Significance

• Novel Therapeutic Modality: ATB1071 represents the first chemical N-degron capable of inducing selective mitophagy via the p62 N-recognin pathway, bypassing the need for global mTOR inhibition.
• Clinical Potential: The compound's ability to cross the BBB, its oral bioavailability, and its high safety profile make it a strong candidate for clinical translation.
• Broad Applicability: The study demonstrates efficacy in two distinct pathological contexts: a genetic mitochondrial disease (Leigh Syndrome) and an acute vascular injury (Stroke), suggesting utility for a wide spectrum of mitochondrial disorders.
• Mechanistic Insight: The work redefines the role of p62 as a critical, non-redundant N-recognin in mitochondrial quality control and identifies NIPSNAP1/2 and EBP1 as essential adaptors for p62 recruitment under different stress conditions.

In conclusion, this paper provides a robust preclinical validation for ATB1071 as a disease-modifying therapy for mitochondrial neuropathies, offering a new paradigm for targeting damaged organelles with high specificity.