Mitochondrial 6mA Epigenetics Drives Adipose Fibrosis via Succinate‑Mediated HIF1α/NLRP3 Crosstalk

The mitochondrial epigenome, not its sequence, sets the tempo of adipose tissue aging.

Hypothesis

Chronic hypoxia in obese adipose tissue increases METTL4‑mediated N6‑methyldeoxyadenosine (6mA) methylation of mtDNA. This epigenetic mark suppresses mitochondrial transcription and shifts metabolite export toward succinate. Elevated succinate stabilizes HIF1α and activates the NLRP3 inflammasome, creating a feed‑forward loop that couples mitochondrial epigenetic silencing to both collagen cross‑linking (fibrosis) and inflammasome‑driven inflammation. Conversely, demethylation of mtDNA 6mA by ALKBH1 restores oxidative phosphorylation, lowers succinate, and attenuates both fibrotic and inflammatory phenotypes.

Mechanistic Model

• Hypoxia → METTL4 ↑ → mtDNA 6mA ↑ (cite [2])
• mtDNA 6mA ↑ → TFAM binding ↓ → OXPHOS ↓ → succinate efflux ↑
• Succinate ↑ → inhibition of prolyl hydroxylases → HIF1α stabilization → lysyl oxidase ↑ → collagen cross‑linking ↑ (cite [4]; succinate‑HIF1α link [5])
• Succinate ↑ → ROS production → NLRP3 inflammasome activation → IL‑1β/IL‑18 release ↑ (cite [7])
• ALKBH1 ↑ → mtDNA 6mA ↓ → TFAM binding restored → OXPHOS ↑ → succinate ↓ → HIF1α/NLRP3 ↓ → fibrosis and inflammation attenuated

Testable Predictions

1. Adipocyte‑specific METTL4 knockdown under hypoxic conditions will reduce mtDNA 6mA, increase mtDNA copy number, lower extracellular succinate, decrease HIF1α protein and lysyl oxidase activity, and diminish collagen deposition.
2. Conversely, METTL4 overexpression will exacerbate 6mA accumulation, succinate release, HIF1α lysyl oxidase signaling, and fibrotic markers even in normoxia.
3. Pharmacological inhibition of succinate export (e.g., with MCAT blockers) or HIF1α prolyl hydroxylase activation (e.g., DMOG) will blunt the fibrotic response despite high mtDNA 6mA.
4. Adipocyte‑specific ALKBH1 overexpression in obese mice will reduce mtDNA 6mA, normalize succinate levels, lower NLRP3 inflammasome activation (ASC speck formation, caspase‑1 cleavage), and improve both fibrosis and insulin resistance.
5. Succinate supplementation to METTL4‑deficient adipocytes will rescue HIF1α stabilization and lysyl oxidase induction, demonstrating sufficiency of the metabolite.

Experimental Approach

• Generate adipocyte‑specific METTL4 KO and ALKBH1 OE mice using Adipoq‑Cre.
• Subject mice to high‑fat diet (HFD) for 16 weeks to induce obesity‑associated hypoxia.
• Measure mtDNA 6mA levels by dot‑blot with 6mA‑specific antibody, mtDNA copy number by qPCR, and extracellular succinate by LC‑MS.
• Assess fibrosis via Sirius Red staining, lysyl oxidase activity assay, and collagen‑1 mRNA.
• Evaluate inflammasome activation by western blot for cleaved caspase‑1 and IL‑1β in adipose tissue lysates.
• Perform metabolic phenotyping (glucose tolerance, insulin tolerance) to link mechanistic changes to systemic outcomes.
• In parallel, isolate primary adipocytes, treat with hypoxia mimetic (CoCl₂) ± METTL4 siRNA or ALKBH1 adenovirus, and quantify succinate, HIF1α, lysyl oxidase, and NLRP3 readouts.

Potential Implications

If validated, this hypothesis reframes mitochondrial DNA not as a static aging script but as an epigenetic hub whose reversible modifications dictate adipocyte fate. It suggests that targeting the METTL4‑6mA‑succinate axis could uncouple mitochondrial dysfunction from fibrosis and inflammation, offering a therapeutic avenue distinct from nuclear genome editing. Such interventions would act upstream of both HIF1α‑driven ECM remodeling and NLRP3‑mediated inflammation, potentially ameliorating adipose tissue dysfunction without altering the nuclear DNA sequence.