Failing mitochondria trigger a peroxisome response that backfires and fuels cellular fat buildup

Why it matters: Mitochondria and peroxisomes share the job of breaking down fats, and each depends on the other. Complex I of the respiratory chain is the most frequent site of mitochondrial disease defects, including Leigh syndrome, a severe paediatric encephalopathy with no cure. Fat accumulation in lipid droplets is a recurring feature of mitochondrial dysfunction, but it is unknown whether it helps or harms cells, and how peroxisomes respond has been poorly characterised.

What was studied: The team used mouse embryonic fibroblasts lacking NDUFS4, an established model of complex I deficiency. They confirmed reduced respiratory chain activity, complex I activity and mitochondrial membrane potential, with unchanged mitochondrial hydrogen peroxide and morphology. They then combined whole-cell lipidomics, a triglyceride assay, BODIPY imaging of lipid droplets, immunoblotting of fatty acid metabolism enzymes, and gene expression analysis. They also used an ACLY inhibitor (SB204990) and a very-long-chain fatty acid challenge to probe peroxisomal function.

Key results: Lipidomics separated knockout from wild-type cells clearly, with 83 lipid species increased and 30 decreased. Triacylglycerols were enriched and every significantly changed TAG species rose, which the triglyceride assay confirmed. Cholesteryl esters were lower, consistent with the group's earlier finding of reduced cholesterol synthesis in respiratory-deficient cells. Lipid droplet number was unchanged, but average droplet area increased. ACSL1, CPT-I and CPT-II protein levels were lower, while most fatty acid catabolism genes were not significantly changed (ACADVL was slightly decreased), suggesting post-translational regulation. The fatty acid pool showed selective accumulation of polyunsaturated fatty acids with four or more double bonds and of C22 very-long-chain fatty acids, and TAGs shifted toward longer, more unsaturated species. According to the abstract and graphical abstract, peroxisome numbers rose but β-oxidation markers (ABCD transporters, ACOX1) fell, acylcarnitines accumulated, and ATP production dropped. When cells were overloaded with a very-long-chain fatty acid, peroxisomes proliferated, but compromising peroxisomal β-oxidation prevented lipid droplet expansion.

Implications: The findings suggest that peroxisomes try to compensate for failing mitochondrial fat oxidation but cannot, so lipids are shunted into storage. Functional peroxisomal processing appears necessary for lipid droplet remodelling and for supporting mitochondria. This points to peroxisomal fatty acid handling as a possible therapeutic target in mitochondrial disease.

Caveats: The work is in one mouse fibroblast model with relatively small numbers of independent experiments (for example, N=4 for lipidomics). It does not test whether peroxisomal changes occur in patients or in whole organisms. This summary draws on the abstract and the portions of the text available, which covered the lipid and mitochondrial results in detail. Peroxisome-specific experiments are described mainly from the abstract.