Respiratory complex III₂ assembles complex I via toxic intermediate in mitochondrial disease.

Area of Science:

• Biochemistry
• Mitochondrial Biology
• Molecular Medicine

Background:

• Mutations in mitochondrial complex I lead to severe metabolic disorders.
• Leigh syndrome, a mitochondrial disease, results from NDUFS4 mutations.
• Current treatments for complex I deficiencies are lacking, but chronic hypoxia shows promise in mouse models.

Purpose of the Study:

• To elucidate the molecular mechanisms behind NDUFS4 mutant pathophysiology.
• To understand how chronic hypoxia rescues function in complex I deficiencies.
• To investigate the structural basis of complex I assembly and dysfunction.

Main Methods:

• Isolation of respiratory supercomplexes from NDUFS4 mutant mice.
• Structural analysis of complex I assembly intermediates.
• Investigation of oxygen-dependent reverse electron transfer pathways.

Main Results:

• Complex I assembly intermediates were identified bound to complex III2, supporting a cooperative assembly model.
• An accumulated complex I intermediate is consistent with pathological reverse electron transfer.
• Pathophysiology is linked to the accumulation of toxic intermediates, not solely reduced complex I levels.

Conclusions:

• The accumulation of toxic intermediates is a key driver of mitochondrial complex I deficiency diseases.
• Pathological reverse electron transfer contributes to NDUFS4 mutant pathophysiology.
• Chronic hypoxia may offer a therapeutic strategy by mitigating toxic intermediate accumulation.