The alternative enzymes-bearing tunicates lack multiple widely distributed genes coding for peripheral OXPHOS...

Area of Science:

• Mitochondrial respiration
• Evolutionary biology
• Biochemistry

Background:

• Alternative enzymes (AEs) NDX and AOX from Ciona intestinalis offer potential for mitochondrial disease bypass therapies.
• These AEs are absent in vertebrates and insects, prompting investigation into tunicate respiratory chain composition.
• Mitochondrial diseases arise from defects in respiratory chain complexes I, III, and IV.

Purpose of the Study:

• To investigate the unique composition of the tunicate respiratory chain and its accommodation of alternative enzymes (AEs).
• To assess the functional interaction of Ciona AEs with the coenzyme Q pool in heterologous systems.
• To evaluate the applicability of Ciona AEs for human bypass therapies for mitochondrial diseases.

Main Methods:

• Xenotopic expression and characterization of Ciona NDX and AOX in human cells, Drosophila melanogaster, and mouse.
• Bioinformatic analysis of tunicate genomes to identify orthologous respiratory chain subunits.
• Structural modeling of Ciona respiratory chain polypeptides.
• Functional assays using Ciona AOX expressed in Drosophila to determine coenzyme Q pool accessibility.

Main Results:

• Fifteen orthologous respiratory chain subunit genes, crucial for complex interactions and supercomplexes (SCs) in mammals, were absent in tunicate genomes.
• Structural modeling revealed significant divergence in Ciona respiratory chain subunits, consistent with the absence of these orthologs.
• Ciona AOX, when expressed in Drosophila, could not access coenzyme Q reduced by Complex I but could oxidize coenzyme Q reduced by glycerophosphate oxidase (a non-SC dehydrogenase).

Conclusions:

• Ciona AEs likely evolved in a distinct mitochondrial inner membrane environment, potentially lacking supercomplexes (SCs).
• The functional interaction of Ciona AEs with non-SC dehydrogenases in heterologous systems supports their unique evolutionary path.
• Findings impact the understanding of animal respiratory chain evolution and the potential application of Ciona AEs in human mitochondrial disease therapies.