SUMMARY The assembly and function of the mitochondrial respiratory chain (RC) involve the organization of RC enzyme complexes in supercomplexes or respirasomes through an unknown biosynthetic process. This leads to structural interdependences between RC complexes, which are highly relevant from biological and biomedical perspectives, because RC defects lead to severe human disorders. We show that in human cells, respirasome biogenesis involves a complex I assembly intermediate acting as a scaffold for the combined incorporation of complexes III and IV subunits, rather than originating from the association of preassembled individual holoenzymes. The process ends with the incorporation of complex I NADH dehydrogenase catalytic module, which leads to the respirasome activation. While complexes III and IV assemble either as free holoenzymes or by incorporation of free subunits into supercomplexes, the respirasomes constitute the structural units where complex I is assembled and activated, thus explaining the functional significance of the respirasomes for RC function.

For decades mitochondria have been considered static round shaped organelles in charge of energy production. On the contrary, they are highly dynamic cellular components that undergo continuous cycles of fusion and fission influenced, for instance, by oxidative stress, cellular energy requirements, or the cell cycle state. New important functions beyond energy production have been attributed to mitochondria, such as the regulation of cell survival due to their role in the modulation of apoptosis, autophagy and aging. Primary mitochondrial diseases due to mutations in genes involved in these new mitochondrial functions, and the implication of mitochondrial dysfunction in multifactorial human pathologies like cancer, Alzheimer's and Parkinson's diseases, or diabetes has been demonstrated. Therefore, mitochondria are set at a central point of the equilibrium between health and disease, and a better understanding of mitochondrial functions will open new fields to explore the role of these mitochondrial pathways in human pathologies. The present review will dissect the relationships between the activity and assembly defects of the mitochondrial respiratory chain, oxidative damage, and alterations in mitochondrial dynamics, with special focus at their implications in neurodegeneration.

No abstract