Function Follows Form: Differential respiratory capacity of mitochondrial complex I mutants in C. elegans. M.J. Falk^1,2, J. Rosenjack³, M.A. O'Riordan², M.M. Sedensky^1,3, P.G. Morgan^1,3,4. 1) Depts of Genetics; 2) Pediatrics; 3) Anesthesiology; 4) Pharmacology, CASE SOM and Univ Hosp of Cleveland, Cleveland, OH.
   Nuclear gene mutations are postulated to be the major cause for mitochondrial oxidative phosphorylation disorders. However, the majority of causative genes are not yet identified and the mechanisms by which their dysfunction results in clinical disease are not known. The nematode, C. elegans, is a useful translational model in which to study the genetic and biochemical basis of mitochondrial dysfunction. We used this model to investigate whether respiratory dysfunction is differentially caused by particular nuclear DNA encoded components of mitochondrial complex I, the largest and most commonly implicated complex in human mitochondrial disease. At least 82% of the 39 nuclear genes encoding human complex I subunits share extensive homology in C. elegans. Utilizing a feeding RNA interference gene knockdown approach, we generated C. elegans mutants for 13 complex I structural subunits. RNA knockdown was confirmed by qRT-PCR. Integrated respiratory capacity of freshly isolated mitochondria from complex I mutants was assayed by polarography. We show that individual nuclear-encoded complex I subunits vary significantly in their impact on integrated complex I-dependent oxidative phosphorylation capacity. Reactive oxygen species damage, as assayed by mitochondrial protein 4-hydroxynonenol antibody staining, also varies by subunit. Importantly, function appears to follow form. Subunits with the greatest impact on respiratory capacity appear to localize to subcomplexes involved in electron transport through complex I. In contrast, subunits known to localize to the membrane-bound arm of complex I minimally impact respiratory capacity. This model organism approach is useful to aid in the rational identification of candidate gene subsets to investigate in human patients with biochemical evidence of mitochondrial complex I dysfunction. It further permits investigation into mechanisms by which individual nuclear genes, when mutated, contribute to human mitochondrial disease.