We explored the role of low mitochondrial membrane potential (⌬⌿m) and the lack of oxidative phosphorylation in apoptosis by assessing the susceptibility of osteosarcoma cell lines with and without mitochondrial DNA to staurosporine-induced death. Our cells without mitochondrial DNA had low ⌬⌿m and no functional oxidative phosphorylation. Contrary to our expectation, these cells were more resistant to staurosporine-induced death than were the parental cells. This reduced susceptibility was associated with decreased activation of caspase 3 but not with the mitochondrial permeability transition pore or cytochrome c release from the mitochondria. Apoptosis in both cell lines was associated with an increase in ⌬⌿m. Bcl-x L could protect both cell types against caspase 3 activation and apoptosis by a mechanism that does not appear to be mediated by mitochondrial function or modulation of ⌬⌿m. Nevertheless, we found that Bcl-x L expression can stimulate cell respiration in cells with mitochondrial DNA. Our results showed that the lack of functional oxidative phosphorylation and/or low mitochondrial membrane potential are associated with an antiapoptotic effect, possibly contributing to the development of some types of cancer. It also reinforces a model in which Bcl-x L can exert an antiapoptotic effect by stimulating oxidative phosphorylation and/or inhibiting caspase activation.

The co-evolution of nuclear and mitochondrial genomes in vertebrates led to more than 100 specific interactions that are crucial for an optimized ATP generation. These interactions have been examined by introducing rat mtDNA into mouse cells devoid of mitochondrial DNA (mtDNA). When mtDNA-less cells derived from the common mouse (Mus musculus domesticus) were fused to cytoplasts prepared from Mus musculus, Mus spretus, or rat (Rattus norvegicus), a comparable number of respiring clones could be obtained. Mouse xenomitochondrial cybrids harboring rat mtDNA had a slower growth rate in medium containing galactose as the carbon source, suggesting a defect in oxidative phosphorylation. These clones respired approximately 50% less than the parental mouse cells or xenomitochondrial cybrids harboring Mus spretus mtDNA. The activities of respiratory complexes I and IV were approximately 50% lower, but mitochondrial protein synthesis was unaffected. The defects in complexes I and IV were associated with decreased steady-state levels of respective subunits suggesting problems in assembly. We also showed that the presence of 10% mouse mtDNA co-existing with rat mtDNA was sufficient to restore respiration to normal levels. Our results suggest that evolutionary distance alone is not a precise predictor of nuclear-mitochondrial interactions as previously suggested for primates.Nuclear-mitochondrial interactions play a fundamental role in cellular homeostasis. The nuclear genome encodes more than 95% of all proteins located in the mitochondria, whereas only 13 polypeptides (all subunits of the oxidative phosphorylation system, OXPHOS) are encoded by the mitochondrial genome. An optimal interaction between nuclear and mitochondrial encoded factors is essential for transcription and translation of mitochondrial DNA (mtDNA) 1 and also for the correct assembly and function of the OXPHOS system (1). Various attempts have been made to understand the interactions between the nuclear and mitochondrial genomes and their respective contributions to the expression of different phenotypes such as tumorigenicity (2, 3) and cell differentiation (4, 5). One approach that has facilitated the study of nuclear-mitochondrial interactions is the construction of interspecific hybrids and cybrids. Because the mtDNA sequence divergence is about 5-13 times more rapid than in nuclear DNA (nDNA), a general incompatibility between nuclear and mitochondrially coded gene products is expected between pairs of even recently diverged taxa (6).Previously, Kenyon and Moraes (7) demonstrated that OX-PHOS function of a human cell line lacking mtDNA ( 0 ) could be restored by inserting mitochondria from other humanoid primates including common chimpanzee (Pan troglodytes), pigmy chimpanzee (Pan paniscus), or gorilla (Gorilla gorilla). These studies suggested that at least in primates, mitochondrial/nuclear compatibility has been retained over approximately 5-12 million years (Myr) period. On the other hand, mtDNA from orangutan (Pongo pygmaeus), a species that div...

Cytochrome c oxidase (COX) deficiency is one of the major causes of Leigh Syndrome (LS), a fatal encephalopathy of infancy or childhood, characterized by symmetrical lesions in the basal ganglia and brainstem. Mutations in the nuclear genes encoding COX subunits have not been found in patients with LS and COX deficiency, but mutations have been identified in SURF1. SURF1 encodes a factor involved in COX biogenesis. To date, 30 different mutations have been reported in 40 unrelated patients. We aim to provide an overview of all known mutations in SURF1, and to propose a common nomenclature. Twelve of the mutations were insertion/deletion mutations in exons 1, 4, 6, 8, and 9; 10 were missense/nonsense mutations in exons 2, 4, 5, 7, and 8; and eight were detected at splicing sites in introns 3 to 7. The most frequent mutation was 312_321del 311_312insAT which was found in 12 patients out of 40. Twenty mutations have been described only once. We also list all polymorphisms discovered to date.

We report a case of neonatal congenital lactic acidosis associated with pyruvate dehydrogenase E3-binding protein deficiency in a newborn girl. She had a severe encephalopathy, and magnetic resonance imaging of the brain showed large subependymal cysts and no basal ganglia lesions. She died 35 days after birth. We detected a novel homozygous deletion (620delC) in the PDX1 gene, which encodes for the E3BP subunit of the pyruvate dehydrogenase complex.

Respiring mitochondria require many interactions between nuclear and mitochondrial genomes. Although mitochondrial DNA (mtDNA) from the gorilla and the chimpanzee are able to restore oxidative phosphorylation in a human cell, mtDNAs from more distant primate species are functionally incompatible with human nuclear genes. Using microcell-mediated chromosome and mitochondria transfer, we introduced and maintained a functional orangutan mtDNA in a human nuclear background. However, partial oxidative phosphorylation function was restored only in the presence of most orangutan chromosomes, suggesting that human oxidative phosphorylation-related nuclear-coded genes are not able to replace many orangutan ones. The respiratory capacity of these hybrids was decreased by 65%-80%, and cytochrome c oxidase (COX) activity was decreased by 85%-95%. The function of other respiratory complexes was not significantly altered. The translation of mtDNA-coded COX subunits was normal, but their steady-state levels were approximately 10% of normal ones. Nuclear-coded COX subunits were loosely associated with mitochondrial membranes, a characteristic of COX assembly-defective mutants. Our results suggest that many human nuclear-coded genes not only cannot replace the orangutan counterparts, but also exert a specific interference at the level of COX assembly. This cellular model underscores the precision of COX assembly in mammals and sheds light on the nature of nuclear-mtDNA coevolutionary constraints.